Adsorption of Glyphosate in Water Using Iron-Based Water Treatment Residuals Derived from Drinking Water Treatment Plants

Glyphosate, a broad-spectrum herbicide, poses a potential threat to human health and the ecosystem due to its toxicity. In this study, iron-based water treatment residuals (Fe-WTRs) were employed for glyphosate removal. The adsorption kinetics, isotherms, and thermodynamics, as well as the effects of pH, Fe-WTR particle size, and temperature, were explored. The results show that Fe-WTRs are an effective adsorbent for glyphosate adsorption, and the maximum uptake capacity was recorded as 30.25 mg/g. The Fe-WTR surface was positively charged, and low-valent iron dominated under acidic conditions, favoring glyphosate adsorption. Furthermore, smaller Fe-WTR particles (<0.125 mm) showed a faster absorption rate and 20% higher adsorption capacity than larger particles (2-5 mm). The kinetic analysis indicated that the adsorption process exhibits a two-step profile, conforming to the pseudo-second-order model, and the thermodynamic analysis indicated that it is a spontaneous, endothermic, and entropy-driven reaction. Finally, the Fourier transform infrared spectral analysis revealed that this process is mainly associated with the formation of metal phosphate through the ligand exchange of the phosphate groups of glyphosates with the hydroxyl groups of iron present in Fe-WTRs. In this study, we demonstrated the potential of Fe-WTRs as a cost-effective and efficient adsorbent for glyphosate removal.