Research On The Recovery Of Nitrogen And Phosphorus In Phosphorus-containing Wastewater By Red Mud-modified Biochar And Its Potential Application

In the process of phosphate fertilizer production,a large amount of wastewater containing nitrogen,phosphorus,and fluorine is produced.The removal of fluoride from phosphorus-containing wastewater and the efficient recovery of nitrogen and phosphorus are of great significance to avoid fluoride pollution and the resource reuse of nitrogen and phosphorus.Because of its convenient operation and good recyclability,adsorption method has attracted widespread attention in the recovery of nitrogen and phosphorus from water.Among them,biochar has great adsorption potential for nitrogen and phosphorus due to its wide source of feedstocks,low cost,and porous structure.However,due to the negative charge on the surface of pristine biochar,the adsorption performance of phosphate by pristine biochar is relatively poor.Therefore,modification treatments are needed to improve the adsorption performance and application potential of biochar for different pollutants.As an industrial solid waste produced in the process of producing alumina from bauxite,red mud contains many metal oxides.Using it as a modifier can increase the number of metal active sites on the surface of biochar,thereby promoting the adsorption of nitrogen and phosphorus.However,there are relatively few studies on the use of red mud modified biochar to recover nitrogen and phosphorus from water,and its adsorption performance and mechanism are still not clear.To effectively recover nitrogen and phosphorus in phosphorus-containing wastewater and realize the resource utilization of typical industrial and agricultural solid wastes,distillers grains,rape straw,and pine sawdust were used as feedstocks in this study.They were co-pyrolyzed with red mud by one-step pyrolysis method to prepare red mud-modified biochar,which were used to adsorb ammonium and phosphate in water.The factors affecting the adsorption capacity of red mud-modified biochar for ammonium and phosphate were discussed.The main adsorption mechanisms were discussed by means of XRD and XPS,adsorption models,and site energy analysis.The adsorption performance of red mud-modified biochar for ammonium and phosphate under dynamic conditions were investigated by fixed-bed column experiments.On this basis,red mud-modified biochar was applied to the recovery of nitrogen and phosphorus from flotation tailings wastewater to evaluate the application potential of red mud-modified biochar in practical wastewater.The slow-release properties of nitrogen and phosphorus and the effects on plant germination and growth of the red mud-modified biochar loaded with nitrogen and phosphorus were also studied.The main results were as follows:（1）The characterization results of pristine and red mud-modified biochar showed that the p H of pristine/red mud-modified biochar gradually increased with the increase of pyrolysis temperature,and the Zeta potential gradually decreased with the increase of solution p H.Compared with the pristine biochar,the content of functional groups such as C–O,C–C,and Fe–O increased,as well as the metal elements such as Ca,Si,Fe,and K in the red mud-modified biochar.In the meanwhile,the CEC,specific surface area,and total pore volume also increased,indicating that the red mud was successfully loaded onto the biochar.（2）The results of different influencing factors on the adsorption of nitrogen and phosphorus by red mud-modified biochar showed that the p H of the solution could significantly affect the adsorption effect of red mud-modified biochar on ammonium and phosphate.The most suitable solution p H for ammonium and phosphate adsorption was 8.0 and 3.0,respectively.The coexistence of Ca²⁺and Mg²⁺had the greatest influence on the adsorption of ammonium,while HCO₃^-and F^-had the greatest effect on the adsorption of phosphate.Combined with the physicochemical properties of biochar,adsorption kinetics,adsorption isotherms,and site energy analysis,the adsorption mechanism of ammonium on red mud-modified biochar was mainly based on the interaction between surface functional groups and ammonium,and the adsorption mechanism of phosphate was mainly surface precipitation.（3）The results of fixed-bed column experiments showed that,when the addition amount of red mud-modified rape straw biochar（RM/RSBC）was 0.5 g,the phosphorus concentration was 50 mg/L（corresponding to the nitrogen concentration of 67.85 mg/L）,and the flow rate was 1 m L/min,the dynamic adsorption performance of RM/RSBC packed column for ammonium and phosphate was better than other situations.The results of the study on the removal of fluoride in simulated wastewater showed that the precipitation process could effectively remove 90%of the fluoride in the wastewater.After filtering the resulting precipitate,adding 0.9 g of RM/RSBC effectively recovered ammonium and phosphate from the solution.After the pre-precipitation of fluoride,the RM/RSBC fixed-bed column had a relatively obvious effect on the treatment of wastewater quality,which provides a solid theoretical basis for the treatment of practical phosphorus-containing wastewater.（4）The research on the release properties of nitrogen and phosphorus showed that the RM/RSBC loaded with nitrogen and phosphorus had good stability.The study on the effects of plant germination and growth showed that the germination and growth of mung beans could be significantly promoted（P<0.05）after adding more than 0.5 wt%of RM/RSBC adsorbed with nitrogen and phosphorus.