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Environmental fate & pathways

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Potassium isoamyl xanthate (potassium isopentyl dithiocarbonate; PIAX) is used as a flotation agent in mining industry. The residues of the substance in process waters are discharged and treated in tailings pond. Therefore, part of the discussion in this section also focuses on the fate and distribution of the substance in the tailings pond treatment.

PIAX is a solid substance marketed in pellet form and used in 10 - 25 % water solutions (typical concentration of 20 %). It is very soluble in water (350 g/l at 25 ˚C, Kirk-Othmer 2000). There are several other xanthates which are analogue substances to PIAX and which all decompose in contact with moisture or water. The loss of xanthates per day is measured to be approximately 0.5 to 4.6 % depending on the concentration of the water solution, pH and temperature (NICNAS 2000). During hydrolytical decomposition, this substance will release mainly alcohols, carbon disulphide (CS2) and carbonates and dithiocarbonates in water. Therefore, the fate and distribution of the substance in the environment is also dependent on the degradation rate and amounts and properties of the degradation products.

The purity of the registered substance is > 80 %. It contains 84 % of potassium isopentyl dithiocarbonate (PIAX) and 11 % of potassium o-pentyl dithiocarbonate (PAX) and 5 % of unknown impurities (section 2 of the CSR). While potassium isoamyl xanthate is the target substance of the chemical safety assessment, information on other structurally analogous xanthates and the most critical degradation products was collected for completeness. Read-across data from PAX is also used as it is the constituent of the target substance. The read-across justifications are presented in Annex I of this CSR.

Based on the most recent toxicity studies of the substance to Daphnia magna (48-h EC50) and Desmodesmus subspicatus (72 -h EC50) toxicity of PIAX is between 3.67 to 10.51 mg/l, the most sensitive species being daphnids. In addition, the 96-h LC50 of an analogue substance PIBX (potassium isobutyl xanthate) to fish (Danio rerio) have been reported to be 10 mg/l. 72-h NOEC (growth rate) to D. subspicatus of 1 mg/l and the 28-d NOEC (reproduction) of 0.79 mg/l has been reported.

There are no biodegradation studies available for the target substance PIAX. However, the results of structural analogue substance (potassium isobutyl xanthate) have been reported to be inherently biodegradable in modified Zahn-Wellens test (OECD 302 B). As discussed below in more detailed, the most important fate process of the substance is hydrolysis. However, based on the knowledge of the degradation reactions of xanthates and the half-lives at the relevant environmental conditions, the hydrolysis cannot be considered to be rapid. In addition, some of the degradation products are more toxic to aquatic organisms (e.g hydrogen sulphide) than the parent substance. Based on the toxicity results and the biodegradation, PIAX is classified as possessing long-lasting toxic effects in aquatic environment (Aquatic Chronic Cat 2, H411).

The degradation processes of the substance

In neutral and alkaline media, xanthates decompose by hydrolytic decomposition. During use in mining processes, the principal decomposition products are carbon disulphide and alcohols. Part of the carbon disulphide formed may decompose further to carbonate and thiocarbonate salts, some of it may evaporate and some may build up in the xanthate solution.

Further hydrolysis of trithiocarbonate to carbonate and hydrogen sulphide and carbon disulphide to carbon dioxide and hydrogen sulphide may occur. The reaction is catalysed by the alcohol formed from the xanthic acid and is self accelerating. Part of the carbon disulphide formed may decompose further to carbonate and thiocarbonate salts, some of it may evaporate and some may build up in the xanthate solution. Once the solubility of carbon disulphide is exceeded it forms a separate layer below the xanthate solution. Formation of dixanthogens is also possible under favourable conditions. Some of the decomposition products are also effective as flotation agents and are known as active impurities. These are hydrosulphide (HS) and trithiocarbonate (CO32 –) ions. (Rao 1971)

Based on the composition and the known decomposition reactions above, potassium isoamyl xanthate will decompose in contact with water to (1) 3 -methylbutan-1 -ol, pentan-1 -ol, (2) carbon disulphide (CS2), and (3) other reaction products such as potassium carbonate and trithiocarbonate. Formation of other decomposition products is dependent on the pH, such as evolution of hydrogen disulphide (H2S) gas.

The rate of decomposition of xanthates is dependent on several factors, the most important being concentration, pH of the solution and temperature. Decomposition of xanthate is the most rapid in acidic conditions and decreases as the pH increases, being much slower in strong alkaline solutions. Other factors which affect rate of decomposition include aging of the solution and presence of metal salts (Rao 1971). Cold climate (as in Scandinavia) significantly affects the degradation of the substance by increasing the stability. According to Fällman (1988) hydrolysis half-life for potassium isoamyl xanthate in tailings pond is 45 days at 17 deg. C

and over 274 days at 0 deg. C at pH 7.5. Sun and Forsling (1997 cited at Lam 1991) investigated the hydrolysis half-lives from supernatants of the flotation tailings at pH 8. The half-lives of xanthates were 72 days at 5 ˚C and 13 days at 20 ˚C, respectively.

