Materials
N6,6 from Sigma-Aldrich and DMF from Sigma-Aldrich were purchased and all other chemicals (NaOH, HCl, formic acid, chloroform) were purchased from Merck Company. We produced GO in our laboratory. Figure 1 shows the structure of N6,6, graphene, and GO. Ultrapure water was used for preparation of Cr (VI) stock solution and other solutions. The stock Cr (VI) solution was originated by dissolving K2Cr2O7 salt (purity: ≥ 99.9%) (Merck) in pure water. For pH adjustment of solution phase, 0.1 M NaOH and HCl solutions were added in the prepared Cr (VI) solutions.
N6,6/GO electrospun nanocomposite production
Glassman High Voltage Inc. brand power supply unit and AC/DC current power supply (in Fig. 2) were used to obtain 1–50 kV and 0.1–100 mA. Universal 801 syringe pump was used to receive a fixed quantity of solution from the syringe tip. Electrospinning polymer is a technique for long and continuous nanofiber production. The high voltage power supply consists of two main sections; electrospinning and a syringe pump. After preparing the polymer solution in a suitable container, it was placed inside a syringe or capillary tubing and then connected to a high voltage power supply. An electrical field was created between the grounded collector (anode) and the tip of the syringe (negative pole). The high voltage power supply is critical to provide the force of electrospinning and continuity of spinning. A syringe pump (feeders) is employed to send the polymer solution in the syringe at a selected flow rate of the polymer.
Polymer drops at feeder unit hanging at the tip of the syringe were in a spherical form up to a critical voltage, due to the surface tension force applied. The applied potential difference (high electric field) reaches a threshold worth and also the surface electrostatic forces are synchronic to tension forces. Polymer drops changes to cone shapes at this time. This cone is named Taylor cone. A jet shoots through the tip of Taylor cone and follows totally different paths once it moves between the syringe needle and the collector. On the way, the solvent in the polymer solution evaporates in the air. As the polymer jets get closer to the collector, new polymer jets in nano-diameter forms and they were collected on the grounded conductive collector (Shin et al. 2001).
In this study, the solution preparation method has been started initially by solving of N6,6. As seen in Fig. 3, N6,6 solution was prepared by adding formic acid to 3 g pellets form of N6,6 and the solution was stirred continuously by a magnetic stirrer till they were mixed fully. At that point, the graphene nanoparticles (GNP) (5 wt%) have been included into 30 mL of formic acid mixed with 9 mL chloroform (Fig. 3a). We added chloroform to the formic acid solution to homogenize the solution before adding N6,6 (Fig. 3b). In that way, the particles can distribute homogeneously throughout the solution. After 10 min of mixing this solution with an ultrasonic blender, N6,6 was added and to the solution then the solution was mixed with a probe appended sonicator (50 kHz frequency) for 1 h. Heat is evolved because of working of the sonicator with sound waves. This heat causes the temperature of the solution to increase. As shown in Fig. 3c, an ice bath was placed at the bottom of the solution vessel to balance the temperature. This prepared solution was taken in 5 mL syringe to obtain nanofiber. In the experimental set-up, a supply pump was adjusted to 1 mL/s speed and the syringe tip is set to be a 12 cm distance from the drum. The high voltage power supply can set to a voltage value of 22 kV to start the nanofiber production. The nanofiber production was carried out at 25 °C in the laboratory.
FT-IR studies
The FT-IR is an important analysis for the evaluation of functional groups. The FT-IR spectra of the material were recorded and given in Fig. 4. FT-IR proves the chemical structure of GO showing the metal binding chemical groups such as O-H bonds at 3310 cm−1, the stretching vibrations of -COOH bonds at 1715 cm−1, stretching vibration of C=C bonds at 1630 cm−1 (Liu et al. 2018), and -C-O stretching vibration at 1250 cm−1 (He et al. 2015). FT-IR indicated the chemical structure of the nylon 6,6, with the certain chemical groups such as N–H stretching peaks at 3299 cm−1, C–H stretching at 2861–2933 cm−1, amide-I at 1637 cm−1, amide-II at 1536 cm−1 (Haggenmueller et al. 2006), and amide-III at 1370 cm−1, O=C–H at 581 cm−1, C–C at 687 cm−1 (Charles et al. 2009). FT-IR confirmed the chemical groups related to the N6,6/GO nanocomposite such as the bands at 1638 cm−1 are assigned to C=C skeletal stretching. The broad peak at 2954 cm−1 shows the stretching vibrations of -OH bonds. The peaks at 1371 cm−1 reflect the carboxylic acid groups due to the O–H in-plane deformation and C=O stretching vibration (Shin et al. 2001; Chen et al. 2010). The O=C–H broad peak and C–C stretching peaks appeared at 592 cm−1 and 689 cm−1, respectively.
