Article Cite This: Environ. Sci. Technol. 2018, 52, 775−782
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Incorporation of Cadmium and Nickel into Ferrite Spinel Solid Solution: X‑ray Diffraction and X‑ray Absorption Fine Structure Analyses Minhua Su,†,∥ Changzhong Liao,§,∥ Tingshan Chan,⊥ Kaimin Shih,*,∥ Tangfu Xiao,‡ Diyun Chen,*,† Lingjun Kong,†,∥ and Gang Song†
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†
Guangdong Provincial Key Laboratory of Radionuclides Pollution Control and Resources, School of Environmental Science and Engineering and ‡Key Laboratory for Water Quality Security and Conservation of the Pearl River Delta, Ministry of Education, School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China § Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management, Guangdong Institute of Eco-environmental Science & Technology, Guangzhou 510650, China ∥ Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong SAR, China ⊥ National Synchrotron Radiation Research Center, Hsinchu Science Park, Hsinchu, Taiwan S Supporting Information *
ABSTRACT: The feasibility of incorporating Cd and Ni in hematite was studied by investigating the interaction mechanism for the formation of CdxNi1−xFe2O4 solid solutions (CNFs) from CdO, NiO, and α-Fe2O3. X-ray diffraction results showed that the CNFs crystallized into spinel structures with increasing lattice parameters as the Cd content in the precursors was increased. Cd2+ ions were found to occupy the tetrahedral sites, as evidenced by Rietveld refinement and extended X-ray absorption fine structure analyses. The incorporation of Cd and Ni into ferrite spinel solid solution strongly relied on the processing parameters. The incorporation of Cd and Ni into the CNFs was greater at high x values (0.7 < x ≤ 1.0) than at low x values (0.0 ≤ x ≤ 0.7). A feasible treatment technique based on the investigated mechanism of CNF formation was developed, involving thermal treatment of waste sludge containing Cd and Ni. Both of these metals in the waste sludge were successfully incorporated into a ferrite spinel solid solution, and the concentrations of leached Cd and Ni from this solid solution were substantially reduced, stabilizing at low levels. This research offers a highly promising approach for treating the Cd and Ni content frequently encountered in electronic waste and its treatment residues.
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INTRODUCTION
processes, often fail to satisfy the increasingly stringent standards.9,12 Developing novel, reliable, and environmentally friendly technologies to effectively control and eliminate the release of Cd and Ni from this type of electronic waste and its treatment residues is of great importance. Recent research has demonstrated a promising strategy to effectively and reliably convert metal-containing wastes into a variety of robust crystalline products.13,14 The incorporation of these waste-borne metals into widely available ceramic matrices (e.g., alumina, hematite, and kaolinite, etc.) under thermal treatment has been studied mechanistically.13−16 The metal leachability of the stabilized products can be substantially
Due to rapid advances in technology, the demand for electronic devices is growing significantly.1−3 Ni−Cd batteries, as portable power sources, are widely utilized in cordless electric utensils, cordless telephones, airplane engine starters, electric and hybrid electric vehicles, and communication-distribution systems.4−6 At the end-of-life of these products, numerous spent Ni−Cd batteries are generated, most of which are disposed of directly.2,4,7 This causes severe environmental pollution because of both the large quantity of these batteries and their high contents of Cd and Ni.8−10 Most compounds of Cd and Ni are highly toxic, and the release of these metals into the environment poses extreme risks to human health and the ecosystem. In view of the toxicological significance of Cd and Ni, special care in the disposal of Ni−Cd batteries is now mandatory in many places.11 However, conventional techniques, including pyrometallurgical and hydrometallurgical © 2017 American Chemical Society
Received: Revised: Accepted: Published: 775
August 24, 2017 October 31, 2017 December 19, 2017 December 19, 2017 DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
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Environmental Science & Technology
