Carbonated ferric green rust as a new material for efficient phosphate removal

Abstract

Phosphate uptake from aqueous solutions by a recently discovered ferric oxyhydroxide is investigated. Carbonated ferric green rust {GR(CO32-)∗} is prepared by varying two synthesis parameters, which are (1) the aging period after the ferrous–ferric green rust {GR(CO32-)} synthesis step and (2) the rate of the hydrogen peroxide addition to oxidize GR(CO32-) into GR(CO32-)∗. These two parameters permit the control of the size, morphology and cristallinity of the synthesized particles. As prepared GR* samples are then evaluated, in batch experiments, as possible low-cost efficient phosphate removal materials. Firstly, kinetic experiments reveal that a fast sorption step initially occurs and equilibrium is reached at ∼500min. The adsorption kinetics data at pH=7 can be adequately fitted to a pseudo-second order model. Secondly, the Freundlich model provides the best correlation and effectively describes phosphate sorption isotherms for all GR(CO32-)∗ samples synthesized. Finally, the phosphate adsorption capacity decreases when pH increases. The highest adsorption capacity is 64.8mgg−1 at pH=4 and corresponds to the GR(CO32-)∗ sample displaying the smallest and least crystallized particles thus reflecting the importance of the synthesis conditions. Overall, all sorption capacities are higher than the main iron oxide minerals, making GR(CO32-)∗ a potentially attractive phosphate adsorbent.