Copper(II) adsorption on the kaolinite(001) surface Insights from first-principles calculations and.

Applied Surface Science 389(2016)316–323Contents lists available at ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scCopper(II)adsorption on the kaolinite(001)surface:Insights from first-principles calculations and molecular dynamics simulationsXiang-Ping Kong,Juan Wang ∗College of Chemistry and Pharmacy,Qingdao Agricultural University,Qingdao 266109,Shandong,PR Chinaa r t i c l ei n f oArticle history:Received 6April 2016Received in revised form 21June 2016Accepted 17July 2016Available online 22July 2016Keywords:Kaolinite Cu(II)Adsorption First-principlesMolecular dynamicsa b s t r a c tThe adsorption behavior of Cu(II)on the basal hydroxylated kaolinite(001)surface in aqueous environ-ment was investigated by first-principles calculations and molecular dynamics simulations.Structures of possible monodentate and bidentate inner-sphere adsorption complexes of Cu(II)were examined,and the charge transfer and bonding mechanism were bining the binding energy of complex,the radial distribution function of Cu(II)with oxygen and the extended X-ray absorption fine structure data,monodentate complex on site of surface oxygen with “upright”hydrogen and bidentate complex on site of two oxygens (one with “upright”hydrogen and one with “lying”hydrogen)of single Al center have been found to be the major adsorption species of Cu(II).Both adsorption species are four-coordinated with a square planar geometry.The distribution of surface hydroxyls with “lying”hydrogen around Cu(II)plays a key role in the structure and stability of adsorption complex.Upon the Mulliken population anal-ysis and partial density of states,charge transfer occurs with Cu(II)accepting some electrons from both surface oxygens and aqua oxygens,and the bonding Cu 3d -O 2p state filling is primarily responsible for the strong covalent interaction of Cu(II)with surface oxygen.©2016Elsevier B.V.All rights reserved.1.IntroductionCopper (Cu)is an essential trace element for living organisms.However,it becomes extremely toxic when presented at a high concentration.Excessive copper intake can cause severe damage to liver and kidney,and may be responsible for hemolysis,neu-rological dysfunction and gastrointestinal disturbance [1,2].The problem of removing Cu(II)from contaminated water is becoming an important process that closely related to the ecological security,and the mineral adsorption has been found to be one of the most economical and effective approaches [3,4].Kaolinite is a 1:1layered aluminosilicate mineral that is com-mon in soil systems.It plays an important role in the transport of metals,protons and organic molecules in soils,and has been widely used as adsorbent for the retention or removal of various metal ions [5,6].Kaolinite has a higher selectivity to Cu(II)than most of the other metal ions such as Zn(II)and Cd(II)[7].Removal of Cu(II)with kaolinite from aqueous solution has been extensively investi-gated experimentally through batch tests and the empirical surface modeling approaches [8–10].While a lot of empirical models based∗Corresponding author.E-mail addresses:kxp2004@ (X.-P.Kong),juaner80@ (J.Wang).on the outer-sphere and inner-sphere complexation are developed,several questions arise with respect to the inner-sphere complex of Cu(II)on kaolinite surfaces.(i)Is the inner-sphere complex mon-odentate or bidentate?(ii)What is the coordination number of the adsorption complex?(iii)Is there one preferred adsorption complex or an equilibrium among several of them?