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Published online 7 May 2007
Published in J Environ Qual 36:874-886 (2007)
DOI: 10.2134/jeq2006.0327
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Mechanisms Controlling the Leaching Kinetics of Fixated Flue Gas Desulfurization (FGD) Material under Neutral and Acidic Conditions

Chin-Min Chenga, Harold W. Walkera,* and Jerry M. Bighamb

a Dep. of Civil and Environ. Eng. and Geo. Sci., The Ohio State Univ., 470 Hitchcock Hall, 2070 Neil Ave., Columbus, OH 43210
b School of Environ. and Nat. Res., The Ohio State Univ., 210 Kottman Hall, 2021 Coffey Rd., Columbus, OH 43210. C.-M. Cheng, current address, Institute for Combustion Science and Environmental Technology, Western Kentucky Univ., 2413 Nashville Rd., Bowling Green, KY 42101

* Corresponding author (walker.455{at}osu.edu)

Received for publication August 21, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A number of agricultural and engineering uses for fixated flue gas desulfurization (FGD) material exist; however, the potential for leaching of hazardous elements has limited widespread application and the processes controlling the leaching of this material are poorly understood. In this study, a flow-through rotating-disk system was applied to elucidate the relative importance of bulk diffusion, pore diffusion, and surface chemical reaction in controlling the leaching of fixated FGD material under pH conditions ranging from 2.2 to 6.8. Changing the hydrodynamics in the rotating disk system did not affect the leaching kinetics at both pH 2.2 and 6.8, indicating that bulk diffusion was not the kinetic-limiting step. Application of the shrinking core model (SCM) to the data suggested a surface reaction-controlled mechanism, rather than a pore diffusion mechanism. The leaching of fixated FGD material increased with decreasing pH, suggesting it can be described by a combination of an intrinsic hydration reaction and a proton-promoted dissolution reaction. X-ray diffraction (XRD) and elemental composition analyses before and after leaching suggests that for most elements a number of solid phases controlled the leaching process.

Abbreviations: BET, Brunauer, Emmett, Teller • FA, fly ash • FC, filter cake • FGD, flue gas desulfurization • GFAAS, graphite furnace atomic absorption spectrometry • ICP–AES, inductively coupled plasma atomic emission spectrometry • ICP–MS, inductively coupled plasma mass spectrometry • SCM, shrinking core model • SEM, scanning electron microscopy • XRD, X-ray diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
APPROXIMATELY 30 million metric tons of flue gas desulfurization (FGD) material is produced every year in the United States (American Coal Ash Association, 2004), and this production is expected to grow on implementation of Phase II of the Clean Air Act Amendments. FGD material is often treated as a waste product and disposed of in landfills after being fixated with fly ash and quicklime. However, a number of beneficial applications in agriculture and engineering have been suggested, including use as a soil amendment, synthetic wallboard gypsum, and low permeability liner material (Stehouwer et al., 1995; Payette et al., 1997; Crews and Dick, 1998; Butalia and Wolfe, 1999; Chen et al., 2001; Lamminen et al., 2001; Punshon et al., 2001).

Although the use of FGD material reduces consumption of raw materials (gypsum, lime, and clay), concern about the leaching properties of this material has limited some applications. For example, in studies using fixated FGD material for neutralizing acid soils, improved plant growth was observed. However, the application of fixated FGD material also increased the metalloid (i.e., As, Se, Mo, and B) and metal (K, Be, and Mg) concentrations in plant tissue and soil leachates (Crews and Dick, 1998; Chen et al., 2001; Punshon et al., 2001). In engineering applications, fixated FGD material can effectively neutralize minespoil (Stehouwer et al., 1995), provide mechanical strength (Payette et al., 1997), decrease permeability (Butalia and Wolfe, 1999), and immobilize hazardous wastes containing heavy metals (Pb and Cd) or oxyanions (i.e., As and Se) (Solem-Tishmack et al., 1995). But the release of major (Ca and S) and trace elements (As, B, Cd, Cu, Ni, Pb, and Se) in neutral or acidic aqueous environments was also found (Stehouwer et al., 1995; Lamminen et al., 2001). For example, the dissolution of mineral components, especially ettringite (Myneni et al., 1997; Laperche and Traina, 1999), can release immobilized contaminants and affect the structural integrity of fixated FGD material (Laperche and Traina, 1999).

Understanding the leaching characteristics of fixated FGD material is, therefore, important with respect to the use, as well as disposal of this material. While numerous studies have examined the leaching of fly ash, comparatively much fewer studies have studied the leaching properties of FGD material (Zhou and Dayal, 1990; Ibanez et al., 1998; Laperche and Traina, 1999; Kost et al., 2005). In most of these studies, leaching experiments were performed at equilibrium or near-equilibrium conditions. Leaching experiments conducted at near-equilibrium conditions provide useful information about the leachate quality with respect to regulatory standards (Zhou and Dayal, 1990; Kost et al., 2005) as well as insight about the controlling solids (Zhou and Dayal, 1990). However, these types of experiments yield less information about the fundamentals of the leaching process. For example, the leaching process of fixated FGD material is a heterogeneous non-catalytic reaction combining a series of processes, i.e., mass transport, dissolution, adsorption, complexation, and precipitation. As a result, the leaching behavior is affected by the chemical and hydrodynamic conditions of the leaching environment, such as pH, presence of organic ligands, state of equilibrium, and agitation method. Therefore, there is a need to investigate the leaching process from a mechanistic point of view.

