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Release Behavior of Se from Coal into Aqueous Solution Cheng Liu, Changchun Zhou,* Ningning Zhang, and Jinhe Pan Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education, School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China ABSTRACT: The release of harmful trace elements from solid such as coal into the water is closely related to human health. The aim of this paper is to study the release procedure of selenium (Se) from coal in aqueous solution. Effects of contact time, pH, and temperature were investigated via a set of batch release tests. Experimental results were also analyzed from the aspects of dynamics and thermodynamics. Batch release tests showed that the release capacity of Se increasees with the increase of contact time and temperature, and it is higher in strong acid or alkaline conditions than in neutral conditions. Dynamic fitting analyses indicated that the release model of Se from coal into water is different with different pH values, and the applicability of different models suggested that the release mechanism of Se is multiple. The first-order kinetics fitting demonstrated that the release procedures of Se from coal into the water with the initial solution pH of 3 and 11 are two-stage release processes. And the best fitting of a pseudo-second-order model manifested the process is mainly controlled by chemical reaction. Thermodynamics analyses indicated that the release of Se from coal into water is an endothermic reaction, and the release enthalpy of Se is 18.314 and 3.533 kJ/mol at the solution pH 7 and 11, respectively.
1. INTRODUCTION
Previous studies mainly focused attention on the air pollution of Se caused during coal combustion, the content and distribution, modes of occurrence of Se in coal from the aspect of geology, or the removal of it from coal by conventional coal cleaning approaches.13 However, the potential impact of Se release from coal into water are often ignored. The problem should be taken into consideration in the wet coal preparation process, as coal is completely soaked in water during this process. In addition, the increasing amount of processed coal is so large that this problem should not be neglected all the time. So far, several studies had been conducted on the leaching behavior of trace elements from coal or its byproducts, and these studies mainly focused on the amounts of leached trace elements in water or the modes of occurrence. Yang et al.5 studied the leaching behavior of trace elements in coal gangue with different leaching agents and indicated that the release of As, Se, Pb, etc. showed high potential risk to groundwater. Cabon et al.14 researched the leaching behavior of trace elements from coal in seawater considering the pollution of coal on seawater. Ohki et al.15 also investigated the release of heavy metal from coal in water. However, there have been no systematic studies on the release of trace element from coal in water from the aspect of dynamics or thermodynamics analysis to understand the detailed mechanism of the release of trace elements from coal into water solution. Dynamics and thermodynamics analysis are useful tools to provide valuable information about the reaction. They were also applied to study the adsorption and desorption of elements in soil. Li et al.16 studied Ce adsorption and desorption on soil and performed analysis using dynamics and thermodynamics methods. The same approach was used to study the adsorption and desorption of Cu2+ and Ni2+ in coal dust.17 In this study,
Coal is one of the most important energy sources in the world. Nevertheless, with the increasing amount of coal combustion, a series of environmental problems also come about. For example, the emission of hazardous trace elements such as mercury (Hg), fluorine (F), selenium (Se), etc. from coal can cause environmental pollution and ultimately affect human health.1 Generally speaking, hazardous trace elements in coal may simultaneously bring about three aspects of pollution: atmospheric contamination, water contamination, and soil contamination.2,3 For example, fluorine in coal can transform into HF, SiF4, and CF4 during coal burning, and Hg0 or Hg2+ may also discharge into the atmosphere during coal combustion, all of which are toxic materials.4 Once these elements are discharged into the environment, they would be greatly harmful to both the natural biology and human being.5 It is reported the endemic fluorine poison and endemic arsenic poison are at a severe stage in Guizhou province of China, and the source of these poisons is attributed to the use of highfluorine and high-arsenic coal.6,7 Se, one of the essential elements for human beings and plants, is also one of the harmful trace elements in coal. Several selenosis events had happened in the world, including the selenium pollution of Kesterson reservoir in the US happened in the past century, and the congregate selenosis accent occurred in Enshi, China.8 Studies had suggested the potential relationship between these poisonings and the Se-rich stone coal.9 The Clarke values of Se in coal range from 0.2 to 4 ppm,10 and it mainly exists in both organic form (Se-organic compound or elemental Se°) and inorganic form (sulfide or selenide).11 Approximately 10% of Se in coal is also present in a soluble/ion-exchanged state, which should receive special attention, as these kinds of Se would easily release to water during the coal cleaning or rain shower process.12 © XXXX American Chemical Society
