Synthesis Of Modified Ldhs And Research On Adsorption Characteristics Of Nitrate And Phosphate

Wastewater reclamation is an effective way to solve the water crisis.Restricted by the current wastewater treatment technology,the effluent from municipal wastewater treatment plant still contain relatively high concentrations of nitrate and phosphate,and can not reach the standard of recycled water reuse.Therefore,it has become the current hot spots to develop the advanced nitrate and phosphate removal technology in the field of wastewater reuse.Based on the nitrate and phosphate in the effluent mainly exist in the form of anions.the anionic adsorbent LDHs was been chosen and LDO was obtained by roasting modification.The the removal of nitrate and phosphate and adsorption mechanism by LDO were studied.Research contents include:The synthesis conditions of LDHs and LDO were optimized by orthogonal test,and Mg/Al-LDO and Mg/Fe-LDO were synthesized.Mg/Al-LDO was used to adsorb phosphate and Mg/Fe-LDO to adsorb nitrate.The effects of adsorption conditions including temperature,pH,coexisting ions and adsorption characteristics including adsorption kinetics and isothermal adsorption were studied.Basing on the adsorption characteristics,study combined with the results of Zeta potential,XRD and FTIR analysis of the changes of isoelectric point,crystal structure and functional group before and after adsorption,the mechanism of LDO adsorption on nitrate and phosphate was discussed.Finally,the regeneration method of the saturated phosphate adsorbent was studied.The main results of the research are as follows:The adsorption effect of LDO on nitrogen and phosphorus was significantly better than that of LDHs.The three conditions in the process of preparation:metal molar ratio,calcination temperature and calcination time are the main factors affecting the adsorption of LDO.The optimum synthesis conditions of Mg/Fe-LDO were as follows:Mg/Fe=3:1,calcination temperature is 450?,calcination time is 2h;under this condition,the NO₃^--N removal rate is about 95%.The optimum synthesis conditions of Mg/Al-LDO were as follows:Mg/Al=2:1,calcination temperature is 450?,calcination time of is 2h;under this condition,the TP removal rate is 96.9%.The changes of pH and temperature have a certain impact in the adsorption process.The influence of coexisting ions on the adsorption is as follows:high anions have a greater effect on adsorption and low anions have little effect on adsorption,which is in agreement with the anion exchange capacity of LDHs:PO₄^3->CO₃^2->SO₄^2->Cl^->NO₃^-.The adsorption kinetics of nitrogen and phosphorus by LDO were has a better fitting result with pseudo-second-order kinetics model and the isothermal adsorption process were consistant with Langmuir adsorption isotherm model.Combined with the results of Zeta potential,XRD and FTIR,it can be inferred that the main removal mechanism is nitrate intercalation into LDHs in the‘structural memory effect'of Mg/Fe-LDO;and Mg/Al-LDO of adsorption of phosphate mainly through electrostatic attraction,anion in layer,ions exchange,surface co-ordination four kinds of function co-operatively.Desorption-roasting two-step regeneration was more suitable for the regeneration of phosphate adsorbent Mg/Al-LDO compared with the one-step calcination method.It was found that the primary regeneration rate reached 95%,and the regeneration rate still could reach up to 67%after five regenerations.The optimum regeneration conditions are as follows:desorption solution of NaOH-Na₂CO₃ mixture,,roasting temperature of 550?and duration of 6 h.Based on the adsorption mechanism of Mg/Al-LDO and the analysis of XRD and FTIR spectrum,the reasons for the declining regeneration rate include two aspects:?1?incomplete desorption of phosphate due to the chemical adsorption of Mg/Al-LDO led to un-removed residual phosphate in the crystal under high temperature calcination that still occupy the adsorbent adsorption site;?2?the conversion of Mg/Al-LDO into spinel?MgAl₂O₄?after several high-temperature calcinations led to the loss of adsorption capacity.The results indicated that LDO is feasible for the removal of nitrate and phosphate from aqueous solution due to its special‘structure memory effect'.Adsorption process is mainly accomplished through anion intercalation,ion exchange,electrostatic attraction and surface co-ordination.The reuse of LDO and phosphorus recovery can be realized through desorption-roasting two-step regeneration method.The above research results can provide technical reference for the application of hydrotalcite like materials in the removal of nitrate and phosphate and phosphorus recovery.The results of this paper have some novelty and engineering application value.