| dc.description.abstract | Diesel is a product of the medium fraction of the refining process of petroleum, which encompasses a mixture of linear and cyclic paraffins, aromatic compounds and hydrocarbons with chain lengths that range from 10 to 28 carbons. Followed by gasoline, diesel fuel (PEMEX diesel) is the most sold petroleum fuel with sales of nearly 300,000 tons monthly and a production of nearly 246,000 barrels per day (SENER; 2010). Contamination of soil and wáter bodies with diesel fuel can occur due to spills and leaks from underground tanks, representing a significant problem since several diesel components are considered carcinogenic and toxic for the environment (Health Protection Agency UK; 2007). Therefore, it becomes a necessity to implement methods to promote intrinsic remediation of the impacted sites or to apply technologies to accelerate cleanup processes through the use of exogenous mechanisms involving equipment and instrumentation. Biological reactors are widely known for enhancing the removal of contaminants up to a certain degree, where microbial populations are capable of cleaving chemical bonds, and therefore modifying chemical structures. Although there are previous diesel degradation studies with biological reactors (Boopathy; 2000), this work studied diesel biodegradation with a biofilm reactor under nitrate-reducing conditions, using volcanic and alluvial stones as packing medium, thus providing a more cost-effective alternative for bioremediation of impacted sites. In the present project batch and continuous upflow packed bed reactor (CPR) studies were conducted to evaluate the biodegradation of the purgeable fraction of diesel fuel under nitrate-reducing conditions by using both biphasic pseudo-first order (BPF) and Arvin's kinetic models. The evaluation of the BPF at a diesel concentration of 30 mg/L recorded the highest first phase rate of 0.20 h-1 and 82% removal for the whole range of diesel (WRD) hydrocarbons with GAELE, and was up to 5-fold higher than without GAELE in batch assays. A 6-month CPR study recorded BPF rates 0.175-0.412 h-1 among the hydraulic retention times (HRTs) of 1.5-3.0 hour at diesel concentrations between 50 and 70 mg/L and showed up to 95.8% WRD removal at the shorter HRT with GAELE. Without GAELE, effluent ORP ranged below zero and increased above zero with GAELE. The 12-month steady-state CPR study was run at an HRT of 0.5 hour and involved a 3-level diesel concentration range: low, medium, and high, with and without GAELE. At the 3-level diesel concentration ranges, with and without GAELE, the BPF model did not record significant differences on the WRD rate constants. In addition, for the C10-C18 and C20-C22 hydrocarbon ranges, the BPF model recorded slight differences only at the medium-range diesel concentration when GAELE was added and failed to record the effect of GAELE at the low- and high-range diesel concentrations. In contrast, Arvin's model recorded an increase on the WRD overall rates of 2.2-, 2.3- and 1.5-fold higher at the low-, medium- and high-range diesel concentrations when GAELE was added. Furthermore, Arvin's model recorded an increase on the rates of the C10-C18 range of 1.2-, 3.3- and 2.5-fold for the low-, medium- and high-range diesel concentrations with the addition of GAELE. Interestingly, the increase on Arvin's model rates of the C20-C22 range recorded 9.6-, 1.8- and 1.2-fold higher when GAELE was added to the low-, medium- and high-range diesel concentrations, respectively. As for the C10-C18, Arvin's maximum utilization rates recorded an increase of 1.8-, 1.9- and 2.3-fold at the low-, medium- and high-range diesel concentrations when GAELE was added to the assays. Noteworthy, the addition of GAELE increased Arvin's maximum utilization rates of C20-C22 by 36.1-, 1.4- and 1.8-fold for the low-, medium- and high-range diesel concentrations, respectively. Among the bacteria identified were the following: Pseudomonas aeruginosa, Pseudomonas stutzeri, Burkholderia cepacia, Achromobacter xylosoxidans, Citrobacter freundii and Ralstonia picketti. In batch studies ORP and DO ranged from 126 mV and 6.9 mg/L at the start of the assays to -130 mV and 0.0 mg/L at the end. In CPR studies ORP and DO ranged from 142 mV and 6.6 mg/L at the influent to -67 mV and 0.6 mg/L at the effluent. BOD5 removals ranged 96.1-73.1% from the lowest to the highest diesel concentrations without GAELE, and increased from 98.1 to 90.8% with GAELE. On the other hand, COD removals ranged 92.9-41.6% from the lowest to the highest diesel concentrations in the absence of GAELE, and increased from 94.7 to 80.7% in the presence of GAELE. GAELE also contributed to an increase of the non-amenable fraction subject to biodegradation. | |
