Removal of crystal violet by TiO2 loaded alkali-activated carbon hybrid material from Raphia farinifera fruit kernels: surface chemistry, parameters and mechanisms

In this work, two TiO2-loaded alkali-activated carbon hybrid materials (RAFFT-B4.76 and RAFFT-B9.1), with varying fractions of TiO2, obtained from Raphia farinifera fruit kernels (RAFF) were synthesized and applied to eliminate crystal violet (CV) dye at circumneutral pH. The structural and textural properties and surface chemistry of the unmodified activated carbon (AC-B) and RAFF-TiO2 hybrids were characterized by: TGA, FTIR, XRD, BET, SEM/EDX, and Boehm titration. The BET specific surface areas (SSA) and pore volumes decreased with an increase in TiO2 fraction. The adsorption capacities for CV were essentially linked to the SSA and surface chemistry of the adsorbents. Furthermore, CV adsorption showed a strong negative dependence on the number of acidic groups (total acidity) due to two factors: (1) increased adsorption of water molecules as competing species and (2) weakened CV-adsorbent interactions due to the electron-withdrawing effect of acidic functional groups. The adsorption data were best described by the pseudo-second-order (PSO) kinetic model and Langmuir isotherm model with the highest adsorption capacity of 119.02 mg/g corresponding to RAFFT-B9.1. The thermodynamic functions (ΔG<0 and ΔH>0) values indicated that the adsorption processes were spontaneous and endothermic. The magnitude of ΔH values (> 40 kJ/mol), conformity to the PSO order model and dependence of adsorption on functional groups (surface chemistry) make it conceivable that the adsorption of CV on these materials is a chemisorption process.