SEM-EDX analysis
SEM is normally used to characterize an adsorbent by giving some knowledge about its surface topology as well as morphology and was used for determining the nanofiber size and shape. The morphology of the electrospun N6,6/GO nanofibers were randomly oriented in the SEM image (Fig. 5a). The elemental mapping of N6,6/GO nanofibers by EDX showed that carbon, nitrogen, and oxygen are the main elements of the nanofibers (Fig. 5b–d). The analysis displayed the existence of carbon (C), oxygen (O), and nitrogen (N) together with the wt% concentrations of 51.62, 26.18, and 22.18, respectively in EDX spectrum of N6,6/GO nanofibers (Fig. 5e).
The morphology of the electrospun Cr (VI) loaded N6,6/GO nanofibers in the SEM image (Fig. 6a) and the typical EDX spectrum of Cr (VI) loaded N6,6/GO nanofibers. The elemental mapping of N6,6/GO nanofibers by EDX showed that carbon and oxygen are the main elements of the nanocomposite. In addition, Fig. 6b–f shows the elemental mapping image regarding Cr (VI) loaded N6,6/GO nanofibers including an even distribution of C, O, N, and Cr, which further validated the successful adsorption of Cr on the surface of N6,6/GO. The elemental analysis displayed the existence of carbon (C), oxygen (O), chromium (Cr), and nitrogen (N) with the wt% concentrations 35.13, 38.69, 7.44, and 18.73 respectively (Fig. 6g).
Adsorption of Cr (VI) experiments
The Cr (VI)-adsorbent interaction was carried out as follows: the adsorption kinetics at a certain temperature was tried by adding of N6,6/GO at a dose of 0.2 g/L to a series of 10 ppm Cr (VI) solution at an initial pH of 2. The beakers were moved to the shaker (250 rpm) for different times (5 min, 15 min, 30 min, 1 h, 2 h, 4 h, 6 h, 9 h, 12 h, 18 h, and 24 h) and were maintained at 25 °C. The effect of different adsorbent dose on the adsorption of Cr (VI) from the solution phase was studied by shaking 10 ppm Cr (VI) solution with the N6,6/GO (Fig. 7).
N6,6/GO dosages in the solution phase were 0.05, 0.1, 0.2, 0.3, and 0.4 g/L. The different initial concentrations of Cr (VI) (2.5, 5, 7.5, 10, 12.5, 15, and 20 ppm) were studied and initial pH of the solution was adjusted to 2. The effect of initial pH on the adsorption was studied by adding 0.2 g/L N6,6/GO to a series of beakers containing 10 ppm Cr (VI) solution and initial pH was adjusted by adding 0.1 M NaOH and 0.1 M HCl solution from 2 to 6. The beakers were shaken (250 rpm) at room temperature for 5 h. After centrifugation, the residual Cr (VI) concentration in the supernatant liquid was analyzed spectrophotometrically at 540 nm using UV-visible spectrophotometer (ELICO-156) and 1,5 diphenylcarbazide (purity ≥ 98, Merck) as a complexing agent. Diphenylcarbazide reacts with Cr (VI) ions in acidic medium to form a purple Cr (III)-diphenylcarbazone complex (Altun et al. 2016). The maximum capacities of the synthesized N6,6, N6,6-G, N6,6/GO (3%), N6,6/GO (5%), and Cr (VI) ions were given in Fig. 8. N6,6/G has the lowest Cr (VI) adsorption capacity. In case of using GO instead of graphene in the composite, the situation changed. That means as GO ratio increased, Cr (VI) adsorption capacity increased. For that reason, N6,6/GO (5%) was preferred as the adsorbent in the rest of the experiments.