Cadmium Stabilization Efficiency and Leachability by CdAl4O7

Article pubs.acs.org/est

Cadmium Stabilization Efficiency and Leachability by CdAl4O7 Monoclinic Structure Minhua Su,† Changzhong Liao,† Kui-Hao Chuang,‡ Ming-Yen Wey,§ and Kaimin Shih*,† †

Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China Department of Safety, Health, and Environmental Engineering, Central Taiwan University of Science and Technology, Taichung, Taiwan § Department of Environmental Engineering, National Chung Hsing University, Taichung, Taiwan ‡

S Supporting Information *

ABSTRACT: This study investigated the stabilization efficiencies of using an aluminum-rich precursor to incorporate simulated cadmium-bearing waste sludge and evaluated the leaching performance of the product phase. Cadmium oxide and γ-alumina mixtures with various Cd/Al molar ratios were fired at 800−1000 °C for 3 h. Cadmium could be crystallochemically incorporated by γ-alumina into CdAl4O7 monoclinic phase and the reaction was strongly controlled by the treatment temperature. The crystal structure details of CdAl4O7 were solved and refined with the Rietveld refinement method. According to the structural refinement results, the stabilization efficiencies were quantified and expressed as a transformation ratio (TR) with optimized processing parameters. The preferred treatment temperature was found to be 950 °C for mixtures with a Cd/ Al molar ratio of 1/4, as its TR value indicated the cadmium incorporation was nearly completed after a 3 h treatment scheme. Constant-pH leaching tests (CPLT) were conducted by comparing the leachability of the CdO and CdAl4O7 phases in a pH 4.0 environment. A remarkable reduction in cadmium leachability could be achieved via monoclinic CdAl4O7 structure formation to effectively stabilize hazardous cadmium in the waste stream. The CPLT and X-ray photoelectron spectroscopy (XPS) results suggested incongruent dissolution behavior during the leaching of the CdAl4O7 phase.

■

disposed of in landfills.9 However, due to its nondegradable nature and high mobility, cadmium can be easily leached under acidic conditions and bioaccumulate in human organs via food chains (soil−plant−animal−human).12,13 A novel technique for economically and reliably managing waste sludge is required during the rapid development of wasteto-resources strategies.14,15 As an alternative waste management technique, thermal treatment offers the possibility and feasibility of immobilizing and stabilizing hazardous metals.16,17 With the addition of aluminum-rich materials into metalcontaining waste under well-controlled thermal treatment schemes, metals can be stabilized via specific crystal structures and have been found to undergo substantial leachability reductions in acidic environments.18,19 For the Cd−Al−O system, CdAl2O4 and CdAl4O7 have been reported as cadmium aluminates that may provide opportunities for stabilizing cadmium via thermal treatment.18−21 The formation of CdAl2O4 was achieved via a two-step (sol−gel and sintering)

INTRODUCTION Metal contamination has increasingly become a concern in many areas of the world.1−3 Cadmium is considered one of the most toxic metals because of its severe acute and chronic toxicological effects.4 Occurrences of itai-itai (“ouch-ouch”) disease have been reported in Japan due to the high toxicity of cadmium.1,4,5 The global production and consumption of cadmium was 22 200 tons in 2014.6 The release of cadmium from industrial activities (such as electroplating, smelting, alloy manufacturing, pigments, plastic, battery, mining, and refining processes) poses substantial risks to public health and ecosystems.1,6 In Japan, a project to restore cadmiumcontaminated farmland along the Jintsu River began in 1979 at a total cost of 496 million USD (estimated in 2012).7 A reported 20 tons of cadmium were discharged into Longjiang River (Guangxi, China) in 2012, causing the deaths of millions of fish and affecting the water supply of up to 4 million people.8 Cadmium hydroxide is the most common phase when cadmium ions are removed from contaminated wastewater via precipitation processes.9,10 The Danish Environmental Protection Agency indicated that 11−59% of the cadmium in municipal wastewater may end up in waste sludge.11 In general, solidified and stabilized cadmium-bearing sludge is ultimately © XXXX American Chemical Society

Received: April 27, 2015 Revised: September 15, 2015 Accepted: October 29, 2015

A

DOI: 10.1021/acs.est.5b02072 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

