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Environmental fate & pathways

Phototransformation in water

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Reference
Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
supporting study
Study period:
Not reported
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Justification for type of information:
This paper is included as supporting information for completeness and in order to provide further detail on how methyltin compounds are understood to behave in the environment.
Reason / purpose:
other: read across target
Study type:
direct photolysis
Qualifier:
no guideline followed
Principles of method if other than guideline:
The kinetics and mechanism of degradation of methyltins under UV irradiation was studied with a newly developed HPLC-HG-MGLS-QSIL-FPD method. The influenced mechanisms of some environmentally relevant parameters such as pH, salinity and humic acid on the degradation of methyltins were studied in detail.
GLP compliance:
not specified
Specific details on test material used for the study:
- Name of test material (as cited in study report): Trimethyltin chloride (TMT), dimethyltin dichloride (DMT) and monomethyltin trichloride (MMT)
- Purity: TMT 98 %, DMT 97 % and MMT 97 %
- Source: Aldrich Chemical Co. (USA)
Radiolabelling:
no
Analytical method:
other: HPLC-HG-MGLS-QSIL-PFD
Light source:
other: mercury vapour lamp
Light spectrum: wavelength in nm:
253.7
Details on light source:
The ultraviolet light (UV) source for photodegradation was a 40 W low-pressure mercury vapour lamp with the wave length of emission of 253.7 nm and lamp length of 38 cm.
Details on test conditions:
Into a 250 mL beaker was added 200 mL of 500 ng/mL (as Sn) MMT, DMT or TMT solution, and the UV lamp was adjusted to 6 cm over the liquid surface of the beaker. Typically, four samples can be simultaneously processed.
After irradiation for the prescribed time with the UV lamp, 200 µL of the sample solution was taken from the beaker and directly injected into HPLC-HG-MGLS-QSIL-PFD system, and the methyltin compounds completely separated and determined within 20 minutes.
To investigate the effect of pH, methyltin solutions were adjusted to pH 4, 6 and 8, using hydrochloric acid and potassium hydroxide.
Methyltin solutions with 0, 0.5, 1.5 and 3.0 % (m/v) of sodium chloride and pH 6 were used to study the effect of salinity, and solutions with 0, 3, 6 and 9 mg/L humic acid and pH 6 were used to evaluate the effect of humic acid.
To determine also if the degradation takes place through a radical reaction, methanol (0.05 %, v/v), a well-known hydroxyl radical scavenger, was added to the MMT aqueous solution.
DT50:
181 min
Test condition:
MMT at pH 4
DT50:
93 min
Test condition:
MMT at pH 6
DT50:
210 min
Test condition:
MMT at pH 8
DT50:
305 min
Test condition:
MMT with 0.05 % methanol at pH 4
DT50:
815 min
Test condition:
MMT with 0.05 % methanol at pH 6
DT50:
1 645 min
Test condition:
MMT with 0.05 % methanol at pH 8
DT50:
408 min
Test condition:
DMT at pH 4
DT50:
70 min
Test condition:
DMT at pH 6
DT50:
198 min
Test condition:
DMT at pH 8
DT50:
267 min
Test condition:
TMT at pH 4
DT50:
140 min
Test condition:
TMT at pH 6
DT50:
1 386 min
Test condition:
TMT at pH 8
Transformation products:
yes
Remarks:
the methyloltin compound CH2OH–Sn3+
Details on results:
DEGRADATION KINETICS AT DIFFERENT pH
The methyltin compounds can be degraded under the irradiation of ultraviolet light. The degradation rates of MMT, DMT and TMT are strongly dependent on the solution pH. All the degradation curves of the methyltins were in accord with the pseudo-first-order rate equation, except for MMT at pH 4 and 6, which were in accordance with the zero-order rate equation.
For all the studied methyltins the highest degradation rate was obtained at pH 6, while the lowest one was obtained at pH 8. Two major reasons might be responsible for this phenomenon. First, the species of the three methyltin compounds in water are dependent on the pH, which influences the energy needed to break the Sn–C bond. Various species of methyltin cations are formed over a wide pH range. For example, (CH3)2Sn2+ forms five species including (CH3)2Sn(OH)+, (CH3)2Sn(OH)2, (CH3)2Sn(OH)3- , [(CH3)2Sn]2(OH)2++ and [(CH3)2Sn]2(OH)3+ , while (CH3)3Sn+ forms only (CH3)3Sn(OH) and (CH3)3Sn (OH)2- . Monomethyltin(IV) cation undergoes hydrolysis and forms CH3Sn(OH)2+, CH3Sn(OH)3 and CH3Sn(OH)4- species in the pH range 2 to 10.5. Moreover, the species would be hydrated to form more stable structures with a coordination number of 5 or 6. Second, the UV degradation of organic compounds is the free radical reaction, which is influenced by the various anions in aqueous solutions. As an anion, OH- can function as a free radical scavenger in aqueous solutions, it can inhibit and reduce the formation of hydroxyl radical thus decrease the degradation rate of methyltins at pH 8. Since HCl was used to adjust the solution pH, the aqueous solutions at pH 4 possessed a higher concentration of Cl- than that at pH 6 and thus lower degradation rate.
The degradation rate of methyltins has a sequence of TMT < DMT < MMT because of the reverse sequence of methyl groups which hinder the reaction. This study indicates that the degradation route of methyltins might be a stepwise demethylation to inorganic tin, as the detected degradation products were DMT, MMT and an unknown intermediate product for TMT; MMT and the unknown intermediate product for DMT; and the unknown intermediate product for MMT.
A very low concentration of MMT was detected during the degradation process of DMT and TMT at pH 6. High concentration of the intermediate product was formed for MMT and DMT at pH 8, which means that the intermediate product can be easily formed at pH 8; whereas DMT degraded to MMT, then formed the intermediate product and finally tin. For TMT, however, the degradation mechanism might be different from that of MMT and TMT as the intermediate product was not detected even at pH 8.
It was determined that methanol greatly inhibits the degradation of MMT at all the studied pH values. The half-life time at pH 4, 6 and 8 increased from 181, 93 and 210 min to 305, 815 and 1645 min, respectively). Therefore, it can be concluded that the degradation of methyltins under UV irradiation is through the radical reaction.

