The effective removal of Pb2+ by activated carbon fibers modified by l-cysteine: exploration of kinetics, thermodynamics and mechanism

Herein, we developed a low-cost fabrication route to prepare chemically grafted activated carbon fibers, which effectively removed Pb2+ from solution. Multiple characteristic results indicated that l-cyst-ACF had abundant nitrogen-containing and sulfur-containing functional groups. Based on the XPS and EDS analyses, the capture of Pb2+ was attributed to the abundant adsorption sites on the fiber surface. According to the analysis of the pseudo-second-order kinetic model and the Langmuir isotherm model, the adsorption process could be interpreted as monolayer adsorption and chemisorption, and the equilibrium adsorption capacity was determined to be 136.80 mg g−1 by fitting the pseudo-second-order kinetic model. The maximum adsorption capacity of l-cyst-ACF for Pb2+ was calculated to be 179.53 mg g−1 using the Langmuir model. In addition, the adsorption reaction was endothermic and spontaneous, as evidenced by the thermodynamic parameters. The outcomes of this study provide a low-cost and feasible strategy for the remediation of Pb2+ pollution in the environment.


Introduction
The rapid development of modern industry has produced a large amount of wastewater polluted with heavy metals, which has caused great ecological and environmental harm, and nally poses a threat to human health through the food chain. 1,2 Heavy metal ions are non-biodegradable carcinogenic factors and tend to accumulate in organisms. 3,4 Heavy metals can enter the food chain through plants and pose a serious threat to human health. 5 Common heavy metal ions can cause acute and chronic diseases such as digestive inammation, brain injury and cancers. 6 Therefore, the removal of heavy metal ions from industrial wastewater has become a major pro-environmental problem in recent years.
Various technologies to remove heavy metals from water are available, such as solvent extraction, 7 ion exchange, 8 precipitation 9,10 and adsorption. 11 Among them, adsorption has the advantages of simple operation, high efficiency, and strong exibility. Many materials have been used to remove heavy metals such as activated carbon, 12,13 polymers, 14,15 and nanomaterials. 16 However, the practical application of adsorbents is hindered by their low adsorption efficiency for the target metal, the inconvenient process for their synthesis of and their unachievable recycling. Therefore, the development of advanced adsorbents with high adsorption capacity, recyclability and good chemical stability has become a vital research direction in the eld of heavy metal remediation.
Activated carbon bers (ACFs) are a new type of adsorbent developed in recent years to remove pollutants in aqueous solutions. Compared with the traditional activated carbon particles, activated carbon bers have more suitable pore sizes, better adsorption effect and easy recycling. [17][18][19] Javier 20 et al. measured the effect of activated carbon bers to adsorb both Pb 2+ -phenol and Cd 2+ -phenol simultaneously, proving that the selectivity of activated carbon bers for Pb 2+ ions was greater than Cd 2+ ions. Deng 18 et al. found that a novel thiosemicarbazide-modied adsorbent exhibited a high performance towards Cd(II) and Pb (II). This result explained that sulfur-containing groups and nitrogen-containing groups could effectively adsorb metal ions in solution by chelation and coordination.
In the present study, activated carbon ber adsorbents graed with multiple functional groups with high adsorption properties were prepared via a simple amide reaction. The preparation bers were characterization through multiple technologies such as Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Meanwhile, the behavior and mechanism of Pb 2+ adsorption were comprehensively explored via adsorption kinetics, isotherm models, and thermodynamic parameters. Then, the adsorption performance of L-cyst-ACF for Pb 2+ was systematically discussed, which suggested that the adsorption process was spontaneous and endothermic.

