Annually, coal-fired electric power plants produce large volumes of potentially hazardous coal combustion products (CCPs) including fly ash. Since majority of the coal fly ash and other CCPs deposited in dry land fills or wet lagoons, they pose risk of contamination to local environment. In this study, we present results of leaching kinetics for As, Mo, and Se from three acidic fly ash samples collected from coal-fired power plants in the southeastern United States. This study shows that the leachate concentrations of As, Mo, and Se increase over time. Three kinetics equations, pseudo-second order, Elovich, and power-function, are able to adequately describe the experimental leaching kinetics data. Experimental leaching data and modeling results indicate that the rate limiting leaching of As, Mo, and Se is controlled by the diffusional process responsible for transferring these elements from interior to the surface of the particles as well as the dissolution of the fly ash particles. Therefore, it is important to adopt effective containment/treatment schemes to avoid potential and persistent dispersion of trace elements from ash disposal facilities to surrounding environment for a long time.
Coal-fired electric power plants generate large volumes of coal combustion products (CCPs) such as fly ash, bottom ash, boiler slag, flue gas desulfurization (FGD) materials, and various gases. Fly ash is the most voluminous fraction, accounting almost 53.0% (in 2013) and projected to increase to 55.0% (2022) of the total of CCPs produced in the United Sates [
The release of trace elements to the environment is of concern because of their potential toxicity. Several studies show that coal fly ash with elevated concentration of trace elements can readily release these elements into the environment [
Total leachable amount as well as overall leaching behavior of hazardous trace elements from fly ash samples is important for determining the environmental consequences of potential release of fly ash into local environment. A large number of studies on dissolution kinetics have been conducted in the past for several minerals [
Fresh acidic fly ash samples (HA, HB and MA) were collected from three electric power plants located in the southeastern USA. These fly ash samples were the combustion products of the Eastern Bituminous coals. All three fly ash samples were collected dry and were homogenized in the laboratory before using them for kinetics experiments.
The pH of the acidic fly ash samples are reported in the range of slightly acidic to neutral. Details on physical and chemical properties of these fly ash were described elsewhere [
Leaching kinetics of As, Mo, and Se from acidic fly ash samples were conducted using jar leaching and batch leaching experiments. For both schemes of experiments, Barnstead nanopure water (18.2 MΩ) was used as leachant.
Jar leaching experiments for kinetic study were performed using 2 L high density polyethylene (HDPE) bottles at a 1:30 solid:liquid ratio. For these experiments, 60 g of each fly ash was mixed with 1.8 L of nanopure water and agitated on an orbital platform shaker at 200 rpm. About 20 mL of the leachate solution was withdrawn at each sampling event from each experimental bottle at 1, 4, 8, 12, 24, 36, 48, 72, 96, and 120 h. The leachate supernatants were separated by centrifugation at 8500 rpm for 10 minutes and filtration through 0.2 μm syringe filters. After the separation, leachate solutions were acidified to 2% with ultrapure OPTIMA nitric acid and stored in refrigerator until chemical analysis with a Perkin Elmer Optima 3000DV inductively coupled plasma optical emission spectrometer (ICP-OES).
In compliment to the Jar leaching experiment, a separate experimental scheme
Elements | HA | HB | MA | LODa |
---|---|---|---|---|
Al | 21,800 | 14,010 | 9310 | 0.001 |
As | 82 | 167 | 158 | 0.009 |
Ca | 4860 | 7580 | 5730 | 0.015 |
Fe | 19,590 | 18,310 | 12,110 | 0.003 |
K | 3160 | 1690 | 1570 | 0.077 |
Mg | 1890 | 1300 | 1130 | 0.001 |
Mo | 13 | 20 | 12 | 0.002 |
Na | 722 | 460 | 616 | 0.009 |
Se | 7.7 | 23 | 14 | 0.004 |
Si | 7290 | 2180 | 3880 | 0.015 |
aICP-OES limit of detection (LOD) values are given in mg/L.
