- Email: [email protected]
Remediation of chromium(VI) ions as chromium oxide xerogel via gamma-radiolysis of aqueous waste discharge
Journal Pre-proofs Remediation of chromium(VI) ions as chromium oxide xerogel via gammaradiolysis of aqueous waste discharge Komal C. Shrivastava, Shailaja P. Pandey, Sanjukta A. Kumar, Ashok K. Pandey, Anil K. Debnath, Amit P. Srivastava, Geeta R. Patkare, Geogy J. Abraham PII: DOI: Reference:
S1383-5866(19)31977-X https://doi.org/10.1016/j.seppur.2019.116291 SEPPUR 116291
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
11 May 2019 2 November 2019 4 November 2019
Please cite this article as: K.C. Shrivastava, S.P. Pandey, S.A. Kumar, A.K. Pandey, A.K. Debnath, A.P. Srivastava, G.R. Patkare, G.J. Abraham, Remediation of chromium(VI) ions as chromium oxide xerogel via gamma-radiolysis of aqueous waste discharge, Separation and Purification Technology (2019), doi: https:// doi.org/10.1016/j.seppur.2019.116291
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Remediation of chromium(VI) ions as chromium oxide xerogel via gamma-radiolysis of aqueous waste discharge Komal C. Shrivastavaa,*, Shailaja P. Pandeya, Sanjukta A. Kumara,**, Ashok K. Pandeyb, Anil K. Debnathc, Amit P. Srivastavad, Geeta R. Patkaree, Geogy J. Abrahamf aAnalytical
Chemistry Division, bRadiochemistry Division, cTechnical Physics Division,
dMechanical
Metallurgy Division, eFuel Chemistry Division, fMaterial Processing & Corrosion
Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India
ABSTRACT Radiolysis of water during -irradiation produces oxidizing and reducing species which can be used for the bulk oxidation or reduction of target toxic species. In the present work, radiolysis of water has been used for the reduction of Cr(VI) to Cr(III) ions in the simulated chrome tan liquor solution samples, and simultaneously precipitation as a chromium oxide hydrate gel. Sodium formate scavenger has been found to be the best suited, among various scavengers studied, not only for the removal of oxidizing species produced during -radiolysis of water but also simultaneous precipitation of thus formed Cr(OH)3 as chromium oxide hydrated to gel. The chromium oxide hydrate gel has been subjected to thermal treatment for 1
an objective of converting it to the chromium oxide xerogel which has potential applications for catalysis. Various techniques, such as BET, XRD, XPS, HRTEM and Raman spectroscopy, have been employed for studying the changes in the physical, crystalline and oxidation states in each step observed in thermal treatment starting from the hydrate gel to chromium oxide xerogel. The results showed the formation of crystalline mixed valence chromium oxide xerogel having specific surface area 20.9 m2 g-1 and pore volume 0.036 cc g-1 after thermal treatment.
Keywords: Chromium(VI) reduction; Aqueous -radiolysis; Precipitation; Thermal treatment; Mixed valence chromium oxides.
Corresponding Author * Email: [email protected]; Tel. +91-22-25590918 (K.C. Shrivastava) ** Email: [email protected]; Tel. +91-22-25590316 (S.A. Kumar)
1. Introduction The toxicity of Cr species is highly dependent on their oxidation states. Cr(VI) is usually the product of anthropogenic activities and has been documented to be toxic, mutagenic and carcinogens [1,2]. Contrary to Cr(VI), Cr(III) does not bear any toxicological relevance and it is an essential trace element in human nutrition [3]. Therefore, the Cr(VI) containing aqueous waste discharges from concerned industries have to be treated for: (i) removal of Cr(VI), (ii) reduction of Cr(VI) to Cr(III), and (iii) sorption-reduction of Cr(VI). There are several techniques for the recovery of Cr(VI) ions from aqueous media such as ion-exchange, chemical precipitation, membrane filtration, adsorption using tailored sorbents, magnetic nanoparticles, 2
biomass and electrochemical treatment [4-8]. In another strategy, the reduction of Cr(VI) to its less toxic form Cr(III) has been done using chemical, electrochemical, photochemical and bioreductions [9-15]. For developing efficient strategies for the remediation of Cr(VI), the several attempts have been made to combine the sorption and reduction steps simultaneously [16-19]. However, the applications of all these methods of the Cr(VI) remediation in aqueous discharges is not amenable to scale up due to requirements of a large aqueous volume processing. Free radicals based processes have shown a potential possibility of treating different waste/polluted water streams by selectively reducing or oxidizing the toxic species to their less toxic forms. The electron beams and -radiations sources, such as 60Co and
137Cs,
have been
used to generate oxidizing and reducing free radicals for the treatments of the aqueous waste streams [16-21]. The major advantage of the ionizing radiation based water radiolysis is attributed to uniform formation of the free radicals in a large volume waste aqueous stream with fast kinetics. Alrehaily et al. have studied the synthesis of chromium oxide (Cr2O3) nanoparticles by -radiolysis of Cr(VI) dissolved in water [18]. In this process, the solution was deaerated by purging argon gas before -radiolysis. This approach is good for small scale syntheses of Cr2O3 nanoparticles, but difficult for adopting for the large volume waste treatment process. Djouider has reported another route for the -radiolysis based treatment of Cr(VI) containing aqueous waste [17]. He has used nitrous oxide bubbling prior to irradiation of toxic Cr(VI) in formate containing aqueous solutions. This method has been found to be very efficient for the quantitative conversion of Cr(VI) to Cr(III), but the purging of solution by N2O is not desirable. Therefore, there is a need to further study -radiolysis based Cr(VI) reduction and precipitation method under ambient conditions without using any purging gas. In the present work, a systematic study has been carried out to reduce Cr(VI) ions in a simulated chrome tan liquor solution with -radiations induced radiolysis followed by in situ 3
precipitation and thermal conversion to xerogel without using any purging gas. The choice radiations (60Co) for radiolysis has been based on the fact that it does not involve a high value machine and can be applied for the larger volume of flowing solutions. The -radiolysis of water produces both oxidizing and reducing free radicals and, therefore, oxidizing free radical scavengers for converting these to reducing species have been studied using 2-propanol ((CH3)2CHOH), formic acid (HCOOH) and sodium formate (HCOONa). Cr(III) hydrolyzes at neutral pH and get precipitated as Cr(OH)3, while Cr(VI) existing as CrO42- anions remains soluble at neutral pH [20-22]. Hence, the chemical conditions have been optimized to precipitate Cr(OH)3 directly, and subject it to thermal treatment for obtaining xerogel, which has a number of potential applications. Different characterizations techniques such as TGA/DTA, XRD, Raman spectroscopy, XPS and HRTEM have been used to understand the mechanism involved in the formation of the xerogel starting from aqueous Cr2O72-/CrO4anions. The novelty of the present work is that the Cr(VI) reduction by the -radiolysis can be performed at basic pH for a large volume sample where thus formed Cr(III) can be readily precipitated and converted to useful material without involving catalysts.
2. Experimental section All the chemicals such as K2CrO4 (Sigma–Aldrich, Steinem, Switzerland), diphenyl carbazide (DPC) (Merck, Mumbai, India), formic acid (Sigma–Aldrich, Steinem, Switzerland), sodium formate (Merck, Mumbai, India) and 2-propanol (S.D. Fine Chem. Limited, India) were AR grade and were used as received. -Irradiations were carried out using 60Co -source (dose rate: 7.8 kGy h-1 as obtained by Fricke dosimetry) procured from Board of Radiation and Isotope Technology, Mumbai, India. The -irradiations was carried out in borosilicate glass vessels. The spectrophotometric analysis of Cr(VI) contents in the solutions were carried out by UV-Vis spectrophotometer supplied by Ocean Optics Inc. Netherland after developing color 4
using diphenylcarbazide (DPC) as described elsewhere [23]. Total Cr contents of the solutions were determined by Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) (Spectro Arcos, Germany make). High Resolution Transmission Electron Microscopy (HRTEM) studies were carried out using Zeiss Libra 200 FE TEM (Carl Zeiss AG). A drop of the sample dispersion was placed on a carbon coated copper grids (0.3 cm diameter, mesh size of 200 holes/cm) and left to dry overnight at room temperature. HRTEM images were analyzed with the help of Image J software. The oxidation states of Chromium were measured by X-ray Photoelectron Spectroscopy (XPS) using MgK (1253.6 eV) X-ray source and a DESA-150 electron analyzer (M/s. Staib Instruments, Germany). The binding energy scale was calibrated to Au-4f7/2 line of 83.95 eV. The analyzer was operated on 40 eV pass energies and pressure in the chamber during analysis was kept as ~ 7 x 10-9 Torr. The XPS spectra were fitted using XPSPEAK41 software. To study the thermal decomposition of precipitated Cr(OH)3, TG and DTA curves were recorded using Mettler Thermoanalyzer (model: TGA/SDTA851??/MT5/LF1600) in dry air at room temperature to 850 °C at a heating rate of 4 °C min-1. Powder X-ray diffraction (XRD) was recorded at each decomposition step to confirm the formation of products obtained at different temperature. All the heat treated samples of different temperature were cooled to room temperature in the respective environment, and characterized by recording room temperature XRD on Rigaku Miniflex–600 diffractometer (??-2?? geometry) with graphite monochromatized Cu K1 radiation (=1.5406 Å) at a scanning rate of 1° (2) per minute in 2 range of 10-100°. Raman scattering measurements was performed using HR 800 Horiba Jobin Yvon Micro laser Raman spectrometer with a laser excitation wavelength of 633 nm. The -radiolysis of the aqueous samples was carried out by filling in the stopper glass bottle of to the maximum to avoid aerated space in the bottle. Typically, 10 mL of a solution containing 0.43 mM CrO42- and 1.35 M of scavenger (CH3)2CHOH / HCOONa / HCOOH were 5
irradiated in 60Co -source for a fixed time period. The pH changes were recorded by measuring pH before and after -irradiation. The concentrations of CrO42- and scavengers were varied to optimize the maximum conversion efficiency of Cr(VI) to Cr(III). Finally, the Cr(III) was precipitated at basic pH. The precipitate were vacua filtered and green color Cr(OH)3 was formed which was washed with water and dried. This precipitated Cr(OH)3 was subjected to thermal heating in the air to obtained chromium oxide xerogel.
