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An evaluation of different soil washing solutions for remediating arsenic-contaminated soils
Chemosphere 173 (2017) 368e372
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Short Communication
An evaluation of different soil washing solutions for remediating arsenic-contaminated soils Yiwen Wang a, Fujun Ma a, Qian Zhang a, Changsheng Peng b, Bin Wu a, Fasheng Li a, Qingbao Gu a, * a b
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China The Key Lab of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China
h i g h l i g h t s H3PO4, NaOH and EDTA can effectively remove arsenic from a heavily contaminated soil. Soil properties were partially changed after washing. Wheat grew best in NaOH-treated soil sample.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2016 Received in revised form 2 January 2017 Accepted 12 January 2017 Available online 12 January 2017
Soil washing is a promising way to remediate arsenic-contaminated soils. Most research has mostly focused on seeking efficient extractants for removing arsenic, but not concerned with any changes in soil properties when using this technique. In this study, the removal of arsenic from a heavily contaminated soil employing different washing solutions including H3PO4, NaOH and dithionite in EDTA was conducted. Subsequently, the changes in soil physicochemical properties and phytotoxicity of each washing technique were evaluated. After washing with 2 M H3PO4, 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA, the soil samples’ arsenic content met the clean-up levels stipulated in China’s environmental regulations. H3PO4 washing decreased soil pH, Ca, Mg, Al, Fe, and Mn concentrations but increased TN and TP contents. NaOH washing increased soil pH but decreased soil TOC, TN and TP contents. Dithionite in EDTA washing reduced soil TOC, Ca, Mg, Al, Fe, Mn and TP contents. A drastic color change was observed when the soil sample was washed with H3PO4 or 0.1 M dithionite in 0.1 M EDTA. After adjusting the soil pH to neutral, wheat planted in the soil sample washed by NaOH evidenced the best growth of all three treated soil samples. These results will help with selecting the best washing solution when remediating arseniccontaminated soils in future engineering applications. © 2017 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Arsenic Soil washing NaOH H3PO4 Dithionite in EDTA
1. Introduction Arsenic has attracted much concern due to its highly toxic and carcinogenic properties which are dangerous to human beings, animals, and plants. The main anthropogenic sources contributing to arsenic contamination in soils include mining, smelting, agricultural use of pesticides and the disposal of industrial wastes (Mahimairaja et al., 2005; Liao et al., 2005; Chen et al., 2016). In China, a number of arsenic polluted soils have been reported, and the arsenic concentrations in some soils are up to 1217 mg/kg (Liao
* Corresponding author. E-mail address: [email protected] (Q. Gu). http://dx.doi.org/10.1016/j.chemosphere.2017.01.068 0045-6535/© 2017 Elsevier Ltd. All rights reserved.
et al., 2005; Liu et al., 2005; Wei et al., 2009). This has been as high as 243 times larger than worldwide arsenic background in soil (5 mg/kg) (WHO, 2001) and 41 times the arsenic allowance in soil in China (30 mg/kg, National Standard GB15618-1995). Subsequently this represents a serious threat to human health and the environment. To remediate arsenic-contaminated soils, solidification/stabilization (Tyrovola and Nikolaidis, 2009; Yoon et al., 2010), soil washing (Abumaizar and Smith, 1999; Elgh-Dalgren et al., 2009; Sierra et al., 2010, 2011; Gusiatin, 2014; Cao et al., 2016), electrokinetic remediation (Mao et al., 2015) and phytoremediation (Ye et al., 2011; Abioye and Uttam, 2016) have been commonly utilized. Of these techniques, soil washing is an effective way to remediate arsenic contaminated soil because it can permanently
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used to determine the optimal extractant concentrations for arsenic removal. According to previous studies (Im et al., 2015; Jho et al., 2015), 20 g soil sample was put in a 200 mL flask and then 100 mL of reagent solution was slowly added to achieve a ratio of reagent solutions (mL) to soil mass (g) of 5. The suspension was mixed at 20 ± 0.5 C in a shaker at 300 rpm for 24 h. Following that, two hundred grams of each soil sample were mixed with the optimal extractant concentrations in a 2 L flask at the soil to solution ratio of 1:5, which were shaken using the same procedure. The soil slurries were then centrifuged at 5000 g for 30 min and the supernatant was passed through a 0.45 mm filter (Sartorius) for metal analysis. The soil pellets were rinsed with deionized water for 30 min by shaking on a reciprocal mixer and then dried after discarding the supernatant.
remove arsenic from soil (Dermont et al., 2008). The washing solution is a key factor in the successful application of soil washing technique. Several washing solutions, including NaOH, H3PO4, and the combination of dithionite and disodium ethylenediaminetetraacetate dihydrate (EDTA), have been proven to effectively extract arsenic from contaminated soils (Jang et al., 2005; Jho et al., 2015; Kim et al., 2015). However, most research only focused on seeking efficient washing solutions for arsenic removal, but not on any changes in soil properties when using this technique. Documenting the changes in soil properties is equally important in the remediation of a certain contaminated soil. In this study, arsenic removal from contaminated soil utilizing different washing solutions was conducted. Subsequently, the changes in soil physicochemical properties and phytotoxicity of each washing technique were evaluated. The results obtained in this study are expected to provide some insights for remediating arsenic-contaminated soils in future engineering applications.
