Pollution of carbonate soils in a Mediterranean climate due to a tailings spill
M. SIMÓN, C. DORRONSORO, I. ORTIZ, F. MARTÍN & J. AGUILAR
Departamento de Edafología y Quimica Agrícola, Facultad de Ciencias,
Universidad de Granada, 18002 Granada, Spain.
Soil properties and macroelements
When the tailings from a pyrite mine are exposed to oxygen and water, sulphides
oxidise to sulphates, the pH falls markedly due to the formation of sulphuric
acid, and the pollutants solubilize (Förstner & Wittmann, 1983). In
the case of pyrite, the most abundant sulphide in these tailings, the oxidation
can be represented by the following reactions:
2FeS2(s) + 7O2(g) + 2H2O 2Fe2+(aq) + 4SO42-(aq) + 4H+(aq) (1)
4Fe2+(aq) + O2(g) + 4H+(aq) Fe3+(aq) + 2H2O (2)
The Fe3+ released in Reaction (2) may hydrolyze to form ferric hydroxide:
Fe3+(aq) + 3H2O Fe(OH)3 + 3H+(aq) (3)
or may oxidize additional pyrite by the reaction:
FeS2(s) + 14Fe3+(aq) + 8H2O 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq) (4)
Reaction (2) is very slow at pH<4.0 and has been described as the rate-determining
step in pyrite oxidation; nevertheless, Fe-oxidizing bacteria (e.g. Thiobacillus
ferrooxidans, Thiobacillus thiooxidans) increased the oxidation rate of Fe2+
by 105 (Singer & Stumn, 1970) and thus, oxidation rates for pyrite are 10-
to 20-fold higher than those resulting from purely chemical oxidation (Battaglia
et al., 1998; Boon & Heijnen, 1998).
When CaCO3 is present in the soils, the acidity is neutralised, the oxidation
of Fe2+ to Fe3+ proceeds rapidly (Singer & Stumm, 1968), iron precipitates
and the calcium and sulphate ions form gypsum (Ritsema & Groenenberg, 1993;
Kashir & Yanful, 2000):
Fe3+(aq) + 2SO42-(aq) + H+(aq)+ 2CaCO3(s) + 5H2O
Fe(OH)3(s) + 2CaSO4.2H2O(s) + 2CO2(g) (5)
On 25 April 1998 the retention walls in a pond containing the residues from
the pyrite mine of Aznalcóllar (southern Spain) broke open, and approximately
4x106 m3 of polluted water (solution phase) and 2x106 m3 of toxic tailings (solid
phase) were spilled into the Agrio and Guadiamar River Basin, affecting some
55 km2 (Grimalt et al., 1999). The solid phase consisted of different sulphides
such as pyrite (75-80%), sphalerite and galene (5%), chalcopyrite (1.5%), arsenopyrite
(1%), as well as minor amounts of bournonite, boulangerite, nuffieldite, jaskolkiite
and numerous trace metals (Almodóvar et al., 1998; López-Pamo
et al., 1999). The principal pollutants were: Zn, Pb, Cu, As, Sb, Bi, Cd and
Tl (Cabrera et al., 1999; Vidal et al., 1999; Simón et al., 1999).
After the spill, the soils of the basin were covered by a layer of tailings
some 7 cm in average thickness (López-Pamo et al., 1999). At the time
of the deposition, the tailings were saturated with water and afterwards began
to dry progressively. As the tailings dried and aerated, sulphides oxidised
to sulphates, the pH fell and the pollutants solubilized (Alastuey et al., 1999).
The concentration of water-soluble SO42-, Cd2+ and Pb2+ in the tailings increased
logarithmically with declining moisture (Simón et al., 1999). These solubilized
elements, with evaporation, rose by capillary action to the surface, forming
a white salty crust composed of Fe(II)-, Zn- and Mg-hydrated sulphates and high
content in Cu, As, Cd and Sb (López-Pamo et al., 1999). Subsequent rains
would dissolve these soluble salts, forming an acidic solution, which would
then infiltrate the soil, raising the pollution level.
In this scenario, our aim is to determine the impact that the acidic solution
from the oxidation of the tailings exerted on the different soil properties,
estimating both the concentration reached by the pollutants in the soils as
well as their penetration in depth.
