Phosphate removal from wastewaters by a naturally occurring, calcium-rich sepiolite

Developing an easily handled and cost-effective phosphate absorbent is crucial for the control of water eutrophication. In this study, a naturally occurring, calcium-rich sepiolite (NOCS) was evaluated for its feasibility as a phosphate absorbent candidate. Batch studies showed that phosphate sorption on NOCS followed a stepwise isotherm for concentrations between 5 and 1000 mg P/l, and the phosphate sorption was fitted well by the Freundlich equation. The estimated maximum phosphorus sorption capacity was 32.0 mg P/g, which was quite high compared with other natural materials and was comparable to some efficient manmade P absorbents. The NOCS sorption kinetics followed a pseudo-first-order model with an R(2) value of 0.999. The adsorption of phosphate was highly pH dependent. Phosphate adsorption decreased moderately with increasing pH values from 3.0 to 6.0, and it decreased sharply in alkaline conditions. Ionic strength, sulfate, nitrate and chloride anions had no effects on the phosphate removal capacity of NOCS, but fluoride and bicarbonate anions exerted large effects. Phosphorus fractionation indicated that phosphate removed from the solution was primarily formed as a calcium-bound phosphorus precipitation, which was further confirmed by SEM-EDS analysis. Moreover, phosphate was barely (<1.5%) desorbed from the phosphorus-adsorbed sepiolite regardless of pH value.