Modelled phototransformation kinetics of the antibiotic sulfadiazine in organic matter-rich lakes

Xenobiotic compounds are commonly detected in inland waters. Sunlight-induced photochemical reactions contribute to xenobiotic degradation, but the role of different photoreactions on large geographic scales remains poorly understood. Here, we used a combination of photochemical modelling and large-scale field data from 1020 lakes across Sweden to elucidate the photodegradation kinetics of the commonly used antibiotic sulfadiazine (SDZ) in organic matter-rich lakes. SDZ occurs in two forms, namely acidic HSDZ (pKa = 6.5) and basic/deprotonated SDZ-. Both species are oxidised fast by the photogenerated triplet states of natural organic matter (3NOM*). However, they also undergo efficient back reactions because the partially oxidised HSDZ (and SDZ- to a larger extent) can be reduced back to the initial compounds by the phenolic moieties contained in NOM. Typical lakes in Sweden are rich in NOM and have low pH, with the consequence that SDZ photochemistry would be dominated by HSDZ. Our simulation results showed that SDZ photodegradation kinetics in Swedish lakes would become significantly slower with increasing water depth and pH, while it depended little on latitude, which affects irradiance, or on organic matter content. As a consequence, SDZ would be particularly persistent in lakewater in some densely populated areas with relatively deep and high-pH lakes such as, most notably, the Stockholm region. Here the surface waters could be more heavily contaminated by pharmaceuticals compared to the scarcely populated regions in the centre-north of the country, where lakewater could otherwise promote an efficient photodegradation of SDZ.