2.3. Soil sample characterization Soil samples were characterized before and after soil washing as follows: Soil pH was measured with a soil/solution ratio of 1:1 (w/ v), using a 0.01 M CaCl2 solution (McLean, 1982). Total organic carbon (TOC) content was analyzed according to the WalkleyeBlack wet oxidation method (Nelson and Sommers, 1982). The cation exchange capacity (CEC) at neutral pH was determined using the ammonium acetate method with 5 g of soil (Thomas, 1982). Particle size distribution was determined according to the approach proposed by Gee and Bauder (1986). Total nitrogen (TN), total phosphorus (TP) and total potassium (TK) were measured according to the Methods of Soil Analysis (Page et al., 1982). Soil colors were determined by using a Munsell soil color chart. The dissolved Al, Fe, Mn, Mg and Ca concentrations in the extraction solutions were measured via ICP-MS.
2. Materials and methods 2.1. Soil sampling Arsenic-contaminated soil samples were collected from farmland near an old smelter site in Shimen, Hunan province, in China. For comparison, the background soil sample was collected from nearby farmland. Soil samples were taken from a 0e40 cm layer and air-dried at a room temperature of 20 C. The soil samples were then sieved through a 2 mm mesh and thoroughly mixed. The prepared soil samples were stored before sample characterization and soil washing tests. The concentrations of arsenic, Al, Fe, Mn, Mg and Ca in soil samples were analyzed via inductively coupled plasma mass spectrometry (ICP-MS, Agilent Technology, 7500 Series) following aqua regia digestion. Arsenic concentrations in contaminated soil and background soil were determined to be 165.5 mg/kg and 13.0 mg/kg.
2.4. Wheat pot experiment To minimize the destruction of soil properties, the soil samples treated by H3PO4 or NaOH were adjusted to neutral (pH ¼ 7.0) by 0.1 M NaOH or 0.1 M HCl, respectively. The soil samples were blended with deionized water with a soil/solution ratio of 1:1 (w/ v). Neutralizer was added dropwise to the solution and stirred with a glass rod. The pH of soil samples was adjusted every 24 h to keep
2.2. Soil washing procedures A range of NaOH (0.1e5 M), H3PO4 (0.1 M-5 M), and 0.1 M dithionite in EDTA (0.05 M-0.2 M) concentrations were initially
120
120
b
a 100
Extraction efficiency ( % )
100 80
80
30 mg/kg
60
60
NaOH H3PO4
40
30 mg/kg
40 20
20
0
0 0
2 4 Washing solutions ( M )
6
0
0.1 0.2 EDTA ( M )
0.3
Fig. 1. Extraction of arsenic from soil with different concentrations of (a) NaOH, H3PO4 or (b) 0.1 M dithionite in EDTA. Dash line represents environmental quality standard for soils in China (30 mg/kg).
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Table 1 Bulk metals concentrations in soils and washing solutions. Untreated soil
Al (mg/kg) Ca (mg/kg) Fe (mg/kg) Mn (mg/kg) Mg (mg/kg)
Treated soil
30,626.7 ± 1776.8 20,654.0 ± 101.7 53,747.4 ± 4144.4 1665.0 ± 134.4 52,156.3 ± 1936.7
Dissolved in washing solutions
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
30,069.9 ± 1761.1 18,997.4 ± 124.2 50,487.9 ± 3625.5 1521.7 ± 112.2 51,872.2 ± 1754.6
30,030.2 ± 1645.1 20,637.9 ± 97.6 53,711.7 ± 4140.7 1664.3 ± 134.5 52,154.9 ± 1896.2
30,090.4 ± 1604.2 17,068.1 ± 379.8 45,789.5 ± 2843.7 1388.4 ± 66.4 51,838.9 ± 1679.1
556.8 ± 66.8 1656.6 ± 89.7 3259.5 ± 172.6 143.3 ± 10.5 284.1 ± 34.1
596.5 ± 77.7 16.1 ± 1.1 35.7 ± 3.7 0.7 ± 0.0 1.4 ± 0.0
536.3 ± 63.6 3585.9 ± 454.4 7957.9 ± 634.7 276.6 ± 13.4 317.4 ± 45.2
Table 2 Changes in soil properties before and after washing.
