Tricarbonyl(methylcyclopentadienyl)manganese

Distribution modelling

Administrative data

Endpoint:

distribution modelling

Type of information:

calculation (if not (Q)SAR)

Remarks:

Migrated phrase: estimated by calculation

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Well documented publication providing sufficient information on methodology and results to predict the effectinve increase in manganese concentrations in the soil due to mmt use.

Data source

Reference

Reference Type:

publication

Title:

Unnamed

Year:

2005

Materials and methods

Media:

other: air release to soil deposition

Test material

Test material information

Constituent 1

Reference substance name:

Tricarbonyl(methylcyclopentadienyl)manganese

EC Number:

235-166-5

EC Name:

Tricarbonyl(methylcyclopentadienyl)manganese

Cas Number:

12108-13-3

Molecular formula:

C9H7MnO3

IUPAC Name:

tricarbonyl(methyl-η5-cyclopentadienyl)manganese

Details on test material:

Name of the test material: methylcyclopentadienyl manganese tricarbonyl

Study design

Environmental properties:

Average monthly:
- Temperatures: 30 °C
- Precipitation: 82.5 mm
- Wind direction: North Westerly
- Speed direction: 11- 12 km/h

Results and discussion

Any other information on results incl. tables

3.1. Site characteristics and model variables

 

The projections of traffic density and Mn emissions (X15%Mn) between years 2002 and 2010 near Warden Avenue and Brock Road sites are reported in Tables 2 and 3. The slope of the equation used for predicting traffic density from the year 2002–2010 was significant for both sites (R2=0.96 and 0.97, respectively). Based on traffic density and predicted X15%, a significantly higher emission level of Mn was estimated for the Warden Avenue site when compared with the Brock Road site (Table 4). Therefore, it was postulated that the Warden Avenue site should show higher accumulation of Mn in surface soil if these emissions were significantly contributing to soil concentrations. No significant difference was observed in surface soil at either Warden Avenue or Brock Road site (Bhuie and Roy, 2001).

 

3.2. Manganese mass balance: input emissions versus surface soil

 

Table 4 presents a comparison between average Mn emission rates from 1985 to 2010 and the mass of Mn in the surface soil (0–5 cm) at each of the study sites. The annual 15% Mn emission rates ranged from 0.11% to 0.79% of the mass of Mn in surface soils. If all emissions from the adjacent section of the highway were deposited on the study sites, they could not account for the magnitude of the Mn concentrations measured in the surface soils. The mobility and bioavailability of Mn compounds are affected by adsorption on mineral surfaces, precipitation as salts, and formation of stable complexes with organic compounds. In this study, the distribution of Mn appeared to be homogeneous, because no differences were observed in surface soil with respect to distance from highway 401.

A significant difference was observed between the mean values of Mn emissions at Warden Avenue compared to the Brock Road site. This is attributable to difference in traffic density, because the AADT volume of Warden Avenue site was significantly higher than the other sites (Table 4). In the soils, no significant difference was observed between the mean values of Mn accumulation in surface soil at Warden Avenue and Brock Road sites, even though emissions were estimated to be higher at the Warden Avenue site. The percentage difference between the estimated (input) atmospheric emission levels and observed (output) surface soils levels based on 10 mg Mn/l in gasoline was found to be 98.94%, and 99.55% at Warden Avenue, and Brock Road sites, respectively (Table 4). Similarly, the comparisons made using different concentrations of Mn in gasoline were found to be about 99% different at both sites (Table 4). This shows that deposition of Mn from mmt would not account for the measured Mn in surface soils, even at the maximum Mn content of 18 mg/l in gasoline.

The projected doubling times for soil Mn content as a result of automobile emissions at both sites are presented in Table 5. Based on 10 mg Mn /l of gasoline, the doubling time ranged from 95 to 226 years, with the doubling time sensitive to the concentration of mmt (and subsequently the Mn) in gasoline (Table 5). Because of the model assumptions, the estimated doubling times are conservative estimates.

The proportion of emitted Mn introduced into the atmosphere varies from 15% to 45% depending on the age, temperature and physical state of an automobile’s exhaust system. The manner in which the engine is operated also influences tailpipe emissions. For example, under conditions of heavy acceleration, Mn deposits, which have accumulated in the exhaust system under idling or steady cruising conditions, are re-suspended and emitted from the tailpipe. The level of accumulation of Mn in surface soil at both sites was estimated to be similar (Table 4). This indicates that the rate of re-suspension or removal of Mn by other mechanism such as soil erosion, leaching, and plant uptake is negligible when compared with the natural abundance of Mn in soil.

