John Hamilton-Taylor

Al(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation

Published experimental data for Al(III) and Fe(III) binding by fulvic and humic acids can be explained approximately by the Humic Ion-Binding Model VI. The model is based on conventional equilibrium reactions involving protons, metal aquo ions and their first hydrolysis products, and binding sites ranging from abundant ones of low affinity, to rare ones of high affinity, common to all metals. The model can also account for laboratory competition data involving Al(III), Fe(III) and trace elements, supporting the assumption of common binding sites. Field speciation data (116 examples) for Al in acid-to-neutral waters can be accounted for, assuming that 60–70 % (depending upon competition by iron, and the chosen fulvic acid : humic acid ratio) of the dissolved organic carbon (DOC) is due to humic substances, the rest being considered inert with respect to ion binding. After adjustment of the model parameter characterizing binding affinity within acceptable limits, and with the assumption of equilibrium with a relatively soluble form of Fe(OH)3, the model can simulate the results of studies of two freshwater samples, in which concentrations of organically complexed Fe were estimated by kinetic analysis. The model was used to examine the pH dependence of Al and Fe binding by dissolved organic matter (DOM) in freshwaters, by simulating the titration with Ca(OH)2 of an initially acid solution, in equilibrium with solid-phase Al(OH)3 and Fe(OH)3. For the conditions considered, Al, which is present at higher free concentrations than Fe(III), competes significantly for the binding of Fe(III), whereas Fe(III) has little effect on Al binding. The principal form of Al simulated to be bound at low pH is Al3+, AlOH2+ being dominant at pH >6; the principal bound form of Fe(III) is FeOH2+ at all pH values in the range 4–9. Simulations suggest that, in freshwaters, both Al and Fe(III) compete significantly with trace metals (Cu, Zn) for binding by natural organic matter over a wide pH range (4–9). The competition effects are especially strong for a high-affinity trace metal such as Cu, present at low total concentrations (1 nM). As a result of these competition effects, high-affinity sites in humic matter may be less important for trace metal binding in the field than they are in laboratory systems involving humic matter that has been treated to remove associated metals.

Characterizing Colloidal Material in Natural Waters

Sampling has inherent uncertainties when applied tothe sub-micron fraction of natural waters.Processes of aggregation, biological activity andchemical transformation potentially effect changesimmediately after a sample is taken from the waterbody and there is no suitable method of samplestabilization. The size distribution and associatedphysical and chemical parameters of colloids in theaquatic environment can be effectively stable overshort periods of time (about two days under idealconditions, but frequently much shorter timeintervals). To achieve accurate representations of thesize distribution and associated colloidalcharacteristics in situ techniques are required,although adequate approximations may be obtained undersome circumstances if separation is done immediatelyafter sampling. This paper reviews the currentlyavailable strategies for separation and analysis ofcolloids from natural waters (primarily filtration andcentrifugation) and discusses their uses andlimitations, as well as potential uses of promisingtechniques (voltammetry, gels, field-flowfractionation, SPLITT). For small colloids, thetechniques of voltammetry, dialysis, DET and DGT maybe used to obtain in situ information. Forlarger colloids it is more difficult to performmeasurements in situ and a combination of rapidfractionation procedures, including filtration,field-flow fractionation and SPLITT, may still berequired.

Trace metal sorption by natural particles and coarse colloids

The effects of size and geochemical properties on the binding of trace metals to natural colloids and particles have been investigated. Suspended particulate matter (SPM) from the River Mersey in NW England was fractionated by centrifugation to give three size fractions (nominally 0.05–0.5 μm, 0.5–1.0 μm and >1.0 μm). The SPM was characterized by scanning electron microscopy and by carbon and nitrogen analysis. Large proportions of the particles were microbial in origin, dominated by diatoms in the largest size fraction and bacteria in all fractions. Acid-base titrations indicated a significant difference between the proton binding characteristics of the three samples. The smallest fraction had the greatest charge per unit mass whereas the largest fraction had the least charge: 2.0 and 1.0 meq g−1 charge developed between pH 4 and 10, respectively. Experimental sorption studies with Cd and Cu indicated that metal binding per unit mass of SPM varied little between the three size fractions, although Cd was more strongly bound to the two smallest fractions. A simple one-site binding model provided a good description of the data and showed that the observed Cd and Cu sorption constants were consistent with literature values. The findings indicate that metal binding to the three size fractions is controlled mainly by the mass concentration and pH. The dependence on mass suggests that the surface area effective for binding is substantially independent of the size class. The results question the importance of the role played by the sub-micron fraction in trace metal binding by natural particle assemblages.

