- Email: [email protected]
Hexavalent chromium reduction in contaminated soil: A comparison between ferrous sulphate and nanoscale zero-valent iron
Accepted Manuscript Title: HEXAVALENT CHROMIUM REDUCTION IN CONTAMINATED SOIL: A COMPARISON BETWEEN FERROUS SULPHATE AND NANOSCALE ZERO-VALENT IRON Author: L. Di Palma M.T. Gueye E. Petrucci PII: DOI: Reference:
S0304-3894(14)00633-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.07.058 HAZMAT 16151
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
19-2-2014 27-7-2014 28-7-2014
Please cite this article as: L.D.I. PALMA, M.T. GUEYE, E. PETRUCCI, HEXAVALENT CHROMIUM REDUCTION IN CONTAMINATED SOIL: A COMPARISON BETWEEN FERROUS SULPHATE AND NANOSCALE ZERO-VALENT IRON, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.07.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
HEXAVALENT CHROMIUM REDUCTION IN CONTAMINATED SOIL: A COMPARISON BETWEEN FERROUS SULPHATE AND NANOSCALE ZERO-VALENT
DI PALMA L.*, GUEYE M. T., PETRUCCI E.
ip t
IRON
Eudossiana 18 – 00184, Roma, Italy.
an
ABSTRACT
us
*[email protected]; tel: +390644585571
cr
Dipartimento di Ingegneria Chimica Materiali Ambiente - Sapienza Università di Roma, via
M
Iron sulphate (FeSO4) and colloidal nano zero-valent iron (nZVI) as reducing agents were compared, with the aim of assessing their effectiveness in hexavalent chromium [Cr(VI)] removal
d
from a contaminated industrial soil. Experiments were performed on soil samples collected from an
te
industrial site where a nickel contamination, caused by a long-term productive activity, was also
Ac ce p
verified. The influence of reducing agents amount with respect to chromium content and the effectiveness of deoxygenation of the slurry were discussed. The soil was fully characterized before and after each test, and sequential extractions were performed to assess chemico-physical modifications and evaluate metals mobility induced by washing.
Results show that both the reducing agents successfully lowered the amount of Cr(VI) in the soil below the threshold allowed by Italian Environmental Regulation for industrial reuse. Cr(VI) reduction by colloidal nZVI proved to be faster and more effective: the civil reuse of soil [Cr(VI)<2 mg/kg] was only achieved using colloidal nZVI within 60 minutes adopting a nZVI/Cr(VI) molar ratio of 30. The reducing treatment resulted in an increase in the amount of chromium in the oxidehydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation. 1
Page 1 of 24
In addition, a decrease of Nickel (Ni) and Lead (Pb) content in soil was also observed when acidic conditions were established.
ip t
Keywords: hexavalent chromium; soil remediation; chemical reduction; ferrous sulfate; nano zero-
cr
valent iron; carboxymethyl cellulose
HIGHLIGHTS:
us
- FeSO4 and colloidal nZVI are compared in the remediation of an industrial soil contaminated by
an
hexavalent chromium
- Colloidal nZVI resulted in a faster and more efficient Cr(VI) reduction with respect to FeSO4
M
- The effect of the reduction treatment on metal mobilization and bioavailability was assessed by
d
sequential extractions
Ac ce p
te
- The mechanisms and the factors affecting the process are discussed
1. INTRODUCTION
A growing number of polluted sites are continuously identified in Italy: 57 of them have been first considered as the most dangerous and classified as Sites of National Interest (SIN), corresponding to about 3% of the whole Italian territory (about 1800 km2 of marine, coastal and lake areas, and about 5500 km2 of terrestrial areas). Because of adverse and long-term effects on human health and the environment, heavy metals originated by the uncontrolled disposal of industrial wastes and residues, are commonly considered among the pollutants of greatest concern [1]. In recent years a high concentration of chromium in areas close to galvanic industries and steel mill has been verified [2]. As known, Chromium in soil occurs primarily in two different redox states: the immobile trivalent 2
Page 2 of 24
form, Cr(III), and the more mobile hexavalent forms, as chromate (CrO42-) or dichromate (Cr2O72-). While Cr(III) is a low toxicity nutrient for plant growth, the hexavalent form is a well known dangerous species and human carcinogen [3].
ip t
It has been shown that Cr(VI) is the oxidation product of Cr(III) reaction with atmospheric oxygen: as the natural oxidation of Cr(III) is extremely low, most of the Cr(VI) found in soil and
cr
groundwater results from pollution [4]. The equilibrium between the two chromium forms in soil depends upon soil physical and chemical characteristics. The oxidation process is only controlled
us
by the reaction kinetics, due to Cr(III) species immobility and insolubility [5]. Cr(III) tends to be
an
strongly bound by soil humic acid polymers, and this affinity restricts the availability of Cr(III) to be oxidized. However, the Cr(III) reactivity increases when the inert crystals and amorphous
M
minerals are transformed to organic and hydroxide forms, which are smaller and more mobile [6]. It has been reported that the presence of manganese oxide in soils favours trivalent chromium
d
oxidation, thus increasing the hazards connected to hexavalent chromium contamination of
te
groundwater [7]. MnO2 works, in fact, as an electron link between Cr(III) and the atmospheric oxygen, and it was found that the amount of MnO2 reduced in soil is proportional to the amount of
Ac ce p
oxidized Cr(III) [5]. As a result, soil Cr(III) oxidation capacity is strongly increased under acidic conditions in the presence of manganese dioxide (MnO2). Several studies performed during the past two decades have already assessed that immobilization technologies can be successfully used for the remediation of contaminated soil [8-9] and groundwater [10]. They can be used as an alternative to the most common extraction techniques [11-12], that commonly involve the use of potentially toxic chelating agents. Moreover, the high overall costs of the extraction treatment, mainly determined by the cost of the chelating agent itself [13], its complexation by soil organic matter [14], or by the needs of enhancing mobilization [15], often limit its full scale application. In addition, mobilization of Cr(III) by organic chelant complexation, could result in an increase of its availability for oxidation to the hexavalent form [2], especially in the presence of a significant amount of Mn-oxides [16]. 3
Page 3 of 24
Furthermore, as a consequence of the extraction process, a strong modification of soil chemical and physical characteristics generally occurs [17-18] and this could affect the equilibrium between the two chromium species.
ip t
Among the immobilization techniques, the in situ manipulation of redox status by chemical reduction using reactive solutions offers a promising alternative for the treatment of contaminated
cr
soil. This technique generally deals with the reduction to Cr(III), providing a source of electrons, followed by pH adjustment to neutrality to favour precipitation of Cr(OH)3 or mixed oxyhydroxide
us
phases [19]. To this aim several studies in the recent years have investigated the use of nano zero
an
valent iron [20-21] or bivalent iron [22-23] as ferrous sulphate. However, most of them were performed in the aqueous phase [24] or in spiked soil [25] and additional studies are required before
M
implementing this technology on a real scale.
The aim of the present research was to compare the effectiveness of iron sulfate and colloidal nano
d
zero valent iron (nZVI) in the chemical reduction of Cr(VI). The experiments were performed with
te
soil samples collected at an industrial site in Northern Italy. The tests were performed under several operating conditions: the main parameter investigated was
Ac ce p
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 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 Materials
The soil was a sandy-loamy soil, collected at an industrial site in Italy, at depth between 0.50 and 2.50 m. Further information about sampling are not available, due to not disclosure agreement. 4
Page 4 of 24
In the tests performed using Fe(II) as reducing agent, the reducing solution was prepared by dissolving 0.548 g of iron sulfate 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
ip t
prepared from a 1 g/L Fe2+ aqueous solution, by reacting with sodium borohydride (NaBH4) at room temperature and in a free oxygen atmosphere [26]. As dispersing agent sodium CarboxyMethyl
cr
Cellulose (CMC) was used, at a CMC/Fe2+=0.005 mol/mol. Before use, deionized (DI) water and the CMC solution were purged with purified N2 for 30 min to remove dissolved oxygen. A 0.86 g/L
us
solution of colloidal nZVI was obtained, and use for the reduction tests. The average size of the
an
obtained nanoparticles (determined with a Nanoparticle Size Analyzer 90Plus, Brookhaven, Wien, Austria) was 13.3 nm, with a standard deviation of 5 nm, very close to the size obtained in similar
M
experiments [27-28].
All reagents were supplied from Sigma Aldrich and used as commercially available without any
te
d
further purification
Ac ce p
2.2 Experimental procedures
2.2.1 Soil characterisation
Soil organic carbon content and organic matter were determined by the Walkley-Black method. Soil pH, cation exchange capacity (CEC), and MnO2 content were determined using standard methods [29]. pH was measured after mixing 10 g of soil samples with 25 ml of a 0.01 M solution of CaCl2. Soil acid digestion was performed to determine the initial chromium and heavy metals content in soil, using hydrogen peroxide (30% v/v), concentrated hydrochloric acid and nitric acid (50% v/v) according to EPA Method 3050B [30]. 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 HNO3 were then added to the solution. The mix was then kept to 95°C for 2 hours and subsequently cooled 5
Page 5 of 24
down to 25°C. After the addition of 2 mL of H2O and 10 mL of H2O2, 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.
ip t
Since metal extraction effectiveness depends upon the leachability of the different metal forms [31], sequential extractions using the Tessier’s method were performed to investigate metals distribution
cr
into five fractions: exchangeable, bound to carbonate, Fe-Mn oxides, bound to organic matter and residual [32].
us
The exchangeable fraction was determined through extraction with 8 mL of 1 M MgCl2 at pH 7 for
an
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
M
determined after extraction with 20 mL of 0.04 M NH2OH.HCl in 25% vol. acetic acid (pH=2) for 6 h at 96°C. The fraction bound to organic matter was determined after extraction with 3 mL of 0.02
d
M HNO3 and 5 mL of 30% H2O2 (pH=2) for 2 h at 85°C, followed by 3 mL of 30% H2O2 (pH=2)
temperature for 30 min.
te
for 3 h at 85°C and then 5 mL of 3.2 M NH4OAc in 20% vol. HNO3 diluted to 20 mL at room
Ac ce p
The residual fraction was determined after digestion at 90°C with 25 mL of diluted aqua regia (50 mL HCl, 200 mL HNO3 and 750 mL of distilled water) for 3 h.
