Registration Dossier

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

Environmental fate and pathways

 

Environmental Fate/Exposure Summary: 

Environmental exposure

Sodium isobutyl xanthate is used as a collector during the processing of sulphide ores by flotation, a process that involves addition of the reagent to aqueous slurries ofcrushed and finely ground ore contained in flotation tanks. Air is blown through theslurry. In general, a series of such tanks is used. During the use of Sodiumisobutylxanthate in the flotation process the mineral particles become separated as a frothfrom the tailings, which settle at the bottom of the flotation tank.

The froth (float) is collected and dried, either in air under ambient conditions or atelevatedtemperatures in an oven,while the tailings are conveyed as a slurry to atailings dam where they settle, dry and consolidate. Spills and washings wouldalso be directed to tailings dams. Tailings typically have a solids content of about30%. In some operations, tailings may be intercepted in settling tanks so thatwastewater containing low concentrations of xanthates can be recovered for reuse inflotation. Xanthates in the tailings are not monitored, but most would be expected to beretained in the froth. Xanthate residues in the ore concentrate are expected to decomposeduring drying or smelting.

 

Environmental fate

Hydrolysis will be a significant factor in determining the environmental fate of Sodium isobutyl xanthate. In neutral or mildly alkaline solutions, Sodium isobutyl xanthate 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 pHrange 8 to 11.

 

Sodium isobutyl xanthate is hydrolytically unstable when exposed to acidic conditions, reverting rapidly to isobutyl, carbon disulphide and caustic soda, 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.Sodium isobutyl xanthate is not expected to contaminate the environment where oretailings are confined to well constructed tailings dams. Most will be retained onsulphide minerals and destroyed when they are dried after flotation. Minor residues that remain associated with tailings will be destroyed by hydrolysis intailings dams.

 

 Stability

 

Phototransformation in air

 

If released to air, a vapor pressure of 1.26E-009 mm Hg at 25 deg C (1.26E-009 mm Hg is equivalent to 1.68E-007 Pa ) indicates Sodium isobutyl xanthate will exist solely as a vapor in the atmosphere. Vapor-phase Sodium isobutyl xanthate will be degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life for this reaction in air is estimated to be 0.667 days, calculated from its rate constant of 16.0392 E-12 cm3/molecule-sec at 25 deg.

Xanthates do not contain chromophores that absorb at wavelengths >290 nm and therefore Sodium isobutyl xanthate is not expected to be susceptible to direct photolysis by sunlight.

Using the AOPWIN QSAR model, the photochemical degradation rate of Sodium isobutyl xanthate in the atmosphere is 16.0392 E-12 cm3/molecule-sec, with a resultant predicted half live of 8.002 Hrs (0.667 Days (12-hr day; 1.5E6 OH/cm3)

OVERALL OH Rate Constant = 16.0392 E-12 cm3/molecule-sec

HALF-LIFE = 0.667 Days (12-hr day; 1.5E6 OH/cm3)

HALF-LIFE = 8.002 Hrs

Phototransformation in water

It is not applicable for a compound wich dissociates.

When water is added to Sodium isobutyl xanthate it reacts with water to form the others substances: alcohol, sodium carbonate, trithiocarbonate and carbon disulphide because of its high water solubility.

Phototransformation in soil

If released to soil,Sodium isobutyl xanthate is expected to have very high mobility based upon an estimated Koc of 11.7.

Volatilization from moist soil surfaces is not expected to be an important fate testing for Phototransformation in soils does not need to be performed.

 

Hydrolysis

 

Hydrolysis will be a significant factor in determining the environmental fate of Sodium isobutyl xanthate. In neutral or mildly alkaline solutions, Sodium isobutyl xanthate decomposes to the alcohol, carbon disulphide, sodium carbonate and sodium trithiocarbonate, the two salts arising from neutralisation of carbondisulphide 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.

Sodium isobutyl xanthate is hydrolytically unstable when exposed to acidic conditions, reverting rapidly to ethanol, carbon disulphide and caustic soda, and therefore will not persist in the acidic environment of tailings dams. If discharged to waterways, the chemical would be likely to persist for at least some days, hydrolysing only slowly in this more neutral environment. However, it is not expected to bioaccumulate in view of its ionic character.

