ABSTRACT
Rice husk, a potential source of activated carbon, is locally available in Nigeria but underutilized thereby constituting solid waste menace in the environment. Incidentally, large volume of wastewater containing high concentrations of toxic contaminants such as phenol and other contaminants are continuously being generated as a result of increased industrial activities. Agricultural waste materials including rice husk, when suitably modified could serve as activated carbon for efficient removal of wastewater contaminants. In this study, rice husk activated carbon was produced by carbonization followed by activation with phosphoric acid. Batch adsorption experiment was conducted on effect of process variables (carbonization temperature, initial phenol concentration, adsorbent dosage, contact time, solution pH and temperature) on the adsorption of phenol onto rice husk activated carbon. Equilibrium isotherm, kinetics and thermodynamics of the adsorption process were studied. Response surface methodology was then employed for modeling and optimization of the process variables. Under the optimum condition for the production of rice husk activated carbon developed, characterization of the rice husk activated carbon was carried out and column adsorption experiment was conducted in two modes: Rapid Small Scale Column Test (RSSCT) and Large Scale Column Test (LSCT) using process wastewater collected from Kaduna refinery and Samaru stream water. RSSCT was utilized to study the effect of process variables: flow rate and bed depth and the experimental data obtained were fitted into column models (BDST, Thomas and Yoon-Nelson), while the LSCT was employed to obtain the peak adsorption performance. For the batch adsorption, process variables were observed to have significant effect on the performance of the adsorption process. Adsorption capacity and removal efficiency of 2.4 mg/g and 96 % were attained and equilibrium was found to occur between 10-20 vii minutes. Adsorption capacity and removal efficiency were also observed to be maximum at pH=4 and better favoured at lower temperatures. The adsorption process was found to be inconsistent with the assumption of Langmuir monolayer but conformed to Freudlinch assumption of multilayer and physical adsorption. The data followed pseudo-second order kinetic model and intra-particle diffusion was not the only rate-limiting step. The adsorption process was also found to be exothermic. The model equations developed have correlation coefficient (R 2 ) values of 0.9979 and 0.9981 for removal efficiency and adsorption capacity respectively. Optimization revealed that the optimum temperature for the production of rice husk activated carbon is 441.46 oC. The models were experimentally validated under optimum condition. Phosphoric acid modification of rice husk was also observed to enhance the surface area from 12.47 to 102.4 m 2 /g and micropores from 2.4 to 1.82 nm. This was confirmed from the SEM micrographs while FTIR analysis revealed the existence of oxy- and phosphorous-oxy-containing functional groups. RSSCT shows that performance of the column adsorption was significantly affected by bed depth and flow rate. The column experimental data fitted well to Thomas and Yoon-Nelson models. LSCT shows that the column experiment attained relatively high adsorption capacity of 55.106 mg/g at exhaustion point of 0.9; this implies the peak adsorption capacity of the rice husk activated carbon.