Geochemical partitioning of iron and trace elements in acid sulfate soils

Salirian R Claff

The objective of this study was to examine the behaviour of Fe and Cr, Cu, Mn, Ni and Zn in acid sulfate soil materials. Acid sulfate soil chemistry is strongly affected by Fe, and Fe mineralogy is highly dependent on the redox conditions. Under reducing conditions Fe(II) minerals such as pyrite dominate soil mineralogy, whereas under oxic conditions Fe(III) minerals such as schwertmannite and jarosite dominate. Ideally, any method used to assess metal partitioning should be able to distinguish the Fe(II) and Fe(III) mineral pools present under both oxidising and reducing conditions. A novel sequential extraction procedure was developed, assessed, and then used to examine metal partitioning in acid sulfate soil materials. The procedure employs six steps to quantify (i) “labile” (magnesium chloride extractable) (ii) “acid-soluble” (hydrochloric acid extractable) (iii) “organic-bound” (pyrophosphate extractable) (iv) “crystalline oxide” (citrate buffered dithionite extractable) (v) “pyritic” (nitric acid extractable) and (vi) “residual” (acid/peroxide digestible) pools of Fe and trace elements. The sequential extraction procedure was shown to adequately partition metals into operationally defined fractions of environmental significance in ASS landscapes, including Fe associated with pyrite, a key mineral in acid sulfate soil chemistry. Standard laboratory methods use dried and ground soil on which to perform routine analysis, procedures which greatly alter the redox conditions of reduced acid sulfate soil materials, potentially altering solid-phase metal partitioning. To address these potential artifacts of oxidation, metal partitioning was examined in both the oven-dried and ground and field condition soil samples. Extensive partitioning changes occurred for many metals in the oven-dried and ground soil compared to soil in field condition. Consequently, field condition soils were analysed during all subsequent metal partitioning studies. Metal partitioning changes during short-term oxidation and acidification of ASS materials were also examined in detail. In general, it was found that oxidation resulted in the repartitioning of metals from oxidisable fractions (i.e. “pyritic” and “organic”) to the “acidsoluble” fraction. Following the onset of extremely acid conditions, metals associated with the “acid-soluble” were fraction were re-partitioned into the “labile” (i.e. environmentally available) fraction. Finally, the sequential extraction procedure was used to determine how metals were repartitioned in an acidic ASS landscape following the re-introduction of tidal inundation. The acidified soil profile displayed extensive mobilisation of Cu, Mn and Ni, whilst sequestration of these same metals was observed in the tidally re-inundated site. The majority of the metals were sequestered in the “acid-soluble” fraction, which is dominated by poorly ordered Fe(III) minerals, although there was also an observable increase in both the “organic” and “pyritic” fractions following the shift to reducing, circum-neutral geochemical conditions. The new sequential extraction procedure provides an important tool to examine the partitioning of iron and trace elements in acid sulfate soil landscapes. Furthermore, it is a simple and routine means able to evaluate likely trace element partitioning behaviour under the dynamic redox conditions that characterise many acid sulfate soil environments.

Geochemical partitioning of iron and trace elements in acid sulfate soils

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Geochemical partitioning of iron and trace elements in acid sulfate soils

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