Adsorptive Removal Of Phosphate From Secondary Effluents In Sewage Treatment Plants By ZnAl Layered Double Hydroxides

In traditional or enhanced biological phosphorus removal processes, secondary effluent still contain tiny amount of phosphate, and its discharge to receiving waters could result in eutrophicatopn of rivers, lakes and bays. Therfor, new technologies of phosphorus removal may need to be considered and combined. In this study, ZnAl layered double hydroxides were synthesized by urea hydrolysis-based coprecipitation. Granular Layered double hydroxides (LDHs) were prepared by using polyvinyl alcohol (PVA) as a binding agent. ZnAl LDHs were prepared to study the possible application of the material in phosphate removal from secondary effluents in wastewater treatment plants (WWTPs). The structural and phosphate adsorption properties of LDHs granules were compared with those of powder LDHs.The adsorbent was characterized by XRD, SEM, BET, and FTIR spectroscopy. The results show that PVA as a binding agent did not change the interlayer distance of ZnAl LDHs or interact with the hydroxyl layers or interlayer anions. Although the distribution of pore size in granular ZnAl LDHs was similar to that in powder samples, the total pore volume and specific surface area (BET) decreased by68.2%and63.1%, respectively, after granulation.The adsorption kinetics with initial concentrations of2,10and50mg-P/L and adsorption isotherms at15,25and35℃were were carried out. The phosphate adsorption capacity of granular ZnAl LDHs was to some extent lowered compared with that of LDHs powder, nevertheless the decrease was not proportional to the reduction in the specific surface area, suggesting that physical surface adsorption was not a major pathway of phosphate uptake by ZnAl LDHs. Phosphate adsorption onto ZnAl LDHs powder was basically a two-step process:a fast uptake in1h and a slower uptake afterwards, which well fit to pseudo-first-order models and pseudo-second-order models, respectively. Phosphate uptake by granular LDHs, however, had better agreements with intra-particle diffusion models during the entire contact time, indicating that mass transfer became the rate-limiting step in phosphate adsorption after LDHs granulation. The isotherms of phosphate adsorption by both powder and granular LDHs have better agreements with Freundlich models than with Langmuir models. Phosphate adsorption capacities of both powder and granular ZnAl LDHs increased with increases in temperature, revealing the endothermic nature. At25℃, the maximum adsorption capacities of ZnAl powder and granules were30.296mg/g and20.874mg/g, respectively. By studying the relationship among the LDHs structure, the phosphate adsorption capacities and the characteristics, it was concluded that the phosphate adsorption by the ZnAl LDHs was attributed to a combination of different mechan isms, e.g., surface adsorption, ion exchange, surface complexation.This paper also analyzes application prospect of phosphate removal from wastewaters by ZnAl LDHs granules. By fixed-bed adsorption experiment,2200bed volumes of phosphate adsorption (2mg/L) can be treated at the breakthrough point of0.5mg-P/L as the discharge limit of phosphate for newly built or reconstructed WWTPs in China.25℃, pH=11and5%NaOH could be a suitable solution for phosphate desorption from the used LDHs granules; the desorption efficiency was-45.58%. The adsorbent regenerated ratio up to45.3%after being used3times. Based on the results, ZnAl LDHs can be a promising alternative in advanced treatment of urban sewage as an efficient adsorbent for phosphate removal.