with x values of 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 were then prepared by sintering. XRD Analysis. After cooling to room temperature, the sintered pellets were ground into fine powders for phase transformation analysis by XRD. The step-scanned XRD pattern of each powder sample was recorded at room temperature by a Bruker D8 Advance X-ray powder diffractometer equipped with Cu Kα1,2 X-ray radiation (λ = 1.54059 Å) and a LynxEye detector. The scanning angle 2θ ranged from 20° to 120° with a step size of 0.02° and a scanning speed of 0.5 s per step. Phase identification was executed by matching the experimental XRD patterns with those retrieved from the standard powder diffraction database of the International Centre for Diffraction Data (ICDD, PDF-2 Release 2008). The Rietveld refinement of the crystal structures was performed via the TOPAS V4.0 program (Bruker AXS). EXAFS Measurements and Analysis. The Cd K-edge EXAFS spectra of the CNFs were collected at Beamline 01C at the National Synchrotron Radiation Research Center (NSRRC) of Taiwan, with the storage ring operated at 1.5 GeV and 300 mA. The EXAFS spectra were recorded in transmission mode, with all ion chambers filled with argon. The EXAFS data were analyzed using Athena and Artemis software.24 Characterization and Treatment of SNCB Sludge. Waste sludge enriched in Cd and Ni was collected from spent Ni−Cd batteries (SNCBs) dried at 105 °C for 24 h in a vacuum oven (the resulting sample being denoted SNCBS105). Ball milling was carried out to homogenize SNCBS-105 for sample analysis and immobilization. The mineral phases of SNCBS-105 were identified by XRD (Figure S2). Graphite (C, ICDD PDF no. 26-1079), cadmium hydroxide (Cd(OH)2, ICCD PDF no. 71-2137), and theophrastite (Ni(OH)2, ICCD PDF no. 73-1520) were the major crystalline phases identified. SNCBS-105 was further calcined at 600 °C for 12 h to achieve decarbonization (removal of graphite),25,26 and the resulting sample was denoted SNCBS-600. In SNCBS-600, only cadmium oxide (CdO, ICDD PDF no. 75-0594) and nickel oxide (NiO, ICDD PDF no. 89-5881) were detected by XRD. The elemental composition of SNCBS-600 was probed by scanning electron microscopy−energy-dispersive X-ray spectroscopy (SEM−EDX) (Hitachi S4800 FEG SEM) and by inductively coupled plasma optical emission spectrometry (ICP-OES) (ICP-OES 800, PerkinElmer) through the digestion method using aqua regia. The EDX results are summarized in Figure S3. In the digestion method, 20 mL of aqua regia was slowly added into 1 g of SNCBS-600 powder, boiled for 1 h, cooled, and diluted with doubly distilled deionized water, and the metal concentrations were then measured by an ICP-OES 800 (PerkinElmer). Cd and Ni were found to be the two major elements in SNCBS-600, and the molar ratio of Cd to Ni was about 1.1. To immobilize the Cd and Ni in SNCBS-600, mixtures of SNCBS-600 and hematite with a Cd/Fe molar ratio of ∼1:3 (with excess hematite available for Cd and Ni incorporation) were prepared. The treatment for SNCBs was conducted via an identical homogenization and palletization process as for the CNFs. The final products after sintering were subjected to CPLT to evaluate the stabilization of Cd and Ni in the CNF solid solutions, compared to the untreated SNCBS-600. Constant-pH Leaching Test (CPLT). The leachability of single-phase Cd-bearing samples (i.e., CdO, CdFe2O4, and CdxNi1−xFe2O4), Ni-bearing samples (i.e., NiO and NiFe2O4),
reduced, achieving the detoxification of hazardous metals in the waste stream. This is mainly attributed to the irreversible crystallographic transformation occurring in the mineral phases during the sintering process. Cd−Ni ferrite solid solutions have a wide variety of applications in industry and their properties and synthesis have attracted strong interest.17,18 For instance, in one study, Cd−Ni ferrite solid solutions were synthesized through a typical ceramic manufacturing process involving the sintering of finely milled oxides in compacted forms at high temperatures.19 Other processes, such as double sintering and co-precipitation, are also commonly used in the fabrication of ferrite solid solutions.20,21 These methods offer a technically feasible approach to incorporating Cd and Ni into a Fe-rich precursor under thermal conditions. Owing to its relatively low cost and wide availability, hematite (α-Fe2O3) is known as a feasible matrix for the immobilization of various hazardous metals in waste.17,22,23 In this work, α-Fe2O3 was used to incorporate Cd and Ni into spinel structures. The incorporation reaction pathways through the sintering of CdO + NiO + α-Fe2O3 mixtures at various temperatures were studied by X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) techniques in detail. The incorporation parameters (i.e., sintering temperature and time) for the formation of CdxNi1−xFe2O4 spinel solid solutions were systematically investigated and were then used to optimize the processing parameters for stabilizing waste sludge containing Cd and Ni. Furthermore, the chemical durability of ferrite spinel solid solutions should be evaluated via leaching experiments under acidic conditions (e.g., pH 4.0) to reflect the metal stabilization effect. Constant-pH leaching tests (CPLTs) can avoid the change of leaching behavior due to the corresponding change of leaching solution pH and were adopted to evaluate the metal stabilization effects of the final products.
