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Environmental fate & pathways

Additional information on environmental fate and behaviour

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Endpoint:
additional information on environmental fate and behaviour
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.
Cross-reference
Reason / purpose:
other: read across target
Reference
Endpoint:
additional information on environmental fate and behaviour
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: study conducted on read across material
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:
read-across source

The methyltins and stannic chloride were found to have no significant inhibitory effects on the activity of landfill micro-organisms.

Conclusions:
The methyltins and stannic chloride were found to have no significant inhibitory effects on the activity of landfill micro-organisms.
Executive summary:

The potential transformation of methyltin chlorides and stannic chloride in landfills was investigated, by incubating the target substances at concentrations relevant to landfill conditions (100 and 500 µg Sn/L). The amounts of methane formed in all treatment bottles, and controls, were measured to evaluate the general microbial activity of the inocula and possible effects of methyltins on the degradation of organic matter.

The methyltins and stannic chloride were found to have no significant inhibitory effects on the activity of landfill micro-organisms, and the methanol used to disperse the tin compounds was completely degraded. In some experimental bottles, the methanol degradation gave rise to larger methane yields than expected, which was attributed to enhanced degradation of the waste material.

Alkyltin analyses showed that monomethyltin trichloride at an initial concentration of 500 µg Sn/L promoted methylation of inorganic tin present in the inoculum. No methylation activities were detected in the incubations with 100 µg Sn/L methyltin chlorides (mono-, di- or tri-methyltin), but demethylation occurred instead. Levels of soluble inorganic tin increased during the incubation period, due partly to demethylation and partly to a release of tin from the waste inocula.

Data source

Reference
Reference Type:
publication
Title:
Transformation of methyltin chlorides and stannic chloride under simulated landfill conditions
Author:
Björn A, Hörsing M, Ejlertsson J & Svensson BH
Year:
2011
Bibliographic source:
Waste Manage. Res. 29(12): 1327 - 1336

Materials and methods

Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
The potential transformation of methyltin chlorides and stannic chloride in landfills was investigated, by incubating the target substances at concentrations relevant to landfill conditions (100 and 500 µgSn/L). The amounts of methane formed in all treatment bottles, and controls, were measured to evaluate the general microbial activity of the inocula and possible effects of methyltins on the degradation of organic matter.
GLP compliance:
not specified

Test material

Reference
Name:
Unnamed
Test material form:
not specified
Specific details on test material used for the study:
- Name of test material (as cited in study report): Methyltin trichloride (MMT), dimethyltin dichloride (DMT), trimethyltin chloride (TMT) and stannic chloride (SnCl4*5H2O)
- Purity: MMT 97 %, DMT 97 %, TMT 99 % and stannic chloride 98 %
- Source: Sigma-Aldrich (Stockholm, Sweden)

Results and discussion

Any other information on results incl. tables

Methane formation

The methyltin concentrations used in this study, selected for their relevance to landfill conditions, did not add substantial quantities of organic material to the incubations relative to the indigenous organic matter in the inocula. The CH4 formed from the organic matter of the inocula amounted to approximately 120 µmol for the LSR waste and 600 µmol for the landfill waste, assuming complete degradation to CH4 and CO2. Thus, even the theoretical CH4 yields that could have been obtained from the target methyltin chlorides (0.03 - 0.37 µmol) were too low to be discriminated by the methodology used. However, the methanol used to add the tin compounds to the experimental bottles gave rise to considerably more CH4 formation than control levels. The added methanol corresponded to theoretical CH4 yields ranging from 43 to 252 µmol. Thus, the CH4 formed in addition to the control levels was mainly derived from the methanol. In all cases the methanol added to the MMT-, DMT-, TMT- or SnCl4-containing bottles was completely degraded, >75 % of the expected CH4 was accounted for. In this experiment, the CH4 yields from the bottles with the LSR inoculum ranged from 80 - 108 %, and nearly all of the CH4 production occurred within the first 50 days of the incubations

Notably, the amount of CH4 formed was even larger in some bottles than expected from the theoretical calculations for methanol transformation, especially for bottles inoculated with landfilled MSW in which CH4 yields as high as 483 (±47) % were recorded. These findings suggest that the presence of methanol initiated the degradation of more organic matter from this inoculum, probably by increasing its accessibility for the microbial population which also contributed to the variations in received CH4 yields. Methylotrophic organisms can readily use methanol and several studies have reported that the presence of methanol can either stimulate or inhibit the degradation of various kinds of organic compounds under anaerobic conditions. The difference in induced MSW degradation between the two inocula was also probably partly due to the differences in their dry-solid contents.