2.2.2 Reduction tests
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 of nZVI was calculated according to the following equation: 3 Fe0 + Cr2O72- + 7 H2O → 3 Fe2+ + 2 Cr(OH)3 + 8 OHFour different reducing solutions were prepared, by adding Fe(II) or Fe0 at a stoichiometric concentration, or at selected molar ratios with respect to the Cr(VI) amount in the sample concentration (5:1, 10:1, 30:1). 6
Page 6 of 24
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.
ip t
At selected times, aliquot of the suspension was sampled and filtered through a 0.45 µm Whatman membrane filter, and the reaction stopped by washing the soil with distilled water. The liquid phase
cr
was analysed for pH, redox potential and metal content. Results were reported as mg/kg, considering the L/S ratio adopted in the each test.
us
All the experiments were performed in triplicate: when a standard deviation higher than 5% was
an
calculated, further repetitions were performed until that target value was obtained basing on three
M
values.
2.3 Analyses
d
The metal content of soil was determined by atomic absorption spectrophotometry, using an Agilent
te
AA DUO 240 Fs instrument, after acid digestion according to the procedure previously described. Cr(VI) amount of the liquid phase was determined with the diphenylcarbazide method after alkaline
Ac ce p
digestion according to the method EPA 3060A with a PG Istruments T80+ UV-VIS spectrophotometer measuring the absorbance at 540 nm [33]. 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.
3. RESULTS AND DISCUSSION
3.1 Soil characterization
The main chemical and physical characteristics of the soil are reported in Table 1. Results show the presence of a substantial amount of Cr(VI), thus suggesting a high natural oxidizing power of the 7
Page 7 of 24
soil. This is in accordance with the values of organic matter content and manganes oxide content, whose presence, as above reported, promotes Cr(III) oxidation. This is confirmed by the results of sequential extractions of the soil, reported in Table 2, where it is indicated that Cr is mainly present
ip t
bound to the organic matter fraction, in the trivalent form. In addition, the preliminary soil characterization highlights a low contamination by other heavy
cr
metals, with the exception of Ni, whose amount is almost totally found in the more labile fractions, thus indicating the occurrence of contamination from anthropic source. In addition, the measured Ni
us
concentration was above the threshold allowed according to Italian Environmental Regulation [34].
an
The results of the sequential extractions also showed that the amount of chromium in the exchangeable fraction was negligible with respect to the other fractions: as a consequence, in the
te
d
3.2 Reduction tests with iron sulphate
M
following extraction tests, the simple washing with water was not investigated.
Figure 1 shows the results obtained in the tests performed using Fe(II) as the reducing agent. The
Ac ce p
concentration of Cr(VI) and Cr over time are reported. The progressive reduction of Cr(VI) was observed, until the simultaneous oxidation of Fe(II) was completed.
Results show that the deoxygenation of the solution proved to be effective in enhancing the reduction efficiency, by preventing reagent consumption caused by the 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 Fe(II):Cr(VI)=5:1 or 10:1, Cr(VI) continuously decreased, thus suggesting that the oxidation-reduction process was not completed in the investigated reaction time. Only when the largest excess of reducing agent was adopted (30:1), after around 40 h of treatment, both in the presence and in the absence of oxygen, the residue Cr(VI) concentration attained an almost constant value. 8
Page 8 of 24
A different behaviour was observed as regards the total Cr concentration in the soil. In all the tests, a quick decrease 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
ip t
iron oxides [35], in the tests performed with Fe(II):Cr(VI)=5:1 and 10:1 the chromium amount in the solid phase increased up to the initial value. Conversely, in the tests performed with the largest
cr
Fe(II) excess with respect to Cr(VI) amount (30:1) a substantial decrease of also total chromium in soil was detected at the end of the treatment. This particular behaviour can be explained considering
us
that, when the largest excess of reducing agent was adopted, the pH of the liquid phase quickly
an
achieved acidic conditions (in the range between 5.3 and 5.8 in the test without oxygen removal and between 5.1 to 5.6 in the test performed after deoxygenation), as shown in Figure 2: at such
M
conditions, the precipitation of the chromium and iron oxides formed was not completed [36]. In the tests performed with 5:1 or 10:1 ratio between Fe(II) and Cr(VI), pH trend was instead quite
d
different, reaching alkaline conditions (up to 7.8) at the end of each experiment.
te
However, in all the tests performed, the residue amount in the soil at the end of the treatment could allow the industrial reuse of the soil according to Italian Environmental Regulation, 15 mg/kg
Ac ce p
Cr(VI) [34].
In the tests performed at Fe(II):Cr(VI)=30:1 without deoxygenation, and the test at 10:1 after deoxygenation, the final Cr(VI) concentration was below the civil reuse of the soil (2 mg/kg). The complete removal of Cr(VI) was only obtained after 45 h of treatment in the tests at Fe(II):Cr(VI)=30:1.
The results of sequential extraction tests performed after the treatment are shown in Table 3. Due to Cr(VI) solubilization, reduction and the following Cr(III) precipitation, both the reducing treatments determined an increase of the amount of chromium bound to the oxide-hydroxide fraction, thus confirming a mechanism of Cr(III)-Fe(III) hydroxides precipitation [23], though Cr(III) sorption onto iron hydroxides and complexation as Cr(III)-organic complexes or adsorbed and precipitated as Cr(OH)x species cannot be excluded [5]. Table 3 also shows that a slight increase of Chromium 9
Page 9 of 24
in the labile form was also observed when acidic conditions were established, due to the mobilization of the more stabile forms. In addition, the reflectometric measurement also showed
ip t
that the iron is essentially present as trivalent iron in the liquid phase.
In addition, the treatment resulted in a significant decrease of selected heavy metals amount in the
cr
soil (about 5% wt. for nickel and for about 35% wt. for lead), while copper solubilization was negligible. This result was consistent with the pH values observed in the tests performed at the 30:1
us
ratio, and also confirmed by the slight dissolution observed in literature where alkaline conditions
an
occurred during soil washing treatment [37].
M
3.3 Reduction tests with nano zero valent iron
d
The results obtained in the tests performed using colloidal nZVI as the reducing agent are shown in
te
Figure 3. The study reveals that treating polluted soil with a colloidal nZVI aqueous solution, chromium removal was time dependant and increasing with nanoparticles concentration. After 180
Ac ce p
minutes of contact time, Cr(VI) concentration always attained a constant value. In particular, in the test performed at the stoichiometric concentration, Cr(VI) removal was about 57%: the residue amount in the soil was higher than the limit allowing both residential and industrial reuse according to Italian Regulation [34]. Increasing the colloidal 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 colloidal nZVI concentration allowed the reduction of more than 90% of the initial amount. However, also in those cases, the residue level of Cr(VI) in the soil was not suitable for a civil reuse of the soil after the treatment, though after treatment with a ten times excess, the industrial reuse could be allowed. The results indicate a quite lower efficiency with respect to the data reported by Singh et al., (2011): 10
Page 10 of 24
at the same molar ratio, they found a complete Cr(VI) removal within 180 min of treatment on an artificially contaminated soil [25]. This difference can be reasonably attributed both to the long term contamination of the soil used in the present study, and to its specific characteristics, mainly the
ip t
high content of manganese oxide. It is known, in fact, that manganese may be reduced by Fe(0), thus consuming the reducing agent. In addition, Cr(III) generated by Cr(VI) reduction may form
cr
various Cr(OH)x species in a wide pH range. They could block active organic sites thus preventing further reduction of Cr(VI) by organic matter [5].
us
The same authors, in another study, reported that only adopting a 20:1 molar ratio, complete Cr(VI)
an
reduction from a tannery wastes contaminated soil occurred starting from an initial Cr(VI) concentration of 43 mg/kg [28].
M
The quick initial reduction observed when the higher nZVI:Cr(VI) ratios were tested (10:1 and 30:1), also suggests that the reaction was controlled by adsorption [38]. When adopting lower molar
te
contact time.
d
excess, a progressive Cr(VI) concentration reduction occurred, thus indicating a positive effect of
Another crucial factor affecting Cr(VI) reduction efficiency was Fe0 oxidation occurring over time
Ac ce p
[25]. During the treatment, in fact, a progressive lowering of Fe0 concentration was observed. In the tests performed adopting a 30:1 molar ratio between Fe0 and Cr(VI), starting from a Fe0 concentration of 262.7 mg/L, a final total iron concentration of 82.7 mg/l was measured after 120 min of treatment, while ferrous iron and ferric iron concentration were 12.4 and 18.2 mg/L respectively. The corresponding final concentration of Fe0 (equal to about 52.1 mg/L) was too low to ensure a further reduction of Cr(VI). Complete removal of Cr(VI) was obtained only adopting a larger excess of reducing agent (30:1 with respect the Cr(VI) molar amount). In this case a final amount of less than 5 mg/kg was reached just after 30 minutes of treatment. In particular, after 60 minutes of treatment Cr(VI) in the soil was not detected.
