The sulfidation of nanoscale zerovalent iron (nZVI) has received increasing attention for reducing the oxidizability of nZVI and improving its reactivity toward heavy metal ions. Here, a sulfide (S)-modified attapulgite (ATP)-supported nanoscale nZVI composite (S-nZVI@ATP) was rapidly synthesized under acidic conditions and used to alleviate Cd2+ toxicity from an aqueous solution. The degree of oxidation of S-nZVI@ATP was less than that of nZVI@ATP, indicating that the sulfide modification significantly reduced the oxidation of nZVI. The optimal loading ratio was at an S-to-Fe molar ratio of 0.75, and the adsorption performance of S-nZVI@ATP for Cd2+ was significantly improved compared with that of nZVI@ATP. The removal of Cd2+ by S-nZVI@ATP was 100% when the adsorbent addition was 1 g/L, the solution was 30 mL, and the adsorption was performed at 25 °C for 24 h with an initial Cd2+ concentration of 100 mg/L. Kinetics studies showed that the adsorption process of Cd followed the pseudo-second-order model, indicating that chemisorption was the dominant adsorption mechanism. The adsorption of Cd2+ by S-nZVI @ATP is dominated by the complexation between the iron oxide or iron hydroxide shell of S-nZVI and Cd2+ and the formation of Cd(OH)2 and CdS precipitates.

  • S-nZVI was loaded on ATP, effectively preventing nZVI agglomeration, and the sulfide modification significantly reduced the oxidation of nZVI.

  • Cd2+ removal was mailly attributed to complex with the iron oxides or iron hydroxide shell of S-nZVI.

  • S-nZVI@ATP showed a stable adsorption effect on Cd2+ over a wide pH range.

  • The removal of Cd2+ by S-nZVI@ATP follows the quasi-second-order kinetic adsorption model.

Graphical Abstract

Graphical Abstract
Graphical Abstract

With the drastic increase in industrial production, many heavy metal elements enter water bodies through industrial practices, such as electroplating, mining, metallurgy, and tanning, causing their levels to exceed the standard and seriously polluting water resources. Cadmium is considered the most potentially carcinogenic substance by American environmental organizations (Sobhanardakani & Zandipak 2015). For humans, Cd is a nonessential element that usually enters the body in the form of Cd2+ and accumulates in the kidneys, liver, and other organs; it can harm the human body even in trace amounts (Namasivayam & Ranganathan 1995). Cd is widely used in the electroplating industry because of its excellent ability to prevent corrosion by alkaline substances. It is also widely used in fabricating batteries, such as the commonly used Ni–Cd batteries. High concentrations of Cd-containing wastewater from the electroplating and battery industries are common sources of Cd pollution. Additionally, Cd-containing waste gases from the zinc refining industry and chemical production can spread to the surrounding environment through rainfall, thus expanding the contaminated area (Soltani et al. 2021). Moreover, in agricultural activities, many fertilizers and pesticides contain Cd. Thus, the enormous use of fertilizers and irrational spraying of pesticides can intensify Cd contamination in the soil, causing crop safety problems (Ma et al. 2022a, 2022b).

The adsorption method is used to remove heavy metal ions from water bodies through the physical and chemical interactions between adsorbent materials and heavy metal ions by enriching them on the adsorbent through electrostatic gravitation, complexation, ion chelation, and coordination (Nguyen et al. 2021). As a green, ecofriendly, selective, cost-effective, and efficient heavy metal treatment technology, the wide applicability of the adsorption method – in terms of operating conditions and treatment efficiency – makes it increasingly suitable for heavy metal treatment (Kwak et al. 2019). Adsorbent development is the key difficulty faced in the adsorption method, and the search for an environmentally friendly, economical, and efficient adsorbent is the critical point of the adsorption method (Dang et al. 2021).