On 4 May 1998 (9 days after spill), in a project aimed at studying the pollution
of the soils affected by the toxic spill, we sampled the soils in various sectors
throughout the basin, analysing the concentration of trace elements in the soils
(0-10 cm in depth). In one of these sectors, at 18 km from the origin of the
spill, the colour of the soil was uniformly pale brown (10YR 6/3). Nevertheless,
by 25 July 1999 (15 months after spill), the first 12 mm of the soil of this
sector had changed to reddish-yellow (7.5YR 6/8) in colour, presumably due to
the infiltration into the soil by the strongly acidic solution from the oxidation
of the tailings, in which the different elements were dissolved. On this date,
we sampled four locations in this sector which were still covered by a layer
of tailings some 3 cm thick. At each location, we made a detailed study of the
soil properties and their pollutant concentrations, sampling the reddish-yellow
band every 2 mm (0-2, 2-4, 4-6, 6-8, 8-10, 10-12 mm in depth) and sampling the
underlying soil at different depths to 101 mm (12-14, 19-21, 29-31, 39-41, 59-61
and 99-101 mm in depth).
In each sample, the particle-size distribution was determined by the pipette
method after removal of organic matter with H2O2 and dispersion with sodium
hexametaphosphate (Loveland & Whalley, 1991). The pH was measured potentiometrically
in a 1:2.5 soil-water suspension. The CaCO3 equivalent was determined by a manometric
method (Williams, 1948). Total carbon was analysed by dry combustion with a
LECO instrument, and the organic carbon was determined by the difference between
total carbon and inorganic carbon from CaCO3. Extracts of tailings:water and
soil:water (1:10) were prepared and the sulphates were precipitated as BaSO4
(U.S. Salinity Laboratory Staff, 1954). Total iron (Fed) and aluminium (Ald)
oxides of the soil samples were extracted with citrate-dithionite (Holmgren,
1967) and the amorphous or poorly crystallized oxides (Feo and Alo) with ammonium
oxalate (Schwertmann & Taylor, 1977). Iron and aluminium in the extracts
were measured by atomic absorption spectroscopy. Pills of soil and lithium tetraborate
(0.6:5.5) were prepared and the total content in silica (SiO2), iron (Fet) and
aluminium (Alt) was measured by X-ray fluorescence in a Philips PW-1404 instrument.
Iron and aluminium within the structure of silicates were estimated by the differences
Fet-Fed and Alt-Ald, respectively; meanwhile, well-crystallized oxides were
estimated by the differences Fed-Feo and Ald-Alo, respectively (Arduino et al.,
1984). X-ray diffraction patterns of powdered soil samples were made with a
Philips PW-1700 instrument, using CuKa radiation. Samples of the soils, very
finely ground (<0.05 mm), were digested in strong acids (HNO3 + HF + HCl).
In each digested sample and in each extract of tailings:water, the Cu, Zn, Cd,
As, Pb, Sb, Bi and Tl content were measured by ICP-MS with a PE SCIEX ELAN-5000A
spectrometer. The accuracy of the method was corroborated by analyses (6 replicates)
of Standard Reference Material: SRM 2711 (soil with moderately elevated trace-element
concentrations; Gills & Kane, 1993) (Table
1). A Zeiss-950 scanning electron microscope with a Tracor Northern 523
X-ray energy-scattering microanalyser (SEM-EDS) was used to examine the morphology
and analyse the composition of certain minerals present in the first 12 mm of
the soil.
The climate of this area is typically Mediterranean (hot, dry summers; cold,
wet winters; temperate autumns; and springs with variable rainfall). The rainfall
from 4 May 1998 to 25 July 1999 was 695 mm, the average temperature 17.9 °C,
and the potential evapotranspiration 975 mm (taken from the Aznalcázar
weather station, at 21 km from the spill).