Preparation of L-Cysteine-modied activated carbon ber
Activated carbon bers were washed with deionized water several times and dried for 12 h at 80 C until a constant weight. Then, the clean bers were cut into small pieces and soaked in a HNO 3 /H 2 SO 4 (3 : 1, v : v) solution for acidication. The bers were washed several times and dried overnight. ACFs were put in an appropriate amount of KH 2 PO 4 /Na 2 HPO 4 buffer solution with a pH of 8.5. EDC (1.8 g) and NHS (1.2 g) were added to the solution with constant stirring at room temperature, and then thiourea (1.52 g) was added. Aer 6 h, ACFs were washed and dried.
Firstly, 1.38 g of sodium bisulfate (NaHSO 4 ) was added to 100 mL of 0.1 m L À1 dilute hydrochloric acid solution, and then 80 mL of ethanol was added. Subsequently, ACF and L-cysteine (1.7563 g) were added to the above mixture, and the system was heated at reux at 95 C for 6 h. The product was washed with ethanol and deionized water several times, and then dried at 80 C in vacuo for 24 h. To reduce the oxidized sulydryl group (-SH), ACFs were added to an organic solvent (dichloromethane:ethyl alcohol, v : v ¼ 1 : 2), Na 2 S was added to the mixture as a reducing agent, and then it was stirred at room temperature for a certain of time. Finally, ACFs were ltered and washed several times, and then stored aer drying. The specic experimental steps are shown in Fig. 1.

Characterization of L-cyst-ACF
The properties of L-cyst-ACF were characterized via a series of performance tests. Fourier-transform infrared spectra (FTIR) were recorded in pressed KBr using a Vertex 70 spectrophotometer (Bruker) in the range of 500-4000 cm À1 at room temperature. The morphological characteristics of L-cyst-ACF were observed via SEM (Hitachi, SU8010). The elemental components of L-cyst-ACF before and aer adsorption were identied by X-ray photoelectron spectroscopy (XPS, Thermo Fisher: ESCALAB 250Xi, single Al Ka as the light excitation source). An Oxford X-MaxN scanning electron microscope (SEM)/energy dispersive X-ray spectroscopy (EDS) was used to observe the surface morphology and element distribution of the ber, respectively. Inductively coupled plasma atomic emission spectrometry (ICP-MS Agilent: 7800, China) was used to detect the Pb(II) concentration.

Batch adsorption experiments
A sequence of batch adsorption experiments was conducted to investigate the performance of the synthesized adsorbents for the elimination of Pb 2+ from aqueous solution. The adsorption experiments were performed in an SHA-B water bath oscillator (RongHua Instrument Manufacturing Co., Ltd, China) at a shaking speed of 120 rpm. Batch experiments were performed to investigate the kinetics, isothermal models, and thermodynamic parameters during the adsorption process. In this investigation, the initial metal ion concentration range was 0-500 mg L À1 , adsorbent dose range was from 50 to 150 mg and the system temperatures were 303 K, 313 K and 323 K, respectively. The pH range of 2-6 was adopted in the experiment to investigate the inuence of pH. All batch experimental solution concentrations were detected through inductively coupled plasma atomic emission spectrometry. Before the solution concentration test, the test liquids were ltered with a 0.22 mm lter to remove impurities.
The adsorption amount q e (mg g À1 ) of Pb 2+ in adsorption equilibrium was calculated using eqn (1): The removal efficiency of Pb 2+ at equilibrium was calculated using eqn (2): where C 0 and C e are the initial concentration and equilibrium concentration (mg L À1 ), respectively. V (L) is the volume of the solution and M (g) is the weight of the absorbent.

