Chemosphere 138 (2015) 253–258
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Effect of different soil washing solutions on bioavailability of residual arsenic in soils and soil properties Jinwoo Im a, Kyung Yang a, Eun Hea Jho b,⇑, Kyoungphile Nam a a b
Department of Civil and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea Department of Environmental Science, Hankuk University of Foreign Studies, 81 Oedae-ro, Mohyeon-myeon, Cheoin-gu, Yongin-si, Gyeonggi-do 449-791, Republic of Korea
h i g h l i g h t s Acidic and near-neutral washing solutions reduced the soil As level by 32–62%. The (NH4)2SO4 , NH4H2PO4 , (NH4)2C2O4-extractable As were greatly reduced. But seed germination, growth, and enzyme activities were lower in the washed soils. Greater toxic effects are largely due to changes in soil properties after washing. Proper post-treatment techniques need to be selected for reuses of the washed soils.
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Article history: Received 11 February 2015 Received in revised form 24 April 2015 Accepted 2 June 2015
Keywords: Soil washing Arsenic Residual arsenic Bioavailability Soil properties Toxic effect
a b s t r a c t The effect of soil washing used for arsenic (As)-contaminated soil remediation on soil properties and bioavailability of residual As in soil is receiving increasing attention due to increasing interest in conserving soil qualities after remediation. This study investigates the effect of different washing solutions on bioavailability of residual As in soils and soil properties after soil washing. Regardless of washing solutions, the sequential extraction revealed that the residual As concentrations and the amount of readily labile As in soils were reduced after soil washing. However, the bioassay tests showed that the washed soils exhibited ecotoxicological effects – lower seed germination, shoot growth, and enzyme activities – and this could largely be attributed to the acidic pH and/or excessive nutrient contents of the washed soils depending on washing solutions. Overall, this study showed that treated soils having lower levels of contaminants could still exhibit toxic effects due to changes in soil properties, which highly depended on washing solutions. This study also emphasizes that data on the As concentrations, the soil properties, and the ecotoxicological effects are necessary to properly manage the washed soils for reuses. The results of this study can, thus, be utilized to select proper post-treatment techniques for the washed soils. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Various remedial techniques such as stabilization/solidification (Miretzky and Cirelli, 2010), soil washing (Tsang and Hartley, 2014), phytoremediation, specifically, phytoextraction using hyperaccumulator species, (Mandal et al., 2014), and electrokinetic remediation (Kim et al., 2013) can be applied for remediation of arsenic (As)-contaminated soils. In Korea, however, soil washing is often considered as a remedial technique for As-contaminated sites (e.g., a former Janghang smelting site, Dalchen mine), as it removes As from soils in a relatively short time compared to other ⇑ Corresponding author. E-mail addresses: [email protected] (J. Im), [email protected] (K. Yang), [email protected] (E.H. Jho), [email protected] (K. Nam). http://dx.doi.org/10.1016/j.chemosphere.2015.06.004 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
remedial techniques rather than just reducing bioavailability of As in soils via immobilization (Han and Shin, 2008; Moon et al., 2011). Since the treated soils are largely reused at the site and the current Korean regulation evaluates remedial efficiency based on the total As concentrations in soils, As removal techniques (e.g., soil washing) may be preferred over As immobilization techniques. During soil washing, contaminated soils are mixed with a washing solution, and agitated to remove contaminants. The washing treatment process or the washing solution used in washing treatment is likely to affect soil properties that are related to soil qualities (Ko et al., 2005; Yi et al., 2012). For example, soil properties such as soil texture, water holding capacity, organic matter (OM) content, and total nitrogen concentration were changed after soil washing (Yi et al., 2012). Such changes in soil properties may change soil permeability and shear strength, which will
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consequently affect long-term soil functions (Zihms et al., 2013). Changes in soil properties could also mean possible changes in bioavailability of As in soils as bioavailability of metals or metalloids is highly dependent on soil properties (e.g., pH, OM content, redox potential) (Masscheleyn et al., 1991; Huang et al., 2006). Soil washing of contaminated soils usually involves strong acids as washing solutions. For example, a number of patented washing solutions targeting As removal from soils in Korea involve HCl or H3PO4 – 1 M hydrochloric acid (HCl) (Choi et al., 2007), 0.5 M phosphoric acid (H3PO4) (Cheong and Kang, 2012), and 2% sodium (Na) dithionite in 0.01 M HCl (Kim et al., 2012). When strong acid-based washing solutions are used for As removal from soils, acids containing oxyanions (e.g., H3PO4 or H2SO4) are more effective than acids without oxyanions (e.g., HCl). This is because of competitive oxyanions (PO34 or SO24 ) that may decrease the re-adsorption of anionic species of As on reactive surfaces of soil (Goh and Lim, 2005; Ko et al., 2005; Lee et al., 2007). However, with increasing awareness of post-remediation soil qualities, mild washing solutions may be preferred over strong acid-based washing solutions, because strong acids could adversely affect soils by dissolving soil components during washing treatment (Ko et al., 2005). Thus, washing solutions prepared with phosphate containing salts that have neutral or near-neutral pH can be used as alternatives to minimize detrimental effects (e.g., losses of soil OM and mineral substances (e.g., Si, Ca, Mg, Fe, Al), decreases in soil pH) of soil washing solutions (Alam et al., 2001; Tokunaga and Hakuta, 2002; Zeng et al., 2008). The performance of washing solutions has largely been accessed on As extractability, and what happens to the residual As in soils or soil properties has not been extensively studied. Since there is a growing interest in minimizing negative effects of soil washing on soil qualities, the changes in soil properties and bioavailability of the residual As in soils after soil washing need to be studied. Therefore, this study investigates how bioavailability of the residual As in soils and soil properties of the washed soils are affected by soil washing with different solutions. Selected patented washing solutions for As-contaminated soils in Korea and the neutral phosphate salt-based washing solution (hereafter referred to as neutral phosphate solution), which was developed in order to avoid problems arising from soil acidity after soil washing with strong acids, were used for comparison. Specifically, the effect of using different washing solutions on changes in bioavailability of the residual As in soils and soil properties was determined by using chemical extraction-based methods (e.g., sequential extraction method) and bioassay test-based methods (e.g., seed germination and growth, microbial enzyme activities).
