Aquatic Toxicity Testing: Difficulties With Sparingly Soluble Metal Substances, with examples from Aluminium and Iron

Transcription

1 Aquatic Toxicity Testing: Difficulties With Sparingly Soluble Metal Substances, with examples from Aluminium and Iron Eirik Nordheim, EAA William Adams, Rio Tinto

2 Workshop Objectives In this Workshop we will explore: Difficulties associated with some metals in laboratory toxicity tests Measurements of solubility versus effect levels Measurements of toxicity (intrinsic toxicity versus physical effects) Metal specific issues (examples aluminium and iron) Possible solutions for going forward for Al 13 May 2011 Presentation Title 2

3 Behaviour of Difficult to Test Metal Substances general problems encountered Difficulties have arisen in aquatic testing with some metals and metal compounds presented at Workshop with ECHA January Aluminium, iron, lead, tin, and some minor metals Each of these substances form insoluble compounds in standard ecotox test media At circumneutral ph and above each of these metals form insoluble metal hydroxides (or carbonates - Pb) which come out of solution (rate and extent of precipitation varies with metal) Reported test concentrations vary significantly depending upon whether total, soluble or some other measure was used 13 May 2011 Presentation Title 3

4 Behaviour of Difficult to Test Metal Substances Continued.. Precipitation is common in standard tests used to evaluate the metal effects ( ug/l) Should the samples for analysis be filtered? Results are reported as total, soluble, labile, bioactive, monomeric (single polymer), etc.. Kinetics of formation of insoluble species are not considered or reported Test solutions are not checked for chemical and toxicological stability over time (i.e., aging) Metal species in solution are not evaluated: tests are conducted as if the substance is a stable soluble compound 13 May 2011 Presentation Title 4

5 Behaviour of Difficult to Test Metal Substances Most published toxicity tests are based on concentrations above the solubility of the compound (next two slides) 50 years of data; most of which is not useable.. Reports of gill smothering and oxygen suffocation are common due to gill precipitation Toxicity does not appear to be intrinsic toxicity, but rather due to physical effects (Al, Fe) There are some data for Al and Fe for Atlantic salmon (NIVA) that show gill accumulation, ion-regulatory effects and direct toxicity due to gill accumulation of the two metals at levels below solubility limits, but there are limits of application of these data for REACH as these situations occur in mixing zones associated with natural metal inputs 13 May 2011 Presentation Title 5

6 A lum inium Speciation in US EPA Very Soft W ater: Al speciation in US EPA Al very T = 10 um soft, DOC water = 2 m g/l Al = 10 um, DOC = 2 mg/l a) no solid phase b) Gibbsite present -4 (a) -5 Al Diss Al AlSO - 4 AlF AlOH Al(OH) + 2 Al-DOM Log Concentration (M) -9-4 (b) Al(OH) AlOH Al(OH) + 2 Al 3+ Al Diss AlF Al(OH) AlSO - 4 Al-DOM ph 13 May 2011 Presentation Title 6

7 Solubility simulations with OECD TG 201 medium (Al and Fe) Metal ph Predicted solid phase MINEQL Solubility limit (Ug/L) MINTEQ Solubility limit (Ug/L) Al 6 gibbsite Al 7 gibbsite AL 8 gibbsite Al(OH) Al(OH) Al(OH) Fe (II) 6 Ferrihydrite Fe (II) 7 Ferrihydrite Fe (II) 8 Ferrihydrite May 2011 Presentation Title 7

8 Behaviour of Difficult to Test Metal Substances 100% 90% Species sensitivity distribution for Fe(II): Individual and Species Mean NOECs. All the values are above the solubility of FE II. 80% 70% Percentile 60% 50% 40% 30% 20% 10% Individual NOECs Species Mean NOECs 0% Fe(II) NOECs, mg/l 13 May 2011 Presentation Title 8

9 Behaviour of Difficult to Test Metal Substances 100% 90% Species sensitivity distribution for Fe(III): Individual and Species Mean NOECs All the values are above the solubility of FE III 80% 70% 60% Percentile 50% 40% 30% 20% 10% Individual NOECs Species Mean NOECs 0% Fe(III) NOECs, mg/l 13 May 2011 Presentation Title 9