Received: October 23, 2017 Revised: January 14, 2018 Published: January 23, 2018 A
DOI: 10.1021/acs.energyfuels.7b03249 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Basic Characteristic of Coal Samples (%)a proximate analysis (%)
a
ultimate analysis (%)
trace element (μg/g)
Mad
Aad
Vad
FCad
C
H
N
S
Se
0.67
41.46
21.38
36.49
53.04
3.85
0.98
1.21
5.73
Mad: air-dried moisture. Aad: air-dried ash. Vad: air-dried volatile. FCad: air-dried fix carbon where Qt is the release capacity at time t, μg/kg; Ct is the Se concentration in solution at time t, μg/L; C0 is the initial concentration of Se in the solution, μg/L; V is the volume of the added solution, L; and m is the mass of the coal sample, kg.
we specially focused on the Se in coal and our major aims were to study the influential factors (time, pH, temperature) of the release of Se in aqueous solution and mainly try to conduct kinetic and thermomechanical analysis on the release procedure, to provide a more fundamental understanding of the release mechanism of Se from coal and the potential environmental effect of Se release during the coal cleaning process.
3. RESULTS AND DISCUSSION 3.1. Effect of Contact Time and Initial pH on Se Release from Coal. A set of experiments were initiated to study the effect of contact time and initial solution pH on Se release, and the results of the experiments are shown as Figure 1. From Figure 1 we can see that both the contact time and pH
2. MATERIALS AND METHODOLOGY 2.1. Samples. Coal samples used in this study were the raw coal collected from the Fangezhuang coal preparation plant located in HeBei province, China. Samples were carefully collected and then stored in a refrigerator to avoid pollution and volatilization of Se. They were divided into two equal parts by the method of coning and quartering, one of which was stored as back-up and another was prepared for experiments. All experimental samples were ground to 100% through 200 mesh for further analysis and experiments. The proximate, ultimate, and trace element analyses of the raw coal sample were carried out, and the results are listed in Table 1. The ash and sulfur content are 41.4% and 1.41%, respectively, suggesting this coal belong to high-ash and low medium-sulfur coal according to GB/ 15224.1-2010. The content of Se is 5.73 μg/g which is 218% higher than that in US coal.18 2.2. Determination Method of Se. An Agilent 7900 ICP-MS spectrometer, fitted with an Agilent S10 autosampler, was used to determine Se concentration of both solid and liquid samples. As for the solid sample, it was first digested by a microwave-assisted digestion method.19 The operational parameters of ICP-MS used during the determination are described as follows: RF power of 1550 W; plasma mass flow rate of 15 L/min; carrier gas flow rate of 1.05 L/min; integrated time of 0.3 s; per peak points of 3; sweep number of 100; and the recovery of internal standards are between 80% and 120% which is in the eligible range. 2.3. Experimental Procedure. To study the influence of contact time (the immersion time of coal in aqueous solution), pH, and temperature on the release of Se from coal, batch release tests were conducted in this study. In each test, a 10 g coal sample was transferred into a preprepared flask along with 100 mL of solution. The pH of the solution was adjusted in a range from 3 to 11 by HNO3 or/and NaOH solution. The immersion time was varied from 0.5 to 24 h to study the effect of contact time. Three different temperatures of 293, 303, and 313 K were selected to study the influence of temperature on Se release behavior. Blank tests with no coal sample were conducted simultaneously with each set of experiments, and Se was not detected in blank tests. All tests were carried out in a temperature-controlled shaker with an agitation speed of 110 r/min. After each experiment, a filtration procedure was conducted to separate the lixivium and residue using a Buchner funnel device accompanied by a vacuum pump and the leachate was collected in a precleaned polyethylene tube and put in cold storage for further determination of Se by ICP-MS. The water used in all tests was ultrapure water with an electrical resistivity of 18.24MΩ/cm produced by the Milli-Q Ultrapure Water Polishing System. Reagents utilized in tests were also all guaranteed grade. To estimate the release results, the release capacity is defined and calculated as eq 1:
Qt =
(Ct − C0)V m
Figure 1. Influence of contact time on Se release in solutions with different initial pH values.
