Activated carbon is a well-known material used in an increasing number of environmental applications particularly for water and wastewater remediation, gas filters, and so forth. Large BET surface area, good catalytic activity and chemical stability of activated carbon have made it an indispensable adsorbent for waste water treatment process. In the present work, seeds of Manilkara zapota (sapodilla)- an agro-waste material, were used to prepare activated carbons by zinc chloride activation under anaerobic condition. Experiments were carried out at different chemical ratios (activating agent/precursor). Effect of carbonization temperature and time are the important variables, which had significant effect on the pore structure of carbon. Developed activated carbon was characterized by SEM and FTIR analysis. The activated carbon thus developed shows substantial capability to adsorb phenol from dilute aqueous solutions. Batch adsorption experiments were carried out to study the effects of various physico-chemical parameters viz initial adsorbate concentration, adsorbent dose, contact time, feed volume and temperature. Maximum removal of phenol on activated carbon in batch mode was obtained for an initial phenol concentration of 990 mg/l (about 92.59 % removal of phenol at the room temperature of 301 K). The effect of changing the dose of adsorbent was studied and an optimum value of 4.5 g of adsorbent dose (activated carbon) was obtained. At room temperature (301 K) keeping the initial phenol concentration (495 mg/l) and adsorbent dose (4.5 g) constant the maximum adsorption was attained at a contact time of 70 minutes. Any feed volume equal to or slightly lesser than 550 ml resulted in maximum phenol adsorption by activated carbon. Equilibrium data were fitted to Langmuir and Freundlich isotherm but the Langmuir model fitted the isotherm data well. Enthalpy value was found to be negative suggesting the reaction to be exothermic, justifying the room temperature to give maximum adsorption. Little reduction of randomness was observed (ΔS°<0).The kinetics of adsorption controls the process efficiency. Adsorption has been treated as a first order, pseudo first order and pseudo second order processes. In the present study adsorption of phenol on activated carbon has been described by pseudo first and second order models and also by a diffusion model. The residual values showed that pseudo second order model, an indication of chemisorption mechanism, fitted better the experimental data than the pseudo first order Lagergren model. However, the interparticle diffusion had also some role in adsorption of phenol by activated carbon. Although the results of this study are preliminary in nature, the same can be further utilized to perform column operation in continuous mode.
The study also establishes the feasibility of Manilkara zapota seeds, an agro waste material to be used as precursor for activated carbon. In view of high cost incurred in the conventional regeneration of spent activated carbon, both powdered and granular, used in various waste water treatment processes, the use of single use adsorbent may be an option. In that respect use of agro waste material as precursor of activated carbon of superior quality assumes techno-economic significance under Indian context, where agro-waste management is a priority area.
