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A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils
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JECE 671 1–4 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
1 2
A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils
3 Q1
Signe B. Rasmussen* , Julie K. Jensen, Ole K. Borggaard
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Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 December 2014 Accepted 15 April 2015
Soils contaminated by arsenic (As) and copper (Cu) must be remediated because As and Cu are nondegradable and toxic. On moderately contaminated soils, As and Cu may be removed by in-situ plant uptake (phytoremediation), whereas strongly contaminated soils must be removed and cleaned by soil washing at a soil disposal site (ex-situ). Strong bonding of As and Cu to soil solids requires the use of strong complexants to release the elements. Often synthetic chemicals such as EDTA and NTA are used. Since such chemicals are environmentally problematic, their replacement with natural organic matter (NOM) such as humic substances (HS) and citrate might be attractive. To test this possibility, a moderately contaminated calcareous urban soil from a soil depot (called the CRC soil) and a soil from a wood impregnation site (called the CCA soil) highly contaminated with As and Cu were extracted with citrate, HS and NTA at different concentration and pH, and the results compared. Extracted As and Cu were found to increase at increasing extractant concentration but to decrease at increasing pH. The efficiency of the three extractants generally decreased in the order: NTA > citrate > HS but at pH 4, HS extracted the same amounts of Cu as citrate and NTA, and substantial amounts of As. Instead of replacing one contaminant, As/Cu, with another contaminant, synthetic NTA, it is therefore recommended to use HS at pH 4 or citrate for removing As and Cu from strongly contaminated soils and to use HS at neutral pH to enhance in-situ phytoremediation of moderately contaminated soils. Citrate (and NTA) cannot be suggested for enhancement of on-site phytoremediation because of high mobilization rates caused by these extractants, which through leaching and runoff may lead to contamination of recipient waters with As and Cu. ã 2015 Published by Elsevier Ltd.
Keywords: Humic substances Heavy metals Enhanced phytoremediation Soil washing Citrate NTA
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Introduction Human activities such as wood impregnation and mining cause arsenic (As) and copper (Cu) contamination of soils, which must be removed as As and Cu are non-degradable and toxic posing a threat to human health and ecosystem functioning [1–3]. As and Cu can be removed in-situ by phytoremediation on moderately contaminated soils or ex-situ by soil washing of strongly contaminated soils. Because As and Cu are often strongly bonded to soil solids, both methods often use a strong complexant to increase contaminant solubility, e.g. ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) or similar compounds [4–8]. However, although
Abbreviations: HS, humic substances; NOM, natural organic matter; NTA, nitrilotriacetic acid; As, arsenic; Cu, copper; Fe, iron; CCA soil, chromium, copper and arsenic contaminated soil; CRC soil, Copenhagen Recycling Centre soil; AAS, atomic absorption spectrometry; ICP-OES, inductively coupled plasma-optical emission spectroscopy. * Corresponding author. Tel.: +45 35333394. E-mail address: [email protected] (S.B. Rasmussen).
EDTA, NTA and other synthetic aminopolycarboxylates are effective cleaning agents, they are environmentally problematic [9–11] suggesting a need for their replacement with cheap natural organic matter (NOM) compounds such as soluble humic substances (HS) and citrate forming strong soluble complexes with Cu and other heavy metals [2,4,5,8,9,12,13]. In fact, recent studies have shown that NOM compounds such as HS-like materials formed by oxidative hydrolytic degradation of cow slurry, leonardite HS and Sigma– Aldrich HS as well as citrate can extract As, Cu and some other heavy metals but they are clearly less effective than EDTA and NTA [2,8,13]. In addition to the nature of the extractant, extraction of As, Cu and other heavy metals depends on soil properties, e.g. the pH, clay and organic C contents, and whether spiked or long-term contaminated Q2 (aged) soil is tested [2,5,14,15]. In order to get improved knowledge about the suitability of replacing synthetic NTA by NOM, the amounts of As and Cu extracted by NTA, citrate and HS from two long-term contaminated soils were determined by laboratory batch extraction experiments. The soils tested included an urban calcareous soil and a chromium (Cr), copper and arsenic (CCA) contaminated soil from an abandoned wood impregnation site.
