Chromium Footprint Reduction: Nanocomposites as Efficient

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Chromium footprint reduction: nanocomposites as efficient pretanning agents for cowhide shoe upper leather Bin Lyu, Rui Chang, Dangge Gao, and Jianzhong Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00233 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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ACS Sustainable Chemistry & Engineering

Chromium footprint reduction: nanocomposites as efficient pretanning agents for cowhide shoe upper leather Bin Lyu*†‡§, Rui Chang†‡, Dangge Gao†‡, Jianzhong Ma*†‡§ †

College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science &

Technology, 6 Xuefu Zhonglu, Weiyang District, Xi'an 710021, P. R. China ‡

National Demonstration Center for Experimental Light Chemistry Engineering Education (Shaanxi

University of Science & Technology), 6 Xuefu Zhonglu, Weiyang District, Xi'an 710021, P. R. China § Key Laboratory of Leather Cleaner Production, China National Light Industry, 6 Xuefu Zhonglu, Weiyang District, Xi'an 710021, P. R. China *E-mail: [email protected]. Tel.: +086-029-8613-2559 (Bin Lyu), *E-mail: [email protected]. Tel.: +086-029-8613-2559 (Jianzhong Ma).

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ABSTRACT: We report a green chemistry approach to synthesized nanocomposites as pretanning agents in cowhide shoe upper leather tanning process. Nano ZnO, as-prepared polymer and polymer/ZnO nanocomposites as pretanning agents were respectively taken for the controls. The amount of chrome tanning agents reduced from 6% (conventional process) to 4% was added in tanning process as a control process. Chromium footprints in wastewater and chromium distribution in leather samples were mainly analyzed. The results showed that chromium footprint in the shoe upper leather marking was minimized when using nanocomposites, compared with the control process, and chromium content in leather increased from 13489 mg·kg-1 to 15039 mg·kg-1. Additionally, thermal properties, mechanical properties, softness, and dyeing fastness of the leather pretreated with nanocomposites were close to those of leather treated with the conventional tanning process.

Scanning

electron

microscopy

analysis

indicated

that

leather pretreated

with

nanocomposites can be endowed with expected grain smoothness, good softness and separated fibers. Nanocomposites applied as pretanning agents can effectively enhance chromium uptake in bath and fix firmly in leather, which will reduce emissions of chromium footprint in leather making. KEYWORDS: Chromium footprint, Nanocomposites, Tanning, Thermal properties, Shoe upper leather


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INTRODUCTION Cowhides are widely used in the production of shoe upper leather with excellent mechanical properties. Similar to other leather producing processes, process for making shoe upper leather includes three parts: pretanning, tanning and finishing. The key tanning process can be considered as one of the ﬁrst industries to use and recycle secondary raw materials1. Tanning is also the main process in leather manufacturing which protects leather against environmental problems such as microbial degradation, heating damage, sweat or moisture, etc2. Approximately 90% of raw hides and skins are tanned by chrome in the world3. However, only 60% of the total chromium reacts with collagen of hides4, which leads to higher concentrations of chromium-contained wastewater5 and higher chromium content in solid wastes6. Moreover, chromium combined with collagen unfasten will be released from crust leather in the post tanning processes such as setting out, washing, chrome retanning, neutralizing, dyeing, fatliquoring and so on7. Hexavalent chromium in tannery wastewater is a potentially hazardous part of the wastes. Its compounds have negative effects on human health and some hexavalent chromium salts are considered carcinogens8. Furthermore, Chromium is a kind of heavy metal which causes oxidative damage by destruction of membrane lipids and DNA damage, it may even cause the death of a few plants species9, 10. This poses a threat to environmental sustainability and hinders the efforts of many industries to adopt cleaner production through zero-discharge and subsequent wastewater reuse. And tannery wastewater and sludge due to the complexity of chemical composition are difficult to remove through conventional treating technologies11. We focus on current situation of the technology and process of tanning effluent’s treatment in China, and end-of-pipe treatment only scratches the surface. The best wastewater management program in tanning industry should include cleaner production12. A cleaner production is obtained by using chrome-free tanning or high exhaust chrome (or less chrome) tanning systems. At present, the cleaner production technology is mainly concentrated in chrome-free tanning agents13,14. However, chrome-free tanning agents have various problems, such as pungent odor, free formaldehyde, low hydrothermal stability, yellowing, high costs, etc15. So，high exhaustion chrome tanning gets a lot of attention, mainly because the unique feature of it lies in reducing producing costs as well as eco-friendly approach with improved organoleptic properties of leather. The current high exhaustion auxiliary including small molecule carboxylic acids16, oxazolidine17, aldehyde acids18, polymer materials assisted chrome tanning have been attempted during leather



