Preparation of Cellulose/Chitin Blend Materials and Influence of Their

sustainability

Article Preparation of Cellulose/Chitin Blend Materials and Inﬂuence of Their Properties on Sorption of Heavy Metals

Dao Zhou 1,*, Hongyu Wang 1 and Shenglian Guo 2

1 School of Civil Engineering, Wuhan University, Wuhan 430072, China; [email protected] 2 School of Hydraulic Engineering, Wuhan University, Wuhan 430072, China; [email protected] * Correspondence: [email protected]

Abstract: A series of biodegradable cellulose/chitin materials (beads and membranes) were successfully prepared by mixing cellulose with chitin in an NaOH/thiourea–water system and coagulation in

a H2SO4 solution. The effects of chitin content on the materials’ mechanical properties, morphology, structure, and sorption ability for heavy metal ions (Pb2+, Cd2+, and Cu2+) were studied by tensile tests, scanning electron micrographs, Fourier transform infrared spectroscopy, and atomic absorption spectrophotometry. The results revealed that the cellulose/chitin blends exhibited relatively good mechanical properties, a homogeneous, microporous mesh structure, and the existence of strong hydrogen bonds between molecules of cellulose and chitin when the chitin content was less than 30 wt%, which indicated a good compatibility of the cellulose/chitin materials. Furthermore, in the same chitin content range, Pb2+, Cd2+, and Cu2+ can be adsorbed efﬁciently onto the cellulose/chitin

beads at pH0 = 5, and the sorption capacity of the beads is more than that of chitin ﬂakes. This shows that the hydrophilicity and microporous mesh structure of the blends are favorable for the kinetics of sorption. Preparation of environmentally friendly cellulose/chitin blend materials provides a simple and economical way to remove and recover heavy metals, showing a potential application of chitin Citation: Zhou, D.; Wang, H.; Guo, S. as a functional material. Preparation of Cellulose/Chitin Blend Materials and Inﬂuence of Keywords: cellulose; chitin; blend materials; sorption; heavy metal removal Their Properties on Sorption of Heavy Metals. Sustainability 2021, 13, 6460. https://doi.org/10.3390/su13116460

1. Introduction Academic Editors: Junfeng Su, Qian Zhang and Sicheng Shao Although the utilization of synthetic polymers extracted from petroleum resources has had a sizable influence on the world economy, the problem of petroleum resource Received: 30 April 2021 shortages and environmental pollution from the waste of non-biodegradable synthetic Accepted: 4 June 2021 polymers, which are mainly derived from petroleum resources, is becoming increasingly Published: 7 June 2021 serious [1]. Therefore, the importance of research and exploitation of new materials based on natural polymers has been recognized in many countries [2]. Abundant biodegradable Publisher’s Note: MDPI stays neutral polymers in nature such as cellulose, chitin, lignin, starch, and various animal and plant with regard to jurisdictional claims in proteins, which possess various functional groups and are available in large quantities, may published maps and institutional affil- have potential for the development of new materials by physical or chemical modification. iations. Polymer blending is an easy, simple, and convenient method to develop new materials with excellent properties for practical use. Furthermore, the cost could be greatly reduced when using polymer blending for research and exploitation of new materials [3]. Nowadays, polymer mixtures or blends are widely used materials in modern industry. They represent Copyright: © 2021 by the authors. one of the most rapidly growing areas in polymer material science [4]. Licensee MDPI, Basel, Switzerland. Today, the global environmental problem is becoming increasingly serious with rapid This article is an open access article population growth, the acceleration of industrialization, and the use of new toxic chemicals. distributed under the terms and The issue of heavy metal pollution has attracted great attention. Heavy metals are widely conditions of the Creative Commons used in many industries, such as electroplating, printing and dyeing, oil paints, electrolysis, Attribution (CC BY) license (https:// pesticides, and medicine. Most heavy metals are toxic, carcinogenic, and can be accumu- creativecommons.org/licenses/by/ lated and transferred in living organisms. Wastewater containing heavy metals, when 4.0/).

