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Enrichment and heterogeneity of trace elements at the redox-interface of Fe-rich intertidal sediments

- Keene, Annabelle, Johnston, Scott, Bush, Richard, Burton, Edward, Sullivan, Leigh, Dundon, Matthew, McElnea, Angus, Smith, C. Douglas, Ahern, Col, Powell, Bernard

  • Authors: Keene, Annabelle , Johnston, Scott , Bush, Richard , Burton, Edward , Sullivan, Leigh , Dundon, Matthew , McElnea, Angus , Smith, C. Douglas , Ahern, Col , Powell, Bernard
  • Date: 2014
  • Type: Text , Journal article
  • Relation: Chemical Geology Vol. 383, no. (2014), p. 1-12
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  • Description: Redox-interfacial sediments can undergo radical geochemical changes with oscillating tides. In this study, we examine trace element enrichment and availability, at both landscape and pedon-scales, in the surface sediments of a remediating acidic tidal wetland. Fe-rich sediments at the surface-water interface (0-10. mm in depth) were collected across an elevation gradient spanning the supratidal to subtidal range. These sediments were analysed for solid phase Fe fractions and trace elements (As, Pb, Cr, Cu, Mn, Ni, Zn, V, B, Co, Mo, Ba and U) via dilute HCl-extractions and total digests. Their concentrations were compared with those of underlying (0.05-0.65. m in depth) former sulfuric horizon sediments of a coastal acid sulfate soil (CASS). Reactive Fe was enriched at the redox interface by up to 16 times (197. g. Fe/kg) that of the former sulfuric horizon. The proportion of total trace elements associated with reactive phases was high in interfacial sediments, representing over 90% of B and U and 50% of Pb, Cu, Zn, V and Ba extractable by dilute HCl. The interfacial sediments were particularly enriched in reactive Cr, Cu, Ni, Zn, B, Mo and U, with reactive B, Mo and U concentrations between 5 and 10 times greater than in the former sulfuric horizon. Surface enrichment of trace elements is strongly co-associated with Fe(III) mineralisation, likely via sorption and co-precipitation processes. Enrichment is highly spatially heterogeneous and is strongly influenced by elevation and tidal zonation at a landscape-scale and by sediment micro-topography and preferential advective transport via surface connected macropores at the pedon-scale. The results from this study provide new insights to the processes influencing trace element enrichment in Fe-rich redox-interfacial sediments across a remediating acidic tidal wetland. © 2014 Elsevier B.V.

Liberation of acidity and arsenic from schwertmannite : Effect of fulvic acid

- Vithana, Chamindra, Sullivan, Leigh, Burton, Edward, Bush, Richard

  • Authors: Vithana, Chamindra , Sullivan, Leigh , Burton, Edward , Bush, Richard
  • Date: 2014
  • Type: Text , Journal article
  • Relation: Chemical Geology Vol. 372, no. (2014), p. 1-11
  • Full Text: false
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  • Description: Schwertmannite is one of the major components that produces acidity in acid mine drainage (AMD) and acid sulfate soils (ASS) and is also known to be an effective scavenger of Arsenic (As) in such environments. Fulvic acid (FA) is an active component of natural organic matter (NOM) and is known to interact strongly with both schwertmannite and As. Two main environmental hazards related to schwertmannite are acidity liberation and potential re-mobilization of adsorbed or co-precipitated As upon hydrolysis. This study focused on understanding the behaviour of As-substituted schwertmannite with regard to the potential of acidity liberation, the effect of FA on acidity liberation from both pure and As-substituted synthetic schwertmannites, and the effect of FA on arsenic mobilization from As-substituted synthetic schwertmannite. This was investigated by means of short-term (48. h) titrations. The liberation of acidity from As-substituted schwertmannite and the effect of FA were examined at two pH values (i.e. 4.5 and 6.5) typical for ASS environments.As-substituted schwertmannite liberated a greater amount of acidity in comparison to pure schwertmannite at both pHs. Concentration of FA and pH each showed a strong influence on the liberation of acidity from both pure and As-schwertmannite. At the acidic pH (4.5), FA inhibited acidity liberation from schwertmannite. At the near neutral pH of 6.5, the concentration of FA played a critical role in affecting the liberation of acidity from schwertmannite. The initial liberation of acidity was enhanced from pure schwertmannite at pH6.5 by low FA concentration (1mgL-1) and from As-schwertmannite by both low (1mgL-1) and moderate (10mgL-1) FA concentrations. Interestingly, higher FA concentrations (25mgL-1) inhibited acidity liberation from both types of schwertmannite in comparison to the control (pure/As-schwertmannite titrated without added FA). FA enhanced the liberation of As from the As-schwertmannite at both pHs under oxidising conditions and the rate of As liberation was greater at the near neutral pH. The present study provides new insights on the effect of As-substitution on acidity liberation from schwertmannite and the role of FA on: a) liberation of acidity, and b) As mobilization, from schwertmannite. © 2014.
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