Desorption of sulfamethoxazole and ciprofloxacin from long-term wastewater-irrigated soils of the Mezquital Valley as affected by water quality

As irrigation with untreated wastewater often leads to an accumulation of contaminants in soils, nowadays, treated wastewater is increasingly used for irrigation. Here, we investigated whether and to which degree irrigation with treated wastewater might cause desorption of antibiotics from three soils (Leptosol, Phaeozem, Vertisol) and a sediment (Endh reservoir) of the Mezquital Valley, Mexico, that were exposed to untreated wastewater in the past. We performed sequential batch desorption experiments with treated wastewater, artificial wastewater with anionic surfactants, and artificial wastewater without surfactants. We observed no desorption, but net sorption of ciprofloxacin in contact with treated wastewater containing 3.475 mu g L-1 of ciprofloxacin. Sulfamethoxazole was desorbed from the Leptosol, the Phaeozem, and the Vertisol in a similar degree with and without surfactant, but not from sediment, where no sulfamethoxazole was detected. In contact with treated wastewater containing 1.045 mu g L-1 of sulfamethoxazole, it was desorbed from the Leptosol and the Phaeozem with low clay, Fe oxide, and organic matter contents, whereas the Vertisol and sediment showed a net sulfamethoxazole sorption. Desorption could be described with a bi-phasic kinetic desorption model, with most sulfamethoxazole being desorbed via a rate-limited process from poorly accessible binding sites, where it had been accumulated during the long-term irrigation in the past. We conclude that a potential release of pharmaceuticals as a result from changes in wastewater irrigation is soil specific: Leptosols and Phaeozems of the Mezquital Valley might act as long-term sources of the sulfonamide sulfamethoxazole, though not of the fluoroquinolone ciprofloxacin.