Neutralization of pH and removal of heavy metals from acid mine water by using low-cost biosorbents in batch and column studies

The present research explored raw oyster shell (RO), oyster shell biochar (OB), and ginkgo leaves biochar (LB) as low-cost biosorbents for heavy metal removal and pH neutralization from acid mine water. Five adsorbents were tested in batch mode. The results showed that OB, RO, and LB effectively removed Cd²⁺ (57–98 %), Cu²⁺ (93–99 %), and Fe²⁺ (96–99 %), while Mn²⁺ (7–57 %) and Zn²⁺ (18–97 %) exhibited lower removal efficiencies. RO and OB also increased solution pH to ∼6.3 due to their alkaline buffering capacity. LB, OB, and RO were further tested in series columns. RO and OB with 31.1, 93.5, and 185 min HRTs were tested in primary columns. A hybrid column with LB, OB, and RO was tested at 93.5 min HRT to enhance removal efficiency. Increasing HRT improved both metal removal and breakthrough times. RO and OB with 185 min HRT removed ∼71 % Cd²⁺, ∼93 % Cu²⁺, ∼6 % Mn²⁺, ∼52 % Fe²⁺, and ∼11 % Zn²⁺ from the primary column. In the secondary hybrid column, 99.53 % Cd²⁺, 100 % Cu²⁺, 55.20 % Mn²⁺, 100 % Fe²⁺, and 74.03 % Zn²⁺ were removed. Cu²⁺ > Fe²⁺ > Cd²⁺> Zn²⁺> Mn²⁺ was the column mode metal removal order. The columns' pH profiles changed significantly during metal sorption, suggesting buffering processes and acidic metal ion elimination. Column modeling using Thomas and Yoon–Nelson equations confirmed high adsorption capacities and extended breakthrough times, particularly in the hybrid system. Mechanistic analysis via SEM–EDS revealed surface deposition and co-precipitation of Cd²⁺, Cu²⁺, and Fe²⁺ on RO and OB, while FTIR spectra and XRD patterns confirmed the roles of carbonate, hydroxyl, phosphate, and amine groups in metal binding. LB's porous structure and functional groups enhanced Mn²⁺ and Zn²⁺ removal through complexation and diffusion-driven sorption. The findings support the potential of oyster shell-based composites as sustainable biosorbents for the remediation of metal-contaminated, acid mine water, and highlight future opportunities for optimization through surface functionalization and hybrid treatment designs.