Synthesis and evaluation of activated carbon from rice husks for removal of humic acid from water

Activated carbon was synthesized from the New Rice for Africa (NERICA) rice husk variety, followed by its evaluation for removal of humic acid from water. Product values of carbon yield, C and total specific surface area, a_s,BET were employed as the performance criterion. The best physically activated carbon resulted from char activation at 800 °C for 10 min, leading to a_s,BET and C×a_s,BET values of 756.8 and 402.7 m² g⁻¹, respectively. The best chemically activated carbon resulted from 30 wt% H₃PO₄ activation of rice husk at 400 °C for 30 min, leading to a_s,BET and C×a_s,BET values of 2258.4 and 1058.7 m² g⁻¹, respectively. Despite the higher a_s,BET value, the maximum adsorption capacity of the best chemically activated carbon (5.3 mg g⁻¹) was found lower than 8.9 and 27.2 mg g⁻¹ exhibited by the chemically activated carbons prepared at 500 and 600 °C, respectively. The best adsorption conditions included carbon dose of 0.5 g, solution pH of 2, and contact time of 60 min. The adsorption capacity of the prepared activated carbons (27.2 mg g⁻¹) was comparable to that of the commercial activated carbon (30.40 mg g⁻¹). The analyses of the adsorption isotherms and kinetics revealed that the experimental data fits well the Langmuir isotherm model, as well as the pseudo-second-order kinetic model. The latter suggests that the adsorption of humic acid onto the activated carbon was controlled by the chemisorption process. Overall, the study revealed that the NERICA rice husk variety has good prospects for preparation of activated carbons for humic acid adsorption.