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Chelator-assisted washing for the extraction of lead, copper, and zinc from contaminated soils: A remediation approach
Applied Geochemistry 109 (2019) 104397
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Chelator-assisted washing for the extraction of lead, copper, and zinc from contaminated soils: A remediation approach
T
Hiroshi Hasegawaa,∗, M. Abdullah Al Mamunb,c,∗∗, Yoshinori Tsukagoshib, Kento Ishiib, Hikaru Sawaid, Zinnat A. Begume,f, Mashio S. Asamia, Teruya Makia, Ismail M.M. Rahmang,∗∗∗ a
Institute of Science and Engineering, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan c Department of Soil Science, Hajee Mohammad Danesh Science and Technology University, Dinajpur, 5200, Bangladesh d Department of Industrial Engineering, National Institute of Technology, Ibaraki College, 866 Nakane, Hitachinaka City, Ibaraki, 312-8508, Japan e Venture Business Laboratory, Advanced Science and Social Co-Creation Promotion Organization, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan f Department of Civil Engineering, Southern University, 739/A Mehedibag Road, Chittagong, 4000, Bangladesh g Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima City, Fukushima, 960-1296, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Chelator Extraction Mobility Potentially toxic elements Remediation Soil washing
Chelators have proven to be effective extractants in soil washing and represent a suitable remediation technology for potentially toxic elements (PTEs) from contaminated sites. In this paper, the extraction of lead (Pb), copper (Cu), and zinc (Zn) from contaminated real and reference soils using both biodegradable (EDDS and HIDS) and persistent (EDTA and DTPA) chelators was investigated. Different metal–chelator complex formation constants, pH, extraction temperature, and mechanochemical energy were tested. The PTEs were extracted at a relatively higher rate under acidic to neutral conditions (pH 5 and 7), with the greatest extraction being observed at pH 5. The metal extraction efficiencies of the chelators for the real sample at pH 5 were as follows: Pb – EDTA > DTPA > EDDS > HIDS; Cu – DTPA > EDTA > EDDS > HIDS; and Zn – DTPA > EDTA > HIDS > EDDS. The data suggested that EDDS was more effective than HIDS for Cu and Zn extraction in neutral to slightly alkaline solutions, as well as at raised temperature. Furthermore, EDTA washing using mechanochemical agitation was found to enhance the removal efficiency of Cu and Zn. Sequential chemical extraction showed that the apparent metal mobilities were reduced by soil washing. EDTA and DTPA appeared to offer greater potential than the biodegradable chelators in extracting metals from the relatively mobile carbonate- and Fe–Mn oxidebound fractions. The residual fraction of Pb displayed considerable stability (≥70%), and was not entirely amenable under washing treatments, and hence could be considered non-bioavailable and non-toxic.
1. Introduction Soil polluted with potentially toxic elements (PTEs) is one of the most critical environmental problems worldwide. Industrialization, urbanization, and modern agricultural practices are considered to be the sources of PTEs in soil. Plants continuously accumulate PTEs from the soil, which results in unacceptable toxicity to human beings and animals through dietary routes. Therefore, a large number of studies on metal remediation of contaminated soils have been conducted to reduce the risk of PTEs to human health and the ecosystem. These remediation technologies are categorized into three broad groups: physical,
chemical, and biological remediation, and have been thoroughly described in several reviews (Shahid et al., 2012; Saifullah et al., 2015; Begum et al., 2016; Rahman et al., 2016; Khalid et al., 2017). The treatment of soil is of great importance in polluted areas and sites. Soil washing is the most promising remediation technique, and provides a viable and permanent option for obtaining clean soil from polluted sites. The metals extracted through washing can also be recycled and easily re-introduced into the material cycle rather than being landfilled (Fedje et al., 2013). In such washing techniques, many different chemical species, including acids, bases, salts, oxidizing or reducing agents, chelating agents, surfactants, and solvents, are used for
Corresponding author. Corresponding author. Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (H. Hasegawa), [email protected] (M.A.A. Mamun), [email protected], [email protected] (I.M.M. Rahman). ∗
∗∗
https://doi.org/10.1016/j.apgeochem.2019.104397 Received 9 November 2018; Received in revised form 28 June 2019; Accepted 6 August 2019 Available online 08 August 2019 0883-2927/ © 2019 Elsevier Ltd. All rights reserved.
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separating contaminants from the contaminated soils (Begum et al., 2016). The performance of chelators in washing processes has been widely assessed, and they have been found to efficiently remove PTEs and to have a less severe effect on the environment (Peters, 1999; Tandy et al., 2004; Begum et al., 2012a). Chelators also have high complex formation constants under weakly acidic to alkaline conditions, and do not require the use of heating or strong acids during extraction. Chelators are thus considered to be suitable extractants for a wide variety of PTEs, and are expected to reduce the cost and the environmental burden of the metal extraction process (Hasegawa et al., 2011; Pinto et al., 2014; Begum et al., 2016). EDTA (2,2ʹ,2ʺ,2ʹʹʹ-(ethane-1,2-diyldinitrilo)tetraacetic acid) and its homologs are most frequently reported to be used for the decontamination of PTEs due to their high potential for this application, relatively low cost, and wide availability (Leštan et al., 2008; Begum et al., 2016). However, EDTA and its homologs are characterized by low biodegradability and relatively high environmental persistence (Leštan et al., 2008; Begum et al., 2013a). Accordingly, further research has focused on the use of biodegradable alternatives like EDDS (2-[2(1,2-dicarboxyethylamino) ethylamino]butanedioic acid), GLDA (2-[bis (carboxymethyl)amino]pentanedioic acid), IDSA (2-(1,2-dicarboxyethylamino)butanedioic acid), MGDA (2-[bis(carboxymethyl)amino] propanoic acid) and HIDS (2-(1,2-dicarboxyethylamino)-3-hydroxy-butanedioic acid) rather than EDTA (Pinto et al., 2014; Begum et al., 2016). Multiple factors affect the effectiveness of the washing process, and the selection of washing additives for specific contaminated sites is of critical importance to develop technically and economically feasible remediation technologies (Dermont et al., 2008). The critical determinants of metal toxicity are its complexation ability and oxidation state, which also affect the bioavailability of the metals (Kakkar and Jaffery, 2005). For an accurate estimation of the potential environmental impacts, toxicity, mobility, and bioavailability of PTEs, particularly those found in soil (e.g., Pb, Zn, Cd, Ni, and As), it is necessary to identify the chemical forms of the contaminant elements, in addition to their total contents. In this regard, several operationallydefined extraction procedures have been introduced for partitioning metal fractions, with the Tessier protocol (Tessier et al., 1979) or the BCR scheme (Rauret et al., 1999) being widely used for metal fractionation in diverse environmental samples (Desaules, 2012; Mizutani et al., 2016; Rahman and Begum, 2019). In this study, a wet washing method based on the separation and removal of PTEs by chelators was applied to contaminated soils. Iminoacetic acid-type chelators, which exhibit high metal-chelator complexation constants and smooth biodegradability with low environmental loads, were used to extract the PTEs Pb, Cu, and Zn. The extraction performance was evaluated using different chelator species, solution pH, operating temperatures/times, and mechanochemical energies. Moreover, the species distribution of the PTEs in the soils before and after chelator-assisted extraction was investigated using the Tessier sequential extraction procedure. The extraction experiments were carried out under controlled conditions and with particular attention to the environmental regulations of Japan.
