title: Donnan membrane speciation of Al, Fe, trace metals and REEs in coastal lowland acid sulfate soil-impacted drainage waters.
abstract: Donnan dialysis has been applied to forty filtered drainage waters collected from five coastal lowland acid sulfate soil (CLASS) catchments across north-eastern NSW, Australia. Despite having average pH values<3.9, 78 and 58% of Al and total Fe, respectively, were present as neutral or negatively-charged species. Complementary isotope dilution experiments with (55)Fe and (26)Al demonstrated that only soluble (i.e. no colloidal) species were present. Trivalent rare earth elements (REEs) were also mainly present (>70%) as negatively-charged complexes. In contrast, the speciation of the divalent trace metals Co, Mn, Ni and Zn was dominated by positively-charged complexes and was strongly correlated with the alkaline earth metals Ca and Mg. Thermodynamic equilibrium speciation calculations indicated that natural organic matter (NOM) complexes dominated Fe(III) speciation in agreement with that obtained by Donnan dialysis. In the case of Fe(II), however, the free cation was predicted to dominate under thermodynamic equilibrium, whilst our results indicated that Fe(II) was mainly present as neutral or negatively-charged complexes (most likely with sulfate). For all other divalent metals thermodynamic equilibrium speciation calculations agreed well with the Donnan dialysis results. The proportion of Al and REEs predicted to be negatively-charged was also grossly underestimated, relative to the experimental results, highlighting possible inaccuracies in the stability constants developed for these trivalent Me(SO4)2(-) and/or Me-NOM complexes and difficulties in modeling complex environmental samples. These results will help improve metal mobility and toxicity models developed for CLASS-affected environments, and also demonstrate that Australian CLASS environments can discharge REEs at concentrations an order of magnitude greater than previously reported.
authors.affiliation: School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia.
authors.forename: Adele M
authors.initials: AM
authors.lastname: Jones
authors.affiliation: School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia.
authors.forename: Youjia
authors.initials: Y
authors.lastname: Xue
authors.affiliation: School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia; Institute for Environmental Research (IER), Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, NSW 2234, Australia.
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Kinsela
authors.affiliation: Institute for Environmental Research (IER), Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, NSW 2234, Australia.
authors.forename: Klaus M
authors.initials: KM
authors.lastname: Wilcken
authors.affiliation: School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia. Electronic address: richard.collins@unsw.edu.au.
authors.forename: Richard N
authors.initials: RN
authors.lastname: Collins
mesh_terms: D001315:Australia; D004784:Environmental Monitoring; D008670:Metals / Q000032:analysis*; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis*; D014872:Water Movements
chemical_list: D008670:Metals; D012987:Soil; D012989:Soil Pollutants
keywords: Acid sulfate soil; Donnan dialysis; Donnan membrane technique; Metal speciation; Rare earth elements