pH-Sensitive and Chromium-Loaded Mineralized Nanoparticles as

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pH-Sensitive and Chromium-Loaded Mineralized Nanoparticles as Tanning Agent for Cleaner Leather Production Kaijun Li, Ruiquan Yu, Ruixin Zhu, Ruifeng Liang, Gongyan Liu, and Biyu Peng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00482 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Sustainable Chemistry & Engineering

pH-Sensitive and Chromium-Loaded Mineralized Nanoparticles as Tanning Agent for Cleaner Leather Production Kaijun Li,†, ‡ Ruiquan Yu, ‡ Ruixin Zhu,†, ‡ Ruifeng Liang,⊥ Gongyan Liu, *, †,‡ Biyu Peng,†, ‡ †National

Engineering Laboratory for Clean Technology of Leather Manufacture,

Sichuan University, Chengdu 610065, China ‡The

Key Laboratory of Leather Chemistry and Engineering of Ministry of Education,

Sichuan University, Chengdu 610065, China ⊥

The State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan

University, Chengdu 610065, China Corresponding Author *Gongyan Liu E-mail: [email protected].

ABSTRACT: In this work, a pH-sensitive and chromium-loaded mineralized nanoparticles (Cr-PPA NPs) were developed by self-assembly of poly [poly (ethylene glycol) methyl ether acrylate-co-acrylic acid] [poly(PEG-co-AA)] copolymers templated Cr(OH)3 mineralization. The Cr-PPA NPs exhibited excellent colloidal stability in water with high salt concentration, protein and pH above 3.0. As a novel chrome tanning agent, such Cr-PPA NPs could effectively protected and deliver 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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chromium into pickled pelt without reacting with collagen fibers during the penetration process, due to its antifouling PEG surface and nano-size. When the interior pH of the pickled pelt was lower than 2.5, the dissolution of the mineralized Cr(OH)3 core would cause disassembly of Cr-PPA NPs and rapid release of Cr3+ ions, leading to uniform chromium distribution in leather. After basifying, the released Cr3+ ions would cross-link the carboxyl groups both on collagen fibers and poly(PEG-co-AA) copolymers, resulting in chromium adsorption efficiency in leather higher than 90% and less chromium discharge in wastewater. Simultaneously, the resultant leather was given enhanced hydrothermal stability and physical properties based on above efficient chromium cross-linking. Compared with traditional chrome tanning agent, this pH-sensitive Cr-PPA NPs can really break through the existing balance limitation between chromium penetration and combination in the conventional tanning process and becomes a promising strategy for cleaner leather production.

KEYWORDS: pH-Sensitive, Nanoparticles, Chrome tanning agent, Leather, Clean production

INTRODUCTION

Leather is a kind of durable material that is produced by tanning animal rawhide and is used to make various goods, including footwear, clothing, automobile seats and furniture.1 Worldwide, approximately 90% of leather products are manufactured by

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the chrome tanning method, due to the excellent hydrothermal stability and physical properties of the chrome-tanned leather.2,3 Currently, the most commonly used chrome tanning agent in the leather industry is the trivalent chromium salt (Cr3+), which is mainly composed of chromium hydroxide sulfate [Cr(OH)SO4].3,4 In the conventional chrome tanning process, the rawhide is usually pretreated into a pickled pelt with interior pH value of 2.5 ~ 3.0.5 At this stage, most of the carboxyl groups on skin collagen are protonated, which only have weak interaction with Cr3+ ions and thus promote the penetration of the chrome tanning agent into the pickled pelt. Subsequently, the pH value is increased to 4.0 by a basifying process for improving the formation of strong coordination bonds between deprotonated carboxyl groups and Cr3+ ions, resulting in cross-linking of collagen fibers and converting animal skin to stable leather.6-10 In the past few decades, this pH-controlled Cr3+ penetration and combination have been not only two key factors of the chrome tanning mechanism, but also a pair of contradictions. However, the perfect balance between uniform penetration and efficient combination of Cr3+ in leather is very difficult to achieve in practical leather production.11 In order to obtain qualified leather products, the dosage of the chrome tanning agent is usually excessive to ensure sufficient chrome penetration and combination. Generally, only 60~70% of the total chrome tanning agent is absorbed by leather, resulting in high concentration of chromium in wastewater.12

Unfortunately,

the

pollution

of

chromium-containing

tannery

wastewater and sludge is still difficult to curb by current treatment technologies.13-16 Even worse, the discharged trivalent chromium salts are potentially oxidized into 3 ACS Paragon Plus Environment


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toxic hexavalent chromium [Cr(VI)], which not only threats our natural environment but also human health.15,17 In fact, increased chromium pollution has seriously restricted the sustainable development of the leather industry and become an urgent problem that needs to be solved.18 In recent years, many efforts have been directed toward clean leather production technologies based on reducing chromium discharge during the tanning process without sacrificing leather quality, which has been considered as a reasonable and sustainable strategy for the leather industry.19-30 Among these clean leather production technologies, great attention has been paid to high-exhaustion chrome tanning. Where, varied high-exhaustion auxiliaries, such as acrylic copolymer,22-24 hyperbranched polymer25 and nanocomposites27-30 have been developed to promote the absorption of chrome by hide, resulting in a chrome-less tanning process.31 A common feature of these high-exhaustion auxiliaries is that they possess multiple carboxyl groups, which can introduce more binding sites onto collagen fibers that react with Cr3+. In particular, nanocomposites with multidentate carboxyl groups have been reported to significantly enhance chromium uptake and reduce chromium discharge in wastewater, by taking advantage of their nano-size and high surface area.27-30 However, these high exhaustion auxiliaries still failed to help chrome tanning agents to overcome the balance limitation between uniform penetration and efficient combination in leather.31 Due to the carboxyl groups introduced by high-exhaustion auxiliary onto collagen fibers, which can simultaneously hinder chrome penetration, thus usually causing excessive chrome combination near the leather surface. Therefore, the protection of 4 ACS Paragon Plus Environment

