Cellulose based electrospun nanofilters: perspectives on tannery effluent waste water treatment

Development of nanofilters with the ability to remove toxic metal ions from effluent wastewater will be of immense help to the leather industry. In this study, fibrous nanofilter (FNF) was prepared using microfibrillated cellulose and tea leaf microparticles blended in poly (vinyl) alcohol. FNF was analysed for its efficacy to remove hazardous metals from tannery effluent wastewater. The FNF had promising traits of tensile strength (19.24 + 0.05 Mpa), elongation at break (22.31 + 0.12%), flexibility (10.88 + 0.05%), water absorption (37.86 + 0.14%) and desorption (32.54 + 0.33%). The metal adsorption studies clearly reflected the removal of toxic Cr (VI) ions from the effluent water by FNF. The study establishes an economically feasible and highly efficient way to remove hazardous metal ions from effluent wastewater.


Introduction
Leather industries pose a severe threat to the environment owing to the discharge of effluent waste water which contains a wide range of pollutants. In one way leather industries benefit mankind by converting raw hide into leather by the process of tanning; but on the other hand, the process involved generates air, water and solid pollution (Stanisław 2020). The tanning industry generates 145 billion gallons of waste water annually (Sathish et al. 2016), which when discharged into the viable water sources without proper treatment becomes detrimental to all the lifeforms. Water contamination especially with heavy metals has become a major ecological issue as it creates adverse impacts on living organisms particularly by accumulating in the food chain (Bhateria and Singh 2019). Chromium (III) salts are used as tanning chemicals in 80-90% of tanneries worldwide due to time and monetary advantages. Unused chromium salts present in tannery waste water (Orukoa et al. 2020) on oxidation get converted to chromium (VI). Hexavalent chromium (Cr VI) causes potent health hazards, and some salts are considered to be carcinogens (Kolomaznik et al. 2008). In view of this alarming fact, it becomes essentially important to treat the effluent waste water in a cost-effective manner to remove heavy metals and other contaminants, before discharge into the environment.
Around ''n'' number of strategies are practised globally to combat this heavy metal polluted waste water problem viz., adsorption, membrane filtration, chemical precipitation, electrochemical treatment, activated carbon etc. But these technologies are highly expensive and time consuming which becomes a great hurdle in their implementation. In this context, nanotechnology has emerged as a viable solution to protect the environment and life forms. Nanotechnology is the branch of science dealing with the engineering of nanosize particles (averaging less than 100 nm in length) and involves the fabrication of nanomaterials using nanoparticles. Nanomaterials such as membranes, films, scaffolds, colloids and quantum dots prepared in different shapes and sizes (Senthil et al. 2020) are characterized with greater surface area, surface action and specific affinity. Efficient nanomaterials based on carbon, silica, metal oxides etc. have been developed for heavy metal removal from wastewater (Singh et al. 2021).
Cellulose is used as a natural low-cost biopolymer for the removal of heavy metal impurities owing to the presence of carboxyl and hydroxyl functional groups (Saito et al., 2007). Large surface area, high hydrophilicity, good strength and transparency are the promising traits enabling the use of nanocellulose in a wide range of applications. Poly vinyl alcohol (PVA) is a biocompatible and biodegradable film forming multi-hydroxyl polymer with superior flexibility and chemical stability used in the preparation of functional membrane materials (Huo et al. 2019). Nanomembranes prepared using the blend of PVA and nanocellulose possessed improved thermal and chemical stability (Peng et al. 2017). Removal of heavy metals from waste water using agro wastes is evolving as a cost-effective strategy to overcome the limitations of existing processes using activated carbon. Tea waste is considered a promising option due to its low cost, abundant availability and its efficiency in adsorbing heavy metals like Ni, Cr (VI), Cu, Pb etc. (Nandal et al. 2014).
Electrospinning is a simple and versatile technique to produce nanofibers with diameters below 100 nm (Subbiah et al. 2005). Nanofibers can be fabricated using electrospinning with different structural designs. The filtering effectiveness of nanofiber membranes is attributed to the presence of high surface area which effectively adsorbs pollutants from air or water (Maneal et al. 2018). Electrospun nanofibrous membranes were proved to be efficient for the removal of chromium from contaminated water (Liu et al. 2015).
The study aims to prepare and characterize a fibrous nanofilter using a combination of microfibrillated cellulose, polymeric solution (PVA) and tea leaf microparticles and it will be evaluated for its efficacy to adsorb heavy meatal from tannery waste water.