In this investigation we studied kinetic leaching of fixated FGD material to examine the relative importance of mass transport and surface chemical processes. To this end, a continuous-flow rotating disk system was used. This system is advantageous because (i) the flux of reagents (protons) to the FGD by-product surface can be conveniently controlled by changing the rotation speed of the disk; (ii) the transport-controlled rates of reactants and products to and from the disk surface can be calculated based on an exact solution to the Navier-Stokes equation; and (iii) a constant leaching condition, including pH, temperature, and state of equilibrium, can be maintained during the leaching process. Because pH is a major variable controlling the leachability of major, minor, and trace elements contained in fly ash, we examined the leaching kinetics of fixated FGD material as a function of pH under acidic conditions. X-ray diffraction (XRD) and elemental composition analyses are used to evaluate the solid phases controlling the leaching of various elements at steady-state.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Fixated Flue Gas Desulfurization Material
The fixated FGD material used in this study was mixed in the laboratory by combining filter cake (FC) and fly ash (FA) from a coal-fired power plant at a FA/FC ratio of 1.5:1 (dry weight basis). An additional 6% by weight of quicklime (Carmeuse Lime, IL) and deionized water (Millipore, MA) were added to produce a final mixture with 30% moisture content. Fixated FGD material with the above composition corresponds closely to material used in engineering applications (Butalia and Wolfe, 1999). The mixture was then compacted in the shape of a disk with diameter of 3.18 cm and thickness of 0.8 cm, and then cured in a 100% humidity chamber. After curing for 28 d, the fixated FGD material disk was oven-dried at 60°C overnight and then stored in a vacuumed desiccator containing diphosphorus pentoxide (P2O5, Fisher Scientific, NJ) to terminate the curing process.

Chemical Composition
A complete elemental analysis of fixated FGD material was accomplished by microwave digestion according to EPA method 3052. About 300 mg of fixated FGD material was first digested in a 20-mL solution which contained 10 mL of deionized water, 6 mL of concentrated nitric acid, 2 mL of concentrated hydrochloric acid, and 2 mL of concentrated hydrofluoric acid. Then, the sample was heated a second time with an additional 20 mL of 30% boric acid. Coal fly ash, 1633b, provided by the National Institute of Standards and Technology, was digested along with fixated FGD material samples for analytical quality control (A standard fixated FGD material was not available). All acids used in this study were trace metal grade and were obtained from Fisher Scientific.

Mineralogical Characterization
The mineral phases and morphology of the fixated FGD material disks were determined using XRD and scanning electronic microscopy (SEM). Randomly oriented, back-filled sample mounts were prepared for XRD analysis. XRD analysis was conducted with a Philips X-ray diffractometer (Philips Analytical, Natick, MA) using CuKa radiation at 35kV and 20mA. Step-scanned data was collected from 6 to 60° 2{theta} with a fixed time of 3 s per 0.05° 2{theta}. All data were analyzed using semi-quantitative data reduction software (WinJade, version 2.0) from Materials Data Inc. (Livermore, CA). A scanning electron microscope (Philips XL-30 or JEOL JSM-A20) was used to observe changes in the morphology of the surface on leaching. Samples were mounted on stainless steel stubs using double-stick carbon tape and then coated with C or Au.

Quantitative mineral characterization of fixated FGD material was conducted by a sequential four-step selective dissolution procedure developed by Laperche and Bigham (2002). The major mineral forms examined using this procedure were hannebachite, gypsum, ettringite, hematite, glass, mullite, and magnetite. Briefly, the amounts of hannebachite and gypsum were determined by thermogravimetric analysis with a Seiko SS5000 instrument (Seiko Inc., Japan). The amount of ettringite was calculated by subtracting the amounts of hannebachite and gypsum from the soluble fraction of fixated FGD material. The soluble fraction (consisting of hannebachite, gypsum, and ettringite) was determined by oxidizing fixated FGD material with H2O2 in a Na acetate-acetic acid buffer followed by dialysis. The quantity of hematite ({alpha}-Fe2O3) was determined by the citrate–bicarbonate–dithionite method (Laperche and Bigham, 2002). After the citrate–bicarbonate–dithionite step, the remaining solid was further etched with 1% HF solution for 16 h to determine the amounts of magnetite and glass. Microwave-assisted digestion was used on the remaining solid to determine the amounts of mullite and quartz by analyzing the concentrations of Al and Si in the extract. To assure experimental quality, each dissolution step was conducted at least in duplicate. Mass recoveries were calculated by comparing the mass of a particular element obtained from the selective dissolution procedure to the mass of an element determined by microwave-assisted digestion (USEPA Method 3052). Mass recoveries between 71 and 103% were observed for all elements, except for Cr, Cu, and Zn, which had recoveries of 68, 42, and 58%, respectively.

