CO-TRANSPORT OF NATURAL PARTICULATE MATTERS AND SORBING CONTAMINANTS IN GROUNDWATER

Natural particulate matters such as mineral colloids and humic substances often co-exist in subsurface environment and substantially influence the transport of sorbing contaminants in groundwater. However, the combined effects of mineral colloids and humic substances on contaminant transport are not clear. This study is aimed at quantifying uranium (U) transport and elucidating geochemical processes that control U transport when both mineral colloids and humic acid (HA) are present. U-spiked solutions were injected into water-saturated sand columns, and U and illite colloid concentration in column effluent was monitored. We found that HA promoted U transport via competition with U for adsorptive sites on geologic materials and formation of aqueous U-HA complexes. Illite colloids had no influence on U transport at pH 5 due to low mobility of the colloids. At pH 9, illite colloids decreased U transport due to U desorption from the colloids. High U transport occurred when both illite colloids and HA were present, which was attributed to enhanced U adsorption to illite colloids via formation of ternary illite-HA-U surface complexes, and enhanced illite transport due to HA attachment to illite and geologic media. This study demonstrates the combined effects of mineral colloids and HA on contaminant transport is different from simple addition of the individual effects.