Sustainable production of WTR-derived adsorbents: Integrating life cycle assessment, hotspot analysis, and process optimization

Municipal drinking water treatment residuals are commonly landfilled despite containing aluminosilicate and metal hydroxide phases that can be converted into functional adsorbents. However, waste-derived materials are not inherently environmentally beneficial, particularly when chemical activation and thermal processing are required. This study developed ZnCl₂-activated adsorbents from water treatment residuals (WTR) and integrated material characterization, cradle-to-gate life cycle assessment, and machine-learning-based multi-objective optimization to identify synthesis conditions that balance adsorbent characteristics with environmental burden. Five synthesis scenarios were evaluated by varying pyrolysis temperature and ZnCl₂/WTR ratio. The optimal condition, 500 °C and ZnCl₂/WTR = 1.5, increased BET surface area from 107.97 to 271.38 m 2 g −1 and produced a micro-mesoporous structure while retaining surface hydroxyl functionality. ReCiPe 2016 midpoint and endpoint results showed that adsorbent production imposed higher impacts than direct landfilling per kilogram of WTR managed, mainly due to ZnCl₂ production, thermal energy demand, and drying operations. However, when expressed per kilogram of adsorbent, the optimized material showed a global warming potential of 5.35 kg CO₂ eq kg −1 , below reported coal-derived activated carbon benchmarks. Gaussian Process Regression coupled with Differential Evolution identified the 500 °C, 1.5 ratio condition as the sole Pareto rank-1 solution within the investigated design space. This study highlights that waste valorization is not inherently sustainable and becomes environmentally defensible only when process design is constrained by life cycle performance rather than driven solely by material optimization.