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

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Johnston, SG, Burton, ED, Bush, RT, Keene, AF, Sullivan, LA, Smith, CD, McElnea, AE, Ahern, CR & Powell, B 2010, 'Abundance and fractionation of Al, Fe and trace metals following tidal inundation of a tropical acid sulfate soil', Applied Geochemistry, vol. 25, no. 3, pp. 323-335.

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Publisher's version of article available at http://dx.doi.org/http://dx.doi.org/10.1016/j.apgeochem.2009.11.015


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.

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