Water Air Soil Pollut (2009) 197:49–60 DOI 10.1007/s11270-008-9790-0
Reduction of Hexavalent Chromium in Soil and Ground Water Using Zero-Valent Iron Under Batch and Semi-Batch Conditions Débora V. Franco & Leonardo M. Da Silva & Wilson F. Jardim
Received: 27 March 2008 / Accepted: 24 June 2008 / Published online: 31 July 2008 # Springer Science + Business Media B.V. 2008
Abstract Chemical remediation of soil and groundwater containing hexavalent chromium (Cr(VI)) was carried out under batch and semi-batch conditions using different iron species: (Fe(II) (sulphate solution); Fe0G (granulated elemental iron); ZVIne (nonstabilized zerovalent iron) and ZVIcol (colloidal zerovalent iron). ZVIcol was synthesized using different experimental conditions with carboxymethyl cellulose (CMC) and ultra-sound. Chemical analysis revealed that the contaminated soil (frank clay sandy texture) presented an average Cr(VI) concentration of 456±35 mg kg−1. Remediation studies carried out under batch conditions indicated that 1.00 g of ZVIcol leads to a chemical reduction of ∼280 mg of Cr(VI). Considering the fractions of Cr(VI) present in soil (labile, exchangeable and insoluble), it was noted that after treatment with ZVIcol (semi-batch conditions and pH 5) only 2.5% of these species were not reduced. A comparative study using iron species was carried out in order to evaluate the reduction
D. V. Franco (*) : W. F. Jardim Institute of Chemistry, LQA—UNICAMP, Cidade Universitária Zeferino Vaz, 13083-970 Campinas, SP, Brazil e-mail:
[email protected] L. M. Da Silva Department of Chemistry, FACESA—UFVJM, Rua da Glória 187, 39100-000 Diamantina, MG, Brazil
potentialities exhibited by ZVIcol. Results obtained under batch and semi-batch conditions indicate that application of ZVIcol for the “in situ” remediation of soil and groundwater containing Cr(VI) constitutes a promising technology. Keywords Contaminated soil . Hexavalent chromium . Carboxymethyl cellulose . Colloidal zerovalent iron . Chromium immobilization
1 Introduction Chromium and other heavy metals can contaminate soil and groundwater by means of several anthropogenic processes involving industrial activities, e.g., electroplating process, wood, pulp and tannery processing, corrosion inhibition and pigment production (Tokunaga et al. 2001; Xu and Zhao 2007). Chromium can be considered a pollutant in its different oxidation states (Cr(0)–Cr(VI)) (Calder 1988; Palmer and Wittbrodt 1991). However, in natural environments, the more stable forms of chromium are available as soluble and insoluble compounds containing the Cr(IV) and Cr(III) species (Calder 1988; Palmer and Wittbrodt 1991). Hexavalent chromium (Cr(VI)) presents the higher standard oxidation potential and exhibits toxic and carcinogenic properties for different biological systems (Calder 1988; Palmer and Wittbrodt 1991; Kimbrough et al. 1999).
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According to the literature (Calder 1988; Palmer and Wittbrodt 1991) most compounds containing the Cr(III) species present a very low solubility in water and, therefore, their transport in soil and groundwater is limited when compared with Cr(VI) species. Besides, the toxicity exhibited by compounds containing Cr(III) is considerably lower than that of compounds containing Cr(VI) (Calder 1988; Palmer and Wittbrodt 1991; Kimbrough et al. 1999). The anthropogenic occurrence of chromium in different polluted environments is regulated and controlled by the different environmental protection agencies around the world. For instance, the U.S. Environmental Protection Agency (EPA) established a maximum concentration value (MCV) for total chromium in potable water of 0.1 mg l−1, while in Brazil the National Sanitary Agency (ANVISA— resolution MS n° 518) established a MCV for total chromium in potable water of 0.05 mg l−1. Hexavalent chromium, in the anionic form, is soluble in water at different pH values (Calder 1988; Palmer and Wittbrodt 1991); the chromate anion (CrO42−(aq)) is the predominant form of Cr(VI) in natural environments presenting a pH in the range of 6 to 9. On the contrary, the ion Cr(III) remains stable in natural environments