http://dx.doi.org/10.1016/j.jece.2015.04.029 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: S.B. Rasmussen, et al., A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.04.029
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Materials and methods
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Soils
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The urban soil sample (the CRC soil) was collected at a soil disposal site at the Copenhagen Recycling Centre (CRC), which receives contaminated soil from the greater Copenhagen area and Q3 is described in more details in Ref. [5]. The soil investigated consisted of five mixed subsamples randomly taken at different parts of the soil depot. The CCA soil sample was from an old wood impregnation site, which were active in the years 1936–1976, using Cr, Cu and As impregnation solutions. The site is named Collstrup, and is located at the town, Hillerød in north Sealand. Two samples were taken from the top 20 cm at the same spot; hereafter called CCA1 soil and CCA2 soil. Based on a map of the Collstrup site [16] supported by observed modest vegetation, the sampled spot was selected as a very contaminated place (spot) due to impregnation solution spills during wood trunk impregnation and storage. After air-drying, the soil samples were sieved (2 mm) and the fine earth characterized as indicated in Table 1. The clay content was determined by the hydrometer method [17], pH by glass electrode in a 1:2.5 soil:water suspension and CaCO3 by calcimeter [18]. Organic carbon was measured by dry combustion at 1250 C in oxygen [19] and corrected for CaCO3. Total Al, As, Cu and Fe were determined by atomic absorption spectrometry (AAS) or inductively coupled plasma-optical emission spectroscopy (ICP-OES) in the digest obtained by boiling the soil for 8 h with 7.3 M HNO3[20]. Oxalateextractable Al and Fe (AlOxalate, FeOxalate) were determined by AAS in the extract obtained by extraction for 2 h with 0.2 M ammonium oxalate at pH 3 in the dark [21] and citrate–bicarbonate–dithioniteextractable Al and Fe (AlCBD, FeCBD) determined by AAS in the extract obtained by three extractions for 15 min at 70 C as described by Mehra and Jackson [22].
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Extractants
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The extractants tested included NTA, citrate and soluble HS. All extractants were made to contain 0.05 M KNO3, which was used as control and pH was adjusted to the desired value by 1 M HNO3 or 1 M KOH. The NTA extractant was prepared by dissolution of Titriplex I (C6H9NO6) in KOH and citrate by dissolution of citric acid (C6H8O7) in KOH. The HS extractant was prepared from a compost percolate collected from a reservoir at a composting plant at CRC, where rainwater has percolated through piles of park and garden wastes collected from the urban areas of Copenhagen. The percolate was evaporated to dryness, dissolved in water again and filtered resulting in a clear brown solution.
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Table 1 Selected characteristics of the CCA and CRC soil samples. Characteristic
CCA soil
CRC soil
Clay (%) CaCO3 (%) Org. C (%) pH Altotal (mg/kg) Fetotal (mg/kg) AlCBD (mg/kg) FeCBD (mg/kg) AlOxalate (mg/kg) FeOxalate (mg/kg) Astotal (mg/kg) Cutotal (mg/kg)
16.0 1.0 0 1.89 0.05 5.4 0.1 11,740 500 7240 150 599 19 3365 38 571 10 1784 20 a 990 30 and b860 20 a 2100 45 and b1500 30
24.0 1.0 9.1 0.1 1.62 0.05 8.1 0.1 16,200 1500 16,600 1800 606 14 2660 50 667 12 1094 33 180 10 140 5
a b
As and Cu contents in CCA1. As and Cu contents in CCA2.
The C concentration was determined using a Shimadzu TOCvcpn total carbon analyser. All extractant concentrations were expressed as dissolved organic carbon (DOC) to ease comparison, i.e. 100 mM DOC concentration of citrate and NTA corresponds to 16.7 mM NTA or citrate as both citrate and NTA contain 6 C per molecule. The content of active groups including carboxylic acid groups (COOH) and phenolic groups (Ar OH) were determined in HS by potentiometric titration, where COOH and Ar OH were taken as titratable acidity between pH 3 and 7 and between pH 7 and 11, respectively [13]. Accordingly, HS contained 140 18 mmol COOH/mol C and 145 24 mmol Ar OH/mol C. For comparison, citrate and NTA contain 500 mmol COOH per mol C but no Ar OH groups.
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Extractions
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The extractions were performed in the batch mode as multi-step extractions with 10 steps, where 2.5 g of soil was shaken horizontally with 25 ml of extractant solution in a 50 ml centrifuge tube for 24 h. Afterwards the tube was subjected to centrifugation at 3000 g for 15 min, two portions of 10 ml of the clear supernatant were pipetted into two 15 ml test tubes, one containing 0.5 ml concentrated HNO3 (for As and Cu determination) and one without HNO3 (for DOC and pH measurements). Then 20 ml of fresh extractant solution was added to the centrifuge tube and the procedure repeated 9 more times. In the non-acidified extracts, pH and the DOC concentration were measured as described above. The contents of As and Cu were determined in the acidified extracts by AAS. Accumulated releases (Yn) of As and Cu in mg/kg were calculated by the following formula:
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Y n ¼ 10 X n 2 X n1 þ Y n1 where X is the element concentration in mg/l and n is number of extractions (= days). The term 2 Xn1 accounts for the As or Cu content in the 5 ml solution per 2.5 g of soil left over from the previous extraction. All the extractions were carried out as triplicates. All chemicals were of analytical grade or ultrapure, the water was triple deionized and all glass- and plastwares were acid-washed before used. The limits of detection were 0.5 mg/l for As and 0.04 mg/l for Cu. Statistical analysis of the data was performed in Microsoft Office Excel.
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Results and discussion
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Soil characteristics
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The contents of clay, CaCO3 and total Al, Fe, As and Cu as well as pH of the CCA and CRC soil samples were clearly different, while org. C, DCB- and oxalate-extractable Al and Fe were comparable (Table 1). The CCA soil consisted of CCA1 and CCA2 with the same general (basic) characteristics but with different As and Cu contents. Regarding the contents of As and Cu, the CCA soil is strongly contaminated while the CRC soil is less contaminated but above the Danish threshold value of 20 mg As/kg; for Cu the Danish threshold is 500 mg/kg [23]. The great variability of As and Cu in soil at the wood impregnation sites is clearly demonstrated by the different contents of the CCA1 and CCA2 samples even though they were sampled very close (1 m) to each other.
