INFLUENCE OF MINERAL COLLOIDS AND HUMIC SUBSTANCES ON ARSENATE TRANSPORT IN WATER- SATURATED SEDIMENT COLUMN

Adsorption of contaminants to natural colloids is an important geochemical process that controls the speciation and aqueous concentration of many water-borne contaminants. This implies the mobility of dissolved contaminants in groundwater under natural conditions could be very different from that estimated using laboratory experiments conducted under simplified conditions in the absence of natural colloids. The objective of this study is to examine the effects of illite colloids and humic substances (representing natural organic colloid) on arsenate transport. Measurements made in experiments with re-packed columns revealed distinct characteristics of colloid-facilitated transport of arsenate at acidic and alkaline pH. Although illite adsorption of arsenate was high at pH 5, arsenate transport was not enhanced in the presence of illite, due to the very low mobility of illite at low pH. At pH 9, arsenate and illite transport was high, however, illite decreased arsenate transport. In contrast, humic substances substantially increased arsenate transport at pH 5, which was attributed to high mobility and high affinity of humic substances for arsenate. At pH 9, humic substances only moderately enhanced arsenate transport due to the relatively low adsorption of arsenate to humic substances. When illite and humic substances co-existed in water, arsenate transport at pH 5 was greatly increased, due to the large increase in illite transport and high adsorption of arsenate to illite. However the co-existence of humic substances and illite at pH 9 did not influence arsenate transport, indicating arsenate adsorption to either illite or humic substances was weak. The results of this work showed the importance of water chemistry and interfacial geochemical reactions in controlling contaminant transport in groundwater environments.