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EXPERIMENTAL SECTION Materials and Sample Preparation. A series of experiments was designed and performed to probe the optimal conditions for Cd and Ni to be incorporated by hematite (αFe2O3). The optimum processing parameters, i.e., sintering temperature and time, for preparing the single-phase CdxNi1−xFe2O4 solid solution samples (CNFs) were systematically investigated. The as-received cadmium oxide (CdO, Fisher Scientific), nickel oxide (NiO, Fisher Scientific), and hematite (α-Fe2O3, Sigma-Aldrich) were used as the reaction precursors. All of the precursor materials were of analytical grade, and used without further purification. The mineralogical compositions of these materials were confirmed by XRD, as shown in Figure S1. The reaction precursors (i.e., CdO, NiO, and α-Fe2O3) were mixed by ball-milling for 1.0 h with different stoichiometric molar ratios of Cd to Ni. The ball milling was conducted in a planetary ball mill consisting of four tungsten carbide grinding jars (100 mL each) at a rotation speed of 500 rpm. Sequentially, the mixtures were dried in a vacuum oven at 105 °C for 24 h and then further homogenized by extended mortar grinding for 10 min in an agate mortar. Before thermal treatment, the dried mixtures were compacted into 20 mm diameter circular pellets under an axial pressure of 250 MPa. For the incorporation reaction, a well-control thermal treatment scheme at the targeted temperature was applied. In most cases, the dwell time was fixed at 3 h for sintering temperatures ranging from 700 to 950 °C. CdxNi1−xFe2O4 solid solutions 776
DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
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Environmental Science & Technology the SNCBS-600 sintered product, and the untreated SNCBS600 sample was assessed by the constant-pH leaching test (CPLT) method with a leaching fluid of pH 4.0 HNO3 solution.27 During the leaching process, the pH value was continually monitored by a pH meter and kept at 4.0 ± 0.1 by adding 1 M HNO3 aqueous solution in volumes ranging from 10 to 20 μL for each adjustment. The CPLT was performed the controlled room temperature (i.e., 25 ± 1 °C) using 500 mL of leaching fluid and 0.5 g of powders with mechanical stirring throughout each test. At 10 min intervals, 5 mL leachate together with suspended solids was sampled and filtered through a 0.2 μm syringe filter. The leached metal concentrations were determined by an ICP-OES 800 (PerkinElmer) with a calibration range of 10−2000 ppb (R2 > 0.9995).
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RESULTS AND DISCUSSION Formation of CdxNi1−xFe2O4 Spinel Solid Solutions. Because the composition of metals in waste streams can
Figure 2. (a) XRD patterns of the single-phase Cd−Ni−Fe spinel solid solutions (CdxNi1−xFe2O4, x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0), and (b) lattice parameters of CdxNi1−xFe2O4 spinel solid solutions, refined from the XRD patterns presented in panel a with the Rietveld refinement method, showing a linear increase of the lattice parameters of the single-phase CdxNi1−xFe2O4 products as x increased.
Figure 1. XRD patterns of the sintered samples of (a) NiO + α-Fe2O3, (b) 0.1CdO + 0.9NiO + α-Fe2O3, (c) 0.3CdO + 0.7NiO + α-Fe2O3, (d) 0.5CdO + 0.5NiO + α-Fe2O3, (e) 0.7CdO + 0.3NiO + α-Fe2O3, (f) 0.9CdO + 0.1NiO + α-Fe2O3, and (g) CdO + α-Fe2O3 for the formation of CdxNi1−xFe2O4 spinel (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) at 950 °C for 3 h. H is for hematite (α-Fe2O3, ICDD PDF no. 890597); N is for bunsenite (NiO, ICDD PDF no. 71-1179); S0, S1, S3, S5, S7, S9, and S10 are for the newly formed spinel in the (a−g) sintered samples, respectively.
