Sustainable disposal of Cr(VI): adsorption-reduction strategy for

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Sustainable disposal of Cr(VI): adsorption-reduction strategy for treating textile wastewaters with aminofunctionalized boehmite hazardous solid wastes Hailin Zhang, Ping Li, Zheming Wang, Wenwen Cui, Yang Zhang, Ying Zhang, Shili Zheng, and Yi Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2018 Downloaded from http://pubs.acs.org on April 15, 2018

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ACS Sustainable Chemistry & Engineering

Sustainable disposal of Cr(VI): adsorption-reduction strategy for treating textile wastewaters with amino-functionalized boehmite hazardous solid wastes Hailin Zhang,a,bPing Li,*,a Zheming Wang, c WenWen Cui, a,b Yang Zhang,aYingZhang,a Shili Zheng,a and Yi Zhanga a

National Engineering Laboratory for Hydrometallurgical Cleaner Production, Techno

-logy, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, 1 North 2nd Street, Zhongguancun, Haidian District, Beijing, 100190, People’s Republic of China b

University of Chinese Academy of Sciences, No.19A Yuquan Road, Shijingshan

District, Beijing, 100049, People’s Republic of China c

Physical and Computational Science Directorate, Pacific Northwest National

Laboratory, 902 Battelle Boulevard, Richland, WA, 99354, USA *

Corresponding author, *Ping Li, E-mail: [email protected]

Tel: +86-10-82544856; fax: +86-10-82544856 Hailin Zhang, E-mail: [email protected]; Zheming Wang, E-mail: [email protected]; Wenwen Cui, E-mail: [email protected]; Yang Zhang, E-mail: [email protected]; Ying Zhang, E-mail: [email protected]; Shili Zheng, E-mail: [email protected]; Yi Zhang, E-mail: [email protected] Hailin Zhang, Ping Li, Zheming Wang，Wenwen Cui, Yang Zhang, Ying Zhang, Shili Zheng and Yi Zhang

ABSTRACT The present

work

reports

a

sustainable

adsorption-reduction

strategy

for

disposal of Cr(VI)-bearing wastes. By crosslinking between boehmite and chitosan to prepare amino-functionalized boehmite adsorbents, Cr(VI) could be adsorbed from the Cr(VI)-containing solutions by the adsorbents with the maximum adsorption capacity of 120.2 mg/g which was improved about 3 times compared to that of 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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boehmite. Adsorption mechanism showed that the Cr(VI) was adsorbed by complexing with –NH2 bonds and exchanging with –OH groups of adsorbens. About 31% Cr(VI) was simultaneously reduced to Cr(III) by the reactive –OH and –NH2 electron acceptor of adsorbents. After adsorption, the Cr(VI/III) and boehmite of adsorbents were dissolved in 25% H2SO4 solution, the remaining Cr(VI) was reduced to Cr(III) with 100% reduction yield, and the CrxAl2-x(OH)2y(SO4)8-y∙nH2O was prepared as product, while the insoluble chitosan was recycled in a closed loop. In application, column experiments results showed that the AlOOH hazardous wastes could effectively dispose the Cr(VI)-containing textile wastewaters, and aluminum and chromium in the wastes could be comprehensively used to prepare chromium-aluminum tanning agent. The developed ―waste-treating-waste‖ method leads to minimum pollutant emission and avoids the complex post-treatment problems. KEYWORDS:

Sustainable

disposal,

Adsorption-reduction

method,

amino-functionalized boehmite hazardous wastes, Textile wastewaters, Chromiumaluminum tanning agent, Chitosan

INTRODUCTION As one of most common contaminants of industrial wastewaters, chromium(VI), soluble and mobile in the natural environment, exerts highly damaging toxic effects in biological systems.1-3 A permissible limit of Cr(VI) concentration in the discharged wastewaters, ranging from 0.02 to 0.5 mg/L depending on its origin and location, has been etablished.4-5 Industrially, Cr(VI) is removed by chemical reduction of Cr(VI) to Cr(III) and precipitation of Cr(III) with the addition of chemical reagents. Reaction equations are as follows: Cr2O72- + Men+ +14H+  2Cr3++ Men+1 + 7H2O

(1)

Cr3+ + 3OH-  Cr(OH)3(s)

(2)

