Modelling the environmental degradation of water contaminants. Kinetics and mechanism of the riboflavin-sensitised-photooxidation of phenolic compounds

The aerobic visible-light-photosensitised irradiation of methanolic solutions of either of the phenolic-type contaminants model compounds (ArOH) p-phenylphenol (PP), p-nitrophenol (NP) and phenol (Ph), and for two additional phenolic derivatives, namely p-chlorophenol (ClP) and p-methoxyphenol (MeOP), used in some experiments, was carried out. Employing the natural pigment riboflavin (Rf) as a sensitiser, the degradation of both the ArOH and the very sensitiser was observed. A complex mechanism, common for all the ArOH studied, operates. It involves superoxide radical anion (O₂^√−) and singlet molecular oxygen (O₂(¹Δ_g)) reactions. Maintaining Rf in sensitising concentrations levels (≈0.02 mM), the mechanism is highly dependent on the concentration of the ArOH. Kinetic experiments of oxygen and substrate consumption, static fluorescence, laser flash photolysis and time-resolved phosophorescence detection of O₂(¹Δ_g) demonstrate that at ArOH concentrations in the order of 10 mM, no chemical transformation occurs due to the complete quenching of Rf singlet excited state. When ArOH is present in concentrations in the order of mM or lower, O₂^√− is generated from the corresponding Rf radical anion, which is produced by electron transfer reaction from the ArOH to triplet excited Rf. The determined reaction rate constants for this step show a fairly good correlation with the electron-donor capabilities for Ph, PP, NP, ClP and MeOP. In this context, the main oxidative species is O₂^√−, since O₂(¹Δ_g) is quenched in an exclusive physical fashion by the ArOH. The production of O₂^√− regenerates Rf impeding the total degradation of the sensitiser. This kinetic scheme could partially model the fate of ArOH in aquatic media containing natural photosensitisers, under environmental conditions.     

Within-plant allocation of a chemical defense in Secale cereale. Is concentration the appropriate currency of allocation?

Summary. The among-leaves allocation of DIBOA, a hydroxamic acid associated with plant resistance, in the shoot of rye (Secale cereale) was evaluated over the vegetative development of the plant. The appropriateness of using the concentration of secondary metabolites, DIBOA in this case, as the parameter to evaluate defense allocation in plants is discussed. Both biological and statistical arguments are put forward to suggest that allocation of chemical defenses should refer to absolute content and not to concentration. Results showed that leaf age was significantly linked to leaf concentration of DIBOA, young leaves having higher concentrations. In contrast, leaf content of DIBOA, our proposed currency of allocation, was not significantly higher in younger leaves. Furthermore, a regression analysis showed that the DIBOA content of leaves was better explained by the leaf relative biomass (proportion of shoot biomass) than by leaf biomass itself. It is suggested that, rather than leaf age, leaf relative biomass is the major factor determining DIBOA allocation in rye shoots. It is proposed that studies addressing within-plant defense allocation should use chemical defense content as the currency, emphasizing the major factors driving this process and its underlying mechanisms. Likewise, it is proposed that studies aiming at characterizing optimal patterns of plant defense should use chemical defense concentration as the currency, and be accompanied by evaluations of the actual resistance against herbivores of the plant parts analyzed, together with the effect on plant fitness. Received 19 February 1999; accepted 28 April 1999.     

CFD simulation of pressure piling

The explosion of flammable mixtures in interconnected compartments is commonly defined as “pressure piling”. Peak pressures much higher than the predictable thermodynamic values are likely to be generated in this geometry, yielding the phenomenon of major interest in industrial safety. In this paper, a CFD model was implemented, aiming at understanding the major factors affecting pressure piling in two cylindrical interconnected vessels, by varying the volume ratio between the two interconnected vessels and the ignition position. A combustion model was specifically developed to follow the flame propagation in any combustion regimes as a function of the local conditions: laminar, flamelet and distributed reaction zone.The model was validated by comparison with experimental results. The agreement between the experiments and the simulations has allowed the interpretation of the pressure piling phenomenon and the understanding of the mechanisms involved. More precisely, the results have showed that the pressure peak intensity is mainly affected by the coupling between the pre-compression of the mixture in the secondary vessel and the violence of explosion in the same vessel as related to the venting time, the latest quantified by the turbulent Bradley number, Br_t i.e. by the reaction time to the venting time ratio.     

