Polyvinyl alcohol nanofibrous mat impregnated with ClCNTs/Fe2O3 nanocomposite for absorption of diesel oil in water

Oil contamination is one of the main sources of water pollution in the world. In this study, a sorption material that showed great promise as an absorbent for diesel oil in aqueous solution was developed. Chlorinated carbon nanotubes (CNTs) previously synthesized using a chemical vapor deposition (CVD) method were loaded with iron oxide nanoparticles via a co-precipitation method. The sorption materials were prepared by embedding ClCNTs/Fe2O3 nanocomposite into a polyvinyl alcohol (PVA) polymer matrix via electrospinning. The PVA mat containing only ClCNTs was also prepared for comparison and the maximum sorption capacity of 9.7 g g−1 was obtained. The optimum concentration of ClCNTs/Fe2O3 nanocomposite that gave uniform, and well-distributed nanofibers was 0.5 wt%. Crosslinking the PVA/ClCNTs/Fe2O3 nanofiber mat with glutaraldehyde (GA) resulted in increased absorption capacity for oil of ∼ 9.4 g g−1 in comparison with an absorption capacity of ∼ 7.6 g g−1 attained with a pure nanofiber mat. The crosslinked nanofiber mat remained stable even after 60 min of oil absorption which proves that crosslinking assisted in increasing the affinity of PVA for oil by reducing the amount of OH groups through acetal formation making PVA less soluble to water. The oil sorption capacity of the prepared materials was not maintained even after two cycles indicating poor reusability.