(iv)What is the bonding nature of Cu(II)with surface oxygen?Understanding the structure of Cu(II)adsorption complex from the molecular or atomic level would be helpful in answering these questions.Among the experimental techniques,only spectroscopic data from the extended X-ray absorption fine structure (EXAFS)deter-mination provide average structural parameters on the molecular level.But a few direct EXAFS spectroscopic investigations of Cu(II)-kaolinite adsorption have been documented.Peacock and Sherman [11]identified the mononuclear and binuclear inner-sphere com-plexes of Cu(II)on the variable charge sites of kaolinite based on the spectra of EXAFS and ab initio calculations.Arai [12]reported the outer-sphere Cu(II)dimer surface species on kaolinite from the EXAFS determination.Computational chemistry has become a useful tool to com-plement experimental data recently.Adsorptions of heavy metal Cd(II),Cu(II),Hg(II),Ni(II)[13]and Pb(II)[14]atoms on the kaolinite(001)surface have been investigated using the density-functional approach.However,these investigations were carried/10.1016/j.apsusc.2016.07.1120169-4332/©2016Elsevier B.V.All rights reserved.X.-P.Kong,J.Wang/Applied Surface Science389(2016)316–323317out with free metal ions adsorbed.Coordination of the metal ion with other ligands such as the water molecules of aqueous medium was not considered.Structure of Cu(II)adsorption complex could not be presented in their work.Thus,a surface complexation model based on actual surface species will be more reliable in improving our understanding of Cu(II)adsorption in real water environment.In this work,a combination offirst-principles calculations and molecular dynamics(MD)simulations was used to investigate the adsorption of Cu(II)on the basal hydroxylated(001)surface of kaolinite in aqueous environment.Structures of inner-sphere, mononuclear,mono-and bi-dentate complexes of Cu(II)were examined,with two different types of surface hydroxyl sites con-sidered.The preferred adsorption sites and adsorption species of Cu(II)were presented based on the value of binding energy,the cal-culation of radial distribution function(RDF)and the similarity of structure parameters to experimental spectroscopic data.Mulliken population and partial density of states(PDOS)were also analyzed in detail to evaluate the related charge transfers and to explore the bonding nature of Cu(II)with surface oxygen.2.Theoretical methods and modelsputational detailsFirst-principles calculations were performed with the CASTEP code[15]in Materials Studio 6.0[16],using the plane-wave ultrasoft pseudopotential method[17]based on density func-tional theory(DFT).The electronic exchange-correlation energy was treated within the generalized gradient approximation(GGA), using the Perdew-Burke-Ernzerhof(PBE)functional[18].Condi-tions similar to those of our previous study[19]were adopted for the optimizations of kaolinite bulk and kaolinite(001)surface models,including the cutoff energy of380eV,the Broyden-Fletcher-Goldfarb-Shanno(BFGS)scheme[20],the Brillouin zone sampling of(4×2×3)and(2×2×1)k-points[21]respectively and the setting of tolerances for self-consistence.Hydration of Cu(II) in aqueous system was investigated with all possible species of [Cu(H2O)n]2+(1≤n≤7)examined in a10×10×10Å3periodic box, using1×1×1grid of k-point sampling.As conventional DFT func-tionals have difficulty in describing the strong correlation effect among the partiallyfilled Cu3d shell[22],the well-known DFT+U method[23,24]was used for the calculations of Cu(II)complexes. The value of effective U for Cu(II)(U eff=U−J=6.5eV)was adopted according to the related references[23,25]as well as our compar-ative calculations across different parameter U values.MD simulations were carried out using the Forcite program of Materials Studio6.0[16].The Universal forcefield[26],based on the single point charge(SPC)water model[27],was adopted.The kaolinite-Cu-H2O system wasfirst simulated in a NPT ensemble for 200ps with constant pressure of0.1MPa and temperature of298K. An extra NVT period of200ps was then performed under the tem-perature of298K.A time step of1fs was used for the velocity Verlet algorithm to integrate the particle motion.The trajectory frame was recorded every20fs.The Ewald summation method was adopted to calculate the long-range electronic interactions.The Nose thermo-stat and Berendsen barostat were used to maintain the temperature and pressure,respectively.Atomic charges were assigned using the charge equilibration method[28].All atoms were allowed to move freely during the simulations.2.2.Surface modelsA one-layer-thick(2×1)slab supercell of kaolinite,composed of six atomic“sublayers”of“H O Al O Si O”[19,29],was adopted for the staticfirst-principles calculations.The slab wasrepeated Fig.1.Top view of the basal hydroxylated(001)surface of kaolinite(i)and the possible adsorption sites for Cu(II)on the cyan AlO6octahedra(ii).Atoms of H,O, Al and Si are in white,red,cyan and yellow,respectively.