Cr(?) Reduction And Adsorption By The Shewanella Oneidensis MR-1 And Biochar Loaded Alginate Gel

In this study,two kinds of natural polymers,alginate and chitosan,were used to modify biochar and impregnate it with Shewanella oneidensis MR-1 for the preparation of the S.oneidensis MR-1 and unmodified biochar impregnated alginate gel(BC-Ca-SA)and the S.oneidensis MR-1 and modified biochar impregnated alginate gel(mBC-Ca-SA).The morphology and surface functional groups of the S.oneidensis MR-1 and biochar impregnated alginate gel were characterized.The loading of microorganisms and chromium remediation ability of these two hydrogel composites were investigated by the Cr(?)reduction kinetics and the growth experiment of mung bean and soy bean.The main results are as follows.(1)The S.oneidensis MR-1 and biochar impregnated alginate gels(BC-Ca-SA and m BC-Ca-SA)were prepared by ion cross-linking method.The surface of the alginate gels are rough with irregular folds,and the interior structure is interpenetrating and porous honeycomb 3D network.S.oneidensis MR-1 is embedded into the gel,and the biochar particles are fixed into the polysaccharide skeleton of calcium alginate.After embedding S.oneidensis MR-1 and biochar,the characteristic carboxyl peak of alginate polysaccharide did not change,indicating that the molecular structure of the calcium alginate gel complex is stable.(2)The capacity of calcium alginate gel and biochar impregnated alginate gels is weak for Cr(?)adsorption.The 7 h reduction ratio of Cr(?)for the S.oneidensis MR-1impregnated alginate gels is 29.6%.After loading with unmodified biochar and chitosan-modified biochar,the reduction ratio of Cr(?)by the two hydrogel composites increased to 2.3 and 2.5 times,respectively.The Cr(?)removal ratio could reach 73.4%after 7 h.The increase of the initial Cr(?)concentration significantly reduced the reduction ratio of Cr(?)by the alginate gel,while the p H value had little effect on the reduction ratio of Cr(?).The Cr(?)reduction by these two hydrogel composites conforms to the pseudo-second-order kinetic equation.The reduction rate of m BC-Ca-SA is 84.8% higher than that of BC-Ca-SA.The persistent free radicals in the biocharaccelerate the reduction of Cr(?)by promoting the extracellular electron transfer of S.oneidensis MR-1.Chitosan could provide amine groups with high affinity to chromium,which is beneficial to the intact contact among S.oneidensis MR-1,Cr(?)and biochar.Thus accelerate the reduction rate of Cr(?).With the reduction of Cr(?),the total chromium concentration in the solution after 7 h decreased by 60.8% and 73.6%,respectively.The formation of metal oxygen bonds on the surface of calcium alginate gel indicates that the reduced Cr(?)was adsorbed by the gel,which could mainly attributed to the chelation of Cr(?)by the unique carboxyl peak of alginate polysaccharide.(3)In the experiment of the growth of mung bean and soy bean sprouts,the growth of mung bean sprouts and soybean sprouts was inhibited in the presence of 1 m M Cr(?)solution.After treated by the S.oneidensis MR-1 and biochar impregnated alginate gel,the mung beans and soy beans in the Cr(?)solution can germinate,and complete root system,cotyledons and stems can be formed after 7-day culture period.Microwave digestion and atomic absorption spectrometry were used to detect the chromium residues in the roots,stems and leaves of mung beans and soy beans.The chromium residues in the roots,stems and leaves of the mung beans treated with BC-Ca-SA were 51%,28% and 45% of the untreated cases.After treatment with m BC-Ca-SA,the chromium residues in the roots,stems and leaves of mung beans decreased by 29%,60% and 13%,respectively.After treatment by BC-Ca-SA,the chromium residues in the roots and leaves of soy beans were 22% and 26% of the untreated ones,while the chromium residues in the stems increased by 15%.After treatment with m BC-Ca-SA,the chromium residues in the roots,stems and leaves of soy beans decreased by 21%,82% and 17%,respectively.The reduction of Cr(?)and the adsorption of Cr(?)by the S.oneidensis MR-1 and biochar impregnated alginate gel reduces the biological toxicity of chromium in water,and also reduces the chromium residues in mung bean and soy beans.The S.oneidensis MR-1 and biochar impregnated alginate gel is a promising choice for the remediation of heavy metal chromium in the environment.