Green Synthesis of S- and N-Codoped Carbon Nanospheres and Application as Adsorbent of Pb (II) from Aqueous Solution

In this paper, green and facile synthesis of sulfurand nitrogen-codoped carbon nanospheres (CNs) was prepared from the extract ofHibiscus sabdariffa L by a direct hydrothermal method. Finally, sulfur-carbon nanospheres (CNs) were used as the adsorbent to remove Pb ions from aqueous solutions because of the high surface area of S-CNs from CNs and N-CNs. .e synthesized nanospheres were examined by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy, transmission electron microscopy (TEM), and nitrogen adsorption-desorption isotherms. .e results show spherical shapes have a particle size of up to 65 nmwith a high surface area capable of absorbing lead ions efficiently. Additionally, the factors affecting the process of adsorption that include equilibrium time, temperature, pH solution, ionic intensity, and adsorbent dose were studied. .e equilibrium removal efficiency was studied employing Langmuir, Freundlich, and Temkin isotherm forms. .e kinetic data were analyzed with two different kinetic models, and both apply to the adsorption process depending on the values of correlation coefficients. .e thermodynamic parameters including Gibbs free energy (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°) were calculated for the adsorption process.


Introduction
After discovering various forms of carbon, for instance, fullerenes and carbon nanotubes, researchers went on to use the carbon material in a broader form [1]. Including these nanodesigned carbon substances, the carbon nanospheres have drawn large considerations owing to their various uses in catalyst supports, anodes for lithium-ion batteries, electrodes for supercapacitors, lubricants, polymer, and rubber additives, which result from their exceptional properties, for instance, superior chemical stability, thermal insulation, low density, and high compressive strength [2][3][4][5][6][7][8]. ere are many different methods to synthesis the carbon nanospheres involving arc plasma technique [9], catalyzed pyrolysis [10], self-generated template manner [11], hydrothermal reaction [12], spray pyrolysis [13] and chemical vapor deposition (CVD) [14], etc. Currently, friendly environment synthesis technique has been advanced including transformation of easily obtainable precursors, for example, glucose [15] and cyclodextrins to carbon nanospheres beneath moderate conditions (hydrothermal reaction at 160-180°C) [16]. is preparation method is clear as it does not use organic solvents and poisonous reagents. e surface of the resulting CNs contains rich hydrophilic functional groups, for example, C-OH, C-O, and C-OOH. ere has been just a little research on an investigation of CNs in water remediation due to the presence of oxygen functional groups [17]. e carbon nanospheres could present good adsorption efficiency towards heavy metals. For activated carbon and different kinds of carbon nanomaterials, there has been exceedingly preparation to increase that the adsorption capacities for the sake of heavy metals can be reinforced by chemical remediation with acids and bases [18,19]. is hydrothermal synthesis method has numerous benefits, for instance, modest temperatures, effective produces, and modest temperatures, good produce, and ultimate importantly a friendly environmentally operation, a "green" technique. Nevertheless, the study of the development procedure of CNs manufactured by hydrothermal methods is comparatively elusive. e major problem in conception of this mechanism is that the reaction takes place in a closed inaccessible vessel, and some experiments have been made to study the hydrothermal reaction in situ using Raman spectroscopy, but further performance is necessary to gain a conception of the growth process [20]. Several methods have been used for the removal of heavy metals from wastewater, involving physical, chemical, and biological methods. Nevertheless, the greatest, suitable, and effective procedure for the removal of heavy metals from aqueous environment is the adsorption process [21].
In the current work, the carbon spheres, sulfur-and nitrogen-doped carbon nanospheres, were prepared by hydrothermal technique and from the green material Hibiscus sabdariffa L extract. And finally we used sulfur-carbon as the surface to remove Pd +2 ion from the contaminated water. Batch studies were performed by varying the contact time, temperature, pH solution, and ionic intensity, where the size of the samples prepared and the morphology were identified using the characterization techniques mentioned.

Experimental Section
2.1. Chemicals. Sulphuric acid, sodium hydroxide, and HCl, which were bought from Scharlau, were used directly without primary purification. Each solution was prepared utilizing deionized water.

Characterization.
e crystalline property of the samples can be identified using X-ray technology with a wavelength of 1.54056Å in the range of from 5°to 80°and with a scanning velocity of up to 0.03 s −1 . e functional groups of the compounds are shown as bands by FT-IR analysis in the range 4000-400 cm −1 with KBr tablets as substance holders.
e Brunauer-Emmett-Teller (BET) surface areas were measured from the N 2 adsorption using a Quadrasorb SIMP. e outer shape of the nanoparticles prepared can be identified by the field emission scanning electron microscopy and quickening voltage up to 8.0 kV, as well as transmission electron microscopy and quickening voltage up to 200 kV. ermal gravimetric analysis (TGA) and a high-temperature thermal analyzer were used to identify the thermal stability of prepared materials with a heating speed of up to 10°C·min −1 in range (50-600)°C using Arcon gas. e mechanical properties and the threedimensional shape of the sample surface were obtained from atomic force microscopy (AFM) of the Park Systems XE-70.

