Geoffrey E. Millward

Modelling metal desorption kinetics in estuaries.

The kinetics of desorption of Cd, Cu, Ni and Zn from riverine and estuarine sediments have been determined in deionised water and seawater (salinity=30). The time-dependent release of Cd, Cu and Ni from riverine sediments in both media showed an increase in dissolved metal concentrations within a few minutes, followed by a plateau after several hours of incubation. Desorption of Zn from riverine sediments in seawater was characterised by a maximum dissolved concentration after several minutes, followed by readsorption onto the suspended sediment. The extent of metal desorption from estuarine sediment (relative to a chemical digest of available particulate metal), in both deionised water and seawater, was less than that from the fluvial sediments, even though the available particulate metal concentrations were higher. The desorption kinetics of the metals were modelled using a mechanism consisting of reversible, first-order consecutive reactions. The dissolved metals were assumed to desorb into the bulk solution from exchangeable surface sites and from binding sites located within the particle matrix. A reasonable fit to the observed time-dependent desorption behaviour was simulated by integrating the differential equations resulting from the mechanism, assuming that the exchangeable particulate metal was defined by the chemical digest. The results are discussed within the context of improved understanding of the non-conservative behaviour of dissolved metals in estuaries.

Resuspension, reactivity and recycling of trace metals in the Mersey Estuary, UK

Distributions of dissolved and non-detrital (available to 1 M HCl) particulate Cd, Co, Cu, Ni, Pb and Zn have been determined in samples collected from the Mersey Estuary, UK, during four high-resolution axial transects. With the exception of Cu, dissolved metal concentrations displayed positive deviations from conservative behaviour. Cobalt and Ni appeared to have a common source, while dissolved Pb peaks coincided with maximum turbidities in the upper estuary. Concentrations of Cd, Co, Ni and Pb in the <63-μm grain size fraction of bed sediment were greater than those in suspended particulate matter (SPM). Magnitudes of dissolved metal peaks reflected the extent of metal enrichment in fine bed sediment compared with SPM, suggesting that addition of dissolved metal was effected primarily by desorption from resuspendable bed particles. An empirical sorption model, based on the mass balance of metal resuspended from the bed, was able to reproduce the axial distributions of dissolved metals that exhibited the most pronounced peaks (namely, Co and Pb) with reasonable success. More generally, model results indicated that bed sediment need only be moderately enriched in trace metal compared with SPM to cause measurable addition of dissolved metal to the overlying water column. Addition of dissolved metals on estuarine mixing is a common observation in macrotidal environments of western Europe, and it is proposed that resuspension of metal-enriched, fine bed sediment may be a significant agency for metal remobilisation in contaminated, turbid estuaries.

Surface areas and porosities of iron(III)- and iron(II)-derived oxyhydroxides.

This paper reports the surface areas and pore characteristics of iron oxyhydroxide precipitates derived from ferric and ferrous iron by using conditions very similar to those encountered in natural waters. Precipitates derived from Fe(II1) had surface areas in the range 159-234 m2 g- with hysteresis loops which suggested pores of the narrow-necked, wide-bodied type. The identification experiments involving X-ray diffraction, infrared spectroscopy, and Mossbauer spectroscopy showed that the precipitates were amorphous in the initial phases but on aging for 12 days they contained 10-20% a-FeOOH. Precipitates derived from Fe(I1) had surface areas in the range 97-120 m2 g-l, and they were identified as poorly crystalline yFeOOH. The natural samples from iron-rich sources showed surface areas in the range 6.4-164 m2 g- with both porous and nonporous characteristics. The results of this study are of relevance to the adsorption of dissolved trace constituents by iron oxyhydroxides in natural waters.

