Trace Metal Concentrations in the Liquid Phase of Phosphate Rock

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A G R O K É M I A É S T A L A J T A N 55 (2006) 1
203–212
Trace Metal Concentrations in the Liquid Phase of
Phosphate Rock-Treated Soils
J. CSILLAG, A. LUKÁCS, E. OSZTOICS, P. CSATHÓ and GY. BACZÓ
Research Institute for Soil Science and Agricultural Chemistry (RISSAC) of the
Hungarian Academy of Sciences, Budapest
Introduction
The application of slowly dissolving, reactive, basic phosphate rocks – which
are favourable alternative crop P-sources in the case of acidic soils – may cause
environmental hazards, as (similarly to commercial P fertilizers), depending on
their origin they may contain potentially toxic contaminants (SAUERBECK, 1992;
SOLER SOLER & SOLER ROVIRA, 1996; VAN KAUWENBERGH, 1997; OSZTOICS et
al., 2005). Special attention has been focussed on cadmium, which has serious human toxicological concerns. P fertilizers produced from sedimentary phosphate
rocks having a high Cd content are considered as a major source of Cd contamination in agricultural soils (HAMON et al., 1998; MERRINGTON et al., 2002; BOLAN et
al., 2003). In the Rothamsted field experiment Cd uptake of plants increased due to
the long-term application of superphosphate fertilizers (NICHOLSON et al., 1994). In
Hungary, however, the intensive P fertilization in the 1970s–1980s was carried out
with superphosphate made from Russian magmatic Kola phosphate rock practically
free of Cd. Therefore, Hungarian agricultural soils have not been polluted with Cd
due to the application of P fertilizers (KÁDÁR, 1991, 2003; DEBRECZENI et al.,
2000).
An indicator of the environmentally hazardous accumulation of potentially toxic
elements may be their appearance and increased concentration in the soil solution.
Few studies are available on the impact of P fertilizer (including phosphate rock)
application on toxic element concentrations in the soil solution (TAYLOR &
PERCIVAL, 2001; CSILLAG et al., 2005). The objectives of this study were
– to determine heavy metal and other toxic element concentrations in the liquid
phase of two acidic soils at increasing phosphate rock doses applied in a pot experiment, and
– to evaluate the potential contaminating, and – simultaneously – the pH elevating and toxic metal immobilizing effects of the applied phosphate rocks.
Correspondence to: Julianna CSILLAG, Research Institute for Soil Science and Agricultural
Chemistry (RISSAC) of the Hungarian Academy of Sciences, H-1022 Budapest, Herman
Ottó út 15. Hungary. E-mail: [email protected]
CSILLAG et al.
204
Materials and Methods
The bulk samples for the pot experiment were collected from the ploughed layer
of an acidic sandy soil and a slightly acidic clay loam soil (from Nyírlugos and
Ragály, resp., Hungary) (Table 1).
Table 1
Main chemical and physical properties of the studied soils
H2O
KCl
Organic
matter,
%
5.0
3.8
0.6
3.0
5.0
2.2
5.7
4.5
3.4
20
60
27
pH
Soil
Acidic sandy soil
(Nyírlugos)
Slightly acidic clay
loam soil (Ragály)
Clay+silt
Clay
Cation ex<0.02
<0.002
change capacity
cmolc·kg-1
mm, %
Table 2
Phosphate rock (PR) doses applied in the experiment
Sign
mg P2O5·kg-1 soil
d1
d2
d3
d4
1400
1600
3200
6400
g PR·kg-1 soil
Senegal
Morocco
4.2
4.9
9.7
19.4
5.4
6.1
12.3
24.5
Increasing doses of two sedimentary phosphate rocks with high Cd content
(Senegal and Morocco), particle size < 160 μm, were added and mixed into the airdried soil samples at the beginning of the experiment. The detailed description of
the pot experiment and phosphate rocks’ properties is published by OSZTOICS et al.