was also employed to evaluate the leaching kinetics of As, Mo, and Se from acidic fly ash samples with a long-period leaching. A different (1:15) solid: liquid ratio was selected for this series of experiments to investigate whether the initial loading scheme has any effect on leaching kinetics. For each fly ash sample, multiple batch leaching sets were prepared by mixing 3 g fly ash and 45 mL of nanopure water in 50 mL centrifuge tubes. The fly ash-water mixtures were continuously agitated until sample collection. At each sampling time, duplicate aliquots of each fly ash leachate solution were collected for chemical analysis by sacrificing two tubes. The last samples for this series of experiments were collected after a leaching period of 30 weeks. Concentration of As, Mo, Se, and pH from leaching experiment is presented in Supplementary
Leaching of trace elements from fly ash could be explained by assuming that this process is similar to desorption/dissolution of elements from solid surface/ma- terials (Ash et al., 2013). Desorption kinetics of different sorbates from several sorbents including soils, metal oxides, and others have been previously described using zero order, first order, second order, and their derivative equations [
In this study, leaching kinetics of As, Mo, and Se are analyzed using three kinetic equations: pseudo-second order, Elovich, and power-function equations. The attempt of using other kinetic models failed because of their inability to adequately describe the experimental data. Kinetic equations with their linear forms and fitting parameters are presented in
Kinetic equations | linear form | Kinetic parameters | |
---|---|---|---|
Pseudo-second ordera | k, | ||
Elovichb | α, β | ||
Power functionc | a, n |
aHo and McKay (1999) [
leaching kinetics of As were to obey the Elovich equation, the plot of qt vs. ln(t) should yield a linear relation with a slope of (1/β) and an intercept of (1/β) ln(αβ). Similarly, if log transformed kinetic data plotted as log(qt) vs. log(t) result in a linear line, then the leaching kinetic was said to be in compliance with power function equation. Power function rate constant and its order were then obtained from slope (n) and intercept (logα) of the fit. The coefficient of determination (R2) for each model was obtained by using experimental and model derived data. The goodness of model fit was also evaluated by calculating two additional parameters, normalized deviation (ND) and normalized standard deviation (NSD) using Equations (1) and (2), respectively [
where n is number of experimental measurements, qt(exp) is the experimental concentration of element at time t, and qt(model) is the model predicted concentration of element at time t. The smaller the values of ND and NSD, the better is the fit of experimental data for the kinetic model.
The leachate pH and concentrations of As, Mo, and Se mobilized during long- term leaching kinetic experiments are presented in
The early relatively rapid leaching of As, Mo, and Se from these acidic fly ash samples are potentially related to their mobility from fly ash particles’ surface enrichment or association of these elements in the finest fraction of the fly ash. It has been well established in literature that such early rapid mobility of elements
generally associates with fast dissolution of sub-micron sized particles in the sample during mineral-water interaction [
The pseudo-second order, Elovich, and power-function kinetic models were used to describe the leaching behavior of As, Mo, and Se from acidic fly ash samples. Linear plots for these kinetic models for both leaching schemes are shown in
