KINETIC AND THERMODYNAMIC STUDY OF THE ADSORPTION OF NATURAL ORGANIC MATTER ON GRANULAR ACTIVATED CARBON
Advancing the chemical engineering fundamentals
Electrochemical Engineering (T2-14P)
Keywords: Drinking water treatment; Disinfection by-products; Natural organic matter; Granular activated carbon; Adsorption kinetics; Isotherms.
During disinfection processes of drinking water the reaction between disinfectant and natural organic matter (NOM) generates disinfection by-products (DBPs). When chlorine is used as disinfectant, these harmful products include trihalometanes (THMs) and haloacetic acids (HAAs). Nowadays, numerous rules make provisions for minimizing the formation of undesirable DBPs.
Adsorption by activated carbon is regarded as one of the best available technologies for removing dissolved contaminants such as natural organic matter (NOM) from drinking water. At the moment, NOM adsorption equilibrium data are available in the literature, but residence times at water treatment plants are not long enough and the process seldom reaches equilibrium. Therefore, not only equilibrium but also kinetic data were evaluated in this study, looking for the activated carbons with both high removal capacity and increased kinetics.
Two different GACs were used for DBPs precursors elimination, one coal based (GACF) and one coconut based (GACA) carbon. Firstly, physical and chemical properties were evaluated. Significant differences were observed between both carbons.
It was evidenced that surface specific area, pore size distribution, point of zero charge (pHpzc), superficial basicity and inorganic content controlled the NOM adsorption process. NOM removal was evaluated by measurements of UV absorbance spectra for wavelengths between 200 and 500 nm. Moreover, the NOM initial concentration and adsorbent particle size were demonstrated to be significant factors on adsorption rate. This behaviour was attributed to mass transfer phenomena.
Finally, adsorption equilibrium data were correlated with two well-known equilibrium isotherm models, Freundlich and Langmuir equations. In addition, the adsorbed organic matter per carbon milligram versus contact time curves were fitted to several adsorption models. The pseudo-first-order equation (PPO), the pseudo-second-order equation (PSO), the intraparticle diffusion model (ID) and the modified pseudo-first-order equation (MPPO) were used. The results showed that the adsorption mechanism in the NOM/GACF and NOM/GACA systems followed the pseudo-second-order equation (PSO) throughout all adsorption curve. Moreover, PSO generates the best agreement with the experimental equilibrium data. This behaviour was maintained regardless of the initial NOM concentration.
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Presented Wednesday 19, 13:30 to 15:00, in session Electrochemical Engineering (T2-14P).