3. Results and discussion 3.1.
Cr(VI) reduction by -radiolysis of water
Cr(VI) speciation depends mainly upon pH of the solution. For example, H2CrO4 is the predominant species at pH<1, CrO42-/Cr2O72- exists at pH>8, and both HCrO4- and Cr2O72- exist on intermediate pH (2 to 6) as suggested by speciation modeling using pKa and other relevant parameters [24]. The pH values of the solutions having 0.43 mM Cr(VI) and 1.35 M of oxidizing free radical scavengers such as HCOOH, HCOONa, and (CH3)2CHOH were found to be 1.9, 7.3 and 8.1, respectively. This seemed to suggest that the Cr(VI) species existing would be HCrO4- and Cr2O72- in a solution having HCOOH. However, it would be single Cr2O72- species in solutions having HCOONa or (CH3)2CHOH. After -irradiation, the pH of solutions having HCOOH, HCOONa, and (CH3)2CHOH were changed to 2.1, 9.0, and 11.0, respectively. The increase in pH was attributed to formation of hydroxyl ions during radiolysis of water. The -radiolysis of water produces several species such as eaq-, OH•, H•, H2, H2O2, H+, and OH-. Among these, the two reducing transient species are solvated electron (eaq-) and hydrogen atom (H•). Hydroxyl radical (OH•) is the major transient oxidizing radical. The stable molecular products are formed by these transient species, which can also to act as reducing or oxidizing species for Cr(VI). However, it is important to note that the radiation chemical yields (G-values) of -radiolysis of water vary as a function of pH of water. As reported by Spinks et 6
al., the G-values for OH• and e-aq are maximum at pH 6.8 to 9.5 [25]. The scavengers for the oxidizing radical OH• reported to the literature are 2-propanol and formate, which convert OH• to reducing species as given below [17-19, 26-27]. CH3)2CHOH + •OH → (CH3)2C•OH + H2O
(1)
OH• + HCO2−(HCO2H)→H2O + CO2•−(CO2H•)
(2)
H• + HCO2− (HCO2H)→H2 + CO2•− (CO2H•)
(3)
Cr(VI) + 3CO2•− → Cr(III) + 3CO2
(4)
Cr(VI) quantitative conversion to Cr(III) as a function of -irradiation time in the presence of scavengers such as HCOOH, HCOONa and 2-propanol are shown in Fig. 1. The conversion of Cr(VI) to Cr(III) was obtained using following an equation:
Conversion (%)
[Cr]t [Cr]Cr(VI) [Cr ]t
100
(5)
Where [Cr]t is total concentration of Cr in acidified aqueous sample obtained ICP-AES (specific to elemental Cr), and [Cr]Cr(VI) is obtained by spectrophotometry (specific to Cr(VI)DPC colored complex) of the same sample after -irradiation. Before -irradiation, it was observed that [Cr]t was equal to [Cr]Cr(VI) indicating the absence of Cr(III) in the aqueous sample. As can be seen from Fig.1, the Cr(VI) conversion to Cr(III) increased form -irradiation time and completed at 10 min in the case of solutions containing HCOOH. In the presence of (CH3)2CHOH or HCOONa, the quantitative conversion (>98%) of Cr(VI) to Cr(III) was achieved after -irradiation time of 25 min and 15 min, respectively. It was also seen from Fig. 1 that the onset of reduction of Cr(VI) had a lag time of 5 min and 2 min in the presence of scavenger (CH3)2CHOH and HCOONa, respectively. This could be attributed to pH of HCOOH, HCOONa, and (CH3)2CHOH as 2.1, 9.0, and 11.0, respectively. At lower pH, hydrate electrons are rapidly scavenged by hydrogen ions and get converted to hydrogen atoms, thereby increasing the effective chemical yield of reducing species. The G-values for OH• and e-aq are 7
reduced above pH 9.5 [25]. The observed trend in the time required for the Cr(VI) quantitative conversion to Cr(III) by the scavenger was: (CH3)2CHOH > HCOONa > HCOOH. It was observed from the literature that Cr(III) should be remained soluble as aqueous Cr3+ at lower pH, but it would get precipitated as Cr(OH)3 maximum at pH 7 to 9.72 [28]. Therefore, HCOONa was found to be better choice of the three scavengers studied for the Cr(VI) conversion to Cr(III) and subsequent precipitation as Cr(OH)3 during -radiolysis of water.
100
Conversion (% )
80
60
40
20
0 0
10
20
30
40
Time (min)
Fig. 1. Cr(VI) quantitative conversion to Cr(III) in -radiolysis (dose rate 7.8 KGy/h) of aqueous solutions containing 0.43 mM Cr(VI) and 1.35 M HCOOH/HCOONa/ (CH3)2CHOH as a function of irradiation time. To examine the effect of HCOONa concentration, the mole proportion of chromate to HCOONa was varied as 1:3000, 1:1500, 1:760, 1:380, and the solutions were -irradiated for the time ranging from 0 to 30 min. The choice of HCOONa was based on a fact that it gave basic pH where Cr(III) got readily precipitated. It can be seen from the plot given in Fig. S1 (Supplementary Information, S.I.), the reduction of Cr(VI) to Cr(III) could be achieved quantitatively within 25 min in all the four cases. However, the lag time for onset of the reduction in solutions, having Cr(VI): HCOONa mole proportion up to 1:760, was nearly same 8
2.5 min. Whereas, the lag time was increased significantly to 10 min in the solution having 1:380 mole proportion. This could be related to drop in G-value of reducing species formed in -radiolysis when formate concentration was reduced to a threshold limit, which affected eventually the kinetics of Cr(VI)-reduction. On varying concentration of Cr(VI) at a fixed optimized concentration of HCOONa and -ray dose, the Cr(VI) conversion to Cr(III) was reduced up to 45 % at a 0.70 mM concentration but did not decrease thereafter, see Fig. S2(S.I.). Also, thus formed green colored Cr(OH)3 did not precipitate below 0.70 mM concentration in -irradiation time of 30 min. However, the conversion could be improved gradually to 60%, 76%, 86% and 97% by increasing irradiation time from 30 min to 3 h, 4h, 5h and 6 h, respectively, under the similar conditions. The increase in conversion with irradiation time seemed to suggest that the reduction of Cr(VI) was limited by the kinetic of production of reducing species during -radiolysis when concentration of Cr(VI) increased in the solution for a given -ray dose rate. However, the Cr(VI) quantitative conversion to Cr(III) could be achieved by increasing the -irradiation time (or increasing the -dose). Irradiation time of 6 h was found to be sufficient not only for 97% conversion but also for 100% precipitation of Cr(III) ions as Cr(OH)3, see Table S1 (S.I.). A simulated effluent was prepared to have the composition similar to Chrome tan effluent obtained from a private leather tanning industry in Vaduganthangal district, Vellore, Tamil Nadu, India. The simulated chrome tan liquor had similar composition of sodium, sulphate, chloride and pH (4.4) as described elsewhere [29]. Chromium concentration was kept as 70 ppm in the prepared chrome tan liquor solution. The stimulated chrome tan liquor solution having pH 4.4 was brought to the alkaline condition having pH ~ 8 by adding HCOONa and 25-100 µL of ammonia (5 wt.% in water). NaOH can also be used for adjusting the pH but avoided in the present work to maintain the fixed salt concentration. The prepared chrome tan 9
liquor solution was then subjected to -radiolysis (dose rate 7.8 kGy/h) for 6 h. After irradiation, a green color precipitates was formed which got settled at the bottom of the tan liquor solution. Total Cr (Cr(III)+Cr(VI)) in solution was determined by ICP-AES and Cr(VI) by spectrophotometry. Cr(III) concentration was determined by the mass balance. The solution was analyzed after precipitation for Cr(VI) and total Cr for obtaining residual Cr(III) remained in the solution. From this value, the amount of Cr(III) precipitated could be obtained by using the mass balance. Similarly, the precipitates was also dissolved in acid to analyze Cr concentration by ICP-AES analysis to confirm the amount of Cr(III) precipitated. Total Cr(VI) conversion to Cr(III) ions was found to be 100% and precipitation of thus formed chromium(III) oxide hydrate gel [(Cr(OH)3.xH2O] was 92 %. 3.2.
Formation of chromium oxide xerogel
Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) were carried out in the temperature range of 25–850 °C under air atmosphere to study the formation of chromium oxide xerogel from the precipitate of Cr(OH)3 from during -radiolysis. During thermal treatment, the crystalline phases of various intermediate products formed were identified by isolating them at various intermediate thermal steps and recording their powder X-ray diffraction (XRD) pattern. The weight loss curve as a function of temperature in dry air is shown in Fig. 2 and interpretations are summarized in Table S2 (S.I.). TG/DTG in the temperature range 25°C – 150 °C showed mass loss of 2.7 wt. % with corresponding endothermal peak in DTA curve indicating loss of water content from the sample. The XRD pattern of the product isolated form 150 °C did not show any crystalline phase, see Fig. 3. From 150 °C to 250 °C, TG/DTG showed a weight loss of 15 % which was due to the loss of H2O molecule from Cr(OH)3 to form CrO(OH). It has been reported in literature that hydrous chromium oxide gels undergo partial hydrothermal conversion to the orthorhombic CrOOH at temperatures around 230 °C [30]. As can be seen from Fig. 3, the 10
XRD pattern of the sample isolated at 250 °C exhibited the similar pattern as that of the sample below 150 °C. From 300 °C to 450 °C, DTA curve showed exothermic peak at 409 °C which was due to the CrO(OH) conversion to CrO2. In this temperature range, TG/DTG showed observed mass loss of 2.5% whereas Table S2 (S.I.) showed calculated mass loss as 1.2%. This difference was attributed to the burning of impurity materials trapped during precipitation process. Thus, the observed mass loss was more than the calculated one. It is reported in the literature that the chromium oxide gels undergo an exothermic transformation or the ''glow phenomenon" when heated in the air at temperature close to 400 °C [31]. At 450 °C - 620 °C, there was an exothermic peak attributed to the conversion of CrO2 to Cr2O3 [31]. The XRD pattern of the product isolated form 500 °C indicated that crystalline phases begin to form, see Fig. 3. Thus, the overall reactions from 150 to 500 °C can be expressed as: 1
(6)
2CrO(OH) + 2O2→2CrO2 + H2O 1
(7)
2CrO2→Cr2O3 + 2O2
8
0
0.000
DTG
4 -0.004
2
-20
TG
-30
-0.008
0 -2
DTA
DTG
Wt. Change (%)
-10
6
-4 -40
DTA -0.012
-6 -8
-50
-10 0
200
400
600
800
Temperature (°C)
11
Fig. 2. Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) of chromium oxide hydrate gels heated up to 850 °C in the air. The labels with both sides of Y-axes are shown indifferent colors to identify TG, DTA and DTG curves. The calculated mass loss due to the conversion of CrO2 to Cr2O3 was 9.5 wt.% whereas the observed mass loss during this conversion is only 3.17 wt.%. This indicated that only partial conversion had occurred during the TG/DTA condition. It is evident from the XRD pattern of the sample isolated form 850 °C (Fig. 3) that the intensities of peaks corresponding to mixed chromium oxides phases were significantly increased with respect to the background indicating quantitative amorphous conversion to crystalline phase. The detailed characterizations of this crystalline product isolated form 850 °C are given below including interpretation of the powder XRD pattern.
