REDUCTION OF Cr(VI) TO Cr (III) BY ASCORBIC ACID IN AN HEAVELY CONTAMINATED

SOIL

Luz Alicia Galeana Corrales (1) , Jorge Javier Ramírez García(1), Lázaro Raymundo Reyes Gutiérrez(2),

Elizabeth Teresita Romero Guzmán(3)

1) Universidad Autónoma del Estado de México. Facultad de Química, Laboratorio de Análisis Instrumental.

Paseo Colón esquina Paseo Tollocan; Colonia. Residencial Colón. Toluca, Estado de México. C.P. 50180.

Tel: +52 (722) 2 17 51 09 Ext. 113, Fax: +52 (722) 2 17 38 90. e-mail: jjramirezg@uaemex.mx

2) Instituto Potosino de Investigación Científica y Tecnológica. División de Geociencias Aplicadas, Camino a

la Presa San José 2055, Col. Lomas 4a. sección C.P. 78216, San Luis Potosí, San Luis Potosí, México

3) Instituto Nacional de Investigaciones Nucleares. Departamento de Química, Carretera México-Toluca Km.

36.5, AP 18-1027. C.P. 52750, Salazar, Estado de México, México

Abbreviations: XPS, X-ray photoelectron spectroscopy · XRD, X-ray diffraction · HPLC, high performance

liquid chromatography · SEM, Scanning electron microscopy · EDS, X-ray energy dispersive spectrometry ·

CCr(VI), chromium VI concentration · t, time · kobs, observed second-order rate constant · T, reaction

temperature · A, pre-exponential factor · R, universal constant of gases · Ea, activation energy

Abstract

This study was conducted to evaluate the reduction of Cr(VI) to Cr(III) by ascorbic acid in samples of a

Cr(VI) contaminated soil. The characterization of contaminated soil shows a concentration of 5886.2 mg/kg

of Cr(VI) determined by the diphenylcarbazide method, the XPS analysis indicates the presence of Cr(VI) as

chromate, and XRD analysis indicate that the mineral species is CaCrO4. We obtained the reduction of Cr(VI)

to 100% in a ratio of Cr(VI): ascorbic acid concentration in units of mg/kg 1:5. The characterization of the

soil after remediation process shows the reduction of the Cr(VI). In the XPS analysis the signals detected,

only correspond to related species to Cr(III), and the signal of CaCrO4 is not detected in the XRD analysis. In

the soluble fraction was possible to observe the reaction progress by HPLC with the formation of

dehydroascorbic acid and Cr(III), and the decline and subsequent disappearance of the signals corresponding

to the Cr(VI) and ascorbic acid.

Keywords. XPS

1 Introduction

The presence of heavy metals such as chromium in soils can be geogenic or anthropogenic origin. The

anthropogenic source refers to the hazardous materials from industrial, agricultural, mining, and urban solid

waste. (Galan and Romero 2008). It is said that a soil is contaminated when one or more pollutants exceed the

maximum permissible level, these level are defined as concentrations which above them, undesirable effects

are shown in the environment and/or in the surrounding living organisms. Unlike other environmental

compartments (atmosphere and water) the pollutants present in soil have long residence periods. The

decontamination of the ground could represent a social problem due to the importance of protecting the

environment, human health and because of its economic relevance. It is clear the need to find techniques for

soil remediation that offer the proper and correct elimination and disposal of the pollutants.

The potential effects of chromium on health depend on a variety of factors, such as the chemical state in

which is found, the amount, exposure time and the way chromium is incorporated to the organism (ingestion,

inhalation and absorption through skin). Although the Cr(III) (in low doses) is an essential trace element in

animals (Schwartz and Mertz 1959; Mertz 1975), Cr(VI) is not, and it is considered to be carcinogen (ATSDR

2000; U.S. EPA 1984, 1990, 1998; IARC 1990; OMS 1988). The main human activities that increase Cr(VI)

concentrations are steel, chromite processing industry, leather and textile manufacturing, pigments,

electroplating and other industrial applications of Cr(VI). It is common that many of these residues remain in,

or around the sites of factories, especially in the case of wastes and foundry salts, whose toxicity, generally

low, tends to increase along the time (Palmer and Wittbrodt 1991).