2. Materials and methods 2.1. Soil sampling Historically As-contaminated soil samples were taken from the vicinity of a former smelter in Janghang, Korea. The soil samples were air-dried at room temperature, and sieved through a 2 mm sieve. The soil pH was 6.1, and the OM content determined by the Walkley–Black procedure (Walkley and Black, 1934) and the iron oxide concentration determined by the bicarbonate -citrate-dithionite (DBC) method (Mehra and Jackson, 1958) were 2.1% and 8240 mg kg 1, respectively. The concentrations of available phosphorus and the total nitrogen were determined by using the Bray’s method (Bray and Kurtz, 1945) and the micro-Kjeldahl method (Bremner, 1960), respectively. The detection limits for the Bray’s and the micro-Kjeldahl methods were 0.01 mg L 1 and 0.1 mg L 1, respectively. The soil texture was silt loam consisting of 28% sand, 54% silt, and 18% clay. The background concentrations
of Pb, Cd, Cu, Zn, and As determined following the aqua regia extraction method with slight modifications (International Standard Organization, 1995), complying with Korea Ministry of Environment (Korea Ministry of Environment, 2009), were 97, 0.5, 52, 89, and 59 mg kg 1, respectively. The As concentration (59 mg kg 1) exceeded the cleanup level (25 mg kg 1 for As) of the Korean Soil Environment Conservation Act. 2.2. Soil washing procedures The soil washing condition and procedure using a neutral phosphate solution were developed in our previous study (Im et al., 2014). Briefly, 24 h soaking of As-contaminated soils in a 0.5 M phosphate solution (pH 6.0), prepared from ammonium dihydrogen phosphate (NH4H2PO4) and diammonium hydrogen phosphate ((NH4)2HPO4), at soil-to-solution (S/L) ratio of 1:5 was followed by 1 h washing at 200 rpm. Since conventional washing procedures involve a single-step soil washing treatment, a 1 h washing period was used with the three selected patented washing solutions for As removal from soils in Korea. The selected washing solutions are 1 M HCl (Choi et al., 2007), 0.5 M H3PO4 (Cheong and Kang, 2012), and 2% Na dithionite in 0.01 M HCl (Kim et al., 2012). All the washing experiments were carried out in triplicate using 1 L jars with 120 g soils and 600 mL washing solutions. After 1 h soil washing at 200 rpm, the washed soils were rinsed twice with deionized water. The washing and rinsing solutions can be treated either by neutralization and precipitation or by sorption on to adsorbents such as granular ferric oxide (Choi et al., 2007; Cheong and Kang, 2012; Yang et al., 2013). Also, As in the solutions can be removed by adding oxidants (e.g., hydrogen peroxide) and coagulants (Kim et al., 2012). 2.3. Extraction and analyses of As For As analysis in soils, soil slurries in the washing experiments were left to settle in order to separate soils from washing solutions. The residual As in the separated and then dried soils (3 g) were extracted following the method described in Section 2.1. Chemical forms of As (i.e., fractionation of As) in soils were assessed using the sequential extraction procedure of Wenzel et al. (2001), which divides As in soils into five fractions, with a slight modification – (1) ammonium sulfate ((NH4)2SO4; 0.05 M)-extractable As (i.e., non-specifically sorbed As); (2) NH4H2PO4 (0.05 M)-extractable As (i.e., specifically sorbed As); (3) ammonium oxalate ((NH4)2C2O4; 0.2 M)-extractable As (i.e., As associated with amorphous hydrous oxides of Fe and Al); (4) (NH4)2C2O4 (0.2 M)/ascorbic acid (0.1 M)-extractable As (i.e., As associated with crystalline hydrous oxides of Fe and Al); and (5) residual As fraction extracted with the USEPA method 3052, which digest siliceous and organically based matrices such as soils and sediments in a mixture of nitric acid, hydrofluoric acid, HCl, and hydrogen peroxide using microwave heating (US Environmental Protection Agency, 1996). The As concentrations in the extracts were analyzed by using inductively coupled plasma-optical emission spectrometry (ICP-OES; ICP-730ES, Varian, USA). The method detection limit for As was 0.072 mg kg 1. Statistical analysis was conducted using Excel 14.0 (Microsoft Corporation, USA) and the statistical significance of differences in As concentrations was determined by using the Student’s t-test at 5% significance level. 2.4. Bioassay tests using plants and soil microbial enzymes Seed germination and growth tests were carried out with ten seeds of Indian mustard (Brassica juncea L.) sown in each petri dish (diameter = 90 mm) containing dried soil equivalent to 30 g. The tests were carried out in triplicate under controlled environment
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of a growth chamber (23 ± 3 °C, 80% humidity, 16 h light period). All the petri dishes were watered daily to 80% of the soil water holding capacity based on weight. The number of the germinated seeds exhibiting >2 mm growth were counted after 7 d (Liu et al., 2005). The shoot length after 14 d from the sowing day was used as a plant growth parameter. Soil dehydrogenase activity and acid phosphatase activity were measured following the methods described by Yi et al. (2013). The dehydrogenase activity can represent microbial activity associated with organic matter breakdown (Ross, 1971), and can be used as an index of endogenous soil microbial activity (Moore and Russell, 1972; Bolton et al., 1985). The phosphatase activity can represent the soil microbial activity associated with phosphorus circulation (Yi et al., 2013). For dehydrogenase activity determination, soil (6 g) incubation (37 °C, 24 h) with 2,3,5-triphenlytetrazolium chloride (1 mL) and distilled water (2.5 mL) was followed by absorbance measurement of methanol extracts at 485 nm. Triphenyl formazan (TPF) was used to prepare standard solutions for absorbance measurement. For acid phosphatase activity determination, a mixture of 1 g soil, 0.2 mL toluene, 4 mL modified universal buffer (MUB, pH 6.5), and 1 mL of 0.025 M p-nitrophenyl phosphate (p-NPP) was incubated at 37 °C for an hour, and then, 1 mL of 0.5 M CaCl2 and 4 mL of 0.5 M NaOH were added to the mixture. The absorbance of p-nitrophenol in the filtered extract was measured at 400 nm. All the absorbance was measured using a UV spectrophotometer (OPTIZEN 2120UV, Mecasys, Korea). All the enzyme assays were run in triplicate. 3. Results and discussions 3.1. Effect of soil washing on chemical forms of residual As in soils Fig. 1(a) compares the As extraction efficiency (i.e., proportion of the extracted As to the total As in soils) for different washing solutions. The As extraction efficiency was the greatest when H3PO4 (0.5 M) was used as a washing solution, and this was followed by 2% Na dithionite in HCl (0.01 M) and HCl (1 M). The acid-based washing solutions extracted more As than the neutral phosphate solution, which removed approximately 32% of As in soils (Fig. 1(a)). Soil washing also changed the chemical forms of the residual As in soils (Fig. 1(b)). Arsenic in soils is largely associated with amorphous and crystalline hydrous oxides of Fe and Al (Smith et al., 2008) and the dominant chemical forms of As in the original soil sample (i.e., unwashed soil) was As associated with
3.2. Effect of soil washing on soil properties The seed germination was lower in the washed soils than in the original soil, regardless of the As removal efficiency by soil washing (Fig. 2(a)). The shoot growth of the germinated seeds was similarly lower in the washed soils than in the original soil (Fig. 2(b)). Similarly, the toxic effects determined using bioluminescent bacteria showed that the toxic effects after soil washing were either
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amorphous hydrous oxides of Fe and Al. This implies that the success of soil washing is highly dependent on removal of (NH4)2C2O4-extractable As according to the Wenzel’s sequential extraction procedure. On that account, all washing solutions used in this study can be used for As removal, because all the washing solutions removed significant amounts of (NH4)2C2O4-extractable As (Fig. 1(b)). Another major changes in the chemical forms of As after soil washing are the decreases in the NH4H2PO4-extractable As and increases in the residual As fraction (Fig. 1(b)). Interestingly, for the soils washed with phosphate-containing washing solutions (i.e., H3PO4 and neutral phosphate solution), the (NH4)2SO4-extractable As increased slightly relative to that of the original soil (Fig. 1(b)). This may be due to the presence of excess amount of phosphate, which, during soil washing, can replace As bound to soils by competition between As and phosphate (Hongshao and Stanforth, 2001). In addition, the less harsh condition created after washing with the neutral phosphate solution, compared to when the acid-based washing solutions were used, is less likely to dissolve soil fractions including metallic fractions (e.g., hydrous oxides of Fe and Al), thus, more soil fractions for resorption of the extracted As may be available (Tokunaga and Hakuta, 2002). Nonetheless, the sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As was reduced after soil washing (Fig. 1(b)). The (NH4)2SO4-extractable and NH4H2PO4-extractable As are readily labile fractions and they could represent bioaccessible As in soils (Whitacre et al., 2013), which is strongly correlated with bioavailable As (Kelley et al., 2002). Furthermore, the amount of the bioaccessible As in the soils used in this study, which was determined by using the solubility/bioavailability research consortium method, was greater than the sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As (Yang et al., 2015). Therefore, the decreases in the sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As could mean decreases in bioaccessible As, hence, bioavailable As.
