chemical extraction of arsenic from contaminated soil under subcritical conditions
TRANSCRIPT
Science of the Total Environment 409 (2011) 3066–3072
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Chemical extraction of arsenic from contaminated soil under subcritical conditions
Seok-Young Oh a,⁎, Myong-Keun Yoon a, Ick-Hyun Kim a, Ju Yup Kim b, Wookeun Bae b
a Department of Civil and Environmental Engineering, University of Ulsan, Ulsan 680 749, South Koreab Department of Civil and Environmental Engineering, Hanyang University, Gyunggi-Do 425 791, South Korea
⁎ Corresponding author. Tel.: +82 52 259 2752; fax:E-mail address: [email protected] (S.-Y. Oh).
0048-9697/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.scitotenv.2011.04.054
a b s t r a c t
a r t i c l e i n f oArticle history:Received 3 February 2011Received in revised form 25 April 2011Accepted 29 April 2011Available online 23 May 2011
Keywords:ArsenicChemical extractionTemperatureSubcritical conditionSequential extraction
In this research, we investigated a chemical extraction process, under subcritical conditions, for arsenic (As)-contaminated soil in the vicinity of an abandoned smelting plant in South Korea. The total concentration of Asin soil was 75.5 mg/kg, 68% of which was As(+III). X-ray photoelectron spectroscopy analysis showed thatthe possible As(+III)-bearing compounds in the soil were As2O3 and R-AsOOH. At 20 °C, 100 mM of NaOHcould extract 26% of the As from the soil samples. In contrast, 100 mM of ethylenediaminetetraacetic acid(EDTA) and citric acid showed less than 10% extraction efficiency. However, as the temperature increased to250 and 300 °C, extraction efficiencies increased to 75–91% and 94–103%, respectively, regardless of theextraction reagent used. Control experiments with subcritical water at 300 °C showed complete extraction ofAs from the soil. Arsenic species in the solution extracted at 300 °C indicated that subcritical water oxidationmay be involved in the dissolution of As(+III)-bearing minerals under given conditions. Our results suggestthat subcritical water extraction/oxidation is a promising option for effective disposal of As-contaminated soil.
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1. Introduction
Arsenic (As) is a well-known, toxic metalloid found in soils andnatural waters. Arsenic can exist in various mineral forms in thenatural environment. Among them, approximately 60% are arsenates,As(+V)O4
3−, and the remaining 40% includes other forms, such asarsenite, As(+III)O3
3−; arsenide, As(− III); sulfides; elemental As; andorganic arsenic compounds (Alloway, 1994). Depending on thedosage and exposure time, As can cause acute and chronic toxicityin human beings, such as diarrhea, hypertension, vascular diseases,anemia, cancer, and leukemia (Alloway, 1994; Henke, 2009). Accord-ing to Agency for Toxic Substances and Disease Registry (ATSDR,2007), As was ranked no. 1 in the 2007 priority list of hazardoussubstances by Comprehensive Environmental Response, Compensa-tion and Liability Act (CERCLA). Because of the high toxicity andmobility, remediation of As-contaminated soils and groundwater hasbecome an important issue in the last few decades.
Soil washing is an ex-situ treatment method for separatingcontaminants from soil via physical, chemical or physicochemicalprocedures (Evanko and Dzombak, 1997; Dermont et al., 2008a). Soilwashing method mostly uses physical separation, such as sizeseparation, gravity concentration, froth flotation, attrition scrubbing
and magnetic separation (Dermont et al., 2008b; Dermont et al.,2010). Combined with chemical extraction, physical separation canalso be used (Dermont et al., 2008a). In chemical extractionprocedure, washing reagents may include leaching agents, surfac-tants, chelating agents, diluted acid/base solutions, and pH-adjustedwater to enhance the removal of contaminants from the soil (Semerand Reddy, 1996; Peters, 1999; U.S. EPA, 2007; Dermont et al., 2008a).Soil washing has several advantages, including rapid kinetics,operational ease, treatability of fine particles, and economic efficiency(Dermont et al., 2008a; Peters, 1999). Due to these advantages, manyefforts have been made worldwide over the past decades to apply soilwashing in pilot facilities and full-scale demonstration facilities toremediate As-contaminated soil (Dermont et al., 2008a). In thesefacilities, the extraction efficiency of As from contaminated soil rangedfrom 6 to 95% (Legiec et al., 1997; Dermont et al., 2008a). As Peters(1999) concluded, the extraction efficiency of As in soil washing isstrongly dependent upon the forms of the As in the soil.