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the chrome tanning agent to avoid reaction with skin collagen during the penetration stage and the subsequent enhancement of chrome combination becomes a key issue for a chrome-tanning process at lower percentage offer. In this work, we report for the first time the preparation and use of chromium-loaded NPs self-assembled by poly(PEG-co-AA) copolymers (PPA) templated Cr(OH)3 mineralization (Scheme 1). Such core-shell structured chromium(Ⅲ)-loaded (Cr-PPA) NPs could effectively deliver trivalent chromium into pickled skin with interior pH value of 3.0, thus avoiding reaction with skin collagen during the penetration process. When the pH value was decreased to 2.5, the Cr(OH)3 dissolution would cause disassembly of Cr-PPA NPs and rapid release of Cr3+ ions, leading to uniform chromium distribution. After basifying (pH ~ 4.0), coordination complexation occurring between released Cr3+ and carboxyl groups both on collagen fibers and PPA copolymers, results in higher chromium combination in leather and a cleaner tanning process. Besides, the immobilization of synthetic PPA copolymers on collagen fibers was helpful for improving physical properties, such as tear strength, breaking elongation and tensile strength of the resultant leather. Different from exhaustion auxiliaries, such pH-sensitive and chromium-loaded NPs can really overcome the existing balance limitation between chromium penetration and combination in the conventional tanning process and open an exciting avenue for cleaner leather production. EXPERIMENTAL SECTION



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Materials. Poly (ethylene glycol) methyl ether acrylate (PEG, Mn ~ 480) was purchased from Aladdin Industrial Corporation (Shanghai, China). Acrylic acid (AA), Azo-bis-isobutyronitrile (AIBN), Sodium hydroxide (NaOH), and ethanol were purchased from Kelong Chemistry Co., Ltd. (Chengdu, China). Chromium chloride hexahydrate (CrCl3·6H2O, 99%) was purchased from Adamas Reagent, Ltd. (Shanghai, China). Chromium standard solution with the chromium concentration of 1000 µg·mL-1 was purchased from Zhongbiao Technology Co., Ltd. (Jinan, China). Commercial chrome tanning agent, mainly basic chromium [Cr(OH)SO4], was purchased from Sichuan Yinhe chemistry Co., Ltd. (Mianyang, China). Bovine serum albumin (BSA) was purchased from Guangzhou saiguo biotech CO., Ltd. (Guangzhou, China) Pickled pelt was made by treating goat skin following the process of degreasing, unhairing, liming, deliming, bating and pickling in our own laboratory. Synthesis of Poly (PEG-co-AA) Copolymer (PPA). PPA copolymer was synthesized via normal free radical polymerization method. Briefly, 10 g PEG, 1.5 g AA, 250 mg AIBN, and 40 mL ethanol were added to a one-neck round-bottom flask with magnets (the molar ratio of PEG:AA = 1:1). After degassing by running a freeze-pump-thaw procedure one time, the mixture was allowed to react in oil-bath at 70 °C for 24 h under stirring. After cooling to room temperature, the mixture was concentrated and precipitated into ethyl ether twice to obtain purified PPA copolymer. The molecular weight of PPA determined by gel permeation chromatography (GPC, Waters 410, Waters corporation USA) was about 11600 g·mol-1 with PDI of 1.43.


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Preparation of Chromium(Ⅲ)-loaded PPA Nanoparticles (Cr-PPA NPs). Cr-PPA NPs were prepared according to previous report,32-34 by a PEG-co-AA templated in situ mineralization technique. Briefly, 0.2 g PPA copolymer was dissolved into 50 mL distilled water with pH value of pH 7.0. Then, 0.62 g CrCl3 · 6H2O was dissolved into 25 mL distilled water and mixed with the PPA aqueous solution. After equilibration for 4 h under stirring at 800 rpm, 25 mL aqueous solution of NaOH (0.28 g) was slowly dropped into the reaction mixture. For a stoichiometric ratio, the molar ratio of [AA] to [Cr3+]/[OH-] was 1:1:3. The final mixed solution was magnetically stirred with a speed at 800 rpm for 20 h at room temperature. Chromium Loading Content and Loading Efficiency of Cr-PPA NPs. To determine the chromium loading content and loading efficiency, 5 mL Cr-PPA NPs solution was dialyzed against deionized water to remove unloaded Cr3+ ions. Then, the nitrolysis of the dialysate was performed by using 10 mL mixture of concentrated nitric acid (HNO3) and 20% (v/v) hydrogen peroxide (H2O2) (8:2, v:v). This acidic solution was diluted with water to 1 L, and the chromium concentration was measured by inductively coupled plasma optical atomic emission spectroscopy (ICP-OES, Optima 2100DV, PerkinElmer, USA) and calculated from the chromium calibration curve. The chromium calibration curve was made by diluting chromium standard solution with chromium concentration of 0, 10, 20, 30, 40, 50 µg·mL-1, respectively. Characterization of Cr-PPA NPs. The particle size and size distribution of Cr-PPA NPs was measured by dynamic light scattering analysis (DLS, Malver 7 ACS Paragon Plus Environment