850°C isolated
Intensity (A.U.)
500°C isolated
350°C isolated
150°C isolated
Rt
20
40
60
12
Fig. 3. Room temperature X-ray diffraction (XRD) patterns of heated products of chromium oxide hydrate gels isolated from different temperatures. 3.3.
Characterizations of chromium oxide xerogel
TEM and HRTEM images of the intermediate products obtained by heating the Cr(OH)3 precipitate, formed during -radiolysis, at 150 °C, 350 °C and 850 °C and cooled to ambient temperature are shown in Fig. 4.
(a)
(b)
(c) Fig. 4. Representative HRTEM images at two magnifications of samples heated at 150 oC (a), 350 oC (b), and 850 oC (c). 13
These images of the intermediate products, obtained by heating at 150-350 °C, indicated the formation of amorphous porous gel like materials. It became crystalline at 850 °C and lattice fringes were clearly visible in the representative HRTEM image shown in Fig.5. The HRTEM analyses showed that the lattice fringe distances were approximately 2.663Å, 2.4798 Å & 3.555 Å which coincided well with the spacing of the (104) planes (2.665 Å), (110) planes (2.479 Å) and (012) planes (3.630 Å) of Cr2O3 [32-33]. Similarly, the fringe distance 2.453 Å matched with the spacing of (101) planes (2.434 Å) of CrO2 [34] and the fringe distances of 3.251 Å, 3.455Å, 2.453Å, 4.371Å and 4.253Å were in a good agreement with the lattice fringe distances of (111) planes (3.383 Å), (120) planes (3.435 Å), (031) planes (2.4540 Å), (020) planes (4.2800Å) and (011) planes (4.1900Å) of CrO3 [35].
(a)
5nm
(b)
(c)
Fig.5. (a) The representative HRTEM image of chromium oxide hydrate gels [Cr(OH)3] heated at 850 oC and cooled to room temperature, (b) Fast Fourier transform (FFT) of HRTEM image, (c) Inverse FFT. The formation of several chromium oxides on heating at 850 oC was also confirmed by the analyses of powder XRD pattern as shown in Fig.6. Thus, the final chromium oxide gel formed at 850 oC contained mixed chromium oxide comprising Cr2O3, CrO2 and CrO3. However, XRD analyses seem to suggest that Cr2O3 was major constituent of the xerogel formed at 850 0C. XRD pattern shown in Fig. 6 represents JCPD data of Cr2O3 to a major extent. The number of peaks observed for CrO2 and CrO3 were lesser due to low concentrations of these oxides. 14
300
(101) CrO / (104)Cr O 2 2 3 Cr O (110) 2 3
250
Intensity
200
(211)CrO / (116) Cr O 2 2 3
(012) Cr O3 2
150 (113) Cr O 2 3
100
CrO /Cr2O 3 3
50
(200)
(024) Cr O 2 3
Cr2O 3
CrO 3
0 10
15
20
25
30
35
40
45
50
55
60
65
2
Fig. 6. The XRD pattern of Cr2O3 obtained from chromium oxide hydrate gels [Cr(OH)3] after heating at 850 oC for 3 h and cooled to room temperature. X-ray photoelectron spectroscopic (XPS) analyses of the mixed chromium oxide xerogel formed after heated at 850 °C was carried out to confirm the oxidation states of Cr. It is seen from the deconvoluted core level binding energy spectra of Cr2p and O1s given in Fig.7 that Cr2O3, Cr, CrO2, and CrO3 were formed. Also, some residual formate (HCOO-) was entrapped within the mixed chromium oxide matrix. According to Baker et al., chromium oxide hydrate gel undergoes the oxidation reduction cycle of Cr3+→Cr6+→Cr3+ on heating at higher temperature in the air [30]. They suggested the considerable oxidation on the surface layer of gel heated in the air at temperatures between 200 and 300 °C with the formation of CrO3 (and sometimes CrO2). The oxidation of Cr3+ to Cr6+ is directly related to the reduced stability of the Cr3+ ion as the H2O ligands are driven out by heating. By prolonged heating at temperatures between 250 and 300 °C, the gel transformed into the stable crystalline α-Cr2O3 phase. As the temperature approached 400 °C, the thermal decomposition of the mixed oxide system became 15
fast and the exothermic transformation was manifested as the ''glow phenomenon". It appeared that a very small extent of Cr0 was also formed due to reduction by trapped formate.
Cr-2p3/2
12000 11500
C
B
Cr-2p1/2 D
11000
Intensity (cps)
A 10500 10000
BE 574.6 576.1 577.8 579.5
FWHM 1.97 1.83 2.22 1.96
Area 3333 3464 6718 2146
584.2 585.5 587.2 589.0
2.00 1.89 2.54 2.43
1662 1684 3446 1253
9500 9000
A: Cr B:CrO2
8500
C:Cr2O3 D:CrO3
8000 570
580
575
590
585
595
BE (eV)
(a)
13000 12000
CrO2
CrO3
BE 527.7 528.8 530.0 531.4 533.0
Cr2O3
Intensity (cps)
11000
*
10000
-C-OH
*
FWHM 1.35 1.41 1.45 1.82 1.67
Area 4594 5946 5766 7235 2695
oxygen atom with patitial negatve charge
9000 8000 7000 6000 524
526
528
530
532
534
536
BE (eV)
(b) Fig. 7. Deconvoluted XPS core-levels Cr2p (a) and O1s (b) binding energy spectra of chromium oxide hydrate gels [Cr(OH)3] heated at 850 oC for 3 h and cooled at ambient temperature. XPS is a surface characterization technique (3-4 nm depth) whereas XRD is a bulk process (5 m depth). Surface and bulk composition of materials is not always the same. Adsorption of oxygen from the ambient environment on the surface of material is a common phenomenon. Hence, the XPS O-1s peak would originate not only from the oxide components but also from 16
an adsorbed oxygen component. Although the deconvoluted peak heights in Fig. 7b of the chromium oxides are apparently looking in equivalent proportion, but actually they are different. From the fitting data, the area under the peak of the CrO2, CrO3 and Cr2O3 were found to be 5946, 5666 and 7235, respectively, and FWHM of the Cr2O3 peak was higher than other two oxides (see the fitting parameters of the Fig.7b). O-1s data and Cr-2p data are in a concordant. The quantitative trend is also similar to that observed in the XRD data studies. Therefore, there is no contradiction between XPS and XRD data. To further confirmation, the Raman spectrum of the sample heated at 850 °C was studied. It was clearly seen from Fig. 8 that the mixed chromium oxides phases in the xerogel were formed but Cr2O3 concentration was relatively higher. The major Raman shift peaks at 360 cm-1, 550 cm-1 and 600 cm-1 correspond to Cr2O3 as reported by Alrehaily et al. [19]. Raman shift peaks at 700 cm-1 and 850 cm-1 were assigned to CrO2 and CrO3 based on the matching with the respective standards.
180
Scattering Intensity (au)
160 140 120 Cr 2O3
100 80 60
Cr 2O3
Cr 2O3
40
CrO3
CrO2
20 0 -20 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 -1
Raman Shift (cm )
Fig. 8.
Raman spectrum of chromium oxide hydrate gels [Cr(OH)3] heated at 850 °C and
cooled at ambient temperature.
17
BET surface analyses of the chromium oxides formed at different temperature were done and results obtained are shown in Fig. S3 (S.I.). The results of these analyses are given in Table 1. As can be seen from this Table 1, the specific surface area of the product formed at 150 °C was 96.63 m2/g (pore volume = 0.129 cc/g and average pore size = 0.3 nm). At 350 °C, the specific area of chromium oxide gel was increased marginally to 104.73 m2/g but pore volume and pore size were increased significantly to 0.207cc/g and 2.4 nm, respectively. This could be due to loss of water and aggregation of chromium oxide particles to begin the formation of crystalline phases. On heating at 850 °C, the crystalline phases of different chromium oxides were formed that reduced the specific surface area from 104.73 m2/g (at 350 °C) to 20.93 m2/g. The average pore volume and average pore size were reduced to 0.036 cc/g and 2.1 nm, respectively, as a result of formation of the crystalline chromium oxide xerogel. The reduction of pore volume and specific surface area could be attributed to shrinking of the gel during the formation of crystalline phases. Table 1. BET analyses of specific surface area, pore volume and average pore-size of the chromium oxide gel formed at 150 °C, 350 °C and 850 °C temperature and cooled down to ambient temperature.
