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Environmental fate & pathways

Mode of degradation in actual use

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Endpoint:
mode of degradation in actual use
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
This report reviews available degradation data on xanthates focusing on the seasonal variation in Swedish tailing ponds in subarctic climate. The results summarise available scientific infromation applying it to realistic conditions in tailing ponds and are therefore rated as scientifically acceptable. Read-across justification: Target substance belongs into the group of substances called xanthates. The alkali metal xanthates, as the target substance also is, are generally prepared from the reaction of the alkoxide, which reacts with carbon disulphide to give the xanthate. These substances contain common functional group which is dithiocarbonate (-OCSS-). Though they are structural analogues with the target substance. All these analogue substances are used in similar use application as water solutions. All xanthates decompose in the presence of water. In neutral to alkaline media, they will release carbon disulphide, particular alcohol(s), 3-methylbutan-1-ol and pentan-1-ol, and carbonates and dithiocarbonates. Carbon disulphide is the major and the most volatile and the most hazardous decomposition products of xanthates. As the xanthates can be considered as a group of substances which have structural similarity and similar behaviour in contact with water and in the physiological processes, their hydrolysis and biodegradation as well as ecotoxicological adverse effects to aquatic organisms are expected to be similar. Therefore, and in order to avoid the unnecessary animal testing, the read-across data from the analogue xanthates is used to evaluate the short-term and long-term toxicity to fish. In addition, the decomposition rate and the most important pathways are evaluated based on the available data on the analogues when there is no data available for the target substance.
Cross-referenceopen allclose all
Reason / purpose:
reference to same study
Reason / purpose:
reference to other study

Data source

Reference
Reference Type:
publication
Title:
Unnamed
Year:
1988

Materials and methods

Principles of method if other than guideline:
The degradation of xanthates were simulated using realistic conditions (monthly water temperatures and inflow rates) in tailing ponds in central Sweden. The impact of alternative technologies to xanthate concentrations in the tailings pond water and discharge was estimated by a numerical model. For xanthates, both a (conservative) constant degradation half-life of 47 days (B), and a calculated temperature-dependant half-life (C) was used. They were compared to a conservative non-degradable substance (A) with t1/2 of 100000 days. Re-generation from dixanthogen was excluded from the modelling. The simulations were not verified with measurements (as it was not known how much dixanthogen was formed and at which rate at it was degraded).
GLP compliance:
no
Type of study / information:
A review of available publications on (hydrolytical) degradation of xanthates. The impact of pH and temperature on degradation half-lives in pure solutions have been recalculated based on literature data, and compared to modelled data in tailings water. Abiotic and/or biotic reaction processes in tailings pond are additionally simulated based on the key parameters obtained from literature.

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
Examples of alkyl xanthates used: sodium isopropyl xanthate, (potassium) isobutyl xanthate, (potassium) amyl xanthate. A single rate is proposed for all xanthates.

Results and discussion

Any other information on results incl. tables

In summary, the modelled concentrations of xanthates in the tailings pond 0.14 - 0.22 mg/L

for a conventional treatment method at dose of 1 g xanthate/ton ore (see Table below).

A very coarse calculation for the concentration in discharge water would end up in concentrations < 1 µg/L.

With three different treatment tehcniques, the concntrations of xanthates in the tailings pond can either increase or decrease ranging 0.05 - 0.23 mg/L. More importantly, these techniques decreased the amount of xanthates discharged 0 - 50 %.

The author referred to measured levels of Walterson (1984) in Swedish tailing ponds. Levels of 0.02 - 42 mg/L xanthates in tailing ponds and 0 - 16800 µg/L in discharge water (calculated as –OCS2equivalent xanthate weight without the carbon chain). This kind of modelling provides a good tool to study the impact of key parameters like temperature, annual precipitation of rain and snow and their seasonal variations. The possibility of a separate biological tratment was pointed our as one possibility to further reduce the xanthate discharges to the surface water.

Table. Summary of xanthate concentrations measured and simulated at the Swedish tailing ponds.

