EXPERIMENTAL ASSESSMENT OF CHROMIUM IMMOBILIZATION IN CONTAMINATED SOIL BY IN SITU CHEMICAL REDUCTION

Transcription

1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 EXPERIMENTAL ASSESSMENT OF CHROMIUM IMMOBILIZATION IN CONTAMINATED SOIL BY IN SITU CHEMICAL REDUCTION DI PALMA L., D ONOFRI R., GUEYE M. T. and PETRUCCI E. Dipartimento di Ingegneria Chimica Materiali Ambiente - Sapienza Università di Roma, via Eudossiana , Roma, Italy. *luca.dipalma@uniroma1.it EXTENDED ABSTRACT As a result of its high water solubility, Cr (VI) may easily move into the soil, thus inducing groundwater pollution. Due to its relatively low toxicity, the extraction of Cr (III) is not generally required, while Cr (VI) removal represents the main environmental concern. Among the technologies available for Cr (VI) removal from polluted soils and groundwater, chemical reduction is known to remove Cr(VI) rapidly and effectively using reducing agent such as ferrous sulfate, sulfur dioxide, or sodium bisulfate followed by precipitation as Cr(III). The main disadvantages of this method are the low efficiency in the presence of low levels of Cr (VI) in the soil, and the costs of the chemicals used in the process. The aim of the present research was to compare the effectiveness of iron sulfate and nano zero valent iron in the chemical reduction of Cr (VI). The experiments were performed on a soil collected at an industrial site: it also resulted polluted by nickel, as a result of a long-term industrial activity. The experimental tests were performed under several operating conditions: the main parameter investigated were the amount of chemical with respect to chromium content, the presence of oxygen in the slurry, and the liquid (L) vs. solid (S) ratio. The soil was fully characterized after each tests, to perform mass balances, and sequential extractions were carried out to assess the metals mobility induced by the washing with the solution containing the reducing agent. Results show that iron sulfate successfully reduced Cr (VI): within 16 hours of treatment the residue amount of all the investigated metals was below the threshold allowed by Italian Environmental Regulation for an industrial reuse. Due to Cr (VI) solubilization, reduction and the following Cr (III) precipitation, in both cases, the treatment resulted in an increase of the amount of chromium bound to the oxide-hydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation. In addition, depending upon the L/S adopted in the tests, heavy metals (mainly Ni, Pb and Zn) solubilization was also observed during the experiments. In the tests performed using Fe 0 nanoparticles, prepared by iron chloride reduction with borohydride, the reduction of Cr(VI) was found to be time dependant, and increasing with the concentration of nanoparticles. Keywords: hexavalent chromium; soil remediation; chemical reduction; iron sulphate; nano zero-valent iron; carboxymethyl cellulose 1. INTRODUCTION A growing number of polluted sites are continuously identified in Italy: 57 of them are considered as the more dangerous and classified as Sites of National Interest (SIN), corresponding to about 3% of the whole Italian territory (about 1800 km 2 of marine, coastal and lake areas, and about 5500 km 2 of terrestrial areas). A strong contamination by the uncontrolled disposal of industrial wastes and residues has been identified as one of the major source of such huge pollution, thus determining a widespread contamination by heavy metals. In particular, a high concentration of chromium in areas close to

