Enhanced phosphorus removal using acid-treated magnesium slag particles

Abstract

Magnesium-enriched magnesium slag particles (MSPs) can be used as an adsorption substrate as well as the magnesium source for struvite precipitation. In this study, an HCl treatment was used to enhance MSPs for phosphorus removal. After soaking in 1 mol/L HCl, an 11.27% decrease in median diameter (D50) and a 6.73% increase in specific surface area were observed when compared with the original MSPs. The improvement of the MSP surface properties resulted in 188.96 mg/kg increase in the PO₄³⁻ adsorption capacity. Irrespective of HCl treatment, the phosphorus adsorption process followed the Dubinin–Radushkevich (D–R) model much more accurately than the Langmuir and Freundlich equations with correlation coefficients higher than 0.94. The adsorption free energy obtained through the D–R model revealed a 9.75% decrease after HCl treatment. Sequential fraction extraction results indicated that 96% of the Mg²⁺ released from the HCl-treated MSPs came from acid-soluble magnesium (exchangeable and carbonate-bound). Mg²⁺ obtained from HCl-treated solutions provided a reliable magnesium source for struvite precipitation. The PO₄³⁻ removal rate can reach 53.63% with the optimal pH value of 10.0 and molar ratio of NH₄⁺ to PO₄³⁻ of 1:1. Struvite precipitation and adsorption can simultaneously occur in HCl-treated MSP solution. It contributed 63.19% to the overall PO₄³⁻ removal and is a major contributor compared with adsorption. Thus, HCl treatment greatly enhanced the potential of MSPs for phosphorus removal due to an improved adsorption capacity and is a reliable Mg²⁺ source for struvite precipitation.