Synthesis and comparative analysis of moringa and rice husk biochar via iodine adsorption method for surface area and pore structure characterization

Abstract Surface area is a critical parameter governing biochar’s adsorption performance. Although the Brunauer-Emmett-Teller method is the standard for surface area determination, its requirement for specialized, expensive instrumentation limits accessibility, particularly in resource-constrained laboratories. The iodine adsorption method offers a simple, cost-effective alternative for estimating surface area and micropore volume, yet its applicability across different feedstocks and pyrolysis conditions remains insufficiently validated. This study addresses this gap by systematically comparing Moringa oleifera stem biochar (MBC) and rice husk biochar (RHB) using iodine adsorption across a wide pyrolysis temperature range (300–700 °C) to characterize surface area and pore structure in both materials. Results indicated that MBC pyrolyzed at 400 °C exhibited superior performance, achieving a maximum iodine adsorption capacity of 507 mg g⁻¹ and a specific surface area of 481 m² g⁻¹. In contrast, RHB pyrolyzed at 500 °C showed a maximum capacity of 240 mg g⁻¹ and a surface area of 228 m² g⁻¹, attributed to its silica-rich composition. Optimal adsorption for both biochars occurred at 0.0265–0.04 M iodine concentrations. Adsorption kinetics revealed that MBC reached equilibrium faster due to its well-developed microporous structure, achieving 90% capacity within 90 h, while RHB exhibited slower, sustained adsorption. Isotherm analysis confirmed MBC’s enhanced capacity and surface heterogeneity compared to RHB. This study provided the first systematic comparison of MBC and RHB using iodine adsorption for surface area and pore structure characterization. Beyond reporting adsorption capacities, this study presented detailed kinetic and isotherm analyses that reveal mechanistic differences between lignocellulosic (MBC) and silica-rich (RHB) biochars. The comparative analysis shows MBC consistently outperforms RHB, achieving approximately two-fold higher iodine-derived surface area. These findings establish iodine adsorption as a validated, low-cost characterization tool for evaluating surface area and pore structure across contrasting feedstock types.