Characterization of ACF and L-cyst-ACF
The FT-IR spectra of ACF and L-cyst-ACF are shown in Fig. 2. In the spectrum of the activated carbon bers, the band at 3442 cm À1 is ascribed to the -OH stretching vibration 21 and that at 1654 cm À1 is ascribed to the C]O stretching. 22 The peaks at 1541 cm À1 and 1388 cm À1 correspond to the C-C and C-O bonds, respectively. 23 Similarly, the peak at 3446 cm À1 in the spectrum of L-cyst-ACF represents the -OH stretching vibration, and the two peaks at 1527 cm À1 and 1386 cm À1 can be ascribed to the C-C and C-O groups, respectively. 24 The peaks at 2813 cm À1 and 2933 cm À1 are attributed to the stretching vibrations of -CH 2 and -CH 3 , respectively. The sharp peak at 3471 cm À1 and the broad band at around 3100-3300 cm À1 are attributed to the -NH 2 vibration. 25 Obviously, the sharp peak at 1655 cm À1 can be assigned to the carbonyl stretch of the -NHCOgroup, not just the carbonyl group (C]O). 24 Meanwhile, a weak peak located at 2401 cm À1 was observed, which can be attributed to the -SH bending vibration. 18,26 The small peak at 1267 cm À1 is ascribed to the C]S bond. 12 The band at 1000 cm À1 may be caused by the C-N stretching. 27 In addition, the peaks at 740 cm À1 and 632 cm À1 correspond to the bending vibration of -NH 2 and vibrational C-S bonds, respectively. 28

Characterization of ACF-SH
The SEM images of ACF and L-cyst-ACF are shown in Fig. 3. The SEM image of ACF (Fig. 3a) displays a regular smooth surface; meanwhile, that of L-cyst-ACF (Fig. 3b) shows an irregular rough surface aer modication, which illustrates that organofunctional groups were successfully graed. Alternatively, a larger specic surface area is also conducive to an improvement in adsorption capability. The changes on the surface of the modied ACF aer adsorption are showed in Fig. 3(c and d).
It can be observed that some irregular tiny particles covered the surface of the bers aer adsorption, demonstrating the further improved effective adsorption ability of L-cyst-ACF to remove Pb 2+ ions. The energy dispersive X-ray spectroscopy mapping of L-cyst-ACF is displayed in Fig. 4, which authenticated the presence of elements including carbon (C), oxygen (O), sulfur (S) and nitrogen (N) related to the presence of L-cysteine modied on ACF. As shown as Fig. 4(B-F), the successful graing reaction of functional groups occurred on ACF. According to the data, it can be concluded that O and N are not co-localized with Pb. Fig. 4E and F show strong S and Pb signals, respectively, which are colocalized, suggesting that the primary adsorbent for Pb is S and that N and O did not signicantly contribute to the adsorption.   Additionally, the ratio of different elements illustrates that the organic functional groups were successfully graed, as shown in Fig. 5B. EDS mapping, FT-IR, and XPS analyses were employed to conrm the adsorption of Pb 2+ on L-cyst-ACF.

Lead adsorption
3.3.1 Contact time. The exploration of the adsorption equilibrium time is a crucial part of measuring the performance of an adsorbent and it is signicance for subsequent research. 29 The change in the adsorption of the material with contact time is presented in Fig. 6. It shows that the adsorption capacity of Pb 2+ increased rapidly within the rst hour, which can be ascribed to the effective complexation of organic groups with Pb 2+ . 30,31 Then, it remained at approximately 125 mg g À1 for Pb 2+ aer 6 h, indicating that adsorption equilibrium had been achieved.
3.3.2 Different adsorbent dosages. The removal efficiency of Pb 2+ with different adsorbent dosages was also investigated. As shown in Fig. 7, the removal rate of Pb 2+ increased signicantly as the dosage increased. When the dosage increased from 0.25 g L À1 to 0.5 g L À1 , the removal of Pb 2+ increased from 23.78% to 51.55%. Simultaneously, the degradation rate of Pb 2+ did not increase signicantly with an increase in the adsorption dosage. However, the removal efficiency of L-cyst-ACF was limited when the adsorption dosage was greater than 0.5 g L À1 , indicating that a larger adsorption dosage had a greater impact on the further removal of Pb 2+ . Thus, we chose the dosage of L-cyst-ACF to be 0.5 g L À1 for the subsequent removal process.
3.3.3 Initial pH. Research has shown that the surface charge of the absorbent material, the degree of ion migration and the state of the adsorbed substance are affected by the pH of the solution during the adsorption process. 32 Fig. 8a shows that the amount of adsorption observably increased with an increase in pH. This phenomenon demonstrated that the existence of a large amount of H + ions competed with Pb 2+ ions for the adsorption sites in the material in an acidic environment. 31 It is generally believed that lead ions exist as cations in aqueous solutions. 33   (precipitation) with OH À when the pH is greater than 6. 35 Thus, zeta potential (z) measurements at different pH were performed to explore the surface charge of the absorbent, as shown in Fig. 8b. Fig. 8b shows that the surface of the material exhibited a negative charge state as the pH increased. Because the functional groups such as -NH 2 and -SH on the material surface lost H À ions as the OH À gradually increased in the solution, and they became the active state. 36 This active state was conducive to the adsorption of Pb 2+ ions by the material. On the contrary, the hydroxyl, carboxyl, amine and sulydryl groups on the surface of L-cyst-ACF tended to be protonated, and they were positively charged when the pH value was low, which led to electrostatic repulsion between the binding site and Pb 2+ ions. 15,37,38 Therefore, the pH of the solution and the surface charge of the material were the key factors that affected its adsorption capacity.