In soils, Cr(III) is relatively static due to its high adsorption capacity, particularly for iron and manganese

oxides, clay minerals and sand (Santone 2009). Cr(III) as hydroxides, once again sedimented this compounds

are hardly moved, because the oxidation of Cr(III) compounds to form Cr(VI) compounds practically does not

occur naturally (Lee and Hering 2005; Unceta 2010). Cr(VI), has great mobility in the subsurface due to its

high solubility, and even in relatively low concentrations is toxic, being pH of the soil a determinant factor

(Palmer and Wittbrodt 1991).

The technologies that incorporate sorption mechanisms have demonstrated a limited sorption capacity of

Cr(VI) and to increase those values it is required to generate an acid environment (Carro Navarro et al. 1995;

Gupta and Babu 2006; Devaprasath et al. 2007; Otiniano et al. 2007; Popuri et al. 2007; Acosta et al. 2008).

Also it is need to remark that if migration of Cr(VI) is not consider from those adsorbents materials to the

environment, only the speed of advection of Cr(VI) will be retard.(Palmer and Wittbrodt 1991).

Chemical treatment is the main aim of reducing the bioavailability/mobility of pollutants through reactions

with specific reagents, it is the case for chromium in which redox reactions are aimed at reducing the Cr(VI)

to Cr(III), which is less toxic and mobile. Bioremediation of soils contaminated by Cr(VI) studies show that

the reduction of Cr(VI) by microorganisms is possible when the concentration of Cr(VI) is low and in

anaerobic conditions, however, for highly contaminated soils (>4000 mg/kg) reduction is inhibited, possibly

because the amount of chromium is toxic for microorganisms and generate low population (Tseng and

Bielefeldt 2002; Tokunaga et al. 2003). Studies of reduction by chemical agents as Fe(II), C6H6FeNO6,

C12H22O11, H3PO3, C4H6O6 and TiO2 have shown extreme conditions of acidity (< 1 - 3) and in some cases a

radiation source so the reduction is carried out at an appropriate level (Batchelor et al. 1998; Wang et al.

1999; Tzou et al. 2003; Khan et al. 2006; Abida et al. 2010; Wang et al. 2010).

Research involving standard solutions of K2Cr2O7 indicates that the reduction of Cr(VI) to Cr(III) by vitamin

C (ascorbic acid) occur both under the irradiation and in the dark, present low reaction time, in a wide range

of temperature and pH, with the added bonus that the reaction product, dehydroascorbic acid, can be degraded

by microorganisms in groundwater or soil (Xu et al. 2004; Xu et al. 2005; Liu et al. 2005). Its applicability to

environmental conditions has not yet been explored.

The present research shows the evaluation of the reduction of Cr(VI) by ascorbic acid in an heavily

contaminated soil, the samples were collected from an industrial landfill located in the field of Química

Central, a company engaged in the manufacture and marketing of chemical products derived from chromium,

located in the State of Guanajuato, Mexico. The company is located adjacent to the highway and to the

railroad León-San Francisco del Rincón, and to the river Los Gómez-León-Turbio and the northwestern edges

of the San Germán dam. Also, the lands of Química Central are located very close to the urban nucleus of

Buenavista. The areas situated to the east, northeast and southeast are used for agriculture purposes, mainly

sorghum and lucerne (Reyes-Gutierrez et al. 2009).

The specific objectives of this study were: a) to evaluate the kinetic of the reduction in samples taken from the

chromium-contaminated soil at different temperatures, b) to characterize morphological and mineralogical the

soil before and after treatment, c) to identify the chemical species in the solid and soluble fraction of each

sample in order to evaluate the reduction method of Cr(VI) by using ascorbic acid.

