The Almadén mining district (Ciudad Real, Spain) was the largest cinnabar (mercury sulphide) mine in the world. Its soils have high levels of mercury a consequence of its natural lithology, but often made much worse by its mining history. The present work examines the thermal desorption of two contaminated soils from the Almadén area under non-isothermal conditions in a N2 atmosphere, using differential scanning calorimetry (DSC). DSC was performed at different heating rates between room temperature and 600
Mercury (Hg) is one of the most toxic of global pollutants (Gochfeld,
The mining districts of Almadén (Spain), Idrija (Slovenia) and Monte Amiata (Italy) have together produced more than half of the total Hg extracted and commercialized in the world. Although operations have now ceased, Almadén was the world's largest and oldest Hg mining operation, its production making up a full one third of the total Hg ever extracted (Hylander and Meili,
In others countries, small-scale gold (Au) mining activities are harmful to the environment in part because of the widespread use of Hg in the extraction process. After thoroughly grinding Au-containing ore or silt, Hg is added, creating an amalgam. Subsequent heating concentrates the gold into a pellet but releases elemental mercury into the environment (Paruchuri et al.,
The technologies used to remove or stabilize Hg in contaminated solid waste or soil include solidification/stabilization, soil washing, thermal treatment, solar thermal desorption, vitrification and electrokinetic remediation. A comparison of the different technologies is provided by Wang et al. (
Thermal desorption treatment usually involves an
Thermal decomposition techniques have been used to identify Hg compounds in soil, sediment samples, iron-based sorbents, and even in Hg lamp waste. The Hg species thermally released from contaminated soils can be analysed in several ways, including temperature-controlled continuous heating of samples in a furnace coupled to an atomic absorption spectrophotometer (AAS) (Windmoller et al.,
Desorption temperatures of different mercury phases in contaminated soils (Kunkel et al.,
Phase | Desorption temperature of phase Hg (°C) |
---|---|
Hg0 | <100 |
Non Cinnabar Hg | 150–250 |
Hg2Cl2 | <180 |
HgCl2 | <250 |
HgS (Cinnabar) | 310–350 |
HgO | 420–550 |
HgSO4 | 450–500 |
Hg in Pyrite | >450 |
Hg in Sphalerite | 600 |
In the present work, the thermal desorption of Hg was analyzed by differential scanning calorimetry (DSC) in a N2 atmosphere, with the aim of elucidating the associated reaction mechanisms and kinetics.
The material analysed consisted of two Hg-contaminated soils (S1 and S2) from the Almadén area (
Location of soil collection sites.
Physico-chemical characteristics of contaminated soils
Soil | pH | EC (1:5) |
ECES
|
OrganicMatter (wt.%) | Texture | Hg |
---|---|---|---|---|---|---|
S1 | 6.9 | 164 | 1.05 | 2.8 | Sandy loam | 34.40±7.20 |
S2 | 6.1 | 614 | 3.93 | 3.7 | Sandy loam | 10.497±1.564 |
SRM 2709a | – | – | – | – | – | 1.40±0.08 |
CRM 051 | – | – | – | – | – | 29.90±5.96 |
Furthermore, reference materials with known concentrations of Hg were used: CRM 051 from an Hg-contaminated area (29.9±5.96 mg kg−1 of total Hg) in the western USA, and SRM 2709a, an agricultural soil (1.40±0.08 mg kg−1 of total Hg) from San Joaquín (Querétaro, Mexico).
The pH (H2O, 1:2.5) and electrical conductivity (EC) (H2O, 1:5) of the soils were measured according to the official methods of the Spanish Ministry of Agriculture, Fisheries and Food (MAPA,
The Hg concentration of the soil samples was directly measured using an atomic absorption spectrophotometer specifically designed for Hg determination (Advanced Mercury Analyser - AMA254 – LECO Company). The certified reference material CRM 051 (soil from USA contaminated area, 29.90±5.96 mg kg−1 of total Hg) was used as a standard to determine the accuracy and precision of the measurements and to validate the applied method. The mean value of total Hg determined for 10 measurements of the certified material using the AMA254 equipment was 29.90±2.89 mg kg−1 of total Hg. At a 95% confidence level, no significant differences were detected between the certified value and the experimental one, this method was therefore considered to be reliable for the determination of total Hg.