Preparation of
Hibiscus sabdariffa L Extract. 10 g of plant sepal powder was added to 50 ml of deionized water with heating and stirring on a magnetic stirrer hot plate for 30 min till the temperature of boiling was achieved. en, the mixture was filtered and the filtrate was cooled to room temperature. Furthermore, the mixture was diluted to 100 ml with deionized water to achieve 10 % extract solution.

Synthesis of the Carbon Nanospheres, Sulfur-and
Nitrogen-Doped Carbon Nanospheres Using Hibiscus sabdariffa L Extract. e carbon spheres, sulfur-and nitrogen-doped carbon nanospheres, were synthesized via the one-pot hydrothermal method. CNs were synthesized using Hibiscus sabdariffa L extract as a carbon source. In an ideal experiment, 50 mL of extract was put in a 100 mL teflon-lined stainless steel autoclave. e autoclave was closed cautiously and heated to 180°C for 5 hours in an oven. rough this method, a liquid different from red to sable black was obtained, which confirmed the formation of CNs. en, the liquid was spontaneously cooled to room temperature and centrifuged at 10000 rpm for 15 min to eliminate the greater or conglomerate atoms. e suspension was collected and dried employing an oven for abstraction of water. e produced solid substance was saved for the other purpose. Pure sulfurand nitrogen-doped carbon nanospheres were obtained via a similar process, but 2 mL of NH 3(aq) solution was added in extract to prepare N-CNs and 1 mL of concentrated sulfuric acid (98%) was added in extract to prepare S-CNs [22,23] Table 1. +2 . Prepare a set of lead ion solutions in the range (1-20 ppm) to the prepared three surfaces (0.2 g of S-and N-co-doped carbon nanospheres) to determine the best surface giving high adsorption efficiency. All solutions are stirred using a shaker water bath for 60 min at a speed of up to 150 rpm. After the adsorption reaction is finished, separate all the solutions using the centrifuge for 10 min with a speed of up to 3000 rpm. en, measure the absorbance (A) of all solutions using an atomic absorption spectrophotometer. Finally, calculate the amount of material absorbed (q e ) by the following equation [24]:

e Study of Adsorption of Pd
where q e (mg/g) is the quantity of absorbent material, m (g) is the weight of surface, V (L) is the volume of mixture, C o (mg/L) is the original concentration, and C e (mg/L) is the equilibrium concentration, while the other factors affecting the adsorption process are studied in the same way, for instance, pH, ionic intensity, time, and temperature.

FTIR Analysis.
e functional groups of the prepared nanospheres were detected by Fourier transform infrared (FTIR). Figure 1 shows the FTIR spectrum of carbon nanospheres and its derivatives. e hydroxyl peak of CNs shows in the stretch (3300-3540 cm −1 ). e 2 International Journal of Chemical Engineering carbonyl peak appears in the stretch (1710 cm −1 ), and the C�C peak appears in the stretch (1648 cm −1 ). Finally, two peaks at 1190 and 1036 cm −1 attributed to the stretch to C-O and C-O-C bonds, respectively [25]. e FT-IR spectrum of N-CNs shows distinctive absorption peaks of hydroxyl and amine appear stretch at 3257 cm −1 and peak at 2919 cm −1 assigned to C-H group. e bands appearing at 1299, 1105, and 1027 cm −1 demonstrate carboxyl groups [25,26]. e FT-IR analysis of S-CNs exhibits the band on stretch at 1038 cm −1 is attributed to C-O, S-O, and C-O-C bonds, while the C-O and C-S bonds appear on stretch at 1194 cm −1 and COO − groups appear between 1396-1496 cm −1 .

XRD Analysis.
e phase structure and degree of crystallinity of prepared nanospheres were determined by XRD. Figure 2 shows the XRD pattern of the CNs, N-CNs, and S-CNs. e pattern of CNs shows an intense XRD peak broad in range (2θ = 19-21°), which applies on d-spacing of (4.50, 4.34, and 4.08) A°, while the XRD pattern of S-CNs appear a wide diffraction band at 23.6°that applies on d-spacing of 5.48 A°, where the presence of functional groups on the surface of S-CNs makes it larger interlayer spacing than of CNs. Also, the presence of ammonia groups on the N-CNs surface makes it a more sharp peak than CNs [23,29].

FE-SEM Images of Nanospheres.
e surface morphology of the carbon nanospheres was noticed employing FE-SEM analysis. Figure 3 shows balls of different sizes with similar morphology and are composed of blocks with a uniform homogeneity and good connectivity between the balls. e sphere sizes of the CNs, N-CNs, and S-CNs are almost 2 μm [28].