Experiments/fly ash/elements | Model parameters | Goodness of fit | ||||||
---|---|---|---|---|---|---|---|---|
k (kg∙mg−1∙h−1) | h (mg∙kg−1∙h−1) | qe (mg∙kg−1) | R2 | ND | NSD | |||
Jar leaching | HA | As | 0.0015 | 0.080 | 7.25 | 0.9384 | 6.7 | 7.9 |
Mo | 0.0229 | 1.470 | 8.01 | 0.9965 | 3.3 | 4.1 | ||
Se | 0.0178 | 0.119 | 2.58 | 0.9829 | 10.9 | 15.2 | ||
HB | As | 0.0479 | 1.928 | 6.34 | 0.9972 | 7.1 | 12.2 | |
Mo | 0.0004 | 0.165 | 20.55 | 0.9326 | 5.3 | 7.9 | ||
Se | 0.0241 | 0.178 | 2.72 | 0.9548 | 21.6 | 30.8 | ||
MA | As | 0.1210 | 4.984 | 6.42 | 0.9994 | 5.7 | 10.3 | |
Mo | 0.0001 | 0.052 | 13.31 | 0.9230 | 1.7 | 1.9 | ||
Se | 0.0881 | 0.052 | 0.77 | 0.9561 | 25.2 | 32.6 | ||
Long-term leaching | HA | As | 0.392 | 7.55 | 4.39 | 0.9986 | 4.58 | 7.37 |
Mo | 0.358 | 29.42 | 9.06 | 0.9986 | 8.04 | 14.47 | ||
Se | 0.662 | 2.93 | 2.10 | 0.9970 | 10.47 | 17.48 | ||
HB | As | 0.0657 | 0.291 | 2.11 | 0.9534 | 19.02 | 23.28 | |
Mo | 0.0727 | 23.96 | 18.2 | 0.9996 | 4.14 | 6.32 | ||
Se | 0.0459 | 2.30 | 7.08 | 0.9931 | 11.43 | 16.72 | ||
MA | As | 0.0201 | 0.168 | 2.89 | 0.9283 | 10.10 | 12.05 | |
Mo | 0.1029 | 4.145 | 6.35 | 0.9978 | 6.55 | 13.28 | ||
Se | 0.2181 | 0.447 | 1.43 | 0.9850 | 15.19 | 21.06 |
Experiments/fly ash/elements | Model parameters | Goodness of fit | |||||
---|---|---|---|---|---|---|---|
α (mg∙kg−1∙h−1) | β kg∙kg−1 | R2 | ND | NSD | |||
Jar leaching | HA | As | 0.21 | 0.75 | 0.9873 | 9.3 | 16.0 |
Mo | 5.0 | 0.74 | 0.9593 | 8.8 | 14.9 | ||
Se | 0.37 | 2.09 | 0.9425 | 32.1 | 64.2 | ||
HB | As | 18.3 | 1.21 | 0.9620 | 4.1 | 5.2 | |
Mo | 0.81 | 0.46 | 0.8531 | 155.4 | 381.8 | ||
Se | 1.34 | 2.71 | 0.8285 | 15.5 | 19.3 | ||
MA | As | 2.4 × 104 | 2.40 | 0.8334 | 4.1 | 5.6 | |
Mo | 0.09 | 1.22 | 0.9163 | 35.4 | 56.3 | ||
Se | 0.40 | 9.48 | 0.8129 | 17.4 | 22.0 | ||
Long-term leaching | HA | As | 111 | 1.90 | 0.9003 | 6.86 | 8.21 |
Mo | 8.3 × 108 | 2.78 | 0.8589 | 0.86 | 1.24 | ||
Se | 252 | 5.04 | 0.9756 | 2.02 | 2.75 | ||
HB | As | 0.92 | 2.78 | 0.9777 | 35.01 | 58.39 | |
Mo | 175 | 0.40 | 0.9120 | 8.90 | 11.69 | ||
Se | 6.81 | 0.79 | 0.9777 | 9.30 | 16.33 | ||
MA | As | 0.57 | 2.17 | 0.9357 | 44.42 | 89.69 | |
Mo | 12.9 | 0.92 | 0.9686 | 6.47 | 9.06 | ||
Se | 1.38 | 3.96 | 0.9487 | 14.29 | 20.02 |
Experiments/fly ash/elements | Model parameters | Goodness of fit | |||||
---|---|---|---|---|---|---|---|
α (mg∙kg−1) | n (mg∙kg−1∙h−1) | R2 | ND | NSD | |||
Jar leaching | HA | As | 0.12 | 0.76 | 0.9649 | 11.4 | 13.1 |
Mo | 1.89 | 0.34 | 0.8467 | 19.3 | 23.2 | ||
Se | 0.18 | 0.57 | 0.9505 | 18.2 | 19.0 | ||
HB | As | 2.69 | 0.20 | 0.9209 | 7.4 | 8.7 | |
Mo | 0.20 | 0.85 | 0.9943 | 7.7 | 9.4 | ||
Se | 0.71 | 0.24 | 0.9092 | 9.7 | 11.0 | ||
MA | As | 4.55 | 0.08 | 0.7916 | 4.8 | 6.0 | |
Mo | 0.03 | 0.94 | 0.9985 | 2.8 | 3.3 | ||
Se | 0.20 | 0.25 | 0.8608 | 11.7 | 15.4 | ||
Long-term leaching | HA | As | 2.71 | 0.17 | 0.8581 | 9.30 | 10.28 |
Mo | 7.76 | 0.04 | 0.9654 | 0.89 | 1.22 | ||
Se | 1.40 | 0.12 | 0.9604 | 2.87 | 3.71 | ||
HB | As | 0.30 | 0.54 | 0.9756 | 6.76 | 8.01 | |
Mo | 9.88 | 0.21 | 0.8348 | 12.54 | 14.44 | ||
Se | 1.92 | 0.40 | 0.9756 | 7.91 | 9.20 | ||
MA | As | 0.17 | 0.76 | 0.9624 | 15.44 | 18.75 | |
Mo | 2.41 | 0.33 | 0.9052 | 13.28 | 16.25 | ||
Se | 0.39 | 0.39 | 0.9553 | 9.72 | 12.69 |
The pseudo-second order kinetic model appears to fit the leaching data for both leaching schemes for all fly ash samples (
