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Extraction characteristics of heavy metals from marine sediments
Accepted Manuscript Extraction Characteristics of Heavy Metals from Marine Sediments Jong-Chan Yoo, Cha-Dol Lee, Jung-Seok Yang, Kitae Baek PII: DOI: Reference:
S1385-8947(13)00648-7 http://dx.doi.org/10.1016/j.cej.2013.05.029 CEJ 10762
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
29 January 2013 3 May 2013 10 May 2013
Please cite this article as: J-C. Yoo, C-D. Lee, J-S. Yang, K. Baek, Extraction Characteristics of Heavy Metals from Marine Sediments, Chemical Engineering Journal (2013), doi: http://dx.doi.org/10.1016/j.cej.2013.05.029
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Extraction Characteristics of Heavy Metals from Marine Sediments Jong-Chan YooaㆍCha-Dol LeeaㆍJung-Seok YangbㆍKitae Baeka* 5
a
Department of Environmental Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeollabuk-do, Republic of Korea b
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KIST-Gangneung Institute, Gangneung, Gangwon-do 210-340, Republic of Korea
* Corresponding author Tel.: +82-63-270-2437; Fax +82-63-270-2449- ; E-mail address: [email protected] (K. Baek)
ABSTRACT In this study, the feasibility of a washing process to extract Cd, Cu, Ni, Pb, and Zn from three real 15
dredged marine sediments (Namhang (NH), Bangeojin (BE), and Haengam (HA) bay sediments, South Korea) were investigated using various washing agents (ethylenediaminetetraacetic acid (EDTA), NaCl, HCl, sodium citrate, and HNO3). Even though the removal efficiencies of heavy metals were not high, which is thought to be largely due to the high concentrations of Fe and Ca, EDTA was a more efficient extraction agent for heavy metals compared to the other agents, because
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EDTA formed more stable complexes with metals. EDTA removed : 30%, 43%, and 9% of Cu; 48%, 66%, and -8% of Pb; and 31%, 60%, and -14% of Zn from NH, BE, and HA sediments, respectively. Additionally, fractionation analysis showed that EDTA converted the strongly bound fractions of Cu and Pb to an exchangeable fraction. Based on the experimental results, the washing technique using EDTA is determined to be efficient for the removal Cu, Pb, and Zn from NH and BE sediment
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samples. On the contrary, residual concentrations of heavy metals were not decreased from HA samples compared to extraction with washing solution because of the high content of fine particles and organic matter in samples. Furthermore, the fractionations of metals highly influenced the chemical extraction of metals in sediments, and the fractionation changes after chemical washing increased the total extractable amount of metals. As a result, the fractionation change caused changes
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in the bioavailability and potential mobility of metals.
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Keywords: marine sediment; chelation; complexation; fractionation; heavy metals; 1. INTRODUCTION 35
The contamination of sediments by heavy metals is a prevalent issue today, due highly to industrial growth and the expansion of key human activities. Marine sediments, both coastal and open-ocean, are polluted by heavy metals originating from ships, dockyards, rivers, and harbor facilities, mainly concentrated around ports, harbors, and estuaries. Even a low load of pollutants in marine sediments can cause secondary environmental pollution through causing various environmental changes in
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aqueous systems. Furthermore, such pollution can negatively impact human health and ecosystems through a range of accumulatory processes within the food chain [1, 2]. In Korea, marine sediments in many areas are dredged in order to maintain a minimum depth of water, and huge amounts of dredged materials have been dumped further out to sea without undergoing treatment. Dredged sediments must be treated before disposal inland in order to protect public health, and this has now become mandatory
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for all dredged sediments, as the London and Basel conventions restrict ocean dumping [3, 4]. Measures prohibiting the ocean dumping of such wastes will be instigated in Korea from 2014. Consequently, it is advisably to start preparing treatment techniques for dredged marine sediments which are to be disposed inland or which are to attain approval for dumping at sea. Generally, the physical and chemical characteristics of marine sediments are considerably different
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from those of inland soil. Dredged marine sediments contain very high proportions of organic matter, carbonates, sulfides, and chlorides [4, 5]. Also, sediments have comparatively large contents of fine particles, such as silt and clay, and exhibit high alkalinity values. These unique characteristics directly influence the mobility of heavy metals [6]. Organic matter combines with heavy metals, forming metal–organic complexes which are very stable [6, 7]. The high proportion of carbonates contributes
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to the high alkalinity and buffering capacity of the sediments, and as a consequence, large amounts of acids are generally required to extract the metals. Hence, sediments contaminated with heavy metals are very difficult to treat by applying normal soil remediation techniques. The treatment alternatives (alternatives to landfilling) for metal-contaminated dredged sediments include: solidification/stabilization [6]; bio-chemical stabilization; soil washing (including physical 2
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separation, chemical leaching/washing, and bioleaching) [1]; electrokinetic remediation [4]; and thermal treatment for Hg [6]. Stabilization/solidification technology makes the contaminants more stable and reduces their solubility, thus, reducing their mobility. In Korea, however, the legal regulations relating to contaminants are based on pseudo-total concentrations, as extracted by aqua regia. Even though stabilization/solidification has been widely used to treat metal-contaminated soil
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[8] , it may not meet regulation levels after application [9, 10]. Soil/sediment washing technologies include chemical extraction, bioleaching procedures, and physical separation methods, or a combination of such methods. Among the washing processes which incorporate physical separation, flotation technology appears to be a suitable technique for the removal of metals from fine-grained matrices such as dredged sediments [11]. Some sediment
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washing techniques which apply flotation for the removal of metals, have been studied on a laboratory scale [12, 13], some of which have been applied on a field-scale [14]. Also, sediment washing via bioleaching and biosurfactant extraction has been investigated [15, 16]. Soil/sediment washing is a suitable technique for the removal of contaminants such as lead, copper, nickel, zinc, cadmium, and chromium [17-20]. Most contaminants are bound to finer particles in a soil or sediment, and washing
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techniques separate the finer particles from the sediments, substantially decreasing the contamination through physical separation of the most heavily contaminated component, dramatically increasing the efficiency of further treatment processes applied to the coarser particles [21]. In the soil/sediment washing technique, the proper selection of washing agents is very important to improve the extraction efficiency. Chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and sodium citrate, form
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strong metal–ligand complexes and enhance the extractability of metals from contaminated soil/sediments [1]. HCl and HNO3 are known as effective strong acids to extract heavy metals from soil/sediments [8]. Additionally, it is widely thought that chloride might form various metal-chloride complexes, which can enhance the extractability of metals. Sea water contains 3.5% NaCl and might contain sufficient chloride for the formation of metal-chloride complexes. Therefore, sea water could
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be applied to treat metal-contaminated marine sediments as NaCl is an effective extracting agent for metals. 3
Generally, heavy metals are bound to soil/sediment in a variety of forms, all of which are closely related to the bioavailability of metals. The SM&T (formerly BCR) three-step sequential chemical extraction procedure has been widely used with Tessier’s method for analysis of the fractionations of 90
metals in soil/sediment samples [22-24]. The SM&T (formerly BCR) three-step sequential chemical extraction procedure has been designed to target the soluble and exchangeable fraction (F1), reducible fraction (F2), and the oxidizable fraction (F3), leaving the residual fraction (F4). The sequential chemical extraction procedure separates each fraction based on three different extractants, and it is a suitable means of evaluating the remediation economics of sediments contaminated with heavy metals.
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The efficiency of metal removal from artificially contaminated marine sediment has previously been investigated, where the initial contamination level was too low to draw significant conclusions [25]. Di Palma et al. [1] reported that the extraction efficiencies of Cu and Pb by EDTA washing/extraction from Italian harbor sediments were higher than those by citric acid washing/extraction. Nystroem et al. [26] reported that a high concentration of 0.6M HCl was most effective for the extraction of heavy
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metals from Norwegian harbor sediment through chemical extraction. In addition, NaCl could extract 20% of Pb and 55% of Cd from harbor sediment, while it was unable to extract Cu and Zn. Marine sediments have been deposited slowly on the ocean floor over many years, and have unique characteristics of containing a high proportion of carbonates, sulfides, and organic matter. Additionally, aging is a key factor which influences metal extractability. Hence, it is necessary to
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investigate the applicability of various remediation techniques, while using specific grades of metalcontaminated marine sediments. Furthermore, the mobility and bioavailability of the metals, as well as their total concentration, should be considered in order to accurately assess the potential risk of metal-contaminated sediments. However, most researchers have merely focused on the extraction and removal efficiencies of metals.
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In this study, therefore, the washing characteristics of heavy metals from three real marine sediments were investigated, using various washing agents, such as EDTA, NaCl, HCl, sodium citrate, and HNO3. We evaluated the potential mobility and bioavailability of heavy metals based on the change in metal fractionation after washing procedure. 4
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2. MATERIALS AND METHODS 2.1. Characterization of sediments Samples were collected from three different harbor sediments located in; Namhang (NH) of Busan city, Bangeojin (BE) of Ulsan city, and Haengam bay (HA) of Jinhae city, South Korea. Table 1 shows the physico-chemical characteristics of the sediments. After oven-drying at 75 °C for 24 h, samples
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were pulverized using a mortar and pestle, and were then sieved through a 2mm (size 10) mesh. The concentration of metals was then measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent, 720ES, USA) after aqua regia extraction. As the concentration of metals extracted by aqua regia is not the total concentration contained in the sediment, but a standardized extractable fraction of the pseudo-total concentration in Korea [3], the total heavy metals
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concentration was analysed compared with X-ray fluorescence (XRF) and a total metal digestion (incorporating HNO3, HClO4 and HF) (Table 2). The exchangeable cation concentration was extracted using 1N ammonium acetate, and was then adjusted to pH 7 and mixed to a L/S (liquid and solid) ratio of 1:10. The cation exchange capacity (CEC) was determined by Method 9081 (SW-846 USEPA Method 9081) [27].
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The sediment pH was determined according to the Korean Standard Test Method (KSTM) as follows: A 5:1 ratio of DI water to-sediment was shaken for 1 h at room temperature, and was then allowed to stand. The mixture was filtered using 5B filter paper, and the pH of the filtrate was measured using a pH/ISE meter (Istek 735P, Korea). The soil texture and organic matter were determined according to ASTM D421 and ASTM D2974, respectively. The carbonate content was
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determined by the calcination method at 900°C [7]. In addition, the surface morphology of NH, BE, and HA sediments were analyzed using a field emission scanning electron microscope and energy dispersive spectroscopy (FESEM-EDS, JEOL, JSM-6701F, Japan) (Fig.1). Figure 1 shows the SEM-EDS analysis results of NH, BE, and HA marine sediment surfaces, respectively. The sediments contain high portions of Al and Si as aluminosilicate minerals are a basic
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component of soil. Additionally, all of the samples have a high portion of carbon, which is likely 5
heavily influenced by the carbonate minerals therein. Data for Na and Mg were included in the composition because they contacted with sea water.
2.2. Sediment washing experiments 145
Sediment washing experimental conditions are shown in Table 3. Solutions of EDTA (Ethylenediaminetetraacetic acid; C10H16N2O8), sodium chloride (NaCl), hydrochloric acid (HCl), sodium citrate (Na3C6H5O7), or nitric acid (HNO3) at a concentration of 0.1 M were used as washing agents. Sediments (20 g) dried at 75°C for 24 h were mixed with the washing solution (60 mL) in a flask, and the mixture was then shaken in a horizontal shaker at 150 rpm at 20°C. After a designated
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time, from 6 to 120 h, the samples were removed and filtered through 5B filter paper. The filtrate was then analyzed for its metal content. The metals in the sediment residue were further extracted by the aqua regia method, and the extractants were analyzed for heavy metals. All experiments were carried out in triplicate. The SM&T (formerly, BCR) three-step sequential extraction procedure was used to evaluate the fractionation changes of metals in the sediment.
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The method provides four fractionation results: soluble and exchangeable (F1) using 0.11M of acetic acid for 16 h, bound to Fe-Mn oxide (F2) using 0.5 M of hydroxylammonium chloride for 16 h, bound to organic matter and sulfides (F3) using 8.8 M of hydrogen peroxide at 85 ± 2°C then 1 M of ammonium acetate for 16 h, and residual (F4) by aqua regia [22, 28]. Table 4 shows the three-step sequential extraction procedure schemes.