as an insoluble hydroxide (Cr (OH)3(S)) (Calder 1988; Palmer and Wittbrodt 1991). The “in situ” chemical reduction of soil containing Cr(VI) following the direct application of the reductant (solution or suspension) has been reported in the literature using Fe(II) and ZVIcol (Tokunaga et al. 2001; Su and Ludwig 2005; Seaman et al. 1999; Ponder et al. 2000; EPA 2000). When compared to the passive redox remediation process using permeable reactive barriers that uses Fe0, the alternative remediation technology comprising the direct application of the redox solution onto the soil containing Cr(VI) can offer several advantages (He et al. 2007). For instance, the redox agent solution can be applied directly in the aquifer zone without further modifications in the natural flux of the groundwater, thus reducing costs and remediation time (He et al. 2007). Studies have revealed that Cr(VI) can be reduced “in situ” and precipitated as an insoluble hydroxide compound (Cr(OH)3 and/or FexCr1-x(OH)3) following the direct application in soil of a suspension containing ZVIcol (Xu and Zhao 2007; Ponder et al. 2000). Some reports (He et al. 2007; He and Zhao 2005; Schrick et al. 2004) revealed that the direct applica-
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tion of a suspension containing ZVIcol in soil can be accompanied by an undesirable agglomeration of the ZVI particles. In this case both the redox power and the mobility in soil of the redox agent are considerably reduced or inhibited due to particle agglomeration. Several studies have been carried out in order to obtain stable suspensions of metallic particles (e.g. Au0, Ag0, Fe0 and iron oxide) (Xu and Zhao 2007; He et al. 2007; Schrick et al. 2004; Kataby et al. 1997, 1999; Sun and Zeng 2002; Kim et al. 2003; Si et al. 2004; Magdassi et al. 2003) and different stabilizing agents have been used to prevent agglomeration of the iron oxide nanoparticles, including thiols, carboxylic acids, surfactants and different polymers (Xu and Zhao 2007; He et al. 2007; He and Zhao 2005; Schrick et al. 2004; Kataby et al. 1997, 1999; Sun and Zeng 2002; Kim et al. 2003). Despite of these efforts, only a few stabilizing agents are indeed available for stabilization of the ZVI particles when this species is devoted to environmental applications, since for in situ application only environmentally friendly species that result in a low cost-effect ratio for the remediation process are viable alternatives (He and Zhao 2005; He et al. 2007). An ideal stabilizing agent for ZVI nanoparticles must present the following characteristics (He and Zhao 2005; He et al. 2007): (1) be able to interact specifically with the ZVIcol nanoparticles and to suppress the agglomeration process; (2) be environmentally friendly; (3) low cost and (4) provide a high mobility of the ZVIcol nanoparticles in the soil microstructure. Most of these pre-requisites can be indeed attained in practice using stabilizing agents based on natural polymers (e.g. amide, modified amide, alginates, xantanes, etc) (He and Zhao 2005; He et al. 2007). In fact, these species have been used with success during preparation and stabilization of supermagnetic iron oxide nanoparticles (SPIONs), Fe0 and Ag0 (Xu and Zhao 2007; He et al. 2007; He and Zhao 2005; Schrick et al. 2004; Kataby et al. 1997, 1999; Sun and Zeng 2002; Kim et al. 2003; Si et al. 2004; Magdassi et al. 2003). Concerning the laboratory preparation of stable suspensions containing metallic nanoparticles, the synthesis assisted by ultra-sound has proved to be a powerful tool in order to obtain ZVI nanoparticles presenting a low average size and, in some cases, causing the formation of porous ZVI particles with
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very high active surface area for the redox process (Suslick et al. 1996). This paper describes a comparative redox treatment study using different iron species (Fe(II) as well as macro and nanoparticles of Fe0) for the reduction of the hexavalent chromium present in soil and groundwater. The remediation study was carried out under batch (slurry reactor) and semi-batch (packed bed column reactor) conditions.
