The degradation of atrazine by soil minerals : effects of drying mineral surfaces

Thesis (MScAgric)--Stellenbosch University, 2014.

ENGLISH ABSTRACT: The herbicide atrazine (ATZ, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) has been identified as an environmental endocrine disruptor and possible human carcinogen. The presence of atrazine, along with its degradation products, in soils and water supplies therefore raises concern. Atrazine biodegradation in soils is well-covered to date, however, atrazine degradation by abiotic mineral surfaces, and the chemical mechanism by which it occurs, is not fully understood. Furthermore, with a changing global climate, the effects of wetting and drying cycles on soil processes (e.g. atrazine degradation) is largely unknown, but increasing in importance. This study therefore investigated atrazine degradation on six common soil mineral surfaces, namely birnessite, goethite, ferrihydrite, gibbsite, Al3+-saturated smectite and quartz, as well as the effects that drying these surfaces has on atrazine degradation. In the first part, a comparison was conducted between the reactivity of fully hydrated and drying mineral surfaces toward atrazine, by reacting atrazine-mineral mixtures under both moist and ambient drying conditions, in parallel, for 14 days. Under moist conditions, none of the mineral surfaces degraded atrazine, but under drying, birnessite and goethite degraded atrazine to non-phytotoxic hydroxyatrazine (ATZ-OH, 2-hydroxy-4-ethylamino-6-isopropylamino-1,3,5-triazine) as major product and phytotoxic deethylatrazine (DEA, 2-chloro-4-amino-6-isopropylamino-1,3,5-triazine) as minor product. The mineral surface reactivity was birnessite (66% degradation) > goethite (18% degradation) >> other mineral surfaces (negligible degradation), indicating possible atrazine oxidation. In the second part, the effects of drying rate were investigated on birnessite only (the most reactive surface), by conducting the drying (1) gradually at ambient rates, (2) rapidly under an air stream, and (3) gradually in the absence of water using only organic solvent. After 30 days of ambient drying, 90% of the atrazine was degraded to ATZ-OH and DEA, but the same extent of degradation was achieved after only 4 days of rapid drying with an air stream. Thirty days of gradual drying using only organic solvent did not increase atrazine degradation compared to the water-moist drying surface. In each case, degradation initiated at a critical moisture content of 10% of the original moisture content. In the third part, the degradation mechanism was further investigated. To test for the possible oxidation of atrazine by the birnessite surface, moist atrazine-birnessite mixtures were dried under a nitrogen (N2) stream to eliminate possible oxidation by atmospheric oxygen (O2). Dissolved Mn2+ was extracted at the end of the experiment to observe any reduction of birnessite. Under N2, the same products were formed as before, with no appreciable Mn2+ production, indicating non-oxidative atrazine degradation by birnessite. The final part investigated the effects ultraviolet (UV) radiation has on the degradation of atrazine by drying mineral surfaces. The UV-radiation enhanced the degradation of atrazine, but no other degradation products were formed. It was therefore concluded that atrazine degradation on redox-active soil mineral surfaces is enhanced by drying, via a net non-oxidative mechanism. Furthermore, this drying-induced degradation is an atrazine detoxification mechanism which could be easily applied through agricultural practices such as windrowing, ploughing and any other practice that (rapidly) dries a Mn- or Fe-oxide rich agricultural soil.

