Adsorption Of Ions Onto Carbonaceous Materials:a Density Functional Theory Study

Soil is the material basis and wealth source for human beings,while the soil system has been seriously contaminated during the burgeoning development of industry and agriculture and the even-increasing process of urbanization,which further threthens the ecological safety and public health.Among the various contaminants,inorganic substances constitute a major source that can be classified mainly as heavy metal ions and anions.In the process of exploring mehtodolgoies to manipulate the inorganic pollutants,researchers realize that carbonaceous materials show great promise in this respect.Currently,the adsorption of ions on carbonaceous materials is being conducted with high enthusiasms;nevertheless,most of the studies focus on the macroscopic phenomena such as adsorption capacity,adsorption strength and process.In this thesis,theoretical calculations were employed to unravel the mechanistic aspects of ion adsorption on carbonanceous materials at the molecular level.In this thesis,density functional theory（DFT）calculations were conducted to have an in-depth and systematic inveisgation on the interaction mechanisms between carbonaceous materials and ions,using polycyclic aromatic hydrocarbons as the prototype for carbonaceous materials,halide ions（X-= F^-,Cl^-,Br^-,I^-）as representatives of anions,alkali metal ions（M⁺ = Li⁺,Na⁺ and K⁺）,alkaline earth metal(M²⁺ = Be²⁺,Mg²⁺ and Ca²⁺)and trivalent lanthanum ion(La³⁺)as representatives of metal ions as well as LiF,NaF,KF,LiI,NaF and KF as representatives of ion pairs.The main findings are summarized as follows:（1）The adsorption characteristics of halide ions and metal ions onto carbonaceous material have cerntain similarities,but they are distinct in essence.Whether for halide ions or for metal ions,the adsorption strengths reduce with increase of ion radius,in the orders of F^-> Cl^-> Br^-> I^-and Li⁺ > Na⁺ > K⁺(Be²⁺ > Mg²⁺ > Ca²⁺),respectively.It can be deduced from the comparisons of binding energies that among the various halide ions,only F^-results in an appreciable interaction with carbonaceous materials while the others are feebly adsorbed so that can be faciely detached.Instead,all metal ions can be firmly adsorbed onto carbonaceous materials.The essentical difference between the interacitons of halide ions and metal ions lies in that carbonaceous materials are electron acceptors when interacting with halide ions wherein electron transfers occur from halide ions to carbonaceous materials,while are electron donors when interacting with metal ions wherein electron transfers occur from carbonaceous materials to metal ions;（2）This thesis further showed how the electronic properties of carbonaceous materials affect the adsorption performances of ions,through the edge-functionalization using the F atom that is typically electron acceptors.It indicated that edge-functionalization enhances pronouncedly the adsorption for all halide ions;however,the adsorption strength of metal ions decreases gradually with increase of the functionization degree（Θ）.When Θ = 100%,the binding energy of F^-reaches as large as-186.3 kJ mol^-1 that is twice as that of pristine carbonaceous materils,while the binding energies of Li⁺,Na⁺ and K⁺ reduce to-95.7,-44.8 and-25.9 kJ mol^-1,which are obviously inferior to that of F^-.Accordingly,edge-functionalization alters the adsorption priority of metal ions vs.halide ions and the preferential adsorption of metal ions or anions can be regulated by choice of an appropriate functionalization degree（Θ）;（3）The adsorption results of hydrated ions show that the hydration significantly attenuates the adsorption characteristics.When F^-is adsorbed,water molecules form H-bonds with F^-and at the same time interact with carbonaceous material through their O atoms;however,metal ions form direct ionic bonds with the O atoms of water molecules,and only the H atoms of water molecules are available to interact with carbonaceous materials.Therefore,hydration significantly affects the adsorption of halide ions on pristine carbonaceous materials while significantly affects the adsorption of metal ions on edge-functionalized carbonaceous materials;（4）All ion pairs can be adsorbed onto carbonaceous materials via the ipsilateral vertical and bilateral configurations.However,the adsorption strength of vertical configurations are stronger than those of bilaterally ones,and when metal ions directly couple with carbonaceous materials,the adsorption configuration is more stable.Further studies showed that the relative stability of two vertical configurations is determined by the binding-affinity differences of anions vs.metal cations.When the adsorption of metal ions is obviously stronger,the configuration of metal ions directly coupling with carbonaceous materials is preferred;vice versa.When the binding energies of the two configurations are nearly equalivlaent,no obvious preference occurs for two vertical configurations,and their stability is close to each other;（5）Atomic doping of carbonaceous materials renders the horizontal configurations to be the most stable,and the stabilities of two vertical configurations reduce significantly that at some conditions are non-existent.The interaction strengths of NaF can be enhanced by all dopants and increase as N < O < B < Be.The enhanced interactions between Na F and carbonaceous materials with electron-deficient dopants（Be and B）are caused by direct bonding between dopants and F^-.However,the condition is different for electron-rich dopants（N and O）where the enhancement should be due to the destruction of π-conjugations and electron redistribution upon N and O doping that also accounts for the infferor interactions than those of Be-or B-dopants;To summarize,the interaction mechanisms of carbonaceous materials with metal ions,anions and ion pairs were studied in an in-depth and systemtic way,considering the electronic properties of carbonaceous materials,doping,hydration and other effects.Results thus obtained provide good theoretical support and scientific basis for the adsorption of ions on carbonaceous materials.Moreover,it also provides some reference direction for the targeted selection and preparation of carbon materials in future.