Experimental Investigation on Co-transport of Chromium (Cr) and Lead (Pb) with CML Colloids in Quartz Sand Column Comprising Different Grain Sizes

Colloids present in the subsurface environment can be identified as potential pollutant carriers that enable rapid mobility of contaminants like heavy metals and pesticides through the interconnecting pores of the aquifer medium. They exhibit strong adsorbing tendencies to interact with the existing contaminants because of microscopic sizes enabling high specific surface area. Underestimating the rapid migration of colloid-facilitated pollutants possess potential threat to the quality degradation of groundwater resources. In this research, an experimental investigation is performed to inspect the transport behavior of carboxylate-modified polystyrene latex microspheres (CML) and colloid-facilitated Chromium (Cr) and Lead (Pb) in one-dimensional sand column having 60 cm length for pulse input concentration. Quartz sand of four different grain sizes ranging from 75 μm to 2 mm is considered as the porous medium in the column experiments. The influence of the straining mechanism on the migration potential of colloid and colloid-facilitated contaminants in the porous medium of different sand grain sizes is the prime concern of the experimental study. From the column study, it is observed that the maximum effluent concentrations of CML colloid concentration reduce with a decrement in sand grain sizes. The straining or entrapment of colloids at grain-grain contact points significantly enhances the colloid retention in the porous medium having small particle sizes. The maximum relative concentration of Cr also reduces from 0.81 for porous medium having the largest sand grains to 0.47 for that having the smallest sand grains and it indicates co-transport of Cr with CML colloids. In comparison, CML colloid-facilitated Pb achieves the maximum relative concentration of 0.62 for the porous medium having the sand largest grains to 0.24 for that having the smallest grains. The two-kinetic site chemical non-equilibrium model along with irreversible first-order straining reaction are incorporated for parameter optimization using the Levenberg-Marquardt algorithm. The numerical simulation indicates that the strained concentration of CML colloids reduces with increase in column depth from inlet and grain sizes of the porous medium. The study can aid to improve strategy of groundwater contaminant remediation.