Interpretation of the differential UV-visible absorbance spectra of metal-NOM complexes based on the quantum chemical simulations for the model compound esculetin.

In this study, the model compound esculetin that has functional groups typical for natural organic matter (NOM) was used to ascertain the nature of the characteristic bands in the differential UV-visible absorbance spectra (DAS) associated with the formation of metal-NOM complexes. The binding of ten different metal ions (Cu(II), Ni(II), Co(II), Fe(III), Cr(III), Al(III), Zn(II), Ca(II), Mg(II) and Pb(II)) with esculetin generate four bands in the DAS. These bands are similar to those present in the DAS of metal-NOM complexes. The UV-visible absorbance spectra of the metal-esculetin systems were calculated using time-dependent density functional theory (TD-DFT). The TD-DFT results demonstrate that the prominent features of the DAS of esculetin are primarily associated with the electron transitions between the molecular orbitals near the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the metal-esculetin complex. Charge decomposition analysis (CDA) results demonstrated that these electron transitions originate from the esculetin fragment to the Zn(II) fragment in the complex. Covalent indexes [(χm)2rc] of the metal ions were found to be correlated with the metal-specific features of the DAS of metal-esculetin systems. The strength of the linear correlations between the quantitative parameters of the electron density of the bond critical points (BCP) is indicative of the strength of the metal-esculetin interactions.

Quantum Chemical Examination of the Sequential Halogen Incorporation Scheme for the Modeling of Speciation of I/Br/Cl-Containing Trihalomethanes.

The recently developed three-step ternary halogenation model interprets the incorporation of chlorine, bromine, and iodine ions into natural organic matter (NOM) and formation of iodine-, bromine-, and chlorine-containing trihalomethanes (THMs) based on the competition of iodine, bromine, and chlorine species at each node of the halogenation sequence. This competition is accounted for using the dimensionless ratios (denoted as γ) of kinetic rates of reactions of the initial attack sites or halogenated intermediates with chlorine, bromine, and iodine ions. However, correlations between the model predictions made and mechanistic aspects of the incorporation of halogen species need to be ascertained in more detail. In this study, quantum chemistry calculations were first used to probe the formation mechanism of 10 species of Cl-/Br-/I- THMs. The HOMO energy (EHOMO) of each mono-, bi-, or trihalomethanes were calculated by B3LYP method in Gaussian 09 software. Linear correlations were found to exist between the logarithms of experimentally determined kinetic preference coefficients γ reported in prior research and, on the other hand, differences of EHOMO values between brominated/iodinated and chlorinated halomethanes. One notable exception from this trend was that observed for the incorporation of iodine into mono- and di-iodinated intermediates. These observations confirm the three-step halogen incorporation sequence and the factor γ in the statistical model. The combined use of quantum chemistry calculations and the ternary sequential halogenation model provides a new insight into the microscopic nature of NOM-halogen interactions and the trends seen in the behavior of γ factors incorporated in the THM speciation models.

Optical property of dissolved organic matters (DOMs) and its link to the presence of metal ions in surface freshwaters in China.

It is believed that dissolved organic matter (DOM) plays an important role in carrying metal ions through global cycle and distribution, while this point has not yet been elucidated clearly as DOM is extremely heterogeneous and site-specific. In this study, optical properties of DOM in 53 surface freshwater sites all over China were recorded by UV-Vis absorbance spectroscopy in pH range 3-11. The subtle processes of protonation-deprotonation of DOM were quantified by tracking the changes of DOM spectra. The binding capacities of DOMs-binding sites and affinity constants-were interpreted by introducing a spectral parameter, differential log-transformed differential absorbance at 400 nm (DlnA400) in combination with Non-Ideal Competitive Adsorption (NICA) Model. It is found that the presence of dissolved Zn, Hg and Ni in the examined waters show strong correlation with total binding sites in DOMs. However, the presence of some other metals with high affinities to DOM, e.g. Cu, Fe and so on, have not demonstrated strong correlation with the total binding sites in DOMs. This indicates the presence of these metals may be seriously influenced by other factors besides DOM. This study demonstrates that the spectroscopic titration approach could potentially provide more structure-specific in situ information about DOM and help to understand the role of DOM in the speciation and bioavailability of toxic metal ions.

Investigating the features in differential absorbance spectra of NOM associated with metal ion binding: A comparison of experimental data and TD-DFT calculations for model compounds.

In this study, seven model compounds containing typical functional groups (phenolic and carboxylic groups) present in nature organic matter (NOM) were used to ascertain the nature of the characteristic bands in differential absorbance spectra (DAS) of NOM that are induced by metal ion binding. Some similarities were found between the DAS of the examined model compounds, caffeic acid, ferulic acid, sinapic acid, terephthalic acid, isophthalic acid, esculetin and myricetin and those of NOM. The binding of Cu(II) with carboxylic group might produce two peaks, A1 and A2, while the binding of Cu(II) with phenolic group might produce all four Gaussian peaks, from A1 to A4 displayed in the DAS of NOM. The UV-visible spectra predicted using time-dependent density functional theory (TD-DFT)-based methods met well with the experimental DAS of model compounds at different stages of Cu(II) binding. It demonstrates that the features in absorbance spectra are chiefly caused by HOMO (Highest Occupied Molecular Orbital) - LUMO (Lowest Unoccupied Molecular Orbital) transitions in the molecule and that the appearance of peaks in DAS reflects the changes of the molecular orbitals around reactive functional groups in a molecule before and after metal ion binding. The basis of the DAS features of NOM that are induced by metal ion binding could be identified primarily by the frontier molecular orbital theory.

Behaviour of I/Br/Cl-THMs and their projected toxicities under simulated cooking conditions: Effects of heating, table salt and residual chlorine.

This study examined the effects of heating, residual chlorine and concentration of table salt on the generation of iodine-, bromine- and chlorine-containing trihalomethanes (THMs) under simulated cooking conditions. In the case of addition of either KI- or KIO3-fortified salt, total I-THM concentrations increased with increasing iodine concentration, while total Cl/Br-THM concentrations decreased. CHCl2I, CHBrClI, CHBrI2, CHBr2I and CHI3 were formed in the presence of KI salt, while only CHCl2I was formed in the presence of KIO3 salt. CHCl2I was unstable under cooking conditions, and >90% of this DBP was removed during heating, which in some cases increased the concentrations of the other I-THMs. The calculated cytotoxicity increased with addition of KI- or KIO3-fortified salt due to the generation of I-THMs, whose impact on the cytotoxicity at room temperature was equal to or five times higher, respectively, than the cytotoxicity of the simultaneously formed Cl/Br-THMs for the cases of salts. Heating decreased the cytotoxicity, except for the case of addition of KI salt, in which the calculated cytotoxicity of I-THMs increased above 150% as the temperature was increased up to 100°C. The reported results may have important implications for epidemiologic exposure assessments and, ultimately, for public health protection.