Materials and methods
Poly (vinyl alcohol)-PVA (molecular weight 110,000-120,000 g/mol) and de-ionized water were purchased from Sigma Aldrich, Turkey. Cellulosic raw boards were collected from Viking Paper and Cellulose A.S. Industry, Aliaga-Izmir, Turkey. Tea waste was obtained from teashops, Bornova-Izmir, Turkey. The chrome-containing tannery waste water was collected from Bursa leather industrial area of Turkey.

Tannery wastewater analysis
Physicochemical characteristics were analyzed according to standard methods. Chemical oxygen demand (COD), and biological oxygen demand (BOD) are measured using photometric cuvette test.
Chromium content was estimated in tannery waste water according to Swarnalatha et al. (2008).

Preparation of microfibrillated cellulose (MFC)
Raw wood boards produced from Oak hardwood (Quercus) were used for the study Riley (2012). Cellulose pulps made from hard wood contained [ 97% cellulose. The soft pulp was transformed to dry pulp, then refined and homogenized under high pressure. The average fiber length was 0.87 mm (hardwood). MFC was produced from cellulose raw board through pulverization process. In brief, raw cellulosic materials were cut into small pieces and converted into microfibrillated cellulose by two-stage pulverization process. Pulverisation was done at room temperature for 10 min using Retsch grinder machine (Retsch GmbH 5657 HAAN WEST -GERMANY & SK1). The microfibrillated cellulose was filtered after the pulverization process (450 lm sized filter) and stored in containers until further use.

Tea leaf microparticle (TLM)
Preparation of TLM was done using waste tea powder. Tea powder was washed with boiling water to remove the soluble and colored components. Then it was washed with distilled water and dried using hot air oven at 80-100°C for 2 to 5 h. Finally, the resulting powder was crushed into fine particles using mortar and pestle.
Preparation of fibrous nanofilter (FNF) PVA (10 g) was dissolved in 100 mL distilled water using an agitator at 80°C for 3 h. PVA solution was then blended with 3.0 g of MFC and 0.9 wt % TLM, by vigorous stirring for 12 h. The spinning solution was carefully placed in the capillary position (New Era Pump Systems, USA) and linked to a positive electrode of high-voltage power supply with a flow rate of 0.5 mL/min. Electrospinning was done using E-Spin Nano equipment (Iseg Spezialelektronik GmbH, Germany). The obtained FNF membranes were vacuum-sealed and stored at 65°C for 12 h and used for further experiments. The preparation FNF is depicted in the schematic diagram ( Fig. 1).

Cross-linking of fibrous nanofilter
Themostabilization of FNF was done in electrothermostatic oven according to Beck et al. (2017). In brief, the electrospun membrane was heated in a constant air flow from 50 to 260°C at a rate of 2°C/ min, and then maintained at the set point for 1 h. The structure of fibrous nanofilter obtained by chemical crosslink is shown in Fig. 2. Cellulose is a linear polymer composed of b (1 ? 4) linked glucose residues. It exhibits intramolecular and intermolecular interactions with van der Waals interactions and hydrogen bonding (French 2017).

