C.J. Milne

Metal ion binding to humic substances: application of the non-ideal competitive adsorption model.

The application of a new model to describe metal ion binding by humic acids is discussed. Metal ion binding is always of a competitive nature since the proton is always present. Although of great practical importance, the combination of a chemically heterogeneous system with competitive binding poses difficult problems from both experimental and theoretical points of view. The new Non-Ideal Competitive Adsorption model (NICA model) used here is able to account for the non-ideal binding to heterogeneous ligands. A good description of the binding of H, Ca, Cd, and Cu to a purified peat humic acid is achieved over a wide range of free metal ion concentrations (-2 > log Me 2+ > -14) and pH (2 < pH < 10). The results show that binding of metal ions to humic acid is strongly influenced by the intrinsic chemical heterogeneity of the humic material itself as well as by ion-specific non-ideality. The results indicate that copper competes much more efficiently with protons bound to the phenolic type groups than calcium and cadmium.

Relating Ion Binding by Fulvic and Humic Acids to Chemical Composition and Molecular Size. 2. Metal Binding

Binding of Cu(II) and Pb(II) to a soil fulvic acid, humic acid, and two different size fractions of the humic acid was investigated with metal titration experiments at pH 4, 6, and 8. Proton and free metal ion activities in solution were monitored after each titration step using pH and ion selective electrodes (ISE), respectively. The amounts of base required to maintain constant pH conditions were recorded and used to calculate stoichiometric proton-to-metal ion exchange ratios. Despite clear differences in chemical composition and protonation behavior, the fulvic acid and all humic acid fractions exhibited very similar metal binding behavior. Binding of Cu(II) and Pb(II) generally increased with increasing pH and total metal concentration. At low to moderate metal ion concentrations, Cu(II) was bound more strongly to the humic substances than Pb(II). Only at high free metal concentrations, the amounts of metal ions sorbed were higher for Pb(II) than for Cu(II). The molar proton-to-metal ion exchange ratios ranged from 1.0 to 1.8 for Cu(II) and from 0.6 to 1.2 for Pb(II), suggesting that Cu(II) was bound as monodentate and bidentate complexes, while Pb(II) was bound predominantly as monodentate complexes. The metal ion binding data were quantitatively described with the consistent NICA-Donnan model. The best description of an entire multicomponent data set consisting of proton titration, Cu(II), and Pb(II) binding data was achieved when the entire data set was fitted simultaneously. To reduce the number of fitting parameters, results from size exclusion chromatography and solid state 13C NMR spectroscopy were used to estimate two of the NICA-Donnan model parameters. The values of the remaining NICA-Donnan parameters for the humic substances are within a narrow range, suggesting that generalized model parameters may be useful in geochemical modeling involving humic substances.

Metal ion binding by natural organic matter: from the model to the field.

With the newly developed NICCA-Donnan model, we estimate the activity of toxic metal ions from simple measurements like total metal concentration and organic matter content. The model evaluates Cu and Cd binding from three field systems, a mountain lake and two sandy soils, using model parameters calibrated for natural organic matter analogues with laboratory measurements. This is possible because the model includes site binding heterogeneity, electrostatic effects, competitive binding, and ion specific nonideality. The predictions derived closely matched the field observations when site binding densities are adjusted. The partition coefficients between soil and soil solution were also predicted for Cd and Cu under conditions where the organic matter controls the metal binding in both soil and soil solution. The model calculations show that in soil solutions 50% of the Cd and 99.99% of the Cu is bound to the dissolved organic matter. The model can be used to evaluate the effects of variations in the chemical conditions (e.g., acidification or total metal loading) on the free metal ion concentration in solution.

Analysis of proton binding by a peat humic acid using a simple electrostatic model

Detailed potentiometric titration data were collected for a purified peat humic acid (PPHA) over a range of pH (pH 3.5–10.5) and KNO3 background electrolyte concentrations (0.001–0.3 M). The data were analyzed following the master curve approach which includes both an electrostatic double layer model and a model for the intrinsic heterogeneity of the PPHA. Spherical and cylindrical double layer models gave equally good fits to the data. A salt dependence observed around pH 5 could not be completely removed by taking into account the electrostatic interactions. Hysteresis was observed to a much greater extent in the first titration cycle compared with the second cycle. This suggested that some slow and only partly reversible aggregation was occurring possibly as a result of the aggregation created during the purification of the humic acid. Titration curves for fully redispersed samples fitted the master curve approach (surface charge vs. surface pH) reasonably well but still displayed an ionic strength dependence at a pH of less than 5 which could not be accounted for using the simple electrostatic model. Heterogeneity analysis of the master curve showed that the affinity distribution had two peaks centred at log KHint ∼ 4 and log KHint ∼ 8 to 9. The total number of weak acid sites titrated between pH 3.5 and 10.5 was approximately 3.5 eq kg−1 but the total number of sites estimated from the isotherm analysis was 5.3–5.8 eq kgt1¯. Double Toth and double Langmuir-Freundlich isotherms fitted the data almost equally well but the implied distribution of sites between the more acidic “car☐ylic” sites and the weakly acidic “phenolic” sites varied with the isotherm chosen. An important source of uncertainty in the analysis was in estimating the charge on the humic acid at its initial pH of about pH 3.

