Slow Degradation Phase Research Articles

Deciphering the Novel Picolinate-Mn(II)/peroxymonosulfate System for Sustainable Fenton-like Oxidation: Dominance of the Picolinate-Mn(IV)-peroxymonosulfate Complex.

A highly efficient and sustainable water treatment system was developed herein by combining Mn(II), peroxymonosulfate (PMS), and biodegradable picolinic acid (PICA). The micropollutant elimination process underwent two phases: an initial slow degradation phase (0-10 min) followed by a rapid phase (10-20 min). Multiple evidence demonstrated that a PICA-Mn(IV) complex (PICA-Mn(IV)*) was generated, acting as a conductive bridge facilitating the electron transfer between PMS and micropollutants. Quantum chemical calculations revealed that PMS readily oxidized the PICA-Mn(II)* to PICA-Mn(IV)*. This intermediate then complexed with PMS to produce PICA-Mn(IV)-PMS*, elongating the O-O bond of PMS and increasing its oxidation capacity. The primary transformation mechanisms of typical micropollutants mediated by PICA-Mn(IV)-PMS* include oxidation, ring-opening, bond cleavage, and epoxidation reactions. The toxicity assessment results showed that most products were less toxic than the parent compounds. Moreover, the Mn(II)/PICA/PMS system showed resilience to water matrices and high efficiency in real water environments. Notably, PICA-Mn(IV)* exhibited greater stability and a longer lifespan than traditional reactive oxygen species, enabling repeated utilization. Overall, this study developed an innovative, sustainable, and selective oxidation system, i.e., Mn(II)/PICA/PMS, for rapid water decontamination, highlighting the critical role of in situ generated Mn(IV).

Study on the Failure Process and Acoustic Emission Characteristics of Freeze–Thawed Sandstone under Cyclic Loading and Unloading

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of “sparse → dense”. In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson’s ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain Δεcv+ reach the maximum value and tend to stabilize, while the crack volume extensional strain Δεcv− tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study’s findings contribute to our knowledge of the mechanical characteristics and sandstone’s degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling.

Biodegradation of Polycaprolactone (PCL) with Different Molecular Weights by Candida antarctica Lipase

Biodegradation characteristics of polycaprolactone (PCL) films with three different molecular weights after being degraded by Candida antarctica lipase were investigated. The changes of the PCL films before and after degradation were characterized. The results showed that the degradation process of PCL films could be roughly divided into two phrases: a rapid degradation phase and a slow degradation phase. Molecular weights of PCL films had no influence on the performance of lipase; however, increasing molecular weight caused the numbers of pores on the degraded PCL films to increase, and the pores became deeper as the degradation time increased. The results also showed that the crystallinity of the films decreased with increasing degradation time, and both the crystalline and the amorphous regions of the films were simultaneously degraded. The thermal stability of the films also gradually decreased with the increase of degradation time. Despite the above changes, the degradation did not cause the chemical composition and structure of PCL to change, and PCL remained the main component.

Bacterial degradation of dissolved organic matter released by Planktothrix agardhii (Cyanobacteria).

Although Planktothrix agardhii often produces toxic blooms in eutrophic water bodies around the world, little is known about the fate of the organic matter released by these abundant Cyanobacteria. Thus, this study focused in estimating the bacterial consumption of the DOC and DON (dissolved organic carbon and dissolved organic nitrogen, respectively) produced by axenic P. agardhii cultures and identifying some of the bacterial OTUs (operational taxonomic units) involved in the process. Both P. agardhii and bacterial inocula were sampled from the eutrophic Barra Bonita Reservoir (SP, Brazil). Two distinct carbon degradation phases were observed: during the first three days, higher degradation coefficients were calculated, which were followed by a slower degradation phase. The maximum value observed for particulate bacterial carbon (POC) was 11.9 mg L-1, which consisted of 62.5% of the total available DOC, and its mineralization coefficient was 0.477 day-1 (t½ = 1.45 days). A similar pattern of degradation was observed for DON, although the coefficients were slightly different. Changes in the OTUs patterns were observed during the different steps of the degradation. The main OTUs were related to the classes Alphaproteobacteria (8 OTUs), Betaproteobacteria (2 OTUs) and Gammaproteobacteria (3 OTUs). The genus Acinetobacter was the only identified organism that occurred during the whole process. Bacterial richness was higher at the slower degradation phase, which could be related to the small amounts of DOM (dissolved organic matter) available, particularly carbon. The kinetics of the bacterial degradation of P. agardhii-originated DOM suggests minimal loss of DOM from the Barra Bonita reservoir.