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3. RESULTS AND DISCUSSIONS 3.1. pH Change Figure 2 shows the pH changes of the solutions during the washing experiments. The initial pH values of extracting solution for each experiment were shown in Table 3. The initial sediment pH of 165
the NH, BE, and HA were 7.4, 8.6 and 8.1, respectively. In all the experiments, the pH changed sharply within 6 h, and then stabilized, which indicates that the reactions between the sediment and 6
washing agents were completed within that time period. In experiments with EDTA, NaCl and sodium citrate, the change in pH value was negligible over time, because those agents are weakly acidic or neutral. In the BE sediment, however, the pH of the EDTA extraction increased to natural conditions. 170
In the HA sediment, however, the equilibrium pH increased gradually through extraction with HNO3, due to the higher carbonate content in the HA sediment. In the cases of HCl and HNO3 extractions, the strongly acidic pH increased instantly due to the high alkalinity of the sediments and the high carbonate content. The strong acid enhanced the dissolution of alkaline and carbonate matter. Neale et al. reported that 0.1M HCl and HNO3 extracted about 99% of Pb under strong acidic pH, which the
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weak acidic solution removed only 1.5-8.6% of Pb from soils, due to the high buffering capacity of soil. In general, the pH of NH and HA sediments was sharply increased to neutral or weakly acidic condition upon treatment with acid, which indicates that the equilibrium pH of the solution increased because NH and HA sediments have a greater buffering capacity compared to BE sediment. Generally, the extraction of metals is enhanced in acidic conditions as to the large amount of H+ increases the
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solubility of sediment minerals, as well as the ion-exchange reaction between the solution and the sediment. Elliott et al. reported [29] that solution pH strongly influenced the extraction of Pb from soils, as the extraction of Pb was negligible below pH 6 -7, with a 0.08 M of EDTA concentration. Based on the experimental results, it was expected that strong inorganic acids would extract more metals from sediments compared to the other extractants.
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3.2. Metal removal and fractionations In order to calculate the speciation distribution of heavy metals in the presence of washing agents, a simulation, using Visual MINTEQ [26], was carried out. The Visual MINTEQ program is useful for prediction of the equilibrium distributions of chemicals in the aqueous systems to apply various input 190
parameters such as chemical species included organic components, concentrations, pH, temperature, etc. Because of the complicated characteristics of the sediment, we simulated the speciation using simplified conditions. In the simulation, the composition of sea water, washing agents (0.1 M), and 7
solution pH were considered, based on the washing experiments. The speciation changes were 195
calculated according to the equilibrium pH. Divalent metals (M) might combine with EDTA, forming [M–EDTA]2-, [MH2–EDTA], [MH– EDTA]- and [MOH–EDTA]3-. Among the various forms of metal–EDTA complexes, the [Me– EDTA]2- form was predominant, considering the pH of the washing experiments. Chloride ions could be combined with metals, producing complexes such as [MCl]+, [MCl2(aq)], [MCl3]-, and [MCl4]2-,
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where the [MCl]+ form was predominant at the specified washing solution pH. Of the citrate forms, [M–(citrate)2]4-, [M–(citrate)]-, [MH–citrate(aq)] and [MH2–citrate]+, the [M–(citrate)2]4- form was the major species in the washing solution. In case of Cd, Pb, and Zn, the proportion of [MCl]+ was similar to that of [M–EDTA]2- below pH 1.5, however, [M–EDTA]2- was predominant above pH 1.5. In contrast, in the cases of Cu and Ni, [M–EDTA]2- was predominant throughout all pH ranges. These
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results were associated with the stability constants of metal–ligand complexes. Table 5 shows the stability constants of each metal–ligand complex [30]. Generally, the stability constants of metal– ligand complexes are in the order EDTA > citrate > chloride, where the higher constants indicate greater stability. Even though the stability constant of Zn-EDTA is relatively low, large amounts of Zn were extracted from sediments because of the higher initial concentration of Zn compared to other
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metals. Although the amounts of extracted metals are highly dependent on their initial concentrations, EDTA showed the highest extraction efficiency of all heavy metals for all sediments. The initial concentration of target heavy metals in dredged marine sediments was evaluated by applying three different analytical methods, as described in Table 2, and the determined concentrations were compared with guideline values. In Korea, guidelines for heavy metals in
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sediments for ocean dumping are determined by a total digestion using HNO3, HClO4, and HF, which was carried out, along with a total metals determination using a strong acid digestion. The concentration based on the total digestion of Cu, Zn and Pb exceeded the guideline value for ocean dumping, however, based on the aqua regia extraction, the metals concentration were less than the guideline for soil quality, and the dredged sediments could be recycled without any further treatment
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after washing with EDTA. Necessarily, such inconclusive methods for setting legislative limits are not 8
a sufficient method of protecting our oceans, as the mobility of metals can be changed in marine environments, influencing the fractionation of metals. Though the original sediments are under reducing conditions, dredged sediments have been exposed to oxidizing conditions, which directly increases the leaching potential for further fractionation and mobility of heavy metals. As a result, the 225
fractionation change affects the amount of aqua regia extractable metals. In addition, significant differences in the amount of metals extracted by aqua regia were observed between replicate samples, due to the severe heterogeneity, which indicated that the extractability of metals by aqua regia is highly dependent on the amount of organic matter, carbonate content, buffering capacity, and fractionation of metals. Therefore, we further discussed metal removal based on the extracted amount
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of metals due to the of severe variability initial metal concentration. Figures 3 and 4 show the extraction kinetics of heavy metals in the sediment, using various chemical extractants. Furthermore, the fractionations of metals before and after the washing experiment using Na2EDTA are presented in Figure 5. Generally, washing was not effective for the extraction of Cd and Ni from sediments, and the
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residual concentrations of Cd and Ni in the sediments were almost constant during the experiments. Cd and Ni extraction by Na2EDTA increased sharply until 6 h and then reached a plateau. However, the Cd and Ni extracted into solutions were negligible amount compared to the total, as concentrations determined by aqua regia extraction. In all sediments, the fraction 2 (Fe/Mn bound fraction, F2) decreased and the residual fraction (F4) slightly increased after extraction by Na2EDTA. It was
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especially difficult to extract Ni from sediments. The reason for this is the strength of the binding. It is well known that chelating agents can extract more metals under acidic conditions than under neutral or alkaline pH condition. Begum et al. [31] reported that 0.5M EDTA under an acidic condition (pH 4) extracted 90% of Cd and 80% of Pb, however, it was difficult to extract Ni compared to other metals, because it was more strongly bound to particles within the sediment [31, 32].
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The residual concentrations of Cu, Pb and Zn after extraction with Na2EDTA decreased dramatically until 6 h in NH and BE sediments. At the same time, the concentration of metals in solution increased sharply until 6h, after which it stabilized without any significant change. On the contrary, the 9
concentrations of metals did not decrease in residual HA sediment. The reason for this might be that the contents of fine particles, carbonate and organic matter in HA sediment are higher than those of 250
NH and BE sediments. In all of the samples, a large amount of Cu was bound to the organic/sulfide fractions (F3) on soils/sediments, and Cu was continuously extracted into solution until 120 h in the BE sediment. However, the organic/sulfide bound fraction (F3) of Cu in BE sediment did not change highly after washing with Na2EDTA, compared to NH and HA sediments. This indicates that metalorganic complexes were destroyed during washing, and especially, BE sediments had relatively lower
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organic contents compared to NH and HA sediments. The residual concentrations of Pb and Zn from NH and BE sediments sharply decreased until 6 h upon application of Na2EDTA, then reaching a plateau. The concentration of Zn in solution from BE sediment sharply increased until 6 h, as a large amount of Zn was bound to F1 in the initial sediment. Most Pb and Zn was extracted from the BE sediment due to its high sand content, and relatively low
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organic matter content. The residual concentration of Pb and Zn in the HA sediment did not decreased as much as the concentration increased in solution, which was a major source of error in the mass balance. This was the reason for the gap of initial concentration of Pb and Zn between aqua regia analysis and the increase in the residual fraction of Zn after the washing experiment. Figure 6 shows the overall mass balance during sediment washing experiments. The mass balance of the metals,
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except for Pb in the HA sediment using Na2EDTA, ranged from 70 to about 130%, compared to the initial metals concentration. Pb was extracted into solution by Na2EDTA as a result of a fractional change to the soluble, exchangeable, and carbonated fractions (F1). However, this can increase the potential risk of environment problems and human health by redox reactions as surrounding environmental changes though increasing F1. In particular, a significant amount of Pb was extracted
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by EDTA, because EDTA has a great affinity for Fe(III), and most Pb was fractionated on Fe–Mn oxides [33]. Figure 7 shows the correlation between Pb and Fe extracted into Na2EDTA solution in the HA sediment. This result indicates that the extraction of Pb is strongly related to the extraction of Fe, and the molar ration of Pb/Fe during the extraction was almost constant. Therefore, a large amount of Pb bound to F2 could be extracted into solution through the extraction of Fe. 10
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Citrate, an organic acid salt, is a trivalent anionic non-toxic chelating agent. It forms metal–citrate complexes, which are weak compared to those of EDTA. This was the reason for the relatively low removal of heavy metals when sodium citrate was used as a washing agent. The chelating agent desorbed heavy metals from the sediment and formed strong metal–ligand complexes. The stability constants with metals might be indicators for the complexation affinity between metals and ligands
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(Table 5). Ca and Fe might compete with heavy metals for ligand complexation, and decrease the complexation of heavy metals [32, 34]. In the case of heavy metals bound to F2, however, they could be extracted by dissolution reaction of Fe via ligand complexation. Actually, citrate was able to extract Fe from sediments about 3 – 4 times more efficiently than EDTA (Figure 8). In addition, the order of extraction efficiency of heavy metals is related to the stability constants between heavy metals and
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chelating agents [4, 33, 35]. Di Palma et al. [1] reported that 0.1M of citric acid could extract Zn regardless of Fe content (Italian harbor sediment), because of a lower solution pH. In this study, however, 0.1M of sodium citrate was not suitable to extract the heavy metals from marine sediments because citrate ions are able to compete with Fe at a higher solution pH. Even though the initial pH of the NaCl solution was weakly acidic (5.3), the equilibrium pH after 6 h
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became weakly alkaline or natural. Therefore, metal extraction was not effective because of a poor ion-exchange reaction between protons and heavy metals. Sediments contain a large amount of organic matter, which consists of humic and fulvic acids. Under acidic conditions, the acids might be protonated and lose their charges, and adsorption or complexation with metals might be decreased [36-38]. HCl and HNO3 washing solution maintained an acidic pH through the washing experiments.