11
Page 11 of 24
The results of sequential extractions performed after the treatment (Table 4) show that there was a strong decrease of Cr in the organic matter and residual fraction in favor of the fraction bound to the oxides and hydroxides fraction; more than 50% with respect to treatment with Fe(II). This is in
ip t
accordance to the measured pH (~7.5) value in the liquid phase which corresponds to a minimum theoretical solubility of chromium and to the mechanisms of Cr(VI) removal by nZVI, which is
cr
based on the reduction of chromium from hexavalent form [Cr(VI)] to trivalent form [Cr(III)], subsequently followed by precipitation of Cr(III) on the surface of nZVI in the form of a layer of
us
chromium iron oxides/hydroxides/oxyhydroxides [39]. Also in this case, a slight increase of
an
Chromium in the labile form was observed, mainly due to the partial transformation of Cr(III) to its mobile forms due to complex formation with humus substances present in the soils immediately
M
after reduction, and to the presence of precipitated products only sligthly adsorbed to the soil matrix. Moreover, the standard reduction potentials of EFe(II)/Fe(0) and ECr(VI)/Cr(III) are −0.44 and
d
1.33V, respectively and that of ECr(III)/Cr(0) is −0.74 V, which is lower than that of Fe0. This suggests
te
that the Cr(III) or Cr(III)-hydroxide might be the steady product in the reaction which confirmed by the values of the redox potential of the extracts (about -0.4V)
Ac ce p
Regarding the residual heavy metals after the treatment, as expected, the results shows that the reducing treatment did not significantly influenced the total metals concentration in the soil (data not shown).
According to previous studies [25, 39], the kinetic model of Cr(VI) reduction by Fe0 nanoparticles can be described using the pseudo-first order kinetic equation: ln [Cr(VI)] = ln [Cr(VI)]0.kobs .t
where [Cr(VI)] and [Cr(VI)]0 are the instantaneous and initial concentration of Cr(VI) in mg kg-1, respectively, and kobs is the kinetic rate constant representing the overall removal rate for remediation (min-1). As shown in Figure 4, analysis of the data obtained in the tests performed with the larger Fe0 excess with respect to the initial Cr(VI) amount (30:1 molar ratio) revealed that the overall removal rate of Cr(VI) from soil followed a pseudo-first order kinetic model. The calculated 12
Page 12 of 24
kinetic constant was 6.9.10-2 min-1.
ip t
4. CONCLUSIONS
The effectiveness of iron sulfate and colloidal nano zero valent iron in the chemical reduction of
cr
Cr(VI) in a real contaminated soil in Italy was compared.
Results of the treatment with iron sulfate show that the residual amount Cr(VI) after 16 hours was
us
below the limit of industrial reuse in accordance with Italian Environmental Regulation. Only after
an
45 h of treatment the complete Cr(VI) removal was achieved using a large excess of Fe(II). Oxygen stripping before reagent addition proved to be a fundamental prerequisite to ensure bivalent iron
M
effectiveness over time. These requirements, together with the mobilization of heavy metals towards more labile forms observed during the reduction process, constitute the main issues in the
d
implementation of such remediation process.
te
In the tests performed using colloidal nZVI nanoparticles, the reduction of Cr(VI) was time dependent and increased with the concentration of nZVI.
Ac ce p
This study demonstrated that colloidal nZVI nanoparticles can be used for effective reduction of Cr(VI) in a real contaminated soil. The reaction was faster than in the case of using Fe(II), though a huge excess of nZVI with respect to the initial Cr(VI) content (30:1) was necessary to achieve a negligible Cr(VI) residue amount in the soil within 1 h of treatment. A pseudo first order kinetic was observed: the calculated kinetic constant was 6.9.10-2 min-1. The lower efficiency with respect to literature tests performed on spiked soil or groundwater was mainly attributed to the high amount of manganese oxides in the soil. Due to Cr(VI) solubilization, reduction, and following Cr(III) precipitation, in both cases, the reducing treatment resulted in an increase in the amount of chromium bound to the oxide-hydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation.
13
Page 13 of 24
References 1. R.A. Wuana, F.E. Okieimen, Heavy metals in contaminated soils: a review of sources, chemistry, risks and best available strategies for remediation, ISRN Ecol. (2011) Article ID
ip t
402647, 20 pages. 2. L. Di Palma, D. Mancini, E. Petrucci, Experimental Assessment of Chromium Mobilization
cr
from Polluted Soil by Washing Chem. Eng. Trans. 28 (2012) 145-150.
3. M. Megharaj, S. Avudainayagam, R. Naidu, Toxicity of hexavalent chromium and its
us
reduction by bacteria isolated from soil contaminated with tannery waste, Curr. Microbiol.
an
47 (2003) 51–54.
4. R.J. Bartlett, Chromium cycling in soils and water: links, gaps, and methods, Environ.
M
Health Perspect. 92 (1991) 17-24.
5. N. Kozuh, J. Stupar, B. Gorenc, Reduction and oxidation processes of chromium in soils,
d
Environ. Sci. Technol. 34 (2000) 112-119.
te
6. S.E. Fendorf, Surface reactions of chromium in soil and waters, Geoderma 67 (1995) 55-71. 7. R.J. Bartlett, B.R. James, Oxidation of chromium in soils, J. Environ. Qual. 8 (1979) 31-35.
Ac ce p
8. D. Dermatas, X. Meng, Utilization of fly ash for stabilization/solidification of heavy metal contaminated soils, Eng. Geol. 70 (2003) 377-394.
9. D. Dermatas, D.H. Moon, Chromium leaching and immobilization in treated soils, Environ. Eng. Sci. 23 (2006) 75-85.
10. C.N. Mulligan, R.N. Yong, B.F Gibbs, Remediation technologies for metal-contaminated soils and groundwater: an evaluation, Eng. Geol. 60 (2001) 193-201.
11. L. Di Palma, N. Verdone, Metals extraction from contaminated soils: model validation and parameters estimation, Chem. Eng. Trans. 28 (2012) 193-198. 12. M.W.H. Evangelou, M. Ebel, A. Schaeffer, Chelate assisted phytoextraction of heavy metals from soil. Effect, mechanism, toxicity, and fate of chelating agents, Chemosphere 68 (2007) 989–1003. 14
Page 14 of 24
13. L. Di Palma, P. Ferrantelli, C. Merli, F. Biancifiori, Recovery of EDTA and metal precipitation from soil flushing solutions, J. Hazard. Mater. B103 (2003) 153-168. 14. L. Di Palma, P. Ferrantelli, C. Merli, E. Petrucci, I. Pitzolu, Influence of Soil Organic Matter
ip t
on Copper Extraction from Contaminated Soil, Soil Sediment Contam. 16 (2007) 323–335. 15. O. Gonzini, A. Plaza, L. Di Palma, M.C. Lobo, Electro-bioremediation of gasoil
cr
contaminated soil, J. Appl. Electrochem. 40 (2010) 1239-1248.
16. C. Negra, D.S. Ross, A. Lanzirotti, Oxidizing Behavior of Soil Manganese: Interactions
us
among Abundance, Oxidation State, and pH, Soil Sci. Soc. Am. J. 69 (2005) 87–95
an
17. N. Manouchehri, S. Besancon, A. Bermond, Major and trace metal extraction from soil by EDTA: equilibrium and kinetic studies, Anal. Chim. Acta 559 (2006) 105-112.
Hazard. Mater. 170 (2009) 96-102.
M
18. L. Di Palma, Influence of indigeneous and added iron on copper extraction from soil, J.
d
19. N. Bolan, A. Kunhikrishnan, R. Thangarajan, J. Kumpiene, J. Park, T. Makino, M.B.
te
Kirkham, K. Scheckel, Remediation of heavy metal(loid)s contaminated soils – To mobilize or to immobilize? J. Hazard. Mater. 266 (2014) 141– 166.
Ac ce p
20. J. Du, J. Lu, Q. Wu, C. Jing, Reduction and immobilization of chromate in chromite ore processing residue with nanoscale zero-valent iron, J. Hazard. Mater. 215-216 (2012) 152– 158.
21. M. Chrysochoou, C.P. Johnston, G. Dahal, A comparative evaluation of hexavalent chromium treatment in contaminated soil by calcium polysulfide and green-tea nanoscale zero-valent iron, J. Hazard. Mat. 201-202 (2012) 33-42.
22. J. C. Seaman, P. M. Bertsch, L. Schwallie, In Situ Cr(VI) Reduction within CoarseTextured, Oxide-Coated Soil and Aquifer Systems Using Fe(II) Solutions, Environ. Sci. Technol. 33 (1999) 938–944.