The addition of the methyl donor mixture to the experimental bottles after stabilisation of CH4 accumulation resulted in further CH4 formation. For the bottles containing landfill MSW the final difference in accumulated CH4, between controls with and without the added methyl donor mixture, amounted to 256 µmol CH4, which was almost identical to the maximum calculated theoretical amount (254 µmol) for added mixture of syringic acid and dimethyl phthalate. This indicated that the compounds in the mixture were completely degraded.

In the corresponding bottles containing LSR waste partial apparent degradation (58 - 66 %) of the syringic acid and dimethyl phthalate mixture was observed 126 days after addition. However, due to the possibility that methanol may have triggered further degradation of organic compounds in the landfilled MSW it was not possible to assess with certainty how much of the methyl donor mixture had been degraded in the bottles initially supplied with each target tin compound.

Neither the methyltin compounds nor SnCl4 inhibited CH4 formation; that is, the amount of CH4 formed in their presence was never lower than the amounts in the corresponding controls.

 

Transformation of methyltin chlorides and stannic chloride under methanogenic conditions

The results showed that the methyltin chlorides added to the experimental bottles were subjected to both methylating and demethylating reactions, and methylation of inorganic tin also occurred. The degree of methylation appeared to depend on the concentrations of methyltins, particularly MMT, and available inorganic tin. In bottles spiked with MMT, DMT or TMT (100 µg Sn/L) reductions in the concentrations of the added methyltin compounds were observed with time, indicating that demethylation had occurred. In TMT-treated bottles, both MMT and DMT appeared, and TMT concentrations declined to 60 and 63 % of initial levels in the presence of LSR and landfill MSW inocula, respectively. The formation of MMT, DMT and SnCl4, along with simultaneous reductions in TMT concentrations, is highly consistent with demethylation activities. In the bottles to which MMT (100 µg Sn/L) and the landfill inoculum were added, levels of MMT fell, via demethylation, to 28 % of the initial concentration during the first 92 days and to a residual 9 % (9 µg Sn/L) at the end of incubation, compared to 47 % in corresponding LSR bottles. DMT appeared as an initial contaminant at 3 µg Sn/L in these bottles. Similarly, residual levels of 14 % DMT and traces of MMT were found at the end of the incubation in bottles to which landfill MSW and 100 µg Sn/L DMT were added. These findings further strengthen the inference that demethylation occurred.

In addition, DMT levels in bottles to which 500 µg Sn/L of the compound were added with landfill MSW initially declined to 201 µg Sn/L within 19 days. However, the final concentration, of 289 µg Sn/L, indicates that significant remethylation or a different degree of demethylation occurred in the separate bottles. 16 (±3) µg Sn/L MMT was detected in these bottles, further supporting the hypothesis that demethylation occurred.

The most noteworthy findings in this experimental series were the increases in levels of MMT with time in bottles to which 500 µg Sn/L MMT and LSR inoculum were added. Over the 211-day incubation period, methylation of tin occurred and the concentration of MMT increased by a factor of about five. A transient increase in MMT levels (of approximately 100 µg Sn/L) occurred during the first 90 days in corresponding bottles with landfill inoculum. However, by the end of the experiment only about 20 % of the initial concentration remained. Thus, there was probably potential for methylation, but it was either inhibited or counteracted by increasing demethylation.

In the DMT (500 µg Sn/L)-containing bottles with LSR inoculum, a decrease in DMT was observed at the end of the incubation with a simultaneous occurrence of MMT. Similarly, a decrease in DMT occurred in the DMT (500 µg Sn/L)-containing bottles with landfill MSW inoculum.

The supply of the methyl donor mix did not result in increased methylation activity in bottles inoculated with landfill waste. However, for LSR bottles with MMT or TMT (100 µg Sn/L), higher concentrations of MMT (79 compared to 47 µg Sn/L) and TMT (92 compared to 60 µg Sn/L) occurred in the bottles with the methyl donor mix than in the corresponding bottles with no added methyl donor mix.

