| The chemical transformation of the herbicide, atrazine [2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine], in soil environments has become a matter of renewed concern because of recent data indicating wide spread of transport and atmospheric deposition as well as potential carcinogenic effects at aqueous concentration. Surface catalyzed degradation of organic pollutants adsorbed to metal oxides, particularly manganese oxides it well established, but abiotic transformation of herbicides by this pathway have rarely been investigated. This study reports the first investigation of abiotic transformation of atrazine by synthetic manganese oxides. The degradation reactions of adsorbed atrazine on manganese oxides were quantified and identified by combined calorespirometry, gas chromatography, liquid chromatography and UV resonance Raman spectroscopy from dry deposition, solid-solid (mechanochemical) interaction and suspension batch reactor experiments. These combined methodologies gave clear evidence for the formation of three N-dealkylated forms of atrazine (DEA, DIA and DDA), hydroxylated atrazine (HA, DEHA, DIHA and ammeline) and CO2. Calorespirometry identified an exothermic reaction for the MnO2 and atrazine interaction. A novel dealkylation mechanism is proposed involving proton transfer to Mn(IV)-stabilized oxo and imido bonds. There was no direct oxygen effect on the mechanism of the N-dealkylation reaction, but O2 was involved only in a secondary oxidation reaction of the alkyl fragments from the deposition experiment. Birnessite and cryptomelane-II gave a relatively higher efficiency to degrade atrazine than other types of manganese oxides from mechanochemical interactions and suspension batch reactor experiments. The rate of degradation by manganese oxide suspension was dependent on the pH, types of manganese oxides and its concentration, and it can be represented by rate = k[MnO2] 0.43[H+]0.11--0.14[atrazine]. The degradation rates were increased with increasing manganese oxide concentration and decreasing pH. The order of reactivity for the manganese oxides towards atrazine was attributed to the degree of crystallinity and its unit surface area. The result obtained in this study may prove useful not only in the design of remediation scenarios, but also to the calibration of in situ biological studies, which heretofore have considered abiotic degradation to be an insignificant pathway for atrazine degradation in soils. |