TEM Images of Nanospheres.
TEM technique provides information about the external surface of prepared nanomaterials. e technique works by passing a beam of electrons during the material to give the picture of FE-SEM [30]. Figure 4 shows TEM image of CNs, N-CNs and S-CNs multiple conglomerates of the carbon spheres are abundant and, in some cases, chains of beads-like accretions of the carbon spheres. No obvious spherical assemblies are found in the TEM image of the S-CNs, which is indicative of their amorphous nature [23].   Acid red 57 (AR) and basic fuchsin (BF) adsorption [40] International Journal of Chemical Engineering  e specific surface area is estimated by using the Brunauer-Emmett-Teller (BET) manner, and the pore size distributions are found by means of the Barrett-Joyner-Halenda (BJH) method equation using the adsorption isotherm model, which are shown in Figure 5.
e surface area (BET) and the pore diameter of carbon nanospheres (CNs) were obtained to be 122.2 m 2 /g and 22.55 nm, respectively. e value of the pore diameter suggested that CNs were a mesoporous material. e isotherm displays a typical type-III shaped curve and no hysteresis loop, while the N-CNs were found to be 179.1 m 2 /g and 8.78 nm, respectively. e isotherm exhibits a typical type-IV shaped curve and a H2 present hysteresis loop, pointing out the presence of slit-shaped pores between the parallel layers of nanospheres. Finally, the S-CNs have the surface area up to 196.3 m 2 /g and pore diameter 12.41 nm. Figure 5 exhibits a typical type-V shaped curve and a H1 present hysteresis loop, and this isotherm is convex to the relative pressure axis and is the characteristic of weak adsorbate-adsorbent interactions. Hysteresis loop H1 refers to porous materials which consist of well-aligned spheres and briquettes, and materials of this type tend to have relatively narrow distributions of pore size [21,31].
3.1.6. TGA Analysis. TGA is a technique that gives information on the thermal stability of the prepared carbon nanospheres and is exhibited in Figure 6, when 100°C weight loss of 4% is due to the subtraction of water molecules, in the range of 100°C to 200°C weight loss of 6%. Finally, when the temperature increases to 435°C, the weight loss of 90% is owing to the gradual removal of functional groups on the surface of CNs, while the TGA curve of N-CNs shows two stages of weight loss: first stage of weight loss of 70.2% in range from 100°C to 350°C as a result of removal of water from the sample with oxidation of the functional groups and their transformation into volatile gases, while the second stage weight loss is 28.9% in the range from 350°C to 593.1°C due to thermal decomposition of the remaining groups with carbon oxidation. Finally, in the TGA curve of S-CNs, all weight losses become less compared with N-CNs. e mass loss was 58.7% at 200-380°C, owing to the elimination of water molecules, sulfur, and oxygen functional groups, while mass loss was about 40.8% in the range 380-600°C, owing to thermal decomposition of the functional groups of the S-CNs. A TGA analysis of the above samples shows that the thermal stability of S-CNs is higher than those of N-CNs [32].

AEM Images of Nanospheres. AFM is a technique that
gives information on the thickness and morphology of the prepared carbon nanospheres. e AFM image in Figure 7 clearly indicates the presence of nanospheres. e average diameter of carbon nanospheres is 4 nm that due to the presence of oxygen-containing groups that increase the diameter of the substance. e average diameter of N-CNs is up to 2.39 nm (Table 2) owing to the removal of the remaining oxygen groups. Finally, the AFM image of the S-CN sample shows heterogeneous aggregated balls with an average diameter of up to 2.87 nm.

Effect of Adsorbent
Dosage. An adsorption experiment of Pb (II) removal is performed using different weights (0.005-0.25 g) of the S-CNs. It was noticed that the lead ion removal percentage increased with increasing the weight of the nanospheres. is can be attributed to the greater surface area of S-CNs and the increase in the available active sites for metal ion adsorption. Consequently, the uptake of metal ion reaches a saturation level at 0.2 g nanospheres as the active sites present on the surface of S-CNs become saturated with Pb (II). For subsequent experiments, 0.2 g of sorbent dose was selected (Figure 8).

