『Abstract
An Inceptisol A-horizon from Hawaii was subjected to a series
of reduction-oxidation cycles - 14 d cycle length over a 56 d
duration - across the “soil-Fe” [Fe(OH)3.Fe2+(aq), log K ゜= 15.74] equilibrium in triplicate
redox-stat reactors. Each reducing event simulated the flush of
organic C and diminished O2 that accompanies
a rainfall-induced leaching of bioavailable reductants from the
forest floor into mineral soil. The soil contained considerable
amounts of short-range ordered (SRO) minerals (e.g., nano-goethite
and allophane) and organic matter (11% org-C). Room temperature
and cryogenic 57Fe Mossbauer(oの頭に¨)
spectroscopy showed that the iron-bearing minerals were dominated
by nano- to micro-scale goethite, and that ferrihydrite was not
present. Over the four full cycles, fluctuations in Eh
(from 200 to 700 mV) and pFe2+ (from 2.5 to 5.5) were
inversely correlated with those of pH (5.5 to 4). Here, we focus
on the solubility dynamics of the framework elements (Si, Fe,
Ti, and Al) that constitute 35% of the oxygen-free soil dry mass.
Intra-cycle oscillations in dissolved (<3 kDa) metals peaked during
the reduction half-cycles. Similar intra-cycle oscillations were
observed in the HCl and acid ammonium oxalate (AAO) extractable
pools. The cumulative response of soil solids during multiple
redox oscillations included: (1) a decrease in most HCl and AAO
extractable metals and (2) a transformation of SRO Fe (as nano-goethite)
to micro-crystalline goethite and micro-crystalline hematite.
This may be the first direst demonstration that Fe oxide crystallinity
increases during redox oscillations - an a priori unexpected
result. 』
1. Introduction
2. Materials and methods
2.1. Overall approach
2.2. Soil selection
2.3. Initial soil characterization
2.4. Redox reactor design
2.5. Redox oscillation experiment
2.5.1. Reactor operation
2.5.2. Reactor sampling scheme
2.6. Chemical analysis
2.7. Mossbauer(oの頭に¨) spectroscopy
2.8. Calculations
3. Results
3.1. Initial soil characterization
3.2. Iron dynamics
3.3. 57Fe Mossbauer(oの頭に¨) characterization
3.3.1. Soil Fe mineral composition
3.3.2. Changes in Fe mineral composition resulting from redox
oscillations
3.4. Metal partitioning
4. Discussion
4.1. Solid phase Fe speciation
4.2. Iron redox cycles as a biogeochemical forcing function
4.3. Fe redox cycle evaluation
4.3.1. Eh, pH, and FeaqII
considerations
4.3.2. Solid phase Fe distribution and mineral ripening
4.3.3. Solubility product analysis
4.3.4. Fe reduction/oxidation rates
4.3.5. Solid phase behavior of Al, Si, and Ti
5. Conclusions
Acknowledgments
Appendix A
References