The process yields appreciable amounts of Cr(III)-containing solid wastes as by-products.6 Various methods have been developed to remove Cr(VI) from the 2


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Cr(VI)-containing wastewaters, such as ion exchange, membrane filtration, solvent extraction, and adsorption with the goal to avoid the generation of the secondary Cr(III)-bearing solid wastes.7-9 Among these methods, adsorption method is one of most sustainable options. The representative adsorbents include activated carbon, fly ash, agricultural wastes, boehmite, etc.10-12 For example, boehmite-AlOOH) and agricultural

waste

biomass

adsorbents,

could

adsorb

Cr(VI)

from

the

Cr(VI)-containing wastewaters with the adsorption capacity of a few and dozens of milligrams per gram, respectively.13,14 However, the relatively limited adsorption capacity makes it ineffective for Cr(VI) removal. Moreover, the adsorption methods often involve the uneconomic regeneration of adsorbents and incomplete desorption of Cr(VI) onto adsorbents, limiting their application under many conditions. Chitosan is a linear polymer which is mainly produced from N-deacetylation of chitin. Its large number of hydroxyl groups and primary amino groups enable it effectively complex with metal ions. In this way, the adsorption capacity for Cr(VI) can be improved by modifying the surface structure of inorganics with chitosan. Lately, inorganics crosslinked by the chitosan including Ti-chitosan composites, Fe-chitosan composites, etc. have shown their improved Cr(VI) adsorption performance.15-18 However, -AlOOH which was crosslinked by the chitosan was seldom reported. Furthermore, the chitosan is hardly dissolved into some desorption medium, such as high concentrated H2SO4 and NaOH solution.19 The distinctive dissolution characteristics make it possible for separation from adsorbents. In this paper, amino-functionalized -AlOOH (AF--AlOOH) was prepared by the cross-linking between -AlOOH and chitosan to improve the adsorption capacity for Cr(VI). Synthesis of AF--AlOOH was conducted, and key factors and mechanism that influenced the Cr(VI) adsorption onto the AF--AlOOH were studied. The effective adsorption removal of Cr(VI) from the textile wastewaters was also investigated in a fixed–bed column set-up with the adsorbents which were obtained by crosslinking between chitosan and mass produced -AlOOH hazardous solid wastes.20 After adsorption, the adsorbents were firstly dissolved into the H2SO4 solution, the insoluble chitosan was separated, the Cr(VI) was reduced by adding the reducing 3



Page 4 of 27

agent, and the Cr(III)-Al(III) containing H2SO4 solutions were obtained to prepare chromium-aluminum tanning agent (CrxAl2-x(OH)2y(SO4)8-y∙nH2O) which had been widely applied in the leather industry to improve the robustness and viability of leather products.21 The process can be expressed as follows: -AlOOH-chitosan + Cr(VI)  -AlOOH-chitosan- Cr(VI)

(3)

-AlOOH-chitosan-Cr(VI) + H2SO4 + Reducer CrxAl2-x(OH)2y(SO4)8-y∙nH2O (4)

+chitosan

Therefore, the following purposes are realized: (i) adsorption separation of Cr(VI) to eliminate the pollution of Cr(III)-containing solid wastes; (ii) recycling of chitosan and preparation of the chromium-aluminum tanning agent, avoiding the complex post-treatment process; (iii) comprehensive utilization of the textile wastewaters and -AlOOH hazardous solid wastes.

RESULTS AND DISCUSSION Adsorbent Preparation and Characterization. The AF--AlOOH was successfully synthesized, as shown in Figure 1A, where it could be found that the obtained sample displayed the major characteristic peaks of -AlOOH and chitosan, which was also supported by the XRD results (Figure S1).22 The overlapping bands resulting from two symmetrical peaks at 3450 cm-1 were attributable to –OH and –NH stretching vibrations.23-25 Compared to those of -AlOOH and chitosan, the distinct difference in the FTIR spectrum of AF--AlOOH was the appearance of new band at 1564 cm-1, related to the monodentate (C–(O–Al)O) or bidentate (C–(O–Al)2) ligand obtained by bonding -AlOOH with C=O stretching of formyl group.26,27 Another possibility is the formation of covalent bond between the O–C–O group and surface of -AlOOH, producing –C(=O)OAl– groups, similar to that of CuO interacted with amino acids.28 The disappearance of some amide groups, included amide I and amide II bands at 1654 and 1596 cm-1, respectively, due to the consumption of NH2 groups by glutaraldehyde. Note that the occurrence of cross-linking reaction between -AlOOH and chitosan in the presence of glutaraldehyde as cross-linker.29,30 This is consistent with the fly ash/chitosan and CdS/chitosan composites.31,32 The 4