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Extraction of As and Cu
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Accumulated As and Cu extracted during the 10 extractions from the soil samples at pH 6 are shown in Fig. 1. The efficiency of the three extractants decreases in the order NTA > citrate > HS
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CRC soil
CCA soil
Extracted As (%)
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A)
B)
0 100
CCA soil
CRC soil Extracted Cu (%)
80 60 40 20
C)
D)
0 1
2
3
4
5
6
7
8
9
10
1
2
Number of extracons HS
Citric acid
3
4
5
6
7
8
9
10
Number of extracons NTA
HS
Citric acid
NTA
Fig. 1. Accumulated extraction of As and Cu by 50 mM DOC of HS, citrate and NTA as a function of number of extractions, at pH 6 and expressed as percentage of total As and total Cu. Panel A: as extracted from the CRC soil sample; Panel B: as extracted from the CCA1 soil sample; Panel C: Cu extracted from the CRC soil sample; Panel D: Cu extracted from the CCA1 soil sample. Vertical bars indicate standard errors. 138
and the extraction curves are somewhat different. As and Cu extracted by NTA and citrate are largest in the first extraction leading to convex extraction curves, whereas the HS extractions curves are almost linear with similar releases during the 10 extractions. In particular the first extraction with NTA and citrate is very efficient compared both to the following NTA/citrate extractions and to As and Cu extracted by HS. In the first extraction, NTA removed 50–80% and citrate 25–70% of total Cu from the CRC sample and the CCA1 sample, respectively while HS only extracted about 10–15%. The extractability of As was less marked with about 20–25% extracted with NTA and citrate in the first extraction from the two soil samples, and only few percent extracted with HS. After 10 extractions, NTA removed all or almost all Cu and about 60% of As (Fig. 1). Citrate removed about the same percentage of As as NTA and 50–90% of total Cu from the two soil samples while HS extracted about 50% Cu and 20–40% of total As. The precise percentages are shown in Table 2. As indicated in Table 2, the efficiency of HS increases slightly with increasing concentration, in particular the extraction of Cu is in good agreement with previous studies [9,13,14]. The influence of the soil factors on the extractability is demonstrated by the higher percentages of As and Cu extracted with 50 mM DOC from the
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Q4
Table 2 Influence of extractant and its concentration (as mM DOC) on accumulated percentage of total As and Cu extracted during 10 extractions from the two CCA soil samples at pH 6; solution to soil ratio 10 in each extraction.
CCA1 soil sample compared to the smaller percentages extracted by 100 mM DOC from CCA2 (Table 2). This is in good agreement with the importance of the soil factor on the extractability as pointed out before [2,4,6]. Also the different percentages extracted from the CRC and CCA samples in Fig. 1 show the importance of the soil factor. The pH also affects the extractability. Although the shapes of the accumulated extraction curves resembled those in Fig. 1, the amounts extracted of the two elements decreased with increasing pH, especially when pH was changed from 4 to 6 (Table 3). This is in line with previous studies [4,6,7,14]. The pH effect was most pronounced for HS, where extracted As increased from 16% to 60% and Cu from 61% to 95% by lowering pH from 6 to 4. This is an important finding in relation to the possibility of using HS in remediation of As and Cu contaminated soils. Copper, which is a cation (Cu2+), is extracted because of formation of stable soluble complexes with carboxylic acid groups of the citrate, NTA and HS molecules [13,14]. In contrast, As occurs in well-aerated soils as anions (arsenate HAsO42/H2AsO4), which are strongly sorbed onto the surfaces of variable charge soil components, in particular iron (Fe) oxides [1–3]. Extraction of As is therefore indirectly resulting from Fe oxide dissolution leading to As release [8] in agreement with the close correlation between dissolved As and Fe in Fig. 2. Table 3 Influence of pH on accumulated percentage of total As and Cu extracted during 10 extractions from the CCA2 soil sample by 100 mM DOC of the three extractants; solution to soil ratio 10 in each extraction.