Determination of arsenic species in fish, crustacean and sediment samples from Thailand using high performance liquid chromatography (HPLC) coupled with inductively coupled plasma mass spectrometry (ICP-MS)

Suitable techniques have been developed for the extraction of arsenic species in a variety of biological and environmental samples from the Pak Pa-Nang Estuary and catchment, located in Southern Thailand, and for their determination using HPLC directly coupled with ICP-MS. The estuary catchment comprises a tin mining area and inhabitants of the region can suffer from various stages of arsenic poisoning. The important arsenic species, AsB, DMA, MMA, and inorganic arsenic (As III and V) have been determined in fish and crustacean samples to provide toxicological information on those fauna which contribute to the local diet. A Hamilton PRP-X100 anion-exchange HPLC system employing a step elution has been used successfully to achieve separation of the arsenic species. A nitric acid microwave digestion procedure, followed by carrier gas nitrogen addition- (N2)-ICP-MS analysis was used to measure total arsenic in sample digests and extracts. The arsenic speciation of the biological samples was preserved using a Trypsin enzymatic extraction procedure. Extraction efficiencies were high, with values of 82-102%(As) for fish and crustacean samples. Validation for these procedures was carried out using certified reference materials. Fish and crustacean samples from the Pak Pa-Nang Estuary showed a range for total arsenic concentration, up to 17 microg g(-1) dry mass. The major species of arsenic in all fauna samples taken was AsB, together with smaller quantities of DMA and, more importantly, inorganic As. For sediment samples, arsenic species were determined following phosphoric acid (1 M H3PO4) extraction in an open focused microwave system. A phosphate-based eluant, pH 6-7.5, with anion exchange HPLC coupled with ICP-MS was used for separation and detection of AsIII, AsV, MMA and DMA. The optimum conditions, identified using an estuarine sediment reference material (LGC), were achieved using 45 W power and a 20 minute heating period for extraction of 0.5 g sediment. The stability and recovery of arsenic species under the extraction conditions were also determined by a spiking procedure which included the estuarine sediment reference material. The results show good stability for all species after extraction with a variability of less than 10%. Total concentrations of arsenic in the sediments from the Pak Pa-Nang river catchment and the estuary covered the ranges 7-269 microg g(-1)and 4-20 [micro sign]g g(-1)(dry weight), respectively. AsV was the major species found in all the sediment samples with smaller quantities of AsIII. The presence of the more toxic inorganic forms of arsenic in both sediments and biota samples has implications for human health, particularly as they are readily available.

Kinetics of phosphate adsorption by iron oxyhydroxides in aqueous systems

Abstract The rate of uptake of phosphate onto synthetic Fe(III)- and Fe(II)-derived oxyhydroxides has been studied using reaction conditions similar to those encountered in natural waters. Kinetic analyses were carried out on the adsorption profiles and both first-order and second-order conditional rate constants were obtained. The temperature dependence of some of the rate constants was investigated and corresponding apparent activation energies calculated. Similar experiments and analyses were undertaken using Fe from natural sources and in general the conditional rate constants obtained in seawater were in agreement with the synthetic ones. The results of this study are of value when comparing the time scales of adsorption processes in natural waters with the time scales of mixing and advection.

Shipboard flow injection determination of sea water pH with spectrophotometric detection

Abstract The use of a flow injection system with spectrophotometric detection for the measurement of sea water pH is described. The acid-base absorption properties of phenol red injected into a sea water stream are measured. The performance of the technique in a shipboard environment is evaluated and shown to have a working precision of ±0.005 pH units with a sampling frequency of ca. 25 h−1. The internal consistency of carbon dioxide measurements throughout an expedition to the Southern Ocean is estimated to be commensurate with precisions of ±0.005 pH unit, ±2 μatm pCO2 and ± 1 μmol kg−1 TCO2. This level of precision, combined with the simplicity and low indicator consumption rate of the pH technique and the fact that no standard is needed for calibration make the method ideally suited for in situ pH measurement in marine waters.