Introduction
Water pollution is becoming a supreme area of scientific research [1], which results in most communities lacking access to clean water.Water is one of the most abundant natural resources on earth that is known to men, with 71% of the earth's surface covered by water [2].Approximately 96.5% of the earth's water resource is accounted for by oceans [3].Surprisingly, with all this abundance, only a minute fraction of water is available as fresh water.Recently in 2022 devastating effects of the floods that occurred in KwaZulu Natal province in South Africa, left most communities without access to potable water, moreover the available water was contaminated due to damaged infrastructure.In 2023 neglected infrastructure that was not maintained for years also led to the recent cholera outbreak that was experienced by Hammanskraal residents at the Gauteng province in South Africa.Water pollutants which include bacteria, viruses, pharmaceutical products, personal care products, pesticides, fertilizers, plastics, nitrates, phosphates, metals, diesel oil, mineral oils, crude oils, and vegetables oils may find way into water streams and channels.Oil is one pollutant of water that is hard to remove and can cause devastating effects to the species living in water.The primary cause of water pollution by oil involve human practices such as petroleum refining [4], petrochemical plant wastes [5], oil runoffs [6], accidental spills [7], and drilling incidents [8].
Various techniques have been developed and applied for treating water that is polluted by oil; namely, membrane filtration [9][10][11], gravity separation [12], air flotation [13], bioprocesses [14,15], membrane bioreactor [16], chemical coagulation [17], adsorption [18][19][20], electrocoagulation [21], absorption [9,[22][23][24], and electro-flotation [25].Of these, absorption is one of the most efficient methods of removing oil pollutants from water.This choice is driven by the extrinsic benefits offered by sorbent materials used for absorption which include high efficiency and selectivity, ease of use, reusability, being environmentally friendly and of low cost [26].Intrinsic features of these materials including high porosity and tuneable morphology are also controlling factors.Adsorption refers to the adhesion of pollutants to the surface of the adsorbent, while absorption describes the phenomenon where the pollutant is drawn inside the pores of the absorbent [27].In the case of absorption, the porosity and morphology of the material plays an important role since the pores produce capillary forces that pull the pollutant into the material resulting in the pores being filled [28].
Most sorbents are selected and developed based on the availability, cost, and safety of their raw materials [29].Sorbents with high sorption capacities for oil uptake in water are usually those with high carbon and oxygen contents [29].Sorbents like carbon nanotubes (CNTs) in organic-inorganic hybrid materials possess optical transparency, high thermal stability, and good mechanical properties [30].Incorporation of iron oxide nanoparticles in hybrid materials introduces new properties to the material; namely, high surface area-volume ratio [31], super-paramagnetic behaviour [30], good electrical conductivity [30], high stability in catalytic applications, good mechanical stability, and convenient recycling [32].The addition of nanoparticles into polymer matrices can result in improvement of membrane separation properties [33][34][35].The inclusion of Fe 3 O 4 nanoparticles into polymer matrix during treatment of oil-water emulsions was found to improve the hydrophilicity of the membrane surface, thereby enhancing the water permeability and resistance to fouling [36].
Organic sorbents like polyvinyl alcohol (PVA) possess flexibility and film forming abilities [30], but its application to water treatment is limited because it is hydrophilic.Therefore, to render PVA useful for water purification it is important to cross-link it before application [37].Water insoluble PVA nanofibers have been fabricated by in situ cross-linking with glutaraldehyde and heat [38].The cross-linking improved the physical properties of the PVA nanofibers [39].The porosity of PVA nanofibers was also improved by encapsulating them with iron oxide nanoparticles and crosslinking with glutaraldehyde [40].Addition of iron oxide nanoparticles has shown to be beneficial for further fine-tuning the properties of electrospun nanofiber sorbent materials.For instance, in the study by Zhang et al (2010) significant increases in the recoveries of experimental PHAs (∼90%) were obtained when carbon coated Fe 3 O 4 nanoparticles were used as adsorbents, compared to the pure Fe 3 O 4 nanoparticles [41].Fard et al (2016) reported carbon nanotubes loaded with iron oxide (Fe 2 O 3 ) nanoparticles to be ideal adsorbents for oil during oil-water separations [29].Multi-walled carbon nanotubes (MWCNTs) and MWCNTs/Fe 3 O 4 derivatives showed the removal efficiency of 85% for oil when employed as adsorbents for oil in water [42].In another study, the mechanical properties of PVA were improved by reinforcing its films with single-walled carbon nanotubes (SWCNTs) and Fe 2 O 3 nanoparticles [43].The performance of an electrospun nanofibrous membrane of polyether sulfone (PES), a widely used polymer in membrane technology, has been enhanced by incorporation of iron oxide nanoparticles [44].The resultant absorbent exhibited outstanding oil elimination (94.01%) and excellent water flux recovery (79.50%) in synthetic oil solution (12,000 ppm) [44].A [(4-((4-((11-ferroceneundecyl)oxy)phenyl)diazinyl)phenoxy)diethylene triamine/poly-ethyleneimine]-decorated CNT-based membrane was developed [45].This membrane had stimuli-switchable separation fluxes that showed strong hydrophilicity to water in the air and high oleophobicity to oil underwater thereby endowing the membrane with the potential to separate oil and water [45].
In this study we aim to fabricate PVA nanofibrous mats that are impregnated with chlorinated carbon nanotubes that were previously loaded with iron oxide nanoparticles and use them as absorbents for oil in prepared oil water solutions.The role of chlorine was fully investigated in the previous study by Maboya and coworkers, whereby chlorine addition resulted in creation of defects on the outer walls of the CNTs [46].In this study we wanted to utilize this defect sites for easy loading or coating of the surface of CNTs with iron oxide nanoparticles.Engine oil (Total SAE 40 diesel) was purchased from Builders Warehouse in Vanderbijlpark South Africa.All reagents and materials were used as supplied, without any further purification unless otherwise stated.

