CO2-enhanced recovery of fulvic acid via activated carbon adsorption: a novel approach to sustainable wastewater treatment

Fulvic acid (FA), a widely existed contaminant in organic wastewater, also represents a significant chemical asset. Recovery using activated carbon (AC) faces limitations due to FA’s high solubility, and the concomitant recovery of other contaminants undermines the value of the recovered FA. In this study, by carefully designed experiments, we have demonstrated that introducing a gaseous CO₂ flow can selectively enhance the FA recovery capacity of AC at room temperature. Extensive experiments and characterizations were conducted to validate the enhancing capability, explain its working mechanism, and optimize the recovery performance for FA. Results showed that the optimal conditions for FA recovery were found to be an FA concentration of 2 g/L, an AC dosage of 1 g, and a CO₂ flow rate of 0.1 L/min. Compared with the sole AC recovery system, the AC + CO₂ system significantly increased FA recovery capacity from 28.7 mg_FA/g_AC to 98.9 mg_FA/g_AC with negligible enrichment of other pollutants. The adsorption process in the AC + CO₂ system followed a pseudo-second-order kinetic model, indicating a predominance of chemical adsorption mechanism. Further experiments were designed to figure out the recovery-enhancing mechanism in respects to gas flow disturbance, pH, and ion species. It was found that the existence of HCO₃^– played an important role in recovery enhancement of AC. Furthermore, the enhancing effect could work for different kinds of AC but only work for FA recovery. The discoveries are promising to develop an effective and economic FA recovery technique in aqueous systems.