Sustainability 2021, 13, 6460. https://doi.org/10.3390/su13116460 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, x FOR PEER REVIEW 2 of 13

Sustainability 2021, 13, 6460 electrolysis, pesticides, and medicine. Most heavy metals are toxic, carcinogenic, and2 of can 11 be accumulated and transferred in living organisms. Wastewater containing heavy metals, when discharged into aquatic bodies, poses a serious threat to human health and dischargedthe survival into of animals aquatic and bodies, plants, poses even a seriousif the heavy threat metals to human are presen healtht in and trace the quantities survival of[5]. animals To remove and heavy plants, metals even iffrom the heavywastewater, metals various are present conventional in trace treatment quantities methods [5]. To removehave been heavy employed, metals from including wastewater, chemical various precipitation conventional [6], treatmentdistillation methods [7], ion have exchange been employed,[8], adsorption including [9], reverse chemical osmosis precipitation [10], elec [trolysis6], distillation [11], etc. [7 ],However, ion exchange these [ 8techniques], adsorp- tionhave [9 ],certain reverse deficiencies osmosis [10 such], electrolysis as high [cost11],, etc.low However, efficiency, these high techniques energy requirements, have certain deficienciesoperational suchcomplexity, as high cost,and lowlarge-scale efficiency, genera hightion energy of sludge requirements, or toxic operationalwaste materials, com- plexity,which may and large-scalerequire special generation treatment of sludge[12]. Henc or toxice, environmentally waste materials, friendly which methods may require with speciallow cost treatment and high [12 efficiency]. Hence, are environmentally necessary for friendlyremoving methods toxic metals with low from cost wastewater. and high efficiencyBiosorption are of necessary heavy metal for removingions has been toxic regarded metals from as one wastewater. of the most Biosorption reliable and of efficient heavy metalmethods. ions Natural has been polymer regarded materials as one ofthat the are most obtained reliable in great and efficient quantities, methods. or certain Natural kinds polymerof waste materialsproducts thatfrom are industrial obtained or in agricu greatltural quantities, processes, or certain including kinds various of waste bacteria prod- ucts[13], from yeast industrial [14], fungi or [15], agricultural algae [16], processes, etc., may includinghave potential various as inexpensive bacteria [13 ],biosorbents. yeast [14], fungiCellulose [15], algae is [ 16the], etc.,most may abundant have potential natural aspolymer inexpensive material biosorbents. in nature and is mainly foundCellulose in trees, is cotton, the most hemp, abundant cereal plants, natural and polymer other materialhigher plants. in nature As a andbiocompatible is mainly foundcarrier, in cellulose trees, cotton, has special hemp, cerealproperties, plants, such and as other being higher non-toxic, plants. safe, As ahydrophilic, biocompatible and carrier,biodegradable cellulose and has possessing special properties, high chemical such stability, as being good non-toxic, mechanical safe, hydrophilic,strength, and and low biodegradablecost [17]. It is andwidely possessing used in highfood, chemicalmedicine, stability, construction, good mechanicalpapermaking, strength, wastewater and lowtreatment, cost [17 ].printing, It is widely electronics, used in food,and medicine,other fields construction, [18,19]. Chitin papermaking, is the second wastewater most treatment,abundant printing,resource electronics,(after cellulose) and otherin nature, fields has [18 ,a19 similar]. Chitin chemical is the second structure most to abundant cellulose, resourceand may (after be considered cellulose) incellulose nature, haswith a C-2 similar hydroxyl chemical replaced structure by toan cellulose, acetamido. and Chitin may bemainly considered comes cellulose from the with exoskeletons C-2 hydroxyl of arthro replacedpods by (shrimp, an acetamido. crabs, Chitinand insects) mainly and comes the fromcell walls the exoskeletons of some fungi, of arthropodsalgae, etc. Figure (shrimp, 1 shows crabs, the and chemical insects) andstructures the cell of walls cellulose of some and fungi,chitin. algae, etc. Figure1 shows the chemical structures of cellulose and chitin.

FigureFigure 1.1.Chemical Chemical structuresstructuresof ofcellulose celluloseand and chitin. chitin.