Characterization of MFC and FNF
FTIR measurements were carried out to determine the formation and changes in the functional groups of MFC and FNF. Nicolet 360 FTIR spectrometer was used to measure the spectra with a resolution of 4 cm -1 in the frequency range of 4000-500 cm -1 . SEM analysis was carried out using Thermo Scientific Apreo SEM at 15 kV accelerating voltage. The samples were coated with gold ions using an ion coating unit and then micrographs were taken. TGA was done using High resolution TGA 2950 (TA Instruments). Samples weighing 10 to 20 mg were placed in a platinum pan and tested in a programed temperature range of 0-800°C at a heating rate of 5°C/min in a nitrogen atmosphere with a flow rate of 50 mL/min. AFM was done to have a deep understanding of the surface morphology of FNF electrospun fibers. The electrospun fibers were deposited on silicon wafers and evaluated in air at room temperature (25 ± 1°C) using a BRUKER Dimension Edge with Scan Analysis AFM. Calibrating binding energy to C1s, X-ray photoelectron spectroscopy (XPS) analysis was performed with a PHI 5000 Versa Probe-Scanning ESCA Microprobe and a monochromatized A1 Ka X-ray source (h] = 1486,6 eV, 15 kV, 39,3 W, diameter beam spot: 200 lm) (285,1 eV). Surface contaminants were extracted using a moderate sputtering process with Ar ? ions at 2 kV.

Mechanical properties
Three dumbbell-shaped specimens, each 4 mm wide and 10 mm in length, were used to test mechanical characteristics. At a rate of 5 mm/min, tensile strength (MPa) and elongation at break (%) were evaluated using a Universal testing machine (INSTRON model 1405). The STM 129 test method was used to determine flexibility (%) utilizing a fiber board flexing (TER 74) machine. According to Senthil et al (2015), the water absorption and desorption (%) properties of PVA, PVA:MFC and PVA: MFC:TLM fibrous nanofilter were determined.

Metal adsorption studies
Batch method was used to investigate the adsorption of hazardous metals on FNF. The adsorption test was performed at room temperature by mixing 50 mg of FNF with 25 mL of chrome-containing wastewater in 150 mL Erlenmeyer flasks on a shaking incubator at 120 rpm. Experiments were conducted at varied pH levels in the range of 3.0-11.0 to determine the effect of solution pH. After allowing the FNF to settle for 12 h, it was tested for physico-chemical parameters and adsorption mechanism (APHA method 1998 & APHA method 2005).  The results are presented as mean ± standard deviation (SD) for three individual experiments (n = 3). ANOVA (Analysis of variance) and Duncan's multiple range analysis were done to determine the significant differences among the different groups. P values of \ 0.05 were considered significant.

Results and discussion
According to the standards for industrial effluents, 2 mg/mL of chromium is permitted in waste water; but the concentration of chromium varies between 2656-5420 mg/mL in the spent chrome liquor, which is highly alarming (Hashem et al. 2015). So, we are in definite need of cost-effective strategies which can make leather production eco-friendly. Adsorption of heavy metals on nanomembranes based on agro wastes offer us benefits of surface area, higher affinity of numerous functional groups for heavy metals, reuse, recycling and cost effectiveness (Wadhawan et al. 2020).
In view of this, MFC was prepared by mechanical process using raw wood boards containing cellulose, hemicellulose and lignin. It had inherent physical structure with polymeric ionic surface, and it acted as a functional agent in hazardous wastewater treatment. Microfibrillated cellulose was blended with PVA and TLM to enhance its functional property and stability.

Tannery wastewater analysis
The pH of tannery waste water was 4.03. The COD and BOD values of tannery waste water were 2787 ± 262 mg/L and 508 ± 86 mg/L, respectively. The Cr 3? content of tannery waste water was 6,298 mg/L. The Cr 6? content of tannery waste water was 5,356 mg/L.

Preparation of microfibrillated cellulose (MFC)
Raw wood boards (Fig. 3a) were used to prepare MFC (Fig. 3b). MFC was light weight in nature with smooth fibers.
Tea leaf microparticles (TLM) TLM (Fig. 4a) was brown in colour with smooth texture. SEM analysis also ascertained the microparticle size and smooth surface of TLM. Presence of carbon and oxygen in TLM was revealed by EDX spectrum (Fig. 4b).
Fourier transform infrared spectroscopy (FTIR) FTIR spectra of MFC and FNF are given in Fig. 5a and b respectively. The significant absorption peak at 1087 cm -1 is due to C-O stretching and O-H bending of PVA (Peresin et al. 2014). The stretching peaks at 2942 and 2908 cm -1 are typical O-H and C-H peaks. The stretching of the O-H bond due to intermolecular and intramolecular hydrogen bonding corresponds to the broad band from 3200 to 3550 cm -1 . The C-H stretch from alkyl groups causes the vibrational band between 2840 and 3000 cm -1 , whereas the C-O and C=O stretches from the remaining acetate groups in PVA cause the peaks between 1730 and 1680 cm -1 (Cho et al. 2000). Structural alterations of cellulose were observed in the range of 850-1500 cm -1 (Nelson and Connor, 1964). The peak observed at 1058 cm -1 in FNF could be attributed to cellulose C-OH stretching. The characteristic bands of tea leaf could be observed in the spectra between 1800 and 1300 cm -1 (Peresin et al., 2010).

