WWRC 95-16
Potential Solid Phases Contolling Dissolved Aluminium and Iron Concentrations in Acidic Soils
Abstract
Acidic soils often have high concentrations of dissolved aluminium (Al) and iron (Fe) in their solutions. The
objective of this study was to determine potential solid phases controlling dissolved Al and Fe concentrations in
pyritic (FeS2) soils undergoing oxidation. Soil samples were extracted with distilled-deionised H2O after reacting
for 14, 21 and 28 days (d) at 25±1°C. Soil water extracts were analysed for pH, Eh, total concentrations of Al and
Fe as well as other cations and anions. From these, ion activities and ion activity products (IAPs) were calculated
and compared with solubility products (KspS). The importance of dissolved organic carbon (DOC) complexes of
Al3+ and Fe3+ in soil extracts was also evaluated. The pH of the samples ranged from 6.6 to 2.3 and pe+pH ranged
between 8.9 and 13.4. Aluminium concentrations ranged from 0.04 to 196 mg L-1 and Fe from 0.38 to 337 mg
L-1. The SO42- concentrations were found to be 253 to 4,924 mg L-1 As pyrite (FeS2) oxidised, pH decreased
and soluble concentrations ofAl3+ Fe3+ and SO42- increased in soil water extracts. Chemical speciation indicated
that DOC complexes of Fe3+ were significant in low pH soil water extracts. For near neutral to slightly acidic soils,
the IAPs suggested saturation with respect to Al(OH)3, (gibbsite). At extreme low pH, the IAPs were in equilibrium
with Al(SO4)(OH).5H2O (jurbanite). Results obtained on iron solid phases indicated that Fe(OH)3 (amorphous
iron) before oxidation of FeS2, FeOOH (goethite) in slightly acidic environments, and FeSO4.7H2O (melanite) in
extreme acidic environments may have potential for controlling Fe3+ concentrations.
Key words: acid sulfate, chemistry of Al and Fe, chemical speciation, solid phase control
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