15
Page 15 of 24
23. K. Kostarelos, E. Rao, D. Reale, D.H. Moon, Reduction of Cr(VI) to Cr(III) in artificial, contaminated soil using ferrous sulfate heptahydrate and sodium thiosulfate, Practice Periodical of Hazardous, Toxic, and Radioactive, Waste Manag. 13 (2009) 135-139.
ip t
24. G. Qin, M.J. McG uire, N.K. Blute , C. Seidel, L. Fong, Hexavalent chromium removal by reduction with ferrous sulfate, coagulation and filtration: a pilot-scale study, Environ. Sci.
cr
Technol. 39 (2005) 6321-6327.
25. R. Singh, V. Misra, R.P. Singh, Synthesis, characterization and role of zerovalent iron
us
nanoparticle in removal of hexavalent chromium from chromium-spiked soil, J. Nanopart.
an
Res. 13 (2011) 4063-4073.
26. F. He, D. Zhao, J. Liu, C.B. Roberts, Stabilization of Fe-Pd Nanoparticles with Sodium
M
Carboxymethyl Cellulose for Enhanced Transport and Dechlorination of Trichloroethylene in Soil and Groundwater, Ind. Eng. Chem. Res. 46 (2007) 29-34.
d
27. F. He, D. Zhao, Manipulating the Size and Dispersibility of Zerovalent Iron Nanoparticles
6221.
te
by Use of Carboxymethyl Cellulose Stabilizers, Environ. Sci. Technol. 41 (2007) 6216-
Ac ce p
28. R. Singh, V. Misra, R.P. Singh, Removal of Cr(VI) by Nanoscale Zero-valent Iron (nZVI) From Soil Contaminated with Tannery Wastes, Bull. Environ. Contam. Toxicol. 88 (2012) 210–214.
29. C. Liu, J.B. Evett, Soil properties, Testing, Measurement, and Evaluation, 5th ed., (2002), Prentice-Hall, New York, USA.
30. US EPA , Method 3050 B: Acid digestion of sediments, sludges and soils (1996). 31. E. Petrucci, D. Montanaro, C. Merli, Sequential extraction analysis provides decisionmaking tools for the use of contaminated sediments, Chem. Ecol. 27 Supp (2011) 107–118. 32. S. Vilar, A. Gutierrez, J. Antezana, P. Carral, A. Alvarez, A comparative study of three different methods for the sequential extraction of heavy metals in soil, Toxicol. Environ. Chem. 87 (2005) 1-10. 16
Page 16 of 24
33. U.S. EPA, Method 3060A: Alkaline digestion for hexavalent chromium (1996). 34. Italian Environmental Regulation (2006), Environmental standards assessment, G.U. n. 88 of April 14th 2006.
ip t
35. I.J. Buerge, S.J. Hug, Kinetics and pH Dependence of Chromium(VI) Reduction by Iron(II), Environ. Sci. Technol. 31 (1997) 1426-1432.
cr
36. D.H. Moon, M. Wazne, A. Koutsospyros, C. Christodoulatos, H. Gevgilili, M. Malik, D.M., Evaluation of the treatment of chromite ore processing residue by ferrous sulfate and
us
asphalt, J. Hazard. Mater. 166 (2009) 27-32.
an
37. L. Di Palma, P. Ferrantelli, Copper leaching from a sandy soil: mechanisms and parameters affecting EDTA extraction, J. Hazard. Mater. B122 (2005) 85-90.
M
38. Q. Wang, H. Qian, Y. Yang, Z. Zhang, C. Naman, X. Xu, Reduction of hexavalent chromium by carboxymethyl cellulose-stabilized zero-valent iron nanoparticles J. Cont.
d
Hydrol., 114 (2010) 35–42.
te
39. D.V Franco, L.M. Da Silva, W.F. Jardim, Reduction of hexavalent chromium in soil and ground water using zero-valent iron under batch and semi-batch conditions, Water Air Soil
Ac ce p
Poll. 197 (2009) 49-60.
17
Page 17 of 24
LIST OF TABLES Table 1 – Soil characterization
ip t
Table 2 – Sequential extractions of the untreated soil Table 3 – Soil sequential extractions after treatment with Fe(II) (nZVI:Cr(VI)=30:1after oxygen
cr
stripping; reaction time=72 h)
Table 4 – Soil sequential extractions after treatment with colloidal nZVI (nZVI:Cr(VI)=30:1; 60
us
min of treatment)
an
Table 5 – Metals content in the solid phase after colloidal nZVI treatment (reaction time=180 min)
M
and Fe(II) treatment (reaction time=72h)
LIST OF FIGURES
d
Figure 1 – Results of the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B:
te
nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen
Ac ce p
stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Figure 2 – pH evolution in the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B: nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Figure 3 – Results of the reducing tests with colloidal nZVI (A: nZVI:Cr(VI)=1:1; B: nZVI:Cr(VI)=5:1; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=30:1)
Figure 4 – Reducing tests with colloidal nZVI: determination of kinetic constant for initial concentrations of Cr(VI) (C0) as the pseudo-first order reaction (nZVI:Cr(VI)=30:1)
18
Page 18 of 24
Tables
pH
7.54
C.E.C (meq/100g)
9.6
Organic carbon (g/kg)
14.4
Organic matter (g/kg)
24.9 770 25
MnO2 (mg/kg)
630 10
CrTOT (mg/kg)
155 25
Cr(VI) (mg/kg)
94 7
Cu (mg/kg)
26 5
an 36267 40
Fe (mg/kg)
156 20
Zn (mg/kg)
M
Ni (mg/kg) Pb (mg/kg)
us
Mn (mg/kg)
ip t
Value
cr
Parameter
21 3 86 5
ce pt
ed
Table 1 – Soil characterization
Exchangeable (%)
Bound to carbonates (%)
Bound to oxideshydroxides (%)
Bound to the organic matter (%)
Residue (%)
Cr
<0.5
<0.5
35.5
46.4
18.1
8.1
6.2
77.6
8.1
<0.5
<0.5
0.8
73.1
7.2
18.9
<0.5
<0.5
18.7
1.0
80.3
Pb
1.9
9.2
18.4
18.4
52.1
Cu
1.0
1.7
55.7
4.4
37.2
Ni Mn Fe
Ac
Metal
Table 2 – Sequential extractions of the untreated soil
Page 19 of 24
Bound to carbonates (%)
Bound to oxideshydroxides (%)
Bound to the organic matter (%)
Residue (%)
Cr
8.9
3.4
53.5
17.6
16.6
Ni
1.0
1.4
75.3
22.3
<0.5
Mn
<0.5
0.7
78.9
0.7
19.7
Fe
<0.5
<0.5
34.6
3.9
61.5
Pb
1.6
3.2
5.5
9.8
79.9
Cu
0.6
1.0
26.4
18.3
53.7
cr
ip t
Metal
Exchangeable (%)
Bound to oxideshydroxides (%)
Bound to the organic matter (%)
Residue (%)
Cr
4.5
2.3
69.9
5.8
17.5
Ni
6.7
12.1
13.4
12.2
55.5
Mn
<0.5
<0.5
78.6
<0.5
21.4
Fe
<0.5
1.2
4.0
1.9
92.8
Pb
4.8
9.6
16.2
28.7
40.7
Cu
<0.5
1.5
38.7
17.6
42.1
M
Bound to carbonates (%)
ce pt
ed
Metal
Exchangeable (%)
an
us
Table 3 – Soil sequential extractions after treatment at Fe(II):Cr(VI)=30:1, after oxygen stripping (reaction time: 72 h)
Table 4 – Soil sequential extractions after treatment with colloidal nZVI, at Fe(0):Cr(VI)=30:1
Ac
(reaction time: 60 min)
Page 20 of 24
Ac
ce pt
ed
M
an
us
cr
ip t
Figure 1
Figure 1 - Results of the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B: nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Page 21 of 24
an
us
cr
ip t
Figure 2
Figure 2 – pH evolution in the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B:
M
nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen
Ac ce pt e
d
stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Page 22 of 24
30
60
90 120 150 180 Time (min)
180
ip t 30
60
90 120 150 180 Time (min)
180
D
160 140 120
M
100
Cr tot Cr VI
80
d
60 40 20 0 0
Ac ce pt e
Chromium (mg/kg)
140 120
Cr tot Cr VI
0
C
160
B
an
0
180 160 140 120 100 80 60 40 20 0
cr
Cr tot Cr VI
Chromium (mg/kg)
A
us
180 160 140 120 100 80 60 40 20 0
Chromium (mg/kg)
Chromium (mg/kg)
Figure 3
30
60
90 120 150 180 Time (min)
100
Cr tot Cr VI
80 60 40 20 0
0
30
60
90 120 150 180 Time (min)
Figure 3 – Results of the reducing tests with colloidal nZVI (A: nZVI:Cr(VI)=1:1; B: nZVI:Cr(VI)=5:1; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=30:1)
Page 23 of 24
Figure 4
0
Time (min) 50 100
150
0 y = -0.069x R² = 0.9238
nZVI:Cr(VI)=30:1
-3 -4
linear nZVI:Cr(VI)=30:1
-5
cr
Ln (C/C0)
-2
ip t
-1
-6
us
-7 -8
an
-9
M
Figure 4 – Reducing tests with colloidal nZVI: determination of kinetic constant for initial
Ac ce pt e
d
concentrations of Cr(VI) (C0) as the pseudo-first order reaction (nZVI:Cr(VI)=30:1)
Page 24 of 24
S0304-3894(14)00633-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.07.058 HAZMAT 16151
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
19-2-2014 27-7-2014 28-7-2014
Please cite this article as: L.D.I. PALMA, M.T. GUEYE, E. PETRUCCI, HEXAVALENT CHROMIUM REDUCTION IN CONTAMINATED SOIL: A COMPARISON BETWEEN FERROUS SULPHATE AND NANOSCALE ZERO-VALENT IRON, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.07.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
HEXAVALENT CHROMIUM REDUCTION IN CONTAMINATED SOIL: A COMPARISON BETWEEN FERROUS SULPHATE AND NANOSCALE ZERO-VALENT
DI PALMA L.*, GUEYE M. T., PETRUCCI E.