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marking. In order to deal with this contradiction, as far as possible, considering how to use less chrome tanning agents are paid attention on. Pelt is pretanned with chrome-free materials firstly, so that the collagen fibers are modified and cross linked gently. At the same time, collagen fibers are grafted onto a number of active groups which can coordinate with chrome effectively. Then, the pretanned pelt is tanned by a small amount of chrome in usual process. This method not only keeps outstanding properties of chrome tanned leather, but also minimizes the pollution and potential danger with chrome water and solid19. Recently, increasing interests have been directed towards incorporating nanotechnologies with leather manufacturing20. Thus nanocomposites applied in tanning process have improved collagen crosslinking effect21, and endowed leather with some special properties22. In addition, it can adsorb metal ion from tanning wastewater such as chromium23, 24. In our previous work, the amphoteric vinyl polymer/ZnO nanocomposites (NCM) were prepared by in situ sol-gel method and the as-prepared NCM was core-shell in structure and had carboxyl groups25, 26. In addition, the qualities and properties of leather made by nanocomposite materials were tested and observed. Results showed that nanocomposite applied in the suede leather27 and goat garment leather making process28 could improve properties of the resultant leather, which could help to realize the cleaner production. In this research, amphoteric polymer/ZnO nanocomposites were prepared as pretanning agent and applied in shoe upper leather making process for reducing chrome content and emissions’ content in tanning and post tanning processes.

EXPERIMENTAL SECTION Materials. Acrylic acid (AA) and hydroxyethyl acrylate (HEA) were purchased from Beijing Chemical Reagent Manufacturing (China). Dimethyl diallyl ammonium chloride (DMDAAC, 60%) was supplied by Shandong Luyue Chemical Industry (China). Acrylamide (AM), sodium hydrogen sulfite (NaHSO3) and ammonium persulfate (APS) were provided by Tianjin Hongyan Chemical Reagent Plant (China). Nano ZnO was purchased from Xiamen lujiali nanophase materials Co., Ltd. (China). Wet-salted USA cowhide was used. Polymer/ZnO nanocomposite materials (NCM) were prepared in our lab. Chrome tanning agent, mainly basic chromium sulfate was used by Sisecam China Limited Company. All chemicals used for leather manufacturing process were of commercial


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grade. All chemicals used for the analysis and detection were of laboratory grade. Synthesis of polymer/ZnO nanocomposite materials (NCM). NCM was fabricated by in-situ polymerization method. In a one-necked round-bottom flask (250ml) 2.086g of NaHSO3 was dissolved with 20g of deionized water and 35.608g of DMDAAC (60%) were introduced. The flask was moved into water bath at 70℃. Then 0.607g of nano ZnO was added to system and kept for 30min. 7.6g of AM, 5.6g of HEA and 18.50g of AA were mixed as mixed monomer. Mixed monomers and an aqueous solution of APS (8.33g, 7.85 wt%) were simultaneously dropped into the reactor in 30min. The reaction mixture was kept at 70℃ for another 4h. Then the solution was cooled to room temperature and pH of the solution was adjusted to about 5.0. Leather process. Pretanning and tanning process. Bated pelt was prepared by wet-salted USA cowhide at beamhouse as usual. Experimental sampling strategies were shown in Fig.1 (repeated experiments have been done for different tanning processes). Experiments were carried out as given in tanning scheme illustrated as Fig.2. The specific pretanning and tanning process parameters were shown in Tab.1. Experimental process and control process were taken for the control factor (pretanning agents). NCM as pretanning agents was experimental process, nano ZnO as pretanning agents was marked as ZnO only process, polymer as pretanning agents was marked as polymer only process and control process meant process without pretanning agents. Conventional process and experimental process were compared. Technically, the chrome tanned leather is called “wet blue”. Wet blue is the semi-finished product in leather making.

Fig.1 Experimental Sampling Strategies（1,1' :ZnO only process; 2,2' :Polymer only process; 3,3' : Experimental process; 4,4' : Control process; 5,5' : Conventional process）



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Fig.2 Tanning scheme Tab.1 The specific operations of conventional and experimental processes

Process Pretanning Pickling

Experimental process （ 2% NCM+4% Basic

Conventional

Chromium Sulfate）

Chromium Sulfate）

Chemicals

Quantity/%

Duration/min

Water

50

NCM

2.0

60

NaCl

1.0

10

Formic acid

0.5

20*3

Remarks

Basic

Chromium

Duration/min

pH:3.5~3.6， 1:10 dilution

8.0

10

0.5

20*3

1.0

60*2

4.0

120

6.0

150

Sodium formate

1.0

60

1.0

60

Sodium bicarbonate

1.0

3*30+30

2.0

6*30+30

Sulfate

Basic

Remarks

50

Sulphuric acid Tanning

Quantity/%

process （ 6%

pH:4.0，1:20 dilution

pH:3.0

pH:4.0

Note: ZnO only process and polymer only process were same with the other parameters of the experimental process except for the different types of pretanning agents.