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Fig. 1. Effect of different soil washing solutions on (a) arsenic (As) extraction and (b) chemical forms of residual As in the washed soils. H3PO4: 1 h washing with 0.5 M phosphoric acid; HCl: 1 h washing with 1 M hydrochloric acid; HCl/Na dithionite: 1 h washing with 2% sodium dithionite dissolved in 0.01 M hydrochloric acid; neutral phosphate: 24 h soaking followed by 1 h washing with 0.5 M phosphate solution prepared with ammonium dihydrogen phosphate and diammonium hydrogen phosphate (pH 6.0); original: contaminated soils that were not washed. For soil washing, solid-to-liquid ratio of 1:5 was used for all the washing solutions. For Fig. 1(b), the (NH4)2SO4extractable As concentrations were 0.45, 0.77, 0.23, 0.19, 3.9 mg kg 1 for original, H3PO4, HCl, HCl/Na dithionite, and neutral phosphate, respectively. The error bars represent the standard error of the mean from triplicate samples.
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Fig. 2. Effect of soil washing with different washing solutions on (a) seed germination of Indian mustard (Brassica juncea L.), (b) growth of Indian mustard, (c) soil dehydrogenase activity, and (d) soil acid phosphatase activity in the washed soils. The error bars represent the standard error of the mean from triplicate samples.
similar to or greater than that before soil washing, despite the reduced level of As in soils after soil washing (Jho et al., 2015). The lower germination and growth in the washed soils than in the original soil (Fig. 2(a) and (b)) did not reflect the reduced As concentrations in the washed soils suggesting that other factors may exhibit higher relationship with ecotoxicological effects (Wang et al., 2009b; Yi et al., 2012, 2013). For example, the ecotoxicological effects of the remediated soils might be better correlated with some of soil properties such as pH rather than contaminant concentrations (Wang et al., 2009b), because the bioavailable amount of contaminants is affected by changes in soil properties. Among the selected soil properties that might affect bioavailability of As, and hence, germination, the soil OM content and cation exchange capacity were similar before and after soil washing. The OM contents were 21 g kg 1 and cation exchange capacities were 17 cmol kg 1, on average. On the other hand, significant changes were observed with soil pH after soil washing. After washing soils with 1 M HCl (pH = 0.3), 0.5 M H3PO4 (pH = 1.0), 2% Na dithionite in 0.01 M HCl (pH = 5.6), and the neutral phosphate solution (pH = 6.0), the pH of the soils changed to 2.8, 4.0, 4.9, and 6.8, respectively, from 6.1 of the original soil pH. The soils washed with HCl and H3PO4 showed a significant decrease in pH compared to the soils washed with Na dithionite in HCl and the neutral phosphate solution. This is likely due to residual acid of the initially acidic washing solutions adhering to soil even after two-times rinsing (Tokunaga and Hakuta, 2002). The As bioaccessibility, which is in correlation with bioavailability, seems to decrease with decreasing soil pH (Yang et al., 2002; Juhasz et al., 2007); however, the germination was lower in the relatively strongly acidic soils generated after washing
with HCl and H3PO4 than in the less acidic and near-neutral soils (Fig. 2(a)). This suggests that the acidic nature of the washed soils had more significant effects on seed germination than the changes in bioavailability of the residual As with changes in soil pH, although there was not a clear correlation between the soil pH and the germination (Fig. 2(a)) due to the multiple factors that might affect the toxic effects of the washed soils. In addition, microbially-mediated processes that may affect nutrient availability to plants may be suppressed in acidic soils, and hence, germination (Robson and Abbott, 2012). The seed germination in the soils washed with Na dithionite in HCl and the neutral phosphate solution were greater than that in the soils washed with acidic washing solutions. The lower germination in the soil washed with the neutral phosphate solution than in the soil washed with Na dithionite in HCl may be ascribed to the relatively higher concentrations of nutrients in the soil washed with the neutral phosphate solution (Fig. 2(a)). The concentrations of available phosphate in the soils (43 mg kg 1) increased significantly after washing with H3PO4 and the neutral phosphate solution (2800–2900 mg kg 1), which exceeded the recommended available phosphate concentration ranges for agricultural soils in Korea (80–550 mg kg 1) (Korean Soil Information System, 2015). These were also significantly higher than that in the soils washed with other washing solutions (43–71 mg kg 1). Also, the total nitrogen concentration was significantly greater in the neutral phosphate solution washed soils (3100 mg kg 1) than the original soil (1150 mg kg 1) and the other soils (1100–1200 mg kg 1). Although the soils washed with the neutral phosphate solution had the pH values in the ideal pH range (5.2–8.0) for plant growth (Truog, 1947; Lake, 2000), the shoot
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growth was lower than that of the original soil (Fig. 2(b)). It may be attributed to an excess amount of phosphorus that can compete with As for sorption, which could lead to increases of available As in soil solutions, and of soil salt content resulting in osmotic stress (Havlin et al., 2005). Also, an excess amount of phosphorus may induce deficiencies of iron and zinc, which are soil micronutrients, and of potassium, which is needed in enzyme activation, gas exchange, and osmotic regulation (Williams, 1948; Bingham, 1963; Osman, 2012). In addition, an excess amount of nitrogen could lead to yellowing of plants and lower yields of plants (van Dijk and Roelofs, 1988; Back et al., 2005; Osman, 2012). Furthermore, the greater residual amount of readily labile As (i.e., sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As) in the neutral phosphate solution washed soil than in the soil washed with Na dithionite in HCl may have affected the germination (Fig. 1(b)). Similarly, the removal of heavy metals by soil washing could result in loss of soil microorganism, soil OM, and nutrients. As a result, the changes in soil properties are likely to lead to changes in soil enzyme activities, which can be used to represent soil microbial activities (Schloter et al., 2003). The significantly lower dehydrogenase and acid phosphatase activities in the washed soils than in the original soil could indicate such losses (Fig. 2(c) and (d)). Since the dehydrogenase activity can represent microbial activity associated with organic matter breakdown (Ross, 1971) and the phosphatase activity can represent the soil microbial activity associated with phosphorus circulation (Yi et al., 2013), the lower dehydrogenase and acid phosphatase activities after soil washing could mean lower microbial activity associated with organic matter breakdown and phosphorus circulation, which are important for growth of plants. The significantly lower dehydrogenase activity in the HCl washed soil than the soils washed with the other washing solutions may be likely due to the strongly acidic nature of the washed soil (Fig. 2(c)). The phosphatase activities, on the other hand, were similarly lower in the washed soils regardless of the washing solutions (Fig. 2(d)). Previous studies also showed that the changes in soil properties such as soil pH were more closely related to microbial activities and enzyme activities (Kim et al., 2010), while reduction in contaminant concentration was not reflected in soil enzyme activities (Wang et al., 2009a).