Among the soil washing techniques, the chemical extractionmethods were recently investigated for the removal of As from thecontaminated soils using various washing reagents. Alam andTokunaga (2006) showed that As was extracted most efficiently by4% H3PO4 with more than 99% from the contaminated soil. Sulfuricacid and citric acid also showed high percentage of extractionefficiency. Though NaOH showed higher extraction efficiency, onlyarsenate (As(+V)) was extracted from the soil. Alam and Tokunaga(2006) also found that most of arsenite (As(+III)) was oxidized toarsenate during mineral acid and alkaline extraction. The extractionefficiency also varied with the concentration of the acid and alkalinesolution. These results suggest that the pH and redox reactions may
Table 1Sequential extraction method used to examine chemical fractionation of As.(Wenzel et al., 2001a).
Arsenic analysis
Steps Extractants Conditions
I. Ionically boundsoluble
1 M MgCl2 (pH 8),25 mL
2 h shaking, 25 °C
II. Adsorbed 1 M NaH2PO4
(pH 5), 25 mL20 h shaking, 25 °C
III. Amorphous andpoorly crystallinehydrous oxide of Feand Al
0.2 M NH4+-oxalate
(PH 3.2), 25 mL4 h shaking, 25 °C
IV. Crystalline Feoxyhydroxide
0.5 M Na-citrate, 8 mL/1.0 MNa-bicarbonate,8 mL/Na-dithionite, 0.4 g
1) Add 8 mL ofNa-citrate
2) Add 1 mL ofNa-bicarbonate
3) Heat the tube in awater bath at75–80 °Cb
4) Add 0.2 g ofNa-dithionitepowder, stir for5 min
5) Add a second 0.2 gof Na-dithionitepowder, stir for 10 min
V. Residual Aqua regia (HNO3+HCl) 1 h, 70 °C
a Modified according to Kim et al. (2003), Ahn et al. (2005), and Keon et al. (2001).b Do not over 80 °C.
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control the extraction of As from soil. Yang et al. (2009) showed that,compared to HCl, H3PO4, and H2SO4, NaOH was favorable in removingAs from mine tailings. A combination of chelant extraction and ironreducing bacteria also increased the extraction efficiency of As fromcontaminates soils from 35% to 90% (Vaxevanidou et al., 2008). Todate, several attempts have beenmade to remediate As-contaminatedsoils in South Korea via chemical extraction, and some successfulresults have been reported. Lee et al. (2004) showed, via batchexperiments with 0.1 M ethylenediaminetetraacetic acid (EDTA), thatmore than 80% of the As could be removed from abandoned minetailings and soils; they also showed that the extraction efficiencyincreased when the pH was less than five or more than ten. Jang et al.(2005) showed that alkaline washing with 0.2 M NaOH was moreeffective for removing As from mine tailings and soils than washingwith citric acid. In contrast, Lee et al. (2007) used 0.2 M citric acid towash stream sediments near an abandoned mine and reported thatthe removal efficiency of metals including As was greater than 95%after 1 h of washing. In bench-scale demonstration and fieldapplication of soil washing for As-contaminated soils (Korea Ministryof Environment, 2009a; Ko et al., 2005, 2006), it was also reportedthat, due to the limitation of chemical extraction, the physicalseparation of particles could enhance the overall efficiency of soilwashing.
