Effective Extraction of Cr(VI) from Hazardous Gypsum Sludge via

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Effective Extraction of Cr(VI) from Hazardous Gypsum Sludge via Controlling the Phase Transformation and Chromium Species Weizhen Liu,†,‡ Jiayi Zheng,†,‡ Xinwen Ou,† Xueming Liu,†,‡,|| Yao Song,†,‡ Chen Tian,†,‡,|| Wencong Rong,|| Zhenqing Shi,†,‡,|| Zhi Dang,†,‡ and Zhang Lin*,†,‡,|| †

School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), Guangzhou, 510006, China || Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, Guangzhou, 510006, China Downloaded via UNIV OF SUNDERLAND on October 25, 2018 at 08:44:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Through controlling the phase transformation and Chromium species under hydrothermal condition, the Cr(VI) was extracted fully from hazardous Cr(VI)-containing gypsum sludge, with a very high efficiency of more than 99.5%. Scanning transmission electron microscopy, X-ray absorption fine structure, and density functional theory calculation results revealed that the dissolution-recrystallization of CaSO4·2H2O into CaSO4 was the key factor to fully release the encapsulated Cr(VI). Moreover, the mineralizer (persulfate salt) provided H+ and SO42− ions, the former made an acidic condition to transform the released CrO42− into the specie (Cr2O72−) with less similarity to SO42−, which further prevented the recombination of the released Cr(VI) with gypsum; and the latter is essential to accelerate crystal growth of calcium sulfate so as to enhance Cr(VI) extraction. This work would provide an instructive guidance to fully extract heavy metals from hazardous solid wastes via the control over crystal transformation and the pollutant species.

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INTRODUCTION

closely related to the properties of the solid phase, such as size, surface defects, and aggregation state. For instance, Cr(VI)-containing gypsum (CaSO4·2H2O) sludge is a typical hazardous solid waste widely derived from chemical industry due to the high mobility, strong toxicity, and carcinogenicity of Cr(VI).18,19 The common used disposal methods based on reduction and immobilization of chromium would likely cause the reoxidation and rerelease of the toxic Cr(VI).20−22 Therefore, directly extracting Cr(VI) from solid is highly desired to eventually avoid the environmental risk. However, fully extract Cr(VI) from the gypsum sludge is very difficult for the following reasons: (i) the CrO42− can theoretically substitute SO42− in lattice during the formation of gypsum crystals, due to their identical charge, similar tetrahedral structure and comparable thermochemical radii;23,24 (ii) the presence of CrO42− as an impurity could influence the growth of gypsum into small, defective and aggregated crystals,25 during which CrO42− would be firmly adsorbed on the gypsum surface or entrapped in the aggregated particles. Therefore, the techniques to fully release

As the most common contaminants in environment, heavy metals have attracted global concern owing to their toxicity, nonbiodegradability, and consequent persistence.1 Therefore, extracting heavy metals from industrial or municipal hazardous solid wastes to reduce their discharges is extremely essential to human health and resource recovery. This strategy is particularly important for developing countries, such as China, where the heavy metals pollution is unparalleled and serious.2,3 Generally, the hazardous solid wastes mainly consist of crystalline or amorphous inorganic compounds contaminated with highly toxic heavy metals, which are adsorbed on the surface of solid phase, entrapped in aggregated particles or incorporated into crystal lattice.4,5 Over the last decades, many methods have been studied to extract heavy metals from solid waste,6−8 including chemical extraction with various leaching agents,9−12 bioleaching,13,14 vacuum chlorinating,15 supercritical fluid extraction with CO2 (SFC)16,17 and so on. However, the widespread practical application of current methods is hindered by insufficient extract efficiency,6,10 especially for the heavy metals in encapsulated state (including physical-entrapped and lattice-incorporated states). Few studies recognized that, besides the occurrence states of heavy metals in solid wastes, the extract efficiency is also © XXXX American Chemical Society

Received: May 22, 2018 Revised: September 23, 2018 Accepted: October 16, 2018

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DOI: 10.1021/acs.est.8b02213 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Characterization. The characterization of the initial gypsum sludge and the solid products after hydrothermal treatment was described in detail in the SI, including XRD, XPS, SEM, and STEM-XEDS. The leaching procedure and concentration test methods of Cr(VI), together with the details of the Cr K-edge XAFS spectra analysis and DFT (density functional theory) calculation were also presented in the SI.