Post tanning process. After the bated pelt was tanned by different methods, the wet blue was shaved and treated by post tanning process separately. Tab.2 shows the operation of post tanning. Wastewater is collected in every step when draining out except for washing process.


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Tab.2 The specific post tanning operation Process

Chemicals

Quantity/%

T/℃

Rewetting

Water

200

45

Degreasing agent

0.2

Penetrant

0.5

Oxalic acid

0.5

60

Washing

Water

300

10

Retanning

Water

100

Aliphatic aldehyde

2.0

30

Basic Chromium Sulfate

4.0

60

Sodium formate

1.0

30

Sodium bicarbonate

0.5

60

Washing

Water

300

10

Neutralizing

Water

150

Neutralizing tanning

2.0

Sodium bicarbonate

0.3

60

Water

300

10

Water

50

Sulphonated oil

1.0

Dispersed syntan

1.0

Acrylic resin retanning agent

5.0

Amino tanning agent

4.0

Withe sytan

4.0

Sytan

4.0

Dicyandiamide tanning agent

3.0

Quebracho extract

4.0

Dispersed syntan

1.0

Dyestuff

2.0

Water

100

Formic acid

0.5

Washing Filling, and dyeing

Duration/min

Remarks

pH: 3.5 Drained and collected wastewater Drained

35

pH: 4.0-4.1 Drained and collected wastewater Drained

30

30 10

90

90 50

10 20


pH: 5.0± Drained and collected wastewater Drained


Fatliquoring

Formic acid

1.0

30

Water

150

Softening fatliquor

2.0

Radical lipid agents lecithin

2.0

Sulphonated oil

2.0

Electrode stable fatliquor

1.0

90

Formic acid

2.0

20×3

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pH:4.0-4.5

55

pH:3.5± Drained and collected wastewater

Washing

Water

300

10

Drained

Note: the weight of chemicals is based on the weight of shaved wet blues.

After post-tanning process, leather samples were further treated by samming, setting, vacuum drying (50℃/4min), air drying, damping back (4h), vibration staking, and tumbling together. Thermal properties. Shrinkage temperature (Ts). The wet blue samples were prepared by the mold after tanning, and then Ts of each tanned leather samples were determined by a shrinkage tester (HY-852; Heng Yu Instrument (China) Limited) following the standard ISO 3380: 2002, MOD. The vertical and horizontal samples near the spine were both measured in each experiment, and the average values were the last values. Differential scanning calorimetric (DSC). A DSC-Q2000 differential scanning calorimeter (America) was used for the determination, curves were recorded. DSC measured heat flow (from temperature differences). The constant weight samples (5mg) were accurately weighed into alumina pans and hermetically sealed. DSC was measured in the presence of without water. The samples were heated with 3℃/min heating rate under N2 atmosphere and the range of temperature was from 25℃ to 250℃. Thermo gravimetric analysis (TGA). A TGA Q500 Thermo gravimetric analysis (America) was used for the determination, TG and DTG curves were recorded. TGA measures weight changes. The constant weight samples (5mg) were accurately weighed into ceramic crucibles and hermetically sealed. The samples were heated with 3℃/min heating rate under N2 atmosphere and the range of temperature was from 25℃ to 750℃. Chromium footprint. Chromium content in wastewater. Data of the chromium content in wastewater was collected and analyzed following per IUC 8 of IULTCS methods29. The data of chromium content in wastewater was measured two times and averaged each experiment.


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Chromium content in leather. Leather samples were digested by a microwave digestion instrument (MDS-10, Shanghai Sineo Microwave Chemistry Technology Co., Ltd.) firstly, and then the chromium content in leather was analyzed by a source atomic emission spectrometer (AAnalytst 400; Perkinelmer Corporation, USA). In order to measure effectively, three positions were gained near the spine that measured the chrome content of leather, and the final result is obtained by average the testing values of measurements. Chromium distribution in leather. Chromium distribution in cross-section of leather samples was analyzed by Energy Dispersive Spectrometer (EDS) which was equipped in a field emission scanning electron microscopy (S-4800, Rigaku Corporation, Japan). Leather properties. Leather samples were cut in standard molds and conditioned under standard atmospheric conditions for 48 h prior to analyze its properties. In order to measure effectively, six samples were obtained separately in perpendicular to the spine and parallel to spine. Tensile strength, tear strength and elongation at break of the leather samples were measured by a universal testing machine (AI-3000; Gotech Testing Machines Limited, China) following the standard ISO 3376: 1976. Bursting strength of the leather samples were measured by leather cracking test machine (HY-835, Heng Yu Instrument (China) Limited) following the standard ISO 3379: 1976, MOD. Softness of the leather samples were determined by a softness tester following the standard procedures (GT303, Gotech testing machines limited, China). Dyeing and finishing fastness of the leather samples were measured by a leathers fastness testing machine (HY-759; Heng Yu Instrument (China) Limited) following the standard ISO 11640: 1993. Under specified pressure, the leather surface color was transferred to the lining cloth in the process of reciprocating rubbing the leather surface. Compared with the grey card, fastness was determined by visual inspection. Morphological analysis. SEM characterization. The microstructure of leather was analyzed using scanning electron microscope (S-4800, Japan Rigaku Corporation). AFM characterization. The roughness of gain surface was observed by Atomic force microscopy (SPI3800/SPA400, Japan Saiko).

