γ-AlO selectively adsorbs transition group metals from contaminated waters to produce bi-metallic catalysts for efficient nitrate reduction.

Metals present in industrial or mining wastewater are hazardous to human health and the environment, and their remediation is costly. In this study, we used γ-AlO to capture transition metals (e.g., Pd, Cu, Ni, and Co) from wastewater and used the spent adsorbent as catalysts for nitrate reduction. Based on a pH-dependent surface-mediated redox reaction mechanism, Pd and Cu ions are selectively adsorbed onto γ-AlO at ambient temperature without involving the use of chemical reductants, thus transforming these pollutants into active catalytic sites. We particularly focused on Pd/Cu bimetallic catalysts, finding that the Pd and Cu ion concentrations in contaminated water impacted both nitrate reduction efficiency and nitrogen selectivity. The best nitrate reduction occurred with 0.74 wt% Pd and 0.66 wt% Cu on γ-AlO, achieving high nitrate reactivity and ∼81 % selectivity for N formation over 10 cycles. Ni and Co, which have smaller redox potential differences than Pd and Cu, showed limited interference with Pd and Cu adsorption, allowing γ-AlO to form Pd-Cu bimetallic catalysts selectively in batch and column tests. Overall, metal-based catalysts can be fabricated under ambient conditions while remediating metal-contaminated wastewater, thereby producing functional products (e.g., hydrogenation catalysts), which is consistent with circular economy principles.