(For interpretation of the references to colour in thisfigure legend,the reader is referred to the web version of this article.)periodically with a vacuum region of15Å.Only atoms in the two bottom“sublayers”were keptfixed during the geometry optimiza-tions.The two different types of hydroxyl groups on the surface, O u H(oriented perpendicularly to the surface)and O l H(oriented parallel to the surface),were considered separately.As shown in Fig.1,monodentate complex of Cu(II)may occur on the surface site of“O u”or“O l”of any Al center.The obtained complexes can be des-ignated as configurations A and B,respectively.Bidentate complex of Cu(II)may form on the“O u–O u”or“O u–O l”site of single Al cen-ter(black solid lines a and b)or two neighboring Al centers(black dashed lines c and d),corresponding to configurations C,D,E and F,respectively.All possible adsorption complexes with general for-mula of[Al(OH)3−x(O)x Cu(H2O)m](2−x)+(1≤x≤2,1≤m≤6)were tested.Only reasonable adsorption complex with maximum aqua ligands was adopted and analyzed.The coordination number of Cu(II)was mainly determined by the optimized Cu O bond length (R Cu O<2.8Å)[30]and the Cu O bonding characteristics in PDOS curves.Binding energies(E b)of the hydrated Cu(II)species and the corresponding adsorption complexes are calculated based on the energy difference of Reactions(1)and(2),respectively.Cu2++n H2O→[Cu(H2O)n]2+E b=E[Cu(H2O)n]2+−nE H2O−ECu2+(1)Al(OH)3+[Cu(H2O)n]2++x OH−→Al(OH)3−x(O)x Cu(H2O)m(2−x)++(n−m+x)(H2O)E b=E com+(n−m+x)E H2O−xE OH−−E[Cu(H2O)n]2+−E Al(OH)3(2)where E com represents the calculated total energy of the kaolin-ite(001)slab with Cu(II)adsorbed on the surface.The deprotonation of surface hydroxyl is considered in Reaction(2)by the parameter x,as inner-sphere complex of Cu(II)has been found to be formed at the pH region of larger than the point of zero charge of kaolinite [31,32].The“OH−”acts as an example group in aqueous system, which can react with the missed“H+”of Al(OH)3here.A periodic supercell(a=20.85Å,b=18.10Å)consisted of8 (4×2×1)unit cells of kaolinite with a total of272atoms was cre-ated for the MD simulations.Consistent with thefirst-principles318X.-P.Kong,J.Wang /Applied Surface Science 389(2016)316–323Fig.2.Equilibrium geometries of [Cu(H 2O)1−6]2+from the optimization conditions of PBE functional,ultrasoft pseudopotential and the cutoff energy of 380eV.Atom of Cu is colored blue.Symbols of (a)and (b)stand for complex of [Cu(H 2O)5]2+in geometries of square pyramid and trigonal bipyramid,respectively.(For interpre-tation of the references to colour in this figure legend,the reader is referred to the web version of this article.)calculations,lattice parameters for the unit cell of kaolinite were adopted from the neutron powder diffraction determination [33].The water molecules and the Cu(II)atom were packed in a cubic box with the lengths a and b matching pretty well with the constructed supercell of kaolinite.Two defect sites were obtained by the depro-tonation of two surface OH bonds,which can also neutralize the positive charge of Cu(II).The RDF (also called pair distribution func-tion or pair correlation function)is analyzed [34],and the average coordination number of Cu(II)in the complex is calculated accord-ing to Formula (3):n ij (r )=4 jrr 2g ij (r )dr(3)where n ij (r ), j and g ij (r )denote the average coordination number,the average number density and the RDF,respectively.Parameters i and j represent the two different types of