(AlOOH), which converted to the γ-Al2O3 phase upon thermal treatment at 650 °C for 3 h (Figure S1). The BET surface area of the fabricated γ-Al2O3 was measured at 180.88 ± 1.24 m2/g. To evaluate the capability of γ-Al2O3 to incorporate cadmium under thermal conditions, we mixed the CdO and γ-Al2O3 precursors to a total dry weight of 60 g at Cd/Al molar ratios of 0.5 (1/2) and 0.25 (1/4), respectively. The mixtures were ballmilled in the water slurry environment for 18 h, and the dried mixtures (at 105 °C for 24 h) were further homogenized by mortar grinding and then pelletized into Φ 20 mm pellets at 250 MPa. The 3 h dwelling time at the targeted temperature was applied for the subsequent thermal treatments (Figure S2). XRD Analysis. The fired pellets were air-cooled and ground into powder form for the powder XRD analysis. The stepscanned XRD pattern of each powder sample was recorded using a Bruker D8 Advance X-ray powder diffractometer equipped with Cu Kα1,2 X-ray radiation and a LynxEye detector. The 2θ scanning range covered 10° to 80°, and the step size was 0.02° with a scan speed of 0.5 s per step. Qualitative-phase identification was executed by matching the powder XRD patterns with those retrieved from the standard powder diffraction database of the International Centre for Diffraction Data (ICDD PDF-2 Release 2008). The identified crystalline phases were used to quantify the transformation efficiency of the cadmium incorporated into specific crystal structure(s). The refinement of the crystal structure(s) and the quantification of the phase compositions were performed via the Rietveld method using the TOPAS V4.0 program (Bruker AXS, Karlsruhe, Germany). The refinement of diffraction patterns allowed the weights of all of the crystalline phases in the samples to be determined, and these weights were expressed as percentages. For the amorphous and poorly crystalline (such as γ-Al2O3) phases, a refinement method using CaF2 as the internal standard was applied to quantify such content in the samples. The validity of the refinement analysis was assessed by testing powder mixtures containing authentic CdO, CdAl4O7, CaF2, and glass (as the amorphous content) in known weight fractions. Table S1 presents the derived reliability factors (the pattern factor (Rp), weighted pattern factor (Rwp) and goodness of fit (GOF)). Leaching Experiment. The leachability of the single-phase cadmium-bearing samples (CdO and CdAl4O7) observed in the products was tested via CPLT to evaluate the stability of cadmium in an acidic environment. Before the leaching test, BET surface area and particle-size distribution of the leaching samples were measured. The particle size distribution measurement was conducted on a LS 13 320 laser diffraction particle size analyzer (Beckman Coulter, Inc.). For CPLT, the initial leaching fluid was a pH 4.0 nitric acid (HNO3) aqueous solution. The pH value was real-time monitored. Once the pH in the leachate reached 4.2, approximately 20 μL of 1 M HNO3 aqueous solution was added to adjust the pH to be 4.0. The leaching test was carried out in a jar filled with 500 mL of leaching fluid and 0.5 g of the test powders, which was magnetically stirred (200 rpm) to keep the mixture homogeneous throughout the leaching process. At each 10 min interval, 5 mL of leachate mixed with corresponding solid sample was withdrawn and filtered (with a 0.2 μm syringe filter) for the subsequent determination of the cadmium concentration via ICP-OES 800 (PerkinElmer). Both the Cd and Al standards were run before and periodically during each sampling event to generate a satisfactory calibration curve range (Cd: 1−2000 ppb, R2 = 0.9999, detected wavelength =214.44

synthesis procedure.20 CdAl4O7 can be obtained courtesy of the traditional solid-state reaction21 from oxide mixtures (cadmium oxide and alumina). However, identifying the cadmium-hosting phase(s) and exploring the crystal structure features under different thermal conditions remain key challenges.22,23 A clear understanding of crystal structure details (e.g., lattice parameters, atomic positions, and occupancies) is required before metal transformation efficiency can be quantified via the X-ray diffraction (XRD) technique.18,19 The stabilized products may be beneficially used as construction materials. The chemical durability of products should be evaluated via leaching experiments to reflect the metal stabilization effect. The toxicity characteristic leaching procedure (TCLP) is a batch test developed by the U.S. Environmental Protection Agency to assess the leachability of toxic metals from wastes.24−26 Column-leaching tests were also commonly used to evaluate the leaching performance of wastes.27−30 The solid sample can largely remain in the column where the pH will change but usually cannot be immediately observed.29 During the leaching test, the pH value strongly affects the mineral dissolution, proton competition on surface-binding sites, and substance surface potential.31 A nonconstant pH may thus retard the leaching process or cause the reprecipitation of metal compound(s).26,32−34 Constant-pH leaching tests (CPLT) can overcome such shortcomings.31,35,36 The TCLP regulatory level of Cd is 1 mg/L.24 The effluent requirements for Cd vary from 0.01 to 1.2 mg/L, depending on the type of wastewater discharged.25 Although currently there is no regulatory limit of Cd for CPLT procedure, CPLT is considered to be highly beneficial in revealing the leachability and leaching behavior of wastes.35,36 Alumina (Al2O3) is a common industrial material that is abundant in the natural environment.37 A total of seven alumina phases, including metastable γ, η, δ, θ, κ, χ-aluminas, and stable α-alumina, have reportedly shown technological importance in a wide range of applications.37−40 Among these alumina phases, γ-Al2O3 is a low-cost precursor material due to its unique crystal structure and highly reactive property.37,38,41,42 In this study, γ-alumina was examined for its ability to incorporate cadmium under thermal conditions. The product structure details were determined via Rietveld refinement analysis of observed diffraction data to produce the crystal structure model required for the phase quantification task. The reaction pathways and incorporation efficiencies of cadmium were then quantitatively evaluated under different thermal conditions. Constant-pH leaching tests were conducted at pH 4.0 to assess the stability of cadmium-bearing phases in the products.

■

EXPERIMENTAL SECTION Materials and Sample Preparation. Cadmium oxide (CdO) and γ-alumina (γ-Al2O3) were used as reactants in the experiment. Mineralogically pure CdO powder (Figure S1) was purchased from Fisher Scientific. Doubly distilled deionized water (DIW) was used throughout the study. The surface area of the CdO powder was measured using the nitrogen adsorption−desorption isotherms at liquid nitrogen temperature (77 K) on a Beckman Coulter SA3100 surface area and pore-size analyzer, yielding 2.63 ± 0.05 m2/g according to the Brunauer−Emmett−Teller (BET) method. The γ-Al2O3 was prepared using alumina powder purchased from Pural SB (Sasol). The qualitative-phase identification of the as-received alumina powder was confirmed by powder XRD as boehmite B


Article


Figure 1. X-ray diffraction patterns of the treated CdO + γ-Al2O3 samples with Cd/Al molar ratios of (a) 1/2 and (b) 1/4 at 800−1000 °C for 3 h and the (c) treated CdO + γ-Al2O3 samples with a Cd/Al molar ratio of 1/4 at 950 °C under different treatment times (0.25−9 h). The standard patterns were derived from the International Centre for Diffraction Data database, including CdO (ICDD PDF no. 75-0594), γ-Al2O3 (ICDD PDF no. 50-0741) and CdAl4O7 (ICDD PDF no. 22-1061).

