Effect of surface chemistry and porosity of activated carbons on the adsorption of water and organic pollutants

Three samples were used for this study: two of wood and one of coal origin. The samples were further oxidized to study the effect of oxidation on surface chemistry and porosity. The microstructure of the carbon was studied using nitrogen adsorption isotherms. The surface chemistry was characterized using Boehm and potentiometric titrations, temperature programmed desorption (TPD), inverse gas chromatography (IGC), and diffuse reflectance FTIR. The results showed that oxidation introduces a variety of functional groups to the surface. The combination of TPD, FTIR and titration methods led to the detection of non-acidic functional groups such as nitro groups, introduced to the carbon matrix via a nitration mechanism during oxidation with HNO ₃. Water and methanol adsorption isotherms were measured at various temperatures close to ambient. From these isotherms, heats of adsorption were calculated. The results showed that the isosteric heats of adsorption are affected by surface chemical heterogeneity only at low surface coverage. The shapes of the isosteric heats indicated strong water-water interactions as a result of adsorption on secondary sites and cluster formation. The results also showed that washing the carbon with methanol significantly modifies the surface chemistry of carbon creating very easily hydrolyzed esters. This prevents a correct calculation of the heat of adsorption. Heats of methanol adsorption showed that pore size and pore volume play major roles in the adsorption process. However, surface chemistry also contributes to the process. Adsorption of diethyl ether was studied by means of inverse gas chromatography at finite concentration. Adsorption isotherms were obtained from the chromatographic peaks using the characteristic-peak elution method. The results showed that the difference in the uptake of diethyl ether depends on the porosity of the samples and their surface chemistry. Analysis of heats of adsorption indicated that ether molecules are adsorbed on the carbon surface via significant contribution of hydrogen bonding on functional groups present in narrow pores and interactions of the hydrocarbon moiety with the pore walls. Phenol adsorption from solution was studied and the results showed a strong dependence on the presence of carboxylic functional groups which act as ester formation sites and as electron withdrawing groups.