Anaerobic biodegradation of soybean biodiesel and diesel blends under sulfate-reducing conditions.

Biotransformation of soybean biodiesel and its biodiesel/petrodiesel blends were investigated under sulfate-reducing conditions. Three blends of biodiesel, B100, B50, and B0, were treated using microbial cultures pre-acclimated to B100 (biodiesel only) and B80 (80% biodiesel and 20% petrodiesel). Results indicate that the biodiesel could be effectively biodegraded in the presence or absence of petrodiesel, whereas petrodiesel could not be biodegraded at all under sulfate-reducing conditions. The kinetics of biodegradation of individual Fatty Acid Methyl Ester (FAME) compounds and their accompanying sulfate-reduction rates were studied using a serum bottle test. As for the biodegradation of individual FAME compounds, the biodegradation rates for the saturated FAMEs decreased with increasing carbon chain length. For unsaturated FAMEs, biodegradation rates increased with increasing number of double bonds. The presence of petrodiesel had a greater effect on the rate of biodegradation of biodiesel than on the extent of removal.

Anaerobic biodegradation of soybean biodiesel and diesel blends under methanogenic conditions.

Biotransformation of soybean biodiesel and the inhibitory effect of petrodiesel were studied under methanogenic conditions. Biodiesel removal efficiency of more than 95% was achieved in a chemostat with influent biodiesel concentrations up to 2.45 g/L. The kinetics of anaerobic biodegradation of soybean biodiesel B100 (biodiesel only) with different petrodiesel loads was studied using biomass pre-acclimated to B100 and B80 (80% biodiesel and 20% petrodiesel). The results indicated that the biodiesel fraction of the blend could be effectively biodegraded, whereas petrodiesel was not biodegraded at all under methanogenic conditions. The presence of petrodiesel in blends with biodiesel had a greater inhibitory effect on the rate of biodegradation than the biodegradation efficiency (defined as the efficiency of methane production). Both the biodegradation rate coefficient and the methane production efficiency increased almost linearly with the increasing fraction of biodiesel. With the increasing fraction of petrodiesel, the biodegradation rate and efficiency were correlated with the concentration of soluble FAMEs in the water.

Aerobic biodegradation kinetics and mineralization of six petrodiesel/soybean-biodiesel blends.

The aerobic biodegradation kinetics and mineralization of six petrodiesel/soybean-biodiesel blends (B0, B20, B40, B60, B80, and B100), where B100 is 100% biodiesel, were investigated by acclimated cultures. The fatty acid methyl esters (FAMEs) of biodiesel were found to undergo rapid abiotic transformation in all experiments. The C10-C21 n-alkanes of petrodiesel were metabolized at significantly higher microbial utilization rates in the presence of biodiesel. The rates of mineralization of the blends were also enhanced in the presence of biodiesel; yet a similar enhancement in the extent of mineralization was not observed. Abiotic fuel-blends/aqueous-phase equilibration experiments revealed that the FAMEs of biodiesel were capable of cosolubilizing the n-alkanes of petrodiesel, a mechanism that fully explains the faster utilization and mineralization kinetics of petrodiesel in the presence of biodiesel without necessarily enhancing the extent of biomineralization. The biodegradation of six targeted aromatic compounds present in petrodiesel was also influenced by the amount of biodiesel in a blend. While toluene, o-xylene, and tetralin were not degraded in the B0 and B20 treatments, all of the targeted aromatic compounds were degraded to below detection limits in the B40 and B80 treatments. Biomass acclimated to B60, however, was unable to degrade most of the aromatic compounds. These results indicate that the amount of biodiesel in a blend significantly affects the absolute and relative abundance of the dissolved and bioavailable constituents of biodiesel and petrodiesel in a way that can considerably alter the biodegrading capacity of microbial cultures.

Microbial kinetic model for the degradation of poorly soluble organic materials.

