Adsorptive removal of phosphate from secondary effluents in WWTPs by ZnAl layered double hydroxides granules

Abstract

Granular ZnAl layered double hydroxides (LDHs) were prepared to study the possible application of the material in phosphate removal from secondary effluents in wastewater treatment plants. The structural and phosphate adsorption properties of LDHs granules were compared with those of powder LDHs. The results of powder X-ray diffraction and Fourier transform infrared spectra show that polyvinyl alcohol 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 Brunauer–Emmett–Teller specific surface area decreased by 68.2% and 63.1%, respectively, after granulation. 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 in 1 h and a slower uptake afterward, which fit well 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.