2 galvanic industries and steel mill is commonly found. Chromium in soil occurs primarily in one of two redox states, the immobile trivalent form, Cr(III), and the toxic, more mobile, hexavalent forms, chromate (CrO 4 2- ) or dichromate (Cr2O7 2- ). Cr(III) is a low toxicity nutrient for plant growth, instead the hexavalent form is more dangerous and human carcinogen (Kozuh et al., 2000). Bartlett (1991) showed that Cr(VI) is capable of moving towards groundwater due to its extreme mobility, and it is in thermodynamic equilibrium with the atmosphere. Cr (VI) is the product of oxidation of Cr(III) with atmospheric oxygen and its presence is greater into the earth crust. Since the natural oxidation of Cr(III) is extremely low, most of the Cr(VI) found in soil and groundwater results from pollution. The Cr(III) reactivity increase when the inert crystals and amorphous mineral are transformed in organic and hydroxide forms, smaller and more mobile. The equilibrium between the two chromium forms in soil depends upon soil physical and chemical characteristics. The oxidation process is only controlled by the reaction kinetics, due to Cr(III) species immobility and insolubility (Bartlett, 1991). Cr(III) tends to be strongly bound by soil humic acid polymers, and this affinity restricts the availability of Cr(III) to be oxidized and reduce the organic matter decomposition. Bartlett and James (1979) have reported that the presence of manganese oxide in soils favours trivalent chromium oxidation, thus increasing the hazards connected to hexavalent chromium contamination of groundwater. The various oxide surface characteristics and the amount of mobile Cr(III) in contact with the surface are the controlling parameters of the Cr(III) oxidation reaction. MnO 2 works, in fact, as an electron link between Cr(III) and the atmospheric oxygen, and it was found that the amount of oxidized Mn in soil is proportional to the amount of oxidized Cr(III) (Bartlett, 1991). As a result, soil Cr(III) oxidation capacity is strongly increased under acidic conditions in the presence of manganese dioxide (MnO 2). Several studies performed during the past two decades have already assessed that immobilization technologies can be successfully used for the remediation of heavy metal contaminated as an alternative to the most common extraction techniques, whose application is often limited by the potential toxicity of the most common chelating agents and the high overall costs of the treatment (Peters, 1999; Mulligan et al., 2001; Dermatas and Meng, 2003; Di Palma et al., 2003; Dermatas and Moon, 2007; Evangelou et al., 2007). In addition, solubilization of Cr(III) by organic chelant complexation, could result in an increase of its availability for oxidation to the hexavalent form (Bartlett, 1991; Di Palma et al., 2012). Furthermore, as a consequence of the extraction process, a strong modification of soil chemical and physical characteristics has been generally observed (Manouchehri et al., 2006; Di Palma, 2009) and this could affect the equilibrium between the two chromium species. However, Cr remains in place and, in the presence of a significant amount of Mn-oxides may undergo to reoxidation, thus becoming a health or regulatory concern at a later date (Seaman et al., 1999). Among the immobilization techniques, the in situ manipulation of redox status by chemical reduction using reactive solutions offers a promising alternative. This technique, generally deals with the reduction to Cr(III) followed by ph adjustment to neutrality to favour precipitation of Cr(OH) 3 or mixed oxyhydroxide phases (Singh et al., 2011). The main disadvantages of this method are the low efficiency in the presence of low levels of Cr (VI) in the soil, and the costs of the chemicals used in the process. The aim of the present research was to compare the effectiveness of iron sulphate and nano zero valent iron (nzvi) in the chemical reduction of Cr (VI). The experiments were performed on a soil collected at an industrial site in Northern Italy: it also resulted polluted nickel, as a result of a long-term industrial activity. The experimental tests were performed under several operating conditions: the main parameter investigated were the amount of reducing agent with respect to chromium content and the presence of oxygen in the slurry. The soil was fully characterized after each tests, to perform mass balances, and