pH TOC (g/kg) CEC (cmol/kg) TN (mg/kg) TP (mg/kg) TK (mg/kg) Sand (%) Silt (%) Clay (%) Soil color
Untreated soil
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
7.4 ± 0.6 74.4 ± 3.9 10.4 ± 0.3 1220 ± 49 70.9 ± 6.2 9795 ± 856 11.0 ± 0.1 35.9 ± 4.0 53.2 ± 4.1 2.5YR 3/2
2.5 ± 0.3 82.5 ± 1.7 10.9 ± 0.9 1762 ± 49 92.3 ± 3.7 10,200 ± 141 11.4 ± 2.6 38.2 ± 5.2 50.4 ± 7.9 2.5YR 5/2
11.5 ± 0.7 49.5 ± 3.2 9.4 ± 0.3 744 ± 18 58.0 ± 0.4 10,600 ± 141 15.5 ± 1.4 35.6 ± 0.9 48.9 ± 2.2 2.5YR 3/2
7.3 ± 0.2 45.8 ± 1.4 10.3 ± 1.3 1408 ± 2 50.6 ± 0.6 11,900 ± 1556 14.6 ± 2.5 44.4 ± 2.1 41.0 ± 4.6 2.5YR 6/2
stable. The neutralized soils were washed by deionized water to remove high content of salts. Each pot was initially filled with 120 g of soil samples and repeated in triplicate. The water-holding capacity was adjusted to 80% by adding distilled water and then the pot was placed overnight. Afterwards, five germinated wheat seeds were sown in each pot. Wheat growth tests were executed in a growth chamber set to a control temperature at 18 ± 1 C during 10-h dark periods and 24 ± 1 C during 14-h light periods. Each pot was irrigated by weight method every 2e4 d according to the amount of soil moisture lost during the experiment. The wheat was
a
b
c
d
harvested at 28 d and measured for plant height, root length, and biomass (dry weight). 3. Results and discussion 3.1. Effect of different soil washing solutions on arsenic removal Fig. 1 shows the arsenic extraction efficiency for different washing solutions. Generally, arsenic extraction efficiency increased when increasing the concentration of washing solutions. Since the soil samples required approximately 82% arsenic removal to reach the clean-up level of 30 mg/kg based on the initial arsenic concentration of the soil sample, 2 M H3PO4, 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA functioned well in extracting arsenic from polluted soils, with an approximate removal rate of 90%. Using larger washing solutions concentrations only slightly increased the proportion of arsenic removed. H3PO4 can be used to specifically extract arsenic from soils due to the structural similarity of PO34 and AsO34 (Lim and Goh, 2005) . Sodium hydroxide’s ability to extract arsenic is mainly attributed to the ligand displacement reaction of hydroxyl ions with arsenic species (Jang et al., 2005). As a reductant, dithionite can dissolve soil minerals (oxide of iron, aluminum, and manganese), which are considered to be the most important sinks of arsenic. EDTA can form strong surface iron-chelate complexes and enhance arsenic extraction (Kim et al., 2015). 3.2. Changes in soil properties caused by soil washing
Fig. 2. Soil color before and after washing. (a) untreated soil, (b) 2 M H3PO4, (c) 2 M NaOH, (d) 0.1 M dithionite in EDTA.
In this study, the residual contents of arsenic in three soil samples treated by 2 M H3PO4, 2 M NaOH and 0.1 M dithionite in 0.1 M EDTA, were successfully reduced to <30 mg/kg. To evaluate whether the treated soil samples are suitable for agricultural reuse, the changes in soil properties of these three soil samples were tested. Significant Ca, Mg, Al, Fe and Mn contents were extracted by H3PO4 or EDTA (Table 1), which was due to their high extraction capacity. EDTA is known to act as a non-specific chelating agent and is therefore capable of extracting a variety of metal ions from the
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Table 3 Wheat growth situation after a 28 d growth period in untreated soil, clean soil and treated soils.
Plant height/cm Root length/cm Dry mass weight/g
Untreated soil
Clean soil
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
19.3 ± 2.6 5.0 ± 0.0 0.2 ± 0.0
36.0 ± 2.7 10.0 ± 1.0 0.7 ± 0.2
30.3 ± 1.5 6.7 ± 0.6 0.3 ± 0.0
33.0 ± 2.0 9.7 ± 0.6 0.6 ± 0.2
30.7 ± 0.6 7.0 ± 1.0 0.4 ± 0.1
soil matrix. H3PO4 can also dissolve metal ions due to its acidity and chelating capacity. Only Al was extracted by NaOH because amorphous aluminum oxide has high solubility in high pH solution (Snoeyink and Jenkins, 1980). After extraction with 2 M H3PO4, 2 M NaOH and 0.1 M dithionite in 0.1 M EDTA, the soils’ pH levels were determined to be 2.5, 11.5 and 7.3, respectively, from 7.4 of the original soil pH (Table 2). The soils washed with H3PO4 or NaOH indicated a significant change in pH compared to the soils washed with dithionite in EDTA. This may due to residual acid or alkaline of the initially acidic or basic washing solutions adhering to the soil even after rinsing (Tokunaga and Hakuta, 2002). TOC contents in the soil samples treated by 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA were 66.5% or 61.6% that of the untreated soil, whereas there was no change of TOC in the soil samples treated by 2 M H3PO4. Since NaOH and EDTA were often used to extract humus from soil (Campbell et al., 1967; McLaren et al., 1975), a decline in TOC contents after washing with NaOH or EDTA can be expected. Similarly, as an important component of humus, organicN can also be extracted by NaOH, which may explain the decrease of TN contents in the soil samples washed by NaOH. TN contents in the soil samples treated by 2 M H3PO4 increased to 1.44 times that of the untreated soil. This phenomenon was also observed by Im et al. (2015). NaOH and EDTA are also employed to extract phosphate from soil (Bowman and Moir, 1993; He et al., 2008; Jarosch et al., 2015), and therefore TP contents in the soil samples treated by 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA decreased significantly. The increase of TP content in the soil sample treated by 2 M H3PO4 may be due to residual phosphate adhering to soil. No significant changes in particle size distribution were observed for the soil sample treated by H3PO4 or NaOH. In contrast, the soil treated by 0.1 M dithionite in 0.1 M EDTA increased in the sand and silt fractions and decreased in the clay fraction due to its high chelating capacity. No soil color changes occurred when the soil sample was washed with NaOH (2.5YR 3/2) (Fig. 2), whereas a drastic color change was observed when the soil sample was washed with H3PO4 (2.5YR 5/2) or 0.1 M dithionite in 0.1 M EDTA (2.5YR 6/2). The observed color change was possibly attributed to the removal of Fe and Mn compounds during soil washing. No significant changes in CEC and TK content were observed for the soil samples treated by the three washing solutions.
