Determination of metal ion binding parameters for humic substances

Effect of dissolved humic acid on the Pb bioavailability in soil solution and its consequence on ecological risk

Current risk characterization in ecological risk assessment does not consider bioavailability of heavy metals, which highly depends on physicochemical properties of environmental media. This study was set to investigate the effect of humic acid (HA), used as a surrogate of organic matter, on Pb toxicity and the subsequent effect on risk characterization in ecological risk assessment. Pb toxicity was assessed using Microtox(®) in the presence and absence of two different forms of HA, particulate HA (pHA) and dissolved HA (dHA). With increasing contact time, the EC10 values increased (i.e., the toxic effects decreased) and the dissolved Pb concentrations of the filtrates decreased. The high correlation (R = 0.88, p < 0.001) between toxic effects determined using both the mixture and its filtrate as exposure media leads us to conclude that the Pb toxicity highly depends on the soluble fraction. Also, reduced Pb toxicity with increasing dHA concentrations, probably due to formation of Pb-dHA complexes, indicated that Pb toxicity largely comes from free Pb ions. Overall, this study shows the effect of HA on metal toxicity alleviation, and emphasizes the need for incorporating the bioavailable heavy metal concentrations in environmental media as a point of exposure in ecological risk assessment.

Chemodynamics of soft nanoparticulate complexes: Cu(II) and Ni(II) complexes with fulvic acids and aquatic humic acids

The dynamics of metal complexation by small humic substances (fulvic acid and aquatic humic acid, collectively denoted as “fulvic-like substance”, FS) are explored within the framework of concepts recently developed for soft nanoparticulate complexants. From a comprehensive collection of published equilibrium and dissociation rate constants for CuFS and NiFS complexes, the association rate constant, ka, is determined as a function of the degree of complexing site occupation, θ. From this large data set, it is shown for the first time that ka is independent of θ. This result has important consequences for finding the nature of the rate limiting step in the association process. The influence of electric effects on the rate of the association process is described, namely (i) the accelerating effect of the negatively charged electrostatic field of FS on the diffusion of metal ions toward it, and (ii) the extent to which metal ions electrostatically accumulate in the counterionic atmosphere of FS. These processes are discussed qualitatively in relation to the derived values of ka. For slowly dehydrating metal ions such as Ni(H2O)6 2+ (dehydration rate constant, kw), ka is expected to derive straight from kw. In contrast, for rapidly dehydrating metal ions such as Cu(H2O)6 2+, transport limitations and electric effects involved in the formation of the precursor outer-sphere associate appear to be important overall rate-limiting factors. This is of great significance for understanding the chemodynamics of humic complexes in the sense that inner-sphere complex formation would not always be the (sole) rate limiting step.

Influence of bicarbonate and humic acid on effects of chronic waterborne lead exposure to the fathead minnow (Pimephales promelas)

Historically, the USEPA has only considered water hardness when establishing acute and chronic water quality criteria (WQC) for lead (Pb) in freshwater. Yet, recent evidence suggests that hardness may not be protective during chronic Pb exposure and that other factors (e.g., dissolved organic carbon (DOC) and alkalinity) influence toxicity. In fact, we have recently shown that Ca(2+) (as CaSO(4)) does not protect against Pb accumulation in fathead minnows (Pimephales promelas) during chronic exposures whereas DOC as humic acid (HA) clearly does. To more clearly define the water chemistry parameters mediating chronic Pb toxicity we carried out 300 d exposures to study the influence of DOC and alkalinity on Pb accumulation and toxicity to fathead minnows at 2 different Pb concentrations (170 and 580 nM (35 and 120 microg/L)). Alkalinity was adjusted by addition of 500 microM NaHCO(3) and DOC by addition of 4 mg/L HA. Fish were collected at 4, 30, 150 and 300 d of exposure to measure growth and Pb accumulation. Breeding assays (21 d) were performed at the end of these exposures to assess reproductive and larval behavioral endpoints. To determine whether effects were acute or chronic, switched breeding exposures were performed in which control breeders were transferred to either high or low Pb conditions and Pb-exposed breeders transferred to tap water without Pb. Mortality and growth effects were observed primarily in the high Pb treatments and within the first 10 d of exposure. Strong protection against Pb accumulation was afforded by increased DOC at both Pb concentrations. Increased alkalinity also appeared to moderately reduce Pb accumulation although not to the level of statistical significance. Tissue distribution of Pb was analyzed at 300 d and was found to accumulate mostly in bone, gill, intestine and kidney. Unexpectedly, high Pb reduced total reproductive output and increased average egg mass in the HCO(3)(-) and DOC treatments but not in the control water (+Pb) treatments. No statistically significant differences in egg hatchability or egg Pb accumulation were observed. Results from switched exposures suggest that embryo Pb accumulation arose from acute exposure to embryos rather than parental transfer. Finally, prey capture assays revealed potential Pb-induced motor/behavioral impairment in 10-d-old F1 larvae exposed to high Pb in all water treatments.

