Effect of Clay Colloid Particles on Formaldehyde Transport in Unsaturated Porous Media

This study examines the effects of two representative colloid-sized clay particles (kaolinite, KGa-1b and montmorillonite, STx-1b) on the transport of formaldehyde (FA) in unsaturated porous media. The transport of FA was examined with and without the presence of clay particles under various flow rates and various levels of saturation in columns packed with quartz sand, under unsaturated conditions. The experimental results clearly suggested that the presence of clay particles retarded by up to ~23% the transport of FA in unsaturated packed columns. Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energy calculations demonstrated that permanent retention of clay colloids at air-water interfaces (AWI) and solid-water interfaces (SWI) was negligible, except for the pair (STx-1b)–SWI. The experimental results of this study showed that significant clay colloid retention occurred in the unsaturated column, especially at low flow rates. This deviation from DLVO predictions may be explained by the existence of additional non-DLVO forces (hydrophobic and capillary forces) that could be much stronger than van der Waals and double layer forces. The present study shows the important role of colloids, which may act as carriers of contaminants.

Colloidal Transport and Deposition Through Dense Vegetation

Background This paper investigates the effectiveness of dense vegetation on removal of colloids, which are important nonpoint source pollutants. In small scale colloid transport experiments, the deposition rate of colloids in dense vegetation is often taken as constant. Results This assumption was tested by experiments and modeling aimed at quantifying changes in colloid retention during transport in dense vegetation, in particular how colloid retention changes with travel distance. Flume experiments using a 10-m long dense vegetated strip were conducted under various conditions with combinations of different flow velocities, initial colloid concentrations, solution pH and ionic strength. The advection-dispersion model coupled with first-order deposition kinetics simulated the experimental data satisfactorily. Conclusions It showed that the colloid deposition rate decreased with travel distance, with the decline following a power law, and reached steady state after about 10 m. Diffusion and interception processes in colloid filtration theory were sufficient to describe the observed deposition kinetics. The decreasing trend in colloid deposition could be attributed to decreased diffusion along the travel path.

Transport of formaldehyde in water unsaturated laboratory columns packed with quartz sand: Effect of colloid-sized clay particles.

<p>Colloid-sized clay particles are in great abundance in the unsaturated or vadose zone and are capable of binding a variety of contaminants, which in turn either facilitate or hinder their migration in the subsurface. Also, FA has relatively strong affinity for kaolinite colloid particles (Fountouli et al., 2019). This study examines the effects of two representative colloid-sized clay particles (kaolinite, montmorillonite) on the transport of formaldehyde (FA) in unsaturated porous media. Transport experiments were performed in columns packed with quartz sand, under unsaturated conditions. The transport of FA was examined with and without the presence of suspended clay particles under various flow rates and various levels of saturation. DLVO interaction energies and the capillary potential energy associated with colloid retention at air-water and solid-water interfaces were calculated. The experimental results clearly suggested that the presence of suspended clay particles hindered the transport of FA in unsaturated packed columns. Moreover, as expected, it was shown that clay particle retention in the packed column increased with decreasing level of water saturation.</p><p> </p><p>Reference</p><p>Fountouli, T.V., C.V. Chrysikopoulos, and I.K. Tsanis, Effect of salinity on formaldehyde interaction with quartz sand and kaolinite colloid particles: batch and column experiments. Environmental Earth Sciences 78, 152, 2019.</p><p> </p>