Performance and mechanisms of active attapulgite-supported sulfidated nanoscale zero-valent iron materials for Pb(II) removal from aqueous solution

To inhibit the oxidation, passivation, and agglomeration of nano-zero-valent iron (nZVI), a liquid phase reduction method was used to load sulfurized nZVI onto acid-modified ATP with attapulgite (ATP) as the carrier (S-nZVI@ATP). The performance and mechanism of this material were studied for Pb2+ removal in water. The S-nZVI@ATP preparation prevents the agglomeration of nZVI particles and reduces nZVI oxidation. Pb2+ removal proceeds efficiently and stably when using S-nZVI@ATP at pH values ranging from 2.5–5.5. According to the electron sharing and transfer-based pseudo-second-order kinetic model, the Pb2+ is adsorbed onto S-nZVI@ATP, and the speed control step is completed by liquid film diffusion and intraparticle diffusion. The S-nZVI@ATP mediated Pb2+ adsorption is well-described by Freundlich’s isothermal adsorption model, which is a multilayer chemical adsorption process. The temperature and initial Pb2+ concentration were varied, and it was determined that Pb2+ adsorbs on S-nZVI@ATP in an endothermic reaction. This S-nZVI@ATP composite material has high reducibility, high surface activity, and good adsorption properties for Pb2+. Tests were performed for 24 h using adsorbent (1 g l−1) in Pb2+ solution (30 ml). For an initial Pb2+ concentration of 700 mg l−1, S−1-nZVI@ATP removes 57.37% of the Pb2+ and has an adsorption capacity of 401.60 mg g−1. In addition to forming PbS and Pb(OH)2 precipitates, Pb2+ also complexes with the Fe/H oxide shell of S-nZVI@ATP, and Fe0 reduces some Pb2+ on the nZVI to Pb0. The results exhibited that S-nZVI@ATP has excellent potential as an adsorbent for the removal of Pb2+ from the industrial wastewater.


Introduction
Heavy metals are emitted throughout all walks of life due to the ongoing acceleration of global industrialization.Wastewater contaminated with these heavy metals threatens human and environmental health [1][2][3][4][5][6].The most common heavy metal used to manufacture batteries is lead (Pb).Pb is used to manufacture paint pigments, radiation-shielding devices, automobiles, and planes [7].Pb pollution is mainly caused by dust, slag, and wastewater containing lead [8].The removal of Pb from the environment is extremely challenging.Upon entering the environment, Pb retains its toxicity, posing a possible risk to human health.Pb is considered one of the most harmful and carcinogenic pollutants because of its high toxicity and carcinogenicity [9].Lead entering the human body causes severe damage to the neurological, immune, circulatory, and skeletal systems.Due to their developing nervous systems, children are particularly vulnerable to lead poisoning, which causes intellectual disabilities and irreversible symptoms [10].In light of the increasing severity of heavy metal pollution, researchers are increasingly focusing on effectively removing lead from the environment.
Several industries have benefited from the development of nanotechnology in recent years.One example of an inorganic nanoscale material is core-shell structured (Fe 0 -FeO/FeOOH) nano-zero-valent iron (nZVI, nano-Fe 0 ).These nanoparticles have good chemical reactivity, large specific surface areas, low-cost preparation, and good adsorption properties [11,12].nZVI is rich in Fe 0 .Furthermore, it is strongly reducible and has a small particle size, which produces a small size effect and a surface effect unique to the microscale.nZVI also shows good solid adsorption characteristics for heavy metals in water [13], and it has magnetic properties.However, nZVI is easily agglomerated, which reduces its reactivity and limits its practical application as a catalyst [14].To solve this problem, some researchers have modified nZVI to improve its dispersibility and stability.Modification methods include solid loading, bimetallic modification, vulcanization, and surface modification.Loading nZVI onto a porous solid surface with a large specific surface area is a good strategy for reducing agglomeration and improving reactivity and stability.Several common carriers have been reported, including resins, activated carbon, biochar, bentonites, montmorillonites, zeolites, and attapulgite [15].These carriers are highly stable and possess excellent adsorption properties.Therefore, combining nZVI with a carrier can synergistically increase the adsorption capacity of the prepared composite for contaminants.
Another strategy for enhancing the properties of nZVI particles is sulfidation.A vulcanizing agent, such as sodium sulfide (Na 2 S), sodium sulfide (Na 2 S 2 O 3 ), or sodium dithionite (Na 2 S 2 O 4 ), is added to the solution.The surface of nZVI is chemically modified via the formation of iron sulfide compounds (including pyrite, tetragonal pyrite, etc) [16].Unlike unmodified nZVI, the sulfidated nZVI (S-nZVI) shell combines iron oxide and FeS [17].It is possible to enhance the electronegativity of FeS by accelerating the electrons to be released from the Fe 0 core to the material surface.Therefore, the sulfidation modification of nZVI can improve its reactivity, dispersion, and electron selectivity for pollutants [18].
Attapulgite (ATP) is a natural magnesium-rich and 2:1 layered chain-like aluminosilicate mineral [19].As a negative carrier of S-nZVI, porous attapulgite has excellent mechanical properties and stability.In this study, a liquid phase reduction method was used to combine solid loading as well as sulfide modification for the preparation of a sulfide-modified nZVI composite on an attapulgite support (S-nZVI@ATP).It is worth noting that the sulfide modification significantly improves the adsorption selectivity of nZVI to Pb 2+ , and enhances the removal efficiency of Pb 2+ in industrial wastewater by nZVI@ATP, which provides a new idea for how to efficiently and reasonably solve the problem of lead pollution in industrial wastewater in the future.The primary focuses of this work were: (1) examining the Pb 2+ adsorption kinetics, thermodynamics, and isotherms on S-nZVI@ATP; (2) studying the S-nZVI@ATP adsorption performance of Pb 2+ using various sulfur-iron ratios and pH values; (3) analyzing the morphology, crystal structure, functional groups, and elemental valence of S-nZVI@ATP by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy (SEM-EDS), and Fourier Transform infrared spectroscopy (FTIR).The obtained results were used to establish the Pb 2+ adsorption mechanism by S-nZVI@ATP.Analytical-grade reagents were used.Attapulgite raw ore used in this experiment came from Gansu Hanxing Environmental Protection Technology Co., Ltd (Lanzhou, China).Sodium nitrate (NaNO 3 , 99.0%) was purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China).The lead solution used 0.01mol l −1 NaNO 3 as the supporting electrolyte.

