Chromium contents linked to iron oxide at areas with tannery sludge disposal

ABSTRACT The tannery sludge disposal in the soil has been a serious environmental problem due to the high chromium concentrations. The aim of this study was to evaluate the iron oxides dynamic and chromium concentrations linked to these minerals in areas of tannery sludge disposal. So, concentration of iron and chromium extracted with ammonium oxalate from oxides with low crystallinity and with dithionite-citrate-bicarbonate from the crystalline oxides in the clay fraction was evaluated. The concentration of chromium extracted from the iron oxides with low crystallinity varied between 3.5 and 4.1 mg kg-1 in the treatments without application of tannery, and between 19.9 and 56.8 mg kg-1 in the treatments with tannery application. The concentration of chromium extracted from the crystalline oxides varied between 21.3 and 23.3 mg kg-1 in the treatments without application of tannery sludge, and between 46.4 and 103.0 mg kg-1 in the treatments with tannery. The application of tannery promoted an increase in the relation between the iron extracted from the iron oxides and the iron extracted with dithionite (Feo/Fed) and the oxides from the disposal area, which is an evidence of the oxide crystallinity decrease. A reduction was found in the relation between the iron extracted with dithionite and the chromium extracted with dithionite (Fed/Crd), what shows the substitution of the iron by chromium in the oxides, emphasizing an isomorphic substitution. This isomorphic substitution can be an alternative to maintain the chromium in the soil with strong retention.