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Mechanism and Kinetics of Mineral Weathering Under Acid Conditions

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Mechanism and Kinetics of Mineral Weathering Under Acid Conditions Mechanisman dkinetic so f mineral weathering under acid conditions C. Anbeek 2 4 FEB. 1994 UB-OARnr~' CENTRALE LANDBOUWCATALOGU S 0000 0575 8392 Uoqr Promotor: Dr. ir. N. van Breemen hoogleraar in de bodemvorming en ecopedologie Co-promotor: Dr. L. van der Plas hoogleraar in de bodemmineralogie AW o?2o', tf3^> Mechanism and kinetics of mineral weathering under acid conditions C. Anbeek Proefschrift ter verkrijging van degraa d van doctor in de landbouw- en milieuwetenschappen, op gezag van de rector magnificus, Dr. C.M.Karssen , in hetopenbaa r teverdedige n op woensdag 16 februari 1994 des namiddags tevie r uuri nd e Aula van de Landbouwuniversiteit te Wageningen. ,0 5*nM Cover: The scanning electron microscope image (right) shows a polished surface of adularia feldspar after 10 minutes immersion in a2 % HF solution. Etch pits have developeda t crystallographically controlled surface sites. The 3-D matrix surface shows the result ofa Monte Carlo simulation ofetc h pit development at acrysta l defect. BIBUOTFHEE«^ ^DBOUWUNI^OUWUNIVEKSIIBV » DOENINGEN CIP-DATA KONINKLIJKE BIBLIOTHEEK, DEN HAAG Anbeek, C. Mechanism andkinetic s of mineral weathering under acid conditions/ C. Anbeek. - [S.I. :s.n.] .- III. Thesis Wageningen. - With ref. - With summary in Dutch. ISBN 90-5485-215-1 Subject headings: mineral weathering / feldspar / quartz. Stellingen 1. Earth is the cradle of mankind, but one should not live inth e cradle forever. K. Tsiolkovsky(1920) Beyond the Planet Earth 2. Alle aandacht is gericht op het kappen van het tropisch regenwoud, maar ook in China, Siberië, Canada en de Verenigde Staten maakt men korte metten met de oerbossen. WageningsUniversiteitsblad, 16jan. 1992 3. De vermeende Cambrische explosie van levensvormen na 214miljar d jaar blauwwieren duidt veeleer op natuurlijke selektie en steeds grotere soortenrijkdom in een periode waarin uiteindelijk de eerste harde skeletdelen werden ontwikkeld. 4. Het belangrijkste doel van de geldverslindende "Postbus 51" campagnes is het wekken van de schijn dat de Rijksoverheid beleid voert. 5. De nauwkeurige afstemming van vele fysische grootheden op een heelal vol diversiteit en met ontwikkelingsmogelijkheden voor organisch leven bewijst het bestaan van een Hogere Macht, óf het bestaan van talloze afzonderlijke heelals - inclusief een aantal met planeten van pizzadeeg -waarui t wij per definitie een "winnend lof' hebben getrokken. 6. Een tweemaal zo sterke Unterseebootdienst aan het begin van de oorlog zou, zowel voor Der Kaiser als voor Der Führer, het pleit binnen twee jaar in hun voordeel hebben beslecht. 7. Alle bestaande literatuur inzake de oplossingskinetiek van "vers" gemalen minerale monsters dient opnieuw te worden geïnterpreteerd. Dit proefschrift 8. Voor het onderling vergelijken van verweringssnelheden is normalisatie naar het geometrisch oppervlak leerzamer en eenvoudiger te realiseren dan normalisatie naar het BET oppervlak. Dit proefschrift 9. De verweringssnelheid van vers gemalen veldspaat, genormaliseerd naar het geometrisch of naar het BET oppervlak, is evenredig met de korrelgrootte. Dit proefschrift 10. Door natuurlijke verwering vormt zich een uiterst poreuze oppervlaktelaag op veldspaatkorrels. De vorming van deze laag bij veldspaat - en het achterwege blijven ervan bij kwarts - kan worden beschreven met behulp van een thermodynamische theorie voor kristalfouten in deze mineralen. Dit proefschrift 11. De lage verweringssnelheid van veldspaat in de natuur, vergeleken met die van vers gemalen veldspaat in gangbare laboratorium-experimenten, wordt gedeeltelijk verklaard doordat het oppervlak van de poriën in verweerde veldspaat niet-reaktief is. Ditproefschrift 12. For feldspar at low pH, the presence of a Si-rich surface layer and the increasing plagioclase dissolution rate with increasing anorthite content imply that, under acid conditions, AI is the destabilizing component in the tetrahedral framework. Alex E. Blum (inpress) In: Feldsparsand TheirReactions. 13. Het feit dat de verweringssnelheid van natuurlijk verweerde veldspaat, genormaliseerd naar BET oppervlak, onafhankelijk is van de korrelgrootte bewijst nog niet dat het aantal reaktjeve plaatsen per eenheid van geometrisch oppervlak onafhankelijk is van de korrelgrootte. Ditproefschrift 14. De verzadigingsgraad van de buik-oplossing is een slechte schatter van de verzadigingsgraad waarbij de verwering van veldspaat daadwerkelijk plaatsvindt. Ditproefschrift 15. Helaas is voor het reizen Sneller Dan Licht meer nodig dan de vermakelijke, quasi- wetenschappelijke praatjes van SF-auteurs over graviton-interaktie, hyperruimte en illiniumstaven in een krachtveld vanvrij e mesonen. W.F. Temple(1968) TheFleshpots ofSansato. J. Blish(1970) Citiesin Flight. J.P. Hogan (1978) TheGentle Giantsof Ganymede. 