Complex degradation rate

The next level of complexity is to maintain the assumptions of the fundamental model, that compartments are well mixed and rapidly equilibrated, and consider degradation rates within compartments. If this is done, the half-life of the chemical in the system can be estimated along with an estimated amount degraded in each compartment. 

A second shortcoming that arises at this stage of evaluation is that in order to conduct the evaluation much more information is required, i.e. soil and sediment degradation rates and hydrolysis and photolysis rates. At this point, more complex nonequilibrium models may be more useful. If and when methods of estimating degradation process become available, this level of evaluation will become more useful. 

The TG and DTG curves of Figure 12 show that melamine polyphosphate undergoes a complex degradation process between 330-650 C. In step 3 of the DTG curve (max. rate 390 C), water, ammonia and melamine are evolved. In this step the thermal behaviour of polyphosphate is somewhat similar to that of the sulphate in the same range of temperature (300-400 0. Indeed evolution of melamine indicates that thermal dissociation of polyphosphate giving free melamine takes place above 330 C. However, evaporation of melamine competes with its condensation as shown by evolution of ammonia. 

Polyesters offer multiple options to meet the complex world of degradable polymers. All polyesters degrade eventually, with hydrolysis being the dominant mechanism. Degradation rates range from weeks for aliphatic polyesters (e.g. polyhydroxyalkanoates) to decades for aromatic polyesters (e.g. PET). Specific local environmental factors such as humidity, pH and temperature significantly influence the rate of degradation. 

These results indicate that our scaled-up model ecosystems are more useful for studying system processes than processes that function in individual components of the environment. In this regard, a preliminary large scale ecosystem study could be very useful to indicate parameter limits such as overall degradation rates and likely concentrations of parent compounds plus metabolites over time. Such information would be useful in the design of metabolic studies in various components of the ecosystem. In addition, the large scale ecosystem study could also be used to determine if processes derived under laboratory conditions continue to function and/or predominate when combined in a complex system. 

In order to further profit from the dual complexing ability of CDs, we have studied Fenton-type processes in the presence of CDs. Several classes of compounds showed enhanced degradation rates in the presence of CDs in aqueous solution PCBs, PAHs, TNT, and chlorinated phenoxyacetic acids [38,102]. Dissolved natural organic matter typically inhibits Fenton degradation by sequestering the iron away from the pollutant [31,32]. However, addition of cyclodextrins overcame the inhibitory effect of the NOM and resulted in enhanced degradation rates [38]. 

Fluorescence spectroscopy and mass spectrometry have suggested that a ternary complex of iron-cyclodextrin-pollutant exists in solution. Such a complex is believed to play a key role in increasing pollutant degradation rates. Studies using added scavengers also support this theory [38]. A schematic illustration of such a theorized ternary complex is depicted in Figure 5. 

---