Thermogravimetric analysis (TGA)
The thermal stability of MFC and FNF (Fig. 5c) was determined using TGA. MFC, had a two-step weight loss at 300 and 390°C, while 87% remained as residue. Cellulose pyrolysis could be responsible for the primary degradation of cellulosic fiber at lower temperatures ranging from 200 to 450°C. Above 450°C cellulosic fiber was degraded into carbon dioxide depolymerizing the lignin molecules (Mascheroni et al., 2016). In FNF, the evaporation of water caused the first weight loss of 6% at approximately 100°C. In the temperature range of 250-450°C, a weight loss of 87% occurred due to the dehydration of polyvinyl alcohol. FNF showed no loss of mass between 380-800°C, indicating thermal stability, which is a useful attribute for effluent wastewater treatment applications (El Miri et al. 2015). The results indicate the stability of FNF at high temperatures and hence it can be heat sterilized. The hydroxyl groups of PVA interact with the hydrophilic surfaces of cellulose and leads to the formation of hydrogen bonds. The thermal performance of FNF could be attributed to the hydrogen bonds, which is in accordance with the earlier report on PVA based scaffolds (Peresin et al. 2014). Pectin is an anionic polysaccharide present in tea leaves which forms electrostatic, steric, and covalent interactions, contributing to thermal stability (Bhateria and Singh 2019).
The structure and sample distribution of FNF was revealed by AFM (Fig. 5d). The scanning area was shown to be 1.2-0.1 lm. The phase and amplitude image of FNF revealed the presence of bundles of long cellulose fibers, as well as microfibrils layer. The AFM image of FNF showed that there was a closely packed uniform layer of MFC and TLM on the electrospun fiber surfaces and also confirmed the presence of a layer on the PVA surfaces. AFM could offer enough mechanical data to guide the development of scaffolds by stretching individual fibers and monitoring parameters such as elasticity and extension capabilities under both dry and wet circumstances (Spurlin et al. 2009).
Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) The SEM and EDX images of MFC and FNF are given in Fig. 6a and b respectively. MFC had a diameter  ranging from 10 to 10.5 lm, with an average diameter of 10.67 ± 0.26. In terms of diameter range, and uniformity, the blended electrospun materials were of high quality. The combination of MFC and TLM in FNF aids in the decrease of nanofiber diameter size and distribution from 100 to 250 nm. The pore size distribution of FNF is mainly determined by the morphology and size of the nanofibers. 6.35 m 2 /g surface area, 0.15 cm 3 /g total pore volume and 75.90 nm average pore width was observed in FNF. The small pore size, surface to volume area and narrow distribution, as well as the significantly high porosity, enable electrospun membranes to efficiently separate contaminants in water and thereby aid in wastewater treatment (Mohammad et al. 2020). The electrospinning of plant-derived microparticles resulted in a smoother surface with reduced shrinking (Yingngam et al. 2018). In our study, we found no distinguishable PVA micropores, and MFC-TLM exhibited smooth surface area. Excellent adhesion between the MFC-TLM and the polymeric layer was observed. This could be due to the hydroxyl groups in both cellulose and TLM, which could contribute to the comparatively strong interfacial interaction that would result in close adhesion between the two materials (Huda et al. 2008). The SEM analysis reveals precise specific surface area and pore area, as well as specific pore volume, which is useful in analyzing the impact of surface porosity and particle size in a variety of applications.

