The effects of MnO2 on sorption and oxidation of Cr(III) by soils
Introduction
Chromium is one of the heavy metals that can be hazardous. Therefore, knowledge of the transport and transformation of this element in the environment can be important for predicting threats from its presence. Chromium action and toxicity depend on its oxidation state. Its commonly existing forms are trivalent and hexavalent chromium. Compounds containing chromium in these two oxidation states differ with respect to chemical and physical properties and also in toxicity Bartlett and James, 1979, Bartlett and James, 1983.
Hexavalent chromium is very toxic, and it can be an inhaled carcinogen, poisonous to humans and other mammals Burrows, 1983, Bartlett and James, 1988, James, 1996 whereas trivalent chromium is an essential nutrient as a mineral supplement (Saner, 1980).
In the natural conditions, which we describe here in terms of pH and redox potential (Eh), the more stable oxidation state of chromium is trivalent chromium. In biological systems (pH 3 to 7 and Eh for −200 to 500 mV) compounds of hexavalent chromium tend to reduce to more thermodynamically stable Cr(OH)3 Barnhart, 1997, Massacheleyn et al., 1992. In more oxidizing environment at Eh>600 mV and pH above 6.0 Cr(VI) is stable Chin, 1994, James, 1994, James and Bartlett, 1983. Our interest is connected with chromium transformation in soils. Chromium (III) can be oxidized in soil especially by manganese (IV) oxides and hydroxides James et al., 1997, Yingxu et al., 1997 according to the reaction:
This reaction strongly depends on pH, and is the most effective in pH 4.5 to 6.0 (Yingxu et al., 1997). Chromium oxidation occurs at MnO2 solution/solid interface, and does not follow solution stoichiometry (Chin, 1994).
There are many reduction agents present in soils, such as organic compounds, compounds of divalent iron and elemental iron. In soils rich in organic matter, oxidation of chromium is limited because chromium bound with organic compounds is not easily oxidized (Chin, 1994).
The aim of this work was to determine the sorption capacity of several soils for trivalent chromium sorption, and to measure their potential to oxidize Cr(III) to Cr(VI) with and without added by MnO2.
Section snippets
Area description, methods and material studied
The investigations were carried out using four different soils from the soil bank maintained by the Institute of Agrophysics of Polish Academy of Sciences in Lublin, Poland. The basic grain-size characteristics of these soils are presented in Table 1. These soils differed in organic matter content: soil 1: Eutric Cambiosol developed from loess (0.72% of organic carbon), soil 2: Phaeozem soil developed from loess, soil 3: Fluvisol developed from silty loam and soil 4: Histosol developed from
Results and discussion
The results of Cr(III) sorption on the soils and Cr(III) oxidation by the soils are presented in Table 2. Soil was introduced to a 200 ml solution of 0.001 M CrCl3·6H2O that corresponds to an initial Cr3+ concentration of 52 mg/kg. Initial trivalent chromium levels introduced to the system are designated as Cr(III)int and Cr(III)res is a residual chromium (III) present in the soil solution after the process of the sorption. Analysis of solutions found that almost all chromium (III) was adsorbed
Conclusions
- 1.
Addition of 52 ppm of Cr(III) to soil–water suspensions resulted in almost complete sorption of trivalent chromium. Chromium (III) retention by these soils ranged from 97.22% to 99.53% of the introduced Cr(III).
- 2.
Soil capacities for chromium (III) sorption are similar for all four soils and equal to 9.72×10−4–9.95×10−4 mol of Cr3+/100 g of soil.
- 3.
Oxidation of trivalent chromium by manganese (IV) oxide progresses with efficiency of maximum 2.66% in these conditions.
- 4.
The sorption of Cr(III) on soil
References (19)
- et al.
Determination of optimal chromium oxidation conditions and evaluation of soil oxidative activity in soils
J. Geochem. Explor.
(1998) Chromium chemistry and implications for environmental fate and toxicity
J Soil Contam.
(1997)- et al.
Behavior of chromium in soils: III. Oxidation
J. Environ. Qual.
(1979) - et al.
Behavior of chromium in soils: VI. Interactions between oxidation–reduction and organic complexation
J. Environ. Qual.
(1983) - et al.
Chromium Metabolism and Toxicity
(1983)Fate and transport of chromium through soil in migration and fate of pollutants in soils and subsoils. Theory and practice
Ist. Rec. Sulle Acque
(1994)- et al.
Problems of the speciation of chromium in soil samples
Fresenius' J. Anal. Chem.
(1995) Hexavalent chromium solubility and reduction in soils enriched with chromite ore processing residue
J. Environ. Qual.
(1994)