Synthesis of chlorinated CNTs using the chemical vapor deposition method
Chlorinated CNTs were produced following the method by Maboya and co-workers [46].Briefly, a 10 wt% Fe-Co/CaCO 3 catalyst (1.084 g) prepared using the method by Mhlanga et al [47] was spread on a quartz boat (26 × 10 mm 2 ) and placed at the center of a quartz tube (0.5 m, 5 mm ID).The quartz tube was inserted into a high temperature furnace.The furnace was heated to 700 °C stepwise by adjusting the temperature at a rate of 10 °C/min under N 2 atmosphere (50 ml min −1 ).Once the required temperature was reached, the N 2 flow rate was increased to 240 ml min −1 and flown through 1,2-dichlorobenzene together with acetylene (C 2 H 2 ) gas with a flow rate of 90 ml/min.The vapors generated by N 2 and C 2 H 2 gases flowing through 1,2-dichlorobenzene were continuously flown into the quartz tube.After 60 min of the reaction, the C 2 H 2 gas flow and bubbling were stopped, and the system was left to cool down to room temperature under a continuous flow of N 2 at a reduced flow rate of 50 ml min −1 .The quartz boat was removed from the furnace and carbon deposits (chlorinated-CNTs ∼2.055 g) were obtained.The carbon deposits were purified by stirring in nitric (HNO 3 ) (prepared 30% in distilled water) acid at room temperature for two hours.This was followed by filtration of the black powdered product and washing the material with distilled water several times until the pH of the filtrates reached 6.5.The purified ClCNTs were then dried in an oven at 110 °C, for overnight, leaving 1,985 g of the product.

Synthesis of Fe 2 O 3 nanoparticles using the co-precipitation method
The method used for synthesis of iron oxide nanoparticles was adopted from Mascolo and co-workers [48].About 1.340 g of FeSO 4 • 7 H 2 O salt was weighed in a beaker and dissolved in 50 ml of previously deoxygenated water (to make a 0.015 mol Fe 2+ solution).In another separate beaker, 2.340 g of FeCl 3 • 6 H 2 O salt was also weighed and dissolved in 50 ml of previously deoxygenated distilled water (to make a 0.030 mol Fe 3+ solution).The two aqueous solutions were transferred into a 250 ml three-necked flask equipped with a cooling condenser, dropping funnel and an N 2 gas inlet.The mixture was placed on a hotplate that was preheated to 80 °C with continuous stirring.Following this, 16 ml of aqueous NH 4 OH solution (27%) was added dropwise to the reaction mixture via the dropping funnel over a period of 40 min.The mixture was continuously stirring and heated at 80 °C under N 2 atmosphere for an additional 40 min.The system was left to cool overnight.The black precipitates that formed were filtered, washed with distilled water until a clear filtrate was obtained, followed by washing with ethanol (50 ml), and finally dried in an oven at 60 °C overnight.An amount of 1.2289 g of Fe 2 O 3 nanoparticles was obtained.

Synthesis of ClCNTs/Fe 2 O 3 nanocomposite using the wet impregnation method
The method used for synthesis of iron oxide nanoparticles and CNT loaded iron oxide nanoparticles were adopted from Mascolo and co-workers [48] and Elnabawy and co-workers [49].About 1.340 g of FeSO 4 • 7 H 2 O salt was weighed in a beaker and dissolved in 50 ml of previously deoxygenated water (to make a 0.015 mol Fe 2+ solution).In another separate beaker, 2.340 g of FeCl 3 • 6 H 2 O salt was also weighed and dissolved in 50 ml of previously deoxygenated distilled water (to make a 0.030 mol Fe 3+ solution).The two aqueous solutions were transferred into a 250 ml three-necked flask equipped with a cooling condenser, dropping funnel and an N 2 gas inlet.The mixture was placed on a hotplate that was preheated to 80 °C with continuous stirring.Following this, 16 ml of aqueous NH 4 OH solution (27%) was added dropwise to the reaction mixture via the dropping funnel over a period of 40 min.About 0.510 g of ClCNTs was added to the contents of the three-necked flask.The resultant mixture was heated and stirred for another 40 min at 80 °C under N 2 atmosphere.The system, which now contained a black precipitate, was cooled overnight.The black precipitate that formed was filtered, washed with distilled water until a clear filtrate was obtained followed by washing with ethanol (50 ml), and finally dried in an oven at 60 °C overnight.An amount of 2.1204 g of Cl-CNT/Fe 2 O 3 nanocomposite was obtained.
2.5.Synthesis of PVA/ClCNTs-Fe 2 O 3 nanocomposite fibers using the electrospinning process About 1.0586 g of PVA was weighed and dissolved in a mixture of distilled water and DMF (9 ml, 8:1 v/v).The PVA solution was stirred at 80 °C until the PVA was completely dissolved in the solvent mixture.A selected amount of ClCNTs/Fe 2 O 3 composites (0.1041, 0.5089, 0.7045 or 1.0856 g) was added into the PVA solution and stirred overnight at room temperature.The resulting reaction mixture was then loaded into a 10 ml plastic syringe for the electrospinning process.Electrospinning of the resulting PVA/ClCNTs-Fe 2 O 3 solution was performed using slightly modified parameters obtained from Huang and co-workers, applying a voltage of 15 kV, tip-collector distance of 10 cm and at a flow rate of 0.050 μl h −1 [50].
2.6.Chemical crosslinking of PVA/ClCNTs-Fe 2 O 3 nano-composite fibers A 25% glutaraldehyde (GA) stock solution was diluted with distilled water to give a 0.25% final solution (GA-25%:H 2 O, 1:9 v/v) [40].Following this, 1 ml of GA solution (0.25%) was placed in a beaker containing 1 ml of acetone and a single drop of HCl (35% v/v in water).A 25 cm 2 of the electrospun mat was immersed into the GA solution and kept in it overnight.The glutaraldehyde cross-linked mat was removed from the solution and allowed to dry at room temperature overnight.2.7.Heat crosslinking of PVA/ClCNTs-Fe 2 O 3 nano-composite fibers A 25 cm 2 electrospun mat was placed on a watch glass and heated in an oven at 40 °C for 24 h.The mat was cooled down before use.

