This research work was designed to compare the Zn distribution in a long-term sludge-amended soil with that in a control soil. Two complementary approaches were performed: 1) a geochemical approach at the metric scale of the bulk soil horizons and 2) a mineralogical approach at the micrometric scale of the primary minerals weathering microsites. The geochemical approach revealed that Zn in the control soil was inherited from the weathering parent-rock. Its concentration was always lower than in the amended soil where Zn was supplied at the surface by the spread sludges and moves downwards. The mineralogical approach showed that the clay minerals, produced by the weathering of the primary minerals (amphiboles and plagioclases), or filling the fissure network were made up of smectites (saponite and montmorillonite) at the bottom and kaolinite at the top of the two soil profiles. Each clay mineral, with its specific sorption capacity, controlled the Zn distribution within the soil: the smectites produced by the amphiboles had high sorption capacity and favored Zn retention in the upper horizons of the soil. Conversely, the kaolinites produced by the plagioclases had lower sorption capacity, did not retain Zn in the surface horizons, and allowed it to migrate to deeper horizons where it was sorbed onto the montmorillonites.
The spreading of sewage sludges onto farmlands can be a beneficial method for soil amendment because the sludges can be a source of plant nutrients (especially N, P) and organic matter. However, much of these sludges results from the treatment of industrial and urban by-products and may contain toxic organic and inorganic compounds. Among the inorganic components, the heavy metals have been critically examined since some of them can be toxic to the biosphere at very low concentrations: for instance, Cd concentration cannot exceed 2 µg・g−1 for soils to be selected for sludge spreading [
The adsorption of heavy metals onto clay minerals operates in particular sites with permanent or variable negative charges [
The major part of the clay minerals found in the soils results from the weathering of the parent-rock in two simultaneous processes: i) the dissolution of the primary rock-forming minerals with the release of the cations and the heavy metals; and ii) the crystallization of clay minerals from the soil solutions with the sorption of the previously released heavy metals. Mineralogical studies have shown that the chemical weathering reactions and their associated metals release are active in specific soil microsystems with their own solid and solution chemical properties [
Up to now, studies dealing with polluted soils described heavy metals contaminations at the soil scale, i.e. in the fine earth (<2 mm) of each soil horizon [
Even though the bulk concentrations of heavy metals can be decisive in assessing global soil pollution, they result from the addition of the partial heavy metals concentrations in each of the weathering microsystems. As a consequence, the detailed analysis of a soil contaminated with heavy metals requires both the knowledge of its bulk metals concentration but also the partial metals contents in the microsystems in which they are retained. By taking advantage of a long-term spreading on soil of highly Zn concentrated sewage sludges (498 mg・kg−1), this study is undertaken to estimate i) the impact of the sludge application upon Zn distribution and migration in the soil and ii) the influence of the clay mineral species upon Zn retention in the weathering microsystems.
The soils investigated in this study were selected and sampled during the geological and pedological field survey of the “Saint-jean-Ligoure” diorite massif in Haute-Vienne (Limousin, France), 25 km in the South of Limoges (
The two soils are moderately drained, deep with the weathered rock found at 100 - 150 cm depth and show the typical A, Bw and C horizons sequence of inceptisols (
very similar physicochemical properties (
Each soil was carefully sampled using plastic cylindrical corers to prevent the soil metal contamination. The cylinder, 10 cm long with a diameter of 10 cm, had cutting edges. It was smoothly and continuously pushed into the soil to collect undisturbed blocks which retained the original soil structure. Each core was divided into two subsamples. The first was disaggregated by gentle shaking in water, sieved to 2 mm, air dried and reserved for the mineral grains separation, the bulk chemical and the X-ray diffraction (XRD) analyses.
The second subsample was preserved undisturbed for thin sections preparation, electron probe microanalyses (EPMA) and scanning electron microscope (SEM) study.
The organic matter in the first <2 mm subsample was removed using 33% hydrogen peroxide. 200 grams of this sample were shaken in water to destroy the aggregates, sieved into the sandy fractions 50 - 100 µm, 100 - 150 µm, 150 - 250 µm, 250 - 500 µm, and dried at 50˚C for 24 h. The observation of each sieved fraction under optical microscopy (OM) revealed that most euhedral monomineralic grains were found in the 250 - 500 µm fraction. The smaller sized fractions were made up of broken primary minerals. About 500 monomineralic grains, 250 to 500 µm in size, of amphiboles and plagioclases (the two most important minerals in the parent-rock and soils) were then needle-sorted under the stereomicroscope for the study of their specific weathering products. Separation efficiency was checked by XRD.
The bulk chemical analyses were obtained from the first subsample, freed of its organic matter. An aliquot quantity of matter (5 g) was collected by quartering with a riffle splitter and crushed down to less than 50 µm size into agate mortar. 300 mg were taken from the total powder, fused with LiBO2 at 1050˚C and dissolved in 1N HNO3 for the bulk chemical analyses. The major and trace elements were analysed using ICP-AES and ICP-MS at the Service d’Analyse des Roches et des Minéraux, (SARM), CRPG-CNRS (Vendoeuvre-lès-Nancy, France). The major and trace elements concentrations were expressed in wt.% and mg・kg−1, respectively.