Temperature (°C)
Sp. Surface area (m2/g)
Pore Volume (cc/g)
Average Pore-size
150
96.63
0.129
0.3 nm
350
104.73
0.207
2.4 nm
850
20.93
0.036
2.1 nm
4. Conclusions
18
The quantitative reduction of Cr(VI) to Cr(III) was carried in the synthetic chrome tan liquor solution samples by -radiolysis of water in the presence of sodium formate, which was simultaneously precipitated to chromium oxide hydrate gel. The formed Cr(OH)3 hydrate gel was subjected to thermal treatment for converting it to chromium oxide xerogel which has potential applications for catalysis. It was observed that the heating up to 850 oC in the air converted chromium oxide hydrate gel to crystalline chromium oxide having mixed phases of Cr2O3 (major), CrO2 and CrO3. XPS studies also showed the formation of Cr0. BET analyses indicated specific surface area of xerogel as 20.9 m2 g-1, average pore-size 2.1 nm and pore volume 0.036 cc g-1 after heating at 850 °C for 3 h in the air. Thus, the toxic Cr(VI) could be removed from the waste stream and converted to chromium oxide xerogel which would find several applications as the catalyst. Thus, the -radiolysis and thermal treatment process reported to the present work provides sustainability to Cr(VI) conversion in the aqueous waste discharges from the various applications of useful chromium oxide material whose properties can be further tuned depending upon the applications. The crystalline chromium oxide exhibits good catalytic activity in many gas phase reactions. Xerogel enhances accessibility of catalytic sites to the reactants.
Acknowledgment Authors are thankful to Dr. P.D. Naik, Head, Analytical Chemistry Division for his keen interest and encouragement in the present work. Mr. T. V. Vittalrao, FCD, BARC is acknowledged for his help in BET surface analyses.
Appendix A. Supplementary data Supplementary data (Plots and Tables for Cr(VI)conversion (%) to Cr(III) during radiolysis under different conditions, Table containing TGA/DTA data, BET plots) related to this article can be found at: 19
SupportingInformation-SI-Komal.pdf
References [1] S.Langard, One hundred years of chromium and cancer: a review of epidemiological evidence and selected case reports, Am. J. Ind. Med. 17 (1990) 189–215. [2] M. Cieslak-Golonka, Toxic and mutagenic effects of chromium(VI): A review, Polyhedron 15 (1995) 3667-3689. [3] W. Mertz, Chromium in human nutrition: A review, J. Nutr. 123 (1993) 626–633. [4] J. Hu,G. Chen, I. M. C. Lo, Removal and recovery of Cr(VI) from wastewater by maghemite nanoparticles, Water Res. 39 (2005) 4528-4536. [5] Y. Xing, X. Chen, D. Wang, Electrically regenerated ion exchange for removal and recovery of Cr(VI) from wastewater, Environ. Sci. Technol. 41 (2007) 1439-1443. [6] M. K.Aroua, F. M.Zuki, N. M.Sulaiman, Removal of chromium ions from aqueous solutions by polymer-enhanced ultrafiltration, J. Hazard. Mater.147(2007) 752-758. [7] E.M. Contreras, A. M. F. Orozco,N.E.Zaritzky,Biological Cr(VI) removal coupled with biomass growth, biomass decay, and multiple substrate limitation, Water Res. 45 (2011) 3034-3046. [8] J. A. Korak, R. Huggins, M. Arias-Paic, Regeneration of pilot-scale ion exchange columns for hexavalent chromium removal, Water Res. 118 (2017) 141-151. [9] M. Pettine, L. Campanella, F. J. Millero, Reduction of hexavalent chromium by H2O2 in acidic solutions, Environ. Sci. Technol. 36 (2002) 901-907. [10]
M. Gaberell, Y.-P. Chin, S. J.Hug, B. Sulzberger, Role of dissolved organic
matter composition on the photo-reduction of Cr(VI) to Cr(III) in the presence of iron, Environ. Sci. Technol. 37 (2003) 4403-4409. [11]
A. Rauf, Md. S. A. S. Shah, G. H. Choi, U. B. Humayoun, D. H. Yoon, J. W.
Bae, J. Park, W. -J. Kim, P. J. Yoo, Facile synthesis of hierarchically structured 20
Bi2S3/Bi2WO6 photocatalysts for highly efficient reduction of Cr(VI), ACS Sustainable Chem. Eng. 3 (2015) 2847-2855. [12]
X. Wang, M. Hong, F. Zhang, Z. Zhuang, Y. Yu, Recyclable nanoscale
zerovalent iron doped g-C3N4/MoS2 for efficient photocatalysis of RhB and Cr(VI) driven by visible light, ACS Sustainable Chem. Eng. 4 (2016) 4055-4063. [13]
C. E.Barrera-Díaza, V. Lugo-Lugoa, B. Bilyeuc, A review of chemical,
electrochemical and biological methods for aqueous Cr(VI) reduction, J. Hazard. Mater. 223-224 (2012) 1-12. [14]
D. Pradhan, L. B. Sukla, M. Sawyer, P. K. S. M. Rahman, Recent bioreduction
of hexavalent chromium in wastewater treatment: A review, J. Indus. Engin. Chem. 55 (2017) 1-20. [15]
R. Marks, T. Yang, P. Westerhoff, K. Doudrick, Comparative analysis of the
photocatalytic reduction of drinking water oxoanions using titanium dioxide, Water Research 104 (2016) 11-19. [16]
K. Pikaev, New data on electron-beam purification of wastewater, Rad. Phys.
Chem. 65 (2002) 515-526. [17]
F. Djouider, Radiolytic formation of non-toxic Cr(III) from toxic Cr(VI) in
formate containing aqueous solutions: A system for water treatment, J. Hazard. Mater. 223-224 (2012) 104-109. [18]
L. M. Alrehaily, J. M. Joseph, J. C. Wren, Radiation-Induced Formation of
Chromium Oxide Nanoparticles: Role of Radical Scavengers on the Redox Kinetics and Particle Size, J. Phys. Chem. C 119 (2015) 16321-16330. [19]
L. M. Alrehaily, J. M. Joseph, A. Y. Musa, D. A. Guzonas, J. C. Wren, Gamma-
radiation induced formation of chromiumoxide nanoparticles from dissolved dichromate, Phys. Chem. Chem. Phys. 15 (2013) 98-107. 21
[20]
X. Wang, S. O. Pehkonen, A. K. Ray, Removal of aqueous Cr(VI) by a
combination of photocatalytic reduction and coprecipitation, Ind. Eng. Chem. Res. 43 (2004) 1665-1672. [21]
Y. Kumagai, R. Nagaishi, R. Yamada, Y. Katsumura, Effect of silica gel on
radiation-induced reduction of dichromate ion in aqueous acidic solution, Rad. Phys. Chem.80 (2011) 876-883. [22]
G. Chen, J. Feng, W. Wang, Y. Yin, H. Liu, Photocatalytic removal of
hexavalent chromium by newly designed and highly reductive TiO2 nanocrystals, Water Res. 108 (2017)383-390. [23]
P. Nagraj, N. Aradhana, A. Shivakumar, A. K. Shrestha, Spectrophotometric
method for the determination of chromium (VI) in water samples, Environmental Monitoring and Assessment 157 (2008) 575-582. [24]
M. Owlad, M. K.Aroua, W. A. W. Daud, S. Baroutian, Removal of hexavalent
chromium-contaminated water and wastewater: A review, Water Air Soil Pollut. 200 (2009) 59-77. [25]
W. T. Spinks, R. J. Woods, An Introduction to Radiation Chemistry. 3rd edition
(1990) 243-312, A WILEY- INTERSCIENCE publication, John Wiley & sons, Inc. [26]
J. Jeong, J. Yoon, Dual roles of CO2•− for degrading synthetic organic
chemicals in the photo/ ferrioxalate system, Water Res. 38 (2004) 3531–3540. [27]
H. A. Schwarz, R. W. Dodson, Reduction potentials of CO2•− and the alcohol
radicals. J. Phys. Chem. 93 (1989) 409-414. [28]
D. Rai, B. M. Sass, D. A. Moore, Chromium(III) hydrolysis constants and
solubility of chromium(III) hydroxide, Inorg. Chem. 26 (1987) 345-349.
22
[29]
K. M. S. Sumathi, S. Mahimairaja, R. Naidu, Use of low-cost biological wastes
and vermiculite for removal of chromium from tannery effluent, Bioresource Technol. 96 (2005) 309–316. [30]
F. S. Baker, J. D. Carruthers, R. E. Day, K. S. W. Sing, L. J. Stryk, Micropore
Structure of Chromium Oxide Gels, Discuss. Faraday Soc. 52 (1971) 173-186. [31]
S. Kittaka, T. Morooka, K. Kitayama, Thermal Decomposition of Chromium
Oxide Hydroxide I. Effect of Particle Size and Atmosphere. J. Solid State Chem. 58 (1985) 187-193. [32]
M. Baster, F. Bouree, A. Kowalska, Z. Latacz, The change of crystal and
exchange parameters in the vicinity of TN in Cr2O3, J. Alloys Comp. 296 (2000) 1-5. [33]
H. Sun, L. Wang, D. Chu, Z. Ma, A. Wang, Synthesis of porous Cr2O3 hollow
microspheres via a facile template-free approach, Mater. Lett. 140 (2015) 35–38. [34]
G. Tirao, S. Ceppi, A. L. Cappelletti, E. V. Pannunzio Miner, Oxidation state
characterization in Cr oxides by means of Cr-K beta emission spectroscopy, J. Phys. Chem. Solids 71 (2010) 199-205. [35]
J. S. Stephens, D. W. J. Cruickshank, The crystal structure of (CrO3)infinite, Acta
Crystallographica: Section B 26 (1970) 222-226.
23
Highlights
Cr(VI) reduction was carried out in chrome tan liquor using γradiolysis.
Formate was used to scavenge oxidizing radicals leaving behind reducing species.
Basic pH of solution resulted to simultaneous precipitation of Cr(OH)3.
Cr(III) precipitate was subjected to thermal treatment for converting to xerogel.
Crystalline mixed valence oxide xerogel with surface area 20.9 m2 g-1 was formed.
