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

Abstract

Bioavailability and mobility of cadmium (Cd 2+ ) and arsenate (As 5+ ) in soils can be effectively lowered through the dissolution of brushite (dicalcium phosphate dihydrate, CaHPO 4 • 2H 2 O) 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 (PO 4 ) (3−y) (AsO 4 ) 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 As 5+ reduction into As 3+ with HA and [HA-Cd] complexation occurs, modulating the concentrations of free Cd 2+ and As 5+ ions to inhibit subsequent precipitation of a Cd (5−x) Ca x (PO 4 ) (3−y) (AsO 4 ) 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.

Simultaneous measurement of aqueous redox-sensitive elements and their species across the soil-water interface

“…The time-dependent curves were consistent with our previous report for As, antimony, cadmium, lead, and nickel (Yuan et al, 2019). Considering the presence of dissolved organic matters, which could slow down the diffusion of solutes (Dočekalová et al, 2002;Reynolds et al, 2004;Yuan et al, 2019;Zhai et al, 2018), we deployed the IPI sampler at 24 hr equilibration period in saturated soils.…”

Tracing the Dynamic Changes of Element Profiles by Novel Soil Porewater Samplers with Ultralow Disturbance to Soil–Water Interface

“…26 Due to the small pores (50 nm) on the IPI sampler, large HA–metal complexes maybe rejected during the sampling. 32−34 Therefore, 24 h equilibration time is recommended for the using of IPI sampler in soils, especially for the environments containing large metal complexes.…”

“…Humic acids are ubiquitous in soil environment, and they may act as natural chelates to complex metals. ,, The large HA–metal complex may be rejected by the porewater samplers as a result of their large size. ,, The influences of HA on Ni, As, Cd, Sb, and Pb are shown in Figure S3.…”

“…The longer equilibration time for metals after adding HA is because HA–metal complex has much smaller diffusion coefficient than the free metal . Due to the small pores (50 nm) on the IPI sampler, large HA–metal complexes maybe rejected during the sampling. − Therefore, 24 h equilibration time is recommended for the using of IPI sampler in soils, especially for the environments containing large metal complexes.…”

In situ observations of the occlusion of a clay-sugar compound within calcite

“…1D), and these results could semi-quantify the content of sugar combined with LAPONITE® under different experimental conditions. 46 The characteristics peaks of sugars (located at 521 cm −1 for Glu or 540 cm −1 for Dex-5/20) and LAPONITE® (at 683 cm −1 ) were selected because the Raman intensities at these positions were obvious and easy to be observed with same treatment. The sugar signals of the LAPONITE®-sugar complexes were weakened after elution, to different extents based on molecular weight (Fig.…”