Chia M. Chang

Transport modeling of copper and cadmium with linear and nonlinear retardation factors

The predictive accuracy of using the one-dimensional advection-dispersion equation to evaluate the fate and transport of solute in a soil column is usually dependent on the proper determination of chemical retardation factors. Typically, the distribution coefficient (Kd) obtained by fitting the linear sorption isotherm has been extensively used to consider general geochemical reactions on solute transport in a low-concentration range. However, the linear distribution coefficient cannot be adequately utilized to describe the solute fate at a higher concentration level. This study employed the nonlinear equilibrium-controlled sorption parameters to determine the retardation factor used in column leaching experiments. Copper and cadmium transportation in a lateritic silty-clay soil column was examined. Through the explicit finite-difference calculations with a third-order total-variation-diminishing (TVD) numerical solution scheme, all results of the theoretical copper and cadmium breakthrough curves (BTCs) simulated by using the Freundlich nonlinear retardation factors revealed good agreement with the experimental observations.

A geometric approach to determine adsorption and desorption kinetic constants

A geometric method based on Langmuir kinetics has been derived to determine adsorption and desorption kinetic constants. In the conventional procedure, either the adsorption kinetic constant (k(a)c) or desorption kinetic constant (k(d)c) is found from kinetic experiments and the other is calculated by their correlation with the equilibrium constant, i.e, k(d)c = Kcon/k(a)c, where Kcon has been known from equilibrium studies. The determined constants (Kcon, k(a)c, k(d)c), if based only on the conventional procedure, may not be accurate due to their mathematical dependence. Therefore, the objectives of this study are applying a geometric approach to directly determine Langmuir kinetic constants and describe adsorption behavior. In this approach, both adsorption kinetic constant (k(a)g) and desorption kinetic constant (k(d)g) are obtained only from data of kinetic experiments, and a geometric equilibrium constant (Kgeo) is calculated by Kgeo = k(a)g/k(d)g. The deviation between Kgeo and Kcon can prove the accuracy of k(a)g and k(d)g which were determined by this method. This approach was applicable to selenate, selenite and Mg2+ adsorption onto SiO2 regardless of whether the adsorbate formed inner- or outer-sphere complexes. However, this method showed some deviation between Kcon and Kgeo for Mn2+ adsorption because of the formation of surface Mn(II)-hydroxide clusters, which was inconsistent with the basic assumption of this method of monolayer adsorption.

Linear relationship for acidity and stability in hexaaqua metal ions — density functional studies

Abstract Linear correlation between the acidity constant and structural stability in hexaaqua metal ions has been rationalized by applying density functional theory. Six-coordinated aqueous cations of Mg2+, Ca2+, Mn2+, Zn2+, Cd2+, Al3+, Sc3+, Cr3+, Fe3+, Ga3+ and In3+ were investigated. Through local (Vosko–Wilk–Nusair) and nonlocal (Becke–Perdew and Becke–Lee–Yang–Parr) density functional binding energy calculations, the structural stability of hexaaqua metal ions is strongly affected by the cationic valence, electronic configuration and ionic radius of the metallic cations. This complete density functional result has confirmed the existence of a global linear relationship for acidity and stability in both main and transition group hexaaqua metal ions.

DFT-based linear solvation energy relationships for the infrared spectral shifts of acetone in polar and nonpolar organic solvents.

Linear solvation energy relationships (LSER) established using solvation free energy and density functional theory (DFT)-based reactivity descriptors are for the first time documented in this study. The solvent-induced shifts of the carbonyl (C=O) stretching frequency of acetone in 21 organic solvents including polar protic, dipolar aprotic, and nonpolar solvents are examined. Results of the multiple regression analysis have shown that four descriptors, namely, (1) the solvation free energy of solute in continuous dielectric medium, (2) the global interaction energy of the solute-solvent system, (3) the maximum electrostatic potential on the hydrogen atom of the solvent molecule, and (4) the maximum condensed nucleophilic Fukui function (or nucleophilic condensed local softness) of the solvent molecule, those which considered both the nonspecific and specific effects of solute-solvent interactions, can be incorporated in a multiparameter equation for constructing the present DFT-based LSER.

Comparison of synthetic and soil al-substituted lepidocrocite

The nature of Fe-oxides in the magnetic clay fractions of the Bt and Bs horizons of alpine forest soils in subtropical and tropical areas is not well defined. The objective of this study was to synthesize lepidocrocite incorporating Al in the laboratory and to compare its properties with soil lepidocrocite. Lepidocrocite and goethite were identified by differential X-ray diffraction and Mossbauer spectroscopy at 77 K in the magnetic clay fractions of the Bsm horizons and exhibited results similar to those of the products from a synthetic Fe(II) system with an Al/Fe molar ratio of 0.03. Several individual soil lepidocrocite particles can be identified in the magnetic clay fractions using high resolution transmission electron microscopy and selected area electron diffraction with the same (010) orientation as that of laboratory synthetic lepidocrocite. Weight percent of Al and Fe in the synthetic and soil lepidocrocite particles was determined by energy dispersive spectroscopy. The Al/Fe molar ratios in the soil lepidocrocite particles ranged from 0.005 to 0.095. This indicates the participation of Al(III) in the formation of lepidocrocite under acidic, udic, and mesic forest soil environments.