24
Graphical Abstract Thermal treatment
-rays Cr(VI) basic pH
formate
Cr(OH)3 Precipitation
Chromium Oxide Xerogel
25
Conflict of Interest and Authorship Conformation Form
o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
The authors declare no conflict of interest. Our Institute is Govt. organization and Govt. provides fund for carrying out the research.
26
S1383-5866(19)31977-X https://doi.org/10.1016/j.seppur.2019.116291 SEPPUR 116291
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
11 May 2019 2 November 2019 4 November 2019
Please cite this article as: K.C. Shrivastava, S.P. Pandey, S.A. Kumar, A.K. Pandey, A.K. Debnath, A.P. Srivastava, G.R. Patkare, G.J. Abraham, Remediation of chromium(VI) ions as chromium oxide xerogel via gamma-radiolysis of aqueous waste discharge, Separation and Purification Technology (2019), doi: https:// doi.org/10.1016/j.seppur.2019.116291
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Remediation of chromium(VI) ions as chromium oxide xerogel via gamma-radiolysis of aqueous waste discharge Komal C. Shrivastavaa,*, Shailaja P. Pandeya, Sanjukta A. Kumara,**, Ashok K. Pandeyb, Anil K. Debnathc, Amit P. Srivastavad, Geeta R. Patkaree, Geogy J. Abrahamf aAnalytical
Chemistry Division, bRadiochemistry Division, cTechnical Physics Division,
dMechanical
Metallurgy Division, eFuel Chemistry Division, fMaterial Processing & Corrosion
Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India
ABSTRACT Radiolysis of water during -irradiation produces oxidizing and reducing species which can be used for the bulk oxidation or reduction of target toxic species. In the present work, radiolysis of water has been used for the reduction of Cr(VI) to Cr(III) ions in the simulated chrome tan liquor solution samples, and simultaneously precipitation as a chromium oxide hydrate gel. Sodium formate scavenger has been found to be the best suited, among various scavengers studied, not only for the removal of oxidizing species produced during -radiolysis of water but also simultaneous precipitation of thus formed Cr(OH)3 as chromium oxide hydrated to gel. The chromium oxide hydrate gel has been subjected to thermal treatment for 1
an objective of converting it to the chromium oxide xerogel which has potential applications for catalysis. Various techniques, such as BET, XRD, XPS, HRTEM and Raman spectroscopy, have been employed for studying the changes in the physical, crystalline and oxidation states in each step observed in thermal treatment starting from the hydrate gel to chromium oxide xerogel. The results showed the formation of crystalline mixed valence chromium oxide xerogel having specific surface area 20.9 m2 g-1 and pore volume 0.036 cc g-1 after thermal treatment.
Keywords: Chromium(VI) reduction; Aqueous -radiolysis; Precipitation; Thermal treatment; Mixed valence chromium oxides.
Corresponding Author * Email: [email protected]; Tel. +91-22-25590918 (K.C. Shrivastava) ** Email: [email protected]; Tel. +91-22-25590316 (S.A. Kumar)
1. Introduction The toxicity of Cr species is highly dependent on their oxidation states. Cr(VI) is usually the product of anthropogenic activities and has been documented to be toxic, mutagenic and carcinogens [1,2]. Contrary to Cr(VI), Cr(III) does not bear any toxicological relevance and it is an essential trace element in human nutrition [3]. Therefore, the Cr(VI) containing aqueous waste discharges from concerned industries have to be treated for: (i) removal of Cr(VI), (ii) reduction of Cr(VI) to Cr(III), and (iii) sorption-reduction of Cr(VI). There are several techniques for the recovery of Cr(VI) ions from aqueous media such as ion-exchange, chemical precipitation, membrane filtration, adsorption using tailored sorbents, magnetic nanoparticles, 2
biomass and electrochemical treatment [4-8]. In another strategy, the reduction of Cr(VI) to its less toxic form Cr(III) has been done using chemical, electrochemical, photochemical and bioreductions [9-15]. For developing efficient strategies for the remediation of Cr(VI), the several attempts have been made to combine the sorption and reduction steps simultaneously [16-19]. However, the applications of all these methods of the Cr(VI) remediation in aqueous discharges is not amenable to scale up due to requirements of a large aqueous volume processing. Free radicals based processes have shown a potential possibility of treating different waste/polluted water streams by selectively reducing or oxidizing the toxic species to their less toxic forms. The electron beams and -radiations sources, such as 60Co and
137Cs,
have been
used to generate oxidizing and reducing free radicals for the treatments of the aqueous waste streams [16-21]. The major advantage of the ionizing radiation based water radiolysis is attributed to uniform formation of the free radicals in a large volume waste aqueous stream with fast kinetics. Alrehaily et al. have studied the synthesis of chromium oxide (Cr2O3) nanoparticles by -radiolysis of Cr(VI) dissolved in water [18]. In this process, the solution was deaerated by purging argon gas before -radiolysis. This approach is good for small scale syntheses of Cr2O3 nanoparticles, but difficult for adopting for the large volume waste treatment process. Djouider has reported another route for the -radiolysis based treatment of Cr(VI) containing aqueous waste [17]. He has used nitrous oxide bubbling prior to irradiation of toxic Cr(VI) in formate containing aqueous solutions. This method has been found to be very efficient for the quantitative conversion of Cr(VI) to Cr(III), but the purging of solution by N2O is not desirable. Therefore, there is a need to further study -radiolysis based Cr(VI) reduction and precipitation method under ambient conditions without using any purging gas. In the present work, a systematic study has been carried out to reduce Cr(VI) ions in a simulated chrome tan liquor solution with -radiations induced radiolysis followed by in situ 3
precipitation and thermal conversion to xerogel without using any purging gas. The choice radiations (60Co) for radiolysis has been based on the fact that it does not involve a high value machine and can be applied for the larger volume of flowing solutions. The -radiolysis of water produces both oxidizing and reducing free radicals and, therefore, oxidizing free radical scavengers for converting these to reducing species have been studied using 2-propanol ((CH3)2CHOH), formic acid (HCOOH) and sodium formate (HCOONa). Cr(III) hydrolyzes at neutral pH and get precipitated as Cr(OH)3, while Cr(VI) existing as CrO42- anions remains soluble at neutral pH [20-22]. Hence, the chemical conditions have been optimized to precipitate Cr(OH)3 directly, and subject it to thermal treatment for obtaining xerogel, which has a number of potential applications. Different characterizations techniques such as TGA/DTA, XRD, Raman spectroscopy, XPS and HRTEM have been used to understand the mechanism involved in the formation of the xerogel starting from aqueous Cr2O72-/CrO4anions. The novelty of the present work is that the Cr(VI) reduction by the -radiolysis can be performed at basic pH for a large volume sample where thus formed Cr(III) can be readily precipitated and converted to useful material without involving catalysts.
2. Experimental section All the chemicals such as K2CrO4 (Sigma–Aldrich, Steinem, Switzerland), diphenyl carbazide (DPC) (Merck, Mumbai, India), formic acid (Sigma–Aldrich, Steinem, Switzerland), sodium formate (Merck, Mumbai, India) and 2-propanol (S.D. Fine Chem. Limited, India) were AR grade and were used as received. -Irradiations were carried out using 60Co -source (dose rate: 7.8 kGy h-1 as obtained by Fricke dosimetry) procured from Board of Radiation and Isotope Technology, Mumbai, India. The -irradiations was carried out in borosilicate glass vessels. The spectrophotometric analysis of Cr(VI) contents in the solutions were carried out by UV-Vis spectrophotometer supplied by Ocean Optics Inc. Netherland after developing color 4
using diphenylcarbazide (DPC) as described elsewhere [23]. Total Cr contents of the solutions were determined by Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) (Spectro Arcos, Germany make). High Resolution Transmission Electron Microscopy (HRTEM) studies were carried out using Zeiss Libra 200 FE TEM (Carl Zeiss AG). A drop of the sample dispersion was placed on a carbon coated copper grids (0.3 cm diameter, mesh size of 200 holes/cm) and left to dry overnight at room temperature. HRTEM images were analyzed with the help of Image J software. The oxidation states of Chromium were measured by X-ray Photoelectron Spectroscopy (XPS) using MgK (1253.6 eV) X-ray source and a DESA-150 electron analyzer (M/s. Staib Instruments, Germany). The binding energy scale was calibrated to Au-4f7/2 line of 83.95 eV. The analyzer was operated on 40 eV pass energies and pressure in the chamber during analysis was kept as ~ 7 x 10-9 Torr. The XPS spectra were fitted using XPSPEAK41 software. To study the thermal decomposition of precipitated Cr(OH)3, TG and DTA curves were recorded using Mettler Thermoanalyzer (model: TGA/SDTA851??/MT5/LF1600) in dry air at room temperature to 850 °C at a heating rate of 4 °C min-1. Powder X-ray diffraction (XRD) was recorded at each decomposition step to confirm the formation of products obtained at different temperature. All the heat treated samples of different temperature were cooled to room temperature in the respective environment, and characterized by recording room temperature XRD on Rigaku Miniflex–600 diffractometer (??-2?? geometry) with graphite monochromatized Cu K1 radiation (=1.5406 Å) at a scanning rate of 1° (2) per minute in 2 range of 10-100°. Raman scattering measurements was performed using HR 800 Horiba Jobin Yvon Micro laser Raman spectrometer with a laser excitation wavelength of 633 nm. The -radiolysis of the aqueous samples was carried out by filling in the stopper glass bottle of to the maximum to avoid aerated space in the bottle. Typically, 10 mL of a solution containing 0.43 mM CrO42- and 1.35 M of scavenger (CH3)2CHOH / HCOONa / HCOOH were 5
irradiated in 60Co -source for a fixed time period. The pH changes were recorded by measuring pH before and after -irradiation. The concentrations of CrO42- and scavengers were varied to optimize the maximum conversion efficiency of Cr(VI) to Cr(III). Finally, the Cr(III) was precipitated at basic pH. The precipitate were vacua filtered and green color Cr(OH)3 was formed which was washed with water and dried. This precipitated Cr(OH)3 was subjected to thermal heating in the air to obtained chromium oxide xerogel.