Monitoring trichloroethene remediation at an iron permeable reactive barrier using stable carbon isotopic analysis

Stable carbon isotopic analysis, in combination with compositional analysis, was used to evaluate the performance of an iron permeable reactive barrier (PRB) for the remediation of ground water contaminated with trichloroethene (TCE) at Spill Site 7 (SS7), F.E. Warren Air Force Base, Wyoming. Compositional data indicated that although the PRB appeared to be reducing TCE to concentrations below treatment goals within and immediately downgradient of the PRB, concentrations remained higher than expected at wells further downgradient (i.e. >9 m) of the PRB. At two wells downgradient of the PRB, TCE concentrations were comparable to upgradient values, and delta13C values of TCE at these wells were not significantly different than upgradient values. Since the process of sorption/desorption does not significantly fractionate carbon isotope values, this suggests that the TCE observed at these wells is desorbing from local aquifer materials and was present before the PRB was installed. In contrast, three other downgradient wells show significantly more enriched delta13C values compared to the upgradient mean. In addition, delta13C values for the degradation products of TCE, cis-dichloroethene and vinyl chloride, show fractionation patterns expected for the products of the reductive dechlorination of TCE. Since concentrations of both TCE and degradation products drop to below detection limit in wells within the PRB and directly below it, these downgradient chlorinated hydrocarbon concentrations are attributed to desorption from local aquifer material. The carbon isotope values indicate that this dissolved contaminant is subject to local degradation, likely due to in situ microbial activity.     

Cellular localisation of secondary metabolites isolated from the Caribbean sponge <Emphasis Type="Italic">Plakortis simplex</Emphasis>

The Caribbean sponge, Plakortis simplex, is known to contain a large array of secondary metabolites, including the antimalarial polyketide plakortin, several unusual glycolipids, and some hopanoids, which closely resemble typical bacterial metabolites. The hypothesis that they could be products of bacterial metabolism was tested by localizing specific metabolites in cells using physical separation of sponge cells, bacterial symbionts and supernatant by differential centrifugation. The obtained fractions were analysed separately for the typical P. simplex metabolites by NMR and mass spectrometry, and most of them were shown to be present in the bacterial cells but not in the sponge cells. In addition, PCR screening showed that the biosynthetic pathway for glycosphingolipids was present in the bacterial cells. Isolation of a Sphingomonas strain PS193 from P. simplex and subsequent glycosphingolipid analysis resulted in the detection of a known glycosphingolipid, GSL-1, that did, however, not match the glycosphingolipid profile of P. simplex. Therefore, it is unlikely that Sphingomonas strain PS193 is an abundant member of the microbial community associated with P. simplex. Other glycosphingolipid producing bacteria in P. simplex remain to be identified. In conclusion, this study provides experimental evidence that the glycolipids and hopanoids and possibly also the polyketide plakortin are produced by microbial symbionts rather than the sponge from which the metabolites were originally isolated.     

Comparative aspects of sexual reproduction in the Caribbean coral genus Diploria (Scleractinia: Faviidae)

