Multiple chemical modifications and Cd2+ adsorption characteristics of sludge-based activated carbon

Sludge resource utilization is commonly realized through carbonization, but the use of direct carbonization to obtain sludge-based activated carbon (SAC) is not functional yet. The multiple chemical modifications were carried out to achieve N-doping and pore-making to modify SAC. The SACU–PF′ was synthesized by activating sludge simultaneously with uric acid and potassium ferrate. Moreover, SACN′, SACU, and SACPF′ were prepared with no additives, uric acid, and potassium ferrate, respectively. The results indicated that the different modifications affected the chemical properties and structure of SAC. The BET of SACU–PF′ was 56.73 m2 g−1, which was higher than that of SACN′ and SACPF′. SACU–PF′ possessed abundant functional groups, such as C 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 N and C–O. The adsorption capacity of SACU–PF′ for Cd2+ was 9.69 mg g−1, 5.5 times that of SACN′, the adsorption process of Cd2+ by SACU–PF′ fitted well for the second-order kinetic model and Langmuir isothermal adsorption model. The XPS and chemical analysis revealed that SACU–PF′ and Cd2+ were bonded by functional groups, and the Cd2+ removal by SACU–PF′ was through complexation, anion exchange, electrostatic attraction, and pore filling. The SACU–PF′ was exhibited different removal capacities for different metals, Pb2+ and Mn2+ correspond to adsorption capacities of 4.9 and 8.1 mg g−1. In addition, the adsorbed SACU–PF′ can be regenerated by sodium hydroxide. The study highlights the importance of multiple chemical modifications performed on SAC, the double coupled chemical modifications to ensure its good performance in the treatment of heavy metals in wastewater treatment.


Introduction
The wastewater treatment process will produce large quantities of sludges. The current ways of sludge treatment and disposal are mainly landll or incineration; in these processes, the resource utilization rate of sludge is low. It is well known that the sludge contains abundant organic matter, like tung wood and other materials, the characteristic suggest that sludge can be used to prepare sludge-based activated carbon (SAC). The SAC is like activated carbon, which can be used in adsorption, catalysis, and in other elds. Therefore, the technology of preparing activated carbon or carbon-based carrier from sludge has become an important alternative for future urban sludge treatment, disposal, and resource utilization. 1,2 However, SAC formed by direct carbonization of sludge is associated with some demerits such as small specic surface area, small pore volume, and poor adsorption capacity. To solve these problems, chemical modication has been widely used to change the structural characteristics of SAC and improve its application performance. Chemical treatment is an excellent approach for modifying SAC, wherein the precursor is activated with chemical reagents, and then heat treatment is performed at a relatively high temperature, which can signicantly improve the structure of SAC and change the pore size distribution of SAC. This, in turn, increases the functionality of SAC. Chemical activation mainly includes acid activation (HCl, HNO 3 , H 2 SO 4 , H 3 PO 4 , urea), alkali activation (NaOH, KOH, Fe(OH) 3 ), salt activation (ZnCl 2 , FeCl 3 ), nitridation, and graphitization. [3][4][5] Wu 6 has utilized HCl, HNO 3, and NaOH to modify sludge. Kang 7 used KOH to activate sludge obtained from a paper mill and prepared activated carbon to adsorb heavy metals, and the results showed monolayer adsorption. Salt activation is also commonly used in the carbonization process of sludge, and the literature suggests that ZnCl 2 plays a major role in dehydration and vaporization, converting sludge into a high specic area SAC. Kong 8 developed a new process of pickling and pyrolysis using citric acid and ZnCl 2 mixed pore-forming agent to improve the performance of SAC. In the process, ZnCl 2 was used as the activator to obtain the maximum surface area. Studies have shown that adding pyrolusite by chemical activation method can result in a new SAC and can effectively remove heavy metal ions in wastewater. 9,10 Recently, many researchers have shown interest in the adsorption properties of carbon-based materials. Among them, graphite-phase carbon nitride materials exhibited high chemical stability, good compatibility, more concentrated pore structure, high porosity, rich hydroxyl and carboxyl groups on the surface, and possessed great potential in the adsorption of heavy metals from wastewater. 11 For example, Yang 12 studied the removal of Cu 2+ , and the adsorption capacity was up to 46.6 mg g À1 . Zhu 13 obtained carbon nitride by directly roasting melamine and adsorbing Pb 2+ with this material. The maximum adsorption capacity exhibited by the carbon nitride material for Pb 2+ was 7.4 mg g À1 in the experiment. Beygli 14 used hydroxyapatite modied carbon nitride powder to adsorb Cu 2+ , and a striking increase in the adsorption capacity (74.5 mg g À1 ) was observed. With the continuous acceleration of the industrial processes, many heavy metals are discharged into the environment together with industrial wastewater. Even a low concentration of heavy metals (Cd, Pb, Mn) are potentially toxic and carcinogenic, and their arbitrary discharge will seriously affect human health. 11 The adsorption method is considered to be the simplest and most effective method to remove heavy metals. Particularly, the adsorption of low-concentration heavy metals from wastewater has obvious advantages. The adsorbent, aer adsorption, is also easy to recover or remove from the water medium, making it the most common water treatment method. 15 Therefore, SAC can be used as an adsorption material, and chemical modication can improve its adsorption capacity. Certain types of functional groups on the surface of SAC can effectively combine with toxic metal ions.
In the present work, two common, simple, and green activators of K 2 FeO 4 and urea (CH 4 N 2 O) were used to change the structure of SAC. This was mainly to prepare a graphitized nitriding sludge carbon material from sludge carbon by nitrogen doping method, to improve its adsorption capacity. In this work, CH 4 N 2 O and K 2 FeO 4 were used as chemical activators to prepare four kinds of SAC, and their properties were analyzed. The adsorption behavior of toxic metal ion Cd 2+ was subjected to study, and the adsorption mechanism was explored by model analysis. An insight is given into the practical application of the SAC.