MMT was formed at low concentrations (5 - 20 µg Sn/L) in the LSR bottles with added SnCl4. The highest concentration (20 µg Sn/L MMT) was found at the end of the incubation of the bottles with 500 µg Sn/L supplied as SnCl4, in which 6 µg Sn/L DMT was also detected. This observation indicates that the LSR inoculum has the capacity to methylate inorganic tin. Smaller amounts (3 µg Sn/L) or no methyltins were observed in LSR controls. In corresponding bottles containing SnCl4 (500 µg Sn/L) with landfill inoculum, only 4 µg Sn/L MMT was detected in one of the analysed samples at day 19. As observed in the experimental bottles inoculated with LSR waste, concentrations of inorganic tin also increased during the incubation period in landfill waste bottles. The increase was due not only to demethylation, but also to a release of tin from the organic matter in the inocula. The difference in the results obtained with the two inocula in this respect is likely to be due to differences in their organic matter contents, which affect the release kinetics of inorganic tin, as well as in their initial contents of inorganic tin. Tin is probably retained in the waste matrix by adsorption processes.

Significant amounts of SnCl4 were detected in LSR bottles to which no SnCl4 had been added, including 33 (±16) µg Sn/L in time-zero bottles. No SnCl4 was detected in corresponding time-zero bottles inoculated with landfill MSW. Heterogeneity among waste samples, together with difficulties of extraction and analysis of inorganic tin, make calculations of tin contents difficult. However, although considerable variation occurred between samples, the total tin concentrations indicate that they generally increased in LSR bottles over the incubation period.

 

Conclusions

The results showed that micro-organisms present in the investigated landfill waste have the potential to transform methyltin chlorides to methylate stannic chloride. The net outcome of methylation and demethylation in those experimental bottles depended on the initial MMT concentrations and the inorganic tin content. The results of experiments with 500 µg Sn/L MMT clearly showed that methylation occurred, especially in the presence of the LSR inoculum, while demethylation was promoted at an initial concentration of 100 µg Sn/L. Levels of soluble inorganic tin increased during the course of the incubations. This was due partly to demethylation, but mainly to the release of tin from the inocula materials. The addition of methyltin chlorides or stannic chloride was observed to have no significant inhibitory effect on the degradation of indigenous organic matter in any of the experimental bottles. In the presence of methanol, added to disperse the target tin compounds, the landfill MSW inoculum gave rise to higher CH4 yields than expected from the calculated theoretical yields.

These experiments suggest that slow releases of methyltin chlorides from PVC, resulting in concentrations of ≤100 µg Sn/L MMT, DMT or TMT, might result in these compounds being demethylated in anaerobic landfill environments. Consequently, methyltin chlorides should not contribute substantially to the toxicity of landfill leachates.

However, at higher release rates, and presumably higher concentrations, net methylation may occur. It is also possible that de novo methylation may occur in cases where a supply of inorganic tin is available. It could also be argued that methylation is most likely to occur during the initial fermentation and establishment of the methanogenic phases of the microbiological development of a landfill.

Applicant's summary and conclusion

Conclusions:
The methyltins and stannic chloride were found to have no significant inhibitory effects on the activity of landfill micro-organisms.
Executive summary:

The potential transformation of methyltin chlorides and stannic chloride in landfills was investigated, by incubating the target substances at concentrations relevant to landfill conditions (100 and 500 µg Sn/L). The amounts of methane formed in all treatment bottles, and controls, were measured to evaluate the general microbial activity of the inocula and possible effects of methyltins on the degradation of organic matter.

The methyltins and stannic chloride were found to have no significant inhibitory effects on the activity of landfill micro-organisms, and the methanol used to disperse the tin compounds was completely degraded. In some experimental bottles, the methanol degradation gave rise to larger methane yields than expected, which was attributed to enhanced degradation of the waste material.

Alkyltin analyses showed that monomethyltin trichloride at an initial concentration of 500 µg Sn/L promoted methylation of inorganic tin present in the inoculum. No methylation activities were detected in the incubations with 100 µg Sn/L methyltin chlorides (mono-, di- or tri-methyltin), but demethylation occurred instead. Levels of soluble inorganic tin increased during the incubation period, due partly to demethylation and partly to a release of tin from the waste inocula.