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HNO3, as an oxidizing acid, could destroy more metal-organic complexes than HCl, while nitrate is a relatively inert anion [4, 39], however, HCl showed a similar or lower heavy metal removal efficiency than HNO3. This result indicates that the chloride-complexes contributed only slightly to the extraction of metals, and abundant cation such as Fe competed with heavy metals to form chloride complexes. Generally, HCl could extract slightly more Fe from sediment than HNO3 (Figure 8). In
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addition, a very weak chloride-complex is not effective for the desorption of heavy metals from sediment. 11
3.3. Assessment of potential mobility and bioavailability of metals It is not always appropriate to evaluate the potential risk of metals to human health and the 305
environment by using the total concentration of heavy metals in sediment, as potential risks are related to the bioavailability and mobility of metals rather than their total contents. Therefore, we investigated the potential mobility of metals by analyzing various chemical fractions. Zhang et al. [40] introduced a Mobility Index (MI) of heavy metals after soil washing process by EDTA in order to evaluate the potential risk of treated soil by residual concentration, and regulate the levels of heavy
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metals in the batch leaching test. However, we demonstrated the bioavailability and potential mobility of metals by analyzing the fractionation and concentration threshold of metals in sediments. The fractionation of metals by sequential chemical extraction is suitable for the assessment of bioavailability and mobility of metals from metal-contaminated sediments. Heavy metals bound to the exchangeable fraction (F1) in the sequential extraction are easily extracted into the surrounding
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environments because they are more weakly bound to soil surface. Therefore, this fraction could be regarded as a bioavailable fraction [41]. Furthermore, the mobility index should be considered with bioavailability simultaneously, because heavy metals bound to Fe-Mn oxyhydroxides fraction (F2; reducible), and organic matters and the sulfides fraction (F3; oxidisable) could be converted to the exchangeable fraction (F1; water soluble and carbonate) by altering the redox potential of the
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surrounding environment. In this study, therefore, the metals in residual sediments were evaluated for their risk to human health and to the environment based on their Bioavailability Index (BIM). The BIM of each residual metal in sediments was investigated by SM&T (formerly BCR) three-step sequential chemical extraction procedure, as follows;
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where, CF1 is the concentration of metals in F1 of sediments after the washing process, CR is the 12
maximum permissible level of a metal according to Korean Soil Quality Legislation (KSQL). In this study, the bioavailability of heavy metals was evaluated based on the ratio of heavy metals bound to 330
the F1 and regulation levels [41]. The metals in the sediment have a higher bioavailability and greater potential risk in the case of BIM > 1, while the metals have less bioavailability and potential risk in the case of BIM < 1. Furthermore, heavy metals bound to F2 and F3 could be converted to F1 by redox potentials of the surrounding environment. Therefore, the potential exchangeability of heavy metals other than F1 was evaluated through analysis of heavy metals concentration ratios of F1 with F2, F3,
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and regulation levels, as follows;
PMIM is a similar indicator to BIM. In the case of PMIM > 1, the metal could be changed to the F1 340
according to alterations in the surrounding conditions, while the potential to be changed is low in case of PMIM < 1. As a result, the increase of PMIM leads to an increase of BIM. Furthermore, the total BIM (BIT) of metals in residual sediments was evaluated based on the following equation;
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Also, BIT and BIM are similar to PMIT and PMIM, respectively. In the case of BIT and PMIT > 1 (higher bioavailability of metals), toxic metals lead directly to harmful effects on human health and the environment, while metals have less toxic potential in the case of BIT and PMIT < 1. Figure 9 350
shows the BIT and PMIT of heavy metals in residual sediments before and after washing with EDTA for 6 and 120 h. BIT, both before and after washing, was lower than 1, however, this slightly increased 13
after all washing process in all samples. On the other hand, PMIT in initial BE sediments was higher than 1, which was reduced below 1 after the washing process. This indicates that low PMIT might decrease BIT due to changes in the surrounding environment, because it was maintained below 1. 355
Furthermore, most Pb bound to F2 in the initial HA sediment was changed to F1 after the washing process with EDTA, however, BIPb did not increased due to the low initial concentration of Pb, as well as BIT being maintained below 1. As a result of the experiments, the impacts to the environmental and to human health were found to be decreased by the washing process due to the low residual concentration of metals, despite changes
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to the fractionation of metals. Therefore, it is necessary to assess the potential bioavailability of each metal through investigation of BIT and PMIT, as changes of the fractionation of heavy metals occur when metal-contaminated sediments are treated by various remediation techniques.
4. CONCLUSIONS 365
The washing characteristics of heavy metals from real marine sediments were evaluated through batch washing experiments and fractionation analysis using five different extracting agents (EDTA, sodium citrate, HCl, HNO3 and NaCl). Among the washing agents tested in this study, EDTA was the most effective extracting agent for Cu, Pb, and Zn from sediments. It is well known that EDTA is able to extract metals from solid matrices based on the complex it forms with metals. Furthermore,
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washing by EDTA converted the form of Cu and Pb bound to F2 or F3 into the exchangeable fraction (F1). However, the amount of extracted metals was not changed significantly because of a high portion of carbonates, organic matter, fine particles, as well as the high content of Fe and Ca. However, citrate was not effective for the extraction of metals from sediments because of relatively high pH and high content of Fe, and HCl and HNO3 were not effective for the extraction of heavy metals because
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of the weak complexation of metals-ligands even though they have a low pH in solution. The total bioavailability indexes of metals (BIT) were calculated before and after the washing process with EDTA, and the indexes were lower than 1 in all samples. In addition, a high total potential mobility index (PMIT) in BE sediment was reduced below 1 through the washing process. It 14
is necessary to assess the potential bioavailability through investigation of BIT and PMIT as total 380
concentrations as well as changes in the fractionation of heavy metals when metal-contaminated sediment is treated by various remediation techniques. In addition, further separation techniques, such as step-wise washing and electrokinetic treatment, might be combined with EDTA washing to enhance the extraction of metals from dredged marine sediments.
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ACKNOWLEDGEMENTS This research is financially supported by Republic of Korea Ministry of Environment as "Green Remediation Research Center for Organic-Inorganic Combined Contamination (The GAIA Project2012000550003)". 390
REFERENCES
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400
405
410
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[1] L. Di Palma, R. Mecozzi, Heavy metals mobilization from harbour sediments using EDTA and citric acid as chelating agents, J Hazard Mater, 147 (2007) 768-775. [2] J.L. Bolton, C.M. Stehr, D.T. Boyd, D.G. Burrows, A.V. Tkalin, T.S. Lishavskaya, Organic and trace metal contaminants in sediments and English sole tissues from Vancouver Harbour, Canada, Mar Environ Res, 57 (2004) 19-36. [3] S.Y. Park, G.Y. Park, D.H. Kim, J.S. Yang, K. Baek, Electrokinetic Separation of Heavy Metals from Wastewater Treatment Sludge, Sep Sci Technol, 45 (2010) 1982-1987. [4] K.J. Kim, D.H. Kim, J.C. Yoo, K. Baek, Electrokinetic Extraction of Heavy Metals from Dredged Marine Sediment, Sep Purif Technol, 79 (2011) 164-169. [5] J.F. Peng, Y.H. Song, P. Yuan, X.Y. Cui, G.L. Qiu, The remediation of heavy metals contaminated sediment, J Hazard Mater, 161 (2009) 633-640. [6] C.N. Mulligan, R.N. Yong, B.F. Gibbs, An evaluation of technologies for the heavy metal remediation of dredged sediments, J Hazard Mater, 85 (2001) 145-163. [7] J. Thoming, B.K. Kliem, L.M. Ottosen, Electrochemically enhanced oxidation reactions in sandy soil polluted with mercury, Sci Total Environ, 261 (2000) 137-147. [8] G. Dermont, M. Bergeron, G. Mercier, M. Richer-Lafleche, Soil washing for metal removal: A review of physical/chemical technologies and field applications, J Hazard Mater, 152 (2008) 1-31. [9] D.H. Kim, B.G. Ryu, S.W. Park, C.I. Seo, K. Baek, Electrokinetic remediation of Zn and Nicontaminated soil, J Hazard Mater, 165 (2009) 501-505. [10] J.S. Yang, J.Y. Lee, K. Baek, T.S. Kwon, J. Choi, Extraction behavior of As, Pb, and Zn from mine tailings with acid and base solutions, J Hazard Mater, 171 (2009) 443-451. [11] G. Dermont, M. Bergeron, M. Richer-Lafleche, G. Mercier, Remediation of metal-contaminated urban soil using flotation technique, Sci Total Environ, 408 (2010) 1199-1211. [12] P. Cauwenberg, F. Verdonckt, A. Maes, Flotation as a remediation technique for heavily polluted dredged material. 2. Characterisation of flotated fractions, Sci Total Environ, 209 (1998) 121-131. [13] P. Cauwenberg, F. Verdonckt, A. Maes, Flotation as a remediation technique for heavily polluted dredged material. 1. A feasibility study, Sci Total Environ, 209 (1998) 113-119. 15
420
425
430
435
440
445
450
455
460
465
470
[14] M. Vanthuyne, A. Maes, P. Cauwenberg, The use of flotation techniques in the remediation of heavy metal contaminated sediments and soils: an overview of controlling factors, Miner Eng, 16 (2003) 1131-1141. [15] C. Loser, A. Zehnsdorf, P. Hoffmann, H. Seidel, Remediation of heavy metal polluted sediment by suspension and solid-bed leaching: Estimate of metal removal efficiency, Chemosphere, 66 (2007) 1699-1705. [16] B. Dahrazma, C.N. Mulligan, Investigation of the removal of heavy metals from sediments using rhamnolipid in a continuous flow configuration, Chemosphere, 69 (2007) 705-711. [17] I. Ko, Y.Y. Chang, C.H. Lee, K.W. Kim, Assessment of pilot-scale acid washing of soil contaminated with As, Zn and Ni using the BCR three-step sequential extraction, J Hazard Mater, 127 (2005) 1-13. [18] R.J. Abumaizar, E.H. Smith, Heavy metal contaminants removal by soil washing, J Hazard Mater, 70 (1999) 71-86. [19] A. Moutsatsou, M. Gregou, D. Matsas, V. Protonotarios, Washing as a remediation technology applicable in soils heavily polluted by mining-metallurgical activities, Chemosphere, 63 (2006) 16321640. [20] W. Zhang, I.M.C. Lo, EDTA-enhanced washing for remediation of Pb-and/or Zn-contaminated soils, J Environ Eng, 132 (2006) 1282. [21] R.A. Griffiths, Soil-washing technology and practice, J Hazard Mater, 40 (1995) 175-189. [22] P. Quevauviller, G. Rauret, J.F. LopezSanchez, R. Rubio, A. Ure, H. Muntau, Certification of trace metal extractable contents in a sediment reference material (CRM 601) following a three-step sequential extraction procedure, Sci Total Environ, 205 (1997) 223-234. [23] A. Tessier, P.G.C. Campbell, M. Bisson, Sequential Extraction Procedure for the Speciation of Particulate Trace-Metals, Anal Chem, 51 (1979) 844-851. [24] Z. Mester, C. Cremisini, E. Ghiara, R. Morabito, Comparison of two sequential extraction procedures for metal fractionation in sediment samples, Anal Chim Acta, 359 (1998) 133-142. [25] L. Di Palma, O. Gonzini, R. Mecozzi, Use of different chelating agents for heavy metal extraction from contaminated harbour sediment, Chem Ecol, 27 (2011) 97-106. [26] G.M. Nystroem, A.J. Pedersen, L.M. Ottosen, A. Villumsen, The use of desorbing agents in electrodialytic remediation of harbour sediment, Sci Total Environ, 357 (2006) 25-37. [27] USEPA, Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, SW-846, National Technical Information Service (NTIS), Port Royal Road Springfield, (1986). [28] G. Rauret, J. Lopez-Sanchez, A. Sahuquillo, R. Rubio, C. Davidson, A. Ure, P. Quevauviller, Improvement of the BCR three step sequential extraction procedure prior to the certification of new sediment and soil reference materials, J Environ Monit, 1 (1999) 57-61. [29] H.A. Elliott, G.A. Brown, Comparative-Evaluation of Nta and Edta for Extractive Decontamination of Pb-Polluted Soils, Water Air Soil Poll, 45 (1989) 361-369. [30] F. Morel, J.G. Hering, Principles and applications of aquatic chemistry, in, Wiley-Interscience, Boboken, New Jersey, 1993. [31] Z.A. Begum, I.M.M. Rahman, Y. Tate, H. Sawai, T. Maki, H. Hasegawa, Remediation of toxic metal contaminated soil by washing with biodegradable aminopolycarboxylate chelants, Chemosphere, 87 (2012) 1161-1170. [32] A. Polettini, R. Pomi, E. Rolle, D. Ceremigna, L. De Propris, M. Gabellini, A. Tornato, A kinetic study of chelant-assisted remediation of contaminated dredged sediment, J Hazard Mater, 137 (2006) 1458-1465. [33] C. Kim, Y. Lee, S.K. Ong, Factors affecting EDTA extraction of lead from lead-contaminated soils, Chemosphere, 51 (2003) 845-853. [34] A. Polettini, R. Pomi, E. Rolle, The effect of operating variables on chelant-assisted remediation of contaminated dredged sediment, Chemosphere, 66 (2007) 866-877. [35] V. Bianchi, G. Masciandaro, D. Giraldi, B. Ceccanti, R. Iannelli, Enhanced heavy metal phytoextraction from marine dredged sediments comparing conventional chelating agents (citric acid and EDTA) with humic substances, Water Air Soil Poll, 193 (2008) 323-333. [36] K. Baek, J.W. Yang, Humic-substance-enhanced ultrafiltration for removal of heavy metals, Sep 16
475
480
Sci Technol, 40 (2005) 699-708. [37] K. Baek, J.W. Yang, Sorption and desorption characteristics of cobalt in clay: Effect of humic acids, Korean J Chem Eng, 21 (2004) 989-993. [38] H.J. Kim, K. Baek, B.K. Kim, J.W. Yang, Humic substance-enhanced ultrafiltration for removal of cobalt, J Hazard Mater, 122 (2005) 31-36. [39] K.J. Kim, J.M. Cho, K. Baek, J.S. Yang, S.H. Ko, Electrokinetic removal of chloride and sodium from tidelands, J Appl Electrochem., 40 (2010) 1139-1144. [40] W.H. Zhang, H. Huang, F.F. Tan, H. Wang, R.L. Qiu, Influence of EDTA washing on the species and mobility of heavy metals residual in soils, J Hazard Mater, 173 (2010) 369-376. [41] T. Batjargal, E. Otgonjargal, K. Baek, J.S. Yang, Assessment of metals contamination of soils in Ulaanbaatar, Mongolia, J Hazard Mater, 184 (2010) 872-876.