 

3.3. Model uncertainties

 

The concentration of Mn in gasoline has been declining since 1976 when first introduced in Canadian gasoline. The levels have been reduced from 18 to 10 mg/l of Mn in the gasoline. Recently, Mn concentration in gasoline has been further reduced to less than 5 mg/l by the petroleum refineries in Canada. If the estimates of the model presented here are based on 5 mg/l of Mn in the gasoline, the emission levels would be reduced further by 50% and the doubling time for Mn content in surface soil would be twice as much as reported in Tables 4 and 5, respectively. The calculation based on 15% tailpipe emission levels for 10 mg of Mn/l of gasoline used in automobiles, which represented 1290 g/year for site 1 and 556 g/year for site 2, will add approximately 6 and 2 mg/kg of Mn annually, respectively. These input levels are considered negligible when compared to the natural abundance of Mn in soil (7000 mg/kg) and it will take longer than several hundred years to reach the output level of Mn observed in surface soil. This suggests that if mmt were continuously used in Canadian gasoline for next 10 years or more, its contribution may not transform the equilibrium that exists between the total (inactive) and mobile (active) content of Mn in the soil. Therefore continuous use of mmt is not likely to excessively increase the levels of soil Mn adjacent to major highways in GTA.

Uncertainties are associated with four of the parameters used in the numerical model: the traffic volume, the fuel consumption, the concentration of Mn used in the gasoline and the fraction of the Mn consequently emitted from the tailpipe. Estimated emission at the tailpipe and the meteorological factors are associated with high variation in the emission and accumulation of Mn on surface soil beside the highways in GTA. Particulate Mn behaves very differently from gaseous form and can be trapped on various media (i.e. grass, snow), absorbed onto larger particles (i.e. roadway dust or soil), and react chemically with rainwater or products exhausted at the tailpipe. Therefore, receptor modeling verified with selective air sampling devices, a mass balance estimation and particle characterization with electron microscopy could lead to a more accurate estimate of the contribution of mobile sources to atmospheric Mn contamination. Considering the history of mmt, the use Mn in gasoline since 1976 represents an increasing but small percentage of gasoline additives, as Pb was slowly phased out and completely eliminated out of the gasoline by 1990. Thus the historical loading may be responsible for some of the uncertainties in the model. Secondly, the concentration of Mn used per liter of gasoline, decreased from 18 mg of Mn/l(1984) to 10 mg of Mn/l(1995). There is approximately 20% uncertainty in the current estimate of the fuel consumption based on the traffic density, and another 20% uncertainty in the estimate of automobile emissions near the sites selected for this study. The daily traffic volume (AADT) could not vary more than 10% from the estimated mean value. The total level of uncertainty in the model is approximately 30%. Future air and soil sampling at the selected sites and the exact fraction of the Mn emitted from automobile tailpipes are necessary to develop a more comprehensive model to characterize the behavior and distribution of Mn from mmt in various soils.

Applicant's summary and conclusion

Conclusions:

Based on the model developed, the annual increase of Mn concentrations in the environment was estimated to be 5.73 and 2.47 mg/kg of Mn for site 1 and site 2, respectively. These input levels are negligible based on the natural manganese content in the soil of 7000 mg/kg. An abnormal increase (doubling) of the Mn content in surface soil would take between 95 to 256 years to occur.

Executive summary:

The study estimated the automotive deposition of Mn from mmt relative to the traffic volume at sites near a major highway in the Greater Toronto Area of Canada, when mmt was used. Manganese emission levels were estimated for two sites that varied according to Annual Average Daily Traffic (AADT) density, fuel consumption, distance traveled by automobiles, and Mn concentration (mg/l) in gasoline. The predicted 15% tailpipe emission levels for 10 mg of Mn/l of gasoline used in automobiles, which represented 1290.03 g/year for site 1 and 555.94 g/year for site 2, will add 5.73 and 2.47 mg/kg of Mn to the soil annually, respectively. These input levels are considered negligible when compared to the natural abundance of Mn in soil, 541.33 and 556.67 mg/kg, respectively. Based on these data, it would take 95–256 years of continuous mmt usage at 10 mg Mn/L of gasoline in the region to double the content of Mn in surface soils at the respective sites.