『Abstract
Vivianite (Fe3(PO4)2・8H2O) may precipitate in
anoxic wetland soils where it may control orthophosphate (Pi) equilibrium solution concentrations at micromolar
levels, and thus be of key importance in reducing excessive P
from agricultural sources and eutrophication. However, vivianite
equilibria and kinetics under in situ conditions are not fully
understood and the occurrence of vivianite in wetland soils is
rarely documented. In the present investigation we have monitored
the temporal (November to June) variation in the pore water chemistry
of a wet meadow soil (Sapric Medihemist) including a vivianite-containing
gyttja layer. Pore water concentrations of Ca, FeII,
HCO3, and NH4 in the
gyttja layer were higher than in adjacent horizons. In contrast,
dissolved Pi concentrations were the lowest
observed in the profile and showed only minor fluctuations (between
0.1 and 8μM). Pore water composition in the gyttja layer was close
to equilibrium with vivianite (saturation index, SIviv,
2.01±0.53) at constant pH (〜6.8). Dissolution and precipitation
experiments in the laboratory with soil suspensions from the gyttja
layer demonstrated that vivianite solubility equilibria were only
slowly restored. Even after 120 days following perturbation the
supersaturation was still high (SIviv 〜6).
It seems that vivianite does contribute to Pi
immobilization in anoxic soil horizons, but due to slow precipitation
kinetics such soils cannot maintain Pi concentrations
at levels below critical thresholds foe eutrophication (〜1μM),
except if pore water geochemistry is kept stable.
Keywords: Wetland; Gyttja; Vivianite; Sorption; Iron (II); Phosphate』
1. Introduction
2. Materials and methods
2.1. Study site and profile description
2.2. Soil pore water sampling and analysis
2.3. Laboratory studies of vivianite dissolution and supersaturation
2.4. Geochemical equilibrium calculations
3. Results
3.1. Soil chemical and mineralogical characteristics
3.2. In situ temporal changes of pore water chemical composition
3.3. Vivianite dissolution and precipitation
4. Discussion
4.1. Field pore water chemistry, redox conditions and equilibrium
considerations
4.2. Vivianite occurrences and possible mechanism for vivianite
formation by microbial reduction of goethite
4.3. Dissolution and precipitation kinetics
5. Summary and conclusion
Acknowledgements
References