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Direct Imaging of Nanoscale Dissolution of Dicalcium Phosphate Dihydrate by an Organic Ligand: Concentration Matters
journal contribution
posted on 2013-12-03, 00:00 authored by Lihong Qin, Wenjun Zhang, Jianwei Lu, Andrew G. Stack, Lijun WangUnraveling 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
[1̅00]Cc and [101̅]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.
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Nanoscale Dissolutionsoil solution conditionsrhizosphere environmentsrhizosphere environmentstep retreat rateforce microscopycalcium orthophosphateimage dissolutionfluid reaction cellacidstep retreat ratesAFMroot exudatescitrate concentrationOrganic LigandDirect ImagingCcconcentration rangeDicalcium Phosphate DihydratepH valuesstep retreat speedsmineralConcentration MattersUnraveling
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