Activation of carbonized biomass materials as potential agents prepared for methylene blue adsorption

Piliostigma thonningii (PSH), Parquetina nigrescens (PNS), and Hildegardia barteri (HB) activated carbon were prepared as adsorbents for methylene blue (MB) dye removal from aqueous solutions. The activation of raw materials with orthophosphoric acid (H3PO4) leads to carbons with high adsorption capacities. The biomasses were carbonized at 400 °C, 500 °C, and 600 °C, and the highest percentage yields were achieved at 400 °C. The biomasses were characterized using scanning electron microscope (SEM), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR), and carbon, hydrogen, and nitrogen (CHN). They were used for batch adsorption of MB dye from aqueous solutions. Effects of initial concentration, contact time, pH, adsorbent dosage, and temperature were studied to determine the optimal condition of the activated carbon (AcC) adsorbents. The adsorption capacities of PSH, PNS, and HB were 55.25, 20.16, and 19.27 mg/g of MB, respectively. The negative value of ΔH of adsorption for PSH (-0.065 KJ/mol) shows that the adsorption was exothermic, while the positive value of ΔH for HB (+0.017 KJ/mol) and PNS (+0.128 KJ/mol) indicates that the adsorption of MB was endothermic. The positive values of ΔS (7.30 J/mol/K for PSH, 9.77 J/mol/K for HB, and 11 J/mol/K for PNS) revealed randomness of the adsorption processes at the sorbent-sorbate interfaces. Langmuir fitted better as the R2 values were all close to unity, while the kinetics studies followed the Pseudo-Second Order for all the adsorbents. The result demonstrated that the AcC sets from the three biomasses have the potential to be employed as low-cost adsorbents.