17
485
Figure captions Figure 1. Scanning Electron Microscope and Energy Dispersive Spectroscopy (SEM-EDS) analysis result of NH, BE, and HA marine sediment surface, respectively. Figure 2. Change of solution pH in time during sediment washing; (a) NH, (b) BE, (c) HA 490
sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. Figure 3. Change of metal concentration of sediments in time during washing procedure; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3.
495
Figure 4. Change of metal concentration in solutions as a function of time during washing procedure; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. Figure 5. Fractionation of metals in sediments before and after washing procedure with EDTA; (a) Cd, (b) Cu, (c) Ni, (d) Pb, (e) Zn
500
Figure 6. The overall results of mass balance after sediment washing experiments; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. Figure 7. Correlation between Pb and Fe extracted into EDTA solution in HA sediment. Figure 8. Comparison of the extracted Fe into extracting agent solutions in HA sediment Figure 9. The BIT and PMIT of metals in residual sediments by SM&T (formerly BCR)
505
sequential extraction before and after washing process using EDTA.
18
Fig. 1. Scanning Electron Microscope and Energy Dispersive Spectroscopy (SEM-EDS) analysis results of NH, 510
BE, and HA marine sediment surfaces, respectively. 19
14
(a)
(b)
(c)
Exp. 1 Exp. 2 Exp. 3 Exp. 4 Exp. 5
12
pH
10 8 6 4 2 0
20
40
60
80
Time(h)
100 120 0
20
40
60
80
Time(h)
100 120
0
20
40
60
80
100 120
Time(h)
Fig. 2. Change of solution pH in time during sediment washing; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3.
20
Concentration of Cd (mg/kg)
1.0
Concentration of Cu (mg/kg)
Exp. 1 Exp. 2 Exp. 3 Exp. 4 Exp. 5
0.8
1.5
2
1.0 1 0.2
0.5 0.0
0.0 40
20
40
60
80
100
0 0
120
20
40
60
80
100
120
0 30
(b)
(a)
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
(c)
100 25
30
80 20 60
20
15
40
10
10 20
5
0
0 20
20
40
60
80
100
0 0
120 20
(a)
20
40
60
80
100
120
0 20
(b)
15
15
15
10
10
10
5
5
5
40
20
40
60
80
100
0
120 60
(a)
(c)
0
0 0
Concentration of Pb (mg/kg)
3
0.4
0
20
40
60
80
100
0
120 40
(b)
(c)
50 30
30 40
20
30
20
20 10
10 10 0
0 0
Concentration of Zn (mg/kg)
(c)
2.5
0.6
0
100
20
40
60
80
100
0 0
120
20
40
60
80
100
120
0 100
(b)
(a)
(c)
200 80
80 150
60
60 100
40
40 50
20
20
0
0 0
515
4
(b)
2.0
0
Concentration of Ni (mg/kg)
3.0
(a)
20
40
60
80
Time(h)
100
120
0 0
20
40
60
80
Time(h)
100
120
0
Time(h)
Fig. 3. Changes of metal concentration of residual sediments in time during washing procedure; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium 21
Concentration of Cd (mg/kg)
citrate, HNO3.
0.3
(a)
Concentration of Cu (mg/kg)
0.4 0.2
0.2 0.1
0.1 0.1
0.0 5
40
60
80
100
120
0 50
(a)
20
40
60
80
100
0
120 10
(b)
4
40
8
3
30
6
2
20
4
1
10
2
0 20
40
60
80
100
120
0 1.5
(a)
20
40
60
80
100
0
120 1.5
(b)
0.8
1.2
1.2
0.6
0.9
0.9
0.4
0.6
0.6
0.2
0.3
0.3
20
40
60
80
100
120
0 50
(a)
8
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
(c)
0.0
0.0
10
20 (c)
0
0
1.0
0
Concentration of Pb (mg/kg)
0.0
0.0 20
0.0
20
40
60
80
100
0
120 20
(b)
(c)
40 15
6
30
4
20
2
10
10
5
0
0 0
Concentration of Zn (mg/kg)
(c)
0.3
0
15
20
40
60
80
100
120
0 0
100
(a)
20
40
60
80
100
120
0 15
(b)
12
80
12
9
60
9
6
40
6
3
20
3
0
0 0
520
0.3
(b)
0.2
0
Concentration of Ni (mg/kg)
0.5
Exp.1 Exp.2 Exp.3 Exp.4 Exp.5
20
40
60
80
Time(h)
100
120
(c)
0 0
20
40
60
80
Time(h)
100
120
0
Time(h)
Fig. 4. Change of metal concentration in solutions as a function of time during washing procedure; 22
(a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3.
23
(F4) Residual (F3) Organic matter and sulfides (F2) Fe-Mn oxyhydroxides (F1) Soluble and exchangeable cations, and carbonate
Fractionation(NH)
1.2
(a)
(b)
(c)
(d)
(e)
1.0 0.8 0.6 0.4 0.2 0.0
Fractionation(BE)
1.2
al iti In
r 0h 12
(a)
al iti In
r 0h 12
(b)
al iti In
r 0h 12
(c)
al iti In
r 0h 12
(d)
al iti In
r 0h 12
(e)
1.0 0.8 0.6 0.4 0.2 0.0
Fractionation(HA)
1.2
r 0h 12
(a)
al iti In
r 0h 12
(b)
al iti In
r 0h 12
(c)
al iti In
r 0h 12
(d)
al iti In
r 0h 12
(e)
1.0 0.8 0.6 0.4 0.2 0.0
525
al iti In
al iti In
r 0h 12
al iti In
r 0h 12
al iti In
r 0h 12
al iti In
r 0h 12
al iti In
r 0h 12
Fig. 5. Fractionation of metals in sediments before and after washing procedure with EDTA; 24
(a) Cd, (b) Cu, (c) Ni, (d) Pb, (e) Zn
1.8
Residual sediment Solution
Recovery (HA)
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
1.4 Residual sediment Solution
Recovery (NH)
1.2 1.0 0.8 0.6 0.4 0.2 0.0 Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
1.2 Residual sediment Solution
Recovery (BE)
1.0 0.8 0.6 0.4 0.2 0.0 Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Exp. 1
Exp. 2
Exp. 3
Exp. 4
Exp. 5
Fig. 6. The overall results of mass balance before and after sediment washing experiments; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. 530
25
Extracted Fe/Pb Molar ratio
4 Pb/Fe
3
2
1
0 0
20
40
60
80
100
120
140
Time (h) Fig. 7. Correlation between molar ratio of Pb and Fe extracted into EDTA solution in HA sediment
26
160 6 hours 120 hours
Extracted Fe (mg/L)
140 120 100 80 60 40 20 0 SC 535
EDTA
HCl
HNO3
Extracting agents Fig. 8. Comparison of the extracted Fe into extracting agent solutions in HA sediment
27
1.6 NH sediment - BIT NH sediment - PMIT
1.4
BE sediment - BIT
BIT & PMIT
1.2
BE sediment - PMIT HA sediment - BIT
1.0
HA sediment - PMIT
0.8 0.6 0.4 0.2 0.0 Initial
6h
120 h
Fig. 9. The BIT and PMIT of heavy metals in residual sediments by SM&T sequential extraction 540
before and after washing process in Exp. 1
28
Table 1. The initial physicochemical characteristics of sediments Parameter pH
N 7.4
Value B 8.6
H 8.1
4.0 89.4 6.6 50.2 3.1
36.0 60.8 3.2 43.76 3.5
19.2 71.3 9.5 55.5 4.5
8.2
5.3
9.1
27.7
23.7
33.6
31000 30210
22940 24000
27600 40400
Sediment texture (wt%) (ASTM D421) Sand Silt Clay Water content (wt%) Carbonate content (wt%) Organic matter (wt%) (ASTM D2974) CEC (cmol(+)/kg) (EPA 9081) Major elements Ca (mg/kg) Fe (mg/kg)
Table 2. Heavy Metals concentration as three analysis procedures of sediments; Samples
545
550
Methods
Cd 0.6 n.d 1.8 2.8 n.d 1.7 3.4 n.d 1.6
Concentrations (mg/kg) Cu Ni Pb 21.8 11.1 23.8 49.0 48.0 66.0 33.4 24.7 30.4 93.3 13.0 53.7 124.0 40.0 116.0 106.8 20.7 57.6 22.5 13.0 27.1 79.0 50.0 51.0 75.2 23.5 86.6
Zn 1a) 74.8 N 2b) 136.0 125.5 3c) 1 197.5 B 2 280.0 3 249.0 1 65.8 H 2 209.0 3 247.0 Korean Ocean Dumping 2.5 65 35 50 200 Limiting Valuesd) Legal Regulation of 4 150 100 200 300 Korean Soil Qualitye) a) Aqua regia extraction b) Analysis by XRF c) Total concentration by HNO3, HClO4 and HF d) The regulation is based on total concentration of heavy metals in the soil e) The regulation is based on pseudo-total concentration of heavy metals extracted by aqua regia. Table 3. The conditions of sediment washing experiments Washing experiment Exp. 1 Exp. 2 Exp. 3 Exp. 4 Exp. 5
Reagents Na2EDTA NaCl HCl sodium citrate HNO3
Concentration 0.1M 0.1M 0.1M 0.1M 0.1M
Initial pH 4.41 5.32 1.09 8.88 1.05 29
Table 4. Summary of three-step SM&T sequential extraction procedure Step 1
Fraction (F1) Soluble and exchangeable, carbonate
0.11M CH3COOH for 16h at 22±5℃
Solution
2
(F2) Bound to Fe-Mn oxides, reducible
0.5M NH2OH·HCl added 2M HNO3 for 16h at 22±5℃
3
(F3) Bound to organic matter and sulfides, oxidisable
8.8M H2O2 at 85±2℃ then 1M CH3COONH4 adjusted
4
(F4) Residual
to pH 2±0.1 with concentrated HNO3 HCl : HNO3 = 3 : 1, aqua regia
555 Table 5. Stability constants (Ks) for formation of complexes from metals with EDTA, chloride and citrate [24]. Metals Cd Cu Ni Pb Zn Fe Ca
Chloride CdCl+ CuCl+ NiCl+ PbCl+ ZnCl+ FeCl2+ CaCl2
Ks 2 0.5 0.6 1.6 0.4 2.1 -
EDTA Cd-EDTA2Cu-EDTA2Ni-EDTA2Pb-EDTA2Zn-EDTA2Fe-EDTACa-EDTA2-
Ks 18.2 20.5 20.4 19.8 18.3 27.7 12.4
citrate +
CdH2-citrate CuOH-citrate2NiH2-citrate+ PbH2-citrate+ ZnH2-citrate+ Fe2(OH)2-citrate22CaH2-citrate+
Ks 12.6 16.4 12.9 8.1 13.3 56.3 12.3
30
Research Highlights 560
> Extractability of metals from marine sediments was investigated using various agents > EDTA covered the strongly bound fraction of Cu and Pb to exchangeable fraction.. > Potential hazard of the sediments was evaluated.