When samples showed a high Hg content, out of equipment range limit (>600 ng), they were pre-processed by an acidic digestion using a MARS5 microwave oven (VERTEX Technics) following EPA Method 3052 (USEPA,
S1, S2, CRM 051 and SRM 2709a were subjected to DSC analysis in a N2 atmosphere (flow rate 20 mL min−1). About 60 mg of each sample were placed in a 175 µL sealed aluminium crucible and heated at a rate of 10 °C min−1 between 25 and 600 °C or 650 °C in a Setaram Model 3D-EVO analyser, which also recorded the temperature peaks associated with Hg desorption.
The kinetics of Hg desorption were then studied by DSC using the apparatus mentioned above. DSC experiments were performed at four different heating rates (5, 10, 15, and 20 °C min−1) between room temperature and 600 °C. Temperature calibration was achieved using ICTAC-recommended DSC standards. The accuracy of the reported temperatures was estimated to be ±2 °C. The sample mass used was again about 60 mg, and all experiments were performed in a N2 atmosphere (flow rate 20 mL min−1).
For mercury desorption, it is generally assumed that the rate of conversion is proportional to the concentration of reacted material. The rate of conversion can be expressed by the following basic rate equation (
where α is the degree of advance of reaction,
where
By combining equations (
where
where
Friedman analysis (Friedman,
where
The Flynn-Wall-Ozawa method (Flynn and Wall,
where β is the heating rate and
The Coats-Redfern method (Coats and Redfern,
allows equation (
The Coats-Redfern method is one of the most widely used procedures for the determination of reaction processes. The
Algebraic expressions of functions of the most common reaction mechanisms
Mechanism | f(α) | g(α) |
---|---|---|
Autocatalytic | (1- α)n. αm | – |
Avarani-Erofe've (A1.5) | 1.5(1- α) [-ln(1- α)]1/3 | [-ln(1- α)]1/3 |
Avarani-Erofe've (A2) | 2(1- α) [-ln(1- α)]1/2 | [-ln(1- α)]1/2 |
Avarani-Erofe've (An) | n(1- α) [-ln(1- α)](1-1/n) | [-ln(1- α)](1-1/n) |
First-order (F1) | (1- α) | -ln(1- α) |
Second-order (F2) | (1- α)2 | (1- α)−1-1 |
Third-order (F3) | (1- α)3 | [(1- α)−2-1]/2 |
Contracting sphere (R2) | 2(1- α)1/2 | [1- (1-α)1/2] |
Contracting Cylinder (R3) | 3(1- α)2/3 | [1- (1-α)1/3] |
Power law (P2) | 2α1/2 | α1/2 |
Power law (P3) | 3α2/3 | α1/3 |
Power law (P4) | 4α3/4 | α1/4 |
One-dimensional diffusion (D1) | 1/2α | α2 |
Two-dimensional diffusion (D2) | [-ln(1- α)]−1 | [(1-α) ln(1-α)]+ α |
Three-dimensional diffusion (D3) | 3(1-α)(2/3)]/ |
[1-(1- α)1/3]2 |
Giustling-Brounsthein (D4) | 1.5 ((1-α)(-1/3) -1) | 1-(2α/3)-(1- α)2/3 |
Three peaks can be seen for S1, with maximum temperatures (Tm) of 109.5 °C, 304.8 °C and 533.8 °C. S2 has only two peaks of Tm 121.8 °C and 305.7 °C.
DSC curves of thermal decomposition of contaminated soils and references (Heating rate: 10 °C min−1).
DSC results for the thermal desorption of Hg from the contaminated and references soils (heating rate: 10 °C min−1)
Peak 1 | Peak 2 | Peak 3 | |||||||
---|---|---|---|---|---|---|---|---|---|
Soil | To (°C) | Tm (°C) | Te (°C) | To (°C) | Tm (°C) | Te (°C) | To (°C) | Tm (°C) | Te (°C) |
S1 | 74.3 | 109.5 | 141.3 | 266.7 | 304.8 | 327.2 | 506.4 | 533.8 | 554.8 |
S2 | 72.3 | 121.8 | 182.1 | 269.0 | 305.7 | 330.0 | – | – | – |
SRM | 78.0 | 122.5 | 172.5 | 265.3 | 279.2 | 296.2 | 505.6 | 528.5 | 559.5 |
CRM | 50.6 | 93.4 | 127.8 | – | – | – | 512.8 | 546.4 | 564.5 |
(To: Start of peak temperature; Tm: maximun temperature; Te: End of peak temperature)
CRM 051 has two peaks of Tm 93.4 °C and 546.4 °C, while SRM 2079a has three of Tm 122.5 °C, 279.2 °C and 528.5 °C.
Using the data in
The peak of Tm=93.4 °C in the DSC curve for CRM 051 could be due to the desorption of metallic Hg (Hg0), according to the reaction Hg0(s) → Hg(g). This peak is not seen in any other DSC curve.
