should metal bioavailability be considered in the evaluation...
TRANSCRIPT
Should metal bioavailability be considered in the evaluation of
treatment and remediation methodologies for acid mine
drainage?Vigneault, B.*, Neculita, C.M.**, Zagury, G.J.**, Kwong, Y.T.J.* and Tisch, B.** CANMET-Mining and Mineral Sciences Laboratories** École Polytechnique de Montréal
5th World Congres of the Society of EnvironmentalChemistry and Toxicology, Sydney, Australia, August 2008
Introduction : Context
• Treatment efficiency is often assessed based on metal removal alone
• More jurisdictions are implementing effluent toxicity requirements and discharge objective or limits based on generic water quality criteria
Introduction : AMD Treatment• Acid Mine Drainage (AMD) is the most
significant environmental issue related to mining world-wide
• AMD treatment most often involves of neutralization with lime
• AMD prevention and / or passive treatment are more preferable
• Prevention and treatment options include:- Water covers- Soil covers- Passive bioreactors
The current presentation highlights the relevance of metal bioavailability in assessing alternative methods for AMD prevention and treatment:
I. Shallow water coverII. Papermill sludge coverIII. Passive bioreactor
Introduction : Objective
AMD Prevention : Water Cover• Subaqueous disposal of reactive sulphide
tailings is an effective way to prevent AMD
• Shallow water cover of ~ 30 cm is preferable for geotechnical considerations
• Formation of a biofilm has been found at the surface of the tailings
Materials and Methods: Water Cover
• Geochemical measurements were completed at the water/tailings interface to assess the impact of biofilm:- Micro-profiles of O2 and pH- Profiles of metals, major ions,
sulphides and DOC - Biotic Ligand Model
(Hydroqual) to estimate the effect of DOC
Vigneault et al. (2001) Wat. Res. 35(4): 1066-1076.
-6
-4
-2
0
2
4
[ ] (µg · L-1)
0.0 0.1 0.2 0.3 0.4-6
-4
-2
0
2
4
0 10 20 30 40
Dep
th (c
m)
-6
-4
-2
0
2
4
Copper
Zinc
Cadmium
Water Cover: Trace metal profiles
AMD Prevention : Paper Mill Biosolids
• Paper mill biosolids are used to reclaim mine tailings
• If possible, paper mill biosolid covers will be use to grow energy crops
• Besides suppressing metal mobilization, the utilization of paper mill biosolids may impact effluent composition
Materials and Methods: Biosolid Covers
• Pulp and paper sludge were obtained from two Canadian mills
• Biosolids leachates tested for:
- Direct toxicity- Copper complexation capacity- Effect on copper toxicity
Materials and Methods: Metal Speciation
• Ion selective electrode for the determination of copper complexationcapacity
• Effect of leachates on copper toxicity to C. dubia (7 day survival and reproduction) tested in laboratory water
Biosolid Covers: Cu Complexation
Domtar
[Cu]Total (µM)12.9 33.8 71.3
{Cu2+
} (µM
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Measured (ISE)Predicted (WHAM VI)
Biosolid Covers: Cu Toxicity
Survival Reproduction
Cu
LC50
/ IC
25 (µ
g · L
-1)
0
50
100
150
200
250
Control Domtar St. Marys
~
C. dubia
AMD Treatment : Passive bioreactors
• Field passive bioreactors are effective to increase pH and remove metals
• Bioreactors require an organic amendment and can release DOC in the treated effluent
Materials and Methods: Passive Bioreactor
• Bioreactor with a hydraulic retention time up to 10 days used to treat artificial AMD
• Reactive mixture composed of 60% organic materials (10% maple wood chips, 20% sawdust, 10% poultry manure and 20% leaf compost) and 40% inorganic materials.
Materials and Methods: Metal Speciation
• Ultrafiltration used to determine truly dissolved metals
• Cadmium, nickel and zinc speciation estimated using WHAM VI (Tipping 1998)
Neculita, Vigneault and Zagury (2008) Env. Toxicol. Chem. 27(8): 1659-1667.
Passive Bioreactor: Metal Speciation
• Treated effluent had a pH of 6.2 and a DOC of 7 mg · L-1
• The majority of cadmium and zinc was < 1 Kda
• Predicted fraction of metal bound to dissolved and particulate organic matter was only 0.2% for nickel and 0.4% for zinc and cadmium
Passive bioreactor: DOC Protective Effect
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Copper Cadmium Zinc
DOC
Effe
ct R
atio
s
Conclusions• Shallow water cover: metal toxicity can
potentially be reduced by 4 x to 25 x
• Biosolid covers: copper toxicity in limed effluent could be reduced by ~ 10 x as supported by the copper toxicity tests and copper speciation measurements
• Passive bioreactor: metal toxicity is not expected to be largely affected by the released organic matter
Conclusions• Expected organic matter releases are
significant (~ 101 mg · L-1) for the 3 tested methods
• Data suggest that the released organic matter can significantly affect metal toxicity and should be considered when using water quality guidelines to establish discharge objectives
Dissolved oxygen (µM)
0 100 200 300 400 500 600 700
Dep
th (m
m)
-20
-15
-10
-5
0
5
10
15
5700 Lux 18500 Lux >20000 Lux
Water Cover: Oxygen micro-profiles
Cu Zn Cd
Diff
usiv
e flu
x to
the
wat
er c
over
(m
ol ·
cm-2
· s-1
)
10-19
10-18
10-17
10-16
10-15
10-14
1998 2005
No flux
Water Cover: Trace metal fluxes
Materials and Methods: Toxicity Tests
• Canadian Metal Mining Effluent Regulations:- acute lethality to Daphnia magna
and rainbow trout
- sublethal toxicity to Pseudokirchneriellasubcapitata, Lemna minor, Ceriodaphnia dubia and Pimephalespromelas
Biosolid Covers: Leachate Toxicity
Rainbow trout D. magna
LC50
(mg
C ·
L-1)
0
50
100
150
200
Domtar St. Marys
>
>
>
~
P. prom
elas
C. dub
ia
L. mino
rP. s
ubca
pitata
IC25
(mg
C ·
L-1)
0
20
40
60
80
100
120
Domtar St. Marys >
>
Biosolid Covers: Leachate Toxicity
Biosolid Covers: Cu Complexation
St. Marys
[Cu]Total (µM)15.0 37.0 73.0
{Cu2+
} (µM
)
0.0
0.1
0.2
0.3
0.4
0.5
Measured (ISE)Predicted (WHAM VI)