4. Conclusions Soil washing can be effective in reducing the soil As concentrations; however, such reduction was not reflected in the ecotoxicological effects of the washed soils, which were determined by the bioassay test methods. The ecotoxicological effects of the washed soils seemed to exhibit higher relationships with the changes in the soil properties such as soil pH and nutrient concentrations. In particular, the lower soil pH after soil washing with acidic washing solutions seemed to be the major factor affecting the germination and growth, while the germination and growth in the soils with near-neutral pH after washing were greatly affected by the higher concentrations of nutrients. The similarly lower enzyme activities after soil washing suggest that the washed soils may need some recovery time for soil microorganism. Overall, this study signifies that washed soils may still exhibit toxic effects, even the contaminant concentrations are reduced, due to changes in soil properties, which highly depend on washing solutions. Therefore, the post-soil washing treatments may be required to improve the fertility of the washed soils, and the data on the changes in the different soil properties after soil washing with various washing solutions and
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the consequential changes in the ecotoxicological effects can then be utilized to effectively and economically post-treat the washed soils. Acknowledgements This project was supported by the Korea Ministry of Environment as the GAIA (Geo-Advanced Innovative Action) project. Also, this work was supported by Hankuk University of Foreign Studies Research Fund. References Alam, M.G.M., Tokunaga, S., Maekawa, T., 2001. Extraction of arsenic in a synthetic arsenic-contaminated soil using phosphate. Chemosphere 43, 1035–1041. Back, N.-H., Choi, W.-Y., Ko, J.-C., Nam, J.-K., Park, H.-K., Choung, J.-I., Kim, S.-S., Park, K.-G., 2005. Proper nitrogen fertilizer level for improving the rice quality at reclaimed saline land in the southwestern area. Korean J. Crop Sci. 50, 46–50. Bingham, F.T., 1963. Relation between phosphorus and micronutrients in plants1. Soil Sci. Soc. Am. J. 27, 389–391. Bolton Jr., H., Elliott, L.F., Papendick, R.I., Bezdicek, D.F., 1985. Soil microbial biomass and selected soil enzyme activities: effect of fertilization and cropping practices. Soil Biol. Biochem. 17, 297–302. Bray, R.H., Kurtz, L., 1945. Determination of total, organic, and available forms of phosphorus in soils. Soil Sci. 59, 39–46. Bremner, J., 1960. Determination of nitrogen in soil by the Kjeldahl method. J. Agric. Sci. 55, 11–33. Cheong, J.G., Kang, W.H., 2012. System and Method for Remediating Contaminated Soil by Separation of Contaminant-Concentrated Coarse Soil. Korean Intellectual Property Office, Korea (KR10-2011-0116871). Choi, S.I., Hwang, J.S., Kim, K.H., Kwon, Y.K., Kim, Y.D., 2007. Remediation Method for Heavy Metal Contaminated Soil. Korean Intellectual Property Office, Korea (KR10-2006-0078779). Goh, K.H., Lim, T.T., 2005. Arsenic fractionation in a fine soil fraction and influence of various anions on its mobility in the subsurface environment. Appl. Geochem. 20, 229–239. Han, K.-W., Shin, H.-M., 2008. Fractionation and the removal of arseniccontaminated soils around Dalchn mine using soil washing process. J. Environ. Sci. 17, 185–193. Havlin, J., Beaton, J.D., Tisdale, S.L., Nelson, W.L., 2005. Soil Fertility and Fertilizers: An Introduction to Nutrient Management. Pearson Prentice Hall, Upper Saddle River, NJ. Hongshao, Z., Stanforth, R., 2001. Competitive adsorption of phosphate and arsenate on goethite. Environ. Sci. Technol. 35, 4753–4757. Huang, R.-Q., Gao, S.-F., Wang, W.-L., Staunton, S., Wang, G., 2006. Soil arsenic availability and the transfer of soil arsenic to crops in suburban areas in Fujian Province, southeast China. Sci. Total Environ. 368, 531–541. Im, J., Kim, Y.-J., Yang, K., Nam, K., 2014. Applicability of soil washing with neutral phosphate for remediation of arsenic-contaminated soil at the former Janghang smeleter site. J. Soil Groundw. Environ. 19, 45–51. International Standard Organization, 1995. ISO 11466:1995 Soil quality – Extraction of trace elements soluble in aqua regia. International Standard Organization, Geneva, Switzerland. Jho, E.H., Im, J., Yang, K., Kim, Y.J., Nam, K., 2015. Changes in soil toxicity by phosphate-aided soil washing: effect of soil characteristics, chemical forms of arsenic, and cations in washing solutions. Chemosphere 119, 1399–1405. Juhasz, A.L., Smith, E., Weber, J., Rees, M., Rofe, A., Kuchel, T., Sansom, L., Naidu, R., 2007. In vitro assessment of arsenic bioaccessibility in contaminated (anthropogenic and geogenic) soils. Chemosphere 69, 69–78. Kelley, M.E., Brauning, S.E., Schoof, R.A., Ruby, M.V., 2002. Assessing Oral Bioavailability of Metals in Soil. Battelle Press. Kim, S.H., Han, H.Y., Lee, Y.J., Kim, C.W., Yang, J.W., 2010. Effect of electrokinetic remediation on indigenous microbial activity and community within diesel contaminated soil. Sci. Total Environ. 408, 3162–3168. Kim, J.G., Lee, J.H., Cho, Y.C., Ahn, J.S., Lee, C.O., Song, H.C., 2012. Remediation Method of Soil Contaminated with Arsenic. World Intellectual Property Organization (PCT/KR2010/007446). Kim, B.-K., Park, G.-Y., Jeon, E.-K., Jung, J.-M., Jung, H.-B., Ko, S.-H., Baek, K., 2013. Field application of in situ electrokinetic remediation for As-, Cu-, and Pbcontaminated paddy soil. Water Air Soil Pollut. 224, 1–10. Ko, I., Chang, Y.Y., Lee, C.H., Kim, K.W., 2005. Assessment of pilot-scale acid washing of soil contaminated with As, Zn and Ni using the BCR three-step sequential extraction. J. Hazard. Mater. 127, 1–13. Korea Ministry of Environment, 2009. Official test methods of soil quality. Korea Ministry of Environment, Gwacheon, Korea. Korean Soil Information System, 2015. Soil Composition. Rural Development Administration. Lake, B., 2000. Understanding soil pH. NSW Agriculture Acid Soil Action Leaflet No.2. Lee, M., Paik, I.S., Do, W., Kim, I., Lee, Y., Lee, S., 2007. Soil washing of Ascontaminated stream sediments in the vicinity of an abandoned mine in Korea. Environ. Geochem. Health 29, 319–329.
