Study On Preparation,Characterization And Phosphate Removal Properties Of Red Mud-Based Granular Materials

Red mud is a solid waste discharged from alumina industry,which is not noly in large quantity but also pollutes the environment.For the purpose of treating waste with waste,this paper summarizes the researches on the use of red mud for phosphate adsorption in water treatment.Red mud,which located on the slope of alumina dam and has been weathered for long period,was used as the main raw material to produce red mud-based granular phosphate removal materials.Based on the effective components and activity of the long-term weathering red mud was lost or reduced,the thermal activation was used to improve the phosphate removal capacity.The phosphate removal experiments and characteristics of raw red mud,raw granular and materials obtained by thermal activation methods were investigated in detail.The researches on phosphate removal performances and machanisms of the red mud-based granular materials,which were obtained under the optimal parameters,were performed under different water treatment environments.In addition,the desorption-adsorption regeneration stability of this material was discussed,which was of great significance for the development and utilization of red mud.and.In the study of preparation of red mud-based granular materials,red mud,fly ash and cement was used as the materials and further activited by microwave and sintering.Compared with raw red mud,the pH in solution after phosphate removal process using activated granulars was lower.These two activation methods played different role in materials preparation:microwave activation mainly expanded the pore volume and the specific surface area,while sintering activation mainly activated raw material and formed functional group with-OH.Based on the advantages of two activation methods,microwave-sintering-granular（MSG）with the mass ratio of 85:10:5 that microwaved at 700 W for 15 min and sintered at 800℃for 15 min was selected for further study.Phosphate removal performances and mechanisms of selected MSG were studied under different water treatment conditions.Rusults showed adsorbent dosage,initial phosphate concentration,contact time,and initial pH in solution were important to phosphate removal efficiency of MSG.When the adsorbent dosage was 4.0 g/L,the phsophate removal capacity was largest,which was 35.11%higher than raw red mud;phosphate removal capacities of MSG increased with the increase of initial phosphate concentration in solution,and phosphate solution with high concentration could play as a buffer on the pH;The adsorption of phosphate on MSG performed a relatively faster initial adsorption stage,and the solution was dynamic equilibrium after contact time of 240 min;Due to the phosphate removalcapacity of MSG would decrease for structure destruction at strong acid and less leaching of metal,surface precipitation and competitive adsorption with OH^-at alkali solution,MSG performed well at pH of3.0⁷.0;the common anions Cl^-,NO₃^-,CO₃^2-and SO₄^2-in wastewater weakly affected the removal performance.Thermodynamics presented that increasing of temperature was beneficial to the phosphate removal and this adsorption behavior was spontaneous,endothermic and entropy increasing process,which could be described by Langmuir isothermal model.Kinetics studies presented that pseudo second-order described the phosphate removal well.The mechanisms of phosphate removal by MSG are as follows:ion-exchange occurred between phosphate and-OH groups;precipitation reacted by alkali metal ions and phosphate;phosphate diffused throw the pores in MSG.In addition,the desorption-adsorption regeneration characteristics of MSG was investigated.The results showed that the desorption capacity of different desorbents was acid>alkali>deionized water.The desorption efficiency increased with acid concentration incresing,but strong acid would cause serious mass loss and structure destruction on MSG,which decrease the phosphate resorption capacity.The results showed phosphate resorption capacity of MSG desorpted by deionized water and 0.01mol/L HCl were better than those by high-concentration HCl and NaOH solutions.When 0.01 mol/L HCl was selected as desorbent,MSG could still performe well on phosphate removal after the phosphate desorption-adsorption experiment persist three times.Generally,red mud which has been weathered for long period was used as the main material,microwave and sintering activation methods were combined,the characterization and adsorption mechanism were studied,which were of great significance for the development and resource utilization of red mud.