The Characteristics And Mechanism Of Phosphate Adsorption By ZnAl Layered Double Hydroxides

On the purpose of removing phosphorus from wastewater, ZnAl layered double hydroxides (LDH) with different precipitation time were developed and their phosphate adsorption characteristics and mechanisms were analyzed.A series of ZnAl LDH were prepared and the effect of reaction time of precipitation on the structure and phosphate adsorption capacity was studied. The results show that all the samples had a typical structure of LDH. With the reaction time extending from12h to96h, the Zn/Al molar ratio decreased from2.06to0.70, average pore size reduced from104.02to59.55nm and specific surface area increased from14.12to112.68m2·g-1.Phosphate adsorption capacity of the ZnAl was in general increased gradually with the reaction time extending, which can be attributed to the surface area rising as well as the increased positive charge of the LDHs layer caused by a higher proportion of Al. With a preparation time of24h, however, a high amount of exchangeable interlayer anions was seen, giving rise to a highest phosphate uptake of34.5mg P·g-1by the ZnAI-24. This reveals that ion exchange would have played an important role during the phosphate adsorption. When the preparation time was96h, the obtained LDH also had a high capacity of phosphate of25mg P·g-1at25℃. Isotherms of phosphate adsorption by ZnAI-24(LDH with a preparation time of24h) fit well with a Langmuir equation; whereas the isotherms with ZnAl-96(LDH with a preparation time of96h) had a good agreement with a Freundlich equation. The adsorption kinetics of both ZnAl fit well with pseudo-second-order models. For the phosphate adsorption on ZnAl-24, ΔH0is less than0suggesting an exodothemic process; as for ZnAl-96the adsorption is endothermic sinceΔH0is positive. pH level of the solution did not impose an markedly influence on the dissolution of ZnAl, but obviously affected the phosphate adsorption capacity. In alkaline environment, phosphate adsorption capacity of ZnAl decreased dramatically. All the coexisting ions investigated had noticeable inhibition on the phosphate adsorption on ZnAl, which was more obvious with the concentration of ions rose. The reason could be the competition of adsorption sites for phosphate and coexisting ions.Analyses of solid and liquid phases during phosphate adsorption show that the variation of XRD patterns of LDH was negligible, which indicates that surface precipitation of phosphate did not contribute much to the total phosphate removal. FT-IR spectra confirm phosphate ions on the phosphated LDH. By comparing the composition of solid and liquid phases and pH during the phosphate adsorption, and the results from desorption assays, it is suggested that phosphate uptake on ZnAl-24is mainly due to ion exchange between the interlayer anions and phosphate ions while the phosphate uptake on ZnAI-96is largely attributed to physico-chemical adsorption.