Contaminant
Extractant/concentration
CCA1 25 mM
CCA1 50 mM
CCA2 100 mM
Contaminant
Extractant
pH 4
pH 6
pH 8
As As As
HS Citrate NTA
10% 53% 59%
18% 64% 60%
16% 63% 50%
As As As
HS Citrate NTA
60% 100% 81%
16% 63% 50%
16% 52% 53%
Cu Cu Cu
HS Citrate NTA
33% 97% 96%
62% 89% 96%
61% 77% 80%
Cu Cu Cu
HS Citrate NTA
95% 95% 96%
61% 77% 80%
58% 81% 85%
Please cite this article in press as: S.B. Rasmussen, et al., A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.04.029
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References
Fig. 2. Relationship between accumulated release/extraction of As and Fe by 100 mM DOC of NTA, citrate and HS from the CCA2 soil sample at pH 6. 183
Extractant comparison
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At neutral pH, NTA and citrate are clearly more effective As and Cu extractants than HS (Table 2; Fig. 1). The superiority of NTA and citrate may be attributed to their higher carboxylic acid density with 500 mmol COOH per mol C compared to 140 mmol COOH/mol C in HS. Furthermore, the NTA and citrate form more stable complexes with Cu and Fe than HS [13,14]. However, at pH 4, HS extracts the same amount of Cu as NTA and citrate and a substantial amount of As (Table 3). Accordingly, HS at pH 4 as well as NTA and citrate may be used as washing agents for strongly As and Cu contaminated soils. Although citrate and NTA can extract substantial amounts of As, Cu and other heavy metals [2,13,14], they are environmentally problematic as they are toxic [9,11]. In contrast, HS merely seems to be a naturally occurring environmentally non-impacting bioagronomic amendment stimulating plant seed germination and soil microfauna [9]. Furthermore, HS at neutral pH seems attractive to enhance in-situ phytoremediation as indicated by the almost linear releases of moderate amounts of As and Cu as shown in Fig. 1. In contrast, the high initial releases of As and Cu to citrate and NTA extractants (Fig. 1) may be phytotoxic and affect water quality if transported to ground and surface waters by leaching and runoff. On the other hand, the CCA soil of the present study (Table 1) is far too contaminated for phytoremediation unless a very resistant plant can be introduced. However, phytoremediation on this site is pure speculation as the soil is too contaminated to be left in place according to Danish legislation [23]. The soil must be removed and place in an approved contaminated soil depot, where it may be remediated by soil washing using NOM such as HS at pH 4 or citrate to be environmentally friendly.
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Conclusions
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Extraction of As and Cu from the very contaminated CCA soil and from the moderately contaminated calcareous urban soil depended on the soil, extractant (NTA, citrate, HS), extractant concentration and pH. At neutral pH, the As and Cu amounts extracted decrease in the order: NTA > citrate > HS, but at pH 4, the three extractants dissolved almost all Cu and most As. To replace synthetic NTA by naturally occurring NOM, HS at pH 4 and citrate can be recommended for soil washing of very contaminated soils. HS at neutral pH seems attractive for enhanced phytoremediation of moderate contaminated soil, whereas use of citrate (and NTA) may negatively affect plant growth and lead to recipient water pollution if used on site.
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Borggaard, A laboratory assessment of potentials and limitations of using EDTA, rhamnolipids, and compost-derived humic substances (HS) in enhanced phytoextraction of copper and zinc polluted calcareous soils, Soil Sediment Contam. 20 (7) (2011) 777–789, doi: http://dx.doi.org/10.1080/15320383.2011.609198. [6] D. Leštan, C.L. Luo, X.D. Li, The use of chelating agents in the remediation of metal-contaminated soils: a review, Environ. Pollut. 153 (1) (2008) 3–13, doi: http://dx.doi.org/10.1016/j.envpol.2007.11.015. 18155817. [7] S. Tandy, K. Bossart, R. Mueller, J. Ritschel, L. Hauser, R. Schulin, B. Nowack, Extraction of heavy metals from soils using biodegradable chelating agents, Environ. Sci. Technol. 38 (3) (2004) 937–944, doi:http://dx.doi.org/10.1021/ es0348750. 14968886. [8] D.C.W. Tsang, N.R. Hartley, Metal distribution and spectroscopic analysis after soil washing with chelating agents and humic substances, Environ. Sci. Pollut. Res. 21 (5) (2014) 3987–3995, doi:http://dx.doi.org/10.1007/s11356-0132300-y. 24297462. [9] 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 Pollut. 193 (1–4) (2008) 323–333, doi:http://dx.doi.org/10.1007/ s11270-008-9693-0. [10] H. Gr9 cman, Š. Velikonja-Bolta, D. Vodnik, B. Kos, D. Leštan, EDTA enhanced heavy metal phytoextraction: metal accumulation, leaching and toxicity, Plant Soil 235 (1) (2001) 105–114, doi:http://dx.doi.org/10.1023/A:1011857303823. [11] C.K. Schmidt, H.J. Brauch, Impact of aminopolycarboxylates on aquatic organisms and eutrophication: overview of available data, Environ. Toxicol. 