Detoxification Of Chromium(â…¥) By Bacteria

Chromium and its compounds are important industrial raw materials. Large amount ofsolid wastes and effluents containing hexavalent chromium (Cr(Ⅵ)) which has beenrecognized as a carcinogen internationally, are discharged off from their production andapplications. Cr(Ⅵ) poses a great threat to the environmental safety and human health.Therefore, detoxification of Cr(Ⅵ) is commonly concerned by domestic and internationalresearchers. A large number of studies have been conducted and a variety of methods havebeen suggested. However, due to complexity of the issue, up to the present there has no anyways which are acknowledged and can be used in any cases.A biochemical engineering method is attempted to detoxify Cr(Ⅵ) in this study. Firstly,sewage and sludge samples containing Cr(Ⅵ) were obtained from a plant which produceschromium-related products. Secondly, strains of Cr(Ⅵ)-resistant bacteria were enriched inbeef extract and peptone liquid culture medium (CC medium) and were screened by plateculturing and separating technology. Thirdly, experimental strains were obtained byacclimating in M9minimal salt medium. The results of shaking-flask-cultivation showed thatsix strains isolated grew in the CC medium with Cr (Ⅵ) up to1000mg/L and the M9minimalsalts medium containing200mg/L Cr(Ⅵ). One strain named as CRB-3with high Cr(Ⅵ)toleration was found later and it removed90%of Cr(Ⅵ) from a solution of10mg/L Cr(Ⅵ) in24h. The bacterium is gram negative and its growth experienced no distinct lag phase andreached to steady after18h.The factors affecting Cr(Ⅵ) reduction of the bacteria, such as carbon source, initial pHvalue, concentration of Cr(Ⅵ), inoculum amount of the cells, and temperature wereinvestigated. The results showed that10g/L glucose and pH6were the best, and CRB-3wereable to reduce Cr(Ⅵ) in the range of Cr(Ⅵ)5-20mg/L and temperature20-37℃. But theCr(Ⅵ) removal percentage decreased with the increase of initial Cr(Ⅵ) concentration. Theincrease of cells amount leaded to the increase of Cr(Ⅵ) removal.Detoxification of chromite ore processing residue (COPR) using CRB-3was studied.Orange peels were added as the neutralizer of COPR, assistant reducer Cr(Ⅵ), and nutrient ofthe bacteria. Orthogonal experiments were carried out to probe main factors affectingdetoxification efficiency and optimize detoxification conditions. The dosages of bacteria andorange peel, treatment temperature, and treatment time were selected as the factors. The results showed that it is proper to add3grams of orange peel for each gram of COPR, and thelarger the amount of bacteria, the better the Cr (Ⅵ) reduction, but the change was notsignificant. The results also showed that pH value of6-8and temperature of around37℃were beneficial to Cr(Ⅵ) removal, The best result—the concentration of leaching Cr(Ⅵ)from treated COPR,1.38mg/L, was reached after duration of75days, which met therequirement of GB18598–2001,2.5mg/L.