3 sequential extractions were carried out to assess the metals mobility induced by the washing with the solution containing the reducing agent. 2. MATERIALS AND METHODS 2.1 Soil characterization The soil was a sandy-loamy soil, collected at an industrial site in Italy. Soil acid digestion was performed to determine the initial chromium content in soil, using hydrogen peroxide (30% v/v), concentrated hydrochloric acid and nitric acid (50% v/v) (all provided by Sigma Aldrich), according to EPA Method 3050b (Liu and Evett, 2002). 1 g of soil was dried at 110 C and placed in a test glass tube with a reflux system. After adding 10 ml of concentrated HCl, the sample was heated to 95 C and kept in agitation for 15 min. The test glass tube was cooled down to 25 C and 15 ml of HNO 3 were then added to the solution. The mix was then kept to 95 C for 2 hours and subsequently cooled down to 25 C. After the addition of 2 ml of H 2O and 10 ml of H 2O 2, the solution was heated at 95 C for 2 hours and then sampled to determine metal content, after filtration through a 0.45 µm Whatman membrane filter, by atomic absorption spectrophotometry, using an Agilent AA DUO 240 Fs instrument. Cr(VI) concentration was determined by the diphenylcarbazide colorimetric method (Bartlett, 1991) using a UV-VIS spectrophotometer. The initial Cr(III) and Cr(VI) concentration were mg/kg, and mg/kg respectively. The amount of MnO 2 into the investigated soil, determined according to the procedure described by Liu and Evett (2002) was mg/kg. Soil organic carbon content and organic matter were determined by the Walkley-Black method. Soil ph, cation exchange capacity (CEC), were determined using standard methods (Italian Environmental Regulation, 1999). ph was measured after mixing 10 g of soil samples with 25 ml of a 0.01 M solution of CaCl 2. The main chemical and physical characteristics of the soil are reported in Table 1. Table 1 Soil characterization Parameter Value ph 7.54 C.E.C (meq/100g) 9.6 Organic carbon (g/kg) Organic matter (g/kg) Mn 770 Cr Cu 25.9 Fe Ni Pb 21 Zn Since metal extraction effectiveness depend upon the leachability of the different metal form, preliminary sequential extraction using the Tessier s method were performed to investigate metals distribution into five fractions: exchangeable, bound to carbonate, Fe- Mn oxides, bound to organic matter and residual (Vilar et al., 2005). The exchangeable fraction was determined through extraction with 8 ml of 1 M MgCl 2 at ph 7 for 1 h. The fraction bound to carbonates was determined after extraction with 8 ml of 1 M NaOAc adjusted to ph 5 with acetic acid for 5 h. The fraction bound to oxides and hydroxides was determined after extraction with 20 ml of 0.04 M NH 2OH. HCl in 25% vol. acetic acid (ph=2) for 6 h at 96 C. The fraction bound to sediment organic matter was

4 determined after extraction with 3 ml of 0.02 M HNO 3 and 5 ml of 30% H 2O 2 (ph=2) for 2 h at 85 C, followed by 3 ml of 30% H 2O 2 (ph=2) for 3 h at 85 C and then 5 ml of 3.2 M NH 4OAc in 20% vol. HNO 3 diluted to 20 ml at room temperature for 30 min. The residual fraction was determined after digestion at 90 C with 25 ml of diluted aqua regia (50 ml HCl, 200 ml HNO 3 and 750 ml of distilled water) for 3 h. The results of sequential extractions of the soil are reported in Table 2. Table 2 Soil sequential extractions Metal Exchangeable Bound to carbonates Bound to oxideshydroxides Bound to the organic matter Residue Cr <0.1 < Ni <0.1 Mn < <0.1 Fe Pb Cu Zn < Experimental procedure In the tests performed using Fe(II) as reducing agent, the reducing solution was prepared by dissolving g of iron sulphate heptahydrate (Carlo Erba Reagents, Milano, Italy) in 10 ml of distilled water, after bubbling for 30 min with nitrogen. Nano zero valent iron particles were prepared from a 1 g/l Fe 2+ aqueous solution, by reacting with sodium borohydride (NaBH 4) at room temperature and in a free oxygen atmosphere, according to the procedure reported by He et al., As dispersing agent sodium CarboxyMethyl Cellulose (CMC) was used, at a CMC/Fe 2+ =0.005 mol/mol. Before use, deionized (DI) water and the CMC solution were purged with purified N 2 for 30 min to remove dissolved oxygen. A 0.86 g/l solution of nzvi was obtained, and use for the reduction tests. The average size of the obtained nanoparticles was 13.3 nm, with a standard