Validation of an updated fractionation and indirect speciation procedure for inorganic arsenic in oxic and suboxic soils and sediments

A sequential extraction procedure (SEP) for the speciation analysis of As(III) and As(V) in oxic and suboxic soils and sediments was validated using a natural lake sediment and three certified reference materials, as well as spike recoveries of As(III) and As(V). Many of the extraction steps have been previously validated making the procedure useful for comparisons to similar previous SEP studies. The novel aspect of this research is the validation for the SEP to maintain As(III) and As(V) species. The proposed five step extraction procedure includes the extraction agents (NH₄)₂SO₄, NH₄H₂PO₄, H₃PO₄ + NH₂OH·HCl, oxalate + ascorbic acid (heated), and HNO₃ + HCl + HF, targeting operationally defined easily exchangeable, strongly sorbed, amorphous Fe oxide bound, crystalline Fe oxide bound, and residual As fractions, respectively. The third extraction step, H₃PO₄ + NH₂OH·HCl, has not been previously validated for fraction selectivity. We present evidence for this extraction step to target As complexed with amorphous Fe oxides when used in the SEP proposed here. All solutions were analyzed on ICP-MS. The greatest concentrations of As were extracted from the amorphous Fe oxide fraction and the dominant species was As(V). Lake sediment materials were found to have higher As(III) concentrations than the soil materials. Because different soils/sediments have different chemical characteristics, maintenance of As species during extractions must be validated for specific soil/sediment types using spiking experiments.