The bulk clay fractions, together with the clay minerals extracted from the weathered monomineralic grains by sonification, were analysed by XRD on Ca-saturated oriented preparations, air dried and glycolated. The diffractometer was a PHILIPS PW 1730 (40 kV, 40 mA) with a Fe-filtered CoKa radiation and a stepping motor driven with a DACO-MP recorder and the Diffrac-AT software (Socabim, Munich, Germany). The bulk clay fraction <2 µm was extracted from 10 g of the <2 mm first subsample. The 10 grams sampled were first dispersed in 350 ml deionized water, sonicated for 2 min at 300 W/20 kHz, then mechanically shaken for 3 h. The stable suspension was then centrifugated at 20˚C and 1000 rpm for 2 mn 30 s with a Jouan JR4.22 centrifuge in order to separate the <2 µm fraction. The effectiveness of this clay separation was then checked using a laser diffraction granulometer IP Malvern Mastersizer.
The second subsample was devoted to the EPMA and the SEM studies. Thin sections were first prepared according to the procedure of Camuti and McGuire [
Representative pedofeatures (clay coatings, clay cutans, etc.) and weathering microsites were first located on the thin sections, marked with black circle under an optical microscope and then “in situ” analysed for major and trace elements. EPMA were obtained using a CAMECA SX 50 electron microprobe (Service CAMPARIS, Université Paris VI) equipped with wavelength-dispersive spectrometers (WDS). The microprobe was calibrated using synthetic and natural oxides. Corrections were made with a ZAF program. A specific trace program with the electron microprobe was developed [
For each dominant primary rock forming mineral, i.e. amphibole and plagioclase, ten grains were selected from the 250 - 500 µm needle-sorted fraction and affixed to glass plates with double-sided adhesive tape. The two glass plates were then gold-coated for the micromorphological SEM study of the weathering products.
The geochemical mass balances were obtained from the calculation of enrichment factors (EF) using the method of Hernandez et al. [
where the content of element X is normalized to the content of the reference element R. A value of 1 for EF indicates no element enrichment or depletion. EF values >1 indicate element enrichment and when <1 indicate element depletion. The aluminium, with its narrow range of variation in the two soils (17.53% to 20.08% in
Sample | CS8 | CS7 | CS6 | CS5 | CS4 | CS3 | CS2 | CS1 | AS7 | AS6 | AS5 | AS4 | AS3 | AS2 | AS1 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Depth (cm) | 485 | 450 | 180 | 160 | 130 | 65 | 40 | 20 | 190 | 170 | 140 | 110 | 70 | 40 | 20 |
SiO2 | 48.79 | 49.00 | 47.99 | 47.53 | 47.15 | 49.12 | 47.48 | 46.07 | 51.2 | 50.21 | 51.27 | 52.34 | 50.83 | 51.23 | 50.95 |
TiO2 | 1.08 | 1.32 | 1.14 | 1.32 | 1.33 | 1.30 | 1.22 | 1.37 | 1.28 | 1.24 | 1.27 | 1.31 | 1.42 | 1.48 | 1.51 |
Al2O3 | 18.78 | 18.25 | 19.10 | 19.37 | 19.22 | 19.84 | 20.08 | 18.98 | 18.76 | 18.47 | 18.41 | 17.93 | 18.67 | 17.53 | 18.74 |
Fe2O3a | 9.58 | 10.04 | 10.03 | 10.08 | 10.29 | 9.41 | 10.13 | 9.75 | 8.87 | 9.53 | 8.99 | 9.47 | 9.26 | 9.65 | 9.13 |
MnO | 0.17 | 0.21 | 0.18 | 0.19 | 0.21 | 0.20 | 0.17 | 0.16 | 0.14 | 0.15 | 0.15 | 0.12 | 0.11 | 0.12 | 0.11 |
MgO | 4.47 | 4.55 | 4.20 | 4.06 | 3.89 | 3.88 | 4.09 | 3.52 | 3.25 | 3.44 | 3.19 | 3.29 | 3.36 | 3.36 | 3.08 |
CaO | 7.52 | 6.97 | 6.97 | 7.05 | 6.69 | 6.19 | 4.15 | 6.26 | 5.7 | 5.83 | 5.24 | 2.11 | 3.55 | 2.41 | 4.26 |
Na2O | 3.54 | 3.29 | 3.36 | 3.13 | 2.94 | 3.25 | 3.88 | 2.82 | 3.19 | 3.08 | 3.19 | 3.5 | 2.96 | 3.18 | 3.25 |
K2O | 1.44 | 1.40 | 1.50 | 1.19 | 1.37 | 1.27 | 1.51 | 1.47 | 1.54 | 1.35 | 1.69 | 1.68 | 1.66 | 1.64 | 1.63 |
P2O5 | 0.59 | 0.70 | 0.60 | 0.66 | 0.70 | 0.67 | 0.47 | 0.57 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
LOIb | 3.77 | 3.94 | 4.68 | 5.21 | 5.04 | 4.62 | 6.50 | 8.53 | 5.91 | 6.48 | 6.19 | 7.77 | 7.74 | 8.49 | 6.72 |
Total | 99.73 | 99.67 | 99.75 | 99.80 | 98.85 | 99.73 | 99.62 | 99.50 | 99.84 | 99.78 | 99.59 | 99.52 | 99.56 | 99.09 | 99.38 |
Zn | 111.18 | 124.46 | 113.06 | 119.32 | 126.08 | 120.42 | 108.24 | 115.56 | 115.36 | 103.04 | 80.64 | 72.15 | 79.64 | 68.27 | 69.29 |
aTotal iron expressed as Fe2O3. bLOI: Loss on ignition.
