Nanocellulosic fibers are typically made by a mechanical process using raw cellulosic boards containing cellulose, hemicellulose and lignin. It had an inherent physical structure with polymeric ionic surface, and it acted as a functional agent in hazardous wastewater treatment.
Tannery Wastewater Analysis
The pH profile of tannery waste water was 4.03. The COD and BOD results of tannery waste water, 2787±262 mg/L and 508±86 mg/L, respectively. The Cr3+content of tannery waste water was 6,298 mg/L. The Cr6+content of tannery waste water was 6,298mg/L.
Preparation of Micro Cellulosic Fibers (MCF)
MCF prepared using cellulosic raw board are shown in Fig.3a and their corresponding surface MCF are given in Fig.3b. The MCF had smooth surface fibers and they were light weight in nature.
Tea leaves Microparticles (TLM)
TLM had a smooth texture and it was microparticle size in nature (Fig.4a). Surface morphology of TLM (Fig.4b) was revealed by scanning electron microscopy. It was confirmed from the micrograph that microparticles. From the EDX spectrum, it was confirmed that elements viz., C and O on the TLM surfaces.
Fourier transform infrared spectroscopy (FTIR)
Fig.5 (a and b) shows the FTIR spectra of MCF and FNF. 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 seen at 1058 cm-1 in cellulosic nanofiber was attributed to cellulose C-OH stretching. The tea leaf characteristics band was visible on spectra between 1800 and 1300 cm-1 (Peresin et al., 2010).
Thermo gravimetric analysis (TGA)
Fig. 5c Show the thermal stability of MCF and FNF as a function of temperature. MCF, had a two-step weight loss at 300 and 390 °C, while 87% remained as residue. The primary degradation of cellulosic fibers, which was related with cellulose pyrolysis, occurred in the lower temperature range of 200 to 450°C. Cellulosic fiber was degraded into carbon dioxide at temperatures exceeding 450°C, depolymerizing the lignin molecules (Mascheroni et al., 2016). The evaporation of water caused the first weight loss of 6% in cellulosic fiber at approximately 100°C. In the temperature range of 250 to 450°C, a weight loss of 87% occurred due to the dehydration of polyvinyl alcohol. FNF showed no loss of mass below 380-800°C, indicating thermal stability, which is a useful attribute for effluent wastewater treatment applications (El Miri et al., 2015). The FNF was quite stable at environmental and industrial temperatures and can be sterilized by heat. The thermal performance of FNF could be attributed to the hydrogen bond formation by PVA, which was consisted 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 (Rajaphasa and Shimizu, 2020
Atomic absorption spectroscopy (AFM).
Fig.5d shows the structure and size distribution of an FNF as studied by AFM. The scanning area was shown to be 1.2 µm to 0.1 µm. The phase and amplitude image of FNF revealed the presence of nanometer scale bundles of long cellulose fibers, as well as a particle layer. Electrospun FNF had a diameter ranging from 22.0 to 100 nm, with an average diameter of 39.0+14.5. 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 MCF and FNF are given in Fig.6 a and b respectively. MCF shows diameter of a fibers around one micron. In terms of diameter range, and uniformity, the blended electrospun materials were of high quality. The combination of MCF and TLM in FNF aids in the decrease of nanofiber diameter size and distribution from 100 to 250 nm. 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 MCF-TLM exhibited smooth surface area. Excellent adhesion between the MCF-TLM and the polymeric layer was observed. This could be due to the hydroxyl groups in both cellulosic 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
Table 1 summarizes the mechanical properties of electrospun scaffolds (PVA, PVA: MCF and PVA: MCF: TLM), including tensile strength, elongation at break, flexing index, water absorption and desorption. The results demonstrate that FNF had better values than PVA and PVA: MCF. Water filters must have appropriate mechanical qualities to carry out their intended functions. Individual nanofiber mechanical properties control deformations, dynamic and static reactions, interactions, and resistance in nanofiber channels (Fang et al., 2011). Study on PVA/cellulosic acetate composites, revealed that the tensile strength of PVA was 3.5 MPa, whereas the tensile strength of PVA and cellulosic acetate was 8 MPa. This finding supports the hypothesis that adding cellulose improves mechanical characteristics (Shunya et al., 2018).
Table 1. Mechanical properties of PVA, PVA:MCF, and PVA:MCF:TLM.
Samples
|
Tensile strength (MPa)
|
Elongation at break (%)
|
Flexing Index (%)
|
Water absorption (%)
|
Water desorption (%)
|
PVA
|
15.14+0.04
|
20.22+0.19
|
8.30+0.10
|
35.78+0.12
|
30.91+0.07
|
PVA: MCF
|
18.21+0.03
|
21.25+0.09
|
9.26+0.14
|
36.36+0.30
|
31.40+0.29
|
PVA: MCF: TLM
|
19.24+0.05
|
22.31+0.12
|
10.88+0.05
|
37.86+0.14
|
32.54+0.33
|
The data are presented as mean +SD of three individual experiments
* p ˂ 0.05. as compared to PVA, using Duncan’s multiple range analysis
XPS analysis
XPS analysis is particularly useful for determining the elemental composition and chemical structure on the surface of FNF. Fig.7 depicts the C1s, N1s and O1s spectra. PVA precursor fiber 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) present in C1s 288.2eV. 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 a direct bearing on the materials overall performance. Fig.8a shows the hazardous metal adsorption of PVA, PVA: MCF, and PVA: MCF: 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 MCF 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. Fig.8b shows the function curves of Cr (VI) adsorption capacity Qe (mg/g) and removal efficiency ɳ (%) at pH levels ranging from 3 to 11. The relationship between adsorption temperature and FNF adsorption performance is shown in Fig.8c. In Fig 8d, the results are determined as a function of adsorption time. FNF after adsorbing Cr (VI) from chrome-containing solution is shown in the SEM metal mapping image in Fig.8 (e&f). 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 as well (Li and Aiqin, 2007). In sewage treatment by adsorption, the contact time between the adsorbate and the adsorbent is essential (Chafik, 2014).
The diameter of MCF 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 MCF to be used in leather industry effluent wastewater treatment.