Characterization of ClCNTs and ClCNTs/Fe 2 O 3 nanocomposites
Functional groups of the prepared materials were determined using Fourier Transform Infrared (FTIR) spectroscopy (Perkin Elmer Frontier spectrometer) operating at a range from 4000 to 500 cm −1 wave number at room temperature.The phase compositions and crystallinity of the materials were determined using a powder X-ray diffraction (XRD) spectroscopy (Rigaku Ultima III X-ray diffraction system).The instrument was conducted with a wide-angle goniometer, high intensity Cu X-ray tube (1.54 Å wavelength), and a scintillation counter detector.The scans were carried in 2θ with a range of 10 °to 90 °and 1 s count time per step.Elemental composition and binding energies of various elements present in the materials were determined using an X-ray photoelectron spectroscopy (XPS) PHI 5000 Scanning ESCA Microprobe.The powder sample was sputtered with a 2 kV 2 μA 1 × 1 mm raster-Ar ion gun at a sputter rate of about 18 nm min −1 for 60 seconds.The structural morphology of the materials was determined using transmission electron microscopy (TEM) (FEI TECNAI G 2 SPIRIT).The samples were prepared by adding a spatula tip of material in an Eppendorf tube and sonicating in ethanol until the material was well dispersed.The sample was then deposited on a holey carboncoated TEM Cu grid using a Pasteur pipette.
2.9.Characterization of PVA, PVA/ClCNTs, and PVA/ClCNTs-Fe 2 O 3 nanofibrous mats The morphology and size distribution of the nanofibers were obtained by scanning electron microscopy (SEM) (JSM-7500F JEOL-Japan).The SEM was operated with a Hitachi S-4800 ultra-high-resolution accelerated with a voltage of 300 kV, a point-to-point resolution of 0.18 nm and a lattice resolution of 0.10 nm.The samples were prepared by mounting the nanofibers onto an Al metal stub using a sticky carbon disc and then the nanofibers were coated twice with carbon and once with gold-palladium to prevent sample charging.FTIR was also used to determine the functional groups and to check that the nanofibers were successfully cross-linked.

Removal of diesel oil
A 25 cm 2 electrospun mat was cut from the thick mat prepared over 3 days.The dry mat was weighed, and its initial mass was recorded.The mat was then placed into a beaker containing a mixture of water (20 ml) and diesel oil (5 ml, 4.48 g).The contents of the beaker were gently swelled for 5 min before removing the mat from the water-oil mixture.The mat was held above the water-oil mixture for 2 min to remove the residual water and diesel oil from the surface of the mat.The mat was weighed, and its final mass was recorded.The schematic diagram of the fabrication of the nanofiber mat via electrospinning and an absorption experiment is shown in figure 1.