(Cd, Cr, Cu, Ni, Pb, and Zn) contents were certified following collaborative procedures, as discussed elsewhere (Pueyo et al., 2001; Rauret et al., 2001). 2.2. Chelators Two representative persistent chelators, EDTA and DTPA (2-[bis[2[bis(carboxymethyl) amino]ethyl]amino]acetic acid) (Kanto Chemicals; Tokyo, Japan), and two biodegradable chelators, EDDS (Chelest; Mie, Japan) and HIDS (Nippon Shokubai; Tokyo, Japan), were used in the current work. The chemical structure, acid dissociation constants (pKa), and stability constants (log KML) of those chelators are shown in Table S2 (Appendix A. Supplementary Information). 2.3. Other materials Analytical grade chemicals were used throughout the study without further purification. Purified water was used to dilute the chelators to 0.01 mol L−1. Metal standards were prepared in the μmol to mmol range from a PlasmaCAL multi-element solution in 5% HNO3 (SCP Science; Quebec, Canada) by dilution on a weight basis. The pH values of the experimental solutions were adjusted using either HCl or NaOH (1 mol L−1) (Kanto Chemicals; Tokyo, Japan). Acetic acid/sodium acetate (CH3COOH/ CH3COONa; 0.1 mol L−1), CAPS (N-cyclohexyl-3-aminopropanesulfonic acid; 0.01 mol L−1), TAPS (N-[tris (hydroxymethyl) methyl]-3-amino-propanesulfonic acid; 0.01 mol L−1) (Kanto Chemicals; Tokyo, Japan), and HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; 0.01 mol L−1) (Nacalai Tesque; Kyoto, Japan) were used as buffer reagents for pH 3–5, 9, 11, and 7, respectively. During sequential extraction (SE), magnesium chloride (MgCl2) (Merck; Darmstadt, Germany), hydroxylamine hydrochloride (HONH2·HCl) (Wako Pure Chemical; Tokyo, Japan), and hydrogen peroxide (H2O2) (Kanto Chemicals; Tokyo, Japan) were used. Low-density polyethylene bottles (Nalge Nunc; Rochester, NY) and perfluoroalkoxy tubes and micropipettes (Nichiryo; Tokyo, Japan) were used throughout the experiments. All the laboratory equipment was subjected to a cleansing treatment following the protocol as described elsewhere (Hasegawa et al., 2018). 2.4. Equipment The metal concentration was quantified using inductively coupled plasma optical emission spectrometry (ICP-OES; iCAP 6300; Thermo Fisher Scientific; Waltham, MA). A microwave reaction system (Multiwave 3000; Anton Paar GmbH; Graz, Austria) was used for the digestion of samples. An energy dispersive X-ray spectroscope (EDX; INCA Energy) attached to a field emission scanning electron microscope (FE-SEM; Oxford Instruments; Abingdon, Oxfordshire) was used for mapping the elements in the sample. Automated high-performance liquid chromatography (HPLC; Tosoh 8020; Tosoh; Tokyo, Japan) was used for analysis of the chelator concentrations. A heat-block type thermal decomposition system (DigiPREP Jr; SCP Science; Quebec, Canada) and a compact high-pressure autoclave (MLS 3750; Sanyo Electric; Osaka, Japan) were used to perform the temperature-controlled extraction experiments. A planetary mono mill (Pulverisette 6; Fritsch Laborgerätebau; Idar-Oberstein, Germany) was used to conduct the experiments in which remediation was carried out using mechanochemical energy. A constant temperature Bioshaker (BR30 L; TAITEC; Tokyo, Japan) was used for the rotary shaking of the samples. A centrifugal separator (H-701FR; Kokusan; Tokyo, Japan) was used to isolate the residue and filtrate in a solid–fluid suspension. A pH meter (Navi F-52; Horiba Instruments; Kyoto, Japan) was used for pH measurements. Cellulose membrane filters (0.45 μm; Advantec; Tokyo, Japan) were used for the vacuum filtration of samples. An Arium Pro water purification system (Sartorius Stedium Biotech GmbH; Gottingen, Germany) was used to produce purified water with a resistivity of > 18.2 MΩ cm.
2. Experimental 2.1. Soil samples A real contaminated fine-grain sludge sample (TCS-18) from a soil treatment facility in Japan (Yamazaki Jari Shoten Co. Ltd.; Otsu, Japan) was used in this experiment. Additionally, an organic-rich soil standard substance (BCR-700) from the European Community Standards Bureau was used to validate the metal extraction. The real sample was dried to a constant weight in an oven at 65 °C, carefully homogenized to a size of 2 mm or less, and stored in a glass bottle. The physico-chemical characters of TCS-18 are presented in Table S1 (Appendix A. Supplementary Information). BCR700 was collected from a German agricultural research station, homogenized, and bottled. Its EDTA and acetic acid-extractable trace element 2
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2.5. Methods
Table 1 Contents of the PTEs (Pb, Cu, and Zn) and other elements in the TCS-18 soil after microwave-assisted acid-digestion and weak-acid induced extraction.
2.5.1. Microwave-assisted digestion Microwave decomposition was used for three types of samples: (a) raw soil, to measure the total metal content; (b) the residue from SE treatment before chelate extraction; and (c) the residue from SE treatment after chelate extraction. The precleaned PTFE reaction vessels were fed with 0.05 g of sample and 1 mL 65% HNO3; the mixture was allowed to stand for 10 min to decompose the organic matter. Subsequently, 3 mL HCl and 1 mL HF were added, and the reactor was sealed with a Teflon seal, and microwave treatment was applied for decomposition. After that, 10 mL of 50 g L−1 boric acid was added to mask the HF in the solution, and microwave treatment was again applied. The optimized operation conditions were chosen according to the manufacturer's recommendations. The mixtures obtained after the microwave reaction were diluted to 50 mL with purified water and filtered. A blank was treated using the same procedure as the samples, and each of the treatments was performed in triplicate.
Elements
Al Cu Fe Mn Pb Zn
MWDa
WEEb
(mg kg−1)
(mg kg−1)
85051 ± 2439 104 ± 4 41523 ± 495 838 ± 12 522 ± 41 309 ± 7
8290 ± 230 77.0 ± 2.6 7025 ± 170 408 ± 14 97.9 ± 3.6 220 ± 9
a MWD: Element contents derived from microwave-assisted acid digestion (extractable with mixed acids: HNO3, HCl, HF), n = 3. b WEE: Element contents derived from weak-acid induced extraction (HCl, 1 mol L−1; liquid to solid ratio, 100:3; duration, 2 h; T, 25 ± 2 °C), n = 3.
extraction with the chelators is presented in Fig. S1 (Appendix A. Supplementary Information). The Pb, Cu, and Zn contents in TCS-18 or BCR-700 detected after microwave digestion were higher than those detected after weak-acid induced extraction.
2.5.2. Chelator-assisted extraction of PTEs The samples were subjected to batch extraction with EDTA, DTPA, EDDS, and HIDS at a 1:10 sample-to-chelator ratio and shaken at 200 rpm for 24 h (T, 25 °C). The soil–chelator suspensions were also treated at 105, 115, 125, and 135 °C (duration, 1 h) and 40, 60, and 80 °C (duration, 6 h) to study the effect of temperature on the extraction of the PTEs by the chelators. The solid–fluid suspension was then centrifuged and filtered to separate the residue; the elemental content of the filtrate was analyzed using ICP-OES. In the test of extraction induced by mechanochemical energy, the mill pot of the planetary mono mill was fed with 10 g sample, 90 g of zirconia balls (ϕ, 8), and 100 mL of EDTA (0.01 mol L−1; pH 7). Wetgrinding was carried out in the ball mill at 200 rpm, and 1 mL of the supernatant was collected at 1, 2, 3, 4, 5, 6, 24, and 48 h. The supernatant samples were analyzed 24 h and one week after chelator-assisted extraction to measure the residual chelator. The samples were thoroughly mixed once per day and then allowed to stand overnight to ensure uniform mixing of the suspensions. All the supernatants were filtered to separate trace residues, and the filtrates were analyzed using ICP-OES.
3.2. Investigation of the optimum conditions for the extraction of the PTEs from TCS-18 3.2.1. Effect of the conditional metal-chelator complexation constant on the extraction of PTEs The extraction of PTEs from TCS-18 with various chelators at pH 7 was studied. The conditional metal-chelator complexation constants (logK'ML; pH, 7) of each chelator were calculated following the protocol described in Begum et al. (2012b). The logK'ML values for the metalchelator complexes of each chelator are listed in Table S2 (Appendix A. Supplementary Information). The amount of PTEs extracted using the chelators was much higher than the amount extracted using the control. The Pb-extraction rates with different chelators followed the order DTPA > EDDS > EDTA > HIDS. The Cu-extraction efficiency of the chelator species followed the order EDDS > EDTA > DTPA ≈ HIDS, and the Zn-extraction rates showed the following trend: EDDS > EDTA > DTPA > HIDS (Fig. 1a). EDDS thus appeared to be the superior extractant for Pb, Cu, or Zn, followed by EDTA. Tandy et al. (2004) concluded that at a chelator-to-metal ratio of 1 and pH ≥ 6, EDDS was a superior extractant compared to EDTA because it
2.5.3. Determination of the weak-acid-extractable PTEs in the soils The content of weak-acid-extractable PTEs in the soils before the chelator-assisted washing treatment and in the residues after chelatorassisted extraction was determined following the protocol suggested by MOE (2003), based on the Soil Contamination Countermeasures Law of Japan (Anonymous, 2002). Soil (1.0 g) was mixed with 1 mol L−1 HCl in a polyethylene decomposition vessel (liquid-to-solid ratio, 100:3) and shaken at 200 rpm for 2 h (T, 25 °C). The soil-fluid suspension was centrifuged at 3000 rpm for 20 min, after which the supernatants were separated from the residue via filtration, and their PTE contents were analyzed using ICP-OES.
Table 2 Contents of the PTEs (Pb, Cu, and Zn) and other elements in the BCR-700 soil after microwave-assisted acid-digestion, weak-acid induced extraction, and a comparison of the experimental and certified concentrations of EDTA-extractable PTEs. Elements
2.5.4. Distribution of the metals by sequential extraction (SE) The mobility of the metals in TCS-18 before and after washing with the chelators was determined using the sequential extraction method following the Tessier protocol (Tessier et al., 1979; Begum et al., 2013b; Rahman et al., 2019). The compositions of the extractants used to demarcate the different operationally defined soil solid phases are shown in Table S3 (Appendix A. Supplementary Information).
