Title : Some studies on the adsorptive removal of phenol from wastewater
Abstract: Phenol is considered as one of the severe environmental pollutants in the present time. Therefore the study for its removal from the wastewaters is very important. There are many techniques developed for removal of phenol. However adsorption process is considered to be cheapest and simplest one. Recently the phenol removal by adsorption using several new green and low-cost adsorbents has been taken up by many researchers. This research aimed to study the phenol removal percentage using four green bio-adsorbents viz. guava tree bark, rice husk, neem leaves and activated carbon from coconut coir as well as four low-cost industrial waste materials viz. rice husk ash, red mud, clarified sludge from basic oxygen furnace and activated alumina. Initially, the surface topography and characterization of the adsorbents were carried out using SEM, XRD, FTIR and BET analyzers. The phenol removal percentage was investigated using all the adsorbents in the batch experiments with the change in typical process variables such as initial phenol concentration (5-500 mg/L), initial pH (2-12), adsorbent dose (0.10-20 gm/L), temperature (25-50°C) and contact time (30-600 min). The results exhibited that the removal percentage was in the range from 8.67% to 97.50% depending on the nature of the adsorbents. Using the experimental observations, the testing of several kinetic models, isotherm models and the thermodynamic study were carried out to determine the adsorption mechanism, spontaneity, randomness and endothermic/exothermic nature of the adsorbents which consequentially would help in the process design for its commercial application. The analysis of kinetic study showed that based on the higher values of correlation coefficients (r2), the pseudo-second order form was most excellently fitted in case of all the adsorbents except red mud. This suggested that the adsorption process was supportive of chemisorption and heterogeneous for all the adsorbents except red mud which was supportive of physisorption and homogeneous. The analysis of kinetic mechanism showed that the linearity (r2) for Reichenberg model, Fick’s model, Furusawa and Smith model, Elovich model and Boyd model were above 0.8 and therefore the adsorption mechanism for all the adsorbents was supportive of film diffusion, intra-particle diffusion and chemisorption. During chemisorption, the phenol adsorption might have taken place through surface exchange interactions and hydrogen bonding. The isotherm analysis suggested that Freundlich isotherm model was best supportive for guava tree bark, rice husk, neem leaves, activated carbon from coconut coir, red mud and activated alumina, but Langmuir isotherm model was best supportive for rice husk ash and D-R isotherm was best supportive for clarified sludge from basic oxygen furnace. The thermodynamic study showed that the spontaneity, randomness and endothermic/exothermic nature were different but conclusive for all the adsorbents. The experimental observations were further used for the application of ANN modeling using two popular algorithms, which established that the experimental and predictive data were within the allowable range. The scale-up design and the safe dumping of used adsorbents were also deliberated in this study. The study concluded that the neem leaves are the most suitable low-cost green bio-adsorbent for phenol removal from wastewater.