Humic Acids Limit the Precipitation of Cadmium and Arsenate at the Brushite-Fluid Interface

Bioavailability and mobility of cadmium (Cd²⁺) and arsenate (As⁵⁺) in soils can be effectively lowered through the dissolution of brushite (dicalcium phosphate dihydrate, CaHPO₄·2H₂O) coupled with the precipitation of a more stable mineral phase containing both Cd and As. Due to the ubiquitous presence of humic acid (HA) in soil environments, it is more complex to predict the fate of dissolved Cd and As during such sequestration. Here, we used in situ atomic force microscopy (AFM) to image the kinetics of simultaneous precipitation of Cd and As at the brushite-fluid interface in the presence of HA. Results show that HA inhibits the formation of both amorphous and crystalline Cd_{(5- x)}Ca _x(PO₄)_{(3- y)}(AsO₄) _y(OH) on the (010) face of brushite. A combination of X-ray photoelectron spectroscopy (XPS) and real-time surface-enhanced Raman spectroscopy (SERS) reveals that part of As⁵⁺ reduction into As³⁺ with HA and [HA-Cd] complexation occurs, modulating the concentrations of free Cd²⁺ and As⁵⁺ ions to inhibit subsequent precipitation of a Cd_{(5- x)}Ca _x(PO₄)_{(3- y)}(AsO₄) _y(OH) phase on the dissolving brushite surface. A combination of AFM imaging, SERS analyses, and PhreeqC simulations suggests that environmentally relevant humic substances can limit the precipitation of Cd and As at mineral surfaces through a mechanism of oxidation/reduction and aqueous/surface complexation. This may exacerbate the transportation of these contaminants into waters by subsurface fluid flow, and research attempts to weaken the negative effect of HA are needed.