Cr(VI) though controlling not only the occurrence states of Cr(VI) but also the properties of solid phase is highly desired. Hydrothermal treatment is an effective method to accelerate the crystallization, crystal growth and lattice perfection.26 In our previous study, it was revealed that the fast crystal growth under hydrothermal treatment could dramatically increase the crystal size and decrease the specific area of the nanoparticles in industrial waste, which further facilitated the desorption of heavy metals from the nanowaste (Scheme 1-I).27,28 Moreover,

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RESULTS AND DISCUSSION Analysis of Initial Sludge. The Cr-containing gypsum sludge in this work is a classic solid waste generated from a

Scheme 1. Extracting Cr(VI) of Different Present States in Industrial Solid Wastes

it was also reported that hydrothermal treatment can control the phase transformation and the aggregation state of the particles.29 Therefore, heavy metals adsorbed on surface defects or encapsulated in the aggregated crystals could potentially be fully extracted with hydrothermal treatment from industrial solid wastes. Herein, we proposed a facile and effective method to extract encapsulated Cr(VI) from gypsum sludge. It is noticed that calcium sulfate has three forms, including dihydrate (CaSO4· 2H2O, DH), hemihydrate (CaSO4·0.5H2O, HH) and anhydrite (CaSO4, AH), among which DH is stable at low temperature and AH is stable at high temperature.30 Hence, we hypothesized that, the crystal reforming process under different hydrothermal conditions might facilitate the release of the encapsulated Cr(VI) from gypsum (Scheme 1-II). In this study, Cr extraction based on the phase transformation was designed and studied systematically. Moreover, the addition of mineralizer into the above process would probably affect the behavior of Cr, thus the effect of mineralizer on Cr extraction efficiency was also investigated.

Figure 1. (a) SEM image of the initial sludge. (b) Cr 2p XPS spectra of initial sludge and chromium compounds. Vertical dotted lines are the binding energies of Na2CrO4 for Cr 2p1/2 and Cr 2p 3/2 at 589.0 and 579.8 eV, respectively.31 (c) Cr(VI) removal efficiency after each washing (column) and the cumulative removal efficiency (circle and curve) with (red) or without (gray) milling. (d) SEM image of the gypsum sludge after quick rinse. (e) STEM-XEDS analysis of rinsed initial sludge: HAADF image and corresponding EDS elemental mappings. The line profile (blue) shows the intraparticle distribution of Cr.

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chlorate industry in Inner Mongolia, China, and its detailed formation process is described in the SI. With the moisture content of 25% and total Cr content of 5726 mg·kg−1 (Table S1), the dried initial sludge was composed of 85% calcium sulfate (43.18% DH, 41.46% HH) and 15.36% NaClO3 (detailed in SI, Figure S1a). The SEM images of the initial sludge (Figure 1a) show that the particles are aggregated in the size of several μm. The rough surface of the particles (pointed by the arrows) indicated the incomplete crystallization during the formation. The Cr 2p XPS spectra show that the Cr exists as CrO42− and is distributed evenly with depth (Figure 1b). Moreover, the Cr(VI) removal efficiency was only 83.6% even after 10 times of water washing, meanwhile the sample was still hazardous waste (as shown in SI). In contrast, the Cr(VI)

EXPERIMENTAL SECTION Cr(VI) Extraction under Hydrothermal Conditions. Hydrothermal treatment with mineralizers was used to extract Cr(VI) from the initial sludge as detailed in the Supporting Information (SI). Various reagents were used as mineralizers in order to assess their effects on Cr(VI) extraction and screen out the optimal mineralizer. Parallel experiments were also carried out to study the effects of heating time and standing time on the extraction efficiency of Cr(VI). Herein, the “heating time” is defined as the soaking time at a constant hydrothermal temperature, whereas the “standing time” is the time interval between taking samples (reactors) out of oven until centrifugal separation. B

DOI: 10.1021/acs.est.8b02213 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 4. Phase evolution and the SEM images of the samples under different hydrothermal conditions with K2S2O8 as mineralizer.

Figure 2. Optical and SEM images of the samples: (a) the initial sludge, (b) after hydrothermal treated without mineralizer, (c) using K2S2O8 as mineralizer. Pictures from the top row to the bottom are the optical images, SEM images and magnified SEM images, respectively. (hydrothermal temperature of 120 °C, heating time of 4 h, standing time of 24 h).

