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Environmental fate & pathways

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Description of key information

Additional information

Lead is a natural element and post-transition metal with more than one oxidation state. Lead in its metallic form is not bioavailable. Lead needs to be transformed to its ionic forms to become available for uptake by living organisms.

The available reliable data selected for the environmental fate and behaviour of lead are all based on either monitoring data of prevailing lead concentrations in water, soil, sediment, suspended matter and organisms or on experimental results with lead (di) nitrate and lead chloride. All reliable data are expressed based on elemental Pb concentrations and grouped together in a read-across approach.

Stability and biodegradation

The classic standard testing protocols on hydrolysis and photo-transformation are not applicable to lead and inorganic lead compounds. This was recognized in the Guidance to Regulation (EC) No 1272/2008 Classification, Labelling and Packaging of substances and mixtures (metal annex):

“Environmental transformation of one species of a metal to another species of the same metal does not constitute “degradation” as applied to organic compounds and may increase or decrease the availability and bioavailability of the toxic species. In addition naturally occurring geochemical processes can partition metal ions from the water column while also other processes may remove metal ions from the water column (e.g. by precipitation and speciation). Data on water column residence time, the processes involved at the water – sediment interface (i.e. deposition and re-mobilisation) are fairly extensive for some metals. Using the principles and assumptions discussed above in section IV.1 of this document, it may therefore be possible to incorporate this approach into the classification”

As outlined in the CLP Guidance (2012), the understanding of the transformation of lead into more or less bioavailable species is relevant to the environmental hazard assessments and this is described below.

Transformation/dissolution of Pb°

Lead in its metallic form (Pb°) needs to be transformed to its ionic forms to become available for uptake by living organisms and therefore the rate and extend of the transformation/dissolution of lead in its massive and powder forms have been assessed from transformation/dissolution tests (in accordance to the OECD guidance, Annex 10 of the GHS). The data demonstrates that the release of soluble lead- ions from Pb° depends on various characteristics including metal forms (massive or powder), loading rates and pH. The release rate is higher at lower pH.

The results of the transformation/dissolution tests are described in details in “additional information fate and pathways” and are summarized as follows:

For massive lead materials, the transformation/dissolution test was carried out at pH 6 and 7, in accordance to the OECD protocol on transformation/dissolution. The results were used to derive the release of lead-ions from 1 mm particles at loadings of 1,10 and 100 mg/L.

7-days transformation/dissolution of a massive particle of 1mm diameter at , a loading of 100 mg/L corresponds to a total release of 428.9 µg Pb/L at pH 6 and 109 µg Pb/L at pH 7.

The results from 28 days transformation/dissolution of a massive particle of 1mm diameter, at pH 6, and a loading of 1 mg/L, corresponds to 14.2 µg Pb/L. The results are carried forward to the hazard classification.

The lead massive was also tested on aged samples (subjected to 28 days of wet/dry cycles under standardised atmospheric conditions before testing) at pH 6, but the dissolution for the aged sample was comparable with that of the non-aged sample.

For lead powders, transformation/dissolution tests were carried out on fine lead powders (<75µm,) in accordance to OECD protocol at pH 6, 7 and 8.

The release of lead to the aqueous medium at 24h for the 100 mg/L loading at pH 6 was 3211.2 µg/L. For the 100 mg/L loading, the average released concentrations of Pb at 24h was 607 and 187.5 µg/L respectively at pH 7 and pH 8. The results are carried forward to the hazard classification.

Transformation of Pb-ions released in the environment –lead speciation

Once released to the environment, lead ions have more than one oxidation state and lead is characterized as ‘post-transition’ metal. The principal ionic form is Pb(II) (Pb2+), which is more stable than Pb(IV) (Pb4+). In all environmental compartments (water, sediment, soil), the binding affinities of Pb(II) with inorganic and organic matter is dependent on pH, the oxidation-reduction potential in the local environment, and the presence of competing metal ions and inorganic anions.

Lead attenuation, removal from water column, geochemical cycling- quantitative assessment

As described above, after the release of Pb(II) in the environment, further transformations occur thereby changing the potential for toxicity, induced by the free Pb(II) ions. The concentrations of “active” Pb(II) ions, that remains available for uptake by biota depends on different processes: precipitation, dissolution, adsorption, desorption, complexation and competition for biological adsorption sites (ligands).  These processes are critical for the fate of lead in the environment. This was recognized in the Guidance to Regulation (EC) No 1272/2008 Classification, Labelling and Packaging of substances and mixtures (metal annexes, CLP Guidane 2012) as stated above.

The use of the Tableau Input Coupled Kinetics Equilibrium Transport (TICKET) model – Unit World Model (UWM) for evaluating removal of soluble metal species through precipitation/partitioning processes over a range of environmentally relevant conditions have been assessed for evaluating the removal of soluble lead species (equivalent to ‘rapid degradation” for organic compounds). The information is reported in the section "additional information on environmental fate" and summarized below

-In the water compartment,lead is rapidly and strongly bound to the suspended solids of the water column. This binding and subsequent settling to the sediment allows for rapid metal removal of lead from the water column as demonstrated by a decrease in soluble lead concentrations by >70% in 28 days in a range of simulations according to the TICKET-UWM calculations (Rader et al., 2010). This information is reported in the section:additional information on environmental fateand is relevant to the environmental classification.

