Complete report (PDF-file in German, 1.6 MB)
1. Introduction and objectives
The cooperative project of the Technical University
of Berlin with the Environmental Research Center in Leipzig/Halle
(UFZ), called "Heavy metal mobilization and migration in
complex mining and smelting heaps by the example of the smelting
location Helbra", developed an extensive database which should
be qualitatively analyzed regarding the leaching behaviour of
different waste dump components from the production of the Kupferschiefer
and its smelting. The results should contribute to an overall
view of material flows in these waste dump bodies.
For this purpose five lysimeter boxes with the components
Kupferschiefer, Theisen-sludge, low-temperature carbonized Theisen-sludge,
slag and host rocks (e.g. limestones and minor sandstones) were
set up in a field test. This study conducted a three-dimensional
process analysis. In a first step variations of element concentrations
within these groups were examined. Thus the natural range of variation
and inhomogeneity of the element concentrations in the raw material
within the lysimeter boxes should be determined. In the second
step the element concentrations in relation to a depth profile
(top-center-bottom) in the lysimeter boxes have been examined.
The results of the leaching experiment provided in the last step
the possibility to regard the dynamics of the mobilization of
the heavy metals over a time span. This work tried to determine
a dynamic reaction profile in the end and to create thereby the
hazard potential of pollutant discharges from the waste dumps.
Since the quantity of the leachate was not measured during the
test series, unfortunately, and climatic data is only incompletely
recorded, a complete quantitative balance on heavy metal contents
could not be executed.
2. Discussion of the results, summary and outlook
Discussion
In this study a detailed investigation of heavy
metal mobility behaviour from waste dump material of typical smelting
products of the Kupferschiefer has been taken place. From these
investigations conclusions for long-term behaviour of components,
an estimation of the endangerment and also the possible application
as building materials (in particular the slag in road construction,
and hydraulic engineering) had to be discussed.
Firstly, it was stated that variations in the leaching
boxes represent generally mobilization processes. Some elements,
such as sulphur in the Kupferschiefer; calcium, potassium, cobalt
and manganese in the host rocks; manganese in the slag; cadmium,
cobalt and lead in the low-temperature carbonized Theisen-sludge
and subordinated cadmium, copper, lead and zinc in the Theisen-sludge,
too, could represent partly variances by layering or an inhomogeneity
by loading the boxes, since their variance is close to the natural
variability of the samples. These were evaluated as not critical
for the discussion of mobilization types.
In the second step the distribution of element concentrations
in the lysimeter boxes has been regarded in comparison before
and after the leaching experiments. Generalized mobilization types
could be determined. The depletion type is the dominating distribution
for almost all parameters in the Theisen-sludge and low-temperature
carbonized Theisen-sludge. This could have been caused by the
high reactivity of the samples due to their small grain-size and
larger mobility of the heavy metals under oxidizing and sour conditions
(sulphide oxidation). A substantial vertical displacement of heavy
metals does not seem to have taken place in the Kupferschiefer
and in the slag. Generally, thus the indifferent mixing type of
the mobilization types for these components outweighs. In the
case of the slag this could be due to its small reactivity (sulphide
droplets are included in a glass matrix) and in the case of the
Kupferschiefer due to its higher carbonate content, which buffers
the release of acids from sulphide oxidation and provides partial
adjustment of heavy metals. The host rocks, which possess a still
higher carbonate content, show highest element concentrations
at the bottom of the lysimeter box (enrichment-type). This could
be explained partially by the buffering effect of the higher carbonate
content. Blow-outs of Theisen-sludge particles could have been
effective for a up to 7-fold enrichment in the case of arsenic.
In the third step hydro-geochemical investigations
on the leachates have been executed. It applies to these that
no water or heavy metal balance could have been executed, since
the quantity of the leachates had not been documented. Thus, only
relative investigations on the leachates have been executed by
the author.
Firstly, a closer look at the temporal development
of pH value and electrical conductivity has been taken. Because
of the high sulphide concentrations, especially in the Theisen-sludge
and low-temperature carbonized Theisen-sludge, it came - due to
sulphide oxidations of the formerly under rather reducing conditions
stored material - to the formation of a very low pH value (3-4),
which constantly rose in the Theisen-sludge in the following leachates
up to a value around 6, whereas the low-temperature carbonized
Theisen-sludge remained hardly changed at pH values around 3.
This shows particularly the very high reactivity of Theisen-sludge
samples and low-temperature carbonized Theisen-sludge. Little
by little the lumpy components are encased by an oxide coating
and transformation of sulphide to sulphate is almost stopped.