2 Materials and Methods

2.1 Sampling

Four points were selected for sampling in the Química Central landfill (Fig. 1): an unaltered area of 1 m 2 was

cleaned and a one meter deep hole was drilled. In the vadose zone, a fine to medium texture sediment was

collected. Soil profiles, respectively 30, 60 and 100 cm deep were investigated to assess Cr content. Samples

of 2 kg were collected into polyethylene bags, transported in an ice pail to the laboratory and stored in a

refrigerator at 4 °C in an in situ moisture condition (0 to ~ 0.1 kPa of water potential). The samples were

air dried to retard the activity of Cr(VI) (U.S. EPA, 1996, method 3060A). Besides, a composite sample was

prepared by mixing several of the collected samples (100 g), and passing them through a No. 40 polyethylene

mesh to obtain a subsample with a relatively homogeneous particle size; this sample was used in the

experiments (Reyes-Gutierrez et al. 2009).

2.2 Cr(VI) sample determination and effect of the [Cr(VI)]:[ascorbic acid] mg/kg ratio on soil treatment

1 g of sample was weighted and transferred to a 100 mL volumetric flask, dissolved with distilled water and

kept in agitation for 24 hours. The concentration of Cr(VI) was determined by the diphenylcarbazide method.

The samples were analyzed in a UV-Vis spectrophotometer PERKIN-ELMER model Lambda 25. All

reagents used for the quantification of Cr(VI) in samples were reactive grade.

Once determined the concentration of Cr(VI) in the sample, a reduction test was performed by duplicate in 1 g

of soil by direct contact at room temperature, for 24 hours to determine the concentration of ascorbic acid that

carries out 100% of reduction, in assessing concentrations Cr(VI) - ascorbic acid 1: 1, 2, 3, 4, 5 and 6 in

mg/kg.

2.3 Kinetic analysis

Reduction kinetics was carried out by duplicate, using the batch equilibrium technique which consists to in

direct contact ascorbic acid solution with contaminated soil during specific periods of time (1, 3, 5, 10, 20,30,

45, 60, 90, 120 y 180 min). Experiments took place without adjustment of pH and at temperatures of 10, 20,

25, 30, 40 y 50 °C. Fermont A.C.S. ascorbic acid standard solution was used for the reduction tests.

2.4 Physicochemical characterization

The morphology and elemental chemical analysis of samples was performed by SEM: scanning electron

microscope model JSM-6510LV JEOL brand, a device coupled with energy dispersive spectroscopy X-ray

(EDS) model X3 Incapet Oxford brand. To determine XRD mineralogical species was used: an X-ray

diffractometer Bruker D8 Advance, Cu lamp with a tube voltage and current of 30 kV and 25 mA

respectively, continuously in a range of 5 to 80 ° 2θ and a step size of 0.03 ° 2θ for 40 min. Speciation of

chromium in the soil surface by XPS. The XPS wide and narrow spectra was aquired using a JEOL JPS-9200,

eqquiped with a Mg X-ray source (1253.6 eV) at 200 W, the area of analysis was 1mm, and the vacuum was

in the order of 10-8 Torr for all samples. The spectra was analyzed using the specsurfTM software included with

the instrument, all spectra was charge corrected by means of the adventitious carbon signal (C1s) at 284.5 eV.

The Shirley method was used for the background substraction, whereas for the curve fitting the Gauss-

Lorentz method was used. For speciation of chromium in the soluble fraction was used a HPLC technique

with RP18 column Lichospher Metachem 5 m 250 x 4.0 mm, 25 mM potassium phosphate monobasic pH

3.0, detection wavelength dual Waters 2487, Waters 1515 isocratic pump and Breeze software for data

processing. The analysis of the present species in samples was made by comparing obtained signals (retention

time) with standard solutions of Cr(VI), Cr(III), ascorbic acid and dehydroascorbic acid.

3 Results and Discussion

3.1 Cr(VI) sample determination and effect of the [Cr(VI)]:[ascorbic acid] mg/kg ratio on soil treatment

The content of Cr(VI) was determined in 50 parts of the sample obtained by the method of diphenylcarbazide,

yielding an average concentration of 5886.2 mg/kg of dry soil with a coefficient of variation of 9.6% (Table

1), this concentration exceeds the limit establish by the NOM-147-SEMARNAT-SSA1-2004 for industrial

land use (510 mg/kg).