incorporation process was observed. As the proportion of Ni increased (Figure 1a), the diffraction signals of residual reactants (i.e., NiO and α-Fe2O3) became more intense in the sintered samples. This implied different incorporation abilities of Cd and Ni, which might have been due to the difference in the required activation energies. With a greater amount of Ni, a higher energy input was required to overcome the high activation barrier for the formation of the final products.28 For a solid-state reaction, the driving force for mass transfer is related to the difference in chemical potential, and, therefore, Gibbs free energy, between the reactants and products.29 The standard Gibbs free energy of formation for CdFe2O4 is greater than that of NiFe2O4 by +6.98 kJ/mol (Text S1),30,31 indicating that the reaction between CdO and αFe2O3 is energetically favored over that between NiO and αFe2O3 in spinel structure formation. Therefore, at a sintering temperature of 950 °C, Cd had a much-greater affinity for incorporation into spinel than did Ni. Given optimized processing parameters, thermal-treatment schemes can achieve complete phase transformation in solid-
fluctuate, the relative incorporation of Cd and Ni depends critically on the Cd-to-Ni molar ratio. Previous experiments performed at temperatures of 700−950 °C showed that 950 °C was a preferred temperature for Cd and Ni incorporation, and the observed weight loss of CdO was insignificant. Figure 1a depicts the XRD patterns of the samples sintered at 950 °C for 3 h. The predominant product phase in the sintered samples of CdO + NiO + α-Fe2O3 exhibited characteristic Bragg reflections of the Fd3m ̅ cubic spinel group. Both Cd and Ni were able to be crystallographically incorporated into the Cd− Ni ferrite spinel solid solutions after thermally reacting with hematite. No other oxidation state of Cd during the 777
DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
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state reactions. Elevating the sintering temperature can stimulate the incorporation reaction, and intensive grinding together with an extended sintering time can enhance the homogeneity and crystallinity of the products.28,29,32 Thus, we performed a multistage sintering process in an effort to achieve the full incorporation of Cd and Ni into the Cd−Ni ferrite spinel solid solutions. The multistage sintering process included homogenization, grinding, drying, pelletizing, and multistage heat treatment. The heat treatment consisted of the following stages: first round, sintering at 950 °C for 3 h; second round, sintering at 975 °C for 12 h; third and fourth rounds, both sintering at 1000 °C for 12 h. For each sample, the fired pellet was ground into powder and then repelletized at 250 MPa for the next cycle of sintering. All of the final products obtained by sintering the CdxNi1−xFe2O4 (0 ≤ x ≤ 1) samples were identified as single-phase Cd−Ni ferrite spinel solid solutions, as inferred from their XRD patterns’ containing no secondary crystalline phases (Figure 2a). Thus, a series of CdxNi1−xFe2O4 spinel solid solutions was successfully produced. The overall reaction is expressed in eq 1. The formation reactions for the CdFe2O4 and NiFe2O4 phases are given in eqs 2 and 3, respectively:
Figure 3. EXAFS data acquired at the Cd K-edge of CdxNi1−xFe2O4 samples (x = 0.5, 0.9, and 1.0). Solid line: experimental data. Dotted line: best fit.