RESULTS AND DISCUSSION Thermal properties. Shrinkage temperature (Ts) analysis. Wet blues are normally shaved to get



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a required thickness before post-tanning, and therefore it should possess an enough thermal stability to avoid damage from these mechanical operations30. Thermal stability of collagen can be used as a criterion to measure the merits of tanning process. Hydrothermal stability is an important symbol and key factor of tanning effect and is often characterized by Ts for convenient measurement, simple and easy opertion31. Higher shrinkage temperature of wet blues means a better thermal stability, which benefit the follow-up processes during shoe upper leather’s production. Fig.3 shows Ts of leather samples tanned by different tanning processes. Ts of the tanned leather was 93.9℃ for control process. Compared with Ts of leather sample for control process, Ts of leather tanned by ZnO only process reduced from 93.9℃ to 91.1℃ and Ts of leather tanned by ZnO only process increased from 93.9℃ to 95.8℃. However, when 2% of nanocomposites combined with 4% of basic chromium sulfate were applied in tanning process, Ts of the sample was 99.8℃which was close to Ts (99.9℃) of traditional tanned leather using 6.0% of basic chromium sulfate (Conventional).

Fig.3 the shrinkage temperature of wet-blue for different tanning process (Note: ZnO only is 2% nano ZnO and 4% basic chromium sulfate; Polymer only is 2% polymer and 4% basic chromium sulfate; Experimental is 2.0% NCM and 4% basic chromium sulfate; Control is 4% basic chromium sulfate; Conventional is 6.0% basic chromium sulfate.)

The interaction mechanisms of the different tanning agents with collagen in tanning process were shown in scheme 1. Combined with the results of Fig.3, the effect of tanning process on the thermal properties of leather was explained.


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Scheme 1 Interaction mechanisms of the different tanning agents with collagen in tanning process: a chrome tanning (control process and conventional); b ZnO only process; c polymer process; d experimental process

Among the tanning agents, basic chromium sulfate is the most widely used tanning agent due to the quality and high stabilization ability they impart to leather. In conventional and control process, basic chromium sulfate was used as tanning agents, reacted with the ionized carboxyl groups of collagen via covalent bonding and chromium nuclei undergo a self-polymerization via hydroxyl bridges which formed in stable -Cr-O-Cr- bridges between the protein chains (Scheme 1a)32. Final polynuclear chromium-collagen cross-linking was obtained, which was a very stable network structure. Therefore, the leather tanned by traditional process could reach a higher shrinkage temperature. To reduce the harm caused by chromium to the environment, consensus view at home and abroad is as much as possible to reduce the amount of chrome tanning agents, the main practice at present is to increase chrome tanning agent uptake.



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Tanning agents that require low dosages and high utilization rates should be developed and nanomaterials used as additives possess exactly these33. In ZnO only process, nano ZnO was used as pretanning agents in tanning process. Nano ZnO reacted with ionized amino groups of collagen via electrovalent bond in pretanning process and condition for combination was the lower pH (Scheme 1b). However when nano ZnO was used in the leather making directly, it was added in water actually, which was a disadvantage for nanoparticle’s dispersion. In this case, nanoparticle as aggregation penetrated into collagen fibers, without nano-meter characteristics, hence which produced certain adverse effects on chromium uptake in subsequent tanning process. For polymer process, the amphoteric vinyl polymer was used as pretanning agent. The -COOH of the side chain in the polymer molecule and the -NH2 reactive group in collagen had a certain interaction. When adding chrome powder, a large number of chromium ions could not only react with carboxyl groups in collagen, but also deposited between the collagen and form multiple crosslinks, and further formed nanocomposites with polymer through coordination bond and electrostatic adsorption; finally inter-triple helix cross-linking was obtained to achieve the effect of stabilizing collagen(Scheme 1c). For “NCM”, little ZnO was added into the polymer through physical stirring and ultrasonic method. Polymer with hydrophilic segments and hydrophobic segments had certain surface activity, which was beneficial to nanoparticle dispersing. Nanocomposites with ZnO as the core and carboxyl chain as the arm could be chelated and adsorbed on chromium ions, and the stronger bond formed in nanocomposite between collagen and chromium, thus formed NCM-chromium-collagen structure, which endowed leather with high hydrothermal stability (Scheme 1d) 27. Similarly inter-triple helix cross-linking structure was gained in experimental process. Compared with the polymer process, cross-linked structure of the leather tanned by experimental process was more stable due to the unique effect of nanomaterials and formed multi-point binding centered on nanomaterials, which was also verified from Fig.3.


