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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance is readily biodegradable
other:
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
Qualifier:
according to
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
GLP compliance:
not specified
Radiolabelling:
no
Analytical monitoring:
not specified
Preliminary study:
Hydrolysis will be a significant factor in determining the environmental fate of Reaction mass of SEX . In neutral or mildly alkaline solutions, Reaction mass of SEX decomposes to the alcohol, carbon disulphide, sodium carbonate and sodium trithiocarbonate, the two salts arising from neutralisation of carbon disulphide with the sodium hydroxide liberated. In more strongly alkaline media, hydrogen sulphide is liberated. However, strongly alkaline conditions are unlikelyto be encountered under the conditions of use in the mining industry. The half-life atpH 7 at 25°C is reportedly about 260 hours, increasing to over 500 hours in the pH range 8 to 11.
Reaction mass of SEX is hydrolytically unstable when exposed to acidic conditions, reverting rapidly to ethyl, carbon disulphide and sodium hydroxide, and therefore will not persist in the acidic environment of tailings dams. If dischargedto waterways, the chemical would be likely to persist for at least some days,hydrolysing only slowly in this more neutral environment. However, it is notexpected to bioaccumulate in view of its ionic character.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
No.:
#4
Details on hydrolysis and appearance of transformation product(s):
In neutral and alkaline media, xanthates decompose by hydrolytic decomposition.
6ROCS–2 + 3H2O →6ROH + CO3 2 – + 3CS2 + 2CS3 2 –
Further hydrolysis of sodium trithiocarbonate to sodium 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.

Reaction C is the main reaction in alkaline solution while A and B occur in acidic solutions. During use in mining processes, reaction C is the principal decomposition pathway and carbon disulphide the principal decomposition product.
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 Reaction mass of SEX solution.
Reactions A and B are minor and require acidic conditions. Reaction C proceeds in neutral or alkaline pH and is self-accelerating, as it is catalysed by the alcohol formed as a product. Its rate increases with concentration of the reagents and with temperature, from 1.1%/day at 20 °C to 4.6%/day at 40 °C for a 10% solution at pH=10. A decrease in pH from 10 to 6.5 increases the decomposition rate from 1.1%/day to 16%/day. Decomposition is also accelerated by the presence of metals, such as copper, iron, lead or zinc, which act as a catalyst.
pH:
7
Temp.:
25 °C
DT50:
260 h
Type:
other: abiotic
pH:
11
Temp.:
25 °C
DT50:
500 h
Type:
other: abiotic
Details on results:
Hydrolysis will be a significant factor in determining the environmental fate of Reaction mass of SEX . In neutral or mildly alkaline solutions, Reaction mass of SEX decomposes to the alcohol, carbon disulphide, sodium carbonate and sodium trithiocarbonate, the two salts arising from neutralisation of carbon disulphide with the sodium hydroxide liberated. In more strongly alkaline media, hydrogen sulphide is liberated. However, strongly alkaline conditions are unlikely to be encountered under the conditions of use in the mining industry. The half-life at pH 7 at 25°C is reportedly about 260 hours, increasing to over 500 hours in the pH range 8 to 11
Validity criteria fulfilled:
yes
Conclusions:
The half-life at pH 7 at 25°C is reportedly about 260 hours, increasing to over 500 hours in the pH range 8 to 11
Executive summary:

Hydrolysis will be a significant factor in determining the environmental fate of Reaction mass of SEX. In neutral or mildly alkaline solutions, Reaction mass of SEX decomposes to the alcohol, carbon disulphide, sodium carbonate and sodium trithiocarbonate, the two salts arising from neutralisation of carbon disulphide with the sodium hydroxide liberated. In more strongly alkaline media, hydrogen sulphide is liberated. However, strongly alkaline conditions are unlikelyto be encountered under the conditions of use in the mining industry. The half-life atpH 7 at 25°C is reportedly about 260 hours, increasing to over 500 hours in the pH range 8 to 11.