This is more obvious in the samples Kupferschiefer,
host rocks and slag. A pH value between 6 and 8 has been adjusted
and rose only slightly - however constantly - during the investigations.
This could be due to the low reactivity in the case of the slag,
the Kupferschiefer and the host rocks could have buffered the
acid release due to their higher carbonate content. The low pH
value in the Theisen-sludge and low-temperature carbonized Theisen-sludge
in interaction with oxidizing conditions, from which one can proceed
because of the constant contact of the leachate with atmospheric
oxygen, provide large mobilities for heavy metals.
Therefore a reversed picture results concerning
the conductivity, too. Very high electrical conductivities showed
all five samples in the first four leachates, however, particularly
the Theisen-sludge and the low-temperature carbonized Theisen-sludge.
Afterwards, a prominent decrease of the electrical conductivity
in all leachates could be observed and thereupon a continuous
slow decrease. Since the sulphate anion concentration is the dominating
species in all samples, it has to be assumed that sulphide oxidation
controls the electrical conductivity, at a smaller scale chlorides
and fluorides in the Theisen-sludge, too.
This is acknowledged by good negative correlation
of pH values with electrical conductivities especially in the
low-temperature carbonized Theisen-sludge and in the Theisen-sludge
itself. The Kupferschiefer and the host rocks follow with lower
correlation coefficients. The slag shows only a low negative correlation
and clarifies thereby the indifferent and almost inert behaviour.
This could happen because of the fact that the sulphide droplets,
which could set sulphate ions free, are inside a glass matrix
and can only react with atmospheric agents at breaking edges.
Further investigations on correlative relations
in between pH values, temperature, the amount of precipitation
and reaction times of atmospheric agents resulted in best positive
correlations of conductivity and temperature in the leachate out
of the low-temperature carbonized Theisen-sludge. This could be
explained by the fact that the lysimeter boxes are very small
in size and changes of temperature can influence reaction flows
from outside. I.e. that exotherm and endotherm reactions can be
influenced by changes of temperature. This might rather be effective
in the larger dumps, since their own microclimate might prevail
in their core due to their size and compactness. This must be
considered during the transfer of results from lysimeter experiments
to dump scales.
The low-temperature carbonized Theisen-sludge has
shown the best negative correlation of the amount of precipitation
to the electrical conductivity, too. The time span between the
samplings is generally not effective on the other parameters,
since low correlation coefficients are pointed out. An exception
represents the slag sample. This could be because of the low reactivity
of the slag, whose effect could have been removed by the longer
time span spent by the precipitate and the lower infiltration
pressure by following percolation water.
The chapter about threshold values has dealt with
the question, which temporal development the leaching process
will take and how could the results be evaluated regarding threshold
value lists (in this study the threshold value lists from Netherlands
and Berlin). At the same time species conditions for each component
in the leachate has been discussed. This was important for further
investigations such as ion balances. Almost all components exceed
the category C of the Netherlands or Berlin threshold value list
for SO42- and NO3-. This is especially true for zinc and cadmium
in the Theisen-sludge and low-temperature carbonized Theisen-sludge
and for copper in the low-temperature carbonized Theisen-sludge
even 100- to 10.000-fold, respectively.
These high concentrations are due to the high reactivity
of the fine-grained samples, the low pH value resulting from it
and the oxidizing conditions, which particularly provide a large
mobility of the heavy metals from the Theisen-sludge and the low-temperature
carbonized Theisen-sludge. The concentrations decrease constantly
in almost every sample and have a local minimum in the fourth
leachate. Only Ca2+ and SO42- in the host rocks and in the slag
possess a local maximum there. The existence of a minimum can
be explained with little precipitation, in addition, small temperatures
(frost) in this time, which caused a strong decrease of the availability
of the solvent (rain water). Generally, many reactions might run
slower at lower temperatures, too.
Precipitations and incrustations could have occurred
(see secondary minerals such as gypsum and anglesite), which provide
a fixation of heavy metals. Their mobilization could hardly be
renewed or there is a delay due to kinetic inhibitions. The formation
of the local maximum for Ca2+ and SO42- in the fourth leachate
could be explained by possible formation from the dissolution
of gypsum crusts in host rocks and slag (Matheis and Jahn, 1996).
Gypsum is stable only up to a pH value of 6.2. The rising pH value
in the leachates could indicate a more effective dissolution of
these gypsum crusts. Since the origin of the nitrate in this investigation
cannot be derived from anthropogenetic loads (fertilizer, faeces),
the source of the nitrate does not remain clarified.
