Chemical extraction of sedimentary iron oxy(hydr)oxides using ammonium oxalate and sodium dithionite revisited - an explanation of processes in coastal sediments

Abstract

Iron oxides promote aggregation, adsorb nutrients and pollutants and serve as electron acceptor; hence it is desirable to understand the composition of the soil and sedimentary iron pool. Here we tested if existing chemical extraction approaches using solely ammonium oxalate and sodium dithionite deliver consistent information on the allocation of amorphous and crystalline iron oxides for coastal sediments, which experience alternating redox conditions. For that purpose iron was extracted using citrate dithionite solution buffered with sodium bicarbonate and acid ammonium oxalate solution (pH 3.25). The content of dithionite extractable iron (Fe_d) and oxalate extractable iron (Fe_o) ranged from 1.7 to 7.4 g kg^-1 and 1.0 to 11.3 g kg^-1, respectively. We calculate the content of crystalline iron oxide as usual from the difference between Fe_d and Fe_o, however, we failed for nearly half of the investigated soil samples, because the Fe_o content exceeded Fe_d. It is assumed that the oxidation of Fe(II) phases into oxalate extractable Fe(III) phases as well as the catalyzed dissolution of Fe(III) minerals by Fe(II) oxalate complexes sophisticate the results gained by chemical extractions using only ammonium oxalate and dithionite as leachates, but that these interferences do not account for oxalate in excess of dithionite extractable iron. Likely, dissolution of magnetite contributed to excess oxalate extractable iron. We conclude that conventional Fe_o and Fe_d extraction schemes may be misleading for coastal soils, i.e., more research is needed that can assign standardized Fe extraction schemes to defined mineral phases including the rapid physicochemical changes in such environments to attain reliable and comparable Fe data, particularly in the transition to terrestrial environments.