Investigating the effect of granular activated carbon characteristics on desorption of vancomycin and rifampicin

Presence of antibiotics in concentrations lower than their minimum inhibitory concentration (MIC) in aqueous environments feasibly imposes selection pressure on bacteria favouring the development of resistant strains. Therefore, adsorption onto granular activated carbon (GAC) is widely used to remove antibiotics from water. Paradoxically, desorption of adsorbed pharmaceuticals from GAC may provide suitable conditions for emergence and spread of antimicrobial resistance but there have been relatively few studies of this phenomenon. In this regard, the effect of surface parameters of different GACs on desorption of vancomycin and rifampicin to the aqueous environment were investigated under a variety of operational conditions. Effects of pH and contact time on antibiotic adsorption and desorption were assessed in experiments. Concentrations of antibiotics were measured by high performance liquid chromatography (HPLC) and UV-visible spectroscopy. All GACs demonstrated a partial release of adsorbed antibiotics into aqueous solutions (from 0.95 to 1.65 % for vancomycin and 0.92 to 1.26% for rifampicin, based on the initially adsorbed antibiotic content in neutral conditions). In addition, GAC with higher micro-pore content and larger specific surface area demonstrated an enhanced capacity to adsorb and retain antibiotic from aqueous solution and were thus less prone to release antibiotics into ambient water. Moreover, pH studies demonstrated significantly higher desorption in basic conditions (i.e. 59.74% for vancomycin and 76.98% for rifampicin) than acidic (i.e. 2.53% and 2.23% for vancomycin and rifampicin, respectively). Contact time studies revealed that vancomycin and rifampicin are desorbed from activated carbon as soon as it is exposed to an aqueous environment for up to 120 hours; however, when contact time was extended to four weeks, gradual re-adsorption onto GAC was observed. Moreover, a larger pore diameter of the GAC appeared to increase the control of the ambient environment on the desorption process from GAC.