Reaction mass of SEX is hydrolytically unstable when exposed to acidic conditions, reverting rapidly to ethyl, carbon disulphide and sodium hydroxide, and therefore will not persist in the acidic environment of tailings dams. If dischargedto waterways, the chemical would be likely to persist for at least some days,hydrolysing only slowly in this more neutral environment. However, it is notexpected to bioaccumulate in view of its ionic character.

Description of key information

Hydrolysis will be a significant factor in determining the environmental fate of Reaction mass of SEX . In neutral or mildly alkaline solutions, Reaction mass of SEX  decomposes to the alcohol, carbon disulphide, sodium carbonate and sodium trithiocarbonate, the two salts arising from neutralisation of carbon  disulphide with the sodium hydroxide liberated. In more strongly alkaline media, hydrogen sulphide is liberated. However, strongly alkaline conditions are unlikelyto be encountered under the conditions of use in the mining industry. The half-life atpH 7 at 25°C is reportedly about 260 hours, increasing to over 500 hours in the pH range 8 to 11.
Reaction mass of SEX is hydrolytically unstable when exposed to acidic conditions, reverting rapidly to ethyl, carbon disulphide and sodium hydroxide, and therefore will not persist in the acidic environment of tailings dams. If dischargedto waterways, the chemical would be likely to persist for at least some days,hydrolysing only slowly in this more neutral environment. However, it is notexpected to bioaccumulate in view of its ionic character.

Key value for chemical safety assessment

Half-life for hydrolysis:
260 h
at the temperature of:
25 °C

Additional information

Hydrolysis is a chemical reaction during which molecules of water (H2O) are split into hydrogen cations (H+, conventionally referred to as protons) and hydroxide anions (OH−) in the process of a chemical mechanism).

 

When water is added to Reaction mass of SEX it reacts with water to form the others substances: alcohol, sodium carbonate, trithiocarbonate and carbon disulphide.

 

On this basis, Reaction mass of SEX does not have a potential for Hydrolysis and sodium ion will not hydrolise.

 

On the other basis hydrolysis may proceed with the others active substances:

 Further hydrolysis of sodium trithiocarbonate to sodium 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.

 

On this basis hydrolysis proceed with the others active substances: sodium trithiocarbonate, carbon disulphide, hydrogen sulphide.

 

Xanthates decompose in aqueous solution by dissociation, oxidation and hydrolysis. Hydrolytic decomposition is the main reaction in alkaline solutions while the other two reactions occur in acidic solutions.

 

Aqueous solution

There are three decomposition pathways of xanthates in aqueous solution:

A. Xanthates dissociate forming alkali metal cations and xanthate anions. The solution undergoes further hydrolysis to xanthic acid which decomposes into carbon disulphide and alcohol.

ROCS2Na + H2O → ROCS2H + NaOH

 

ROCS2H → CS2 + ROH

 

B. Xanthate is oxidised to dixanthogen. The extent of this reaction is very small and dependent on the pH. Equilibrium is reached after about 5–10% of the xanthate is oxidised, and the reaction increases with a fall in the pH.

 

2ROCS–2 + H2O + _O2 →(ROCS2)2 + 2OH–

 

C. In neutral and alkaline media, xanthates decompose by hydrolytic decomposition.

6ROCS–2 + 3H2O →6ROH + CO3 2 – + 3CS2 + 2CS3 2 –

Further hydrolysis of sodium trithiocarbonate to sodium 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.

 

Reaction C is the main reaction in alkaline solution while A and B occur in acidic solutions. During use in mining processes, reaction C is the principal decomposition pathway and carbon disulphide the principal decomposition product.

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 Reaction mass of SEX solution.

Reactions A and B are minor and require acidic conditions. Reaction C proceeds in neutral or alkaline pH and is self-accelerating, as it is catalysed by the alcohol formed as a product. Its rate increases with concentration of the reagents and with temperature, from 1.1%/day at 20 °C to 4.6%/day at 40 °C for a 10% solution at pH=10. A decrease in pH from 10 to 6.5 increases the decomposition rate from 1.1%/day to 16%/day. Decomposition is also accelerated by the presence of metals, such as copper, iron, lead or zinc, which act as a catalyst.