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EC number: 260-828-5
CAS number: 57583-34-3
The methyltins and stannic chloride were found to have no significant
inhibitory effects on the activity of landfill micro-organisms.
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.
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
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
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
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
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
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.
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