Removal Efficiency And Mechanism Of Cr(VI) And Se(IV) In Water By Natural Organic Matter-stabilized Nano-FeS

With the mining of chromium(Cr)and selenium(Se)minerals and their wide application in the industrial and agricultural production process,a large amount of Cr and Se elements enter the environmental water body,polluting the ecological environment,and also producing human health and development potential threat.Efficient reduction,adsorption or precipitation has been considered as a good effective method for removing chromium and selenium in water.Therefore,finding stable and efficient reduction and adsorption materials is one of the research hotspots in this field.Nano-FeS(n-FeS)not only has excellent reducing ability,but also has good surface characteristics,it is an environment-friendly reducing repair material.However,since n-FeS is very easy to agglomerate and is easily oxidized,it will have great limitations in practical applications.Therefore,looking for an environmentally friendly and excellent performance n-FeS stabilizer and stabilization method can solve the bottleneck problem in the application process of n-FeS.In this paper,natural organic tannic acid(TA)and phytic acid(PA,IP6)were used as stabilizers for n-FeS.Stabilized n-FeS(TA-n-FeS)and phytic acid stabilized n-FeS(IP6-n-FeS)are prepared by coprecipitation method.The removal efficiency and mechanism of stabilized n-FeS on Cr(?)and Se(?)were characterized by FT-IR,TEM,SEM,XRD,XPS,Zeta potential and other means.The adsorption kinetics and thermodynamics of the reaction process were studied,and the influence of influencing factors such as the proportion of stabilizer,dosage,pH,coexisting ions,and O2 on the removal efficiency of Cr(?)and Se(?)was investigated.TA-n-FeS with excellent stability was selected as the reduction-adsorbent,and its removal efficiency and mechanism of Se(?)were investigated.The main research findings are as follows:(1)TA complexes with FeS through various structures of its own hydroxyl group,phenolic hydroxyl group and benzene ring to form stable TA-n-FeS;IP6 complexes FeS with active groups such as hydroxyl groups,PO43-,HPO42-,and bonds such as Fe—O,P=O,and P—O,so that FeS is evenly distributed in IP6 to form IP6-n-FeS.Compared with FeS,TA-n-FeS and IP6-n-FeS both carry more active groups and have stronger reactivity.Both TA-n-FeS and IP6-n-FeS formed after stabilization can increase the electronegativity of the particle surface,increase the electrostatic repulsion between particles,enhance the dispersibility of the material,and reduce the occurrence of agglomeration and sedimentation of FeS,making them has smaller particle size and higher stability.The stability of TA-n-FeS,IP6-n-FeS,and FeS were also investigated.The order of the stability of the three is TA-n-FeS>IP6-n-FeS>FeS.(2)The removal efficiency and reaction mechanism of Cr(?)by IP6-n-FeS and TA-nFeS were studied.The removal of Cr(?)by FeS,IP6-n-FeS and TA-n-FeS is more consistent with pseudo-second-order kinetics,indicating that all reactions are chemisorption and the order of their saturated adsorption capacity is TA-n-FeS>IP6-nFeS>FeS>TA>IP6.The process of IP6-n-FeS and TA-n-FeS removal of Cr(?)is better fitted by Langmuir model,which indicates that the reaction process is monolayer adsorption.When the FeS concentration in stabilized FeS is 0.5 g/L,the optimal concentration of IP6 in IP6-n-FeS is 0.10%(w/w),and the optimal concentration of TA in TA-n-FeS is 0.05%(w/w).The best addition ratio of IP6-n-FeS and TA-n-FeS to remove Cr(?)is 2.5:1(calculated as FeS:Cr).The increase of pH will have a certain negative impact on the removal efficiency of Cr(?).The presence of coexisting ions can inhibit the removal efficiency of Cr(?)by TA-n-FeS,but promote the removal of Cr(?)by IP6-n-FeS.The presence of O2 will not cause the re-release of Cr(?)removal,and due to the coupling reaction of material stabilization and O2,it will even increase the removal rate of Cr(?)by TA-n-FeS and IP6-n-FeS.TA-n-FeS and IP6-n-FeS can effectively remove Cr(?)in aerobic and anaerobic environments.The removal mechanism of Cr(?)by IP6-n-FeS and TA-n-FeS can be summarized as the steps of adsorption,reduction,and precipitation.That is,the material first adsorbs Cr(?)to its surface,and then quickly reduces it.Reduction products of Cr(?)such as FeCr2O4 and Cr2O3 and oxidation products of Fe(?)and S(-?)such as Fe2O3 and S8 are formed.Compared with IP6-n-FeS,TA-n-FeS has higher stability,reactivity and higher Cr(?)removal efficiency due to its excellent reducing properties due to the characteristics of TA electron shuttle.(3)The removal efficiency and reaction mechanism of Se(?)by TA-n-FeS were studied.The removal of Se(?)by FeS and TA-n-FeS is more consistent with pseudosecond-order kinetics,indicating that all reactions are chemisorption,and saturated adsorption capacity order is TA-n-FeS>FeS.The removal process of Se(?)by TA-nFeS was better fitted by Freundlich model,indicating that the reaction was multilayer adsorption.In TA-n-FeS,when the FeS concentration is 0.5 g/L,the optimal concentration of TA is 0.05%(w/w),and the optimal dosage of TA-n-FeS to remove Se(?)is 5:1.(Calculated as FeS:Se).Unlike most studies,an increase in pH will increase the removal rate of Se(?)by TA-n-FeS.Coexisting ions accelerate the destabilizing agglomeration of TA-n-FeS,thereby inhibiting the removal rate of Se(?).The presence of O2 will not cause the re-release of Se(?)removal.TA-n-FeS can remove Se(?)well under aerobic and anaerobic conditions.TA-n-FeS has a bright application prospect in treating Se(?).The removal process of Se(?)by TA-n-FeS includes the steps of adsorption,reduction,replacement,and generation of precipitation.That is,TA-n-FeS first adsorbs Se(?)to its surface,and then quickly reduces it in a short time.A series of reactions such as substitution and substitution generate Se(?)reduction products such as FeSe and Se(0)and oxidation products of Fe(?)and S(-?)such as Fe2O3 and Ss.