deviation of 5 nm, very close to the size obtained in similar experiments (He and Zhao, 2007; Singh et al., 2012). The reduction tests with nzvi were performed in batch mode, by mixing 5 g of soil in an orbital shaker at 120 rpm with 50 ml of the reducing solution. The stoichiometric amount fo nzvi was calculated according to the following equation: 3 Fe 0 + Cr 2O H 2O 3 Fe Cr(OH) OH - Four different reducing solutions were prepared, by adding Fe(II) or Fe 0 at a stoichiometric concentration, or at selected excess concentration (x5, x10, x30) with respect to the Cr(VI) amount in the treated sample. The experiments were performed at room temperature (20±1 C) without any ph adjustment. In the tests performed using bivalent iron, the effectiveness of oxygen stripping before the treatment was also evaluated. At selected intervals of time, the soil sample was filtered through a 0.45 µm Whatman membrane filter, and the reaction stopped by washing the soil with distilled water. The concentration of Cr(VI) and metals in the liquid phase was determined by UV-VIS spectrophotometry and atomic absorption spectrophotometry, respectively, as previously described. The residue amount of metals in soil after treatment was determined by atomic absorption spectrophotometry, after acid digestion performing according to the procedure previously described, while Cr(VI) amount was determined with the diphenylcarbazide method after alkaline digestion according to the method EPA 3060A (US EPA, 1996).

5 Fe(II) concentration in the liquid phase was determined reflectometrically by means of a Merck specific analytical test based on a Ferrospectral 2.20 bipyridine reagent. All the experiments were performed in triplicate: when a standard deviation higher than 5% was calculated, further repetitions was performed until that target value was obtained basing on three values. 3. RESULTS AND DISCUSSION Figure 1 shows the results obtained in the tests performed using Fe(II) as a reducing agent. Figure 1 Results of the reducing tests with iron sulphate (a: x5 excess; b: x5 excess after oxygen stripping; c: x10 excess; d: x10 excess after oxygen stripping; e: x30 excess; f: x30 excess after oxygen stripping). Results show that the deoxygenation of the solution proved to be effective in enhancing the reduction efficiency, by preventing reagent consumption by reaction with oxygen. In all the tests, Cr(VI) removal was faster within the first 3-5 hours of treatment, then proceeded slowly. In the tests performed with x5 or x10 excess, equilibrium conditions were not attained during the experiments. Only at the largest excess (x30) after around 40 h of treatment, both in the presence and in the absence of oxygen, the residue Cr(VI) concentration achieved an almost constant value.

6 A different behaviour in the tests was observed for total Cr concentration in the soil. In all the tests, a quick reduction of Cr in soil was first observed, due to the initial solubilization of the hexavalent form. As a consequence of its reduction, that lead to the formation of low soluble chromium and iron oxides (Buerge and Hug, 1997), the chromium amount in the solid phase increased up to the initial value in the tests performed with x5 and x10 excess of reducing agent. Conversely, in the tests performed with the largest Fe(II) excess (x30 excess) a substantial reduction of also total chromium in soil was detected at the end of the treatment. This particular behaviour can be explained considering that, when a large excess of reducing agent was adopted, the ph of the liquid phase during mixing was always in the range between 5.3 and 5.8 in the test without oxygen removal and 5.1 to 5.6 in the test performed after oxygenation: at such conditions, the precipitation of the chromium and iron oxides formed was not completed. In the tests performed with x5 or x10 excess, ph trend was quite different, reaching alkaline conditions (up to 7.8) at the end of each experiment. In all the tests, the residue amount in the soil could allow the industrial reuse of the soil according to Italian Environmental Regulation (15 mg/kg, Italian