ChitinChitin and and chitosan chitosan (deacetylation (deacetylation of chitin)of chitin) are regardedare regarded as good as good metal metal ligands. ligands. They canThey form can coordination form coordination complexes complexes with heavy with metalheavy ions.metal It ions. has beenIt has reported been reported that stable that complexesstable complexes are formed are formed between between the N-acetyl the N-ac groupetyl in group chitin andin chitin the heavy and the metal heavy ions metal [20]. However,ions [20]. thereHowever, are some there problems are some with problems directly with applying directly chitin applying in flake chitin shape, in flake such asshape, the separationsuch as the of separation chitin after of sorption, chitin after mass sorption, loss after mass regeneration, loss after low regeneration, strength, and low small strength, size, whichand small make size, it difficult which make to employ it difficult in column to employ utilizations in column [21]. utilizations Furthermore, [21]. the Furthermore, solubility of chitinthe solubility is verylow of chitin in lots is of very solvents, low in and lots it of is solvents, extremely and brittle, it is causingextremely limitations brittle, causing in its reactivitylimitations and in processabilityits reactivity and for processability application. In for a previousapplication. study, In a a previous novel aqueous study, solvent a novel system,aqueous NaOH/thiourea–water solvent system, NaOH/thiourea–wate solution, was successfullyr solution, developed, was successfully which can developed, dissolve cellulosewhich can to obtaindissolve a transparent cellulose to solution. obtain Regenerateda transparent cellulose solution. membranes Regenerated obtained cellulose in this solution system exhibited good mechanical properties and microporous structure with high porosity [22]. These characteristics can help us to develop cellulose-based bioaffinity functional materials with good performance. In a previous work, cellulose/chitin blend Sustainability 2021, 13, 6460 3 of 11

membranes containing 4.6 to 19.3 wt% chitin were prepared, which had a certain degree of compatibility within the experimental weight ratios [23]. In the present work, we aim to prepare cellulose/chitin blend materials (beads and membranes) containing 10 to 70 wt% chitin in an NaOH/thiourea–water solution system. The mechanical properties of the cellulose/chitin membranes were tested, and the morphology and structure of the blend beads were studied by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The ability of the cellulose/chitin beads to adsorb Pb2+, Cd2+, and Cu2+ in aqueous solutions was also investigated by atomic absorption spectrophotometry (AAS) and discussed. This work should provide an easy and economical way to potentially apply chitin as a functional material.

2. Materials and Methods 2.1. Materials Cotton linters (cellulose) were provided by Hubei Chemical Fiber Group Ltd., China. The intrinsic viscosity ([η]) of the cellulose at 25 ± 0.1 ◦C was measured in cadoxen by using viscometry, and the value of viscosity-average molecular weight (Mη) was calculated to be 5 −2 0.76 1.03 × 10 according to the equation [η] = 3.85 × 10 Mw (mL/g) [24]. Chitin ﬂakes was provided by Zhenjiang Yuhuan Co. Ltd., China, and their intrinsic viscosity ([η]) at 25 ± 0.1 ◦C was tested by using viscometry in N,N-dimethylacetamide (DMAc) containing 6 5 wt% LiCl. The viscosity-average molecular weight (Mη) of the chitin was 1.4 × 10 in −2 0.69 terms of the following equation [η] = 2.4 × 10 Mw (mL/g) [25]. According to the equation DA (%) = 1 − [(WC/WN − 5.14)/1.72] × 100% from the nitrogen content, where WC/WN is the weight ratio of carbon to nitrogen in chitin, the degree of acetylation (DA) of the chitin was 73% [26]. The chemical reagents used in the experiments were of analytical grade and were supplied by Shanghai Chemical Reagent Co., Ltd., Shanghai, China.

2.2. Preparation of Cellulose/Chitin Membranes According to previous works, we prepared about 4 wt% cellulose solution and 2 wt% chitin solution [22,23]. Eight grams of cotton linters was dispersed completely in 192 g–6 wt% NaOH/5 wt% thiourea–water solution, and it was then placed below −8 ◦C for about 8 h. After that, it was thawed with intense agitation at room temperature for 1 h to obtain a 4 wt% transparent cellulose solution. Two grams of chitin ﬂakes was immersed entirely in 30 g–46 wt% NaOH–water solution, and it was then placed below −0 ◦C for about 10 h. Afterwards, 68 g of ice pieces was added, and the mixture was thawed with strong stirring at room temperature for 1 h to obtain 2 wt% clear chitin solution. A mixture of the cellulose and chitin solutions, containing 0, 10, 20, 30, 40, 50, 60, and 70 wt% chitin, was subjected to uniform mixing and total degassing. Then, the resulting mixture was cast on a glass plate to form a gel layer with a thickness of 0.2–0.3 mm, and immersed soon after in a 5 wt% H2SO4 water solution for coagulation for about 5 min. The resulting membranes were washed in running water, and then dried in air. The ratio of chitin content was calculated according to the following equation:

W × 2 wt% P (%) = chitin × 100 % (1) Wcellulose × 4 wt% + Wchitin × 2 wt%

where P is the ratio of chitin content in the membrane. Wcellulose and Wchitin are the weight of the cellulose solution and the chitin solution, respectively. Accordingly, the prepared membranes containing 0, 10, 20, 30, 40, 50, 60, and 70 wt% chitin were coded as RCM, M-1, M-2, M-3, M-4, M-5, M-6, and M-7.