Effect of Contact Time and Kinetics of Adsorption.
Lead ion adsorption has been studied at different times after (1-120 min). In Pb (II), adsorption capacity was increased with increasing the time to its maximum value (saturation   International Journal of Chemical Engineering state), but sometimes the adsorption capacity decreases with increasing time due to a desorption process (Figure 9). In order to investigate the mechanism of adsorption and potential rate controlling steps, two kinetic models, namely, pseudo-first-order and pseudo-second-order equation models were analyzed. e pseudo-first-order equation is expressed as follows: log q e − q t � log q e − k 1 2.303 t.
(2) e equation above shows that k 1 is the first-order velocity constant (1/min), where the drawing log (q e − q t ) with time gives a straight line. rough this, we obtain the amount of absorbent material and the first-order velocity constant of the intercept and slope, respectively, while the equation below describes the pseudo-second-order model: where drawing t/q t with time gives a linear relationship, from which we obtain the amount of the absorbent material and the second-order velocity constant (g/mg min) from the intercept and slope (Figure 10). e kinetic parameters of all models and the correlation coefficients are shown in Table 3. e results show that the lead ion adsorption process on the surface of S-CNs is applied to the pseudo-second-order model because the value of R 2 is equal to 1. e high R 2 value is obtained for the pseudo-first-order model, indicating that the experimental data are well described by the pseudo-first-order model.

Influence of the Acidic Function (pH).
e most important factor affecting lead ion adsorption is the acidic function due to the change in surface charge of the adsorbent material. e influence of the solution pH on Pb (II) removal by the S-CNs is displayed in Figure 11, where we notice that the rate of removal of Pb (II) on the surface of S-CNs increases with the increase of acid function. Because the lead ion has a positive charge at pH � 2, it decreases adsorption efficiency of the small hydrogen ions which have the ability to cover the lead ions as they are associated with the surface of S-CNs. However, increasing the value of pH � 12 increases the negative charges and increases the surface ability to absorb the lead ion very efficiently.   ree classic adsorption models, Langmuir, Freundlich, and Temkin, were used to describe the adsorption equilibrium. e mathematical representations of the Langmuir, Freundlich, and Temkin models are given below: e equations above show that the adsorption constants for all models are K L , K F , and K T , q m represents the maximum adsorption capacity (mg g −1 ), and n represents the linear factor. e Langmuir model describes multilayer adsorption and homogenous and is a good model, while the Freundlich model is monolayer adsorption and is the most widely used model. Finally, the Temkin model describes adsorption as a strong interference between the surface of the adsorbent material and the absorbed cations [33]. Figure 12 shows the adsorption results for all the models used and the isotherm constants shown in Table 4, where we observe that the adsorption process of the lead ion on the surface of S-CNs applies to the Langmuir model of the value R 2 = 0.993. While adsorption does not apply to the Freundlich and Temkin models because the values of the correlation coefficients are slightly. is indicates that adsorption is multilayered.

Effect of Temperature on Adsorption Process.
Temperature is one of the factors that greatly affect the adsorption capacity. e temperature is studied in the range (20-50)°C. Figure 13 shows that the adsorption capacity increases gradually with increasing temperature to the high adsorption efficiency at 50°C. Observe that the       [34]. e equations below show the use of thermodynamic parameters to identify the spontaneous reaction and determine the reaction of exothermic or endothermic and random system.
where ∆G°represents the Gibbs energy, ∆H°represents the enthalpy change, ∆S°represents the change in entropy, T is the absolute temperature (K), and R is the general constant of gases (8.314 J/mol·K). By drawing ln K with inverted time (T −1 ), we obtain values (ΔH°and ΔS°) from the slope and intercept, respectively. Table 5 shows that the value of the Gibbs energy for each temperature is negative, and this indicates that the adsorption process is spontaneous reaction. Also, the exothermic reaction depends on the negative value ΔH°. Finally, the high positive value of the change in entropy takes place in the highly randomized system [35].

Effect of Ionic Strength.
Ionic intensity is one of the factors influencing on the adsorption process of Pb (II) on the surface of S-CNs. Different weights from salt are used in the range 0.001-0.03 g to know the effect of salt weight on the efficiency of pollutant removal. Figure 14 shows increase in salt weight with the decrease in the adsorption capacity. In previous research, low adsorption capacity with increased salt weight due to the ability of sodium ions to block lead ions for active sites on the surface of S-CNs [36].

Literature
A list of some previous work prepared of CNs is given in Table 1.

Conclusions
Carbon nanospheres (CNs) were prepared using a natural extract, and then sodium hydroxide (NaOH) solution and sulfuric acid were added to the preparation of N-CNs and S-CNs, respectively. e extract adds functional groups to the carbon nanosphere surface to increase the adsorption capacity of the Pd (II) from aqueous solution. e resulting CNs showed homogenous nanospheres with an interlayer distance of 4.5 nm, as shown by TEM. e lead ion adsorption process based on the thermodynamic study is spontaneous and exothermic, while kinetic studies show that adsorption process follows both models. It can be expected that the carbon nanospheres (CNs) are additionally appropriate for numerous other uses, for example, drug delivery, sensors, heterogeneous catalysis, and manufacture of functional polymer composites.

Data Availability
All data are enclosed within the manuscript.

Conflicts of Interest
e authors declare that they have no conflicts of interest.