الفهرس 
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Abstract
Hexavalent chromium Cr(VI) is considered a mutagenic, teratogenic and has been listed as class-A human carcinogen by the United States Environmental Protection Agency US-EPA. It is discharged into the environment from several industries such as leather tanning, metal cleaning, alloy formation and textile dyeing.
Biological adsorption and reduction of Cr(VI) to relatively less toxic Cr(III) by microbial biomass is regarded as one of the most practical and useful methods for detoxification of Cr(VI).
In this study 49 Cr(VI) resistant bacterial isolates were isolated from different contaminated and non-contaminated soils (Misr Al Qadima tanneries, beside Alsaad aluminum factory and Ain Shams University garden soil). They were selected on nutrient agar and starch nitrate agar media. Screening was done to select the most potent Cr(VI) reducing isolates. Among the 49 isolates three potent Cr(VI) reducing isolates (Ia3, Ia4, and Sa1) showed remarkable ability to reduce Cr(VI) at high concentration.
The three bacterial isolates were identified by16S rRNA gene sequence analysis as Bacillus sp., Streptomyces rochei and Pseudomonas chlororaphis. Several factors were examined to improve Cr(VI) reduction. These factors included incubation periods, inoculums size, Cr(VI) concentration, pH, temperature and agitation. The optimum inoculums size, pH, temperature and agitation were 140, 285x107 CFU/mL for Bacillus sp. Ia3 and Pseudomonas chlororaphis Ia4 respectively and 51x105 CFU/disc (using 3 discs) for Streptomyces rochei Sa1, 7, 30 °C and 200 rpm respectively. While efficiency of Cr(VI) reduction decreases with the increase of Cr(VI) concentration and increases with the increase of incubation period Cr(VI). Sonication was performed to determine the fate and distribution of Cr forms. After 24 h of incubation of Bacillus sp. Ia3 bacterium in a nutrient broth containing 200mg/L Cr(VI). It was found that 61.3 % of Cr(VI) was reduced to Cr(III), 24.5 % of Cr(VI) was found unreduced in the cell free supernatant and 14.1 % accumulated inside the bacterial cells as Cr(VI). On the other hand after 7 days of incubation of Streptomyces rochei Sa1 in a starch nitrate broth containing 400 mg/L Cr(VI). It was found that 56 % of Cr(VI) was found
unreduced in the cell free supernatant, 22.3 % accumulated inside the actinobacterial cells as Cr(VI) and 21.7 % Cr(VI) was reduced to Cr(III). Depending on the preceding results, optimum environmental conditions were applied to a microcosm experiment using a consortium of the three bacterial isolates. A promising result with complete reduction of Cr(VI) concentrations 200 mg/Kg and 400 mg/Kg in 4 and 7 days respectively was reported for sterile soils amended with Cr(VI) and consortium and 5 and 9 days respectively for unsterile soils amended with Cr(VI) and consortium. Also a slower rate of reduction observed in unsterile soils with Cr(VI) and without consortium and all sterile controls remains with no reduction. Scanning electron microscopy and EDX was done to the most potent bacterium Bacillus sp. Ia3 and actinobacterium Streptomyces rochei Sa1. It showed that for Bacillus sp. Ia3 the Cr(VI) accumulates on cell surface, the cells are smaller and the growth is less heavy in cells treated with Cr(VI) than the non-tread cells. While for actinobacterium Streptomyces rochei Sa1decreased sporulation with hollow spore sheath and enlarged mycelia were observed in culture treated with Cr(VI) in comparison with control culture.
A chromium responsible peak in EDX was observed in chromium treated cultures of both bacteria and absent from control cultures.
Transmission electron microscope photos showed hollow dead cells of Bacillus sp. Ia3 and presence of small particles of Cr precipitates found inside cells in Cr(VI) treated cultures. These precipitates were also found in case of Streptomyces rochei Sa1 mycelia