Mutual effects of pH and ionic strength on the mobility of metoprolol in saturated quartz sand

Emerging micropollutants such as lifestyle drugs, pesticides, and pharmaceuticals are increasingly detected in the aquatic environment. To counter possible threats posed by these pollutants, their transport behaviour needs to be thoroughly understood.

In this study, 22 laboratory column transport experiments were conducted at selected pH and Na⁺ concentrations with natural quartz sand as sorbent to determine transport behaviour of the beta-blocker metoprolol (MTP, pK_a=9.67) at various conditions. MTP breakthrough curves were measured at pH 3, 6, and 11, as well as NaCl concentrations of 1, 10, and 100 mM/l to account for coupled effects. The observed mean R ranges from R=1.04 (pH 11; 100 mM/l NaCl) to R=10.5 (pH 5.6; 1 mM/l NaCl).

An ion exchange equation was used to model CEC and exchange coefficients of Na⁺, H⁺ and MTP using a least square refinement routine considering the whole dataset of 43 retardation values.

With the model, a high R-regime at low cation concurrence (c(NaCl)~1 mM/l) and neutral pH (5<pH<9) could be identified. Decreasing of retardation can be attributed to: (1) increasing Na⁺ concurrence, (2) decreasing pH at pH<5 due to lowered zeta potential and i.e. negatively charged sites covered by H⁺, and (3) increasing pH at pH>8.5 as a result of changing MTP speciation.