Figure 3. Extraction efficiency of Cr(VI) under different hydrothermal temperature and heating time (K2S2O8 0.5M, standing time of 12 h).

removal efficiency was improved after sample was milled, indicating the presence of encapsulated Cr(VI) (Figure 1c). In order to study the spatial distribution of Cr(VI) on the phases of calcium sulfate in gypsum sludge, the initial sludge was rinsed quickly with the mixture of ethanol and distilled water to remove soluble NaClO3. As shown in the X-ray powder diffraction (XRD), DH was the predominant phase (SI Figure S1b) with remained Cr(VI) of 45.6%. For random small gypsum particle after milled, the high angle annular dark field (HAADF) images and elemental mappings obtained on scanning transmission electron microscopy with X-ray energy dispersive spectroscopy (STEM-XEDS) show that the Cr distribution matched well with the shapes of gypsum particles (Figure 1e and SI Figure S2). The line profile (Figure 1e) confirmed the presence of Cr inside the particle. These results proved that Cr was not only adsorbed on the surface but also encapsulated in the gypsum.

Figure 5. Cr K-edge EXAFS data of initial gypsum sludge and the supernatant after treatment: (a) unfiltered k3-weighted K-space EXAFS data, (b) Magnitude part and of the FT R-space, (c) Real part of the FT R-space. C

DOI: 10.1021/acs.est.8b02213 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Table 1. Cr K-Edge EXAFS Fitting Results (Fit Range: 2.5 Å−1 < k < 9.5 Å−1 and 1 Å < R < 3 Å)a sample

atomic path

Na2CrO4 initial sludge Na2Cr2O7

Cr−O Cr−O Cr−O Cr−Cr Cr−O Cr−Cr

solution

CNb 3.4 3.8 3.0 0.9 2.6 0.9

Rc (Å)

(0.1) (0.2) (0.2) (0.1) (0.2) (0.1)

1.65 1.65 1.60 3.16 1.61 3.16

(0.02) (0.03) (0.02) (0.05) (0.02) (0.05)

σ2d (Å)

R-Factore

0.002 (0.001) 0.002 0.003 (0.001) 0.004 (0.002) 0.003 0.004

0.01 0.01 0.004 0.008

a The structural parameters comfirm that the Cr in initial sludge is CrO42− and in treated supernatant is Cr2O72−. bCN: coordination number. cR: bond distance. dσ2: Debye−Waller, eR-Factor: goodness of fitting.

sludge consisted of aggregated small particles (Figure 2a). After the sludge was treated without mineralizer, it changed to yellow crystal clusters consisting of self-assembled needle-like crystals with significantly increased size (Figure 2b). In contrast, with persulfate as mineralizer, the product was white powders consisting of dispersive crystals of hundreds-μm with smooth surfaces (Figure 2c). As mentioned above, the hydrothermal temperature and heating time would influence the phase transformation between different forms of calcium sulfate in gypsum sludge. The effects of these two factors were studied with K2S2O8 as the mineralizer (Figure 3). Obviously, the extraction efficiency increased with increasing hydrothermal temperature and heating time. However, temperature seems to be a key factor. When the temperature was below 100 °C, the efficiency was unsatisfied no matter how long the heating time was. Once the temperature reached 120 °C, Cr was completely extracted (>99.5%) within 12 h, meanwhile the phase DH transformed into AH (Figure S4). After AH was formed at 120 °C, it was transformed gradually into DH during cooling and standing. These phase evolutions were agreed with the temperaturedependent Ksp and phase diagram of DH and AH (SI Figure S5).32 Mechanism Study. (i) Phase Transformation and Micromorphological Change. As shown in Figure 4, at 100 °C without phase transformation, the crystal size of DH was polarized to either small size of several μm or large size of hundreds μm (Figure 4b), but the final DH crystals still had rough surface with numerous defects (Figure 4c). In contrast, at 120 or 180 °C, DH was transformed into regular cuboid AH crystals (Figure 4d and f) which were also confirmed by XRD. Subsequently, the AH crystals formed at 120 °C changed back to well-dispersed and dense DH crystals with smooth surface after standing. The crystal size was more than 50 times larger than that of the crystals in initial sludge (Figure 4e). The morphologies of DH and AH were quite different according to their SEM images (Figure 4), and the crystal structures of DH and AH were also distinct (SI Figure S6). These indicate that the phase transformation between DH and AH is a dissolution-

Figure 6. Binding energy (Eb) of different ion species (SO42−, CrO42−, and Cr2O72−) with calcium sulfate (DH/AH) calculated by DFT method.

