Adsorption of methylene blue from aqueous solutions using activated carbon derived from sand box seed shell (Hura crepitans): Isothermal, thermodynamic, and kinetic studies

This study explores the use of activated carbon derived from sandbox seed shell (SSS) for the removal of methylene blue (MB⁺) from aqueous solutions. The SSS was impregnated individually with KOH and H₃PO₄ at 85% of the biomass weight using a 1:3 solution ratio to enhance porosity and surface functionality. Carbonisation was performed at 700 °C under nitrogen following a 24-h soaking period. Characterisation using SEM, FTIR, EDS, BET, XRD, TGA, XRF, and PZC verified notable chemical and structural changes. BET surface areas increased from 209.454 m2/g (RSSS) to 543.378 m2/g (SSS-KOHAC) and 433.493 m2/g (SSS-H₃PO₄AC), with corresponding iodine numbers of 482.334, 977.361, and 1142.37 mg/g, respectively. Batch adsorption experiments demonstrated that the removal efficiency of MB⁺ increased with rising pH and peaked near neutral values, consistent with PZC measurements of 6.9, 7.12, and 7.83 for SSS-KOHAC, SSS-H₃PO₄AC, and RSSS, respectively. Thermodynamic modelling indicated spontaneous adsorption for activated samples (ΔG° < 0), whereas RSSS exhibited non-spontaneous behaviour (ΔG° > 0) at elevated temperatures. SSS-H₃PO₄AC showed the highest adsorption energy and selectivity, attributed to phosphate-derived surface groups. Adsorption followed favourable isotherm and kinetic models, confirming the potential of chemically activated biomass as an efficient, low-cost adsorbent for dye removal in water treatment applications.