19 (6) (2004) 620–637, doi:http://dx.doi.org/10.1002/tox.20071. 15526262. [12] P. Conte, A. Agretto, R. Spaccini, A. Piccolo, Soil remediation: humic acids as natural surfactants in the washings of highly contaminated soils, Environ. Pollut. 135 (3) (2005) 515–522, doi:http://dx.doi.org/10.1016/j.envpol.2004.10.006. 15749548. [13] M. Soleimani, M.A. Hajabbasi, M. Afyuni, S. Akbar, J.K. Jensen, P.E. Holm, O.K. Borggaard, Comparison of natural humic substances and synthetic ethylenediaminetetraacetic acid and nitrilotriacetic acid as washing agents of a heavy metal-polluted soil, J. Environ. Qual. 39 (3) (2010) 855–862, doi:http:// dx.doi.org/10.2134/jeq2009.0292. 20400581. [14] A. Polettini, R. Pomi, G. Calcagnoli, Assisted washing for heavy metal and metalloid removal from contaminated dredged materials, Water Air Soil Pollut. 196 (1–4) (2009) 183–198, doi:http://dx.doi.org/10.1007/s11270-0089767-z. [15] A. Romero-Freire, M. Sierra-Aragón, I. Ortiz-Bernad, F.J. Martín-Peinado, Toxicity of arsenic in relation to soil properties: implications to regulatory purposes, J. Soils Sediments 14 (5) (2014) 968–979, doi:http://dx.doi.org/ 10.1007/s11368-014-0845-0. [16] Skov- og Naturstyrelsen, Kemikalieaffaldsdepot 219-3, Stenholtsvang, Forureningsundersøgelse, Fase 1-3, A/S Samfundsteknik, Allerød, Denmark, 1989 (in Danish). [17] P.R. Day, Particle fractionation and particle-size analysis, in: C.A. Black, D.D. Evans, L.E. Ensminger, J.L. White, F.E. Clark (Eds.), Methods of Soil Analysis, ASA and SSSA, Madison, WI, 1965, pp. 545–567. [18] L.E. Allison, C.D. Moodie, Carbonate, in: C.A. Black, D.D. Evans, L.E. Ensminger, J. L. White, F.E. Clark (Eds.), Methods of Soil Analysis, ASA and SSSA, Madison, WI, 1965, pp. 1379–1396. [19] ELTRA, CS500 Simultaneous Carbon/Sulphur Determinator, ELTRA Gmbh, Neuss, Germany, 1995. [20] J.C. Tjell, M.F. Hovmand, Metal concentrations in Danish arable soils, Acta Agric. Scand. 28 (1) (1978) 81–89, doi:http://dx.doi.org/10.1080/ 00015127809435159. [21] U. Schwertmann, Differenzierung der Eisenoxide des Bodens durch Extraktion mit Ammoniumoxalat-Lösung, Zeitschrift für Pflanzenernährung, Düngung, Bodenkunde 105 (3) (1964) 194–202, doi:http://dx.doi.org/10.1002/ jpln.3591050303. [22] O.P. Mehra, Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate, Clays Clay Miner. 7 (1) (1958) 317– 327, doi:http://dx.doi.org/10.1346/CCMN.1958.0070122. [23] Miljøstyrelsen, Grænseværdier for jord. Available from http://www.mst.dk/ virksomhed-myndighed/kemikalier/graensevaedier-og-kvalitetskriterier/ graensevaerdier-for-jord (in Danish) (assessed 7.10.14.).
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JECE 671 1–4 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
1 2
A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils
3 Q1
Signe B. Rasmussen* , Julie K. Jensen, Ole K. Borggaard
4
Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 December 2014 Accepted 15 April 2015
Soils contaminated by arsenic (As) and copper (Cu) must be remediated because As and Cu are nondegradable and toxic. On moderately contaminated soils, As and Cu may be removed by in-situ plant uptake (phytoremediation), whereas strongly contaminated soils must be removed and cleaned by soil washing at a soil disposal site (ex-situ). Strong bonding of As and Cu to soil solids requires the use of strong complexants to release the elements. Often synthetic chemicals such as EDTA and NTA are used. Since such chemicals are environmentally problematic, their replacement with natural organic matter (NOM) such as humic substances (HS) and citrate might be attractive. To test this possibility, a moderately contaminated calcareous urban soil from a soil depot (called the CRC soil) and a soil from a wood impregnation site (called the CCA soil) highly contaminated with As and Cu were extracted with citrate, HS and NTA at different concentration and pH, and the results compared. Extracted As and Cu were found to increase at increasing extractant concentration but to decrease at increasing pH. The efficiency of the three extractants generally decreased in the order: NTA > citrate > HS but at pH 4, HS extracted the same amounts of Cu as citrate and NTA, and substantial amounts of As. Instead of replacing one contaminant, As/Cu, with another contaminant, synthetic NTA, it is therefore recommended to use HS at pH 4 or citrate for removing As and Cu from strongly contaminated soils and to use HS at neutral pH to enhance in-situ phytoremediation of moderately contaminated soils. Citrate (and NTA) cannot be suggested for enhancement of on-site phytoremediation because of high mobilization rates caused by these extractants, which through leaching and runoff may lead to contamination of recipient waters with As and Cu. ã 2015 Published by Elsevier Ltd.