An ion balance has been executed examining the quality
of the chemical analysis. The Kupferschiefer component showed
a very good cation/anion equilibrium, or the analysis was good,
since it measured all parameters correctly. For the slag and the
host rocks an anion deficit or a cation surplus has been determined,
which has been probably caused by the alkalinity (HCO3-), which
has not been measured. However, it can be assumed that this species
is already led into the system by rain water. The Theisen-sludge
and the low-temperature carbonized Theisen-sludge indicate a cation
deficit. The reason could be that more anions were possibly needed
for complexing reactions.
Since high ion concentrations are available and
these influence each other mutually, ion activities were calculated,
with which the following hierarchical cluster analyses were executed.
A simple hierarchical cluster analysis according
to the weighted average value method (Davis, 1986) was executed
in order to distinguish statistically homogeneous groups due to
their relative similarity. Besides the activities of the species,
temperature, the amount of precipitation, pH values and electrical
conductivities were parameters, which were considered in the analysis.
Firstly, it should be mentioned that the examination of the executed
grouping by formation of cophenetic correlation coefficients and
their comparison with the original correlation generally led to
good results. Subsequently, a closer look at the heavy metals
has been taken. In the Kupferschiefer the hierarchical cluster
analysis resulted in grouping zinc, calcium and pH value together.
This clarifies the influence, which the carbonate content in the
Kupferschiefer sample has on the pH value and thus on the release
of zinc. In the slag sample no heavy metals were leached and there
is no genuine grouping recognizable, too. This reflects again
the indifferent behaviour of the slag during these leaching experiments,
which became already clear with the mobilization trends. The exceptional
position of the parameter pH value has particularly to be mentioned
in the cluster analysis of parameters in the slag leachates. The
parameters in the Theisen-sludge sample show a high correlation
among themselves. Environmentally relevant metals like cadmium,
zinc, and nickel form a group with sulphate as an anion. This
shows preferential occurrence with sulphate from sulphide oxidation
in this sample. Copper forms a subgroup with manganese and lead
is independent of the remaining heavy metals. This might be because
of the fact that lead and at a smaller scale also copper are less
soluble than zinc under these conditions. This clarifies that
lead and zinc, which behave similarly in the primary dispersion
as sulphides, can differ with respect to same Eh-/pH-conditions
in the secondary dispersion. In contrast to the Kupferschiefer
leachate, calcium, and thus basic properties of gypsum, does not
have an influence. This is shown in the very small relative similarity
to heavy metal clusters and to pH values, too. Something similar
results from the leachate out of the low-temperature carbonized
Theisen-sludge, in which calcium and lead form a cluster. Copper,
however, forms the cadmium-zinc-nickel cluster. This higher mobility
is probably due to significantly lower pH values in the low-temperature
carbonized Theisen-sludge. The leachate from the host rocks does
not indicate heavy metal contents. The calculated calcium-magnesium-sulphate
cluster points out the dolomitic component in the host rocks,
which essentially are dolomitic limestones as country rocks of
the Kupferschiefer, and thus clarifies the effectiveness of this
grouping method.
Holmstrom (1999) determined during investigations
on sulphide bearing waste dump components of different types (with
and without carbonate) that the carbonate-free component achieved
a very low pH value with time and released high heavy metal concentrations.
Contrasting to it the pH value remained high in the carbonate
bearing component and prevented releases of heavy metals. The
components in this investigation behaved similarly. However, the
carbonate-free samples Theisen-sludge and the low-temperature
carbonized Theisen-sludge achieved very low initial pH values
due to their high reactivity and therefore a very high heavy metal
release, which - as in the mentioned investigation - was reduced
with time.
Summary and outlook
In order to execute a final estimation of endangerment,
the parameters which influence the mobility of pollutants are
summarized again. An attempt to discuss the hazard potential of
the individual groups has been made. The factor permeability dominates
the physical parameters, and has a crucial influence on the solution
and transportation of pollutants. Compaction, swelling capacity
of the deposited material and the precipitation rate control the
availability of the solvent, water. This is of special importance
for the extremely fine-grained samples of the Theisen-sludge and
low-temperature carbonized Theisen-sludge.
This permits good wetting with water and a high
chemical reactivity due to large surfaces. The formation of jointing
in the slag dumps enlarges permeability, too. The slag, however,
did not react with the atmospheric agents for the sulphide droplets
are included in the glass. However, the distribution of Theisen-sludge
suspensions in the dumps (pond IX and pond X) results in an endangerment
with possible long-term effects of heavy metal release. The Kupferschiefer
shows a good cleavage parallel to layering, achieved thereby a
gradually enlarging surface and thereby could be an endangering
potential due to the physical parameters. The host rocks do not
represent physically caused endangerment due to their lumping.