Reusability
To test if the two mats that exhibited the highest oil sorption can be reusable, their absorption-desorption properties and reusability experiments were conducted.First, the sorbents were weighed and then contacted with diesel oil.After they reached their maximum oil absorbency, they were dipped in an organic solvent (petroleum ether), which was followed drying in air for 6 h.The new weight of each mat after drying was recorded as the initial mass and the oil sorption was repeated for four cycles.

Sorption capacity of the nanocomposite fibrous materials
The sorption capacities of the materials were expressed as grams of diesel oil absorbed per gram of the mat (g/g) (equation 1): where: Q t (g/g) is the sorption capacity on the nanocomposite fibrous material at a certain time (min), m t (g) is the weight of the nanocomposite fibrous material after absorption and m 0 (g) is the initial weight of the nanocomposite fibrous material [51].2: Scherrer relation where θ is the full-width at half maximum of the XRD peak appearing at the diffraction angle θ, A is the Scherrer constant (0.154), and the wavelength of X-ray is 0.91 nm.An iron oxide crystallite size of 2θ = 14.26 nm was calculated for pure iron oxide nanoparticles that were free of ClCNTs.Iron oxide nanoparticles crystallite size changed from 14.26 nm to 28.06 nm in ClCNTs/Fe 2 O 3 nanocomposite.The increase in size from pure to loaded Fe 2 O 3 was attributed to ClCNTs acting as nucleation sites due to the presence of defects on their outer walls created by chlorine.The survey scan XPS spectrum of ClCNTs/Fe 2 O 3 nanocomposite showed the presence of C1s, O1s, Cl2p, Fe3p and Fe2p peaks at binding energy of 286, 530, 202, 711 and 724 eV respectively (figure 4(a)).The C1s deconvoluted XPS spectra correlated with the FTIR assignments where the carboxyl (COOH), hydroxyl, and sp 3 hybridized carbon environments were observed (figure 4(b)).The C-Cl peak was also observed from these spectra indicating successful functionalization with chlorine.The O1s spectrum was deconvoluted into three components at 532, 530.8 and 530.4 eV, corresponding to C-O, C=O and the O 2-of iron oxide nanoparticles (figure 4(c)) [52].The binding energy of the O1s state at 530.6 eV is presumably coupled to Fe 2 O 3 , while the peak at 532 eV is consistent with the binding energy attributed to the presence of FeO(OH) [53,54].The weakness of the Cl2p peak which could not be deconvoluted, could indicate their blockage by Fe 2 O 3 nanoparticles coated on the surface of the ClCNTs and their low functionalization.The amount of Cl detected by XPS was less than 1 atomic percent.Two main peaks located at 711 and 724 eV appeared on the Fe2p deconvoluted XPS spectra and were assigned to Fe2p 3/2 and Fe2p 1/2 , respectively (figure 4(d)).The spin-orbit coupling of the 2p bands was 13 eV [55].The Fe2p 3/2 at 711 eV was deconvoluted into three peaks located at 710, 711 and 712 eV (figure 4(d)).These results confirmed the presence of the 3+ valence state of an Fe species in ClCNTs/Fe 2 O 3 nanocomposite.Meanwhile, the existence of the peak associated with a satellite peak located at 716 eV could be correlated to the characteristic XPS spectra of Fe 2 O 3 or Fe 3+ ion [56].Charge-transfer-related to the satellite peaks were indicative of the actual oxidation state of iron.Magnetite, Fe 3 O 4 containing both Fe 2+ and Fe 3+ (in 1:2 ratio) in air causes the nanoparticles to undergo a partial oxidation to maghemite (γ-Fe 2 O 3 ) [57,58].Maghemite (γ-Fe 2 O 3 ) are considered as an oxidized form of magnetite (Fe 3 O 4 ) where the previous form coexisted in a complex crystalline arrangement [59].

