Direct Observation of Simultaneous Immobilization of Cadmium and Arsenate at the Brushite–Fluid Interface

Abstract

Cadmium (Cd) and Arsenate (As) are the main toxic elements in soil environments and are easily taken up by plants. Unraveling the kinetics of the adsorption and subsequent precipitation/immobilization on mineral surfaces is of considerable importance for predicting the fate of these dissolved species in soils. Here we used in situ atomic force microscopy (AFM) to image the dissolution on the (010) face of brushite (dicalcium phosphate dihydrate, CaHPO·2HO) in CdCl- or NaHAsO-bearing solutions over a broad pH and concentration range. During the initial dissolution processes, we observed that Cd or As adsorbed on step edges to modify the morphology of etch pits from the normal triangular shape to a four-sided trapezium. Following extended reaction times, the respective precipitates were formed on brushite through a coupled dissolution-precipitation mechanism. In the presence of both CdCl and NaHAsO in reaction solutions at pH 8.0, high-resolution transmission electron microscopy (HRTEM) showed a coexistence of both amorphous and crystalline phases, i.e., a mixed precipitate of amorphous and crystalline CdCa (AsO)(PO) OH phases was detected. These direct dynamic observations of the transformation of adsorbed species to surface precipitates may improve the mechanistic understanding of the calcium phosphate mineral interface-induced simultaneous immobilization of both Cd and As and subsequent sequestration in diverse soils.

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

“…Sequestration of cadmium (Cd 2+ ) and arsenate (As 5+ ) can be an important process to lower their bioavailability and mobility through coupled dissolution−precipitation reactions at various mineral surfaces, 1−3 such as calcite (CaCO 3 ), 4 goethite (FeOOH), 5 and brushite (dicalcium phosphate dihydrate, DCPD, CaHPO 4 •2H 2 O). 6 The dissolution of minerals in Cd/ As-bearing fluids provides a reliable ion source for nucleation and growth of Cd/As-bearing precipitates at the mineral−fluid interface to decrease the concentration of free Cd/As ions. 7 The chemical and physical properties of minerals (solubility and wettability) and Cd/As-bearing fluids (redox potential, ionic strength (IS), and pH) can greatly affect contaminant sequestrations by modulating the dissolution of minerals and/ or the species distribution of Cd and As under various environmental conditions.…”

“…21 Our previous study has demonstrated that Cd and As can be simultaneously precipitated at the brushite−fluid interface under specific solution conditions. 6 HA is present widely in real soils/water and might form complexes with many other compounds. This will greatly impact the dissolution of brushite and the possible subsequent reprecipitation of potential new phases; corresponding dynamic processes remain largely unknown.…”

“…■ EXPERIMENTAL SECTION Brushite Crystal Synthesis. Brushite single crystals were synthesized by a gel method 6,22 and subsequently were collected and characterized by Bruker D8X-ray diffraction (Billerica, Massachusetts) to identify the brushite crystals as a single phase (Figure S1).…”

Inhibition of Spiral Growth and Dissolution at the Brushite (010) Interface by Chondroitin 4-Sulfate

“…A 1.0 M potassium dihydrogen phosphate (KH 2 PO 4 ) solution was incorporated into the gel and 0.5 M calcium chloride (CaCl 2 ) was added at the top of the tubes. All the gel tubes were kept at room temperature (22 ± 2 °C) for about 30 days . The brushite crystals were harvested and then characterized by Bruker D8 X-ray diffraction (Billerica, Massachusetts) to identify the crystals as a single phase.…”

“…All the gel tubes were kept at room temperature (22 ± 2 °C) for about 30 days. 27 The brushite crystals were harvested and then characterized by Bruker D8 X-ray diffraction (Billerica, Massachusetts) to identify the crystals as a single phase.…”