Al Cu Fe Mn Pb Zn
MWDa
WEEb
EDTA-E-Ec
EDTA-E-Cd
(mg kg−1)
(mg kg−1)
(mg kg−1)
(mg kg−1)
41082 ± 2579 184 ± 3 40120 ± 3596 1451 ± 360 463 ± 8 1592 ± 100
1860 ± 70 108 ± 5 11020 ± 240 831 ± 25 182 ± 7 1050 ± 40
– 84.4 ± 2.1 – – 109 ± 5 582 ± 13
– 89.4 ± 2.8 – – 103 ± 5 510 ± 17
a MWD: Element contents derived from microwave-assisted acid digestion (extractable with mixed acids: HNO3, HCl, HF), n = 3. b WEE: Element contents derived from weak-acid induced extraction (HCl, 1 mol L−1; liquid to solid ratio, 100:3; duration, 2 h; T, 25 ± 2 °C), n = 3. c EDTA-E-E: Experimentally-derived EDTA-extractable element contents in BCR-700 (EDTA, 0.05 mol L−1; liquid to solid ratio, 10:1; duration, 2 h; T, 25 ± 2 °C), n = 3. d EDTA-E-C: Certified values of EDTA-extractable element contents in BCR700 (Pueyo et al., 2001; Rauret et al., 2001).
3. Results and discussion 3.1. PTE content in the soils The PTE (Pb, Cu, and Zn) contents in TCS-18 and BCR-700 after microwave-assisted acid digestion and weak-acid induced extraction are shown in Tables 1 and 2. The elemental mapping of TCS-18 before 3
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Fig. 1. (a) Extraction behavior of the PTEs (Pb, Cu, and Zn) in TCS-18 using different chelators. (b) Correlation between the conditional metal-chelator complexation constant (logK'ML) and the extraction of the PTEs by the chelators. Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid-fluid suspension mixing speed, 200 rpm; process duration, 24 h.
compromises the interactions of metals with Ca and Fe in soils more effectively than EDTA. The higher Pb extraction ability of DTPA compared to the other chelators could be attributed to its ability to compete with soil organics to create soluble complexes (Li and Shuman, 1997). The correlations between logK'ML and the chelator-assisted extraction rates of Pb, Cu, and Zn yielded inconsistent trends (Fig. 1b). The observed behavior could be attributed to the co-existence of several variables within this system, which might alter the expected extraction performance. Such variables include the percent ratio of the dominant metal–chelator species in the solution, the co-precipitation behavior of
the metal ions, and the re-sorption ratio of the metal-chelator complexes, among others (Nowack, 2002). 3.2.2. Effect of pH on Pb, Cu, and Zn extraction The chelator-assisted extraction of the PTEs in the soils were significantly influenced by the solution pH, which influences several factors, e.g., the aqueous species concentration of the PTEs, the solubilization of the chelators, the sorption and desorption of the PTEs, the ionic exchange behavior between PTEs, and the re-adsorption mechanism of the metal–chelator complexes, among others (Polettini 4
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control solution indicated that the relative contents of the water-soluble fractions in the soil were low. A similar observation has been reported by Niinae et al. (2008) for Pb extraction using chelators. The efficacy of the chelators in acidic (pH 3 and 5), neutral (pH 7), and alkaline (pH 9 and 11) conditions can be summarized as follows: (a) the highest extraction efficiency was achieved at an acidic pH of 5; (b) the efficiency at a neutral pH of 7 was lower than that at pH 5, but higher than that at pH 3; and (c) the efficiency was lower in alkaline environments than at neutral conditions. The Cu extraction performances of EDTA and EDDS were similar at pH 5, 7, 9, and 11. The formation of stable complexes of Cu and EDTA or EDDS would explain this result, as the EDTA–Cu and EDDS–Cu complexes have similar stability constants of 18.8 and 18.4, respectively (Martell and Smith, 2001; Guo et al., 2010). Although the dissolution of the PTEs attached to solid phases increases with the solution acidity, higher PTE extractions were observed at pH 5 than pH 3. This might have been due to the protonation of the chelator in the more acidic solution (HL, H2L, …, HnL), which would lead to lower conditional complexation constant than at the other pH levels measured. The PTEs in the solution tend to form nearly insoluble hydroxides with increasing pH at pH 7 to 11 (M (OH), M (OH)2, …, M(OH)n), which might have been responsible for the lower extraction rates at basic conditions. EDTA was superior to DTPA for extracting Pb, while the opposite was found for the extraction of Cu and Zn at pH 5. However, DTPA was the best option for Pb extraction at pH 7. EDDS showed higher Pb extraction efficiency under neutral to slightly alkaline conditions (pH 7–9); its efficiency was comparable with that of EDTA and HIDS. EDDS also exhibited relatively higher extraction of Cu and Zn at pH 7 and 9 compared to EDTA and DTPA. The optimal PTE extraction efficiency of EDDS at pH 7–9 was also reported in earlier works (Tandy et al., 2004; Begum et al., 2013b). EDDS and HIDS displayed similar performance in the extraction of Cu and Zn at pH 3 and 5. HIDS was the least efficient in Pb extraction at pH 11. 3.2.3. Effect of the extraction temperature The mobility and complex formation of PTEs can be enhanced due to the effect of temperature change on the behavior of the humic substances and the rate of desorption of the PTEs from the Fe–Mn oxide fraction of the soil (Weng et al., 2002). Fig. 3 shows the effect of applying different temperatures and extraction times on the extraction of PTEs from TCS-18 using the chelators. Pb and Zn extraction performance of the control solution did not improve even when the reaction temperature was raised to 135 °C. The EDTA washing results indicated that the extraction performance of Pb and Zn was improved when the temperature was increased from 60 to 135 °C. DTPA and EDDS washing also showed a similar trend, except in the case of Pb. A gradual increase in the extraction of Cu with EDTA, DTPA, and EDDS was observed with increasing temperature up to 115, 115 and 125 °C, respectively. The extraction of Pb by HIDS was relatively insensitive to temperature, while the extraction of Pb by EDDS decreased with increasing temperature in the range 80–135 °C. The extraction of Cu by HIDS increased with temperature up to 80 °C, but decreased remarkably at ≥105 °C, while the EDDS-assisted extraction of Cu started to decrease at ≥125 °C. The Zn-extraction rates either showed an increasing trend (EDTA, DTPA, or EDDS) or were unaffected (HIDS) with increasing system temperature. The lower Pb-extraction efficiency of EDDS and HIDS and reduced Cu-extraction of HIDS at higher temperatures can be attributed to the thermal denaturation of the biodegradable chelators, which diminished their complexation ability. The Cu–HIDS complex formation constant was low compared to that of EDDS, which might be the reason for the low Cu extraction of HIDS.
Fig. 2. Effect of the solution pH on the chelator-assisted extraction of the PTEs (Pb, Cu, and Zn) in TCS-18. Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid-fluid suspension mixing speed, 200 rpm; process duration, 24 h.
et al., 2007; Zou et al., 2009; Qi et al., 2011). The extraction of Pb, Cu, and Zn from TCS-18 by the chelators at various pH values, along with results of the blank experiments, are shown in Fig. 2. The extraction efficiencies using the chelators increased considerably as the pH was varied compared to the extraction efficiency of the control solution. The minimal extraction of PTEs in the
3.2.4. Mechanochemical effect The introduction of mechanical force is a simple and economical approach to achieve enhanced metal recovery from solid wastes (Tan 5
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Fig. 4. Comparative impacts of normal shaking and mechanochemical treatment on the extraction of the PTEs (Pb, Cu, and Zn) with EDTA for varying process durations. Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid–fluid suspension mixing speed, 200 rpm.
Fig. 3. Effect of the system temperature on the chelator-assisted extraction of the PTEs (Pb, Cu, and Zn) in TCS-18 for varying process durations (T, 25 °C: 24 h; T, 40–80 °C: 6 h; T, 105–135 °C: 1 h). Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; solid–fluid suspension mixing speed, 200 rpm.
process durations of up to 48 h. Additionally, the amount of extraction under normal shaking conditions was measured for comparison, and is shown in Fig. 4. The amount of Pb, Cu, and Zn extracted in the mechanochemical process increased with increasing process time, and the total amount extracted was higher than that obtained by regular shaking. The improved removal of PTEs via mechanochemical processing could be attributed to the grinding action of the ball mill, which
and Li, 2015). The addition of mechanical energy can have a beneficial effect on chemical reactions via lattice defects, strain energy, and surface energy (Maki et al., 2011; Ou et al., 2015; Tan and Li, 2015). We expected that the extraction efficiency of the chelators could be improved using mechanochemical reaction, and thus investigated the effect of EDTA on metal removal by wet pulverization (pH 7) for varying 6
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Fig. 5. Comparison of the extracted and residual contents of the PTEs (Pb, Cu, and Zn) in the standard (BCR-700) and real (TCS-18) samples after washing treatment with different chelators at different pHs (5, 7, and 9). Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid–fluid suspension mixing speed, 200 rpm; process duration, 24 h.