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Fig.4 DSC thermogram of leather samples tanned in four different tanning processes

Differential scanning calorimetric analysis. The analysis of thermosability is able to offer an insight into the stability of tanning agents’ binding sites on collagen fiber34. And the interaction between collagen and tanning agent may be destroyed when temperature is enough high35. Normally Ts represents the hydrothermal stability of leather and whether it is heat resistance of leather in wet state. The high hydrothermal stability of leather usually means it has protease resistance, good formability, and breathability. Besides Ts, thermal denaturing temperature of the chromium tanned leather is also the direct evidence of thermosability of leather. Degeneration is a transition that three helix of collagen was translated into random coil and it occur cross-linking region. Therefore, the stability of tanning agents binding sites on collagen fiber can be studied by degradation temperature (Td) temperature36. Fig.4 is the DSC thermogram of leather samples tanned in different tanning processes. The peak value of DSC represents Td of collagen. As shown in Fig.4, Td of leather reached 131.45℃ for conventional process. With the amount of chrome tanning agents reducing, Td of leather for control process reduced from 131.45℃ to 117.36℃. Differently, Td of leather for ZnO only process and polymer only process increased slightly compared to Td of leather for control process. However, 2% of nanocomposites combined with 4% of basic chromium sulfate were applied in the tanning process, Td of tanned leather reached 124.17℃. This indicated that the stability of tanning agents binding sites on collagen fiber for experimental process was more stable compared with ZnO only process and polymer only process. These conclusions were confirmed by the results of Ts.



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Thermo gravimetric analysis. Thermo gravimetric (TG) analysis and differential thermal gravity (DTG) analysis show the thermal stability of leather samples. The TG and DTG curves obtained for the original samples by different methods were shown in Fig.5 and Tab.3 collected the data from it. It can be seen that five leather samples had similar TG curves with three stages: (ⅰ) 50~150℃: In this stage, the weight loss was approximately 10%, mainly caused by evaporation of water molecules that stabilized the conformation of collagen37. (ⅱ) 150~500℃: The stage was main decomposition step of which weight loss was approximately 70%, mainly caused by the decomposition of collagen fibers. (ⅲ) 500℃~750℃: In this stage the material underwent a slow decomposition and it was probably because restructuring of the char, desorption of some volatiles previously retained by the char, etc38.

Fig.5 TG (a) and DTG (b) curves of leather samples tanned by three different tanning processes Tab.3 the data collected from TG and DTG curves Process

Tmax (℃) in second stage

Residue at Tmax ℃ in second stage (%)

Residue at 750℃ (%)

ZnO only

354.96

60.61

22.70

Polymer only

333.37

66.90

24.07

Experimental

337.43

73.98

31.58

Control

334.43

70.79

28.62

335.43

74.95

32.07

Conventional

Peak temperature of leather samples decomposition (Tmax) reflects structural stability of leather, and high Tmax is benefit to stability of leather’s microstructure. By comparing five samples’ Tmax in the second stage, Tmax of leather pretanned with 2% of nano ZnO (ZnO only) was higher than those of other samples. It was mainly due to the high inorganic component of leather samples in ZnO only process. In addition, Tmax of tanned leather for experimental process was a little higher than these of


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polymer only, control and conventional process samples, and residue at Tmax in second stage was similar to that of leather sample for conventional process. This indicated that pretanning with NCM could improve the leather’s thermal stability. Because for one thing NCM with negative charge was beneficial to uptake more chromium, hence inorganic components in collagen was increased; for another thing stable network structure formed by combining NCM with chromium and collagen. These conclusions were further confirmed by the results of Ts and Td. In the third stage the residue at 750℃ of leather was mainly inorganic substance that cannot be pyrolysis39. Compared with five samples, the residue at 750℃ of leather for ZnO only process was less than those of other samples (Tab.3). The residue at 750℃ of leather for control process was 28.62%, which was composed of chromium compound. Compared with the control process, the residue at 750℃ of leather treated with polymer tanning process was lower. However, the residue at 750℃ of leather for experimental process increased from 28.62% to 31.58%, mainly caused by the decomposition of chromium compound and residual nanoparticles of NCM. And it was close to the traditional method - using 6.0% of basic chromium sulfate (Conventional process) and the content of the residue reached 32.07%, which was mainly resulted by decomposition of chromium compound. Monitoring of chromium footprint. Chromium content in wastewater. During tanning, absorption of hide for chrome tanning agents is limited. So after tanning, some chrome tanning agents are not combined with leather and it will leave some residual chromium in wastewater. In addition, chromium combined unstably with crust leather will be washed out during every process of post tanning at varying degrees40. Tab.4 Chromium content in wastewater of each process Cr2O3/mg·L-1