The scale and causes of the groundwater arsenic problem in Bangladesh

Groundwater is now extensively used for drinking water in Bangladesh and present estimates indicate that there are some 6–11 million tubewells in Bangladesh. It is now apparent that approximately 1/4 of these wells contain arsenic at concentrations exceeding the Bangladesh drinking water standard (50 μg L −1 ). As many as 35 million people may be drinking arsenic-affected groundwater. We discuss a national survey of groundwater quality in Bangladesh that attempted to map the distribution and nature of affected wells. Other solutes measured included Na, K, Ca, Mg, Fe, Mn, P and SO4. The worst-affected part of Bangladesh lies in the south-east of the country where the sediments are of Holocene age and where concentrations of arsenic frequently exceeded 200 μg L −1 . Where sampled, deep groundwaters (>150 m) were only rarely affected as were shallower groundwaters from older sediments including the aquifers underlying the Barind and Madhupur Tracts. Seven groundwater samples from the capital city of Dhaka also suggest that the city is not affected. The arsenic is undoubtedly of natural origin and the problem arises even though the sediments do not contain abnormal quantities of total arsenic. There is no evidence to suggest that the dissolved arsenic is derived from the oxidation of pyrite as some have suggested. Rather it appears that the high concentrations reflect a combination of factors: (i) young sediments undergoing rapid change from an oxidizing to a reducing environment following sedimentburial; (ii) the release of arsenic by one or more mechanisms which are poorly understood at present but which probably involve the desorption and dissolution of arsenic from iron oxides which are quite abundant in many of the worst-affected sediments; (iii) the very low hydraulic gradients throughout much of Bangladesh mean that groundwater flow is very slow which, combined with the ‘young’ age of many of the sediments, means that the natural flushing of the shallow aquifer will be slow allowing any released arsenic to accumulate. The rapid rate of deposition of sediments in Bangladesh and the Bengal Basin means that the chance of a well intercepting arsenic-rich water is likely to be relatively high compared with smaller deltas and other alluvial environments where the sedimentation rate is much lower.

Assessing urinary flow rate, creatinine, osmolality and other hydration adjustment methods for urinary biomonitoring using NHANES arsenic, iodine, lead and cadmium data

BackgroundThere are numerous methods for adjusting measured concentrations of urinary biomarkers for hydration variation. Few studies use objective criteria to quantify the relative performance of these methods. Our aim was to compare the performance of existing methods for adjusting urinary biomarkers for hydration variation.MethodsCreatinine, osmolality, excretion rate (ER), bodyweight adjusted ER (ERBW) and empirical analyte-specific urinary flow rate (UFR) adjustment methods on spot urinary concentrations of lead (Pb), cadmium (Cd), non-arsenobetaine arsenic (AsIMM) and iodine (I) from the US National Health and Nutrition Examination Survey (NHANES) (2009–2010 and 2011–2012) were evaluated. The data were divided into a training dataset (n = 1,723) from which empirical adjustment coefficients were derived and a testing dataset (n = 428) on which quantification of the performance of the adjustment methods was done by calculating, primarily, the correlation of the adjusted parameter with UFR, with lower correlations indicating better performance and, secondarily, the correlation of the adjusted parameters with blood analyte concentrations (Pb and Cd), with higher correlations indicating better performance.ResultsOverall performance across analytes was better for Osmolality and UFR based methods. Excretion rate and ERBW consistently performed worse, often no better than unadjusted concentrations.ConclusionsOsmolality adjustment of urinary biomonitoring data provides for more robust adjustment than either creatinine based or ER or ERBW methods, the latter two of which tend to overcompensate for UFR. Modified UFR methods perform significantly better than all but osmolality in removing hydration variation, but depend on the accuracy of UFR calculations. Hydration adjustment performance is analyte-specific and further research is needed to establish a robust and consistent framework.