Subcritical water is liquid water under pressure at temperaturesbetween the usual boiling point (100 °C) and the critical temperature(374.1 °C) (LaGrega et al., 2000). Under subcritical conditions, thedielectric constant, surface tension, and viscosity of water moleculesare dramatically decreased (Siskin et al., 1990; Kuhlmann et al., 1994;LaGrega et al., 2000). Also, under subcritical conditions, concentra-tions of H+ and OH− are greatly increased via self-ionization of watermolecules (Kuhlmann et al., 1994). In addition, the combination ofsubcritical water with oxidizing agents, such as H2O2, permanganate,O2, and air, can effectively oxidize organic compounds that arenormally very difficult to oxidize (Kronholm et al., 2001; Dadkhah andAkgerman, 2002). Thus, over the last few decades, attempts have beenmade to utilize subcritical water to extract or decompose polycyclicaromatic hydrocarbons, pesticides, polychlorinated biphenyls (PCBs),dioxin, and other organic contaminants in soils (Hawthorne et al.,2000; Lagadec et al., 2000; Weber et al., 2002; Kubátová et al., 2002;Hashimoto et al., 2004). In contrast, only limited efforts have beenmade to use subcritical water to extract metals from contaminatedsoils (Priego-López and de Castro, 2002).
Currently, the Ministry of Environment of South Korea is planningto take action to remediate the Janghang area located in thesouthwestern Korean peninsula. The Janghang area is heavilycontaminated with As and other metals, which were released froma Au–Cu smelting plant that was operated from 1936 to 1989 (KoreanEnvironment Corporation (KEC), 2009). Previous investigations bythe Ministry of Environment of South Korea have reported that areaswithin approximately 1.5–2.0 km from the abandoned smelting plantare contaminated with As. Similar to As distribution from othersmelting plants (Helsen, 2005), it was reported that SO2 and As2O3
were released from a stack and distributed to the soil throughatmospheric dispersion (KEC, 2009). In order to select a remediationprocess, national research projects are being conducted to selectoptimal remediation technologies.
In this study, the chemical extraction of soil under subcriticalconditions was investigated as one of the potential processes to treatAs-contaminated soils in the Janghang area. It was hypothesized thatthe elevated temperatures and pressures associated with subcriticalconditions would enhance the extraction of As due to the increasedsolubility of As compounds and the oxidation of As(+III)-bearingminerals under these conditions. Batch experiments at varioustemperatures with various types of washing reagents were performedto determine the optimal temperature to maximize the extraction ofAs from the soil.
2. Materials and methods
2.1. Chemicals
NaOH (98%, Dae Jung Chemical, Korea), citric acid (99.5%, OrientalChemical, Korea), EDTA (99.5%, Yakuri Pure Chemical, Japan), HNO3
(35%, DO Chemical, Korea), HCl (69–70%, Junsei Chemical, Japan), andacetic acid (99%, Dae Jung Chemical, Korea) were purchased and usedas received.
2.2. Sampling and characterization of soils
As-contaminated soil was sampled from a locationwithin 1 kmof anabandoned smelting plant in the Janghangarea. The samplewasdried atroom temperature out of the direct sunlight. The completely dried soilwas crushed using amortar and pestle and screened by a 2-mm sieve toremove any coarse particles. Particle size analysiswas conductedusing alaser particle size analyzer (Mastersizer 2000, Malvern, U.K.). Mineralsin the soil were qualitatively identified via X-ray diffraction (XRD)analysis using an X-ray diffractometer (RAD-3C, Rigaku, Japan). X-rayphotoelectron spectroscopy (XPS, Model K-Alpha, Thermo Scientific,Waltham,MA,USA) analysiswas performed to qualitatively identify theAs-containing forms in the soil. Loss on ignition was determined byweighing the sample before and after it was heated at 600±25 °C for3 h using amuffle furnace (Model 184A, Fisher Scientific, Pittsburgh, PA,USA). The pHof the soilwasmeasured by anAccumet 925pH/ionmeter(Fisher Scientific, Pittsburgh, PA, USA) after 5 g of soil were mixed with25 mL of deionized water and agitated for 1 h.
2.3. Elemental analysis and determination of chemical forms of As
Elemental analysis was conducted through an aqua regia extractionfollowing a Korean standard method (Korea Ministry of Environment,2009a). One milliliter of HNO3 and 3 mL of HCl were added to a 0.25-g
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sample, which was heated to 70 °C for 1 h with shaking, and 6 mL ofdeionized water were added to make the final solution for analyticaldetermination by an atomic absorption spectrophotometer (AAS, 5100ZL, Perkin Elmer, USA) (Korea Ministry of Environment, 2009a). Also,quantitative determination of the species of As, i.e., As(+III) and As(+V), was conducted by using anion extraction cartridges (Supelco,Bellefonte, PA, USA) (Yalçin and Le, 2001). After pre-conditioning withthe mixture of methanol and deionized water (50:50 v/v), 20 mL ofextracted samplewas pumped through the anion exchange cartridge ataflow rate of 1.5 mL/min using a peristaltic pump (Cole-Parmer, VernonHills, IL, USA). As(+V) was selectively retained on the anion exchangecartridge and the concentration of As(+III) in the cartridge effluentwasdetermined by the AAS. In order to determine the fractionation of As inthe soil, sequential extraction was also conducted according to themodified method of Wenzel et al. (2001) (Table 1).