Partitioning of mercury onto suspended sedimentsin estuaries

Radiochemical partitioning experiments using 203Hg have been undertaken with mixtures of river, seawater and sediment samples taken from three geochemically contrasting UK estuaries: the Plym, Beaulieu and Mersey. Species of dissolved Hg were determined using reversed-phase C18 chelating columns and particulate species were determined by sequential leaching with 1 M NH4OAc and 1 M HCl. Mercury had a high particle reactivity with partition coefficients, KDs, ranging from 10(4) to 5 x 10(5) ml g(-1), depending on salinity, the chemical composition of the end-member waters, and on the physico-chemical characteristics of the sediment. Dissolved organic matter present in the waters (humic substances and/or anthropogenic compounds) was found to be the main factor governing the forms of dissolved Hg and their reactivity. From the spiked 203Hg, up to 95% of the dissolved metal was retained on the C18 columns for the Mersey waters, whereas this fraction was < 60% in the Plym and Beaulieu waters. Quasi-irreversible adsorption of Hg onto particles from each estuary was observed over a time-scale of a few hours and < 20% of total particulate Hg was released by the sequential leach. In this paper, physico-chemical processes are proposed to explain the estuarine behaviour of Hg and the results are discussed in terms of Hg availability in estuarine systems.

Tissue-specific incorporation and genotoxicity of different forms of tritium in the marine mussel, Mytilus edulis.

Marine mussels (Mytilus edulis) were exposed to seawater spiked with tritiated water (HTO) at a dose rate of 122 and 79xa0μGyxa0h(-1) for 7 and 14 days, respectively, and tritiated glycine (T-Gly) at a dose rate of 4.9xa0μGyxa0h(-1) over 7 days. This was followed by depuration in clean seawater for 21 days. Tissues (foot, gills, digestive gland, mantle, adductor muscle and byssus) and DNA extracts from tissues were analysed for their tritium activity concentrations. All tissues demonstrated bio-accumulation of tritium from HTO and T-Gly. Tritium from T-Gly showed increased incorporation into DNA compared to HTO. About 90% of the initial activity from HTO was depurated within one day, whereas T-Gly was depurated relatively slowly, indicating that tritium may be bound with different affinities in tissues. Both forms of tritium caused a significant induction of micronuclei in the haemocytes of mussels. Our findings identify significant differential impacts on Mytilus edulis of the two chemical forms of tritium and emphasise the need for a separate classification and control of releases of tritiated compounds, to adequately protect the marine ecosystem.

Distribution of tritium in estuarine waters: the role of organic matter

Tritium is an important environmental radionuclide whose reactivity with ligands and solids in aquatic systems is assumed to be limited. We studied the fractionation and sorption of tritium (added as tritiated water) in river water and seawater, and found that its distribution appears to be influenced by its affinity for organic matter. Tritium rapidly equilibrates with dissolved organic ligands that are retained by a reverse-phase C18 column, and with suspended sediment particles. Significantly, a measurable fraction of sorbed tritium associates with proteinaceous material that is potentially available to sediment-feeding organisms. These characteristics have not been reported previously and cannot be accounted for solely by isotopic exchange with hydrogen. Nevertheless, they are in qualitative agreement with available measurements of tritium in estuarine and coastal waters where its principal discharge is as tritiated water. Further research into the estuarine biogeochemical behaviour of tritium is required and radiological distribution coefficients and concentration factors that are assumed for this radionuclide may require reconsideration.

Tissue-specific assimilation, depuration and toxicity of nickel in Mytilus edulis

The tissue-specific accumulation and time-dependent depuration of radioactive (63)Ni by the byssus, gut, foot, gills, kidney, adductor muscle and faeces of Mytilus edulis has been investigated using a pulse-chase technique. The rate and extent of depuration of (63)Ni varied between tissues and, after 168 h, the concentration factors and assimilation efficiencies ranged from 1 to 35 L kg(-1) and 5%-13%, respectively. Mussels were also exposed to a range of environmentally-realistic concentrations of dissolved Ni, prior to the analysis of biological endpoints. The clearance rate was concentration-dependent and at the highest concentration decreased by 30%. Neutral red retention (NRR) assays indicated a cytotoxic response and DNA strand breaks were observed in the haemocytes. The association of DNA damage with that of physiological and cytotoxic effects suggests that Ni exerts a significant impact on Mytilus edulis at cellular and genetic levels.