13

C-NMR spectra of

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chitosan and AF--AlOOH were further shown in Figure 1B. It could be found the chemical shift values of C=O (173.9 ppm) , C1 (105.2 ppm), C2 (57.4 ppm), C4 (82.8 ppm), C5/C3 (75.3 ppm), C6 (61.5 ppm) and CH3 (23.3 ppm) in methyl and carbonyl groups of pure chitosan.33-35 After the crosslinking, the C4 and C2 peaks of AF--AlOOH were overlapped and disappeared owing to the enlargement of C5 and C6 peaks. The peak of C=O was relatively shifted to 180.1 ppm and the line width was enlarged because of the incorporation of -AlOOH graft chain or carboxylic acid.33,36 The appearance of new wide peak at 149.3 ppm was assigned to the imine carbon (C=N) bond.37-40 It thus could be speculated that the –NH2 groups on chitosan and –OH groups of -AlOOH as cross-link sites reacted with formyl groups of glutaraldehyde, realizing the functionalization of -AlOOH with amino41 in the cross-linking process, as shown in Scheme S1. SEM examination indicated that the AF--AlOOH was agglomerated (Figure S2), while the cross section exhibited the irregular porous structure (Figure S3A), after grinding and polishing of particles by a sample thinning method.42 The relative EDX results showed that Al, C, N and O elements with uniform distribution were clearly observed in the cross section, confirming the incorporation of -AlOOH into chitosan (Figure S3B).

5



Figure 1. (A)FTIR, (B) 13C solid-state NMR spectra and (C) Zeta potential of the -AlOOH, chitosan and AF--AlOOH.

A plot of zeta potential (ξ) of AF--AlOOH, -AlOOH and chitosan versus pH (3-13) is shown in Figure 1C. As observed, the AF--AlOOH had a significant positive ξ value toward the acid region due to the protonation of the amino group in comparison with those of -AlOOH and chitosan. A maximum value of around 23 mV at pH 4, while those of -AlOOH and chitosan had 5 mV and 16 mV, respectively， were recorded in Cr(VI)-containing solutions (50 mg/L of Cr(VI)) whereupon it started to decrease when the pH increased. The increase of pH resulted in a negative ξ value among the -AlOOH, chitosan and AF--AlOOH allowing the increase of electrostatic repulsion in the solutions. The isoelectric point of AF--AlOOH occurred at pH 9.5, higher than those of -AlOOH and chitosan, also suggesting that there was crosslinking reaction between chitosan and -AlOOH. The textural parameters of AF--AlOOH, -AlOOH and chitosan were also listed in Table S1. As observed, the BET surface of -AlOOH reached 412.6 m2/g, however, the AF--AlOOH was strikingly decreased to 20.56 m2/g, which could be ascribed to the presence of chitosan macromolecular chain and flocculation behavior, resulting in the micropore blockage of -AlOOH.43-45 Note that the crosslinking process was required to impart a high internal surface area to the chitosan. Cr(VI) Adsorption. Adsorption isotherms were determined and the obtained data (Figure S4A) were simulated by both the Langmuir and the Freundlich models46 (supporting information, Part 1.1). Table 1 suggested that the Langmuir model properly described the adsorption equilibrium of Cr(VI), indicating that the adsorption of Cr(VI) onto the -AlOOH, AF--AlOOH and chitosan belonged to dominantly monolayer adsorption.15 The maximum adsorption capacities (qm) for Cr(VI) onto the chitosan, -AlOOH and AF--AlOOH were 50.3, 61.5, and 120.2 mg/g, respectively. It thus could be calculated that the Cr(VI) adsorption capacity was increased by about 3 times in the AF--AlOOH as compared to both chitosan and -AlOOH. The presence of –OH groups and –NH2 groups from the chitosan moiety enables strong surface complexation between the groups and AF--AlOOH, leading to the improved 6


