Removal Of Anionic Polluntants From Water By Biochar Derived From Paper Mill Sludge And Its Mechanisms

Biochar is a high carbon content solid prepared by pyrolysis of biomass under no oxygen or oxygen-limited conditions.It has a good adsorption and removal capacity for heavy metals and organic pollutants in water with low cost.Therefore,it is considered as an efficient multi-functional environmental material.However,due to the strong electrostatic repulsion from the solid surface of biochar,the adsorption of anionic contaminants in water is not ideal,which limits its wide application.It has been found that metal or metal oxide modified biochar can improve its ability to adsorb anionic contaminants.In this paper,an iron-rich waste biomass,paper mill sludge,was selected as raw material,and the one-step pyrolysis was used to prepare the iron modified paper mill sludge biochar.Phosphate and hexavalent chromium were used as the representatives of anionic pollutants to evaluate its adsorption capacity of anion pollutants in water and study its adsorption mechanism.The main results are as follows:（1）Preparation of iron-modified biochar composite material paper mill sludge biochar by the one-step pyrolysis can effectively remove phosphate and hexavalent chromium from water.Adsorption kinetic experiments showed that the adsorption equilibrium time of phosphate and hexavalent chromium for paper mill sludge biochar was 36h and 120h,respectively,which can be fitted by a psuedo-second-order kinetic model,indicating that the adsorption process was mainly controlled by chemical adsorption.The isotherms showed that the adsorption of phosphate and hexavalent chromium by paper mill sludge biochar are non-linear,which can be fitted by Langmuir adsorption isotherm model well.The maximum adsorption amounts of the two pollutants on the paper mill sludge biochar were 25.19mg/g and 13.87mg/g,respectively.In addition,the paper mill sludge biochar had strong magnetic properties,which can effectively be separated by external magnetic field.（2）The environmental conditions have a great influence on the removal of two pollutants from paper mill sludge biochar.High p H facilitate the removal of phosphate,while low p H facilitate the removal of hexavalent chromium.The increase of the concentration of the co-existing anion inhibited the adsorption of phosphate and hexavalent chromium.The order of inhibition to phosphate is:HCO₃^->NO₃^->Cl^-;while it is CO₃^2->NO₃^->Cl^-for hexavalent chromium.The removal effect of the two pollutants increased with the increase of the ambient temperature,indicating that the adsorption process of both pollutants was a spontaneous endothermic process.（3）The adsorption mechanism research showed that the iron oxide on the surface of paper mill sludge biochar can be used as an active adsorption site to play an important role in the adsorption of phosphate and hexavalent chromium.In addition,when the p H of the solution was less than p H_pzc,the groups on the surface of the adsorbent were positively charged due to protonation,reducing electrostatic repulsion and enhancing the adsorption and removal of phosphate.