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

Abstract

A commercial activated carbon (AC), obtained from peanut shells, was characterized and tested as adsorbent forthe removal of the pharmaceutical products caffeine (CF) and diclofenac (DIC), which were used as modelemerging contaminants. Nitrogen adsorption, XRD, SEM, FT-IR spectroscopy, and chemical analyses were typicalof 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. Atheoretical equation derived from the Langmuir isotherm is proposed to explain the adsorption percentage oradsorbed fraction (fads) as a function of the adsorbent dose (D, adsorbent ?concentration?). Good fittings of thefads vs. D curves and the normal adsorption isotherms were obtained with the same Langmuir parameters. Animportant and practical application of this new equation is to permit a straightforward calculation of the soliddose needed to achieve a required adsorption percentage. With the aim of describing the adsorption processes ofCF and DIC and their competition for surface sites under an ample range of concentrations, the adsorption of theemerging contaminants was investigated in single adsorbate experiments and with binary mixtures, and thecompetitive Langmuir model was applied. CF adsorption was high and independent of pH, whereas DICadsorption was high between pH 4 and 6 and showed a continuous decrease from pH 6 to 10.5. The use of thecompetitive Langmuir isotherm for binary mixtures indicated that there was no pure competition between CF andDIC for surface sites. Instead, there was influenced competition, meaning that the presence of one substance at thesurface modified the adsorption parameters of the other, either through lateral interaction forces or by changingthe molecular orientation at the surface. In both cases, one substance favored the adsorption of the other,compared to pure competition.