The decomposition of cinnabar (HgS) occurs between 267 and 327 °C, according to the reaction HgS (s) → Hg(g) + S(g) (peak 2 in the DSC curves; see
Finally, HgO decomposes at 505–565 °C according to the reaction HgO(s) → Hg(g)+½ O2(g) (peak 3 in the DSC curves; see
The Tm values for desorption of Hg from the different phases of the contaminated soils follow the order Hg0<HgCl2<HgS<HgO. These temperatures agree quite well with those reported by other authors using other techniques (Windmoller, 1996 and López et al.,
Hg is subject to a wide array of chemical and biological transformation processes, such as Hg0 oxidation, Hg2+ reduction, and methylation, depending on the soil pH, temperature, and humus content. The formation of organic Hg2+ complexes is known to be a dominant process, largely due to the affinity of Hg2+ and its inorganic compounds for sulphur-containing functional groups (Skyllberg et al.,
Besides the above mentioned peaks, all samples show a sharp endothermic peak at 575 °C, that could be attributed to the polymorphic transformation of hypothermic quartz to hyperthermic quartz which starts at this temperature. It is due to the phase transition in quartz (α-Quartz trigonal to β-Quartz hexagonal) (Karathanasis et al.,
DSC curves for the decomposition of contaminated soils at different heating rates. a) for S1 and b) for S2.
DSC data for the thermal decomposition of the soils at different heating rates
Peak 1 | Peak 2 | Peak 3 | |||||||
---|---|---|---|---|---|---|---|---|---|
β (°C min−1) | To (°C) | Tm (°C) | Te (°C) | To (°C) | Tm (°C) | Te (°C) | To (°C) | Tm (°C) | Te (°C) |
S1 | |||||||||
5 | 60.8 | 95.1 | 122.7 | 251.6 | 284.9 | 309.0 | 493.1 | 507.6 | 524.2 |
10 | 74.3 | 109.5 | 142.3 | 266.7 | 304.8 | 327.2 | 495.0 | 526.0 | 549.7 |
15 | 74.9 | 110.9 | 149.7 | 261.7 | 300.9 | 325.8 | 566.9 | 572.3 | 575.9 |
20 | 73.5 | 108.6 | 151.6 | 262.6 | 301.0 | 324.6 | 564.2 | 572.1 | 577.9 |
S2 | |||||||||
5 | 61.5 | 103.6 | 141.5 | 260.6 | 294.1 | 314.3 | – | – | – |
10 | 72.3 | 121.8 | 146.7 | 269.0 | 305.7 | 330.0 | – | – | – |
15 | 85.7 | 122.5 | 149.4 | 274.8 | 312.3 | 335.9 | – | – | – |
20 | 96.9 | 129.8 | 154.3 | 279.2 | 318.2 | 343.0 | – | – | – |
Both soils present the same two firs peaks showing similar chemical bonds. However soil S1 has a third peak which does not appear in the case of S2. Therefore, S1 seems to have significantly more HgO than S2 has.
The Friedman (FR) and the Flynn-Wall-Ozawa (FWO) methods were first used to calculate the
With the FR method, the
Equation (
Activation energies for S1 and S2, as determined by the Friedman and the Flynn-Wall-Ozawa methods
FR |
FWO |
|||
---|---|---|---|---|
Peak / Reaction | Soil | Ea (kJ mol−1) | A (s−1) | Ea (kJ mol−1) |
Peak 1: |
S1 |
44.6±3.9 |
7.74×103
|
53.0±4.8 |
Peak 2: |
S1 |
138.6± |
4.73×1010
|
145.0±7.6 |
Peak 3: |
S1 |
255.2± |
5.95×1014
|
284.4±2.7 |
The
For all the decomposition reactions studied, the variation in
Apparent activation energy, as determined by the Friedman method for a) Soil S1 and b) Soil S2.