interacted atoms,and r refers to the distance from surrounding interacted atoms of j to the central atom of i .3.Results and discussion3.1.Structures of hydrated Cu(II)complexesThe hydration of Cu(II)has been investigated by experimen-tal spectroscopic techniques [35,36]and theoretical calculations [37,38].The numbers of hydrated water molecules in the first solva-tion shell are mainly four,five and six.Consistent with the previous results,reasonable structures of [Cu(H 2O)n ]2+were obtained at hydration number lower than seven from the DFT +U optimizations in our work.One or more aqua ligands of [Cu(H 2O)7]2+were found to be excluded from the coordination sphere of Cu(II)and went into the second hydration shell or solution ultimately.The equilibrium geometries of [Cu(H 2O)n ]2+with n ranging from 1to 6were shown in Fig.2,where geometries of both square pyramid and trigonal bipyramid were examined for the complex of [Cu(H 2O)5]2+.As seen from Fig.2,the hydrated complex of Cu(II)changed from the linear structure at n =1,2to a square planar geometry at n =4and an elongated octahedron at n =6.With the increase of the num-ber of aqua ligands,the mean Cu O bond length (Cu O m )extended from 1.86Åat n =1to 1.97Åat n =4,2.04Åat n =5and 2.12Åat n =6(Table 1).Repulsive interactions among the aqua ligands of Cu(II)should be responsible for these enlarged pared with complexes of [Cu(H 2O)]2+,[Cu(H 2O)2]2+and [Cu(H 2O)3]2+,differ-ence among the binding energies of [Cu(H 2O)4]2+,[Cu(H 2O)5]2+and[Cu(H 2O)6]2+was relatively small.Moreover,comparable Mulliken charges of Cu(II)in complexes of [Cu(H 2O)4]2+,[Cu(H 2O)5]2+(a),[Cu(H 2O)5]2+(b)and [Cu(H 2O)6]2+(1.47,1.46,1.48and 1.46|e|,respectively)were obtained (Table 1).These results indicate that the hydrated complex of Cu(II)in aqueous solution may be tetra-,penta-or hexa-coordinated.Structure transformation has been reported to occur dynamically among the three of them [39].Furthermore,mean equatorial and axial Cu O bond lengths in [Cu(H 2O)6]2+were calculated to be 2.02and 2.34Å(Table 1),in con-sistent with the experimental data of 1.94–2.00and 2.40±0.10Åfrom various research techniques [37,40],respectively.Thus,com-plex of [Cu(H 2O)6]2+featuring the highest hydration number was adopted as the dominant adsorbate on kaolinite surface in aqueous environment,and the value of six was adopted for n when the bind-ing energy of Cu(II)adsorption complex was calculated by Reaction (2).3.2.Monodentate adsorption complexes of Cu(II)The metal ion of Cu(II)can react with the hydroxyl group of kaolinite(001)surface in a monodentate manner.The maximum number of aqua ligands in the possible complex of surface “O u H”site was obtained at 4(Fig.3A 1),and the adsorption complex fea-turing the common coordination number of 4was also presented (Fig.3A 2).With respect to the “O l H”site,the most probable complex containing three aqua ligands was shown in Fig.3B.The coordi-nation number of Cu(II)in monodentate complex was lower than that in [Cu(H 2O)6]2+in aqueous solution.When the hydrated Cu(II)complex approached the hydroxylated kaolinite surface,some of the aqua ligands were crowded out of the coordination shell of Cu(II)due to the large steric hindrance of kaolinite.Also,repulsive interactions from surrounding “upright”surface hydroxyl groups (O u H)to the aqua ligands played less of a factor here.Hydrogen bonds (black dashed lines in Fig.3)have been observed to be the major interactions between kaolinite surface hydroxyls and aqua ligands of complex.Three types of hydro-gen bonds formed,surface oxygen of “O l ”or “O u ”with hydrogen of aqua ligand (designated as “O l ···H w ”and “O u ···H w ”,respec-tively)and oxygen of aqua ligand with hydrogen of surface “O u H”group (“O w ···H u ”).Compared with atom of surface “O u ”,surface “O l ”connecting with the “lying”hydrogen has the position advan-tage to interact with the aqua ligands.The strong interaction can be confirmed by the short bond distance of 1.39–1.60Åfor the hydrogen bond of “O l ···H w ”type (Table 2).Long bond lengths of 1.48–1.61and 1.96–2.45Åfor bonds of “O u ···H w ”and “O w ···H u ”were found (Table 2),respectively,indicating the relatively weak interaction between surface “O u H”groups and the ligating waters.Thus,hydrogen bonding interaction of “O l ···H w ”played a key role in the stability of Cu(II)adsorption complex.Configuration A 1featured Cu O s bond length of 1.91Åand Cu O w lengths of 2.01,2.01,2.19and 2.36Å(Table 2).A distorted square pyramidal geometry was obtained in Fig.3A 1.The geometry has been frequently reported for five-coordinated Cu(II)complexes [41,42].Configuration A 2displayed a slightly short Cu O s bond of 1.89Å,with the longest Cu O w bond of 2.12Åbeing observed.The approximately square planar geometry of configuration A 2was consistent with many other four-coordinated Cu(II)complexes [43,44].Both complexes were stabilized by the hydrogen bonding interactions of “O l ···H w ”and “O w ···H u ”types.The Cu O s lengths of 1.91and 1.89Åin the two configurations were consistent with the value of 1.89Åin the Cu(II)adsorption complex on kaolinite at pH 6.5from the EXAFS technique [11].Comparable shortest Cu Al separations of 3.38and 3.36Åin the two configurations have also been observed,which were close to the corresponding distance ofX.-P.Kong,J.Wang /Applied Surface Science 389(2016)316–323319Table 1Structural parameters (in Å),coordination geometries,binding energies (in kcal/mol)and Mulliken charges (in |e|)of [Cu(H 2O)n ]2+.nCu OCu O m aGeometryE bQ Cu1 1.861.86straight line −128.98+1.762 1.83,1.841.84broken line −218.18+1.663 1.89,1.90,1.921.90trigonal planar −280.85+1.664 1.95,1.98,1.98,1.961.97square planar −324.92+1.475(a)2.00,1.98,2.00,2.04,2.19 2.04square pyramid −352.33+1.465(b) 2.12,2.04,2.07,1.98,1.972.04trigonal bipyramid −353.30+1.4862.03,2.03,2.00,2.02,2.33,2.342.12elongated octahedron−374.89+1.46aThe mean value of Cu–O bondlengths.Fig.3.Optimized structures of monodentate Cu(II)complexes adsorbed on “O u ”(A 1and A 2)and “O l ”(B)sites of the hydroxylated kaolinite(001)surface.Black dashed lines are representative of hydrogen bonding interactions between aqua ligands and surface hydroxyls.Table 2Structure parameters (distances in Å)and binding energies (in kcal/mol)of mono-and bi-dentate Cu(II)adsorption complexes on the hydroxylated kaolinite(001)surface.ComplexN a Cu O s bCu O w cO l ···H wO u ···H wO w ···H uCu Al dE bA 15 1.91 2.01,2.01,2.19,2.36 1.40,1.42,1.60– 2.38,2.43,2.45 3.38−119.63A 24 1.89 1.96,1.98,2.12 1.39,1.40–2.363.36−103.74B 4 1.862.00,1.99,2.04–1.48,1.59,1.61 1.96 3.25−96.42C 4 1.96,1.872.03,2.18 1.56,1.66–– 2.68−86.04D 4 1.86,2.03 2.11,2.04 1.692.32– 2.76−89.61E41.88,1.872.20,2.291.64,1.65––3.06−87.72a Coordination number of the complex.b Distance of Cu(II)to surface oxygen center.c Distance of Cu(II)to oxygen center of aqua ligand.dSeparation of Cu(II)to the nearest aluminum center of octahedra.3.34Åin the Cu(II)adsorption complex on the surface of gibbsite from the EXAFS determination [45].Configuration B,the monodentate complex of “O l ”site,exhib-ited Cu O s bond of 1.86Åand Cu O w bonds at lengths of about 2.0Å.The short Cu O s length was close to that in complex of [Cu(H 2O)]2+or [Cu(H 2O)2]2+.Repulsive interactions from sur-rounding “upright”surface hydroxyl groups to the aqua ligands may act as the major cause.As seen from Fig.1,there were six sur-face “O u H”groups evenly distributing around every “O l H”group with a distance of approximately 3.4Å[19].When the adsorption took place on the “O l ”site,only “O u ···H w ”and “O w ···H u ”types of hydrogen bonds formed between surface “O u H”and aqua ligands of Cu(II).No interaction of “O l ···H w ”bond occurred,which might lead to the weak stability of the complex.The binding energies of complexes A 1and A 2were calcu-lated to be −119.63and −103.74kcal/mol (Table 2),respectively.Both values were higher than that of complex B (–96.42kcal/mol),revealing the less likely adsorption of Cu(II)on the surface “O l ”site.From the Reaction (2),binding