Adsorption kinetics
To quickly and accurately interpret the adsorption behavior of materials, four types of kinetic models were adopted in this investigation. The linear types of models are expressed as eqn (3)-(6). 39,40 ln(q e À q t ) ¼ ln where q e and q t are the adsorption capacity at equilibrium and time t (mg g À1 ). 36 K 1 (min À1 ) and K 2 [g$(mg min) À1 ] are the rate constant. K i is the rate constant of diffusion particles. 41 a [mg (g min) À1 ] and b (g mg À1 ) represent the initial constant adsorption and desorption constants, respectively. 42 According to the tting results in Fig. 9 and Table 1, it can be seen that the R 2 values of the pseudo-second-order and intraparticle-diffusion kinetic models were better than that of the Fig. 8 Effect of initial pH on Pb 2+ adsorption and zeta potential of L-cyst-ACF during removal process at different pH.  pseudo-rst-order kinetic models and Elovich model, indicating that the possible mechanism of adsorption is chemisorption and the adsorbent surface corresponds to a heterogeneous system. Furthermore, this result reveals that the adsorption is related to the electron exchange between the surface functional groups and Pb 2+ ions. The experimental equilibrium adsorption capacity, q e (136.80 mg g À1 ), obtained by tting the pseudo-second-order kinetic equation maintained high consistency with the actual test value ($125 mg g À1 ).
To investigate the internal diffusion mechanism during the adsorption process, the intra-particle diffusion model proposed by Weber and Morris 43 was used to discuss the rate-controlling step. Multi-linearity was observed, as shown in Fig. 9c, and the crucial information obtained is presented in Table 1. As shown in Fig. 9c, the Pb 2+ adsorption process involved three steps. The initial linear part was the result of adsorption on the external surface, namely, the attachment of Pb 2+ to the surface of L-cyst-ACF. 44 The second linear part represents the progressive internal diffusion of particles. 12 In the third part, the decreased diffusion rate is due to the reduction in Pb 2+ concentration and the enhanced electrostatic repulsion on the material surface. 45 The intra-particle diffusion and complex or ion exchange may play a certain role in the adsorption process given that none of the three linear curves passed through the origin of the coordinate. 23 Combined with the characterization test and mechanism investigation, we realized that the relatively rough surface on the ber surface and the partially oxidized thiol functional groups generated sulfonic acid groups, which reduced contact probability between the Pb 2+ in solution and active sites on the material surface, affecting the transfer rate of Pb 2+ in the surface liquid layer. Comparatively, the ion transport rate in the pore was larger. The a and b constants also indicated that the  material had a low adsorption rate and a non-homogeneous surface structure.

