Duncan P.H. Laxen

Influence of blood lead on the ability and attainment of children in Edinburgh.

The effect of blood-lead on childrens ability and attainment was investigated in a sample of 855 boys and girls aged 6-9 years from eighteen primary schools within a defined area of central Edinburgh. The geometric mean blood-lead value was 10.4 micrograms/dl. In a stratified subsample, 501 children completed individual tests of cognitive ability and educational attainment from the British Ability Scales (BAS). An extensive home interview with a parent was also done. Multiple regression analyses showed a significant negative relation between log blood-lead and BAS combined score, number skills, and word reading when thirty-three possible confounding variables were taken into account. There was a dose-response relation between blood-lead and test scores, with no evidence of a threshold. The size of the effect was small compared with that of other factors. Lead at low levels of exposure probably has a small harmful effect on the performance of children in ability and attainment tests.

Trace metal adsorption/coprecipitation on hydrous ferric oxide under realistic conditions

Abstract Adsorption of Cd, Cu and Ni (and possibly Pb) onto hydrous ferric oxide is significantly modified in the presence of humic substances. Employing realistic concentrations in a synthetic freshwater, it is found that adsorption/coprecipitation of Cd and Ni (and less certainly Pb) is enhanced in the presence of humics. Cu adsorption may also be enhanced, but any effect is masked by a strong competitive complexation of Cu with soluble humics. The pH dependency of the adsorption process is modified in a manner consistent with “ligand like” metal adsorption. The data are considered in terms of four models which might account for the enhanced adsorption. The most plausible one involves a complexation of metals with adsorbed humics, which is stronger than that with soluble humics. A series of experiments using natural waters of widely differing composition demonstrates the generality of the observations made with synthetic freshwaters. Cu adsorption is dominated by the competitive formation of soluble Cu-humic complexes. It is also found to be independent of alkalinity, indicating that neither Ca/Mg nor HCO3/CO3 ions influence the adsorption or complexation reactions. Cd adsorption, on the other hand, is essentially independent of the humic concentration, but dependent upon alkalinity. As alkalinity increases so Cd adsorption decreases, a phenomenon probably due to competition from Ca/Mg ions. These findings have important implications for the development of realistic speciation models.

Size distribution of iron and manganese species in freshwaters

Abstract A recently established technique for size fractionation of particulate matter in freshwaters, based on low volume filtration through Nuclepore filters in the field, has been applied to the study of Fe and Mn species in 3 freshwater lakes and a stream feeding one of the lakes. The technique has also been used in a series of laboratory experiments to provide further insight into the process of particle formation. The results are complemented with scanning electron microscope examination of the particles. The raw data are transformed into mass size distributions, which are generally unimodal, with the major part of the mass confined to a single log unit size range. These size distributions are compared and contrasted with a theoretical model of particle behaviour in a lake. The results suggest that particulate Fe and Mn behave according to particle theory. The observed size distributions represent a balance between hydraulic input/output, aggregation, and gravitational settling, as well as chemical precipitation and dissolution processes. Overall particle aggregation is shown to be a slow process in the waters examined, with a time constant of the order of days. There is evidence for a fairly stable particle regime in the 0.05–0.4 μm size range. Deviations from the unimodal distribution in the epilimnion of the lakes indicate the association of Fe, but not Mn, with phytoplankton. In the waters examined Mn was most frequently found in solution, while Fe was predominantly particulate. These findings are interpreted in terms of the differing redox behaviour of the metals.

Manganese chemistry in rivers and streams

The speciation of Mn has been determined in 15 rivers and streams representing a wide variety of physico-chemical conditions. Using the technique of anodic stripping voltammetry (asv), specific for reduced Mn(II) species, it is found that a major part of the <0.015 μm Mn size fraction is present in a reduced Mn(II), asv-labile, form. In some waters there is also a significant asv inactive Mn fraction considered to be present as a ‘small colloidal’ species. The soluble (<0.015 μm) Mn fraction represents 15–95% of total Mn and does not appear to be dependent upon pH, alkalinity, specific conductance or humic substance concentration in the water. It is argued that under the dynamic, short residence time, conditions that apply in most rivers the paniculate and soluble Mn fractions are decoupled, their respective presence being dependent principally upon the catchment hydrogeological conditions. This contrasts with a previously held view that the paniculate phase is coupled to the dissolved phase by the pH dependent oxidation of dissolved Mn(II) to highly insoluble Mn(IV) species (Graham et al., 1976). Consideration of manganese speciation in waters which were incubated for five months showed that pH becomes the controlling factor when equilibrium is approached.

