Density Functional Theory Studies On The Hydrolysis-Complexation Reactions Of Aqueous Aluminum Ions And Aluminum Mineral Dissolution Mechanism

Aluminum(Al)chemistry studies have important scientific significance in geochemistry,environmental science,and ecotoxicology.The investigation of Al speciation and transformation mechanism at the molecular level is very important for accurately grasping the migration mechanisms and reactivity changes of Al in Earth's crust.Focusing on the key hot issues in current environmental Al chemistry studies,the molecular mechanisms of the hydrolysis,solvent exchange and complexation reactions of Al3+ in aqueous solution,as well as the Al mineral dissolution mechanism are systematically investigated by using the density functional theory-quantum chemical cluster model(DFT-CM)method.The purpose is to deepen our understanding of the speciation and transformation mechanism of environmental Al as well as their practical applications.The dissertation has ten chapters as follows:1.IntroductionThe scientific significance of the study on the speciation and transformation mechanism of environmental Al is expounded.The research progresses for the hydrolysis-complexation reaction mechanisms of aquated Al ions in aqueous solution,the dissolution mechanism of Al minerals and the computational modeling methods are summarized.2.The spontaneous hydrolysis reaction mechanism of Al3+in aqueous solutionThe kinetic mechanism of spontaneous Al3+hydrolysis reaction in aqueous solution is investigated using the DFT-CM method.Three typical reaction pathways for the spontaneous Al3+ hydrolysis reaction are modeled,including(1)the traditional spontaneous proton dissociation on the Al3+ inner-shell coordinated waters;(2)the conventional bulk water-assisted proton dissociation;and(3)the second-shell water-assisted synergistic dissociation of the protons on the Al3+ inner-shell waters.The results show that the electrostatic effects between Al3+ and its coordinated waters alone cannot fully account for the proton loss on an inner-shell coordinated water.It is suggested that the main reaction pathway for natural hydrolysis of aqueous Al3+is the second-shell water-assisted synergistic proton dissociation,in which the participation of the second hydration shell is crucially important.The calculated synergistic proton dissociation rate constant,kH+=1.14×105 s-1,is in close agreement with the experimental results(1.09×105 s-1 and 7.9×104 s-1).The first hydrolysis equilibrium constant pKal of Al3+ is calculated as 5.82,also consistent with the literature value of 5.00.This work elucidates the molecular mechanism of the spontaneous Al3+hydrolysis reaction in natural waters and has important environmental implications3.The forced hydrolysis reaction mechanism of Al3+ in aqueous solutionThe forced hydrolysis reaction of aqueous Al3+ is of critical importance in Al chemistry,but its microscopic mechanism has long been neglected.Herein,density functional calculations reveal an external OH--induced barrierless proton dissociation mechanism for the forced hydrolysis of Al3+(aq).Dynamic reaction pathway modeling results show that the barrierless deprotonations induced by the second-or third-shell external OH-proceed via the concerted proton transfer through H-bond wires connected to the coordinated waters,and the inducing ability of the external OH-decreases with increasing hydration layers between Al(H2O)63+and the external OH-.The OH--induced forced hydrolysis mechanism of Al3+(aq)is quite different from its self-hydrolysis mechanism without OH-.The inducing ability is a unique characteristic of OH-,rather than other anions such as F-or Cl-.4.The coordination structures and stabilities of aqueous monomeric Al3+ hydrolytic speciesThe 27Al NMR chemical shifts and relative stabilities of monomeric Al3+ hydrolytic species with different coordination structures in aqueous solution are systematically investigated by using the DFT-CM method.The main work includes:the static configurations of 20 possible existing monomeric Al3+ hydrolytic species from Al3+ to Al(OH)4-are optimized,and their 27Al NMR shieldings are calculated;the dehydration reaction pathways for typical monomeric Al3+ hydrolytic species are modeled,and the dominant forms of the intermediate hydrolytic species of Al(OH)2+,Al(OH)2+,and Al(OH)3O are analyzed based on the Gibbs free energy changes of the dehydration reactions.The important role of the tetracoordinated Al(H2O)(OH)30 in the formation mechanism of the polynuclear Keggin-Al13 is discussed.This work provides valuable references for further studying the formation and transformation mechanisms of the aqueous monomeric and polymeric Al species.5.The real and apparent water-exchange reaction kinetics of Al3+ in aqueous solutionThe dissociative(D)mechanistic water-exchange kinetics of aquated Al3+in aqueous solution is systematically studied using the density functional theory-quantum chemical cluster model(DFT-CM)method.The modeled pathways include dehydration of the hexacoordinated Al(H2O)63+and successive hydration of the pentacoordinated Al(H2O)53+.For the hydration of Al(H2O)53+,the attacking pathways corresponding to the second-shell solvent water molecules at different sites are investigated.The gas phase-supermolecule-polarizable continuum model(GP-SM-PCM)is used to simulate the explicit and bulk solvation effects.The reactant,transition state,and product geometries of the modeled reaction pathways are optimized at the B3LYP/6-311+G(d,p)level of theory.The real and apparent water-exchange reaction mechanisms of Al3+ are analyzed based on the Gibbs free energy changes for the dehydration and hydration pathways.The possible ligand competition with the solvent water molecules in the formation of aqueous Al3+-ligand complexes is discussed,which provides a useful reference for further analysis of the molecular transformation mechanisms of Al species in the natural environment.6.The formation mechanisms of Al-salicylate complexes in aqueous solutionThe formation mechanisms,thermodynamic stabilities,and water-exchange reactivities of 1:1 monomer aluminum-salicylate(Al-salicylate)complexes in acidic aqueous solution are investigated using the DFT-CM method(1)The formation pathways for possible monodentate and bidentate Al-salicylate configurations are modeled with GP-SM-PCM.It shows that the formation pathways for the Al-salicylate complexes follow the Eigen-Wilkins mechanism,where the dissociation of an inner-shell coordinated water of Al3+ is the rate-determining step.