Sample preparation
The attapulgite ore was passed through a (200-mesh-sieve and then modified by adding 3000 ml of 3 mol l −1 hydrochloric acid in a 5 l beaker.This mixture was stirred at 1000 r/min for 3 h utilizing an electric stirrer, then ultrasonicated for 1 h.Next, the supernatant was poured out, and a neutral pH was achieved by dousing the precipitate with distilled water.The obtained precipitate was dried in the oven, crushed and screened by 200 mesh sieve to obtain acid-modified attapulgite. A three-necked flask with a volume of 1000 ml was used to prepare a series of S-nZVI@ATP composites through liquid-phase reduction.First, the acid-modified attapulgite and FeSO 4 •7H 2 O were dissolved in deionized (DI) water (100 ml) in the three-necked flask and allowed to stir for 2 h, after that, it was added with 100 ml of anhydrous ethanol and provided 30 min further stirring.To obtain S-nZVI @ ATP by centrifugation, 0.2 mol NaBH 4 and Na 2 S (specific amount) were added to 400 ml ultrapure water at molar ratios of 0.75, 0, 0.1, 0.25, or 0.5.After screening with a 200 mesh seve followed by vacuum-drying, S-nZVI@ATP was rinsed with alcohol and DI water, and then sealed under vacuum and stored in the freezer.2+ adsorption by S-nZVI@ATP 2.3.1.Pb 2+ adsorption as a function of Fe/attapulgite ratios The Pb 2+ adsorption performance of different adsorbents (nZVI, ATP, and S-nZVI@ATP) was examined.For this purpose, an adsorbent (1 g l −1 ) was transferred to a centrifuge tube, which then added 30 ml of Pb 2+ solution (700 mg l −1 ).NaOH and HNO 3 (0.1 mol L −1 each) were added to achieve pH = 5.0.A thermostatic water bath oscillator was used to shake this mixture for 24 h at 200 rev/min and 298 K to ensure a complete reaction between the adsorbent and Pb 2+ .A filter membrane (0.45 μM) and a flame atomic absorption spectrometer were used to measure the residual Pb 2+ concentrations in the solution.In a follow-up experiment, the optimal sulfur-iron ratio was determined based on actual conditions.