16. Mijn vrouw noemt mij een model-echtgenoot: thuiskomend van het modelleren van bodemverweringsprocessen ga ik verder met het modelleren van Amerikaanse spoorwegen. Stellingen behorend bij het proefschrift "Mechanism and kinetics of mineral weathering under acid conditions". Chris Anbeek, Wageningen, 16februar i 1994. De burgemeester keek versuft naar de deur, die zich achter de wetenschapsman sloot. "Wat wilde hij?" vroeg hij zich af. "Hij is tenslotte een geleerde en men kan zijn woorden niet verwaarlozen.Al s men ze maar begreep." M.Toonde r (1978) Degeweldige wiswassen Aan mijn ouders Aan Gerrie zonder jullie steun zou dit proefschrift niet zijn verschenen Abstract Anbeek, C. (1994) Mechanism and kinetics of mineral weathering under acid conditions. Doctoral thesis, Agricultural University of Wageningen, The Netherlands. This study deals with the relationships between crystal structure, grain diameter, surface morphology and dissolution kinetics for feldspar and quartz under acid conditions. Intensively ground samples from large, naturally weathered mineral fragments are frequently used in dissolution studies. The surface area of such samples, estimated from their gas adsorption isotherm (BET method), is normally implied to be all freshly created by grinding. This study revealed that: (1) during natural weathering, micropores (diameters » 2 nm) develop in feldspar but not in quartz grains; (2) the micropores account for virtually all BET surface area of naturally weathered feldspar grains; and (3) due to the micropores, grinding of large, naturally weathered feldspar fragments is highly ineffective in creating samples with only freshly ground BET surface area. By assuming all BET surface area of ground feldspar samples to be freshly created, experimental dissolution data have been explained from dissolution rates essentially independent of the grain diameter. For ground feldspar samples this study revealed that: (1) the dissolution rate of the freshly created BET surfaces is essentially proportional to the grain diameter; and (2) the dissolution rate of the naturally weathered BET surfaces, still present after grinding, is most likely independent of the grain diameter. Moreover, the dissolution rate, normalized to BET surface area, of unfractured, naturally weathered feldspar grains was essentially independent of the grain diameter. These findings can be explained if: (1) the average density of dissolution sites on freshly created feldspar surfaces is approximately proportional to the grain diameter; (2) micropores develop at dissolution sites during natural weathering; and (3) the BET surface area of the micropore "walls" (i.e. the area perpendicular to the grain surface) is essentially non-reactive. Thermodynamical considerations and Monte Carlo simulations showed that: (1) the formation of micropores in feldspar but not in quartz grains during natural weathering can be explained from enhanced dissolution at crystal defects; and (2) the BET surface area of micropore "walls" from enhanced dissolution at crystal defects is essentially non-reactive. A kinetic model is developed, showing for feldspar that the non-reactivity of the micropore "walls" helps to explain the discrepancy, reported in the literature, between laboratory and field dissolution rates. Keywords: mineral weathering, feldspar, quartz, BET surface area, geometric surface area, surface roughness, crystal defects, etch pits, micropores. Voorwoord Na zelf ook weer vier en een half jaar verder verweerd te zijn, is thans mijn proefschrift over mineraalverwering afgerond. Dat dit mogelijk werd dank ik allereerst aan mijn ouders, die mij altijd stimuleerden en in staat stelden om de opleiding van mijn keuze te volgen. Ik ben blij dat zij dit moment kunnen meebeleven. En ik weet zeker dat zij hetzelfde voelen, al zou met name mijn vader liever een Nederlandstalige versie in handen hebben gehad. Naast de bijzondere rol van mijn ouders hierin dankt dit proefschrift zijn bestaan bovenal aan de inspanningen en interesse van mijn promotor Nico van Breemen. Nico, bedankt voor het in mij gestelde vertrouwen en voor het kernachtig herformuleren van mijn soms amorfe gedachtenspinsels. Verder wil ik hier speciaal noemen de steun en hulp die ik ontving van mijn co-promotor en voormalig professor in de bodemmineralogie Leendert van der Plas, en van mijn collega Ed Meijer. Leendert, bedankt voor het met raad en daad terzijde staan tijdens momenten waarop met name mijn onvoldoende ervaring op optisch­ mineralogisch gebied mij parten speelde. Ed, naast alle dingen die je voor me hebt gedaan dank ik je speciaal voor de vele lessen logisch redeneren die je me - wellicht onbewust - hebt gegeven tijdens onze discussies. Enorm veel werk is mij uit handen
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