Development and evaluation of rapid image-based and turbidimetric methods for quantifying colloidal activated carbon...

Colloidal activated carbon (CAC) sorption barriers created in groundwater via well injections have emerged as a promising in situ approach for immobilizing and trapping recalcitrant organic pollutants. This emergence necessitates the need for fast and reliable methods to verify in situ CAC distributions using soil cores. Thermal-based carbon measurement methods are the most widely used but are limited by the testing complexity, time demands, bias in the presence of background natural organic matter, and the need for specialized equipment. To address these limitations, three alternative methods are evaluated in this study. All three methods rely on using a novel sonication step that separates CAC from the sand in a water solution, followed by CAC quantification using either gravimetric, turbidimetric, or image analysis. Method performance is first evaluated using CAC validation experiments with known CAC additions to sand and through CAC breakthrough experiments by quantifying CAC loadings in column effluent samples and in sand samples taken from columns post-breakthrough. Gravimetric analysis proved unreliable, whereas image analysis and turbidimetry showed strong agreement with combustion-based measurements, providing rapid and low-cost CAC quantification in clean sandy materials. These methods are intended as a first-phase methodological development and require further validation for natural soils containing clay-sized particles, mobilizable colloids, or elevated organic matter, which may influence CAC release and measurement accuracy.