CONFIRMATION OF THE UNKNOWN INTERMEDIARY PRODUCT
It was considered to be very important to elucidate the structure of the intermediate product to understand the mechanism of methyltin degradation. According to the chromatograms of methyltins, the retention times of TMT, MMT, DMT and the intermediate product are 9.6, 13.7, 17.3 and 11.2 min, respectively. As MMT, DMT and TMT have the same intermediate product and MMT has the simplest structure, MMT was used to deduce the structure of the intermediate product. The peak height of the intermediate product increases at first and then decreases during the degradation process of MMT, which means that the intermediate product is an organotin compound because organotin compounds have higher sensitivity using the sulfur interference filter (390 nm) in this study. However, the maximum emission wavelength of Sn–H bonds is at 610 nm and the sensitivity of FPD for inorganic tin is poor at 390 nm, even no signal for the end product, inorganic tin. The retention time of the intermediate product is shorter than that of MMT in the reversed phase HPLC chromatogram, suggesting that the polarity of the intermediate product is stronger than that of MMT.
Thus, the intermediate product might be CH2OH–Sn3+, (CHO)–Sn3+or CH3O–Sn3+. As Sn–C bond is detected at 390 nm by QSIL-FPD, the intermediate product should not be CH3O–Sn3+. During the degradation process, the amount of the formed intermediate product was strongly dependent on the solution pH because its content at pH 8 is greatly higher than that at pH 4 and 6. This suggests that this intermediate product is more easily formed and is more stable in alkaline solutions, thus the methyloltin compound CH2OH–Sn3+ has higher possibility.
To detect the intermediate product and thus elucidate its structure, direct electrospray ionization-Quadrupole-Time of flight-mass spectrometry (ESI-Q-TOF-MS) was used to determine the samples taken from a completely degraded MMT solution which had a higher concentration of the intermediate product. The result suggests the intermediate product at pH 3.5 and 8 has equal stability as the same peak height and retention time were obtained from the chromatograms. Therefore, the structure of intermediate product was elucidated at pH 3.5 in order to get stronger signal using ESI-Q-TOF-MS.
Both the positive and negative ionization modes of Electrospray ionization (ESI) were investigated by ESI-Q-TOF-MS, and results suggested the negative mode has stronger signals than the positive mode. Moreover, experiments show that, with negative ionization mode, better signal was obtained at pH 3.5 than at pH 8 as organotin compounds are easier to complex with Cl- at pH 3.5 to produce a series of charged species. Various charged species were detected in this study with negative ESIQ-TOF-MS at pH 3.5. MMT and the intermediate product exhibited stronger signals at pH 3.5 and the negative ionization mode. The characteristic isotopic distribution of the tin atom allows clear proof of the presence of tin in these fragment ions because elemental chlorine has two isotopes with the relative isotopic abundance of m/z 35 (75.78 %) and 37 (24.22 %), while elemental tin has 10 isotopes, m/z 116 (14.54 %), 118 (24.22 %), 120 (32.58 %), 122 (4.63 %) and 124 (5.79 %).
Comparing the spectra of MMT and the intermediate product, it can be found that there are no peaks at m/z 239, 241 and 243, the molecular weight of monomethyltin trichloride (CH3SnCl3), which is a neutral molecule and cannot produce the signal at the negative mode. There are strong signals at m/z 273, 275, 277 and 279 ([CH3SnCl4]-). Although the intermediate product and MMT have the same ratio of mass/charge at m/z 273, 275, 277 and 279, the isotopic intensity of m/z 275 is stronger than m/z 277 for MMT than for the intermediate product, suggesting they are different compounds and contain different numbers of chlorine atoms. According to the calculation of a series of molecular weights, the intermediate product can be concluded to be methyloltin compound (CH2OH)Sn3+ and it is [Sn(CH2OH) (OH)Cl3]- that gives m/z 273, 275, 277 and 279.

