Research Progress of Enhanced Microbial Remediation Techniques for Chromium-Contaminated Sites
Chromium is a toxic metal element with carcinogenicity and teratogenicity.Human activities such as chromium salt production,chromium plating and leather manufacturing cause a large amount of chromium to be released into the environment,caused serious environmental pollution,which has aroused wide attention of scholars in various countries.Firstly,the present situation of chromium pollution at home and abroad and its distribution characteristics in soil,water and atmosphere were introduced.Chromium mainly come from natural sources such as rock and mineral decay,volcanic eruption and atmospheric subsidence,and human activities such as chromium smelting,chromium plating and leather manufacturing.Chromium ore was mainly distributed in Mediterranean region and Pacific Rim region,and South Africa was the largest chromium storage area.Chromium pollution in China was mainly concentrated in the central region,followed by the west.Chromium mainly existed in the form of Cr(Ⅵ)and Cr(Ⅲ)in the environment.Under oxidation conditions,when pH was 0～4,HCrO4-was a common inorganic form.Under reducing conditions,when pH was 5～14,Cr(OH)4-was a common inorganic form.Oxides/hydroxides,organic matter and microorganisms affected the migration and transformation of chromium in soil through redox and adsorption.Microbial remediation mainly relied on bioadsorption,bioaccumulation and biotransformation,and Cr(Ⅵ)was adsorbed and reduced by microorganisms such as bacteria and fungi.The surface of microbial cell membrane and cell wall contained a large number of functional groups,which could adsorb Cr(Ⅵ)by electrostatic adsorption and complexation.Cr(Ⅵ)could be reduced by secreting extracellular polymers and chromium reductase.It was important to strengthen the microbial remediation technology to apply mineral materials,fixed materials and irritants to improve the removal efficiency of Cr(Ⅵ)by improving the electron transfer efficiency and providing a suitable living environment for microorganisms.At present,the commonly used chromium repair microorganisms were Pseudomonas,Bacillus,Shiwanella and other bacteria and fungi such as mold and yeast.Fe-based materials and carbon-based materials could accelerate the reduction of Cr(Ⅵ)by promoting the extracellular electron transfer process of microorganisms.Bentonite,biochar and other materials provided suitable living space for microorganisms.Trehalose and other stimulants could improve the soil environment,promote the growth of microorganisms and strengthen the ability of microbial repair.Enhanced microbial remediation technology could enhance the reduction efficiency of Cr(Ⅵ)and adapt to the harsh soil environment,which had gradually become the focus of current research.
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