Materials and reagents
The sludge was collected from the sewage treatment plant of the Hefei economic development zone. Reagents, including K 2 FeO 4 , CH 4 N 2 O, HCl (36.5%), NaOH, NaCl, Cd(NO 3 ) 2 , Pb(NO 3 ) 2 , Mn(NO 3 ) 2 , and ethylenediamine tetraacetic acid (EDTA) were purchased from Sinopharm Chemical Reagent Co, Ltd. All chemicals were employed without further purication, and the solutions were prepared using deionized water.

Preparation of SAC
The sludge was dried at 105 C, crushed, and then screened (80 mesh) to obtain the pretreatment sludge samples. The pretreated sludge samples were carbonized into sludge charcoal by heating at 5 C min À1 to 500 C for 120 min in an N 2 atmosphere. Then, 25 g of the so treated sludge charcoal was weighed and immersed in pure water, 250 mL of 1 mol L À1 CO(NH 2 ) 2 solution, 250 mL of 1 mol L À1 K 2 FeO 4 solution, a mixture of 250 mL of 1 mol L À1 CO(NH 2 ) 2 and 1 mol L À1 K 2 FeO 4 , respectively, for 6 h. The samples were then dried at 80 C, and the dried samples were heated to 800 C at a rate of 5 C min À1 under an N 2 atmosphere and maintained under the same condition for about 120 min. Then the four samples were screened through 200 mesh and soaked in 0.5 mol L À1 HCl for 2 h. The samples were then washed with deionized water repeatedly until neutral and dried until constant weight. The samples are denoted as SAC N 0 , SAC U , SAC PF 0 , and SAC U-PF 0 , respectively. A schematic diagram of preparation and reaction is represented in Fig. 1.