Comparison of degradation properties of carbohydrate and protein during anaerobic digestion of activated sludge at mesophilic temperature

The degradation of carbohydrate and protein is vital to the sludge anaerobic digestion process. This study investigated the degradation properties of carbohydrate and protein to gain an insight into organic matter removal during anaerobic digestion. Results show that carbohydrate was more efficiently degraded than protein, and also degraded before it. The final removal efficiencies of volatile solids, carbohydrate, and protein were 36.18%, 52.04%, 31.2%, respectively. The first four days of digestion represented a lag phase for protein degradation because the rapid degradation of carbohydrate during this phase led to suppression of protease formation. Kinetic results for protein degradation demonstrated that, after the initial lag phase, it followed first-order kinetics, with rate constants of 0.016 d−1 and 0.002 d−1 during the rapid degradation phase (four to 24 days), and slow degradation phase (24-52 days), respectively. Carbohydrate degradation followed first order kinetics at a constant rate of 0.007 d−1 following the initial rapid degradation phase. Cumulative biogas production increased linearly with protein degradation, and exponentially with carbohydrate degradation.

Comparison of degradation properties of carbohydrate and protein during anaerobic digestion of activated sludge at mesophilic temperature

The degradation of carbohydrate and protein is vital to the sludge anaerobic digestion process. This study investigated the degradation properties of carbohydrate and protein to gain an insight into organic matter removal during anaerobic digestion. Results show that carbohydrate was more efficiently degraded than protein, and also degraded before it. The final removal efficiencies of volatile solids, carbohydrate, and protein were 36.18%, 52.04%, 31.2%, respectively. The first four days of digestion represented a lag phase for protein degradation because the rapid degradation of carbohydrate during this phase led to suppression of protease formation. Kinetic results for protein degradation demonstrated that, after the initial lag phase, it followed first-order kinetics, with rate constants of 0.016 d−1 and 0.002 d−1 during the rapid degradation phase (four to 24 days), and slow degradation phase (24-52 days), respectively. Carbohydrate degradation followed first order kinetics at a constant rate of 0.007 d−1 following the initial rapid degradation phase. Cumulative biogas production increased linearly with protein degradation, and exponentially with carbohydrate degradation.

Degradation properties of protein and carbohydrate during sludge anaerobic digestion

Degradation of protein and carbohydrate is vital for sludge anaerobic digestion performance. However, few studies focused on degradation properties of protein and carbohydrate. This study investigated detailed degradation properties of sludge protein and carbohydrate in order to gain insight into organics removal during anaerobic digestion. Results showed that carbohydrate was more efficiently degraded than protein and was degraded prior to protein. The final removal efficiencies of carbohydrate and protein were 49.7% and 32.2%, respectively. The first 3days were a lag phase for protein degradation since rapid carbohydrate degradation in this phase led to repression of protease formation. Kinetics results showed that, after initial lag phase, protein degradation followed the first-order kinetic with rate constants of 0.0197 and 0.0018d−1 during later rapid degradation phase and slow degradation phase, respectively. Carbohydrate degradation also followed the first-order kinetics with a rate constant of 0.007d−1 after initial quick degradation phase.