Soil properties and macroelements
The chemical characteristics of the acidic solution from the oxidation of the
tailings, although these must have changed over time, can be estimated from
extracts of tailing:water on 25 July 1999 (Table
2). The infiltration of this solution into the soil resulted in the total
decarbonation and strong acidification of the reddish-yellow band (Table
3). The mean pH for the four locations analysed fell to a minimum of 3.2
compared with a maximum of 8.1 in the underlying calcareous soil. This strong
acidification led to the hydrolysis of the finest mineral particles, reducing
the mean clay content by roughly 65%, fine silt by 60% and coarse silt by 30%,
with respect to the underlying soil. The X-ray of the soil-powder samples (Figure
1) revealed that, within the first 12 mm, the reflections of the calcite
disappeared, the relative intensities of feldspars diminished, and the intensity
and sharpness of the basal reflections at 1.0 (illite) and 1.4 nm (smectite)
of the clay minerals also decreased; by contrast, the reflections of the minerals
resistant to hydrolysis, such as quartz, increased in relative intensity. The
content in organic carbon was hardly affected by the acid solution, the values
remaining nearly constant in the first 100 mm of the soil.
The SO42- ions in the acid solution precipitated upon making contact with the
soil, accumulating in the reddish-yellow band (Figure
2a), where their mean concentration increased by approximately 30 g kg-1
with respect to the underlying soil at 100 mm in depth. This precipitation occurred
primarily in the form of gypsum (Miedema et al., 1974; Ritsema & Groenenberg,
1993; Kashir & Yanful, 2000), as reflected by the X-ray diffraction diagram
showing the peaks for gypsum (Figure 1A).
The SEM-EDS analysis, in addition to crystals composed of S and Ca and tabular
habit (Figure 3a,b) typical of gypsum
(Eswaran & Shoba, 1983), also shows needle-like crystals composed of S and
Fe projecting from a mass made up of S, Zn, Fe, Mn and Al (Figure
3c), as well as raised scaly crystals composed of S and Al (Figure
3b) and aggregate microcrystals with pseudo-cubic morphology and composed
of S, Fe and K (Figure 3d), presumably
jarosite (Wagner et al., 1982; Eswaran & Shoba, 1983). These results indicate
that, in addition to the precipitation as gypsum, sulphate may also be adsorbed
by various Fe and Al oxides and hydrous oxides, this being assumed to be the
primary mechanism for SO42- retention in a variety of soils (Parfitt & Smart,
1978; Johnson & Todd, 1983; Singh, 1984).
The iron in the acid solution also precipitated in this band (Figure
2c,d), increasing the concentration of Fet by approximately 36 g kg-1 with
respect to the soil at 100 mm in depth. This precipitation must have resulted
from the neutralization of the acidic solution on contact with the calcareous
soil, accelerating the oxidation of Fe2+ to Fe3+ (Singer & Stumm, 1968).
The Fe oxidized eventually reached saturation with respect to a variety of less
soluble oxyhydroxides and hydroxysulphate minerals forming ochreous precipitates
(Bigham & Nordstrom, 2000) responsible for the reddish-yellow colour of
the uppermost soil layer. From the 36 g kg-1 of the Fet accumulated in the first
12 mm, almost all (33 g kg-1) was extracted with citrate-dithionite (Fed), and
Fet-Fed remained practically constant with depth. The iron extracted with ammonium
oxalate (Feo) was approximately 28 g kg-1, indicating that the iron precipitated
basically in the amorphous or poorly crystallized form (85%) and only a small
portion (15%) in a well-crystallized form (Fed-Feo).
The Fed content proved to be closely related to the sulphate content, both in
g kg-1, by the linear regression:
Fed = 8.925 + 1.071 SO42- r2=0.973 p<0.0001
supporting the idea that iron and sulphate precipitate together, this also being
reflected in the SEM-EDS analysis. The poorly crystallized iron hydroxysulphate
formed appeared to be schwertmannite (Nordstrom, 1982; Bigham et al., 1996;
Bigham & Nordstrom, 2000); also, a minor fraction of the iron precipitated
as aggregate microcrystals composed exclusively of iron (Figure
3b). In addition, the colour 7.5YR 6/8 of the uppermost 12 mm coincides
with that characteristic of schwertmannite (Scheinost & Schwertmann, 1999).
The well-crystallized form could be jarosite, also visible in the SEM-EDS analysis.