3. Results and discussion 3.1.
Cr(VI) reduction by -radiolysis of water
Cr(VI) speciation depends mainly upon pH of the solution. For example, H2CrO4 is the predominant species at pH<1, CrO42-/Cr2O72- exists at pH>8, and both HCrO4- and Cr2O72- exist on intermediate pH (2 to 6) as suggested by speciation modeling using pKa and other relevant parameters [24]. The pH values of the solutions having 0.43 mM Cr(VI) and 1.35 M of oxidizing free radical scavengers such as HCOOH, HCOONa, and (CH3)2CHOH were found to be 1.9, 7.3 and 8.1, respectively. This seemed to suggest that the Cr(VI) species existing would be HCrO4- and Cr2O72- in a solution having HCOOH. However, it would be single Cr2O72- species in solutions having HCOONa or (CH3)2CHOH. After -irradiation, the pH of solutions having HCOOH, HCOONa, and (CH3)2CHOH were changed to 2.1, 9.0, and 11.0, respectively. The increase in pH was attributed to formation of hydroxyl ions during radiolysis of water. The -radiolysis of water produces several species such as eaq-, OH•, H•, H2, H2O2, H+, and OH-. Among these, the two reducing transient species are solvated electron (eaq-) and hydrogen atom (H•). Hydroxyl radical (OH•) is the major transient oxidizing radical. The stable molecular products are formed by these transient species, which can also to act as reducing or oxidizing species for Cr(VI). However, it is important to note that the radiation chemical yields (G-values) of -radiolysis of water vary as a function of pH of water. As reported by Spinks et 6
al., the G-values for OH• and e-aq are maximum at pH 6.8 to 9.5 [25]. The scavengers for the oxidizing radical OH• reported to the literature are 2-propanol and formate, which convert OH• to reducing species as given below [17-19, 26-27]. CH3)2CHOH + •OH → (CH3)2C•OH + H2O
(1)
OH• + HCO2−(HCO2H)→H2O + CO2•−(CO2H•)
(2)
H• + HCO2− (HCO2H)→H2 + CO2•− (CO2H•)
(3)
Cr(VI) + 3CO2•− → Cr(III) + 3CO2
(4)
Cr(VI) quantitative conversion to Cr(III) as a function of -irradiation time in the presence of scavengers such as HCOOH, HCOONa and 2-propanol are shown in Fig. 1. The conversion of Cr(VI) to Cr(III) was obtained using following an equation:
Conversion (%)
[Cr]t [Cr]Cr(VI) [Cr ]t
100
(5)
Where [Cr]t is total concentration of Cr in acidified aqueous sample obtained ICP-AES (specific to elemental Cr), and [Cr]Cr(VI) is obtained by spectrophotometry (specific to Cr(VI)DPC colored complex) of the same sample after -irradiation. Before -irradiation, it was observed that [Cr]t was equal to [Cr]Cr(VI) indicating the absence of Cr(III) in the aqueous sample. As can be seen from Fig.1, the Cr(VI) conversion to Cr(III) increased form -irradiation time and completed at 10 min in the case of solutions containing HCOOH. In the presence of (CH3)2CHOH or HCOONa, the quantitative conversion (>98%) of Cr(VI) to Cr(III) was achieved after -irradiation time of 25 min and 15 min, respectively. It was also seen from Fig. 1 that the onset of reduction of Cr(VI) had a lag time of 5 min and 2 min in the presence of scavenger (CH3)2CHOH and HCOONa, respectively. This could be attributed to pH of HCOOH, HCOONa, and (CH3)2CHOH as 2.1, 9.0, and 11.0, respectively. At lower pH, hydrate electrons are rapidly scavenged by hydrogen ions and get converted to hydrogen atoms, thereby increasing the effective chemical yield of reducing species. The G-values for OH• and e-aq are 7
reduced above pH 9.5 [25]. The observed trend in the time required for the Cr(VI) quantitative conversion to Cr(III) by the scavenger was: (CH3)2CHOH > HCOONa > HCOOH. It was observed from the literature that Cr(III) should be remained soluble as aqueous Cr3+ at lower pH, but it would get precipitated as Cr(OH)3 maximum at pH 7 to 9.72 [28]. Therefore, HCOONa was found to be better choice of the three scavengers studied for the Cr(VI) conversion to Cr(III) and subsequent precipitation as Cr(OH)3 during -radiolysis of water.
100
Conversion (% )
80
60
40
20
0 0
10
20
30
40
Time (min)
Fig. 1. Cr(VI) quantitative conversion to Cr(III) in -radiolysis (dose rate 7.8 KGy/h) of aqueous solutions containing 0.43 mM Cr(VI) and 1.35 M HCOOH/HCOONa/ (CH3)2CHOH as a function of irradiation time. To examine the effect of HCOONa concentration, the mole proportion of chromate to HCOONa was varied as 1:3000, 1:1500, 1:760, 1:380, and the solutions were -irradiated for the time ranging from 0 to 30 min. The choice of HCOONa was based on a fact that it gave basic pH where Cr(III) got readily precipitated. It can be seen from the plot given in Fig. S1 (Supplementary Information, S.I.), the reduction of Cr(VI) to Cr(III) could be achieved quantitatively within 25 min in all the four cases. However, the lag time for onset of the reduction in solutions, having Cr(VI): HCOONa mole proportion up to 1:760, was nearly same 8
2.5 min. Whereas, the lag time was increased significantly to 10 min in the solution having 1:380 mole proportion. This could be related to drop in G-value of reducing species formed in -radiolysis when formate concentration was reduced to a threshold limit, which affected eventually the kinetics of Cr(VI)-reduction. On varying concentration of Cr(VI) at a fixed optimized concentration of HCOONa and -ray dose, the Cr(VI) conversion to Cr(III) was reduced up to 45 % at a 0.70 mM concentration but did not decrease thereafter, see Fig. S2(S.I.). Also, thus formed green colored Cr(OH)3 did not precipitate below 0.70 mM concentration in -irradiation time of 30 min. However, the conversion could be improved gradually to 60%, 76%, 86% and 97% by increasing irradiation time from 30 min to 3 h, 4h, 5h and 6 h, respectively, under the similar conditions. The increase in conversion with irradiation time seemed to suggest that the reduction of Cr(VI) was limited by the kinetic of production of reducing species during -radiolysis when concentration of Cr(VI) increased in the solution for a given -ray dose rate. However, the Cr(VI) quantitative conversion to Cr(III) could be achieved by increasing the -irradiation time (or increasing the -dose). Irradiation time of 6 h was found to be sufficient not only for 97% conversion but also for 100% precipitation of Cr(III) ions as Cr(OH)3, see Table S1 (S.I.). A simulated effluent was prepared to have the composition similar to Chrome tan effluent obtained from a private leather tanning industry in Vaduganthangal district, Vellore, Tamil Nadu, India. The simulated chrome tan liquor had similar composition of sodium, sulphate, chloride and pH (4.4) as described elsewhere [29]. Chromium concentration was kept as 70 ppm in the prepared chrome tan liquor solution. The stimulated chrome tan liquor solution having pH 4.4 was brought to the alkaline condition having pH ~ 8 by adding HCOONa and 25-100 µL of ammonia (5 wt.% in water). NaOH can also be used for adjusting the pH but avoided in the present work to maintain the fixed salt concentration. The prepared chrome tan 9
liquor solution was then subjected to -radiolysis (dose rate 7.8 kGy/h) for 6 h. After irradiation, a green color precipitates was formed which got settled at the bottom of the tan liquor solution. Total Cr (Cr(III)+Cr(VI)) in solution was determined by ICP-AES and Cr(VI) by spectrophotometry. Cr(III) concentration was determined by the mass balance. The solution was analyzed after precipitation for Cr(VI) and total Cr for obtaining residual Cr(III) remained in the solution. From this value, the amount of Cr(III) precipitated could be obtained by using the mass balance. Similarly, the precipitates was also dissolved in acid to analyze Cr concentration by ICP-AES analysis to confirm the amount of Cr(III) precipitated. Total Cr(VI) conversion to Cr(III) ions was found to be 100% and precipitation of thus formed chromium(III) oxide hydrate gel [(Cr(OH)3.xH2O] was 92 %. 3.2.
Formation of chromium oxide xerogel
Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) were carried out in the temperature range of 25–850 °C under air atmosphere to study the formation of chromium oxide xerogel from the precipitate of Cr(OH)3 from during -radiolysis. During thermal treatment, the crystalline phases of various intermediate products formed were identified by isolating them at various intermediate thermal steps and recording their powder X-ray diffraction (XRD) pattern. The weight loss curve as a function of temperature in dry air is shown in Fig. 2 and interpretations are summarized in Table S2 (S.I.). TG/DTG in the temperature range 25°C – 150 °C showed mass loss of 2.7 wt. % with corresponding endothermal peak in DTA curve indicating loss of water content from the sample. The XRD pattern of the product isolated form 150 °C did not show any crystalline phase, see Fig. 3. From 150 °C to 250 °C, TG/DTG showed a weight loss of 15 % which was due to the loss of H2O molecule from Cr(OH)3 to form CrO(OH). It has been reported in literature that hydrous chromium oxide gels undergo partial hydrothermal conversion to the orthorhombic CrOOH at temperatures around 230 °C [30]. As can be seen from Fig. 3, the 10
XRD pattern of the sample isolated at 250 °C exhibited the similar pattern as that of the sample below 150 °C. From 300 °C to 450 °C, DTA curve showed exothermic peak at 409 °C which was due to the CrO(OH) conversion to CrO2. In this temperature range, TG/DTG showed observed mass loss of 2.5% whereas Table S2 (S.I.) showed calculated mass loss as 1.2%. This difference was attributed to the burning of impurity materials trapped during precipitation process. Thus, the observed mass loss was more than the calculated one. It is reported in the literature that the chromium oxide gels undergo an exothermic transformation or the ''glow phenomenon" when heated in the air at temperature close to 400 °C [31]. At 450 °C - 620 °C, there was an exothermic peak attributed to the conversion of CrO2 to Cr2O3 [31]. The XRD pattern of the product isolated form 500 °C indicated that crystalline phases begin to form, see Fig. 3. Thus, the overall reactions from 150 to 500 °C can be expressed as: 1
(6)
2CrO(OH) + 2O2→2CrO2 + H2O 1
(7)
2CrO2→Cr2O3 + 2O2
8
0
0.000
DTG
4 -0.004
2
-20
TG
-30
-0.008
0 -2
DTA
DTG
Wt. Change (%)
-10
6
-4 -40
DTA -0.012
-6 -8
-50
-10 0
200
400
600
800
Temperature (°C)
11
Fig. 2. Thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) of chromium oxide hydrate gels heated up to 850 °C in the air. The labels with both sides of Y-axes are shown indifferent colors to identify TG, DTA and DTG curves. The calculated mass loss due to the conversion of CrO2 to Cr2O3 was 9.5 wt.% whereas the observed mass loss during this conversion is only 3.17 wt.%. This indicated that only partial conversion had occurred during the TG/DTA condition. It is evident from the XRD pattern of the sample isolated form 850 °C (Fig. 3) that the intensities of peaks corresponding to mixed chromium oxides phases were significantly increased with respect to the background indicating quantitative amorphous conversion to crystalline phase. The detailed characterizations of this crystalline product isolated form 850 °C are given below including interpretation of the powder XRD pattern.