565
31
S1385-8947(13)00648-7 http://dx.doi.org/10.1016/j.cej.2013.05.029 CEJ 10762
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
29 January 2013 3 May 2013 10 May 2013
Please cite this article as: J-C. Yoo, C-D. Lee, J-S. Yang, K. Baek, Extraction Characteristics of Heavy Metals from Marine Sediments, Chemical Engineering Journal (2013), doi: http://dx.doi.org/10.1016/j.cej.2013.05.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Extraction Characteristics of Heavy Metals from Marine Sediments Jong-Chan YooaㆍCha-Dol LeeaㆍJung-Seok YangbㆍKitae Baeka* 5
a
Department of Environmental Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeollabuk-do, Republic of Korea b
10
KIST-Gangneung Institute, Gangneung, Gangwon-do 210-340, Republic of Korea
* Corresponding author Tel.: +82-63-270-2437; Fax +82-63-270-2449- ; E-mail address: [email protected] (K. Baek)
ABSTRACT In this study, the feasibility of a washing process to extract Cd, Cu, Ni, Pb, and Zn from three real 15
dredged marine sediments (Namhang (NH), Bangeojin (BE), and Haengam (HA) bay sediments, South Korea) were investigated using various washing agents (ethylenediaminetetraacetic acid (EDTA), NaCl, HCl, sodium citrate, and HNO3). Even though the removal efficiencies of heavy metals were not high, which is thought to be largely due to the high concentrations of Fe and Ca, EDTA was a more efficient extraction agent for heavy metals compared to the other agents, because
20
EDTA formed more stable complexes with metals. EDTA removed : 30%, 43%, and 9% of Cu; 48%, 66%, and -8% of Pb; and 31%, 60%, and -14% of Zn from NH, BE, and HA sediments, respectively. Additionally, fractionation analysis showed that EDTA converted the strongly bound fractions of Cu and Pb to an exchangeable fraction. Based on the experimental results, the washing technique using EDTA is determined to be efficient for the removal Cu, Pb, and Zn from NH and BE sediment
25
samples. On the contrary, residual concentrations of heavy metals were not decreased from HA samples compared to extraction with washing solution because of the high content of fine particles and organic matter in samples. Furthermore, the fractionations of metals highly influenced the chemical extraction of metals in sediments, and the fractionation changes after chemical washing increased the total extractable amount of metals. As a result, the fractionation change caused changes
30
in the bioavailability and potential mobility of metals.
1
Keywords: marine sediment; chelation; complexation; fractionation; heavy metals; 1. INTRODUCTION 35
The contamination of sediments by heavy metals is a prevalent issue today, due highly to industrial growth and the expansion of key human activities. Marine sediments, both coastal and open-ocean, are polluted by heavy metals originating from ships, dockyards, rivers, and harbor facilities, mainly concentrated around ports, harbors, and estuaries. Even a low load of pollutants in marine sediments can cause secondary environmental pollution through causing various environmental changes in
40
aqueous systems. Furthermore, such pollution can negatively impact human health and ecosystems through a range of accumulatory processes within the food chain [1, 2]. In Korea, marine sediments in many areas are dredged in order to maintain a minimum depth of water, and huge amounts of dredged materials have been dumped further out to sea without undergoing treatment. Dredged sediments must be treated before disposal inland in order to protect public health, and this has now become mandatory
45
for all dredged sediments, as the London and Basel conventions restrict ocean dumping [3, 4]. Measures prohibiting the ocean dumping of such wastes will be instigated in Korea from 2014. Consequently, it is advisably to start preparing treatment techniques for dredged marine sediments which are to be disposed inland or which are to attain approval for dumping at sea. Generally, the physical and chemical characteristics of marine sediments are considerably different
50
from those of inland soil. Dredged marine sediments contain very high proportions of organic matter, carbonates, sulfides, and chlorides [4, 5]. Also, sediments have comparatively large contents of fine particles, such as silt and clay, and exhibit high alkalinity values. These unique characteristics directly influence the mobility of heavy metals [6]. Organic matter combines with heavy metals, forming metal–organic complexes which are very stable [6, 7]. The high proportion of carbonates contributes
55
to the high alkalinity and buffering capacity of the sediments, and as a consequence, large amounts of acids are generally required to extract the metals. Hence, sediments contaminated with heavy metals are very difficult to treat by applying normal soil remediation techniques. The treatment alternatives (alternatives to landfilling) for metal-contaminated dredged sediments include: solidification/stabilization [6]; bio-chemical stabilization; soil washing (including physical 2
60
separation, chemical leaching/washing, and bioleaching) [1]; electrokinetic remediation [4]; and thermal treatment for Hg [6]. Stabilization/solidification technology makes the contaminants more stable and reduces their solubility, thus, reducing their mobility. In Korea, however, the legal regulations relating to contaminants are based on pseudo-total concentrations, as extracted by aqua regia. Even though stabilization/solidification has been widely used to treat metal-contaminated soil
65
[8] , it may not meet regulation levels after application [9, 10]. Soil/sediment washing technologies include chemical extraction, bioleaching procedures, and physical separation methods, or a combination of such methods. Among the washing processes which incorporate physical separation, flotation technology appears to be a suitable technique for the removal of metals from fine-grained matrices such as dredged sediments [11]. Some sediment
70
washing techniques which apply flotation for the removal of metals, have been studied on a laboratory scale [12, 13], some of which have been applied on a field-scale [14]. Also, sediment washing via bioleaching and biosurfactant extraction has been investigated [15, 16]. Soil/sediment washing is a suitable technique for the removal of contaminants such as lead, copper, nickel, zinc, cadmium, and chromium [17-20]. Most contaminants are bound to finer particles in a soil or sediment, and washing
75
techniques separate the finer particles from the sediments, substantially decreasing the contamination through physical separation of the most heavily contaminated component, dramatically increasing the efficiency of further treatment processes applied to the coarser particles [21]. In the soil/sediment washing technique, the proper selection of washing agents is very important to improve the extraction efficiency. Chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and sodium citrate, form
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strong metal–ligand complexes and enhance the extractability of metals from contaminated soil/sediments [1]. HCl and HNO3 are known as effective strong acids to extract heavy metals from soil/sediments [8]. Additionally, it is widely thought that chloride might form various metal-chloride complexes, which can enhance the extractability of metals. Sea water contains 3.5% NaCl and might contain sufficient chloride for the formation of metal-chloride complexes. Therefore, sea water could
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be applied to treat metal-contaminated marine sediments as NaCl is an effective extracting agent for metals. 3
Generally, heavy metals are bound to soil/sediment in a variety of forms, all of which are closely related to the bioavailability of metals. The SM&T (formerly BCR) three-step sequential chemical extraction procedure has been widely used with Tessier’s method for analysis of the fractionations of 90
metals in soil/sediment samples [22-24]. The SM&T (formerly BCR) three-step sequential chemical extraction procedure has been designed to target the soluble and exchangeable fraction (F1), reducible fraction (F2), and the oxidizable fraction (F3), leaving the residual fraction (F4). The sequential chemical extraction procedure separates each fraction based on three different extractants, and it is a suitable means of evaluating the remediation economics of sediments contaminated with heavy metals.
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The efficiency of metal removal from artificially contaminated marine sediment has previously been investigated, where the initial contamination level was too low to draw significant conclusions [25]. Di Palma et al. [1] reported that the extraction efficiencies of Cu and Pb by EDTA washing/extraction from Italian harbor sediments were higher than those by citric acid washing/extraction. Nystroem et al. [26] reported that a high concentration of 0.6M HCl was most effective for the extraction of heavy
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metals from Norwegian harbor sediment through chemical extraction. In addition, NaCl could extract 20% of Pb and 55% of Cd from harbor sediment, while it was unable to extract Cu and Zn. Marine sediments have been deposited slowly on the ocean floor over many years, and have unique characteristics of containing a high proportion of carbonates, sulfides, and organic matter. Additionally, aging is a key factor which influences metal extractability. Hence, it is necessary to
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investigate the applicability of various remediation techniques, while using specific grades of metalcontaminated marine sediments. Furthermore, the mobility and bioavailability of the metals, as well as their total concentration, should be considered in order to accurately assess the potential risk of metal-contaminated sediments. However, most researchers have merely focused on the extraction and removal efficiencies of metals.
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In this study, therefore, the washing characteristics of heavy metals from three real marine sediments were investigated, using various washing agents, such as EDTA, NaCl, HCl, sodium citrate, and HNO3. We evaluated the potential mobility and bioavailability of heavy metals based on the change in metal fractionation after washing procedure. 4
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2. MATERIALS AND METHODS 2.1. Characterization of sediments Samples were collected from three different harbor sediments located in; Namhang (NH) of Busan city, Bangeojin (BE) of Ulsan city, and Haengam bay (HA) of Jinhae city, South Korea. Table 1 shows the physico-chemical characteristics of the sediments. After oven-drying at 75 °C for 24 h, samples
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were pulverized using a mortar and pestle, and were then sieved through a 2mm (size 10) mesh. The concentration of metals was then measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent, 720ES, USA) after aqua regia extraction. As the concentration of metals extracted by aqua regia is not the total concentration contained in the sediment, but a standardized extractable fraction of the pseudo-total concentration in Korea [3], the total heavy metals
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concentration was analysed compared with X-ray fluorescence (XRF) and a total metal digestion (incorporating HNO3, HClO4 and HF) (Table 2). The exchangeable cation concentration was extracted using 1N ammonium acetate, and was then adjusted to pH 7 and mixed to a L/S (liquid and solid) ratio of 1:10. The cation exchange capacity (CEC) was determined by Method 9081 (SW-846 USEPA Method 9081) [27].
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The sediment pH was determined according to the Korean Standard Test Method (KSTM) as follows: A 5:1 ratio of DI water to-sediment was shaken for 1 h at room temperature, and was then allowed to stand. The mixture was filtered using 5B filter paper, and the pH of the filtrate was measured using a pH/ISE meter (Istek 735P, Korea). The soil texture and organic matter were determined according to ASTM D421 and ASTM D2974, respectively. The carbonate content was
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determined by the calcination method at 900°C [7]. In addition, the surface morphology of NH, BE, and HA sediments were analyzed using a field emission scanning electron microscope and energy dispersive spectroscopy (FESEM-EDS, JEOL, JSM-6701F, Japan) (Fig.1). Figure 1 shows the SEM-EDS analysis results of NH, BE, and HA marine sediment surfaces, respectively. The sediments contain high portions of Al and Si as aluminosilicate minerals are a basic
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component of soil. Additionally, all of the samples have a high portion of carbon, which is likely 5
heavily influenced by the carbonate minerals therein. Data for Na and Mg were included in the composition because they contacted with sea water.
2.2. Sediment washing experiments 145
Sediment washing experimental conditions are shown in Table 3. Solutions of EDTA (Ethylenediaminetetraacetic acid; C10H16N2O8), sodium chloride (NaCl), hydrochloric acid (HCl), sodium citrate (Na3C6H5O7), or nitric acid (HNO3) at a concentration of 0.1 M were used as washing agents. Sediments (20 g) dried at 75°C for 24 h were mixed with the washing solution (60 mL) in a flask, and the mixture was then shaken in a horizontal shaker at 150 rpm at 20°C. After a designated
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time, from 6 to 120 h, the samples were removed and filtered through 5B filter paper. The filtrate was then analyzed for its metal content. The metals in the sediment residue were further extracted by the aqua regia method, and the extractants were analyzed for heavy metals. All experiments were carried out in triplicate. The SM&T (formerly, BCR) three-step sequential extraction procedure was used to evaluate the fractionation changes of metals in the sediment.
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The method provides four fractionation results: soluble and exchangeable (F1) using 0.11M of acetic acid for 16 h, bound to Fe-Mn oxide (F2) using 0.5 M of hydroxylammonium chloride for 16 h, bound to organic matter and sulfides (F3) using 8.8 M of hydrogen peroxide at 85 ± 2°C then 1 M of ammonium acetate for 16 h, and residual (F4) by aqua regia [22, 28]. Table 4 shows the three-step sequential extraction procedure schemes.