Keywords: Humic substances Heavy metals Enhanced phytoremediation Soil washing Citrate NTA
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Introduction Human activities such as wood impregnation and mining cause arsenic (As) and copper (Cu) contamination of soils, which must be removed as As and Cu are non-degradable and toxic posing a threat to human health and ecosystem functioning [1–3]. As and Cu can be removed in-situ by phytoremediation on moderately contaminated soils or ex-situ by soil washing of strongly contaminated soils. Because As and Cu are often strongly bonded to soil solids, both methods often use a strong complexant to increase contaminant solubility, e.g. ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) or similar compounds [4–8]. However, although
Abbreviations: HS, humic substances; NOM, natural organic matter; NTA, nitrilotriacetic acid; As, arsenic; Cu, copper; Fe, iron; CCA soil, chromium, copper and arsenic contaminated soil; CRC soil, Copenhagen Recycling Centre soil; AAS, atomic absorption spectrometry; ICP-OES, inductively coupled plasma-optical emission spectroscopy. * Corresponding author. Tel.: +45 35333394. E-mail address: [email protected] (S.B. Rasmussen).
EDTA, NTA and other synthetic aminopolycarboxylates are effective cleaning agents, they are environmentally problematic [9–11] suggesting a need for their replacement with cheap natural organic matter (NOM) compounds such as soluble humic substances (HS) and citrate forming strong soluble complexes with Cu and other heavy metals [2,4,5,8,9,12,13]. In fact, recent studies have shown that NOM compounds such as HS-like materials formed by oxidative hydrolytic degradation of cow slurry, leonardite HS and Sigma– Aldrich HS as well as citrate can extract As, Cu and some other heavy metals but they are clearly less effective than EDTA and NTA [2,8,13]. In addition to the nature of the extractant, extraction of As, Cu and other heavy metals depends on soil properties, e.g. the pH, clay and organic C contents, and whether spiked or long-term contaminated Q2 (aged) soil is tested [2,5,14,15]. In order to get improved knowledge about the suitability of replacing synthetic NTA by NOM, the amounts of As and Cu extracted by NTA, citrate and HS from two long-term contaminated soils were determined by laboratory batch extraction experiments. The soils tested included an urban calcareous soil and a chromium (Cr), copper and arsenic (CCA) contaminated soil from an abandoned wood impregnation site.
http://dx.doi.org/10.1016/j.jece.2015.04.029 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: S.B. Rasmussen, et al., A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.04.029
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Materials and methods
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Soils
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The urban soil sample (the CRC soil) was collected at a soil disposal site at the Copenhagen Recycling Centre (CRC), which receives contaminated soil from the greater Copenhagen area and Q3 is described in more details in Ref. [5]. The soil investigated consisted of five mixed subsamples randomly taken at different parts of the soil depot. The CCA soil sample was from an old wood impregnation site, which were active in the years 1936–1976, using Cr, Cu and As impregnation solutions. The site is named Collstrup, and is located at the town, Hillerød in north Sealand. Two samples were taken from the top 20 cm at the same spot; hereafter called CCA1 soil and CCA2 soil. Based on a map of the Collstrup site [16] supported by observed modest vegetation, the sampled spot was selected as a very contaminated place (spot) due to impregnation solution spills during wood trunk impregnation and storage. After air-drying, the soil samples were sieved (2 mm) and the fine earth characterized as indicated in Table 1. The clay content was determined by the hydrometer method [17], pH by glass electrode in a 1:2.5 soil:water suspension and CaCO3 by calcimeter [18]. Organic carbon was measured by dry combustion at 1250 C in oxygen [19] and corrected for CaCO3. Total Al, As, Cu and Fe were determined by atomic absorption spectrometry (AAS) or inductively coupled plasma-optical emission spectroscopy (ICP-OES) in the digest obtained by boiling the soil for 8 h with 7.3 M HNO3[20]. Oxalateextractable Al and Fe (AlOxalate, FeOxalate) were determined by AAS in the extract obtained by extraction for 2 h with 0.2 M ammonium oxalate at pH 3 in the dark [21] and citrate–bicarbonate–dithioniteextractable Al and Fe (AlCBD, FeCBD) determined by AAS in the extract obtained by three extractions for 15 min at 70 C as described by Mehra and Jackson [22].
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Extractants
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The extractants tested included NTA, citrate and soluble HS. All extractants were made to contain 0.05 M KNO3, which was used as control and pH was adjusted to the desired value by 1 M HNO3 or 1 M KOH. The NTA extractant was prepared by dissolution of Titriplex I (C6H9NO6) in KOH and citrate by dissolution of citric acid (C6H8O7) in KOH. The HS extractant was prepared from a compost percolate collected from a reservoir at a composting plant at CRC, where rainwater has percolated through piles of park and garden wastes collected from the urban areas of Copenhagen. The percolate was evaporated to dryness, dissolved in water again and filtered resulting in a clear brown solution.
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Table 1 Selected characteristics of the CCA and CRC soil samples. Characteristic
CCA soil
CRC soil
Clay (%) CaCO3 (%) Org. C (%) pH Altotal (mg/kg) Fetotal (mg/kg) AlCBD (mg/kg) FeCBD (mg/kg) AlOxalate (mg/kg) FeOxalate (mg/kg) Astotal (mg/kg) Cutotal (mg/kg)
16.0 1.0 0 1.89 0.05 5.4 0.1 11,740 500 7240 150 599 19 3365 38 571 10 1784 20 a 990 30 and b860 20 a 2100 45 and b1500 30
24.0 1.0 9.1 0.1 1.62 0.05 8.1 0.1 16,200 1500 16,600 1800 606 14 2660 50 667 12 1094 33 180 10 140 5
a b
As and Cu contents in CCA1. As and Cu contents in CCA2.