The two soils are derived from the same dioritic parent rock which outcrops at the bottom of the quarry exposing the control soil profile (R horizon in
The enrichment factors calculated using the Equation (1) and
Amphibole | Plagioclase | Orthoclase | Albite | |||||
---|---|---|---|---|---|---|---|---|
n = 14a | SEb | n = 10 | SE | n = 10 | SE | n = 10 | SE | |
Zn | 180 | 4 | 17 | 2 | 20 | 2 | 14 | 6 |
aEach analysis is already the mean of ten measurements. bStandard error.
more than twice from the sample reference CS8 (EF-L.O.I. = 1) to the top of the soils (EF-L.O.I. reaches 2.41 in the sample AS2 at the top of the amended soil). This trend reflects the general increase in the bulk water content (loss on ignition at 1050˚C, i.e. the hydrate content in the absence of the CO2 found in carbonates and after organic matter destruction), and is related to the increase in the hydrated clay minerals crystallizations as weathering proceeds. In the same way as aluminium that was chosen for reference element, the measured SiO2 and Fe2O3 concentrations (
The chemical analyses of Zn in the bulk soil horizons (
The clay minerals identified in the <2 µm granulometric fractions are similar in the control and amended soils. The XRD on Ca-oriented preparations reveals the typical 001 reflections of smectite, at 15.10 Å in air-dried state and shifting to 17.05 Å after ethylene-glycol solvation. The smectite is associated with a kaolinite which is typified, in air-dried and ethylene-glycol solvated states, by 001 and 002 reflections at 7.15 Å and 3.55 Å respectively. These reflections disappear after 450˚C heating. No reflection of iron oxide or hydroxide was detected on the XRD patterns of the random powders.
The Zn contents in the soil clay fractions were measured using the electron microprobe and are given with their mean standard errors in
AS7 | AS6 | AS5 | AS4 | AS3 | AS2 | AS1 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Depth (cm) | 190 | 170 | 140 | 110 | 70 | 40 | 20 | |||||||||
n = 4a | SEb | n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | |||
Zn | 150 | 12 | 158 | 12 | 144 | 11 | 153 | 10 | 177 | 9 | 203 | 13 | 195 | 12 | ||
Sample | CS8 | CS7 | CS6 | CS5 | CS4 | CS3 | CS2 | CS1 | ||||||||
Depth (cm) | 485 | 450 | 180 | 160 | 130 | 65 | 40 | 20 | ||||||||
n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | n = 4 | SE | |
Zn | 15 | 2 | 14 | 1 | 14 | 2 | 17 | 2 | 18 | 3 | 19 | 3 | 24 | 5 | 10 | 3 |
aEach analysis is already the mean of ten measurements. bStandard error.
(
The first signs of amphiboles weathering are found in the C horizons of the two soils. The OM observation of the thin sections reveals that euhedral amphibole prismatic crystals begin to weather along the enlarged cleavage planes (
The clay minerals, extracted from the amphiboles grains by sonication, were identified by XRD. The XRD patterns are similar in the C, Bw, and A horizons with the same typical reflections of smectite. The Ca-oriented preparations reveals strong 001 reflections ranging from 14.50 Å to 15.49 Å in air-dried state, shifting to 16.87 - 17.14 Å with higher orders at 8.46 Å and 3.40 Å after ethylene-glycol solvation. This expansion to 17 Å with ethylene-glycol is typical of smectite layers.
The EPMA of the amphibole weathering products characterize two chemically distinct smectite populations. The first is detected in the C horizons of soils and is made up of magnesian smectites belonging to the trioctahedral group of saponites. The second population is found in more advanced weathering stages (Bw and A horizons) as aluminous smectites belonging to the dioctahedral smectite group of montmorillonites.
The Zn contents in the clay minerals produced by the weathering of amphiboles were measured using the electron microprobe and are given in
fresh amphibole (
When taking into account the mean standard errors, the Zn concentrations in saponites and montmorillonites appear almost constant throughout the control soil profile (