Children's blood lead and exposure to lead in household dust and water: a basis for an environmental standard for lead in dust

Good quantitative evidence on the role of lead in household dust as a source of exposure to children has been lacking. A study of 495 children in Edinburgh, Scotland shows a significant relationship between lead in dust vacuumed from the floors of the childrens homes and their blood lead levels. A multiple regression analysis incorporating drinking water and household dust estimates that a 1,000 micrograms g-1 increase in dust lead concentration would increase blood lead by 1.9 micrograms dl-1, for a child with the median population blood lead of 10.1 micrograms dl-1. Dust lead concentration is a more useful predictor of blood lead than lead loading. The sanding or blow-lamp stripping of old paint is found to be an important source of the higher household dust lead concentrations. Finally, the dust lead-blood lead relationship is used to derive a standard for lead in house dust, as no such standard exists for this exposure route.

Adsorption (co‐precipitation) of trace metals at natural concentrations on hydrous ferric oxide in lake water samples

Abstract Adsorption (co‐precipitation) of Ca, Cd, Cu, Ni, Pb on hydrous ferric oxide (HFO) has been investigated under natural conditions. Ferrous iron (ca 6.8 × 10−5 M) present in samples of anoxic lake water was oxidised under controlled conditions to form insoluble HFO. Adsorption of metals at their natural concentrations (10−9‐10−8M; Ca 4 × 10−4M) was determined in the pH range pH 6.5 ‐ 8.5. A large fraction of the Cd (>80%), Cu (ca 50%) and Pb (ca 60%), but not Ni (< 20–30%) and Ca (ca 1%), was adsorbed.

Cadmium adsorption in freshwaters — a quantitative appraisal of the literature

Abstract Information from 33 studies of cadmium adsorption on amorphous and crystalline hydrous iron oxides, clays, silica, humic solids, manganese oxides and natural sediments has been collated. The results have been transferred to a unified data base using a surface complexion model, and are presented in terms of conditional adsorption constants as a function of pH. The adsorption constants for a given substrate at a fixed pH generally agree within an order of magnitude. This is reasonable given the wide range of experimental conditions employed. The affinity of the substrates (on a weight basis) for cadmium appears to follow the order Mn > Fe am > chlorite (montmorillonite?) > Fe cryst. = illite = humics > kaolinite > silica. Application of the experimental conditional adsorption constants for the individual substrates to a model suspended sediment predicts adsorption levels similar to those found in experiments with real sediments. The results confirm the potentially important role played by hydrous oxides of iron and manganese. They also emphasise that cadmium is in a very dynamic regime with respect to adsorption in many freshwaters. Deficiencies in our present information on cadmium adsorption are highlighted and suggestions made for future studies.

Lead from dust and water as exposure sources for children

Data from the Edinburgh Lead Study are used to estimate the respective contributions of water and dust lead to blood lead in 6–9 year old children. Both sources are significantly related to blood lead. An exposure of 100 μg/l in kitchen cold water is estimated to be equivalent to 2700 μg/g of lead in dust. In this population water is a more important source of lead than dust for the bulk of the population.

Adsorption of Cd, Pb and Cu during the precipitation of hydrous ferric oxide in a natural water

Abstract Adsorption of Cd, Pb and Cu (pH 7) has been studied as a function of time during the slow oxidation (24 hr.) of ferrous iron naturally present in samples of anoxic lake water. A simple surface complexation model fits the Cd and Pb behaviour. Initial uptake of these metals is fairly evenly distributed across all size fractions of the precipitate. There appears, though, to be a transfer of Pb, and to a lesser extent Cd, from the smaller to the larger-size fractions during the course of the precipitation. This is explained in terms of slow desorption kinetics coupled with the slow aggregation of the precipitate. Cu behaves unexpectedly when spiked at a low concentration at the beginning of the experiment. An initial high uptake is focussed on the small-size fraction (0.1–0.4 μm). This is followed by a steady desorption from this size fraction. Three hypotheses to account for this behaviour are explored, but none provides a wholly satisfactory answer.

The variability of lead in dusts within the homes of young children.

The variability of household dust lead concentration and loadings over the period of about a year has been examined in 10 homes. The overall uncertainty on a single sample is ± 65% for lead concentration and ± 81% for lead loading. Redecoration involving electric sanding and/or blow lamp preparation of painted surfaces is the major cause of variation in lead levels. The effect is short lived, levels returning to normal within 2 months of redecoration ending. There is evidence of a seasonal trend for dust loading and lead loading but not for lead concentration. The implications of these findings for blood lead — dust lead exposure studies are considered.