7
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decreases the particle size and increases the surface area of the soil particles (Hasegawa et al., 2013).
7, and 9. The residual PTEs in the chelator-washed samples were then extracted with 1 mol L−1 HCl. The extraction of PTEs from BCR-700 or TCS-18 followed the order pH 5 > 7 > 9 (Fig. 5) for all extractants. At pH 5, EDTA extracted 27.9 times more Pb, 5.6 times more Cu, and 1.7 times more Zn than the control. The residual Pb content and removal percentage after the EDTA washing of BCR-700 were 50.6 mg kg−1 (72.3%), 127.1 mg kg−1 (30.0%), and 151.2 mg kg−1 (16.4%) at pH 5, 7, and 9 respectively. In TCS-18, the amount of Pb extracted by EDTA
3.3. Comparison of Pb, Cu, and Zn extraction behavior between TCS-18 and BCR-700 Each extractant (EDTA, DTPA, EDDS, and HIDS) was applied to BCR-700 and TCS-18 to compare their extraction performances at pH 5,
Fig. 6. Comparison of the distribution of the PTEs (Pb, Cu, and Zn) in TCS-18 before and after washing with the chelators at different pH values (5 and 7). Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid-fluid suspension mixing speed, 200 rpm; process duration, 24 h. Abbreviations used: BW, before wash; F1, exchangeable; F2, carbonate-bound; F3, Fe–Mn oxide bound; F4, organic matter-bound; F5, residual; CW, chelator-washed fraction. 8
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was 60.9 mg kg−1, 28.6 mg kg−1, and 14.8 mg kg−1 at pH 5, 7, and 9, respectively. The residual content of Pb and the reduction percentage after EDTA washing were 26.2 mg kg−1 (73.2%), 68.4 mg kg−1 (30.1%), and 82.7 mg kg−1 (15.5%) at pH 5, 7, and 9 respectively. The Cu and Zn contents in both BCR-700 and TCS-18 decreased greatly after extraction with the chelators. A noticeable effect was found using EDTA and DTPA at pH 5; EDTA, DTPA, and EDDS at pH 7; and DTPA and EDDS at pH 9. The residual Cu content (mg kg−1) after washing the BCR700 sample followed the order DTPA > EDDS > EDTA > HIDS > Control at pH 5; DTPA > EDDS > EDTA > HIDS > Control at pH 7; and DTPA > EDDS > HIDS > EDTA > Control at pH 9. The same trends were observed in the residual Cu content of TCS-18. DTPA appeared to be an effective extractant for Cu separation from both the samples. For BCR-700, the extraction efficiency of Zn followed the order DTPA > EDTA > EDDS > HIDS > Control at pH 5; EDTA > DTPA > EDDS > HIDS > Control at pH 7; and DTPA > EDTA > EDDS > HIDS > Control at pH 9. The extraction efficiency at low concentrations of chelators depends on the pH and the abundance of major cations, especially Ca and Fe, in the soil (Chrastný et al., 2008). The extractability of PTEs with chelators usually follows the order Zn > Cu > Pb (Bermond and Varrault, 2004; Wasay et al., 2007; Xia et al., 2009). Pb is highly immobile and more difficult to extract due to its stronger adsorption to soil particles due to its small hydrated radius and higher ionic charge, whereas Cu and Zn are intermediate in terms of hydrated radii and extractability. However, comparatively higher removal of Cu from artificially contaminated soil was observed under neutral and alkaline conditions using biodegradable chelators (Begum et al., 2012a). The extraction amount showed a correlation with the decrease in the residual metal content of the chelator-extracted soil. From the above results, the trends in the extraction of the PTEs from BCR-700 were found to be comparable with the results obtained from the real sample TCS-18.
to be extracted easily (Tessier et al., 1979). Elless and Blaylock (2000) and Tandy et al. (2004) reported that strongly bound metal fractions could not be desorbed even using highly potent chelators, such as EDTA. Therefore, extraction with a chelator is believed to be useful for the PTEs bound to the F1 to F4 fractions of soils. The higher extraction from the F1 to F3 fractions confirmed that mobilization of PTEs occurred due to changes in the ionic strength or reduced pH and redox potential. 3.5. Evaluation of the retainability of the chelators Chelators with low biodegradability remain in the environment for a long time, and necessitate apprehension for further metal elution. Microorganisms or other processes can decompose biodegradable chelators, which have amino acid skeletons, reducing the environmental burden. Therefore, the residual concentration of the chelators used in this study was measured in TCS-18 (Fig. S2; Appendix A: Supplementary Information). No significant change was observed in the concentrations of the persistent chelators, and the average concentration after 24 h and 1 week was 9.5 and 9.3 mmol L−1 for EDTA and DTPA, respectively (initial concentration 10 mmol L−1). On the contrary, a remarkable decrease was observed in the concentrations of the biodegradable chelators after a week. The concentration and percent decrease after one week were 6.7 mmol L−1 (32.5%) for EDDS and 8.2 mmol L−1 (17.7%) for HIDS. It was believed that decomposition by microorganisms was responsible for the decrease in the concentration of the biodegradable chelators over time. Tandy et al. (2004) showed that the half-life of EDDS was 4.2–5.0 days, while degraded after a lag of 7–11 days. However, in this experiment, no significant decrease was observed in the concentrations of EDDS and HIDS after 24 h. Thus, it is conceivable that degradation by microorganisms did not occur. Furthermore, the HPLC measurement takes about 40 min per sample; part of the sample solution may have evaporated and thus concentrated the chelators due the open state of the sample. These factors might result in the measurement of high concentrations of chelators, especially EDDS and HIDS, even if biodegradation by microorganisms did occur.
3.4. Analysis of the mechanism of chelator-assisted extraction using sequential extraction Determining the fractionation of the PTEs in the soil solid-phases is essential to explore the binding affinity between the PTEs and the soil particles, and the extent to which they can be remobilized into the environment (Turki, 2007). The Tessier protocol defines five fractions of soil solid-phases: exchangeable (F1), carbonate-bound (F2), Fe–Mn oxide-bound (F3), organic matter-bound (F4), and residual (F5). The relative metal concentrations of TCS-18 before and after extraction with the chelators (pH 5 and 7) as determined using sequential chemical extraction are shown in Fig. 6. These two pH values (5 and 7) were chosen due to the higher extraction efficiencies achieved at these pHs. The Pb contents of F2 and F3 decreased remarkably after being washed with the chelators. In the case of EDTA and DTPA-assisted washing at pH 5, the Pbcontent decreased by 89.6–93.3% (F2) and 68.9–77.8% (F3). EDTA, DTPA, and EDDS-assisted washing at pH 7 induced reductions of 63.6–74.9% Pb from F2, and 42.7–53.4% from the F3. The extraction tendencies of the other metal elements (Cu and Zn) were similar, but their rates of dissolution were higher than that of Pb. The higher extractability of Cu and Zn by the chelators can be explained by their relatively larger concentrations in the weakly-bound fractions (50% and 64% of Cu and Zn were present in F1, F2, or F3). The contents of Cu and Zn in F2 and F3 were reduced when compared with pH 7. The results also indicated that the Cu and Zn in the residual fraction of the chelator-washed samples increased after sequential extraction. The tendency of Zn to concentrate in the residual fraction after sequential extraction was in agreement with previous researches (Rivero et al., 2000; Kabala and Singh, 2001). The extraction of Pb by the chelators was lower than that of Cu and Zn, which might have been due to the abundant incorporation of Pb (78.3%) in the residual fraction of the soil. PTEs attached to the residual fraction represent the persistent content in soil phases, and are unlikely
4. Conclusion The extraction of PTEs (Pb, Cu, and Zn) with the persistent chelators EDTA and DTPA and the biodegradable chelators EDDS and HIDS was studied in real contaminated soil. The chelators achieved higher extraction of the PTEs than the control treatment of water-only washing. The effectiveness of the washing treatment was most noticeable at weakly acidic to neutral solution pH values. Among the tested chelators, EDTA, DTPA, and EDDS showed good extraction performance. The extraction of PTEs with the chelators was improved at elevated temperatures, although the effectiveness of the biodegradable chelators was limited above 80 °C. The introduction of mechanochemical energy also improved the chelator-assisted extraction of the PTEs compared to regular rotary shaking. The PTEs released from the chelators mostly bound to the carbonate and Fe–Mn oxide phases of the contaminated soil. The retention of EDTA and DTPA in soil was high even after one week, whereas that of the biodegradable chelators EDDS and HIDS was considerably reduced. Acknowledgment The research was partially supported by Grants-in-Aid for Scientific Research (17K00622 and 18H03399) from the Japan Society for the Promotion of Science. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apgeochem.2019.104397. 9
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Chelator-assisted washing for the extraction of lead, copper, and zinc from contaminated soils: A remediation approach
T
Hiroshi Hasegawaa,∗, M. Abdullah Al Mamunb,c,∗∗, Yoshinori Tsukagoshib, Kento Ishiib, Hikaru Sawaid, Zinnat A. Begume,f, Mashio S. Asamia, Teruya Makia, Ismail M.M. Rahmang,∗∗∗ a
Institute of Science and Engineering, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan c Department of Soil Science, Hajee Mohammad Danesh Science and Technology University, Dinajpur, 5200, Bangladesh d Department of Industrial Engineering, National Institute of Technology, Ibaraki College, 866 Nakane, Hitachinaka City, Ibaraki, 312-8508, Japan e Venture Business Laboratory, Advanced Science and Social Co-Creation Promotion Organization, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan f Department of Civil Engineering, Southern University, 739/A Mehedibag Road, Chittagong, 4000, Bangladesh g Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima City, Fukushima, 960-1296, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Chelator Extraction Mobility Potentially toxic elements Remediation Soil washing
Chelators have proven to be effective extractants in soil washing and represent a suitable remediation technology for potentially toxic elements (PTEs) from contaminated sites. In this paper, the extraction of lead (Pb), copper (Cu), and zinc (Zn) from contaminated real and reference soils using both biodegradable (EDDS and HIDS) and persistent (EDTA and DTPA) chelators was investigated. Different metal–chelator complex formation constants, pH, extraction temperature, and mechanochemical energy were tested. The PTEs were extracted at a relatively higher rate under acidic to neutral conditions (pH 5 and 7), with the greatest extraction being observed at pH 5. The metal extraction efficiencies of the chelators for the real sample at pH 5 were as follows: Pb – EDTA > DTPA > EDDS > HIDS; Cu – DTPA > EDTA > EDDS > HIDS; and Zn – DTPA > EDTA > HIDS > EDDS. The data suggested that EDDS was more effective than HIDS for Cu and Zn extraction in neutral to slightly alkaline solutions, as well as at raised temperature. Furthermore, EDTA washing using mechanochemical agitation was found to enhance the removal efficiency of Cu and Zn. Sequential chemical extraction showed that the apparent metal mobilities were reduced by soil washing. EDTA and DTPA appeared to offer greater potential than the biodegradable chelators in extracting metals from the relatively mobile carbonate- and Fe–Mn oxidebound fractions. The residual fraction of Pb displayed considerable stability (≥70%), and was not entirely amenable under washing treatments, and hence could be considered non-bioavailable and non-toxic.