2.3. Preparation of Cellulose/Chitin Beads The cellulose/chitin beads were obtained by uniform mixing of 4 wt% transparent cellulose solution and 2 wt% clear chitin solution according to the above procedure. A mixture of cellulose and chitin solutions, containing 0, 10, 20, 30, 40, 50, 60, and 70 wt% chitin, was prepared by stirring and degassing. The resulting mixture was injected into a Sustainability 2021, 13, 6460 4 of 11

cylinder to extrude a fiber with a diameter of 0.3 mm. The fibers obtained were coagulated in a 5 wt% H2SO4 solution for 5 min and then washed using distilled water. After that, the fibers were cut into small beads with length ranging from 0.3 mm to 2.0 mm. In this way, the cellulose/chitin beads were prepared. The beads were stored in a refrigerator before use. Accordingly, the beads obtained containing 0, 10, 20, 30, 40, 50, 60, and 70 wt% chitin were coded as RC, B-1, B-2, B-3, B-4, B-5, B-6, and B-7.

2.4. Characterization

The tensile strength (σb) and elongation at breaking (εb) of the cellulose/chitin membranes (RCM, M-1, M-2, M-3, M-4, M-5, M-6, M-7) in dry and wet states were determined by a universal electronic tensile testing machine (CMT 6503, Shenzhen new Sansi Test Machine Co., Ltd., Shenzhen, China) according to ISO 6239-1993 (E). The test conditions were as follows: 10 mm of sample width, 70 mm of sample length (50 mm between the grips), 5 mm/min of elongation rate. The wet cellulose/chitin membranes were tested soon after by placing them in water for 1 h. The water resistivity of the membranes was calculated in terms of the following equation:

Rb = [σb (wet)/σb (dry)] × 100% (2)

where Rb is the water resistivity of the membrane. σb (dry) and σb (wet) are the tensile strength (σb) of membranes in dry state and in wet state. The morphology of the cellulose beads (RC), the chitin ﬂakes, and the cellulose/chitin beads (B-1, B-2, B-3, B-4, B-5) was investigated by scanning electron micrographs on a scanning electron microscope (SEM, Hitachi S-570, Hitachi. Ltd., Tokyo, Japan). The samples were prepared for the SEM experiments by the following steps. The wet samples were frozen in liquid nitrogen, snapped immediately, and then freeze-dried under a vacuum. The cross-section and surface of resulting samples were coated with gold, watched carefully, and photographed. Fourier transform infrared spectroscopy (FTIR) of the cellulose beads (RC), the chitin ﬂakes, and the cellulose/chitin beads (B-1, B-2, B-3, B-4, B-5) was recorded by a FITR spectrometer (FITR, Perkin-Elmer 1600, PerkinElmer Co., Ltd., Waltham, MA, USA). The samples for FTIR measurement were dried under a vacuum for a day and then cut into powder. The test specimens were prepared by the KBr-disk method.

2.5. Static Sorption The efficiency of sorption for heavy metal ions in water solutions depends largely upon the initial metal ion concentration. Therefore, the sorption ability of the cellulose/chitin beads for heavy metal ions in water solution was investigated as a function of initial metal ion concentration in this study. The concentrations of heavy metal ions in the solutions were measured by using an atomic absorption spectrophotometer (AAS, Perkin-Elmer AAS 800, PerkinElmer Co., Ltd., Waltham, Mass., USA). The Pb2+, Cd2+, and Cu2+ stock solutions of 1.0 mg/mL were obtained by dissolving Pb(NO3)2, CdCl2·2.5H2O, and CuCl2·2H2O in distilled water. Then, suitable solution concentrations of the heavy metal ions were obtained by diluting the stock solution (Pb2+, Cd2+, Cu2+). The experiments were performed in some 250 mL conical flask with cover, which contained 0.1 L solution with different concentrations of heavy metal ions. The same amount of sorbents (0.1 g) was added into each conical flask with cover. In order to minimize evaporation of the solution, all the conical flasks were sealed with a cap. The tests were carried out at pH0 = 5 and room temperature for 24 h. The amount of sorption qe (mmol/g), which is the uptake of heavy metal ions adsorbed at equilibrium, was calculated as follows: (c − c )V q = 0 e (3) e m Sustainability 2021, 13, x FOR PEER REVIEW 5 of 13