Prediction of landfarming period using degradation kinetics of petroleum hydrocarbons: test with artificially contaminated and field-aged soils and commercially available bacterial cultures

This study predicts the optimal landfarming period for the total petroleum hydrocarbons (TPHs)-contaminated field soils that are subject to the combined landfarming and Fenton oxidation treatment. The TPHs degradation in the artificially contaminated model soils and the field-aged contaminated soils were compared in a laboratory scale. The soils were bioaugmented with the 16 petroleum hydrocarbon-degrading microbial cultures that are commercially available in Korea. The TPHs degradation by bioaugmentation in the model soils was 81 ± 2% after 56 days, while it was only 27 ± 2% after 74 days in the field-aged soils. The lower degradation in the field-aged soils can be attributed to the removal of a large part of the biodegradable and volatile TPHs fractions during the weathering process. The two-compartment model that can depict a fast-degradation phase followed by a slow-degradation phase predicted that the remedial goal of 475 mg kg−1 of TPHs could be achieved in the model soils within the conventional landfarming period of 60–120 days in Korea; however, the predicted period for the field-aged soils (710–4,086 days) was not practical requiring a combined biological and chemical treatment approach. Based on the kinetics study, the optimal landfarming period for the field-aged soils was 3 weeks and pre-Fenton oxidation can be used to meet the remedial goal. The two-compartment model can be useful for predicting landfarming period for the combined landfarming and Fenton oxidation treatment of field-aged TPHs contaminated soils. The prediction of the optimal landfarming period could lead to the reduction in both the treatment cost and period.

Effect of high porosity on biodegradation of poly (4-hydroxybutyrate)in vivo

Poly-4-hydroxybutyrate, P(4HB), is a biosynthetic thermoplastic polymer that has been studied as a bio-absorbable medical material. In order to explore the in vivo biodegradation behaviour of porous P(4HB) membranes with specified apertures (89-150 µm), membranes with different porosities were implanted subcutaneously into the backs of 27 eight-week-old Sprague Dawley® rats. The implanted specimens were examined with Masson and hematoxylin and eosin staining. Masson staining indicated that the P(4HB) membranes were encased in fibrous cysts and that more collagen fibers were present within the sections of the hyper-porosity group. Hematoxylin and eosin staining showed that the residual area of the P(4HB) membranes in the hyper-porosity group decreased sharply compared to the hypo-porosity group, which implied that the P(4HB) membranes with higher porosity degraded faster than those with lower porosity. A slow degradation phase persisted for approximately 14 weeks during the degradation process. After the 16th week, the P(4HB) scaffolds fell into a fast degradation phase. The residual areas of the hyper-porosity P(4HB) membranes at the 32nd week were reduced by 39.76% compared with the second week after implantation. We concluded that P(4HB) membranes manifest a special biodegradation behavior in vivo and that the increased porosity of these membranes is an important factor favoring their biodegradation rates.

Kinetic Modeling and Error Analysis for Decontamination of Different Petroleum Hydrocarbon Components in Biostimulation of Oily Soil Microcosm

A microcosm study was constructed to investigate the effect of complex co-substrate (corn steep liquor, CSL) addition on indigenous bacterial community, rate and extent of petroleum hydrocarbons (PH) degradation in an oily soil with total petroleum hydrocarbons (TPH) content of 63353 mg kg−1. TPH degradation was found to be characterized by a rapid phase of degradation during the first three weeks where 76% removal of TPH occurred, followed by a slower degradation phase, where further 7% of the initial TPH was removed by the end of incubation period, 35 d. Branched alkanes are more resistant to microbial degradation than n-alkanes. Furthermore, the unresolved complex mixtures (UCM) of hydrocarbons are less degradable than n- and iso-alkanes. Pristane (Pr) was the most recalcitrant aliphatic compound studied in this work. These results in addition to the extensive bacterial growth observed (from 107 to 1010 CFU g−1 soil) give strong support that the addition of CSL resulted in increased degradation rates. The indigenous bacteria grew exponentially during the incubation period of 35 d with a growth rate of 0.26 d−1. Kinetic modeling was performed to estimate the rates of biodegradation of each hydrocarbon type component in the studied system. Five different error functions were used in this study to evaluate the fitness of the model equation to the obtained experimental data. This showed that the degradation of ∑nC20-nC24, ∑nC35-nC42 and nC18 can be better represented by a second order model, whereas the TPH, total resolvable peaks (TRP), nC17, UCM, ∑nC10-nC14, ∑nC15-nC19, ∑nC25-nC29, ∑nC30-nC34, ∑nCn, and ∑isoCn and isoprenoids Pr and phytane (Ph) were similarly following the first order model.