Characterization methods
The SAC was analyzed by scanning electron microscopy (SEM) using SU8010 cold-eld emission scanning electron microscope (Hitachi Company, Japan). The SAC was also analyzed by FEI-Talos-F200X (Thermo FEI, USA) transmission electron microscope (TEM). TD-3500 X-ray diffractometer (Dandong Tongda) was utilized to obtain X-ray diffraction (XRD) patterns of the SAC. The X-ray photoelectron spectroscopic (XPS) data were obtained using the Thermo Fisher K-alpha XPS instrument (Thermo Field). The SAC was scanned by using the Nicolet IS 50+ Continuum Fourier Transform Infrared Spectrometer (FTIR) (Thermo Fisher, USA). Raman spectra were obtained by the DXR laser confocal micro-Raman spectrometer (Thermo Fisher, USA). The pore size of the SAC was quantied by the nitrogen adsorption/desorption curves using a fully automated surface and porosity tester (Quantachrome, USA) and BET analysis. The zeta potential was measured by particle size and zeta analyzer (Zetasizer Nano-ZS 90, Malvern, USA). The thermogravimetric (TG) analysis used Q500 Thermo Gravimetric Analyzer (TA, USA).

Batch adsorption study
2.4.1 Adsorption property measurement. 0.1 g of each of the four different kinds of SAC samples (SAC N 0 , SAC U , SAC PF 0 , and SAC U-PF 0 ) were added to 100 mL of Cd 2+ solution with an initial concentration of 10 mg L À1 . The samples were placed in a water bath with an oscillating chamber at 25 C and a speed of 100 r per min. During the oscillation process, samples were taken out at different times. The concentration of Cd 2+ was measured at 228 nm using ame atomic absorption spectrometry (ZEEnit700P). The adsorption capacity and rate were calculated using eqn (1) and (2), respectively.
q (mg g À1 ) is the adsorption capacity at that instant, V (L) is the volume of adsorbent solution, C 0 (mg L À1 ) is the initial mass concentration of adsorbent in the solution, C t is the mass concentration of adsorbent in the solution at the reaction time, m (g) is the mass of adsorbent, and R is the removal efficiency.

2.4.2
Research methods of adsorption kinetics. Dilute 1 g L À1 of Cd 2+ reserve solution to 10 mg L À1 and take 100 mL of Cd 2+ solution into a 150 mL conical ask. The solution pH will be the initial pH. Then add 0.1 g each of four kinds of SAC, and shake at 25 C, 100 r per min at a constant temperature. The samples were measured at different time intervals (4,8,16,30,60, 90, 120, 240, 360, 480, 600, 720 min).
Pseudo-rst-order model (eqn (3)), pseudo-second-order kinetic model (eqn (4)) and intra particle diffusion model (eqn (5)) were used to t: 16 where q e (mg g À1 ) represents the equilibrium adsorption capacity, q t (mg g À1 ) is the adsorption capacity at time t, k 1 (min À1 ) and k 2 (g mg À1 min À1 ) are the rst-and second-order adsorption rate constants, t (min) is the time, k (mg g À1 min À0.5 ) is the internal diffusion rate constant, and the intercept b corresponds to the boundary layer effect.

Adsorption isotherm study.
The 100 mg L À1 Cd 2+ reserve solution was successively diluted to 2.5, 5, 7.5, 10, and 12.5 mg L À1 , respectively. 100 mL was placed in a 150 mL conical ask, and 0.1 g of the four SAC samples were added. The oscillation box was maintained at 25 C for 12 h at 100 r per min.
The adsorption isotherm tting models include the Langmuir type (eqn (6)) and Freundlich type (eqn (7)), which are the commonly used isotherms to describe multilayer adsorption: 17 where q e (mg g À1 ) represents the equilibrium adsorption capacity, Q m (mg g À1 ) represents saturated adsorption capacity, C e is the equilibrium concentration (mg L À1 ), and K L is the Langmuir adsorption constant. The Q m value is proportional to the adsorption capacity. K L represents the binding capacity to the adsorption site. 1/n is a constant related to adsorption, and K f is the Freundlich adsorption constant.