It is important to note that the leaching of any solute (e.g. trace elements) from a solid phase (e.g., fly ash) involves displacement of former from the latter into the leachant. This process is assumed to be consisted of multiple steps including chemical interactions such as dissociation of chemical bonds and transport of slackened trace elements from solid phase to the leachant. Once the ions of the trace elements reach to the particle surface from interior or become loose
at the surface by dissolution of host material, it is assumed that they instantaneously mix with the leachant owing to the consistent agitating environment during experiment. Therefore, the rate limiting leaching kinetics during the experiment is related only to the either gradual dissolution of fly ash particles or diffusional transport from particle interior to the surface. However, in natural environments where a comparable agitating condition is absent, the transport related processes active in the exterior milieu of the particles are also important for rate limiting leaching of trace elements from fly ash. In general, diffusive transport within the solid particles and/or dissolution of particles at the surface, transfer of elements from solid-leachant interface to leachant, and dispersive as well as diffusive transport in the leachant are important rate limiting processes in natural environment.
Similarly, Elovich equation also shows a strong ability to describe the leaching behavior of these trace elements from fly ash samples during batch leaching (
The power-function equation is also able to describe most of the leaching data for both leaching schemes with few exceptions (
The good agreement of experimental data with the model equations indicate the presence of one or multiple rate limiting mechanisms controlling the release of trace elements from fly ash samples [
The leaching tests of three acidic fly ash samples collected from coal-fired power plants in the southeastern United States show the increasing leachate concentrations of As, Mo, and Se with time. The jar leaching experiments show that the leachate concentrations of these elements are relatively low during the early phase of leaching; however, their concentrations increase with increasing time. Similar leaching trends are observed during long-term leaching experiments.
Kinetic leaching of As, Mo, and Se from acidic fly ash samples can be described by pseudo-second order, Elovich, and power function kinetic equations. Although all three kinetic equations are able to fit experimental data, relatively, pseudo-second order model represents the experimental data strongly than the other two models. The experimental as well as modeling results indicate that the rate limiting release of As, Mo, and Se from fly ash samples is largely controlled by dissolution of fly ash particles. Such leaching behavior could make fly ash a persistent source of contaminates in the environment should a catastrophic event akin the TVA fly ash spill occur either from operational or old fly ash disposal facility. Therefore, it is desirable to adopt containment as well as treatment plans for fly ash deposited in present/past fly ash disposal facilities.
We would like to thank Dr. Z. Yue and Ms. E.Y. Graham for their help during laboratory experiments and analytical work. Funding for this research was partially provided by Hooks Fund (Department of Geological Sciences), Geological Society of America (GSA), Gulf Coast Association of Geological Societies (GCAGS), and Graduate Student Association (GSA) at UA. The Graduate School at UA also provided one year research fellowship to GN to conduct a part of this research.
The authors declare no competing financial interest.