From the evidence of reduction at 24 h with different Cr(VI): ascorbic acid ratios in mg/kg, it was observed

that at 1:5 ratio, 100% of the initial Cr(VI) were reduced at natural pH and room temperature (Table 2).

3.2 Kinetic analysis

The efficiency and rate in the reduction of Cr(VI) increase as the reaction temperature increases as is shown

in Fig. 2. In 180 min 100 % of reduction is reached at 40 and 50 °C, at the other hand, 89 % of reduction is

reached at 10 °C. The initial pH was 8.2 ± 0.5, while treated soil pH was 7.1 ± 0.2 during all experiments; no

acidification of the ground is generated.

The experimental results have not demonstrated a first order dependence as reported in studies with potassium

dichromate solution (Xu et al. 2004, Xu et al. 2005, Liu et al. 2005). The experimental data satisfies second

order kinetics:

(1)

The kobs values calculated by equation (1) are listed in Table 3 (0.973 < r 2 < 0.997). The coefficients of

determination values indicate that the reduction follows second order kinetics model during the first hour of

the reaction.

Assumming that kobs has Arrhenius behavior, the activation energy (Ea) can be calculated from the relation

below:

(2)

Where A is the pre-exponential factor, R is the universal constant of gases (8.314 J/mol ∙K), and T is the

reaction temperature (K). According to that, the minimum energy required for the reduction of Cr(VI) by

ascorbic acid in the contaminated soil is 104.1 kJ/mol (Fig. 3).

3.3 Physicochemical characterization

The transition elements are characterized for their capacity to change color when they are reduced. This was

observed in the reduction test by ascorbic acid. The yellow color presented in Cr(VI) contaminated sample

turns to green, indicating the presence of Cr(III) compounds in soil.

The obtained micrographs by SEM of the Cr(VI) contaminated soil and treated soil show a heterogeneous

soil, as well as, small aggregates adhered to particles of major size, indicating that the material is a

heterogeneous powder. The size of discrete soil particles ranges from 250 m to less than 1 m (Fig. 4).

Elemental chemical analysis by energy dispersive spectroscopy X-ray (EDS), Table 4 shows the results from

content in weight % of each element present in the sample, both contaminated and treated soils show the usual

composition of aluminosilicates (Si, Al, O, Na and Ca) and indicate that the main constituents of the soils are

O, Si and C. Cr represents 0.75% by weight in the treated samples, similar to the Cr content in the

contaminated sample (0.66%). There is no decreasing in the amount of Cr, due to the process only reduce the

oxidation state of Cr(VI) to Cr(III), and not imply a remotion process.

The treated and contaminated soil samples were analyzed by XRD, the diffractogram of the contaminated soil

(Fig. 5a) shows that the principal minerals found are quartz, SiO2 (JCPDS card 33-1161), albite (Na, Ca)(Si,

Al)4O8 (JCPDS card 20-0554), and chromatite CaCrO4 (JCPDS card 08-0458); as they were previously

reported by Reyes-Gutiérrez et al. 2009. The chromatite evidence the presence of Cr(VI). Fig. 5b shows the

diffractogram of the soil after remediation process. These samples have a mineralogical composition of

quartz, SiO2 (JCPDS card 33-1161) and albite (Na, Ca)(Si, Al)4O8 (JCPDS card 20-0554). It should be noted

that no chromatite was identified by XRD in this sample.

The XPS surface analysis was used to determine the oxidation state of chromium present in the polluted and

treated soil. The narrow scan of Cr 2p3/2 from the contaminated and treated soil samples are shown in Fig. 6.

