Removal Mechanism Of Hexavalent Chromium Using Biogenic Mackinawite(FeS)-coating Kaolinite

Chromium as a raw material is widely used in metal finishing and dye industries.Chromium pollution has become a growing problem and performs a great threat to the security of environment.Mackinawite?FeS?has attracted more attention to treat chromium contamination due to its high reductibility,high reactivity and high specific surface area.However,FeS is susceptible to aggregation during the synthesis,which suppresses the reactivity of FeS.It is critical to provide an effective method to enhance the stability and reactivity of FeS.Kaolinite is a common clay mineral in the environment,which might act as the nucleation site for biogenic FeS during the biosynthetic process and reduce the aggregation of FeS particles.Therefore,we explored the feasibility of kaolinite as a supporting material for biogenic FeS in present study.The capability of biogenic FeS coated on kaolinite?FeS-kaolinite?and naked FeS to Cr??? removal was compared.Meanwhile,we investigated the removal mechanism of Cr??? using FeS-kaolinite and the tolerance of FeS-kaolinite towards different enviromental factors.The main results were as following:1.Preparation and characterization of kaolinite acts as a supporting material for biogenic FeS?FeS-kaolinite?.The aggregation of biogenic FeS efficiently decreased by selecting kaolinite as a supporting material,forming thin,dispersive and looser FeS particles.The size of FeS particles decreased from 10?m to 2?m.The specific surface area FeS on FeS-kaolinite was 101.18 m²/g,which was much higher than naked FeS?74.44 m²/g?.The rate and capability of FeS-kaolinite to Cr??? removal were 1.3 times and 2.2 times higher than that of naked FeS at pH=7,respectively,illustrating that FeS-kaolinite enabled to increase the rate and capability of Cr???removal than that of FeS only.2.Removal mechanism of Cr??? using FeS-kaolinite.The maximium removal capability of FeS-kaolinite reached 399 mg/g when the concentration of kaolinite was 3 g/L added into the SRB medium where the percentage of FeS on FeS-kaolinite was 5%.The removal kinetics were well fitted with the pseudo-second-order model at pH=5,7,and 9.The removal rate decreased with increase pH.Cr???could be completely removed at pH 3-8 within 1 h using FeS-kaolinite,whereas the capacity was significantly reduced at pH>9.The mechanism of Cr??? removal by FeS-kaolinite was as followed:Cr???was absorbed on the FeS-kaolinite surface and then Cr???was reduced into Cr??? by Fe???partially released and absorbed on FeS-kaolinite surface.Subsequently,Cr??? was precipitated as Cr?OH?₃and Cr_xFe_1-x?OH?₃.Fe??? was reduced to Fe??? by S?-??which was oxidized to SO₄^2-and SO₃^2-.The contribution of reduction and absorption of Cr??? by FeS-kaolinite were 70.8%and 29.3%,respectively.3.Effect of various environmental factors on the removal of Cr??? by FeS-kaolinite.The reduction of Cr??? was suppressed with oxygen in aerobic condition,which were attributed to the oxidation of Fe²⁺ and the inhibition of Fe???/Fe??? cycle induced by S^2-in the presence of O₂.The reduction of Cr??? and the precipitation of Cr??? were not depended on the ion strength?Cl^-,0-1000 m M?and organic matter?HA,0-20 mg/L?at pH=7.The co-exsting cation ions wouldn't inhibite the reduction of Cr???by occupying the reaction sites on FeS-kaolinite surface at pH 4.The Cr???precipitation was favored at low concentration of co-existing cations?0.3 m M?but significantly suppressed at high concentration?30 m M?.The reduction of Cr???was significantly suppressed with MnO₂ whereas the Cr??? precipitation was promoted with the addition of MnO₂ by absorbing Cr???,which increased with the concentration of MnO₂ increase.