2.4. Chemical extraction experiments
In the chemical extraction experiments, three different types ofwashing reagents were used, i.e., EDTA, citric acid, and NaOH. EDTAand citric acid, which are weak acids, are typical chelating agents, andNaOH is inexpensive and effective for As extraction (Semer andReddy, 1996; Peters, 1999; Mulligan et al., 2000; Dermont et al.,2008a,b). In order to evaluate washing efficiency, duplicate 250-mLErlenmeyer flasks containing 4 g of sediment and 80 mL of washingsolution were shaken at 250 rpm. Preliminary kinetic experimentswith 10 mMof the washing reagents showed that an equilibrium timeof 6 h is required for EDTA, citric acid, and NaOH to extract As from thesoil. To investigate the effect of temperature, the washing tempera-ture was set to 20, 50, and 100 °C using a water bath shaker (HB-205SW, Hanbaek, Korea). Chemical extraction experiments wereperformed at 150, 200, 250, and 300 °C, using a pre-designed,temperature-controlled, pressurized reactor system, including a500-mL main reactor, a temperature controller, a shaking speedcontroller, pressure controllers, and gage and gas sampling ports(Fig. 1). The temperature was set to 150, 200, 250 and 300 °C, and,accordingly, the internal pressure of the reactor wasmaintained at thevapor pressure of steam at those temperatures. Thus, the internalpressures of the reactor at 150, 200, 250, and 300 °C wereapproximately 4.7, 15, 39, and 85 atm (corresponding to 0.48, 1.52,3.95, and 8.61 MPa), respectively. Twelve grams of soil and 240 mL ofwashing solution were put into a reactor and shaken at 150 rpm. Theconcentration of the washing reagents was adjusted to 1, 10, and100 mM to determine the effect of concentration. After 6 h of shaking,the solution was separated by a GF/C filter and analyzed by the AAS.
Fig. 1. Schematic diagram of a temperat
Control experiments were also performed without the washingreagents under identical conditions.
3. Results and discussion
3.1. Characterization of soil
The soil showed an acidic pH of 5.1, and the loss on ignition (LOI)was 4.3%. The particle size distributions showed that clay, silt, andsand made up 17.7%, 77.1%, and 5.2% of the soil, respectively,indicating that the soil can be classified as silt by the U.S. Departmentof Agriculture Classification (Das, 2006). X-ray powder diffractionanalysis identified kaolinite, quartz, feldspar, and goethite as themajor minerals in the soil. The concentrations of metals in the soil aresummarized in Table 2. The total concentration of As was 75.5 mg/kg,which is much higher than the natural concentrations of As found insoil, which average 1.5 mg/kg (Sparks, 2003). Based on the Koreanregulation level (Table 2), the total concentration of As (75.5 mg/kg)was found to be about three times higher than the regulation level of25 mg/kg. The concentration of Pb was also slightly higher than theregulated level of 200 mg/kg. Except for As and Pb, all other metalshad concentrations that were much lower than the regulated levels.Chemical analysis using a sequential extraction procedure showedthat the concentration of As in phases 1–3 (ionically bound soluble,adsorbed, and amorphous and poorly crystalline hydrous oxide of Feand Al), which are relatively mobile phases, was 19.65 mg/kg, whichwas 25.4% of the total concentration of As in the soil (Table 3). Incontrast, 57.84 mg/kg, amounting to 74.6% of the total As concentra-tion, existed in phases 4–5 (crystalline Fe oxyhydroxide and residual),which is much more stable than other phases. By using anionexchange cartridges, it was shown that 68% and 32% of the As in thesoil existed as As(+III) and As(+V), respectively. Because theextractability of As from mineral phases is dependent upon themineral solubility and dissolution kinetics (Matera et al., 2003; Wangand Mulligan, 2008), X-ray diffraction (XRD) and scanning electronmicroscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analy-sis were conducted. However, the total concentration of As in soil(75.5 mg/kg)was low, it was very difficult to identify mineral forms ofAs in the soil. Alternatively, we performed XPS analyses to find outchemical forms of As in the soil. Although the peak was not sharp dueto the low concentration of As, XPS analysis showed that the peak ofthe spectrum (As 3d) was observed at 45.0 eV of binding energy,possibly representing As2O3 and R(alkyl)-AsOOH (Fig. 2). Based onthese results and previous results reported by KEC (2009), it can beinferred that As2O3 was released from the stack of the smelting plant
ure-controlled pressurized reactor.