10 Behaviour of Difficult to Test Metal Substances DOC, hardness and ph interactions have been shown to be very important for Al, Fe, and Pb. Al & Fe hydroxide polymers above water solubility interact with DOC & hardness ions! DOC binding to Al and Fe explains why surface water concentrations in the 500 ug/l to 2000 ug/l are often not a concern. Death Valley, CA, USA Background water concentrations of Al and Fe are typically quite high ( ug/l) compared to other cationic metals; for tin they are very low (<5 ug/l) controlled by solubility limits. Al and Fe in natural waters are controlled by precipitation reactions and loss by binding to suspended solids. 13 May 2011 Presentation Title 10

11 Behaviour of Difficult to Test Metal Substances Recent reviews of Al and Fe data indicate that toxicity is due to: (1) toxicity from large hydroxide polymers that prevent oxygen exchange or (2) transient low molecular weight hydroxide species Transient species appear to be gill reactive species (i.e. they accumulate on the gills) The species are small polymers that result from the formation of hydroxide species Death Valley, CA, USA These polymers grow in size over time (minutes to hours) and precipitate from solution and may cause effects in lab studies; less so in natural environments 13 May 2011 Presentation Title 11

12 Behaviour of Difficult to Test Metal Substances Extensive modelling efforts to date indicate that it is possible to develop a biotic ligand model (BLM) for predicting toxicity of Al across a range of concentrations and water quality parameters. An empirical model for algae, & BLM model for daphnids and fathead minnows were prepared for the REACH dossier (next slide) Death Valley, CA, USA Preliminary assessment suggest that it may also be possible for iron but iron has two redox states so it is more complicated. An acute BLM for lead is now available, chronic is close to completion. 13 May 2011 Presentation Title 12

13 The Solution

14 The Solution for Aluminium Initial BLM model was built using data for Atlantic salmon, ph range Chronic tests were performed with ceriodaphnia dubia, green algae, and fathead minnows at ph 6 and 8, ph stabilized with buffers Test solutions were aged for 3 hours to stabilize toxicity Test concentrations were measured as total, dissolved and monomeric Al. Across the entire ph range, total aluminium was the best indicator of effects The algal model is an empirical model derived from multiple laboratory tests where ph, DOC and hardness were varied Model was used to predict a PNEC for various water types for Europe PNEC values are preliminary and reflect only 3 species. [next slide] Data are being developed for 7 other species to meet guidelines of 10 species 13 May 2011 Presentation Title 14

15 Summary of the phys.-chem. characteristics of selected water types with corresponding Auminium PNEC values based on BLM modeling N Scenario Type ph Hardness (mg CaCO3/l) 1 Worst Case Scenario Acid river with low hardness & DOC DOC (mg C/l) PNEC Fish (ug/l) PNEC Inverte. (Ug/L) PNEC Algae (ug/l) Reasonable Hypothetical lake worst Case 3 Average case Neutral-Acidic lake in Sweden Acidic system Ditch - Small (ditches with Netherlands flow rate of ± 1,000 m³/d) 6 River Rhine - Large (rivers with Netherlands flow rate of ± 1,000,000 m³/d) 7 River Ebro -Italy Mediterranean river Lake Monate Oligotrophic systems River Otter in UK Medium (rivers with flow rate of ± 200,000 m³/d) May 2011 Presentation Title 15

16 Summary for Difficult to Test Metal Substances Several metals (Al, Fe, Mn, Pb, Sn) form insoluble metal species in standard OECD toxicity test media (metal hydroxides or carbonates) To obtain useful data from toxicity tests: - ph and toxicity need to be stable - metal species in solution as a function of ph must be known - Measurements of metal concentration must be related to effect (the exact type of measure, i.e. total, soluble, other, may vary by metal) Interactions with DOC, hardness and ph are important for AL, Fe, Pb Remember all the tests performed to date used soluble metal substances, i.e., metal chloride or metal sulfate etc. Application of these data to other sparingly soluble substances, i.e., oxides or sulfides or metal powders must be done in conjunction with Transformation dissolution studies for those substances 13 May 2011 Presentation Title 16

17 Summary for Difficult to Test Metal Substances Each of these substances must be evaluated as a specific case which do not follow general guidance and they are different from other metals. Aluminium and iron are the two most common metals in the earth s crust and any concentration found in the aquatic environment will be completely dominated by geogenic sources.