Kinetic, isotherm, and thermodynamic adsorption experiments
Pseudo-second-and -first-order kinetic models were applied to assess the Pb 2+ adsorption by S-nZVI@ATP.An intraparticle diffusion model was used for the determination of the rate-controlling step and the best adsorption time (table 1).Anaerobic solutions of 700 mg l −1 Pb 2+ and S-nZVI@ATP (1 g l −1 ) were placed into a centrifuge tube, after which the pH was adjusted to 5.0.A thermostatic oscillator was used to oscillate this solution at 200 r min −1 and 298 K for 0.25, 0.5, 1, 2, 3, 4, 6, 8, 12, 20, or 24 h.The treated solutions were filtered through a microporous membrane (0.45 μM) to determine their residual Pb 2+ concentrations.
Further adsorption experiments were performed for analysis using Langmuir, Freundlich, and Temkin models (table 1). 1 g l −1 of S-nZVI@ATP was added to anaerobic Pb 2+ solution (30 ml) (500-900 mg l −1 Pb 2+ ) in a centrifuge tube, and adjusted to pH 5.0.This solution was oscillated for 24 h at 200 r min −1 and a temperature of 308 K, 298 K, or 318 K.The Pb 2+ content was measured after filtering the solution with a 0.45 μM microporous membrane.
The Van 't Hoff equation was used to calculate the enthalpy (ΔH 0 ), the Gibbs free energy (ΔG 0 ), and the entropy (ΔS 0 ) values to study the temperature influence on Pb 2+ adsorption.
Equations (1) and (2) can be used to derive the Van 't Hoff equation, as displayed in equation (3): where R is the gas constant, K d (ml g −1 ) is the adsorption distribution coefficient and T (K) is the temperature.

Instrumental and characterization techniques
SEM (Karl Zeiss MERLIN) was utilized for the evaluation of morphological changes in the prepared adsorbents.Analysis for elemental compositions and crystal structures was carried out by EDS (Oxford Instruments X-Max, UK) and x-ray diffractometry (XRD; RIGAKU, Japan), respectively.FT-IR was performed using a Bruker VERTEX 70.S-nZVI@ATP ζ-potentials were determined utilizing a ζ-potential tester (Nano ZS90, Malvern).Atomic absorption spectrophotometry (AAS; Varian 220FS, USA) was used to analyze the Pb 2+ concentration in the solution.XPS (Kratos AXIS Ultra DLD) was applied to study the surface chemistry.The spectra were calibrated using a carbon peak at 284.4 eV, which was then compared with the NIST database (https://srdata.nist.gov/xps/Default.aspx,access date is April 9, 2022).

Results and discussion
3.1.Characterization results EDS analysis and SEM images of S-nZVI@ATP before the treatment showed that its surface possessed a scaly structure (see figure 1(a)).After the reaction, S-nZVI@ATP showed some flake structures and fine particles, while the scaly morphology on the surface was gone (figure 1(b)).A comparison of figures 1(c) and (d) shows the presence of multiple Pb peaks after adsorption, with a total Pb content of 13.41%.Therefore, Pb 2+ was adsorbed successfully onto the S-nZVI@ATP surface.It is worth noting that EDS measurements on light elements such as C and O are only qualitative, not quantitative.These SEM-EDS findings show that the flake structures were generated by Pb 2+ adsorption.The S-nZVI@ATP crystal structure before and after the adsorption of Pb 2+ was studied by XRD (figure 2).The diffraction peak dispersion reveals that the crystallinity of S-nZVI@ATP deteriorated due to Pb 2+ adsorption.Characteristic PbS peaks (#) were detected at 25.96°, 30.05°, 43.06°, 51.01°, and 71.0°.The PbS solubility (K sp = 8 × 10 −28 ) is lower than the FeS (Ksp = 6.3 × 10 −18 ).Therefore, S-nZVI@ATP removed Pb 2+ via the coprecipitation of S 2− and Pb 2+ to form PbS. The peak at 2θ = 24.64°wasascribed to Pb(OH) 2 (¤).Furthermore, distinct Fe 3 O 4 (+) and FeOOH (∇) peaks were also observed.The formation of Pb(OH) 2 was ascribed to the oxidation of Fe 0 during the reaction to Fe 2+ /Fe 3+ .A Pb 0 diffraction peak was observed at 2θ = 36.4°,which was probably due to the low content of Pb 0 or its poor crystallinity (making it difficult to detect by XRD).
The -OH absorption FT-IR bands recorded for S-nZVI@ATP before and after Pb 2+ adsorption were observed at 3404 cm −1 and 1614 cm −1 (see figure 3) Fe-O-OH was generated by the reaction between the Fe 0 in S-nZVI@ATP and oxygen or water.This facilitated the adsorption of Pb 2+ .The Fe-O band at 665 cm −1 became stronger after Pb 2+ adsorption, which was due to changes in the valence state of Fe 0 [20].