Attapulgite (ATP) is a natural clay mineral with a 2:1 type of layered chain of water-rich Mg–Al silicates belonging to the sepiolite family (Fu et al. 2021). ATP has many distinctive physicochemical properties because of its special chain-layered crystal structure, including adsorption, cation exchange capacity, and other physical and chemical properties. The adsorption properties of ATP mainly depend on its own specific surface area, surface physical and chemical structures, and ionic state (Haden & Schwint 1967). The nontoxicity, cost-effectiveness, cation exchangeability, and universal applicability of ATP make it a potentially excellent adsorbent (Chen & Zhao 2009). However, natural ATP clay materials existing in natural environments are limited (Anang et al. 2022). The mineral composition of ATP raw minerals is complex, and in addition to containing the main component of ATP, it is often interspersed with other mineral impurities of variable content, including clay minerals, such as montmorillonite, kaolinite, seafoam, hydromica, and nonclay minerals, such as quartz and dolomite (Liu et al. 2012). Notably, there are significant morphological and chemical component differences that exist between ATP raw minerals formed in different environments (Nasedkin et al. 2009); these differences are additionally accompanied by differences in adsorption properties, which has become a bottleneck in promoting the application of ATP.

Nanoscale zerovalent iron (nZVI; also abbreviated as Fe0) particles range from 1 to 100 nm in size. Because of the small particle size, a unique surface effect and a small-sized effect can be generated, resulting in better adsorption properties. However, Fe0 is inherently reductive, and nanomaterials have surface effects that make nZVI more reductive (Santhosh et al. 2016). Therefore, it is used to remove various pollutants from water, such as heavy metals, organic compounds, chlorinated organics, and radioactive substances, and has considerable potential for application in wastewater treatment (Xu et al. 2019b; Reginatto et al. 2020). However, the application of nZVI in the practical treatment of heavy metal pollution is unsatisfactory for removal, attributed to agglomeration, easy aging, and poor electron selectivity (Li et al. 2017). To avoid the agglomeration of nZVI particles and enhance the stability and mobility of nZVI, various modifications have been explored. The most cost-effective approach is the solid-loading method, which can effectively solve the problems of easy agglomeration and aging of nZVI (Wu et al. 2018). In recent years, a new revolution of modification has emerged with the introduction of vulcanizing agents for nZVI that can improve the reactivity and dispersion of nZVI as well as the electron selectivity for the target contaminants (Han & Yan 2016). In addition, the research on nZVI mainly focuses on metal ions with standard reduction potential higher than Fe2+/Fe, such as Cr6+, and less on Cd2+ with standard reduction potential similar to that of zero-valent iron and lower reduction efficiency. In order to better develop nZVI, it is urgent to investigate the removal mechanism of Cd in water by clay mineral-loaded nZVI complex. Considering the increasingly serious pollution of heavy metals in water bodies, investigating efficient, environmentally friendly, and inexpensive adsorbents has become a significant research focus. Herein, inexpensive Gansu ATP was selected as the carrier, and a sulfide modified attapulgite supported nanoscale zero-valent iron composite (S-nZVI@ATP) was prepared by combining the liquid-phase reduction and sulfidation methods. Then, the adsorption performance of the fabricated composite material for Cd2+ in water was investigated through static adsorption experiments. Finally, based on the optimal S-to-Fe ratio, the effects and patterns of pH, temperature changes, and aging time on the adsorption of Cd2+ were discussed, and the adsorption performance of S-nZVI@ATP on Cd2+ in water was investigated using adsorption kinetic, isothermal adsorption, and adsorption thermodynamic models. The morphological and structural changes of the adsorbent before and after adsorption were also investigated via scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy to investigate the adsorption mechanism.

Experimental materials

Attapulgite raw ore, ferrous sulfate heptahydrate (FeSO4·7H2O, ≥ 99.0%), sodium sulfide nine water (Na2S·9H2O), anhydrous ethanol (C2H5OH, ≥ 99.7%), sodium borohydride (NaBH4, ≥ 98.0%), sodium hydroxide (NaOH, ≥ 96.0%), nitric acid (HNO3), hydrochloric acid (HCl), cadmium nitrate (Cd(NO3)2·4H2O, ≥ 99.0%) were all purchased from Sinopharm Chemical Reagents Co., Ltd. Sodium nitrate (NaNO3, ≥ 99.0%) was purchased from Tianjin Damao Chemical Reagent Factory. All chemical reagents were of analytical grade. Attapulgite from Tianshui County, Gansu Province was selected. Its mineral composition was as follows: 19.3% attapulgite, 15.1% quartz, 5.3% sepiolite, 19.8% feldspar, 10.9% dolomite, and 6.9% mica. The attapulgite pH was 8.47.

