Hydrothermal metal recovery of metal-contaminated wastewater with forest residue: a zero waste discharge process†

Hydrothermal technology emerges as a cutting-edge approach for utilizing liquid effluent and waste biomass into valuable products. Simulated zinc metal effluent (Zn-1758 ppm) and real zinc electroplating effluent (Zn-765 ppm and Cr-506 ppm in high concentration) with pine needles as an adsorbent, aiming for zero waste discharge were investigated. A comprehensive study was performed to analyze the impact of several critical parameters, such as temperature (100–600 °C), time (0–60 min), and biomass to simulated metal effluent ratio (1 : 4 to 1 : 10), on metal recovery from metal-contaminated wastewater. The metal ions in the effluent are bound to the carbon matrix and reduced to lower valence metal oxide or pure metal during the hydrothermal process, later recovered as a metal–carbon composite. Parameters such as temperature and time positively impact the recovery of metal ions from wastewater. Under operating conditions of 400 °C, 30 minutes, and a biomass-to-effluent ratio of 1 : 100 utilizing pine needle-infused real zinc electroplating effluent, a recovery exceeding 99.9% of metal ions has been attained, concurrently yielding a metal loading of 303.4 mg g⁻¹ of the carbon composite. Under similar operating conditions with pine needles and simulated zinc metal effluent, a maximum metal loading of 623.3 mg g⁻¹ of carbon composite was achieved. The generated carbon composite has nanometals with a quasi-spherical morphology and a significant surface area (max: 221.1 m² g⁻¹), rendering it suitable for fabricating sensors and energy storage devices.