One of the advantages of biodiesel and some of the other biofuels is that they are non-toxic and degrade more rapidly than fossil fuels. This is an important feature in the case of accidents and spillages. Marine environments, freshwater, soil and various

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Fig. 8.21. Carbon dioxide produced per unit of energy for gaseous fuels compared with some fossil fuels. CNG, compressed natural gas; LPG, liquid petroleum gas; DME, dimethyl ether; hydrogen produced from electrolysis, natural gas and coal. (From Gustavsson et al, 1995; Matthews, 2001; Gielen and Unander, 2005; IEA, 2005a.)

sediments have been contaminated with oil components throughout the world as a result of accidents, leaks, spills and disposal. Oil components can cause considerable environmental disruption and the most spectacular are those accidents involving oil tankers.

One definition of biodegradability is where a fuel is 90% or more degraded within 21 days under fixed conditions (Sendzikene et al., 2007). The determination of degradation can be followed by the reduction in the concentration of the fuel as estimated by analysis such as gas liquid chromatography or the release of breakdown products such as carbon dioxide (Zhang et al., 1998). The two methods give some­what different results. Using carbon dioxide emissions rapeseed methyl esters degrade by 77-89% in 28 days compared with 18% for mineral diesel. Using gas liquid chromatography (GLC) the values were 88% for rapeseed methyl esters and 26% for mineral diesel (Zhang et al., 1998). This is not surprising as there should be a delay for the fuels to be fully mineralized to carbon dioxide compared with the metabolism of the methyl esters.

More recently the concentrations of oils and fatty acid methyl esters have been estimated using infrared spectroscopy and Fourier-transformed infrared spectroscopy (FTIR). In Europe, the biodegradability of fuel has been measured using infrared with the absorption of the C-M stretch of CH2—CH3 at 2930 cm-1 (Sendzikene et al., 2007). In other determinations the band at 1573 cm-1 (COO-) was used (Al-Alawi et al., 2006).

The biodegradation of a number of plant oil — and animal fat-derived biodiesel has been determined using a microbial mixture obtained from a wastewater system. The results are shown in Fig. 8.23. Diesel was about 60% degraded by day 21 whereas linseed-, tallow-, lard — and rapeseed-derived biodiesel were over 90% degraded in the same time. The rate of degradation was greatest with tallow — and lard-derived biodiesel.

Mixing biodiesel with mineral diesel increases the rate of degradation of mineral diesel and as the concentration of biodiesel increases so does the rate of degradation (Table 8.3). The reason for the increase in degradation is not known but may be due

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Fig. 8.23. Biodegradation of various biodiesel preparations over 21 days (Redrawn from Sendzikiene et al., 2007.)

either to the provision of a more accessible substrate biodiesel or the solubilization of mineral diesel. Biodiesel has been shown to solubilize mineral diesel and has been used to remove crude oil from contaminated sand (Pereira and Mudge, 2004).