Distribution and mobility of Cr in tannery waste amended semi-arid soils under simulated rainfall.

This study was conducted to evaluate the distribution and mobility of Cr in tannery waste that has been added to semi-arid soils. In addition, the amount of total oxidizable soil Cr (III), Cr (VI), pH and soil microbial activities were determined. Tannery sludge alone or mixed with fleshing waste was added to two types of soils, which were then incubated at 25 degrees C for 6 months and subsequently subjected to simulated rainfall. The highest total amount of Cr loss occurred due to infiltration, regardless of the treatments. The Cr loss ranged from 0.452 to 0.825 microg g(-1) soil for all soils from 1 to 3 months, with the exception of those that were located under the canopy and treated with tannery sludge and fleshing waste, which had the highest runoff (from 1.312 to 1.667 microg Crg(-1) soil). The pH of the soil increased from 1 to 3 months (from 7.35 to 8.46), while the total oxidizable soil Cr (III) (2.12-4.31 mg g(-1) soil) peaked after 1 month of treatment. The majority of the fractionated chromium was bound to carbonates at initial time, 3 and 6 months prior to and after application of the simulated rainfall. The microbial activities decreased in each of the treated soils from initial time to 3 months; however, after 6 months bacterial activity increased, while pH decreased (from 8.03 to 7.63). Overall, these data suggest that pH is responsible for Cr loss in response to infiltration and runoff, Cr speciation and the equilibrium of Cr fractionation.

Effects of Cr3+, Cr6+ and tannery sludge on C and N mineralization and microbial activity in semi-arid soils.

The aim of this study was to evaluate the effect of two Cr species (Cr(3+) and Cr(6+)) on N and C mineralization and dehydrogenase activity in semi-arid soils. The Cr species (250 mg kg(-1)soil) were either added alone or mixed with tannery sludge (0.0125 g g(-1)) to three soils: cultivated soils, and outside and under the canopy of mesquite trees were then incubated for 180 days at 25 degrees C. Sole Cr(6+) addition had a higher inhibition of CO(2) production rate in cultivated soil (58-73%) than in soils under the canopy and outside the canopy. Soil outside the canopy amended with Cr(6+) showed the highest inhibition of dehydrogenase activity (40-100%) followed by cultivated and under the canopy soils. However, Cr(6+) added alone increased the inhibition of nitrification in soil outside the canopy (68-84%, from 30 to 120 days), followed by under the canopy and cultivated soils. The addition of tannery sludge to Cr(6+) significantly reduce the CO(2) production rate and dehydrogenase activity in all three soils, and increased the inhibition of nitrification in the following order: outside the canopy, cultivated and under the canopy soils. The addition of Cr(3+) or Cr(3+) plus tannery sludge either stimulated or inhibited CO(2) production rate, dehydrogenate activity and ammonification in the three soils in no clearly defined order. Measurement of dehydrogenase activity was the best tool for assessing the harmful effect of Cr(6+) on soil microbial activity in semi-arid soils exposed for an extended period.