Synthesis Of EPS/nZVI Composites And Its Sb(Ⅴ) Reductive Adsorption Efficiency And Mechanisms

Antimony（Sb）is widely used in various industrial manufacturing,but as a toxic pollutant,the treatment of antimony wastewater has attracted worldwide attention.The reductive adsorption method is an effective approach to treat Sb（V）wastewater.Extracellular polymer（EPS）is a a green and renewable biosorbent,EPS has broad prospects in wastewater treatment.However,the EPS reduction ability is limited,the separation cost is high,and the EPS production process of pure bacteria is complicated,which limit the application of EPS.Nano zero-valent iron（n ZVI）has excellent removal ability on various pollutants as a strong reducing chemical reducing agent.But n ZVI is easy to aggregate,and it can generate passivation layer on the surface when contacting with oxygen,which can reduce the reactivity of n ZVI.This study was based on the microbial extracellular polymeric substances produced by Klebsiella pneumoniae J1.The EPS was combined with n ZVI,to prepare new materials that can reduce the difficulty of separating EPS from water,can alleviate the problems of agglomeration and passivation of n ZVI,and can enhance the Sb（V）reduction and adsorption efficiency.The Sb（V）reduction and adsorption mechanisms by EPS composite n ZVI materials were studied,which provided a theoretical basis for the Sb（V）reduction and adsorption in wastewater.The application of waste sludge EPS simplifies the production process of EPS,enables the recycling of waste sludge,and provides technical support for the treatment of actual antimony wastewater.EPS and magnetic microbial extracellular polymeric substances（EPS@Fe₃O₄）were used to adsorb Sb（V）.EPS had a high Sb adsorption capacity（120 mg/g）,while EPS@Fe₃O₄（216 mg/g EPS）exhibited a better adsorption capacity due to its higher specific surface area and dispersibility.The pseudo second order kinetic model and the Redlich-Peterson isotherm model fitted the adsorption process well.Sb（V）was reduced to Sb（OH）₃ that is easier to be adsorbed.The amino,hemiacetal and phenolic hydroxyl groups on EPS and EPS@Fe₃O₄ had reductive functions,reducing Sb（V）to the Sb（OH）₃ that is easier to be adsobed.To further improve the reducibility of EPS,two kinds of microbial extracellular polymeric substances and nano zero-valent iron composites were synthesized:EPS coated n ZVI（EPS@n ZVI）,and EPS@Fe₃O₄ supported n ZVI（magnetic EPS@n ZVI）.Compared with the bare n ZVI,the two kinds of EPS composite n ZVI had better dispersibility and inhibited the generation of passivation layer of n ZVI to a certain extent.EPS@n ZVI presented an obvious core-shell structure,and EPS can be the protective shell of n ZVI.In magnetic EPS@n ZVI,m-EPS can be used as the supported material of n ZVI.The optimal adsorption conditions of n ZVI,EPS@n ZVI,and magnetic EPS@n ZVI for Sb（V）was p H=5,the adsorption time wass 2 h,and the maximum adsorption capacity was respectively 180 mg/g;202 mg/g;160 mg/g.The pseudo second order kinetic model and the Redlich-Peterson isotherm model fitted the adsorption process well.For EPS@n ZVI,in most p H ranges（p H=3～13）,the Sb（V）adsorption capacity of EPS@n ZVI was greater than that of n ZVI.There are several reasons:EPS was wrapped outside n ZVI,which increased the dispersibility of n ZVI;From the aspect of zeta potential,the main Sb（V）ion was Sb（OH）₆^-when p H>5,so the repulsion between EPS@n ZVI and Sb（V）was smaller,which is more conducive to adsorption;n ZVI was passivated at p H=6～13,but the hydroxyl,amino,carboxyl and other functional groups in EPS can still play the role of adsorption;EPS was likely to promote the corrosion of n ZVI and enhance n ZVI reactivity;EPS can protect n ZVI in the air,and can reduce the direct contact between n ZVI and oxygen to a certain extent,and strengthen the reduction of Sb（V）to Sb（III）which is easier to be adsorbed.For magnetic EPS@n ZVI,the adsorption capacity of magnetic EPS@n ZVI was greater than the sum of half the adsorption capacity of m-EPS and n ZVI,which means that the synthetic material increased the adsorption capacity of the original material.The Fenton reaction under aerobic conditions inhibited the reduction of Sb（V）to Sb（III）to a certain extent,resulting in a smaller adsorption capacity of magnetic EPS@n ZVI under aerobic conditions than under anaerobic conditions,but the gap was not obvious.Compared with EPS@n ZVI,EPS in n ZVI@m-EPS did not promote the dissolution of Fe,and EPS did not inhibit the Fenton reaction of n ZVI in the presence of dissolved oxygen in water as significantly as that in EPS@n ZVI.However,the separation of EPS@n ZVI from the adsorption solution mainly depends on the sedimentation of adsorbent,but n ZVI@m-EPS is magnetic and can realize rapid separation and recovery under magnetic field conditions.To simplify the EPS production process,the extracellular polymeric substances extracted from waste sludge combined with nano zero valent iron（EPS*@n ZVI）was synthesized.EPS*@n ZVI is the core-shell structure and the waste sludge EPS wrapped n ZVI.The EPS extracted from the sludge can slow down the oxidation of the inner layer n ZVI by air.Compared with ordinary n ZVI,EPS*@n ZVI（188 mg/g）showed slightly better Sb（V）adsorption in experimental water than n ZVI（180 mg/g）.In the treatment of actual antimony industrial wastewater,EPS*@n ZVI still had excellent performance.The adsorption capacity of Sb by n ZVI was less than 15 mg/g,while the adsorption capacity of Sb by EPS*@n ZVI was up to 32 mg/g.It was because the waste sludge EPS can enhance the dispersibility of n ZVI,reduce the contact of n ZVI with oxygen,inhibit its passivation,and enhance the reductive adsorption effect of n ZVI on Sb（V）,and the waste sludge EPS has relatively rich functional groups,which can provide adsorption sites for Sb（V）.The adsorption of Sb（V）by EPS*@n ZVI fitted the pseudo-second-order kinetic model and the Redlich-Peterson adsorption isotherm model.Compared with EPS@n ZVI,sludge EPS is not reductive and can not inhibit the Fenton reaction produced by n ZVI.Therefore,EPS@n ZVI has a better reduction and adsorption effect on Sb（V）than EPS*@n ZVI.However,EPS*@n ZVI was extracted from the waste sludge of domestic sewage treatment plant,which simplifies the extraction process of EPS and realizes the resource utilization of the waste sludge.Based on microbial extracellular polymeric substances,combined with nano zero valent iron.New materials were synthesized to solve the problem of difficult separation of microbial extracellular polymeric substances from wastewater,to enhance the reactivity of nano zero valent iron,to improve the Sb（V）adsorption efficiency.The reduction and adsorption mechanisms of Sb（V）by extracellular polymeric substances were analysed.The production process of microbial extracellular polymeric substances was simplified and the waste sludge was recycled.This study forms a theoretical basis for the reduction and adsorption of Sb（V）,provides technical support for the treatment of actual antimony wastewater,and lay a foundation for the development of efficient,green and economical adsorbents.