(2006). Phosphorus doses applied on the non-cropped, control soils and used for the
soil solution studies are shown in Table 2.
The moisture level of the control soils, similarly to the cropped samples, was
initially adjusted to ~ 60% of field capacity. They were periodically watered with
deionised water to maintain this moisture level for two months (the vegetation period of the plant in the pot trial).
In order to obtain soil solution, at the end of the pot experiment the control samples were air dried, 1–1 kg soil was placed in air tight plastic vessels, then they
were rewetted to field capacity with deionised water (to 26.5 % and 28.3 % gravimetric water content of Nyírlugos and Ragály soil, resp.). After one week incubation, during which the wet soil samples were homogenized three times, the soil solution was extracted by 1 hour centrifugation. A rotor speed (5000–5500 rpm) corresponding to -1500 kPa (the suction power exerted by the plant at the conventional
Trace Metal Concentrations in the Liquid Phase of Phosphate Rock-Treated Soils
205
wilting point) was applied, so the separated solution could be regarded as “plant
available” (CSILLAG et al., 1999).
“Total” element contents in the untreated soils and in the applied phosphate
rock samples were determined in their 1:50 extracts made with concentrated HNO3
+ H2O2 solution by applying microwave wet digestion in teflon bomb. As, Ba, Cd,
Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sr and Zn concentrations in the soil solutions and
in the extracts were determined by ICP spectrometry. The total P content of the
phosphate rock samples was also measured with ICP in the extract of the acid digestion. The 10% HCl soluble carbonate content of the phosphate rock samples was
determined in Scheibler calcimeter, and expressed as CaCO3 %.
Results and Discussion
Trace metal and other potentially toxic element contents of the initial (untreated)
soils and the applied phosphate rock samples
The potentially toxic element contents of the untreated sandy soil – determined
by acid digestion before the set-up of the pot trial – were several times lower than
those of the clay loam soil (Table 3). In the sandy soil they were much lower than
the background concentrations given in the corresponding Hungarian standard: e.g.
the Cd content was more than one order of magnitude less than the national background threshold level. In the clay loam soil Cr and Ni contents were somewhat
higher than the background concentrations and the Ba content considerably exceeded it (Table 3). Cd content in the clay loam soil was several times higher (0.2
mg·kg-1) than in the sandy soil (0.04 mg·kg-1) and both values were in the lower
part of/or below the 0.1–0.5 mg “total” Cd·kg-1 range measured in unpolluted Hungarian field soils in the 2001–2003 period (MÁTHÉ-GÁSPÁR et al., 2004).
In both phosphate rock samples the Cr and especially Cd contents considerably
exceeded the previous strict Hungarian limit values for P fertilizers (Table 3).
(Since May 1, 2004, when Hungary became an EU member state, no threshold values have been specified for toxic elements in case of fertilizers distributed as “EU
fertilizer”, in accordance with the 2003/2003/EK decree of the European Parliament
and Council.)
Concentrations of trace metals and other potentially toxic elements in the soil solution of the initial (untreated) soils
Concentrations of the elements in the liquid phase of the untreated sandy soil
were, with the exception of Co, lower than or near to the allowed tolerable concentrations in subsurface waters, but most were higher than the corresponding background concentrations (Table 4). In case of the clay loam soil Ba and Ni concentrations exceeded the allowed threshold values.
206
CSILLAG et al.
Trace Metal Concentrations in the Liquid Phase of Phosphate Rock-Treated Soils
207
CSILLAG et al.
208
Although element contents determined by acid digestion were higher in the clay
loam soil than in the sandy soil (Table 3), this tendency was true only for Ba, Cd,
Cu, Ni and Sr concentrations in the soil solution (Table 4). When soil solution concentrations (cS) were expressed as a percentage of the “total” amounts in the soil
measured by acid digestion (cSOIL), the relative metal concentrations in the soil solution (s% = 100 cS/cSOIL) were, with the exception of Sr, higher in the colloid-poor
sandy soil (Table 4) as was expected in a soil retaining metals weakly. (cS was related to mass unit of the air-dried soil: μg·kg-1 = (μg·L-1 × gravimetric water content
of the soil)·100-1.)