Results and discussion
Following characterization of the ClCNTs and ClCNTs/Fe 2 O 3 nanocomposites prepared above, the nanocomposites were then embedded into a polyvinyl alcohol (PVA) matrix using the electrospinning method for preparation of mats that were later used to absorb oil.Firstly, a pure PVA mat with a concentration of 10 wt% PVA in a H 2 O-DMF mixture (9 ml, 8:1 v/v) (concentration chosen was obtained from literature [60,61]) was prepared by electrospinning, to enable us to compare its fiber morphologies to the one prepared after addition of the nanocomposite.The surface of the PVA mat was reasonably smooth, but the nanofiber diameters were irregular with an average diameter of 457 ± 140 nm, and some beads and fused fibers were also observed (figure 5(a)).The addition of the nanocomposite material had a significant impact on the surface and diameter of the nanofibers.A decrease in the average outer diameter of the nanofibers was observed with an increase in the quantity of the nanocomposite added, from 457 nm for the pure PVA mat to 413 nm after addition of 0.2 wt% ClCNTs (figure 5(b)) and 366, 211, 209 and 169 nm after addition of 0.1, 0.5, 0.7 and 1 wt% ClCNTs/Fe 2 O 3 nanocomposite, respectively (figures 5(c)-(f), figures 6(b)-(e)).The reduced nanofiber diameter could be attributed to an increased conductivity of the solution facilitated by the addition of nanocomposites.The absence of beads after the addition of nanocomposites indicates stable fiber formation [62].A minor decrease in nanofiber diameter was obtained for nanocomposites with concentration of 0.5 and 0.7 wt% which dropped quite a bit from a 0.1 wt%, however notably, the 0.5 wt% gave a more uniform porous structure and hence it was chosen as an optimum that will be used to prepare a mat to be used in oil absorption studies.A further reduction in nanofiber diameter was observed for the matrix containing 1 wt% of nanocomposite, however, beads also emerged at this concentration.Moreover, some clustered nanocomposite materials that could not be embedded inside the polymer matrix were observed on the surface of the nanofibers (figure 5(e)).Figure 5(g) presents the variation of average outer diameter as a function of nanocomposite concentration, and the trend follows the above discussions.
The PVA/ClCNTs (0.2 wt%) and PVA/ClCNTs-Fe 2 O 3 (0.5 wt%) nanofibers were subjected to crosslinking by adding glutaraldehyde and HCl to stabilize PVA, as it is well known that PVA is a water-soluble polymer (figures 7(a) and (b)).HCl acts as a catalyst in the process as it promotes the crosslinking reaction between the hydroxyl groups of PVA and aldehyde groups of glutaraldehyde to form an acetal bond [40].For comparison the other PVA/ClCNTs-Fe 2 O 3 (0.5 wt%) material was crosslinked with heat (figure 7(c)).The nanofiber morphology did not change significantly after crosslinking with glutaraldehyde for ClCNTs loaded mat (figure 7(a)), whereas few flat nanofibers were observed for ClCNTs-Fe 2 O 3 loaded mat which indicate increased effective molecular entanglement and molecular weight due to the presence of Fe 2 O 3 and GA (figure 7(b)).Flat nanofibers have been previously observed after crosslinking and were attributed to high molecular weights [38].A brown color was observed from the as-prepared PVA/ClCNTs-Fe 2 O 3 (0.5 wt%) nanofibers which changed to light brown after crosslinking indicating dissolution of some Fe 2 O 3 nanoparticles.The fiber morphology was  also maintained after crosslinking the nanofibers with heat, with only a few beaded nanofibers observed (figure 7(c)).
The GA crosslinked PVA/ClCNTs (0.2 wt%) mat, still had visible pores even after cross-linking (figure 7(a)).Fused fibers were observed from a GA crosslinked PVA/ClCNTs-Fe 2 O 3 (0.5 wt%) nanofibers mat, whereas beads were observed from the heat crosslinked mat, which could be due to dissolution or agglomeration of Fe 2 O 3 nanoparticles (figures 7(b) and (c)).FTIR spectra was also recorded to see if crosslinking occurred and the reduction of the O-H bands intensity was observed for GA crosslinked mats, which could be the result of formation of acetal bridges, which indicate that crosslinking has occurred (figures 8 and 9).The O-H band reduction was also observed from the heat crosslinked mat, which shows that excess water was removed from the mat by heating (figure 9(b)).The reduction also indicates that the hydroxyl groups reacted with the aldehyde groups to form acetyl functional groups GA was used as a crosslinking agent [63].