Screening of Mineralizer and Hydrothermal Conditions. Hydrothermal treatment with different mineralizers at optimized temperature (120 °C, see below) and with heating time of 4 h was conducted to extract the encapsulated Cr(VI), and the corresponding extraction efficiency was compared (SI Figure S3a). It was found that, without mineralizer, the extraction efficiency of Cr(VI) was only 67%. However, it was increased significantly with mineralizers, following the sequence of carbonate < bicarbonate < sulfate < persulfate salts. After treated with the mineralizer of carbonate, bicarbonate, or sulfate, the extraction efficiency of Cr(VI) was between 90%−93%, however the product has Cr(VI) leaching concentration of more than 5 mg·L−1, indicating it is still a hazardous solid. By contrast, K2S2O8 exhibited the highest extraction efficiency of Cr(VI) of 99% (SI Figure S3a), and the Cr(VI) leaching concentration of product was around 1.8 mg·L−1, meeting the Chinese national standard for general industrial solid waste ( 1 pH > 7 (8)

In this study, the Cr K-edge EXAFS analysis (Figure 5 and Table 1) shows that the EXAFS fitting parameters of Cr in the initial sludge are similar to that of Na2CrO4 reference. Because the CrO42− is of perfect tetrahedral symmetry, it has only one Cr−O bond distance and the fitting local first-shell Cr−O bond distance is around 1.65 Å. In contrast, EXAFS fitting result of Cr of the supernatant solution after hydrothermal treatment by persulfate is similar to that of Na2Cr2O7 reference. For the Cr2O72− ion, the Cr atom is surrounded by four tetrahedrally arranged oxygen atoms at a distance ∼1.60 Å, and Cr−Cr single scattering contributions are also to be expected in this R range, with bind distance ∼3.16 Å. This is confirmed that the dominating Cr species in the initial sludge was CrO42−, but it changed to Cr2O72− in the supernatant after the hydrothermal treatment with persulfate. In addition, there was no Cr signal in Cr K-edge XANES of the solid product (SI Figure S7), indicating that Cr was extracted completely. Meanwhile, the SO42− ions from persulfate are essential ions for crystal growth of calcium sulfate so as to enhance Cr(VI) extraction. This is because that the crystal growth rate follows a power law expression: G = kgΔCg, where G is the growth rate (in μm/min) of crystals with the volume median mean size, g is reaction order, kg is kinetic growth rate constant and ΔC is the supersaturation (which is the SO42− concentration in excess of the solubility value at the test conditions in mg/L).37 It means that increase SO42− concentration can accelerate

(1) At the appropriate hydrothermal temperature, CaSO4· 2H2O is converted into CaSO4 phase, leading to a dissolution-recrystallization process which releases the encapsulated CrO42− ions. (2) Persulfate is decomposed at hydrothermal temperature, providing H+ and SO42−. Under the acidic condition, CrO42− changes into Cr2O72− ions, which is less similar to SO42− and easier to be released from calcium sulfate, preventing Cr(VI) recombination with calcium sulfate. (3) The provided SO42− by persulfate further accelerate crystal growth of calcium sulfate so as to enhance Cr(VI) extraction. (4) Authough the sulfate and hydroxyl radicals produced by heat-activated persulfate can oxidize and eliminate the trace organic matter in the system, it appears to be not the major factor for complete extraction of Cr(VI) from gypsum sludge. E

DOI: 10.1021/acs.est.8b02213 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology To the best of our knowledge, this is the first study to provide a feasible strategy to extract encapsulated Cr(VI) completely from solid waste. The kilo-scale experiment (SI Table S3) confirms the practicability and feasibility of this strategy. And the extracted Cr(VI) in solution can be recycled after treated by normal chemical methods. Additionally, the simplicity and low cost of hydrothermal method are beneficial for its wide application in industry. Therefore, this study might provide a theoretical guidance to extract pollutants from solid wastes through manipulating crystal states of the solid phase and the pollutant species.

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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.8b02213.

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Additional data and experimental details are presented (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: 86-20-39380503; fax: 86-20-39380508; e-mail: zlin@ scut.edu.cn. ORCID

Zhenqing Shi: 0000-0003-1721-5369 Zhang Lin: 0000-0002-6600-2055 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21607045, 21836002, and 21477129), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569), Guangzhou Science and Technology Project (No. 201804010189) and the Fundamental Research Funds for the Central Universities (No. 2018MS42). Lan Ling and Xiuyu Gong in Tongji University (Shanghai) are greatly acknowledged for STEM-XEDS analysis. We also thank the beamline 4W1B (Beijing synchrotron radiation facility) for providing the beam time.

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DOI: 10.1021/acs.est.8b02213 Environ. Sci. Technol. XXXX, XXX, XXX−XXX