In situ speciation measurements of trace metals in headwater streams

Concentrations of Al, Fe, Mn, Ni, Cu, Cd, Pb, and Zn were measured using DGT (diffusive gradients in thin-films) devices deployed in situ in 34 headwater streams in Northern England. Mean values of filtered samples analyzed by ICP-MS (inductively coupled plasma mass spectrometry) were used, along with DOC (dissolved organic carbon), pH and major ions, to calculate the distribution of metal species using the speciation code WHAM. DGT-measured concentrations, [Me]DGT, of Zn and Cd were generally similar to concentrations in filtered samples, [Me]filt. For the other metals, [Me]DGT was similar to or lower than [Me]filt. Calculation of the maximum dynamic metal from the speciation predicted using WHAM showed that most of the lower values of [Cu]DGT could be attributed to the dominance of Cu-fulvic acid complexes, which diffuse more slowly than simple inorganic species. Similar calculations for Al, Pb, and Mn were consistent with appreciable proportions of these metals being present as colloids that are not simple complexes with humic substances. Differences between WHAM predictions and the measured [Ni]DGT indicated that WHAM used with the default binding parameters underestimates Ni binding to natural organic matter. Plots of [Me]DGT versus the ratio of bound metal to DOC provided slight evidence of heterogeneous binding of Pb and Cu, while results for Mn, Cd, and Zn were consistent with weak binding and complete lability.

Copper and other trace elements strongly bound to humic acids along sediment cores in the Ross Sea, Antarctica

Marine sediment cores sampled in three different areas, during the 1997/98 Italian Antarctic Expedition, were studied. The cores (40 cm) were immediately subdivided into 13 or 14 layers (2-4 cm) in order to obtain a high-depth resolution in the metal content and humic substance analyses. The results obtained for the amount of metals strongly bound to humic acids showed the following order: Cu >> Zn >> Co > As >or= Mn. The fraction of Cu, Zn and As bound to humic acid (microg g(-1) of HA) increases in the first 10-12 cm of cores, then a decrease is observed. This observation could be related both to the presence of a mobile fraction of metals in sediments successively bound to humic acids and to different structural features showed by humic acids along the cores, due to transformation processes. For the other metals a variable pattern along the cores was found.

The role of organic matter on metal toxicity and bio-availability

A short review concerning the role of organic matter on metal toxicity and bio-availability in aqueous systems is carried out. The complexity of the issue derives both from the high number of natural and anthropogenic organic compounds and from the variability of their structures. In fact, the binding capacity and affinity is dependent on the number and type of ligands, on their position in the structures, on the ligand/metal ratio. It is also necessary to develop analytical protocol in order to carry out speciation studies of organic carbon and of metals bound to organic compounds, and at the same time to characterise the nature of the complexes.

