Effectiveness of phosphate and hydroxide for desorption of arsenic and selenium species from iron oxides

Phosphate and OH- are often used for the extraction of As and Se from soils, either as single extractants or as part of a sequential extraction scheme. However, the recovery of As and Se species and the integrity of the resulting solution speciation merit investigation. In this study the relative effectiveness of PO4 at 0.1 and 0.5 XI and pH values of 3 and 6.7 and 0.1 Ai OH- to extract As(III), As(V), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), p-arsanilic acid (p-ASA), roxarsone (ROX), Se(IV) and Se(VI) sorbed to goethite and an amorphous Fe oxide mere compared, and the speciation in the resulting extract was determined. The extent to which 0.1 M PO4 added to 0.25 iii NH2OH . HCl or 0.175 M Na oxalate/0.1,II oxalic acid prevents readsorption of As(V) or Se(IV) to goethite during the dissolution of an amorphous Fe oxide was also assessed. Hydroxide was the most effective extractant for desorption of all species except As(III) from both oxide surfaces. Arsenite was extracted most efficiently by 0.5 M PO4 at low pH; however, amorphous Fe oxide exhibited a strong affinity for As(III) with a maximum of 18% of As(III) extracted by 0.5 M PO4 at pH 2.8. Partial oxidation of As(III) to As(V) occurred in all extractions where an Fe oxide solid phase was present, but only in the hydroxide extract in the absence of a Fe solid phase. Addition of 0.1 M PO4 to extractants used for the dissolution of the amorphous Fe oxide prevented the readsorption of As(V) and Se(IV) to goethite.