Leaching Experiments
The continuous-flow rotating disk system used to examine leaching kinetics is shown in Fig. 1. In this system, a disk-shaped fixated FGD material sample was attached to a Teflon-lined rod (Fisher Scientific, NJ) with a tailor-made acrylic sample holder. The sample holder allowed only one side of the disk surface to be exposed to the leaching solution. The distance from the disk surface to the bottom of the vessel (Nalgene, NY) was kept constant at 8 cm. The rotation speed of the disk was controlled by a motorized mixer (Series 20, Barnant, IL). Temperature was controlled by a thermo-regulated water bath (Thermo Forma, OH) at 25.0 ± 0.1°C. The ionic strength of the leaching solution was controlled by 0.01 M sodium nitrate. The pH in the reactor was adjusted by a pH-stat autotitrator (PHM290 pH-Stat Controller, Radiometer Analytical, France) with trace-metal grade nitric acid.


Figure 1
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  		Fig. 1. Schematic diagram of the rotating disk system.

 
Five different acidic leaching conditions ranging from pH 2.2 to 6.8 were tested. A rotation speed of 60 rpm was applied to each acidic leaching condition. Other rotation speeds were tested at pH 2.2 and 5.0. The initial surface roughness of the fixated FGD material disk was kept constant by polishing the surface with 320 grit size waterproof silicon carbide paper (3M, MN) before every experiment. After polishing, disks were rinsed with deionized water several times and submerged into a 250-mL high density polyethylene beaker containing deionized water for at least 30 min. According to Cornet et al. (1969), the hydrodynamics near the disk surface are laminar for the surface roughness and rotation speeds in this study. Solution samples were collected periodically until the leaching process reached steady state. Steady state was defined as the time after which the relative standard deviation of the last five successive samples, equivalent to 24 h of leaching, was less than 8%. In selected leaching experiments at pH 5.0 and pH 2.2, both filtered (0.45 µm syringe filters, Millipore, MA) and unfiltered samples were analyzed and no concentration difference was observed. All other samples were unfiltered.

After reaching steady state, fixated FGD material disks were carefully submerged into 1000 mL deionized ultra-purified water for 10 min and then oven-dried at 60°C. The specific surface area of each leached fixated FGD material disk was characterized using a FlowSorb 2300II (Micromeritics, GA) system using the Brunauer, Emmett, Teller (BET) equation. A tailor-made monolith holder was constructed that allowed for surface area analysis without breaking disk samples.

The experimentally determined steady-state leaching rate (Ri) (µmol m–2 min–1) of element i was calculated as:

Formula 1[1]
where Formula 1i, eff is the mean concentration of the last four successive samples collected after steady-state was reached (µmol L–1), Formula 1 is the average flow rate during collection of the last four samples (L min–1), and A is total surface area of fixated FGD material after leaching (cm2). The error in the calculated rate was estimated by the Gaussian error propagation method (Barrante, 1974; Cama et al., 2000).

Analytical Methods
Inductively coupled plasma–atomic emission spectrometry (ICP–AES, Vista Pro, Varian Inc., Australia) was used to analyze the levels of Al, Ba, Ca, Cu, Cr, Fe, K, Mg, Mn, Ni, Pb, Si, and Zn in solutions collected from both mineralogical characterization and leaching tests. To correct the matrix effect caused by the high electrolyte content of the solution, the technique of standard additions was used. High resolution inductively coupled plasma–mass spectroscopy (ICP–MS, ThermoFinnigan Element 2, Thermo Electron Co., CA) or graphite furnace atomic adsorption spectroscopy (GFAAS, Varian 880Z, Varian Inc., Australia) was used to analyze As and Se. For both ICP–AES and GFAAS analyses, the continuing calibration verification test was used to ensure the validity of the calibration throughout the analysis and was performed every 9 analytical samples. An internal standard was applied for ICP–MS measurements. In select experiments, the redox potential was monitored throughout the leaching experiments by a ThermoOrion EA940 multi-channel bench top meter with a ThermoOrion 9678BNWP Redox/ORP Electrode.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Chemical and Mineralogical Characterization of Fixated Flue Gas Desulfurization Material
Initial analyses focused on characterizing the fixated FGD material. The elemental composition of the fixated FGD material and its ingredients; i.e., filter cake, fly ash, and quicklime, are listed in Table 1. The morphology and mineral phases of fixated FGD material are shown in Fig. 2. Flake-shaped hannebachite (CaSO3·0.5H2O) was the major mineral phase in filter cake (Fig. 2A-I and-II). Spherical- or irregular-shaped particles containing mullite (Al6Si2O13), magnetite (Fe3O4), hematite (Fe2O3), quartz (SiO2), and amorphous glass were the main components in fly ash (Fig. 2B-I and-II). In addition to lime, portlandite was also found in quicklime (Fig. 2C-I and-II). After curing of fixated FGD material, a secondary mineral form, ettringite (Ca6Al2(SO4)3(OH)12·26H2O), was found along with the mineral phases identified in fly ash and filter cake (Fig. 2D-I and-II). The needle-shaped ettringite formed a net-like structure between spherical fly ash particles and flake-shaped hannebachite. Lime was not detected in cured fixated FGD material, perhaps due to dilution in the presence of other mineral forms. Also, formation of ettringite occurs at a pH higher than 10 (Myneni et al., 1997), suggesting lime dissolution occurred to reach this pH.