ZEN3600n, England). The morphology of Cr-PPA NPs were observed by transmission electron microscope (TEM, ZEISS Libra 200 FE, Germany) and TEM samples were prepared by drying a drop of Cr-PPA NPs solution onto a carbon-coated copper grid. The element composition of lyophilized Cr-PPA NPs was analyzed by energy dispersive X-ray spectrometry (EDS, ProX, Netherlands). X-ray diffraction measurements (XRD, EMPYREAN, Netherlands) of lyophilized Cr-PPA NPs were performed with a Rigaku D/max-RB apparatus powder diffractometer and image-plate photography using graphite-monochromatized Cu KR radiation (λ = 1.542 Å). Data were collected from 10 ° to 90 °. Stability and pH-Sensitivity of Cr-PPA NPs. First, the stability of Cr-PPA NPs with respect to storage time was tested via monitoring the changes of particle size distributions within 30 days; To determine the colloidal stability of Cr-PPA NPs in presence of salt, the average size changes of Cr-PPA NPs in aqueous solution with NaCl concentration of 1000 mM and pH of 7.0 were tested and data were collected in the range of 0-24 h; To determine the colloidal stability of Cr-PPA NPs with respect to protein, the average size changes of Cr-PPA NPs in aqueous solution with BSA concentration of 5 mg·mL-1 and pH of 7.0 were tested and data were collected in the range of 0-24 h. The pH-sensitivity of Cr-PPA NPs was tested via monitoring the changes of average particle size within 24 h under pH 2.0, pH 3.0, pH 5.0 and pH 7.0, respectively. pH-Dependent Behavior of Cr3+ Ions from Cr-PPA NPs. The release behavior of chromium from Cr-PPA NPs was measured by ICP-OES under different pH 8 ACS Paragon Plus Environment

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conditions. Briefly, 5 mL Cr-PPA NPs solutions with polymer concentration of 2 mg·mL-1 at pH 2.0, pH 3.0, pH 5.0 or pH 7.0 in dialysis bag (MWCO=3500) were put into 30 mL deionized water with pH of 2.0, 3.0, 5.0 and 7.0, respectively. At predetermined time intervals, 5 mL aqueous solution was withdrawn from the release media and another 5 mL fresh deionized water was added. Then, the accumulated release of chromium was measured by ICP-OES and calculated by the chromium calibration curve. Application of Cr-PPA NPs as Chrome Tanning Agent in Leather Productive Process. Cr-PPA NPs were used as chrome tanning agent for tanning pickled sheep skin, and commercial chrome tanning agent was used as control. Before tanning, the pickled sheep skin was divided into three equal samples (Shown in Figure S1 and Figure S2 in the Supporting Information). Control A and Control B were tanned by commercial chrome tanning agent, and the dosages of commercial chrome tanning agent were 4 wt% and 8 wt% of pickled skin weight, respectively. Experiment C sample was tanned by Cr-PPA NPs solutions, in which the mass of chromium (III) was equal to that in commercial chrome tanning agent for Control A sample (4 wt%). Tanning process was carried out as Table S1 and Table S2 in the Supporting Information. Characterization of Cr-PPA NPs Tanned Leather. The Distribution of Cr-PPA NPs in Leather. The distribution of Cr-PPA NPs in leather was observed by scanning electron microscopy (SEM, Quanta250, FEI, USA). Briefly, after the penetration of Cr-PPA NPs into pickled skin, leather sample was lyophilized, parallelly split from 9 ACS Paragon Plus Environment


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middle, and observed by scanning the parallel section using SEM. Distribution of Chromium in Leather. The distribution of chromium was measured by an energy-dispersive X-ray spectroscopy (EDS) equipped in a field-emission scanning electron microscope (SEM, JSM-7500F, Japan). Chromium distribution in the full cross-section of the leather samples were analyzed. The micro-structure of longitudinal section of leather was observed by scanning electron microscopy (SEM, Quanta250, FEI, USA). Chromium Content in Tanning Wastewater. The chromium content in tanning wastewater as determined by ICP-OES. The chromium content in wastewater was measured three times and averaged for each experiment. The total volume of wastewater collected was 300% of the pickled sheep skin weight. the absorption efficiency of chromium (%) =

B A 100 B

Where A is the chromium content in tanning wastewater and B is the total chromium in tanning material. Shrinkage Temperature (Ts) of Tanned Leather Samples. The Ts of each leather sample was determined by a shrinkage temperature tester (MSW-YD4, Sunlight Electronics Institute, China) according to standard ISO 3380 : 2002. The vertical and horizontal samples near the spine were both measured in each experiment, and the average values were the last values.