3.3. Germination trial For wheat planted in untreated soil, the average plant height, root length, and dry mass weight were 19.3 cm, 5.0 cm, and 0.2 g, respectively, about half of that planted in clean soil (Table 3). These results indicate that the growth of wheat was highly impacted by arsenic contamination. After soil washing, the wheat grew better than that grew in untreated soil. For the H3PO4 treated soil, average plant height, root length, and dry mass weight of wheat were 84.2%, 67.0%, and 42.5%, respectively, of that planted in clean soil. Nutrient deficiencies such as Fe and Mn following H3PO4 treatment were harmful to enzyme activation, gas exchange, and osmotic regulation of wheat (Bingham, 1963; Osman, 2012). In addition, an excessive amount of nitrogen after H3PO4 treatment could lead to
yellowing of plants and smaller yields of plants (van Dijk and Roelofs, 1988; Osman, 2012). Similar wheat growth was found for EDTA treated soil, which was also possibly due to nutrient deficiencies (Brock et al., 2003; Nannipieri et al., 2011). Of the three treated soils, the growth of wheat planted in NaOH treated soil was the best, and in fact the average plant height, root length, and dry mass weight of wheat planted in untreated soil were 91.7%, 97.0%, and 85.4% of that planted in clean soil. This suggests that the change in soil properties induced by NaOH treatment had little effect on the growth of wheat. In summary, this study demonstrates that all three washing solutions including H3PO4, NaOH and dithionite in EDTA can effectively remove arsenic from soil. Soil properties were only partially changed after washing. After adjusted the soil pH to neutral, wheat grew best in the soil sample washed by NaOH. Therefore, when ecotoxicological aspects are considered for the remediation of arsenic-contaminated soils by soil washing, it is more appropriate to use NaOH than H3PO4 or dithionite in EDTA. These results will assist in choosing the best washing solution when remediating arsenic-contaminated soils in future engineering applications. Acknowledgements Financial support from the National High Technology Research and Development Program of China (863 Program) (2013AA06A207) is gratefully acknowledged. References Abioye, O.F., Uttam, K.S., 2016. Arsenic hyperaccumulating fern: implications for remediation of arsenic contaminated soils. Geoderma 284, 132e143. Abumaizar, R.J., Smith, E.H., 1999. Heavy metal contaminants removal by soil washing. J. Hazard. Mater. 70, 71e86. Bingham, F.T., 1963. Relation between phosphorus and micronutrients in plants. Soil Sci. Soc. Am. J. 27, 389e391. Bowman, R.A., Moir, J.O., 1993. Basic EDTA as an extractant for soil organic phosphorus. Soil. Sci. Soc. Am. J. 57, 1516e1518. Brock, T.D., Madigan, M.T., Martinko, J.M., Parker, J., 2003. Biology of Microorganisms, tenth ed. Pearson Prentice Hall, London. Campbell, C.A., Paul, E.A., Rennie, D.A., Mccallum, K.J., 1967. Applicability of the carbon-dating method of analysis to soil humus studies. Soil. Sci. 104, 217e224. Cao, M.H., Ye, Y.Y., Chen, J., Lu, X.H., 2016. Remediation of arsenic contaminated soil by coupling oxalate washing with subsequent ZVI/Air treatment. Chemosphere 144, 1313e1318. Chen, W.Q., Shi, Y.L., Wu, S.L., Zhu, Y.G., 2016. Anthropogenic arsenic cycles: a research framework and features. J. Clean. Prod. 139, 328e336. che, M., 2008. Soil washing for Dermont, G., Bergeron, M., Mercier, G., Richer-Lafle metal removal: a review of physical/chemical technologies and field applications. J. Hazard. Mater 152, 1e31. €berg, R., Ribe , V., Waara, S., Elgh-Dalgren, K., Arwidsson, Z., Camdzija, A., Sjo Allard, B., von Kronhelm, T., van Hees, P.A.W., 2009. Laboratory and pilot scale soil washing of PAH and arsenic from a wood preservation site: changes in concentration and toxicity. J. Hazard. Mater. 172 (2e3), 1033e1040. Gee, G.W., Bauder, J.W., 1986. Particle size analysis. In: Klute, A. (Ed.), Methods of Soil Analysis, Part 1: Physical and Mineralogical Methods, second ed., Am. Soc. Agron. WI., Madison, p. 191. Gusiatin, Z.M., 2014. Tannic acid and saponin for removing arsenic from brownfield soils: mobilization, distribution and speciation. J. Environ. Sci. China 26 (4), 855e864. He, Z., Honeycutt, C.W., Cademenun, B.J., Senwo, Z.N., Tazisong, I.A., 2008. Phosphorus in poultry litter and soil: enzymatic and nuclear magnetic resonance characterization. Soil. Sci. Soc. Am. J. 72, 1425e1433. Im, J., Yang, K., Jho, E.H., Nam, K., 2015. Effect of different soil washing solutions on bioavailability of residual arsenic in soils and soil properties. Chemosphere 138,
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Short Communication
An evaluation of different soil washing solutions for remediating arsenic-contaminated soils Yiwen Wang a, Fujun Ma a, Qian Zhang a, Changsheng Peng b, Bin Wu a, Fasheng Li a, Qingbao Gu a, * a b
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China The Key Lab of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China
h i g h l i g h t s H3PO4, NaOH and EDTA can effectively remove arsenic from a heavily contaminated soil. Soil properties were partially changed after washing. Wheat grew best in NaOH-treated soil sample.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2016 Received in revised form 2 January 2017 Accepted 12 January 2017 Available online 12 January 2017