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van Dijk, H.F.G., Roelofs, J.G.M., 1988. Effects of excessive ammonium deposition on the nutritional status and condition of pine needles. Physiol. Plant. 73, 494–501. Walkley, A., Black, I.A., 1934. An examination of the Degtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Sci. 37, 29–38. Wang, Q.-Y., Zhou, D.-M., Cang, L., Li, L.-Z., Wang, P., 2009a. Solid/solution Cu fractionations/speciation of a Cu contaminated soil after pilot-scale electrokinetic remediation and their relationships with soil microbial and enzyme activities. Environ. Pollut. 157, 2203–2208. Wang, Q.-Y., Zhou, D.-M., Cang, L., Sun, T.-R., 2009b. Application of bioassays to evaluate a copper contaminated soil before and after a pilot-scale electrokinetic remediation. Environ. Pollut. 157, 410–416. Wenzel, W.W., Kirchbaumer, N., Prohaska, T., Stingeder, G., Lombi, E., Adriano, D.C., 2001. Arsenic fractionation in soils using an improved sequential extraction procedure. Anal. Chim. Acta 436, 309–323. Whitacre, S.D., Basta, N.T., Dayton, E.A., 2013. Bioaccessible and non-bioaccessible fractions of soil arsenic. J. Environ. Sci. Health A 48, 620–628. Williams, R., 1948. The effects of phosphorus supply on the rates of intake of phosphorus and nitrogen and upon certain aspects of phosphorus metabolism in gramineous plants. Aust. J. Biol. Sci. 1, 333–361. Yang, J.-K., Barnett, M.O., Jardine, P.M., Basta, N.T., Casteel, S.W., 2002. Adsorption, sequestration, and bioaccessibility of As(V) in soils. Environ. Sci. Technol. 36, 4562–4569. Yang, J.-S., Hwang, J.-M., Baek, K., Kwon, M.J., 2013. Soil washing and effluent treatment for contaminated soil with toxic metals. Korean Chem. Eng. Res. 51, 745–754. Yang, K., Im, J., Jeong, S., Nam, K., 2015. Determination of human health risk incorporating experimentally derived site-specific bioaccessibility of arsenic at an old abandoned smelter site. Environ. Res. 137, 78–84. Yi, Y.-M., Oh, C.-T., Kim, G.-J., Lee, C.-H., Sung, K.-J., 2012. Changes in the physicochemical properties of soil according to soil remediation methods. J. Soil Groundw. Environ. 17, 36–43. Yi, Y., Kim, G., Sung, K., 2013. Effects of soil remediation methods on the biological properties of soils. J. Soil Groundw. Environ. 18, 73–81. Zeng, M., Liao, B., Lei, M., Zhang, Y., Zeng, Q., Ouyang, B., 2008. Arsenic removal from contaminated soil using phosphoric acid and phosphate. J. Environ. Sci. 20, 75– 79. Zihms, S., Switzer, C., Tarantino, A., 2013. Remediation and reuse of soils. EGU General Assembly 2013, Vienna, Austria, 7–12 April 2013.
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Effect of different soil washing solutions on bioavailability of residual arsenic in soils and soil properties Jinwoo Im a, Kyung Yang a, Eun Hea Jho b,⇑, Kyoungphile Nam a a b
Department of Civil and Environmental Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea Department of Environmental Science, Hankuk University of Foreign Studies, 81 Oedae-ro, Mohyeon-myeon, Cheoin-gu, Yongin-si, Gyeonggi-do 449-791, Republic of Korea
h i g h l i g h t s Acidic and near-neutral washing solutions reduced the soil As level by 32–62%. The (NH4)2SO4 , NH4H2PO4 , (NH4)2C2O4-extractable As were greatly reduced. But seed germination, growth, and enzyme activities were lower in the washed soils. Greater toxic effects are largely due to changes in soil properties after washing. Proper post-treatment techniques need to be selected for reuses of the washed soils.
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 24 April 2015 Accepted 2 June 2015
Keywords: Soil washing Arsenic Residual arsenic Bioavailability Soil properties Toxic effect
a b s t r a c t The effect of soil washing used for arsenic (As)-contaminated soil remediation on soil properties and bioavailability of residual As in soil is receiving increasing attention due to increasing interest in conserving soil qualities after remediation. This study investigates the effect of different washing solutions on bioavailability of residual As in soils and soil properties after soil washing. Regardless of washing solutions, the sequential extraction revealed that the residual As concentrations and the amount of readily labile As in soils were reduced after soil washing. However, the bioassay tests showed that the washed soils exhibited ecotoxicological effects – lower seed germination, shoot growth, and enzyme activities – and this could largely be attributed to the acidic pH and/or excessive nutrient contents of the washed soils depending on washing solutions. Overall, this study showed that treated soils having lower levels of contaminants could still exhibit toxic effects due to changes in soil properties, which highly depended on washing solutions. This study also emphasizes that data on the As concentrations, the soil properties, and the ecotoxicological effects are necessary to properly manage the washed soils for reuses. The results of this study can, thus, be utilized to select proper post-treatment techniques for the washed soils. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Various remedial techniques such as stabilization/solidification (Miretzky and Cirelli, 2010), soil washing (Tsang and Hartley, 2014), phytoremediation, specifically, phytoextraction using hyperaccumulator species, (Mandal et al., 2014), and electrokinetic remediation (Kim et al., 2013) can be applied for remediation of arsenic (As)-contaminated soils. In Korea, however, soil washing is often considered as a remedial technique for As-contaminated sites (e.g., a former Janghang smelting site, Dalchen mine), as it removes As from soils in a relatively short time compared to other ⇑ Corresponding author. E-mail addresses: [email protected] (J. Im), [email protected] (K. Yang), [email protected] (E.H. Jho), [email protected] (K. Nam). http://dx.doi.org/10.1016/j.chemosphere.2015.06.004 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.