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Physical Properties of Leather. Tensile strength, tear strength, and elongation at break and of the leather samples were measured by the universal testing machine (AI-7000 SN, Gotech Testing Machines Co., Ltd, China) according to standard ISO 3376 : 1976. RESULTS AND DISCUSSION Preparation and Characterization of Cr-PPA NPs. As shown in Scheme 1, chromium(III)-loaded NPs were prepared by a mineralization induced self-assembly of Cr3+ ions and PPA copolymer in the presence of hydroxyl ion (OH-), with inorganic Cr(OH)3 as core and organic PEG as shell. In this self-assembly process, metal-polymer complexes are fist formed via the coordinated complexation between chromium cations (Cr3+) and anionic carboxyl groups (-COO-) of PPA. Then, aqueous NaOH solution is added to generate Cr(OH)3 mineralization around the AA moieties, thereby inducing the self-assembly of this PPA templated mineralization into NPs (Cr-PPA NPs). Compared with the aqueous solution of commercial chrome tanning agent [Cr(OH)SO4], Tyndall effect of colloidal particles for Cr-PPA NPs solution was clearly observed (the inset photograph in Scheme 1), suggesting the successful formation of the mineralized NPs.



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Scheme 1. Schematic illustration of fabrication process of Cr-PPA NPs During the preparation of such Cr-PPA NPs, we found that the optimal stoichiometric feed molar ratio of [AA] to [Cr3+]/[OH-] was 1:1:3. Lower or higher molar ratio of [AA] to [Cr3+]/[OH-] would lead to either formation of irregular aggregates or unstable mineralized particles. The transmission electron microscopy (TEM) image displayed in Figure 1a shows the regular spherical morphology of this Cr-PPA NPs, with an average diameter of 13.3 ± 4.4 nm (shown in Figure S3). However, the average hydrodynamic size of Cr-PPA NPs measured by dynamic light scattering (DLS) was 61.1 ± 6.5 nm (Figure 1b), which is much higher than that determined by TEM. This result proved the core-shell structure of such mineralized NPs, with a Cr(OH)3 core size of 13.3 nm and a PEG shell thickness of around 23.9 nm. The hydrophilic PEG chains can stabilize the Cr-PPA NPs and thus provide good


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dispersibility, resulting in a narrow size distribution with polydispersity index (PDI) of 0.17. To further confirm the inorganic Cr(OH)3 core of Cr-PPA NPs, energy-dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction (XRD) analysis were used to analyze the lyophilized Cr-PPA NPs. In Figure 1c, the EDX spectrum shows peaks corresponding of Cr and O, which were attributed to Cr(OH)3. The XRD pattern (Figure 1d) shows characteristic peaks that can be distinctly indexed to a hexagonal phase for nanocrystalline Cr(OH)3 according to the international powder diffraction file (PDF No. 16-0817).35 The loading content and loading efficiency of chromium in the Cr-PPA NPs was 35.1 wt% and 83.4 wt%, respectively.

Figure 1. (a) TEM image, (b) DLS analysis, (c) EDX spectra and (d) the powder XRD pattern of Cr-PPA NPs.



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Colloidal Stability and pH-Sensitivity of Cr-PPA NPs. Due to the hydrophilic PEG shell, the Cr-PPA NPs are expected to have good stability in aqueous solution. Accordingly, the colloidal stability of Cr-PPA NPs was evaluated under various conditions. As shown in Figure 2a and 2b, the average diameter of Cr-PPA NPs remained almost unchanged for 30 days, demonstrating good stability during storage at room temperature. In addition, the DLS analysis results presented in Figure 2c, 2d, 2e and 2f indicated the excellent colloidal stability of Cr-PPA NPs in an aqueous solution of high concentration of NaCl or protein, showing a steady mean diameter and PDI value for up to 24 h. Such excellent colloidal stability was mainly attributed to the steric exclusion effect and surface hydration of PEG polymers, which can not only prevent aggregation of Cr-PPA NPs but also resist the adsorption of salt or protein onto NPs surface.36,37 The above results also suggested that the Cr-PPA NPs could be applied in industries with complex water environment.


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Figure 2. (a, b) Size changes of Cr-PPA NPs in water within 30 days at room temperature; (c, d) Size changes of Cr-PPA NPs in water with 1000 mM NaCl and (e, f) 5mg•mL-1 BSA during 24 h. Since the micelle core of Cr-PPA NPs was formed by the mineralization of Cr(OH)3 around AA moieties, such inorganic core may be dissolved under acidic condition. Accordingly, the pH-sensitivity of Cr-PPA NPs was investigated in water at various pH values, by DLS analysis to monitor the size changes within 24 hours. The results presented in Figure 3a reveal that Cr-PPA 15 ACS Paragon Plus Environment


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NPs maintained their average size without obvious change for 24 h at pH value of 7.0, 5.0 and 3.0. However, when the pH was adjusted to 2.5, the size of Cr-PPA NPs dramatically increased from ~ 60 nm to ~ 1600 nm after 12 h. This result showed that the Cr(OH)3 core could be dissolved at acidic pH lower than 3.0, which would lead to the disassembly of the micellar structure of Cr-PPA NPs with concomitant Cr3+ ions release. Consistent with the DLS results shown in Figure 3a, the release of Cr3+ from Cr-PPA NPs was significantly faster at pH 2.5 compared with that at pH 3.0 (Figure 3b). Within 12 h, more than 95% of Cr3+ was released at pH 2.5, while only about 20% of Cr3+ was released at pH 3.0. At pH value of 5.0 or 7.0, almost no release of Cr3+ ions was detected by inductively coupled plasma atomic emission spectroscopy (ICP-OES) analysis. It is clear that the Cr3+ release from Cr-PPA NPs exhibited a pH-dependent behavior.

Figure 3. (a) The pH-sensitivity of Cr-PPA NPs determined by DLS and (b) Cr3+ ions release behavior from Cr-PPA NPs under acidic conditions determined by ICP-OES.