Absorption studies for removal of diesel oil
The oil sorption capacities of fabricated electrospun nanocomposite mats for diesel oil are presented in figure 10(a).The oil sorption capacity of 7.6 g g −1 was obtained when an un-crosslinked PVA/ClCNTs-Fe 2 O 3 (0.5 wt%) mat was used as an absorbent.After crosslinking with heat and glutaraldehyde the sorption capacities  increased to ∼ 7.9 g g −1 and 9.4 g g −1 , respectively.It was hypothesized that the increase in sorption capacity emanated from the increase in hydrophobicity of the mats because of reduced number of hydroxyl groups that resulted from crosslinking.Whereas the slight decrease in oil sorption capacity for the un-crosslinked mat could be a result of the presence of Fe 2 O 3 nanoparticles on the surface of the ClCNTs masking the hydrophobic carbon structure, thereby reducing the hydrophobicity of the mat.In the heat driven cross-linking, etherification of the hydroxyl groups occurred, whereas in the glutaraldehyde driven cross-linking acetalization of these hydroxyl groups occurred.Comparison with the PVA/ClCNTs GA crosslinked mat, the absorption capacity exceeded all the absorption capacities obtained with PVA/ClCNTs-Fe 2 O 3 mats (figure 10(a)).Oil sorption capacity improved up to 9.7 g g −1 when using pure 0.2 wt% ClCNTs were embedded into PVA, and it was postulated that the increase was due to the hydrophobic nature of ClCNTs [64].A similar trend was also observed with percent removal of diesel oil, whereby PVA/ClCNTs exhibited the highest percent removal of 55.4% compared to 39% obtained using PVA/ClCNTs-Fe 2 O 3 before and after crosslinking with heat (47%) and GA (52%), respectively (figure 10(b)).Perhaps the non-dissolution of Fe 2 O 3 nanoparticles during crosslinking with GA can be attributed to the fact that Fe 2 O 3 nanoparticles formed a stable composite with ClCNTs.It should be noted that the addition of Fe 2 O 3 is still beneficial since they can improve the strength of the nanocomposite.
The absorption behavior was described by plotting the absorption capacity versus the square root of time (figure 11).The plot revealed well-defined initial linear regimes, according to classical time-dependence,  followed by final regimes departing from the linear behavior up to saturation.The steeper linear regime of the absorbent capacities of the PVA/ClCNTs-Fe 2 O 3 (0.5 wt%) mats un-crosslinked, crosslinked and PVA/ClCNTs crosslinked revealed that there were differences in the nanostructure and chemical composition of the porous network.The GA crosslinked PVA/ClCNTs (0.2 wt%) and PVA/ClCNTs-Fe 2 O 3 (0.5 wt%) mats evidently underwent significant structural and chemical modification, allowing them to exhibit superior absorption capacities.The heat crosslinked mat only has a lower (r) square fit because of the data point at 10 min 1/2 .The highest correlation coefficient (r) of almost an order of one was shown in plots obtained using GA crosslinked mats.
The nanocomposite mat prepared in this study was compared with other electrospun nanofibers that were used in other studies towards absorption of oil (table 1).The nanofiber composites prepared in this study (PVA/ ClCNTs and PVA/ClCNTs-Fe 2 O 3 ) compares well with PS/CNT and PS/SiO 2 nanocomposites, although they presented less absorption capacities when compared to the PVDF/Fe 3 O 4 /PS and PVP/MWCNT-Fe 3 O 4 nanocomposites.PVDF is a more hydrophobic polymer, with high chemical resistance, good mechanical strength, excellent thermal stability, and steady performance for long-term application as compared to PVA [65].However, PVA is attractive because it is biodegradable, water soluble and less likely to contribute to secondary pollution.
To evaluate the surface integrity of the nanofiber mats after oil absorption, their SEM images were recorded after the mats had reached saturation (figure 12).The pores on the membranes disappeared and the fibers were no longer visible after absorption, which showed that the mats were completely saturated by oil (figure 12).The findings agree with literature studies by Yifan and co-workers, where it was postulated that a strong chemical bond and interaction exist between the membranes and diesel oil which may prevent them from being reused [69].
Reusability of sorbent plays a crucial role in the field of oil spill applications since reusable sorbents can save costs.The reusability performance was examined using PVA/ClCNTs and PVA/ClCNTs-Fe 2 O 3 nanofiber mats crosslinked with GA, for four sequential cycles in diesel oil (figure 13).The reusability shows that the maximum   absorption decreased in every cycle (dried for 6h), revealing that the sorbents have poor reusability in diesel oil.
In future we will investigate the use of different desorption liquids and increase the drying time for the sorbents.