Adsorption isotherms
Two common isothermal models were used to evaluate the adsorption behavior between Pb 2+ and L-cyst-ACF. The linear form of the Langmuir isotherm is represented by the following equation: 46 where q e and q max represent the equilibrium adsorption capacity and monolayer maximum adsorption capacity (mg g À1 ), respectively. 18 The constant K L describes the affinity between the adsorbent and the target substance. 12 The Freundlich model is described by the following equation: 46,47 where K F and 1/n are empirical constants expressed the adsorption intensity and adsorb rate desorb rate . 48 The experimental results obtained by tting the Langmuir and Freundlich models are shown in Fig. 10. The affinity and adsorption capacity of L-cyst-ACF for Pb 2+ at different temperatures were evaluated. The constants for the two models are summarized in Table 2. The Langmuir model is normally used to evaluate the monolayer adsorption characteristics and energy level of a homogeneous material surface. 49 The R 2 value from the Langmuir model was higher than that from the Freundlich model, which indicates the adsorption of Pb 2+ was primarily homogeneous and monolayer adsorption. The low K L value suggests a relatively strong interaction between Pb 2+ and L-cyst-ACF. Compared with the results in other works, as shown in Table 3, that herein exhibited a higher adsorption effect. The Freundlich model generally describes the multi-layer adsorption of the adsorbent on a heterogeneous surface and that the energy level of the adsorption site is not constant. 43 Meanwhile, the poor tting data showed that the adsorption was not physical adsorption or multilayer adsorption. Furthermore, the value of 1/n in the three ranges of 0 < 1/n < 1, 1/n ¼ 0 and 1/n > 1 implies favorable, unfavorable and irreversible adsorption, respectively. 50 Thus, it can be predicted that the adsorption behavior was favorable and spontaneous according to the tting results.

Adsorption thermodynamics
The standard Gibbs free energy change (DG q ), entropy change (DS q ) and enthalpy change (DH q ) were calculated using eqn (9) and (10) to recognize the energetics and spontaneity involved in the procedure for the combination of Pb 2+ and the adsorbent.
where K q represents the constant associated with the thermodynamic equilibrium, which is replaced by K L from the Langmuir tting. 43 R and T are the universal gas constant [8.314 J (mol À1 K À1 )] and the temperature, respectively. 13 The thermodynamic parameters of adsorption provide an effective method for in-depth understanding of the nature of the adsorption process. The changes on Gibbs free energy (DG), enthalpy (DH), and entropy (DS) values during Pb 2+ adsorption was given in Table 4. Accordingly, the calculated adsorption enthalpy and entropy for of Pb 2+ adsorption were positive, which indicated that the affinity between metal ions and the  material surface was sufficient to make it adhere, and the randomness of the solid-liquid interface increased during this process. Otherwise, negative DG value showed the feasibility and spontaneity of the adsorption process, indicating endothermic process which was also predicted before. The endothermic nature of adsorption could be attributed to the increasing temperature that accelerated the diffusion of Pb 2+ and the increase of system disorder, which lead to the positive progress of adsorption.