ip t
IRON
Eudossiana 18 – 00184, Roma, Italy.
an
ABSTRACT
us
*[email protected]; tel: +390644585571
cr
Dipartimento di Ingegneria Chimica Materiali Ambiente - Sapienza Università di Roma, via
M
Iron sulphate (FeSO4) and colloidal nano zero-valent iron (nZVI) as reducing agents were compared, with the aim of assessing their effectiveness in hexavalent chromium [Cr(VI)] removal
d
from a contaminated industrial soil. Experiments were performed on soil samples collected from an
te
industrial site where a nickel contamination, caused by a long-term productive activity, was also
Ac ce p
verified. The influence of reducing agents amount with respect to chromium content and the effectiveness of deoxygenation of the slurry were discussed. The soil was fully characterized before and after each test, and sequential extractions were performed to assess chemico-physical modifications and evaluate metals mobility induced by washing.
Results show that both the reducing agents successfully lowered the amount of Cr(VI) in the soil below the threshold allowed by Italian Environmental Regulation for industrial reuse. Cr(VI) reduction by colloidal nZVI proved to be faster and more effective: the civil reuse of soil [Cr(VI)<2 mg/kg] was only achieved using colloidal nZVI within 60 minutes adopting a nZVI/Cr(VI) molar ratio of 30. The reducing treatment resulted in an increase in the amount of chromium in the oxidehydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation. 1
Page 1 of 24
In addition, a decrease of Nickel (Ni) and Lead (Pb) content in soil was also observed when acidic conditions were established.
ip t
Keywords: hexavalent chromium; soil remediation; chemical reduction; ferrous sulfate; nano zero-
cr
valent iron; carboxymethyl cellulose
HIGHLIGHTS:
us
- FeSO4 and colloidal nZVI are compared in the remediation of an industrial soil contaminated by
an
hexavalent chromium
- Colloidal nZVI resulted in a faster and more efficient Cr(VI) reduction with respect to FeSO4
M
- The effect of the reduction treatment on metal mobilization and bioavailability was assessed by
d
sequential extractions
Ac ce p
te
- The mechanisms and the factors affecting the process are discussed
1. INTRODUCTION
A growing number of polluted sites are continuously identified in Italy: 57 of them have been first considered as the most dangerous and classified as Sites of National Interest (SIN), corresponding to about 3% of the whole Italian territory (about 1800 km2 of marine, coastal and lake areas, and about 5500 km2 of terrestrial areas). Because of adverse and long-term effects on human health and the environment, heavy metals originated by the uncontrolled disposal of industrial wastes and residues, are commonly considered among the pollutants of greatest concern [1]. In recent years a high concentration of chromium in areas close to galvanic industries and steel mill has been verified [2]. As known, Chromium in soil occurs primarily in two different redox states: the immobile trivalent 2
Page 2 of 24
form, Cr(III), and the more mobile hexavalent forms, as chromate (CrO42-) or dichromate (Cr2O72-). While Cr(III) is a low toxicity nutrient for plant growth, the hexavalent form is a well known dangerous species and human carcinogen [3].
ip t
It has been shown that Cr(VI) is the oxidation product of Cr(III) reaction with atmospheric oxygen: as the natural oxidation of Cr(III) is extremely low, most of the Cr(VI) found in soil and
cr
groundwater results from pollution [4]. The equilibrium between the two chromium forms in soil depends upon soil physical and chemical characteristics. The oxidation process is only controlled
us
by the reaction kinetics, due to Cr(III) species immobility and insolubility [5]. Cr(III) tends to be
an
strongly bound by soil humic acid polymers, and this affinity restricts the availability of Cr(III) to be oxidized. However, the Cr(III) reactivity increases when the inert crystals and amorphous
M
minerals are transformed to organic and hydroxide forms, which are smaller and more mobile [6]. It has been reported that the presence of manganese oxide in soils favours trivalent chromium
d
oxidation, thus increasing the hazards connected to hexavalent chromium contamination of
te
groundwater [7]. MnO2 works, in fact, as an electron link between Cr(III) and the atmospheric oxygen, and it was found that the amount of MnO2 reduced in soil is proportional to the amount of
Ac ce p
oxidized Cr(III) [5]. As a result, soil Cr(III) oxidation capacity is strongly increased under acidic conditions in the presence of manganese dioxide (MnO2). Several studies performed during the past two decades have already assessed that immobilization technologies can be successfully used for the remediation of contaminated soil [8-9] and groundwater [10]. They can be used as an alternative to the most common extraction techniques [11-12], that commonly involve the use of potentially toxic chelating agents. Moreover, the high overall costs of the extraction treatment, mainly determined by the cost of the chelating agent itself [13], its complexation by soil organic matter [14], or by the needs of enhancing mobilization [15], often limit its full scale application. In addition, mobilization of Cr(III) by organic chelant complexation, could result in an increase of its availability for oxidation to the hexavalent form [2], especially in the presence of a significant amount of Mn-oxides [16]. 3
Page 3 of 24
Furthermore, as a consequence of the extraction process, a strong modification of soil chemical and physical characteristics generally occurs [17-18] and this could affect the equilibrium between the two chromium species.
ip t
Among the immobilization techniques, the in situ manipulation of redox status by chemical reduction using reactive solutions offers a promising alternative for the treatment of contaminated
cr
soil. This technique generally deals with the reduction to Cr(III), providing a source of electrons, followed by pH adjustment to neutrality to favour precipitation of Cr(OH)3 or mixed oxyhydroxide
us
phases [19]. To this aim several studies in the recent years have investigated the use of nano zero
an
valent iron [20-21] or bivalent iron [22-23] as ferrous sulphate. However, most of them were performed in the aqueous phase [24] or in spiked soil [25] and additional studies are required before
M
implementing this technology on a real scale.
The aim of the present research was to compare the effectiveness of iron sulfate and colloidal nano
d
zero valent iron (nZVI) in the chemical reduction of Cr(VI). The experiments were performed with
te
soil samples collected at an industrial site in Northern Italy. The tests were performed under several operating conditions: the main parameter investigated was
Ac ce p
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 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 Materials
The soil was a sandy-loamy soil, collected at an industrial site in Italy, at depth between 0.50 and 2.50 m. Further information about sampling are not available, due to not disclosure agreement. 4
Page 4 of 24
In the tests performed using Fe(II) as reducing agent, the reducing solution was prepared by dissolving 0.548 g of iron sulfate 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
ip t
prepared from a 1 g/L Fe2+ aqueous solution, by reacting with sodium borohydride (NaBH4) at room temperature and in a free oxygen atmosphere [26]. As dispersing agent sodium CarboxyMethyl
cr
Cellulose (CMC) was used, at a CMC/Fe2+=0.005 mol/mol. Before use, deionized (DI) water and the CMC solution were purged with purified N2 for 30 min to remove dissolved oxygen. A 0.86 g/L
us
solution of colloidal nZVI was obtained, and use for the reduction tests. The average size of the
an
obtained nanoparticles (determined with a Nanoparticle Size Analyzer 90Plus, Brookhaven, Wien, Austria) was 13.3 nm, with a standard deviation of 5 nm, very close to the size obtained in similar
M
experiments [27-28].
All reagents were supplied from Sigma Aldrich and used as commercially available without any
te
d
further purification
Ac ce p
2.2 Experimental procedures
2.2.1 Soil characterisation
Soil organic carbon content and organic matter were determined by the Walkley-Black method. Soil pH, cation exchange capacity (CEC), and MnO2 content were determined using standard methods [29]. pH was measured after mixing 10 g of soil samples with 25 ml of a 0.01 M solution of CaCl2. Soil acid digestion was performed to determine the initial chromium and heavy metals content in soil, using hydrogen peroxide (30% v/v), concentrated hydrochloric acid and nitric acid (50% v/v) according to EPA Method 3050B [30]. 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 HNO3 were then added to the solution. The mix was then kept to 95°C for 2 hours and subsequently cooled 5
Page 5 of 24
down to 25°C. After the addition of 2 mL of H2O and 10 mL of H2O2, 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.
ip t
Since metal extraction effectiveness depends upon the leachability of the different metal forms [31], sequential extractions using the Tessier’s method were performed to investigate metals distribution
cr
into five fractions: exchangeable, bound to carbonate, Fe-Mn oxides, bound to organic matter and residual [32].
us
The exchangeable fraction was determined through extraction with 8 mL of 1 M MgCl2 at pH 7 for
an
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
M
determined after extraction with 20 mL of 0.04 M NH2OH.HCl in 25% vol. acetic acid (pH=2) for 6 h at 96°C. The fraction bound to organic matter was determined after extraction with 3 mL of 0.02
d
M HNO3 and 5 mL of 30% H2O2 (pH=2) for 2 h at 85°C, followed by 3 mL of 30% H2O2 (pH=2)
temperature for 30 min.
te
for 3 h at 85°C and then 5 mL of 3.2 M NH4OAc in 20% vol. HNO3 diluted to 20 mL at room
Ac ce p
The residual fraction was determined after digestion at 90°C with 25 mL of diluted aqua regia (50 mL HCl, 200 mL HNO3 and 750 mL of distilled water) for 3 h.