It should be noted that this finding differed from those of previous studies,43−45 which often used CdAl2O4 as the product phase to discuss the capture and stabilization behavior of cadmium during combustion processes. Crystal Structure Determination. To obtain the high quality XRD data for the CdAl4O7 crystal structure, we prepared a single-phase CdAl4O7 sample by heating a CdO + γAl2O3 pelletized mixture with Cd/Al = 1/4 at 950 °C for 36 h. The homogenization, pelletization, and thermal treatment were repeated twice to facilitate the fabrication of the single-phase sample. The diffraction pattern was collected via a 2θ scan ranging from 10° to 120° with a step size of 0.02° and a scan speed of 1.0 s per step. Figure 2 presents the obtained XRD data and the corresponding Rietveld refinement results for this single-phase CdAl4O7 sample. All of the Bragg diffraction reflections could be well indexed on the basis of the monoclinic structure system (space group C12/c1). High-quality XRD

nm; Al: 1−2000 ppb, R2 = 0.9999, detected wavelength =396.153 nm). After the leaching experiments, the powder samples were dried, and some were further analyzed via X-ray photoelectron spectroscopy (XPS) (PHI 5600 Multi-Technique XPS system, Physical Electronics, Inc.) to assist in the investigation of the surface leaching behavior.

■

RESULTS AND DISCUSSION Formation of Product Phases. Figure 1a,b shows the XRD patterns of the samples treated at 800−1000 °C for 3 h with Cd/Al molar ratios of 1/2 and 1/4. The diffraction signals of the CdAl2O4 phase were not observed in the XRD patterns. A predominate CdAl4O7 (ICDD PDF no. 22-1061) phase appeared in both sample systems. The crystallization of CdAl4O7 was first observed at 900 °C when the Cd/Al molar ratio was 1/2, but a large amount of CdAl4O7 was formed at 900 °C when the Cd/Al molar ratio was 1/4. The Cd/Al = 1/2 system had insufficient γ-alumina to promote the growth of CdAl4O7. Unlike the reaction pathway through the sol−gel method, CdAl2O4 was not found unless heating the oxide precursors at temperatures above 1050 °C. As the CdAl4O7 phase dominated both Cd/Al ratio systems, the formation of CdAl4O7 was the preferred reaction between CdO and γ-Al2O3 at 800−1000 °C. A similar circumstance was also observed in the CdO excessive systems with Cd/Al ratios of 3/4 or 2/1 (Figure S3). Therefore, the CdO signal was observed for all of the Cd/Al = 1/2 samples (with excess cadmium for the CdAl4O7 phase) even at the highest treatment temperature (1000 °C) (Figure 1a). The effect of the treatment time is reflected by the XRD patterns shown in Figure 1c. The CdO + γ-Al2O3 (Cd/Al = 1/4) samples were treated at 950 °C for 0.25−9 h. Crystallographic CdAl4O7 remained the only cadmium aluminate phase in the products after prolonged heating. This solid-state reaction (eq 1) was found to be highly efficient, requiring only 15 min (0.25 h) to reach substantial formation. CdO + 2γ −Al 2O3 → CdAl4O7

Figure 2. Observed (red cross symbol), calculated (blue line), and difference profiles (green line) derived from Rietveld refinement analysis of the XRD data. The pink vertical marks indicate the reported Bragg diffraction positions (ICDD PDF no. 22-1061) of the CdAl4O7.

(1) C


Article

Environmental Science & Technology Table 1. Atomic Coordinates and Equivalent Isotropic Thermal Parameters (Beq, Å2) of CdAl4O7 site Cd1 Al1 Al2 O1 O2 O3 O4

Wyckoff site 4 8 8 4 8 8 8

X 0.000 00 0.157 91 0.120 73 0.000 00 0.113 60 0.113 90 0.191 67

Y

Z

0.793 08 0.083 68 0.438 66 0.523 36 0.063 34 0.251 15 0.449 05

0.250 00 0.316 24 0.232 12 0.250 00 0.584 14 0.149 30 0.556 94

atom 2+

Cd Al3+ Al3+ O2− O2− O2− O2−

occupancy

Beq (Ǻ 2)

1 1 1 1 1 1 1

1.811 1.195 1.195 2.022 2.022 2.022 2.022

Note: Space group of C12/c1 with refined lattice parameters (a = 12.65 Å, b = 8.86 Å, c = 5.40 Å, β = 105.91°, volume = 581.86 Å3, density = 3.79 g/ m3).

Figure 3. Quantitative comparison of phase compositions (wt %) in the treated CdO + γ-Al2O3 samples (Cd/Al = 1/4) at (a) 800−1000 °C for 3 h and (b) 950 °C for 0.25−18 h. The transformation ratios (TR, %) for cadmium incorporated into the CdAl4O7 monoclinic structure treated at (c) 800−1000 °C for 3 h and (d) 950 °C for 0.25−18 h.