The C concentration was determined using a Shimadzu TOCvcpn total carbon analyser. All extractant concentrations were expressed as dissolved organic carbon (DOC) to ease comparison, i.e. 100 mM DOC concentration of citrate and NTA corresponds to 16.7 mM NTA or citrate as both citrate and NTA contain 6 C per molecule. The content of active groups including carboxylic acid groups (COOH) and phenolic groups (Ar OH) were determined in HS by potentiometric titration, where COOH and Ar OH were taken as titratable acidity between pH 3 and 7 and between pH 7 and 11, respectively [13]. Accordingly, HS contained 140 18 mmol COOH/mol C and 145 24 mmol Ar OH/mol C. For comparison, citrate and NTA contain 500 mmol COOH per mol C but no Ar OH groups.
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Extractions
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The extractions were performed in the batch mode as multi-step extractions with 10 steps, where 2.5 g of soil was shaken horizontally with 25 ml of extractant solution in a 50 ml centrifuge tube for 24 h. Afterwards the tube was subjected to centrifugation at 3000 g for 15 min, two portions of 10 ml of the clear supernatant were pipetted into two 15 ml test tubes, one containing 0.5 ml concentrated HNO3 (for As and Cu determination) and one without HNO3 (for DOC and pH measurements). Then 20 ml of fresh extractant solution was added to the centrifuge tube and the procedure repeated 9 more times. In the non-acidified extracts, pH and the DOC concentration were measured as described above. The contents of As and Cu were determined in the acidified extracts by AAS. Accumulated releases (Yn) of As and Cu in mg/kg were calculated by the following formula:
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Y n ¼ 10 X n 2 X n1 þ Y n1 where X is the element concentration in mg/l and n is number of extractions (= days). The term 2 Xn1 accounts for the As or Cu content in the 5 ml solution per 2.5 g of soil left over from the previous extraction. All the extractions were carried out as triplicates. All chemicals were of analytical grade or ultrapure, the water was triple deionized and all glass- and plastwares were acid-washed before used. The limits of detection were 0.5 mg/l for As and 0.04 mg/l for Cu. Statistical analysis of the data was performed in Microsoft Office Excel.
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Results and discussion
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Soil characteristics
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The contents of clay, CaCO3 and total Al, Fe, As and Cu as well as pH of the CCA and CRC soil samples were clearly different, while org. C, DCB- and oxalate-extractable Al and Fe were comparable (Table 1). The CCA soil consisted of CCA1 and CCA2 with the same general (basic) characteristics but with different As and Cu contents. Regarding the contents of As and Cu, the CCA soil is strongly contaminated while the CRC soil is less contaminated but above the Danish threshold value of 20 mg As/kg; for Cu the Danish threshold is 500 mg/kg [23]. The great variability of As and Cu in soil at the wood impregnation sites is clearly demonstrated by the different contents of the CCA1 and CCA2 samples even though they were sampled very close (1 m) to each other.
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Extraction of As and Cu
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Accumulated As and Cu extracted during the 10 extractions from the soil samples at pH 6 are shown in Fig. 1. The efficiency of the three extractants decreases in the order NTA > citrate > HS
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CRC soil
CCA soil
Extracted As (%)
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A)
B)
0 100
CCA soil
CRC soil Extracted Cu (%)
80 60 40 20
C)
D)
0 1
2
3
4
5
6
7
8
9
10
1
2
Number of extracons HS
Citric acid
3
4
5
6
7
8
9
10
Number of extracons NTA
HS
Citric acid
NTA
Fig. 1. Accumulated extraction of As and Cu by 50 mM DOC of HS, citrate and NTA as a function of number of extractions, at pH 6 and expressed as percentage of total As and total Cu. Panel A: as extracted from the CRC soil sample; Panel B: as extracted from the CCA1 soil sample; Panel C: Cu extracted from the CRC soil sample; Panel D: Cu extracted from the CCA1 soil sample. Vertical bars indicate standard errors. 138
and the extraction curves are somewhat different. As and Cu extracted by NTA and citrate are largest in the first extraction leading to convex extraction curves, whereas the HS extractions curves are almost linear with similar releases during the 10 extractions. In particular the first extraction with NTA and citrate is very efficient compared both to the following NTA/citrate extractions and to As and Cu extracted by HS. In the first extraction, NTA removed 50–80% and citrate 25–70% of total Cu from the CRC sample and the CCA1 sample, respectively while HS only extracted about 10–15%. The extractability of As was less marked with about 20–25% extracted with NTA and citrate in the first extraction from the two soil samples, and only few percent extracted with HS. After 10 extractions, NTA removed all or almost all Cu and about 60% of As (Fig. 1). Citrate removed about the same percentage of As as NTA and 50–90% of total Cu from the two soil samples while HS extracted about 50% Cu and 20–40% of total As. The precise percentages are shown in Table 2. As indicated in Table 2, the efficiency of HS increases slightly with increasing concentration, in particular the extraction of Cu is in good agreement with previous studies [9,13,14]. The influence of the soil factors on the extractability is demonstrated by the higher percentages of As and Cu extracted with 50 mM DOC from the
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Q4
Table 2 Influence of extractant and its concentration (as mM DOC) on accumulated percentage of total As and Cu extracted during 10 extractions from the two CCA soil samples at pH 6; solution to soil ratio 10 in each extraction.