amount of sorption qe (mmol/g), which is the uptake of heavy metal ions adsorbed at Sustainability 2021, 13, 6460 equilibrium, was calculated as follows: 5 of 11

(𝑐 −𝑐)𝑉 𝑞 = (3) 𝑚

wherewhere cc00 (mmol/L) and and cce e(mmol/L)(mmol/L) are are the the initial initial and and th thee equilibrium equilibrium concentrations concentrations of ofheavy heavy metal metal ions, ions, respectively. respectively. V (L)V (L)is the isthe solution solution volume, volume, and and m (g)m (g)is the is themass mass of the of thesorbents. sorbents.

3. Results and Discussion The effect of chitin content (W ) on the tensile strength (σ ) and elongation at The effect of chitin content (Wchitinchitin) on the tensile strength (σbb) and elongation at breaking (ε ) of the membranes in dry state is shown in Figure2. breaking (εbb) of the membranes in dry state is shown in Figure 2.

16 80

14 70

12 60

10 50

8 40 (%) (MPa) b b ε σ 6 30

4 20

2 10

0 0 -10 0 10 20 30 40 50 60 70 80 w (%) chitin

Figure 2. Effect of chitin content ( (WWchitinchitin) on) on the the tensile tensile strength strength (σ (b,σ ●b), and•) and elongation elongation at breaking at breaking (εb, ) of the membranes in dry state. (εb, ) of the membranes in dry state.

The resultsresults showshow thatthat thethe tensiletensile strengthstrength ((σσbb) andand elongationelongation atat breakingbreaking ((εεbb)) ofof thethe membranes increasedincreased with with an an increase increase in in chitin chitin content content (W (chitinWchitin) in) in the the range range of of 0–10 0–10 wt% wt% in dryin dry state. state. When When the theW chitinWchitinwas was more more than than 10 10 wt%, wt%, the theσ σb band andε εbb valuesvalues ofof thethe membranesmembranes werewere 72.4172.41 MPaMPa andand 5.41%,5.41%, eveneven higherhigher thanthan thatthat ofof thethe RCRC membranesmembranes preparedprepared underunder thethe samesame conditions, whichwhich werewere 52.8352.83 MPaMPa and 2.01%, respectively. TheseThese resultsresults suggestsuggest thatthat therethere isis aa strongstrong interactioninteraction betweenbetween moleculesmolecules ofof chitinchitin andand cellulosecellulose inin thethe blendblend membranes. As the the WWchitinchitin waswas more more than than 10 10 wt%, wt%, the the σb valuesσb values decreased decreased gradually gradually in dry in drystate. state. The Theεb valuesεb values decreased decreased with with an increase an increase in W inchitinW chitinin thein range the range of 10–20 of 10–20 wt%, wt%, and andthen thenremained remained nearly nearly constant constant as the as W thechitinW waschitin morewas than more 20 than wt%. 20 In wt%. the range In the of range 0–30 ofwt%0–30 chitin wt% content,chitin content,the tensile the strength tensile of strength the membranes of the membranes was relatively was high, relatively exhibiting high, exhibitinggood mechanical good mechanical properties. properties. However, However, in the range in the of range chitin of content chitin content from 0 from wt% 0 to wt% 70 towt%, 70 wt%,the tensile the tensile strength strength of the of blend the blendmembranes membranes decreased decreased with an with increase an increase in the inchitin the chitincontent content in wet in state wet (Table state (Table 1). 1).

Table 1. The mechanical properties of the membranes.