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The Application of Cr-PPA NPs as Tanning Agent for Cleaner Production of Leather. The pH-sensitive and chromium-loaded Cr-PPA NPs were applied as tanning agent in the tanning of pickled sheep skin, using commercial chrome tanning agent as control. The steps of the tanning process are summarized in Table 1 and Table S1. The data shown in Table 1 revealed that the interior pH value of the pickled sheep skin was about 3.0 before tanning. In the chrome tanning process, 16% NaCl is usually used to inhibit the acidic swelling of the pickled skin, which will result in the high salt concentration of the tanning solution. The results presented in Figure 2 and Figure 3 indicate that Cr-PPA NPs have good colloidal stability under high salt concentration or at pH 3.0. Therefore, Cr-PPA NPs were expected to exhibit good dispersibility in tanning solution and effectively penetrate into the skin interior (as illustrated in Scheme 2). After penetration of the Cr-PPA NPs, the pickled skin was split in the middle into two layers which were further freeze-dried and examined by SEM. The SEM image shown in Figure 4 reveals that many Cr-PPA NPs, without aggregation, were uniformly distributed around the skin collagen bundles, indicating the successful penetration of Cr-PPA NPs into the pickled skin. Such uniform distribution of Cr-PPA NPs inside the pickled skin was mainly attributed to the antifouling PEG shell of Cr-PPA NPs which can repel the interaction with the skin collagen during the penetration process. This is due to the high density and hydrophilic PEG surface, which can effectively resist protein adsorption (as shown in Figure 2e and 2f).36,37



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Table 1. The steps of the tanning process using Cr-PPA NPs for pickled sheep skin.

Process

Penetration

Tanning

Cr3+ release

Chemical materials

Dosage a)

NaCl

16%

Cr-PPA NPs

Time

interior

pH 3.0 200%

300 min

H2SO4

2%

120 min

HCOONa

2%

60 min

NaHCO3

4%

60 min

H2O

100%

120 min c)

solution b)

pH of leather

pH 2.5

pH 4.0

Cross-linking

a) The

dosage of chemical materials in the tanning process was according to the weight

of the pickled sheep skin; b) the chromium content of 200% Cr-PPA NPs solution was equal to 4% of the commercial chrome tanning agent. c) Kept the temperature at 40 °C for 120 min and then stayed overnight.


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Scheme 2. Schematic illustration of chrome tanning process by application of Cr-PPA NPs.

Figure 4. SEM images of parallel section of pickled sheep skin after penetration of Cr-PPA NPs. 19 ACS Paragon Plus Environment


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After the penetration of Cr-PPA NPs, H2SO4 was added to adjust the interior pH of the pickled sheep skin from 3.0 to 2.5. Due to the pH-dependent release behavior of Cr-PPA NPs, shown in Figure 3b, Cr3+ ions would be released into the collagen fiber gap with concomitant disassembly of the Cr-PPA NPs.8 At pH 2.5, most carboxyl groups on the collagen fibers and PPA copolymers are protonated and cannot form effective coordination complexation with Cr3+ ions, which is helpful for the uniform distribution of chromium in the leather. When the interior pH of the pickled skin is increased to 4.0 by basifying, the released Cr3+ ions would cross-link the deprotonated carboxyl groups both on the collagen fibers and PPA copolymers, thus exhibiting similar chrome tanning mechanism as that of the commercial tanning agent and converting the animal skin into stable wet-blue leather (shown in Scheme 2 and Scheme S1).The cross sections of wet-blue leather samples tanned by Cr-PPA NPs, and 4 and 8% commercial chrome tanning agent were each examined by SEM. The SEM images in Figure 5a, 5c and 5e reveal that the collagen fiber bundles of the leather sample tanned by Cr-PPA NPs exhibited similarly dispersed structure as that of leather samples tanned by the commercial chrome tanning agent. This result provided good evidence for the chrome tanning effect of Cr3+ ions released from Cr-PPA NPs onto the collagen fibers. However, the EDS analysis results showed that the chromium distribution in the leather sample tanned by Cr-PPA NPs (Figure 5b) was more uniform than that in leather samples tanned with the commercial chrome tanning agent (Figure 5d and 5e). This was mainly 20 ACS Paragon Plus Environment

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due to the uniform penetration of Cr-PPA NPs into the leather helped by its antifouling PEG surface, which eventually leaded to uniform chromium cross-linking. Instead, in the conventional chrome tanning process, commercial chrome tanning agent tended to combine with collagen fibers at the start of the penetration stage, which usually led to higher chromium content in the flesh side and grain side regions than that in the regions in the middle of the leather. In fact, uniform chromium distribution in the cross-section of leather is necessary and determinant for the leather quality. Particularly since the absorption and distribution of anionic dyes, fatliquoring agents and retanning agents in the post-tanning process are significantly dependent on chromium distribution. The pH-sensitive Cr-PPA NPs could easily solve the chromium distribution problem by overcoming the balance limitation between chromium penetration and combination in leather. Furthermore, during the cross-linking process, the PPA copolymers from disassembled Cr-PPA NPs are helpful in forming more stable coordination bonds with Cr3+ ions than by the conventional process, which could increase the absorption of chromium in leather and reduce chromium discharge in wastewater. As shown in Figure 6a and 6b, the chromium absorption efficiency of Cr-PPA NPs tanned leather was higher than 90% with chrome content in tanning waste water of about 0.126 g·L-1. In contrast, the chromium absorption rate of leather tanned with the commercial tanning agent was only 60~70% due to the absorption equilibrium limitation of Cr3+ ions on collagen fibers. The 21 ACS Paragon Plus Environment


chrome content in tanning wastewater for 4 and 8% commercial tanning agent was 0.405 and 0.895 g·L-1, respectively. Compared with the 4 and 8% commercial chrome tanning wastewater, the chromium content in the Cr-PPA NPs tanning wastewater was reduced by 68.9 and 85.9%, respectively. These results demonstrated that Cr-PPA NPs effectively promoted the cross-linking between chromium and collagen fibers and resulted in a much cleaner leather tanning process.

