Selective sorption of organic phosphonate HEDP by steel slag: Efficiency and mechanism

The utilization of steel slag as an adsorbent for wastewater treatment (e.g., phosphorus removal) is a promising direction for the simultaneous disposal of solid waste and eutrophication control. In this study, 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) was selected as a representative phosphonate and its adsorption performance and mechanism on steel slag were explored. The experiment results show that for the HEDP solution with a concentration of 0–50 mg/L, the removal efficiency of steel slag was 85–88% and the adsorption capacity was 0.0835–8.2793 mg-P/g. The HEDP adsorption behavior on steel slag conformed to the pseudo-second-order adsorption kinetics model. Both the Freundlich and Langmuir adsorption isotherm models can describe the experimental data well. And the theoretical maximum adsorption capacity at 25°C was calculated as 23.74 mg-P/g. Combined with our SEM, XRD, FTIR, and XPS analyses, the adsorption mechanism involved ligand exchange, hydrogen bond interaction, and electrostatic attraction. After 5 reuse cycles, the regeneration efficiency of steel slag remained 77% by using 1 mol/L NaOH. The research work in this paper proves that steel slag, as a solid waste with large emissions in China, can effectively remove HEDP, which supplies a new way for the resource utilization of steel slag.