2 Experimental 2.1 Soil Analysis Chemical analysis of the contaminated site (an industrial waste landfill located in Brazil) revealed an average total chromium concentration in groundwater and soil (frank clay sandy texture) of 550± 12 mg dm−3 and 456±35 mg kg−1, respectively. Hexavalent chromium extracted from groundwater and soil was analyzed via the colorimetric technique using the diphenylcarbazide complexation procedure (maximum absorption at 540 nm) following the method 7196A of EPA. Determination of total Cr and Fe was carried out using atomic absorption (AAS) according to method 3050B of EPA. Analytical grade chemicals were used throughout: ferrous sulphate (FeSO4·7H2O—Synth); sodium boronhydride (NaBH4—Acros Organic); sodium carboxymethyl cellulose (CMC—Synth); sec-diphenylcarbazide (Synth); granulated iron (Fe 0 G — Aldrich) and acetone (Mallinkrodt). 2.2 Synthesis of Zero-valent Iron (ZVI) Non-stabilized (ZVIne) and stabilized (ZVIcol) ZVInanoparticles were synthesized in the aqueous phase via chemical reduction of a Fe(II) solution in the absence and the presence of CMC and ultra-sound. In all cases, NaBH4 was used as the reductant (Xu and Zhao 2007; He et al. 2007; He and Zhao 2005; Schrick et al. 2004; Suslick et al. 1996). 2.2.1 Synthesis of ZVIcol A Fe(II) solution prepared using FeSO4·7H2O was added to a reaction flask containing the CMC solution in order to obtain a colloidal suspensions of Fe0 in the
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desired concentration: 1–2 g l−1 Fe0 stabilized with 0.25% CMC (w/w). The flask containing the colloidal suspension was allowed to stand for 20 min. After that, the ZVIcol suspensions were prepared by adding NaBH4 directly to the reaction flask using the [Fe(II)]: [NaBH4] molar ratio of 1:2. This procedure was assisted by ultra-sound for 5 min. 2.2.2 Synthesis of ZVIne ZVIne suspensions were prepared as described previously (see item 2.2.1), except for the fact that CMC was not used. Figure 1 shows the different macroscopic appearance presented by the ZVIcol and the ZVIne suspensions. 2.3 Chemical Remediation of Groundwater Containing Hexavalent Chromium via Redox Process Using ZVI Under Batch Conditions Comparative batch experiments were carried out in order to evaluate the redox performance for the remediation process exhibited by ZVIcol in comparison with the other iron species (Fe0G and ZVIne). A study concerning the performance of Fe(II) for the redox remediation process was also evaluated in the presence and in the absence of CMC (see Table 1). In this study a fixed volume of contaminated groundwater was added to the solutions containing the reductant in order to obtain the appropriate molar ratio to result in [Cr(VI)]=1,000 mg l−1 (see Table 1). Fe(II) solutions and ZVI suspensions were placed in an orbital shaker (120 rpm for 24 h). All suspensions were centrifuged (3,600 rpm for 10 min) and filtered using a cellulose acetate membrane (0.2 μm). The supernatant was characterized based on the parameters: pH, redox potential (EH), [Cr(VI)] and [Cr(total)]. 2.4 Chemical Remediation of Soil Containing Hexavalent Chromium via Redox Process Using ZVI Under Semi-Batch Conditions Semi-batch studies were carried out to investigate the overall removal rate of Cr(VI) under flow conditions using a packed bed column reactor containing the contaminated soil. A scheme representing the experimental setup is presented in Fig. 2. The vertically disposed column reactor (height: 5.5 cm, diameter: 3.55 cm and volume of 52.9 ml) was
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Water Air Soil Pollut (2009) 197:49–60
Fig. 1 Macroscopic appearance presented by the ZVIcol and the ZVIne suspensions. a ZVIne (absence of ultrasound). b ZVIne (presence of ultrasound). c ZVIcol (absence of ultrasound). d ZVIcol (presence of ultrasound)
carefully filled with previously dried and homogenized contaminated soil. The density ρ(packed soil) was ≅1.4± 0.2 g ml−1, thus confirming reproducible conditions concerning the packing procedure was rather good. Table 1 Experimental conditions adopted during chemical reduction of hexavalent chromium using different iron species Reductanta Fe(II) Fe(II) + CMC Fe0G ZVIne—natural pH ZVIne—pH 5 ZVIcol—natural pH ZVIcol—pH 5
a
Reductant mass/g 0.20 0.20 2.50 0.20 0.20 0.20 0.08 0.16 0.20
[Cr(VI)]:[Red] 1:3 1:3 1:50 1:4 1:4 1:4 1:1.5 1:3 1:4
Stoichiometric ratios for the redox process: Cr(IV):Fe(II)=1:3 and Cr(VI):Fe0 =1:1.
