Thesis (MScAgric)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: In the humid Graskop region of Mpumalanga Province, South Africa, there is an
anomalous body of highly weathered black, manganiferous oxisols derived from dolomite.
With Mn contents as high as 17%, potential large-scale Mn release is an environmental
concern under current, acid generating, forestry practices. This study aims at establishing
the factors which may affect the stability of the manganiferous oxisols of Graskop and in
the process, investigating some of the morphological, mineralogical and chemical
properties of these unique soils.
Typically, the soils show a reddish, nodule-rich horizon, containing 3-4% Mn, grading
through a red and black mottled zone into a black (5YR 2.5/1) apedal subsoil with >7%
Mn. The Mn gradient down the profile as well as the abundant nodule content of the upper
subsoil horizons implies that Mn mobilization and redistribution are active pedogenic
processes. The exceptional Mn content of these soils is complemented with Fe and Al
concentrations of up to 10 and 8%, respectively, and anomalously high trace element levels
in particular Ni and Zn (as high as 541 and 237 mg kg-1, respectively) which are at the
upper limit of cited world natural maxima for soils. The Mn mineral lithiophorite
[(Al,Li)MnO2(OH)2], dominates the mineralogy of the soils with accessory amounts of
birnessite, gibbsite, goethite, hematite, maghemite, kaolinite, aluminous chlorite and mica -
a mineral suite reflecting that of well weathered soils.
With the pH of the soil being at or close to the point of zero charge (4.5-5.5) the soils show
isoelectric equilibrium. The very low buffer capacity results in metal dissolution
commencing with the first increment of titrated acidity. Manganese dissolution is relatively
minor considering the large potential for release and is highly overshadowed by Al release.
The apparent resilience of the Mn phase to added acidity may relate to the overwhelming
poise of the soils which maintains robust, oxic conditions despite the usual instability of
Mn oxides at low pH.
Manganese release and soil redox properties are substantially affected by drying especially
in the organic rich topsoils. Using various redox analyses, evidence is shown for
involvement of Mn(III)-organic complexes in the drying reactions. Using this and
information gained in a real time, attenuated total reflectance Fourier transform infrared(ATR-FTIR) spectroscopic study, a mechanism is suggested which may account for the
observed Mn release and the loss of Cr oxidising capacity commonly observed in dried
soils. The information provided by the ATR-FTIR study showed the decrease in surface
pH of a clay film, from 5 to below 2, as well as the shift in coordination nature of sorbed
oxalate from a more outer-sphere association to a more inner-sphere association
concomitant with the removal of free water from the clay surface. This spectroscopic
evidence for these chemical changes which accompany surface drying not only provides
further insight into the reactions involving Mn oxides in soils but also highlights the
suitability of ATR-FTIR for real time, in situ investigation into the chemistry of the drying
water interface.
From these results it is concluded that Mn release from the manganiferous oxisols, under
acid generation of the kind known to occur in pine plantations, is less that anticipated. On
the other hand, desiccation of the topsoil results in substantial Mn release with a suggested
mechanism which involves a Mn(III) intermediate. / AFRIKAANSE OPSOMMING: ‘n Onreelmatige grondliggaam van hoogs verweerde, swart, mangaanhoudende oxisols
wat uit dolomiet ontwikkel het, word in die humiede Graskop streek van die Mpumalanga
Provinsie van Suid-Afrika aangetref. Die hoë Mn-inhoud (tot 17%) van hierdie oxisols is
van groot omgewings-belang weens die potensiële grootskaalse Mn-vrystelling onder
huidige, suur-genererende bosbou praktyke. Hierdie studie beoog om die faktore wat die
stabiliteit van die mangaanhoudende oxisols van Graskop affekteer, vas te stel. Tesame
hiermee word die morfologiese, mineralogiese en chemiese eienskappe van hierdie
unieke gronde ondersoek.
