Control of organic and iron colloids on arsenic partition and transport in high arsenic groundwaters in the Hetao basin, Inner Mongolia

Due to the importance of colloids in regulating element transport and mobility in aquifers, As distribution in the colloidal fraction needs to be identified in high As groundwaters. Groundwater samples were filtered in the field through a progressively decreasing pore size (0.45 μm, 100, 30, 10, 5 kDa) using a filtration technique under a N2 atmosphere. Major and trace elements and organic C (OC) were measured in (ultra)filtrates. The studied groundwater samples have typical physio-chemical characteristics of the basin waters. Declines in concentrations of alkali (Na, K), alkaline-earth (Mg, Ca, Sr, Ba) elements, Mo, Si and Se during ultrafiltration are smaller relative to other elements. Arsenic, Cu, Cr, U and V are generally about 30% lower in 5 kDa ultrafiltrates in comparison with 0.45 μm filtrates. Around 50% of Fe, OC and Al are bound to colloids with grain size between 5 kDa and 0.45 μm. Two types of colloids, including large-size Fe colloids and small-size organic colloids, have been identified. Results indicate that As would be more likely to be associated with small-size organic colloids than Fe colloids. SEM images and EDS analysis and synchrotron XRF analyses confirm the association of As with NOM with molecular weights of 5–10 kDa. The better correlation between As(V) and OC in the 5–10 kDa fraction indicates that the small-size organic colloids have a greater affinity for As(V) than As(III). Arsenic associated with organic complexes may not be immobilized by adsorption, and, therefore, easily transported in the aquifer. Thus, the presence of As-containing colloidal complexes in high As groundwaters must be considered in the modeling of As transport in the aquifers.