energy of complex A 1incorpo-rated hydration energy of one proton from surface hydroxyl group,dissociation energies of two aqua ligands from [Cu(H 2O)6]2+and adsorption energy of Cu(II).Obviously,dissociation energy of one more water molecule from [Cu(H 2O)6]2+was incorporated in the binding energy of complex A 2.Thus,the adsorption energies of Cu(II)in the two complexes might be comparable.Both configu-rations A 1and A 2can be the preferred monodentate complex of “O u ”site on the kaolinite(001)surface.3.3.Bidentate adsorption complexes of Cu(II)Complexation of Cu(II)with the hydroxylated kaolinite(001)surface in bidentate way was shown in Fig.4.The possible adsorp-tion complexes on sites of “O u –O u ”,“O u –O l ”of single Al center and on “O u –O u ”of neighboring Al centers were corresponding to the configurations C,D and E,respectively.Site of “O u –O l ”on neigh-boring Al centers was found hard for the Cu(II)adsorption to occur,where long Cu O w bond length of greater than 2.80Åand short Cu O s length of about 1.84Åwere observed during the DFT +U optimization.Configurations C,D and E all exhibited the four-coordinated structure of Cu(II).A distorted tetrahedral coordination geometry was observed for configurations C and E,whereas an approximately square planar geometry was found for configuration D.According to the crystal field theory (CFT)[46],the square planar structure of Cu(II)complex featuring much higher crystal field stabilization energy (CFSE)should be more stable than the tetrahedral complex.No significant difference was found among the structural parame-ters of these bidentate adsorption complexes,except for the nearest Cu–Al separation.Configuration D displayed two Cu O s bonds of 1.86and 2.03Å,two Cu O w bonds of 2.11and 2.04Åand a nearest320X.-P.Kong,J.Wang /Applied Surface Science 389(2016)316–323Fig.4.Optimized structures of bidentate Cu(II)complexes adsorbed on “O u –O u ”(C)and “O u –O l ”(D)sites of the same Al center and on “O u –O u ”(E)site of neighboring Al centers of the hydroxylated kaolinite(001)surface.Hydrogen bonding interactions are indicated by black dashed lines.Cu–Al distance of 2.76Å.Configuration E exhibited relatively long Cu–Al separation of 3.06Å.The calculated binding energies of con-figurations C,D and E were −86.04,−89.61and −87.72kcal/mol,respectively.The formation of bidentate Cu(II)adsorption complex has been reported by a number of related spectroscopic investigations.A combination of EXAFS spectra and DFT calculations showed the existence of bidentate Cu(II)complex on kaolinite surface [11].A XAFS study reported the edge-sharing bidentate Cu(II)com-plex on the oxide-water interface of ␣-Al 2O 3(0001)surface,with Cu O s length of 1.92–1.94Åand the nearest Cu–Al separation of 2.78–2.93Å[47].An electron nuclear double resonance (ENDOR)determination gave the structure of bidentate complex of Cu(II)on hydrous oxide surface of ␦-Al 2O 3with Cu O s length of 2.0Å[48].Among the three possible configurations of bidentate Cu(II)complexes on kaolinite(001)surface,configuration D featured the highest binding energy,the experimentally determined Cu O s length of 1.92–1.94Å,Cu–Al separation of 2.78–2.93Åand the reported edge-sharing adsorption mode.Correlating well with the XAFS study [47],ion of Cu(II)in configuration D bonded to the Al(OH)3groups through the equatorial bonds and displayed a sim-ilar square planar coordination geometry.Therefore,configuration D should be the preferred bidentate adsorption species of Cu(II)on the hydroxylated kaolinite(001)surface.3.4.Charge transfer and bond populationCharge transfers of different atoms and Cu O s bond populations in the possible mono-and bi-dentate Cu(II)adsorption complexes were obtained from the Mulliken population analysis [49].As shown in Table 3,charges of atoms O u and O l in the clean kaolin-ite(001)surface before Cu(II)adsorption were −1.06and −1.05|e|,respectively,indicating that few charge difference existed between the “upright”and the “lying”oxygens.In the adsorption