Transport and Deposition of Colloidal Activated Carbon (CAC) in Saturated Sand Columns: Impacts of input CAC concentration, transient ionic strength, and multiple CAC injections

Injection of colloidal activated carbon (CAC) into the subsurface is an innovative low-cost technology for remediation of legacy and emerging contaminants. It is typically used as a permeable barrier for removing contaminants via sorption and/or followed by microbial/chemical degradation. In addition, CAC has also been used as a catalyst for oxidative degradation of organic contaminants as well as a carrier for subsurface delivery of nano zerovalent iron. A growing application of CAC in the subsurface is its use for sorption and plume control of per-/polyfluorinated alkyl substances (PFAS). With a growing suite of remediation technologies for PFAS, CAC offers the advantage of not producing unknown and/or toxic intermediates while limiting further spread of PFAS in the subsurface. The performance of CAC largely depends on its ability to transport to and deposit at the desired location in the contaminated aquifer under environmentally relevant groundwater conditions. Two such conditions of utmost interest are: (1) injection of CAC with or without downgradient injection of CaCl₂ which restricts CAC mobility by aggregation and (2) multiple injections of CAC in the event of breakthrough of sorbed contaminants. Under these conditions, variations in CaCl₂ concentrations over time are expected due to its potential post-injection downstream migration as well as potential changes in hydraulic conductivity from repeated CAC injections. Thus, it is critical to understand how these conditions impact retention, release, and remobilization of not only the CAC but also of the sorbed contaminants. Our study has examined the effects of input CAC concentration, transient changes in CaCl₂ concentration, and multiple injections of CAC on its transport and deposition in 1-D saturated sand columns. The breakthrough curves and retention profiles generated for the CAC in this study are primary inputs for 1-D transport models which are necessary for prediction of CAC mobility in groundwater.