The Kinetics And Mechanisms For Nanoscale Dissolution At The Calcium Phosphate-Organic Acid Solution Interface

Phosphorus（P） is an essential nutrient that is required for plant development and reproduction. Inorganic phosphate（orthophosphate, Pi）, the only form of P that can be assimilated by plants, can be sequestered by cations in soil(such as Ca²⁺ and Mg²⁺ in alkali soil or Fe3+ and Al3+ in acid soil). The exudation of OAs by root, such as citrate, enhances Pi availability in highly Pi-fixing soil. However, there is still a lot of uncertainty and variation among these macroscopic studies due to the existence of complex factors in soils and the rhizosphere. The dissolution mechanisms and the interactions between organic acids and Ca-P mineral surfaces at microscopic levels have still to be identified. In this study, we investigate the fluence of the concentration of OAs and the alohol-OH groups in OAs on the dissolution of DCPD-water interface, and how citrate modify the halides induced counterion effects on the dissolution of negatively charged DCPD-water interface, by using in situ atomic force microscopy（AFM） coupled with a fluid reaction cell through which solutions were flowed with varying compositions relevant to rhizospheric solution conditions. And the main results are summarized as follows:（1） We find that low concentrations of citrate（10-100 μM） induced a reduction in step retreat rates. However, at higher concentrations（exceeding 100 μM）, this inhibitory effect was reversed with step retreat speeds increasing rapidly. Theseresults 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.（2） By using in situ AFM to visualize the nanoscale dissolution of DCPD（010） face in the presence of three OAs which share the same carbon backbone but with the different number of alcohol-OH groups（succinic acid,（SA）, L-malic acid,（L-MA）, and L-tartaric acid（L-TA））, we systematically investigate the role of alcohol-OH groups in organic acids. It is observed that alcohol-OH groups in fully deprotonated dicarboxylic acids can play a critical role in ligand-promoted dissolution, and only L-TA bearing two alcohol-OH groups can adsorb along specific directions on the DCPD exposed（010） surface, which depend on the extent of negatively charged at DCPD/solution interface. In addition, we also visualize the proton from deprotonation of –COOH groups on the surface participating in dissolving the DCPD exposed（010） surface at low pH（4.0）. The deprotonation of –COOH groups on DCPD（010） surface depends on the pKa and alcohol-OH dependent stereochemical conformity between OAs and DCPD（010） cleavage surfaces.（3） By using in situ AFM combined with XPS and the measure of zeta potential, we directly visualize how citrate（50 μM） modify the nanoscale dissolution of negatively charged DCPD（CaHPO₄.2H₂O）-water interface influenced by the cooperativity between halide anions and counterions（Na⁺ ions）. It is observed that at low salt concentrations, larger size of halide（such as I-） evaluates the direct adsorption of counterions（Na⁺ ions） on DCPD surface which hinder the step retreat rate, and the introduction of citrate（50 μM） can remove the adsorbed dehyrated Na⁺ ions, causing the disappearance of special salt effects on dissolution at interface. We also visualize a balance of two forces in a higher salinity of solution, determining crystal surface dissolution kinetics: the tightly adsorbed counterions（Na⁺ ions） stabilizing the crystal surface structure to hinder the retreat rate of pit, and a higher concentration of solvated counterions（Na⁺ ions） at interface having influences on the activity of structure to significantly initiate the deepening velocity of pits. And the amount of adsorbed Na⁺ ions（dehydrated） and solvated Na⁺ with water shell at negatively charged interface is related with halide anions. In addition to push out the adsorbed Na⁺ ions on the surface), the introduction of citrate（50 μM） in a higher salinity of solution, also eliminate the deep pits due to the immediate disappearance of the deepening velocity of pits, suggesting citrate acting as a protective shelter which weakens the attack strength of active structure of interfacial water.