remedial techniques rather than just reducing bioavailability of As in soils via immobilization (Han and Shin, 2008; Moon et al., 2011). Since the treated soils are largely reused at the site and the current Korean regulation evaluates remedial efficiency based on the total As concentrations in soils, As removal techniques (e.g., soil washing) may be preferred over As immobilization techniques. During soil washing, contaminated soils are mixed with a washing solution, and agitated to remove contaminants. The washing treatment process or the washing solution used in washing treatment is likely to affect soil properties that are related to soil qualities (Ko et al., 2005; Yi et al., 2012). For example, soil properties such as soil texture, water holding capacity, organic matter (OM) content, and total nitrogen concentration were changed after soil washing (Yi et al., 2012). Such changes in soil properties may change soil permeability and shear strength, which will
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consequently affect long-term soil functions (Zihms et al., 2013). Changes in soil properties could also mean possible changes in bioavailability of As in soils as bioavailability of metals or metalloids is highly dependent on soil properties (e.g., pH, OM content, redox potential) (Masscheleyn et al., 1991; Huang et al., 2006). Soil washing of contaminated soils usually involves strong acids as washing solutions. For example, a number of patented washing solutions targeting As removal from soils in Korea involve HCl or H3PO4 – 1 M hydrochloric acid (HCl) (Choi et al., 2007), 0.5 M phosphoric acid (H3PO4) (Cheong and Kang, 2012), and 2% sodium (Na) dithionite in 0.01 M HCl (Kim et al., 2012). When strong acid-based washing solutions are used for As removal from soils, acids containing oxyanions (e.g., H3PO4 or H2SO4) are more effective than acids without oxyanions (e.g., HCl). This is because of competitive oxyanions (PO34 or SO24 ) that may decrease the re-adsorption of anionic species of As on reactive surfaces of soil (Goh and Lim, 2005; Ko et al., 2005; Lee et al., 2007). However, with increasing awareness of post-remediation soil qualities, mild washing solutions may be preferred over strong acid-based washing solutions, because strong acids could adversely affect soils by dissolving soil components during washing treatment (Ko et al., 2005). Thus, washing solutions prepared with phosphate containing salts that have neutral or near-neutral pH can be used as alternatives to minimize detrimental effects (e.g., losses of soil OM and mineral substances (e.g., Si, Ca, Mg, Fe, Al), decreases in soil pH) of soil washing solutions (Alam et al., 2001; Tokunaga and Hakuta, 2002; Zeng et al., 2008). The performance of washing solutions has largely been accessed on As extractability, and what happens to the residual As in soils or soil properties has not been extensively studied. Since there is a growing interest in minimizing negative effects of soil washing on soil qualities, the changes in soil properties and bioavailability of the residual As in soils after soil washing need to be studied. Therefore, this study investigates how bioavailability of the residual As in soils and soil properties of the washed soils are affected by soil washing with different solutions. Selected patented washing solutions for As-contaminated soils in Korea and the neutral phosphate salt-based washing solution (hereafter referred to as neutral phosphate solution), which was developed in order to avoid problems arising from soil acidity after soil washing with strong acids, were used for comparison. Specifically, the effect of using different washing solutions on changes in bioavailability of the residual As in soils and soil properties was determined by using chemical extraction-based methods (e.g., sequential extraction method) and bioassay test-based methods (e.g., seed germination and growth, microbial enzyme activities).
2. Materials and methods 2.1. Soil sampling Historically As-contaminated soil samples were taken from the vicinity of a former smelter in Janghang, Korea. The soil samples were air-dried at room temperature, and sieved through a 2 mm sieve. The soil pH was 6.1, and the OM content determined by the Walkley–Black procedure (Walkley and Black, 1934) and the iron oxide concentration determined by the bicarbonate -citrate-dithionite (DBC) method (Mehra and Jackson, 1958) were 2.1% and 8240 mg kg 1, respectively. The concentrations of available phosphorus and the total nitrogen were determined by using the Bray’s method (Bray and Kurtz, 1945) and the micro-Kjeldahl method (Bremner, 1960), respectively. The detection limits for the Bray’s and the micro-Kjeldahl methods were 0.01 mg L 1 and 0.1 mg L 1, respectively. The soil texture was silt loam consisting of 28% sand, 54% silt, and 18% clay. The background concentrations
of Pb, Cd, Cu, Zn, and As determined following the aqua regia extraction method with slight modifications (International Standard Organization, 1995), complying with Korea Ministry of Environment (Korea Ministry of Environment, 2009), were 97, 0.5, 52, 89, and 59 mg kg 1, respectively. The As concentration (59 mg kg 1) exceeded the cleanup level (25 mg kg 1 for As) of the Korean Soil Environment Conservation Act. 2.2. Soil washing procedures The soil washing condition and procedure using a neutral phosphate solution were developed in our previous study (Im et al., 2014). Briefly, 24 h soaking of As-contaminated soils in a 0.5 M phosphate solution (pH 6.0), prepared from ammonium dihydrogen phosphate (NH4H2PO4) and diammonium hydrogen phosphate ((NH4)2HPO4), at soil-to-solution (S/L) ratio of 1:5 was followed by 1 h washing at 200 rpm. Since conventional washing procedures involve a single-step soil washing treatment, a 1 h washing period was used with the three selected patented washing solutions for As removal from soils in Korea. The selected washing solutions are 1 M HCl (Choi et al., 2007), 0.5 M H3PO4 (Cheong and Kang, 2012), and 2% Na dithionite in 0.01 M HCl (Kim et al., 2012). All the washing experiments were carried out in triplicate using 1 L jars with 120 g soils and 600 mL washing solutions. After 1 h soil washing at 200 rpm, the washed soils were rinsed twice with deionized water. The washing and rinsing solutions can be treated either by neutralization and precipitation or by sorption on to adsorbents such as granular ferric oxide (Choi et al., 2007; Cheong and Kang, 2012; Yang et al., 2013). Also, As in the solutions can be removed by adding oxidants (e.g., hydrogen peroxide) and coagulants (Kim et al., 2012). 