SAC regeneration.
SAC was regenerated by the solvent method, and the effects of different regenerators on the regeneration of SAC U-PF 0 were analyzed. 0.5 g of the adsorbed SAC U-PF 0 was placed in several beakers, and 0.1 mol L À1 of HNO 3 solution, 0.1 mol L À1 of NaOH solution, and 2 mmol L À1 of EDTA solution were congured accordingly. 50 mL of the regeneration solution was added to the beaker lled with saturated SAC for adsorption for a duration of 12 h. The regenerated SAC was washed with deionized water to neutralize. It was then dried and made to adsorb Cd 2+ again. The process was repeated 5 times, and the results were compared. Fig. 2 shown the SEM and TEM images of four kinds of SAC samples. The surface of SAC N 0 was rough and not smooth, and the surface particles showed no obvious pore accumulation structure. In Fig. 2(b1 and b2), some lamellar or occulent pore structures appeared in SAC U , indicating that CH 4 N 2 O could modify the surface of SAC to some extent. SAC PF 0 in Fig. 2(c1) had an obvious uniform granular structure. From Fig. 2(d1) corresponding to SAC U-PF 0 , it was obvious that the material surface obtained by mixing CH 4 N 2 O and K 2 FeO 4 with nitrogen doping also had many uniform granular pore structures, and these structures tend to stack and aggregate, leading to an obvious pore structure. 4,12 Fig. 2 showed high-resolution TEM images for the four samples. Aer comparison, it was found that the modied SAC had obvious characteristics, the shape of SAC N 0 was irregular, the shape of SAC U was uffy, and SAC PF 0 and SAC U-PF 0 exhibited a ake square-like shape. Fig. 3(a) shown the XRD spectra of four kinds of SAC. It can be seen from the gure that all the four SAC had diffraction peaks at 2q ¼ 26.57 , which can be considered as a 002 crystalline graphite structure plane. The peak values of about 2q ¼ 36 represent amorphous carbon. According to the comparison of JCPDS card no. (41-1487) standard card, it was found that the diffraction peak near 21 and 26 of 2q could be related to the diffraction of quartz (SiO 2 ), and the signal still existed aer modication. Thus suggesting that SiO 2 is the main crystalline state of SAC. 6,18 The XPS results are shown in Fig. 3(b). The peaks at 283.6 (AE0.3 eV), 530.7 (AE0.3 eV), and 399.7 (AE0.3 eV) in the spectra, respectively, represented the binding energies of C1s, O1s, and N1s, whereas the O/C ratio of the unmodied material was increased from 0.77 to 4.65. The oxygen content in SAC U-PF 0 reached up to 73.92%, indicating that the surface of the material contains many oxygencontaining functional groups, which was consistent with the analysis of FTIR results. 19 The FTIR of the four SAC was shown in Fig. 4(a). It can be seen from the spectrum that there were many carbon and oxygen functional groups present in the sample. The peak value at 527 cm À1 belonged to SiO 4 2À asymmetric angular motion, the peak value at 626 cm À1 was caused by C-O-H vibration, and the one at 777 cm À1 represented the stretching vibration of Si-C. 970 cm À1 and 1068 cm À1 peaks corresponded to the C-O-C stretching vibration, 1298 cm À1 belonged to the diffraction peak caused by C-N stretching vibration, and 1293 cm À1 and 1541 cm À1 belonged to C]C stretching vibration. 20 SAC N 0 was characterized by the antisymmetric stretching vibration of CO 3 2À at 1390-1510 cm À1 . In SAC U , aer modication by CH 4 N 2 O, the diffraction peak of C]N stretching vibration appeared at 1370-1610 cm À1 , and it possessed a carbonyl C]O stretching vibration at 1756 cm À1 . The peak at 2892 cm À1 can be ascribed to the symmetrical motion of CH 2 alkane. Similarly, the peak at 3430 cm À1 corresponded to O-H stretching vibration of liquid water, and 3005 cm À1 belonged to ¼ C-H. 21 The strength of the O-H peak corresponding to 3688 cm À1 was enhanced aer modication, indicating that the hydrophilic groups on the surface of SAC increased aer modication. 11 Raman spectrum analysis further studied the structure of SAC, and the results are shown in Fig. 4(b). SAC N 0 , SAC U , and SAC U-PF 0 samples all exhibited a D band at 1320 cm À1 , which suggested the presence of defect sites or disordered sp2 hybridization related to graphitized carbon atoms and plane vibration G band (1600 cm À1 ) of SP 2 bonded carbon atoms in a two-dimensional hexagon. 18 The degree of graphitization was directly proportional to the ratio of the intensity of D to G-band; that is, the The SAC U-PF 0 was prepared via graphitization and nitriding. 22 The BET and BJH plots of the four SAC samples were clas-sied as type IV according to IUPAC comparison results and were mainly mesoporous. Single-layer to multi-layer adsorption on the mesoporous wall led to slow growth of the low-pressure section. When P/P 0 ¼ 0.5-0.8, the adsorption amount increased signicantly, reecting the size of the sample aperture and suggesting whether the width of the change corresponded to the size of the mesopore. 23 A hysteresis loop was observed in all materials, corresponding to the H 3 hysteresis type, which was a slit hole of ake particle accumulation consistent with the results of BJH. 18,24 The results in Fig. 5(a2-d2) and Table 1 showed that the BET and pore sizes of SAC N 0 , SAC U , SAC PF 0 , and SAC U-PF 0 were 35.19, 69.16, 55.99, 56.73 m 2 g À1 , and 13.42, 47.99, 21.36, 27.85 nm, respectively, moreover, all SAC were mesoporous. By comparison, it was found that the peak height of SAC N 0 was the lowest, while the other three were almost similar. This observation indicates that the number of SAC N 0 pores was small, and SAC N 0 and SAC PF 0 were microporous structures. While SAC U and SAC U-PF 0 were large mesoporous structures and SAC PF 0 had the narrowest peak, indicating that the pore size was uneven. 25 Fig. 6(a) shown the inuence of pH on the zeta potential of four kinds of SAC. It can be seen from the gure that the zeta potentials of the four SAC were positive at pH ¼ 2 and negative when pH ¼ 6, so the pHPZC value lies between 2 to 6. At the   respective pHPZC value, the amount of positive charge carried by the adsorbent will be equal to the amount of negative charge. This was because K 2 FeO 4 contains acidic groups, while CH 4 N 2 O contains basic groups. As a result, aer modication, the SAC exhibited weak acidity, attracted H + , and had a positive charge, so the zeta potential value was positive. Therefore, in the subsequent single-factor experiments, when the pH was adjusted between 2 to 6, it generated electrostatic repulsion with Cd 2+ in the solution and adsorbed Cd 2+ by electrostatic attraction when the pH was more than 6. 26 In the Fig. 6(b), TG analysis shown that the four of SAC possessed good thermal stability, the TG variation was within 7%. As a carbon material, the decomposition of surface groups on SAC at high temperature leaded to a small amount of weight loss.

