Optimization of heavy metal removal by activated carbon obtained as a co-product from fast pyrolysis of rice husks

The realization of a circular economy calls for maximum utilization of existing resources with no recoverable waste after the process cycle. During fast pyrolysis of biomass to produce bio-oil for energy purposes, solid residues in form of bio-char are generated. In this study, residual char after pyrolytic-oil extraction from rice husks was activated using steam at 800 °C to produce activated carbon (AC). The formed AC was characterized and evaluated for removal of heavy metals from contaminated water. Box Behnken Design of Response Surface Methodology was used to optimize the removal of Cu²⁺, Co²⁺, Zn²⁺, Pb²⁺, and Ni²⁺ from water. The process conditions were: adsorbent dose (2–12 g/L), contact time (30–180 min) and temperature (25–70 °C). Characterization of AC revealed surface area and pore volume of 407 m²g^-1 and 0.22 m³g^-1, respectively. For all developed models, adsorbent dose, and contact time were the most significant terms. A linear model best fits Cu²⁺ remediation, while quadratic models best-fit removals of Co²⁺, Zn²⁺, Pb²⁺, and Ni ²⁺. Heavy metal removal efficiency increased with increasing adsorbent dose, contact time and temperature. Optimum treatment parameters were: adsorbent dose (11.90 g/L), contact time (172.5 min), temperature (54 °C) with removal efficiencies of 98.2%, 84.1%, 75.3%, 98.1%, 75.7% for Cu^2+, Co²⁺, Zn²⁺, Pb²⁺, and Ni²⁺, respectively. Adsorption data best fitted Langmuir isotherm and pseudo second order models. These results confirm the applicability of AC from pyrolytic-oil residual char for adsorption of heavy metals. Use of AC from residual char in water treatment contributes to circular economy.