Page 6 of 27

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adsorption uptake capacity for Cr(VI). Comparison of the adsorption capacity of Cr(VI) onto the AF--AlOOH with those of the other chitosan-crosslinking inorganics adsorbents reported in literature (Table S2) clearly demonstrated that the AF--AlOOH was a relatively good adsorbent for Cr(VI) be next only to Ti or Zr-chitosan composite which possessed Ti4+ or Zr4+ adsorption centers with stronger electrostatic attraction for Cr(VI).15,24,47 Table 1. Langmuir and Freundlich parameters for Cr(VI) ions adsorption onto the -AlOOH, chitosan and the AF--AlOOH. Langmuir

Adsorbent

Freundlich

qm/( mg/g)

KL/(L/mg)

R2

n

-AlOOH

61.5

0.239

0.997

3.661

18.656

0.861

Chitosan

50.3

0.045

0.998

2.000

5.917

0.830

AF--AlOOH

120.2

0.108

0.997

2.284

18.576

0.843

KF/(L·g-1) R2

Table 2. Adsorption rate constant obtained from the pseudo-first-order and pseudo-second-order kinetic models of Cr(VI) ions onto the -AlOOH, chitosan and the AF--AlOOH. pseudo-first-order Adsorbent

pseudo-second-order

k1/h-1

qe (mg·g-1)

R2

k2/(g·mg-1·h-1)

qe (mg·g-1)

R2

-AlOOH

1.327

14.13

0.842

0.090

52.5

0.998

Chitosan

0.351

2.98

0.145

1.077

42.52

0.999

AF--AlOOH

0.528

24.52

0.832

0.127

76.9

0.999

The effect of adsorption time (Figure S4B) showed that the Cr(VI) uptake on AF--AlOOH, -AlOOH and chitosan was rapid in the first 0.5 h, contributing to about 85% of the ultimate adsorption amount for Cr(VI), augmented gradually, and achieved the adsorption equilibrium about 5 h. The rapid adsorption in the beginning could be attributed to the greater concentration gradient and more available sites for adsorption.15,20 The fact that Cr(IV) adsorption reached equilibrium in about the same amount of time on the adsorbents, suggested that the crosslinking process had negligible effect on the adsorption rate of Cr(VI). Pseudo-first-order model and pseudo-second-order kinetic model20 (supporting information, Part 1.2) were used to 7



Page 8 of 27

fit the experimental data (Table 2). The fitting results indicated that the Cr(VI) adsorption

onto

-AlOOH,

AF--AlOOH

and

chitosan

followed

the

pseudo-second-order kinetic model (R2>0.998), indicating that the Cr(VI) adsorption was a chemisorption process and no involvement of a mass transfer in solution.15 The effect of pH (Figure 2A) showed that Cr(IV) adsorption on the AF--AlOOH was more active in the acidic solution. The Cr(VI) adsorption capacity nearly plateaued between pH 3-5, and the maximum adsorption occurred at pH 3 with an adsorption capacity of 81 mg/g. As a comparison, it was only 42.9 mg/g that the adsorption capacity(q) for Cr(VI) onto the mixtures between -AlOOH and chitosan with the mass ratio of 1:1 under the same conditions. Cr(VI) adsorption capacity decreased with increasing the pH from 5 to 12. The Cr(VI) adsorption onto the AF--AlOOH could be ascribed to the ion exchange between –OH and Cr(VI) and complexing Cr(VI) with –NH2 by the electrostatic attraction, considering that the Cr(VI) species mainly existed as HCrO4- and CrO42- at pH 2-6 and pH>6, respectively, in the solution. With rising pH, the extent of protonation of –NH2 group would be reduced and the net surface charge became less positive, resulting in the decrease of Cr(VI) adsorption capacity.

Figure 2. Adsorption of Cr(VI) on the AF--AlOOH as a function of (A): pH, (B): temperature, 8


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(C): initial concentration.