The effect of pH, ion strength and reactant content on the complexation of Cu2+ by various natural organic ligands from water and soil in Hong Kong

The complexation constants for copper associated with different natural organic ligands, including dissolved organic carbon (DOC) extracted from water, water soluble organic carbon (WSOC), fulvic acid (FA) and humic acid (HA) from soil, were determined and then compared based on discrete single site model. Both ion-selective electrode (ISE) and anodic stripping voltammetry were used to determine the content of free copper ions, while the relative number of complexation sites was estimated using a fluorescence quenching (FQ) at the same time. ISE proved to be the most applicable technique when the concentration of copper was above 10(-7) moll(-1). The logk values for two WSOC sample sites (Song Ziyuan and Xin Niangtan) were 4.64 and 4.66; higher than both the DOC and HA values yet lower than the FA values, which were unusually high due to unavoidable pollution from the cation exchange resin used during the purification process. Binding affinities between the copper ions and the organic ligands obtained from streams in Yong Wei (DOC) and Song Ziyuan (WSOC) were influenced by pH, ion strength, and reactant concentration. Values for logk increased with increases in pH (ion strength of 0.1 N). At pH 4, the logk values decreased with increases in the supporting electrolyte concentration and total copper ion additions.

Role of fulvic acid on lead bioaccumulation by Chlorella kesslerii

To better understand the relationship between lead speciation and bioavailability in natural freshwaters, the interaction of lead with the freshwater alga Chlorella kesslerii was studied in the presence of the Suwannee River fulvic acid (SRFA). Special attention was paid to direct interactions of the fulvic acid on the algae, as well as potential physiological (membrane permeability and algal metabolism) influences. Lead-free ion concentration measurements were carried out using a novel ion-selective electrode. Pb uptake decreased in the presence of SRFA with respect to noncomplexed Pb, but uptake fluxes, cellular Pb, Pb bound to the transport sites, and total adsorbed Pb were all higher than predicted from Pb2+ activities, in accordance with the free ion activity model (FIAM). The discrepancies between the observed values and those predicted by the FIAM in the presence and absence of synthetic ligands increased with increasing concentration of SRFA. Several hypotheses were examined to explain the observed differences. No contributions of labile and/or hydrophobic Pb-SRFA complexes were found. Furthermore, direct biological effects, including variations in membrane permeability or algal metabolism, could not account for the observations. On the other hand, changes in the algal surface charge due to SRFA adsorption seemed to account, at least partially, for the observed increase in lead uptake in the presence of SRFA as compared to that corresponding to the same Pb2+ concentration in the presence of synthetic ligands.

Implications of natural organic matter binding heterogeneity on understanding lead(II) complexation in aquatic systems

A critical analysis of data published for complexation (conditional stability constants and complexation capacities) of lead(II) by natural organic matter (NOM) is presented. The same patterns are observed for the different types of NOM, i.e. isolated humic-type substances, isolated algae- and bacteria derived ligands and cell walls, and filtered whole water systems: apparent stronger binding sites are utilised at lower metal ion loadings, progressively weaker sites contribute to metal complexation at higher loadings. Continuous binding functions should thus be used to adequately describe lead-NOM complexation in natural waters. Two important observations arising from our analysis are that: (i) the binding curves (binding affinity as a function of metal ion loading) for whole natural water samples lie between those for compounds representative of the two main NOM types, viz., isolated humic compounds and biota, and (ii) binding constants commonly used in speciation codes probably underestimate the actual extent of lead(II) binding by NOM at natural concentration levels.

Voltammetric analysis of heterogeneity in metal ion binding by humics

The complexation of Cd, Pb, and Cu by fulvic acids at a fixed pH and ionic strength is studied by means of different voltammetric techniques at any metal-to-ligand ratio. When using Reverse Pulse Polarography (RPP) the complex species are electrochemically labile and not subject to significant electrodic adsorption. RPP titrations of fulvic acid with metal ions are interpreted on the basis of a recently proposed analytical expression for limiting currents valid for fully labile heterogeneous complexation. The voltammetric data are transformed into the corresponding binding curve, i.e., the fraction of occupied sites vs free metal concentration. Finally, the competition between metal ions and protons in their interaction with the fulvic binding sites as well as the concomitant polyelectrolytic effects are analyzed in terms of the NICCA-Donnan model. The results show that voltammetric techniques can be applied to the studies of heterogeneous complex systems in a broad range of metal-to-ligand ratios.