1. Introduction Soil polluted with potentially toxic elements (PTEs) is one of the most critical environmental problems worldwide. Industrialization, urbanization, and modern agricultural practices are considered to be the sources of PTEs in soil. Plants continuously accumulate PTEs from the soil, which results in unacceptable toxicity to human beings and animals through dietary routes. Therefore, a large number of studies on metal remediation of contaminated soils have been conducted to reduce the risk of PTEs to human health and the ecosystem. These remediation technologies are categorized into three broad groups: physical,
chemical, and biological remediation, and have been thoroughly described in several reviews (Shahid et al., 2012; Saifullah et al., 2015; Begum et al., 2016; Rahman et al., 2016; Khalid et al., 2017). The treatment of soil is of great importance in polluted areas and sites. Soil washing is the most promising remediation technique, and provides a viable and permanent option for obtaining clean soil from polluted sites. The metals extracted through washing can also be recycled and easily re-introduced into the material cycle rather than being landfilled (Fedje et al., 2013). In such washing techniques, many different chemical species, including acids, bases, salts, oxidizing or reducing agents, chelating agents, surfactants, and solvents, are used for
Corresponding author. Corresponding author. Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (H. Hasegawa), [email protected] (M.A.A. Mamun), [email protected], [email protected] (I.M.M. Rahman). ∗
∗∗
https://doi.org/10.1016/j.apgeochem.2019.104397 Received 9 November 2018; Received in revised form 28 June 2019; Accepted 6 August 2019 Available online 08 August 2019 0883-2927/ © 2019 Elsevier Ltd. All rights reserved.
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separating contaminants from the contaminated soils (Begum et al., 2016). The performance of chelators in washing processes has been widely assessed, and they have been found to efficiently remove PTEs and to have a less severe effect on the environment (Peters, 1999; Tandy et al., 2004; Begum et al., 2012a). Chelators also have high complex formation constants under weakly acidic to alkaline conditions, and do not require the use of heating or strong acids during extraction. Chelators are thus considered to be suitable extractants for a wide variety of PTEs, and are expected to reduce the cost and the environmental burden of the metal extraction process (Hasegawa et al., 2011; Pinto et al., 2014; Begum et al., 2016). EDTA (2,2ʹ,2ʺ,2ʹʹʹ-(ethane-1,2-diyldinitrilo)tetraacetic acid) and its homologs are most frequently reported to be used for the decontamination of PTEs due to their high potential for this application, relatively low cost, and wide availability (Leštan et al., 2008; Begum et al., 2016). However, EDTA and its homologs are characterized by low biodegradability and relatively high environmental persistence (Leštan et al., 2008; Begum et al., 2013a). Accordingly, further research has focused on the use of biodegradable alternatives like EDDS (2-[2(1,2-dicarboxyethylamino) ethylamino]butanedioic acid), GLDA (2-[bis (carboxymethyl)amino]pentanedioic acid), IDSA (2-(1,2-dicarboxyethylamino)butanedioic acid), MGDA (2-[bis(carboxymethyl)amino] propanoic acid) and HIDS (2-(1,2-dicarboxyethylamino)-3-hydroxy-butanedioic acid) rather than EDTA (Pinto et al., 2014; Begum et al., 2016). Multiple factors affect the effectiveness of the washing process, and the selection of washing additives for specific contaminated sites is of critical importance to develop technically and economically feasible remediation technologies (Dermont et al., 2008). The critical determinants of metal toxicity are its complexation ability and oxidation state, which also affect the bioavailability of the metals (Kakkar and Jaffery, 2005). For an accurate estimation of the potential environmental impacts, toxicity, mobility, and bioavailability of PTEs, particularly those found in soil (e.g., Pb, Zn, Cd, Ni, and As), it is necessary to identify the chemical forms of the contaminant elements, in addition to their total contents. In this regard, several operationallydefined extraction procedures have been introduced for partitioning metal fractions, with the Tessier protocol (Tessier et al., 1979) or the BCR scheme (Rauret et al., 1999) being widely used for metal fractionation in diverse environmental samples (Desaules, 2012; Mizutani et al., 2016; Rahman and Begum, 2019). In this study, a wet washing method based on the separation and removal of PTEs by chelators was applied to contaminated soils. Iminoacetic acid-type chelators, which exhibit high metal-chelator complexation constants and smooth biodegradability with low environmental loads, were used to extract the PTEs Pb, Cu, and Zn. The extraction performance was evaluated using different chelator species, solution pH, operating temperatures/times, and mechanochemical energies. Moreover, the species distribution of the PTEs in the soils before and after chelator-assisted extraction was investigated using the Tessier sequential extraction procedure. The extraction experiments were carried out under controlled conditions and with particular attention to the environmental regulations of Japan.