Table 2Metal concentrations in the soil (unit: mg/kg).
As Cd Cu Pb Ni Zn
Extraction by aqua regia(Korean standard method)
75.5 2.5 89.1 218.9 7.8 55.1
Korean regulationfor soil contaminationa
25 4 150 200 100 300
a Standard for agricultural and public areas excluding plants, roads, and railroads(Korea Ministry of Environment, 2009b).
Fig. 2. XPS spectra (As 3d) of As-contaminated soil from Jang Hang area (X-ray source:monochromated Al Kα, spot size: 400 μm, Ar ion gun 1 keV).
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when it was in operation, accumulated in the nearby soil, and part ofthe deposited As2O3 was transformed to As(+V) in the soil over thefew last decades.
3.2. Chemical extraction at room temperature
The results of chemical extraction with citric acid, EDTA, and NaOHat room temperature (20±1 °C) for As removal are shown in Fig. 3. Aspreviously reported (Semer and Reddy, 1996; Peters, 1999; Mulliganet al., 2000; Dermont et al., 2008a,b), NaOH has been shown to extractAs from the soil efficiently. It was suggested that large amounts ofOH− can extract As, which exists as anions (arsenate or arsenite),from soil via a ligand exchange process (Jang et al., 2005). Theefficiency of the extraction of As using NaOH appears to increaselinearly with the NaOH concentration. Using a solution with aconcentration of 100 mM of NaOH, almost 26% of the As in the soilwas extracted (Fig. 3). In contrast, EDTA and citric acid did noteffectively extract As from the soil. Even with 100 mM citric acid andEDTA, only 6.3% of the As was extracted from the soil. Because EDTAand citric acid are chelating agents that extract cationic metals, suchas Cd, Cu, Zn, and Pb, the extraction of As is less effective (Peters,1999). Although chemical extraction with 100 mM of NaOH is mostfavorable for As-contaminated soil, these results indicate that theextraction efficiency of chemical extraction was limited at roomtemperature and that the washing process could not lower the Asconcentration in the soil sufficiently to meet the 25 mg/kg require-ment set by the Korean regulation (Table 2).
3.3. Chemical extraction under subcritical conditions
As temperature was increased, chemical extraction using the threereagents was enhanced. For each reagent, the extraction efficiencygradually increased as the temperature was elevated from roomtemperature to 200 °C. At each temperature, the extraction efficiencyalso increased as the concentration of washing reagent increased(Fig. 3). With 100 mM of citric acid, EDTA, and NaOH, 25.3, 28.4, and34.6 mg/kg of As were extracted at 200 °C, respectively, correspond-ing to the extraction of 33.5%, 37.6%, and 45.8% of the total As in thesoil, respectively (Fig. 3). The differences in the extraction efficienciesof the three reagents at room temperature were somewhat dimin-ished when the extraction was performed at 200 °C (Fig. 4).
Table 3Concentration of As from sequential extraction of the soil (unit: mg/kg).