 

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. Thesolution undergoes further hydrolysis to xanthic acid which decomposes intocarbon disulphide and alcohol.

ROCS2Na + H2O→ ROCS2H + NaOH

 

ROCS2H →CS2+ ROH

 

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

 

2ROCS2+ H2O + _O2→ (ROCS2)2+ 2OH

 

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

6ROCS2+ 3H2O →6ROH + CO32 –+ 3CS2+ 2CS32 –

Further hydrolysis of sodium trithiocarbonate to sodium carbonate andhydrogen sulphide and carbon disulphide to carbon dioxide and hydrogensulphide may occur. The reaction is catalysed by the alcohol formed from thexanthic 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 decompositionpathway and carbon disulphide the principal decomposition product.

Part of thecarbon disulphide formed may decompose further to carbonate and thiocarbonatesalts, 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 belowthe sodium isobutyl xanthate 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

 

Biodegradation

Biodegradation in water: screening tests

 

When water is added to Sodium isobutyl xanthate it reacts with water to form the others substances: alcohol, sodium carbonate, trithiocarbonate and carbon disulphide.

Carbon disulphide (CAS number 75–15–0) it is the major decomposition product, which is liquid and have to be considered.

The biodegradation of CS2 was >80 % after 28 days of exposure, therefore CS2 is readily biodegradable.

Sodium isobutyl xanthate readily decomposes to carbon disulphide, especially in the presence of moisture/water.

Therefore, the Biodegradation in water of carbon disulphide (CS2) need to be considered in the assessment of sodium isobutyl xanthate.

These results suggest that sodium isobutyl xanthate will be readily biodegradable and it is not expected to persist in the environment.

Biodegradation in water and sediment: simulation tests

Sodium isobutyl xanthate readily decomposes to carbon disulphide, especially in the presence of moisture/water. Therefore, the Biodegradation in water of carbon disulphide (CS2) need to be considered in the assessment of sodium isobutyl xanthate.

Due to their structural similarity, it is expected that all xanthates would have similar Biodegradation effects either due to the xanthate or carbon disulphide.

On this basis, testing for Biodegradation in water for Sodium isobutyl xanthate is not applicable

Carbon disulphide (CAS number 75–15–0) it is the major decomposition product, which is liquid and have to be considered.

According to “ANNEX IX- STANDARD INFORMATION REQUIREMENTS FOR SUBSTANCES MANUFACTURED OR IMPORTED IN QUANTITIES OF 100 TONNES OR MORE”, a simulation testing on ultimate degradation in surface water, the study does not need to be performed if the substance is ready biodegradable. As Sodium isobutyl xanthate is ready biodegradable a ready biodegradability study does not need to be conducted.

Therefore testing for Biodegradation in water does not need to be performed.

Biodegradation in soil

If released to soil,Sodium isobutyl xanthate is expected to have very high mobility based upon an estimated Koc of 11.7. Volatilization from moist soil surfaces is not expected to be an important fate process.

Therefore testing for biodegradation in soil does not need to be performed.

Bioaccumulation

 

This substance has a limited potential to bioaccumulate(based on log Kow used :, Log Kow estimated : 1.78, and predicted bioconcentration factors, log BCF =0.839  (EPIWIN/BCF Program).

Bioconcentration/bioaccumulation was estimated using QSAR estimation software (EPISUITE v4.1 BCFBAF v3.00), in accordance with REACH Annex XI. The estimated log BCF is 0.84 (BCF = 6.9 L/kg wet-wt) using the regression-based method. The estimated log BAF is 0.71 (BAF = 5 SMILES : O(C(CC)C)C(S)=S

CHEM  : Carbonodithioic acid, o-(2-methylpropyl) ester, sodium salt

MOL FOR: C5 H10 O1 S2

MOL WT : 150.25

 

BCFBAF Program (v3.01) Results:

