Phosphorus removal from aqueous solution by steel making slag – Mechanisms and performance optimisation

This study aims to evaluate a passive inexpensive process to remove the residual phosphorus (P) from wastewater treatment effluents prior to discharge to the environment. This work also differentiates between surface adsorption and chemical precipitation in the bulk solution as key P removal mechanisms by steel-making slag. Experimental results show that P removal efficiency is governed by steel-making slag particle size, initial P content and ratio of steel-making slag mass to aqueous solution volume. The results demonstrate the potential of steel-making slag for removing dissolved phosphate from wastewater especially as a polishing step. Even at an elevated concentration of P of 5 mg/L, 90% P removal was achieved using 5 kg steel-making slag with particle size of 0.15–0.6 mm for each m3 aqueous solution. Higher removal efficiency was also achievable through process optimisation. In particular, P removal by steel-making slag can be significantly enhanced, and nearly complete removal (>99%) can be achievable by buffering the aqueous solution at pH of 5.6. This study also established the isotherms and kinetics of the adsorption of P to steel-making slag to identify key removal mechanisms. Experimental data systematically indicate that P removal by steel-making slag is governed by both adsorption and chemical precipitation. At the early stage of the removal process, adsorption is the dominating removal mechanism, while the P removal via chemical precipitation can occur once the release of Ca2+ calcium into the aqueous phase is sufficient to form calcium phosphate precipitates. Overall, P removal by chemical precipitation depends on both pH and Ca2+ concentration in aqueous solution.