Theoretical Study On The Association And Adsorption Of Ions In Aqueous Solutions

Arsenic contamination of mine wastes and groundwater causes a expressive human disease and environmental hazard in many countries.The investigation on the treatment technologies for As-containing drinking water and industrial wastewater is currently of great urgency and high priority.The most commonly used technologies for removing arsenate from water is co-precipitated by Fe-based materials,which is identified as the most inexpensive and high efficient method in the treatment of arsenic contamination water.Although co-precipitation and adsorption by Fe-based materials can productively remove As?V?,the mechanism remains unclear at the molecular level.The investigation of the association of Fe-based materials with As?V?species will provide valuable insight into the precipitates mobility of As?V?in the aqueous phase,and help to improve the stability of the As?V?removal techniques.In this work,the stability of Fe-As precipitation and the association of Fe-based materials with As?V?species was systematically explored using density functional theory,Moller-Plesset perturbation method?MP2?and Car-Parrinello molecular dynamics?CPMD?simulations.The results show that the associated species of[FeH2AsO4]2 aq are unstable in an aqueous phase,which indicates that the H₂AsO₄^-species may not be precipitated by Fe³⁺ aq in an aqueous solution.The As???in the aqueous solution can be precipitated by Fe³⁺ aq through HAsO₄^2-species,and thus the precipitation of As???by Fe³⁺ aq in an aqueous solution needs a transformation of H₂AsO₄^-species to HAsO₄^2-species.The theoretical results show that the protonation of arsenate weakens electrostatic attraction,and also partially influences charge transfer?CT?,which has an adverse effect on Fe???-As???association.The hydration tendency of[FeH2AsO4?H2O?n]²⁺clusters is larger than that of[FeAsO4?H2O?n]0 clusters,and hydration hinders Fe???-As???association.The hydroxyl groups coordinated with Fe???hinder Fe-As association,because the CT between Fe???and As???decreases and the transformation from bidentate association structure to the monodentate one will be favorable.Meanwhile,the distribution of the HAsvO₄^2-species will be not dominant under acidic conditions and the amounts of OH-increase with the pH increases,the high efficiency for the removal of arsenate by Fe³⁺ aq only occur under neutral pH or weak alkaline conditions.The adsorption is an another important mechanism of As?V?removal by iron materials.The results show that the association of the colloidal ferric hydroxides with arsenate species is stronger than that of Fe-oxides with arsenate species.Theoretical results indicate that electrostatic interaction is essential in the association of Fe species with As?V?.In addition,CT from Fe?III?to As species cannot be ignored in the association of colloids with As?V?.This CT decreases as the number of OH-coordinated with Fe?III?in colloidal ferric hydroxide complexes increases,while the mono-nuclear mono-dentate structures of the associated complexes with arsenate species become favorable with the OH-number increases.And thus the high efficiency for the removal of arsenate by colloids can occur only under the lower pH and neutral pH.All in all,adjusting the pH of the aqueous solutions to the neutral environment will lead to a higher efficiency of arsenic removal by iron salts or other iron based materials.Mg²⁺ and Ca²⁺ are widely occurred in the fields of biochemistry.The interactions of Mg²⁺ and Ca²⁺ with some amino acid molecules or moieties play very important role in life activities.But the interactions between Mg²⁺ and Ca²⁺ and amino acids have not been revealed at the molecular level.In this work,the associated structures of alanine molecule were investigated using CAM-B3LYP method,and the interactions of Mg²⁺,Ca²⁺ and Cl-with polar groups of alanine in aqueous solutions were studied using the classical molecular dynamics simulation.In the gas phase,the calculated binding energies indicate that the bidentate structures are more stable.The interaction of Mg²⁺with alanine is stronger than that of Ca²⁺ with alanine.In MgC12 and CaC12 aqueous solutions,the interaction between Mg²⁺,Ca²⁺ and alanine will hinder the association of alanine molecules to some extent.In the aqueous solution,the interaction of Mg²⁺ with alanine is still stronger than that of Ca²⁺ with alanine.In dilute salt aqeuous solution,Mg²⁺ directly contacts with alanine molecule,while the association structure of Ca²⁺with alanine molecule is solvent-shared ion pair.As the concentration increases,the association between alanine molecules will become weak in MgC12 aqueous solution.However,as the concentration increases,the association between alanine molecules will enhance,and thus become weak in CaCl₂ aqueous solution.The analyses show that the association between alanine molecules can be more easily effected by Ca²⁺while not Mg²⁺ in aqueous solutions,and such trend can be contributed to the characteristics of Ca²⁺.Compared with Mg²⁺,the hydration of Ca²⁺ need more water molecules,the influence of Ca²⁺ on ions or polar groups in its second shell cannot be neglected.The unique trend of the association of alanine molecules in CaCl₂ aqueous solution as the concentration increases,may be attributed to the hydration characteristics of Ca²⁺.