Microscopic, spectroscopic, and experimental approach towards understanding the phosphate adsorption onto Zn–Fe layered double hydroxide

Abstract

Seizure of phosphorus from wastewater and runoff has been prioritized in order to reclaim phosphorus for sustainable growth and eutrophication remediation. Here, we report an ultrasonication mediated synthesis of Zn–Fe layered double hydroxide by co-precipitation method for the recovery of phosphate. The kinetics of phosphate adsorption followed the pseudo-second-order kinetics and intra-particle diffusion model, whereas the adsorption isotherm was well fitted with the Freundlich model. Ionic strength and pH-dependent studies predicted the involvement of surface complexation in the initial phase at lower pH and interlayer anion-exchange at higher pH in the later phases of the adsorption process which was further confirmed by Raman and XPS analysis. The role of precipitation at pH 5 was confirmed by XRD where an increase in the intensity at 2θ = 10–35° along with a decrease in the proportion of Zn concentration by XPS were observed. However, from XPS analysis, insignificant loss of Fe indicated an absence of Fe precipitates. The phosphate adsorption capacity of ~36 mg g−1 (pH 6, 20 mg L−1 phosphate) within 5 min coupled with an excellent selectivity against competing inorganic and organic anions make Zn–Fe LDH, a suitable adsorbent for the phosphate recovery. Moreover, the adsorbent retained ~72.4% of its P-adsorption capacity even after six adsorption-desorption cycles and thus, making it a highly economical adsorbent for the treatment of contaminated water.