Environmental Regulation, 2006). The tests with x30 excess without deoxygenation, and the test with x10 excess after deoxygenation, also ensured the civil reuse of the soil (2 mg/kg). The complete removal of Cr(VI) was obtained only after 45 h of treatment in the tests with x30 excess. The results of sequential extraction tests performed after the treatment (Table 3), due to Cr(VI) solubilization, reduction and the following Cr(III) precipitation, in both cases, the treatment determined an increase of the amount of chromium bound to the oxidehydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation. Table 3 Soil sequential extractions after treatment with Fe(II) (x30 after oxygen stripping; 72 h of treatment) Bound to Bound to the Bound to Exchangeable oxideshydroxides matter organic Residue Metal carbonates Cr Ni <0.1 Mn Fe Pb Cu Zn Table 3 also shows that the treatment determined a significant reduction of the heavy metals content in the soil, that accounted for about 15-20% for zinc and nickel and for about 30% for lead, while Cu solubilization was negligible. At the same time, the displacement of metals from the more liable to the residue fraction in the soil was observed. This result was consistent with the ph values observed in the x30 excess tests, and confirmed by the slight dissolution observed in the other tests, where alkaline conditions occurred (Di Palma and Ferrantelli, 2005). The results obtained in the tests performed using nzvi as a reducing agent are shown in Figure 2. The study reveals that treating polluted soil with a nzvi aqueous solution the Chromium removal was time dependant and increasing with Fe 0 concentration. The tests were prolonged for 180 minutes: in all the tests, after this time, Cr(VI) reduction to Cr(III)

7 achieved equilibrium conditions. In the test performed at the stoichiometric concentration, the Cr(VI) removal was about 57%: the residue amount in the soil is higher than the limit for both residential and industrial reuse according to Italian Regulation (Italian Environmental Regulation, 2006). Increasing the nzvi concentration, the amount of Cr(VI) in the soil was further reduced: using a five times excess of nzvi with respect to the stoichiometric concentration, the removal of Cr(VI) increased up to about 77%, while a further doubling of nzvi concentration allowed the reduction of more than 90% of the initial amount. However, in both cases, the residue level of Cr(VI) in the soil were not suitable for a civil reuse of the soil, while after the x10 excess, the industrial reuse could be allowed. Figure 2 Cr(VI) residue in the soil treated with nzvi To achieve a complete remediation of the soil, concerning its Cr(VI) content, it was necessary to treat the soil using the larger excess of reducing agent (30 times with respect to the hexavalent chromium amount). In this case a residue amount of less than 5 mg/kg was reached just after 30 minutes of treatment. After 60 minutes of treatment, residue Cr(VI) in the soil was not detected. As regards the residue amount of heavy metals after the treatment, the results reported in Table 4 shows that the reducing treatment did not significantly influenced the total metals concentration in the soil. Table 4 Metals content in the liquid phase after nzvi treatment (reaction time=180 min) Test Mn Cr Cu Fe Ni Pb Zn x x x x CONCLUSIONS The effectiveness of iron sulphate and nano zero valent iron in the chemical reduction of Cr (VI) in a contaminated soil was compared. After each test the soil was fully characterized in order to perform mass balances, and sequential extractions were carried out to assess the metal mobility induced by the reducing solution. Results of the treatment with iron sulphate shows that the residue amount Cr(VI) after 16 hours was below the limit of industrial reuse in accordance with Italian Environmental Regulation. Only after 45 h of treatment the complete Cr(VI) removal was achieved using