σb (MPa) ε (%) Samples R (%) Dry Wet Dry Wet RCM 52.83 28.38 53.72 2.01 1.43 M-1 72.41 7.81 10.78 5.41 1.29 M-2 40.73 6.09 14.95 1.03 1.18 M-3 32.55 1.07 5.22 0.96 1.17 M-4 25.56 0.76 2.97 0.99 1.25 M-5 13.04 0.67 5.14 1.04 0.99 M-6 9.76 0.78 7.99 1.03 1.1 M-7 6.23 0.21 3.37 1.02 0.9 Sustainability 2021, 13, 6460 6 of 11

When the content of chitin was more than 30 wt%, the tensile strength of blend membranes in wet state decreased quickly. Therefore, the mechanical properties of the blend membranes are optimal when the chitin content is less than 30 wt%. Sustainability 2021, 13, x FOR PEER REVIEWScanning electron micrograph (SEM) images of the chitin ﬂakes, the cellulose beads,7 of 13

and the cellulose/chitin blend beads with different chitin contents are shown in Figure3.

Figure 3. SEM photographs: surface (a) and cross-section (b) of the chitin flakes; surface (c) and Figure 3. SEM photographs: surface (a) and cross-section (b) of the chitin flakes; surface (c) and cross-section (d) of the cellulose beads; surface (e) and cross-section (f) of the cellulose/chitin beads cross-sectionB-1; surface ((dg)) ofand the cross-section cellulose beads; (h) of surface B-2; surface (e) and ( cross-sectioni) and cross-section (f) of the (j) cellulose/chitin of B-3; surface beads(k) and B-1;cross-section surface (g )(l and) of B-4; cross-section surface (m (h) )and of B-2; cross-section surface (i )(n and) of cross-sectionB-5. (j) of B-3; surface (k) and cross-section (l) of B-4; surface (m) and cross-section (n) of B-5. The surface and cross-section of the chitin flakes present a dense and almost non- The surface and cross-section of the chitin flakes present a dense and almost non- porous structure, which makes it difficult for the chitin flakes to adsorb heavy metal ions. porous structure, which makes it difficult for the chitin flakes to adsorb heavy metal ions. The surface and cross-section of the cellulose beads and the cellulose/chitin beads display The surface and cross-section of the cellulose beads and the cellulose/chitin beads display a microporous mesh structure, which is apparently different from the structure of the a microporous mesh structure, which is apparently different from the structure of the chitin chitin flakes. It has been reported that cellulose membranes prepared from an flakes. It has been reported that cellulose membranes prepared from an NaOH/thiourea– NaOH/thiourea–water solution system exhibited a homogeneous porous network water solution system exhibited a homogeneous porous network structure and a relatively structure and a relatively narrow pore size [27]. However, Figure 3 shows that the pore size of the cellulose beads is smaller than that of the cellulose/chitin blend beads, and there is a rapid increase in pore size in the cellulose/chitin blend beads with an increase in the chitin content. When the content of chitin is less than 30 wt%, the surface and cross-section of the cellulose/chitin beads show a homogeneous microporous mesh structure, suggesting good compatibility of the blending beads and a relatively high specific surface area. However, when the content of chitin is more than 30 wt%, the cross-section and Sustainability 2021, 13, 6460 7 of 11