Process

ZnO only 330.77

Polymer only 398.05

Experimental 389.08

Control 841.20

Conventional 2294.60

Rewetting

78.33

84.12

67.88

128.09

182.47

Retanning

8844.10

908.26

623.08

1562.07

1875.07

91.16

120.67

119.07

186.18

193.34

298.77

421.80

416.18

450.66

681.08

Tanning

Neutralizing Filling, and Fatliquoring

dyeing&

Tab.4 shows chromium content in wastewater during tanning and post tanning process by different methods. Compared with the experimental process, chromium content in tanning



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wastewater of ZnO only process was less, but the chromium content in retanning wastewater of ZnO only process was much more than that in the other tanning process, which indicated that chrome tanning agent absorbed very little during retanning process. The main reason for this lay in that the nano ZnO reacted with the amino groups of collagen via electrovalent bond n the pretanning process. However nano ZnO was used in the leather making directly. It was added in water actually, which was disadvantage for nanoparticle’s dispersion (scheme 1b). In this case, nanoparticle as aggregation penetrated into collagen fibers, without nano-meter characteristics. Meanwhile, chromium content in retanning wastewater of polymer only process was obviously reduced from 8844.10 to 908.26 compared with ZnO only process. For “ZnO nanocomposite”, little ZnO was added into the polymer through physical stirring and ultrasonic method. Polymer with hydrophilic segments and hydrophobic segments had certain surface activity, which was beneficial to nanoparticle dispersing. Meanwhile -COOH in polymers could help improve the absorption of chromium. Obviously, from Tab.4, it was seen that chromium content during every step in waste discharges in experimental process was minimal. Compared with the control process, chromium content in the experimental process was reduced from 841.20 to 389.0 in tanning process and reduced from 1562.07 to 623.08 in retanning process. That was to say, the absorption efficiency of chrome tanning agents was increased by 50%. It indicated that leather pretanned by NCM could enhance the chrome tanning agents’ fixing among collagen fibers, and it was not easy to be eluted in subsequent processes. This may lie in nanocomposites with ZnO as the core and carboxyl chainas the arm could be chelated and adsorbed on chromium ions, and the stronger bond formed in nanocomposite between collagen and chromium. Compared with the conventional process, a few nanocomposites as pretanning agents could effectively reduce the chromium wastewater discharge in leather making process, and it was good for cleaner production.


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Fig.6 Chromium content in leather

Chromium content in leather. Normally, increasing chromium content in leather can improve the property of leather. Meanwhile, total chromium is conserved - so those leathers with higher retained Cr should result in lower Cr discharge (for the same Cr input). Chromium content in leather was shown in Fig.6. It was clear that chromium content in leather was 13480 mg·kg-1 after 4.0% of chrome tanning agents (control process). Chromium content in leather pretanned with 2% of NCM before tanned with 4.0% chrome tanning agents (experimental process), was increased from 13489 mg·kg-1 to 15039 mg·kg-1, and it was lower than that in traditional tanning process using 6.0% of chrome tanning agents (conventional process) which reached 16990 mg·kg-1. Obviously, chromium content in leather for ZnO only process was less than those in other processes. The chromium content in leather for polymer only process was lower than that of leather for experimental process, but it was higher than that leather for control process. It appeared that chromium absorption of leather as well as chrome binding fastness of leather was improved by pretanning with NCM.



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Fig.7 Chromium distribute in leather as they form flesh side to grain side by EDS

Chromium distribution in leather. It is better that chromium in leather’s cross-section distributed as evenly as possible because that anionic dyes, fatliquoring agents and retanning agents can be absorbed and distributed more uniform in corresponding processes. In our work, EDS liner scanning was used to detect chromium distribution in cross section of leather samples in the different processes (Fig.7). It was seen that chromium content in leather for conventional process was the maximum, which was mainly due to the dosage of chrome tanning agents was higher than those of other processes, and which were in agreement with results of chromium content in leather (Fig.6). However, the chromium distribution in leather was not uniform for conventional process. In contrast, the chromium distribution in leather treated by pretanning process was more uniform. This indicated that pretanned leather was beneficial to chromium permeation uniformly in subsequent handling. Compared control process, chromium content of leather samples for ZnO only process was lower, on the contrary chromium content of leather samples for polymer only process and experimental process were higher. This showed that pretanning agent with carboxyl chain segment more easily could chelate and adsorb chromium ion. And the results were basically the same as the results of chromium content in leather (Fig.6). Leather properties. Since tanned leathers are resource materials for consumer goods like footwear and lifestyle products, certain criteria is demanded for their physical parameters such as strength and fastness to ensure durability and aesthetics of products. Criteria such as tensile, tear and bursting strength are indicators of durability of leather samples. Criteria such as color fastness and


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finishing fastness ensure aesthetics property of products during usage on exposure to various environments41.

