Adsorption of Phenols onto Fly Ash (2008)

ABSTRACT&#13;\nOrganic wastewaters have always been a problem due to their increasing toxic threat to humans and the environment. The removal of organic compounds has become an&#13;\nimportant issue due to stringent measures introduced by various countries to enforce&#13;\nregulations concerning wastes originating from petrochemical industries. Hence,&#13;\nwastewater containing such compounds must be treated.&#13;\nIn this research, the adsorptive capacity of an unclassified (class F) South Africa Coal Fly Ash (SACFA) originating from a power plant was evaluated for the removal of phenol, 2-Nitrophenol and 4-Nitrophenol from aqueous solutions. Fly ash generated in the Lethabo Power Station was collected and tested for its use as a low-cost adsorbent. Batch studies were performed to evaluate the effects of various experimental parameters such as adsorbent dosage, initial pH (pHo), and contact time on the removal of these three adsorbates. The equilibrium isotherms for the adsorption of the adsorbates on the SACFA were analyzed by the Freundlich and Langmuir models at pH 2.22. The desorption of the adsorbates from the FA surface was also carried out using 30 ml of distilled water at 307 K.&#13;\nFixed bed column tests were undertaken to study and evaluate the dynamic adsorption&#13;\nbehaviour of phenol/FA through a dynamic column approach. The performance of a&#13;\nsmall - scale fixed bed column containing FA was evaluated using 20 mg/l of phenol&#13;\nconcentrations. The column with 20 mm diameter, studied bed depths of 30, 40 and 50&#13;\nmm and flow rate of 3.0 ml/min was used in order to obtain experimental breakthrough&#13;\ncurves. The bed depth service time (BDST) model was used to analyze the experimental&#13;\ndata and the design parameters like adsorption capacity, adsorption rate and service time at 30% and 70% breakthrough. BDST was also extended by predicting service times of columns operated under different influent concentrations and flow rates and these theoretical values were compared with the experimental values.&#13;\nThe rate of adsorption follows first order kinetics before attaining equilibrium with the sorption rate ( ad K ) obtained for adsorption on FA being the highest for 4-nitrophenol ( 7.0X10-3 h -1 ). The adsorption isotherm indicates that the Freundlich model effectively fits the experimental data for the adsorbates better than the Langmuir model, with 4-Nitrophenol having the highest adsorption capacity of 6.51 X 10-2 mg/g, 2-Nitrophenol&#13;\n6.00 X 10-2 mg/g and phenol 6.31 X 10-2 mg/g. SACFA was found to adsorb 90.2% of&#13;\nphenol, 88.9% of 2-Nitrophenol and 92.6% of 4-Nitrophenol at an initial concentration of&#13;\n20 mg/l. The desorption studies suggested that the desorption of 4-Nitrophenol was the&#13;\nmost difficult out of the three adsorbates to be desorbed. The desorption efficiency was&#13;\n17.9% for phenol, 18.8% for 2-Nitrophenol and 10.2% for 4-Nitrophenol by using 30 ml&#13;\ndistilled water at 300 rpm for 22 h.&#13;\nThe studies showed that SACFA can be used as an efficient adsorbent material for&#13;\nremoval of organic compounds from water and wastewater.