2.3. Extraction and analyses of As For As analysis in soils, soil slurries in the washing experiments were left to settle in order to separate soils from washing solutions. The residual As in the separated and then dried soils (3 g) were extracted following the method described in Section 2.1. Chemical forms of As (i.e., fractionation of As) in soils were assessed using the sequential extraction procedure of Wenzel et al. (2001), which divides As in soils into five fractions, with a slight modification – (1) ammonium sulfate ((NH4)2SO4; 0.05 M)-extractable As (i.e., non-specifically sorbed As); (2) NH4H2PO4 (0.05 M)-extractable As (i.e., specifically sorbed As); (3) ammonium oxalate ((NH4)2C2O4; 0.2 M)-extractable As (i.e., As associated with amorphous hydrous oxides of Fe and Al); (4) (NH4)2C2O4 (0.2 M)/ascorbic acid (0.1 M)-extractable As (i.e., As associated with crystalline hydrous oxides of Fe and Al); and (5) residual As fraction extracted with the USEPA method 3052, which digest siliceous and organically based matrices such as soils and sediments in a mixture of nitric acid, hydrofluoric acid, HCl, and hydrogen peroxide using microwave heating (US Environmental Protection Agency, 1996). The As concentrations in the extracts were analyzed by using inductively coupled plasma-optical emission spectrometry (ICP-OES; ICP-730ES, Varian, USA). The method detection limit for As was 0.072 mg kg 1. Statistical analysis was conducted using Excel 14.0 (Microsoft Corporation, USA) and the statistical significance of differences in As concentrations was determined by using the Student’s t-test at 5% significance level. 2.4. Bioassay tests using plants and soil microbial enzymes Seed germination and growth tests were carried out with ten seeds of Indian mustard (Brassica juncea L.) sown in each petri dish (diameter = 90 mm) containing dried soil equivalent to 30 g. The tests were carried out in triplicate under controlled environment
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of a growth chamber (23 ± 3 °C, 80% humidity, 16 h light period). All the petri dishes were watered daily to 80% of the soil water holding capacity based on weight. The number of the germinated seeds exhibiting >2 mm growth were counted after 7 d (Liu et al., 2005). The shoot length after 14 d from the sowing day was used as a plant growth parameter. Soil dehydrogenase activity and acid phosphatase activity were measured following the methods described by Yi et al. (2013). The dehydrogenase activity can represent microbial activity associated with organic matter breakdown (Ross, 1971), and can be used as an index of endogenous soil microbial activity (Moore and Russell, 1972; Bolton et al., 1985). The phosphatase activity can represent the soil microbial activity associated with phosphorus circulation (Yi et al., 2013). For dehydrogenase activity determination, soil (6 g) incubation (37 °C, 24 h) with 2,3,5-triphenlytetrazolium chloride (1 mL) and distilled water (2.5 mL) was followed by absorbance measurement of methanol extracts at 485 nm. Triphenyl formazan (TPF) was used to prepare standard solutions for absorbance measurement. For acid phosphatase activity determination, a mixture of 1 g soil, 0.2 mL toluene, 4 mL modified universal buffer (MUB, pH 6.5), and 1 mL of 0.025 M p-nitrophenyl phosphate (p-NPP) was incubated at 37 °C for an hour, and then, 1 mL of 0.5 M CaCl2 and 4 mL of 0.5 M NaOH were added to the mixture. The absorbance of p-nitrophenol in the filtered extract was measured at 400 nm. All the absorbance was measured using a UV spectrophotometer (OPTIZEN 2120UV, Mecasys, Korea). All the enzyme assays were run in triplicate. 3. Results and discussions 3.1. Effect of soil washing on chemical forms of residual As in soils Fig. 1(a) compares the As extraction efficiency (i.e., proportion of the extracted As to the total As in soils) for different washing solutions. The As extraction efficiency was the greatest when H3PO4 (0.5 M) was used as a washing solution, and this was followed by 2% Na dithionite in HCl (0.01 M) and HCl (1 M). The acid-based washing solutions extracted more As than the neutral phosphate solution, which removed approximately 32% of As in soils (Fig. 1(a)). Soil washing also changed the chemical forms of the residual As in soils (Fig. 1(b)). Arsenic in soils is largely associated with amorphous and crystalline hydrous oxides of Fe and Al (Smith et al., 2008) and the dominant chemical forms of As in the original soil sample (i.e., unwashed soil) was As associated with
3.2. Effect of soil washing on soil properties The seed germination was lower in the washed soils than in the original soil, regardless of the As removal efficiency by soil washing (Fig. 2(a)). The shoot growth of the germinated seeds was similarly lower in the washed soils than in the original soil (Fig. 2(b)). Similarly, the toxic effects determined using bioluminescent bacteria showed that the toxic effects after soil washing were either
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amorphous hydrous oxides of Fe and Al. This implies that the success of soil washing is highly dependent on removal of (NH4)2C2O4-extractable As according to the Wenzel’s sequential extraction procedure. On that account, all washing solutions used in this study can be used for As removal, because all the washing solutions removed significant amounts of (NH4)2C2O4-extractable As (Fig. 1(b)). Another major changes in the chemical forms of As after soil washing are the decreases in the NH4H2PO4-extractable As and increases in the residual As fraction (Fig. 1(b)). Interestingly, for the soils washed with phosphate-containing washing solutions (i.e., H3PO4 and neutral phosphate solution), the (NH4)2SO4-extractable As increased slightly relative to that of the original soil (Fig. 1(b)). This may be due to the presence of excess amount of phosphate, which, during soil washing, can replace As bound to soils by competition between As and phosphate (Hongshao and Stanforth, 2001). In addition, the less harsh condition created after washing with the neutral phosphate solution, compared to when the acid-based washing solutions were used, is less likely to dissolve soil fractions including metallic fractions (e.g., hydrous oxides of Fe and Al), thus, more soil fractions for resorption of the extracted As may be available (Tokunaga and Hakuta, 2002). Nonetheless, the sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As was reduced after soil washing (Fig. 1(b)). The (NH4)2SO4-extractable and NH4H2PO4-extractable As are readily labile fractions and they could represent bioaccessible As in soils (Whitacre et al., 2013), which is strongly correlated with bioavailable As (Kelley et al., 2002). Furthermore, the amount of the bioaccessible As in the soils used in this study, which was determined by using the solubility/bioavailability research consortium method, was greater than the sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As (Yang et al., 2015). Therefore, the decreases in the sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As could mean decreases in bioaccessible As, hence, bioavailable As.
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Fig. 1. Effect of different soil washing solutions on (a) arsenic (As) extraction and (b) chemical forms of residual As in the washed soils. H3PO4: 1 h washing with 0.5 M phosphoric acid; HCl: 1 h washing with 1 M hydrochloric acid; HCl/Na dithionite: 1 h washing with 2% sodium dithionite dissolved in 0.01 M hydrochloric acid; neutral phosphate: 24 h soaking followed by 1 h washing with 0.5 M phosphate solution prepared with ammonium dihydrogen phosphate and diammonium hydrogen phosphate (pH 6.0); original: contaminated soils that were not washed. For soil washing, solid-to-liquid ratio of 1:5 was used for all the washing solutions. For Fig. 1(b), the (NH4)2SO4extractable As concentrations were 0.45, 0.77, 0.23, 0.19, 3.9 mg kg 1 for original, H3PO4, HCl, HCl/Na dithionite, and neutral phosphate, respectively. The error bars represent the standard error of the mean from triplicate samples.