Effects of various S/Fe ratios on Pb 2+ adsorption by S-nZVI@ATP
The S/Fe ratio significantly affected the Pb 2+ removal performance of the S-nZVI@ATP composites.Using 0.1, 0.25, 0.5, and 0.75 S/Fe molar ratios resulted in Pb 2+ removal rates of 47.85%, 41.61%, 42.54%, and 57.37%, respectively.As the S/Fe ratio was increased, Pb 2+ adsorption first decreased and then increased (figure 4).The Pb 2+ adsorption capacities of these composites ranged from 309.84 mg g −1 to 401.60 mg g −1 .With the increase of mole ratio of S/Fe, more FeS was generated on the nZVI surface, meaning that less Fe 0 was available to convert  Pb 2+ into Pb 0 .To further study the S-nZVI@ATP performance for the Pb 2+ removal, a 0.75 S/Fe molar ratio was used for the reaction mechanism and adsorption characteristics analysis.

Adsorption kinetics
Kinetic analysis of Pb 2+ adsorption by the S-nZVI@ATP revealed rapid adsorbtion in the first 4 h with a significant increase Pb 2+ adsorption from 160.4 to 356.8 mg g −1 (see figure 5).The adsorption of Pb 2+ slowed down with increasing time from 4 h to 20 h, and equilibrium was reached after 20 h.These results were fitted using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models (tables 2 and 3).The correlation coefficients of the pseudo-first-order (R 1 2 = 0.8567) and pseudo-second-order (R 2 2 = 0.9590) models demonstrate the compatibility of the experimental data with the latter.Moreover, the  Table 2. S-nZVI@ATP kinetic parameters for Pb 2+ adsorption.
theoretical equilibrium adsorption capacity of this model (q m2 = 407.10mg g −1 ) also more closely fits the experimental results.Therefore, the Pb 2+ adsorption by S-nZVI@ATP was rate-limited by chemical adsorption [21].A segmented intra-particle diffusion kinetic fitting result is presented in table 3. Two stages can also be distinguished in the adsorption process: the first stage revealed a rapid adsorption rate (k d1 = 134.4634),while the second stage had a reduced adsorption rate (k d2 = 9.8958).The correlation coefficients these two stages both exceed 0.9, indicating intraparticle diffusion [22].Two-stage fitting curves did not pass through the origin.Thus, intraparticle diffusion is not the main rate-limiting step in Pb 2+ adsorption by S-nZVI@ATP.

Adsorption isotherm
The fitting of the Freundlich, Langmuir, and Temkin isothermals with our data is shown in figure 6 and table 4.This indicates that Pb 2+ was adsorbed on S-nZVI@ATP by many mechanisms.With the temperature increment from 298 to 318 K, K L increased from 0.0170 to 0.0631, K F increased from 139.1995 to 287.5965, and 1/n decreased from 0.1849 to 0.1166, indicating that Pb 2+ was more easily adsorbed on S-nZVI@ATP at higher temperatures.At 298 K, the saturation adsorption capacity reached 580.74 mg g −1 , which was much higher than the adsorption capacity of Pb 2+ (270.27mg g −1 ) by magnetic ATP in the study of Sun et al [23].