(Cd, Cr, Cu, Ni, Pb, and Zn) contents were certified following collaborative procedures, as discussed elsewhere (Pueyo et al., 2001; Rauret et al., 2001). 2.2. Chelators Two representative persistent chelators, EDTA and DTPA (2-[bis[2[bis(carboxymethyl) amino]ethyl]amino]acetic acid) (Kanto Chemicals; Tokyo, Japan), and two biodegradable chelators, EDDS (Chelest; Mie, Japan) and HIDS (Nippon Shokubai; Tokyo, Japan), were used in the current work. The chemical structure, acid dissociation constants (pKa), and stability constants (log KML) of those chelators are shown in Table S2 (Appendix A. Supplementary Information). 2.3. Other materials Analytical grade chemicals were used throughout the study without further purification. Purified water was used to dilute the chelators to 0.01 mol L−1. Metal standards were prepared in the μmol to mmol range from a PlasmaCAL multi-element solution in 5% HNO3 (SCP Science; Quebec, Canada) by dilution on a weight basis. The pH values of the experimental solutions were adjusted using either HCl or NaOH (1 mol L−1) (Kanto Chemicals; Tokyo, Japan). Acetic acid/sodium acetate (CH3COOH/ CH3COONa; 0.1 mol L−1), CAPS (N-cyclohexyl-3-aminopropanesulfonic acid; 0.01 mol L−1), TAPS (N-[tris (hydroxymethyl) methyl]-3-amino-propanesulfonic acid; 0.01 mol L−1) (Kanto Chemicals; Tokyo, Japan), and HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; 0.01 mol L−1) (Nacalai Tesque; Kyoto, Japan) were used as buffer reagents for pH 3–5, 9, 11, and 7, respectively. During sequential extraction (SE), magnesium chloride (MgCl2) (Merck; Darmstadt, Germany), hydroxylamine hydrochloride (HONH2·HCl) (Wako Pure Chemical; Tokyo, Japan), and hydrogen peroxide (H2O2) (Kanto Chemicals; Tokyo, Japan) were used. Low-density polyethylene bottles (Nalge Nunc; Rochester, NY) and perfluoroalkoxy tubes and micropipettes (Nichiryo; Tokyo, Japan) were used throughout the experiments. All the laboratory equipment was subjected to a cleansing treatment following the protocol as described elsewhere (Hasegawa et al., 2018). 2.4. Equipment The metal concentration was quantified using inductively coupled plasma optical emission spectrometry (ICP-OES; iCAP 6300; Thermo Fisher Scientific; Waltham, MA). A microwave reaction system (Multiwave 3000; Anton Paar GmbH; Graz, Austria) was used for the digestion of samples. An energy dispersive X-ray spectroscope (EDX; INCA Energy) attached to a field emission scanning electron microscope (FE-SEM; Oxford Instruments; Abingdon, Oxfordshire) was used for mapping the elements in the sample. Automated high-performance liquid chromatography (HPLC; Tosoh 8020; Tosoh; Tokyo, Japan) was used for analysis of the chelator concentrations. A heat-block type thermal decomposition system (DigiPREP Jr; SCP Science; Quebec, Canada) and a compact high-pressure autoclave (MLS 3750; Sanyo Electric; Osaka, Japan) were used to perform the temperature-controlled extraction experiments. A planetary mono mill (Pulverisette 6; Fritsch Laborgerätebau; Idar-Oberstein, Germany) was used to conduct the experiments in which remediation was carried out using mechanochemical energy. A constant temperature Bioshaker (BR30 L; TAITEC; Tokyo, Japan) was used for the rotary shaking of the samples. A centrifugal separator (H-701FR; Kokusan; Tokyo, Japan) was used to isolate the residue and filtrate in a solid–fluid suspension. A pH meter (Navi F-52; Horiba Instruments; Kyoto, Japan) was used for pH measurements. Cellulose membrane filters (0.45 μm; Advantec; Tokyo, Japan) were used for the vacuum filtration of samples. An Arium Pro water purification system (Sartorius Stedium Biotech GmbH; Gottingen, Germany) was used to produce purified water with a resistivity of > 18.2 MΩ cm.
2. Experimental 2.1. Soil samples A real contaminated fine-grain sludge sample (TCS-18) from a soil treatment facility in Japan (Yamazaki Jari Shoten Co. Ltd.; Otsu, Japan) was used in this experiment. Additionally, an organic-rich soil standard substance (BCR-700) from the European Community Standards Bureau was used to validate the metal extraction. The real sample was dried to a constant weight in an oven at 65 °C, carefully homogenized to a size of 2 mm or less, and stored in a glass bottle. The physico-chemical characters of TCS-18 are presented in Table S1 (Appendix A. Supplementary Information). BCR700 was collected from a German agricultural research station, homogenized, and bottled. Its EDTA and acetic acid-extractable trace element 2
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2.5. Methods
Table 1 Contents of the PTEs (Pb, Cu, and Zn) and other elements in the TCS-18 soil after microwave-assisted acid-digestion and weak-acid induced extraction.
2.5.1. Microwave-assisted digestion Microwave decomposition was used for three types of samples: (a) raw soil, to measure the total metal content; (b) the residue from SE treatment before chelate extraction; and (c) the residue from SE treatment after chelate extraction. The precleaned PTFE reaction vessels were fed with 0.05 g of sample and 1 mL 65% HNO3; the mixture was allowed to stand for 10 min to decompose the organic matter. Subsequently, 3 mL HCl and 1 mL HF were added, and the reactor was sealed with a Teflon seal, and microwave treatment was applied for decomposition. After that, 10 mL of 50 g L−1 boric acid was added to mask the HF in the solution, and microwave treatment was again applied. The optimized operation conditions were chosen according to the manufacturer's recommendations. The mixtures obtained after the microwave reaction were diluted to 50 mL with purified water and filtered. A blank was treated using the same procedure as the samples, and each of the treatments was performed in triplicate.
Elements
Al Cu Fe Mn Pb Zn
MWDa
WEEb
(mg kg−1)
(mg kg−1)
85051 ± 2439 104 ± 4 41523 ± 495 838 ± 12 522 ± 41 309 ± 7
8290 ± 230 77.0 ± 2.6 7025 ± 170 408 ± 14 97.9 ± 3.6 220 ± 9
a MWD: Element contents derived from microwave-assisted acid digestion (extractable with mixed acids: HNO3, HCl, HF), n = 3. b WEE: Element contents derived from weak-acid induced extraction (HCl, 1 mol L−1; liquid to solid ratio, 100:3; duration, 2 h; T, 25 ± 2 °C), n = 3.
extraction with the chelators is presented in Fig. S1 (Appendix A. Supplementary Information). The Pb, Cu, and Zn contents in TCS-18 or BCR-700 detected after microwave digestion were higher than those detected after weak-acid induced extraction.
2.5.2. Chelator-assisted extraction of PTEs The samples were subjected to batch extraction with EDTA, DTPA, EDDS, and HIDS at a 1:10 sample-to-chelator ratio and shaken at 200 rpm for 24 h (T, 25 °C). The soil–chelator suspensions were also treated at 105, 115, 125, and 135 °C (duration, 1 h) and 40, 60, and 80 °C (duration, 6 h) to study the effect of temperature on the extraction of the PTEs by the chelators. The solid–fluid suspension was then centrifuged and filtered to separate the residue; the elemental content of the filtrate was analyzed using ICP-OES. In the test of extraction induced by mechanochemical energy, the mill pot of the planetary mono mill was fed with 10 g sample, 90 g of zirconia balls (ϕ, 8), and 100 mL of EDTA (0.01 mol L−1; pH 7). Wetgrinding was carried out in the ball mill at 200 rpm, and 1 mL of the supernatant was collected at 1, 2, 3, 4, 5, 6, 24, and 48 h. The supernatant samples were analyzed 24 h and one week after chelator-assisted extraction to measure the residual chelator. The samples were thoroughly mixed once per day and then allowed to stand overnight to ensure uniform mixing of the suspensions. All the supernatants were filtered to separate trace residues, and the filtrates were analyzed using ICP-OES.
3.2. Investigation of the optimum conditions for the extraction of the PTEs from TCS-18 3.2.1. Effect of the conditional metal-chelator complexation constant on the extraction of PTEs The extraction of PTEs from TCS-18 with various chelators at pH 7 was studied. The conditional metal-chelator complexation constants (logK'ML; pH, 7) of each chelator were calculated following the protocol described in Begum et al. (2012b). The logK'ML values for the metalchelator complexes of each chelator are listed in Table S2 (Appendix A. Supplementary Information). The amount of PTEs extracted using the chelators was much higher than the amount extracted using the control. The Pb-extraction rates with different chelators followed the order DTPA > EDDS > EDTA > HIDS. The Cu-extraction efficiency of the chelator species followed the order EDDS > EDTA > DTPA ≈ HIDS, and the Zn-extraction rates showed the following trend: EDDS > EDTA > DTPA > HIDS (Fig. 1a). EDDS thus appeared to be the superior extractant for Pb, Cu, or Zn, followed by EDTA. Tandy et al. (2004) concluded that at a chelator-to-metal ratio of 1 and pH ≥ 6, EDDS was a superior extractant compared to EDTA because it
2.5.3. Determination of the weak-acid-extractable PTEs in the soils The content of weak-acid-extractable PTEs in the soils before the chelator-assisted washing treatment and in the residues after chelatorassisted extraction was determined following the protocol suggested by MOE (2003), based on the Soil Contamination Countermeasures Law of Japan (Anonymous, 2002). Soil (1.0 g) was mixed with 1 mol L−1 HCl in a polyethylene decomposition vessel (liquid-to-solid ratio, 100:3) and shaken at 200 rpm for 2 h (T, 25 °C). The soil-fluid suspension was centrifuged at 3000 rpm for 20 min, after which the supernatants were separated from the residue via filtration, and their PTE contents were analyzed using ICP-OES.
Table 2 Contents of the PTEs (Pb, Cu, and Zn) and other elements in the BCR-700 soil after microwave-assisted acid-digestion, weak-acid induced extraction, and a comparison of the experimental and certified concentrations of EDTA-extractable PTEs. Elements
2.5.4. Distribution of the metals by sequential extraction (SE) The mobility of the metals in TCS-18 before and after washing with the chelators was determined using the sequential extraction method following the Tessier protocol (Tessier et al., 1979; Begum et al., 2013b; Rahman et al., 2019). The compositions of the extractants used to demarcate the different operationally defined soil solid phases are shown in Table S3 (Appendix A. Supplementary Information).