Neupane, G., Donahoe, R.J., Bhattacharyya, S. and Dhakal, P. (2017) Leaching Kinetics of As, Mo, and Se from Acidic Coal Fly Ash Samples. Journal of Water Resource and Protection, 9, 890-907. https://doi.org/10.4236/jwarp.2017.98060
ID | Time (hours) | pH | Concentrations (mg/kg) | ||
---|---|---|---|---|---|
As | Mo | Se | |||
HA | 1 | 7.19 | 0.00 | 1.22 | 0.14 |
4 | 6.83 | 0.00 | 3.74 | 0.47 | |
8 | 6.74 | 0.52 | 4.70 | 0.50 | |
12 | 6.71 | 0.79 | 5.44 | 0.87 | |
24 | 6.85 | 1.47 | 6.27 | 1.49 | |
36 | 7.05 | 2.36 | 7.12 | 1.64 | |
48 | 7.09 | 2.74 | 7.47 | 1.88 | |
72 | 7.12 | 3.32 | 7.42 | 1.87 | |
96 | 7.11 | 3.64 | 7.18 | 2.03 | |
120 | 7.15 | 3.97 | 7.91 | 2.25 | |
HB | 1 | 4.53 | 2.30 | 0.19 | 0.87 |
4 | 4.86 | 3.78 | 0.64 | 0.90 | |
8 | 5.17 | 4.36 | 1.05 | 1.13 | |
12 | 5.25 | 4.65 | 1.70 | 1.12 | |
24 | 5.76 | 5.40 | 3.35 | 1.38 | |
36 | 5.78 | 5.81 | 4.82 | 1.65 | |
48 | 5.97 | 5.76 | 5.83 | 1.71 | |
72 | 6.01 | 6.43 | 7.62 | 2.13 | |
96 | 6.08 | 5.90 | 9.07 | 2.32 | |
120 | 6.15 | 6.25 | 9.84 | 2.66 | |
MA | 1 | 4.91 | 4.02 | 0.00 | 0.22 |
4 | 5.25 | 5.44 | 0.00 | 0.26 | |
8 | 5.45 | 5.72 | 0.20 | 0.43 | |
12 | 5.40 | 6.02 | 0.30 | 0.27 | |
24 | 5.68 | 6.01 | 0.58 | 0.39 | |
36 | 5.83 | 6.05 | 0.84 | 0.48 | |
48 | 5.89 | 6.20 | 1.14 | 0.51 | |
72 | 5.99 | 6.18 | 1.67 | 0.58 | |
96 | 6.04 | 6.27 | 2.05 | 0.70 | |
120 | 6.06 | 6.46 | 2.45 | 0.73 |
ID | Time (week) | pH | Concentrations (mg/kg) | ||
---|---|---|---|---|---|
As | Mo | Se | |||
HA | 0.29 | 6.64 | 1.86 | 7.38 | 1.13 |
0.29 | 6.77 | 1.85 | 7.28 | 1.14 | |
0.57 | 6.85 | 2.27 | 7.49 | 1.29 | |
0.57 | 6.92 | 2.29 | 7.63 | 1.28 | |
1.00 | 7.14 | 2.86 | 7.87 | 1.51 | |
1.00 | 7.15 | 2.83 | 7.78 | 1.38 | |
2.00 | 7.11 | 3.25 | 7.93 | 1.56 | |
2.00 | 7.17 | 3.36 | 7.94 | 1.55 | |
3.00 | 7.15 | 3.66 | 8.19 | 1.70 | |
3.00 | 7.17 | 3.79 | 8.18 | 1.74 | |
4.29 | 7.03 | 3.97 | 8.30 | 1.72 | |
4.29 | 7.12 | 4.08 | 8.23 | 1.71 | |
6.00 | 7.07 | 4.09 | 8.45 | 1.80 | |
6.00 | 7.08 | 3.90 | 8.31 | 1.76 | |
10.57 | 6.96 | 4.14 | 8.61 | 1.88 | |
10.57 | 7.06 | 3.93 | 8.65 | 1.89 | |