Figure 5. (a, b) SEM images and EDS results of cross sections of leather samples 22 ACS Paragon Plus Environment

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tanned by Cr-PPA NPs, (c, d) 4% commercial chrome tanning agent and (e, f) 8% commercial chrome tanning agent.

Figure 6. (a)The absorption efficiency of Cr3+ in different tanned leather and the Cr3+ content in tanning wastewater (b). Results are presented as mean ± SE with * P < 0.05. Characterization of the Resultant Leather Properties. Wet-blue leather samples tanned by Cr-PPA NPs, and 4 and 8% commercial chrome tanning agent were further treated by a fatliquoring process according to Table S2. Then, the hydrothermal stability and physical parameters of the resultant leather samples were carefully evaluated and summarized in Figure 7. The hydrothermal stability is a key indicator of the resultant leather and also a significant distinction between leather and animal skin, which can be used as a criterion to evaluate the tanning effect. It can be usually characterized by the shrinkage temperature (Ts), which represents the cross-linking degree of collagen fibers. In Figure 7, the average shrinkage temperature of the Cr-PPA NPs tanned leather sample was 95.2 °C, which was higher than that (89.5 °C) of the 4% commercial agent tanned leather, but close to that (98.1°C) of the 8% 23 ACS Paragon Plus Environment


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commercial agent tanned leather, though the chrome content of Cr-PPA NPs was the same as that of 4% commercial agent. This finding demonstrated the efficient cross-linking of collagen fibers by the Cr3+ ions released from Cr-PPA NPs, based on the high absorption of chromium inside leather. The physical parameters of leather samples, including tear strength, breaking elongation and tensile strength, which are important indicators of leather durability, were also evaluated and compared. Notably, the tear strength of Cr-PPA NPs tanned leather was the highest with an average value of 26.83 N·mm-1, while the breaking elongation and tensile strength of Cr-PPA NPs tanned leather were nearly the same as that of the leather tanned with 8% commercial agent. Except for the tanning effect provided by Cr-PPA NPs, PPA copolymers cross-linked onto collagen fibers may also contribute similar excellent physical properties to the leather sample, as a result of the improvement of the physical strength of collagen fibers by the combined synthetic copolymers.

Figure 7. Hydrothermal stability and physical parameters of resultant leather samples tanned by Cr-PPA NPs, 4% and 8% commercial chrome tanning agent.


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CONCLUSIONS In summary, a chromium loaded NPs with a core-shell structure were fabricated by PPA block copolymer templated Cr(OH)3 mineralization in the presence of Cr3+ and OH-. The produced Cr-PPA NPs exhibited pH-sensitive properties based on the dissolution of its inorganic Cr(OH)3 core in acidic condition. Due to the excellent colloidal stability provided by the hydrophilic and antifouling PEG shell, the Cr-PPA NPs were selected to be used as novel chrome tanning agent. Compared with the commercial tanning agent, at pH 2.5. Cr-PPA NPs effectively penetrated into the interior of pickled skin, subsequently disassembled and released Cr3+ ions, resulting in uniform chromium distribution in the leather. In addition, the PPA copolymers were involved in the cross-linking process and helpful in forming more stable coordination bonds with the released Cr3+ ions, leading to a cleaner tanning process with high chromium absorption onto the leather and less chromium discharge in wastewater. Furthermore, the synthetic PPA copolymers cross-linked onto collagen fibers also conferred good hydrothermal stability and physical properties to the resultant leather. ASSOCIATED CONTENT Supporting Information Sampling diagram, Control A: 4% chrome agent tanned leather. Control B: 8% chrome agent tanned leather. Experimental C: Cr-PPA NPs tanned leather; Leather 25 ACS Paragon Plus Environment


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productive process of tanning; The size distribution histograms of Cr-PPA NPs ； Specific operations of tanning process by using commercial tanning agent for pickled sheep skin; Fatliquoring process of Cr-PPA NPs tanned leather and commercial tanning agent tanned leather; Schematic illustration of chrome tanning process by application of Cr-PPA NPs; This material is available free of charge via the Internet at http://pus.acs.org. AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Key Point and Invention Program of the Thirteenth (2017YFC1104601), and Support Program of Sichuan University-Luzhou City (2017CDLZ-S02).