2.2.2 Reduction tests
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 of nZVI was calculated according to the following equation: 3 Fe0 + Cr2O72- + 7 H2O → 3 Fe2+ + 2 Cr(OH)3 + 8 OHFour different reducing solutions were prepared, by adding Fe(II) or Fe0 at a stoichiometric concentration, or at selected molar ratios with respect to the Cr(VI) amount in the sample concentration (5:1, 10:1, 30:1). 6
Page 6 of 24
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.
ip t
At selected times, aliquot of the suspension was sampled and filtered through a 0.45 µm Whatman membrane filter, and the reaction stopped by washing the soil with distilled water. The liquid phase
cr
was analysed for pH, redox potential and metal content. Results were reported as mg/kg, considering the L/S ratio adopted in the each test.
us
All the experiments were performed in triplicate: when a standard deviation higher than 5% was
an
calculated, further repetitions were performed until that target value was obtained basing on three
M
values.
2.3 Analyses
d
The metal content of soil was determined by atomic absorption spectrophotometry, using an Agilent
te
AA DUO 240 Fs instrument, after acid digestion according to the procedure previously described. Cr(VI) amount of the liquid phase was determined with the diphenylcarbazide method after alkaline
Ac ce p
digestion according to the method EPA 3060A with a PG Istruments T80+ UV-VIS spectrophotometer measuring the absorbance at 540 nm [33]. 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.
3. RESULTS AND DISCUSSION
3.1 Soil characterization
The main chemical and physical characteristics of the soil are reported in Table 1. Results show the presence of a substantial amount of Cr(VI), thus suggesting a high natural oxidizing power of the 7
Page 7 of 24
soil. This is in accordance with the values of organic matter content and manganes oxide content, whose presence, as above reported, promotes Cr(III) oxidation. This is confirmed by the results of sequential extractions of the soil, reported in Table 2, where it is indicated that Cr is mainly present
ip t
bound to the organic matter fraction, in the trivalent form. In addition, the preliminary soil characterization highlights a low contamination by other heavy
cr
metals, with the exception of Ni, whose amount is almost totally found in the more labile fractions, thus indicating the occurrence of contamination from anthropic source. In addition, the measured Ni
us
concentration was above the threshold allowed according to Italian Environmental Regulation [34].
an
The results of the sequential extractions also showed that the amount of chromium in the exchangeable fraction was negligible with respect to the other fractions: as a consequence, in the
te
d
3.2 Reduction tests with iron sulphate
M
following extraction tests, the simple washing with water was not investigated.
Figure 1 shows the results obtained in the tests performed using Fe(II) as the reducing agent. The
Ac ce p
concentration of Cr(VI) and Cr over time are reported. The progressive reduction of Cr(VI) was observed, until the simultaneous oxidation of Fe(II) was completed.
Results show that the deoxygenation of the solution proved to be effective in enhancing the reduction efficiency, by preventing reagent consumption caused by the 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 Fe(II):Cr(VI)=5:1 or 10:1, Cr(VI) continuously decreased, thus suggesting that the oxidation-reduction process was not completed in the investigated reaction time. Only when the largest excess of reducing agent was adopted (30:1), after around 40 h of treatment, both in the presence and in the absence of oxygen, the residue Cr(VI) concentration attained an almost constant value. 8
Page 8 of 24
A different behaviour was observed as regards the total Cr concentration in the soil. In all the tests, a quick decrease 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
ip t
iron oxides [35], in the tests performed with Fe(II):Cr(VI)=5:1 and 10:1 the chromium amount in the solid phase increased up to the initial value. Conversely, in the tests performed with the largest
cr
Fe(II) excess with respect to Cr(VI) amount (30:1) a substantial decrease of also total chromium in soil was detected at the end of the treatment. This particular behaviour can be explained considering
us
that, when the largest excess of reducing agent was adopted, the pH of the liquid phase quickly
an
achieved acidic conditions (in the range between 5.3 and 5.8 in the test without oxygen removal and between 5.1 to 5.6 in the test performed after deoxygenation), as shown in Figure 2: at such
M
conditions, the precipitation of the chromium and iron oxides formed was not completed [36]. In the tests performed with 5:1 or 10:1 ratio between Fe(II) and Cr(VI), pH trend was instead quite
d
different, reaching alkaline conditions (up to 7.8) at the end of each experiment.
te
However, in all the tests performed, the residue amount in the soil at the end of the treatment could allow the industrial reuse of the soil according to Italian Environmental Regulation, 15 mg/kg
Ac ce p
Cr(VI) [34].
In the tests performed at Fe(II):Cr(VI)=30:1 without deoxygenation, and the test at 10:1 after deoxygenation, the final Cr(VI) concentration was below the civil reuse of the soil (2 mg/kg). The complete removal of Cr(VI) was only obtained after 45 h of treatment in the tests at Fe(II):Cr(VI)=30:1.
The results of sequential extraction tests performed after the treatment are shown in Table 3. Due to Cr(VI) solubilization, reduction and the following Cr(III) precipitation, both the reducing treatments determined an increase of the amount of chromium bound to the oxide-hydroxide fraction, thus confirming a mechanism of Cr(III)-Fe(III) hydroxides precipitation [23], though Cr(III) sorption onto iron hydroxides and complexation as Cr(III)-organic complexes or adsorbed and precipitated as Cr(OH)x species cannot be excluded [5]. Table 3 also shows that a slight increase of Chromium 9
Page 9 of 24
in the labile form was also observed when acidic conditions were established, due to the mobilization of the more stabile forms. In addition, the reflectometric measurement also showed
ip t
that the iron is essentially present as trivalent iron in the liquid phase.
In addition, the treatment resulted in a significant decrease of selected heavy metals amount in the
cr
soil (about 5% wt. for nickel and for about 35% wt. for lead), while copper solubilization was negligible. This result was consistent with the pH values observed in the tests performed at the 30:1
us
ratio, and also confirmed by the slight dissolution observed in literature where alkaline conditions
an
occurred during soil washing treatment [37].
M
3.3 Reduction tests with nano zero valent iron
d
The results obtained in the tests performed using colloidal nZVI as the reducing agent are shown in
te
Figure 3. The study reveals that treating polluted soil with a colloidal nZVI aqueous solution, chromium removal was time dependant and increasing with nanoparticles concentration. After 180
Ac ce p
minutes of contact time, Cr(VI) concentration always attained a constant value. In particular, in the test performed at the stoichiometric concentration, Cr(VI) removal was about 57%: the residue amount in the soil was higher than the limit allowing both residential and industrial reuse according to Italian Regulation [34]. Increasing the colloidal 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 colloidal nZVI concentration allowed the reduction of more than 90% of the initial amount. However, also in those cases, the residue level of Cr(VI) in the soil was not suitable for a civil reuse of the soil after the treatment, though after treatment with a ten times excess, the industrial reuse could be allowed. The results indicate a quite lower efficiency with respect to the data reported by Singh et al., (2011): 10
Page 10 of 24
at the same molar ratio, they found a complete Cr(VI) removal within 180 min of treatment on an artificially contaminated soil [25]. This difference can be reasonably attributed both to the long term contamination of the soil used in the present study, and to its specific characteristics, mainly the
ip t
high content of manganese oxide. It is known, in fact, that manganese may be reduced by Fe(0), thus consuming the reducing agent. In addition, Cr(III) generated by Cr(VI) reduction may form
cr
various Cr(OH)x species in a wide pH range. They could block active organic sites thus preventing further reduction of Cr(VI) by organic matter [5].
us
The same authors, in another study, reported that only adopting a 20:1 molar ratio, complete Cr(VI)
an
reduction from a tannery wastes contaminated soil occurred starting from an initial Cr(VI) concentration of 43 mg/kg [28].
M
The quick initial reduction observed when the higher nZVI:Cr(VI) ratios were tested (10:1 and 30:1), also suggests that the reaction was controlled by adsorption [38]. When adopting lower molar
te
contact time.
d
excess, a progressive Cr(VI) concentration reduction occurred, thus indicating a positive effect of
Another crucial factor affecting Cr(VI) reduction efficiency was Fe0 oxidation occurring over time
Ac ce p
[25]. During the treatment, in fact, a progressive lowering of Fe0 concentration was observed. In the tests performed adopting a 30:1 molar ratio between Fe0 and Cr(VI), starting from a Fe0 concentration of 262.7 mg/L, a final total iron concentration of 82.7 mg/l was measured after 120 min of treatment, while ferrous iron and ferric iron concentration were 12.4 and 18.2 mg/L respectively. The corresponding final concentration of Fe0 (equal to about 52.1 mg/L) was too low to ensure a further reduction of Cr(VI). Complete removal of Cr(VI) was obtained only adopting a larger excess of reducing agent (30:1 with respect the Cr(VI) molar amount). In this case a final amount of less than 5 mg/kg was reached just after 30 minutes of treatment. In particular, after 60 minutes of treatment Cr(VI) in the soil was not detected.