Mechanical properties
Mechanical properties are important indices to be considered while designing filtration membranes. Table 1 summarizes the mechanical properties of electrospun scaffolds (PVA, PVA: MFC and PVA: MFC: TLM). The results demonstrate that FNF had better tensile strength, elongation at break, flexing index, water absorption and desorption values compared to PVA and PVA:MFC. These enhanced properties of FNF could be attributed to the inherent strength of cellulose and the formation of hydrogen bonds between cellulose and PVA (Peng et al. 2017).
XPS analysis XPS analysis is particularly useful for determining the elemental composition and chemical structure on the surface of FNF. Figure 7 depicts the C1s, N1s and O1s spectra. PVA C1s binding energy peaked at 281.3 eV. It was made up of two components, one for the main polymer chain and other for the nitrile group. PVA/cellulose membrane had sulfur and nitrogen element, as well as an amide group (O = C-NH) was present in C1s 288.2 eV. Binding Al indicated that 73.50 eV oxidized aluminum in tea micro particles which may improve metal adsorption capabilities of electrospun membrane.

Metal adsorption studies
The presence of phenolic compounds in electrospun membranes has directly impacted the materials overall performance. Figure 8a shows the hazardous metal adsorption of PVA, PVA:MFC, and PVA:MFC:TLM. FNF had a better adsorption capacity than the other two samples which could be due to the combined influence of surface structure alterations and nanomaterial size in nanofiber (Nuri et al. 2015). The presence of hydroxyl and carboxyl groups in MFC play a vital role in the removal of Cr (VI) ions. The functional groups present in TLM viz., carboxylate, aromatic carboxylate, phenolic hydroxyl and oxyl groups also contribute towards efficient heavy metal adsorption (Nandal et al. 2014). The effect of adsorption experimental factors such as solution pH, temperature and contact time on the removal of hazardous metal in a batch system were investigated in this study. Figure 8b shows the function curves of Cr (VI) adsorption capacity Qe (mg/g) and removal efficiency g (%) at pH levels ranging from 3 to 11. The relationship The data are presented as the mean ± SD of three individual experiments * p \ 0.05. compared to PVA, using Duncan's multiple range analysis between adsorption temperature and FNF adsorption performance is given in Fig. 8c. The results determined as a function of adsorption time is depicted in Fig. 8d. The SEM metal mapping ( Fig. 8e and f) clearly shows the adsorption of Cr (VI) from chromecontaining solution onto FNF. The results showed that at low pH, Cr (VI) adsorption was significant, which could be due to effective adsorption between the anionic surfaces on the cellulosic fibers and tea leaves particles (Wang and Ge 2013). As the temperature increases, the electrospun mat adsorption capacity and removal efficiency improved (Li and Aiqin 2007). In wastewater treatment by adsorption, the contact time between the adsorbate and the adsorbent plays an imperative role (Chafik 2014). The results reveal that maximum adsorption of heavy metals could be achieved within 3 h of time.
The diameter of MFC with TLM electrospun nanofibers were lower than those of pure cellulose nanofiber materials, which augmented the adsorption capacity in electrospun mat. Toxic metallic ions such as lead, copper, and cadmium, were removed by the membrane which clearly portrays the promising potential of MFC to be used in leather industry effluent wastewater treatment.

Conclusions
The tanning industry generates 145 billion gallons of wastewater annually, especially rich in heavy metals, which pose a severe threat to living organisms and to the marine ecosystem. Removal of heavy metals especially Cr (VI) from tannery effluent water is a major concern globally. A sustainable solution to this problem will be the use of cost-effective strategies using agro residues. This study aimed to harness the advantages of wood boards (a low-cost material) in heavy metal adsoption using the nanomaterial platform. Addition of PVA and TLM improved the stability, functional and mechanical properties of e Hazardous metal absorption SEM and EDX images of FNF and f Hazardous metal mapping image of FNF microfibrillated cellulose. The prepared nanofilter was able to efficiently remove the heavy metals from effluent waste water thus proving to be a viable and cost-effective strategy.