Al Cu Fe Mn Pb Zn
MWDa
WEEb
EDTA-E-Ec
EDTA-E-Cd
(mg kg−1)
(mg kg−1)
(mg kg−1)
(mg kg−1)
41082 ± 2579 184 ± 3 40120 ± 3596 1451 ± 360 463 ± 8 1592 ± 100
1860 ± 70 108 ± 5 11020 ± 240 831 ± 25 182 ± 7 1050 ± 40
– 84.4 ± 2.1 – – 109 ± 5 582 ± 13
– 89.4 ± 2.8 – – 103 ± 5 510 ± 17
a MWD: Element contents derived from microwave-assisted acid digestion (extractable with mixed acids: HNO3, HCl, HF), n = 3. b WEE: Element contents derived from weak-acid induced extraction (HCl, 1 mol L−1; liquid to solid ratio, 100:3; duration, 2 h; T, 25 ± 2 °C), n = 3. c EDTA-E-E: Experimentally-derived EDTA-extractable element contents in BCR-700 (EDTA, 0.05 mol L−1; liquid to solid ratio, 10:1; duration, 2 h; T, 25 ± 2 °C), n = 3. d EDTA-E-C: Certified values of EDTA-extractable element contents in BCR700 (Pueyo et al., 2001; Rauret et al., 2001).
3. Results and discussion 3.1. PTE content in the soils The PTE (Pb, Cu, and Zn) contents in TCS-18 and BCR-700 after microwave-assisted acid digestion and weak-acid induced extraction are shown in Tables 1 and 2. The elemental mapping of TCS-18 before 3
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Fig. 1. (a) Extraction behavior of the PTEs (Pb, Cu, and Zn) in TCS-18 using different chelators. (b) Correlation between the conditional metal-chelator complexation constant (logK'ML) and the extraction of the PTEs by the chelators. Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid-fluid suspension mixing speed, 200 rpm; process duration, 24 h.
compromises the interactions of metals with Ca and Fe in soils more effectively than EDTA. The higher Pb extraction ability of DTPA compared to the other chelators could be attributed to its ability to compete with soil organics to create soluble complexes (Li and Shuman, 1997). The correlations between logK'ML and the chelator-assisted extraction rates of Pb, Cu, and Zn yielded inconsistent trends (Fig. 1b). The observed behavior could be attributed to the co-existence of several variables within this system, which might alter the expected extraction performance. Such variables include the percent ratio of the dominant metal–chelator species in the solution, the co-precipitation behavior of
the metal ions, and the re-sorption ratio of the metal-chelator complexes, among others (Nowack, 2002). 3.2.2. Effect of pH on Pb, Cu, and Zn extraction The chelator-assisted extraction of the PTEs in the soils were significantly influenced by the solution pH, which influences several factors, e.g., the aqueous species concentration of the PTEs, the solubilization of the chelators, the sorption and desorption of the PTEs, the ionic exchange behavior between PTEs, and the re-adsorption mechanism of the metal–chelator complexes, among others (Polettini 4
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control solution indicated that the relative contents of the water-soluble fractions in the soil were low. A similar observation has been reported by Niinae et al. (2008) for Pb extraction using chelators. The efficacy of the chelators in acidic (pH 3 and 5), neutral (pH 7), and alkaline (pH 9 and 11) conditions can be summarized as follows: (a) the highest extraction efficiency was achieved at an acidic pH of 5; (b) the efficiency at a neutral pH of 7 was lower than that at pH 5, but higher than that at pH 3; and (c) the efficiency was lower in alkaline environments than at neutral conditions. The Cu extraction performances of EDTA and EDDS were similar at pH 5, 7, 9, and 11. The formation of stable complexes of Cu and EDTA or EDDS would explain this result, as the EDTA–Cu and EDDS–Cu complexes have similar stability constants of 18.8 and 18.4, respectively (Martell and Smith, 2001; Guo et al., 2010). Although the dissolution of the PTEs attached to solid phases increases with the solution acidity, higher PTE extractions were observed at pH 5 than pH 3. This might have been due to the protonation of the chelator in the more acidic solution (HL, H2L, …, HnL), which would lead to lower conditional complexation constant than at the other pH levels measured. The PTEs in the solution tend to form nearly insoluble hydroxides with increasing pH at pH 7 to 11 (M (OH), M (OH)2, …, M(OH)n), which might have been responsible for the lower extraction rates at basic conditions. EDTA was superior to DTPA for extracting Pb, while the opposite was found for the extraction of Cu and Zn at pH 5. However, DTPA was the best option for Pb extraction at pH 7. EDDS showed higher Pb extraction efficiency under neutral to slightly alkaline conditions (pH 7–9); its efficiency was comparable with that of EDTA and HIDS. EDDS also exhibited relatively higher extraction of Cu and Zn at pH 7 and 9 compared to EDTA and DTPA. The optimal PTE extraction efficiency of EDDS at pH 7–9 was also reported in earlier works (Tandy et al., 2004; Begum et al., 2013b). EDDS and HIDS displayed similar performance in the extraction of Cu and Zn at pH 3 and 5. HIDS was the least efficient in Pb extraction at pH 11. 3.2.3. Effect of the extraction temperature The mobility and complex formation of PTEs can be enhanced due to the effect of temperature change on the behavior of the humic substances and the rate of desorption of the PTEs from the Fe–Mn oxide fraction of the soil (Weng et al., 2002). Fig. 3 shows the effect of applying different temperatures and extraction times on the extraction of PTEs from TCS-18 using the chelators. Pb and Zn extraction performance of the control solution did not improve even when the reaction temperature was raised to 135 °C. The EDTA washing results indicated that the extraction performance of Pb and Zn was improved when the temperature was increased from 60 to 135 °C. DTPA and EDDS washing also showed a similar trend, except in the case of Pb. A gradual increase in the extraction of Cu with EDTA, DTPA, and EDDS was observed with increasing temperature up to 115, 115 and 125 °C, respectively. The extraction of Pb by HIDS was relatively insensitive to temperature, while the extraction of Pb by EDDS decreased with increasing temperature in the range 80–135 °C. The extraction of Cu by HIDS increased with temperature up to 80 °C, but decreased remarkably at ≥105 °C, while the EDDS-assisted extraction of Cu started to decrease at ≥125 °C. The Zn-extraction rates either showed an increasing trend (EDTA, DTPA, or EDDS) or were unaffected (HIDS) with increasing system temperature. The lower Pb-extraction efficiency of EDDS and HIDS and reduced Cu-extraction of HIDS at higher temperatures can be attributed to the thermal denaturation of the biodegradable chelators, which diminished their complexation ability. The Cu–HIDS complex formation constant was low compared to that of EDDS, which might be the reason for the low Cu extraction of HIDS.
Fig. 2. Effect of the solution pH on the chelator-assisted extraction of the PTEs (Pb, Cu, and Zn) in TCS-18. Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid-fluid suspension mixing speed, 200 rpm; process duration, 24 h.
et al., 2007; Zou et al., 2009; Qi et al., 2011). The extraction of Pb, Cu, and Zn from TCS-18 by the chelators at various pH values, along with results of the blank experiments, are shown in Fig. 2. The extraction efficiencies using the chelators increased considerably as the pH was varied compared to the extraction efficiency of the control solution. The minimal extraction of PTEs in the
3.2.4. Mechanochemical effect The introduction of mechanical force is a simple and economical approach to achieve enhanced metal recovery from solid wastes (Tan 5
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Fig. 4. Comparative impacts of normal shaking and mechanochemical treatment on the extraction of the PTEs (Pb, Cu, and Zn) with EDTA for varying process durations. Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid–fluid suspension mixing speed, 200 rpm.
Fig. 3. Effect of the system temperature on the chelator-assisted extraction of the PTEs (Pb, Cu, and Zn) in TCS-18 for varying process durations (T, 25 °C: 24 h; T, 40–80 °C: 6 h; T, 105–135 °C: 1 h). Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; solid–fluid suspension mixing speed, 200 rpm.
process durations of up to 48 h. Additionally, the amount of extraction under normal shaking conditions was measured for comparison, and is shown in Fig. 4. The amount of Pb, Cu, and Zn extracted in the mechanochemical process increased with increasing process time, and the total amount extracted was higher than that obtained by regular shaking. The improved removal of PTEs via mechanochemical processing could be attributed to the grinding action of the ball mill, which
and Li, 2015). The addition of mechanical energy can have a beneficial effect on chemical reactions via lattice defects, strain energy, and surface energy (Maki et al., 2011; Ou et al., 2015; Tan and Li, 2015). We expected that the extraction efficiency of the chelators could be improved using mechanochemical reaction, and thus investigated the effect of EDTA on metal removal by wet pulverization (pH 7) for varying 6
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Fig. 5. Comparison of the extracted and residual contents of the PTEs (Pb, Cu, and Zn) in the standard (BCR-700) and real (TCS-18) samples after washing treatment with different chelators at different pHs (5, 7, and 9). Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid–fluid suspension mixing speed, 200 rpm; process duration, 24 h.
