Solution thermochemistry of humic substances—I. Acid-base equilibria of humic acid

The technique of titration calorimetry has been used to characterize the acidic functional groups of humic acid. Both the concentrations of acidic functional groups and the thermodynamic parameters (pK_a, ΔH_a, etc.) for ionization of these functional groups were determined. The calculated values confirm the generally accepted opinion that humic acid contains both carboxyl and phenolic hydroxyl groups. Furthermore, at least one-third of the phenolic hydroxyl groups are not ortho to carboxyl groups and thus cannot participate in chelation of metal ions via salicylate-like complexing sites. The nature of the remaining two-thirds of the phenolic hydroxyl groups is still unknown.     

Computational methods for fitting statistical distribution models of multi-site binding equilibria

Statistical distribution models of multi-site binding equilibria have potential applicability in the study of acid-base and metal complexation chemistry of humic substances in soils, sediments, and natural waters. A mathematical derivation is presented for the general continuous model for the case of proton binding; computational methods are described for fitting numerically the parameters in such models. Among models considered are those based on nontruncated, truncated, and bimodal (mixed) distributions. Specific emphasis is placed on Gaussian distribution models.     

Importance of heavy metal-organic matter interactions in natural waters

The role of organic ligands in metal complexing in natural waters has received little attention because of uncertainties regarding both the abundance and nature of dissolved organic carbon compounds. Recent data show that the bulk of dissolved organic matter in natural waters consists of highly oxidized and chemically and biologically stable polymeric compounds closely resembling soil humic substances. Average molar concentrations of these aquatic humics in major U.S. rivers range from 5 × 10^?6to 3 × 10^?5 moles 1^?1. Fractional elution of soil organic matter by meteoric waters may be considered to be the main process contributing to the presence of humic matter in rivers. The aquatic humic polymers participate in complex formation through ionizable functional groups with a range of differential acidities. The stabilities of metal-humic complexes in natural waters are higher than those of the corresponding inorganic metal complexes. Quantitative evaluation of the metal-organic interactions can be approached by applying variable equilibrium functions which take into account the differential physico-chemical characteristics of the active complexing sites on the polymer molecule. Assuming an average humic concentration of 10 mg 1^?1, complexation of trace metals can be significant even in the presence of excess concentrations of major cations.     

Calculation of molecular weights of humic substances from colligative data: Application to aquatic humus and its molecular size fractions

A rigorous mathematical expression for the dependence of colligative properties on acid dissociation of water soluble humic substances is presented. New data for number average molecular weights of a river derived humic material and its gel permeation Chromatographic fractions are compared with $\text{M}\text{?}{}_{\mathrm{n}}$ values obtained by a reevaluation of previously published experimental observations on soil and water fulvic acids. The results reveal a remarkable similarity of fulvic acids from widely different sources with respect to number-average molecular weight.     

Chemical and spectroscopic characterization of marine dissolved organic matter isolated using coupled reverse osmosis-electrodialysis

The coupled reverse osmosis-electrodialysis (RO/ED) method was used to isolate dissolved organic matter (DOM) from 16 seawater samples. The average yield of organic carbon was 75 ± 12%, which is consistently greater than the yields of organic carbon that have been commonly achieved using XAD resins, C₁₈ adsorbents, and cross-flow ultrafiltration. UV-visible absorbance spectra and molar C/N ratios of isolated samples were consistent with the corresponding properties of DOM in the original seawater samples, indicating that DOM samples can be isolated using the coupled RO/ED method without any bias for/against these two properties. Five of the samples were desalted sufficiently that reliable measurements of their ¹³C and ¹HNMR spectra and their Fourier transform ion cyclotron resonance (FTICR) mass spectra could be obtained. The ¹³C and ¹HNMR spectra of RO/ED samples differed distinctly from those of samples that have been isolated in much lower yields by other methods. In particular, RO/ED samples contained a relatively lower proportion of carbohydrate carbon and a relatively greater proportion of alkyl carbon than samples that have been isolated using cross-flow ultrafiltration. From the FTICR mass spectra of RO/ED samples, samples from the open ocean contained a much lower proportion of unsaturated compounds and a much higher proportion of fatty acids than coastal samples.     

