Characterisation of pristine and KOH-modified rice husk biochars for efficient heavy metal removal in wastewater treatment

Biochar-based technology is emerging as a low-cost adsorbent in municipal and industrial wastewater treatment, given its large surface area and highly porous structure. Rice husk presents a significant waste problem as it constitutes disposal challenges and is barely useful for other purposes. In this study, rice husks (RH) from locally grown rice cultivars were obtained from two rice mills in Nigeria and pyrolysed to biochar at 400 and 500 °C before chemical modification using 2 M KOH. The adsorption capacities (Q_e) of the pristine RH biochar pyrolysed at 400 °C for Zn²⁺ and Pb²⁺, following synthetic wastewater treatment, were 462.5 and 142.8 mg/g, and at 500 °C, 1047.5 and 275.5 mg/g, respectively. KOH-modified biochar outperformed its pristine counterparts, as the recorded Q_e for Zn²⁺ on the KOH-modified RH1 pyrolysed at 400 °C (KRH1_400B) showed 1547.75 mg/g, and 1534.25 mg/g at 500 °C (KRH1_500B), respectively, with a 98 % Zn²⁺ removal efficiency. For Pb²⁺, all KOH-modified biochars for RH1 and RH2 showed a 100 % removal efficiency and maximum Q_e of 275.5 mg/g. Scanning Electron Microscopy (SEM) revealed increased poration of 200 µm sized pores, densely distributed across the rough surface of the KOH-modified biochar. Energy Dispersive Spectroscopy showed varying carbon and silicon compositions of the RH1 and KRH1_400B. Fourier Transform Infrared Spectroscopy corroborated structural disparities in SEM, as compositional differences in the observed functional groups were identified in the chemically modified biochar. These include hydroxyl (3354 cm^-1) and silicate ion (1017.6 cm^-1) which enhance sorption capacity. X-ray Fluorescence revealed an increase in MgO, K₂O, and Al₂O₃ upon KOH modification attributable to the improved heavy metal adsorption efficiency. These results highlight the impact of pyrolysis temperature, physicochemical properties of biomass and chemical modification on heavy metal removal efficiency of biochar for sustainable environmental remediation.