The increasing decline of Caribbean reef-building species underlines the importance of more information on their reproductive biology and ecology. This study compares the reproductive biology of the three species of Diploria, an important Caribbean reef-building genus affected by bleaching and disease, by: (a) characterizing their gametogenetic cycles simultaneously, (b) exploring the spatial and temporal variability in reproductive traits within and across species, (c) assessing fecundity and the minimum size of sexual reproduction, and (d) comparing information with other studies in the Caribbean. Starting in 1999, one tissue core was collected every month (for 17 months) from each of at least five large, tagged colonies of each species in La Parguera, Puerto Rico. Three more temporally spaced samplings were done during summer of 2000 and winter–spring of 2001 and 2002. Tissues were fixed in Helly’s solution, rinsed in fresh water, decalcified with 10% HCl, and preserved in 70% ethanol, embedded in paraplast, cut, and thin slides (7 μm) prepared and stained with Heidenhain’s Aniline-Blue method. Maximum egg size, number of eggs and spermaries were measured and gametogenetic cycles assessed. Microscopic observations confirmed that all three species were simultaneous hermaphrodites with a single, annual gametogenetic cycle. Spermatocytes and oocytes developed within the same mesentery but were not intermingled. Female gametes developed several months earlier than that of males, but both reached maturation simultaneously. Minor differences in the onset of gametogenesis were found for D. strigosa and D. clivosa, with both spawning after 10 p.m. between 8 and 10 days after the August and/or September full moons. D. labyrinthiformis on the other hand, began oogenesis in early July and spawned after 11 p.m. between 7 and 10 days after the April and/or May full moons. Spatial and temporal variability in mean number of spermaries and eggs/mesentery among colonies within and among species were found. D. labyrinthiformis had consistently significant higher mesenterial and polyp fecundity compared to the other two species over the 3 years. Average egg diameter varied between 240 and 246 μm and was similar among the three species. D. strigosa and D. clivosa sexual reproductive characteristics were similar to those of other large broadcast spawning Faviidae (i.e. Montastraea spp) in the region. In contrast, D. labyrinthiformis was a spring spawner (similar to Colpophyllia natans in Puerto Rico), which together with its significantly different micromorphology suggests that it might be phylogenetically more distant to the other taxa.     

Development of an enzyme-linked immunosorbent assay for screening contamination by chlorophenols in environmental waters

The development of an immunoassay for screening contamination by chlorophenols is presented. Two haptens were synthesized and conjugated to immunizing proteins to raise rabbit polyclonal antibodies. The antibody-coated format (direct) gave better sensitivity than the conjugate-coated format (indirect) if 2,4,6-trichlorophenol is used as target analyte. The measurement range was 86.4 microg l(-1) to 0.7 microg l(-1), with an average I50 of 7.8 microg l(-1) and a detection limit of 0.2 microg l(-1). The assay detects the presence of trichloropyridinol and other chlorophenols such as di-, tetra- and pentachlorophenols constituting thus a suitable tool for the early warning of the presence of such family contaminants. The optimized method permits the detection of the most important chlorophenols in a fast and reproducible way using no more than one antibody and a single assay. The results achieved with water samples spiked with different chlorophenols fit with a multiple linear regression model when expressing the total concentration of chlorophenols as equivalent of 2,4,6-trichlorophenol (P < 0.01), demonstrating the usefulness of the assay as a screening tool to detect contamination by chlorophenols.     

Multi-causal and integrated assessment of sustainability: the case of agriculturization in the Argentine Pampas

Assessing the sustainability of complex development processes requires multi-causal and integrated analyses. We develop a system-based methodology, rooted in interdisciplinary discussion and consensus building between 15 experts, to construct a multi-causal diagram which examines the sustainability of the Argentine Pampas′ process of agriculturization. The resulting diagram includes 25 factors and provides a big-picture of the multiple dimensions and interrelations affecting sustainability. According to this examination, the increasing concentration of production and the incorporation of technological innovations, triggered by economic and institutional factors, are the cause of environmental distresses and social changes, whose consequences for sustainability are still highly disputed. Nevertheless, the symptoms of both environmental and social unsustainability are more evident in the case of the extra-Pampean regions than in the Pampas. This suggests that the Pampean agriculture model should not be transferred to these regions without substantial modifications. The experts did not reach consensus on whether the agriculturization process is overall sustainable or unsustainable. Lack of consensus revolved mainly around opposing perspectives regarding the significance of the threats to environmental sustainability. The magnitude of socio-distributive unbalance and loss of rural jobs were also contentious. Yet, the paper shows how the exercise of building a joint causal diagram was undoubtedly helpful for linking piece-meal disciplinary facts, brought in from all fronts, into a comprehensive and coherent picture. Readers should send their comments on this paper to: BhaskarNath@aol.com within 3 months of publication of this issue.