Pretreatment of materials

The attapulgite ore screened by a 200 mesh sieve was placed in a 5 L beaker, and modified by adding 3,000 mL of hydrochloric acid with a 3 mol/L concentration. Attapulgite was subjected to ultrasonic wave treatment (ultrasonic power: 500 W) for 1 h after being stirred for 3 h using an electric stirrer at 1,000 rpm. The supernatant was decanted after standing for a period of time. Next, the lower sediment was washed with distilled water until a neutral pH was attained. The obtained sediment was then dried in an oven at 80 °C, ground, and sieved with a 200 mesh sieve to obtain acid-modified attapulgite.

The S-nZVI@ATP composites were prepared through liquid phase reduction in a 1,000 mL three-neck flask reactor. First, a certain amount of attapulgite and FeSO4·7H2O were dissolved in a three-neck flask containing 100 mL deionized water and stirred for 2 h. Next, 100 mL anhydrous ethanol was added and stirred for 30 min. Subsequently, a certain amount of Na2S and 0.2 mol NaBH4 were added to 400 mL ultrapure water according to the S/Fe2+ molar ratios of 0, 0.1, 0.25, 0.5, and 0.75. S-nZVI@ATP was obtained via centrifugation. It was then washed with ethanol and deionized water thrice and vacuum dried at 40 °C for 6 h. All of the abovementioned reaction processes were performed in an N2 atmosphere. S-nZVI@ATP was then vacuum-sealed and stored in the freezing layer of the refrigerator for future use (Li et al. 2021).

Cd2+ adsorption in aqueous solution by S-nZVI@ATP

Screening of S-nZVI@ATP adsorbents with different iron–attapulgite ratios

Two groups of seven identical 50 mL centrifuge tubes were taken. Different adsorbents (i.e., ATP, S-nZVI, and nZVI@ATP with sulfide–iron ratios of 0.1, 0.25, 0.5, and 0.75) with 1 g/L and 30 mL dosages were added one by one with 100 mg/L Cd2+ concentration. The pH value of the solution was adjusted to 5.0 with 0.1 mol/L HNO3 and 0.1 mol/L NaOH before the Cd solution was added. The reaction temperature was set at 25 °C, and the rotation speed was 200 rpm. Oscillation was performed in a constant-temperature water bath oscillator until the material fully reacted with the solution for 24 h. It was then passed through a 0.45 μm filter membrane. The remaining Cd2+ concentration in the solution was measured using a flame atomic absorption spectrometer. The best adsorption performance for Cd2+ was then determined. Considering the actual conditions, the subsequent experiments were performed on the basis of the optimum sulfide–iron ratio.

Adsorption kinetics experiment

For this experiment, 50 mL centrifuge tubes were taken, to which 1 g/L S-nZVI@ATP and 30 mL oxygen-free Cd2+ solution, with 100 mg/L concentration and pH 5.0, were added. Oscillation was performed at a speed of 200 rpm in a constant-temperature oscillator at 25 °C for 0.25, 0.5, 1, 2, 3, 4, 6, 8, 12, 20, and 24 h. At the end of the oscillation, the solution was filtered through a 0.45 μm membrane, and the Cd2+ concentration was determined.

Effect of pH on Cd2+ adsorption by S-nZVI@ATP

Nine identical 50 mL centrifuge tubes were taken, to which 1 g/L S-nZVI@ATP and 30 mL of 100 mg/L oxygen-free Cd2+ solution with pH 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 6.0 were added. Oscillation was performed in the constant-temperature water bath oscillator at 25 °C at a speed of 200 rpm for 24 h. The solution was passed through a 0.45 μm filter membrane to measure the remaining Cd2+ concentration. The zeta potential of the corresponding S-nZVI@ATP solution was measured using a zeta potentiometer.

Characterization

The MERLIN SEM (Carl Zeiss, Germany) was used to observe and analyze the morphological changes in the samples. The X-max extreme X-ray EDS (Oxford Instruments, England) was used to determine their elemental composition. The crystal structure and the composition of the samples were determined using a high-resolution XRD (Rigaku, Japan). The functional groups of the samples were analyzed using a VERTEX 70 FTIR (Bruker Company). nZVI@ATP was measured using a zeta potential tester (Zeta) (Nano-ZS90, Malvern Company, UK). The Cd2+ concentration in the aqueous solution was measured through atomic adsorption spectrophotometry (AAS, 220FS, Varian, USA).

