Single-molecule Mechanisms Of Organo–mineral Interactions And Dynamics Of The Immobilization Of Cadmium And Arsenate At Environmental Mineral Interfaces By In Situ Atomic Force Microscopy

Mineral-water interfacial interactions play a key role in the soil environmental chemical processes including the dissolution,adsorption and precipitation.These reactions can directly affect the release and fixation of heavy metal and metalloid pollutants in the soil.During these reactions,the kinetics is ultimately controlled by the mineral interface,various ionic components and organic molecules?such as soil organic matters?at the mineral-fluid interface.Therefore,studying the mineral-solution interface reaction under different conditions at the nano-scale can allow us to further understand the mechanism of fixing cadmium and arsenic pollutants on the mineral surface and the role of soil organic matter?SOM?in this process.At present,we can directly observe the nano-scale reaction process occurring on the mineral surface in real time with the latest development of in situ observation methods and calculate the bonding energy between SOM and minerals using dynamic force spectroscopy?DFS?,helping to reveal the mechanism underlying reactions occurring at the mineral-fluid interface.In this study,brushite?dicalcium phosphate dihydrate,DCPD,Ca HPO₄?2H₂O?and mica were used as model minerals.The in situ atomic force microscope?AFM?was used to observe the process of cadmium and arsenate immobilization with the regulation of organic components?humic acid?HA?,peptides,and nucleic acids?on the mineral surface.DCPD can simultaneously immobilize cadmium and arsenate through a coupled dissolution-reprecipitation reaction,but HA can inhibit this process.Furthermore,we combined the DFS technique to calculate the binding free energy between organic matters and minerals in solutions containing cadmium and arsenic,and proved that the biomimetic hexapeptide can specifically recognize arsenate and phosphate and that cadmium can promote environmental DNA?e DNA?adsorbing onto the surface of mica.These nanoscale studies provide theoretical foundations and methodological explorations for studying various reactions occurring at mineral interfaces in a complex soil system.The main results of this study are summarized below:1. Direct observation of simultaneous immobilization of cadmium and arsenate at the brushite-Fluid,showing heavy metal pollutants can be immobilized through a coupled dissolution and precipitation mechanism.Cadmium(Cd²⁺)and Arsenate(As⁵⁺)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 AFM to image the dissolution on the?010?face of brushite in Cd Cl₂-or Na₂HAs O₄-bearing solutions over a broad p H 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 Cd Cl₂ and Na₂HAs O₄ in reaction solutions at p H 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 Cd_?5-x?Ca_x?As O₄?_?3-y??PO₄?_yOH 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.2. Humic acids limit the precipitation of cadmium and arsenate at the brushite-fluid fnterface,showing that soil organic matters play an important role in regulating the immobilization of cadmium and arsenate.Bioavailability and mobility of cadmium(Cd²⁺)and arsenate(As⁵⁺)in soils can be effectively lowered through the dissolution of brushite coupled with the precipitation of a more stable mineral phase containing both Cd and As.Due to the ubiquitous presence of HA in soil environments,it is more complex to predict the fate of dissolved Cd and As during such sequestration.Here,we used in situ AFM to image the kinetics of simultaneous precipitation of Cd and As at the brushite-fluid interface in the presence of HA.Results show that HA inhibits the formation of both amorphous and crystalline Cd_?5-x?Ca_x?PO₄?_?3-y??As O₄?_y?OH?on the?010?face of brushite.A combination of X-ray photoelectron spectroscopy?XPS?and real-time surface-enhanced Raman spectroscopy?SERS?reveals that part of As⁵⁺reduction into As³⁺with HA and[HA-Cd]complexation occurs,modulating the concentrations of free Cd²⁺ phase on the dissolving brushite surface.A combination of AFM imaging,SERS analyses,and Phreeq C simulations suggests that environmentally relevant humic substances can limit the precipitation of Cd and As at mineral surfaces through a mechanism of oxidation/reduction and aqueous/surface complexation.This may exacerbate the transportation of these contaminants into waters by subsurface fluid flow,and research attempts to weaken the negative effect of HA are needed.3. Dynamics and molecular mechanism of a biomimetic hexapeptide distinguishing phosphate from arsenate,exploring the mechanism underlying the reactions between organic matters and phosphate or arsenate.Phosphorus and arsenic belong to the same main group and have similar physical and chemical properties.Phosphorus?P?recovery from As-contained water is a challenge for sustainable P management.A biomimetic hexapeptide?SGAGKT?has been demonstrated to bind inorganic P in P-rich environments,however the dynamics and molecular mechanisms of P-binding to the hexapeptide still remain largely unknown.We used dynamic force spectroscopy?DFS?to directly distinguish the P-unbound and P-bound SGAGKT adsorbed to a mica?001?surface by measuring the single-molecule binding free energy??G_b?.Using atomic force microscopy?AFM?to determine real-time step retreat velocities of triangular etch pits formed at the nanoscale on the dissolving?010?face of brushite in the presence of SGAGKT,we observed that SGAGKT peptides promoted in situ dissolution through an enhanced P-binding driven by hydrogen bonds in a P-loop being capable of discriminating phosphate over arsenate,concomitantly forming a thermodynamically favored SGAGKT-HPO₄^2-complexation at p H 8.0 and relatively low ionic strength,consistent with the DFS and isothermal titration calorimetry?ITC?determinations.The findings reveal the thermodynamic and kinetic basis for binding of phosphate to SGAGKT and provide direct evidence for phosphate discrimination in phosphate/arsenate-rich environments.4. Molecular-scale investigations reveal noncovalent bonding underlying the adsorption of environmental DNA on mica in the presence of cadmium, exploring the effect of cadmium on the storage of soil organic matters at the mineral-fluid interface.Mineral-soil organic matter?SOM including DNA,proteins,and polysaccharides?associations formed through various interactions,play a key role in regulating longterm SOM preservation.The mechanisms underlying DNA-mineral and DNA-protein/polysaccharide interactions at nanometer and molecular scales in environmentally relevant solutions remain uncertain.Here,we present a model mineral-SOM system consisting of mineral?mica?-nucleic acid?environmental DNA,e DNA?/protein?bovine serum albumin?/polysaccharide?alginate?,and combine atomic force microscopy?AFM?-based dynamic force spectroscopy and Peak Force quantitative nanomechanical mapping using DNA-decorated tips.Single-molecule binding and adhesion force of e DNA to mineral and to mineral adsorbed by protein/polysaccharide reveal the noncovalent bonds and that systematically changing ion compositions,ionic strength,and p H result in significant differences in organic-organic and organic-mineral binding energies.Consistent with the bond-strength measurements,protein,rather than polysaccharide,promotes mineral-bound DNA molecules by ex situ AFM deposition observations in relatively high concentrations of divalent cation-containing acidic solutions.These molecular-scale determinations and nanoscale observations should substantially improve our understanding of how environmental factors influence the organo-mineral interfacial interactions through the synergy of collective noncovalent and/or covalent bonds in mineral-organic associations.