Sequential phase Concentration
Phase 1: ionically bound soluble 3.08Phase 2: adsorbed 10.02Phase 3: amorphous and poorly crystallinehydrous oxide of Fe and Al
6.55
Phase 4: crystalline Fe oxyhydroxide 33.17Phase 5: residuals 24.67Sum of phases 1–5 77.47Total concentration by aqua regia extraction 75.5Recovery (%) 102.6
As the temperature was increased further to 250 and 300 °C, theextraction efficiency was greatly enhanced for each washing reagent.With citric acid, EDTA, and NaOH, the extraction efficiency of As at250 °C was 75–82%, 78–88%, and 74–91%, respectively (Figs. 3 and 4).No significant difference among the washing reagents was seen at250 °C, implying that the effect of temperature may be moreimportant than the choice of extracting reagent. For each extractingreagent, extraction efficiency increased as the concentration increasedfrom 1 mM to 100 mM. However, when extraction efficiencies werecompared for extractions at 150 and 200 °C, it appears that thedifferences in their efficiencies were reduced. At 300 °C, the extractionefficiency was enhanced even more, showing 95–103%, 94–103%, and96–102% for citric acid, EDTA, and NaOH, respectively. Consideringexperimental errors, these results suggest that As in the soil wasalmost completely extracted at 300 °C.
There are three possible explanations of the increase of extractionefficiency as temperature was increased. First, the increase oftemperature may increase the solubility of As-containing forms inthe soil. In As-contaminated soil, various types of As-containingminerals (e.g., As2O3, FeAsO4, and Ca3(AsO4)2) can exist in the soil.Because the extent and kinetics of dissolution of those minerals areproportional to temperature (Snoeyink and Jenkins, 1980), theelevated temperature may enhance the extraction of As from thesoil. Second, a high amount of H+ and OH− in subcritical water mayenhance the extraction of As from the soil. The solubility product ofwater (Kw) was increased from 10−14 at 25 °C to 10−11.64 and10−11.30 at 150 °C and 200 °C, respectively (Marshall and Franck,1981). The increased amount of H+ and OH− may enhance theextraction of As from soil by accelerating the dissolution of As-bearingforms. However, the effect of increased amounts of H+ and OH− onthe extraction of As could be limited at temperatures less than 200 °Cbecause Kw was not significantly increased at temperatures higherthan 200 °C (Marshall and Franck, 1981). Third, as shown in Figs. 3and 4, the extraction efficiency was enhanced significantly astemperature increased from 200 °C to 250 and 300 °C. The increasedefficiency of As extraction at 250 and 300 °C may be attributed to theoxidative dissolution of As(+III)-bearing minerals, such as As2O3 insoil (Pourbaix, 1974).
As2O3 þ 5H2O→2HAsO2−4 þ 8H
þ ð1Þ
Experiments for the chemical extraction of As were performed byusing a 500-mL pressurized reactor including 240 mL of chemicalsolution and 12 g of soil. When the temperature was increased, theinternal pressure increased as determined by the vapor pressure of
Temperature (oC)
0
As
conc
entr
atio
n (m
g/kg
)
0
20
40
60
80
1mM10mM100mM
As
conc
entr
atio
n (m
g/kg
)
0
20
40
60
80
As
conc
entr
atio
n (m
g/kg
)
0
20
40
60
80
(a) Citric acid
(b) EDTA
(c) NaOH
30025020015010050
Temperature (oC)
0 30025020015010050
Temperature (oC)
0 30025020015010050
1mM10mM100mM
1mM10mM100mM
Fig. 3. Concentrations of released As after washing the soil with (a) citric acid, (b) EDTA,and (c) NaOH at 20–300 °C. Data points and error bars represent averages and standarddeviations of samples from duplicate reactors, respectively.
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steam. Under the given conditions, the dissolution of oxygen from theair into the solution in the reactor could be significantly increased astemperature was elevated. Specially, when the temperature waselevated from 200 to 250 °C, the solubility of oxygen in water wasincreased very abruptly from 0.6–0.7 to 2.6–2.8 cm3/g (Pray et al.,1952). At 300 °C, the solubility of oxygen in water was estimated to be
approximately 8–9 cm3/g, according to extrapolation of the regres-sion curves given by Pray et al. (1952).Therefore, it is plausible thatthe increased amount of oxygen in the subcritical water can enhancethe oxidation of As(+III)-containing minerals (possibly As2O3) toarsenate(As(+V)) in solution at 250 and 300 °C. Regardless of thetype of washing chemicals, the species of As extracted from soil at 250and 300 °C was As(+V), which also supported the hypothesis that Aswas extracted from soil via oxidation under the given conditions.