 

Measured

(Walterson 1984) *

Simulated

conventional

(35 % recycled water)

Simulated

reduced drainage

Simulated

thickened

disposal

Simulated

thickened disposal with

reduced drainage

Dose

11 - 162

1 g/tonne ore

1 g/tonne ore

1 g/tonne ore

1 g/tonne ore

Concentr. in pond (mg/L)

0.02 – 42

 

 

0.14 – 0.22

 

0.15 – 0.23

 

0.05 - 0.14

 

0.06 – 0.18

Concentr.

in discha

rge water (µg/L)

0 - 16800

calc.

(0.07 – 0.36)

10 – 20 % of conventional

23 – 30 % of conventional

0 – 50 % of convention

al

*as –OCS2equivalent xanthate weight without the carbon chain

Attachment:

Figure I-1. Schematic presentation of the water flows at an ore concentrator for an underground mine with conventional deposition technique.

Figure I-9. Mean monthly values of inflow rates and water temperatures for the pond in Figure I-1 (in Central Sweden).

Table I-2. Half-lives (t1/2) for xanthates at water temperatures used in the example at pH 7.5 (from Part II)

Figure I-10. Monthly variations of amounts of substances A, B and C discharged to the receiving water.

Figure I-11. Monthly variations of concentration A, C and B in the pond.

Figure I-12. Scematic presentations of water flows at an ore concentrator with thickened disposal technique.

Figure I_13. Monthly variations in amounts of substance A discharged to the receiving water from the pond for conventional (A) and thickened technique (D).

Figure I-14. Monthly variations of xanthates discharged to the receiving water from the pond for conventional (C) and thickened (D) technique.

Figure I-15. Inflow from the drainage are for both the natural and reduced area.

Figure I-16. Concentrations of a) conservative substance A and b) xanthate at snowmelt using the three techniques.

Figure I-17. Discharged amounts of a) conservative substance A and b) xanhtate at snowmelt using the three techniques.

Table I-3. Influence of accumulation of a recycled conservative substance A on discharged amounts and concentration in the pond using the three techniques

Table I-4. Amounts of substances A recycled to the proecess using the three techniques.

Table II-1. The dosage of xanthates at some Swedish ore concentratirs and outlet of xanthates or xanthate derivatives from the tailing ponds (Walterson 1984).

Applicant's summary and conclusion

Conclusions:
Measured levels of xanthates in Swedish tailing ponds have ranged 0.02 - 42 mg/L in tailing ponds and 0 - 16800 µg/L in discharge water (calculated as –OCS2equivalent xanthate weight without the carbon chain).

The modelled concentrations of xanthates in the tailings pond were 0.14 - 0.22 mg/L for a conventional treatment method at dose of 1 g xanthate/ton ore. A rough calculation of xanthate concentrations was < 1 µg/L.. Three different treatment tehcniques either increased or decreased xanthate concentrations in the tailings water (0.05 - 0.23 mg/L). More importantly, these techniques decreased the amount of xanthates discharged to surface water 0 - 50 %. The simulation confirms that annual temperature and seasonal variations in the rain or snow deposition are extremely important parameters when considering the use of treatment technologies for reducing xanthate levels from tailings water.


Executive summary:

This report reviews available degradation data on xanthates focusing on the seasonal variation in Swedish tailing ponds in subarctic climate. Temperature, pH and the concentration of the xanthate are among the key factors determining the degradation of xanthates in tailing ponds.

Measured levels of xanthates in Swedish tailing ponds have ranged 0.02 - 42 mg/L in tailing ponds and 0 - 16800 µg/L in discharge water (calculated as –OCS2equivalent xanthate weight without the carbon chain).

The modelled concentrations of xanthates in the tailings pond were 0.14 - 0.22 mg/L for a conventional treatment method at dose of 1 g xanthate/ton ore. A rough calculation of xanthate concentrations was < 1 µg/L. Three different treatment tehcniques either increased or decreased xanthate concentrations in the tailings water (0.05 - 0.23 mg/L). More importantly, these techniques decreased the amount of xanthates discharged to surface water 0 - 50 %. The simulation confirms that annual temperature and seasonal variations in the rain or snow deposition are extremely important parameters when considering the use of treatment technologies for reducing xanthate levels from tailings water.

The results summarise available scientific infromation applying it to realistic conditions in tailing ponds in subarctic climate and are therefore rated as scientifically acceptable.