Visualizing Organophosphate Precipitation at the Calcite-Water Interface by in Situ Atomic-Force Microscopy

Direct observations of nanoscale brushite dissolution by the concentration-dependent adsorption of phosphate or phytate

With the development of agricultural intensification, phosphorus (P) accumulation in croplands and sediments has resulted in the increasingly widespread interaction between inorganic and organic P species, which has been, previously, underestimated or even ignored. We quantified the nanoscale dissolution kinetics of sparingly soluble brushite (CaHPO4·2H2O, DCPD) over a broad range of phosphate and/or phytate concentrations by using in situ atomic force microscopy (AFM). Compared to water, we found that low concentrations of phosphate (1-1000 µM) or phytate (1-100 µM) inhibited brushite dissolution by slowing single step retraction. However, with increasing phosphate or phytate concentrations to 10 mM, there was a reverse effect of dissolution promotion at brushite-water interfaces. In situ observations of the coupled dissolution-reprecipitation showed that phosphate precipitated more readily than phytate on brushite surfaces, with the formation of amorphous calcium phosphate (ACP). For a fundamental understanding, zeta potential and in situ Raman spectroscopy (RS) revealed that the concentration-dependent dissolution is attributed to the reverse of outer-sphere to inner-sphere adsorption with increasing phosphate or phytate concentrations. In addition, the mineralization of phytate with outer-sphere adsorption by phytase was higher than that with inner-spere adsorption, and the presence of phytate delayed ACP phase transformation to hydroxylapatite (HAP). These in situ observations and analyses may fill the knowledge gaps of interaction between inorganic and organic P species in P-rich terrestrial and aquatic environments, thereby implicating their biogeochemical cycling and the associated availability.

In situ quantitative determination of the intermolecular attraction between amines and a graphene surface using atomic force microscopy

The adsorption of pollutants on carbonaceous environmental media has been widely studied via batch sorption experiments and spectroscopic characterization. However, the molecular interactions between pollutants and interfacial sites on carbonaceous materials have only been indirectly investigated. To comprehend the adsorption mechanisms in situ, we applied atomic force microscopy force spectroscopy (AFM-FS) to quantitatively determine the molecular interactions between typical amines (methylamines and N-methylaniline) and the surface of highly oriented pyrolytic graphite (HOPG), which was supported by the single molecule interaction derived from density functional theory and batch adsorption experiments. This method achieved direct and in situ characterization of the molecular interactions in the adsorption process. The molecular interactions between the amines and the adsorption sites on the graphite surface were affected by pH and peaked at pH 7 due to strong cation-π interactions. When the pH was 11, the attractions were weak due to a lack of cation-π interaction, whereas, when the pH was 3, the competitive occupation of hydronium ions on the surface reduced the attraction between the amines and HOPG. Based on AFM-FS, the single molecule force of methylamine and N-methylaniline on the graphite surface was estimated to be 0.224 nN and 0.153 nN, respectively, which was consistent with density functional theory (DFT) calculations. This study broadens our comprehension of cation-π interactions between amines and electron-rich aromatic compounds at the micro/nanoscale.

Dissolution and Precipitation Dynamics at Environmental Mineral Interfaces Imaged by In Situ Atomic Force Microscopy

Chemical reactions at the mineral-solution interface control important interfacial processes, such as geochemical element cycling, nutrient recovery from eutrophicated waters, sequestration of toxic contaminants, and geological carbon storage by mineral carbonation. By time-resolved in situ imaging of nanoscale mineral interfacial reactions, it is possible to clarify the mechanisms governing mineral-fluid reactions.In this Account, we present a concise summary of this topic that addresses a current challenge at the frontier of understanding mineral interfaces and their importance to a wide range of mineral re-equilibration processes in the presence of a fluid aqueous phase. We have used real-time nanoscale imaging of liquid-cell atomic force microscopy (AFM) to observe the in situ coupling of the dissolution-precipitation process, whereby the dissolution of a parent mineral phase is coupled at mineral interfaces with the precipitation of another product phase, chemically different from the parent. These nanoscale observations allow for the identification of dissolution and growth rates through systematically investigating various minerals, including calcite (CaCO₃), siderite (FeCO₃), cerussite (PbCO₃), magnesite (MgCO₃), dolomite (CaMg(CO₃)₂), brushite (CaHPO₄·2H₂O), brucite (Mg(OH)₂), portlandite (Ca(OH)₂), and goethite (α-FeOOH), in various reacting aqueous fluids containing solution species, such as arsenic, phosphate, organo- or pyrophosphate, CO₂, selenium, lead, cadmium, iron, chromium, and antimony. We detected the in situ replacement of these parent mineral phases by product phases, identified through a variety of analytical methods such as Raman spectroscopy, high-resolution transmission electron microscopy, and various X-ray techniques, as well as modeling by geochemical simulation using PHREEQC. As a consequence of the coupled processes, sequestration of toxic elements and hazardous species and inorganic and organic carbon, and limiting or promoting recovery of nutrients can be achieved at nano- and macroscopic scales.We also used in situ AFM to quantitatively measure the retreat rates of molecular steps and directly observe the morphology changes of dissolution etch pits on calcium phosphates in organic acid solutions present in most rhizosphere environments. By molecular modeling using density functional theory (DFT), we explain the origin of dissolution etch pit evolution through specific stereochemistry and molecular recognition and provide an energetic basis by calculating the binding energies of chemical functionalities on organic acids to direction-specific steps on mineral surfaces. In addition, we further quantified precipitation kinetics of calcium phosphates (Ca-P's) on typical mineral surfaces at the nanoscale in environmentally relevant solutions with various organic molecules, by measurements obtained from sequential images obtained by liquid-cell AFM. In situ dynamic force spectroscopy (DFS) was used to determine binding energies of single-molecules with different chemical functionalities found in natural organic matter at mineral-fluid interfaces. Quantifying molecular organo-mineral bonding or binding energies mechanistically explains phosphate precipitation and transformation. From DFS measurements, molecular-scale interactions of mineral-natural organic matter (DNA, proteins, and polysaccharides) associations were determined. With this powerful tool, single-molecule determinations of polysaccharide-amorphous iron oxide or hematite interactions provided the mechanistic origin of the phase- or facet-dependent adsorption. These systematic investigations and findings significantly contribute to a more quantitative prediction of the fate of nutrients and contaminants, chemical element cycling, and potential geological carbon capture and nuclear waste storage in aqueous environments.