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  		Table 1. Selected element composition of lime, fly ash (FA), filter cake (FC), and fixated FGD material.

 

Figure 2
Figure 2
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  		Fig. 2. (I) SEM micrographs and (II) X-ray diffraction patterns of (A) filter cake; (B) fly ash; (C) lime; (D) 28 d-cured fixated FGD material before leaching; and (E) fixated FGD material after leaching at pH2.2 for 240 h. P = portlandite; L = lime; M = mullite; Q = quartz; He = hematite; Ma = magnetite; H = hannebachite; E = ettringite.

 
Based on the selective dissolution analysis, the amounts of hannebachite (290 g kg–1), ettringite, (150 g kg–1), magnetite (46 g kg–1), hematite (71 g kg–1), glass (164 g kg–1), mullite (70 g kg–1), quartz (123 g kg–1), and gypsum (CaSO4·2H2O, 9.2 g kg–1) were quantified (Table 2). As can be seen in Table 2, the mineral composition results obtained from this study are comparable to Laperche and Bigham (2002) in their study of an FGD material (1:1 mixture of FA and FC with 5% added lime) obtained from the same power plant. Trace elements present in fixated FGD material originated mainly from fly ash, except for Mo and Sn (Table 1). Based on the composition of fly ash, filter cake, and lime, over 90% of As, Se, Cr, Cu, Ba, Mn, Mo, and Zn in fixated FGD material were contributed by the fly ash.


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  		Table 2. Mineralogical composition of fixated flue gas desulfurization (FGD) material (g kg–1).

 
Kinetic Leaching Experiments
Figure 3 presents the concentration of selected elements as a function of time at pH 2.2 and a rotation speed of 60 rpm. As can be seen, the leaching kinetics of different elements exhibited a very similar pattern: a rapid release at the early stage of the leaching process followed by an exponential decrease. After about 150 h, all leaching rates reached steady-state. Similar qualitative leaching profiles were found for other detectable elements (B, Ba, K, and Mn) and at all other leaching conditions.


Figure 3
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  		Fig. 3. Concentration profiles of selected elements as a function of time at pH 2.2 and angular velocity of 60 rpm.

 
A number of processes may explain the early-stage rapid release (about first 25 h), including (i) the presence and rapid dissolution or release of ultra-fine particles created naturally or from sample preparation (Schott et al., 1981; Eggleston et al., 1989); (ii) a higher density of more active sites on the initial surface than on the surface at steady-state (Wehrli, 1989); (iii) the presence, and subsequent depletion, of more soluble mineral phases; and (iv) formation of a remineralization zone between the outer layer and the unreacted core of the sample acting as a sink for leached elements (Islam et al., 2004a, 2004b). The relatively constant stoichiometric ratios of Ca to S over a wide range of leaching conditions (Fig. 4a) indicated the mineral phases controlling the release of Ca and S were constant over the leaching period. In the case of Al and Si, the slow increase of Al to Si (Fig. 4b) likely reflected a transition from initial incongruent dissolution to congruent dissolution from the same mineral phase (Lerman, 1979; Stumm, 1992), because no mineral containing Al and Si was depleted during the leaching process (Fig. 2). Based on the SEM images, no remineralization zone was formed during leaching (Fig. 2E-I). Therefore, processes (iii) and (iv) are unlikely. However, additional work is needed to better elucidate the relative importance of the different mechanisms outlined above.


Figure 4
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  		Fig. 4. Stoichiometric ratio profiles of (a) Ca to S and (b) Al to Si during various leaching experiments.

 
After prolonged leaching, the absence of connecting mineral structures (i.e., hannebachite and ettringite) on the bottom of the image shown in Fig. 2E-I created a more porous section (leached layer) compared with the section near the top of the image (unreacted core). The physical appearance of the leached layer was darker than the unreacted core, due to the darker color of the remaining fly ash particles after hannebachite, ettringite, and gypsum were depleted. Solid samples from the leached layer at pH 2.2 and pH 2.9 were collected by carefully scratching the darkened surface with a plastic spatula. The elemental composition of these solids is shown in Table 1 and indicates that the levels of Al, Ca, S, Fe, Mg, and Si were very similar to levels in fly ash. The XRD diffraction pattern (Fig. 2E-II) of the leached solid was also very similar to that observed for fly ash (Fig. 2C-II). However, the concentrations of all trace elements, except for Sr, in the leached layer were less than in fly ash. No detectable Se was observed in the leached layer.