have a significant effect on Se release from coal in an aqueous solution. The release capacity of Se is weakest when the initial solution pH is 7 while it is highest when the pH value is 11 followed by pH 3, indicating that both a strong acid and an alkaline condition contribute to the release of Se. This may be attributed to an additional driving force being provided to overcome the resistance of mass transfer of Se when the aqueous solution is under strong acid and alkaline conditions.20 Besides, the ionic strength in solution could also be an important factor, including H+ /OH−, and other leachable inorganic elements such as Al, Fe, Ca, etc. which are also expected to be released from coal considering the inorganic composition of coal.21−23 According to the slope of curves which reflect the release rate of Se release, Se releases rapidly in the first 2 h. Thereafter, the release rate decreases gradually with the increase in time, and the release tends to level off and up to the maximum finally; this may due to the number of release sites in the coal surface being fixed.24 The concentration of Se in aqueous solution ranges from 8.94 to 25.14 μg/L after the coal is soaked, the maximum value of which has exceeded the standard value of Se concentration in water (20 μg/L) according to Chinese Standard GB 3838-2002. If the total amount of coal in the coal preparation plant or in other place of the junction of coal and water is considered, the accumulated pollution effect is huge. Considering that a highly
(1) B
DOI: 10.1021/acs.energyfuels.7b03249 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
fitting results are summarized in Table 2. What is interesting is that the release procedure of Se from coal to water can be regarded as a two-stage first-order kinetic model when the initial pH value is 3 or11 as the log(Qe − Qt) versus t consists of two lines, which had also been found during investigation on the mercury(II)/kaolinite system.27 This suggests that the release mechanism of Se from coal into water is diverse in solution with different pH values. 3.2.2. Pseudo-Second-Order Model. A pseudo-second-order model, which is reported to be well fitted with metal adsorption experimental data,28 was also applied to analyze the release procedure, and the model in a differential form is given as
acidic or highly basic case is more conducive to the release of Se from coal in aqueous solution, pH regulation is particularly important in reducing Se release to water and highly acidic or alkaline conditions should be avoided during coal processing. 3.2. Kinetic Studies of Se Release from Coal. The release process can be considered as a reversible reaction with the equilibrium being established in a two-phase condition.25 So, kinetic study was conducted to analyze the Se release from coal in aqueous solution for further detailed study of the release procedure. The release experimental data were modeled by classical kinetic models including the Lagergren first-order kinetic model, pseudo-second-order model, and Elovich model. To evaluate the fitting results, adjusted linear regression (R2) was chosen as the evaluation index. 3.2.1. Lagergren First-Order Kinetic Model. First, a Lagergren first-order kinetic model which is based on the assumption that the release procedure may be controlled by diffusion was applied and given as below.26 dQ = k1(Q e − Q t ) dt
dQ t dt
(2)
k1 t 2.303
(4)
Rearrange and integrate eq 4 for the boundary cases, t = 0 to t = t and Q = 0 to Q = Qt, and then the equation can be shown in another linear form t 1 1 = + t 2 Qt Qe k 2Q e (5)
The above eq 2 was integrated with the boundary conditions of t = 0 to t = t, Q = 0 to Q = Qt, and a linearized form of pseudo-first-order rate kinetics can be obtained as follow: log(Q e − Q t ) = log Q e −
= k(Q e − Q t )2
From this we can see if the release of Se from coal in water complies with the pseudo-second-order model law, as the plot with t/Qt versus t should be a linear line, and from the slope and intercept, we can calculate the value of k2 and Qe. Figure 3
(3)
where Qt and Qe are the release capacity at time t and equilibrium time, respectively, μg/kg; t is time, h; and k1 is the rate constant reflecting the rate of reaction. The relationship between log(Qe − Qt) and t is plotted in Figure 2, and the
Figure 3. Pseudo-second-order model kinetics plots for release procedure in solutions with different initial pH values. Figure 2. First-order kinetics plots for release procedure in solutions with different initial pH values.
