Modeling the Fate of Metal Concentrates
Using the TICKET Unit World Model
Richard F. Carbonaro 1,2, Patricio H. Rodriguez3, Kevin J. Farley 2, Katrien Delbeke 4, and Kevin J. Rader 1
1
Mutch Associates, LLC, Ramsey, NJ, USA | 2 Manhattan College, Riverdale, NY, USA | 3 Center of Ecotoxicology and Chemistry
of Metals, Universidad Adolfo Ibañez, Santiago, Chile | 4 European Copper Institute, Brussels, Belgium
Introduction
• Mineral ores typically contain low percentage of metals; they must undergo concentration steps to
produce ore “concentrate” which can have up to 30% metal content
• Key challenges for concentrate hazard assessment and classification are their sparingly soluble
nature (which affects metal bioavailability) and long term fate following release (a function of
degradability)
• Since metals do not "degrade” (according to the classic definition for organic substances), a new
approach aligned with Globally Harmonized System (GHS) [1] guidance was proposed based on
rapid removal from the water column
• Transformation/dissolution (T/D) tests assess the rate and extent of transformation and
subsequent dissolution of sparingly soluble metal compounds in standard environmental media for
hazard classification [2]
• T/D tests are worst-case scenarios for metal bioavailability (low pH, limited/no organic carbon or
other binding phases) and don’t consider metal fate after solubilization (i.e., T/D reactive vessels
are stirred)
• TICKET-UWM (http://www.unitworldmodel.net/) [3], a unit world model for metals in lakes,
considers key processes affecting metal transport, fate, and toxicity (see Fig. 1) and can
incorporate information from T/D tests on metal dissolution from concentrates
• OBJECTIVE: apply the TICKET-UWM to the assessment of concentrate bioavailability and
degradability in a generalized water body
Figure 1. Conceptual description of the processes affecting fate of metals during release of
a metal concentrate to a lake.
Materials and Methods
Characterization of Concentrates
Outotec Research in Abo, Finland):
(by
• Total metal composition was determined by ICP-OES
• Mineralogy was determined by x-ray diffraction
• Mineral composition was based on x-ray diffraction,
optical microscopy, scanning electron microscopy with
EDS and WDS analysis of identified phases and
minerals and the analyzed chemical composition of
the sample from both total and selective dissolution
methods
T/D procedure
•
•
•
•
•
pH 6.06 OECD 203 media buffered with CO2
O2 saturation ≥ 70%
Stirring rate: 100 rpm
Sample loading: 1.0 mg/L
28-day duration to assess chronic classification
endpoint
• Measure Cu, As, Pb, Co and Zn
TOTAL METAL = TOTM =
TICKET-UWM Modeling
• Model simulations were made using a generalized lake system
based on the EUSES lake [4-6].
• Two generalized lake simulations:
1. Fate of solubilized Cu from T/D tests
2. Fate of Cu added to a lake directly in
concentrate form
• Concentrate settling rate estimated from Stokes Law
• Water chemistries used in water column simulations were
based on available directives/guidance documents [4,7]
• Water column binding phases: dissolved organic carbon
(DOC), particulate organic carbon (POC) + potential
precipitation of copper oxides, sulfates, and chlorides
• Sediment binding phases: DOC, POC, hydrous ferric oxides,
hydrous manganese oxides + potential precipitation of copper
oxides, sulfates, chlorides, and sulfides
SOLUBILIZED METAL
MT
(Total Dissolved)
+
MP
(Particulate)
+
MC
(Concentrate)
Results
• Lead (Pb) release from Concentrate 1 (C1) was from galena, PbS(s). Rate of release consistent
with oxidative release mechanism [8-9]
• Cu in C2 present as chalcopyrite (CuFeS2), bornite (Cu5FeS4), and chalcocite (Cu2S). Chalcocite
is the most likely phase controlling release of Cu and the likely mechanism is oxidative release
• Metal solubilization from concentrates via oxidative release mechanism is not likely to
occur in anoxic sediment
• Dissolution of copper (Cu) from C2 modeled
w/ first-order limiting-growth rate law (Fig. 2)
• This rate includes (1) instantaneously
solubilized Cu (y-intercept) and (2) kinetically
solubilized Cu
• After 28 days, solubilized Cu was 19.2 µg/L
• Based on typ. pH 6 water chemistry w/ DOC
= 2 mg/L and POC = 1.5 mg/L, most
solubilized Cu would be associated w/
CuT = (TOTCu – CuT,0)[1 – exp(-kdist)] + CuT,0
organic carbon and in the particulate phase
w/ k = 1.32×10 /hr; Cu = 0.112 μM (7.11 μg/L)
(Fig. 3).