narrow pore size [27]. However, Figure3 shows that the pore size of the cellulose beads is smaller than that of the cellulose/chitin blend beads, and there is a rapid increase in pore size in the cellulose/chitin blend beads with an increase in the chitin content. When the content of chitin is less than 30 wt%, the surface and cross-section of the cellulose/chitin beads show a homogeneous microporous mesh structure, suggesting good compatibility of the blending beads and a relatively high specific surface area. However, when the content of chitin is more than 30 wt%, the cross-section and surface of the cellulose/chitin blend beads show a non-homogeneous loose mesh structure, indicating a certain level of phase separation of the blending beads. Therefore, the cellulose/chitin blend beads exhibit a microporous, homogeneous mesh structure when the chitin content is less than 30 wt%, which is consistent with the results of the tensile test. Fourier transform infrared spectroscopy (FTIR) spectra are a helpful tool to distinguish functional groups in a molecule, as each specific chemical bond often has a unique energy absorption band, and we can obtain structural and bond information on a complex to study the strength and the fraction of hydrogen bonding and compatibility. If two polymers are not completely miscible, the FTIR spectra of the blends are only the simple overlying of the spectra of the two polymers. In addition, if the polymers are miscible, there should be significant differences between the IR spectrum of the blend and the co-addition of the spectra of two components. These differences would be derived from chemical interactions resulting in band shifts, intensity changes, and broadening [28]. Figure4 shows FTIR spectra (400–4000 cm −1) of the RC beads, the chitin flakes, and the cellulose/chitin blend beads with different chitin contents. Generally, the –OH stretching vibration sited at around 3436 cm−1 is the major peak of regenerated cellulose [29], and there are three major peaks of α-chitin, which are the –OH stretching vibration sited at around 3446 cm−1, the –NH stretching vibration sited at around 3260 cm−1, and the three sharp bands in the C=O region sited at around 1660 cm−1 (amide I), 1623 cm−1, and 1557 cm−1 (amide II), respectively [30]. Compared with the IR spectra of the cellulose beads and the chitin flakes, it can be found that the –OH bands were broadened and moved to a lower wave number, the –NH bands vanished, and only two sharp bands of amide appeared on the IR spectra of the cellulose/chitin blend beads. The results indicate that relatively strong interactions have occurred between cellulose and chitin in the cellulose/chitin blend beads, which is mainly caused by intermolecular hydrogen bonds. A good compatibility of the blends is reflected by the occurrence of strong intermolecular hydrogen bonding. The FTIR results agree with the conclusion from SEM of the blends. To study the influence of the chitin content on the sorption ability of cellulose/chitin beads for heavy metal ions in water solutions, static sorption experiments of single sorption for heavy metal ions on the cellulose beads, the chitin flakes, and the cellulose/chitin beads with different chitin contents were carried out. The results are given in Figure5, where the cellulose beads exhibited little sorption ability for these heavy metal ions. Nevertheless, for the cellulose/chitin beads, the equilibrium adsorption capacity (qe) of these heavy metal ions increased with an increase in the chitin content in the range of 0–20 wt%. When the chitin content was over 20 wt%, the qe values decreased. In particular, when the chitin content was in the range of 0–30 wt%, the qe values of cellulose/chitin blend beads for Pb2+, Cd2+, and Cu2+ were higher than that of chitin flakes, which were 2+ 2+ 2+ 0.121 mmol/g (Pb ), 0.079 mmol/g (Cd ), and 0.062 mmol/g (Cu ) at pH0 = 5. The hydrophilic cellulose skeleton is beneficial for the kinetics of sorption [17]. In addition, the homogeneous and microporous mesh structure of cellulose/chitin blend beads increases the specific surface areas, which can also improve the adsorption capacity. However, when the content of chitin was more than 30 wt%, the qe values of cellulose/chitin blend beads for these heavy metal ions were even lower than that of the chitin flakes. The reason for this is that as the content of chitin is higher, the specific surface areas of the beads obviously decrease due to the large porous size structure caused by phase separation, which can be clearly seen in the SEM images and corresponds to the tensile test results. SustainabilitySustainability 20212021, 13, 13, x, 6460FOR PEER REVIEW 8 of 11 9 of 13

Figure 4. FTIR spectra of the cellulose beads (a), the chitin ﬂakes (b), and the cellulose/chitin blend Figure 4. FTIR spectra of the cellulose beads (a), the chitin flakes (b), and the beads B-1 (c), B-2 (d), B-3 (e), B-4 (f), and B-5 (g). cellulose/chitin blend beads B-1 (c), B-2 (d), B-3 (e), B-4 (f), and B-5 (g). Generally, the –OH stretching vibration sited at around 3436 cm−1 is the major peak of regenerated cellulose [29], and there are three major peaks of α-chitin, which are the – OH stretching vibration sited at around 3446 cm−1, the –NH stretching vibration sited at around 3260 cm−1, and the three sharp bands in the C=O region sited at around 1660 cm−1 (amide I), 1623 cm−1, and 1557 cm−1 (amide II), respectively [30]. Compared with the IR spectra of the cellulose beads and the chitin flakes, it can be found that the –OH bands were broadened and moved to a lower wave number, the –NH bands vanished, and only two sharp bands of amide appeared on the IR spectra of the cellulose/chitin blend beads. The results indicate that relatively strong interactions have occurred between cellulose Sustainability 2021, 13, x FOR PEER REVIEW 10 of 13

and chitin in the cellulose/chitin blend beads, which is mainly caused by intermolecular hydrogen bonds. A good compatibility of the blends is reflected by the occurrence of strong intermolecular hydrogen bonding. The FTIR results agree with the conclusion from SEM of the blends. To study the influence of the chitin content on the sorption ability of cellulose/chitin beads for heavy metal ions in water solutions, static sorption experiments of single sorption for heavy metal ions on the cellulose beads, the chitin flakes, and the Sustainability 2021, 13, 6460 cellulose/chitin beads with different chitin contents were carried out. The results are given9 of 11 in Figure 5, where the cellulose beads exhibited little sorption ability for these heavy metal ions.

