Insights of competitive adsorption on activated carbon of binary caffeine and diclofenac solutions

A commercial activated carbon (AC), obtained from peanut shells, was characterized and tested as adsorbent for the removal of the pharmaceutical products caffeine (CF) and diclofenac (DIC), which were used as model emerging contaminants. Nitrogen adsorption, XRD, SEM, FT-IR spectroscopy, and chemical analyses were typical of ACs, and Boehm titrations, calculations of surface sites distributions and zeta potential measurements indicated that reactions of deprotonable oxygenated groups at the AC surface lead to an isoelectric point of 3.2. A theoretical equation derived from the Langmuir isotherm is proposed to explain the adsorption percentage or adsorbed fraction (f_ads) as a function of the adsorbent dose (D, adsorbent "concentration"). Good fittings of the f_ads vs. D curves and the normal adsorption isotherms were obtained with the same Langmuir parameters. An important and practical application of this new equation is to permit a straightforward calculation of the solid dose needed to achieve a required adsorption percentage. With the aim of describing the adsorption processes of CF and DIC and their competition for surface sites under an ample range of concentrations, the adsorption of the emerging contaminants was investigated in single adsorbate experiments and with binary mixtures, and the competitive Langmuir model was applied. CF adsorption was high and independent of pH, whereas DIC adsorption was high between pH 4 and 6 and showed a continuous decrease from pH 6 to 10.5. The use of the competitive Langmuir isotherm for binary mixtures indicated that there was no pure competition between CF and DIC for surface sites. Instead, there was influenced competition, meaning that the presence of one substance at the surface modified the adsorption parameters of the other, either through lateral interaction forces or by changing the molecular orientation at the surface. In both cases, one substance favored the adsorption of the other, compared to pure competition.