AFRIKAANSE OPSOMMING: Die onkruiddoder atrasien (ATS, 2-chloro-4-etielamino-6-isopropielamino-1,3,5-triasien) is as 'n omgewings endokriene versteurder en moontlike menslike karsinogeen geidentifiseer. Die teenwoordigheid van atrasien, tesame met sy afbreekprodukte, in grond en water toevoere wek dus kommer. Die bio-afbreking van atrasien in gronde is tot dusver goed gedek, maar die afbreking van atrasien deur abiotiese mineraaloppervlaktes, en die chemiese meganisme waarmee dit plaasvind, word nie heeltemal verstaan nie. Verder, met 'n veranderende globale klimaat, is die effekte van benatting- en drooging-siklusse op grondprosesse (bv. atrasien afbreking) grootliks onbekend, maar toenemend belangrik. Daarom het hierdie studie atrasien afbreek op ses algemene mineraaloppervlaktes, naamlik birnessiet, goethiet, ferrihidriet, gibbsiet, Al3+-versadigde smektiet en kwarts, ondersoek, asook die effekte wat drooging van hierdie oppervlaktes op atrasien afbreking het. In die eerste deel, was 'n vergelyking gedoen tussen die reaktiwiteit van volgehidreerde en droëende mineraaloppervlaktes teenoor atrasien, deur atrasien-mineraal mengsels, in parallel, onder albei nat en omliggende droogings toestande te reageer vir 14 dae. Onder nat toestande, het geeneen van die mineraaloppervlaktes atrasien afgebreek nie, maar onder drooging het birnessiet en goethiet atrasien afgebreek na nie-fitotoksiese hidroksieatrasien (ATS-OH, 2-hidroksie-4-etielamino-6-isopropielamino-1,3,5-triasien) as hoofproduk en fitotoksiese deetielatrasien (DEA, 2-chloro-4-amino-6-isopropielamino-1,3,5-triasien) as minder-produk. Die mineraaloppervlakte-reaktiwiteit was birnessiet (66% afbreking) > goethiet (18% afbreking) >> ander mineraaloppervlaktes (geringe afbreking), wat moontlike atrasien oksidasie aandui. In die tweede deel, is die effekte van droogingstempo ondersoek, op birnessiet alleenlik (die mees reaktiewe oppervlak) deur drooging by (1) 'n omliggende geleidelike tempo, (2) 'n versnelde tempo onder 'n lugstroom, en (3) 'n geleidelike tempo in die afwesigheid van water, deur slegs gebruik te maak van 'n organiese oplosmiddel. Na 30 dae se geleidelike drooging, is 90% van die atrasien afgebreek na ATS-OH en DEA, maar dieselfe hoeveelheid afbreking is bereik na slegs 4 dae onder versnelde drooging met die lugstroom. Dertig dae van geleidelike drooging met slegs organiese oplosmiddel het nie atrasien afbreking vermeerder in vergelyking met die water-nat droëende oppervlak nie. In elke geval, is afbreking geïnisieer by 'n kritiese water inhoud van 10% van die oorspronklike water inhoud. In die derde deel is die afbrekingsmeganisme verder ondersoek. Om te toets vir die moontlike oksidasie van atrasien deur die birnessiet oppervlak, is nat atrasien-birnessiet mengsels onder stikstof (N2) gedroog, om die moontlike oksidasie deur atmosferiese suurstof (O2) te verhoed. Opgeloste Mn2+ was teen die einde van die eksperiment geekstraëer om enige reduksie van birnessiet waar te neem. Onder N2 is dieselfde produkte as voorheen gevorm, met geen aansienlike Mn2+ produksie nie, aanduidend van 'n nie-oksideerende afbreek van atrasien deur birnessiet. Die laaste deel het die effekte van ultraviolet (UV) straling op die afbreek van atrasien op droëende mineraaloppervlaktes ondersoek. Die UV-straling het atrasien afbreek vermeerder, maar geen ander afbreek-produkte is gevorm nie. Die gevolgtrekking is dus dat atrasien afbreking op redoks-aktiewe mineraal-oppervlaktes verhoog word met drooging, deur 'n netto nie-oksidasie meganisme. Verder is hierdie drooging-geinduseerde afbreking 'n atrasien ontgiftingsmeganisme wat eenvoudig toegepas kan word deur landboupraktyke soos windrying, ploeg en ander praktyke wat (vinnig) 'n Mn- of Fe-oksied ryke landbou grond verdroog.

National Research Foundation (NRF)