As expected, Cr(VI) adsorption capacity of AF--AlOOH increased with the increase of temperature and initial Cr(VI) concentration (Figure 2B and 2C). The results showed that a plateau was reached until the saturation point was attained at 100 mg/L. This was attributed to the fact that the available sites for adsorption remained constant for a fixed amount of AF--AlOOH. The values of thermodynamic parameters related to the influence of temperature were also given in Table S3, where it could be found that the negative values of free energy change (∆Gº) at all temperatures revealed that the adsorption of Cr(VI) onto the AF--AlOOH was spontaneous and feasible, while the positive value of enthalpy change (∆Hº) indicated that the adsorption process was endothermic in nature.48,49 Adsorption mechanism. XPS spectra were measured for AF--AlOOH before and after Cr(VI) adsorption to characterize the Cr(VI) adsorption surface information. Figure S5A showed the corresponding C1s, N1s, O1s, Al 2p and Cr 2p.50 Table 3 summarizes the identification of the bands observed on the curve fitted XPS spectra (Figure S5B-F) and the calculated atomic fractions. As observed, the C 1s spectrum of AF--AlOOH could be assigned to peaks at the binding energies of 284.4, 285.7 and 288.6 eV for the carbon atoms in the C-C, C=N and C=O or O-C-O groups, respectively.51-53As for N1s spectrum, the peaks at 398.9, 399.9, 401.4 and 406.5 eV represented the characteristic peaks of –NH2, N, –NH3+ and –N=CH–, respectively.54,55 After the Cr(VI) adsorption, the intensity of C 1s peak at 285.9 eV decreased from 21.9% to 13.37%, indicating that the C=N bonds complexed with Cr(VI) and the occurrence of ion exchange between –OH groups and Cr(VI). The binding energy of –NH2 shifted from 398.9 to 398.4 eV, indicating the formation of –NH2HCrO4- complexes, in which the Cr(VI) donated electron to N atom and thus the electron cloud density of N atom was increased resulting in a decrease of binding energy.56,57The peaks at 586.2 and 577.7 eV belonged to Cr(VI), while the peak at 576.1 eV could be attributed to Cr(III), respectively,58-60 revealing that the Cr(VI) was partially reduced. The intensities that represented Cr(VI) and Cr(III) were 0.78% and 0.35%, demonstrating that about 31% Cr(VI) was reduced to Cr(III) during the 9



Page 10 of 27

adsorption. The reduction of toxic Cr(VI) to less toxic Cr(III) was likely ascribed to the reactive –OH and –NH2 electron acceptor present in the AF--AlOOH.61-63 Table 3. Assignments of main spectral bands based on their energies and atomic concentration for natural chitosan and the AF--AlOOH before and after Cr(VI) adsorption. Element Assignments AF--AlOOH AF--AlOOH -Cr BE(eV)

AC%

BE(eV)

AC%

C 1s

284.4

21.63%

284.5

27.68%

C 1s

285.7

21.90%

285.9

13.37%

C 1s

288.6

6.91%

288.2

2.02%

Total C

50.44%

C-C or adventitious carbon C-N, C=N, C-O or C-O-C C=O or O-C-O

43.07%

N 1s

398.9

1.23%

398.4

2.04%

-NH2

N 1s

399.9

0.70%

399.4

0.76%

N

N 1s

401.4

0.36%

401.2

0.47%

NH3+

N 1s

406.5

0.27%

406.4

0.12%

-N=CH-

Total N

2.56%

O 1s O 1s

531.4

16.84%

O 1s

532.3

21.50%

Total O Al 2p Total Al

3.39% 530.7

24.45%

O2-

531.9

17.09%

OHH2O

38.34% 74.0

8.66%

41.54% 73.5

8.66%

10.05% 10.05%

Cr 2p

576.1

0.35%

Cr(III)

Cr 2p

577.7

0.82%

Cr(VI)

Cr 2p

586.2

0.78%

Cr(VI)

Total Cr

1.95%

Separation of chitosan and preparation of chromium-aluminum tanning agent. A new strategy has been adopted to recycle chitosan and to comprehensively utilize the Cr(VI) and -AlOOH, avoiding the post-treatment process. Specifically, 2 g of Cr(VI)-bearing AF--AlOOH was firstly dissolved into 10 mL 25% H2SO4 solution under mild stirring. The solid residues were separated from the H2SO4 solutions for recycling by filtration at -0.1 MP for 0.5 h. ICP-OES analysis showed that the H2SO4 solutions contained 4.46 g/L Cr and 26.55 g/L Al(III) . A calculated amount of 10


