Insight into the sorption mechanism of metformin and its transformation product guanylurea in pastoral soils and model sorbents

Single solute sorption mechanisms of metformin (MET) and guanylurea (GUA) were investigated in six soils and three model sorbents (kaolinite, bentonite and humic acid) at varying initial pH and background electrolyte (Ca2+) concentrations. Electrostatic interaction and cation exchange were proposed as mechanisms of MET sorption. At initial solution pH between pKa1 and pKa2, electrostatic interaction is the dominating mechanism of MET sorption. However, as pH approaches pKa1, cation exchange becomes a significant mechanism of sorption as evidenced by the increased distribution coefficient (Kd) values in Matawhero (130-fold) and Nelson (2000-fold) soils with high cation exchange capacities (CEC) and permanently negative charged sites and when equilibrium pH < pKa1 where the divalent cationic form dominates in the solution. Furthermore, results showed higher sorption of MET on bentonite with effective distribution coefficient (Kdeff) value of 14.92 L/kg with high permanent negative charges than on kaolinite (Kdeff = 6.70 L/kg), a variable charge clay. Increased MET sorption at low equilibrium pH on kaolinite (Kdeff = 2.3 × 107 L/kg) and humic acid (Kdeff = 20.86 L/kg) further suggest cation exchange is also possible at pH < pKa1. On the other hand, two lines of evidence suggest cation exchange as an important mechanism of GUA sorption: (a) the positive correlation between cation exchange capacity and Kdeff values and (b) decreased Kdeff values as the Ca2+ concentration in solution was increased in all soils. Biosolids amendment of three soils resulted in contrasting effects on sorption affinities with a decrease for MET and increase for GUA, further confirming sorption mechanisms and significance of solution pH and CEC on the sorption of MET and GUA, respectively.