Arsenic mobility in flooded soils
The widespread drainage of wetlands has enhanced the in-situ oxidation of buried iron-sulfide minerals (primarily pyrite, FeS2). During pyrite oxidation, any pyrite-bound arsenic is released along with iron and sulfate. This leads to the formation of arsenic bearing ferric oxide minerals, such as jarosite, schwertmannite and goethite. These minerals are capable of sequestering much of the arsenic released by in-situ pyrite oxidation and are therefore important controls on arsenic mobility.
Pyrite within undrained wetland soils is often rich in arsenic.
Ferric oxides are relatively stable in well-drained soils, yet undergo profound transformations following soil re-flooding. For example, the bacterially-mediated reduction of ferric iron can cause reductive dissolution and the release of previously-bound arsenic. The issue of arsenic mobility in re-flooded soil is important because wetland re-flooding is gaining widespread implementation as a remedy of environmental degradation.
Arsenic mobility during experiment simulation of wetland re-flooding. In this case, we examined a wetland located in the Murray-Darling basin in southern Australia.
Research at Southern Cross GeoScience has aimed at understanding the geochemical processes controlling arsenic mobility in re-flooded soils. In particular, we have focused on acid-sulfate soils and interactions between arsenic and minerals that are common in such soils (e.g. schwertmannite).
Work completed to date has addressed:
- understanding the in-situ geochemistry of arsenic, iron and sulfur in field-based investigations,
- examining coupling of arsenic mobility to iron-sulfur geochemistry in controlled, re-flooding experiments, and
- unraveling arsenic sorption-desorption to key iron-sulfur minerals, with a focus so far on schwertmannite-arsenic interactions.
The arsenic redox state controls sorptive interactions with schwertmannite.
Burton, E. D., Bush, R. T., Sullivan, L. A., Johnston, S. G., Hocking, R. K. (2008) Mobility of arsenic and selected metals during re-flooding of iron- and organic-rich acid-sulfate soil. Chemical Geology 253, 64 - 73.
Burton, E. D., Bush, R. T., Johnston, S. G., Watling, K., Hocking, R. K., Sullivan, L. A., Heber, G. K. (2009) Sorption of arsenic(V) and arsenic(III) to schwertmannite. Environmental Science & Technology 43, 9202 - 9207.
Johnston S.G., Keene A.F., Burton E.D., Bush R.T., Sullivan L.A., McElnea A.E., Ahern C.R., Smith C.D., Powell B. (2010) Arsenic mobilisation in a seawater inundated acid sulfate soil. Environmental Science & Technology 44, 2016 - 2021.
Burton, E. D., Johnston, S. G., Watling, K., Bush, R. T., Keene, A. F., Sullivan, L. A., (2010) Arsenic effects and behaviour in association with the Fe(II)-catalysed transformation of schwertmannite. Environmental Science & Technology 44, 1968 - 1973.