β (105.98°) values for the CdAl4O7 were used for the initial lattice parameters. The CdAl4O7 structure was then determined from the obtained XRD data via the Rietveld refinement method using the TOPAS V4.0 program. More than 40 parameters were refined, including the background, lattice, profile, structural, and individual thermal parameters and atomic coordinates. The refinement results showed that the calculated profile fit well with the experimental data, as reflected by the reliability factors of Rp = 4.59%, Rwp = 6.24% and GOF = 2.61. The refined lattice parameters of the CdAl4O7 unit cell were found to be a = 12.65216 (Å), b = 8.86081 (Å), c = 5.39776 (Å), β = 105.9068°, and V = 581.86 (Å3). The atomic coordinates and individual thermal parameters for the CdAl4O7, which had previously gone unaddressed, were also determined and reported (Table 1). Cadmium-Stabilization Efficiency. Quantitative XRD (QXRD) analysis of the phase compositions in the treated

were obtained via the LynxEye detector. Therefore, the lowintensity peaks not reported in the diffraction database are indexed in Figure 2.21 For instance, the observed peak at 2θ = 29.34° in the XRD pattern was identified to be the (400) lattice plane of the CdAl4O7 (Figure S4). As mentioned in the introduction, a clear understanding of the crystal structure details could have clarified the properties of the new cadmiumhosting phase. However, the crystal structure details of the CdAl4O7 phase remained unavailable despite the positions of the main diffraction peaks recorded in the database. Because the CaAl4O7 was isostructural to the CdAl4O7,21 the CaAl4O7 was selected as the preliminary crystal structural model to determine the CdAl4O7 crystal structure details. With a CaAl4O7 structure (Inorganic Crystal Structure Database (ICSD) no. 34487), the site (4e according to the Wyckoff notation) occupied by calcium was replaced by cadmium, and the estimated values for a (12.68 Å), b (8.86 Å), c (5.40 Å), and D


Article


Figure 4. (a) Leached cadmium normalized by the surface area and cadmium weight percentage of the CdO and CdAl4O7 powders, (b) molar ratios of the measured Cd/Al in the CdAl4O7 leachates, and (c) cadmium and aluminum concentrations in the CdAl4O7 leachates.

might have been thermodynamically feasible for CdAl4O7 to form at a lower temperature, such reactions mainly occurred at the reactant grain boundary and were limited by the slow surface-diffusion process involved in the short-heating scheme.21,54,55 The formation of a new phase usually undergoes a disordered state and then evolves into a ordered state via solid-state diffusion.56 An extended sintering time is one of the most critical factors and can enhance both the homogenization and crystallization of products.56,57 The influence of treatment time on the formation of CdAl4O7 at 950 °C was examined to minimize the treatment temperature and facilitate the incorporation reaction. Figure 3b shows the varying phase compositions in the treated CdO + γ-Al2O3 samples (Cd/Al molar ratio of 1/4) at 950 °C for 0.25−18 h. A significant amount of CdAl4O7 (55 wt %) was formed within a very short treatment time (0.25 h) at 950 °C. A slight increase in the CdAl4O7 phase was observed as the treatment time was extended up to 3 h. With a 9 h treatment, the CdAl4O7 was observed to be 82 wt % in the treated samples, and the 90 wt % CdAl4O7 in the product was produced through the 18 h heat treatment. Given the atomic-level diffusion in solid-state reactions, a higher crystalline product is often required to solve well-defined crystallographic positions.2,56−58 However, an increase in the CdAl4O7 product layer may act as a barrier, hindering further diffusion for product phase formation.57 The efficiency of incorporating cadmium into the crystalline phase by thermally reacting with γ-Al2O3 can be assessed by a transformation ratio (TR, %) index,17−19 which is expressed as follows:

samples was required to further investigate the efficiency of the cadmium incorporation. Figure 3a shows the mass fractions of the crystalline and amorphous phases in the treated CdO + γAl2O3 samples (Cd/Al molar ratio of 1/4) at 800−1000 °C for 3 h. The γ-Al2O3 was a polycrystalline material with abundant defects and vacancies favorable for incorporating cadmium under thermal conditions.42,46−48 Figure 3a shows that only CdO (14 wt %) and the amorphous phase (86 wt %) were observed at 800 °C. At this stage, the diffusion and crystallization might have only occurred on the surface or interface of the reactants. When the treatment temperature increased to 850 °C, the amorphous phase largely continued to dominate in the treated samples and the formation of crystalline CdAl4O7 (nearly 4 wt %) was first observed. Although the crystallization process was still suppressed at this temperature (850 °C), the diffused atoms on the grain boundary started to energetically form crystalline cadmium aluminate.46−49 As the temperature elevated to 900 °C, the cadmium incorporation reaction was enhanced and yielded 51 wt % of crystalline CdAl4O7 due to the stronger mass-transfer and atom-rearrangement processes.46,48 With treatment at 950 °C for 3 h, the cadmium was highly incorporated into the monoclinic CdAl4O7 structure. Higher temperatures generally provide a higher driving force for overcoming the energy barriers present in solid-state reactions.49−51 Increasing the temperature to 1000 °C further developed the CdAl4O7 crystallization degree and harvested 83 wt % of CdAl4O7. Studies have reported that CdAl4O7 formation starts at 800 °C.21 However, it required a temperature higher than 900 °C to effectively incorporate cadmium into the CdAl4O7 monoclinic phase in the 3 h thermal treatment scheme used in this study. Thermodynamic conditions and the diffusion process are two key factors that promote solid-state reactions.52−54 Although it E


Article


TR (%) =

wt % of CdAl4O7 MW of CdAl4O7 wt % of CdAl4O7 wt . % of CdO + MW of CdO MW of CdAl4O7

