Use of low cost adsorbents to treat industrial wastewater

Musapatika, Evans Tongesai
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Advanced wastewater treatment techniques, such as adsorption, are economically and environmentally essential in the removal of non-biodegradable toxic compounds from industrial wastewater. The present study focuses on the use of low cost adsorbents prepared from pine sawdust, coal fly ash and sugarcane bagasse to adsorb recalcitrant compounds such as heavy metals, phenol and direct dyes from synthetic wastewater. Carbonization of the adsorbents was done using a muffle furnace whereas a fluidized bed reactor was used to prepare steam activated sawdust. Both batch and column tests were performed to investigate the use of these alternative adsorbents as potential replacements for the current costly commercial adsorbents. A 23 full-factorial design with four centre points and response surface methodology were used to study the interactive effect of the operating conditions on the adsorption capacity of sawdust and coal fly ash. In addition, the performance of the low cost adsorbents was compared with that of commercial activated carbon (CAC). The maximum adsorption capacities were determined and correlated with the physicochemical properties of the adsorbents. Adsorption capacity was found to vary with initial concentration, adsorbent dose and pH. An increase in pH led to a significant increase in heavy metal removal suggesting the involvement of ion exchange mechanism. In column studies, breakthrough times increased with an increase in bed height and aspect ratio. Conversely, breakthrough times decreased with an increase in initial concentration. Overall, commercial activated carbon was found to be superior; however, sawdust and coal fly ash showed potential as alternative adsorbents for the removal of heavy metals and phenol from petrochemical wastewater. Additionally, the trend followed by the maximum monolayer adsorption capacities, qmax for the four adsorbents used to treat petrochemical wastewater was in agreement with the specific surface area of the adsorbents i.e adsorbents with high specific surface area had high qmax values with a few exceptions. Amongst the low cost adsorbent, steam activated sawdust (SAS) was superior and coal fly ash had the lowest adsorption capacities. Also, the highest adsorbed contaminant by steam activated sawdust was Fe (15.385 mg/g). It has been shown that the performance of steam activated sawdust was comparable to that of commercial activated carbon in the removal of Pb and Ni. Nevertheless, sugarcane bagasse proved to be a better adsorbent than coal fly ash in the uptake of direct red dyes from textile wastewater. Direct red 80 (DR 80) was the highest adsorbed dye by both sugarcane bagasse (6.536 mg/g) and coal fly ash (1.560 mg/g). Equilibrium data for metal removal conformed well to the Freundlich isotherm whereas phenol and direct dye removal complied with the Langmuir adsorption isotherm model. Kinetic data were fitted to Lagergren first-order, pseudo secondorder and the intraparticle diffusion models. Thus, kinetic parameters, rate constants, equilibrium adsorption capacities and related correlation coefficients, for each kinetic model were calculated and discussed. Consequently, the adsorption of cobalt followed pseudo second order kinetics suggesting chemisorption for all the tested adsorbents. Also, results suggested that intraparticle diffusion was not the only step controlling the overall adsorption process of cobalt. Results presented here can help to design an appropriate environmental management strategy to minimize the adverse impacts caused by industrial wastewater.