Xanthates in freshwater and marine water
Toxicant default guideline values for protecting aquatic ecosystems
Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals’ of the ANZECC & ARMCANZ (2000) guidelines.
The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.
Description of chemical
Xanthates are a group of chemicals used in the mining industry for flotation and treatment of sulfide and metallic ores (Cyanamid 1989). They are commonly used in conjunction with formulations of dithiophosphates to improve recoveries, concentrate grades and kinetics of flotation (Cyanamid 1989). Flotation involves separating the mineral particles as froth from the tailings, which settle out.
The general structure of xanthates is as follows:
S- (Na+, K+) where R is an alkyl group
Their molecular weights are around 150 to 200, e.g. SIPX has a MW of 158.2. Xanthates are water-soluble powders or pellets usually with a strong and unpleasant odour and they are usually applied as 10 to 20% solutions. Their water solubilities vary from 110 to 460 g/L and increase at higher temperatures (Cyanamid 1989). They would not be expected to enter the environment in normal, well-run mining operations but may enter through spills or accidents or from tailing dams. The predicted concentration of sodium ethyl xanthate (SEX) in tailing slurry is around 1.5 ppm (NICNAS 1995), which is consistent with measured values of 0.2 to 1.2 mg/L (Hawley 1977).
Xanthates degrade to inorganic sulfides or organo-sulfur compounds. In neutral or mildly alkaline solutions, SEX degrades to the organic alcohol, carbon disulfide, sodium carbonate and sodium trithiocarbonate (NICNAS 1995). The half-life of SEX at pH 7 (25°C) is around 260 hours and doubles at above pH 8. Xanthates are expected to last a few days in the natural environment (NICNAS 1995) and they are not expected to bioaccumulate.
Only limited freshwater data were available for xanthates, covering only one fish and two invertebrate species although Hawley (1977) lists some unscreened ranges. Invertebrates are around 1 to 2 orders of magnitude more sensitive to xanthates than fish for PAX and SIBX but the reverse applies for PEX, SEX and SIPX. Commonly used xanthates with their CAS numbers and available freshwater toxicity data are listed below in Table 8.3.23.
Data not originally considered included 24-hour EC50 (immobilisation) figures for Daphnia magna of around 3.6 mg/L for SIBX and SIPX and 0.35 µg/L for SEX. In addition, some of the figures that Hawley (1977) reported as ranges (Daphnia magna and two species of fish, Notropis atherinoides and Pimephales promelas) were much lower than the screened figures. For this reason it was considered appropriate to base ECL figures (see Section 22.214.171.124 of the ANZECC & ARMCANZ 2000 guidelines) on median values of the lowest unpublished ranges reported by Hawley (1977) and tabulated in NICNAS (1995).
SEX: Some 2 to 7-day algal or macrophyte growth or population growth figures were available, on seven species, 2 to 20 mg/L, but end-points were not specified. These would not affect environmental concern level (ECL) calculations.
Factors that modify the toxicity of xanthates
Two different commercial products of eight xanthates varied in their toxicity (for the same xanthate type) by up to 1 order of magnitude (Hawley 1977).
There were insufficient data to derive reliable trigger values for any xanthates. Low reliability trigger values adopted from ECLs (see Section 126.96.36.199 of the ANZECC & ARMCANZ 2000 guidelines) derived from the screened data in Table 8.3.23 may not protect the most sensitive species. They were calculated from median values for the most sensitive of three species listed in Hawley (1977). These ECLs are listed in the table and range from 0.05 µg/L for PIPX, SEX and SSBX to 500 µg/L for PHX.
Table 8.3.23 Xanthates, freshwater short-term toxicity data (LC50 or EC50 immobilisation mg/L, i.e. 1000 x µg/L; trigger value, TV, fresh and marine, in µg/L)
|CAS no.||Potassium amyl (PAX)||Potassium ethyl (PEX) 140-89-6||Potassium hexyl (PHX) 2720-76-5||Potassium isopropyl (PIPX) 140-92-1||Sodium ethyl (SEX) 140-90-9||Sodium isobutyl (SIBX) 25306-75-6||Sodium isopropyl (SIPX) 140-93-2||Sodium s-butyl (SSBX) 36551-21-0|
|Crustacean data (mg/L screened)||ND||ND||ND||ND||ND||ND||ND||8.5 (n=1)|
|Crustacean (other data*)||0.1-1.0 (n=1)||0.1-1.0 (n=1)||ND||ND||0.35 (n=1)||0.56-3.60 (n=1)||0.1-3.6 (n=1)||0.56-10.00 (n=1)|
|Fish data (mg/L screened)||ND||10-100 (n=1)||ND||32-320||29-37 (n=1)||70-100 (n=1)||217 (n=1)||100-320 (n=1)|
|Fish (other data*)||1.8-180.0 (n=2)||0.01-2.60 (n=2)||100-1000 (n=2)||3.2-32.0 (n=2)||0.01-3.20 (n=2)||10-560 (n=2)||0.01-5.60 (n=2)||1-320 (n=2)|
|TV Low reliability||0.5||0.05||500||15||0.05||5||0.05||5|
*Other data from Hawley 1977 (as ranges rather than single figures) or 24-h data from AQUIRE (1994); ND = no data found.
ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.
AQUIRE (Aquatic Toxicity Information Retrieval Database) 1994. AQUIRE standard operating procedures. USEPA, Washington, DC.
Cyanamid (American Cyanamid Company) 1989. Mining chemicals handbook. Mineral Dressing Notes 26-1, USA.
Hawley JR 1977. The use, characteristics and toxicity of mine-mill reagents in the Province of Ontario. Ministry of the Environment, Ontario. (Cited in NICNAS 1995)
NICNAS (National Industrial Chemical Notification and Assessment Scheme) 1995. Sodium ethyl xanthate: Priority existing chemical No. 5. Public Report, Australian Government Publishing Service, Canberra.