8 a large excess of Fe(II), and oxygen stripping was ensured before reagent addition. Due to Cr (VI) solubilization, reduction and the following Cr (III) precipitation the treatment resulted in an increase of the amount of chromium bound to the oxide-hydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation In both cases, a reduction of the heavy metals content in soil was observed, depending upon the ph of the slurry established during the treatment. In the tests performed using Fe 0 nanoparticles, obtained by iron sulphate reduction with borohydride, the reduction of Cr(VI) was time dependant and increasing with the concentration of iron nanoparticles. The reaction was faster than in the case of using Fe(II), though a large excess of nzvi was necessary to achieve the complete soil remediation according to the Italian Environmental Regulation within 1 h of treatment. References 1. Bartlett R.J. (1991), Chromium cycling in soils and water: links, gaps, and methods, Environ. Health Perspectives, 92, Bartlett R.J., James, B.R. (1979), Oxidation of chromium in soils, J. Environ. Quality, 8, Buerge I.J., Hug S.J., Kinetics and ph Dependence of Chromium(VI) Reduction by Iron(II), Environ. Sci. Technol., 31, Dermatas D., Moon D.H. (2006), Chromium leaching and immobilization in treated soils, Environ. Eng. Science, 23, Dermatas D., Meng X. (2003), Utilization of Fly Ash for Stabilization/Solidification of Heavy Metal Contaminated Soils, Eng. Geology, 70, Di Palma L. (2009), Influence of indigeneous and added iron on copper extraction from soil, J. Hazard. Mater., 170, Di Palma L., Ferrantelli P., Merli C., Biancifiori F. (2003), Recovery of EDTA and metal precipitation from soil flushing solutions, J. Hazard. Mater., B103, Di Palma L., Ferrantelli P. (2005), Copper leaching from a sandy soil: mechanisms and parameters affecting EDTA extraction, J. Hazard. Mater., B122, Di Palma L., Ferrantelli P., Medici F. (2005), Heavy metals extraction from contaminated soil: recovery of the flushing solution, J. Environ. Manage., 77, Di Palma L., Mancini D., Petrucci E. (2012), Experimental Assessment of Chromium Mobilization from Polluted Soil by Washing, Chem. Eng. Trans., 28, Evangelou M.W.H., Ebel M., Schaeffer A. (2007), Chelate assisted phytoextraction of heavy metals from soil. Effect, mechanism, toxicity, and fate of chelating agents, Chemosphere, 68, He, F., Zhao, D. (2007), Manipulating the Size and Dispersibility of Zerovalent Iron Nanoparticles by Use of Carboxymethyl Cellulose Stabilizers, Environ. Sci. Technol. 41, He, F., Zhao, D., Liu, J., Roberts, C.B. (2007), Stabilization of Fe-Pd Nanoparticles with Sodium Carboxymethyl Cellulose for Enhanced Transport and Dechlorination of Trichloroethylene in Soil and Groundwater. Ind. Eng. Chem. Res. 46, Italian Environmental Regulation, G.U. n. 185 of October 21 th Italian Environmental Regulation (2006), Environmental standards assessment, G.U. n. 88 of April 14 th Kozuh N., Stupar J., Gorenc B. (2000), Reduction and oxidation processes of chromium in soils, Environ. Sci. Technol., 34, Liu C., Evett J.B. (2002), Soil properties, Testing, Measurement, and Evaluation, 5 th ed., Prentice-Hall, New York, USA. 18. Manouchehri N., Besancon S., Bermond A. (2006), Major and trace metal extraction from soil by EDTA: equilibrium and kinetic studies, Anal. Chim. Acta, 559, Mulligan C.N., Yong R.N., Gibbs B.F. (2001), Remediation technologies for metal contaminated soils and groundwater: an evaluation, Eng. Geol., 60, Peters R.W. (1999), Chelant extraction of heavy metals from contaminated soils, J. Hazard. Mater., B66, Singh R., Misra V., Singh R. P. (2011), Synthesis, characterization and role of zerovalent iron nanoparticle in removal of hexavalent chromium from chromium-spiked soil, Journal of Nanoparticle Research, 13,

9 22. Singh, R., Misra, V., Singh, R. P. (2012), Removal of Cr(VI) by Nanoscale Zero-valent Iron (nzvi) From Soil Contaminated with Tannery Wastes, Bull. Environ. Contam. Toxicol. 88, U.S. EPA (1996), Method 3060A: Alkaline digestion for hexavalent chromium. 24. Vilar, S., Gutierrez A., Antezana, J., Carral, P., Alvarez, A. (2005), A comparative study of three different methods for the sequential extraction of heavy metals in soil. Toxicol. Environ. Chem., 87, 1-10.