The mineral composition of the leached layer was quantitatively determined by the selective dissolution method and the results can be seen in Table 2. The results shown in the "Observed" column were determined by the selective dissolution method and show that the leached layer was enriched in hematite, magnetite, glass, quartz, and mullite compared with the mineral composition of the FGD material before leaching. In fact, the levels of these minerals were similar to levels expected if all the gypsum, ettringite, and hannebachite were dissolved, as shown in the "calculated" column of Table 2.

Mechanisms Controlling Leaching Kinetics at Steady State
The observation of a porous leached layer on the disk surface (Fig. 2E-I) suggests the leaching process at steady state can be conceptualized by the following sequential steps: (i) diffusion of solutes through a boundary layer created by the rotating motion of the disk; (ii) diffusion of solutes through the depleted porous layer; (iii) adsorption of solutes onto the wall of pore solids; (iv) surface reaction at specific active sites; (v) detachment of the product from the surface; (vi) diffusion of the product through the porous interior layer to the external surface; and (vii) diffusion of the products through the external boundary layer into the bulk solution. The overall leaching kinetics are determined by the rate-limiting process, which can either be bulk diffusion (steps i and vii), pore diffusion (steps ii and vi), or surface chemical reaction (steps iii to v).

In the case of a bulk diffusion controlled mechanism, at steady state, the leaching rate equals the mass transport rate of reactant r or the product (element i) through the diffusion boundary layer on the solid surface. For a rotating disk, Levich (1962) determined that

Formula 2[2]
where Ri is the leaching rate of element (mol L–1 min–1); Ci is the concentration of element i in the bulk solution (mol L–1); J is the mass flux of element i or reactant r (mol dm–2 sec–1); S is the surface area of the disk exposed to the leaching solution (dm2); D is the diffusion coefficients of element i or reactant r in the diffusion boundary layer (dm2 sec–1); and {sigma} is the thickness of the diffusion boundary layer (dm).

A change in hydrodynamic condition varies the thickness of the diffusion boundary layer and thus alters the bulk diffusion process. In the rotating disk system, the thickness of the diffusion boundary layer ({sigma}) is inversely proportional to the square root of the rotation speed, which can be written as (Levich, 1962):

Formula 3[3]
where {upsilon} is kinematic viscosity (dm2 sec–1) and {omega} is angular velocity (rad sec–1). The mass flux of reactant or product to or from the disk surface, J, can be written as (Gregory and Riddiford, 1956; Newman, 1966):

Formula 4[4]
where Sc is the Schmidt number (i.e., {upsilon}/D); Cbulk and Cdisk are the concentrations of reactant r or element i (mol L–1) in the bulk solution and on the disk surface, respectively. When the leaching kinetics are controlled by bulk diffusion, the leaching rate, Ri, is equal to the transport rate of reactant r or element i and is proportional to the square root of the angular velocity with constant concentration difference across the diffusion boundary layer, Cbulk – Cdisk.

To examine whether the leaching kinetics were controlled by a bulk diffusion mechanism, the rotation speed was varied from 6 to 120 rpm at both pH 2.2 and pH 5.0. Figure 5 shows the normalized leaching rates of major elements at pH 2.2 as a function of the square root of the disk rotation speed, {omega}1/2. In this figure, leaching rates were normalized by the leaching rate observed at an angular velocity of 6 rpm, the lowest speed utilized in these experiments. The solid and dashed reference lines show the relationship between the rate and {omega}1/2 when dissolution is purely bulk-diffusion-controlled and reaction-pore-diffusion-controlled, respectively. As can been seen, the increase in rotation speed did not increase the leaching rate of selected elements at pH 2.2. Leaching rates were also independent of {omega}1/2 at pH 5.0 (data not shown). These observations suggest bulk diffusion was not the controlling leaching mechanism.


Figure 5
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  		Fig. 5. Steady-state leaching rates as a function of rotating speed at pH2.2. Ca (•); S ({circ}); Al ({blacktriangledown}); Si ({nabla}); Mg ({blacksquare}); Fe ({square}); As ({diamondsuit}); Se ({lozenge}).

 
To distinguish between pore diffusion and surface chemical reaction mechanisms, the shrinking core model (SCM) was applied. The SCM, first developed by Yagi and Kunii (1955), is applicable when the porosity of the unreacted solid is very small so that the solid is practically impervious to the fluid reactant. The permeability of fixated FGD material similar to that examined here is 3 x 10–7 cm sec–1 (Butalia and Wolfe, 1999). As the reaction proceeds, a porous layer forms between the un-reacted core and the diffusion boundary layer, similar to what was visually observed by SEM following leaching in this study.