The variation in
The
Employing equation (
Activation energies and pre-exponential factors for S1 and S2 as determined by the Coasts-Redfern method for different
S1 | S2 | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
HgCl2→Hg(g)+2Cl(g) | HgS→Hg(g)+S | HgO→Hg(g)+½O2 (g) | HgCl2→Hg(g)+2Cl(g) | HgS→Hg(g)+S | ||||||
Model | Ea
|
A |
Ea
|
A |
Ea
|
A |
Ea
|
A |
Ea
|
A |
Autocatalytic | 33.92 | 2.56×102 | 137.05 | 3.45×1010 | 4.44×10−5 | 1.46×10−2 | 47.34 | 1.15×10−4 | 176.46 | 1.06×1014 |
A1.5 | 14.99 | 3.2×10−1 | 103.97 | 2.31×107 | 231.26 | 1.25×1013 | 24.42 | 5.92 | 123.86 | 1.2×109 |
A2 | 7.02 | 2.17×10−2 | 75.74 | 4.57×104 | 170.35 | 9.61×108 | 15.78 | 3.6×10−1 | 95.07 | 2.37×106 |
An | 21.34 | 2.57 | 139.58 | 5.45×1010 | 4.64×10−5 | 2.61×103 | 39.51 | 6.76×102 | 173.84 | 5.19×1013 |
F1 | 30.93 | 5.73×102 | 160.52 | 4.99×1012 | 353.07 | 1.8×1021 | 41.69 | 1.33×103 | 181.45 | 2.61×1014 |
F2 | 66.43 | 1.02×107 | 272.63 | 3.54×1023 | 601.04 | 1.27×1028 | 78.90 | 2.68×108 | 295.76 | 1.82×1025 |
F3 | 101.93 | 1.82×1012 | 384.83 | 2.51×1034 | 849 | 8.92×1054 | 116.10 | 5.17×1013 | 410.08 | 1.26×1036 |
Fn | 44.43 | 5.69×103 | 151.39 | 6.67×1011 | 359.16 | 4.66×1021 | 53.22 | 5.8×104 | 187.76 | 1.04×1015 |
R2 | 13.18 | 6.79×10−2 | 104.32 | 9.36×106 | 229.09 | 3.39×1012 | 23.09 | 1.49 | 124.29 | 4.94×108 |
R3 | 19.09 | 3.4×10−1 | 123.02 | 4.02×108 | 270.42 | 1.45×1015 | 29.29 | 7.6 | 143.34 | 2.11×1010 |
Rn | 44.43 | -2.16×103 | 151.39 | 5.37×1010 | 359.16 | -1.14×1020 | 53.22 | -1.8×104 | 187.76 | -5.73×1013 |
P2 | -21.76 | 1.23×10−6 | -14.75 | 8.24×10−5 | – | 2.4×10−5 | -14.34 | 1.92×10−5 | 2.83 | 4.29×10−3 |
P3 | -27.49 | 1.62×10−7 | -35.74 | 7.3×10−7 | -74.99 | 1.98×10−8 | -20.62 | 2.28×10−6 | -18.61 | 3.77×10−5 |
P4 | -30.357 | 5.38×10−8 | -46.24 | 6.31×10−8 | -97.51 | 5.21×10−10 | -23.75 | 7.23×10−7 | -29.33 | 3.25×10−6 |
D1 | 29.80 | 1.1×101 | 174.15 | 2.56×1013 | 375.26 | 1.44×1022 | 42.15 | 4.15×102 | 195.75 | 1.43×1015 |
D2 | 43.24 | 5.59×102 | 217.57 | 2.1×1017 | 470.55 | 2.24×1028 | 56.31 | 2.25×104 | 239.89 | 1.14×1019 |
D3 | 61.52 | 6.23×104 | 275.20 | 1.74×1022 | 598 | 2.26×1036 | 75.45 | 2.63×106 | 298.62 | 9.35×1023 |
D4 | 49.68 | 1.11×103 | 237.80 | 4.19×1018 | 515.35 | 5.47×1030 | 63.05 | 4.52×104 | 260.51 | 2.27×1020 |
Activation energies of S1 and S2 as obtained by the Coats-Redfern method
Peak/Reaction | Ea
|
A |
Thermal decomposition mechanism | |
---|---|---|---|---|
Peak 1: |
S1 |
44.4 |
5.69×103
|
Fn: (1-α)1.38 (n=1.38) |
Peak |
S1 |
139.6 |
5.45×1010
|
An=n(1-α)[-ln(1-α)(1-1/n) (n=1.1) |
Peak |
S1 |
231.3 |
1.25×1013
|
An=n(1-α)[-ln(1-α)(1-1/n) (n=1.5) |
Reported an
The
This work was funded by MAYASA and the Centro Tecnológico Nacional para la Descontaminación del Mercurio (CTNDM).
Dr. O. Rodríguez is the recipient of contract JAE-Doc_09-00121 (CSIC), co-funded under the FSE 2007-2013 Multiregional Adaptability and Employment Operational Programme.
DSC: Differential Scanning Calorimetry
α: conversion
n and m: reaction order
R: gas constant (J mol−1 K−1)
T: Temperature (°C)
Tm: Maximum temperature peak (°C)
β: Heating rate (°C min−1)