Determination of Arsenic Speciation in Complex Environmental Samples by the Combined Use of TEM and XPS

Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) have been used to refine the solid-state speciation of As in mine waste materials and stream-bed sediments from the Baccu Locci mine area in Sardinia, Italy. The combined use of TEM and XPS confirmed previous studies and pointed out that: (i) As is contained in plumbojarosite in substitution of S, (ii) As-bearing Fe(III) hydroxides are represented by arsenical 2-line ferrihydrites with most Fe=As molar ratios in the range of 1.6–3.2, (iii) scorodite often lacks long range order. Arsenatian plumbojarosite will tend to dissolve incongruently in ferrihydrite, releasing Pb and As into water. However, Pb is fated to precipitate as Pb(OH)2 or to be adsorbed onto ferrihydrite, whereas As mobility is favoured since the adsorptive capacity of ferrihydrite for As(V) is notably reduced under the neutral to alkaline pH conditions occurring in the Baccu Locci stream water. Arsenical ferrihydrites will tend to be converted into goethite or hematite with time, releasing As into the water as a consequence of the notably lower density of the adsorption sites of the crystalline Fe(III) phases. Also scorodite is relatively unstable under the Baccu Locci system conditions, and its stability is further decreased owing to the higher solubility of the amorphous form compared to the crystalline one.