Importance of cation exchange for colloid-associated transport of organic cations on clays
Final Report Abstract
Groundwater resources are increasingly under pressure. More and more organic compounds are being detected in surface and groundwater. Only a deeper understanding of the processes and systems involved in the entry and mobility of existing and potentially new pollutants in groundwater can ensure its quality and quantity in the long term and thus prevent conflicts of use. However, it is often unclear how quickly ionizable organic pollutants in particular move in groundwater. Uncertainties arise in particular from the possibility of the coupled transport of such substances on the smallest mobile particles (colloids) in groundwater, known as co-transport. It has already been shown in the past that colloids are able to significantly increase the mobility of radionuclides in the subsurface. The ”Co-Trans” project investigated the effect of co-transport of two ionogenic organic substances on selected clay colloids in a typical groundwater aquifer material. For this purpose, the mobility of the β-blocker metoprolol (MTP; organic base, pKa =9.6) was first determined in quartz sand at selected pH values and NaCl concentrations under water-saturated conditions in the absence of colloids in order to comprehensively characterize the transport of MTP alone. It was shown that pH value and NaCl concentration strongly influence mobility. MTP was highly mobile and moved through the aquifer material without significant interaction both at high and low pH values and under high Na+ competition. However, its mobility was strongly reduced in the range of neutral pH values and low Na+ competition. In the next step, the affinity of MTP for montmorillonite colloids was investigated. It became clear that the clay colloids take up significantly more MTP than a comparable mass of quartz sand. A 300-fold higher fixation was observed for the clay colloids. Moreover, even at very low pH values, MTP fixation on the colloids was still observed, unlike in the case of quartz sand. This demonstrated that there is a high potential for co-transport of MTP on clay colloids. Finally, experiments were carried out to determine the mobility of MTP in the presence of clay colloids. It became clear that co-transport or free transport dominates depending on the water composition (pH value and Na competition). Especially under conditions where MTP should be immobilized, it was transported at high velocity and concentration adhering to clay colloids through the quartz sand. In fact, in the presence of clay colloids, MTP was highly mobile under all conditions, either freely or in co-transport, and was not significantly retained in the groundwater aquifer under any of the selected conditions. In the end, the project clearly demonstrated that the presence of mobile clay colloids significantly changes the mobility of ionizable organic substances. This aspect should be given greater importance in the risk assessment of groundwater contamination wherever clay colloids are mobile and occur in increased amounts.
Publications
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Coupled effects of pH and competing cations on metoprolol mobility: column experiments and parameter estimation using reactive transport modeling. Goldschmidt2022 abstracts. European Association of Geochemistry.
Sawall, Joshua & Schiperski, Ferry
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Mutual Effects of pH and Competing Cations on the Mobility of Metoprolol in Saturated Quartz Sand: Parameter Estimation and Mobility Prediction Using Retardation Modeling. ACS ES&T Water, 3(1), 41-50.
Sawall, Joshua & Schiperski, Ferry