Fig.8 Physical and mechanical properties of leather samples for different processes

Tear strength, bursting strength, elongation, and break strength of leather samples for different processes were determined (Fig.8). The results indicated that tear strength of leather samples for polymer only process and experimental process were similar with conventional process. For polymer only process, tensile strength and bursting strength of leather were lower than those of leather treated in other processes. This mainly was due to the polymer chain could increase the degree of lubrication between the collagen fibers, thus reduced the acting force between the collagen fibers. By comparing with experimental, control and conventional process, tear strength and tensile strength of leather tanned by experimental process were similar to that by conventional process, and they were both higher than those of control process. It indicated that leather pretanned by 2.0% of NCM and then tanned by 4% of basic chromium sulfate could reach the same strength of leather when tanned by 6.0% of basic chromium sulfate. This may be due to the fact that NCM cross linked with collagen fibers firmly. Bursting strength of leather tanned in experimental process was the highest in all of leather samples. It indicated that hide pretanned by NCM could efficiently provide tanned leather with strength. Of course it could give leather long life during using for shoes, bag or other leather



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products. Fig.9 shows the softness of leather samples. Comparing conventional chrome tanning process with control chrome tanning process, softness of leather with control chrome tanning process decreased. The amount of chrome tanning agents reduced from 6% (conventional process) to 4% (control process) caused reduction of chromium’s reaction with collagen and even positive charges. Hence ionic bond sites of anion fatliquoring agents and collagen were decreased. Softness of leather tanned in ZnO only process and control process was nearly the same, but it was lower than those for experimental process. This may because lower chromium content in leather for ZnO only process than those in other processes (Fig.6), which caused reduction of positive charges. Therefore ionic bond sites of anion fatliquoring agents and collagen decreased. However, softness of leather samples for polymer only process, experiment process were close to that of conventional tanning. As is known, collagen is a kind of amphoteric substance. Carboxyl group of NCM and polymer could react with amino groups of collagen and increase the sites of carboxyl group, thus formed inter-triple helix cross-linking structure and enhance chromium uptake (Scheme 1c and Scheme 1d). Hence ionic bond sites of anion fatliquoring agents and collagen increased. Additionally, the long alkanol chain of polymer could easily slip inside collagen fiber42. So softness of leather tanned in polymer only and experimental process could be matchable to softness of leather by traditional chrome tanning.

Fig.9 the softness of leather samples

Dyeing fastness of leather refers to dyes of the leather to the external resistance during the processing and usage. Dyeing variations of leather (dye purity, color, brightness) and staining degree of attaching materials are standard of measuring. In fact, dye fastness is measured through dye


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variations. That is to say, color fastness is determined by identifying the changes in leather color. Tab.5 shows dye fastness of crust leather samples. Obviously, different samples showed the same fastness, which meant that different tanning processes had no obvious effects on fastness of dyeing for leather samples. Tab. 5 Dyeing degree of leather Process

Dyeing fastness ZnO only

Polymer only

Experimental

Control

Conventional

Dry rubbing fastness class

4-5

4-5

4-5

4-5

4-5

Wet rubbing fastness class

3-4

3-4

3-4

3-4

4-5

Morphological analysis (SEM). Crust leather samples were assessed by viewing the grain surface and cross section using SEM. SEM was used to assess penetration of tanning agents through leather and into the hierarchy of its structure. And it was a useful technique to evaluate the effects of various treatments on the crust leather. Grain surface of leather samples for different process was shown in Fig.10a, 10b, 10c and 10d. For pores in grain surface, the shallower the pore is, the more the degree of convergence occurs. The pores for experimental process and conventional process were shallower than those of other process, and leather grain surface of them was further smoother. It indicated that NCM was favourable to absorb other agents through pretanning with nanomaterials43. It also reflected the chromium footprint and Ts. SEM analyses of tanned leather samples were showed and cross section in a magnification of 4,000 were depicted and were given in Fig.10a', 10b', 10c' and 10d'. NCM pretanned leather (Fig.10a') showed a larger dispersion in the fiber structure throughout the cross section indicating the uniform structure, which was a good evidence for the penetration of NCM onto the leather fibers44.



