『Abstract
The weathering of discharged mine tailings can contaminate groundwaters,
rivers and floodplains with potentially toxic Cd and Zn, depending
on tailings mineralogy, storage, dispersal and climatic conditions.
The mechanisms of long-term tailings weathering and its influence
on waste piles and floodplain environments were assessed by a
column leaching experiment that incorporated tailings and soil
from Potosi（iの頭は´）, Bolivia, and modelled
20 cycles of wet and dry season conditions over three calendar
years. Chemical analysis of the leachate and column solids, optical
mineralogy, XRD, SEM, EPMA, BCR and water-soluble chemical extractions
and speciation modelling were carried out to determine the processes
responsible for the leaching of Cd, Fe, S and Zn. Over this period,
approximately 50 to 95％ of the original Cd and 50 to 60％ of the
Zn were leached from the columns. Large amounts of leached Cd
and Zn at the beginning of the experiment are attributed to the
dissolution of soluble sulphate minerals present in the original
tailings and formed after the first wetting of the columns. The
Zn/Cd mass ratios of the tailings and soil, initially 429 and
400, respectively, vary considerably over the course of the experiment.
Low values (between 220 and 300) in the early cycles are attributed
to preferential weathering of Cd-rich wurtzite [(Zn,Fe)S』] and
sequestration of Zn in preference to Cd in secondary Fe phases
forming in the columns. In the middle cycles, dissolution of secondary
Fe(OH)3 under low pH (＜3) conditions, and
of ferroan (Cd-poor) sphalerite [(Zn,Fe)S], releases Zn and raises
the Zn/Cd ratio to 550-600 in the tailings-only columns and up
to 1500 in the mixed tailings-soil columns. The very high ratios
in the latter are also ascribed to the formation of low molecular
weight organic ligands that have high affinity for Zn over Cd.
In the later column-cycles, Zn/Cd ratios return to near-initial
values, due to the weathering of Fe-poor sphalerite and secondary
Fe phases, and the declining preference of Zn over Cd in the soil
organic acids under the strongly acidic conditions prevailing
in the columns. The formation and dissolution of secondary soluble
sulphate minerals also play a role in Cd and Zn cycling, especially
at the beginning of the experiment.
Keywords: Mine tailings; Bolivia; Potosi（iの頭は´）;
Zn; Cd; Sphalerite; Wurtzite; Column leaching
1. Introduction
2. Study area
3. Methods and materials
3.1. Collection and characterization of mine tailings and
floodplain soil
3.2. Column leaching experiments
3.3. Characterisation of pre- and post-leaching column solids
3.4. Speciation and reaction-path modelling
4. Results
4.1. Leachate
4.1.1. Leachate volume
4.1.2. Leachate pH
4.1.3. Leachate Fe, S, Cd, Zn and Zn/Cd
4.2. Characterisation of tailings, soil and post-leaching solids
4.2.1. Visual character of pre- and post-leaching column solids
4.2.2. Physical characteristics of pre- and post-leaching column
solids
4.2.3. Bulk chemical analysis of solids
4.2.4. BCR and water-soluble chemical extractions
4.2.5. Optical microscopy and XRD analysis
4.2.6. Electron probe microanalysis (EPMA)
4.2.7. Composition and character of ZnS polymorphs
4.3. Speciation and reaction^path modelling
5. Discussion
5.1.Stability of ZnS polymorphs in Potosi tailings
5.2. Formation and dissolution of secondary phases, and sorption
and de-sorption of Cd and Zn
5.3. Implications for other mine tailings-affected systems
6. Conclusions
Acknowledgements
References