When the soil contains 75.5 mg/kg of As, more than 50.5 mg/kg ofAs must be extracted from the soil to meet the 25 mg/kg level set byKorean regulation, corresponding to at least 67% washing efficiency.At 250 and 300 °C, regardless of reagent type, more than 70%extraction efficiency was achieved (Fig. 4). Therefore, in terms ofremediation of As-contaminated soil, 250 °C under subcritical condi-tions was the temperature needed to meet the regulation.
3.4. Extraction with deionized water under subcritical conditions
The results of chemical extraction of the As-contaminated soil at250 and 300 °C showed that the different types of washing reagentsdid not have notably different extraction efficiencies (Figs. 3 and 4). Itappears that the effect of temperature (specifically, 250 and 300 °Cunder subcritical conditions) is much more important in enhancingextraction efficiency, probably due to subcritical water oxidation of As(+III)-containing minerals (i.e., As2O3) at those temperatures. Toconfirm this analysis, the As-contaminated soil was washed withsubcritical water. As expected, the extraction efficiencies usingsubcritical water were very similar to those obtained by otherwashing reagents at the same temperatures. Although the extent ofincrease of As extraction by deionized water was less than that bywashing chemicals, the extraction of As by subcritical water graduallyincreased until 200 °C, and, thereafter, the extraction efficiencyincreased steeply (Fig. 5). At 200 °C, 20.5 mg/kg of As were extracted,corresponding to an extraction efficiency of 27%. As mentioned above,the increase of As extraction efficiency until the temperature reached200 °C was due mostly to the enhancement of the dissolution of theAs-bearing minerals at elevated temperatures and the H+ and OH−
formed under subcritical conditions. At 250 and 300 °C, 71 and 75 mg/kg of As were removed from the soil, showing that the extractionefficiency was 92% and 99%, respectively. These results clearly supportour hypothesis that the enhancement of extraction efficiency is due tothe subcritical water oxidation of As(+III)-bearing minerals atelevated temperature and water pressure under subcriticalconditions.
4. Conclusions
In this study, chemical extraction under subcritical conditions wasevaluated for remediating As-contaminated soil from the Janghangarea of South Korea. As the temperature increased to 250 and 300 °C,extraction efficiency was greatly enhanced regardless of the type ofwashing reagent, probably due to the enhancement of the dissolutionof As-bearing minerals at elevated temperature and to the subcriticalwater oxidation of As(+III)-bearing minerals. Subcritical waterextraction also showed 92% and 99% extraction of As from soil at250 and 300 °C, indicating that, at 250 and 300 °C, the effect ofelevated temperature and pressure were much more important forefficient extraction of As than the effect of type and concentration ofthe washing reagent. Because the extraction was conducted undersevere subcritical conditions, the changes of properties of the soilsafter chemical extraction are under investigation. We also plan toinvestigate the treatmentmethod of washing effluent for the design ofsoil washing process to treat As-contaminated soils in the Janghangarea. We will report them near future.
Citric Acid
As
rem
oval
effi
cien
cy (
%)
0
20
40
60
80
100 1 mM10 mM100 mM
(a) 150 oC
(c) 250 oC
(b) 200 oC
(d) 300 oC
As
rem
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%)
0
20
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As
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%)
0
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NaOHEDTA
Citric Acid NaOHEDTA
Citric Acid NaOHEDTA
Citric Acid NaOHEDTA
1 mM10 mM100 mM
Fig. 4. Extraction efficiency of As at 150–300 °C. Data points and error bars represent averages and standard deviations of samples from duplicate reactors, respectively.
3071S.-Y. Oh et al. / Science of the Total Environment 409 (2011) 3066–3072
Acknowledgments
This researchwas supported by the KoreaMinistry of Environmentthrough a research grant from the 2009–2010 Geo-AdvancedInnovative Action (GAIA) project.
0
As
conc
entr
atio
n (m
g/kg
)
0
20
40
60
80
30025020015010050
Temperature (oC)
Fig. 5. Concentrations of released As after washing the soil with deionized water at 20–300 °C. Data points and error bars represent averages and standard deviations ofsamples from duplicate reactors, respectively.
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