Toxicological response of a bioluminescent microbial assay to Zn, Pb and Cu in an artificial soil solution: relationship with total metal concentrations and free ion activities.

The relationship between toxicological response and both total concentrations and free ion activities of Pb, Cu and Zn in an artificial soil solution has been investigated using lux-marked Escherichia coli HB101 (pUCD607) as a bioassay. SO4(2-) (as K2SO4) was added as an inorganic complexing agent up to 0.01 M representing the range of ionic strengths found in soil solutions and giving a wide range of free metal ion activities. EC50 values expressed in terms of concentration, varied significantly with K2SO4 molarity for all metals. However, when EC50 values were expressed in terms of free ion activity they were not significantly different for Pb and Zn, supporting the free ion activity model. Conversely, EC50 values expressed as free Cu activity did vary significantly with K2SO4 molarity, possibly due to a greater degree of adsorption of Cu onto inactive sites on the cell surfaces than for Zn and Pb. Linear regression analysis of bioluminescence on free ion activity revealed significant correlations for each metal above the toxicity threshold. In conclusion, lux-marked E. coli is suitable for investigating the toxicity of metal ions and complexes in non saline systems although cell surface adsorption effects could be important for some metals, e.g. Cu.

Effect of bone meal (calcium phosphate) amendments on metal release from contaminated soils--a leaching column study.

Metal-contaminated soil may be remediated in situ by the formation of highly insoluble metal phosphates if an appropriate phosphorus (P) source can be found. Leaching column experiments have been carried out to assess the suitability of bone meal as such a source. Bone meal additions reduced metal release from a contaminated soil, increased soil and leachate pH and decreased soil leachate toxicity. Minimal P leaching occurred from the soil. The data are consistent with a proton consuming bone meal (calcium phosphate) dissolution reaction followed by the formation of metal phosphates. Although, no metal phosphates were observed to form using X-ray diffraction of scanning electron microscopy this could be due to their low concentration. Relatively low (1:50 bone meal:soil) concentrations of fine (90-500 microns) bone meal would appear to be an effective treatment for metal-contaminated soils.

Lead mineral transformation by fungi.

Pyromorphite (Pb5(PO4)3Cl), the most stable lead mineral under a wide range of geochemical conditions [1], can form in urban and industrially contaminated soils [2] [3] [4] [5]. It has been suggested that the low solubility of this mineral could reduce the bioavailability of lead, and several studies have advocated pyromorphite formation as a remediation technique for lead-contaminated land [3] [5] [6], if necessary using addition of phosphate [6]. Many microorganisms can, however, make insoluble soil phosphate bioavailable [7] [8] [9] [10], and the solubilisation of insoluble metal phosphates by free-living and symbiotic fungi has been reported [11] [12] [13] [14] [15]. If pyromorphite can be solubilised by microbial phosphate-solubilising mechanisms, the question arises of what would happen to the released lead. We have now clearly demonstrated that pyromorphite can be solubilised by organic-acid-producing fungi, for example Aspergillus niger, and that plants grown with pyromorphite as sole phosphorus source take up both phosphorus and lead. We have also discovered the production of lead oxalate dihydrate by A. niger during pyromorphite transformation, which is the first recorded biogenic formation of this mineral. These mechanisms of lead solubilisation, or its immobilisation as a novel lead oxalate, have significant implications for metal mobility and transfer to other environmental compartments and organisms. The importance of considering microbial processes when developing remediation techniques for toxic metals in soils is therefore emphasised.

Lead phosphate formation in soils.

Pyromorphite (Pb5(PO4)3Cl) is one of the most thermodynamically stable lead minerals under the geochemical conditions prevailing in the surface environment. It is therefore expected to form in soils contaminated with Pb if sufficient phosphorus is available. Pyromorphite has previously been identified in mine-waste and industrially contaminated soils but has not previously been identified in urban soils. This paper reports on the presence of a Pb phosphate in urban and roadside soils. This phase has formed in the soil as a weathering product of Pb-bearing grains. Quantitative EDX analyses indicated that the Pb phosphate phase is pyromorphite with Ca frequently substituting for Pb between 21-31 atomic percent. However, positive identification of this phase by XRD was hindered by the deviation from pure end-member and possibly also by the poorly crystalline nature of the phase. Pyromorphite accounted for less than 2% of the total Pb in these soils. However, phosphate amendments to the soil could induce further formation of pyromorphite. As pyromorphite is a highly insoluble mineral, this may be effective in reducing the bioavailability of Pb in urban soils.

Lead contamination of UK dusts and soils and implications for childhood exposure: An overview of the work of the Environmental Geochemistry Research Group, Imperial College, London, England 1981-1992.

Over the course of the last decade, research conducted by the Imperial College Environmental Geochemistry Research Group has focused on the nature and effects of lead in UK dusts and soils. An initial nationwide reconnaissance survey demonstrated that approximately 10% of the population is exposed to lead levels in excess of 2,000 µg g(-1) in house-hold dust. Subsequent exposure studies revealed that for 2 year old children in the UK urban environment, approximately 50% of lead intake was from dust ingested as a result of hand-to-mouth activity. Follow-up computer controlled scanning electron microscopy (CCSEM) analysis of urban household dust and particulate material wiped from children's hands showed that important sources of dust lead include lead-based paint, road dust and soils. CCSEM identification of specific soil lead tracer particles (from minewaste contaminated soils) in dusts and on children's hands further documented the important role of soil as a source of exposure. Speciation studies of soil lead of this origin indicated that the form of the lead, which is largely influenced by the soil environment, is the primary control on bioavailability. It appears that although lead of minewaste origin may be present at elevated levels in dusts and soils, it does not necessarily contribute to elevated blood lead levels when the lead is present in relatively insoluble form.

Sources and pathways of environmental lead to children in a Derbyshire mining village.

Garden soil and housedust samples, from households in a Derbyshire village closely associated with historic lead mining, have highly elevated lead levels. Handwipe samples from children also have relatively high lead concentrations suggesting that elevated levels of lead are transferred to the child by the soil-dust-hand-mouth pathway. However, this is not reflected in their blood lead concentrations which are within normal UK ranges and less than predicted by some lead exposure models. SEM analysis of soil grains has revealed that many are composed of pyromorphite [Pb5(PO4)3Cl], a stable soil-lead mineral. This mineral is formed from the weathering of galena [PbS] but it is not clear to what extent weathering has occurred in the soil. Pyromorphite has an extremely low solubility which may contribute to a low human bioavailability of lead in these soils, resulting in the lower than expected blood lead concentrations.