Hybrid flow system for automatic dynamic fractionation and speciation of inorganic arsenic in environmental solids

An integrated flow analysis system and protocol are proposed for the first time for automatic dynamic flow-through fractionation of inorganic arsenic (arsenite and arsenate) in environmental solids in combination with its real-time speciation. Four extractants (i.e., (1) 0.05 M ammonium sulfate, (2) 0.05 M ammonium dihydrogen phosphate, (3) 0.2 M ammonium oxalate, and (4) a mixture of 0.2 M ammonium oxalate and 0.1 M ascorbic acid at 96 °C) are applied sequentially to the sample to measure bioaccessible inorganic arsenic associated with (1) nonspecifically sorbed phases, (2) specifically sorbed phases, (3) amorphous plus poorly crystalline hydrous oxides of iron and aluminum, and (4) well-crystallized hydrous oxides of Fe and Al, respectively. The kinetic extraction profiles of arsenite and total inorganic arsenic are obtained for each extractant by automatic collection of a given number of its aliquots (subfractions) exposed to the solid sample. Arsenite and total inorganic arsenic in each subfraction are converted to arsine sequentially by hydride generation at pH 4.50 and in 1.14 M hydrochloric acid, respectively. Arsine is absorbed into a potassium permanganate solution, the discoloration of which is related to the concentration of the corresponding arsenic species. The proposed method is successfully validated by analyzing a soil reference material (NIST 2710a) and a sediment sample.