Advanced oxidation and mineralization of 3-indole butyric acid (IBA) by Fenton and Fenton-like processes

The degradation and mineralization of 3-indole butyric acid (IBA) in aqueous solution was examined using Fenton and Fenton-like processes. Various operating conditions were evaluated including pH and the concentrations of iron ions (Fe 2+ and Fe 3+) and hydrogen peroxide (H 2O 2). The highest COD removal efficiency was achieved at 0.2 mM/0.6 mM Fe 2+/H 2O 2 ratio and 0.2 mM/1.0 mM Fe 3+/H 2O 2 ratio at pH 3 for Fenton and Fenton-like processes, respectively. IBA degradation and mineralization exhibited pseudo-first-order kinetics while the depletion of H 2O 2 and Fe 2+ or Fe 3+ exhibited zero-order kinetics during both processes in all experiments. 97% of IBA degradation proceeded via two distinctive kinetic regimes. The initial phase of the reaction was directly attributable to the Fenton reaction wherein nearly all of the OH radicals were generated. This was followed by a slower degradation phase, which can be thought of as a series of Fenton-like reactions within a Fenton process. In the Fenton-like process, the initial phase lasted longer than in the Fenton process because the generation of OH radicals proceeded at slower rate; however, 98% degradation of IBA was achieved. The mineralization of IBA was 16.2% and 50% for Fenton and Fenton-like processes, respectively. After 24 h, H 2O 2 was the limiting reagent for further mineralization of IBA intermediates present in the system. The results of the study showed that Fenton Process may be more useful when only removal of IBA is required and mineralization is unnecessary. But if mineralization of IBA is needed, Fenton-like process gains more important than Fenton Process due to its mineralization efficiency.

Fate and transport of Irgarol 1051 in a modular estuarine mesocosm

Fate and transport of Irgarol 1051 were assessed using a modular estuarine mesocosm containing natural seawater, saltmarsh sediments, marsh grass, shrimp, clams, snails, and naturally derived planktonic and benthic microorganisms. The mesocosms were enclosed in a greenhouse under near ambient conditions, and included a saltwater sump and a simulated tidal flux. The exposure was comprised of four replicate treatments of 0, 100, 1,000 and 10,000 ng/L Irgarol. Solid-phase extraction (SPE) was used to extract Irgarol and its major metabolite M1 (aka GS26575) from water samples. Sediment samples were extracted using Accelerated Solvent Extraction (ASE) with 100% dichloromethane. Irgarol and M1 were separated and quantified utilizing Liquid Chromatography Tandem Mass Spectrometry (LC-MS-MS) with ElectroSpray Ionization in Multiple Reaction Monitoring mode. Aqueous concentrations of Irgarol declined rapidly (average 93% loss) over the course of the 35 day experiment while Irgarol was accumulated in the sediments (average mass balance of 75 +/- 5%). Loss of aqueous Irgarol occurred in two distinct phases, a relatively rapid phase up to 96 hours post-dose, and a slower phase following 96 hours. The rate constants of the initial rapid degradation phase (k(1)) for treatments 100 and 1,000 ng/L Irgarol were 2-3 times higher than the rate constants of the subsequent slow degradation phase (k(2)) for these treatments. The average half-life of Irgarol in water was 7 +/- 3 days. The aqueous concentration of Irgarol's metabolite, M1 increased over the course of the experiment. By 35 days post-dose, M1 concentrations in water averaged about 3% of the parent compound. M1 also accumulated in mesocosm sediments (average 16.7 +/- 2.5% of total Irgarol dose). Mass balance calculations showed that by 35 days, Irgarol and M1 amounts in mesocosm water and sediments were close to 100% of the initial Irgarol dose. Average partition coefficient normalized for organic carbon (log K(oc)) calculated for Irgarol was 3.2 +/- 0.1 for 35 days post-dose. Accumulation of Irgarol and M1 in mesocosm sediments may warrant further study to assess toxicity for benthic communities.