==============================

SMILES : O(C(CC)C)C(S)=S

CHEM  : Carbonodithioic acid, o-(2-methylpropyl) ester, sodium salt

MOL FOR: C5 H10 O1 S2

MOL WT : 150.25

--------------------------------- BCFBAF v3.01 --------------------------------

Summary Results:

 Log BCF (regression-based estimate): 0.84 (BCF = 6.9 L/kg wet-wt)

 Biotransformation Half-Life (days) : 0.0777 (normalized to 10 g fish)

 Log BAF (Arnot-Gobas upper trophic): 0.71 (BAF = 5.11 L/kg wet-wt)

 

Log Kow (experimental): not available from database

Log Kow used by BCF estimates: 1.78

 

Equation Used to Make BCF estimate:

  Log BCF = 0.6598 log Kow - 0.333 + Correction

 

     Correction(s):                   Value

      No Applicable Correction Factors

 

  Estimated Log BCF = 0.839 (BCF = 6.899 L/kg wet-wt)

 

Bioaccumulation: terrestrial

This substance has a limited potential to bioaccumulate (based on log Kow used :, Log Kow estimated : 1.78 , and predicted bioconcentration factors, log BCF = 0.84  (EPIWIN/BCF Program).

The estimated BCF of 6.899 L/kg wet-wt was measured by calculation from EPI SuiteTM v4.1 Program.

These values would suggest very low bioaccumulation potential.

This is Exposure Assessment Tools and Models made from EPA (Environmental Protection Agency).

 

BCFBAF Program (v3.01) Results:

==============================

SMILES : O(C(CC)C)C(S)=S

CHEM  : Carbonodithioic acid, o-(2-methylpropyl) ester, sodium salt

MOL FOR: C5 H10 O1 S2

MOL WT : 150.25

--------------------------------- BCFBAF v3.01 --------------------------------

Summary Results:

 Log BCF (regression-based estimate): 0.84 (BCF = 6.9 L/kg wet-wt)

 Biotransformation Half-Life (days) : 0.0777 (normalized to 10 g fish)

 Log BAF (Arnot-Gobas upper trophic): 0.71 (BAF = 5.11 L/kg wet-wt)

 

Log Kow (experimental): not available from database

Log Kow used by BCF estimates: 1.78

 

Equation Used to Make BCF estimate:

  Log BCF = 0.6598 log Kow - 0.333 + Correction

 

     Correction(s):                   Value

      No Applicable Correction Factors

 

  Estimated Log BCF = 0.839 (BCF = 6.899 L/kg wet-wt)

==========================================================

Whole Body Primary Biotransformation Rate Estimate for Fish:

===========================================================

------+-----+--------------------------------------------+---------+---------

 TYPE | NUM | LOG BIOTRANSFORMATION FRAGMENT DESCRIPTION | COEFF | VALUE 

------+-----+--------------------------------------------+---------+---------

 Frag | 2 | Methyl [-CH3]                           | 0.2451 | 0.4902

 Frag | 1 | -CH2- [linear]                          | 0.0242 | 0.0242

 Frag | 1 | -CH-  [linear]                          | -0.1912 | -0.1912

 L Kow| * | Log Kow =  1.78 (KowWin estimate)       | 0.3073 | 0.5458

 MolWt| * | Molecular Weight Parameter               |        | -0.4417

 Const| * | Equation Constant                        |        | -1.5058

============+============================================+=========+=========

  RESULT  |       LOG Bio Half-Life (days)           |        | -1.1097

  RESULT  |           Bio Half-Life (days)           |        | 0.07767

  NOTE    | Bio Half-Life Normalized to 10 g fish at 15 deg C  |

============+============================================+=========+=========

 

Biotransformation Rate Constant:

 kM (Rate Constant): 8.924 /day (10 gram fish)

 kM (Rate Constant): 5.018 /day (100 gram fish)

 kM (Rate Constant): 2.822 /day (1 kg fish)

 kM (Rate Constant): 1.587 /day (10 kg fish)

 

Arnot-Gobas BCF & BAF Methods (including biotransformation rate estimates):

