Direct Imaging of Nanoscale Dissolution of Dicalcium Phosphate Dihydrate by an Organic Ligand: Concentration Matters

Abstract

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.

Interfacial Precipitation of Phosphate on Hematite and Goethite

“…Furthermore, calculated SI also predicts that low concentrations of citrate can promote the formation of strengite, tinticite, or cacoxenite formed on hematite or goethite [5] due to elevated SI (Tables S3 and S4). Organic acids in the soil solution typically range from 0.1 µM to 0.1 mM [5,25,26]. Our experiments also show that higher concentrations of citric acid retard Fe-P precipitation formation.…”

Direct Nanoscale Imaging of Calcium Oxalate Crystallization on Brushite Reveals the Mechanisms Underlying Stone Formation

“…There are a number of proposed mechanisms, including enhancing chelation and complexation by inner/outer-sphere adsorbed species: 36−38 organic ligand anions bind in an inner-sphere, mononuclear, bidentate manner can dramatically enhance the dissolution kinetics of a variety of oxide minerals by polarizing bridging metal−oxygen bonds on mineral surfaces. 36 33 The introduction of low concentration OA for enhancing the step retreat rates along the [101] Cc steps could not be related to a thermodynamic driving force for dissolution because our AFM experiments are far away from equilibrium (as is the case for our solutions). 33 However, an alternative mechanism is that oxalate may alter the water molecular layer near the (010) face, effectively lowering the magnitude of the desolvation barrier, E K , by perturbations that displace water molecules, 39 or of the kinetic barrier, E b , 33,39 to removing Ca atoms along the [101] Cc steps to promote the dissolution.…”

“…36 33 The introduction of low concentration OA for enhancing the step retreat rates along the [101] Cc steps could not be related to a thermodynamic driving force for dissolution because our AFM experiments are far away from equilibrium (as is the case for our solutions). 33 However, an alternative mechanism is that oxalate may alter the water molecular layer near the (010) face, effectively lowering the magnitude of the desolvation barrier, E K , by perturbations that displace water molecules, 39 or of the kinetic barrier, E b , 33,39 to removing Ca atoms along the [101] Cc steps to promote the dissolution. Net increase in the overall dissolution rates depends on retreat and deepening rates as well as the nucleation density of etch pits.…”

“…41 Thus, we reasonably deduce that at the site of oxalate adsorption, especially on the [101] Cc steps, polarizing bridging Ca-oxygen bonds between surface Ca 2+ cation and the oxygen of the underlying HPO 4 2− anion in the DCPD surface layer by adsorbed oxalate, may generate greater ease of surface Ca 2+ detachment by all possible combinations of above processes/mechanisms at different OA concentrations. Furthermore, the specificity of the OA interaction with [101] Cc steps may be due to its bidentate structure containing two carboxylate groups, serving as double binding sites that allow for specific molecular recognition with the Ca atoms 33,42 or stereochemical interacting with step-edges on crystal surfaces and the step geometry that may allow double accommodations of carboxylate groups with the least amount of distortion. 43 For Our previous results have demonstrated that the local binding of negatively charged phosphate side chains to DCPD crystal faces rather than any secondary peptide structure to prevent step movement and increase interfacial/surface energies may be responsible for both the bulk and surface crystallization inhibition.…”

“…33 Measurements were made on more than three crystals per solution composition to ensure reproducibility of the results, and each was imaged in different locations on the crystal (>3 points). All data (including the step retreat rates and the depths of etch pits) with their mean values ± standard deviation (SD) of three independent experiments are presented.…”