Soil washing is a promising way to remediate arsenic-contaminated soils. Most research has mostly focused on seeking efficient extractants for removing arsenic, but not concerned with any changes in soil properties when using this technique. In this study, the removal of arsenic from a heavily contaminated soil employing different washing solutions including H3PO4, NaOH and dithionite in EDTA was conducted. Subsequently, the changes in soil physicochemical properties and phytotoxicity of each washing technique were evaluated. After washing with 2 M H3PO4, 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA, the soil samples’ arsenic content met the clean-up levels stipulated in China’s environmental regulations. H3PO4 washing decreased soil pH, Ca, Mg, Al, Fe, and Mn concentrations but increased TN and TP contents. NaOH washing increased soil pH but decreased soil TOC, TN and TP contents. Dithionite in EDTA washing reduced soil TOC, Ca, Mg, Al, Fe, Mn and TP contents. A drastic color change was observed when the soil sample was washed with H3PO4 or 0.1 M dithionite in 0.1 M EDTA. After adjusting the soil pH to neutral, wheat planted in the soil sample washed by NaOH evidenced the best growth of all three treated soil samples. These results will help with selecting the best washing solution when remediating arseniccontaminated soils in future engineering applications. © 2017 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Arsenic Soil washing NaOH H3PO4 Dithionite in EDTA
1. Introduction Arsenic has attracted much concern due to its highly toxic and carcinogenic properties which are dangerous to human beings, animals, and plants. The main anthropogenic sources contributing to arsenic contamination in soils include mining, smelting, agricultural use of pesticides and the disposal of industrial wastes (Mahimairaja et al., 2005; Liao et al., 2005; Chen et al., 2016). In China, a number of arsenic polluted soils have been reported, and the arsenic concentrations in some soils are up to 1217 mg/kg (Liao
* Corresponding author. E-mail address: [email protected] (Q. Gu). http://dx.doi.org/10.1016/j.chemosphere.2017.01.068 0045-6535/© 2017 Elsevier Ltd. All rights reserved.
et al., 2005; Liu et al., 2005; Wei et al., 2009). This has been as high as 243 times larger than worldwide arsenic background in soil (5 mg/kg) (WHO, 2001) and 41 times the arsenic allowance in soil in China (30 mg/kg, National Standard GB15618-1995). Subsequently this represents a serious threat to human health and the environment. To remediate arsenic-contaminated soils, solidification/stabilization (Tyrovola and Nikolaidis, 2009; Yoon et al., 2010), soil washing (Abumaizar and Smith, 1999; Elgh-Dalgren et al., 2009; Sierra et al., 2010, 2011; Gusiatin, 2014; Cao et al., 2016), electrokinetic remediation (Mao et al., 2015) and phytoremediation (Ye et al., 2011; Abioye and Uttam, 2016) have been commonly utilized. Of these techniques, soil washing is an effective way to remediate arsenic contaminated soil because it can permanently
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used to determine the optimal extractant concentrations for arsenic removal. According to previous studies (Im et al., 2015; Jho et al., 2015), 20 g soil sample was put in a 200 mL flask and then 100 mL of reagent solution was slowly added to achieve a ratio of reagent solutions (mL) to soil mass (g) of 5. The suspension was mixed at 20 ± 0.5 C in a shaker at 300 rpm for 24 h. Following that, two hundred grams of each soil sample were mixed with the optimal extractant concentrations in a 2 L flask at the soil to solution ratio of 1:5, which were shaken using the same procedure. The soil slurries were then centrifuged at 5000 g for 30 min and the supernatant was passed through a 0.45 mm filter (Sartorius) for metal analysis. The soil pellets were rinsed with deionized water for 30 min by shaking on a reciprocal mixer and then dried after discarding the supernatant.
remove arsenic from soil (Dermont et al., 2008). The washing solution is a key factor in the successful application of soil washing technique. Several washing solutions, including NaOH, H3PO4, and the combination of dithionite and disodium ethylenediaminetetraacetate dihydrate (EDTA), have been proven to effectively extract arsenic from contaminated soils (Jang et al., 2005; Jho et al., 2015; Kim et al., 2015). However, most research only focused on seeking efficient washing solutions for arsenic removal, but not on any changes in soil properties when using this technique. Documenting the changes in soil properties is equally important in the remediation of a certain contaminated soil. In this study, arsenic removal from contaminated soil utilizing different washing solutions was conducted. Subsequently, the changes in soil physicochemical properties and phytotoxicity of each washing technique were evaluated. The results obtained in this study are expected to provide some insights for remediating arsenic-contaminated soils in future engineering applications.