Adsorption mechanism
The adsorption of metal ions on the surface of carbon materials is primarily related to their pore structure and surface groups. According to Fig. 11, the XPS spectra of L-cyst-ACF before and aer adsorbing were recorded to gain insight into the mechanism of action between Pb 2+ and L-cyst-ACF. The peak positions of Pb 4f 7/2 and Pb 4f 5/2 were detected at 143.78 and 139.03 eV, respectively, which indicate the successful adsorption of Pb 2+ . 58 As shown in Fig. 11c illustrates that the carboxyl group (O]C-O-) played a key role during the adsorption process. According to reports, a variety of carbonyl-containing oxygen functional groups, carboxylate and epoxide groups in the adsorbent structure act as electron donors, which lead to surface complexation with metal ions, contributing to the uptake of Pb 2+ . 15,37,61 There are three main binding energy positions at 398.63, 399.94, and 401.28 eV in Fig. 11g, which correspond to -NH, C-N, and O]C-NH-, respectively. 62 Obviously, these binding energy positions did not exhibit a visible change, as shown in Fig. 11h, which indicates that nitrogen-containing functional groups did not participate in the adsorption process. There are many chemical environments of S in the modied ber material, 26 such as S-S, C-S, -SH, C]S, and SO 3 2À , which were accurately found at 163.74, 164.61, 167.53, 169.05, and 169.95 eV in Fig. 11i, respectively. 26,27 As shown in Fig. 11j, the spectra of S-S and C-S could be tted with two peaks at 163. 45 and 164.31 eV, which represent that these two bonds did not participate in the adsorption of Pb 2+ . Surprisingly, the binding energy positions at 167.53, 169.05, and 169.95 shied to lower positions, as shown in Fig. 11j ($165.57 eV, $167.85 eV, and $169.08 eV), revealing that -SH, C]S, and SO 3 2À , respectively, produced coordinated chelation and electrostatic attraction toward Pb 2+ . According to the hard so acid base (HSAB) theory, 63 a metal-ligand complex can be formed between Pb 2+ (so acid with empty track) and surface functional groups, such as -SH, O]C-O-, and C]S (so base with free electrons) via covalent bonding. Therefore, the formation of metal-organic complexes greatly improved the adsorption properties of the material.

Reusability and cycling performance
It was very important to evaluate the cycling performance of Lcyst-ACF considering the cost of adsorbents used in practical production. Therefore, adsorption-desorption cycle experiments were carried out on L-cyst-ACF herein, focusing on the reusability and removal efficiency changes. EDTA-2Na salt solution with a concentration of 0.5 mol L À1 was used to desorb L-cyst-ACF saturated with adsorbed species in this study. The changes in the adsorption capacity and adsorption efficiency of the material aer 5 adsorption-desorption cycles are summarized in Fig. 12. With an increase in the number of regeneration cycles, the adsorption of lead ions decreased from  about 130 mg g À1 to only 80 mg g À1 , indicating that the adsorption capacity of L-cyst-ACF for Pb 2+ decreased aer the ber underwent multiple elution cycles. We speculated that the functional groups on the ber surface were lost and some were oxidized and failed during repeated experimental operations. In general, L-cyst-ACF exhibited ideal reusability and has certain prospects in the treatment of lead pollution.

Conclusion
An activated carbon ber material graed with L-cysteine was prepared via an amide reaction and the batch adsorption experiments showed that L-cyst-ACF was effective for the removal of Pb 2+ from aqueous solution. Adsorption experiments were carried out to investigate different factors such as initial concentration, pH, adsorbent dosage, contact time, and temperature. The experimental results showed that the adsorption effect was better when the pH of the solution was between 5 and 6; a higher temperature was conducive to the adsorption process; the adsorption efficiency was the best when the dosage of adsorbent was 0.5 g L; the adsorption reached equilibrium within 6 h, and q e was 136.80 mg g À1 . In addition, Lcyst-ACF had a certain cycling performance aer multiple regeneration treatments. Compared with the pseudo-rst-order kinetic equation, it was concluded that the tting of the pseudo-second-order kinetic equation was more suitable in the adsorption process, implying that the adsorption process was chemisorption. The data from the isothermal models showed that the Langmuir model tted well, which indicated that the material adsorption process was monolayer adsorption. The thermodynamic analysis showed that DG was negative at all temperatures, which indicated that the adsorption process was endothermic and spontaneous. The positive DS indicated that the system disorder gradually increased during the adsorption process. Finally, the XPS analysis illustrated that abundant -SH, -COOH, and C]S groups on the ber surface were the crucial factor for the signicant improvement in the adsorption capacity.

Conflicts of interest
The authors declare no competing nancial interest.