REFERENCES (1) Bailey, A. J.; Light, N. D.; Atkins, E. D. T. Chemical Cross-Linking Restrictions on Models for the Molecular Organization of the Collagen Fibre. Nature 1980, 288, 408-410. 26 ACS Paragon Plus Environment

Page 27 of 33 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


(2) Maier, M.; Oelbermann, AL.; Renner, M.; Weidner, E. Screening of European Medicinal Herbs on Their Tannin Content-New Potential Tanning Agents for the Leather Industry. Ind. Crops Prod. 2017, 99, 19-26. (3) Zhang, C.; Lin, J.; Jia, X.; Peng, B. A Salt-Free and Chromium Discharge Minimizing Tanning Technology: the Novel Cleaner Integrated Chrome Tanning Process. J. Cleaner Prod. 2016, 115, 1055-1063. (4) Cao, S.; Wang, K.; Zhou, S.; Wang, Y.; Liu, B.; Chen, B.; Li, Y. Mechanism and Effect of High-Basicity Chromium Agent Acting on Cr-Wastewater-Reuse System of Leather Industry. ACS Sustainable Chem. Eng. 2018, 6, 3957-3963. (5) Covington, A. D. Tanning Chemistry: the Science of Leather. RSC Publishing, Cambridge 2009. (6) Covington, A. D. Modern Tanning Chemistry. Chem. Soc. Rev. 1997, 26, 111-126. (7) Zhang, Y.; Mansel, B. W.; Naffa, R.; Cheong, S.; Yao, Y.; Holmes, G.; Chen, HL.; Prabakar, S. Revealing Molecular Level Indicators of Collagen Stability: Minimizing Chrome Usage in Leather Processing. ACS Sustainable Chem. Eng. 2018, 6, 7096-7104. (8) WU, B.; Mu, C.; Zhang, G.; Lin, W. Effects of Cr3+ on the Structure of Collagen Fiber. Langmuir 2009, 25, 11905-11910. (9) Zhang, Y.; Ingham, B.; Cheong, S.; Ariotto, N.; Tilley, R. D.; Naffa, R.; Holmes, G.; Clarke, D. J.; Prabakar, S. Real-Time Synchrotron Small-Angle X-ray 27 ACS Paragon Plus Environment


Page 28 of 33

Scattering Studies of Collagen Structure during Leather Processing. Ind. Eng. Chem. Res. 2018, 57, 63-69. (10)

Sundar,

V.

J.;

Rao,

J.

R.;

Muralidharan,

C.

Cleaner

Chrome

Tanning-Emerging Options. J. Cleaner Prod. 2002, 10, 69-74. (11) Zhou, J.; Hu, S.; Wang, Y.; He, Q.; Liao, X.; Zhang, W.; Bi, S. Release of Chrome in Chrome Tanning and Post Tanning Processes. J. Soc. Leather Technol. Chem. 2002, 96, 157-162. (12) Lu, J.; Ma, J.; Gao, D.; Lyu, B.; Zhang, J. Application of an Amphoteric Polymer for Leather Pickling to Obtain a Less Total Dissolved Solids Residual Process. J. Cleaner Prod. 2016, 139, 788-795. (13) Houshyar, Z.; Khoshfetrat, A. B.; Fatehifar, E. Influence of Ozonation Process on Characteristics of Pre-alkalized Tannery Effluents. Chem. Eng. J. 2012, 191, 59-65. (14) Ye, Y.; Jiang, Z.; Xu, Z.; Zhang, X.; Wang, D.; Lv, L.; Pan, B. Efficient Removal of Cr(III)-Organic Complexes from Water Using UV/Fe(III) System: Negligible Cr(VI) Accumulation and Mechanism. Water Res. 2017, 126, 172-178. (15) Goutam, S. P.; Saxena, G.; Singh, V.; Yadav, A. K.; Bharagava, R. N.; Thapa, K. B. Green Synthesis of TiO2 Nanoparticles Using Leaf Extract of Jatropha curcas L. for Photocatalytic Degradation of Tannery Wastewater. Chem. Eng. J. 2018, 336, 386-396.


Page 29 of 33 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


(16) Ashokkumar, M.; Narayanan, N. T.; Reddy, A. L. M,; Gupta, B. K.; Chandrasekaran, B.; Talapatra, S.; Ajayan, P. M.; Thanikaivelan, P. Transforming collagen wastes into doped nanocarbons for sustainable energy applications. Green Chem. 2012, 14, 1689-1695. (17) Zhang, L.; Fu, F.; Tang, B. Adsorption and Redox Conversion Behaviors of Cr(VI) on Goethite/Carbon Microspheres and Akaganeite/Carbon Microspheres Composites. Chem. Eng. J. 2019, 356, 151-160. (18) Bacardit, A.; van der Burgh, S.; Armengol, J.; Olle, L.; Evaluation of a New Environment Friendly Tanning Process. J. Cleaner Prod. 2014, 65, 568-573. (19) Sundarapandiyan, S.; Brutto, P. E.; Siddhartha, G.; Ramesh, R.; Ramanaiah, B.; Mandal, A. B. Enhancement of Chromium Uptake in Tanning Using Oxazolidine. J. Hazard. Mater. 2011, 190, 802-809. (20) Krishnamoorthy, G.; Sadulla, S.; Sehgal, P. K.; Mandal, A. B. Greener Approach to Leather Tanning Process: D-Lysine Aldehyde as Novel Tanning Agent for Chrome-Free Tanning, J. Cleaner Prod. 2013, 42, 277-286. (21) Krishnamoorthy, G.; Sadulla, S.; Sehgal, P. K.; Mandal, A. B. Green Chemistry Approaches to Leather Tanning Process for Making Chrome-Free Leather by Unnatural Amino Acids. J. Hazard. Mater. 2012, 215, 173-182. (22) Jia, L.; Ma, J.; Gao, D.; Tait, W. R. T.; Sun, L. A Star-Shaped POSS-Containing Polymer for Cleaner Leather Processing. J. Hazard. Mater. 2019, 361, 305-311. 29 ACS Paragon Plus Environment