PHOTODEGRADATION MECHANISM
As the result of ESI-Q-TOF-MS suggests the intermediate product to be methyloltin compound,the possible photodegradation mechanism was elucidated.
MMT produces inorganic tin and methyloltin compound through free hydroxyl radical reaction at pH 4 and 6. However, the reaction mechanism is different at pH 8 as methyloltin compound is always the major product until the complete degradation of MMT. This is due to the production of the hydroxyl radical that was partly inhibited at pH 8 and the hydroxyl radical mainly attacks the methyl group to produce the methyloltin compound first, then the hydroxyl radical attack leads to the cleavage of Sn–C bond and finally produces the inorganic tin. DMT has a similar photodegradation pathway to MMT, which produces MMT, methyloltin compound and inorganic tin at pH 4 and 6, as well as the same phenomenon as MMT at pH 8. TMT has a different degradation mechanism from MMT and DMT, as almost no methyloltin compound was detected even at pH 8. Therefore, the mechanism for TMT is that more methyl groups were simultaneously demethylated for TMT degradation under the UV irradiation.

EFFECT OF SALINITY ON DEGRADATION
The degradation rates of methyltin compounds were strongly affected by the salinity of aqueous solutions. For TMT and DMT, the degradation rates at pH 6 decreased with the increase of NaCl concentrations in the range of 0 - 3.0 % (m/v). For MMT, however, the degradation rate in solutions with NaCl was much higher than that without NaCl, though the degradation rate also decreased with the increase of NaCl concentrations in the range of 0.5 - 3.0 % (m/v). It is noteworthy that the NaCl almost completely inhibited the degradation of TMT within 500 minutes. The degradation rate of DMT is greatly reduced by saline. The half-life times in solutions containing NaCl are more than twice of those without NaCl, and it increased with the NaCl concentration in the range of 0.5 - 3.0 % (m/v). This is because Cl- can scavenge the free radical in solutions of TMT and DMT. Furthermore, TMT was firstly degraded to DMT and then to other products, thus TMT degradation was greatly inhibited with the increase of NaCl concentration. However, it is difficult to explain the increase of degradation rate for MMT in the solution containing NaCl in comparison to that without NaCl. It is possible that the chloride anion can form ion pairs with alkyltin(IV) cations and their hydrolysed species, such as the formation of (CH3)Sn(OH)Cl+ and other Cl- mixed species, which greatly decreases the bond energy of C–Sn in MMT and leads to the easier degradation of MMT, even though the scavenger function of the chloride ion is taken into account.