Comparison of adsorption capacities of different SAC
The comparison of the adsorption capacity of Cd 2+ by four different SAC was demonstrated in Fig. 7.
The results showed that K 2 FeO 4 modied and K 2 FeO 4 added with CH 4 N 2 O samples had signicantly higher adsorption capacities than CH 4 N 2 O modied and unmodied SAC, and the maximum adsorption capacity of SAC U-PF 0 was 9.69 mg g À1 , which was 5.5 times than that of SAC N 0 . The adsorption capacities of SAC U and SAC PF 0 was 4.54 and 9.34 mg g À1 , respectively. Before 240 min, the adsorption rates of the four kinds of SAC were fast, and at about 480 min, adsorption equilibrium was attained. The rapid adsorption may be accounted to the availability of sufficient active sites, and the active sites gradually attain saturation during the process of adsorption. These results indicated that the mixed modication of K 2 FeO 4 and CH 4 N 2 O improved the adsorption capacity of SAC. The adsorption capacity of SAC U-PF 0 's was slightly higher than SAC PF 0 's, possibly due to the further increase in the specic surface area caused by nitrogen doping. 27  With the increase of pH of Cd 2+ solution from 2 to 8, the adsorption capacity of SAC also increased gradually. At a low pH value, the adsorption capacity of the material was also low because hydroxyl was not readily available, and the binding sites for adsorption of Cd 2+ became fewer. H + competed with heavy metal Cd 2+ for the active sites. 28 When the pH exceeded the initial value of 6, the adsorption capacity of SAC PF 0 and SAC U-PF 0 exhibited little difference, and the adsorption capacity of SAC U-PF 0 could reach up to 9.96 mg g À1 . However, when the pH of SAC N 0 and SAC U was about 8, the adsorption capacity of SAC N 0 and SAC U showed obvious improvement. At a higher pH, a negative charge was easily formed on the surface of the adsorbent, which generated electrostatic binding with Cd 2+ , leading to a good adsorption efficiency. In addition to electrostatic interaction, metal ion precipitation also existed. 29 Pezoti O. 30 used the same explanation upon exploring the inuence of pH on the adsorbent. Strong acid and strong base affected the  surface charge distribution and properties of sludge-carbon material, thereby changing the electrostatic interaction between the material and Cd 2+ , which could also be explained based on zeta potential analysis. To explore the effect of temperature on the adsorption of Cd 2+ by SAC, three temperature gradients of 10, 25, and 40 C were selected for the study. The results showed that temperature variation on SAC N 0 and SAC U had little effect on the adsorption efficiency, and the adsorption effect of these two materials was poor. The temperature had a certain effect on SAC PF 0 and SAC U-PF 0 . The reason may be that at low temperatures, the activation energy of solute molecules was low, and the active sites were few, which was not conducive to adsorption. As the temperature was initially increased, the active sites also increased, but when the temperature continued to rise, the active sites gradually became saturated. 31