On the basis of the cation−proton exchange mechanism, the destruction of cadmium oxide by the acidic solution can be expressed as follows: + 2+ CdO(s) + 2H(eq) → Cd(eq) + H 2O

where MW is the molecular weight of the phase in the sample (g/mol). In this study, no cadmium incorporation occurred for TR = 0%. However, a complete transformation of cadmium into a CdAl4O7 monoclinic structure occurred for TR = 100%. Figure 3c summarizes the TR values for the evaluation of cadmium incorporation efficiency by γ-Al2O3. Cadmium was transformed into the CdAl4O7 structure at 850 °C for 3 h, resulting in a TR of 12%. When the treatment temperature was elevated to 900 °C, the TR value increased remarkably to 80%. A nearly complete transformation was observed when the sample was treated at 950 °C for 3 h. Figure 3b demonstrates the influence of the treatment time. Short sintering times of 15 and 30 min were found to achieve TR values of 83% and 93%, respectively. The XPS spectra showed that the binding energy of Cd 3d peaks of CdAl4O7 was in good agreement with that of CdO, indicating no further oxidation of Cd during the incorporation process (Figure S5). Within the range of our sintering temperatures, CdAl4O7 does not involve structural evolution or decomposition as reflected in Figure 1 and its phase diagram.21 Furthermore, the insignificant weight change of CdAl4O7 powder calcined at 1025 °C for 3 h indicates that the CdAl4O7 is thermodynamically stable at temperatures below 1000 °C (Table S2). Stabilization-Effect Evaluation. To evaluate the cadmium stabilization effect, we evaluated the inherent leachability of the two cadmium-containing phases, i.e., the CdO (BET surface area =2.63 ± 0.05 m2/g) and as-prepared CdAl4O7 (BET surface area = 2.07 ± 0.03 m2/g) phases, via CPLT at pH 4.0. The average particle size of CdO and CdAl4O7 powders are 22.1 ± 0.5 μm and 23.3 ± 0.2 μm, respectively (Figure S6). Because the surface reactions and available amount of metals in the sample played crucial roles during the leaching of the solid, such influences were expected to be proportional to the sample surface area and molecular weight of the leached phases.17,19 Therefore, the normalized leached cadmium per surface area of sample (NLCdSA; m−2) was calculated as follows: NLCdSA = 10−6 ×

(2)

The concentration of cadmium ions [Cd2+ (eq)] in relation to the Cd(OH)2(s) by the potential precipitation and dissolution reaction can be expressed as follows: 2+ − Cd(OH)2(s) ↔ Cd(eq) + 2OH(eq)

(3)

The solubility constant (Ksp) of eq 3 is 10−14.3.59,60 The cadmium concentrations measured in the CdO and CdAl4O7 leachates were 691.2 mg/L (6.2 × 10−3 M) and 11.8 mg/L (1.1 × 10−4 M) at the end of the CPLT process, respectively. − 2 Therefore, the product of [Cd2+ (eq)] × [OH(eq)] was calculated −22.2 −23.9 as 10 and 10 . These outcomes indicated that the cadmium concentrations in the CdO and CdAl4O7 leachates were undersaturated in relation to the Cd(OH)2(s) and were not limited by the precipitation and dissolution of the Cd(OH)2(s). In the case of a “congruent dissolution,” the leaching behavior of the CdAl4O7 phase can be expressed as follows: + 2+ 3+ CdAl4O7(s) + 14H(eq) → Cd(eq) + 4Al(eq) + 7H 2O

(4)

3+ Although the [Cd2+ (eq)]/[Al(eq)] molar ratio in the leachate should generally maintain a theoretical value of 0.25, the ratio ranged significantly higher from 0.55 to 0.36 in this study (Figure 4b). This may indicate the reprecipitation of aluminum or that the metals incongruently dissolved from the CdAl4O7. The precipitation and dissolution (eq 5) of aluminum hydroxide (Al(OH)3(s)) can be expressed as follows:

3+ − Al(OH)3(s) ↔ Al(eq) + 3OH(eq)

(5)

The solubility constant (Ksp) of eq 5 is 10−31.2.59 Therefore, −2 at pH = 4.0, the [Al3+ M (eq)] should generally be 6.3 × 10 (1701 mg/L) when saturated with Al(OH)3(s). Nevertheless, the measured aluminum concentrations in the CdAl 4O7 leachates were between only 5.9 × 10−5 M (1.6 mg/L) and 2.9 × 10−4 M (7.8 mg/L) (Figure 4c) and significantly undersaturated in relation to Al(OH)3(s). Consequently, the 3+ observed high [Cd2+ (eq)]/[Al(eq)] molar ratios at pH 4.0 were ascribed to the incongruent dissolution behavior of the CdAl4O7. The continuing attack of the CdAl4O7 by protons further increased both the cadmium and aluminum concentrations in the leachates (Figure 4a,c). The XPS spectrum (Figure S7) also reflected the cadmium deficiency and the enrichment of aluminum on the surface of the leached CdAl4O7 samples. These results indicated the preferential release of cadmium from the CdAl4O7 phase through the incongruent dissolution behavior, with the majority of the Al−O bonds probably remaining on the sample surfaces. Although after incongruent dissolution the remaining surface compounds might have reorganized into new phase(s),61,62 no new crystalline phases were observed in the XRD patterns of the leached CdAl4O7 samples (Figure S8). The presence of an aluminum-rich layer on the leached surface might have also further affected the release rate of cadmium from the CdAl4O7 samples in the later stage of the leaching process.