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Na2Cr2O7 was secondly supplemented into the H2SO4 solutions to reach an Al: Cr mass ratio of greater than 2. Thirdly, 0.4 g glucose was subsequently added into the H2SO4 solutions to completely reduce the Cr(VI) into Cr(III) to form green solutions. At last, CrxAl2-x(OH)2y(SO4)8-y∙nH2O as chromium-aluminum tanning agent product was prepared after drying the solutions at 100 oC for 12 h. Under the experiment conditions, the synthesized product with the appropriate compositions (Table S4) met the

requirements

of

industrial

application,

e.g.

more

than

16%

Cr2O3, less than 8% Al2O3 and 33% alkalinity.64 XRD and FTIR results of the solid residues indicated that both the phase and structure resembled those of the natural chitosan (Figure S6). SEM-EDX analysis (Figure S7) showed that the resulting solid residues ranged in size from tens of nanometers to many microns, C and N were evenly distributed in the particles, and their chemical contents were 53.0% and 8.18%, respectively. About 2% S also could be detected, because of the entrainment of S from the H2SO4 solutions. No Al and Cr were detected in the particles of residues, indicating that the Cr(VI) and AlOOH of adsorbents were totally dissolved into the H2SO4 solutions. The separated chitosan was further used to crosslink the AlOOH to prepare the adsorbents, realizing the closed-loop recycling. Five continuous cycles experiments results (Figure S8) of separated chitosan showed that the Cr(VI) removal efficiency of the AF--AlOOH still remained almost 99% of the initial uptake, demonstrating that the separated chitosan could be reused to prepare the AF--AlOOH for Cr(VI) adsorption.

Green application in disposal of waste with waste

11



Page 12 of 27

Figure 3. Experimental and predicted breakthrough curves for Cr(VI) adsorption by the AF-AlOOH(Flow rate: 1ml/min, pH:5, Temperature: 30 oC, Dose:3g).

Column experiments were performed to treat the textile wastewaters65 with the adsorbents which was obtained by crosslinking between chitosan and -AlOOH hazardous solid wastes. Different from the synthesized -AlOOH, the -AlOOH hazardous solid wastes were produced from the synchronous adsorption separation process of Al(III) and V(V) from the Na2CrO4-NaAlO2-NaVO3 solutions in the chromate clean production.20,46 Experimental data is subjected to Thomas model66,67 developed to analyze lab-scale column data for the purpose of industrial applications. Thomas model assumes that rate driving force obeys second order reversible reaction kinetics, and the adsorption equilibrium follows the Langmuir model. The Thomas model is expressed as follows: ln

𝐶0 𝐾𝑇ℎ 𝑞0 𝑚 = − 𝐾𝑇ℎ 𝐶0t 𝐶𝑡 𝑄

(5)

Where KTh is the Thomas model constant (L/g min), q0 is the adsorption capacity(mg/g), and t stands for total flow time (min). The values of KTh and q0 can be 𝐶

determined from the linear plot of ln( 𝐶0-1) against t. 𝑡

As shown in Figure 3 and Table S5, the Cr(VI) concentration predicted from the model was in good agreement with the experimental data. The residual Cr(VI) in the effluent remained below the national discharge standard (NDS, GB4287-2012)68 with the 1 h of breakthrough time (th). Insofar, nearly 60 mL textile wastewaters were effectively disposed by the adsorbents with the 1 mL/min of flow rate, and the 12