complexes,charges of surface oxygens (O u and O l )varied between −1.02and −1.08|e|.Except for atom of O l in complex C and O u in complex D,surface oxygens in the examined adsorption modes exhibited positive charge transfer values ( Q Os )of +0.02to +0.04|e|.The finding indicates that most surface oxygens act as the role of elec-tron contributors during the adsorption process.Meanwhile,atom of Cu(II)accepted electrons from the ligated oxygens with Q Cu ranging from −0.25to −0.42|e|(Table 3).As the absolute value of Q Cu was much larger than that of Q Os ,oxygens of aqua ligands of Cu(II)should also denote electrons to Cu(II)here.Moreover,the charges of Cu(II)in the six possible adsorption configurations were lower than that in the initial adsorbate of [Cu(H 2O)6]2+,revealing the further decreased ionic character of Cu(II)in the adsorption complexes.The Mulliken overlap population can provide an objective cri-terion for bonding between atoms,and has been used frequentlyto assess the covalent or ionic nature of a bond.Values of Mulliken population for bonds of Cu O u and Cu O l in different types of com-plexes were listed in Table 3.All the Cu O s bond populations were positive and relatively large,revealing the strong covalent charac-ter of bond Cu O u or Cu O l in the adsorption complexes of Cu(II)on the kaolinite(001)surface.Populations for bond of Cu O u in all possible complexes varied between 0.24and 0.36,and the only two Cu O l populations in complexes B and D were obtained at 0.30and 0.18,respectively.Populations of Cu O u in monodentate com-plexes A 1and A 2were 0.32,slightly larger than the Cu O l value of 0.30in complex B.This is in consistent with the lower binding energy of complex B than complex A 1or A 2,and further confirms the less likely monodentate complex of Cu(II)on surface “O l ”site.In the preferred bidentate edge-sharing complex D,bond of Cu O u seems to be stronger than Cu O l based on the much higher popu-lation of 0.29than the value of 0.18in Table 3.3.5.RDF analysisIn order to further identify the exact structure of mononuclear inner-sphere complex of Cu(II)on the kaolinite(001)surface,clas-sical MD simulations for the preferred adsorption configurations A and D were carried out.The initial conformation for the simulation of monodentate complex A was illustrated in Fig.5(i),where the two deprotonated O u H bonds located on different Al centers were far away from each other (the green atoms of oxygen).After the NPT and NVT ensembles,the water molecules in the aqueous system moved freely from different directions to interact with the hydrox-ylated kaolinite(001)surface,and the atom of Cu(II)was located just above one “O u ”defective site with a separation about 1.90Å(Fig.5(ii)).The resulted RDF and average coordination number of Cu(II)with oxygen were shown in Fig.6.Two narrow connected peaks were obtained on the g Cu O (r )curve at 1.90and 2.03Å,corresponding to the mean bond lengths of Cu O s and Cu O w ,respectively.These two values were very close to the Cu O s bond of 1.89Åand mean Cu O w length of 2.02Åin adsorption complex A 2(Table 2).Coordination number of n Cu O (r )for the first peak at r of 1.82–1.96Åwas 1.06,in consistent with the Cu(II)complexa-tion with kaolinite(001)surface in monodentate way.Coordination number of n Cu O (r )for the second peak at r of 1.96–2.50Åwas 2.93,indicating that three aqua ligands were involved in the first coor-dination sphere of Cu(II).Thus,it confirmed that the adsorption complex A 2was the dominant monodentate adsorption species of Cu(II)on the kaolinite(001)surface.Conformations of MD simulations for the bidentate adsorption complex D were shown in Fig.7.Two defective sites of “O u ”and “O l ”(green atoms of oxygen)were on the same Al center of kaoli-nite(001)surface,and the atom of Cu(II)was located almost above the middle of the two defective sites with mean Cu O s separa-tion of 1.94Åafter the final NVT ensemble.From the RDF shown in Fig.8,the g Cu O (r )curve at r of 1.84–2.22Åexhibited a rela-。