Influence of pH and Na-concentration on facilitated and hampered transport of metoprolol by montmorillonite colloids in water-saturated quartz sand

The role of clay colloids in facilitating or hampering the transport of cationic contaminants, such as heavy metal cations or radiogenic nuclides, in groundwater is well established. However, it is unclear whether this phenomenon occurs with positively charged organic contaminants to the same extent.

Laboratory column studies were conducted on water-saturated quartz sand for investigating the influence of colloids on the transport of metoprolol (MTP), a β-blocker with an acid dissociation constant (pK_a) of 9.6, under varying pH and ionic strength conditions. The experiments were carried out at pH 3, 6, and 11, with sodium concentrations of 1 and 100 mmol L^–1. Experiments were conducted with and without 0.5 g L^–1 Na-montmorillonite colloids (diameter: 100–1000 nm). The results show that the presence of colloids increases the transport velocity of MTP, particularly at pH 6 and low sodium concentrations, indicating colloid-facilitated transport as the primary mechanism. Upon reducing the pH, the magnitude of colloid-facilitated transport decreases, but the bimodal breakthrough curve still suggests some co-transport of MTP. At pH 6 and high sodium concentration (100 mmol L^–1) and pH 11, where MTP is predominantly uncharged, the presence of montmorillonite did not significantly impact the transport of MTP.

These findings highlight the dependence of MTP transport on the existence of clay colloids, salt concentrations, and the speciation of the contaminant determined by water pH. Thus, considering these variables is crucial for accurately predicting the mobility of positively charged organic contaminants in groundwater.