For a flat plate, the development of the leaching layer can be expressed as a function of leaching time as:

Formula 5[5]
where t is leaching period (hr); n = 1 for a surface reaction mechanism and n = 2 for a pore diffusion mechanism; XB is the fraction of an element leached (g g–1), which is calculated from

Formula 6[6]
where Mi,total is the total mass of element i in the specimen (g), Mi,leached is the cumulative mass of element i leached during the time period for regression. Ci,d, qd, and td are the concentration of element i (µmol L–1), flow rate (mL min–1), and extent of the leaching process (hr), respectively, of the "d" sampling event. Accordingly, Ci,d–1, qd–1, and td–1 are the concentrations of element i, flow rate, and extent of the leaching process, respectively, of the previous sampling event. {tau} in Eq. [6] is the time required for complete leaching, which can be determined by

Formula 7[7]

Formula 8[8]
where {rho}B is molar density of element b in the solid (mol m–3); L is the thickness of the disk (m); b is the stoichiometric coefficient of the chemical reaction; k is the first-order rate constant; CH+ is the concentration of protons on the surface; {Delta}C is the concentration difference of protons across the leached layer, and De is the effective diffusion coefficient which takes into account porosity and tortuosity. A detailed derivation of the model can be found in Levenspiel (1999). By taking the natural logarithm of both sides, Eq. [5] becomes:

Formula 9[9]
The value of n can be obtained from regression analysis.

Table 3 lists the regression results showing a linear relationship (n {approx} 1) between, ln XB, and ln t for every selected element and demonstrates a chemical-controlled leaching mechanism. The fraction of an element leached, XB, was calculated from the leaching data collected after steady state was reached. Therefore, the kinetics of the leaching process of fixated FGD material should be described by a rate law reflecting surface chemistry instead of an internal pore diffusion process. In a previous study, Ibanez et al. (1998) fitted sulfate leaching results with a semi-empirical model proposed by Côté et al. (1986) and concluded that the leaching of a cement-amended, stabilized FGD material was diffusion controlled. However, diffusion was likely more significant in controlling the leaching of the cement-amended samples tested by Ibanez et al. (1998) due to greater formation of crystalline calcium sulfoaluminate hydrates in the forms of ettringite and monosulfate (Gougar et al., 1996), resulting in lower porosity and greater tortuosity. In addition, the chemical potential was not constant during the leaching tests conducted by Ibanez et al. (1998) resulting in more complex interactions and increasing the difficulty in interpreting a kinetic mechanism.


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  		Table 3. Regression results form the shrinking core model.

 
Effect of Proton Concentration on the Leaching Kinetics
In Fig. 6, the steady-state leaching rates of selected elements are plotted as a function of pH. As can be seen, the leaching rates of selected elements were constant at near neutral pH (between pH 6.8 and 5) and the rates increased when pH was below 3.7. In the case of Mg and Fe, the leaching rates increased as the pH of the leaching solution was below 5.0. This observation is similar to the pH effect found for the dissolution of metal (hydr)oxides (Furrer and Stumm, 1986) and feldspars (Welch and Ullman, 1996), which is suggested to involve intrinsic hydration and proton-promoted reaction. Therefore, the overall leaching rate of element i can be written as:

Formula 10[10]
where Ri FGD,H2O represents the kinetics of the intrinsic hydration reaction (µmol m–2 min–1) and Ri,FGD,H+ is the rate due to proton-promoted leaching (µmol m–2 min–1); ki,FGD,H+' is the empirical rate constant for proton-promoted leaching; [H+]bulk is the concentration of protons in the bulk aqueous solution, and j is the reaction order. Equation [10] describes the dissolution of metal (hydr)oxides (Furrer and Stumm, 1986) and metal aluminum silicates (Welch and Ullman, 1996) and has not been applied to other mineral forms. However, patterns for Ca, S, As, and Se observed in Fig. 6 were similar to the patterns for Fe, Al, and Si suggesting the leaching of these elements can also be described by an equation of this form. Ryzhenko and Mironenko (1994) also observed that the dissolution rates of carbonates, sulfates, sulfides, and fluorides increased as pH decreased below 5, whereas the dissolution rates were relatively constant from pH 5 to 9.


Figure 6
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  		Fig. 6. Steady-state leaching rates as a function of pH. Ca ({blacksquare}); S ({square}); Al ({circ}); Si (•); Fe ({blacktriangledown}); As (gray circle); Se (gray upside-down triangle).

 
The leaching rates due to hydration are given in Table 4 and were determined as the constant leaching rate observed in Fig. 6 at near neutral pH conditions (i.e., pH 5.5 and 6.8). The proton-promoted leaching rate of each element (also provided in Table 4) was determined by a linear regression at the ascending part of the logarithmic plot (Fig. 6) at low pH values. It was found that the reaction orders were very close to 0.5 for divalent cations; such as Ca and Mg, and close to 0.75 for trivalent cations; i.e., Al and Fe. The nonintegral reaction orders reflect the fact that more than one elemental reaction, such as the adsorption of proton and formation and detachment of the surface complex, were involved in the surface chemical process during leaching. It should also be noted that the fixated FGD material contained more than one mineral phase, and therefore, the leaching kinetic constants determined in Table 4 reflect the combined release rate of the element from the dissolution of one or more minerals.