Figure 4. Contents of (a) Cd and (b) Ni in the leachates of CdO, NiO, CdFe2O4, NiFe2O4, and CdxNi1−xFe2O4 after 120 min of CPLT. 778
DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
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increase of the lattice parameter from 8.3420(3) Å to 8.7072(1) Å (Table S1) as x increased from 0.0 to 1.0 in CdxNi1−xFe2O4 corresponds well with Vegard’s law.34 Linear fitting of the data generated the following relationship between the lattice parameter (y) and the x value: y = 0.36513x + 8.3439 (R2 = 0.9996). The crystal radius of Cd (r (Cd2+) = 0.97 Å) is larger than that of Ni (r (Ni2+) = 0.72 Å) in the CdxNi1−xFe2O4 solid solution structure.35 This was the cause of the increasing lattice parameters of the products with higher loading of Cd, as observed in Figure 2b. Therefore, the sintered products can be expected to expand when both Cd and Ni are fully incorporated into the CNFs. To reveal the formation mechanism of these CNFs, it is important to determine which sites are occupied by the cations. EXAFS spectroscopy can offer the structural information on many elements, including bond distances and coordination numbers at molecular level.36,37 Figure 3 shows the fitting results of the EXAFS data acquired at the Cd K-edge of three CdxNi1−xFe2O4 samples (x = 0.5, 0.9, and 1.0), all of which are closely consistent with the spinel structure. In general, the octahedral environments in spinel structures contain three major Fourier peaks (at ∼2, ∼3, and ∼3.5 Å), while only two peaks (at ∼2 and ∼3.5 Å) appear for the tetrahedral environments.38 The absence of peaks at 3.0 Å in all of the Fourier forms of the EXAFS spectra for CdxNi1−xFe2O4 (x = 0.5, 0.9, and 1.0) indicates that the Cd2+ ions exclusively occupied the tetrahedral sites (8a sites). Furthermore, we note that NiFe2O4 is an inverse spinel in which all of the Ni2+ ions occupy the octahedral sites. Therefore, the most reasonable basis for Rietveld refinement was to assume that all Ni2+ occupied octahedral sites, while all Cd2+ occupied tetrahedral sites. During the Rietveld analysis, an attempt to allocate Cd atoms into octahedral sites was also conducted, but not surprisingly, this obtained a very low level of occupancy. The profiles calculated on the assumption that Cd2+ ions occupy 8a sites and Ni2+ ions occupy 16d sites (Tables S2−6) agreed very closely with the experimental data (Tables S2−6 and Figures S4−8). Leachability of CdxNi1−xFe2O4 Spinel Solid Solutions. To verify the stabilization of Cd and Ni after the formation of the CNFs, the inherent leachability of CdxNi1−xFe2O4 was assessed by CPLT at pH 4.0. For comparison, the as-received CdO and NiO, and samples of CdFe2O4 spinel and NiFe2O4 spinel synthesized in-house, were also tested by CPLT under the same acidic condition (pH = 4.0). All of the leaching samples were ball-milled into powder. The congruent dissolution of CdO, CdFe2O4, NiFe2O4, and CdxNi1−xFe2O4 in an acidic aqueous solution can be described as:
Figure 5. XRD patterns of (a) the SNCBS-600 + α-Fe2O3 mixtures sintered at temperatures ranging from 700 to 950 °C for 3 h, showing that both Cd and Ni were incorporated into the ferrite spinel solid solution, and (b) the sintered product of SNCBS-600 + α-Fe2O3 after a treatment scheme to incorporate Cd and Ni into the ferrite spinel solid solution. A is for sodium iron oxide (NaFeO2, ICDD PDF no. 76-0243); C is for cadmium oxide (CdO, ICDD PDF no. 75-0594); H is for hematite (α-Fe2O3, ICDD PDF no. 89-0597); K is for potassium iron oxide (KFe11O17, ICDD PDF no. 25-0652); N is for bunsenite (NiO, ICDD PDF no. 71-1179); and S is for the ferrite spinel solid solution formed after sintering the mixture of SNCBS-600 and hematite.
xCdO + (1 − x)NiO + α − Fe2O3(hematite) → CdxNi1 − xFe2O4
+ 2+ CdO(s) + 2H(eq) → Cd(eq) + H 2O
(4)
2+ 3+ + CdFe2O4(s) + 8H(eq) → Cd(eq) + 2Fe(eq) + 4H 2O
(5)
+ 2+ 3+ NiFe2O4(s) + 8H(eq) → Ni(eq) + 2Fe(eq) + 4H 2O
(6)
(1)
CdO + α − Fe2O3(hematite) → CdFe2O4
(2)
NiO + α − Fe2O3(hematite) → NiFe2O4
(3)
As shown in Figure 2a, the diffraction peaks of the singlephase CNFs were shifted toward lower 2θ values as x was increased from 0.0 to 1.0. This indicates that the lattice parameters increased as the Cd content was increased and the Ni content was decreased. Rietveld refinement allowed accurate determination of the lattice parameters of the CNFs, as given in Figure 2b. The lattice parameters measured here were in good agreement with those found in previous studies.17,33 The
+ 2+ 2+ CdxNi1 − xFe2O4(s) + 8H(eq) → xCd(eq) + (1 − x)Ni(eq) 3+ + 2Fe(eq) + 4H 2O
(7)
Figure 4 shows the 120 min CPLT results of the tested phases. The leachates of CdO and NiO contained very high concentrations of Cd and Ni, revealing that both oxides were highly vulnerable to proton-mediated dissolution. In contrast, 779
DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
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Figure 6. Concentrations of (a) Cd, (b) Ni, (c) Co, (d) K, (e) Na, and (f) Fe in the leachates of the untreated and sintered SNCB sludge (SNCBS600). The sintered SNCB sludge was thermally reacted with hematite (α-Fe2O3) at 950 °C for 12 h, followed by another sintering process at 1050 °C for 6 h.