11
Page 11 of 24
The results of sequential extractions performed after the treatment (Table 4) show that there was a strong decrease of Cr in the organic matter and residual fraction in favor of the fraction bound to the oxides and hydroxides fraction; more than 50% with respect to treatment with Fe(II). This is in
ip t
accordance to the measured pH (~7.5) value in the liquid phase which corresponds to a minimum theoretical solubility of chromium and to the mechanisms of Cr(VI) removal by nZVI, which is
cr
based on the reduction of chromium from hexavalent form [Cr(VI)] to trivalent form [Cr(III)], subsequently followed by precipitation of Cr(III) on the surface of nZVI in the form of a layer of
us
chromium iron oxides/hydroxides/oxyhydroxides [39]. Also in this case, a slight increase of
an
Chromium in the labile form was observed, mainly due to the partial transformation of Cr(III) to its mobile forms due to complex formation with humus substances present in the soils immediately
M
after reduction, and to the presence of precipitated products only sligthly adsorbed to the soil matrix. Moreover, the standard reduction potentials of EFe(II)/Fe(0) and ECr(VI)/Cr(III) are −0.44 and
d
1.33V, respectively and that of ECr(III)/Cr(0) is −0.74 V, which is lower than that of Fe0. This suggests
te
that the Cr(III) or Cr(III)-hydroxide might be the steady product in the reaction which confirmed by the values of the redox potential of the extracts (about -0.4V)
Ac ce p
Regarding the residual heavy metals after the treatment, as expected, the results shows that the reducing treatment did not significantly influenced the total metals concentration in the soil (data not shown).
According to previous studies [25, 39], the kinetic model of Cr(VI) reduction by Fe0 nanoparticles can be described using the pseudo-first order kinetic equation: ln [Cr(VI)] = ln [Cr(VI)]0.kobs .t
where [Cr(VI)] and [Cr(VI)]0 are the instantaneous and initial concentration of Cr(VI) in mg kg-1, respectively, and kobs is the kinetic rate constant representing the overall removal rate for remediation (min-1). As shown in Figure 4, analysis of the data obtained in the tests performed with the larger Fe0 excess with respect to the initial Cr(VI) amount (30:1 molar ratio) revealed that the overall removal rate of Cr(VI) from soil followed a pseudo-first order kinetic model. The calculated 12
Page 12 of 24
kinetic constant was 6.9.10-2 min-1.
ip t
4. CONCLUSIONS
The effectiveness of iron sulfate and colloidal nano zero valent iron in the chemical reduction of
cr
Cr(VI) in a real contaminated soil in Italy was compared.
Results of the treatment with iron sulfate show that the residual amount Cr(VI) after 16 hours was
us
below the limit of industrial reuse in accordance with Italian Environmental Regulation. Only after
an
45 h of treatment the complete Cr(VI) removal was achieved using a large excess of Fe(II). Oxygen stripping before reagent addition proved to be a fundamental prerequisite to ensure bivalent iron
M
effectiveness over time. These requirements, together with the mobilization of heavy metals towards more labile forms observed during the reduction process, constitute the main issues in the
d
implementation of such remediation process.
te
In the tests performed using colloidal nZVI nanoparticles, the reduction of Cr(VI) was time dependent and increased with the concentration of nZVI.
Ac ce p
This study demonstrated that colloidal nZVI nanoparticles can be used for effective reduction of Cr(VI) in a real contaminated soil. The reaction was faster than in the case of using Fe(II), though a huge excess of nZVI with respect to the initial Cr(VI) content (30:1) was necessary to achieve a negligible Cr(VI) residue amount in the soil within 1 h of treatment. A pseudo first order kinetic was observed: the calculated kinetic constant was 6.9.10-2 min-1. The lower efficiency with respect to literature tests performed on spiked soil or groundwater was mainly attributed to the high amount of manganese oxides in the soil. Due to Cr(VI) solubilization, reduction, and following Cr(III) precipitation, in both cases, the reducing treatment resulted in an increase in the amount of chromium bound to the oxide-hydroxide fraction, thus confirming a mechanism of chromium-iron hydroxides precipitation.
13
Page 13 of 24
References 1. R.A. Wuana, F.E. Okieimen, Heavy metals in contaminated soils: a review of sources, chemistry, risks and best available strategies for remediation, ISRN Ecol. (2011) Article ID
ip t
402647, 20 pages. 2. L. Di Palma, D. Mancini, E. Petrucci, Experimental Assessment of Chromium Mobilization
cr
from Polluted Soil by Washing Chem. Eng. Trans. 28 (2012) 145-150.
3. M. Megharaj, S. Avudainayagam, R. Naidu, Toxicity of hexavalent chromium and its
us
reduction by bacteria isolated from soil contaminated with tannery waste, Curr. Microbiol.
an
47 (2003) 51–54.
4. R.J. Bartlett, Chromium cycling in soils and water: links, gaps, and methods, Environ.
M
Health Perspect. 92 (1991) 17-24.
5. N. Kozuh, J. Stupar, B. Gorenc, Reduction and oxidation processes of chromium in soils,
d
Environ. Sci. Technol. 34 (2000) 112-119.
te
6. S.E. Fendorf, Surface reactions of chromium in soil and waters, Geoderma 67 (1995) 55-71. 7. R.J. Bartlett, B.R. James, Oxidation of chromium in soils, J. Environ. Qual. 8 (1979) 31-35.
Ac ce p
8. D. Dermatas, X. Meng, Utilization of fly ash for stabilization/solidification of heavy metal contaminated soils, Eng. Geol. 70 (2003) 377-394.
9. D. Dermatas, D.H. Moon, Chromium leaching and immobilization in treated soils, Environ. Eng. Sci. 23 (2006) 75-85.
10. C.N. Mulligan, R.N. Yong, B.F Gibbs, Remediation technologies for metal-contaminated soils and groundwater: an evaluation, Eng. Geol. 60 (2001) 193-201.
11. L. Di Palma, N. Verdone, Metals extraction from contaminated soils: model validation and parameters estimation, Chem. Eng. Trans. 28 (2012) 193-198. 12. M.W.H. Evangelou, M. Ebel, A. Schaeffer, Chelate assisted phytoextraction of heavy metals from soil. Effect, mechanism, toxicity, and fate of chelating agents, Chemosphere 68 (2007) 989–1003. 14
Page 14 of 24
13. L. Di Palma, P. Ferrantelli, C. Merli, F. Biancifiori, Recovery of EDTA and metal precipitation from soil flushing solutions, J. Hazard. Mater. B103 (2003) 153-168. 14. L. Di Palma, P. Ferrantelli, C. Merli, E. Petrucci, I. Pitzolu, Influence of Soil Organic Matter
ip t
on Copper Extraction from Contaminated Soil, Soil Sediment Contam. 16 (2007) 323–335. 15. O. Gonzini, A. Plaza, L. Di Palma, M.C. Lobo, Electro-bioremediation of gasoil
cr
contaminated soil, J. Appl. Electrochem. 40 (2010) 1239-1248.
16. C. Negra, D.S. Ross, A. Lanzirotti, Oxidizing Behavior of Soil Manganese: Interactions
us
among Abundance, Oxidation State, and pH, Soil Sci. Soc. Am. J. 69 (2005) 87–95
an
17. N. Manouchehri, S. Besancon, A. Bermond, Major and trace metal extraction from soil by EDTA: equilibrium and kinetic studies, Anal. Chim. Acta 559 (2006) 105-112.
Hazard. Mater. 170 (2009) 96-102.
M
18. L. Di Palma, Influence of indigeneous and added iron on copper extraction from soil, J.
d
19. N. Bolan, A. Kunhikrishnan, R. Thangarajan, J. Kumpiene, J. Park, T. Makino, M.B.
te
Kirkham, K. Scheckel, Remediation of heavy metal(loid)s contaminated soils – To mobilize or to immobilize? J. Hazard. Mater. 266 (2014) 141– 166.
Ac ce p
20. J. Du, J. Lu, Q. Wu, C. Jing, Reduction and immobilization of chromate in chromite ore processing residue with nanoscale zero-valent iron, J. Hazard. Mater. 215-216 (2012) 152– 158.
21. M. Chrysochoou, C.P. Johnston, G. Dahal, A comparative evaluation of hexavalent chromium treatment in contaminated soil by calcium polysulfide and green-tea nanoscale zero-valent iron, J. Hazard. Mat. 201-202 (2012) 33-42.
22. J. C. Seaman, P. M. Bertsch, L. Schwallie, In Situ Cr(VI) Reduction within CoarseTextured, Oxide-Coated Soil and Aquifer Systems Using Fe(II) Solutions, Environ. Sci. Technol. 33 (1999) 938–944.
15
Page 15 of 24
23. K. Kostarelos, E. Rao, D. Reale, D.H. Moon, Reduction of Cr(VI) to Cr(III) in artificial, contaminated soil using ferrous sulfate heptahydrate and sodium thiosulfate, Practice Periodical of Hazardous, Toxic, and Radioactive, Waste Manag. 13 (2009) 135-139.
ip t
24. G. Qin, M.J. McG uire, N.K. Blute , C. Seidel, L. Fong, Hexavalent chromium removal by reduction with ferrous sulfate, coagulation and filtration: a pilot-scale study, Environ. Sci.
cr
Technol. 39 (2005) 6321-6327.