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3. RESULTS AND DISCUSSIONS 3.1. pH Change Figure 2 shows the pH changes of the solutions during the washing experiments. The initial pH values of extracting solution for each experiment were shown in Table 3. The initial sediment pH of 165
the NH, BE, and HA were 7.4, 8.6 and 8.1, respectively. In all the experiments, the pH changed sharply within 6 h, and then stabilized, which indicates that the reactions between the sediment and 6
washing agents were completed within that time period. In experiments with EDTA, NaCl and sodium citrate, the change in pH value was negligible over time, because those agents are weakly acidic or neutral. In the BE sediment, however, the pH of the EDTA extraction increased to natural conditions. 170
In the HA sediment, however, the equilibrium pH increased gradually through extraction with HNO3, due to the higher carbonate content in the HA sediment. In the cases of HCl and HNO3 extractions, the strongly acidic pH increased instantly due to the high alkalinity of the sediments and the high carbonate content. The strong acid enhanced the dissolution of alkaline and carbonate matter. Neale et al. reported that 0.1M HCl and HNO3 extracted about 99% of Pb under strong acidic pH, which the
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weak acidic solution removed only 1.5-8.6% of Pb from soils, due to the high buffering capacity of soil. In general, the pH of NH and HA sediments was sharply increased to neutral or weakly acidic condition upon treatment with acid, which indicates that the equilibrium pH of the solution increased because NH and HA sediments have a greater buffering capacity compared to BE sediment. Generally, the extraction of metals is enhanced in acidic conditions as to the large amount of H+ increases the
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solubility of sediment minerals, as well as the ion-exchange reaction between the solution and the sediment. Elliott et al. reported [29] that solution pH strongly influenced the extraction of Pb from soils, as the extraction of Pb was negligible below pH 6 -7, with a 0.08 M of EDTA concentration. Based on the experimental results, it was expected that strong inorganic acids would extract more metals from sediments compared to the other extractants.
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3.2. Metal removal and fractionations In order to calculate the speciation distribution of heavy metals in the presence of washing agents, a simulation, using Visual MINTEQ [26], was carried out. The Visual MINTEQ program is useful for prediction of the equilibrium distributions of chemicals in the aqueous systems to apply various input 190
parameters such as chemical species included organic components, concentrations, pH, temperature, etc. Because of the complicated characteristics of the sediment, we simulated the speciation using simplified conditions. In the simulation, the composition of sea water, washing agents (0.1 M), and 7
solution pH were considered, based on the washing experiments. The speciation changes were 195
calculated according to the equilibrium pH. Divalent metals (M) might combine with EDTA, forming [M–EDTA]2-, [MH2–EDTA], [MH– EDTA]- and [MOH–EDTA]3-. Among the various forms of metal–EDTA complexes, the [Me– EDTA]2- form was predominant, considering the pH of the washing experiments. Chloride ions could be combined with metals, producing complexes such as [MCl]+, [MCl2(aq)], [MCl3]-, and [MCl4]2-,
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where the [MCl]+ form was predominant at the specified washing solution pH. Of the citrate forms, [M–(citrate)2]4-, [M–(citrate)]-, [MH–citrate(aq)] and [MH2–citrate]+, the [M–(citrate)2]4- form was the major species in the washing solution. In case of Cd, Pb, and Zn, the proportion of [MCl]+ was similar to that of [M–EDTA]2- below pH 1.5, however, [M–EDTA]2- was predominant above pH 1.5. In contrast, in the cases of Cu and Ni, [M–EDTA]2- was predominant throughout all pH ranges. These
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results were associated with the stability constants of metal–ligand complexes. Table 5 shows the stability constants of each metal–ligand complex [30]. Generally, the stability constants of metal– ligand complexes are in the order EDTA > citrate > chloride, where the higher constants indicate greater stability. Even though the stability constant of Zn-EDTA is relatively low, large amounts of Zn were extracted from sediments because of the higher initial concentration of Zn compared to other
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metals. Although the amounts of extracted metals are highly dependent on their initial concentrations, EDTA showed the highest extraction efficiency of all heavy metals for all sediments. The initial concentration of target heavy metals in dredged marine sediments was evaluated by applying three different analytical methods, as described in Table 2, and the determined concentrations were compared with guideline values. In Korea, guidelines for heavy metals in
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sediments for ocean dumping are determined by a total digestion using HNO3, HClO4, and HF, which was carried out, along with a total metals determination using a strong acid digestion. The concentration based on the total digestion of Cu, Zn and Pb exceeded the guideline value for ocean dumping, however, based on the aqua regia extraction, the metals concentration were less than the guideline for soil quality, and the dredged sediments could be recycled without any further treatment
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after washing with EDTA. Necessarily, such inconclusive methods for setting legislative limits are not 8
a sufficient method of protecting our oceans, as the mobility of metals can be changed in marine environments, influencing the fractionation of metals. Though the original sediments are under reducing conditions, dredged sediments have been exposed to oxidizing conditions, which directly increases the leaching potential for further fractionation and mobility of heavy metals. As a result, the 225
fractionation change affects the amount of aqua regia extractable metals. In addition, significant differences in the amount of metals extracted by aqua regia were observed between replicate samples, due to the severe heterogeneity, which indicated that the extractability of metals by aqua regia is highly dependent on the amount of organic matter, carbonate content, buffering capacity, and fractionation of metals. Therefore, we further discussed metal removal based on the extracted amount
230
of metals due to the of severe variability initial metal concentration. Figures 3 and 4 show the extraction kinetics of heavy metals in the sediment, using various chemical extractants. Furthermore, the fractionations of metals before and after the washing experiment using Na2EDTA are presented in Figure 5. Generally, washing was not effective for the extraction of Cd and Ni from sediments, and the
235
residual concentrations of Cd and Ni in the sediments were almost constant during the experiments. Cd and Ni extraction by Na2EDTA increased sharply until 6 h and then reached a plateau. However, the Cd and Ni extracted into solutions were negligible amount compared to the total, as concentrations determined by aqua regia extraction. In all sediments, the fraction 2 (Fe/Mn bound fraction, F2) decreased and the residual fraction (F4) slightly increased after extraction by Na2EDTA. It was
240
especially difficult to extract Ni from sediments. The reason for this is the strength of the binding. It is well known that chelating agents can extract more metals under acidic conditions than under neutral or alkaline pH condition. Begum et al. [31] reported that 0.5M EDTA under an acidic condition (pH 4) extracted 90% of Cd and 80% of Pb, however, it was difficult to extract Ni compared to other metals, because it was more strongly bound to particles within the sediment [31, 32].
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The residual concentrations of Cu, Pb and Zn after extraction with Na2EDTA decreased dramatically until 6 h in NH and BE sediments. At the same time, the concentration of metals in solution increased sharply until 6h, after which it stabilized without any significant change. On the contrary, the 9
concentrations of metals did not decrease in residual HA sediment. The reason for this might be that the contents of fine particles, carbonate and organic matter in HA sediment are higher than those of 250
NH and BE sediments. In all of the samples, a large amount of Cu was bound to the organic/sulfide fractions (F3) on soils/sediments, and Cu was continuously extracted into solution until 120 h in the BE sediment. However, the organic/sulfide bound fraction (F3) of Cu in BE sediment did not change highly after washing with Na2EDTA, compared to NH and HA sediments. This indicates that metalorganic complexes were destroyed during washing, and especially, BE sediments had relatively lower
255
organic contents compared to NH and HA sediments. The residual concentrations of Pb and Zn from NH and BE sediments sharply decreased until 6 h upon application of Na2EDTA, then reaching a plateau. The concentration of Zn in solution from BE sediment sharply increased until 6 h, as a large amount of Zn was bound to F1 in the initial sediment. Most Pb and Zn was extracted from the BE sediment due to its high sand content, and relatively low
260
organic matter content. The residual concentration of Pb and Zn in the HA sediment did not decreased as much as the concentration increased in solution, which was a major source of error in the mass balance. This was the reason for the gap of initial concentration of Pb and Zn between aqua regia analysis and the increase in the residual fraction of Zn after the washing experiment. Figure 6 shows the overall mass balance during sediment washing experiments. The mass balance of the metals,
265
except for Pb in the HA sediment using Na2EDTA, ranged from 70 to about 130%, compared to the initial metals concentration. Pb was extracted into solution by Na2EDTA as a result of a fractional change to the soluble, exchangeable, and carbonated fractions (F1). However, this can increase the potential risk of environment problems and human health by redox reactions as surrounding environmental changes though increasing F1. In particular, a significant amount of Pb was extracted
270
by EDTA, because EDTA has a great affinity for Fe(III), and most Pb was fractionated on Fe–Mn oxides [33]. Figure 7 shows the correlation between Pb and Fe extracted into Na2EDTA solution in the HA sediment. This result indicates that the extraction of Pb is strongly related to the extraction of Fe, and the molar ration of Pb/Fe during the extraction was almost constant. Therefore, a large amount of Pb bound to F2 could be extracted into solution through the extraction of Fe. 10
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Citrate, an organic acid salt, is a trivalent anionic non-toxic chelating agent. It forms metal–citrate complexes, which are weak compared to those of EDTA. This was the reason for the relatively low removal of heavy metals when sodium citrate was used as a washing agent. The chelating agent desorbed heavy metals from the sediment and formed strong metal–ligand complexes. The stability constants with metals might be indicators for the complexation affinity between metals and ligands
280
(Table 5). Ca and Fe might compete with heavy metals for ligand complexation, and decrease the complexation of heavy metals [32, 34]. In the case of heavy metals bound to F2, however, they could be extracted by dissolution reaction of Fe via ligand complexation. Actually, citrate was able to extract Fe from sediments about 3 – 4 times more efficiently than EDTA (Figure 8). In addition, the order of extraction efficiency of heavy metals is related to the stability constants between heavy metals and
285
chelating agents [4, 33, 35]. Di Palma et al. [1] reported that 0.1M of citric acid could extract Zn regardless of Fe content (Italian harbor sediment), because of a lower solution pH. In this study, however, 0.1M of sodium citrate was not suitable to extract the heavy metals from marine sediments because citrate ions are able to compete with Fe at a higher solution pH. Even though the initial pH of the NaCl solution was weakly acidic (5.3), the equilibrium pH after 6 h
290
became weakly alkaline or natural. Therefore, metal extraction was not effective because of a poor ion-exchange reaction between protons and heavy metals. Sediments contain a large amount of organic matter, which consists of humic and fulvic acids. Under acidic conditions, the acids might be protonated and lose their charges, and adsorption or complexation with metals might be decreased [36-38]. HCl and HNO3 washing solution maintained an acidic pH through the washing experiments.