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exhaust time(te) of Cr(VI) adsorption reached 26 h. A decrease in breakthrough time from 1 h to 0.5 h was also observed as the flow rate increased from 1 mL/min to 2 mL/min, leading to the lower treatment volume of textile wastewaters. Note that the higher Cr(VI) concentration gradient provided an increased mass transfer and Cr(VI) loading rate.69 It also could be found that PO43- and RB were effectively removed by the adsorbents while COD, SO42- and Cl- kept constant in the adsorption process (Figure 3 and Figure S9). Specifically, the treated wastewaters volumes before the breakthrough point were 60 mL and 90 mL, with the breakthrough time (th) of 1 h and 1.5 h, respectively. Additionally, the exhaust time(te) of PO43- and RB adsorption was 17 h and 15 h, respectively. The correlation coefficient values (R2 in Table S5) for Cr(VI), PO43- and RB obtained by the Thomas models indicated a good agreement between the experimental data and the column performance. The maximum bed adsorption capacities that were estimated by the model were 62.77, 2.53 and 11.7 mg/g with the initial concentrations of 50 mg/L of Cr(VI), 7 mg/L of PO43-, and 100 mg/L of RB, respectively. The adsorption capacity for Cr(VI) decreased to 66% in the textile wastewaters in comparison with the Cr(VI)-containing solutions, which was attributed to the active adsorption sites on the adsorbents occupied by PO43- and RB due to their competitive affinity.70 The adsorbents obtained after column adsorption which contained 2.98% of Cr(VI), 0.354% of PO43- and 0.59% of RB, were further used to prepare the chromium-aluminum tanning agent product and recycle the chitosan (see Separation of chitosan and Preparation of chromium-aluminum tanning agent section). The product could be expressed as Cr1.4Al0.6(OH)10.4(SO4)2.8∙nH2O, based on the content of Al, Cr and S, as shown Table S4. About 0.0157% P(V) and 0.0174% V(V) were also detected which was originated from the textile wastewaters and -AlOOH hazardous residues, respectively. It has been reported that traces of P(V) and V(V) does not affect its practical usage.71 Besides, it is believed that the adsorbed RB had the negligible effect on the performance of tanning agent since the RB could be oxidized by the supplemented Na2Cr2O7 at acid conditions, and the R-N=N-R bond of 13



RB could be broken down and be further converted to colorless compounds with small molecular weights and simple structures.72 Based on the finding above, we proposed a cost-effective and sustainable flow sheet of comprehensive utilization of AlOOH hazardous wastes and textile wastewaters, as presented in Scheme 1. Boehmite hazardous residue chitosan

glutaraldehyde

Functionality

Adsorbents

Chitosan recycling

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Textile wastewater

Adsorption Cr(VI) containing-adsorbents H2SO4

Dissolution Al(III)-Cr(VI) -containing solution Na2Cr2O7

C6H12O6

Reduction/Dry

chromium-aluminum tanning agent

Scheme 1. Principal flow sheet of adsorption-reduction method to dispose textile wastewaters with boehmite hazardous solid wastes.

In the adsorption step, AlOOH hazardous solid wastes was functionalized by chitosan with the glutaraldehyde as the cross-linker, and then used to adsorb Cr(VI), PO43- and RB from the textile wastewaters. 100 g AlOOH hazardous wastes could dispose about 4 L textile wastewaters to meeting the textile wastewaters emission standards. After adsorption, nearly 100% AlOOH hazardous solid wastes and its adsorbates were dissolved into 25% H2SO4 solution, and about 99% chitosan, present as solid residues, was separated via centrifugation and then recycled. In the reduction step, after adjusting the ingredient of Al and Cr by Na2Cr2O7, the Cr(VI) of solutions 14


Page 14 of 27

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was reduced to Cr(III) by the addition of C6H12O6 with 100% reduction yield. At last, the chromium-aluminum tanning agent product was obtained after drying the solutions. In conclusion, AlOOH hazardous wastes and textile wastewaters were effectively disposed

by

AF-AlOOH-based


method,

instead

of

conventional chemical reduction-precipitation method, and the Cr and Al in the wastes were fully utilized without pollutant emission, avoiding the complex post-treatment problem. Therefore, potential application of wastes disposal and preparation of valued products could be expected.

CONCLUSIONS Amino-functionalized chitosan-bearing -AlOOH adsorbents were successfully synthesized from -AlOOH in the presence of chitosan via a crosslinking process. After

adsorption

of

Cr(VI),

the

adsorbents

could

be

used

to

prepare

Cr1.4Al0.6(OH)10.4(SO4)2.8∙nH2O and separate the chitosan as medium, by dissolving the adsorbents into the H2SO4 solutions, and then reducing the Cr(VI) to Cr(III) by C6H12O6. -AlOOH hazardous wastes which were functionalized by chitosan were used to dispose the simulated textile wastewaters in continuous fixed–bed column, and performed well in terms of the removal of Cr(VI), color and phosphate. Furthermore, the chromium-aluminum tanning agent product was prepared and the chitosan was recycled. The cost-effective and sustainable adsorption-reduction method strategy is proposed to have promising potential for removal of heavy metals from wastewaters and the comprehensive utilization of adsorbents and adsorbates after adsorption.

