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Environmental fate & pathways

Hydrolysis

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Description of key information

As 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, the estimated half-life (47 d) for hydrolysis in neutral conditions (pH 7, 17 degr. C) was chosen as the key value for chemical safety assessment (Fällman 1988).

Key value for chemical safety assessment

Half-life for hydrolysis:
47 d
at the temperature of:
17 °C

Additional information

There are several studies available on the hydrolytical decomposition on potassium isoamyl xanthate (PIAX), potassium amyl xanthate (PAX) and other structural analogue xanthates. 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.

 

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 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 (Fällman 1988).

Based on the composition and the known decomposition reactions, potassium isoamyl xanthate will decompose in contact with water to (1) carbon disulphide (CS2), (2) 3-methylbutan-1 -ol and pentan-1 -ol, 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 following information is available for xanthates half-lives and the degradation product formed

Acidic conditions: The main degradation products are alcohols and carbon disulphide, and possibly hydrogen disulphide (H2S)

- pH 6: half-life = 1.6 d (25 degr. C) (Rao 1971)

- pH 5.5: half-life = 7 – 14 d (15 degr. C) (Bertillis 1986)

Neutral: The main degradation products are alcohols and carbon disulphide, carbonate and trithiocarbonate ions.

- pH 7: half-life = 11 d (25 degr. C) (Rao 1971)

- pH 7.5: half-life = 47 d (17 degr. C) (Fällman 1988)

- pH 7.5: half-life = 58-67 d (15 degr. C) (Bertillis 1986)

Alkaline: The main degradation products arealcohol, carbon disulphide, carbonate and trithiocarbonate ions.

- Rao: pH 9 = 24 d (25oC)

Strong alkaline: The main degradation products are alcohols, carbonate and sulphide (S2-) anions. Hydrogen sulphide may be liberated.

In addition to pH, temperature has a significant effect on the hydrolytical degradation rate. Sun and Forsling (1997 first cited at Lam 1991) investigated 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.

The stability for 10 % and 25 % (pure) xanthate solutions was determined during two weeks storage (Aero Xanthate Handbook 1972), and estimated amount of CS2released based on a proposed stoichiometric degradation equation. Hydrolytical decomposition half-lives of potassium amyl xanthate (PAX) ranged from 12 to 63 d for 10 % solution and from 10 to 71 d in 25 % solution at 20, 30 and 40oC. pH was not reported. Estimated amount of CS2released ranged between 0.2 - 0.8 g/L (for 10 % solution at 20...40 oC) and between 0.4 - 2.5 g/L (25 % solution at 20...40oC temperature).

As 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, the estimated half-life (47 d) for hydrolysis in neutral conditions (pH 7, 17 degr. C) was chosen as the key value for chemical safety assessment (Fällman 1988).