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HNO3, as an oxidizing acid, could destroy more metal-organic complexes than HCl, while nitrate is a relatively inert anion [4, 39], however, HCl showed a similar or lower heavy metal removal efficiency than HNO3. This result indicates that the chloride-complexes contributed only slightly to the extraction of metals, and abundant cation such as Fe competed with heavy metals to form chloride complexes. Generally, HCl could extract slightly more Fe from sediment than HNO3 (Figure 8). In
300
addition, a very weak chloride-complex is not effective for the desorption of heavy metals from sediment. 11
3.3. Assessment of potential mobility and bioavailability of metals It is not always appropriate to evaluate the potential risk of metals to human health and the 305
environment by using the total concentration of heavy metals in sediment, as potential risks are related to the bioavailability and mobility of metals rather than their total contents. Therefore, we investigated the potential mobility of metals by analyzing various chemical fractions. Zhang et al. [40] introduced a Mobility Index (MI) of heavy metals after soil washing process by EDTA in order to evaluate the potential risk of treated soil by residual concentration, and regulate the levels of heavy
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metals in the batch leaching test. However, we demonstrated the bioavailability and potential mobility of metals by analyzing the fractionation and concentration threshold of metals in sediments. The fractionation of metals by sequential chemical extraction is suitable for the assessment of bioavailability and mobility of metals from metal-contaminated sediments. Heavy metals bound to the exchangeable fraction (F1) in the sequential extraction are easily extracted into the surrounding
315
environments because they are more weakly bound to soil surface. Therefore, this fraction could be regarded as a bioavailable fraction [41]. Furthermore, the mobility index should be considered with bioavailability simultaneously, because heavy metals bound to Fe-Mn oxyhydroxides fraction (F2; reducible), and organic matters and the sulfides fraction (F3; oxidisable) could be converted to the exchangeable fraction (F1; water soluble and carbonate) by altering the redox potential of the
320
surrounding environment. In this study, therefore, the metals in residual sediments were evaluated for their risk to human health and to the environment based on their Bioavailability Index (BIM). The BIM of each residual metal in sediments was investigated by SM&T (formerly BCR) three-step sequential chemical extraction procedure, as follows;
325
where, CF1 is the concentration of metals in F1 of sediments after the washing process, CR is the 12
maximum permissible level of a metal according to Korean Soil Quality Legislation (KSQL). In this study, the bioavailability of heavy metals was evaluated based on the ratio of heavy metals bound to 330
the F1 and regulation levels [41]. The metals in the sediment have a higher bioavailability and greater potential risk in the case of BIM > 1, while the metals have less bioavailability and potential risk in the case of BIM < 1. Furthermore, heavy metals bound to F2 and F3 could be converted to F1 by redox potentials of the surrounding environment. Therefore, the potential exchangeability of heavy metals other than F1 was evaluated through analysis of heavy metals concentration ratios of F1 with F2, F3,
335
and regulation levels, as follows;
PMIM is a similar indicator to BIM. In the case of PMIM > 1, the metal could be changed to the F1 340
according to alterations in the surrounding conditions, while the potential to be changed is low in case of PMIM < 1. As a result, the increase of PMIM leads to an increase of BIM. Furthermore, the total BIM (BIT) of metals in residual sediments was evaluated based on the following equation;
345
Also, BIT and BIM are similar to PMIT and PMIM, respectively. In the case of BIT and PMIT > 1 (higher bioavailability of metals), toxic metals lead directly to harmful effects on human health and the environment, while metals have less toxic potential in the case of BIT and PMIT < 1. Figure 9 350
shows the BIT and PMIT of heavy metals in residual sediments before and after washing with EDTA for 6 and 120 h. BIT, both before and after washing, was lower than 1, however, this slightly increased 13
after all washing process in all samples. On the other hand, PMIT in initial BE sediments was higher than 1, which was reduced below 1 after the washing process. This indicates that low PMIT might decrease BIT due to changes in the surrounding environment, because it was maintained below 1. 355
Furthermore, most Pb bound to F2 in the initial HA sediment was changed to F1 after the washing process with EDTA, however, BIPb did not increased due to the low initial concentration of Pb, as well as BIT being maintained below 1. As a result of the experiments, the impacts to the environmental and to human health were found to be decreased by the washing process due to the low residual concentration of metals, despite changes
360
to the fractionation of metals. Therefore, it is necessary to assess the potential bioavailability of each metal through investigation of BIT and PMIT, as changes of the fractionation of heavy metals occur when metal-contaminated sediments are treated by various remediation techniques.
4. CONCLUSIONS 365
The washing characteristics of heavy metals from real marine sediments were evaluated through batch washing experiments and fractionation analysis using five different extracting agents (EDTA, sodium citrate, HCl, HNO3 and NaCl). Among the washing agents tested in this study, EDTA was the most effective extracting agent for Cu, Pb, and Zn from sediments. It is well known that EDTA is able to extract metals from solid matrices based on the complex it forms with metals. Furthermore,
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washing by EDTA converted the form of Cu and Pb bound to F2 or F3 into the exchangeable fraction (F1). However, the amount of extracted metals was not changed significantly because of a high portion of carbonates, organic matter, fine particles, as well as the high content of Fe and Ca. However, citrate was not effective for the extraction of metals from sediments because of relatively high pH and high content of Fe, and HCl and HNO3 were not effective for the extraction of heavy metals because
375
of the weak complexation of metals-ligands even though they have a low pH in solution. The total bioavailability indexes of metals (BIT) were calculated before and after the washing process with EDTA, and the indexes were lower than 1 in all samples. In addition, a high total potential mobility index (PMIT) in BE sediment was reduced below 1 through the washing process. It 14
is necessary to assess the potential bioavailability through investigation of BIT and PMIT as total 380
concentrations as well as changes in the fractionation of heavy metals when metal-contaminated sediment is treated by various remediation techniques. In addition, further separation techniques, such as step-wise washing and electrokinetic treatment, might be combined with EDTA washing to enhance the extraction of metals from dredged marine sediments.
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ACKNOWLEDGEMENTS This research is financially supported by Republic of Korea Ministry of Environment as "Green Remediation Research Center for Organic-Inorganic Combined Contamination (The GAIA Project2012000550003)". 390
REFERENCES
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400
405
410
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[1] L. Di Palma, R. Mecozzi, Heavy metals mobilization from harbour sediments using EDTA and citric acid as chelating agents, J Hazard Mater, 147 (2007) 768-775. [2] J.L. Bolton, C.M. Stehr, D.T. Boyd, D.G. Burrows, A.V. Tkalin, T.S. Lishavskaya, Organic and trace metal contaminants in sediments and English sole tissues from Vancouver Harbour, Canada, Mar Environ Res, 57 (2004) 19-36. [3] S.Y. Park, G.Y. Park, D.H. Kim, J.S. Yang, K. Baek, Electrokinetic Separation of Heavy Metals from Wastewater Treatment Sludge, Sep Sci Technol, 45 (2010) 1982-1987. [4] K.J. Kim, D.H. Kim, J.C. Yoo, K. Baek, Electrokinetic Extraction of Heavy Metals from Dredged Marine Sediment, Sep Purif Technol, 79 (2011) 164-169. [5] J.F. Peng, Y.H. Song, P. Yuan, X.Y. Cui, G.L. Qiu, The remediation of heavy metals contaminated sediment, J Hazard Mater, 161 (2009) 633-640. [6] C.N. Mulligan, R.N. Yong, B.F. Gibbs, An evaluation of technologies for the heavy metal remediation of dredged sediments, J Hazard Mater, 85 (2001) 145-163. [7] J. Thoming, B.K. Kliem, L.M. Ottosen, Electrochemically enhanced oxidation reactions in sandy soil polluted with mercury, Sci Total Environ, 261 (2000) 137-147. [8] G. Dermont, M. Bergeron, G. Mercier, M. Richer-Lafleche, Soil washing for metal removal: A review of physical/chemical technologies and field applications, J Hazard Mater, 152 (2008) 1-31. [9] D.H. Kim, B.G. Ryu, S.W. Park, C.I. Seo, K. Baek, Electrokinetic remediation of Zn and Nicontaminated soil, J Hazard Mater, 165 (2009) 501-505. [10] J.S. Yang, J.Y. Lee, K. Baek, T.S. Kwon, J. Choi, Extraction behavior of As, Pb, and Zn from mine tailings with acid and base solutions, J Hazard Mater, 171 (2009) 443-451. [11] G. Dermont, M. Bergeron, M. Richer-Lafleche, G. Mercier, Remediation of metal-contaminated urban soil using flotation technique, Sci Total Environ, 408 (2010) 1199-1211. [12] P. Cauwenberg, F. Verdonckt, A. Maes, Flotation as a remediation technique for heavily polluted dredged material. 2. Characterisation of flotated fractions, Sci Total Environ, 209 (1998) 121-131. [13] P. Cauwenberg, F. Verdonckt, A. Maes, Flotation as a remediation technique for heavily polluted dredged material. 1. A feasibility study, Sci Total Environ, 209 (1998) 113-119. 15
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465
470
[14] M. Vanthuyne, A. Maes, P. Cauwenberg, The use of flotation techniques in the remediation of heavy metal contaminated sediments and soils: an overview of controlling factors, Miner Eng, 16 (2003) 1131-1141. [15] C. Loser, A. Zehnsdorf, P. Hoffmann, H. Seidel, Remediation of heavy metal polluted sediment by suspension and solid-bed leaching: Estimate of metal removal efficiency, Chemosphere, 66 (2007) 1699-1705. [16] B. Dahrazma, C.N. Mulligan, Investigation of the removal of heavy metals from sediments using rhamnolipid in a continuous flow configuration, Chemosphere, 69 (2007) 705-711. [17] I. Ko, Y.Y. Chang, C.H. Lee, K.W. Kim, Assessment of pilot-scale acid washing of soil contaminated with As, Zn and Ni using the BCR three-step sequential extraction, J Hazard Mater, 127 (2005) 1-13. [18] R.J. Abumaizar, E.H. Smith, Heavy metal contaminants removal by soil washing, J Hazard Mater, 70 (1999) 71-86. [19] A. Moutsatsou, M. Gregou, D. Matsas, V. Protonotarios, Washing as a remediation technology applicable in soils heavily polluted by mining-metallurgical activities, Chemosphere, 63 (2006) 16321640. [20] W. Zhang, I.M.C. Lo, EDTA-enhanced washing for remediation of Pb-and/or Zn-contaminated soils, J Environ Eng, 132 (2006) 1282. [21] R.A. Griffiths, Soil-washing technology and practice, J Hazard Mater, 40 (1995) 175-189. [22] P. Quevauviller, G. Rauret, J.F. LopezSanchez, R. Rubio, A. Ure, H. Muntau, Certification of trace metal extractable contents in a sediment reference material (CRM 601) following a three-step sequential extraction procedure, Sci Total Environ, 205 (1997) 223-234. [23] A. Tessier, P.G.C. Campbell, M. Bisson, Sequential Extraction Procedure for the Speciation of Particulate Trace-Metals, Anal Chem, 51 (1979) 844-851. [24] Z. Mester, C. Cremisini, E. Ghiara, R. Morabito, Comparison of two sequential extraction procedures for metal fractionation in sediment samples, Anal Chim Acta, 359 (1998) 133-142. [25] L. Di Palma, O. Gonzini, R. Mecozzi, Use of different chelating agents for heavy metal extraction from contaminated harbour sediment, Chem Ecol, 27 (2011) 97-106. [26] G.M. Nystroem, A.J. Pedersen, L.M. Ottosen, A. Villumsen, The use of desorbing agents in electrodialytic remediation of harbour sediment, Sci Total Environ, 357 (2006) 25-37. [27] USEPA, Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, SW-846, National Technical Information Service (NTIS), Port Royal Road Springfield, (1986). [28] G. Rauret, J. Lopez-Sanchez, A. Sahuquillo, R. Rubio, C. Davidson, A. Ure, P. Quevauviller, Improvement of the BCR three step sequential extraction procedure prior to the certification of new sediment and soil reference materials, J Environ Monit, 1 (1999) 57-61. [29] H.A. Elliott, G.A. Brown, Comparative-Evaluation of Nta and Edta for Extractive Decontamination of Pb-Polluted Soils, Water Air Soil Poll, 45 (1989) 361-369. [30] F. Morel, J.G. Hering, Principles and applications of aquatic chemistry, in, Wiley-Interscience, Boboken, New Jersey, 1993. [31] Z.A. Begum, I.M.M. Rahman, Y. Tate, H. Sawai, T. Maki, H. Hasegawa, Remediation of toxic metal contaminated soil by washing with biodegradable aminopolycarboxylate chelants, Chemosphere, 87 (2012) 1161-1170. [32] A. Polettini, R. Pomi, E. Rolle, D. Ceremigna, L. De Propris, M. Gabellini, A. Tornato, A kinetic study of chelant-assisted remediation of contaminated dredged sediment, J Hazard Mater, 137 (2006) 1458-1465. [33] C. Kim, Y. Lee, S.K. Ong, Factors affecting EDTA extraction of lead from lead-contaminated soils, Chemosphere, 51 (2003) 845-853. [34] A. Polettini, R. Pomi, E. Rolle, The effect of operating variables on chelant-assisted remediation of contaminated dredged sediment, Chemosphere, 66 (2007) 866-877. [35] V. Bianchi, G. Masciandaro, D. Giraldi, B. Ceccanti, R. Iannelli, Enhanced heavy metal phytoextraction from marine dredged sediments comparing conventional chelating agents (citric acid and EDTA) with humic substances, Water Air Soil Poll, 193 (2008) 323-333. [36] K. Baek, J.W. Yang, Humic-substance-enhanced ultrafiltration for removal of heavy metals, Sep 16
475
480
Sci Technol, 40 (2005) 699-708. [37] K. Baek, J.W. Yang, Sorption and desorption characteristics of cobalt in clay: Effect of humic acids, Korean J Chem Eng, 21 (2004) 989-993. [38] H.J. Kim, K. Baek, B.K. Kim, J.W. Yang, Humic substance-enhanced ultrafiltration for removal of cobalt, J Hazard Mater, 122 (2005) 31-36. [39] K.J. Kim, J.M. Cho, K. Baek, J.S. Yang, S.H. Ko, Electrokinetic removal of chloride and sodium from tidelands, J Appl Electrochem., 40 (2010) 1139-1144. [40] W.H. Zhang, H. Huang, F.F. Tan, H. Wang, R.L. Qiu, Influence of EDTA washing on the species and mobility of heavy metals residual in soils, J Hazard Mater, 173 (2010) 369-376. [41] T. Batjargal, E. Otgonjargal, K. Baek, J.S. Yang, Assessment of metals contamination of soils in Ulaanbaatar, Mongolia, J Hazard Mater, 184 (2010) 872-876.