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Fig.10 SEM images of grain surface (a, b, c, d, e) and cross section (a', b', c', d', e') of leather a) and a'): ZnO only, b) and b'): Polymer only, c) and c'): Experimental, d) and d'): Control, e) and e'): Conventional

Economic analysis. Constructing a conservation-minded society and circular economy are necessary precondition. The essence of developing circular economy is green economy. Tannery is one of the main bodies that develop circular economy. Hence it needs green production, that is, sustainable production. If a tannery produced 1000kg of cowhide, it would form about 1000kg of chromium-containing wastewater. Because wastewater is forbidden to directly discharge, it must be precipitated mainly by NaOH and formed amount of chromium mud. Tannery needs to pay for the


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cost for mud recovery, which is calculated by the mass weight. Tab.6 Econonmic cost of experimental process or conventional process: 1000 kg of US cowhide Material and Waste disposal

Pretanning agent

Unit price(￥/kg)

NCM(13.0)

Experimental process(2%NCM+4% Basic Chromium Sulfate)

Conventional process （ 6% Basic Chromium Sulfate）

2%NCM

20*13

0

-260

，2% Basic Chromium Sulfate cost（￥）

ZnO(80)

Nacl

0.44

30*0.44

70*0.44

0

BCS

8.5

40*8.5

60*8.5

+170

NaOH

2.3

1*2.3

2.5*2.3

0

Chromium mud

3.0

51*3.0

107*3.0

+105

Total cost(￥)

768.5

867.55

+15

Save cost(￥)

+99.05

(compare to conventional process)

Tab.6 shows the price of tanning materials and waste disposal. We easily calculated the bill for different methods in leather making. Compared to conventional tanning process, nanocomposites pretanning method (Experimental process) could help tannery save about ￥99.05 pre 1000kg hide in production. Furthermore, chromium usages were almost a half of that of classical method, which could save chromium resource that was limited in the earth. Also, chromium waste reduction was a potential wealth to the human beings. Cost of the nanocomposite and 2% of chromium cost as well as waste water treatment cost were compared, 2% of NCM could save ￥15 pre 1000kg hide. The results demonstrated that nanocomposite combined with chrome tanning process is more economical than conventional tanning process.

CONCLUSIONS High concentration of chromium-containing wastewater can be produced when hides and skins are traditionally tanned by chrome tanning agents, which is a potential hazardous to human and plant. Nanocomposites prepared by nanotechnology were used as a novel less chrome tanning agent. When nanocomposites as pretanning agents were applied in shoe upper leather making, thermal properties of wet blues could be enhanced. Ts and Td of wet blues respectively were 99.8℃ and 124.17℃, which was close to those of traditional tanned leather using 6.0% of basic chromium sulfate.



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Meanwhile, chromium content in leather increased from 13489 to 15039 mg·kg-1 compared with the control process (using 4.0% of basic chromium sulfate). Moreover, chromium content in wastewater was reduced by the range of 50~80% in tanning process, which indicated that reduced emissions of chromium footprint in leather production. Furthermore, EDS chromium line scanning results of leather cross section indicated that the chromium distribution in leather samples was more evenly and confirmed the results of chromium content in leather. SEM analysis demonstrated that tanning method which nanocomposites combined with chrome tanning agents gave more separated fibers and better grain smoothness. Mechanical properties, softness and dyeing of the leather pretreated with nanocomposites were close to those of the conventional leather. Additionally, nanocomposites pretanning method (experimental process) could help tannery save about ￥99.05 pre 1000kg hide in production compared to conventional tanning process. The above results indicated that nanocomposites applied in shoe upper leather as pretanning agents can not only reduce emissions of chromium footprint in leather making, but also reduce the costs of company. NCM can reduce emissions of chromium footprint in leather making and is a greener chemistry and economical approach. ASSOCIATED CONTENT Supporting Information Confirming no supporting information files. AUTHOR INFORMATION Corresponding Author *Phone: +86-029-86132559; E-mail: [email protected] (Bin Lyu) *Phone: +86-029-86168006; E-mail: [email protected] (Jianzhong Ma) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Young Science and Technology Star Project of Shaanxi Province (2016KJXX-02), the Key Research Project of Shaanxi Province (2017GY-187), and Shaanxi Provincial Education Department Serves Local Special Project (17JF002). REFERENCES (1) Gil, R. R.; Girón, R. P.; Lozano, M. S.; Ruiz, B.; Fuente, E. Pyrolysis of biocollagenic wastes of


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TOC/Abstract Graphic



Page 30 of 30

Synopsis： A sustainable leather process of nanocomposites as pretanning agents for less chrome tanning systems can reach a good tanning effect and reduce chrome emissions in leather making


Chromium Footprint Reduction: Nanocomposites as Efficient

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