Characterization of composite

Figure 1(a)–1(c) shows the microscopic SEM-obtained morphologies of ATP, nZVI@ATP, and S-nZVI@ATP. After sulfidation, the surface roughness of the composite increased, and the regular spherical nZVI with well-defined particles was basically not observed on the surface, however, some flocculent particles of different sizes, which should be the nZVI particles wrapped by sulfide, were observed. Figure 1(d)–1(f) shows the energy spectra of ATP, nZVI@ATP, and S-nZVI@ATP, respectively. The mass ratio of the spectra shows that compared to acid-washed ATP the mass percentage of Fe in S-nZVI@ATP increased from 1.79 to 36.19%, and the diffraction peak of S was added with the corresponding mass percentage of 2.04%, compared with nZVI@ATP. Na was introduced by Na2S, indicating the successful modification of ATP by nano-ZVI sulfide.
Figure 1

SEM image of (a) ATP, (b) nZVI@ATP, and (c) S-nZVI@ATP; EDS image of (d) ATP, (e) nZVI@ATP, and (f) S-nZVI@ATP.

Figure 1

SEM image of (a) ATP, (b) nZVI@ATP, and (c) S-nZVI@ATP; EDS image of (d) ATP, (e) nZVI@ATP, and (f) S-nZVI@ATP.

Close modal
The ATP characteristic vibrational peaks located at 3,404, 1,022, 879, and 467 cm−1 were unchanged (Liu et al. 2012), indicating that the sulfide modification did not change the internal structure of ATP (Figure 2). A weaker vibrational peak was observed at 665 cm−1 for S-nZVI@ATP (c), which is the characteristic peak of the Fe–O-bonding absorption (Dai et al. 2022), significantly weaker than the Fe–O absorption peak near 620 cm−1 for nZVI@ATP (b), which reflects a certain extent that the sulfide modification reduces the occurrence of nZVI oxidation.
Figure 2

FTIR spectra of ATP (a), nZVI@ATP (b) and S-nZVI@ATP (c).

Figure 2

FTIR spectra of ATP (a), nZVI@ATP (b) and S-nZVI@ATP (c).

Close modal
The crystal structure and composition of the material were determined by high resolution XRD. The scanning range was 5° ∼ 80°, and the step size was 0.01°. It can be seen from the Figure 3(a) that the main phases of ATP are attapulgite (◆), SiO2 (▴) and dolomite (◇): the characteristic diffraction peaks of ATP appear at 2θ = 27.5°, 30.89°, 35.1° and 40.01°, the characteristic diffraction peaks of SiO2 appear at 2θ = 20.81°, 26.58° and 39.5°, 2θ = 29.49° and 33.41° are characteristic diffraction peaks of dolomite (Ren et al. 2021). As shown in the Figure 3(b), the characteristic diffraction peaks of Fe0 (○) appeared at 2θ = 44.86° (Dai et al. 2022), indicating that nZVI was successfully loaded on ATP. In general, with the increase of grain size, the characteristic summit becomes high and sharp. The smaller the grain size is, the wider the corresponding characteristic summit is, or even difficult to identify. The characteristic peak of Fe0 has a wider peak type and a weaker peak strength, indicating the low crystallization of Fe0. The positions of the characteristic ATP peaks at 2θ = 30.89° and 41.01° did not change for nZVI@ATP and S-nZVI@ATP. nZVI@ATP and S-nZVI@ATP both showed diffraction peaks of Fe0 (100) around 2θ = 44.86°; however, the diffraction peak of S-nZVI@ATP broadened, indicating that Fe0 crystallinity becomes weaker after sulfidation (Su et al. 2015) (Figure 3). No obvious diffraction peaks of FeS are observed in the S-nZVI@ATP pattern, which can be attributed to the poor crystallinity of the generated FeS or the overlap with other characteristic peaks that cannot be accurately identified.
Figure 3

XRD spectra of ATP (a), nZVI@ATP (b) and S-nZVI@ATP (c).

Figure 3

XRD spectra of ATP (a), nZVI@ATP (b) and S-nZVI@ATP (c).