25. R. Singh, V. Misra, R.P. Singh, Synthesis, characterization and role of zerovalent iron
us
nanoparticle in removal of hexavalent chromium from chromium-spiked soil, J. Nanopart.
an
Res. 13 (2011) 4063-4073.
26. F. He, D. Zhao, J. Liu, C.B. Roberts, Stabilization of Fe-Pd Nanoparticles with Sodium
M
Carboxymethyl Cellulose for Enhanced Transport and Dechlorination of Trichloroethylene in Soil and Groundwater, Ind. Eng. Chem. Res. 46 (2007) 29-34.
d
27. F. He, D. Zhao, Manipulating the Size and Dispersibility of Zerovalent Iron Nanoparticles
6221.
te
by Use of Carboxymethyl Cellulose Stabilizers, Environ. Sci. Technol. 41 (2007) 6216-
Ac ce p
28. R. Singh, V. Misra, R.P. Singh, Removal of Cr(VI) by Nanoscale Zero-valent Iron (nZVI) From Soil Contaminated with Tannery Wastes, Bull. Environ. Contam. Toxicol. 88 (2012) 210–214.
29. C. Liu, J.B. Evett, Soil properties, Testing, Measurement, and Evaluation, 5th ed., (2002), Prentice-Hall, New York, USA.
30. US EPA , Method 3050 B: Acid digestion of sediments, sludges and soils (1996). 31. E. Petrucci, D. Montanaro, C. Merli, Sequential extraction analysis provides decisionmaking tools for the use of contaminated sediments, Chem. Ecol. 27 Supp (2011) 107–118. 32. S. Vilar, A. Gutierrez, J. Antezana, P. Carral, A. Alvarez, A comparative study of three different methods for the sequential extraction of heavy metals in soil, Toxicol. Environ. Chem. 87 (2005) 1-10. 16
Page 16 of 24
33. U.S. EPA, Method 3060A: Alkaline digestion for hexavalent chromium (1996). 34. Italian Environmental Regulation (2006), Environmental standards assessment, G.U. n. 88 of April 14th 2006.
ip t
35. I.J. Buerge, S.J. Hug, Kinetics and pH Dependence of Chromium(VI) Reduction by Iron(II), Environ. Sci. Technol. 31 (1997) 1426-1432.
cr
36. D.H. Moon, M. Wazne, A. Koutsospyros, C. Christodoulatos, H. Gevgilili, M. Malik, D.M., Evaluation of the treatment of chromite ore processing residue by ferrous sulfate and
us
asphalt, J. Hazard. Mater. 166 (2009) 27-32.
an
37. L. Di Palma, P. Ferrantelli, Copper leaching from a sandy soil: mechanisms and parameters affecting EDTA extraction, J. Hazard. Mater. B122 (2005) 85-90.
M
38. Q. Wang, H. Qian, Y. Yang, Z. Zhang, C. Naman, X. Xu, Reduction of hexavalent chromium by carboxymethyl cellulose-stabilized zero-valent iron nanoparticles J. Cont.
d
Hydrol., 114 (2010) 35–42.
te
39. D.V Franco, L.M. Da Silva, W.F. Jardim, Reduction of hexavalent chromium in soil and ground water using zero-valent iron under batch and semi-batch conditions, Water Air Soil
Ac ce p
Poll. 197 (2009) 49-60.
17
Page 17 of 24
LIST OF TABLES Table 1 – Soil characterization
ip t
Table 2 – Sequential extractions of the untreated soil Table 3 – Soil sequential extractions after treatment with Fe(II) (nZVI:Cr(VI)=30:1after oxygen
cr
stripping; reaction time=72 h)
Table 4 – Soil sequential extractions after treatment with colloidal nZVI (nZVI:Cr(VI)=30:1; 60
us
min of treatment)
an
Table 5 – Metals content in the solid phase after colloidal nZVI treatment (reaction time=180 min)
M
and Fe(II) treatment (reaction time=72h)
LIST OF FIGURES
d
Figure 1 – Results of the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B:
te
nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen
Ac ce p
stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Figure 2 – pH evolution in the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B: nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Figure 3 – Results of the reducing tests with colloidal nZVI (A: nZVI:Cr(VI)=1:1; B: nZVI:Cr(VI)=5:1; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=30:1)
Figure 4 – Reducing tests with colloidal nZVI: determination of kinetic constant for initial concentrations of Cr(VI) (C0) as the pseudo-first order reaction (nZVI:Cr(VI)=30:1)
18
Page 18 of 24
Tables
pH
7.54
C.E.C (meq/100g)
9.6
Organic carbon (g/kg)
14.4
Organic matter (g/kg)
24.9 770 25
MnO2 (mg/kg)
630 10
CrTOT (mg/kg)
155 25
Cr(VI) (mg/kg)
94 7
Cu (mg/kg)
26 5
an 36267 40
Fe (mg/kg)
156 20
Zn (mg/kg)
M
Ni (mg/kg) Pb (mg/kg)
us
Mn (mg/kg)
ip t
Value
cr
Parameter
21 3 86 5
ce pt
ed
Table 1 – Soil characterization
Exchangeable (%)
Bound to carbonates (%)
Bound to oxideshydroxides (%)
Bound to the organic matter (%)
Residue (%)
Cr
<0.5
<0.5
35.5
46.4
18.1
8.1
6.2
77.6
8.1
<0.5
<0.5
0.8
73.1
7.2
18.9
<0.5
<0.5
18.7
1.0
80.3
Pb
1.9
9.2
18.4
18.4
52.1
Cu
1.0
1.7
55.7
4.4
37.2
Ni Mn Fe
Ac
Metal
Table 2 – Sequential extractions of the untreated soil
Page 19 of 24
Bound to carbonates (%)
Bound to oxideshydroxides (%)
Bound to the organic matter (%)
Residue (%)
Cr
8.9
3.4
53.5
17.6
16.6
Ni
1.0
1.4
75.3
22.3
<0.5
Mn
<0.5
0.7
78.9
0.7
19.7
Fe
<0.5
<0.5
34.6
3.9
61.5
Pb
1.6
3.2
5.5
9.8
79.9
Cu
0.6
1.0
26.4
18.3
53.7
cr
ip t
Metal
Exchangeable (%)
Bound to oxideshydroxides (%)
Bound to the organic matter (%)
Residue (%)
Cr
4.5
2.3
69.9
5.8
17.5
Ni
6.7
12.1
13.4
12.2
55.5
Mn
<0.5
<0.5
78.6
<0.5
21.4
Fe
<0.5
1.2
4.0
1.9
92.8
Pb
4.8
9.6
16.2
28.7
40.7
Cu
<0.5
1.5
38.7
17.6
42.1
M
Bound to carbonates (%)
ce pt
ed
Metal
Exchangeable (%)
an
us
Table 3 – Soil sequential extractions after treatment at Fe(II):Cr(VI)=30:1, after oxygen stripping (reaction time: 72 h)
Table 4 – Soil sequential extractions after treatment with colloidal nZVI, at Fe(0):Cr(VI)=30:1
Ac
(reaction time: 60 min)
Page 20 of 24
Ac
ce pt
ed
M
an
us
cr
ip t
Figure 1
Figure 1 - Results of the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B: nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Page 21 of 24
an
us
cr
ip t
Figure 2
Figure 2 – pH evolution in the reducing tests with iron sulphate (A: nZVI:Cr(VI)=5:1; B:
M
nZVI:Cr(VI)=5:1 after oxygen stripping; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=10:1 after oxygen
Ac ce pt e
d
stripping; E: nZVI:Cr(VI)=30:1; F: nZVI:Cr(VI)=30:1 after oxygen stripping).
Page 22 of 24
30
60
90 120 150 180 Time (min)
180
ip t 30
60
90 120 150 180 Time (min)
180
D
160 140 120
M
100
Cr tot Cr VI
80
d
60 40 20 0 0
Ac ce pt e
Chromium (mg/kg)
140 120
Cr tot Cr VI
0
C
160
B
an
0
180 160 140 120 100 80 60 40 20 0
cr
Cr tot Cr VI
Chromium (mg/kg)
A
us
180 160 140 120 100 80 60 40 20 0
Chromium (mg/kg)
Chromium (mg/kg)
Figure 3
30
60
90 120 150 180 Time (min)
100
Cr tot Cr VI
80 60 40 20 0
0
30
60
90 120 150 180 Time (min)
Figure 3 – Results of the reducing tests with colloidal nZVI (A: nZVI:Cr(VI)=1:1; B: nZVI:Cr(VI)=5:1; C: nZVI:Cr(VI)=10:1; D: nZVI:Cr(VI)=30:1)
Page 23 of 24
Figure 4
0
Time (min) 50 100
150
0 y = -0.069x R² = 0.9238
nZVI:Cr(VI)=30:1
-3 -4
linear nZVI:Cr(VI)=30:1
-5
cr
Ln (C/C0)
-2
ip t
-1
-6
us
-7 -8
an
-9
M
Figure 4 – Reducing tests with colloidal nZVI: determination of kinetic constant for initial
Ac ce pt e
d
concentrations of Cr(VI) (C0) as the pseudo-first order reaction (nZVI:Cr(VI)=30:1)
Page 24 of 24