The aluminium incorporated in silicates such as feldspars, illite, and smectite,
which can be estimated by the difference between the total aluminium and the
total aluminium oxides (Alt-Ald), diminished substantially from 40 g kg-1 at
100 mm in depth to 24 g kg-1 in the first 12 mm of the soil (approximately 40%),
reflecting the intense hydrolysis process that affected this sector (Figure
2f). The Al that was dissolved in the acidic solution, although a minor
part accumulated in the first 12 mm (Figure
3b), was more mobile than iron, a large portion being leached from the very
acidic layer and accumulating in the layer between 12 -14 mm in depth with pH=6.0
(Figure 2e). From the total hydrous
Al oxides accumulated (Ald), roughly 80% precipitated in the amorphous or poorly
crystallized form (Alo) and only 20% in a well-crystallized form (Ald-Alo).
The relatively high sulphate content in this layer (18.3 g kg-1), the pH>5.0
and the predominance of the poorly crystallized form, appear to indicate that
this precipitation could have occurred fundamentally as basic aluminium sulphate
(Van Breemen, 1973), presumably basaluminite (Nordstrom & May, 1996). The
well-crystallized form could be jurbanite (Bigham & Nordstrom, 2000). The
Ald content was not related to sulphate content because the majority of the
SO42- precipitated in the first 12 mm as gypsum and iron hydroxysulphate.
The hydrolysis of the aluminium silicates also caused the partial solubilization
and leaching of the silica (SiO2) in the first 12 mm of the soil, where its
concentration diminished by some 100 g kg-1 in relation to 100 mm in depth (Figure
2b). However, in contrast to the Al, silica showed no clear accumulation
within the first 100 mm of depth, and thus the accumulation may have occurred
at a greater depth.
On 4 May 1998 the oxidation rate of the tailings and the concentration of water-soluble
pollutants was found to be negligible (Simón et al., 1999), and no rain
fell between 25 April 1998 (spill) and 4 May 1998 (first sampling). Consequently,
the concentration in the different pollutants of the soils sampled on 4 May
1998 would be attributable only to the soil concentrations prior to the spill
(background) and the amount that penetrated the soil from the solution and solid
phases of the toxic spill (initial pollution). The increase of pollutants in
the soils sampled on 25 July 1999 with respect to those on 4 May 1998, can be
ascribed to the oxidation of the tailings (Alastuey et al., 1999), solubilization
of the pollutants in the rainwater (Table
2) and infiltration into the soils. This increase will be termed oxidative
pollution.
The concentration of the pollutant elements between 0 and 10 cm in depth in
the soil sampled on 4 May (this soil concentration will hereafter be termed
SC4M) was very similar to mean concentrations between 99 and 101 mm in depth
of the soils sampled on 25 July 1999 (Table
3). This indicates that almost all the elements that were dissolved in the
acidic solution of the tailings remained in the first 100 mm of the soil. Only
Zn and Cd, with average concentrations between 99 and 101 mm in depth clearly
greater than SC4M, appear to have penetrated beyond the first 100 mm.
Nevertheless, although all the elements dissolved in the acidic solution tended
to accumulate in the first 100 mm of the soil, there were clear differences
between them. Thus, the mean concentrations of Zn and Cd in the first 12 mm
of the soil were equal to or even less than SC4M and therefore these elements
did not accumulate in this highly acidic layer of soil (pH<4.5); it even
appears that the acidic solution from the tailings solubilized and leached part
of the Zn already present in the soil. The accumulation began in the soil layer
with a pH=6.0 (12-14 mm in depth) and reached its maximum in the soil layer
with a pH>7.0 (19-21 mm in depth). Deeper than 21 mm, concentrations progressively
decreased. The difference between the mean concentration at each depth and that
of SC4M, defined above as oxidative pollution (POZn and POCd), proved to be
significantly related to the clay content and pH (p<0.05). This indicates
that these elements were adsorbed onto or diffused through the clay structure,
this process increasing with the pH (Harter, 1983; Shuman, 1985; Brummer, 1986).
In addition, as with Ald, Cu partially accumulated in the strongly acidic layer
(pH<4.5), where the mean concentration was higher than SC4M, reaching its
highest accumulation at pH=6.0 (12-14 mm in depth) and accumulating almost totally
in the first 40 mm of the soil. The oxidative pollution (POCu) proved to be
significantly related to Ald (p<0.001), indicating that this element is fundamentally
adsorbed by amorphous or poorly crystallized hydrous Al oxides (Baker &
Senft, 1995).