the levels of Cd leached from CdFe2O4 and Ni leached from NiFe2O4 were observed to be very low. Furthermore, in the CdxNi1−xFe2O4 leachates, both the Cd and Ni concentrations were much lower still than those in the CdFe2O4 or NiFe2O4 leachates. The solid solutions with lower Cd and Ni loading produced leachates with substantially decreased concentrations of those metals (Figure 4). It can be seen that the concentrations of Cd and Ni in the CdxNi1−xFe2O4 leachates were 3 and 2 orders of magnitude smaller than those derived from CdO and NiO, respectively. This indicates that the phase transformation of Cd and Ni into solid solutions markedly
reduced their leachability, suggesting a promising strategy for processing industrial waste by adding sufficient Fe to stabilize their Cd and Ni content. SNCB Waste Sludge. Cd−Ni-bearing waste sludge was collected from SNCBs. To fully remove graphite, which can affect the stabilization process and the product texture, the SNCBS-105 sample was further pretreated at 600 °C for 12 h. The elemental composition of SNCBS-600 was analyzed by energy-dispersive X-ray spectroscopy (EDX) (Figure S3). The peaks in the spectrum corresponded well to the characteristic signals of cadmium (Cd), nickel (Ni), cobalt (Co), sodium 780
DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
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Environmental Science & Technology (Na), and potassium (K). Both Na and K were from the alkaline electrolytes in the batteries. The Co content was from an additive to improve the electrical conductivity of the electrodes. The metal contents (i.e., Cd, Ni, Co, Na, and K) in SNCBS-600 were quantified by acid digestion and ICP-OES. Table S7 lists the weight percentages and the molar fractions of those five metals. The Ni (19.07 ± 1.05 wt %) and Cd (40.13 ± 1.86 wt %) contents were relatively high. The molar ratio of Cd to Ni was around 1.1, just above the range of x values in the CdxNi1−xFe2O4 spinel solid solutions. SNCB Sludge Stabilization. The successful production of acid-resistant CNFs enabled the design of a stabilization strategy for Cd- and Ni-bearing waste. Figure 5a shows the XRD patterns of samples of SNCBS-600 + hematite sintered for 3 h at temperatures ranging from 700 to 950 °C. The incorporation reaction was initiated at 700 °C, as reflected by the characteristic Bragg reflection positions corresponding to the typical ferrite spinel structure. As the sintering temperature increased, the diffraction signals of CdO and NiO significantly declined, indicating that both Cd and Ni were transformed into the new spinel phase. Upon further increasing the treatment temperature to 950 °C, the ferrite spinel solid solution came to significantly dominate the final product composition. Eq 8 describes the incorporation reaction:
phases were formed herein after sintering (Figure 5). NaFeO2 is highly soluble in aqueous (eq 9) or acidic media.42 The exchange of Na ions for protons might also have led to the leaching-out of Na from the sintered product, causing the high Na concentration in the leachates. The substantial reduction of the leaching of metals (e.g., Cd and Ni) from the waste sludge of the SNCBs after sintering can be attributed to the successful formation of ferrite spinel solid solutions.
