Toxic metals leaching from columns of mine tailings amended with compost.

Mobilization of toxic metals should be a primary concern in the implementation of mine tailings reclamation with compost or other organic amendments. In fact, dissolved organic carbon (DOC) is a key component involved in metals solubilisation and mobility. In this study we investigated the effect of compost addition to a mine tailing from washing deposits of a sphalerite (ZnS) and galena (PbS) dismissed mine site in repacked columns. The native mine tailings were collected from the Raibl mine (Cave del Predil, Italy) and contained 373 g As g-1, 1935 g Pb g-1, 521 g g-1 Tl and 7180 g Zn g-1 . They were compared with a single profile of compost amended tailings and with a two layer profile: the 10 cm top layer made by compost amended tailings and the 10 cm bottom layer made of native tailings. Columns were irrigated with deionized water simulating average local precipitation events for 100 days and leachates collected by gravity. Leacheates were analyzed for pH, for As, Tl, Pb and Zn by ICP-OES and for DOC by dry oxidation (Shimadzu TOC). Microbial respiration in compost amended tailing was measured in closed jars by CO2 absorption to 0.5 M NaOH solution. Monitored trace elements showed different leaching patterns: significantly more Zn and Tl leached from the composite profile (amended tailings + native tailings) while Pb and As showed a much larger mobility in the amended tailings (single profile). The lower horizon of native tailings was able to drastically reduce metals leaching from the top horizon. This different behaviour is coherent with the fundamental role of DOC in the mobilization of Pb and As and with its much lower influence on that Zn and Tl. Mobilization of Zn and Tl was governed by S oxidation and to a lesser extent by carbonates. These results highlight the need of an appropriate assessment of metals mobilization risks before performing any amendment application to highly contaminated tailings.