Hydrolytic Degradation and Drug Release of Ricinoleic Acid–Lactic Acid Copolyesters

A systematic study on the degradation and drug release from L-lactic acid and ricinoleic-acid-based copolyesters is reported. These copolyesters were synthesized by ring opening polymerization (ROP), melt condensation (COND) and transesterification (TRANS) of high molecular weight poly(lactic acid) (PLA) with ricinoleic acid (PLA-RA), and repolymerization by condensation to yield random and block copolymers of weight average molecular weights (Mw) between 3000 and 13,000. All polymers showed an almost zero-order weight loss, with a 20-40% loss after 60 days of incubation. Lactic acid release to the degradation solution is proportional to weight loss of the polymer samples. The main decrease in molecular weight was observed during the first 20 days, followed by a slow degradation phase, which kept the number average molecular weight (Mn) at 4000-2000 for another 40 days. Water-soluble 5FU was released from ricinoleic-acid-based polymers faster than slightly water-soluble triamcinolone. Drug release into phosphate-buffered saline (pH 7.4, 0.1 M) at 37 degrees C from P(LA-RA) 60:40 prepared by condensation of the acids was faster than from pasty P(PLA-RA) 60:40 synthesized by transesterification for both drugs.

Kinetics of Polycyclic Aromatic Hydrocarbon (PAH) Degradation in Long-term Polluted Soils during Bioremediation

Bioremediation experiments with ten different soil samples from former industrial sites which were long-term polluted with polycyclic aromatic hydrocarbons (PAHs) were carried out using outdoor pot trials. The degradation of 15 PAHs according to the US EPA was investigated for 168 weeks through repeated soil sampling and determination of the total PAH concentration. On average, degradation was largest for acenaphthene (88%; 63 to 99%) and smallest for anthracene (22%; no significant degradation to 89%). For most of the PAH single substances, degradation kinetics were characterised by a first initial phase of fast degradation. In a subsequent second phase, degradation diminished and residual PAH concentrations were approached within 168 weeks, resulting in a similar PAH pattern in the ten soil samples. Degradation kinetics was calculated through the selection of the appropriate differential rate equation from a set of seven equations. Kinetics of PAH degradation was best fitted by single and two coupled first order exponential equations with median R² of 0.71 (0.01 to 1.00). Degradation rate constants of the rapid phase (k 1) ranged from 0.05×10−2 week−1 for benzo[k]fluoranthene to 18.3 week−1 for naphthalene and for the subsequent slow degradation phase (k 2) they ranged from 0.01×10−2 week−1 for benzo[a]anthracene to 2.3×10−2 week−1 for fluoranthene. Degradation was governed by desorption and diffusion processes of different rates, while microbial activity did not influence the kinetics. Median disappearance times (DT50) ranged from 6.1 weeks for naphthalene to 522 weeks for benzo[k]fluoranthene. With the exception of the 6-ring PAHs dibenzo[ah]anthracene and indeno[1,2,3-cd]pyrene, this sequence followed the PAHs’ degree of condensation. The total initial PAH concentration and the residual concentration were correlated with R² of 0.69, with larger initial PAH concentrations leading to larger residual concentrations and degradation rates.

Complex co‐substrate addition increases initial petroleum degradation rates during land treatment by altering bacterial community physiology