  Estimated Log BCF (upper trophic) = 0.709 (BCF = 5.111 L/kg wet-wt)

  Estimated Log BAF (upper trophic) = 0.709 (BAF = 5.111 L/kg wet-wt)

  Estimated Log BCF (mid trophic)  = 0.626 (BCF = 4.23 L/kg wet-wt)

  Estimated Log BAF (mid trophic)  = 0.626 (BAF = 4.23 L/kg wet-wt)

  Estimated Log BCF (lower trophic) = 0.596 (BCF = 3.941 L/kg wet-wt)

  Estimated Log BAF (lower trophic) = 0.596 (BAF = 3.941 L/kg wet-wt)

 

Arnot-Gobas BCF & BAF Methods (assuming a biotransformation rate of zero):

  Estimated Log BCF (upper trophic) = 0.862 (BCF = 7.278 L/kg wet-wt)

  Estimated Log BAF (upper trophic) = 0.870 (BAF = 7.407 L/kg wet-wt)

 

Transport and distribution

Adsorption / desorption

 

 

According to “ANNEX IX- STANDARD INFORMATION REQUIREMENTS FOR SUBSTANCES MANUFACTURED OR IMPORTED IN QUANTITIES OF 100 TONNES OR MORE , an adsorption study need not be conducted if:

— based on the physicochemical properties the substance can be expected to have a low potential for adsorption (e.g. the substance has a low octanol water partition coefficient), or

— the substance and its degradation products decompose rapidly.

 

 

The log of the adsorption coefficient (KOC) of Sodium isobutyl xanthate was estimated to be log KOC = 1.06 which is equal to a KOC value of 11.7 using the KOCWIN v2.00 QSAR method. This value indicates that Sodium isobutyl xanthate will be adsorbed by organic carbon in soil. Sodium isobutyl xanthate can be classified to be of very high mobility in soil according these results and does not have a high potential for adsorption to soil

 

Sodium isobutyl xanthate and xanthates in general adsorbs strongly to sulphide minerals but has less affinity for surfaces in general. The KOC value of 11.7 also suggest this conclusion.

 

 

The estimated Soil Adsorption Coefficient was 11.7 L/kg measured by calculation from EPI SuiteTM v4.1 Program. This is Exposure Assessment Tools and Models made from EPA (Environmental Protection Agency

 

 

SMILES : O(C(CC)C)C(S([Na]))=S

CHEM  :

MOL FOR: C5 H9 O1 S2 Na1

MOL WT : 172.24

--------------------------- KOCWIN v2.00 Results ---------------------------

NOTE: METAL (Na, Li or K) HAS BEEN REMOVED TO ALLOW ESTIMATION via MCI!

 Koc Estimate from MCI:

 ---------------------

        First Order Molecular Connectivity Index ........... : 3.664

        Non-Corrected Log Koc (0.5213 MCI + 0.60) .......... : 2.5098

        Fragment Correction(s):

                 1  Thiocarbonyl (C=S) .................. : -0.5701

                 1  Ether, aliphatic (-C-O-C-) .......... : -0.8716

        Corrected Log Koc .................................. : 1.0681

 

                        Estimated Koc: 11.7 L/kg  <===========

 

 Koc Estimate from Log Kow:

 -------------------------

        Log Kow (Kowwin estimate) ......................... : -1.33

        Non-Corrected Log Koc (0.55313 logKow + 0.9251) .... : 0.1894

        Fragment Correction(s):

                 1  Thiocarbonyl (C=S) .................. : 0.3004

                 1  Ether, aliphatic (-C-O-C-) .......... : -0.0906

        Corrected Log Koc .................................. : 0.3993

 

                        Estimated Koc: 2.508 L/kg  <===========

 

Henry's Law constant

The estimated Henrys Law Constant (25 deg C) measured by calculation from EPI SuiteTM v4.1, HENRYWIN v3.20 Program was  3.491E-016 atm-m3/mole (3.538E-011 Pa-m3/mole) , which is almost zero.

This is Exposure Assessment Tools and Models made from EPA (Environmental Protection Agency).