850°C isolated
Intensity (A.U.)
500°C isolated
350°C isolated
150°C isolated
Rt
20
40
60
12
Fig. 3. Room temperature X-ray diffraction (XRD) patterns of heated products of chromium oxide hydrate gels isolated from different temperatures. 3.3.
Characterizations of chromium oxide xerogel
TEM and HRTEM images of the intermediate products obtained by heating the Cr(OH)3 precipitate, formed during -radiolysis, at 150 °C, 350 °C and 850 °C and cooled to ambient temperature are shown in Fig. 4.
(a)
(b)
(c) Fig. 4. Representative HRTEM images at two magnifications of samples heated at 150 oC (a), 350 oC (b), and 850 oC (c). 13
These images of the intermediate products, obtained by heating at 150-350 °C, indicated the formation of amorphous porous gel like materials. It became crystalline at 850 °C and lattice fringes were clearly visible in the representative HRTEM image shown in Fig.5. The HRTEM analyses showed that the lattice fringe distances were approximately 2.663Å, 2.4798 Å & 3.555 Å which coincided well with the spacing of the (104) planes (2.665 Å), (110) planes (2.479 Å) and (012) planes (3.630 Å) of Cr2O3 [32-33]. Similarly, the fringe distance 2.453 Å matched with the spacing of (101) planes (2.434 Å) of CrO2 [34] and the fringe distances of 3.251 Å, 3.455Å, 2.453Å, 4.371Å and 4.253Å were in a good agreement with the lattice fringe distances of (111) planes (3.383 Å), (120) planes (3.435 Å), (031) planes (2.4540 Å), (020) planes (4.2800Å) and (011) planes (4.1900Å) of CrO3 [35].
(a)
5nm
(b)
(c)
Fig.5. (a) The representative HRTEM image of chromium oxide hydrate gels [Cr(OH)3] heated at 850 oC and cooled to room temperature, (b) Fast Fourier transform (FFT) of HRTEM image, (c) Inverse FFT. The formation of several chromium oxides on heating at 850 oC was also confirmed by the analyses of powder XRD pattern as shown in Fig.6. Thus, the final chromium oxide gel formed at 850 oC contained mixed chromium oxide comprising Cr2O3, CrO2 and CrO3. However, XRD analyses seem to suggest that Cr2O3 was major constituent of the xerogel formed at 850 0C. XRD pattern shown in Fig. 6 represents JCPD data of Cr2O3 to a major extent. The number of peaks observed for CrO2 and CrO3 were lesser due to low concentrations of these oxides. 14
300
(101) CrO / (104)Cr O 2 2 3 Cr O (110) 2 3
250
Intensity
200
(211)CrO / (116) Cr O 2 2 3
(012) Cr O3 2
150 (113) Cr O 2 3
100
CrO /Cr2O 3 3
50
(200)
(024) Cr O 2 3
Cr2O 3
CrO 3
0 10
15
20
25
30
35
40
45
50
55
60
65
2
Fig. 6. The XRD pattern of Cr2O3 obtained from chromium oxide hydrate gels [Cr(OH)3] after heating at 850 oC for 3 h and cooled to room temperature. X-ray photoelectron spectroscopic (XPS) analyses of the mixed chromium oxide xerogel formed after heated at 850 °C was carried out to confirm the oxidation states of Cr. It is seen from the deconvoluted core level binding energy spectra of Cr2p and O1s given in Fig.7 that Cr2O3, Cr, CrO2, and CrO3 were formed. Also, some residual formate (HCOO-) was entrapped within the mixed chromium oxide matrix. According to Baker et al., chromium oxide hydrate gel undergoes the oxidation reduction cycle of Cr3+→Cr6+→Cr3+ on heating at higher temperature in the air [30]. They suggested the considerable oxidation on the surface layer of gel heated in the air at temperatures between 200 and 300 °C with the formation of CrO3 (and sometimes CrO2). The oxidation of Cr3+ to Cr6+ is directly related to the reduced stability of the Cr3+ ion as the H2O ligands are driven out by heating. By prolonged heating at temperatures between 250 and 300 °C, the gel transformed into the stable crystalline α-Cr2O3 phase. As the temperature approached 400 °C, the thermal decomposition of the mixed oxide system became 15
fast and the exothermic transformation was manifested as the ''glow phenomenon". It appeared that a very small extent of Cr0 was also formed due to reduction by trapped formate.
Cr-2p3/2
12000 11500
C
B
Cr-2p1/2 D
11000
Intensity (cps)
A 10500 10000
BE 574.6 576.1 577.8 579.5
FWHM 1.97 1.83 2.22 1.96
Area 3333 3464 6718 2146
584.2 585.5 587.2 589.0
2.00 1.89 2.54 2.43
1662 1684 3446 1253
9500 9000
A: Cr B:CrO2
8500
C:Cr2O3 D:CrO3
8000 570
580
575
590
585
595
BE (eV)
(a)
13000 12000
CrO2
CrO3
BE 527.7 528.8 530.0 531.4 533.0
Cr2O3
Intensity (cps)
11000
*
10000
-C-OH
*
FWHM 1.35 1.41 1.45 1.82 1.67
Area 4594 5946 5766 7235 2695
oxygen atom with patitial negatve charge
9000 8000 7000 6000 524
526
528
530
532
534
536
BE (eV)
(b) Fig. 7. Deconvoluted XPS core-levels Cr2p (a) and O1s (b) binding energy spectra of chromium oxide hydrate gels [Cr(OH)3] heated at 850 oC for 3 h and cooled at ambient temperature. XPS is a surface characterization technique (3-4 nm depth) whereas XRD is a bulk process (5 m depth). Surface and bulk composition of materials is not always the same. Adsorption of oxygen from the ambient environment on the surface of material is a common phenomenon. Hence, the XPS O-1s peak would originate not only from the oxide components but also from 16
an adsorbed oxygen component. Although the deconvoluted peak heights in Fig. 7b of the chromium oxides are apparently looking in equivalent proportion, but actually they are different. From the fitting data, the area under the peak of the CrO2, CrO3 and Cr2O3 were found to be 5946, 5666 and 7235, respectively, and FWHM of the Cr2O3 peak was higher than other two oxides (see the fitting parameters of the Fig.7b). O-1s data and Cr-2p data are in a concordant. The quantitative trend is also similar to that observed in the XRD data studies. Therefore, there is no contradiction between XPS and XRD data. To further confirmation, the Raman spectrum of the sample heated at 850 °C was studied. It was clearly seen from Fig. 8 that the mixed chromium oxides phases in the xerogel were formed but Cr2O3 concentration was relatively higher. The major Raman shift peaks at 360 cm-1, 550 cm-1 and 600 cm-1 correspond to Cr2O3 as reported by Alrehaily et al. [19]. Raman shift peaks at 700 cm-1 and 850 cm-1 were assigned to CrO2 and CrO3 based on the matching with the respective standards.
180
Scattering Intensity (au)
160 140 120 Cr 2O3
100 80 60
Cr 2O3
Cr 2O3
40
CrO3
CrO2
20 0 -20 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 -1
Raman Shift (cm )
Fig. 8.
Raman spectrum of chromium oxide hydrate gels [Cr(OH)3] heated at 850 °C and
cooled at ambient temperature.
17
BET surface analyses of the chromium oxides formed at different temperature were done and results obtained are shown in Fig. S3 (S.I.). The results of these analyses are given in Table 1. As can be seen from this Table 1, the specific surface area of the product formed at 150 °C was 96.63 m2/g (pore volume = 0.129 cc/g and average pore size = 0.3 nm). At 350 °C, the specific area of chromium oxide gel was increased marginally to 104.73 m2/g but pore volume and pore size were increased significantly to 0.207cc/g and 2.4 nm, respectively. This could be due to loss of water and aggregation of chromium oxide particles to begin the formation of crystalline phases. On heating at 850 °C, the crystalline phases of different chromium oxides were formed that reduced the specific surface area from 104.73 m2/g (at 350 °C) to 20.93 m2/g. The average pore volume and average pore size were reduced to 0.036 cc/g and 2.1 nm, respectively, as a result of formation of the crystalline chromium oxide xerogel. The reduction of pore volume and specific surface area could be attributed to shrinking of the gel during the formation of crystalline phases. Table 1. BET analyses of specific surface area, pore volume and average pore-size of the chromium oxide gel formed at 150 °C, 350 °C and 850 °C temperature and cooled down to ambient temperature.
