Elsevier

Geoderma

Volume 122, Issues 2–4, October 2004, Pages 291-296
Geoderma

The effects of MnO2 on sorption and oxidation of Cr(III) by soils

https://doi.org/10.1016/j.geoderma.2004.01.015Get rights and content

Abstract

In the paper, the oxidation of trivalent chromium in soils is presented. The main oxidant responsible for oxidation of Cr(III) compounds in soil is manganese (IV) oxide. The paper examined two aspects of Cr(III) oxidation in soils: (1) the soil oxidation potential and; (2) sorption of Cr(III) on the examples of four soils differing in organic matter content (0.72% to 3.72% of organic carbon) and total and easily reducible Mn. The second consideration was oxidation of Cr(III) by MnO2 itself and in the presence of the soils studied. The experiments were carried out in a batch process for 2 h, in which the soil was mixed with CrCl3 solution at 1:10 ratio. The initial Cr(III) concentration in the solution was 52 ppm. In order to examine Cr(III) oxidation by MnO2, the oxidant was added in three amounts: 0.065, 0.13 and 0.26 g/100 g soil (dry basis). The studies showed that the soil oxidation potential was not detectable in the soils studied. Sorption capacities for chromium (III) were similar for each soil and ranged from 9.72×10−4 to 9.95×10−4mol of Cr(III)/100 g of soil. The oxidation of Cr(III) by MnO2 was slight and progressed with a maximum conversion of 2.66% for the highest dose of oxidant (0.26 g). The presence of the soil decreases Cr(III) oxidation by MnO2; the decrease being related to the soil properties (organic matter, reducible Mn).

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:Cr3++1.5MnO2+H2OHCrO4+1.5Mn2++H+

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)

  • T. Makino et al.

    Determination of optimal chromium oxidation conditions and evaluation of soil oxidative activity in soils

    J. Geochem. Explor.

    (1998)
  • J. Barnhart

    Chromium chemistry and implications for environmental fate and toxicity

    J Soil Contam.

    (1997)
  • R.J. Bartlett et al.

    Behavior of chromium in soils: III. Oxidation

    J. Environ. Qual.

    (1979)
  • R.J. Bartlett et al.

    Behavior of chromium in soils: VI. Interactions between oxidation–reduction and organic complexation

    J. Environ. Qual.

    (1983)
  • R.J. Bartlett et al.
  • D. Burrows

    Chromium Metabolism and Toxicity

    (1983)
  • P.-K.F. Chin

    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)
  • P. Fodor et al.

    Problems of the speciation of chromium in soil samples

    Fresenius' J. Anal. Chem.

    (1995)
  • B.R. James

    Hexavalent chromium solubility and reduction in soils enriched with chromite ore processing residue

    J. Environ. Qual.

    (1994)
There are more references available in the full text version of this article.

Cited by (0)

View full text