CCd × AWCd n × k SW × SA × MWPhase

where n is the cadmium atom number in each test phase, k is the ratio of sample weight (g) to extraction fluid volume (mL), CCd is the cadmium concentration in leachate (mg/L), AWCd represents the atomic weight of cadmium, SW is the sample weight (g), SA is the solid sample surface area (m2/g), and MWPhase represents the molecular weight of the test phase. Figure 4a illustrates the leaching results of the cadmium from the test samples after normalization. The leached cadmium from the CdO and CdAl4O7 phases gradually increased as the leaching process was prolonged. The maximum cadmium concentration observed in the CdO leachates was 691.2 mg/L at the end of the leaching test (120 min). After normalization, it was more than 2 orders of magnitude higher than those from the CdAl4O7 samples. This significant difference indicates that the CdAl4O7 had a much higher inherent resistance to the attack of strong acids than the CdO phase. The leaching result suggested that the phase transformation to the CdAl4O7 monoclinic structure was a highly effective cadmium-stabilization strategy. F


Article


■

(3) Guney, M.; Zagury, G. J. Heavy metals in toys and low-cost jewelry: Critical review of U.S. and Canadian legislations and recommendations for testing. Environ. Sci. Technol. 2012, 46 (8), 4265−4274. (4) Fowler, B. A. Monitoring of human populations for early markers of cadmium toxicity: A review. Toxicol. Appl. Pharmacol. 2009, 238 (3), 294−300. (5) Shiraishi, Y. Cytogenetic studies in 12 patients with Itai-Itai disease. Humangenetik 1975, 27 (1), 31−44. (6) U.S. Geological Survey (USGS). USGS Minerals Information: Cadmium, Mineral Commodity Summaries; USGS: Reston, VA, 2015. (7) http://www.japanfs.org/en/pages/032147.html (accessed August 12, 2012). (8) http://www.china.org.cn/environment/2012-01/29/content_ 24496235.htm (accessed January 29, 2012). (9) Cartledge, F. K.; Butler, L. G.; Chalasani, D.; Eaton, H. C.; Frey, F. P.; Herrera, E.; Tittlebaum, M. E.; Yang, S. Immobilization mechanisms in solidifiction/stabilization of cadmium and lead salts using portland cement fixing agents. Environ. Sci. Technol. 1990, 24 (6), 867−873. (10) Rao, K.; Mohapatra, M.; Anand, S.; Venkateswarlu, P. Review on cadmium removal from aqueous solutions. Int. J. Eng. Sci. Technol. 2011, 2 (7), 81−103. (11) Danish Environmental Protection Agency (DEPA). Massestrømsanalyse for cadmium [Substance flow analysis for cadmium]; Environmental Project No. 557; DEPA: København K, Denmark, 2000. (12) Wei, Y. L. Leaching study of thermally treated cadmium-doped soils. Hazard. Waste Hazard. Mater. 1995, 12 (3), 233−242. (13) Gray, C. W.; McLaren, R. G.; Roberts, A. H. C. Cadmium leaching from some New Zealand pasture soils. Eur. J. Soil Sci. 2003, 54 (1), 159−166. (14) Monteiro, S. N.; Alexandre, J.; Margem, J. I.; Sánchez, R.; Vieira, C. M. F. Incorporation of sludge waste from water treatment plant into red ceramic. Constr. Build. Mater. 2008, 22 (6), 1281−1287. (15) Tay, J.-H.; Show, K.-Y.; Hong, S.-Y.; Chien, C.-Y.; Lee, D.-J. Thermal stabilization of iron-rich sludge for high strength aggregates. J. Mater. Civ. Eng. 2003, 15 (6), 577−585. (16) Wei, Y.-L.; Chiu, S.-Y.; Tsai, H.-N.; Yang, Y.-W.; Lee, J.-F. Thermal stabilization of chromium (VI) in kaolin. Environ. Sci. Technol. 2002, 36 (21), 4633−4641. (17) Shih, K.; White, T.; Leckie, J. O. Spinel formation for stabilizing simulated nickel-laden sludge with aluminum-rich ceramic precursors. Environ. Sci. Technol. 2006, 40 (16), 5077−5083. (18) Tang, Y.; Shih, K.; Wang, Y.; Chong, T.-C. Zinc stabilization efficiency of aluminate spinel structure and its leaching behavior. Environ. Sci. Technol. 2011, 45 (24), 10544−10550. (19) Lu, X.; Shih, K. Phase transformation and its role in stabilizing simulated lead-laden sludge in aluminum-rich ceramics. Water Res. 2011, 45 (16), 5123−5129. (20) Kurihara, L. K.; Suib, S. L. Sol-gel synthesis of ternary metal oxides. 1. Synthesis and characterization of MAl2O4 (M= Mg, Ni, Co, Cu, Fe, Zn, Mn, Cd, Ca, Hg, Sr, and Ba) and lead aluminum oxide (Pb2Al2O5). Chem. Mater. 1993, 5 (5), 609−613. (21) Colin, F. Des phases formees au cours de la reduction de certains oxydes mixtes nAl2O3·MO. Rev. Int. Hautes Temp. Refract. 1968, 5 (4), 267−283. (22) Bouhemadou, A.; Khenata, R.; Zerarga, F. Prediction study of structural and elastic properties under pressure effect of CdX2O4 (X = Al, Ga, In) spinel oxides. Comput. Mater. Sci. 2007, 39 (3), 709−712. (23) Bouhemadou, A.; Zerarga, F.; Almuhayya, A.; Bin-Omran, S. FPLAPW study of the fundamental properties of the cubic spinel CdAl2O4. Mater. Res. Bull. 2011, 46 (12), 2252−2260. (24) United States Environmental Protection Agency. Test Methods for Evaluating Solid Waste, Physical/Chemical Methods. Publication No. SW-846; USEPA: Washington, D.C., 1997. (25) Mislin, H.; Ravera, O. Cadmium in the environment (Experientia supplementum: Vol. 50). Birkhäuser: Basel, Switzerland, 1986.