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Fig. 2. Effect of soil washing with different washing solutions on (a) seed germination of Indian mustard (Brassica juncea L.), (b) growth of Indian mustard, (c) soil dehydrogenase activity, and (d) soil acid phosphatase activity in the washed soils. The error bars represent the standard error of the mean from triplicate samples.
similar to or greater than that before soil washing, despite the reduced level of As in soils after soil washing (Jho et al., 2015). The lower germination and growth in the washed soils than in the original soil (Fig. 2(a) and (b)) did not reflect the reduced As concentrations in the washed soils suggesting that other factors may exhibit higher relationship with ecotoxicological effects (Wang et al., 2009b; Yi et al., 2012, 2013). For example, the ecotoxicological effects of the remediated soils might be better correlated with some of soil properties such as pH rather than contaminant concentrations (Wang et al., 2009b), because the bioavailable amount of contaminants is affected by changes in soil properties. Among the selected soil properties that might affect bioavailability of As, and hence, germination, the soil OM content and cation exchange capacity were similar before and after soil washing. The OM contents were 21 g kg 1 and cation exchange capacities were 17 cmol kg 1, on average. On the other hand, significant changes were observed with soil pH after soil washing. After washing soils with 1 M HCl (pH = 0.3), 0.5 M H3PO4 (pH = 1.0), 2% Na dithionite in 0.01 M HCl (pH = 5.6), and the neutral phosphate solution (pH = 6.0), the pH of the soils changed to 2.8, 4.0, 4.9, and 6.8, respectively, from 6.1 of the original soil pH. The soils washed with HCl and H3PO4 showed a significant decrease in pH compared to the soils washed with Na dithionite in HCl and the neutral phosphate solution. This is likely due to residual acid of the initially acidic washing solutions adhering to soil even after two-times rinsing (Tokunaga and Hakuta, 2002). The As bioaccessibility, which is in correlation with bioavailability, seems to decrease with decreasing soil pH (Yang et al., 2002; Juhasz et al., 2007); however, the germination was lower in the relatively strongly acidic soils generated after washing
with HCl and H3PO4 than in the less acidic and near-neutral soils (Fig. 2(a)). This suggests that the acidic nature of the washed soils had more significant effects on seed germination than the changes in bioavailability of the residual As with changes in soil pH, although there was not a clear correlation between the soil pH and the germination (Fig. 2(a)) due to the multiple factors that might affect the toxic effects of the washed soils. In addition, microbially-mediated processes that may affect nutrient availability to plants may be suppressed in acidic soils, and hence, germination (Robson and Abbott, 2012). The seed germination in the soils washed with Na dithionite in HCl and the neutral phosphate solution were greater than that in the soils washed with acidic washing solutions. The lower germination in the soil washed with the neutral phosphate solution than in the soil washed with Na dithionite in HCl may be ascribed to the relatively higher concentrations of nutrients in the soil washed with the neutral phosphate solution (Fig. 2(a)). The concentrations of available phosphate in the soils (43 mg kg 1) increased significantly after washing with H3PO4 and the neutral phosphate solution (2800–2900 mg kg 1), which exceeded the recommended available phosphate concentration ranges for agricultural soils in Korea (80–550 mg kg 1) (Korean Soil Information System, 2015). These were also significantly higher than that in the soils washed with other washing solutions (43–71 mg kg 1). Also, the total nitrogen concentration was significantly greater in the neutral phosphate solution washed soils (3100 mg kg 1) than the original soil (1150 mg kg 1) and the other soils (1100–1200 mg kg 1). Although the soils washed with the neutral phosphate solution had the pH values in the ideal pH range (5.2–8.0) for plant growth (Truog, 1947; Lake, 2000), the shoot
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growth was lower than that of the original soil (Fig. 2(b)). It may be attributed to an excess amount of phosphorus that can compete with As for sorption, which could lead to increases of available As in soil solutions, and of soil salt content resulting in osmotic stress (Havlin et al., 2005). Also, an excess amount of phosphorus may induce deficiencies of iron and zinc, which are soil micronutrients, and of potassium, which is needed in enzyme activation, gas exchange, and osmotic regulation (Williams, 1948; Bingham, 1963; Osman, 2012). In addition, an excess amount of nitrogen could lead to yellowing of plants and lower yields of plants (van Dijk and Roelofs, 1988; Back et al., 2005; Osman, 2012). Furthermore, the greater residual amount of readily labile As (i.e., sum of the (NH4)2SO4-extractable and NH4H2PO4-extractable As) in the neutral phosphate solution washed soil than in the soil washed with Na dithionite in HCl may have affected the germination (Fig. 1(b)). Similarly, the removal of heavy metals by soil washing could result in loss of soil microorganism, soil OM, and nutrients. As a result, the changes in soil properties are likely to lead to changes in soil enzyme activities, which can be used to represent soil microbial activities (Schloter et al., 2003). The significantly lower dehydrogenase and acid phosphatase activities in the washed soils than in the original soil could indicate such losses (Fig. 2(c) and (d)). Since the dehydrogenase activity can represent microbial activity associated with organic matter breakdown (Ross, 1971) and the phosphatase activity can represent the soil microbial activity associated with phosphorus circulation (Yi et al., 2013), the lower dehydrogenase and acid phosphatase activities after soil washing could mean lower microbial activity associated with organic matter breakdown and phosphorus circulation, which are important for growth of plants. The significantly lower dehydrogenase activity in the HCl washed soil than the soils washed with the other washing solutions may be likely due to the strongly acidic nature of the washed soil (Fig. 2(c)). The phosphatase activities, on the other hand, were similarly lower in the washed soils regardless of the washing solutions (Fig. 2(d)). Previous studies also showed that the changes in soil properties such as soil pH were more closely related to microbial activities and enzyme activities (Kim et al., 2010), while reduction in contaminant concentration was not reflected in soil enzyme activities (Wang et al., 2009a).
4. Conclusions Soil washing can be effective in reducing the soil As concentrations; however, such reduction was not reflected in the ecotoxicological effects of the washed soils, which were determined by the bioassay test methods. The ecotoxicological effects of the washed soils seemed to exhibit higher relationships with the changes in the soil properties such as soil pH and nutrient concentrations. In particular, the lower soil pH after soil washing with acidic washing solutions seemed to be the major factor affecting the germination and growth, while the germination and growth in the soils with near-neutral pH after washing were greatly affected by the higher concentrations of nutrients. The similarly lower enzyme activities after soil washing suggest that the washed soils may need some recovery time for soil microorganism. Overall, this study signifies that washed soils may still exhibit toxic effects, even the contaminant concentrations are reduced, due to changes in soil properties, which highly depend on washing solutions. Therefore, the post-soil washing treatments may be required to improve the fertility of the washed soils, and the data on the changes in the different soil properties after soil washing with various washing solutions and
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