Operating mines apply active-treatment strategies, using BATEA, which can be
engineered to accommodate variable flow rates and effluent quality, but with a costdependent flexibility. With evolving water quality standards and stricter regulations,
new treatment approaches are required to ensure satisfactory performance and stable
sludge for refractory contaminants at high flow rates and high concentrations,
especially under remote and extreme cold-climate conditions. Mine effluents, including gold mining of low grade refractory ores, are complex and notoriously challenging to treat, mainly because of: (1) incomplete characterization (e.g., metastable
contaminants, such as thiosalts, or contaminants in low concentrations, such as Se)
especially with mixed effluents (drainage water from mining works, supernatant and
exfiltrations from tailings ponds, etc.); (2) low temperature (and higher solubility of
carbonates under such condition with accelerated exhaustion) and freeze–thaw
cycles (with enhanced kinetics of some contaminants generation); and (3) poorly
understood transient aquatic toxicity. Chemical treatment systems are usually efficient but tend to generate increasing residual salinity. New and/or adapted treatment
approaches must be developed for these specific conditions.
Gaps and research needs for the four groups of CECs in mine effluents are the
following: (1) Salinity/TDS originates from several sources: ores, chemicals (ore
processing, water treatment), water recycling, saline water pumping, de-icing agents,
permafrost alteration, etc. Salinity, a highly soluble and persistent contamination, is
defined as an integrated measure of major, biologically essential ions (Na
+
, Ca
2+ ,
Mg
2+ , K
+ , Cl
À
, SO 4
2À and HCO 3
À
) and generally viewed to possess low individual
toxicity (van Dam et al. 2014). (2) Thiosalts, as metastable sulfur oxyanions (up to
60% produced during sulfide flotation), are correlated to aquatic toxicity due to the
pH depression following their oxidation to sulfuric acid (Miranda-Trevino et al.
2013) and to chemical degradation of xanthates (dithiocarbonates used in the
flotation processes for most sulfide minerals) under acidic conditions (Alto et al.
1978; Novak and Holtze 2010; Xu et al. 1988). (3) Se, which is characterized as
having a narrow window between essentiality and toxicity (Sharma et al. 2017). A
predicted no-effect concentration (PNEC) for Se is set by Environment Canada, for
freshwater, at 1 ppb (and by British Colombia at 2 ppb for freshwater and marine
environment), while US EPA, Australia, and New Zealand have set values at 5 ppb
or higher (Sharma et al. 2017). In addition, the US EPA criterion maximum
concentrations (CMCs) for freshwater organisms are 257 ppb for Se(IV) and
417 ppb for Se(VI) (Sharma et al. 2017). A 5 ppb Se concentration was also recently
issued as a trigger concentration for Canadian mine effluents (Government of Canada
2017). Precisely, if an effluent has >5 ppb of Se, the mine must conduct fish tissue
sampling in the receiving environment to evaluate tissue Se concentrations. At the
same time, the toxicity differences for Se species, i.e., oxyanions (selenite and
selanite) vs. Se-based nanoparticles (released to the environment and produced by
bioremediation of soluble Se) suggest the needs for additional research (Sharma et al.
2017) for the proper analysis of Se speciation (Pettine et al. 2015), for the environmental alteration of Se nanoparticles and for the correlation with associated toxicity.
(4) Nitrogen compounds (NH 3 , NO 2
À , NO 3
À
) are also toxic (Baker et al. 2017;
Jermakka et al. 2015).
600
C. M. Neculita et al.
engineered to accommodate variable flow rates and effluent quality, but with a costdependent flexibility. With evolving water quality standards and stricter regulations,
new treatment approaches are required to ensure satisfactory performance and stable
sludge for refractory contaminants at high flow rates and high concentrations,
especially under remote and extreme cold-climate conditions. Mine effluents, including gold mining of low grade refractory ores, are complex and notoriously challenging to treat, mainly because of: (1) incomplete characterization (e.g., metastable
contaminants, such as thiosalts, or contaminants in low concentrations, such as Se)
especially with mixed effluents (drainage water from mining works, supernatant and
exfiltrations from tailings ponds, etc.); (2) low temperature (and higher solubility of
carbonates under such condition with accelerated exhaustion) and freeze–thaw
cycles (with enhanced kinetics of some contaminants generation); and (3) poorly
understood transient aquatic toxicity. Chemical treatment systems are usually efficient but tend to generate increasing residual salinity. New and/or adapted treatment
approaches must be developed for these specific conditions.
Gaps and research needs for the four groups of CECs in mine effluents are the
following: (1) Salinity/TDS originates from several sources: ores, chemicals (ore
processing, water treatment), water recycling, saline water pumping, de-icing agents,
permafrost alteration, etc. Salinity, a highly soluble and persistent contamination, is
defined as an integrated measure of major, biologically essential ions (Na
+
, Ca
2+ ,
Mg
2+ , K
+ , Cl
À
, SO 4
2À and HCO 3
À
) and generally viewed to possess low individual
toxicity (van Dam et al. 2014). (2) Thiosalts, as metastable sulfur oxyanions (up to
60% produced during sulfide flotation), are correlated to aquatic toxicity due to the
pH depression following their oxidation to sulfuric acid (Miranda-Trevino et al.
2013) and to chemical degradation of xanthates (dithiocarbonates used in the
flotation processes for most sulfide minerals) under acidic conditions (Alto et al.
1978; Novak and Holtze 2010; Xu et al. 1988). (3) Se, which is characterized as
having a narrow window between essentiality and toxicity (Sharma et al. 2017). A
predicted no-effect concentration (PNEC) for Se is set by Environment Canada, for
freshwater, at 1 ppb (and by British Colombia at 2 ppb for freshwater and marine
environment), while US EPA, Australia, and New Zealand have set values at 5 ppb
or higher (Sharma et al. 2017). In addition, the US EPA criterion maximum
concentrations (CMCs) for freshwater organisms are 257 ppb for Se(IV) and
417 ppb for Se(VI) (Sharma et al. 2017). A 5 ppb Se concentration was also recently
issued as a trigger concentration for Canadian mine effluents (Government of Canada
2017). Precisely, if an effluent has >5 ppb of Se, the mine must conduct fish tissue
sampling in the receiving environment to evaluate tissue Se concentrations. At the
same time, the toxicity differences for Se species, i.e., oxyanions (selenite and
selanite) vs. Se-based nanoparticles (released to the environment and produced by
bioremediation of soluble Se) suggest the needs for additional research (Sharma et al.
2017) for the proper analysis of Se speciation (Pettine et al. 2015), for the environmental alteration of Se nanoparticles and for the correlation with associated toxicity.
(4) Nitrogen compounds (NH 3 , NO 2
À , NO 3
À
) are also toxic (Baker et al. 2017;
Jermakka et al. 2015).
600
C. M. Neculita et al.