Combustion of dried animal dung as biofuel results in the generation of highly redox active fine particulates

Abstract

Background: The burning of biomass in the developing world for heating and cooking results in high indoor particle concentrations. Long-term exposure to airborne particulate matter (PM) has been associated with increased rates of acute respiratory infections, chronic obstructive lung disease and cancer. In this study we determined the oxidative activity of combustion particles derived from the biomass fuel dung cake by examining their capacity to deplete antioxidants from a model human respiratory tract lining fluid (RTLF). For comparison, the observed oxidative activity was compared with that of particles derived from industrial and vehicular sources.

Results: Incubation of the dung cake particle suspensions in the RTLF for 4 h resulted in a mean loss of ascorbate of 72.1 +/- 0.7 and 89.7 +/- 2.5% at 50 and 100 microg/ml, respectively. Reduced glutathione was depleted by 49.6 +/- 4.3 and 63.5 +/- 22.4% under the same conditions. The capacity of these samples to deplete ascorbate was in excess of that observed with diesel or gasoline particles, but comparable to that seen with residual oil fly ash and considerably in excess of all three control particles in terms of glutathione depletion. Co-incubation with the metal chelator diethylenetriaminepentaacetate inhibited these losses, whilst minimal inhibition was seen with superoxide dismutase and catalase treatment. The majority of the activity observed appeared to be contained within aqueous particle extracts.

Conclusion: These data demonstrate that biomass derived particles have considerable oxidative activity, largely attributable to their transition metal content.

Figure 1

AA remaining in synthetic RTLF following a 4 h incubation with three separate dung cake (DC) samples at 50 and 100 μg/ml (grey bars)The impact of co-incubation with the transition metal chelator DTPA (200 μM) is illustrated by the white bars. Pre (0 hr) and post (4 hr) AA concentrations in the particle-free controls are illustrated in the black filled bars. The losses observed with 50 μg/ml doses of fresh diesel and gasoline PM_0.1–2.5, as well as ROFA are also illustrated. Data represent the mean (SD) of 3 separate experiments: 'a' – indicates that AA concentrations after the 4 h incubation were significantly lower (P < 0.05) than the 4 h particle free control values; 'b' illustrates that the losses observed at 100 μg/ml are significantly greater than those seen at the lower 50 μg/ml concentration. The '*' illustrates a significant loss of AA in the particle free control over the 4 h incubation period.

Figure 2

Total (GSx) and reduced glutathione (GSH) remaining in synthetic RTLF following a 4 h incubation with three separate dung cake (DC) samples at 50 and 100 μg/ml. Reduced glutathione concentrations after 4 h are illustrated by dark grey bars, whilst the corresponding total glutathione concentration (GSx) are shown in light grey beneath the corresponding GSH data. The impact of co-incubation with the transition metal chelator DTPA (200 μM) is illustrated by the white bars, otherwise the figure is formatted as outlined in the legend to figure 1 with the following amendments. Notably DTPA incubation prevented not only the PM-induced loss of GSH but the background auto-oxidation seen in the particle-free controls. 'a" indicates that the concentration of GSH remaining after 4 h following co-incubation with DTPA was significantly greater than that in the 4 h particle-free control.

Figure 3

Rate of ascorbate consumption observed from a simple ascorbate-only solution with and without a range of metal chelation and free radical scavenger treatments. All data represent the mean (SD) of three separate experiments: AOX = particle free control; DC = dung cake samples (1–3 at 50 μg/ml); SOD/CAT = superoxide dismutase (50 U/mL) and catalase (150 U/mL); hiSOD/CAT = heat inactivated antioxidant enzymes; CAT = catalase only (150 U/mL), and DTPA (100 μM). 'a' indicates that the rate of ascorbate consumption was significantly reduced (P < 0.05) after SOD/CAT and DTPA treatment; 'b' that the DTPA treatment reduced the rate significantly more than the antioxidant enzyme treatment.

Figure 4

Time dependent loss of ascorbate associated with incubation of DC at 50 μg/ml at varying concentrations of DTPA. Each trace represents the mean of three separate experiments. The SDs on each mean value are not illustrated for graphical simplicity but were less than 5% of the mean values in all cases.

Figure 5

Rates of ascorbate consumption seen with aqueous and organic extracts of dung cake (DC) particle extracts. All data represent the mean (SD) of three separate experiments. '*' Indicates that the rate of AA depletion seen with the complete, aqueous and organic dung cake extracts was significantly greater (P < 0.05) than that observed in the particle free control; 'a' that the activity of the aqueous and organic extracts were significantly less than the complete extract; and, 'b' that the rate of AA depletion by the organic extract, with or without DTPA, was less than that seen with the aqueous fraction.