Situación técnica en conjuntos habitacionales

Although the hydroxylation mechanism is well supported by the surface precursor complex – net non-oxidative mechanism proposed by Shin and Cheney (2005), the net non-oxidative mechanism proposed by Wang et al. (1999) for the dealkylation of atrazine on birnessite does not fully support some of the observations in this study. In all of the experiments where dealkylation occurred, no deisopropylatrazine (DIA) was observed in any of the experiments, despite its formation in all of the studies (Wang et al. 1999; Shin et al. 2000; Shin and Cheney 2004, 2005) where this mechanism was proposed for dealkylation. In all of the abiotic degradation studies conducted where this non-oxidative mechanism was proposed, equal proportions of deethylatrazine (DEA) and deisopropylatrazine (DIA) were formed. Differential formation of one dealkylation product over another are usually observed in the biodegradation of atrazine, where the removal of one of the alkyl chains is energetically favoured over the removal of another (Erickson et al. 1989). The formation of deethylatrazine in preference to deisopropylatrazine was

also observed in some hydroxyl radical (•OH) generating systems (e.g. Acero et al. 2000), and in these cases the formation of deethylatrazine was attributed to the reactivity of oxidation intermediates, such as 2-chloro-4-acetamido-6-isopropylamino-1,3,5-triazine (CDIT), which undergo further reaction to form deethylatrazine. Furthermore, the formation of deethylatrazine by oxidation mechanisms have peroxyl-radicals (C3H7-NH(C3N3Cl)NH-CH(•OO)CH3) as

intermediates (Arnold et al. 1995a; Tauber and von Sonntag 2000). In this context, it is clear that steric hindrance could play a possible role, since molecular oxygen (O2) would more than likely

preferably attack the less bulky ethyl group on atrazine, as opposed to the more bulky isopropyl group. The reaction of these oxidation intermediates to form deethylatrazine is accompanied by the concomitant release of ethanal (acetaldehyde), which can be readily detected.

The authors who proposed the non-oxidative mechanism, namely Wang et al. (1999), Shin et al. (2000) and Shin and Cheney (2004, 2005), detected appreciable amounts of acetaldehyde and acetone, after the formation of deethylatrazine and deisopropylatrazine, respectively. Under a nitrogen (N2) atmosphere, however, they only detected unoxidized ethylene and propylene,

despite detecting very similar amounts of deethylatrazine and deisopropylatrazine as they did in the non-N2 atmosphere experiments. The authors therefore concluded that oxygen was oxidizing

these olefins after their release from atrazine, and their hypothesis was supported by the time lag that existed between dealkylation and formation of acetaldehyde and acetone.

Unfortunately, due to time and equipment constraints, these by-products were not analyzed for in this study. However, a comparison of the air-dried and N2-dried data does reveal very

similar rates of atrazine degradation (as well as hydroxyatrazine and deethylatrazine formation), which means that it is highly likely that the mechanism originally proposed by Wang et al. (1999) for dealkylation (see Figure 5.12 for a slightly modified version) does hold true in this study. However, this mechanism does not account at all for the apparently strong preference for the formation of deethylatrazine over deisopropylatrazine (DIA was not detected at all). In addition, further degradation to the didealkyl- (didealkylatrazine, DDA) and other hydroxy-metabolites (deethylhydroxyatrazine, DEA-OH; deisopropylhydroxyatrazine, DIA-OH; and ammeline, DDA- OH), which was observed for the reaction of atrazine with birnessite in a study by Shin and Cheney (2005), was not observed in this study. Once hydroxyatrazine and deethylatrazine had formed in this study, they only accumulated on the birnessite surface and did not degrade further to any other products. This was confirmed by the conservation of the total moles of s-triazines in the system, and by the lack of degradation observed when these metabolites were reacted with

birnessite (Figure 5.13). It is not clear at this point why only these two metabolites formed in the reaction, why there was a preference for deethylatrazine formation over deisopropylatrazine, and why no further degradation occurred. It could be possible that in the fully hydrated system investigated by Shin and Cheney (2005), the atrazine molecule has more freedom to move and reconfigure itself to align itself with the birnessite surface in the most efficient way for the reaction to occur. Furthermore, in these hydrated systems, it is also possible that the reaction products are free to move off of the birnessite surface and back into the aqueous solution once they have formed, especially since they are often much more polar than the original atrazine molecule, and perhaps are even repelled by the birnessite surface somewhat. In the drying system presented in this study, once all the moisture has evaporated, the atrazine molecule may be fixed in a certain position, well contained within a thin film of water molecules. In this configuration, the atrazine molecule may be configured in such a way that it is sterically aligned to the mineral surface, or aligned in a manner that is energetically most favourable, considering that it is a hydrophobic compound, and that it is generally repelled by charged mineral or sesquioxide surfaces. Since it is forced to make intimate contact with the mineral surface, as a result of the loss of water volume during drying, it would probably do so in the most favourable manner, which is not necessarily the most efficient for its reaction with birnessite, but is favourable for its forced contact with the mineral surface. In this manner, steric hindrance might be an important controlling factor for interaction between the mineral surface and the moieties on the atrazine molecule. In addition to this, once the metabolites have formed, they cannot leave the surface and return to solution in the manner they perhaps do in fully hydrated systems. They are forced to remain on the mineral surface, where they are possibly stabilized by various interactions, preventing further reactions to other metabolites. For example, hydroxyatrazine contains the OH group on position 2 of the triazine ring, rather than the more electron-withdrawing Cl group. Electron donation into the ring, and subsequent resonance rearrangements of electrons to the alkylamino moieties could stabilize the Mn-N bridge that is formed, not allowing sufficient electron transfer to occur, which could possibly explain the lack of products such as deethylhydroxyatrazine (DEA-OH), since the Mn-N bridge electron transfers are critical in dealkylation. With deethylatrazine (DEA), it is possible that since the molecule cannot leave the surface it might rearrange within the thin water film to have its amino (—NH2) (polar) moiety

directed more toward the birnessite surface, perhaps removing the isopropyl moiety out of reach of the birnessite surface and thus preventing the removal of the isopropyl group and subsequent formation of didealkylatrazine (DDA). It is thus possible that the drying surface is necessary for

rapid initial degradation of atrazine, but could be greatly inhibiting to further reactions of the metabolites with the mineral surface. The exact degradation mechanisms of atrazine on a drying birnessite (or soil mineral) surface is thus not fully elucidated in this study, but it is clear that drying certainly has an enhancing effect on atrazine degradation relative to moist systems, and that oxygen is most likely not the oxidant in these reactions. Water appears to have a critical role in the degradation of atrazine (compared with water-absent solvent systems) on drying mineral surfaces. It appears that surficial water probably plays a key role in the formation of a surface atrazine-birnessite complex, and that inherent water content within birnessite is not sufficient to initiate the reaction. Drying forces contact between the mineral surface and atrazine, where interaction most likely occurs between surficial waters and ring N atoms of atrazine, leading to ring-N protonation and subsequent hydroxylation. It is not clear why hydroxyatrazine forms as major product over deethylatrazine, but it could be related to possible preferences of the mineral surface waters and central cation interacting preferably with the ring N atoms over the N atoms within the alkylamino groups. It is also possible that the alkylamino chains are simply more hydrophobic than the triazine ring and thus interact less favourably with the mineral surface.