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It has been reported that the use of high concentrations of SbCl3dissolved in water have resulted in precipitation of antimony (Brooke et al.,1986). What is most likely being observed is an initial formation of chloroantimonate (III) species, which in aqueous solutions are weak, and which then hydrolyse to oxychloride (SbOCl), which has low solubility in water, and is further hydrolysed to Sb2O3(Filella et al.,2002b; Filella and May, 2003). The percentage of Sb lost from solution by precipitation in the study by Brooke et al.(1986) during a 96-h test ranged from a low 6% for the lowest exposure concentration (nominally 25 mg Sb/L) to 76% for the highest concentration (nominally 50 mg Sb/L). The apparent trend was to lose a greater percentage of antimony from solution with increasing nominal concentration. The authors improved the methods by introducing mixing and filtering to remove the precipitate from the solution. The maximum concentration maintained in a solution without organisms for 96 h was 35.0 mg Sb/L, using a nominal concentration of 250 mg Sb/L. Nominal bioassay concentrations at 50 mg Sb/L and 100 mg Sb/L using Sb2O3resulted in measured soluble concentrations of 3.4 and 5.0 mg Sb/L, respectively. Solutions of Sb2O3in laboratory water at nominal concentrations of 28, 58, and 110 mg Sb/L had measured dissolved concentrations during the 96 h experiment period of 1.9, 2.6, and 3.3 mg Sb/L, respectively.

Similar observations of the removal of some antimony species from solution over time have been observed in transformation/dissolution tests on antimony compounds in some cases (I2A 2010).

Nominal concentrations will therefore be very misleading guides to the actual amount of dissolved antimony. At equal nominal concentrations, readily soluble compounds like SbCl3result in more dissolved antimony during the exposure periods used in toxicity experiments than less soluble compounds like Sb2O3. However, even the readily soluble compound SbCl3may at high nominal concentrations result in lower amounts of soluble antimony.

Filella and May (2003) conducted a critical review of all available thermodynamic data on antimony and developed a computer speciation model of antimony in multi-component solutions, representative of different environmental conditions. Based on the limited data set and the subsequent speciation calculations it was shown that antimony is exclusively present as the pentavalent Sb(OH)-6in oxic freshwater systems and as the trivalent Sb(OH)3in anoxic conditions, at all pH values of environmental relevance for aquatic systems. The formation of chloride-antimony species does not appear to be of importance under environmentally relevant conditions, as no Sb(III) -chloride was observed under seawater conditions, and the concentration of possible Sb(V)-chloride could not be calculated due to a lack of data. The very few studies available on Sb(V)-chloride binding had been performed under extremely acidic conditions to prevent hydrolysis and could thus not be used, as it was difficult to establish the strength of such interactions under dilute conditions relative to other antimony species. Therefore no thermodynamic relationship of this kind has been published.

Based on the available information on antimony there is nothing that indicates that the difference observed in toxicity in aquatic systems between different inorganic antimony compounds of the same valence, such as for instance SbCl3and Sb2O3, would be due to different antimony species exerting different degrees/kinds of toxicity. Instead, an observed difference in toxicity at equal nominal doses of antimony is most probably a reflection of differences in solubility, which means that a more soluble antimony compound will result in more dissolved antimony capable of exerting toxicity. However, it may be that higher concentrations of Sb compounds also reflect an increased presence of counter ions and/or protons.

Toxicity studies in the aquatic compartment in which only nominal antimony concentrations are reported will therefore not be considered to be reliable in this report. However, studies will not be rejected based solely on which antimony compound was used in the test, or whether or not a tri- or a pentavalent compound was used, as long as the results are considered reliable and relevant. All effect and no-effect concentrations are reported as a concentration of the antimony ion.

The results from six studies, i. e. Brooke et al. (1986), Kimbal (1978), TAI (1990), Heijerick and Vangheluwe (2003), Heijerick and Vangheluwe (2004), and LISEC (2001), all provide valid EC50s and NOECs for fish, invertebrates and algae.

Toxicity results for marine species are scarce. Only the results from Takayanagi (2001) are considered reliable.

None of the NOECs used to derive the PNECs in the aquatic compartment are considered to be confounded by the additions of counter ions (i.e. chloride) and/or protons resulting from the use of SbCl3.