Occurrence and mobility of As in the Ylöjärvi Cu–W–As mine tailings

Fifteen tailings samples were investigated for mineralogy by scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), 12 for geochemical distribution by sequential extractions successively attacking water-soluble, adsorbed-exchangeable-carbonate, Fe (oxy)hydroxide, Fe oxide and sulfide fractions, as well as 13 tailings pore water and two groundwater samples for geochemical composition and physicochemical conditions at the former Ylöjärvi Cu–W–As mine in SW Finland. Mineralogical observations and sequential extractions demonstrated the depletion of arsenopyrite and other sulfide minerals from the superficial and deeper coarse-grained tailings resulting in high concentrations of As in pore water and groundwater beneath the tailings heap (5140 and 14 500 μg L−1, respectively), and lowering the pore pH (from ca. 7.8 to 5.3). The solubility of As was controlled by pH (and Eh) as well as precipitating Fe(III) (oxy)hydroxides and Fe(III) arsenates. Although these phases are an important sink for As, it accumulates in the percolating water. Further acidification of the tailings after the consumption of scarce neutralizing calcite is probable and will affect the stability of the remaining arsenopyrite and As-bearing secondary phases.

► Arsenopyrite altered under circumneutral to acidic, slightly reducing conditions.
► Fe arsenates and Fe (oxy)hydroxides partially retain As from tailings pore water.
► As correlates well with pH and the precipitation of Fe(III) phases.
► High As concentrations (14 500 µg L−1) have accumulated in the groundwater.
► Acidification may enhance dissolution of arsenopyrite and other As-bearing phases.