ASSOCIATED CONTENT Supporting Information The supporting information is available in another document:

EXPERIMENTAL

SECTION, Supplementary SEM image, EDS, XRD patterns, XPS spectra of samples 15



Page 16 of 27

and detailed information of adsorption kinetics and isotherms, including 9 figures and 5 tables.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected];

Tel: +86-10-82544856

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51574212, U1403195, 51274179) and the Major State Basic Research Development Program of China (No. 2013CB632605).

NOTES AND REFERENCES

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by

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Page 24 of 27

For Table of Contents Use Only

Synopsis a

sustainable,

cost-effective


strategy

for

treating

Cr(VI)-containing textile wastewaters with amino-functionalized boehmite hazardous solid wastes

24


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (A)FTIR, (B) 13C solid-state NMR spectra and (C) Zeta potential of the -AlOOH, chitosan and AF--AlOOH.

Table 1. Langmuir and Freundlich parameters for Cr(VI) ions adsorption onto the -AlOOH, chitosan and the AF--AlOOH. Langmuir

Adsorbent

Freundlich

qm/( mg/g)

KL/(L/mg)

R2

n

-AlOOH

61.5

0.239

0.997

3.661

18.656

0.861

Chitosan

50.3

0.045

0.998

2.000

5.917

0.830

AF--AlOOH

120.2

0.108

0.997

2.284

18.576

0.843

KF/(L·g-1) R2

Table 2. Adsorption rate constant obtained from the pseudo-first-order and pseudo-second-order kinetic models of Cr(VI) ions onto the -AlOOH, chitosan and the AF--AlOOH. pseudo-first-order Adsorbent

pseudo-second-order

k1/h-1

qe (mg·g-1)

R2

k2/(g·mg-1·h-1)

-AlOOH

1.327

14.13

0.842

0.090

52.5

0.998

Chitosan

0.351

2.98

0.145

1.077

42.52

0.999

AF--AlOOH

0.528

24.52

0.832

0.127

76.9

0.999


qe (mg·g-1)

R2


Page 26 of 27

Figure 2. Adsorption of Cr(VI) on the AF--AlOOH as a function of (A): pH, (B): temperature, (C): initial concentration. Table 3. Assignments of main spectral bands based on their energies and atomic concentration for natural chitosan and the AF--AlOOH before and after Cr(VI) adsorption. Element Assignments AF--AlOOH AF--AlOOH -Cr BE(eV)

AC%

BE(eV)

AC%

C 1s

284.4

21.63%

284.5

27.68%

C 1s

285.7

21.90%

285.9

13.37%

C 1s

288.6

6.91%

288.2

2.02%

Total C

50.44%

C-C or adventitious carbon C-N, C=N, C-O or C-O-C C=O or O-C-O

43.07%

N 1s

398.9

1.23%

398.4

2.04%

-NH2

N 1s

399.9

0.70%

399.4

0.76%

N

N 1s

401.4

0.36%

401.2

0.47%

NH3+

N 1s

406.5

0.27%

406.4

0.12%

-N=CH-

Total N

2.56%

O 1s O 1s

531.4

16.84%

O 1s

532.3

21.50%

Total O Al 2p Total Al

3.39% 530.7

24.45%

O2-

531.9

17.09%

OHH2O

38.34% 74.0

8.66% 8.66%

41.54% 73.5

10.05% 10.05%


Page 27 of 27

Cr 2p

576.1

0.35%

Cr(III)

Cr 2p

577.7

0.82%

Cr(VI)

Cr 2p

586.2

0.78%

Cr(VI)

Total Cr

1.95%

Figure 3. Experimental and predicted breakthrough curves for Cr(VI) adsorption by the AF-AlOOH(Flow rate: 1ml/min, pH:5, Temperature: 30 oC, Dose:3g).

Boehmite hazardous residue chitosan

glutaraldehyde

Functionality

Adsorbents Textile wastewater

Chitosan recycling

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Adsorption Cr(VI) containing-adsorbents H2SO4

Dissolution Al(III)-Cr(VI) -containing solution Na2Cr2O7

C6H12O6

Reduction/Dry

chromium-aluminum tanning agent

Scheme 1. Principal flow sheet of adsorption-reduction method to dispose textile wastewaters with boehmite hazardous solid wastes.


Sustainable disposal of Cr(VI): adsorption-reduction strategy for

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