17
485
Figure captions Figure 1. Scanning Electron Microscope and Energy Dispersive Spectroscopy (SEM-EDS) analysis result of NH, BE, and HA marine sediment surface, respectively. Figure 2. Change of solution pH in time during sediment washing; (a) NH, (b) BE, (c) HA 490
sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. Figure 3. Change of metal concentration of sediments in time during washing procedure; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3.
495
Figure 4. Change of metal concentration in solutions as a function of time during washing procedure; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. Figure 5. Fractionation of metals in sediments before and after washing procedure with EDTA; (a) Cd, (b) Cu, (c) Ni, (d) Pb, (e) Zn
500
Figure 6. The overall results of mass balance after sediment washing experiments; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. Figure 7. Correlation between Pb and Fe extracted into EDTA solution in HA sediment. Figure 8. Comparison of the extracted Fe into extracting agent solutions in HA sediment Figure 9. The BIT and PMIT of metals in residual sediments by SM&T (formerly BCR)
505
sequential extraction before and after washing process using EDTA.
18
Fig. 1. Scanning Electron Microscope and Energy Dispersive Spectroscopy (SEM-EDS) analysis results of NH, 510
BE, and HA marine sediment surfaces, respectively. 19
14
(a)
(b)
(c)
Exp. 1 Exp. 2 Exp. 3 Exp. 4 Exp. 5
12
pH
10 8 6 4 2 0
20
40
60
80
Time(h)
100 120 0
20
40
60
80
Time(h)
100 120
0
20
40
60
80
100 120
Time(h)
Fig. 2. Change of solution pH in time during sediment washing; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3.
20
Concentration of Cd (mg/kg)
1.0
Concentration of Cu (mg/kg)
Exp. 1 Exp. 2 Exp. 3 Exp. 4 Exp. 5
0.8
1.5
2
1.0 1 0.2
0.5 0.0
0.0 40
20
40
60
80
100
0 0
120
20
40
60
80
100
120
0 30
(b)
(a)
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
(c)
100 25
30
80 20 60
20
15
40
10
10 20
5
0
0 20
20
40
60
80
100
0 0
120 20
(a)
20
40
60
80
100
120
0 20
(b)
15
15
15
10
10
10
5
5
5
40
20
40
60
80
100
0
120 60
(a)
(c)
0
0 0
Concentration of Pb (mg/kg)
3
0.4
0
20
40
60
80
100
0
120 40
(b)
(c)
50 30
30 40
20
30
20
20 10
10 10 0
0 0
Concentration of Zn (mg/kg)
(c)
2.5
0.6
0
100
20
40
60
80
100
0 0
120
20
40
60
80
100
120
0 100
(b)
(a)
(c)
200 80
80 150
60
60 100
40
40 50
20
20
0
0 0
515
4
(b)
2.0
0
Concentration of Ni (mg/kg)
3.0
(a)
20
40
60
80
Time(h)
100
120
0 0
20
40
60
80
Time(h)
100
120
0
Time(h)
Fig. 3. Changes of metal concentration of residual sediments in time during washing procedure; (a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium 21
Concentration of Cd (mg/kg)
citrate, HNO3.
0.3
(a)
Concentration of Cu (mg/kg)
0.4 0.2
0.2 0.1
0.1 0.1
0.0 5
40
60
80
100
120
0 50
(a)
20
40
60
80
100
0
120 10
(b)
4
40
8
3
30
6
2
20
4
1
10
2
0 20
40
60
80
100
120
0 1.5
(a)
20
40
60
80
100
0
120 1.5
(b)
0.8
1.2
1.2
0.6
0.9
0.9
0.4
0.6
0.6
0.2
0.3
0.3
20
40
60
80
100
120
0 50
(a)
8
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
20
40
60
80
100
120
(c)
0.0
0.0
10
20 (c)
0
0
1.0
0
Concentration of Pb (mg/kg)
0.0
0.0 20
0.0
20
40
60
80
100
0
120 20
(b)
(c)
40 15
6
30
4
20
2
10
10
5
0
0 0
Concentration of Zn (mg/kg)
(c)
0.3
0
15
20
40
60
80
100
120
0 0
100
(a)
20
40
60
80
100
120
0 15
(b)
12
80
12
9
60
9
6
40
6
3
20
3
0
0 0
520
0.3
(b)
0.2
0
Concentration of Ni (mg/kg)
0.5
Exp.1 Exp.2 Exp.3 Exp.4 Exp.5
20
40
60
80
Time(h)
100
120
(c)
0 0
20
40
60
80
Time(h)
100
120
0
Time(h)
Fig. 4. Change of metal concentration in solutions as a function of time during washing procedure; 22
(a) NH, (b) BE, (c) HA sediment; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3.
23
(F4) Residual (F3) Organic matter and sulfides (F2) Fe-Mn oxyhydroxides (F1) Soluble and exchangeable cations, and carbonate
Fractionation(NH)
1.2
(a)
(b)
(c)
(d)
(e)
1.0 0.8 0.6 0.4 0.2 0.0
Fractionation(BE)
1.2
al iti In
r 0h 12
(a)
al iti In
r 0h 12
(b)
al iti In
r 0h 12
(c)
al iti In
r 0h 12
(d)
al iti In
r 0h 12
(e)
1.0 0.8 0.6 0.4 0.2 0.0
Fractionation(HA)
1.2
r 0h 12
(a)
al iti In
r 0h 12
(b)
al iti In
r 0h 12
(c)
al iti In
r 0h 12
(d)
al iti In
r 0h 12
(e)
1.0 0.8 0.6 0.4 0.2 0.0
525
al iti In
al iti In
r 0h 12
al iti In
r 0h 12
al iti In
r 0h 12
al iti In
r 0h 12
al iti In
r 0h 12
Fig. 5. Fractionation of metals in sediments before and after washing procedure with EDTA; 24
(a) Cd, (b) Cu, (c) Ni, (d) Pb, (e) Zn
1.8
Residual sediment Solution
Recovery (HA)
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
1.4 Residual sediment Solution
Recovery (NH)
1.2 1.0 0.8 0.6 0.4 0.2 0.0 Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
1.2 Residual sediment Solution
Recovery (BE)
1.0 0.8 0.6 0.4 0.2 0.0 Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Cd Cu Ni Pb Zn
Exp. 1
Exp. 2
Exp. 3
Exp. 4
Exp. 5
Fig. 6. The overall results of mass balance before and after sediment washing experiments; Extracting agents from Exp. 1 to Exp. 5 are EDTA, NaCl, HCl, sodium citrate, HNO3. 530
25
Extracted Fe/Pb Molar ratio
4 Pb/Fe
3
2
1
0 0
20
40
60
80
100
120
140
Time (h) Fig. 7. Correlation between molar ratio of Pb and Fe extracted into EDTA solution in HA sediment
26
160 6 hours 120 hours
Extracted Fe (mg/L)
140 120 100 80 60 40 20 0 SC 535
EDTA
HCl
HNO3
Extracting agents Fig. 8. Comparison of the extracted Fe into extracting agent solutions in HA sediment
27
1.6 NH sediment - BIT NH sediment - PMIT
1.4
BE sediment - BIT
BIT & PMIT
1.2
BE sediment - PMIT HA sediment - BIT
1.0
HA sediment - PMIT
0.8 0.6 0.4 0.2 0.0 Initial
6h
120 h
Fig. 9. The BIT and PMIT of heavy metals in residual sediments by SM&T sequential extraction 540
before and after washing process in Exp. 1
28
Table 1. The initial physicochemical characteristics of sediments Parameter pH
N 7.4
Value B 8.6
H 8.1
4.0 89.4 6.6 50.2 3.1
36.0 60.8 3.2 43.76 3.5
19.2 71.3 9.5 55.5 4.5
8.2
5.3
9.1
27.7
23.7
33.6
31000 30210
22940 24000
27600 40400
Sediment texture (wt%) (ASTM D421) Sand Silt Clay Water content (wt%) Carbonate content (wt%) Organic matter (wt%) (ASTM D2974) CEC (cmol(+)/kg) (EPA 9081) Major elements Ca (mg/kg) Fe (mg/kg)
Table 2. Heavy Metals concentration as three analysis procedures of sediments; Samples
545
550
Methods
Cd 0.6 n.d 1.8 2.8 n.d 1.7 3.4 n.d 1.6
Concentrations (mg/kg) Cu Ni Pb 21.8 11.1 23.8 49.0 48.0 66.0 33.4 24.7 30.4 93.3 13.0 53.7 124.0 40.0 116.0 106.8 20.7 57.6 22.5 13.0 27.1 79.0 50.0 51.0 75.2 23.5 86.6
Zn 1a) 74.8 N 2b) 136.0 125.5 3c) 1 197.5 B 2 280.0 3 249.0 1 65.8 H 2 209.0 3 247.0 Korean Ocean Dumping 2.5 65 35 50 200 Limiting Valuesd) Legal Regulation of 4 150 100 200 300 Korean Soil Qualitye) a) Aqua regia extraction b) Analysis by XRF c) Total concentration by HNO3, HClO4 and HF d) The regulation is based on total concentration of heavy metals in the soil e) The regulation is based on pseudo-total concentration of heavy metals extracted by aqua regia. Table 3. The conditions of sediment washing experiments Washing experiment Exp. 1 Exp. 2 Exp. 3 Exp. 4 Exp. 5
Reagents Na2EDTA NaCl HCl sodium citrate HNO3
Concentration 0.1M 0.1M 0.1M 0.1M 0.1M
Initial pH 4.41 5.32 1.09 8.88 1.05 29
Table 4. Summary of three-step SM&T sequential extraction procedure Step 1
Fraction (F1) Soluble and exchangeable, carbonate
0.11M CH3COOH for 16h at 22±5℃
Solution
2
(F2) Bound to Fe-Mn oxides, reducible
0.5M NH2OH·HCl added 2M HNO3 for 16h at 22±5℃
3
(F3) Bound to organic matter and sulfides, oxidisable
8.8M H2O2 at 85±2℃ then 1M CH3COONH4 adjusted
4
(F4) Residual
to pH 2±0.1 with concentrated HNO3 HCl : HNO3 = 3 : 1, aqua regia
555 Table 5. Stability constants (Ks) for formation of complexes from metals with EDTA, chloride and citrate [24]. Metals Cd Cu Ni Pb Zn Fe Ca
Chloride CdCl+ CuCl+ NiCl+ PbCl+ ZnCl+ FeCl2+ CaCl2
Ks 2 0.5 0.6 1.6 0.4 2.1 -
EDTA Cd-EDTA2Cu-EDTA2Ni-EDTA2Pb-EDTA2Zn-EDTA2Fe-EDTACa-EDTA2-
Ks 18.2 20.5 20.4 19.8 18.3 27.7 12.4
citrate +
CdH2-citrate CuOH-citrate2NiH2-citrate+ PbH2-citrate+ ZnH2-citrate+ Fe2(OH)2-citrate22CaH2-citrate+
Ks 12.6 16.4 12.9 8.1 13.3 56.3 12.3
30
Research Highlights 560
> Extractability of metals from marine sediments was investigated using various agents > EDTA covered the strongly bound fraction of Cu and Pb to exchangeable fraction.. > Potential hazard of the sediments was evaluated.
565
31