Page 30 of 33

(23) Kanagaraj, J.; Gupta, S.; Baskar, G.; Reddy, B. S. R. High Exhaust Chrome Tanning Using Novel Copolymer for Eco-Friendly Leather Processing. J. Am. Leather Chem. Assoc. 2008, 103, 36-43. (24) Abd El-Monem, F.; Hussain, A. I.; Nashy, E. H. A.; Abd El-Wahhab, H.; Naser, A. M. Nano-Emulsion Based on Acrylic Acid Ester Co-Polymer Derivatives as an Efficient Pre-Tanning Agent for Buffalo Hide. Arabian J. Chem. 2017, 10, S3861-S3869. (25) Qiang, T.; Gao, X.; Ren, J.; Chen, X.;Wang, X. A Chrome-Free and Chrome-Less Tanning System Based on the Hyperbranched Polymer. ACS Sustainable Chem. Eng. 2016, 4, 701-707. (26) Lyu, B.; Chang, R.; Gao, D.; Ma, J. Chromium Footprint Reduction: Nanocomposites as Efficient Pretanning Agents for Cowhide Shoe Upper Leather. ACS Sustainable Chem. Eng. 2018, 6, 5413-5423. (27) Liu, M.; Ma, J.; Lyu, B.; Gao, D.; Zhang, J. Enhancement of Chromium Uptake in Tanning Process of Goat Garment Leather Using Nanocomposite. J. Cleaner Prod. 2016, 133, 487-494. (28) Jaisankar, S. N.; Ramalingam, S.; Subramani, H.; Mohan, R.; Saravanan, P.; Samanta, D.; Mandal, A. B. Cloisite-g-Methacrylic Acid Copolymer Nanocomposites by Graft fromMethod for Leather Processing. Ind. Eng. Chem. Res. 2013, 52, 1379-1387.


Page 31 of 33 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


(29) Ma, J.; Lv, X.; Gao, D.; Li, Y.; Lv, B.; Zhang, J. Nanocomposite-Based Green Tanning Process of Suede Leather to Enhance Chromium Uptake. J. Cleaner Prod. 2014, 72, 120-126. (30) Kanagaraj, J.; Senthilvelan, T.; Panda, R. C.; Kavitha, S.; Eco-Friendly Waste Management Strategies for Greener Environment towards Sustainable Development in Leather Industry: a Comprehensive Review. J. Cleaner Prod. 2015, 89, 1-17. (31) Saravanabhavan, S.; Aravindhan, R.; Thanikaivelan, P.; Rao, J. R.; Nair, B. U. Green Solution for Tannery Pollution: Effect of Enzyme Based Lime-Free Unhairing and Fibre Opening in Combination with Pickle-Free Chrome Tanning. Green Chem. 2003, 5, 707-714. (32) Min, K. H.; Min, H. S.; Lee, H. J.; Park, D. J.; Yhee, J. Y.; Kim, K.; Kwon, I. C.; Jeong, S. Y.; Silvestre, O. F.; Chen, X.; Hwang, Y. S.; Kim, E. C.; Lee, S. C.; pH-Controlled Gas-Generating Mineralized Nanoparticles: A Theranostic Agent for Ultrasound Imaging and Therapy of Cancers. ACS NANO 2015, 9, 134-145. (33) Han, H.; Wang, H.; Chen, Y.; Li, Z.; Wang, Y.; Jin, Q.; Ji, J. Theranostic Reduction-Sensitive Gemcitabine Prodrug Micelles for Near-Infrared Imaging and Pancreatic Cancer Therapy. Nanoscale 2016, 8, 283-291. (34) Li, Z.; Wang, H.; Chen, Y.; Wang, Y.; Li, H.; Chen, T.; Jin, Q.; Ji, J;. pH‐ and NIR Light‐Responsive Polymeric Prodrug Micelles for Hyperthermia‐Assisted Site‐Specific Chemotherapy to Reverse Drug Resistance in Cancer Treatment. Small 2016, 12, 2731-2740. 31 ACS Paragon Plus Environment


Page 32 of 33

(35) Huang, Z.; Chen, C.; Xie, J.; Wang, Z. The Evolution of Dehydration and Thermal Decomposition of Nanocrystalline and Amorphous Chromium Hydroxide. J. Anal. Appl. Pyrolysis. 2016, 118, 225-230. (36) Ren, X.; Feng, Y.; Guo, J.; Wang, H.; Li, Q.; Yang, J.; Hao, X.; Lv, J.; Ma, N.; Li, W. Surface Modification and Endothelialization of Biomaterials as Potential Scaffolds for Vascular Tissue Engineering Applications. Chem. Soc. Rev. 2015, 44, 5680-5742. (37) Shi, H.; Shi, D.; Yao, Z.; Luan, S.; Jin, J.; Zhao, J.; Yang, H.; Stagnaro, P.;Yin, J.;

Synthesis

of

AmphiphilicPoly(cyclooctene)-Graft-Poly(ethylene

glycol)

Copolymers via ROMP and Its Surface Properties. Polym. Chem. 2011, 2, 679-684.


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pH-Sensitive and Chromium-loaded Mineralized Nanoparticles as Tanning Agent for Cleaner Leather Production 84x47mm (300 x 300 DPI)

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pH-Sensitive and Chromium-Loaded Mineralized Nanoparticles as

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