Adsorption thermodynamics
The Pb 2+ adsorption on S-nZVI@ATP was studied at different temperatures to calculate thermodynamic parameters.Figure 7 shows a Van't Hoff plot with the fitted ΔG 0 , ΔH 0 , and ΔS 0 values for the Pb 2+ adsorption  on S-nZVI@ATP.The values for the 298, 308, and 318 K are also reported in table 5. ΔG 0 was less than 0 for all three temperatures.This indicated that Pb 2+ adsorption on S-nZVI@ATP occurred spontaneously.The ΔH 0 values were positive, suggesting the endothermic adsorption process.The positive ΔS 0 values that the randomness and degree of freedom of the solution interface increased when Pb 2+ was adsorbed on S-nZVI@ATP.This was mainly caused by the simultaneous hydrolysis and adsorption of Pb 2+ during adsorption.The hydrolysis of Pb 2+ requires more heat than the adsorption of Pb 2+ by S-nZVI@ATP.Meanwhile, the dissociation of water molecules increases the degree of freedom of the system, while Pb 2+ adsorption on S-nZVI@ATP decreases the degree of freedom.The hydrolysis of Pb 2+ and the adsorption of free Pb 2+ vary in different ranges.The increase of the former is greater than the decrease of the latter, resulting in an endothermic and entropy increase in the whole adsorption process.Table 4. Parameters obtained by fitting the Pb 2+ adsorption on S-nZVI@ATP using various models.

T/K
Langmuir Freundlich Temkin is the maximum adsorbed capacity; K L (l mg −1 ) is the Langmuir constant indicating the affinity of the binding sites for the heavy metal ions; K f (l mg −1 ) is the Freundlich adsorption coefficient; /n 1 is the adsorption intensity; K t is the Temkin isothermal adsorption equilibrium constant (L/mol), A is the constant related to Temkin and adsorption heat (J/mol).Table 5. Thermodynamic parameters for the Pb 2+ adsorption on S-nZVI@ATP.3.6.Pb 2+ removal under various pH conditions An initial Pb 2+ concentration equal to 700 mg l −1 was used to analyze the S-nZVI@ATP adsorption performance at pH = 2-5.5, as shown in figure 8.At pH 2.0 to 2.5, Pb 2+ adsorption by S-nZVI@ATP sharply improved with the increase in pH.In this range, the S-nZVI@ATP adsorption capacity enhanced from 283.32 to 381.12 mg g −1 , while the rise of the Pb 2+ removal rate was observed from 40.47% to 54.44%.When pH was increased from 2.5 to 5.5, the S-nZVI@ATP Pb 2+ adsorption capacity changed from 381.1 to 402.3 mg g −1 , and the Pb 2+ removal rate enhanced from 54.44% to 57.47%.Therefore, the sulfidation of S-nZVI@ATP resulted in efficient and stable Pb 2+ removal performance in a wide pH range of 2.5-5.5.The S-nZVI@ATP isoelectric point in the aqueous solution was 7.43.When pH < 7.43, the functional group's deprotonation on S-nZVI@ATP increased with increasing pH, S-nZVI@ATP became more negative, and S-nZVI@ATP and Pb 2+ became less electrostatically repelled.Consequently, S-nZVI@ATP exhibited the worst Pb 2+ removal effect when the pH was 2. This was mainly because the positive charge on the S-nZVI surface was more significant under strongly acidic conditions.S-nZVI and Pb 2+ , therefore, exhibited greater electrostatic repulsion, resulting in poor adsorption.Moreover, under strongly acidic conditions, H + competed with Pb 2+ for active sites, further reducing the S-nZVI@ATP adsorption capacity.Since at high pH values less, H + was present in the solution, H + competitive adsorption with Pb 2+ also decreased.Therefore, the S-nZVI@ATP surface became more negative, improving its performance for Pb 2+ .During this reaction, Fe 0 reacted with the water or residual oxygen in the water, forming Fe(OH) 2 or Fe(OH) 3 .These species were then hydrolyzed into coordination ions such as Fe(OH) + and Fe(OH) 2+ , promoting the formation of related complexes and the precipitation of Pb(OH) 2 .The XRD analysis reported in section 3.1.2confirms this conclusion.

