Phosphate Elimination From Waters By Lanthanum-based Carbonate Adsorbents And The Related Mechanisms Study

To prevent eutrophication,it is very crucial to deeply eliminate phosphate in sewages and wastewaters to reduce the total phosphorus load in surface waters such as lakes and reservoirs.Because of its simple operation,effectiveness even at low phosphate concentrations,and potential for non-renewable phosphorus resource recovery,adsorption has been widely accepted as an economical and effective method for advanced phosphate removal.Lanthanum（La）based materials,which show a high affinity towards phosphate,have been increasingly employed for phosphate removal from wastewaters.However,due to insufficient sorption capacity and/or difficult separation and recycling,the practical use of many La-based adsorbents remains challenging.To overcome these problems,lanthanum carbonates（LC）-based sorbents with high separation performance and phosphate uptakes were developed by combining LC species possessing abundant CO₃^2-exchange sites as active components with magnetic particles or relatively large carriers.Then,sorption and removal characteristics of phosphate by these sorbents,their applications in advanced phosphate removal from（waste-）waters at low concentrations,the potential of phosphorus recovery,and related sorption mechanisms were investigated.Through this study,A phosphate adsorption and removal reaction system based on LC-sorbents was constructed to achieve efficient phosphate removal and recovery.Herein,a magnetic lanthanum oxycarbonate sorbent（La₂O₂CO₃/γ-Fe₂O₃,M-LDC）was firstly prepared via two-step calcination,by using La/Fe-BTC metal-organic-framework（La/Fe-BTC MOF）prepared by a co-precipitation method as precursors.Because the removal of organic ligands can generate abundant pore structures,M-LDC sorbent has micro/nano secondary structures and rich active sites.M-LDC performed very well in the removal of phosphate from aqueous solutions,its phosphate sorption capacity can reach 134.83 mg-P/g,with the corresponding La usage efficiency（P/La molar ratio）of 1.06,and it can be effectively regenerated after use.In addition,M-LDC sorbent also showed excellent sorption performance in advanced removal of phosphate at low concentrations,it can be well used in the treatment of simulated wastewater,surface water,and secondary effluent,and the phosphate levels in outflows after treatment can be lower than 0.01 mg-P/L,which could meet the strict requirement of P discharge in some regions（<0.05 mg-P/L）.A magnetic hydrated lanthanum carbonate sorbent（La₂（CO₃）₃·x H₂O/Fe₃O₄,M-HLC）was prepared by a simple,green,and easily scaled-up chemical co-precipitation method,which simplified the synthesis process of sorbents and solved the problems of high organic ligand and energy consumption in M-LDC preparation,as well as its p H-dependent phosphate removal behavior.Results showed that M-HLC sorbent has a micro/nano-hybrid structure,and can achieve rapid solid-liquid separation in the presence of an external magnetic field with high efficiencies of over 96.8%.Due to the abundant CO₃^2-site in its structure,M-HLC sorbent showed superior phosphate removal performance in a wide p H range（3-10）,with fast removal rates,and high sorption capacity（102.3 mg-P/g）,strong selectivity,and high La usage efficiency of 1.12.After saturation,P-loaded M-HLC sorbents can be effectively regenerated by Na₂CO₃ solution,and the phosphate enriched in eluents could be recovered by calcium precipitation,showing great application potential for the treatment of wastewaters with relatively high phosphate levels and the recovery of P resources.However,thanks to the relatively large structure size of micron M-HLC sorbent,it is still limited in the use of advanced removal of low-level phosphate.Only when its dosage is≥0.5 g/L or the sorption time is≥3 h,it can achieve the rapid and effective removal of low-level phosphate（0.77 mg-P/L）in secondary effluent,there remains a large space for improvement.Moreover,the relatively high content of La in the M-HLC sorbent will also lead to the high cost of sorbents.Activated carbon-loaded hydrated lanthanum carbonate（C-HLC）sorbent was prepared by a homogeneous co-precipitation,which improved the poor adsorption performance of M-HLC sorbent at low phosphate levels caused by its large structure size,and further improved the utilization efficiency of lanthanum site and reduced the preparation cost of adsorbent.Because C-HLC sorbent is a multistage micro/nano hybrid structure,it has a large specific surface area and rich pore structures（302.4 m2/g,0.267 cm³/g）,and numerous nanosized（<100nm）amorphous hydrated lanthanum carbonate particles are uniformly loaded on the surface of C-HLC,which can provide abundant active sites for phosphate binding.C-HLC sorbent performed very well in the phosphate sorption and removal,besides the fast sorption kinetics,C-HLC sorbent also showed a higher phosphate sorption capacity（75.86 mg-P/g）,its related lanthanum utilization rate（P/La ratio）is 1.76,which was significantly higher than M-LDC and M-HLC sorbents.Moreover,C-HLC sorbents（0.2 g/L）also showed an excellent removal performance for low concentration phosphate in secondary effluent,to decline the phosphate level in outflows from 0.77 mg-P/L to below the detection limit within10 min.In addition,C-HLC sorbent is also well applied in the fixed-bed continuous phosphate removal experiments,showing broad application prospects in the advanced treatment of tailwaters with phosphate at low levels and phosphate from high-concentration wastewaters.The adsorbents before and after phosphate sorption were comprehensively characterized,and the selective removal mechanism of phosphate by three kinds of lanthanum carbonate-based adsorbents was analyzed.Results showed that the phosphate removal by these sorbents was a chemical sorption process.Besides the electrostatic attraction,phosphate was mainly removed by M-LDC sorbent via the ligand exchange interaction between surface hydroxyl group/CO₃^2-and phosphate to form stable inner-sphere La PO₄ complexes,and adsorbed by M-HLC and C-HLC sorbents through surface precipitation and ligand exchange between CO₃^2-and H₂PO₄^-/HPO₄^2-.La₂O₂CO₃-LDC,La₂（CO₃）₃·x H₂O-HLC,and La₂O₃-LO sorbents with normalized structure morphology but different La species were prepared to clarify the influence of La species of La-based sorbents on phosphate removal,and their differences in phosphate removal performance and related mechanism were investigated.The results show that the difference in sorption sites in La-sorbents is the main reason for their different phosphate removal behaviors.Compared to the LO with surface hydroxyl group as sorption sites and the LDC with surface hydroxyl and CO₃^2-as active sites,HLC sorbent with only CO₃^2-exchange site in the structure showed the greatest application potential of phosphate removal.The comparison analysis of M-LDC,M-HLC,and C-HLC sorbents in phosphate removal performance,separation efficiency,and economic cost showed that M-LDC sorbent is suitable for the treatment occasions requiring the phosphate concentration of effluents to be reduced to an extremely low level and requiring high separation efficiency;M-HLC sorbent is suitable for occasions requiring high separation efficiency,but there’s no strict requirement on discharge concentrations of phosphate;C-HLC sorbent is relatively economical and suitable for occasions with stricter phosphate discharge requirements but low separation efficiency or long separation time.