Abiotic degradation is the most important fate process of the substance at tailings pond which is the relevant waste water treatment method in the identified use application. At higher temperature (30 ˚C) and at pH 9, the biodegradation studies of xanthates by the microbes isolated from tailings lagoon have presented the most promising results with Pseudomonas putida and P. stutzeri up to xanthate concentration of 10 mg/l (Lam 1999). Higher xanthate concentrations (> 20 mg/l) and presence of cyanide compounds inhibited the microbial activity of tailings lagoon microbes. However, biodegradation is not expected to be a relevant degradation process in the tailings pond.

PIAX is a non-volatile solid. If released to water it will decompose releasing volatile degradation products (mainly CS2,3 -methyl-1 -butanol and 1 -pentanol) to atmosphere. From the known degradation products the most volatile and the most hazardous substance is carbon disulphide during repeated exposure (classified as STOT RE 1, Repro. Cat 2). Carbon disulphide (CS2) released during the hydrolysis is expected to volatilize to atmosphere based on a Henry’s law constant of 1748 Pa m3/mol at 25 °C and a vapour pressure of 47 kPa at 25 °C. If released to atmosphere, the estimated half life of CS2 is 5.5 - 15 days through reaction with hydroxyl radicals, and about 11 d through photolysis (WHO 2003). The loss of CS2 from 10 to 25 % water solutions at 20°C have been reported to be 0.02 % (Aero Xanthate Handbook 1972). Overall, the emissions of CS2 from mining processes to air are considered insignificant. The measured CS2 emissions in the process area have been undetectable or up to 7.8 mg/m3 (maximum 16 mg/m3). Available air monitoring data for sites where solid xanthates are used indicate that atmospheric levels of CS2 are generally below 31 mg/m3 (10 ppm).

In water, potassium isoamyl xanthate is not expected to bioaccumulate in view of its ionic character. No partition to sediments is assumed based on the log low Kow (-0.76) and Koc (20.24 and 5.183 based on log Kow of the substance) estimated by using EPISUITE (EPA 2013). As it is an unstable compound, the apparent toxicity reflects to the toxicity of the degradation products. The alcohols (3 -methylbutan-1 -ol and pentan-1 -ol) released from the substance are readily biodegradable and have low aquatic toxicity (EC50/LC50 values > 100 mg/l; HSDB 2013, OECD 2006, EPA ECOTOX 2012). These substances are neither persistent nor bioaccumulative (log Kowvalues less than 4).

Hydrogen sulphide (H2S) is a possible degradation product of xanthate which is acutely toxic to aquatic organisms. Toxicity to fathead minnows exposed in flow through bioassays, with 96-hour end points is reported to vary between 7 µg/l and 550 µg/l over the temperature range 6 – 24 ˚C (NICNAS 1995). As a water soluble substance, H2S is expected to remain dissolved in water rather than evaporate to air (ATSDR 2006). Ionization of hydrogen sulphide in water may occur, depending primarily upon pH. Hydrogen sulphide may evaporate easily from water, depending on factors such as temperature and pH. In general, low pH and high temperature tend to favor evaporation (HSDB 2013). As H2S is known to liberate from xanthate solutions in strongly alkaline conditions or as gas when pH is less than 6, the formation of H2S is unlikely to be encountered under the conditions of use of PIAX in the mining industry.

If released to water, hydrogen sulphide dissociates in aqueous solution to form 2 dissociation states involving the hydrosulphide anion (HS-) and the sulphide anion (S2-). In an aqueous solution at pH 5 - 9, the primary product is elemental sulphur (Macaluso, 1969 first cited at WHO 2003). Therefore, it is also expected to precipitate with metals to tailings. If released to surface water, microorganisms in water are involved in oxidation-reduction reactions which oxidize hydrogen sulphide to elemental sulphur. Several aquatic and marine micro-organisms are known to oxidize hydrogen sulphide to elemental sulphur, and its half-life in these environments usually ranges from 1 h to several hours (Jørgensen, 1982 first cited at WHO 2003). Food-chain bioconcentration and biomagnification are unlikely (HSDB 2013).

If PIAX is released to soil, it has low potential to adsorb to soil (log Kow -0.76, estimated Koc 5.183 based on this log Kow value) and is not persistent. The substance is hydrolytically unstable, and laboratory studies on biodegradation also present the substance to be inherently biodegradable. Also the decomposition products have a low potential to adsorb to soil and are not persistent. When the substance comes in contact with water or moisture, hydrolysis will take place, alcohols (3 -methylbutan-1 -ol and pentan-1-ol) and CS2 being the most critical decomposition products. The relevant alcohols are readily biodegradable having also low log Kow values (< 4). If the alcohols are released to soil, they are expected to have very high mobility based upon an estimated Koc < 4. Volatilization from moist soil surfaces is expected based upon a Henry's Law constant of 1.41 Pa m3/mol and the vapor pressure (ca. 0.3 kPa at 25 °C) and low adsorption to soil (HSDB 2013). CS2 has a low adsorption potential (log Koc = 1.8), and is expected to evaporate rapidly based upon a Henry's Law constant of 1748 Pa m3/mol at 25 °C and vapor pressure 47 kPa at 25 °C (WHO 2003).