AbstractA pilot‐scale land treatment unit (LTU) was constructed at the former Guadalupe oil production field with the purpose of investigating the effect of co‐substrate addition on the bacterial community and the resulting rate and extent of total petroleum hydrocarbon (TPH) degradation. The TPH was a weathered mid‐cut distillate (C10‐C32) excavated from the subsurface and stockpiled before treatment. A control cell (Cell 1) in the LTU was amended with nitrogen and phosphorus while the experimental cell (Cell 2) was amended with additional complex co‐substrate—corn steep liquor. During the pilot LTU operation, measurements were taken of TPH, nutrients, moisture, aerobic heterotrophic bacteria (AHB), and diesel oxidizing bacteria (DOB). The bacterial community was also assayed using community‐level physiology profiles (CLPP) and 16S rDNA terminal restriction fragment (TRF) analysis. TPH degradation in both cells was characterized by a rapid phase of degradation that lasted for the first three weeks, followed by a slower degradation phase that continued through the remainder of the project. The initial rate of TPH‐degradation in Cell 1 (−0.021 day−1) was slower than in Cell 2 (−0.035 day−1). During the slower phase, degradation rates in both cells were similar (−0.0026 and −0.0024 respectively). AHB and DOB counts were similar in both cells during the fast degradation phase. A second addition of co‐substrate to Cell 2 at the beginning of the slow degradation phase resulted in an increased AHB population that lasted for the remainder of the project but did not affect TPH degradation rates. CLPP data showed that co‐substrate addition altered the functional capacity of the bacterial community during both phases of the project. However, TRF data indicated that the phylogenetic composition of the community was not different in the two cells during the fast degradation phase. The bacterial phylogenetic structure in Cell 2 differed from Cell 1 after the second application of co‐substrate, during the slow degradation phase. Thus, co‐substrate addition appeared to enhance the functional capacity of the bacterial community during the fast degradation phase when the majority of TPH was bioavailable, resulting in increased degradation rates, but did not affect rates during the slow degradation phase when the remaining TPH may not have been bioavailable. These data show that co‐substrate addition might prove most useful for applications such as land farming where TPH is regularly applied to the same soil and initial degradation rates are more important to the project goals. © 2003 Wiley Periodicals, Inc.

Polymer degradation and in vitro release of a model protein from poly( d, l-lactide- co-glycolide) nano- and microparticles

The objective of the study was to investigate the effect of particle size of nano- and microparticles formulated from poly( d, l-lactide- co-glycolide) (50:50 PLGA) on polymer degradation and protein release. Since the surface area to volume ratio is inversely proportional to the particle size, it is hypothesized that the particle size would influence the polymer degradation as well as the release of the encapsulated protein. PLGA nano- and microparticles of approximate mean diameters of 0.1, 1 and 10 μm, containing bovine serum albumin as a model protein, were formulated using a multiple water-in-oil-in-water emulsion solvent evaporation technique. These particles were incubated at 37 °C in phosphate-buffered saline (pH 7.4, 154 mM) and the particles were characterized at various time points for molecular weight of polymer, surface-associated polyvinyl alcohol content (PVA), and the particle surface topology using scanning electron microscopy. The supernatants from the above study were analyzed for the released protein and PVA content. Polymer degradation was found to be biphasic in both nano- and microparticles, with an initial rapid degradation for 20–30 days followed by a slower degradation phase. The 0.1 μm diameter nanoparticles demonstrated relatively higher polymer degradation rate ( P<0.05) during the initial phase as compared to the larger size microparticles (first order degradation rate constants of 0.028 day −1, 0.011 day −1 and 0.018 day −1 for 0.1, 1 and 10 μm particles, respectively), however the degradation rates were almost similar (0.008 to 0.009 day −1) for all size particles during the later phase. All size particles maintained their structural integrity during the initial degradation phase; however, this was followed by pore formation, deformation and fusion of particles during the slow degradation phase. Protein release from 0.1 and 1 μm particles was greater than that from 10 μm size particles. In conclusion, the polymer degradation rates in vitro were not substantially different for different size particles despite a 10- and 100-fold greater surface area to volume ratio for 0.1 μm size nanoparticles as compared to 1 and 10 μm size microparticles, respectively. Relatively higher amounts of the surface-associated PVA found in the smaller-size nanoparticles (0.1 μm) as compared to the larger-size microparticles could explain some of the observed degradation results with different size particles.

Cationic reactions in the melt

The effect of Lewis acids of different acidities, i.e., AlCl3 and Bu2SnCl2, on polystyrene melt has been investigated using the mixing chamber of a Brabender Plastograph at 180°C. The torque was monitored as a function of time and molecular weights of periodically withdrawn samples were determined. The structure of select samples has-been analyzed by 1H-NMR spectroscopy. A sequence of complex physical and chemical transformations has been identified. The addition of Lewis acids to polystyrene melts causes an immediate drastic decrease in the torque due to rapid partial polymer degradation which is followed by homogenization and a slower degradation phase. There is evidence for the formation and accumulation of unsaturation during degradation. Cationic reactions induced by the Lewis acids readily explain the observed sequence of chemical transformations.