• TICKET-UWM simulations in a generalized
lake with this solubilized Cu as a starting
concentration indicate that after 28 days,
the solubilized Cu concentration dropped
Figure 2. Release of copper (Cu) from C2 over time during to 0.053 µg/L (factor of 360 reduction in Cu
28-day T/D test at pH 6. Solid line represents first-order or 99.7% reduction)
dis
−4
T,0
limiting-growth rate law it to the data.
• Fig. 4 indicates TICKET-UWM results for C2
added directly to the water column of a
generalized lake at 1 mg/L w/ settling and
sediment exchange
• In the generalized lake, the concentrate
settles rapidly, thereby limiting C2 dissolution
and Cu release in the water column
• The limited amount of Cu solubilized in the
water column settles at the slower rate
associated with POC settling
• Since Cu dissolution from C2 is likely to occur
via oxidative dissolution, model simulations
were made both with and without dissolution in
the anoxic sediment
• When C2 was allowed to dissolve in sediment
in the model, some Cu was recycled to the
water column via resuspension and diffusive
exchange (Fig. 4, red line)
Figure 3. TICKET-UWM speciation output for a solubilized • However, as in the simulation described
copper concentration of 19.2 µg/L and pH 6 water
above, the day 28 Cu concentrations in the
chemistry (DOC = 2 mg/L and POC = 1.5 mg/L)
generalized lake water column are orders of
magnitude lower than the value from the T/D
• TICKET-UWM sediment speciation
test (i.e., 19.2 µg/L)
results at day 28 indicate the vast
majority of Cu in sediment is present as
the concentrate
• When dissolution is allowed to occur in
the model sediment layer, any Cu
solubilized from C2 reacts w/ AVS and
precipitates as Cu-sulfide
• Sediment Flux Results:
• Without dissolution in sediment (Fig.
5, black lines), net flux is initially
directed into the sediment; it rapidly
approaches zero as settling and
resuspension
metal
fluxes
equilibrate
• With dissolution in sediment, (Fig. 5,
red lines), net flux is initially directed
toward sediment but then reverses
• Reason for reversal:
Cu
solubilized from C2 in the
sediment reacts w/ AVS and
precipitates as Cu-sulfide. This
Figure 4. TICKET-UWM results for Cu (from C2) in the water
Cu-sulfide is resuspended and
column at pH 6.0. Black dashed line = Cu bound in C2
concentrate (CuC). Black solid line = solubilized Cu (CuT +
re-supplies Cu to water column
CuP) results for generalized lake simulations with settling and
as it oxidizes (note: oxidation of
exchange w/ sediment (without dissolution in the sediment).
resuspended
Cu-sulfide
is
Red line = results for generalized lake with dissolution in the
instantaneous in model = worst
sediment.
case)
Figure 5. TICKET-UWM modeling results for the sediment remobilization analysis for Cu in C2. Settling
and resuspension fluxes are shown for the no dissolution in sediment and dissolution in sediment cases.
Conclusions
• Rapid concentrate settling limited the impact dissolution has on water column Cu concentration
• Continued dissolution of concentrate in the sediment did impact the water column indirectly as
some of the additional Cu was recycled to the water column through resuspension and diffusive
exchange but resulting concentrations were > 350 lower than the day 28 concentration of 19.2 µg/L
from the T/D tests
• In cases where oxidative dissolution is the mechanism for metal release observed in T/D tests,
dissolution in anoxic sediments may not occur
• The day 28 water column concentrations in the generalized lake were significantly lower than the
day 28 concentrations from the T/D tests
References
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