Adsorption of nitrogen heterocyclic compounds (NHC) on soil minerals: Quinoline as an example

Understanding the adsorption behavior of nitrogen heterocyclic compounds (NHCs) on soil minerals underlies more effective design of wastewater treatment and soil/groundwater remediation. We investigated adsorption of quinoline, a representative NHC, on quartz sand (using Berea sandstone as a model), at 3 < pH < 9, in low (0.05 M) and high (0.7 M) ionic strength solutions where NaCl was the background electrolyte. Minor clay (kaolinite) in the sandstone contributed significantly to quinoline uptake. Adsorption peaked at pH∼6. It decreased from 2 equivalent monolayers at low ionic strength to a monolayer at NaCl activity approaching that of seawater. A triple layer surface complexation model fits the data well, where quartz and kaolinite contributed sites for three types of quinoline (Q) complexes: 1) innersphere ≡SiOHQ; 2) outersphere ≡SiO-QH⁺ and 3) innersphere ≡AlOHQ₂. Aluminol kaolinite sites promote multilayer quinoline adsorption, whereas only monolayers form on silanol sites. Site density calculations and molecular dynamics (MD) confirmed that quinoline adsorbs upright, on edge, and multilayer adsorption follows formation of the initial monolayer. Our results confirm the effectiveness of sand(stones) and clays for removing NHCs from waste and groundwaters.