Although numerous reactions between solid and solution phase run
relatively slowly for they are kinetically restrained, the parameter
time was not effective besides in the slag sample. All other components
showed up very reactive, particularly the Theisen-sludge and the
low-temperature carbonized Theisen-sludge. The temperature could
have had an influence due to the small size of the leaching boxes,
like good correlation results in the low-temperature carbonized
Theisen-sludge show. The remaining samples behave indifferently
to the parameter temperature.
Chemical parameters control solution and precipitation
reactions between the fixed phase and the solvent water. Numerous
equilibrium reactions are affected by pH values and redox potentials,
whereby these two parameters dominate leachates. They control
chemical environments and thus the mobility as well as fixation
of heavy metal cation- and anion-complexes. However, redox potentials
were not measured, unfortunately. Probably it is possible to assume
oxidizing conditions, since the leachates were constantly in contact
with atmospheric oxygen. Adsorptive bonding or complex bonding
can be very effective, additionally. Organic compounds can cause
complexing reactions as well and thus lead to additional mobilization.
The first chemical reactions form an initial Eh-pH-environment
by the interaction of dissolution, precipitation and transportation.
This could lead to a reduction of the mobilization of heavy metals,
or an increase (Scheffer & Schachtschabel, 1982). The influence
of the pH value is evident for the samples Theisen-sludge and
low-temperature carbonized Theisen-sludge due to the sulphide
oxidation. On the other hand, this investigation revealed that
the carbonate content ensured especially in the host rocks and
at a smaller scale in the Kupferschiefer higher pH values and
an immobilization of heavy metals occurred (buffering effect).
The slag seems to have behaved chemically inert except for the
edges. Complexing reactions seem to be indicated in the organic-rich
Theisen-sludge, whereby fluorides could have been important, too.
Biological parameters could not be considered in
this study, but might have been effective, additionally, for most
redox equilibria are kinetically restrained reactions (Scheffer
& Schachtschabel, 1982) and would run in extremely long periods
without catalysis.
Particularly favourable geochemical prerequisites
(environmental conditions) for pollutant mobility, availability
and thus the danger of a groundwater contamination with heavy
metals within areas with acidic tendencies show - results presented
in this study - the samples Theisen-sludge and low temperature
carbonized Theisen-sludge. Their reclamation is highly necessary.
For this reason the old Theisen-sludge dump location in the pond
IX was mitigated. However, the Theisen-sludge dispersedly distributed
in the jointed slag dump represents still an endangerment, which
is pointed out by high heavy metal concentrations in the percolation
water ("Stadtborn seepage"). At present these are cleaned
from zinc in a neutralization plant. However, the produced neutralization
mud has to be deposited separately.
Theisen-sludge from the old pond IX has been relocated
in a waste dump of host rocks and low-grade Kupferschiefer-ore
under a geo textile cover (pond X). In the case of a break-through
of solutions, the release of higher heavy metal concentrations
would be rather prevented by high carbonate contents of this dump.
The Kupferschiefer follows the Theisen-sludge and the low-temperature
carbonized Theisen-sludge concerning the endangerment priority,
since heavy metal release is clearly lower and, on the other hand,
it occurs mixed with low-grade Kupferschiefer-ore with dumps of
host rocks, which do not represent a hazard potential. Slag, which
represents volumetrically the largest dump component in Helbra,
behaves chemically almost inertly and therefore it could be used
as secondary raw material (road construction (paving-stones, crushed
stone material)), as long as it is not interspersed by Theisen-sludge
out of the old pond IX. Schreck (1997) mentioned a study which
does not recommend the use of slag for dwellings due to its radio
toxicity.
Further investigations are necessary, which should
particularly deal with the questions for water balance, temperature,
Eh and the seasonal climatic influence. Therefore, experiments
should be continued during the whole year considering all available
climatic data, in order to be able to meet more exact predictions
to mobilization processes and hazard potential. Besides this,
pollutant inputs by precipitation (wet deposition) and atmosphere
(dry deposition) as well as the pH value of the precipitation
should be included into a balance, too. Generally, the transmissibility
of the leaching experiments on the dumps, its heterogeneities
or filling of the lysimeter boxes with the sample material should
be checked.
Thermodynamic considerations and saturation calculations
could be employed on the basis of expected reactions and reactions
indicated by secondary minerals. Moreover, pH, Eh and temperature
dependence of the stability constants should be considered. It
should be noticed particularly for the results of the presented
correlation analysis that correlations rely on 7 values (7 leachates).
A more reliable statistical result would be given by more values
(and thus more leachates). This should be considered in a possible
subsequent project, if a multivariate-statistical analysis of
the leachates should be intended.