EFFECT OF HUMIC ACID ON DEGRADATION
The degradation rates of methyltin compounds were strongly influenced by the humic acid at pH 6 in this study. It is interesting that the degradation rate of MMT is the zero order at pH 6 and various concentrations of humic acid (0 - 9 mg/L), and decreased with increasing humic acid concentration with half-life times of 95, 113, 180 and 254 minutes at 0, 3, 6 and 9 mg/L of humic acid, respectively.
While DMT had slight degradation at the range of 0 - 9 mg/L of humic acid, TMT was almost completely inhibited by the humic acid (3 - 9 mg/L) at pH 6. It is noteworthy that the degradation routes with humic acid are the same as that without humic acid because the same intermediary products were detected. Two major reasons might be responsible for the decrease of methyltin degradation rate by humic acid. First, methyltins can react with humic acid to form polymeric species as humic acid, a ubiquitous class of electrolytes, can form the complexes with organotin compounds in natural waters through chelation and hydrogen bonding, which influence strongly the degradation rates of methyltins. Second, humic acid and its UV degradation products can bind with free radicals and thus function as a powerful free radical scavenger.
However, it is not clear why the degradation rate orders are different from each other for MMT, DMT and TMT. It may be due to forming the different complex structures of methyltins and humic acid.

Table 1: Degradation rate constants and half-life times

Compound

pH

Half-life time (minutes)

Degradation curve and rate constant

MMT

4

181

y = -1.30x + 486.33, R² = 0.978 ( linear)

6

93

y = -2.76x + 506.62, R² = 0.999 ( linear)

8

210

y = 0.003x, R² = 0.989

MMT (with 0.05 % methanol)

4

305

-

6

815

-

8

1645

-

DMT

4

408

y = 0.0017x, R² = 0.965

6

70

y = 0.0101x, R² = 0.996

8

198

y = 0.0035x, R² = 0.992

TMT

4

267

y = 0.0026x, R² = 0.992

6

140

y = 0.0048x, R² = 0.988

8

1386

y = 0.0005x, R² = 0.987

Validity criteria fulfilled:
not applicable
Conclusions:
MMT, DMT and TMT can be degraded under UV irradiation rapidly at different pHs, with a degradation rate sequence of TMT < DMT < MMT.
Executive summary:

The photodegradation of monomethyltin trichloride (MMT), dimethyltin dichloride (DMT) and trimethyltin chloride (TMT) was studied with a newly developed HPLC-FPD hyphenated system, which enables rapid and sensitive detection of methyltins. The half-life times and kinetic rate constants of their degradation at different pH values were calculated.

The results suggest that MMT, DMT and TMT can be degraded under UV irradiation rapidly at different pHs, with a degradation rate sequence of TMT < DMT < MMT. An unknown intermediary product, which is more stable and has higher concentration at pH 8 for MMT and DMT, of methyltin photodegradation was detected for the first time. This unknown intermediary product was identified as methyloltin with electrospray mass spectrometry, and a possible mechanism was proposed based on the intermediary product.

The effects of some environmental parameters such as salinity and humic acid on the degradation rate of methyltins were also investigated. Results suggest that salinity and humic acid have strong effect on their degradation, especially for TMT, which was almost never degraded in the solutions containing NaCl and humic acid.

Description of key information

The results suggest that MMT, DMT and TMT can be degraded under UV irradiation rapidly at different pHs, with a degradation rate sequence of TMT < DMT < MMT.

Key value for chemical safety assessment

Additional information

A paper is included as supporting information for completeness and in order to provide further detail on how methyltin compounds are understood to behave in the environment.

The photodegradation of monomethyltin trichloride (MMT), dimethyltin dichloride (DMT) and trimethyltin chloride (TMT) was studied with a newly developed HPLC-FPD hyphenated system, which enables rapid and sensitive detection of methyltins. The half-life times and kinetic rate constants of their degradation at different pH values were calculated.

The results suggest that MMT, DMT and TMT can be degraded under UV irradiation rapidly at different pHs, with a degradation rate sequence of TMT < DMT < MMT. An unknown intermediary product, which is more stable and has higher concentration at pH 8 for MMT and DMT, of methyltin photodegradation was detected for the first time. This unknown intermediary product was identified as methyloltin with electrospray mass spectrometry, and a possible mechanism was proposed based on the intermediary product.

The effects of some environmental parameters such as salinity and humic acid on the degradation rate of methyltins were also investigated. Results suggest that salinity and humic acid have strong effect on their degradation, especially for TMT, which was almost never degraded in the solutions containing NaCl and humic acid.