Close modal
The specific surface area of S-nZVI@ATP was measured by bet-N2 surface area analyzer, as shown in Table 1. The specific surface area (Brunauer–Emmett–Teller, BET) and pore size of nZVI@ATP were 22.381 m2/g and 10.752 nm, respectively, which is smaller than the BET of nZVI (20–60 m2/g) (Arshadi et al. 2014), By analyzing the N2 adsorption–desorption isotherm curve in Figure 4, S-nZVI@ATP showed a type IV(a) isotherm (short platform, only inflection point). A comprehensive analysis of isotherms, Barrett–Joyner–Halenda (BJH) model, and T-plot micropore model showed that the pore structure of S-nZVI@ATP was mesoporous (2–50 nm).
Table 1

Pore structure parameter of S-nZVI@ATP

SampleS-nZVI@ATP
BET surface area (m2/g) 22.381 
BJH adsorption cumulative surface area of pores (m2/g) 23.404 
Single point adsorption total pore volume of pores (cm3/g) 6.016 × 10−2 
BJH adsorption cumulative volume of pores (cm3/g) 5.842 × 10−2 
Adsorption average pore width (4 V/A by BET) (nm) 10.752 
SampleS-nZVI@ATP
BET surface area (m2/g) 22.381 
BJH adsorption cumulative surface area of pores (m2/g) 23.404 
Single point adsorption total pore volume of pores (cm3/g) 6.016 × 10−2 
BJH adsorption cumulative volume of pores (cm3/g) 5.842 × 10−2 
Adsorption average pore width (4 V/A by BET) (nm) 10.752 
Figure 4

Nitrogen adsorption–desorption curve of nZVI@ATP.

Figure 4

Nitrogen adsorption–desorption curve of nZVI@ATP.

Close modal

Effect of different sulfur to iron ratios on the adsorption of Cd2+ by S-nZVI@ATP

When the S-to-Fe molar ratios are 0.1, 0.25, 0.5, and 0.75, the removal rates of Cd2+ are 61.63, 81.9, 85.7, and 100%, respectively, which increase with the increment in the S-to-Fe molar ratio (Figure 5). Thus, the S-to-Fe molar ratio significantly affects the removal of Cd2+ by the composites. As shown in Figure 5 the S-to-Fe molar ratio with the best adsorption performance for Cd2+ is 0.75. The adsorption performance characteristics of S-nZVI@ATP prepared with different S-to-Fe ratios were significantly stronger for Cd2+ than those of ATP, increasing from 11.66 to 100 mg/g. The adsorption capacity of nZVI@ATP was greater than that of pure nZVI but less than that of S-nZVI@ATP with a 0.75 S-to-Fe ratio. The pure nZVI particles easily agglomerated, which limited their chances of contact with pollutants and prevented them from giving full play to their adsorption properties, thus wasting adsorption sites. However, after ATP loading, nZVI agglomeration improved owing to the spatial barrier effect of ATP. This was the main reason why the Fe content of nZVI@ATP and S-nZVI@ATP was substantially less than that of pure nZVI but the adsorption amount of nZVI@ATP and S-nZVI@ATP remained higher than that of pure nZVI. Considering the reaction performance and economics of S-nZVI@ATP for Cd2+ removal, in the subsequent experiments, an S-to-Fe molar ratio of 0.75 was used to investigate the adsorption characteristics and reaction mechanisms. Table 2 compares the adsorption performance of different adsorbents. The adsorption performance of S-nZVI@ATP on Cd2+ is higher than that of the other adsorbents mentioned above. Therefore, it can be seen that S-nZVI@ATP has important potential in removing Cd2+ from water.
Table 2

Adsorption capacity of cadmium by adsorbent reported in literatures

SampleCarrierC0 (mg/L)T(K)pHDosage (g/L)Qt (mg/g)Ref.
nZVI  300 298 2.0 86.0 Owija et al. (2021)  
nZVI-BC Corn stalk biochar 30 298 5.5 0.25 33.8 Yang et al. (2020)  
GB/nZVI Graphene-like biochar 20 298 7.0 0.4 46.4 Liu et al. (2020)  
Z- nZVI Zeolite 100 298 6.0 0.5 48.6 Li et al. (2018)  
nZVI@mSiO2 Mesoporous hydrated silica 20 298 5.0 0.15 105.2 Ma et al. (2022a, 2022b)  
S-nzVI@ATP Attapulgite 200 298 5.0 1.0 140.6 This work 
SampleCarrierC0 (mg/L)T(K)pHDosage (g/L)Qt (mg/g)Ref.
nZVI  300 298 2.0 86.0 Owija et al. (2021)  
nZVI-BC Corn stalk biochar 30 298 5.5 0.25 33.8 Yang et al. (2020)  
GB/nZVI Graphene-like biochar 20 298 7.0 0.4 46.4 Liu et al. (2020)  
Z- nZVI Zeolite 100 298 6.0 0.5 48.6 Li et al. (2018)  
nZVI@mSiO2 Mesoporous hydrated silica 20 298 5.0 0.15 105.2 Ma et al. (2022a, 2022b)  
S-nzVI@ATP Attapulgite 200 298 5.0 1.0 140.6 This work 
Figure 5

