Ascorbic acid-induced digenite (Cu9S5) formation: A strategy to enhance sulfidation efficiency for copper recovery from acidic wastewater

Sulfide precipitation is an effective method for copper recovery from acidic wastewater. However, excessive use of sulfide reagents leads to secondary pollution, which poses a significant challenge. This study demonstrates that leveraging the reducing properties of ascorbic acid (AA) and adding Na₂S to acidic copper-containing solutions (pH 2-4) at a Cu:S molar ratio of 1.6:0.8 enhances Cu(II) removal efficiency from 50 % to 100 %, doubling sulfide reagent utilization. Solution chemistry, electron microscopy, and X-ray spectroscopy analyses indicate that AA primarily promotes copper sulfide precipitation with higher copper content. Addition of AA at pH 2 at a Cu:S:AA molar ratio of 1.6:0.8:1 lowers the oxidation-reduction potential (ORP) from 0.5 V to 0.1 V, forming digenite (Cu₉S₅). Compared to covellite (CuS) formed without AA, digenite exhibits a higher Cu oxidation state and a lower S oxidation state. Furthermore, coagulation kinetics studies show that solution pH, Na₂SO₄, and FeCl₂ concentrations influence copper sulfide aggregation. At pH 2, Cu₉S₅ coagulates at least 4.2 times faster than CuS within 20 min. Cost analysis shows that the cost per ton of copper recovered from wastewater using this method is about one-third of the cost of conventional methods. More importantly, this study minimizes residual sulfide, offering a novel strategy for dose control in copper sulfide recovery.