A novel mechanistic model is presented that describes the aerobic biodegradation kinetics of soybean biodiesel and petroleum diesel in batch experiments. The model was built on the assumptions that biodegradation takes place in the aqueous phase according to Monod kinetics, and that the substrate dissolution kinetics at the oil/water interface is intrinsically fast compared to biodegradation kinetics. Further, due to the very low aqueous solubility of these compounds, the change in the substrate aqueous-phase concentration over time was assumed to approaches zero, and that substrate aqueous concentration remains close to the saturation level while the non-aqueous phase liquid (NAPL) is still significant. No former knowledge of the saturation substrate concentration (S(sat)) and the Monod half-saturation constant (K(s)) was required, as the term S(sat)/(K(s) + S(sat)) in the Monod equation remained constant during this phase. The n-alkanes C10-C24 of petroleum diesel were all utilized at a relatively constant actual specific utilization rate of 0.01-0.02 mg-alkane/mg-biomass-hr, while the fatty acid methyl esters (FAMEs) of biodiesel were utilized at actual specific rates significantly higher with increasing carbon chain length and lower with increasing number of double bonds. The results were found to be in agreement with kinetic, genetic, and metabolic evidence reported in the literature pertaining to microbial decay rates, uptake mechanisms, and the metabolic pathway by which these compounds are assimilated into microorganisms. The presented model can be applied, without major modifications, to estimate meaningful kinetic parameters from batch experiments, as well as near source zone field application. We suggest the estimated actual microbial specific utilization rate (kC) of such materials to be a better measure of the degradation rate when compared to the maximum specific utilization rate (k), which might be orders of magnitude higher than kC and might never be observed in reality.

Microtox aquatic toxicity of petrodiesel and biodiesel blends: the role of biodiesel's autoxidation products.

The acute Microtox toxicity of the water accommodated fraction (WAF) of six commercial soybean biodiesel/petrodiesel blends was investigated at different oil loads. We analyzed five fatty acid methyl esters (FAMEs), C10-C24 n-alkanes, four aromatics, methanol, and total organic carbon (TOC) content. At high oil loads, the WAFs' toxicity was significantly higher for blends containing biodiesel. At the lowest load, the WAFs' toxicity decreased almost linearly with decreasing biodiesel in the blend. At intermediate loads, the WAFs of all the blends appeared to have a similar toxicity. Analysis of WAFs confirmed the presence of autoxidation byproducts of FAMEs at high oil loads. Pure unsaturated FAMEs and n-alkanes were nontoxic when present in water at their reported solubility limits. However, 24-h equilibrated WAFs of pure FAMEs were highly toxic for C18:1 and C18:3, but not for C18:2. The authors concluded that at high oil loads, the acute toxicity of the WAFs was caused by FAMEs' autoxidation byproducts, whereas at low oil loads, the toxicity appeared to be caused primarily by the aromatic compounds present in petrodiesel. The addition of a synthetic antioxidant in biodiesel did not appear to affect the concentration of autoxidation byproducts in the WAF but resulted in a slight decrease in its toxicity. The major autoxidation byproducts identified in the WAF of commercial biodiesel were present neither in the WAFs of pure unsaturated FAMEs nor in the WAF of a different soybean biodiesel that was transesterified in our laboratory, which was nontoxic. We concluded that the process of transesterification of biodiesel might be a more critical factor in determining the aquatic toxicity of the fuel than the source of feedstock itself.

Partitioning behavior of petrodiesel/biodiesel blends in water.

The partitioning behavior of six petrodiesel/soybean-biodiesel blends (B0, B20, B40, B60, B80, and B100, where B100 is 100% unblended biodiesel) in water was investigated at various oil loads by the 10-fold dilution method. Five fatty acid methyl esters (FAMEs), C10-C20 n-alkanes, and four monoaromatic compounds were targeted for analysis. Only the aromatic compounds were partitioned according to Raoult's law at all oil loads. The partitioning of the FAMEs and n-alkanes at higher oil loads was found to be orders of magnitude higher than the reported aqueous solubilities of these compounds, and directly correlated with the amount of oil load applied. Depth filtration of the water-accommodated fractions (WAFs) significantly reduced the observed concentrations of the FAMEs and n-alkanes, but did not appreciably affect the aromatic compounds. The FAMEs and n-alkanes concentrations in the filtered WAFs agreed with the aqueous solubilities of those compounds reported in the literature, but the n-alkanes showed progressive deviations from those solubilities with the increase in the amount of biodiesel in the blends. Further dilution experiments on pure n-hexadecane confirmed the presence of a metastable colloidal phase that seems to be controlled by hydrophobic interactions and surface phenomena. The addition of biodiesel to the oil blend appeared to have a positive impact on the dissolved concentrations and the colloidal accommodation of the n-alkanes. Autoxidation of the biodiesel constituents was found to be significant, and increased with increasing oil loads. Chemical products such as hexanal, n-butyl acetate, diethylene glycol monobutyl ether, and diethylene glycol monobutyl ether acetate were positively identified among the FAMEs' autoxidation byproducts. Our data suggest a positive enhancement for biodiesel on the formation of the oil in water colloidal phase, possibly by forming a surfactant-cosurfactant-like pair of the FAMEs and their autoxidation byproducts.