2.3. Soil sample characterization Soil samples were characterized before and after soil washing as follows: Soil pH was measured with a soil/solution ratio of 1:1 (w/ v), using a 0.01 M CaCl2 solution (McLean, 1982). Total organic carbon (TOC) content was analyzed according to the WalkleyeBlack wet oxidation method (Nelson and Sommers, 1982). The cation exchange capacity (CEC) at neutral pH was determined using the ammonium acetate method with 5 g of soil (Thomas, 1982). Particle size distribution was determined according to the approach proposed by Gee and Bauder (1986). Total nitrogen (TN), total phosphorus (TP) and total potassium (TK) were measured according to the Methods of Soil Analysis (Page et al., 1982). Soil colors were determined by using a Munsell soil color chart. The dissolved Al, Fe, Mn, Mg and Ca concentrations in the extraction solutions were measured via ICP-MS.
2. Materials and methods 2.1. Soil sampling Arsenic-contaminated soil samples were collected from farmland near an old smelter site in Shimen, Hunan province, in China. For comparison, the background soil sample was collected from nearby farmland. Soil samples were taken from a 0e40 cm layer and air-dried at a room temperature of 20 C. The soil samples were then sieved through a 2 mm mesh and thoroughly mixed. The prepared soil samples were stored before sample characterization and soil washing tests. The concentrations of arsenic, Al, Fe, Mn, Mg and Ca in soil samples were analyzed via inductively coupled plasma mass spectrometry (ICP-MS, Agilent Technology, 7500 Series) following aqua regia digestion. Arsenic concentrations in contaminated soil and background soil were determined to be 165.5 mg/kg and 13.0 mg/kg.
2.4. Wheat pot experiment To minimize the destruction of soil properties, the soil samples treated by H3PO4 or NaOH were adjusted to neutral (pH ¼ 7.0) by 0.1 M NaOH or 0.1 M HCl, respectively. The soil samples were blended with deionized water with a soil/solution ratio of 1:1 (w/ v). Neutralizer was added dropwise to the solution and stirred with a glass rod. The pH of soil samples was adjusted every 24 h to keep
2.2. Soil washing procedures A range of NaOH (0.1e5 M), H3PO4 (0.1 M-5 M), and 0.1 M dithionite in EDTA (0.05 M-0.2 M) concentrations were initially
120
120
b
a 100
Extraction efficiency ( % )
100 80
80
30 mg/kg
60
60
NaOH H3PO4
40
30 mg/kg
40 20
20
0
0 0
2 4 Washing solutions ( M )
6
0
0.1 0.2 EDTA ( M )
0.3
Fig. 1. Extraction of arsenic from soil with different concentrations of (a) NaOH, H3PO4 or (b) 0.1 M dithionite in EDTA. Dash line represents environmental quality standard for soils in China (30 mg/kg).
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Table 1 Bulk metals concentrations in soils and washing solutions. Untreated soil
Al (mg/kg) Ca (mg/kg) Fe (mg/kg) Mn (mg/kg) Mg (mg/kg)
Treated soil
30,626.7 ± 1776.8 20,654.0 ± 101.7 53,747.4 ± 4144.4 1665.0 ± 134.4 52,156.3 ± 1936.7
Dissolved in washing solutions
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
30,069.9 ± 1761.1 18,997.4 ± 124.2 50,487.9 ± 3625.5 1521.7 ± 112.2 51,872.2 ± 1754.6
30,030.2 ± 1645.1 20,637.9 ± 97.6 53,711.7 ± 4140.7 1664.3 ± 134.5 52,154.9 ± 1896.2
30,090.4 ± 1604.2 17,068.1 ± 379.8 45,789.5 ± 2843.7 1388.4 ± 66.4 51,838.9 ± 1679.1
556.8 ± 66.8 1656.6 ± 89.7 3259.5 ± 172.6 143.3 ± 10.5 284.1 ± 34.1
596.5 ± 77.7 16.1 ± 1.1 35.7 ± 3.7 0.7 ± 0.0 1.4 ± 0.0
536.3 ± 63.6 3585.9 ± 454.4 7957.9 ± 634.7 276.6 ± 13.4 317.4 ± 45.2
Table 2 Changes in soil properties before and after washing.