Adsorption of Cd2+ by S-nZVI@ATP with different sulfide-iron ratios and the ratio of iron to attapulgite is fixed at 1:2. Reaction conditions: Cd2+, 30 mL at 100 mg/L; material, 1 g/L; temperature, 25 °C; reaction time, 24 h.

Figure 5

Adsorption of Cd2+ by S-nZVI@ATP with different sulfide-iron ratios and the ratio of iron to attapulgite is fixed at 1:2. Reaction conditions: Cd2+, 30 mL at 100 mg/L; material, 1 g/L; temperature, 25 °C; reaction time, 24 h.

Close modal

Adsorption kinetics

The experimental data of the adsorption capacity of S-nZVI@ATP for Cd2+ with time were fitted by the pseudo-first-order and pseudo-second-order kinetic equations and the intraparticle diffusion equation (Equations (1)–(3)), respectively. Figure 6 shows the fitting curve, and Tables 3 and 4 present the fitting parameters.
formula
(1)
formula
(2)
formula
(3)
Table 3

Kinetic parameters for Cd2+ adsorption on S-nZVI@ATP

C0(mg/L)Pseudo-first-order
Pseudo-second-order
qm1/(mg/g)k1[g/(mg•h)]R12qm2/(mg/g)k2 [g/(mg•h)]R2
200 135 4.07 0.786 141 0.054 0.975 
C0(mg/L)Pseudo-first-order
Pseudo-second-order
qm1/(mg/g)k1[g/(mg•h)]R12qm2/(mg/g)k2 [g/(mg•h)]R2
200 135 4.07 0.786 141 0.054 0.975 

Note: qm1 is the theoretical adsorption capacity obtained by fitting the pseudo-first-order kinetic model; qm2 is the theoretical adsorption capacity obtained by fitting the pseudo-second-order kinetic model; R is the correlation coefficient.

Table 4

Parameters of intra-particle diffusion kinetic for Cd2+ adsorption on S-nZVI@ATP

C0/(mg/L)kd1/[mg/(g•h1/2)]C1/(mmol/g)R2kd2/[mg/(g•h1/2)]C2/(mmol/g)R2
200 31.4 84.7 0.949 1.07 135 0.815 
C0/(mg/L)kd1/[mg/(g•h1/2)]C1/(mmol/g)R2kd2/[mg/(g•h1/2)]C2/(mmol/g)R2
200 31.4 84.7 0.949 1.07 135 0.815 

Note: Kdi is the intra-particle diffusion rate constant. Ci is the intercept of the intra-particle diffusion equation. i represents phase i, i = 1 or 2; R is the correlation coefficient.

Figure 6

Adsorption kinetic model fitting curves (a) and intra-particle diffusion model fitting curve (b) of S-nZVI@ATP for Cd2+.

Figure 6

Adsorption kinetic model fitting curves (a) and intra-particle diffusion model fitting curve (b) of S-nZVI@ATP for Cd2+.

Close modal

In Equations (1)–(3), qe and qt are the adsorption capacities of nZVI@ATP for Cd2+ at the adsorption equilibrium and at any time (mg/g), respectively; k1 is the pseudo-first-order kinetic constant (g/(mg·h)); k2 is the pseudo-second-order kinetic constant, g/(mg·h); and kd is the diffusion rate constant in particles, mg/(g·h1/2), which directly reflects the adsorption rate. The reaction rate increases as kd increases. Ci is the intercept (mmol/g), which is related to the boundary layer thickness. If Ci is not zero, the diffusion process of the boundary layer to the particle's surface cannot be ignored. Besides, the boundary layer influence increases as Ci increases.