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  		Table 4. Numerical results for pH effect on leaching of selected elements. Rates Ri,H and RH2O for Ca, S, Al, Si, Fe, and Mg in nmol m–2 min–1. Rates for As and Se in pmol m–2 min–1. k'I,FGD,H+ is unitless.

 
Controlling Solids
Calcium and Sulfur
Decreasing pH not only increased the leaching rate, it also altered the stoichiometric ratios between elements implying changes in the solids controlling leaching. As can be seen in Fig. 7A, the Ca/S molar ratio in the leachate (1.95 ± 0.19) at steady-state and near neutral pH conditions (pH 6.8 and 5) was similar to ettringite (Ca/S = 2) suggesting this solid likely controlled the release of Ca and S. However, at pH 2.2 and 2.9, the Ca/S ratio decreased to 1.48 ± 0.02, which is close to the ratio of 1.39 for the bulk fixated FGD material (the solid reference line). This change in Ca/S suggests an increase in dissolution rates of hannebachite and gypsum at lower pH. Therefore, the leaching rates of Ca and S were controlled by the dissolution of ettringite at near neutral pH and by the dissolution of ettringite, hannebachite, and gypsum at lower pH values. It should be added that hannebachite can be converted to gypsum in oxidizing and acidic environments (Hao and Dick, 2000), and gypsum is more soluble than hannebachite (Laperche and Bigham, 2002). By conducting a leaching experiment with continuous N2 purging, the leaching rates for major elements were found similar to the leaching rates observed without N2 purging (Table 3). The redox potentials were –70 ± 20 mV with N2 purging and 410 ± 10 mV without. Therefore, conversion of hannebachite to gypsum does not explain the higher leaching rates at lower pH in our experiments.


Figure 7
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  		Fig. 7. The stoichiometric ratios of (A) Ca/S (•), and Al/Si ({blacktriangleup}); (B) As/ettringite-Ca (•) and Se/Ettringite-Ca ({circ}); (C) As/hematite-Fe (•) and As/glass-Al ({circ}) leaching rates at steady state under various acidic leaching conditions with 60rpm rotating speed. Solid reference lines represent the ratios of (A) Ca/S, (B) As/ettringite-Ca, and (C) As/hematite-Fe in the bulk of fixated FGD material before leaching. Dashed reference lines are the ratios of (A) Al/Si, (B) Se/ettringite-Ca, and (C) As/glass-Al in the bulk of fixated FGD material before leaching. The gray solid and dashed lines are the standard deviation of the black lines.

 
Myneni et al. (1998) proposed, under equilibrium conditions, that ettringite dissolves incongruently into gypsum, amorphous Al-hydroxide, Al-hydroxy sulfates, and Ca-Al-hydroxy sulfate at near neutral and acidic pHs; therefore, they concluded the dissolution of these secondary minerals control the activity of Ca, S, and Al in solution. The data in Table 4 suggest the incongruent dissolution of ettringite occurred far from equilibrium as well. The leaching rate of Ca was more than 100-fold higher than Al, instead of threefold. However, the formation of gypsum and Ca-Al-hydroxy sulfate was insignificant in this study based on the observed higher than 1 Ca/S ratios and lower than 1 Al/Ca ratios. It is likely that the release of Ca and S from ettringite was faster than Al, and therefore, it allowed the remaining Al to form amorphous Al-hydroxide, which might have a lower dissolution rate than ettringite within the tested pH range.

Aluminum and Silicon
In addition to ettringite-converted Al-hydroxide, dissolution of mullite and glass may also contribute to the release of Al. In the case of Si, the possible controlling solids were mullite, amorphous glass, and quartz. The Al/Si ratio in leachates varied from 0.42 to 1.3 (Fig. 7A) suggesting the release of Al and Si might be controlled by different solids at different pH values, or due to the incongruent dissolution of amorphous glass (Oelkers and Gislason, 2001; Wolff-Boenisch et al., 2004), especially at acidic pHs (Wolff-Boenisch et al., 2004). Xiao and Lasaga (1994) found the energy for the hydrolysis of oxygen atoms between Al and Si was less than the energy for the hydrolysis of oxygen between Si and Si. Therefore, Al is more easily liberated from the glass framework than Si (Oelkers and Gislason, 2001).