PCB and PAH fluxes to a dated UK peat core

Concentrations of PCBs and PAHs have been determined from the individual sections of peat cores obtained from an ombrotrophic bog in rural north-west England. Chronological intervals throughout the core were determined from both radiometric (210Pb, 137Cs, 241Am) and independent, non-chemical characteristics (pollen, magnetics) information. Net fluxes of 25 individual PCB congeners and 14 PAH compounds to the bog were then derived. PCB inputs were apparent from the late-1930s/ early-1940s, with maximum sub-surface fluxes ( 1300 pg cm−2 year−1) observed at a depth corresponding to 1964. Loadings decreased by 65% over the following 15 year period before showing a surface enrichment. Initial increases in PAH loadings appear to coincide with the beginning of the Industrial Revolution, with fluxes peaking in the early-1930s (305 ng cm−2 year−1). Introduction of emission controls and the decline of heavy industry has led to an 80 % reduction in the net flux of PAHs to the bog over the last three to four decades. Potential effects of postdepositional diagenesis are considered, with particular reference to alteration of contaminant chronologies.

Polychlorinated biphenyl and polycyclic aromatic hydrocarbon deposition to and exchange at the air–water interface of Esthwaite Water, a small lake in Cumbria, UK

Atmospheric concentrations and deposition fluxes of several polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) were measured concurrently over a 1-year period at a shore-based site at Esthwaite Water, to estimate their loadings from the atmosphere to the lake water surface. The PAH deposition fluxes (μg m−2 month−1) to conventional upturned Frisbees varied from 0.19 (anthracene) to 7.8 (phenanthrene), with a ΣPAH deposition of 33.5. Phenanthrene, fluoranthene and pyrene contributed >50% of the ΣPAH deposited. The deposition fluxes of PCBs ranged from 0.03 (octa-PCBs) to 0.38 (tri-PCBs) μg m−2 month−1 with a ΣPCB flux estimated at 0.87 μg m−2 month−1. The coefficients of variation of deposition varied from 75% (octa-PCBs) to 120% (tri-PCBs) and from 90 to 150% for individual PAHs. Temporal variability in fluxes to the collector was large, suggesting highly variable atmospheric concentrations and scavenging processes. Atmospheric PAH concentrations were negatively correlated with temperature over the sampling period, suggesting that the source function—rather than temperature-dependent, air–surface equilibrium partitioning—primarily controlled air concentrations. Deposition fluxes of PAHs correlated well with rainfall between January and September, suggesting that particle washout was the main factor controlling deposition over this period. The controlling factor for deposition between October and December was related to the increased source function of the compounds to the atmosphere. Regression analysis of the data for PAHs with ≥4 rings gave a strong positive correlation between the deposition flux and atmospheric concentrations, suggesting that the transfer of compounds occurred with a similar efficiency. Simultaneous air and water sampling over an annual cycle was used to calculate fugacity quotients for individual PAHs and PCBs. These calculations showed that PCBs were outgassing from the lake water throughout the year (i.e. volatilisation>deposition) whilst PAH transfers varied seasonally with net deposition in winter months when there is no ice cover, and net volatilisation at all other times.

Freshwater DOM quantity and quality from a two-component model of UV absorbance

We present a model that considers UV-absorbing dissolved organic matter (DOM) to consist of two components (A and B), each with a distinct and constant spectrum. Component A absorbs UV light strongly, and is therefore presumed to possess aromatic chromophores and hydrophobic character, whereas B absorbs weakly and can be assumed hydrophilic. We parameterised the model with dissolved organic carbon concentrations [DOC] and corresponding UV spectra for c. 1700 filtered surface water samples from North America and the United Kingdom, by optimising extinction coefficients for A and B, together with a small constant concentration of non-absorbing DOM (0.80 mg DOCL⁻¹). Good unbiased predictions of [DOC] from absorbance data at 270 and 350 nm were obtained (r² = 0.98), the sum of squared residuals in [DOC] being reduced by 66% compared to a regression model fitted to absorbance at 270 nm alone. The parameterised model can use measured optical absorbance values at any pair of suitable wavelengths to calculate both [DOC] and the relative amounts of A and B in a water sample, i.e. measures of quantity and quality. Blind prediction of [DOC] was satisfactory for 9 of 11 independent data sets (181 of 213 individual samples).

Quantification of natural DOM from UV absorption at two wavelengths.