Conclusion
This work reports an environmentally friendly material that will reduce the amount of oil pollution in water.The material makes use of a biodegradable polyvinyl alcohol polymer as a major part of the absorbent composite.
The work avoids secondary contamination by polymers, since PVA nanocomposites can be easily degraded after use by dissolving it in water or burning the mat and the nanomaterials remaining could be recycled for reuse.Loading of iron oxide nanoparticles in the form of maghemite onto the surface of the chlorinated CNTs was successful and it was done to increase the strength of the resultant material.Crosslinking the resultant nanofibrous mat resulted in higher absorption capacities for oil of 9.4 g g −1 as compared to 7.6 g g −1 obtained using un-crosslinked nanofibrous mat.An even higher sorption capacity of 9.7 g g −1 was obtained when using a PVA/ClCNTs GA crosslinked mat and the increase was attributed to the absence of Fe 2 O 3 which blocked some of the pores.The crosslinked mat was also resistant to dissolution by water since the mat did not fall apart when immersed in an oil-water solution for a period of 60 min.The nanocomposites mats lost capacity towards oil sorption with repeated cycles which shows that they are not reusable.The electrospinning method of nanofiber fabrication and the oil absorbent materials used here are environmentally friendly and cost-effective, hence they can be recommended for further industrial applications in water treatment.

3. 1 .
Characterization of the prepared nanocomposites TEM images of ClCNTs, iron oxide nanoparticles and ClCNTs loaded with iron oxide nanoparticles are presented in figure 2. TEM image in figure 2(a) showed ClCNTs that are entangled with a spaghetti-like morphology.Interestingly, the image also depicted secondary growth of carbon nanofibers emanating at the surface of the main ClCNTs.It is believed that the secondary nanofibers grew at defect sites and were induced by the incorporation of chlorine[46].The average outer diameter of the resultant chlorinated carbon nanotubes was ∼ 21.78 ± 7.72 nm (figure2(a)).For comparison and validation purposes, iron oxides nanoparticles were synthesized separately for use as a benchmark, and their TEM image is shown in figure 2(b).Formation of iron oxide nanoparticles was evident from their uniform spherical shape, and they had an average diameter of 14.89 ± 3.77 nm (figure 2(b)).It was later observed from the TEM image of the iron oxide loaded ClCNTs nanocomposite (figure2(c)) the uniform dispersion of iron oxide nanoparticles on the surfaces of ClCNTs.However, the average diameter of the Fe 2 O 3 had increased to 28.36 ± 11.63 nm when loaded onto the ClCNTs, and this was attributed to the fact that ClCNTs acted as nucleation sites which changed the rate and way of their precipitation.It is believed that the observed distribution and localization of iron oxide nanoparticles at the walls of ClCNTs was influenced also by their prior chemical modification with chlorine which resulted in creation of functional groups and attachment sites (defects) at the surface of the ClCNTs.

Figure 1 .
Figure 1.Schematic diagram of the nanofiber fabrication via electrospinning and absorption experiments.

Figure 10 .
Figure 10.(a) Oil sorption capacity and (b) percent removal measurements of different fabricated electrospun mats to diesel oil.

Figure 11 .
Figure 11.Absorbent capacity for different electrospun mats during oil intake versus square root of time.Lines correspond to linear fit parts of each graph.

Table 1 .
Comparison of sorbent capacities of polymer nanocomposites towards oil obtained in literature to the one in the current study.