ENVIRONMENTAL IMPLICATIONS Cadmium-laden waste discarded in landfills presents a severe threat to the environment. A well-controlled thermal treatment process can be a cost-effective strategy for treating the metalcontaminated matrix through the formation of preferable crystalline phases. These product phases often possess a high resistance to acidic attack and significantly decrease the hazards involved in releasing the metal into the environment. This study demonstrated that γ-Al2O3 (a typical aluminum-rich material) could effectively incorporate cadmium into the CdAl4O7 structure at 950 °C. However, processing parameters, such as the Cd/Al molar ratio, treatment temperature, and time, may significantly influence the reaction pathway and the incorporation level via CdAl4O7 crystallization. Given the crystallographic details of the CdAl4O7 revealed in this study via Rietveld refinement analysis, this new crystal information not only contributes to the knowledge of CdO−Al2O3 solid-state reactions but also helps to quantify the efficiency of cadmium transformation into the monoclinic CdAl4O7 structure. The leaching results from the CPLT process also clearly show the strong leaching resistance of CdAl4O7 and identify a promising stabilization strategy among the beneficial uses of cadmiumcontaining products.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02072. Tables showing quantitative-phase analysis results and the calculated transformation ratio, weight variation of CdAl4O7 powder. Figures showing X-ray diffraction (XRD) patterns, schematic diagram of the cadmium stabilization reaction pathway with thermal treatment, a comparison of the observed and calculated XRD patterns, XPS spectra and particle size distributions of CdO and CdAl4O7, and decrease of the Cd 3d signal and increase of the Al 2p signal on the surface of the CdAl4O7 sample leached for 120 min and observed via X-ray photoelectron spectroscopy. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +852 28591973; fax: +852 25595337; e-mail: kshih@hku. hk. Notes

The authors declare the following competing financial interest(s): The authors gratefully acknowledge the funding for this research provided by the General Research Fund Scheme of the Research Grants Council of Hong Kong (715612, 17206714) and HKU Strategic Research Themes on Clear Energy and Earth as a Habitable Planet.

■ ■

ACKNOWLEDGMENTS The authors are thankful to Ms. Hanlu Yan for assisting with the ICP-OES measurement. REFERENCES

(1) Järup, L. Hazards of heavy metal contamination. Brit Med. Bull. 2003, 68 (1), 167−182. (2) Jensen, D. L.; Holm, P. E.; Christensen, T. H. Soil and groundwater contamination with heavy metals at two scrap iron and metal recycling facilities. Waste Manage Res. 2000, 18 (1), 52−63. G


Article

Environmental Science & Technology (26) Fällman, A.-M. Performance and design of the availability test for measurement of potentially leachable amounts from waste materials. Environ. Sci. Technol. 1997, 31 (3), 735−744. (27) ASTM. Standard Test Method for Leaching Solid Waste in a Column Apparatus; D 4874. ASTM: West Conshohocken, PA, 2001. (28) NEN 7343. Leaching characteristics of solid earth and stony building and waste materials. Leaching tests. Determination of the leaching of inorganic components from granular materials with the column test. Nederlandse Normalisatie-instituut: Delft, Netherlands. 1995. (29) Jackson, D. R.; Garrett, B. C.; Bishop, T. A. Comparison of batch and column methods for assessing leachability of hazardous waste. Environ. Sci. Technol. 1984, 18 (9), 668−673. (30) Pendowski, J. J. An Assessment of Laboratory Leaching Tests for Predicting the Impacts of Fill Material on Ground Water and Surface Water Quality; Publication No. 03-09-107; Washington State Department of Ecology: Olympia, WA, 2003. (31) Al-Abed, S. R.; Jegadeesan, G.; Purandare, J.; Allen, D. Arsenic release from iron rich mineral processing waste: Influence of pH and redox potential. Chemosphere 2007, 66 (4), 775−782. (32) Poon, C. S.; Lio, K. W. The limitation of the toxicity characteristic leaching procedure for evaluating cement-based stabilized/solidified waste forms. Waste Manage. 1997, 17 (1), 15−23. (33) Hooper, K.; Iskander, M.; Sivia, G.; Hussein, F.; Hsu, J.; Deguzman, M.; Odion, Z.; Ilejay, Z.; Sy, F.; Petreas, M.; Simmons, B. Toxicity characteristic leaching procedure fails to extract oxoanionforming elements that are extracted by municipal solid waste leachates. Environ. Sci. Technol. 1998, 32 (23), 3825−3830. (34) Halim, C. E.; Scott, J. A.; Natawardaya, H.; Amal, R.; Beydoun, D.; Low, G. Comparison between acetic acid and landfill leachates for the leaching of Pb(II), Cd(II), As(V), and Cr(VI) from cementitious wastes. Environ. Sci. Technol. 2004, 38 (14), 3977−3983. (35) Islam, M. Z.; Catalan, L. J.; Yanful, E. K. Effect of remineralization on heavy-metal leaching from cement-stabilized/ solidified waste. Environ. Sci. Technol. 2004, 38 (5), 1561−1568. (36) United States Environmental Protection Agency. Leachability of Metals from Mineral Processing Waste; Report 600/R-04/051; USEPA: Washington, D.C., 2004. (37) Wefers, K.; Misra, C.Oxides and Hydroxides of Aluminum; Technical Report #19 for Alcoa Laboratories: Pittsburgh, PA, 1987. (38) Zhou, R. S.; Snyder, R. Structures and transformation mechanisms of the, and transition aluminas. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47 (5), 617−630. (39) Wolverton, C.; Hass, K. Phase stability and structure of spinelbased transition aluminas. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 63 (2), 024102. (40) Cai, S.-H.; Rashkeev, S. N.; Pantelides, S. T.; Sohlberg, K. Atomic scale mechanism of the transformation of γ-alumina to θalumina. Phys. Rev. Lett. 2002, 89 (23), 235501. (41) Zhang, Z.; Hicks, R. W.; Pauly, T. R.; Pinnavaia, T. J. Mesostructured forms of γ-Al2O3. J. Am. Chem. Soc. 2002, 124 (8), 1592−1593. (42) Gutiérrez, G.; Taga, A.; Johansson, B. Theoretical structure determination of γ-Al2O3. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 65 (1), 012101. (43) Knacke, O.; Kubaschewski, O.; Hesselmann, K. Thermochemical Properties of Inorganic Substances. Springer-Verlag: Berlin, Germany, 1991. (44) Folgueras, M. B.; Díaz, R. M.; Xiberta, J.; Alonso, M. Effect of inorganic matter on trace element behavior during combustion of coalsewage sludge blends. Energy Fuels 2007, 21 (2), 744−755. (45) Uberoi, M.; Shadman, F. High-temperature removal of cadmium compounds using solid sorbents. Environ. Sci. Technol. 1991, 25 (7), 1285−1289. (46) Yu, H.-C.; Yeon, D.-H.; Van der Ven, A.; Thornton, K. Substitutional diffusion and Kirkendall effect in binary crystalline solids containing discrete vacancy sources and sinks. Acta Mater. 2007, 55 (20), 6690−6704.