Effect of phosphate rock application rates on soil pH and metal concentrations in
the soil solution
As compared to agricultural practice the applied phosphate rock doses were extremely high. The d3 dose (3200 mg P2O5·kg-1 soil) (Table 2), for example, was
more than 100 times greater than the application rate commonly used on fields under intensive fertilization (40 kg P·ha-1 = 90 kg P2O5·ha-1 = 30 mg P2O5·kg-1 soil).
Addition of basic phosphate rocks decreased the acidity of both soils. The
pH(KCl) values measured in the control samples (0 mg P2O5·kg-1 soil) of the sandy
and clay loam soils at the end of the pot experiment were 3.7 and 4.5, resp., lower
than the pH(KCl) values of the phosphate rock-treated soils (Fig. 1). In the d1 to d4
range of increasing phosphate rock doses the pH(KCl) increased by 0.4 and 0.1
(Senegal) and 0.8 and 0.3 (Morocco) units for the sandy soil and the clay loam soil,
respectively (Fig. 1). In the case of the d4 (Morocco) treatment the pH of the two
soils became identical; the initially more acidic sandy soil reached the pH of the
clay loam soil. The somewhat more basic Morocco phosphate rock (pH = 8.1,
CaCO3 content = 14%) had a more stronger pH-increasing effect than the Senegal
phosphate rock (pH = 7.9, CaCO3 content = 4%).
Metal release to the soil solution in the phosphate rock-treated soils at increasing phosphate rock rates was characterized by relative metal concentrations:
s% =
100cs
c SOIL + c PR
(1)
where: cS: soil solution concentrations (related to mass unit of air-dried soil) (μg·kg-1
soil); cSOIL: “total” amounts of elements in the soil measured by acid digestion (μg·kg-1
soil); cPR: metal amounts carried into the soils with the phosphate rocks; (for d2: see
Table 5).
After the two month long experiment during which the soils were subjected to
repeated drying–rewetting periods, the relative metal concentrations (s%) decreased
with increasing phosphate rock doses in the sandy soil (Fig. 1) due to the pH elevating and cation immobilizing effects of phosphate rocks (BOLAN et al., 2003). The
relative concentrations were generally lower in treatments with the more basic Morocco phosphate rock than with the Senegal phosphate rock. Generally s% values
Trace Metal Concentrations in the Liquid Phase of Phosphate Rock-Treated Soils
6
10
8
6
4
2
0
pH (KCl)
5
4
d1
2.5
2.0
1.5
1.0
0.5
0.0
d3
s%
d1
1.5
d2
d4
d2
s%
d3
d4
1.0
0.5
0.0
d1
0.20
d2
d3
s%
Ni
0.15
0.06
s%
Sr
2
0
d2
d3
s%
d4
d1
Co
d2
d3
d4
N-Mor.
N-Sen.
R-Mor.
R-Sen.
d2
d3
s%
d1
d4
8
4
d1
1.0
0.8
0.6
0.4
0.2
0.0
Ba
Mn
6
d1
2.5
2.0
1.5
1.0
0.5
0.0
Cd
s%
209
d4
Zn
d2
d3
s%
0.5
0.4
0.3
0.2
0.1
0.0
d4
Pb
s%
d1
0.008
Cu
d2
d3
s%
d4
Cr
0.006
0.04
0.004
0.10
0.02
0.05
0.00
0.002
0.00
d1
d2
d3
d4
0.000
d1
d2
d3
d4
d1
d2
d3
d4
Fig. 1
Change in soil pH(KCl) and relative amounts (s%) of potentially toxic metals entering the
soil solution in the case of increasing phosphate rock doses.
Phosphate rock doses: d1–d4: 1400, 1600, 3200 and 6400 mg P2O5·kg-1 soil, resp.