Adsorption kinetics
The adsorption kinetics tting results of the adsorption model of Cd 2+ in four different SAC are shown in Fig. 9 and 10. Table 2 presents the parameters of the kinetics model.
The comparison between Fig. 9 and Table 2 indicates that the correlation coefficient, R 2 , of the second-order kinetic model was better than that of the rst-order kinetic model, so it was more consistent with the adsorption process of Cd 2+ . In addition, the correlation coefficient of SAC U-PF 0 was the highest (R 2 ¼ 0.996), and the calculated equilibrium adsorption value (q e ) of SAC U-PF 0 using second-order kinetic simulation was 8.767, which showed the least deviation from the experimental value of 9.69. Therefore, based on the above analysis, the adsorption behavior of SAC U-PF 0 for Cd 2+ was more apt to be tted with the second-order kinetic model, mainly relying on electrostatic attraction for adsorption. 32 As can be seen from Fig. 10, SAC U-PF 0 particle diffusion model in the tting curve is divided into two sections. The rst stage was mainly for the spread of microporous adsorption and diffusion of ions in the solution to the active sites on the surface of the adsorbent. The second stage ions continue to be diffused into microporous and mesoporous channels, with higher rates for SAC PF 0 and SAC U-PF 0 in the second stage, indicating the dominance of mesoporous adsorption. 33

Adsorption isotherms
The tting results of Langmuir type and Freundlich type adsorption thermodynamics are demonstrated in Fig. 11 and Table 3.
According to the tting results, the Langmuir isotherm model was quite consistent with the adsorption process of Cd 2+ , which is evident from the data given in Table 3. The correlation coefficient R 2 of the Langmuir was higher than that of the  Table 2 The kinetic parameters and intra-particle diffusion parameters of four SAC

Adsorption mechanism
According to the structural characteristics and adsorption performance of SAC, the mechanism of adsorption of Cd 2+ by SAC was conjectured and displayed in Fig. 12. The adsorption mechanism mainly included pore lling, complexation, electrostatic attraction, and anion exchange. As a typical carbon structure, SAC had many pores in which Cd 2+ could accumulate by pore lling. The heavy metal, Cd 2+ , and SAC adsorbent surface develop an electrostatic attraction. Due to the electrostatic attraction, metal ions are gathered on the SAC surface by the displacement of other ions, ion exchange, ion accumulation on the surface of the adsorbent, and physical adsorption. The metal ions and the material itself involves in   complexation with oxygen-containing functional groups. The XPS spectra before and aer adsorption were analyzed, and the results are demonstrated in Fig. 13(a). Aer adsorbing Cd 2+ onto SAC, the characteristic peak of C-O exhibited a redshi, indicating the loss of electrons by C and O and consequently gain of electrons by Cd. According to the hybrid orbital theory, it may be observed in Fig. 13(b) that the lone pair of electrons from N 2s 2 p 3 , O 2s 2 p 4 , and hybrid orbital could occupy the Cd 5s 2 d 10 orbital, and a strong force can be established between active groups (C-O, C-N) and the metal ion (Cd 2+ ). This suggests that a coordinate bond (Cd-C, Cd-O, and Cd-N) could be validated between Cd 2+ and the active group on the surface of SAC. 38 According to the analysis of FTIR results, the surface of the adsorbent contained some anions, such as SO 4 2À and CO 3