-In the sediment compartment,lead binds to the anaerobic sulphides resulting in the formation of PbS. The analysis was based on an AVS concentration between 1 and 9 µmol AVS/g. The results show that “insoluble” PbS keeps lead in the anaerobic sediment layers, limiting the potential for remobilization of Pb-ions into the water column. The potential for remobilization from oxic sediments, quantified by comparing water column lead concentrations resulting from sediment feedback to the 70% removal concentration was also insignificant. For conditions favoring remobilization, water column dissolved lead concentrations at pseudo steady state were more than 100 times less than the 70% removal concentration.

All these results therefore suggest that Pb removel from the water column is rapid and its remobilization from the sediment is relatively insignificant, supporting the removal of the “persistent” criterion for soluble lead salts. The relevant information is reported in the section“additional information on environmental fate”and is relevant to the environmental classification.

Transport and distribution

While lead is a naturally-occurring chemical, it is rarely found in its elemental form. It occurs in the Earth’s crust primarily as the mineral galena (PbS), and to a lesser extent as anglesite (PbSO4) and cerussite (PbCO3). Lead minerals are found in association with zinc, copper, and iron sulfides as well as gold, silver, bismuth, and antimony minerals. It also occurs as a trace element in coal, oil, and wood. Lead may enter the environment from both natural (weathering of soil, sea salt spray, volcanos) and anthropogenic sources (mining, ore processing, smelting, refining, recycling or disposal). In these processes, lead may be released to land, water, and air. In the atmosphere, non-organic compounds of lead exist primarily in the particulate form. The median particle distribution for lead emissions from smelters is 1.5 μm with 86% of the particle sizes under
10 μm (Corrin and Natusch 1977). The smallest lead-containing particulate matter (<1 μm) is associated with high-temperature combustion processes. Upon release to the atmosphere, lead particles are dispersed and ultimately removed from the atmosphere by wet or dry deposition.

 

The amount of soluble lead in surface waters depends upon the pH of the water and the dissolved salt content. Equilibrium calculations show that at pH >5.4, the total solubility of lead is approximately
30 μg/L in hard water and approximately 500 μg/L in soft water. Sulfate ions, if present in soft water, limit the lead concentration in solution through the formation of lead sulfate. Above pH 5.4, the lead carbonates, PbCO3 and Pb2(OH)2CO3, limit the amount of soluble lead.A significant fraction of lead carried by river water is expected to be in an undissolved form, which can consist of colloidal particles or larger undissolved particles of lead carbonate, lead oxide, lead hydroxide,or other lead compounds incorporated in other components of surface particulate matters from runoff. Lead may occur either as sorbed ions or surface coatings on sediment mineral particles, or it may be carried as a part of suspended living or nonliving organic matter in water. The fate of lead in soil is affected by the adsorption at mineral interfaces, the precipitation of sparingly soluble solid forms of the compound, and the formation of relatively stable organic-metal complexes or chelates with soil organic matter. These processes are dependent on such factors as soil pH, soil type, particle size, organic matter content of soil, the presence of inorganic colloids and iron oxides, cation exchange capacity (CEC), and the amount of lead in soil. In soil, lead can slowly undergo speciation to the more insoluble sulfate, sulfide, oxide, and phosphate salts. From the literature overview, the following partitioning coefficients (median values) have been derived for Pb:    

- Aquatic compartment

Partition coefficient in freshwater suspended matter: Kd susp= 295,121 L/kg

Partition coefficient in freshwater sediment: Kd sed, fw = 153,848 L/kg

Partition coefficient in marine sediment: Kd sed, mar = 457,088 L/kg

Partition coefficient in estuarine suspended matter: Kd susp= 667,954 L/kg

Partition coefficient in marine suspended matter: Kd susp= 1,518,099 L/kg

- Soil compartment

Median partitioning coefficient: Kd soil: 6,400 L/kg

Bioaccumulation

Secondary poisoning results when toxicant concentrations in an organism reach a level that is toxic to the organisms that feed on it. Substances that bioaccumulate or biomagnify in food webs often are considered to have the greatest potential to cause secondary poisoning. It has been reported that the classic concept of biomagnification and food chain poisoning, based primarily on organic chemicals does not hold for metals. This may be explained in part by the limited bioavailability of the inorganic forms of metals in food and by the regulation of metals that occurs in both aquatic and terrestrial organisms.

Lead is not an essential nutrient, but the observedinverse relationship between the Pb internal concentration and the external concentration indicates that Pb is actively regulated.Within the typical environmental concentration range, the gathered BAFs for fish ranged between 11 and 143 L/kgww(median 23 L/kgww) while the BAFs for molluscs ranged between 18 and 3,850 L/kgww(median 675 L/kgww), for insects between 968 and 4,740 L/kgww(median 1,830 L/kgww) and for crustaceans between 1,583 and 11,260 L/kgww(median 3,440 L/kgww). Lead therefore poses the highest risk to insects and crustaceans, compared to fish or molluscs. The BAF value of 1,553 L/kgww(50th% of the mixed diet BAF for aquatic organisms) is further used for the assessment of secondary poisoning in the aquatic environment.

The bioaccumulation factor for lead in soil is dependent on the effective cation exchange capacity (eCEC) of the soil:

log BSAF = -0.89 * log eCEC +0.55

A generic BSAF factor of 0.048 (fresh weight) is derived for a soil with a median eCEC of 16 cmolc/kg soil. Fresh-weight based BSAF values for a soil with an eCEC of 8 and 30 cmolc/kg soil, corresponding to the 10thand 90thpercentile of eCEC in European arable soils are 0.089 and 0.028, respectively.

More detailed summaries on respectively aquatic and terrestrial bioaccumulation are available from the aquatic and terrestrial bioaccumulation summary sections.