Frontier-molecular-orbital correlations for the acidity constants in aqueous metal ions

Abstract By employing density functional theory (DFT) and coupled cluster (CCSD(T)) calculations, the energy difference between the highest occupied molecular orbital (HOMO) energy of water molecule and the lowest unoccupied molecular orbital (LUMO) energy of metal cation is proven to serve as a good indicator for predicting the experimental acidity constant (p K a ) of metal cations in aqueous solutions. Mono-, di- and tri-valent metal cations comprising main group and transition group elements are investigated. This frontier-molecular-orbital linear correlation is useful to rationalize deprotonation trends of aqueous metal ions without having to perform time-consuming calculations and is useful to clarify the interdependence of various aqueous reactions such as the hydration and deprotonation reactions of metal cations.

The H2CO potential energy surface: advanced ab initio and density functional theory study

Abstract The ground state potential energy surface of formaldehyde (H 2 CO) was investigated with a variety of Gaussian based ab initio methods (G2, CBS-Q, CBS-QB3, G3, G3B3) as well as the B3LYP density functional theory with small (6-31G(d), as implemented in the G3B3 method) to the very large 6-311++G(3df, 3pd) basis set. In this work, we have considered the decomposition of H 2 CO into H 2 +CO (with an average theoretical to experimental error of around 1.5%), as well as the isomerization barriers, activation barriers and heats of reaction.

Novel descriptors based on density functional theory for predicting divalent metal ions adsorbed onto silica—disiloxane cluster model study

Abstract The optimized structures and equilibrium energies of metal aquo-complexes binding with disiloxane ((SiH3)2O*) cluster model on silica surface are studied using the local (Vosko–Wilk–Nusair) and nonlocal (Becke–Perdew–Wang) density functional theory methods. The divalent Mg, Ca, Sr, Ba, Zn, Cd and Hg metal ions are investigated. The results show that the structures of silica surface cluster and adsorbed metal aquo-ion have obvious changes after the adsorptive reaction carried out, especially for the case of inner-sphere adsorption. The atomization energy of metal aquo-ion/disiloxane cluster binding complexes (AEo; AEi) and the inverse of metal-oxide bond length of metal aquo-ions (RMO−1) can be used to supply two novel descriptors for accurately predicting the stability constant ( log K int ,j,k ) of the surface complexation model for divalent metal ions adsorbed on silica in aqueous solutions. However, if the commonly used parameters such as the calculated energy changes (ΔEo; ΔEi) in the binding reactions of disiloxane cluster model+aqueous metal ion→aqueous metal ion/disiloxane cluster binding complexes are employed to succeed the role of previous atomization energies as the predicting descriptors, it is found that the liner regression results are inferior to the previous results predicted by using the atomization energies.

A unified model for predicting the mononuclear first- to fifth-order and the polynuclear hydrolysis constants of metal cations

Abstract This contribution introduces a quite simple method to establish a universal equation for quantitatively estimating the cationic hydrolysis tendency in water. The unified model is constructed on the basis of the general from of the hydrolysis reaction of metal cations: m Me Z + + n H 2 O→Me m (OH) n ( mZ − n )+ + n H + . According to this general form, the metal cation (Me Z + ) and water molecule (H 2 O) are properly regarded as the main reactants of a hydrolysis reaction of metal cations. The universal predicting equation for the thermodynamic hydrolysis constants ( log K mn ) can then be obtained as: log K mn =Ω R − S ×(n/m)×(m IE +n BE )−(m+n) log 55.51+(− Δ G° Extra /2.303RT)(n+1)/2. The parameter IE is the total ionization energy of metal atoms. The parameter BE can be calculated from the energy change of the following reaction: oxygen atom+2 hydrogen atom→water molecule. The enthalpy for stabilizing the hydrolysis reactants in an aquatic system is defined as Ω R − S ×(n/m)×(m IE +n BE ). Here Ω R − S is the reactant-stabilizing enthalpy coefficient, which equals the inverse of (2.303 RT ×dielectric constant of water). The entropy contribution for stabilizing the hydrolysis reactants in an aquatic system can be represented by the total molar numbers of the hydrolysis reactants, ( m + n ), multiplied with − log 55.51. The remaining free energy term, (−Δ G ° Extra /2.303 RT ), accounts for the extraneous effects arisen from adjacent solvent molecules interacting with the metal cation. This unified model finds out a physically meaningful coefficient (i.e. reactant-stabilizing enthalpy coefficient, Ω R − S ) and proposes a novel free energy partioning procedure to make the universal prediction for cationic hydrolysis constants become possible.

Global relationship between the acidity constants and structural stability in hexaaqua metal complexes

Abstract The structural stability related to the first deprotonation constants of six-coordinated hydrated metallic cations has been rationalized by the density functional approach. Hexaaqua complexes of bivalent Mg, Ca, Mn, Ni, Zn, Cd, and trivalent Al, Ga, In, Sc, Cr, Fe cations were examined. The result is a global linear relationship with regression coefficient r 2 = 0.9417 between the pK a values and binding energies in these hexaaqua metal complexes.