2À
, which could exchange ions with Cd 2+ . 39 Similarly, the hydroxyl, carbonyl, and aldehyde groups on the surface of the adsorbent reacted with Cd 2+ . The result had also been conrmed by other studies, and the hydroxyl group in the adsorbent was responsible for the adsorption of heavy metals. 40 In addition, according to the  zeta potential, when pH < pHpzc, the adsorbent will have a more positive charge, leading to competition between Cd 2+ and the adsorbent. When pH was higher, the adsorbent functional group will possess a negative charge, resulting in electrostatic attraction with Cd 2+ , thereby affecting the removal effect. 41

SAC adsorbent regeneration
Strong acids, bases, and surfactants could regenerate metal sorbents. 42 The analytical regeneration of SAC U-PF 0 with three different regenerating agents (HNO 3 , NaOH, and EDTA) and the readsorption of Cd 2+ were subjected to study. The adsorption effect is illustrated in Fig. 14.
As shown in Fig. 14, NaOH showed the best regeneration effect, and the adsorption capacity aer ve cycles reached 7.37 mg g À1 , while EDTA showed the second-best effect, which may be related to the positive charge on the Cd 2+ ion. 43 Aer successive regeneration, the adsorption efficiency also gradually decreased, which may be due to incomplete desorption and ablative. It was worth noting that the performance of SAC U-PF 0 was even damaged to a certain extent. 44 NaOH exhibited a good regeneration effect and could be used as a regenerating agent for recovering SAC U-PF 0 adsorbed with heavy metal ions. This result provided some theoretical support for subsequent studies on the regeneration of metal sorbents.

Application test of SAC
The application test of SAC U-PF 0 for low-concentration heavy metal removal from wastewater was performed. SAC U-PF 0 was employed to test different concentrations and types of metal ions, as displayed in Fig. 15.
The adsorption effect of SAC U-PF 0 at different Cd 2+ concentrations was shown in Fig. 15(a), from which it was evident that the adsorption amount increased with increasing the concentration, but when the concentration of Cd 2+ exceeded 10 mg g À1 , the adsorption effect decreased. Moreover, the adsorption amount at the concentration of 12.5 mg g À1 was lower than that at 10 mg g À1 . Adsorption eventually became difficult and tended to equilibrate over time. 11 Therefore, the SAC possesses a certain application value for low concentration metal removal from wastewater. Fig. 15(b) shows the adsorption effect of SAC U-PF 0 on Cd 2+ , Mn 2+ , and Pb 2+ . The adsorption capacity of Pb 2+ was only 4.88 mg g À1 , Mn 2+ was 8.1 mg g À1 , and Cd 2+ was the best, 9.69 mg g À1 . In addition, over time, the adsorption of Pb 2+ and Mn 2+ gradually slowed down, while the adsorption rate of Cd 2+ was fast in the early stage and gradually attened out later, which may be due to the presence of many adsorption sites in the early stage. As the adsorption progressed, the availability of adsorption sites decreased, and the adsorption became more and more difficult. It can be concluded that the SAC U-PF 0 can be used to adsorb other low concentrations of heavy metal from wastewater as well.

Conclusions
SAC U-PF 0 was prepared by multiple chemical modications employing K 2 FeO 4 and CH 4 N 2 O. Such modications were executed based on the fact that suitable chemical activators could promote the characteristics of the SAC U-PF 0 by generating abundant pores with variable sizes and functional groups, which in turn was benecial to the adsorption of Cd 2+ . The maximum adsorption capacity of SAC U-PF 0 was 9.69 mg g À1 , 5.5 times that of SAC N 0 . The SAC U-PF 0 adsorbent can be used in the removal of low concentrations of heavy metals from wastewater. The approach of multiple chemical modications improved the adsorption performance of heavy metals in wastewater and increased the possibility of sludge resource utilization.

Conflicts of interest
There are no conicts to declare.