Adsorption of different-valence chromium ions by soils of Udmurtia

Industrial waste and sewage sludge are the main sources of receipt of man-made compounds chromium in the environment. The chromium is in different oxidation states in such compounds. In the present study the authors analyzed the behavior of chromium ion in the oxidation state 3+ and 6+ in some types of soil Udmurtia. Under the laboratory conditions a sorption of chromium ions was studied in A-horizons of the most common types of soil Udmurtia: sod-carbonate, dark gray forest, sod-podzolic, sod-weakly podzolic poorly washed away, sod-carbonate leached poorly washed away, gray forest podzolized.
The Initial solutions of chromium 6+ with a concentration about 0.7 mg/dm$^3$ and chromium 3+ with a concentration about 0.3 mg/dm$^3$ was prepared by dissolving the samples of corresponding salts K$_2$Cr$_2$O$_7$ and Cr$_2$(SO$_4$)$_3$, the exact concentration was determined spectroscopically.
Adsorption is a spontaneous process of accumulation substances at the phase boundary. According to experimental data the maximum absorption capacity of chromium cations recorded in dark gray forest soil. Extent of absorption of chromium cations for this type of soil approximates to 100 %. Sod-carbonate soil absorbs metal ions a little less. Sod-podzolic soil absorbs chromium ions less than all the investigated types of soils. The adsorption isotherms of chromium cation sod-carbonate and sod-podzolic soils are of L-shape. In this type of adsorption the adsorbate with low values of concentration is relatively high chemical affinity with the adsorbent surface. Adsorption isotherms of chromium (III) for dark gray forest soil is preferable refers to the H-form, which indicates a very high affinity adsorbate to the surface of the adsorbent. Organic substance, oxides and hydroxides of iron, aluminum and manganese, pH of the soil filtrate significantly affect the adsorption of chromium cations by soil absorbing complex.
According to experimental data among the investigated soil types the highest sorption capacity relative to the dichromate ions was found in sod-carbonate leached poorly washed away soil. Gray forest podzolized and sod-weakly podzolic poorly washed away have about the same absorption capacity in relation to the anion of chromium (VI). With increasing concentration of the injected pollutant there is happening nonmonotonic increase in the amount of sorbed anion by the soil. This process can be caused a shift of the adsorption-desorption equilibrium between the soil and the dichromate ion.
In the investigated soil samples pH soil extract is practically the same value and has no significant effect on the difference in the degree of adsorption of dichromate ion soils. Content of organic substance in the soil is not the main factor determining the soil adsorption capacity for anions. The proximity of the analyzed soil samples for mineralogical and granulometric composition leads to slight differences in their sorption abilities towards dichromate-ion. We have found that the same quantity of applied to the soil pollutant cations absorbed by the soil in larger quantities and stronger compared to its anionic form. This phenomenon can be explained by the fact that in Nonchernozem strip SAC preferably has a negative surface charge, besides sesquioxides oxides of soil are distinguished by high selectivity and significantly greater strength in relation to the heavy metal cations. On the other hand, non-specificity of soil colloids to the dichromate-ion results in a decrease in the extent of absorption of the anion with increasing concentration of pollutant and its transition into the soil filtrate.
On the basis of laboratory experience, it was found that cationic forms of metals absorbed by the soil stronger and in greater amounts compared to its anionic forms. The existence of a periodically changing the equilibrium in the system SAC $\leftrightarrow$ soil solution is typical for anions and, therefore, their high mobility in the soil.