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The absorption of humic acids on iron oxide-coated silicon plates were shown to be dependent on pH, with a decrease in pH increasing humic acid absorption. Desorption was achieved either through a change in pH with no corresponding change in the concentration of the humic acid or upon dilution with no corresponding change in pH. Desorption using the change in pH was found to be reversible, with additional increases in pH accelerating the desorption process.

A model for ion binding to humic substances was developed through the combination of the non-ideal competitive adsorption (NICA) model for specific binding of electrolyte ions with a Donnan-type model for nonspecific binding. The resulting NICA-Donnan model was applied to data for H, Ca, Cd, Cu and Pb binding by a purified peat humic acid. Predictions of the competitive effect of Ca on Cd and Cu binding at various pHs were consistent with experimental data, except for some small differences. The increase in Cd and Cu binding on changing from a 0.1 M KNO3 background electrolyte to 0.01 M KNO3 was also predicted well by the NICA-Donnan model.

The non-ideal competitive adsorption model was developed to describe the competitive binding of metal ions by humic acids in a chemically heterogeneous system. It was used to describe the binding of hydrogen, calcium, copper and cadmium ions on a purified peat humic acid. The results showed that the model fits the data well over a wide range of free metal ion concentrations and pH range, indicating that the binding of metal ions to humic acids is a function of the effects of ion-specific non-ideality and the chemical heterogeneity of humic material.

Different semianalytical methods to determine the affinity distribution functions may be used to derive f(log K) from experimental binding data only if the error problem has been accounted for. The condensation approximation method is ideal for poor data, while the logarithmic symmetrical approximation with the second-order affinity spectrum method is useful for high-quality data. A smoothing spline technique is suitable in determining the derivatives of a binding curve essential to describe the distribution function. The generalized cross-validation technique combined with physical constraints is recommended to unarbitrarily establish the smoothing parameter.

The local decay function approximation-G2 (LODA-G2) for the determination of rate constant distribution functions was used to re-analyze copper-humate dissociation data on humic material from various sampling points in the Ogeechee River, GA. A smoothing spline procedure was applied to the data to explain the experimental error. The appropriate smoothing parameter was taken based on the generalized cross-validation technique adapted to the problem.

Derivatives of the overall adsorption isotherm are subjected to binding heterogeneity analysis to determine the affinity distribution function. Ionic strength due to electrostatic effects determined the apparent affinity distribution. These electrostatic effects were described by a double layer rigid sphere model. The resulting affinity distribution is described by a pair of semi-Gaussian peaks.

Local isotherm approximation (LIA) and differential equilibrium function (DEF) can be used to calculate the affinity of ions to bonding with heterogeneous ligands. LIA methods use estimates of local binding functions to compute the distribution function. This results in a flattened distribution curve. The DEF method uses the law of mass action and produces distribution curves that are asymmetrically distorted.

Binding of lead (Pb) to soil fulvic acid (JGFA), soil humic acids (JGHA, JLHA), and lignite-based humic acid (PAHA) was investigated through binding isotherms and XAFS. Pb binding to humic substances (HS) increased with increasing pH and decreasing ionic strength. The NICADonnan model described Pb binding to the HS satisfactorily. The comparison of the model parameters showed substantial differences in median Pb affinity constants among JGFA, PAHA, and the soil HAs. Milne's "generic" parameters did not provide an adequate prediction for the soil samples. The Pb binding prediction with generic parameters for the soil HAs was improved significantly by using the value nPb1 = 0.92 instead of the generic value nPb1 = 0.60. The nPb1/nH1 ratios obtained were relatively high, indicating monodentate Pb binding to the carboxylic-type groups. The nPb2/nH2 ratios depended somewhat on the method of optimization, but the values were distinctly lower than the nPb1/nH1 ratios, especially when the optimization was based on Pb bound vs log [Pb2+]. These low values indicate bidentate binding to the phenolic-type groups at high Pb concentration. The NICA-Donnan model does not consider bidentate binding of Pb to a carboxylic- and a phenolic-type group. The EXAFS results at high Pb loading testified that Pb was bound in bidentate complexes of one carboxylic and one phenolic group (salicylate-type) or two phenolic groups (catechol-type) in ortho position. [ABSTRACT FROM AUTHOR]

Interactions of purified Aldrich humic acid (PAHA) with the protein lysozyme (LSZ) are studied with dynamic light scattering and isothermal titration calorimetry by mixing LSZ and PAHA at various mass ratios. In solution LSZ is positive and PAHA is negative at the investigated pH values. Up to moderate KCl concentrations no aggregation occurs for LSZ and for PAHA aggregated particles with an average radius of 80 nm are present Complexation of PAHA with LSZ starts as soon as PAHA is added to LSZ and is followed by aggregation when the iso-electric-point (IEP) of the complexes is approached. Aggregation. is gradual for 50mM KCl and sudden for low KCl concentrations. The aggregate size is at its maximum at the IEP of the complexes. At mass ratios beyond the IEP the aggregates partially disaggregate. Positively charged complexes of PAHA and LSZ, formed in the absence of salt, strongly aggregate upon salt addition. Mixing of LSZ and PAHA is initially enthalpically driven. Near the IEP complexation and aggregation are due to hydrophobic forces (structural reorganization) and counterion release. The observations are relevant for other HA-protein systems when the protein is positively charged. [ABSTRACT FROM AUTHOR]

Binding of heavy metal and actinide ions to natural colloids, such as humic substances (HSs) and metal (hydr)- oxides, plays an important role in the ecotoxicological behavior of these ions. Several thermodynamic models have been constructed to predict the speciation of these ions in metal/HS or metal/oxide binary systems. However, in natural environments the adsorption of HSs on oxides can influence the binding of target metals, leading to deviation from the additivity of calibrated binary models. In this study binding of copper (Cu2+) to the purified Aldrich humic acid (PAHA)/goethite complex in the neutral to acidic pH region was investigated by measuring Cu2+ binding isotherms. The measured isotherms were compared with the results obtained for the binary systems under similar conditions. The comparison revealed that Cu2+ binding in the ternary system is enhanced with respect to the sum of Cu2+ binding in the corresponding binary systems. From the analysis of the charging behavior of the adsorbed PAHA as well as the smeared-out potential profile near the PAHA/goethite interface, the increase of Cu2+ binding to the complex was mainly attributed to the decrease of proton competition to the functional groups of the adsorbed PAHA and the change of the electrostatic potential in the vicinity of the goethite surface. [ABSTRACT FROM AUTHOR]