Study On Preparation Of Modified Biochar From Red Mud And Its Adsorption Mechanism For Phosphorus In Water

A range of environmental problems including eutrophication can be caused by excessive amounts of phosphorus entering natural water,which has had significant negative impacts on human society.Therefore,it is of great importance for alleviating water eutrophication to how to simply and efficiently remove phosphorus from wastewater.Biochar,as a common adsorbent,has been widely used in water pollution treatment due to its environmental friendliness,low cost,and ease of production,and its adsorption performance has been greatly enhanced through modification.Red mud,a major solid waste produced by the aluminum industry,is also harmful to environment certainly.Red mud contains abundant mineral components that have multiple factors affecting the adsorption of phosphorus in water,making it a potential modification material.Therefore,it’s particularly necessary to develop a low-cost and high-efficiency adsorbent using red mud.This study used walnut shells and red mud as the base material and the modification material,to prepare modified biochar from red mud（RM-BC）using a dry mixing-co-pyrolysis method.The study also investigated the optimal preparation technology of RM-BC,as well as its adsorption characteristics and mechanism for phosphorus in water.（1）It was found that walnut shells and red mud had high adsorption effects on phosphorus,with adsorption capacities of 0.092 mg·g^-1 and 0.159 mg·g^-1,respectively,through screening experiments.And walnut shells and red mud were chosen as raw materials because of convenient operation.,The ranges of the preparation conditions for RM-BC were determined to be:pyrolysis temperature of 250～350℃,pyrolysis time of 45～75 min,and a ratio of 1:1～3:1 g·g^-1 through single-factor experiments.Using the response surface optimization method,the phosphorus removal rate of RM-BC was 99.74%under the best preparation conditions being a pyrolysis temperature of320℃,a pyrolysis time of 58 min,and a mass ratio of 1:1 g·g^-1.（2）The specific surface area of RM-BC increased to 28.80 m²·g^-1 is 3.6 times that of walnut shell biochar（BC）after modified by red mud,and the pore volume also increases significantly.The average pore size decreases to 18.12 nm,indicating that RM-BC is a mesoporous material.There are a large number of particle-like substances on the surface of RM-BC,which are coarser than the surface of BC.Combined with the results of X-ray diffraction（XRD）,it can be seen that the particle-like substance is Fe₂O₃,which proves that the iron in the red mud is successfully loaded onto RM-BC.There are also abundant functional groups（O-H,C=O,C-H,Fe-O and-COOH）on the surface of RM-BC.（3）The results of adsorption experiments found that the removal rate of RM-BC for phosphorus reached more than 95%under acidic conditions（pH=3～4）,when the dosages were 6 g·L^-1,indicating that the pH of water had a great influence on the adsorption of phosphorus by RM-BC.The four anions added in the solution(NO₃^-,SO₄^2-,CO₃^2-and Cl^-)had little effect on the adsorption of phosphorus by RM-BC,indicating that RM-BC was not sensitive to anions in water.The fitting results of the isothermal adsorption model and adsorption kinetic model show that the adsorption process of RM-BC for phosphorus can be better described by the Redlich-Peterson model（R²=0.995～0.998）and the pseudo-second-order kinetic model（R²=0.991）,indicating that the adsorption process of RM-BC for phosphorus not only has the characteristics of adsorption surface heterogeneity but also has the characteristics of single-molecule layer adsorption,and it is also a chemical adsorption process.The fitting results of the adsorption thermodynamic model show that it is a spontaneous,entropy-reducing,and endothermic reaction process of RM-BC for phosphorus.The iron element in RM-BC was greatly lost due to displacement reaction between Fe and HCl after hydrochloric acid treatment,resulting in a significant reduction in the removal effect of phosphorus,which indicates that hematite in the red mud makes a vast difference in the adsorption process.At the same time,calcite in the red mud can also play a role in the phosphorus removal process through chemical precipitation after decomposition into Ca O during pyrolysis.However,iron-silicon generated by Fe and Si will reduce the phosphorus removal performance.Combined with the characterization analysis results of RM-BC and modified biochar from hematite（H-BC）,the adsorption mechanism of RM-BC for phosphorus is mainly a combined process of chemical adsorption（formation of Fe-O-P bonds,surface coordination,ion exchange,and chemical precipitation）and physical adsorption（pore filling and electrostatic attraction）.Ten kinds of agricultural wastes and five kinds of industrial wastes were used as raw materials to prepare biochar in this study,and the differences of phosphorus removal among single biochar were expounded systematically.At the same time,combined with single factor experiment and response surface optimization method,a new adsorbent with simple operation and good performance was successfully prepared using walnut shell and red mud as raw materials,which provided a feasible scheme for the resource utilization of red mud and also provided a theoretical basis for the application research of red mud.