Environmental context. Dissolved organic matter (DOM) is part of the global carbon cycle, ecologically and geochemically active, and costly to remove in water treatment. Spectroscopic monitoring at a single wavelength provides some indication of DOM concentration, but variations in optical properties mean that accurate determinations currently rely on slow and costly laboratory methods. We show that for water samples containing non-anthropogenic DOM, ultraviolet absorbance at two wavelengths can quantify DOM rapidly, cheaply and accurately, and also indicate its quality. Abstract. The precise simulation of ultraviolet absorption by 23 contrasting surface-water DOM samples was achieved with a model based on two components, one absorbing light strongly (A) and the other weakly (B). The parameterised model can be used to predict (DOC) in water samples simply from absorbance values at two wavelengths, while information on DOM quality is provided by the calculated fractionation into A and B. The model was tested by predicting (DOC) for a separate dataset obtained by combining results for 12 samples each from surface waters in the UK, Canada and the USA, and from UK groundwaters. A close correlation (R 2 = 0.997) was obtained, with only slight underestimation of the true (DOC). The proportions of components A and B varied considerably among the sites, which explains why precise prediction of (DOC) from absorbance data at a single wavelength was not possible. When the model was applied to samples collected from river locations in a heterogeneous UK catchment with areas of industry and high human population, (DOC) was underestimated in many cases, which may indicate the presence of non-absorbing pollutant DOM. Additional keywords: dissolved organic carbon, dissolved organic matter, two-component model, UV spectra.

Methods for preparing synthetic freshwaters.

Synthetic solutions that emulate the major ion compositions of natural waters are useful in experiments aimed at understanding biogeochemical processes. Standard recipes exist for preparing synthetic analogues of seawater, with its relatively constant composition, but, due to the diversity of freshwaters, a range of compositions and recipes is required. Generic protocols are developed for preparing synthetic freshwaters of any desired composition. The major problems encountered in preparing hard and soft waters include dissolving sparingly soluble calcium carbonate, ensuring that the ionic components of each concentrated stock solution cannot form an insoluble salt and dealing with the supersaturation of calcium carbonate in many hard waters. For acidic waters the poor solubility of aluminium salts requires attention. These problems are overcome by preparing concentrated stock solutions according to carefully designed reaction paths that were tested using a combination of experiment and equilibrium modeling. These stock solutions must then be added in a prescribed order to prepare a final solution that is brought into equilibrium with the atmosphere. The example calculations for preparing hard, soft and acidic freshwater surrogates with major ion compositions the same as published analyses, are presented in a generalized fashion that should allow preparation of any synthetic freshwater according to its known analysis.

Europium binding by fulvic acids.

The binding of Eu to fulvic acid (FA) as a function of pH was determined by Schuberts method and by equilibrium dialysis. Data obtained by both methods showed a strong pH dependence, which was reproduced well by the model used to interpret the data (humic ion-binding model VI). The model was also able to account for five other data sets from the literature, and there was reasonable agreement among the values of the characteristic binding parameter for Eu–FA interactions. Data sets investigating the effects of aluminum competition were also explained well by Model VI. The model was used to calculate the speciation of Eu in natural waters as a function of the most-likely competitive solution species (protons, aluminium and calcium). The calculations indicate that under environmentally realistic concentrations both, aluminium and calcium can be significant competitors at low and high pH, respectively, although FA binding appears to dominate Eu speciation under most conditions.

The transport of Chernobyl-derived radiocaesium through two freshwater lakes in Cumbria, UK

The specific activities of 137Cs and 134Cs in the waters and sediments of Windermere North Basin (WNB) and Esthwaite Water (EW) in the English Lake District were determined over an 18-month period, immediately following the catastrophic accident at the Chernobyl nuclear reactor. Input of Chernobyl-derived Cs to the lake surfaces (∼2000 Bq of 137Cs m−2) occurred predominantly through direct atmospheric deposition over the period 3–20 May, 1986. The initial and highest specific activities (Co) of 137Cs were ∼80 Bq m−3 and ∼390 Bq m−3 in WNB and EW, respectively, reflecting the greater mean depth of WNB (25.1 m) compared with EW (6.4 m). Of the initial input, an estimated 37–41% and 32% was hydraulically flushed from WNB and EW, respectively, the remainder accumulating in the sediments. Retention half-life within the lake waters was 70 days in WNB and 15 days in EW. The temporal decline in caesium in surface waters could be modelled by assuming that either direct adsorption to the sediments or association with settling particles occurred in conjunction with hydraulic flushing. Model fits resulted in values of 0.10 ± 0.05 cm for the boundary layer thickness, and 105 litre kg−1 for the partition coefficients. Kd, in both lakes, indicating that transport by particles may be the dominant process. A small fraction (0.25%) of the caesium accumulating in the sediment appeared to be remobilized into the overlying waters when they became anoxic. This was the major source of radiocaesium in EW after April 1987 (ca. 1 year after the initial input).