Efficient removal of arsenic, antimony and nickel from mine wastewaters in Northern treatment peatlands and potential risks in their long-term use

The number of metal mines, the amount of ore processed, and thus the amount of mine wastewaters have been rapidly increasing in recent decades. Pretreated process effluent and drainage waters containing high concentrations of e.g., arsenic (As), antimony (Sb), and nickel (Ni) have been purified for up to 6 years in treatment peatlands at a gold mine in Finnish Lappland prior to their release into downstream water bodies. Retention efficiencies based on monitoring of peatland inflow and outflow waters indicated that retention of As, Sb, and Ni was generally good (up to 95%) but temporarily variable. Other contaminants such as sulfate or iron were retained less efficiently or were even leaching from the peatland. Most of the As, Sb, and Ni was retained close to the wastewater distribution ditch, and there was a near-linear increase in As, Sb, and Ni concentrations in the peatlands over time. Adsorption experiments indicated the high potential of peat soils to adsorb As, Ni and especially Sb. Accumulation of contaminants has led to or will lead to concentrations exceeding higher guideline values for contaminated soils within the next 1-18 years, which indicates a limited lifetime for treatment peatlands. Leaching of contaminants is anticipated in situations where clean natural waters mix with mine waters, e.g., during snowmelt or after mine closure, and desorption experiments suggest that especially easily mobilized (i.e., loosely-bound) fractions of As, Sb, and Ni are likely to leach from the treatment peatlands. The collective data indicate (i) high purification efficiencies for As, Sb, and Ni in the peatlands, (ii) high adsorptive capacity of the peat soil for those contaminants, (iii) increased risk of contaminant leaching e.g., after mine closure, and (iv) a need for further studies of environmental parameters and retention processes affecting contaminant removal efficiencies and potential lifetimes of treatment peatlands.