The operational nature of acidic functional group analyses and its impact on mathematical descriptions of acid-base equilibria in humic substances

Calorimetric and potentiometric titrations have been used to evaluate direct and indirect methods of analysis for the carboxyl content of aquatic humic substances. The effects of cation binding, base strength, and removal of humic reaction products on carboxyl content values have been examined. The results suggest that the most appropriate analytical method for determination of an operationally defined carboxyl content would utilize a weak base that does not contain a polyvalent cation in an indirect titration on a reaction mixture from which all humic reaction products have been removed prior to titration. Titration data have been analyzed in terms of a formation function (δn_OH) and in terms of operationally defined ‘mass action quotients’ ($\text{K}\text{?}$). The dependence of $\text{K}\text{?}$ on pH is more readily and realistically described if no artificial differentiation of carboxyl groups into two or more sub-groups is imposed upon the data.     

A statistical model of proton binding by humus

A model that considers both the carboxyl and weakly acidic groups of humus as continuous binding site distributions in which individual ligand concentrations are normally distributed with respect to the log $\text{K}$ values for proton binding is proposed. The concentration, mean log $\text{K}$ value, and variance for the log $\text{K}$ distribution of each class of functional groups are estimated by nonlinear regression analysis of titration data. The values obtained for those parameters are chemically reasonable and the extent of proton binding between pH 4.0 and pH 10.8 is accurately described by the model.     

Faecal indicators and pathogens in selected New Zealand waterfowl

Freshly excreted droppings from Canada geese (n=80), black swans (n=80), ducks (n=80) and gulls (n=80) were collected from sites around New Zealand. The droppings were enumerated for Escherichia coli, enterococci and Salmonella spp., and for the presence/absence of Cryptosporidium spp. Overall prevalence of E. coli and enterococci in samples was 95% and 94%, respectively. Cryptosporidium spp. was detected in 2% of the samples, whereas no Salmonella spp. were detected in the survey. Preliminary estimates of daily microbial outputs suggest that ducks will produce the highest loadings of E. coli and enterococci per bird, whereas Canada geese will produce the highest loadings of Campylobacter spp. per bird. This study provides the first set of indicator and pathogen counts for one of the largest sources of diffuse faecal contamination of natural waters in New Zealand.     

The application of Cd NMR spectrometry to the study of cadmium complexation by natural organic matter

The results of a comprehensive ¹¹³Cd NMR study of complexation of Cd²⁺ by Suwannee River natural organic matter (NOM) are presented and compared with the more familiar behavior of simple systems containing Cd²⁺ and a single ligand (e.g., ethylenediamine). pH-dependent trends in the chemical speciation of Cd(II), chemical shift of ¹¹³Cd, and line width of resonance peaks lead to the conclusion that the contribution of N donor atoms to the primary coordination sphere of Cd²⁺ increases steadily with increasing pH, so much so that relatively strong downfield resonances can be observed at alkaline pH. Those observations also indicate that there is no accessible, unique combination of magnetic field strength and solution chemistry for which the exchange rates of all forms of Cd are simultaneously fast, intermediate, or slow.     

Analytical constraints on the structural features of humic substances

A rigorous calculation of the maximum fraction of aromatic carbon in humus or any other complex mixture of organic compounds is described. Input parameters include elemental composition, carboxyl content, carbonyl content, and an estimate of number-average molecular weight. The calculated upper limit constrains structural models that are based on interpretations of ¹³C-NMR spectra or on the products of degradation reactions and also directly limits the phenolic content of a sample.Another computational method is described that yields good estimates of the actual levels of aromatic (and aliphatic) carbon in a humus sample. Even though the method is capable of estimating only the most probable level of aromatic carbon, the predicted values are surprisingly close to the experimental values that have been determined by ¹³C-NMR spectroscopy.