is drawn based on this model fitted to the experiments data, and the good linearity of the fitted lines demonstrate that the pseudo-second-order provides a good correlation of data
Table 2. Constant Values of Kinetics Models Lagergren constants pH value
k1 (1/h)
R
3
0.1648 0.9962a 0.2829 0.2509 0.2540 0.1678 0.7013a
0.7073 0.9997 0.9594 0.9847 0.9536 0.9822 0.9986
5 7 9 11
a
2
Pseudo second-order rate constants
Elovich model constants 2
2
k2 (g/mg·h)
Qe (mg/g)
R
R
a
b
3.63 × 10−3
247.52
0.9984
0.9831
129.84
35.63
10−3 10−3 10−3 10−3
188.32 168.06 200.80 269.54
0.9999 0.9999 0.9997 0.9977
0.9842 0.9871 0.9972 0.9965
147.24 118.24 137.53 116.26
13.81 16.29 19.55 44.31
19.4 13.8 8.38 2.23
× × × ×
The constant value of the second-stage of the Lagergren first-order kinetic model. C
DOI: 10.1021/acs.energyfuels.7b03249 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels obtained from the release experiments in solutions with different pH values. The constants of the pseudo-secondorder equation are listed in Table 2. The R2 of the pseudosecond-order kinetics is closer to 1 compared with that of the first-order kinetics model, suggesting the pseudo-second-order kinetics, based on the assumption that the procedure is governed by chemical mechanism and the release rate is up to the number of vacant sites on the surface, provide a better relationship of the data. 3.2.3. Elovich Model. Initially, the Elovich model was a widely applied experiential equation to investigate the mechanisms of those complex reaction mechanisms initially, such as chemisorption of gas in the solid surface.29 Recently, it is also used in studies on adsorption and desorption of elements (such as P and Ce) on soils, as it is suitable for a complicated heterogeneous system. Considering the similarity between a coal−water system and soil−water system, the Elovich model was used to fit our test results. And it can be described linearly as
Q t = a + b ln(t )
Figure 5. Effect of temperature on Se release (with standard error bar).
(6)
an increase not only in release capacity but also in release rate. This may because of the enhancement of the intraparticle diffusion rate of Se. As for the condition of the initial pH of 11, the increase of temperature from 303.15 to 313.15 K makes the release capacity increase by only 26.34 μg/kg. Interestingly, although temperature effects on the release rate and release capacity under different pH conditions are different, the final equilibrium release capacity of Se in different conditions is approximately equal, which indicates that there is a limitation capacity of Se release from coal into water in a conventional environment. 3.4. Enthalpy Analysis of Se Release from Coal. The release enthalpy was also analyzed according to the van't Hoff equation (eq 7) to study the effect of temperature on Se release.31
where a and b are the constants of the Elovich model. The fitting curves are drawn in Figure 4. And the values of R2 are about 0.98, indicating that a multiple mechanism model also
1
ΔH ° = −R
d ln K
d
1 dT
(7)
where ΔH° is the change of enthalpy, kJ/mol; R is the gas constant whose value is 8.314 × 10−3 kJ/(mol·K); T is the absolute temperature, K; and Kd is the distribution constant, mL/g, which can be described as follows:32
Figure 4. Elovich model kinetics plots for release procedure in solutions with different initial pH values.