Distribution modelling.

Sodium isobutyl xanthate has no affinity to be in air and sediment. The direct emissions to soil and surface water are significant, therefore Sodium isobutyl xanthate will be almost exclusively be found in soil and surface water.

 

Mackay fugacity modelling (level 3) indicates that, taking into account degradation and using inflow parameters which are consistent with the known production tonnage of this substance in, fugacity coefficient indicates that environmental concentrations in water are predicted to be 5.84e-021 (atm), in air (atm) 2.03e-019and soil 2.64e-019 (atm) and sediment to be  5.3e-021  (atm).

These are negligible low levels. This can be considered a worse case prediction as it assumes all product is emitted with no emission control systems used.

 

Other distribution data

 

These results suggest for Sodium isobutyl xanthate that direct and indirect exposure from distribution in media is unlikely.

Based on low vapor pressure and low estimated log Pow, expected to partition to water and soil. Not expected to partition to air, sediments or biota.

 

Therefore testing for distribution in media does not need to be performed.

 

The estimated STP Fugacity Model and Volatilization From Water were measured by calculation from EPI SuiteTM v4.1 Program. This is Exposure Assessment Tools and Models made from EPA (Environmental Protection Agency) .

 

                              Volatilization From Water

               =========================

 

Chemical Name: Carbonodithioic acid, o-(2-methylpropyl) ester, sodium salt

 

Molecular Weight   : 172.24 g/mole

Water Solubility      : 8.179E+005 ppm

Vapor Pressure      : 1.26E-009 mm Hg

Henry's Law Constant: 3.49E-016 atm-m3/mole (calculated from VP/WS)

 

                                                   RIVER        LAKE        

                                                ---------        ---------

Water Depth    (meters):             1                1         

Wind Velocity   (m/sec):             5                0.5       

Current Velocity (m/sec):           1                0.05      

 

     HALF-LIFE (hours) :     2.201E+012       2.401E+013

     HALF-LIFE (days ) :     9.17E+010        1E+012    

     HALF-LIFE (years) :    2.511E+008      2.739E+009

 

 

STP Fugacity Model: Predicted Fate in a Wastewater Treatment Facility

======================================================================

  (using 10000 hr Bio P,A,S)

PROPERTIES OF: Carbonodithioic acid, o-(2-methylpropyl) ester, sodium salt

-------------

Molecular weight (g/mol)                             172.24

Aqueous solubility (mg/l)                             17900

Vapour pressure (Pa)                                  1.67986E-007

               (atm)                                            1.65789E-012

               (mm Hg)                                       1.26E-009

Henry 's law constant (Atm-m3/mol)            9132E-016

Air-water partition coefficient                      1.42785E-014

Octanol-water partition coefficient (Kow)    0.0467735

Log Kow                                                           -1.33

Biomass to water partition coefficient          0.809355

Temperature [deg C]                                    25

Biodeg rate constants (h^-1),half life in biomass (h) and in 2000 mg/L MLSS (h):

         -Primary tank       0.04       16.16    10000.00

         -Aeration tank     0.04       16.16    10000.00

         -Settling tank       0.04       16.16    10000.00

 

                                  STP Overall Chemical Mass Balance:

                                   ---------------------------------

                                                    g/h                  mol/h          percent

 

Influent                                  1.00E+001       5.8E-002      100.00

 

Primary sludge                      2.50E-002        1.4E-004       0.25

Waste sludge                        1.51E-001         8.7E-004       1.51

Primary volatilization           1.90E-013        1.1E-015        0.00

Settling volatilization            5.19E-013        3.0E-015        0.00

Aeration off gas                   1.28E-012         7.4E-015        0.00

 

Primary biodegradation       1.76E-003        1.0E-005         0.02

Settling biodegradation       5.27E-004        3.1E-006          0.01

Aeration biodegradation     6.93E-003         4.0E-005          0.07

 

Final water effluent            9.82E+000       5.7E-002         98.15

 

Total removal                      1.85E-001        1.1E-003        1.85

Total biodegradation           9.22E-003        5.4E-005        0.09