Temperature (°C)
Sp. Surface area (m2/g)
Pore Volume (cc/g)
Average Pore-size
150
96.63
0.129
0.3 nm
350
104.73
0.207
2.4 nm
850
20.93
0.036
2.1 nm
4. Conclusions
18
The quantitative reduction of Cr(VI) to Cr(III) was carried in the synthetic chrome tan liquor solution samples by -radiolysis of water in the presence of sodium formate, which was simultaneously precipitated to chromium oxide hydrate gel. The formed Cr(OH)3 hydrate gel was subjected to thermal treatment for converting it to chromium oxide xerogel which has potential applications for catalysis. It was observed that the heating up to 850 oC in the air converted chromium oxide hydrate gel to crystalline chromium oxide having mixed phases of Cr2O3 (major), CrO2 and CrO3. XPS studies also showed the formation of Cr0. BET analyses indicated specific surface area of xerogel as 20.9 m2 g-1, average pore-size 2.1 nm and pore volume 0.036 cc g-1 after heating at 850 °C for 3 h in the air. Thus, the toxic Cr(VI) could be removed from the waste stream and converted to chromium oxide xerogel which would find several applications as the catalyst. Thus, the -radiolysis and thermal treatment process reported to the present work provides sustainability to Cr(VI) conversion in the aqueous waste discharges from the various applications of useful chromium oxide material whose properties can be further tuned depending upon the applications. The crystalline chromium oxide exhibits good catalytic activity in many gas phase reactions. Xerogel enhances accessibility of catalytic sites to the reactants.
Acknowledgment Authors are thankful to Dr. P.D. Naik, Head, Analytical Chemistry Division for his keen interest and encouragement in the present work. Mr. T. V. Vittalrao, FCD, BARC is acknowledged for his help in BET surface analyses.
Appendix A. Supplementary data Supplementary data (Plots and Tables for Cr(VI)conversion (%) to Cr(III) during radiolysis under different conditions, Table containing TGA/DTA data, BET plots) related to this article can be found at: 19
SupportingInformation-SI-Komal.pdf
References [1] S.Langard, One hundred years of chromium and cancer: a review of epidemiological evidence and selected case reports, Am. J. Ind. Med. 17 (1990) 189–215. [2] M. Cieslak-Golonka, Toxic and mutagenic effects of chromium(VI): A review, Polyhedron 15 (1995) 3667-3689. [3] W. Mertz, Chromium in human nutrition: A review, J. Nutr. 123 (1993) 626–633. [4] J. Hu,G. Chen, I. M. C. Lo, Removal and recovery of Cr(VI) from wastewater by maghemite nanoparticles, Water Res. 39 (2005) 4528-4536. [5] Y. Xing, X. Chen, D. Wang, Electrically regenerated ion exchange for removal and recovery of Cr(VI) from wastewater, Environ. Sci. Technol. 41 (2007) 1439-1443. [6] M. K.Aroua, F. M.Zuki, N. M.Sulaiman, Removal of chromium ions from aqueous solutions by polymer-enhanced ultrafiltration, J. Hazard. Mater.147(2007) 752-758. [7] E.M. Contreras, A. M. F. Orozco,N.E.Zaritzky,Biological Cr(VI) removal coupled with biomass growth, biomass decay, and multiple substrate limitation, Water Res. 45 (2011) 3034-3046. [8] J. A. Korak, R. Huggins, M. Arias-Paic, Regeneration of pilot-scale ion exchange columns for hexavalent chromium removal, Water Res. 118 (2017) 141-151. [9] M. Pettine, L. Campanella, F. J. Millero, Reduction of hexavalent chromium by H2O2 in acidic solutions, Environ. Sci. Technol. 36 (2002) 901-907. [10]
M. Gaberell, Y.-P. Chin, S. J.Hug, B. Sulzberger, Role of dissolved organic
matter composition on the photo-reduction of Cr(VI) to Cr(III) in the presence of iron, Environ. Sci. Technol. 37 (2003) 4403-4409. [11]
A. Rauf, Md. S. A. S. Shah, G. H. Choi, U. B. Humayoun, D. H. Yoon, J. W.
Bae, J. Park, W. -J. Kim, P. J. Yoo, Facile synthesis of hierarchically structured 20
Bi2S3/Bi2WO6 photocatalysts for highly efficient reduction of Cr(VI), ACS Sustainable Chem. Eng. 3 (2015) 2847-2855. [12]
X. Wang, M. Hong, F. Zhang, Z. Zhuang, Y. Yu, Recyclable nanoscale
zerovalent iron doped g-C3N4/MoS2 for efficient photocatalysis of RhB and Cr(VI) driven by visible light, ACS Sustainable Chem. Eng. 4 (2016) 4055-4063. [13]
C. E.Barrera-Díaza, V. Lugo-Lugoa, B. Bilyeuc, A review of chemical,
electrochemical and biological methods for aqueous Cr(VI) reduction, J. Hazard. Mater. 223-224 (2012) 1-12. [14]
D. Pradhan, L. B. Sukla, M. Sawyer, P. K. S. M. Rahman, Recent bioreduction
of hexavalent chromium in wastewater treatment: A review, J. Indus. Engin. Chem. 55 (2017) 1-20. [15]
R. Marks, T. Yang, P. Westerhoff, K. Doudrick, Comparative analysis of the
photocatalytic reduction of drinking water oxoanions using titanium dioxide, Water Research 104 (2016) 11-19. [16]
K. Pikaev, New data on electron-beam purification of wastewater, Rad. Phys.
Chem. 65 (2002) 515-526. [17]
F. Djouider, Radiolytic formation of non-toxic Cr(III) from toxic Cr(VI) in
formate containing aqueous solutions: A system for water treatment, J. Hazard. Mater. 223-224 (2012) 104-109. [18]
L. M. Alrehaily, J. M. Joseph, J. C. Wren, Radiation-Induced Formation of
Chromium Oxide Nanoparticles: Role of Radical Scavengers on the Redox Kinetics and Particle Size, J. Phys. Chem. C 119 (2015) 16321-16330. [19]
L. M. Alrehaily, J. M. Joseph, A. Y. Musa, D. A. Guzonas, J. C. Wren, Gamma-
radiation induced formation of chromiumoxide nanoparticles from dissolved dichromate, Phys. Chem. Chem. Phys. 15 (2013) 98-107. 21
[20]
X. Wang, S. O. Pehkonen, A. K. Ray, Removal of aqueous Cr(VI) by a
combination of photocatalytic reduction and coprecipitation, Ind. Eng. Chem. Res. 43 (2004) 1665-1672. [21]
Y. Kumagai, R. Nagaishi, R. Yamada, Y. Katsumura, Effect of silica gel on
radiation-induced reduction of dichromate ion in aqueous acidic solution, Rad. Phys. Chem.80 (2011) 876-883. [22]
G. Chen, J. Feng, W. Wang, Y. Yin, H. Liu, Photocatalytic removal of
hexavalent chromium by newly designed and highly reductive TiO2 nanocrystals, Water Res. 108 (2017)383-390. [23]
P. Nagraj, N. Aradhana, A. Shivakumar, A. K. Shrestha, Spectrophotometric
method for the determination of chromium (VI) in water samples, Environmental Monitoring and Assessment 157 (2008) 575-582. [24]
M. Owlad, M. K.Aroua, W. A. W. Daud, S. Baroutian, Removal of hexavalent
chromium-contaminated water and wastewater: A review, Water Air Soil Pollut. 200 (2009) 59-77. [25]
W. T. Spinks, R. J. Woods, An Introduction to Radiation Chemistry. 3rd edition
(1990) 243-312, A WILEY- INTERSCIENCE publication, John Wiley & sons, Inc. [26]
J. Jeong, J. Yoon, Dual roles of CO2•− for degrading synthetic organic
chemicals in the photo/ ferrioxalate system, Water Res. 38 (2004) 3531–3540. [27]
H. A. Schwarz, R. W. Dodson, Reduction potentials of CO2•− and the alcohol
radicals. J. Phys. Chem. 93 (1989) 409-414. [28]
D. Rai, B. M. Sass, D. A. Moore, Chromium(III) hydrolysis constants and
solubility of chromium(III) hydroxide, Inorg. Chem. 26 (1987) 345-349.
22
[29]
K. M. S. Sumathi, S. Mahimairaja, R. Naidu, Use of low-cost biological wastes
and vermiculite for removal of chromium from tannery effluent, Bioresource Technol. 96 (2005) 309–316. [30]
F. S. Baker, J. D. Carruthers, R. E. Day, K. S. W. Sing, L. J. Stryk, Micropore
Structure of Chromium Oxide Gels, Discuss. Faraday Soc. 52 (1971) 173-186. [31]
S. Kittaka, T. Morooka, K. Kitayama, Thermal Decomposition of Chromium
Oxide Hydroxide I. Effect of Particle Size and Atmosphere. J. Solid State Chem. 58 (1985) 187-193. [32]
M. Baster, F. Bouree, A. Kowalska, Z. Latacz, The change of crystal and
exchange parameters in the vicinity of TN in Cr2O3, J. Alloys Comp. 296 (2000) 1-5. [33]
H. Sun, L. Wang, D. Chu, Z. Ma, A. Wang, Synthesis of porous Cr2O3 hollow
microspheres via a facile template-free approach, Mater. Lett. 140 (2015) 35–38. [34]
G. Tirao, S. Ceppi, A. L. Cappelletti, E. V. Pannunzio Miner, Oxidation state
characterization in Cr oxides by means of Cr-K beta emission spectroscopy, J. Phys. Chem. Solids 71 (2010) 199-205. [35]
J. S. Stephens, D. W. J. Cruickshank, The crystal structure of (CrO3)infinite, Acta
Crystallographica: Section B 26 (1970) 222-226.
23
Highlights
Cr(VI) reduction was carried out in chrome tan liquor using γradiolysis.
Formate was used to scavenge oxidizing radicals leaving behind reducing species.
Basic pH of solution resulted to simultaneous precipitation of Cr(OH)3.
Cr(III) precipitate was subjected to thermal treatment for converting to xerogel.
Crystalline mixed valence oxide xerogel with surface area 20.9 m2 g-1 was formed.
24
Graphical Abstract Thermal treatment
-rays Cr(VI) basic pH
formate
Cr(OH)3 Precipitation
Chromium Oxide Xerogel
25
Conflict of Interest and Authorship Conformation Form
o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
The authors declare no conflict of interest. Our Institute is Govt. organization and Govt. provides fund for carrying out the research.
26