Pb 2+ removal mechanisms by S-nZVI@ATP
The surface valence states and elemental composition of S-nZVI@ATP were analyzed by XPS before and after the adsorption of Pb 2+ .The survey spectra shown in figure 9 In contrast, the FeOOH peak somewhat decreased in intensity after Pb 2+ adsorption.This trend was also observed by FT-IR analysis.This indicated the involvement of Fe in S-nZVI@ATP in Pb 2+ adsorption and was able to react with the Pb 2+ .The S-nZVI@ATP's Fe 2p spectra obtained before as well as after Pb 2+ adsorption are presented in figure 9(d).The peaks at 710.5, 712.9, and 706.9 eV were attributed to Fe (II), Fe (III), and Fe 0 , and the peaks at 723.21 and 727.51 eV respectively correspond to Fe (II) and Fe (III) [27].During the preparation of S-nZVI@ATP, oxygen or water may react with Fe 0 to form iron oxides on nZVI.The S 2p S-nZVI@ATP spectra obtained in both conditions i.e., before adsorption and after Pb 2+ adsorption are presented in figure 9(e).Three characteristic peaks were observed, ascribed to S  This demonstrated the loading of S-nZVI that has been successfully achieved on the ATP surface.The S on the S-nZVI@ATP surface primarily consisted of S 2− , S 2 2− , and SO 4 2− .This SO 4 2− mainly came from the ferrous sulfate synthesized by nZVI [30,31].Figure 9(f) depicts the Pb 4f spectrum of S-nZVI@ATP Pb 2+ adsorption.Pb 4f 7/2 and Pb 4f 5/2 peaks were observed at 143.01 eV and 137.9 eV, and these may be related to the PbS, Pb(OH) 2 , and Pb 0 formations.Consistent with the XRD analysis, Pb 0 was produced via the reduction of Pb 2+ by Fe 2+ generated by S-nZVI@ATP, while lead oxide was produced by the dehydration of Pb(OH) 2 [32].

Conclusions
ATP was modified by acid and loaded with sulfided nano-zero-valent iron.The prepared S-nZVI@ATP was able to efficiently and consistently remove Pb 2+ from water within the pH range of 2.5-5.5.In addition, the pseudo- second-order kinetic model described the adsorption of Pb 2+ by S-nZVI@ATP, implying an electron sharing and transfer process.The Freundlich isothermal adsorption model was utilized to demonstrate the Pb 2+ adsorption by S-nZVI@ATP, indicating a multi-molecular layer chemical adsorption process.The S-nZVI@ATP adsorbent achieved a Pb 2+ adsorption capacity of 401.60 mg g −1 and 57.37% of Pb 2+ removal rate using 1 g l −1 of adsorbent, 30 ml of Pb 2+ solution (700 mg l −1 initial concentration), 25 °C of adsorption temperature with adsorption time of 24 h.Pb 2+ removal was achieved via the formation of PbS and Pb(OH) 2 precipitates.These results demonstrate that the acid-modified attapulgite-loaded vulcanized nZVI composite prepared in this work is a suitable functional material for treating Pb 2+ in water, and provides a new idea for how to efficiently and reasonably solve the problem of lead pollution in industrial wastewater in the future.
(a)  indicate that sulfide was present on the adsorbent surface.Typical Pb 4f peaks were visible after the adsorption reaction, indicating the effective adsorption of Pb 2+ .The characteristic O 1s spectra of S-nZVI@ATP before adsorption as well as after adsorption of Pb 2+ are depicted in figures 9(b)-(c).Four typical peaks were observed, attributed to Fe 3 O 4 at 529.5 eV, Fe-O at 530.4 eV, C-O/FeOOH at 531.2 eV, and C = O at 532.0 eV[24].Therefore, S-nZVI@ATP mainly contained Fe and O on its surface, primarily in the form of Fe 3 O 4 , Fe 2 O 3 , FeOOH, and FeO[25,26].The peak intensities of Fe-O, Fe 3 O 4 , and Fe 2 O 3 slightly increased after Pb 2+ adsorption, indicating the generation of more iron oxide.

Figure 9 .
Figure 9. Scan (a) and and high-resolution (b) O 1s spectra of S-nZVI@ATP before and after the treatment with Pb 2+ .(c) Highresolution O 1s spectrum of S-nZVI@ATP after being adsorbed with Pb 2+ .(d) S-nZVI@ATP high-resolution Fe 2p spectra before adsorption and after being adsorbed with Pb 2+ .(e) S-nZVI@ATP high-resolution S 2p spectra before and after Pb 2+ adsorption.(f) S-nZVI@ATP high-resolution Pb 4f spectrum after Pb 2+ adsorption.Note: * B.E. represents the kinetic energy or electron binding energy, and a.u.represents the relative photoelectron flow intensity.

Table 1 .
Kinetic and isotherm models utilized to describe Pb 2+ removal in the batch experiments.

Table 3 .
Intra -particle diffusion kinetics of Pb 2+ adsorption on S-nZVI@ATP.C 0 (mg l −1 ) di is the diffusion rate constant within a particle; i represents phase i (i = 1 or 2).