Like Fed, As tended to accumulate more or less uniformly in the reddish-yellow,
strongly acidic band (pH 3.0-4.5). The oxidative pollution (POAs) was highly
correlated to Fed and SO42- (p<0.001), indicating that the As, probably as
H2AsO- (the major species in this pH interval), was highly adsorbed by iron
hydroxysulphates (O'Neill, 1995; Waychunas et al., 1995). The Pb accumulated
in the first 2 mm of the soil, values falling sharply below this depth. The
oxidative pollution (POPb) proved not to be related to the hydrous oxides (Ald
or Fed) despite that the amorphous Fe oxides have a high affinity for Pb (Kinniburgh
et al., 1976; McKenzie, 1980). This high affinity, and propensity to precipitate
as PbSO4 (Brookins, 1988), may have caused the Pb to accumulate in the first
2 mm of the soil, while the Fed content was distributed more or less uniformly
throughout the first 12 mm, masking any statistical relationship between the
two. The other elements (Sb, Tl and Bi) showed a behaviour intermediate between
As and Pb, with a maximum accumulation in the first 2 mm of the soil but with
a progressive decline in depth. The oxidative pollution of each element (POSb,
POTl and POBi) proved to be significantly related to Fed (p<0.001) and SO42-
(p<0.01), revealing that they were adsorbed by iron hydroxysulphates (Edwards
et al., 1995). The oxidative pollution of these elements was also significantly
related to POPb (p<0.001), this confirming the adsorption of Pb by Fed.
To estimate the mobility of the different pollutants in the soil, we calculated
a mobility index (M) by the equation:
12
M = ∑ (POin / TPOi) dn
n=1
where n represents the layers sampled, POin is the oxidative pollution of each
element i in each layer sampled n, TPOi is the sum of the oxidative pollution
of each element i of all layers sampled, and dn the depth (mm) of the lower
limit of each layer sampled n. The value of this index would range from 2 (if
the element accumulated entirely in the first 2 mm) to 101 (if the element accumulated
entirely between 99 and 101 mm). The mobility in descending order was (the M
values in parenthesis):
Zn (29.7) > Cd (28.1) >>> Bi (15.6) > Cu (14.7) >> Sb (8.3
)>As (6.6) >Tl (5.4) > Pb (4.8)
For the same spill, comparable results were reported by Vidal et al. (1999)
three months after the accident, with the exception of Bi, which these authors
include among the elements of low mobility such as Pb. This appears to indicate
that Bi can increase in mobility over time.
The infiltration into the soil of the strongly acid solution formed by the oxidation
of pyrite tailings gave rise to acidification, weathering of the carbonates,
and hydrolysis of the mineral particles (more intense the finer the particle
size), but only in the first 12 mm of the soil. The SO42- ions present in the
acidic solution precipitated almost entirely at this depth, forming gypsum,
hydroxysulphates and complex sulphates. The Fe3+ ions also precipitated at this
depth, mainly in amorphous or poorly crystallized forms (probably schwertmannite),
adsorbing to As, Sb, Tl and Pb dissolved in the acidic solution. The Al3+ ions,
though partially precipitating in the acidic layer, accumulated most when the
soil pH exceeded 5.5. As Fe3+, the Al3+ ions precipitated primarily as amorphous
or poorly crystallized forms (probably basaluminite), adsorbing to Cu dissolved
in the acidic solution. The Zn2+ and Cd2+ ions precipitated mainly at pH>7.0,
being adsorbed chiefly by the clay mineral. Most of these pollutants did not
penetrate beyond 100 mm in depth.
After 15 months, the solution from the oxidation of the tailings acidified only
the first 20 mm of the soils, and most of the pollutants did not penetrate deeper
than 100 mm, without contaminating the subsoil or groundwater. This implies
that under a Mediterranean climate in calcareous soils, the speed of the cleanup
is not as important as a thorough removal of the spill without leaving behind
remains of tailings mixed with soils. Also, the upper 10-cm layer of the soils
should be removed together with the toxic tailings.
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