From the deconvolution of chromium, it is obtained that the contaminated soil present two peaks at binding

energies of 578.7 eV and 577.4 eV corresponding to Cr(VI) probably as CrO3 (70.5 %) and Cr(III) as

Cr(OH)3 (29.5 %) respectively, while in the soil after remediation process only shows signals corresponding

to compounds of Cr(III), at 575.6 eV and 577.1 eV as Cr2O3 and Cr(OH)3 respectively. It is clear from the

intensity of the peaks in Fig. 5, that ascorbic acid reduced Cr(VI) to Cr (III) at 100%

Speciation of the chromium in the soluble fraction of contaminated and treated soil, was carried out by HPLC

at 230 nm, this technique allows following the progress of the reaction:

(Liu et al.

2005)

Fig. 7a, presents the chromatogram for the contaminated soil showing the signal of Cr(VI) at 2.4 min, the

chromatogram for the soil after remediation process shows only the signal corresponding to Cr(III) at 2.0 min

(Fig. 7b).

The chromatogram of the soil treated with an excess of ascorbic acid, shows signals of Cr(III) at 2.0 min,

dehydroascorbic acid at 2.8 min, and ascorbic acid at 3.2 min (Fig. 8a). The chromatogram obtained from the

soil sample treated at 10 °C, shows Cr(III) and Cr(VI) signals, where 10 % of Cr(VI) was not reduced (Fig.

8b). In this way it was possible to observe the reaction progress by HPLC with the formation of

dehydroascorbic acid and Cr(III), and the decline and subsequent disappearance of the signals corresponding

to the Cr(VI) and ascorbic acid.

4 Conclusions

The concentration of Cr(VI) in the soil sample was 5886.2 mg/kg. The ascorbic acid in a 1:5 ([Cr(VI)]:

[ascorbic acid], mg/kg) ratio reduces the Cr(VI) at 100 % in 24 hours at temperatures of 20-50 ˚C. The

reduction in the soil follows a second order kinetic reaction in the first 60 minutes and the activation energy is

104.1 kJ/mol. The XPS and HPLC analysis verify the reduction of Cr(VI) to Cr(III) by ascorbic acid,

identified in treated soil only species corresponding to Cr(III).

On the basis of the results present in this study, it can be concluded that the ascorbic acid treatment could be

an efficient method as a part of a global strategy of remediation of heavily contaminated soils. However in

addition to the results obtained in this study, numerical techniques must be applied to take into account the

processes affecting the migration of Cr(VI) as well as the reducing agent on the ground including

groundwater flow, diffusion-controlled mass transfer across heterogeneous boundaries,

dissolution/precipitation, and adsorption/desorption, to predict the rate and percentage of Cr(VI) reduction in

situ.

Acknowledgments

In first place to Química Central de México for the technical support, to the Universidad Autónoma del

Estado de México (UAEM) for the financial support through proyects 2049/2005 y 2566/2007U, and also to

the Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM for the analytical support.

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FIGURE CAPTIONS

Fig. 1 Map of study area, Buenavista, León Guanajuato, México. The location of the sampling sites at the

industrial landfill of the chromium wastes in the facilities of the Química Central chromate factory is shown.

(Reyes-Gutiérrez, et al., 2009)

Fig. 2 Effects of temperature on the Cr(VI) reduction efficiency

Fig. 3 Plot of ln(kobs) versus 1/T under different temperatures. Reactions were performed at natural pH of the

soil sample

Fig. 4 Scanning Electron Microscope (SEM) micrographs. a soil contaminated with Cr(VI). b soil after

remediation process

Fig. 5 X-ray diffraction (XRD) patterns that show the intensity of diffracted X-rays from various planes as a

function of 2θ value for a soil contaminated with Cr(VI). b soil after remediation process. Q: quartz, A: albite,

and C: chromatite

Fig. 6 X-ray photoelectron spectroscopy (XPS) spectrums for a soil contaminated with Cr(VI). b soil after

remediation process. The binding energy peaks are assigned to various valence states of chromium

Fig. 7 Chromatograms obtained using High Performance Liquid Chromatography (HPLC) at 230 nm for the

soluble fraction from a soil contaminated with Cr(VI). b soil after remediation process

Fig. 8 The typical chromatograms of chromium species obtained from real samples a with an excess of

ascorbic acid. b an incomplete Cr(VI) reduction