The speciation and mobility of arsenic (As) in waters are largely influenced by the colloids
however, the impacts of colloids with different molecular weights (MWs) in water fractions remain largely unknown. Herein, the surface water was fractionated into three colloidal fractions and truly dissolved fraction via cross-flow ultrafiltration. Total As (As(T)) presented mainly as As(V) and existed primarily in the truly dissolved fraction. Arsenic(III) proportions of 3.1 %-8.0 % in various colloids sizes indicated reducing conditions within the colloids particles in surface water around the mine. Negative correlations between EC, TDS and As(T) and As(V) in colloids were found, indicating water properties favored As mobilization in surface water. The aromaticity and humification of DOM reflected endogenous sources in truly dissolved fraction and large-MW colloids and exogenous sources in medium-small-MW colloids. Fourier transform infrared spectroscopy detected As-DOM and As-Fe-DOM formation in colloids, highlighting new aspects of colloids in surface water. Arsenic(T) and As(V) concentrations were positively correlated with terrestrial humic-like components and negatively correlated with microbial humic-like component in colloids, suggesting that in the closed As mine, As tend to be transported from land to surface water together with terrestrial DOM, while endogenous DOM in water affected As mobilization.