fits with the Se release from coal, though the value of R2 is a bit smaller than the fitting results of the pseudo-second-order model and not as well as expected. This may be because of both the composition differences between coal and soil and the differences of modes of occurrence of elements in coal and soil. According to the above, the release of Se from coal into water is fitted with several various models, suggesting a multiple mechanism model of the release procedure. Comparatively speaking, the pseudo-second-order model provides the best fitting, and the release of Se from coal may mainly be governed by the chemical reaction. 3.3. Effect of Temperature on Se Release from Coal. The increase in temperature greatly affects the activity of available active surface sites during sorption/desorption.30 However, such an evident function of temperature only happened when the initial pH was 7 in the Se release process from coal into water as shown in Figure 5. As the surrounding temperature increased from 293.15 to 313.15 K, the release capacity of Se increased from 164.92 to 265.37 μg/kg, bringing
Kd =
CS Cw
(8)
in which CS (μg/g) and Cw (μg/mL) is the Se concentration in the solid and in water at equilibrium. As the amount of Se released from coal is relatively small compared with the total amount, CS nearly remains constant during the release tests. Replacing Kd in eq 7 by eq 8 leads to
ΔH ° = −
d ln Cw 1
d RT
(9)
which suggests that the slope of a plot ln Cw versus 1/RT can be used to calculate the value of ΔH°. The plot of ln Cw versus 1 is shown in Figure 6, and the ΔH° values of different tests RT are summarized in Table 3. The good linear relationship between ln Cw and 1/RT is shown in Figure 6, with the value of R2 close to 1. The release enthalpies (ΔH°) of Se from coal are 18.314 and 3.533 kJ/mol D
DOI: 10.1021/acs.energyfuels.7b03249 Energy Fuels XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS The authors greatefully acknowledge the financial support by the Fundamental Research Funds for the Central Universities (2017XKQY036).
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Figure 6. van't Hoff plot of Se release from coal (with standard error bar).
Table 3. Release Enthalpy of Se Release from Coal into Aqueous Solution temp (K)
ΔH° (kJ/mol)
R2
pH
293.15 303.15 313.15
18.314
0.938
11
pH 7
temp (K)
ΔH° (kJ/mol)
R2
293.15 303.15 313.15
3.533
0.987
at pH 7 and 11 respectively. And the positive value of enthalpies demonstrates it is an endothermic reaction. The lower enthalpy at pH 11 suggests that the effect of temperature on the release of Se is relatively slight. This is because the release capacity of Se is close to its maximum even at lower temperatures when the pH value is up to 11.
4. CONCLUSION This paper focused on the release of Se from coal into water; dynamics and thermodynamics analyses methods were applied to study this process. The main conclusions can be summarized as follows: (1) pH, contact time, and temperature have a significant influence on the release of Se from coal. The release capacity of Se increases with the increase of contact time and temperature, and strong acid and alkaline environments enhance the release of Se. (2) The pseudo-second-order model provides the best fit for the resulting data. At the conditions of initial pH value 3 and 11, the release process of Se can be regarded as a two-stage procedure according to the first-order kinetic model. The good fit of the Elovich model also showed this release procedure is a multiple mechanism reaction. (3) Thermodynamics studies showed that the release of Se from coal into an aqueous solution is an endothermic reaction, and the ΔH° of Se release from coal is 18.314 and 3.533 kJ/mol respectively at pH value 7 and 11.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Cheng Liu: 0000-0001-5494-8217 Changchun Zhou: 0000-0002-3913-9718 Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acs.energyfuels.7b03249 Energy Fuels XXXX, XXX, XXX−XXX
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F
DOI: 10.1021/acs.energyfuels.7b03249 Energy Fuels XXXX, XXX, XXX−XXX