CCA1 soil sample compared to the smaller percentages extracted by 100 mM DOC from CCA2 (Table 2). This is in good agreement with the importance of the soil factor on the extractability as pointed out before [2,4,6]. Also the different percentages extracted from the CRC and CCA samples in Fig. 1 show the importance of the soil factor. The pH also affects the extractability. Although the shapes of the accumulated extraction curves resembled those in Fig. 1, the amounts extracted of the two elements decreased with increasing pH, especially when pH was changed from 4 to 6 (Table 3). This is in line with previous studies [4,6,7,14]. The pH effect was most pronounced for HS, where extracted As increased from 16% to 60% and Cu from 61% to 95% by lowering pH from 6 to 4. This is an important finding in relation to the possibility of using HS in remediation of As and Cu contaminated soils. Copper, which is a cation (Cu2+), is extracted because of formation of stable soluble complexes with carboxylic acid groups of the citrate, NTA and HS molecules [13,14]. In contrast, As occurs in well-aerated soils as anions (arsenate HAsO42/H2AsO4), which are strongly sorbed onto the surfaces of variable charge soil components, in particular iron (Fe) oxides [1–3]. Extraction of As is therefore indirectly resulting from Fe oxide dissolution leading to As release [8] in agreement with the close correlation between dissolved As and Fe in Fig. 2. Table 3 Influence of pH on accumulated percentage of total As and Cu extracted during 10 extractions from the CCA2 soil sample by 100 mM DOC of the three extractants; solution to soil ratio 10 in each extraction.
Contaminant
Extractant/concentration
CCA1 25 mM
CCA1 50 mM
CCA2 100 mM
Contaminant
Extractant
pH 4
pH 6
pH 8
As As As
HS Citrate NTA
10% 53% 59%
18% 64% 60%
16% 63% 50%
As As As
HS Citrate NTA
60% 100% 81%
16% 63% 50%
16% 52% 53%
Cu Cu Cu
HS Citrate NTA
33% 97% 96%
62% 89% 96%
61% 77% 80%
Cu Cu Cu
HS Citrate NTA
95% 95% 96%
61% 77% 80%
58% 81% 85%
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References
Fig. 2. Relationship between accumulated release/extraction of As and Fe by 100 mM DOC of NTA, citrate and HS from the CCA2 soil sample at pH 6. 183
Extractant comparison
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At neutral pH, NTA and citrate are clearly more effective As and Cu extractants than HS (Table 2; Fig. 1). The superiority of NTA and citrate may be attributed to their higher carboxylic acid density with 500 mmol COOH per mol C compared to 140 mmol COOH/mol C in HS. Furthermore, the NTA and citrate form more stable complexes with Cu and Fe than HS [13,14]. However, at pH 4, HS extracts the same amount of Cu as NTA and citrate and a substantial amount of As (Table 3). Accordingly, HS at pH 4 as well as NTA and citrate may be used as washing agents for strongly As and Cu contaminated soils. Although citrate and NTA can extract substantial amounts of As, Cu and other heavy metals [2,13,14], they are environmentally problematic as they are toxic [9,11]. In contrast, HS merely seems to be a naturally occurring environmentally non-impacting bioagronomic amendment stimulating plant seed germination and soil microfauna [9]. Furthermore, HS at neutral pH seems attractive to enhance in-situ phytoremediation as indicated by the almost linear releases of moderate amounts of As and Cu as shown in Fig. 1. In contrast, the high initial releases of As and Cu to citrate and NTA extractants (Fig. 1) may be phytotoxic and affect water quality if transported to ground and surface waters by leaching and runoff. On the other hand, the CCA soil of the present study (Table 1) is far too contaminated for phytoremediation unless a very resistant plant can be introduced. However, phytoremediation on this site is pure speculation as the soil is too contaminated to be left in place according to Danish legislation [23]. The soil must be removed and place in an approved contaminated soil depot, where it may be remediated by soil washing using NOM such as HS at pH 4 or citrate to be environmentally friendly.
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Conclusions
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Extraction of As and Cu from the very contaminated CCA soil and from the moderately contaminated calcareous urban soil depended on the soil, extractant (NTA, citrate, HS), extractant concentration and pH. At neutral pH, the As and Cu amounts extracted decrease in the order: NTA > citrate > HS, but at pH 4, the three extractants dissolved almost all Cu and most As. To replace synthetic NTA by naturally occurring NOM, HS at pH 4 and citrate can be recommended for soil washing of very contaminated soils. HS at neutral pH seems attractive for enhanced phytoremediation of moderate contaminated soil, whereas use of citrate (and NTA) may negatively affect plant growth and lead to recipient water pollution if used on site.
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