FigureFigure 5. 5. ExperimentalExperimental data data of of single single sorpti sorptionon for for heavy heavy metal metal ions ions on on the the chitin chitin flakes, ﬂakes, the the cellulose cellulose beads beads ( (aa),), and and the the cellulose/chitin blend beads B-1 (b), B-2 (c), B-3 (d), B-4 (e), and B-5 (f) at pH0 = 5.0 and room temperature. cellulose/chitin blend beads B-1 (b), B-2 (c), B-3 (d), B-4 (e), and B-5 (f) at pH0 = 5.0 and room temperature.

4. ConclusionsNevertheless, for the cellulose/chitin beads, the equilibrium adsorption capacity (qe) of these heavy metal ions increased with an increase in the chitin content in the range of In this study, a series of cellulose/chitin blend materials (beads and membranes) with different chitin contents have been successfully prepared by mixing cellulose with chitin in a 6 wt% NaOH/5 wt% thiourea–water solution and coagulation in a 5 wt%H2SO4 water solution. The tensile strength (σb) and elongation at breaking (εb) of the membranes increased with an increase in the chitin content (Wchitin) in the range of 0–10 wt%, and then decreased when the Wchitin was over 10 wt% in dry state. In particular, when Wchitin was 10 wt%, the σb and εb values of the membranes were 72.41 MPa and 5.41%, even higher than that of the RC membranes prepared under the same conditions, which were 52.83 MPa and 2.01%, respectively. This suggests that a relatively strong interaction occurs between molecules of chitin and cellulose in the blends. The mechanical properties of the cellulose/chitin membranes are optimal as the chitin content is less than 30 wt%. The SEM images indicated Sustainability 2021, 13, 6460 10 of 11

that the porosity and pore size of the cellulose/chitin beads rapidly increased with the increase in chitin content. When the chitin content is less than 30 wt%, the cellulose/chitin blend beads exhibit a homogeneous, dense, microporous mesh structure, which indicates a good compatibility and a relatively larger specific surface area of the blend materials. How- ever, when the Wchitin is over 30 wt%, the blend beads exhibit a non-homogeneous loose mesh structure, showing a certain level of phase separation of the blend materials. The IR spectra revealed that stronger hydrogen bonding existed between molecules of cellulose and chitin in the cellulose/chitin blend beads, especially when the Wchitin was 10 wt%, displaying a good compatibility of the blend materials. The static sorption experiments of single sorption for heavy metal ions (Pb2+, Cd2+, Cu2+) in water solutions showed that the equilibrium adsorption capacity (qe) of these heavy metal ions increased with an increase in the Wchitin in the range of 0–20 wt%, and then the qe values decreased when the Wchitin was over 20 wt%. The cellulose/chitin beads exhibited good sorption ability for heavy 2+ 2+ 2+ metal ions (Pb , Cd , Cu ), when the Wchitin was less than 30 wt%, even higher than that of chitin flakes. This could be explained by the hydrophilicity and microporous mesh structure of the cellulose/chitin beads potentially promoting sorption. The cellulose/chitin blend materials possess good mechanical properties, miscibility, larger specific surface area, and higher affinity for heavy metal ions in the chitin content range of 0–30 wt%. Therefore, in future studies, the mechanisms regarding miscibility of cellulose and chitin could be further researched. Moreover, the sorption properties and mechanisms of cellulose/chitin blend beads containing about 20 wt% chitin for more kinds of heavy metal ions could be explored for potential applications of chitin and cellulose. Preparation of cellulose/chitin materials by blending cellulose with chitin provides a simple and economical way to remove and recover heavy metals from wastewater.

Author Contributions: Conceptualization, D.Z.; methodology, D.Z. and H.W.; formal analysis, D.Z.; investigation, S.G.; writing—original draft preparation, D.Z.; writing—review and editing, D.Z. and H.W.; visualization, D.Z. and H.W. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to thank the referees and journal editors for their constructive and valuable feedback. Conﬂicts of Interest: The authors declare that they have no conﬂict of interest.

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