Detailed sorption characteristics of the anti-diabetic drug metformin and its transformation product guanylurea in agricultural soils

Highlights: • Metformin and guanylurea have contrasting sorption behaviour. • Metformin and guanylurea isotherms exhibited non-linearity at high concentrations. • Sorption of metformin in soils showed high correlation with Si/Al ratio. • Guanylurea sorption correlates well with cation exchange capacity of soils. Detection of metformin, an antidiabetic drug and its transformation product guanylurea in various environmental matrices such as surface water and groundwater, coupled with their effects on aquatic organisms warrant an understanding of the compounds fate and behaviour in the environment. Batch studies were conducted with the aim of evaluating the sorption of these two emerging contaminants in six New Zealand agricultural soils of contrasting physico-chemical properties. Kinetic studies revealed that metformin and guanylurea sorption in Te Kowhai soil was very rapid initially achieving 90% sorption within the first 4 and 13 h, respectively. Fit of several isotherm models to the measured batch sorption data showed that the hybrid models Langmuir-Freundlich and Redlich-Peterson best described the isotherms. Freundlich isotherm showed higher linearity for guanylurea (n_F = 0.58–0.93) in all soils compared to metformin (n_F = 0.25–0.71). A linear isotherm was fitted at environmentally relevant low concentrations (< 3 mg/L) of target compounds and calculated values of sorption distribution coefficient (K_d) were in the range of 8.97 to 53.49 L/kg for metformin and between 10.6 and 37.51 L/kg for guanylurea. Sorption of both metformin and guanylurea was dependent on the soil characteristics, however, no generalisation could be made as to which had higher affinity to soils studied. Pearson's correlation and multiple regression analyses indicate that Si/Al (p = 0.042) and clay (p = 0.015) significantly influenced metformin K_d values, whereas the soil's cation exchange capacity (p = 0.024) is the single most significant factor determining guanylurea sorption in soils. It is likely that the type of minerals present in soils and its ion-exchange capacity could play an important role in metformin and guanylurea sorption, respectively.