Direct Observations of the Occlusion of Soil Organic Matter within Calcite

Global soil carbon cycling plays a key role in regulating and stabilizing the earth's climate change because of soils with amounts of carbon at least three times greater than those of other ecological systems. Soil minerals have also been shown to underlie the persistence of soil organic matter (SOM) through both adsorption and occlusion, but the microscopic mechanisms that control the latter process are poorly understood. Here, using time-resolved in situ atomic force microscopy (AFM) to observe how calcite, a representative mineral in alkaline soils, interacts with humic substances, we show that following adsorption, humic substances are gradually occluded by the advancing steps of spirals on the calcite (1014) face grown in relatively high supersaturated solutions, through the embedment, compression, and closure of humic substance particles into cavities. This occlusion progress is inhibited by phytate at high concentrations (10-100 μM) due to the formation of phytate-Ca precipitates on step edges to prevent the step advancement, whereas phytate at relatively low concentrations (≤1 μM) and oxalate at high concentrations (100 μM) have little effect on this process. These in situ observations may provide new insights into the organo-mineral interaction, resulting in the incorporation of humic substances into minerals with a longer storage time to delay degradation in soils. This will improve our understanding of carbon cycling and immobilization in soil ecological systems.

Imaging Organophosphate and Pyrophosphate Sequestration on Brucite by in Situ Atomic Force Microscopy

In order to evaluate the organic phosphorus (OP) and pyrophosphate (PyroP) cycle and their fate in the environment, it is critical to understand the effects of mineral interfaces on the reactivity of adsorption and precipitation of OP and PyroP. Here, in situ atomic force microscopy (AFM) is used to directly observe the kinetics of coupled dissolution-precipitation on cleaved (001) surfaces of brucite [Mg(OH)₂] in the presence of phytate, glucose-6-phosphate (G6P) and pyrophosphate, respectively. AFM results show that the relative order of contribution to mineral surface adsorption and precipitation is phytate > pyrophosphate > G6P under the same solution conditions and can be quantified by the induction time of OP/PyroP-Mg nucleation in a boundary layer at the brucite-water interface. Calculations of solution speciation during brucite dissolution in the presence of phytate or pyrophosphate at acidic pH conditions show that the solutions may reach supersaturation with respect to Mg₅H₂Phytate.6H₂O as a Mg-phytate phase or Mg₂P₂O₇ as a Mg-pyrophosphate phase that becomes thermodynamically stable before equilibrium with brucite is reached. This is consistent with AFM dynamic observations for the new phase formations on brucite. Direct nanoscale observations of the transformation of adsorption/complexation-surface precipitation, combined with spectroscopic characterizations and species simulations may improve the mechanistic understanding of organophosphate and pyrophosphate sequestration by mineral replacement reactions through a mechanism of coupled dissolution-precipitation occurring at mineral-solution interfaces in the environment.

Molecular Self-Assembly Versus Surface Restructuring During Calcite Dissolution

Organic additives are known to alter the mineral-water interface in various ways. On the one hand, organic molecules can self-assemble into ordered structures wetting the surface. On the other hand, their presence can affect the interfacial morphology, referred to as surface restructuring. Here, we investigate the impact of a class of calcium-complexing azo dyes on the dissolution of calcite (10.4) using high-resolution atomic force microscopy operated in aqueous solution, with a focus on the two constitutional isomers Eriochrome Black T and Eriochrome Black A. A very pronounced surface restructuring is observed in the presence of the dye solution, irrespective of the specific dye used and independent of the pH. This surface restructuring is obtained by the stabilization of both the nonpolar acute and the polar [010] step edges, resulting in a greatly altered, characteristic interface morphology. In sharp contrast to the prevalence of the surface restructuring, an ordered molecular structure on the crystal terraces is observed only under very specific conditions. This formation of an ordered stripe-like molecular structure is obtained from Eriochrome Black A only and limited to a very narrow pH window at a pH value of around 3.6. Our results indicate that such molecular self-assembly requires a rather precise adjustment of the molecular properties including control of the conformation and deprotonation state. This is in sharp contrast to the additive-induced surface restructuring, which appears to be far more robust against both pH changes and variations in the molecular conformation.