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h.2FateAlthough it is expected that little MMT will be released into the environment from its use as a fuel additive (Section 8.1.2) there are a number of relevant papers in the literature addressing the environmental fate of this compound, and these are briefly summarised in the following subsections. As indicated previously, most of the MMT will be destroyed during combustion of the fuel with release of inorganic Mn compounds (Mn phosphates, sulphates and oxides), with almost all the released Mn being associated with and incorporated in small particles. h.2.1AtmosphereMMT is unstable to photochemical degradation in the atmosphere, with a reported atmospheric half-life of 8-18 seconds determined from direct measurement of the content of organic and inorganic Mn (apparently Mn oxides and Mn carbonates) down wind of a device designed to release MMT at a controlled rate (Ter Haar et al., 1975). However, the authors indicated that experimental uncertainties precluded a more precise determination, and while they endeavoured to obtain more accurate measurements through direct photolysis of an MMT/air mixture in a quartz tube under well-controlled conditions, this effort was confounded by deposition of photolysis products on the surface of the tube. Nevertheless, the conclusion from these experiments was that MMT decomposes quickly in the atmosphere and the decomposition mechanism was reported to involve both light (wavelength 340-440 nm) and atmospheric oxygen. However, a more recent study indicated that the first step in the degradation process involves adsorption of a visible-UV photon, which then weakens the bonds between Mn and the CO groups leading to ejection of a CO molecule (Vreugdenhil and Butler, 1998). Regardless of the detailed mechanism for photo-degradation, the ultimate degradation products would most likely be water, CO2 and MnO2. A recent study determined the rate constant for reaction of atmospheric MMT for direct photolysis (with visible-UV light), with hydroxyl radicals and with ozone, and found these to be (1.30.1) x 10-2, (1.10.3) x 10-10 and 7.71.9 x 10-18 cm3 molecule-1 sec-1 respectively. These rate constants provided half-lives of 80 sec, 1.3-2.5 hours and 14-72 hours for photolysis, reaction with OH radicals and ozone respectively (Wallington et al., 1999). h.2.2WaterIn a determination of the ready biodegradation of MMT in a closed bottle test, while 46% degradation was observed after 15 days, no further degradation was observed after this time (Analytical Biochemistry Laboratories Inc., 1990). The test was conducted according to the protocols of OECD TG 301 D by incubating samples of the MMT (equivalent to 2 mg/L carbon) with sewage bacteria, and monitoring the residual biochemical oxygen demand (BOD) after 5, 15 and 28 days. The result of this test indicates that the compound cannot be classified as readily biodegradable. The rate of photolytic decomposition of MMT in distilled water was determined and found to be characterised by a half-life of approximately 1 minute when a solution of the compound was exposed to midday sunlight (Garrison et al., 1995). The authors remarked that this was similar to the result obtained by Ter Haar et al. for direct atmospheric photolysis. A separate part of this study also examined the possibility of degradation of MMT (in the dark) by direct hydrolysis, and found that this process was very slow if indeed it happens at all, with an estimated minimum half-life (at 25 2C) of 500 days (Garrison et al., 1995). It is also of interest that these authors (Garrison et al., 1995) indicated that literature values for water solubility and the n-octanol/water partition coefficient were uncertain. Since these two physico-chemical parameters are important for the determination of environmental fate, these authors presented their own measured values and determined the water solubility at 25C as 293 mg/L and Log Kow as 3.7 (again at 25C). This water solubility together with the vapour pressure of MMT (1.1x10-2 kPa at 25C - Ethyl Corporation) were used to calculate the Henrys Law Constant as 82 Pa.m3.mol-1, indicating that any MMT entering the water compartment (and not degraded through photolysis) would evaporate and would be destroyed through photolysis in the atmosphere (Lyman, Rheel and Rosenblatt, 1990).
h.2.3Soils and sedimentsThe relatively large value for Log Kow (3.7) indicates that MMT would have significant affinity for the organic component of soils and sediments, although the water solubility (29 mg/L) could bestow some mobility to any MMT that enters the soil/sediment compartment. However, in an investigation of the adsorption of MMT to a variety of soil types as well as to the important soil minerals alumina and silica, Vreugdenhil and Butler (1998) found that MMT binds to soils. The mechanism appears to be due to interaction of the carbonyl groups of MMT with silica or alumina surfaces of clay minerals rather than through association of the compound with the organic component of the soils. These authors concluded that MMT can adsorb to and become immobilised in soils and that this would reduce its potential for photo-degradation. Degradation of MMT spiked into a natural anaerobic aqueous sediment was also studied by Garrison et al. (1995), and although the sediment was kept in the dark to prevent photolytic degradation, no measures were taken to either encourage or hinder biodegradation. In this experiment, the rate of disappearance of the MMT was very slow with data fitted to first order kinetics providing a degradation half-life of 0.5-1.5 years (Garrison et al., 1995). h.2.4Fate of inorganic compounds from combustion of MMTMost of the MMT used as an AVSR in fuel within Australia will be combusted and as described above will be converted to inorganic Mn compounds (oxides, sulphate and phosphate), most of which apparently remain in the exhaust train. However, around 20% of these Mn compounds (approximately 9.1 tonnes) could be expected to be emitted with exhaust gases associated with very fine particles (< 2.5 μm). These small particles have very low quiescent air sedimentation velocities of around 1-2 cm/hour and less, and are consequently not expected to settle under gravity prior to being precipitated. For example, the settling velocity of the particles in non-turbulent air can be estimated using Stokes law (CRC, 1977), which gives the settling velocity for a particle of radius r (cm) as: Vset = 2gr2d/9 where g is the acceleration of gravity, d is the density of the particle (gm/cm3) and is the viscosity of air, which is around 180 x 10-6 gm/cm-sec at 25C. Taking r as 1.5 μm (= 1.5 x 10-4 cm), and assuming d is 2gm/cm3, Vset is calculated as 3.75 x 10-4 cm/sec (= 1.35 cm/h). Consequently, the small particles emitted from the exhaust pipes are expected to remain suspended in the air for prolonged periods. Ultimately these fine particles would be precipitated to the ground with rain or through becoming associated with larger particles with higher sedimentation velocities, and would become associated with soils and aquatic sediments. The inorganic Mn residues remaining in the engines and exhaust systems of vehicles would ultimately be placed into landfill with discarded cars and exhaust systems, or if these are recycled for metal recovery, the residues would become associated with slag and other products from the blast furnaces. Directory: data -> assets -> word doc -> 0020 word doc -> Commonwealth of Australia 2002 word doc -> Commonwealth of Australia 2000 word doc -> Rail safety news issue 6 – October 2011 word doc -> Review of Multiple Chemical Sensitivity: Identifying word doc -> Board Endorsed October 2009 amended 13-09-11 with extra units 0020 -> Rail safety news issue 5 may 2011 0020 -> The arts ripple effect: valuing the arts in communities 0020 -> Monash university accident research centre report documentation page 0020 -> Section 1: Working with the Government and the Parliament 6 1Summary 6 0020 -> Bachelor of Computer Science and Bachelor of Economics (61010) Applicability of the general provisions 11 47. 1 Download 0.91 Mb. Share with your friends:
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