(47) Shek, C. H.; Gu, T. S.; Lin, G. M.; Lai, J. K. L. Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina. Appl. Phys. A: Mater. Sci. Process. 1998, 66 (4), 413−418. (48) Van der Ven, A.; Yu, H.-C.; Ceder, G.; Thornton, K. Vacancy mediated substitutional diffusion in binary crystalline solids. Prog. Mater. Sci. 2010, 55 (2), 61−105. (49) Chen, I.-W.; Wang, X.-H. Sintering dense nanocrystalline ceramics without final-stage grain growth. Nature 2000, 404 (6774), 168−17. (50) Herring, C. Effect of change of scale on sintering phenomena. J. Appl. Phys. 1950, 21 (4), 301−303. (51) Bowen, P.; Carry, C. From powders to sintered pieces: forming, transformations and sintering of nanostructured ceramic oxides. Powder Technol. 2002, 128 (2−3), 248−255. (52) Tanaka, H. Thermal analysis and kinetics of solid state reactions. Thermochim. Acta 1995, 267 (0), 29−44. (53) Smart, L. E.; Moore, E. A. Solid state Chemistry: an Introduction; CRC Press, Taylor & Francis Group, Boca Raton, FL, 2012. (54) Tang, Y.; Chui, S. S. Y.; Shih, K.; Zhang, L. Copper stabilization via spinel formation during the sintering of simulated copper-laden sludge with aluminum-rich ceramic precursors. Environ. Sci. Technol. 2011, 45 (8), 3598−3604. (55) Buscaglia, M. T.; Bassoli, M.; Buscaglia, V.; Vormberg, R. Solidstate synthesis of nanocrystalline BaTiO3: Reaction kinetics and powder properties. J. Am. Ceram. Soc. 2008, 91 (9), 2862−2869. (56) Rubio-Marcos, F.; Marchet, P.; Merle-Méjean, T.; Fernandez, J. F. Role of sintering time, crystalline phases and symmetry in the piezoelectric properties of lead-free KNN-modified ceramics. Mater. Mater. Chem. Phys. 2010, 123 (1), 91−97. (57) Branda, F.; Costantini, A.; Lanotte, L.; Matteazzi, P. Crystallization of amorphous Fe73.5Si13.5B9Nb3Cu1. J. Mater. Sci. 1997, 32 (22), 6015−6019. (58) Mehrer, H. Diffusion in Solids: Fundamentals, Methods, Materials, Diffusion-Controlled Processes. Springer: Berlin, Heidelberg, 2007. (59) Schweitzer, G. K.; Pesterfield, L. L. The Aqueous Chemistry of the Elements; Oxford University Press: New York, 2010. (60) Srivastava, V. C.; Mall, I. D.; Mishra, I. M. Equilibrium modelling of single and binary adsorption of cadmium and nickel onto bagasse fly ash. Chem. Eng. J. 2006, 117 (1), 79−91. (61) Valls, S.; Vazquez, E. Leaching properties of stabilised/solidified cement-admixtures-sewage sludges systems. Waste Manage. 2002, 22 (1), 37−45. (62) Ohlin, C. A.; Villa, E. M.; Rustad, J. R.; Casey, W. H. Dissolution of insulating oxide materials at the molecular scale. Nat. Mater. 2010, 9 (1), 11−19.

H


Cadmium Stabilization Efficiency and Leachability by CdAl4O7

Recommend Documents