pH TOC (g/kg) CEC (cmol/kg) TN (mg/kg) TP (mg/kg) TK (mg/kg) Sand (%) Silt (%) Clay (%) Soil color
Untreated soil
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
7.4 ± 0.6 74.4 ± 3.9 10.4 ± 0.3 1220 ± 49 70.9 ± 6.2 9795 ± 856 11.0 ± 0.1 35.9 ± 4.0 53.2 ± 4.1 2.5YR 3/2
2.5 ± 0.3 82.5 ± 1.7 10.9 ± 0.9 1762 ± 49 92.3 ± 3.7 10,200 ± 141 11.4 ± 2.6 38.2 ± 5.2 50.4 ± 7.9 2.5YR 5/2
11.5 ± 0.7 49.5 ± 3.2 9.4 ± 0.3 744 ± 18 58.0 ± 0.4 10,600 ± 141 15.5 ± 1.4 35.6 ± 0.9 48.9 ± 2.2 2.5YR 3/2
7.3 ± 0.2 45.8 ± 1.4 10.3 ± 1.3 1408 ± 2 50.6 ± 0.6 11,900 ± 1556 14.6 ± 2.5 44.4 ± 2.1 41.0 ± 4.6 2.5YR 6/2
stable. The neutralized soils were washed by deionized water to remove high content of salts. Each pot was initially filled with 120 g of soil samples and repeated in triplicate. The water-holding capacity was adjusted to 80% by adding distilled water and then the pot was placed overnight. Afterwards, five germinated wheat seeds were sown in each pot. Wheat growth tests were executed in a growth chamber set to a control temperature at 18 ± 1 C during 10-h dark periods and 24 ± 1 C during 14-h light periods. Each pot was irrigated by weight method every 2e4 d according to the amount of soil moisture lost during the experiment. The wheat was
a
b
c
d
harvested at 28 d and measured for plant height, root length, and biomass (dry weight). 3. Results and discussion 3.1. Effect of different soil washing solutions on arsenic removal Fig. 1 shows the arsenic extraction efficiency for different washing solutions. Generally, arsenic extraction efficiency increased when increasing the concentration of washing solutions. Since the soil samples required approximately 82% arsenic removal to reach the clean-up level of 30 mg/kg based on the initial arsenic concentration of the soil sample, 2 M H3PO4, 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA functioned well in extracting arsenic from polluted soils, with an approximate removal rate of 90%. Using larger washing solutions concentrations only slightly increased the proportion of arsenic removed. H3PO4 can be used to specifically extract arsenic from soils due to the structural similarity of PO34 and AsO34 (Lim and Goh, 2005) . Sodium hydroxide’s ability to extract arsenic is mainly attributed to the ligand displacement reaction of hydroxyl ions with arsenic species (Jang et al., 2005). As a reductant, dithionite can dissolve soil minerals (oxide of iron, aluminum, and manganese), which are considered to be the most important sinks of arsenic. EDTA can form strong surface iron-chelate complexes and enhance arsenic extraction (Kim et al., 2015). 3.2. Changes in soil properties caused by soil washing
Fig. 2. Soil color before and after washing. (a) untreated soil, (b) 2 M H3PO4, (c) 2 M NaOH, (d) 0.1 M dithionite in EDTA.
In this study, the residual contents of arsenic in three soil samples treated by 2 M H3PO4, 2 M NaOH and 0.1 M dithionite in 0.1 M EDTA, were successfully reduced to <30 mg/kg. To evaluate whether the treated soil samples are suitable for agricultural reuse, the changes in soil properties of these three soil samples were tested. Significant Ca, Mg, Al, Fe and Mn contents were extracted by H3PO4 or EDTA (Table 1), which was due to their high extraction capacity. EDTA is known to act as a non-specific chelating agent and is therefore capable of extracting a variety of metal ions from the
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Table 3 Wheat growth situation after a 28 d growth period in untreated soil, clean soil and treated soils.
Plant height/cm Root length/cm Dry mass weight/g
Untreated soil
Clean soil
2 M H3PO4
2 M NaOH
0.1 M dithionite in 0.1 M EDTA
19.3 ± 2.6 5.0 ± 0.0 0.2 ± 0.0
36.0 ± 2.7 10.0 ± 1.0 0.7 ± 0.2
30.3 ± 1.5 6.7 ± 0.6 0.3 ± 0.0
33.0 ± 2.0 9.7 ± 0.6 0.6 ± 0.2
30.7 ± 0.6 7.0 ± 1.0 0.4 ± 0.1
soil matrix. H3PO4 can also dissolve metal ions due to its acidity and chelating capacity. Only Al was extracted by NaOH because amorphous aluminum oxide has high solubility in high pH solution (Snoeyink and Jenkins, 1980). After extraction with 2 M H3PO4, 2 M NaOH and 0.1 M dithionite in 0.1 M EDTA, the soils’ pH levels were determined to be 2.5, 11.5 and 7.3, respectively, from 7.4 of the original soil pH (Table 2). The soils washed with H3PO4 or NaOH indicated a significant change in pH compared to the soils washed with dithionite in EDTA. This may due to residual acid or alkaline of the initially acidic or basic washing solutions adhering to the soil even after rinsing (Tokunaga and Hakuta, 2002). TOC contents in the soil samples treated by 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA were 66.5% or 61.6% that of the untreated soil, whereas there was no change of TOC in the soil samples treated by 2 M H3PO4. Since NaOH and EDTA were often used to extract humus from soil (Campbell et al., 1967; McLaren et al., 1975), a decline in TOC contents after washing with NaOH or EDTA can be expected. Similarly, as an important component of humus, organicN can also be extracted by NaOH, which may explain the decrease of TN contents in the soil samples washed by NaOH. TN contents in the soil samples treated by 2 M H3PO4 increased to 1.44 times that of the untreated soil. This phenomenon was also observed by Im et al. (2015). NaOH and EDTA are also employed to extract phosphate from soil (Bowman and Moir, 1993; He et al., 2008; Jarosch et al., 2015), and therefore TP contents in the soil samples treated by 2 M NaOH or 0.1 M dithionite in 0.1 M EDTA decreased significantly. The increase of TP content in the soil sample treated by 2 M H3PO4 may be due to residual phosphate adhering to soil. No significant changes in particle size distribution were observed for the soil sample treated by H3PO4 or NaOH. In contrast, the soil treated by 0.1 M dithionite in 0.1 M EDTA increased in the sand and silt fractions and decreased in the clay fraction due to its high chelating capacity. No soil color changes occurred when the soil sample was washed with NaOH (2.5YR 3/2) (Fig. 2), whereas a drastic color change was observed when the soil sample was washed with H3PO4 (2.5YR 5/2) or 0.1 M dithionite in 0.1 M EDTA (2.5YR 6/2). The observed color change was possibly attributed to the removal of Fe and Mn compounds during soil washing. No significant changes in CEC and TK content were observed for the soil samples treated by the three washing solutions.
