Mechanisms of phosphate removal from aqueous solution by zero-valent iron: A novel kinetic model for electrostatic adsorption, surface complexation and precipitation of phosphate under oxic conditions

Mechanisms of phosphate removal from aqueous solution by zero-valent iron (ZVI) were examined. Solution pH largely affected phosphate removal efficiency. The phosphate removal decreased from 96.4 to 48.4% with increasing pH from 3 to 4.5 and subsequently increased with further increase in pH. The maximum removal efficiency of 97.3% was obtained at pH 7. When pH was higher than 7, the removal efficiency decreased with increasing pH. At pH 11 the phosphate removal was drastically reduced to 8.6%. To quantify effect of pH on mechanisms of phosphate removal by ZVI, i.e., contributions of adsorption on ZVI surface and precipitation in solution to phosphate removal, the fractions of removals by adsorption and precipitation were experimentally defined and a novel phenomenological kinetic model was developed. In the modelling, electrostatic adsorption, surface complexation and precipitation of phosphate were taken into account as predominant mechanisms of phosphate removal by ZVI. Their contributions linked with each other varied with solution pH. Since the increase in pH resulted in the decrease of ZVI dissolution causing the deceleration of elution of Fe ions from ZVI, the precipitation in solution was inhibited at higher pH. For pH < similar to 9 (the isoelectric point of ZVI), with increasing pH the renewal of ZVI surface was suppressed and the active-sites for phosphate adsorption onto ZVI surface increased. As a result, the overall adsorption onto ZVI surface or the sum of electrostatic adsorption and surface complexation of phosphate on ZVI surface dominated phosphate removal. The proposed kinetic model could successfully predict the contributions of overall adsorption and precipitation to phosphate removal by ZVI in the range of pH from 3 to 11 with the average relative deviations of 7.3 and 15.1%, respectively.