18.14 | 6.94 | 4.15 | 8.76 | 2.01 | |
18.14 | 7.10 | 4.06 | 8.78 | 2.00 | |
24.57 | 6.96 | 4.48 | 8.75 | 1.98 | |
24.57 | 7.06 | 4.28 | 8.85 | 1.96 | |
30.57 | 7.08 | 4.37 | 9.41 | 2.17 | |
30.57 | 7.14 | 4.27 | 8.93 | 2.10 |
ID | Time (week) | pH | Concentrations (mg/kg) | ||
---|---|---|---|---|---|
As | Mo | Se | |||
HB | 0.29 | 6.21 | 0.13 | 5.83 | 1.06 |
0.29 | 6.12 | 0.17 | 5.86 | 1.06 | |
0.57 | 6.35 | 0.24 | 7.88 | 1.46 | |
0.57 | 6.33 | 0.24 | 7.74 | 1.36 | |
1.00 | 6.57 | 0.33 | 11.15 | 1.90 | |
1.00 | 6.55 | 0.34 | 11.80 | 2.00 | |
2.00 | 6.62 | 0.44 | 13.98 | 2.84 | |
2.00 | 6.64 | 0.49 | 14.26 | 2.91 | |
3.00 | 6.68 | 0.56 | 13.31 | 2.88 | |
3.00 | 6.68 | 0.57 | 14.50 | 3.13 | |
4.29 | 6.76 | 0.62 | 14.70 | 3.40 |
4.29 | 6.83 | 0.68 | 15.52 | 4.05 | |
---|---|---|---|---|---|
6.00 | 6.87 | 0.72 | 16.04 | 4.45 | |
6.00 | 6.90 | 0.77 | 16.22 | 4.44 | |
10.57 | 6.84 | 0.98 | 16.27 | 5.01 | |
10.57 | 6.84 | 1.09 | 16.52 | 5.20 | |
18.14 | 6.87 | 1.32 | 17.19 | 5.84 | |
18.14 | 6.96 | 1.43 | 17.48 | 6.15 | |
24.57 | 6.92 | 1.73 | 17.84 | 6.40 | |
24.57 | 6.98 | 1.58 | 17.66 | 6.16 | |
30.57 | 7.11 | 1.87 | 17.80 | 6.59 | |
30.57 | 7.11 | 1.78 | 17.70 | 6.58 |
ID | Time (week) | pH | Concentrations (mg/kg) | ||
---|---|---|---|---|---|
As | Mo | Se | |||
MA | 0.29 | 6.08 | 0.00 | 1.15 | 0.18 |
0.29 | 6.06 | 0.00 | 1.30 | 0.24 | |
0.57 | 6.25 | 0.08 | 2.04 | 0.33 | |
0.57 | 6.23 | 0.10 | 2.04 | 0.31 | |
1.00 | 6.46 | 0.13 | 2.52 | 0.52 | |
1.00 | 6.38 | 0.16 | 2.07 | 0.31 | |
2.00 | 6.51 | 0.29 | 3.83 | 0.55 | |
2.00 | 6.48 | 0.26 | 3.68 | 0.56 | |
3.00 | 6.58 | 0.39 | 4.17 | 0.63 | |
3.00 | 6.55 | 0.50 | 3.75 | 0.54 | |
4.29 | 6.76 | 0.72 | 4.71 | 0.71 | |
4.29 | 6.77 | 0.74 | 4.20 | 0.63 | |
6.00 | 6.84 | 0.85 | 4.93 | 0.83 | |
6.00 | 6.83 | 0.82 | 5.10 | 0.94 | |
10.57 | 6.78 | 1.02 | 5.31 | 1.01 | |
10.57 | 6.76 | 1.08 | 5.56 | 1.08 | |
18.14 | 6.83 | 1.47 | 6.10 | 1.29 | |
18.14 | 6.78 | 1.35 | 5.79 | 1.17 | |
24.57 | 6.85 | 1.56 | 6.01 | 1.24 | |
24.57 | 6.82 | 1.61 | 6.14 | 1.34 | |
30.57 | 6.90 | 1.97 | 6.14 | 1.38 | |
30.57 | 6.88 | 1.98 | 5.82 | 1.24 |