Competitive Effects of Cu(II) on the Adsorption of Cd(II), Mn(II), and Zn(II) from Neutral Mine Drainage Using Low-Cost Adsorbents

Neutral mine drainage may contain potentially toxic metals even under circum-neutral pH conditions. This study investigated the competitive effect of Cu(II) on the removal of Cd(II), Mn(II), and Zn(II) from real neutral mine drainage using unmodified bentonite, natural zeolite, and biomass-derived fly ash. Batch experiments were conducted for 120 min using 0.5 g of adsorbent per 100 cm3 of mine water, with Cu(II) additionally introduced at 20 mg·dm−3. Biomass fly ash increased the solution pH to 9.4 and achieved the highest removal efficiencies in the presence of Cu, reaching 98.8% for Cu(II), 99.3% for Mn(II), and 100.0% for Zn(II). In contrast, Cd(II) removal by fly ash decreased from 57.3% without Cu to 0.0% with Cu, whereas Cd(II) removal increased from −199.8% to 100.0% with bentonite and from −224.6% to 38.4% with zeolite. Cu addition also increased Mn(II) removal from −5.1% to 85.6% for bentonite, from −20.7% to 71.4% for zeolite, and from −40.0% to 99.3% for fly ash, while Zn(II) removal increased to 100.0%, 96.8%, and 100.0%, respectively. Two-way analysis of variance confirmed highly significant Cu × adsorbent interactions for Cd(II) (p < 0.001), Mn(II) (p < 0.001), and Zn(II) (p < 0.001). Langmuir analysis gave the highest calculated qm for Mn(II) with Cu addition: 0.864 mg·g−1 for zeolite, 0.754 mg·g−1 for fly ash, and 0.720 mg·g−1 for bentonite; for fly ash, these values should be interpreted considering simultaneous adsorption and precipitation. No desorption of Cu(II), Mn(II), or Zn(II) was observed after adsorption. Overall, biomass fly ash showed strong potential for Cu(II), Mn(II), and Zn(II) removal from real neutral mine drainage, while the competitive suppression of Cd(II) highlights the importance of multicomponent interactions in practical treatment systems.