Eco-Friendly Utilization of Phosphogypsum via Mechanical Activation for Sustainable Heavy Metal Removal from Wastewater

This study examined significant changes in phosphogypsum, a byproduct of the phosphoric acid industry, induced via mechanical activation through intensive grinding using a planetary ball mill. Alterations in crystallinity, surface area, and zeta potential were monitored using X-ray diffraction, Brunauer–Emmett–Teller analysis, zeta potential measurements, X-ray photoelectron spectroscopy, and scanning electron microscopy. The severe grinding of this mining waste led to the conversion of gypsum (CaSO4·2H2O) to anhydrite (CaSO4), an increase in surface area from 5.8 m2/g to 17.8 m2/g, and a decrease in pore radius from 76.6 nm to 9.3 nm. The zeta potential shifted as the isoelectric point changed from pH 8.5 to pH 4.3. These modifications enhanced the material’s potential as a cost-effective and eco-friendly adsorbent for wastewater treatment. The enhanced adsorption capabilities for Cd and Pb were evaluated, revealing a higher adsorption capacity (~40 mg/g for both) and removal efficiency (~90% for Cd and ~80% for Pb) for activated phosphogypsum. The adsorption process followed the Freundlich isotherm and pseudo-second-order kinetic model, indicating its physisorption nature and spontaneous thermodynamic characteristics, and highlighting its potential for wastewater treatment. The mechanically activated adsorbent demonstrated over 90% desorption efficiency over five cycles, ensuring effective regeneration and reusability for Cd and Pb removal. Real tannery wastewater was treated using mechanically activated phosphogypsum at pH 6 and 70 °C for 60 min, achieving a 94% Cd and 92% Pb removal efficiency, with an overall heavy metal removal efficiency of up to 83%. This study demonstrates the sustainable utilization of phosphogypsum, contributing to green wastewater management and environmental protection.