7
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decreases the particle size and increases the surface area of the soil particles (Hasegawa et al., 2013).
7, and 9. The residual PTEs in the chelator-washed samples were then extracted with 1 mol L−1 HCl. The extraction of PTEs from BCR-700 or TCS-18 followed the order pH 5 > 7 > 9 (Fig. 5) for all extractants. At pH 5, EDTA extracted 27.9 times more Pb, 5.6 times more Cu, and 1.7 times more Zn than the control. The residual Pb content and removal percentage after the EDTA washing of BCR-700 were 50.6 mg kg−1 (72.3%), 127.1 mg kg−1 (30.0%), and 151.2 mg kg−1 (16.4%) at pH 5, 7, and 9 respectively. In TCS-18, the amount of Pb extracted by EDTA
3.3. Comparison of Pb, Cu, and Zn extraction behavior between TCS-18 and BCR-700 Each extractant (EDTA, DTPA, EDDS, and HIDS) was applied to BCR-700 and TCS-18 to compare their extraction performances at pH 5,
Fig. 6. Comparison of the distribution of the PTEs (Pb, Cu, and Zn) in TCS-18 before and after washing with the chelators at different pH values (5 and 7). Extraction conditions: soil-to-chelator ratio, 1:10; pH, 7; T, 25 °C; solid-fluid suspension mixing speed, 200 rpm; process duration, 24 h. Abbreviations used: BW, before wash; F1, exchangeable; F2, carbonate-bound; F3, Fe–Mn oxide bound; F4, organic matter-bound; F5, residual; CW, chelator-washed fraction. 8
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was 60.9 mg kg−1, 28.6 mg kg−1, and 14.8 mg kg−1 at pH 5, 7, and 9, respectively. The residual content of Pb and the reduction percentage after EDTA washing were 26.2 mg kg−1 (73.2%), 68.4 mg kg−1 (30.1%), and 82.7 mg kg−1 (15.5%) at pH 5, 7, and 9 respectively. The Cu and Zn contents in both BCR-700 and TCS-18 decreased greatly after extraction with the chelators. A noticeable effect was found using EDTA and DTPA at pH 5; EDTA, DTPA, and EDDS at pH 7; and DTPA and EDDS at pH 9. The residual Cu content (mg kg−1) after washing the BCR700 sample followed the order DTPA > EDDS > EDTA > HIDS > Control at pH 5; DTPA > EDDS > EDTA > HIDS > Control at pH 7; and DTPA > EDDS > HIDS > EDTA > Control at pH 9. The same trends were observed in the residual Cu content of TCS-18. DTPA appeared to be an effective extractant for Cu separation from both the samples. For BCR-700, the extraction efficiency of Zn followed the order DTPA > EDTA > EDDS > HIDS > Control at pH 5; EDTA > DTPA > EDDS > HIDS > Control at pH 7; and DTPA > EDTA > EDDS > HIDS > Control at pH 9. The extraction efficiency at low concentrations of chelators depends on the pH and the abundance of major cations, especially Ca and Fe, in the soil (Chrastný et al., 2008). The extractability of PTEs with chelators usually follows the order Zn > Cu > Pb (Bermond and Varrault, 2004; Wasay et al., 2007; Xia et al., 2009). Pb is highly immobile and more difficult to extract due to its stronger adsorption to soil particles due to its small hydrated radius and higher ionic charge, whereas Cu and Zn are intermediate in terms of hydrated radii and extractability. However, comparatively higher removal of Cu from artificially contaminated soil was observed under neutral and alkaline conditions using biodegradable chelators (Begum et al., 2012a). The extraction amount showed a correlation with the decrease in the residual metal content of the chelator-extracted soil. From the above results, the trends in the extraction of the PTEs from BCR-700 were found to be comparable with the results obtained from the real sample TCS-18.
to be extracted easily (Tessier et al., 1979). Elless and Blaylock (2000) and Tandy et al. (2004) reported that strongly bound metal fractions could not be desorbed even using highly potent chelators, such as EDTA. Therefore, extraction with a chelator is believed to be useful for the PTEs bound to the F1 to F4 fractions of soils. The higher extraction from the F1 to F3 fractions confirmed that mobilization of PTEs occurred due to changes in the ionic strength or reduced pH and redox potential. 3.5. Evaluation of the retainability of the chelators Chelators with low biodegradability remain in the environment for a long time, and necessitate apprehension for further metal elution. Microorganisms or other processes can decompose biodegradable chelators, which have amino acid skeletons, reducing the environmental burden. Therefore, the residual concentration of the chelators used in this study was measured in TCS-18 (Fig. S2; Appendix A: Supplementary Information). No significant change was observed in the concentrations of the persistent chelators, and the average concentration after 24 h and 1 week was 9.5 and 9.3 mmol L−1 for EDTA and DTPA, respectively (initial concentration 10 mmol L−1). On the contrary, a remarkable decrease was observed in the concentrations of the biodegradable chelators after a week. The concentration and percent decrease after one week were 6.7 mmol L−1 (32.5%) for EDDS and 8.2 mmol L−1 (17.7%) for HIDS. It was believed that decomposition by microorganisms was responsible for the decrease in the concentration of the biodegradable chelators over time. Tandy et al. (2004) showed that the half-life of EDDS was 4.2–5.0 days, while degraded after a lag of 7–11 days. However, in this experiment, no significant decrease was observed in the concentrations of EDDS and HIDS after 24 h. Thus, it is conceivable that degradation by microorganisms did not occur. Furthermore, the HPLC measurement takes about 40 min per sample; part of the sample solution may have evaporated and thus concentrated the chelators due the open state of the sample. These factors might result in the measurement of high concentrations of chelators, especially EDDS and HIDS, even if biodegradation by microorganisms did occur.
3.4. Analysis of the mechanism of chelator-assisted extraction using sequential extraction Determining the fractionation of the PTEs in the soil solid-phases is essential to explore the binding affinity between the PTEs and the soil particles, and the extent to which they can be remobilized into the environment (Turki, 2007). The Tessier protocol defines five fractions of soil solid-phases: exchangeable (F1), carbonate-bound (F2), Fe–Mn oxide-bound (F3), organic matter-bound (F4), and residual (F5). The relative metal concentrations of TCS-18 before and after extraction with the chelators (pH 5 and 7) as determined using sequential chemical extraction are shown in Fig. 6. These two pH values (5 and 7) were chosen due to the higher extraction efficiencies achieved at these pHs. The Pb contents of F2 and F3 decreased remarkably after being washed with the chelators. In the case of EDTA and DTPA-assisted washing at pH 5, the Pbcontent decreased by 89.6–93.3% (F2) and 68.9–77.8% (F3). EDTA, DTPA, and EDDS-assisted washing at pH 7 induced reductions of 63.6–74.9% Pb from F2, and 42.7–53.4% from the F3. The extraction tendencies of the other metal elements (Cu and Zn) were similar, but their rates of dissolution were higher than that of Pb. The higher extractability of Cu and Zn by the chelators can be explained by their relatively larger concentrations in the weakly-bound fractions (50% and 64% of Cu and Zn were present in F1, F2, or F3). The contents of Cu and Zn in F2 and F3 were reduced when compared with pH 7. The results also indicated that the Cu and Zn in the residual fraction of the chelator-washed samples increased after sequential extraction. The tendency of Zn to concentrate in the residual fraction after sequential extraction was in agreement with previous researches (Rivero et al., 2000; Kabala and Singh, 2001). The extraction of Pb by the chelators was lower than that of Cu and Zn, which might have been due to the abundant incorporation of Pb (78.3%) in the residual fraction of the soil. PTEs attached to the residual fraction represent the persistent content in soil phases, and are unlikely
4. Conclusion The extraction of PTEs (Pb, Cu, and Zn) with the persistent chelators EDTA and DTPA and the biodegradable chelators EDDS and HIDS was studied in real contaminated soil. The chelators achieved higher extraction of the PTEs than the control treatment of water-only washing. The effectiveness of the washing treatment was most noticeable at weakly acidic to neutral solution pH values. Among the tested chelators, EDTA, DTPA, and EDDS showed good extraction performance. The extraction of PTEs with the chelators was improved at elevated temperatures, although the effectiveness of the biodegradable chelators was limited above 80 °C. The introduction of mechanochemical energy also improved the chelator-assisted extraction of the PTEs compared to regular rotary shaking. The PTEs released from the chelators mostly bound to the carbonate and Fe–Mn oxide phases of the contaminated soil. The retention of EDTA and DTPA in soil was high even after one week, whereas that of the biodegradable chelators EDDS and HIDS was considerably reduced. Acknowledgment The research was partially supported by Grants-in-Aid for Scientific Research (17K00622 and 18H03399) from the Japan Society for the Promotion of Science. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apgeochem.2019.104397. 9
Applied Geochemistry 109 (2019) 104397
H. Hasegawa, et al.
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