Lead (II) adsorption by KOH-modified rice straw biochar from battery wastewater: Adsorption optimization, isotherm, kinetic and thermodynamic studies

Lead (Pb(II)) contamination from battery manufacturing poses a major threat to aquatic systems in Uganda, necessitating cost-effective and sustainable treatment solutions. This study investigates KOH-modified rice straw biochar (KRSB) as an efficient and eco-friendly adsorbent for Pb(II) removal from battery wastewater. Rice straw was pyrolyzed to produce raw biochar (RSB) and subsequently activated with KOH. Both materials were characterised using X-ray fluorescence (XRF), scanning electron microscopy coupled energy-dispersive X-ray spectroscopy (SEM-EDX), and Fourier transformed infrared spectroscopy (FTIR) analyses. Batch adsorption experiments were conducted and optimized using response surface methodology (RSM), considering adsorbent dosage, initial Pb(II) concentration, contact time, temperature, and pH as key variables. The optimal conditions, 0.8 g/L dosage, 121 mg/L Pb(II), 52 min contact time, 33°C, and pH 5.6, resulted in 85.1% Pb(II) removal from real battery wastewater. XRF and SEM–EDX analyses suggested ion exchange and surface complexation as dominant mechanisms, supported by FTIR evidence of oxygen-containing functional groups (–OH, –COO⁻). Adsorption followed the Langmuir–Temkin isotherms and pseudo-second-order kinetics, indicating chemisorption with a maximum capacity of 129 mg/g. Thermodynamic parameters confirmed that the process was spontaneous and endothermic. Although KRSB demonstrated strong potential for wastewater remediation, further studies on activation energy, co-ion interference, adsorbent regeneration, and scale-up are recommended.