Biosorption Potential of Arachis hypogaea Peel-Derived Biochar for Mercury-Contaminated Wastewater Treatment

Background and Objective: Mercury contamination in water remains a pressing global challenge due to its persistence, toxicity, and ability to bioaccumulate through food chains, posing serious ecological and health risks. Conventional treatment technologies, though effective, are often costly and energy-intensive, limiting their application in resource-constrained settings. This study explored the potential of groundnut (Arachis hypogaea) peel-derived biochar as an economical biosorbent for mercury removal from aqueous solutions. The work aimed to evaluate the influence of operational parameters, adsorption kinetics, and equilibrium behavior, while also considering the broader implications for waste valorization and sustainable water treatment. Materials and Methods: Groundnut peels collected from local sources were washed, oven-dried, and converted into biochar through slow pyrolysis at 450°C. The resulting biochar was ground, sieved, and applied in batch adsorption experiments. Parameters including adsorbent dosage (1.5-5.0 g), initial mercury concentration (5-25 mg/L), pH (2.4-9.1), and contact time (5-120 min) were systematically varied at 30°C. Residual mercury concentrations were determined using atomic absorption spectrophotometry. Adsorption data were modeled using Langmuir and Freundlich isotherms, while kinetic behavior was assessed with pseudo-first-order and pseudo-second-order models. Statistical analyses were performed using ANOVA at p<0.05. Results: Mercury removal efficiency increased steadily with biochar dosage, reaching 84.96% at 5 g. Adsorption was rapid within the first 40 min, then slowed as equilibrium approached, reflecting external film diffusion followed by intraparticle transport. Optimum removal occurred at pH 6.0 (70.78%), while efficiency declined under strongly acidic or alkaline conditions due to proton competition and hydroxide precipitation. Removal remained above 66% up to 20 mg/L but decreased at higher concentrations due to site saturation. The Langmuir model provided the best fit (q_max = 20.01 mg/g, R² = 0.932), indicating monolayer adsorption, while kinetic data aligned more closely with the pseudo-first-order model (k1 = 0.107 min^–1, R² = 0.978), suggesting physisorption dominance. Conclusion: Groundnut peel biochar demonstrated promising potential as a sustainable, low-cost sorbent for mercury removal, combining moderate adsorption capacity with rapid uptake and pH-sensitive performance. While effective under laboratory conditions, further studies on surface modification, regeneration, and column applications are recommended to enhance efficiency and support real-world deployment. This work highlights the dual benefit of mitigating mercury pollution while valorizing agricultural waste within a circular economy framework.