Abundance and fractionation of Al, Fe and trace metals following tidal inundation of a tropical acid sulfate soil

Scott G. Johnston, Southern Cross University
Edward D. Burton, Southern Cross University
Richard T. Bush, Southern Cross University
Annabelle F. Keene, Southern Cross University
Leigh A. Sullivan, Southern Cross University
C Douglas Smith, Department of Natural Resources & Water, Qld.
Angus E. McElnea, Department of Natural Resources & Water, Qld.
Col R. Ahern, Department of Natural Resources & Water, Qld.
Bernard Powell, Department of Natural Resources & Water, Qld.

Abstract

Tidal inundation was restored to a severely degraded tropical acid sulfate soil landscape and subsequent changes in the abundance and fractionation of Al, Fe and selected trace metals were investigated. After 5 a of regular tidal inundation there were large decreases in water-soluble and exchangeable Al fractions within former sulfuric horizons. This was strongly associated with decreased soil acidity and increases in pH, suggesting pH-dependent immobilisation of Al via precipitation as poorly soluble phases. The water-soluble fractions of Fe, Zn, Ni and Mn also decreased. However, there was substantial enrichment (2¨C5¡Á) of the reactive Fe fraction (FeR; 1 M HCl extractable) near the soil surface, plus a closely corresponding enrichment of 1 M HCl extractable Cr, Zn, Ni and Mn. Surficial accumulations of Fe(III) minerals in the inter-tidal zone were poorly crystalline (up to 38% FeR) and comprised mainly of schwertmannite (Fe8O8(OH)6SO4) with minor quantities of goethite (¦Á-FeOOH) and lepidocrocite (¦Ã-FeOOH). These Fe (III) mineral accumulations provide an effective substrate for the adsorption/co-precipitation and accumulation of trace metals. Arsenic displayed contrary behaviour to trace metals with peak concentrations (60 ¦Ìg g−1) near the redox minima. Changes in the abundance and fractionation of the various metals can be primarily explained by the shift in the geochemical regime from oxic¨Cacidic to reducing-circumneutral conditions, combined with the enrichment of reactive Fe near the soil surface. Whilst increasing sequestration of trace metals via sulfidisation is likely to occur over the long-term, the current abundance of reactive Fe near the sediment¨Cwater interface favours a dynamic environment with respect to metals in the tidally inundated areas.