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(9)
ENVIRONMENTAL IMPLICATIONS Wastes containing cadmium and nickel pose a severe threat to the environment. Stabilizing cadmium and nickel by incorporating them into crystalline products is an effective approach to detoxify hazardous wastes. In this study, a series of single-phase CdxNi1−xFe2O4 solid solutions was prepared and demonstrated considerable resistance to strong acid attack. Both Cd and Ni were incorporated in large quantities into the Cd−Ni ferrite spinel structure at 950 °C. The optimal experimental parameters for the incorporation of Cd and Ni into ferrite spinel solid solutions via a solid-state reaction were determined systematically, and the stability of these solutions in an acidic environment was confirmed. The EXAFS results further revealed that Cd2+ ions occupied the tetrahedral sites (8a sites), while Ni2+ occupied the octahedral sites (16d sites) in the CdxNi1−xFe2O4 solid solutions. This information provides an important basis for stabilizing waste bearing Cd and Ni (e.g., SNCB sludge derived from spent Cd−Ni batteries) and a guide to the optimal use of the final products. The major metals in the tested SNCB sludge were Cd, Ni, Co, K, and Na. The SNCB sludge was further sintered with hematite to produce a spinel solid solution capable of incorporating the hazardous metals Cd and Ni. The CPLT results showed a substantial reduction of the leachability of hazardous metals in the final treated products compared to the untreated SNCB sludge.
SNCB sludge + hematite(α − Fe2O3) → ferrite spinel solid solution
NaFeO2 + H 2O → NaOH + Fe(OH)3
(8)
Based on the results in Figure 5a, the optimal temperature for the incorporation of Cd and Ni by hematite is 950 °C. Repeated grinding and thermal treatment are known to assist the full incorporation of metals. To assist the full incorporation of Cd and Ni by hematite, the sample of SNCBS-600 + hematite fired for at 950 °C for 12 h was further subjected to additional grinding followed by sintering at 1050 °C for 6 h. As observed from Figure 5b, the resulting product consisted mainly of the Cd−Ni−Fe spinel phase after this thermal treatment. The lattice parameter of the Cd−Ni−Fe spinel was estimated to be 8.52175(1) Å, and was related to x by the relationship y = 0.36513x + 8.34392. The stabilization of Cd and Ni in the CNF was evaluated by leaching the final sintered product (obtained by heat treatment at 950 °C for 12 h followed by 1050 °C for 6 h) under CPLT conditions at pH 4.0 for 120 min. During the CPLT, 13 aliquots were taken over the 120 min period, and three independent replicates were conducted for each leaching experiment. The measured concentrations of all of the metals (Cd, Ni, Co, K, Na, and Fe) in the leachates of the sintered SNCBS-600 plus hematite mixtures (a ferrite spinel solid solution) are shown in Figure 6. Compared to the untreated SNCBS-600, the sintered sludge products exhibited a higher resistance to strong acid attack. The leachability of the sintered product of SNCB-600 was slightly higher than those of the CdxNi1−xFe2O4 samples prepared in this study. This was likely due to the surface area of the sintered SNCBS-600 (7.39 m2/g) being greater than those of the CdxNi1−xFe2O4 samples (ranging from 1.78 to 1.96 m2/g) (Table S8). In addition to Cd and Ni, the Co and K concentrations in the leachates also decreased significantly after sintering, potentially indicating the transformation of Co and K into more stable crystal structures.39−41 K has been reported to strongly stabilize defect spinel structures by a potassium-substituted magnetite oxidation process.39 Moreover, the KFe11O17 and NaFeO2
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b04350. Additional details on standard Gibbs free energy. Tables showing lattice parameters, Rietveld refinement results, metal contents, and specific BET surface areas. Figures showing XRD patterns, element species determination, and profiles derived from Rietveld refinement analysis. (PDF)
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AUTHOR INFORMATION
ORCID
Kaimin Shih: 0000-0002-6461-3207 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Research Grants Council of Hong Kong (grant nos. 17212015, 17257616, C7044-14G, and T21-771/16R), the National Natural Science Foundations of China (grant nos. 51708143, 41372364, U1501231, and U1612442), the Project of Radioactive Pollution Control and 781
DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
Article
Environmental Science & Technology
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Resource Reuse Key Laboratory (grant no. 2012A061400023), and Guangzhou University’s Training Program for Excellent New-recruited Doctors (grant no. YB201710), High Level University Construction Project (Regional Water Environment Safety and Water Ecological Protection), and GDAS’ Special Project of Science and Technology Development (grant no. 2017GDASCX-0834). The use of facilities from the National Synchrotron Radiation Research Center (NSRRC) of Taiwan is gratefully acknowledged.
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DOI: 10.1021/acs.est.7b04350 Environ. Sci. Technol. 2018, 52, 775−782