3.3. Germination trial For wheat planted in untreated soil, the average plant height, root length, and dry mass weight were 19.3 cm, 5.0 cm, and 0.2 g, respectively, about half of that planted in clean soil (Table 3). These results indicate that the growth of wheat was highly impacted by arsenic contamination. After soil washing, the wheat grew better than that grew in untreated soil. For the H3PO4 treated soil, average plant height, root length, and dry mass weight of wheat were 84.2%, 67.0%, and 42.5%, respectively, of that planted in clean soil. Nutrient deficiencies such as Fe and Mn following H3PO4 treatment were harmful to enzyme activation, gas exchange, and osmotic regulation of wheat (Bingham, 1963; Osman, 2012). In addition, an excessive amount of nitrogen after H3PO4 treatment could lead to
yellowing of plants and smaller yields of plants (van Dijk and Roelofs, 1988; Osman, 2012). Similar wheat growth was found for EDTA treated soil, which was also possibly due to nutrient deficiencies (Brock et al., 2003; Nannipieri et al., 2011). Of the three treated soils, the growth of wheat planted in NaOH treated soil was the best, and in fact the average plant height, root length, and dry mass weight of wheat planted in untreated soil were 91.7%, 97.0%, and 85.4% of that planted in clean soil. This suggests that the change in soil properties induced by NaOH treatment had little effect on the growth of wheat. In summary, this study demonstrates that all three washing solutions including H3PO4, NaOH and dithionite in EDTA can effectively remove arsenic from soil. Soil properties were only partially changed after washing. After adjusted the soil pH to neutral, wheat grew best in the soil sample washed by NaOH. Therefore, when ecotoxicological aspects are considered for the remediation of arsenic-contaminated soils by soil washing, it is more appropriate to use NaOH than H3PO4 or dithionite in EDTA. These results will assist in choosing the best washing solution when remediating arsenic-contaminated soils in future engineering applications. Acknowledgements Financial support from the National High Technology Research and Development Program of China (863 Program) (2013AA06A207) is gratefully acknowledged. References Abioye, O.F., Uttam, K.S., 2016. Arsenic hyperaccumulating fern: implications for remediation of arsenic contaminated soils. Geoderma 284, 132e143. Abumaizar, R.J., Smith, E.H., 1999. Heavy metal contaminants removal by soil washing. J. Hazard. Mater. 70, 71e86. Bingham, F.T., 1963. Relation between phosphorus and micronutrients in plants. Soil Sci. Soc. Am. J. 27, 389e391. Bowman, R.A., Moir, J.O., 1993. Basic EDTA as an extractant for soil organic phosphorus. Soil. Sci. Soc. Am. J. 57, 1516e1518. Brock, T.D., Madigan, M.T., Martinko, J.M., Parker, J., 2003. Biology of Microorganisms, tenth ed. Pearson Prentice Hall, London. Campbell, C.A., Paul, E.A., Rennie, D.A., Mccallum, K.J., 1967. Applicability of the carbon-dating method of analysis to soil humus studies. Soil. Sci. 104, 217e224. Cao, M.H., Ye, Y.Y., Chen, J., Lu, X.H., 2016. Remediation of arsenic contaminated soil by coupling oxalate washing with subsequent ZVI/Air treatment. Chemosphere 144, 1313e1318. Chen, W.Q., Shi, Y.L., Wu, S.L., Zhu, Y.G., 2016. Anthropogenic arsenic cycles: a research framework and features. J. Clean. Prod. 139, 328e336. che, M., 2008. Soil washing for Dermont, G., Bergeron, M., Mercier, G., Richer-Lafle metal removal: a review of physical/chemical technologies and field applications. J. Hazard. Mater 152, 1e31. €berg, R., Ribe , V., Waara, S., Elgh-Dalgren, K., Arwidsson, Z., Camdzija, A., Sjo Allard, B., von Kronhelm, T., van Hees, P.A.W., 2009. Laboratory and pilot scale soil washing of PAH and arsenic from a wood preservation site: changes in concentration and toxicity. J. Hazard. Mater. 172 (2e3), 1033e1040. Gee, G.W., Bauder, J.W., 1986. Particle size analysis. In: Klute, A. (Ed.), Methods of Soil Analysis, Part 1: Physical and Mineralogical Methods, second ed., Am. Soc. Agron. WI., Madison, p. 191. Gusiatin, Z.M., 2014. Tannic acid and saponin for removing arsenic from brownfield soils: mobilization, distribution and speciation. J. Environ. Sci. China 26 (4), 855e864. He, Z., Honeycutt, C.W., Cademenun, B.J., Senwo, Z.N., Tazisong, I.A., 2008. Phosphorus in poultry litter and soil: enzymatic and nuclear magnetic resonance characterization. Soil. Sci. Soc. Am. J. 72, 1425e1433. Im, J., Yang, K., Jho, E.H., Nam, K., 2015. Effect of different soil washing solutions on bioavailability of residual arsenic in soils and soil properties. Chemosphere 138,
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