(2)The formation constants Kaq for different Al-salicylate configurations are estimated based on the total Gibbs free energy changes ?Go for their overall formation pathways.It is indicated that in the acidic aqueous solution at pH?3,the main existence form of the 1:1 monomer Al-salicylate complex is the phenol-deprotonated bidentate Al(Sal)(H2O)4+with six-membered ring.Its log Kaq is calculated as 13.8,in good agreement with the literature values of 12.9-14.5.(3)The water-exchange reactions are modeled for different Al-salicylate configurations.The water-exchange rate constant for Al(Sal)(H2O)4+is estimated as log kH2O=3.9 s-1,close to the experimental value of 3.7 s-1.It proves again that this configuration is the dominant form under experimental conditions.7.The estimation of the formation rate constants for aqueous Al complexesThe DFT-CM method is employed to model static outer-sphere ion-pair configurations and kinetic ligand-exchange reactions in the Eigen-Wilkins-type complex formation processes of six aqueous aluminum systems(Al3+/F-,AlOH2+/F-,A13+/HCOO-,Al3+/HSal-,K-GaAl127+/F-and K-Al137+/HSal-).The ion-pair association constants Kos are estimated from the Fuoss equation.The ligand-exchange rate constants k*are obtained with the transition state theory.The complex formation rate constants kf are estimated by using the formula kf=Kosk*.The results show that(1)reasonable Kos can be estimated based on the "effective ion distances" and "effective ionic charges" from the DFT-CM optimized ion-pair configurations;(2)in the dissociative ligand-exchange reactions,the second-shell anionic ligands only slightly affect the energy barriers for the rate-limiting steps of inner-shell coordination water dissociation.Using k*or pure water-exchange rate constants kex of aquated Al cations as approximations of k',we can obtain kf that are consistent with the experimental data;(3)this study provides a feasible method with which to estimate the Kos,k*and kf for aqueous Al complex formation reactions.It can be extended to the mechanistic studies on the "surface complexation model" of ligand adsorption onto mineral surfaces.8.The carboxylate ligand-promoted Al mineral dissolution mechanismThe stable geometries of a series of mononuclear bidentate chelating Al(?)-carboxylate complexes in aqueous solution are optimized and their transition-states for water-exchange reactions are modeled with the DFT-CM method.The studied carboxylates include oxalate,malonate,succinate,phthalate,salicylate and benzoate.Thermodynamic stability constants Kaq and water-exchange reaction rate constants kex of the tested Al(?)-carboxylate complexes are estimated from calculated Gibbs free energy changes.The estimated kex values suggest that the coordination of dicarboxylate and salicylate ligands to Al3+ leads to the labilization of inner-shell coordinated waters and that the cis waters of chelate rings are more labile than trans waters.From estimated log Kaq and log kex values of aqueous mononuclear Al(?)-carboxylate complexes,linear correlations between log Kaq,log kex values and experimental apparent rate constants kL of carboxylate ligand-promoted ?-Al2O3 mineral dissolution are established.Based on the calculation results,cis labilizing effects of the carboxylate ligands,strong "log kL?log Kaq" correlations,and weak"log kL?log kex" correlations are discussed.This work offers further insight into intrinsic relationships between mineral surface and aqueous metal complexes and furthers understanding of mineral dissolution kinetics at the molecular level.9.The rattling and rotation behaviors of the hydrated excess proton in waterThe rattling and rotation behaviors of the hydrated excess proton(H+)in water are investigated using the DFT-CM method.By constructing GP-SM-PCM and searching for kinetic transition state structures,the following results are obtained:(i)when the target proton*H+has a symmetrical solvation environment,*H+ can rattle between two O atoms in the form of Zundel cation.The nuclear quantum effect of zero-point contribution reduces the activation energy barrier of*H+rattling and enables the rattling process to occur spontaneously at room temperature;(2)the rotational behavior of*H+in the form of*H+·H2O*is found.During the rotation,*H+changes its position accompanied by the concerted displacement of the surrounding solvent water molecules and the breaking-formation of hydrogen bonds;and(3)it is proposed that the main transfer mechanism of the hydrated proton in water is the"*H+·H2O" rotating migration mechanism"—the same target proton*H+ migrates via*H+·H2O*rotation through void in solvent water,rather than the different protons hopping through the water hydrogen bond chains caused by the covalent bond breaking and formation as known as the Grotthuss proton hopping mechanism.The results of this work provide new references for choosing reaction coordinates in future theoretical simulations and experimental detections of the proton transfer mechanism in aqueous environments.10.Conclusion and perspectiveThe research results and main innovations of this dissertation are summarized.The imperfections need to be improved in this study are discussed,and the outlooks for the future research work are put forward.