Application of temporal moment analysis to interpret colloid and colloid-facilitated solute transport under varying size exclusion and attachment coefficient.

Colloidal particles can attach the contaminants like heavy metals and radionuclides and act as contaminant carriers to provide a faster movement of pollutants through the interconnecting pores of the porous medium. The electrostatic repulsive force between the negatively charged colloids and the solid surface restricts the transport of the mobile colloids to the larger pores of the porous medium and initiates the size exclusion mechanism. The temporal moment analysis is treated as an effective tool to interpret the solute breakthrough curves for analyzing the statistical behavior of the contaminants. In past literature, the temporal moments have not been incorporated with the breakthrough curves of colloids and colloid-facilitated contaminants for statistical interpretation. In this research study, the temporal variations of concentrations of mobile colloids, solute attached to the mobile colloids, and the dissolved solute are obtained numerically in a fully saturated one-dimensional column considering a continuous source for varying size exclusion and colloid attachment coefficient. Utilizing the simulated spatially varying breakthrough curves, the temporal moments are estimated to calculate the mass recovery, average residence time, and the spreading of mobile colloids and dissolved solutes. The temporal moment analysis suggests that the velocity enhancement for higher size exclusion reduces the average residence time of the mobile colloids and the solute adsorbed to the mobile colloids significantly. The mass recovery of mobile colloids and the solute attached to the mobile colloids increases at a specific depth for higher size exclusion. The estimated second central moment attributes that the solute spreading follows the nonlinear trend for low size exclusion. The peaks of the relative concentration of mobile colloids and solute attached to mobile colloids drastically decrease with an increase in attachment coefficient. The peak of the relative concentration of dissolved contaminant enhances with attachment coefficient. The high second temporal moment of the dissolved contaminant at a higher attachment coefficient indicates the slow interaction of dissolved solute and porous medium and that enables a greater spreading of solute through the interconnecting porous medium. The study suggests that the faster movement of mobile colloids and the solute attached to the mobile colloids at higher exclusion imparts a potential risk of groundwater contamination and thorough statistical interpretation is needful to analyze the behavior of colloids and colloid-facilitated contaminants. The research work does not consider the transient flow field and the effect of the presence of air phase in the partially saturated soil column in the groundwater system.