Kenmerkend van hierdie gronde is ‘n rooi, nodule-ryke horison met 3-4% Mn aan die
oppervlakte. Bogenoemde horison verander met toename in diepte in ‘n rooi en swart
gevlekte sone wat weer in ‘n swart (5YR 2.5/1) apedale ondergrond met >7% Mn
oorgaan. Die Mn gradiënt in die profiel sowel as die hoë nodule-inhoud van die boonste
grondhorison dui daarop dat Mn-mobilisasie en -herverspreiding huidige aktiewe
pedogenetiese prosesse in die profiele is. Fe en Al, met konsentrasies van 10% en 8%
onderskeidelik, word saam met die onreelmatig hoë Mn inhoud aangetref. Baie hoë
vlakke van Ni en Zn (so hoog as 541 en 237 mg.kg-1 onderskeidelik) wat hoër is as
aangehaalde wêreld natuurlike maksimum waardes, word ook aangetref. Die mineralogie
van die gronde word deur die Mn mineraal litioforiet [(Al,Li)MnO2(OH)2] gedomineer.
Bykomstige hoeveelhede van birnessiet, gibbsiet, goethiet, hematiet, maghemiet,
kaoliniet, aluminiumryke chloried en mika word ook aangetref. Hierdie minerale
samestelling is kenmerkend van hoogs verweerde gronde.
Met die pH van die grond in die omgewing van die punt van geen lading (4.5 – 5.5),
word ‘n iso-elektriese ekwilibrium by die gronde aangetref. Die baie lae bufferkapasiteit
het metaal-oplossing aangehelp wat met die eerste inkrement van titreerbare suurheid ‘n
aanvang geneem het. Mangaan-oplossing is baie klein indien die groot potensiaal vir
vrystelling asook die groot mate van Al-vrystelling in ag geneem word. Die skynbare
teenwerking van die Mn fase tot toegevoegde suurheid, mag toegeskryf word aan diesterk ewewig van die gronde om sterk, oksiese kondisies, ten spyte van die normale
onstabiliteit van Mn oksiedes by lae pH, te onderhou.
Mangaan vrystelling en grond redoks eienskappe word beduidend deur uitdroging
beïnvloed en veral in die organies-ryke bogronde. Deur van verskeie redoks analises
gebruik te maak is daar bewyse van die betrokkenheid van Mn(III)-organiese komplekse
in die uitdroging-reaksies gevind. Dit. en data ingesamel in ‘n “real time, attenuated total
reflectance Fourier transform infrared (ATR-FTIR)” spektroskopiese studie, is gebruik
om ‘n meganisme voor te stel wat die waargenome Mn vrystelling en die verlies aan Cr
oksidasie kapasiteit (algemeen waargeneem in droë gronde) te kan verklaar. Die data
verkry met die ATR-FTIR studie het ‘n afname in oppervlak pH van 5 na 2 van ‘n klei
film asook die verskuiwing in koördinasie toestand van die gesorbeerde oksalaat van ‘n
meer buite-sfeer assosiasie tot ‘n meer binne-sfeer assosiasie, gepaardgaande met die
verwydering van vry water van die klei oppervlaktes, uitgewys. Die spektroskopiese
bewyse vir die chemiese veranderinge wat die oppervlak uitdroging vergesel, gee nie net
meer insig in die reaksies rakende Mn oksiedes in gronde nie maar onderstreep ook die
toepasbaarheid van die ATR-FTIR vir intydse (“real time”), in situ ondersoeke na die
chemie van die uitdrogende water kontakvlak.
Vanuit hierdie resultate kan afgelei word dat Mn vrystelling vanuit mangaanhoudende
oxisols onder suur genererende denne plantasies laer is as wat verwag is. Aan die ander
kant sal uitdroging van die bogrond tot aansienlike Mn vrystelling, met ‘n verwagte
meganisme wat Mn (III) as intermediêre toestand insluit, lei.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16312 |
| Date | 12 1900 |
| Creators | Dowding, Catherine Elaine |
| Contributors | Fey, M. V., University of Stellenbosch. Faculty of Agrisciences. Dept. of Soil Science. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xii, 118 leaves : ill. (some col.) |
| Rights | University of Stellenbosch |
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