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Unraveling the kinetics and mechanisms of sparingly soluble calcium orthophosphate (Ca-P) dissolution in the presence of organic acids at microscopic levels is important for an improved understanding in determining the effectiveness of organic acids present in most rhizosphere environments. Herein, we use in situ atomic force microscopy (AFM) coupled with a fluid reaction cell to image dissolution on the (010) face of brushite, CaHPO4 · 2H2O, in citrate-bearing solutions over a broad concentration range. We directly measure the dependence of molecular step retreat rate on citrate concentration at various pH values and ionic strengths, relevant to soil solution conditions. We find that low concentrations of citrate (10-100 μM) induced a reduction in step retreat rates along both the [100]Cc and [101]Cc directions. However, at higher concentrations (exceeding 0.1 mM), this inhibitory effect was reversed with step retreat speeds increasing rapidly. These results demonstrate that the concentration-dependent modulation of nanoscale Ca-P phase dissolution by citrate may be applied to analyze the controversial role of organic acids in enhancing Ca-P mineral dissolution in a more complex rhizosphere environment. These in situ observations may contribute to resolving the previously unrecognized interactions of root exudates (low molecular weight organic acids) and sparingly soluble Ca-P minerals.

Due to the potential shortage of phosphate (P) rock resources and a faster growth in demand for phosphate fertilizers, unraveling the kinetics of calcium phosphate (Ca–P) crystallization and dissolution is important for understanding the P mobility and bioavailability. Plants have developed different strategies, such as carboxylic acid exudation into the rhizosphere, to cope with low P bioavailability through dissolution of sparingly soluble Ca–P minerals. However, the dissolution kinetics may be more complicated in the presence of both carboxylate and hydroxyl groups in organic acids. Here in situ atomic force microscopy (AFM) is used to directly observe the kinetics of nanoscale dissolution on the (010) surface of dicalcium phosphate dihydrate (brushite, CaHPO4·2H2O) in the presence of succinic acid (SA, 0 alcoholic hydroxyl (−OH)), malic acid (MA, 1 −OH), and tartaric acid (TA, 2 −OH), respectively, over a broad concentration range. We demonstrate that the role of dicarboxylic acids varies with the number of alcoholic hydroxyls and that fully deprotonated hydroxy-dicarboxylic acids play a critical role in controlling the dissolution rate of steps and morphology modification of etch pits. Direct AFM imaging shows that only TA can adsorb along specific directions of the [1̅01̅] Cc steps on the brushite (010) surface at pH ≥ 6 to induce the formation of trapezium-shaped etch pits. This depends on specific molecular recognition and stereochemical conformity between hydroxyl-carboxyl of TA and atomic [1̅01̅] Cc steps by molecular modeling using density functional theory. The effectiveness of alcoholic hydroxyls can be enhanced by deprotonated brushite interfaces with the increase of the solution pH. This combined AFM and molecular modeling study may provide microscopic insights into understanding P mobilization by dissolution in soils.

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.