Characteristics And Role of Colloids Fractions In Arsenic Mobilization in Surface Water Near The Shimen Realgar Mine

Abstract

Arsenic mobility and transport in natural water are substantially influenced by inorganic components and dissolved organic matter (DOM) in colloids, but the characteristics of different molecular weights (MW) colloids and their impacts on As mobilization remain unclear. Therefore, bulk surface water (<0.7 μm) near the Shimen realgar mine was fractionated into colloidal (large MW fraction: 100 kDa-0.7 μm, medium MW fraction: 10-100 kDa, small MW fraction: 3-10 kDa) and truly dissolved (<3 kDa) fractions by cross-flow ultrafiltration. The environmental parameters EC, TDS, Ca2+ and HCO3- showed negative correlations (p < 0.05) with As, As(V) or As(III) in colloids and therefore potentially inhibited As mobility. Arsenic, As(V), As(III) and DOC were primarily in the <3 kDa fraction; Al, Fe and Mn were mainly present in the 100 kDa-0.7 μm fraction. Medium–small-MW DOM was likely of terrestrial origin with high degrees of humification and aromaticity, and the opposite was observed for large-MW colloidal and truly dissolved DOM. Positive correlations (p < 0.05) between As(III) and Mn in the 100 kDa-0.7 μm fraction and between As(V) and Al, Fe, and DOC in the 10-100 kDa and 3-10 kDa fractions indicated that large-MW Mn-rich inorganic colloids scavenged As(III) and medium–small-MW organic/organometallic colloids scavenged As(V). Fourier transform infrared spectroscopy (FTIR) confirmed As-DOM and As-Fe-DOM formations in colloids. Arsenic concentrations were positively (p < 0.05) correlated with C1 and C2 and negatively (p < 0.05) correlated with C3 in colloids, suggesting that terrestrial-derived DOM promoted As mobility, while microbial-derived DOM had the opposite effect. No significant correlations (p > 0.05) between molecular mass of DOM and optical indices were observed except for SUVA254 in the 10-100 kDa fraction, implying that molecular mass of DOM cannot accurately characterize the composition and structure of DOM fractions.

Keywords: Arsenic, Cross-flow ultrafiltration, colloids, Dissolved organic matter, Surface water