N: Acidic sandy soil (Nyírlugos); R: Slightly acidic clay loam soil (Ragály);
Mor.: Phosphate rock from Morocco; Sen.: Phosphate rock from Senegal;
s%: Equation 1
CSILLAG et al.
210
Table 5
Amounts of the studied metals (μg·kg-1 soil) carried into the soil with the
d2 phosphate rock dose (1600 mg P2O5·kg-1 soil)
Phosphate
rock
Ba
Cd
Co
Cr
Cu
Mn
Ni
Pb
Sr
Zn
Senegal
Morocco
380
310
550
310
27
2
580
1320
330
140
850
31
240
240
20
18
3090
10450
2440
3090
were not high in the sandy soil: e.g. the max. amount of Cd that entered the soil
solution was only 2.1% of the total Cd present in the soil + phosphate rock system
(41 μg·L-1 at the d1 dose of the Senegal phosphate rock applied to the sandy soil).
In the clay loam soil s% values, characterizing the mobility of metals, were
much lower than in the more acidic light textured Nyírlugos soil (Fig. 1). With increasing phosphate rock doses they remained practically constant. The two applied
phosphate rocks’ effect on the s% value was similar in the clay loam soil.
Comparing s% of the metals, Mn, Sr and to a smaller extent Cd and Co were the
most, while Pb and Cr the least mobile elements in this experiment (Fig. 1). In the
d1 (1400 mg P2O5·kg-1 soil) treatment with Senegal phosphate rock, for example,
s% values of Mn, Sr, Cd and Co were 7.9%, 7.2%, 2.1% and 1.9%, resp., in case of
the sandy soil. The corresponding values for Ba, Zn, Cu and Ni were lower: 1.2%,
0.7%, 0.16% and 0.15%, resp. These values for Pb and Cr were very low, 0.03%
and 0.005%, resp., in the sandy soil and not measurable in the clay loam soil, which
indicates that these metals were completely bound. The concentrations of the anion
forming As and Mo were below the detection limit in the liquid phase of both
acidic soils even in case of the highest phosphate rock dose.
Conclusions
The phosphate rocks used in the pot experiment contained rather high amounts
of potentially toxic metals, therefore the environmental hazard when applying them
to the soil was considerable. Relative metal concentrations in the liquid phase of the
acidic sandy soil (Nyírlugos), however, generally decreased with increasing phosphate rock rates. This means that under the conditions of the present pot experiment
soil solution concentrations were influenced less by the amount of contaminants
carried into the soil with the increasing phosphate rock doses than by the immobilizing effect of phosphate rocks. In the slightly acidic clay loam soil (Ragály) the
relative metal concentrations were low and did not change with the increase in
phosphate rock rates. This indicates that the two opposing effects (increased input
of metals versus phosphate rocks' immobilizing effect) were compensated in this
soil.
Trace Metal Concentrations in the Liquid Phase of Phosphate Rock-Treated Soils
211
It was concluded that in the experimental time frame the environmental risk (i.e.
the accumulation of toxic metals in the soil solution), depending on the soil properties, decreased or did not change with increasing phosphate rock rate.
Summary
Concentrations of potentially toxic elements were determined in the soil solution of two soils (acidic sandy and slightly acidic clay loam) treated with phosphate
rocks having high Cd content in a pot experiment. Relative concentrations characterizing the mobility of metals (expressed as soil solution concentrations in percentage of their “total” amounts in the phosphate rock-treated soil) decreased with increasing phosphate rock rates in the sandy soil. Mn≅Sr>Cd≅Co were the most,
while Pb and Cr the least mobile elements. The relative concentrations in the clay
loam soil were much lower than in the sandy soil and they practically remained
constant with increasing phosphate rock rates. It was concluded that in the experimental time frame the environmental risk did not increase with the increase of
phosphate rock rate.
The present study was supported by the Hungarian National Scientific Research
Fund (OTKA) under grant No. T 038046.
Key words: soil solution, trace metal, phosphate rock
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