The dissolution rate of quartz at 25°C ranges from about 0.04 nmol m–2 min–1 at near neutral pH (Brady and Walther, 1990) to 0.007 nmol m–2 min–1 at pH 4.43 (Oelkers and Gislason, 2001). This dissolution rate is at least one order of magnitude less than the dissolution rates of natural (0.2–30 nmol m–2 min–1 at around pH 4) (Oelkers and Gislason, 2001; Wolff-Boenisch et al., 2004) and laboratory-prepared glasses (varied from 0.3–755 nmol m–2 min–1 at a pH range of 2.11–7.03) (Oelkers and Gislason, 2001). The promotion of the quartz dissolution rate by alkaline earth cations (Ca and Mg) was likely not significant in our study given the low concentrations of these elements observed (0.1 mmol was the highest concentration observed in our experiments) (Dove and Nix, 1997). Also, the leaching rate of Si determined in Table 4 ranged from 0.12 to 1.0 nmol m–2 min–1 which is in the range reported for dissolution of glass (Nurkeev et al., 1981; Oelkers and Gislason, 2001; Wolff-Boenisch et al., 2004). Therefore, the release of Si was mainly controlled by the dissolution of glass. This conclusion is supported by the mineral composition observed in the leached layer shown in Table 2. It was found that the concentration ratio of glass to quartz decreased from 1.3 to 0.9, which indicated that the dissolution rate of glass was faster than that of quartz during the leaching process.

For the release of Al, mullite was unlikely the controlling solid. Nurkeev et al. (1981) found less than 0.56% of mullite was dissolved in 20% HCl at 105°C in 7 d compared with 85.71 and 2.32% for gibbsite and corundum, respectively. The dissolution rates of gibbsite and corundum at pH 3 were about 0.1 (Dietzel and Boehme, 2005) and 0.01 nmol m–2 min–1 (Nordin et al., 1999), respectively. However, the relative importance of glass and amorphous Al-hydroxide in controlling the release of Al is not easy to determine due to insufficient information on the form of the ettringite-derived amorphous Al-hydroxide. Thus, the leaching rates of Al could be controlled by glass and/or amorphous Al-hydroxide.

Iron
The dissolution rate of magnetite has been shown to be an order of magnitude higher than hematite (Sidhu et al., 1981; White et al., 1994). Therefore, the release of Fe during the leaching process was likely controlled by the dissolution of magnetite. This is supported by the observation in Table 2. The ratio of hematite to magnetite was higher in the leached layer than in the bulk of the fixated FGD material before leaching.

Arsenic and Selenium
As can be seen in Table 1, 31.8 and 28.6 mg kg–1 of As remained in the leached layer of fixated FGD material at pH 2.2 and 2.9, respectively. No detectable Se was observed at either pH. This indicates that about 56% of As and all of the Se were leached. The observed leaching of Se and As correlates with its distribution in fixated FGD material. According to the selective dissolution analysis, Se was completely (108 ± 3%) extracted from fixated FGD material in the dialysis step with the soluble fraction of fixated FGD material. As shown in Fig. 7B, the stoichiometric ratio of Se and Ca in leachates (ranging from 5.3 x 10–5 to 6.4 x 10–5) was very close to the ratio of Se to ettringite-Ca in the bulk of fixated FGD material (6.1 x 10–5, shown as the short-dashed line in Fig. 7B) at near neutral pH. It has been shown in the previous section that the dissolution of ettringite controlled the release of Ca within this pH range. Therefore, the release of Se during leaching was most likely associated with the dissolution of ettringite, with the decrease in Se/Ca ratio at lower pH due to the increased dissolution rate of hannebachite or/and gypsum.

Less than 40% of As was found distributed in the soluble fraction of fixated FGD material meaning glass-trapped and hematite-associated As might also be extracted during the leaching process. As shown in Fig. 7B, the As to Ca ratio in the leachate was also close to the stoichiometric ratio of As and ettringite-Ca in the bulk of fixated FGD material. With higher dissolution of hannebachite and gypsum, the ratio decreased at lower pH. However, the As to Fe and As to Si ratios in the leachates were at least two orders of magnitude higher than the As to hematite-Fe and As to glass-Al ratios indicating the release of As from either glass-trapped or hematite-associated fractions should be insignificant.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Experimental results from a rotating disk system demonstrate that the leaching process of fixated FGD material was controlled by surface chemical reaction, not bulk or internal pore diffusion. The overall leaching kinetics were described by an intrinsic hydration reaction at near neutral pH and a proton-promoted dissolution reaction at lower pH. The release of Ca and S from fixated FGD material was controlled by ettringite at near neutral pH conditions. However, at more acidic conditions, the release of both elements from the dissolution of hannebachite/gypsum was more significant. The dissolution of ettringite incongruently released Al which might be due to the formation of amorphous Al-hydroxide after the fast detachment of Ca and S. Glass was likely the dominating solid for the release of Si in the tested pH range; however, the relative importance of glass and amorphous Al-hydroxide on the release of Al was not determined. The higher stoichiometric ratio of Al/Si at lower pH likely resulted from the incongruent dissolution of glass or the increased dissolution rate of amorphous Al-hydroxide. The dissolution of magnetite controlled the release of Fe during the leaching process of fixated FGD material. In addition, this study shows the amounts of As and Se in the dialysis-soluble fraction of fixated FGD material correlate with their release during the leaching process.


   ACKNOWLEDGMENTS
 
The authors thank the Ohio Coal Development Office and the Ohio State University for funding support.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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