Before the experiments, the packed soil samples were slowly saturated under down flow conditions using synthetic ground water (SGW) pumped at a volumetric flow rate (G) of 9.00±0.60 ml h−1. SGW was synthesized to obtain the following composition (in mg l−1) (Seaman et al. 1999): 1.00 of Ca2+; 0.37 of Mg2+; 0.21 of K+; 1.40 of Na+ and 0.73 of SO42−. The volume of SGW filling the permeable porous soil microstructure was defined as the pore volume (PV) of the packed soil (Seaman et al. 1999). In the light of this approach, an average PV of 25.5±1.9 ml was obtained. Application of Darcy’s law using the falling head test (Domenico and Schwartz 1979) revealed a hydraulic conductivity of the contaminated soil of (3.78±0.71)×10−2 cm h−1. Soil samples containing Cr(VI) were treated under semi-batch conditions using different suspensions of ZVIcol. The [Cr(VI)]:[ZVIcol] molar ratios of 1:4 and 1:8 were used throughout. These molar ratios were
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Fig. 2 Scheme showing the experimental setup used in studies carried out using ZVI under semi-batch conditions
chosen considering a packed soil mass of 70.0±3.2 g. In all cases, due to the instability of the redox suspension in an aerated medium (solution exposed to the atmosphere) the suspension was prepared immediately before its application. ZVIcol suspensions were pumped at 18.0±0.60 ml h−1. In all cases the suspension was pumped using a model Minipuls 3 Peristaltic Pump from Gilson. Aliquots of the aqueous phase were withdrawn at the outlet of the packed bed column as functions of the treatment time (see Fig. 2). These aliquots were filtered using a cellulose acetate membrane (0.45 μm) and measurements of pH, EH and [Cr(VI)] were promptly carried out. 2.5 Extraction of the Residual Hexavalent Chromium Present in Soil after the Redox Treatment Using ZVIcol in Semi-Batch Conditions Three extractions for each soil sample were carried out by gradually increasing the strength of the extractor reagent (James et al. 1995). The first extraction was carried out using distilled water (pH 5.7) in order to obtain the soluble and labile chromium present in soil. The second one was carried out using phosphate buffer (pH 7.0) in order to determine the amount of chromium available in the exchangeable form (the more strongly adsorbed
fraction of chromium present in soil). The fraction defined as total hexavalent chromium, Cr(VI)total, was obtained from a hot extraction procedure (80–90°C) using a solution containing NaOH and CaCO3 (pH> 12) as the extractor phase. The fraction of the hexavalent chromium defined as non-exchangeable (insoluble), which represents the amount of Cr(VI) present as a precipitate and/or chemically adsorbed in soil, was obtained by subtracting the amount of Cr (VI)labile plus Cr(VI)exchangeable from Cr(VI) total.
3 Results and Discussion 3.1 Chemical Reduction and Immobilization of Chromium Present in Groundwater Under Batch Conditions A comparative remediation study using different iron species (Fe(II), ZVIne, ZVIcol and Fe0G) was carried out under batch conditions in order to evaluate their redox performance for chemical reduction of Cr(VI). Analysis of the data presented in Table 2 reveals that lower pH values were obtained using the Fe(II) solution. In the other cases involving the different iron species one can observe that pH changed from 4 to 9. Analysis of the EH-values shows that ZVIcol
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Table 2 Parameters obtained in the redox treatment process carried out under batch conditions (t=24 h; 120 rpm and 25°C) Experiment Reacting medium
pH (final)
1 2 3 4 5 6 7 8 9 10 11
6.00±0.01 5.90±0.01 6.10±0.12 2.35±0.15 2.30±0.10 7.30±0.05 6.30±0.06 8.60±0.06 3.90±0.60 4.60±0.60 4.60±0.60
Distilled water Distilled water + CMC + NaBH4 Fe0G (30×) Fe(II) (1×) Fe(II) + CMC (1×) ZVIne—pHnat (4×) ZVIne—pH 5 (4×) ZVIcol—pHnat (4×) ZVIcol—pH 5 (1.5×) ZVIcol—pH 5 (3×) ZVIcol—pH 5 (4×)
EH (final)/mV [Cr(VI)]/mg l−1 [Cr(total)]/mg l−1 [Fe(total)]/mg l−1
presents better reducing conditions in the reaction medium. It is desirable that the reduction of Cr(VI) present in soil and groundwater is accompanied by immobilization in soil of the corresponding insoluble reduced chromium species. The efficiency presented by ZVI for the remediation of groundwater containing Cr(VI) can be evaluated comparing the different findings
Fig. 3 Experimental findings (chromium fractions) obtained after application of ZVI for the reduction of groundwater containing Cr (VI)
496±30 476±30 470±18 636±7 627±5 390±28 471±17 299±31 700±35 380±16 375±8
932±25 908±25 957±25
