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With oxygen; ozone; In water; |
Example B; Degradation of benzo[a]pyrene; This example focuses on an integrated treatment of benzo[a]pyrene involving sequential chemical oxidation and biological degradation. The objectives are to: 1) provide mechanistic details in the ozone-mediated degradation of benzo[a]pyrene in the aqueous phase, 2) test the biodegradability of resultant intermediates, and 3) test the feasibility for the coupled chemical-biological treatment of the 5-ring PAH. Batch and packed column reactors were used to examine the degradation pathways of benzo[a]pyrene subject to ozonation in the aqueous phase. After different ozonation times, samples containing reaction intermediates and byproducts from both reactors were collected, identified for organic contents, and further biologically inoculated to determine their biodegradability. The O3-pretreated samples were incubated for 5, 10, 15, and 20 days; afterward biochemical oxygen demand (BOD), chemical oxygen demand (COD), and E-Coli toxicity tests were conducted along with qualitative and quantitative determinations of benzo[a]pyrene, intermediates, and reaction products by GC/FID and GC/MS methods. Prevalent intermediates identified at different stages included ring-opened aldehydes, phthalic derivatives, and aliphatics. The degradation of benzo[a]pyrene is primarily initiated via O3-mediated ring-opening, followed by O3 and hydroxyl radical fragmentation, and ultimately brought to complete mineralization primarily via hydroxyl radicals. Intermediates formed during chemical oxidation were biodegradable with a measured first-order rate constant (k0) of 0.18 day-1. The integrated chemical-biological system seems feasible for treating recalcitrant compounds, while pretreatment by chemical oxidation appears useful in promoting soluble intermediates from otherwise highly insoluble, biologically inaccessible benzo[a]pyrene.Materials and MethodsDescriptions of sections on Chemicals, Analytical Methods and Equipment, and Reactors and Procedures were identical to Example A. Only deviations from Example A are highlighted here. <strong>[50-32-8]Benzo[a]pyrene</strong> (BaP) (98%, Aldrich Chemical Co.) in place of pyrene was used and purified as described. A typical sample size for analysis is 150 ml and the storage temperature awaiting analysis -12 C. With the same GC/MS system, a split ratio of 5:1, solvent delay at 6 min, and scan range from m/z 15 to m/z 500 at 1.4 scan/s were used. Comparison of parent compound structure and interpretation of mass spectra of the intermediates from ion fragmentation information were performed particularly for the identification of key intermediates 7-propanal-8-methylpyrene, 7-ethyl-8-ethanalpyrene, and 4-methyl-5-hydroxylchrysene. Reactor systems (FIG. 1) were identical to ones previously used except that 0.15 g benzo[a]pyrene was prepared and loaded into the packed column reactor. Samples during batch reaction were taken at 2, 10, 20, 30, and 50 min. Sample BOD and toxicity were determined in triplicates and duplicates, respectively. Previous analytical efforts for pyrene were redirected toward benzo[a]pyrene.; Results and DiscussionThe degradation pathway, biodegradability of intermediates, and oxidant balance during ozonation of BaP will be addressed in turn.Degradation Pathways of Ozonated <strong>[50-32-8]Benzo[a]pyrene</strong>COD measurements were made for three solutions: 1) a saturated aqueous solution of BaP, 2) the solution after ozonation of a batch of excess BaP suspension (0.150 g/10.7 L), and 3) the effluent of a column packed with excess BaP solid (0.149 g) and glass beads (7.5 in. in bed-length). The saturated BaP solution was prepared by allowing excess BaP solid to reach dissolution equilibrium in water overnight followed by removal of the excess solid using a 0.45-mum filter. The ozonated batch solution was obtained after 50 min of ozonation and filtered, while the column effluent was collected from the packed column fed with an ozonated water over a 4-hr period and filtered. Table B-I shows the results COD measurements of all solutions and one BOD5 measurement for the column effluent. The saturated solution of BaP, due to its very limited aqueous solubility, registered a negligible COD value compared to that of the ozonated batch solution or the ozonated column effluent. In both the batch and column solutions, much higher COD values were measured after ozonation, which indicated dissolution of daughter compounds of BaP into the aqueous phase as a result of ozonation. A relativbiochemical oxygen demand ely high BOD5-to-COD ratio of 0.43 was observed for the column effluent, which suggested the intermediates were susceptible to biodegradation, a point of further discussion later.The COD values in the batch solution were relatively stable at about 15 mg/L during the 50-min ozonation period, as shown in FIG. 11. This seemingly steady-state level of COD could be indicative of the relatively constant quantity of intermediates that were continually added to the aqueous phase via oxidati... |
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