To gain insight into the adsorption behavior of S-nZVI@ATP, the varying effects of S-nZVI@ATP on qe after different reaction times were investigated. Figure 6 shows that the adsorption amount of S-nZVI@ATP on Cd2+ rapidly increased within the initial 0–2 h of adsorption, reaching 131.67 mg/g. The reason is that the sufficient number of vacant sites on the surface of S-nZVI@ATP involved in the initial reaction and the mass transfer driving force because of the large concentration gradient of Cd2+ both contributed to the rapid diffusion of Cd2+ to S-nZVI@ATP. After 2 h, the number of active sites on the surface saturates, and Cd2+ overcomes the large resistance to enter the interior of S-nZVI@ATP in search of adsorption sites, which slowly increases the adsorption amount and finally reaches adsorption equilibrium.

Table 3 presents the detailed kinetic fitting parameters for the adsorption of Cd2+ by S-nZVI@ATP. Evidently, the adsorption process of Cd2+ by S-nZVI@ATP was not well fitted by the pseudo-first-order kinetic equation (R12 = 0.7862). However, the pseudo-second-order kinetic equation (R22 = 0.9748) can better explain the adsorption process of Cd2+ by S-nZVI@ATP. The equilibrium adsorption amount qm2 obtained from the fitting is 140.64 mg/g, which deviated <0.5% from the actual results, indicating that the adsorption process of Cd2+ by S-nZVI@ATP composites was not controlled by simple external diffusion factors, and the adsorption process mainly occurred via electron transfer and sharing.

Figure 6(b) shows the results of the intraparticle diffusion model fitting for Cd2+ adsorption by S-nZVI@ATP, and Table 4 presents the fitted parameters. The entire adsorption process can be divided into two stages to be fitted separately. The first stage is the initial reaction stage, which is mainly the liquid membrane diffusion, i.e., Cd2+ diffusion from the solution to the surface of S-nZVI@ATP, and the adsorption rate kd1 is 31.3913, which is fast because of the presence of more significant mass transfer driving force and more vacant sites with little diffusion resistance. The second stage is the intraparticle diffusion stage, as most of the active sites on the surface of S-nZVI@ATP have been occupied by Cd2+, Cd2+ enters the particle to search for binding sites, and the adsorption rate kd2 is 1.0708, which is less than kd1. The magnitude of E can indirectly reflect the effect of the boundary-layer thickness on the adsorption reaction (E1 < E2), which reflects the increase in the boundary-layer thickness and resistance in the intraparticle diffusion phase, thereby decreasing the adsorption rate. Both fitted curves do not pass through the origin, indicating that the process of Cd2+ adsorption by S-nZVI@ATP is co-controlled by the liquid film diffusion in addition to the intraparticle diffusion influence.

Adsorption isotherms and thermodynamic

Langmuir, Freundlich, Temkin isothermal adsorption equations (Equations (4)–(6)) was used to fit the isothermal adsorption curve of S-nZVI@ATP for Cd2+. Thermodynamic equation (Equations (7)–(10) was used to explore the thermodynamic parameters of S-nZVI@ATP for Cd2+ adsorption.
formula
(4)
formula
(5)
formula
(6)
In Equations (4)–(6), qe is the amount of heavy metals adsorbed at equilibrium (mg/g); qm is the maximum adsorbed capacity (mg/g); KL is the Langmuir constant indicating the affinity of the binding sites for the heavy metal ions (L/mg); KF is the Freundlich adsorption coefficient (L/mg); is the adsorption intensity (0.1 < nf < 1); A, Kt are the Temkin adsorption coefficient.
formula
(7)
formula
(8)
formula
(9)

In Equations (7)–(9), R is the gas constant, 8.314 J/(mol K);), T is the absolute temperature, ΔGo is Gibbs free energy change, kJ/mol, ΔH0 is the enthalpy change, kJ/mol, ΔS0 is the entropy change, J/(mol/K), Kd is the distribution coefficient of adsorption (mg/L).

The equation for calculating the apparent adsorption equilibrium constant Kd (mL/g)
formula
(10)

In Equation (10), Kd is the distribution coefficient of adsorption (mL/g), qe is the amount of heavy metals adsorbed at equilibrium (mg/g), Ce is the remaining concentration (mg/L).

Isothermal adsorption experiments were conducted to investigate the adsorption effect of S-nZVI@ATP on Cd2+ at different initial concentrations at 25, 35 and 45 °C. The fitting results of Langmuir, Freundlich and Temkin isothermal adsorption models are shown in Figure 7 and the fitting parameters are listed in Table 5. The correlation coefficients of Freundlich model R2 of S-nZVI@ATP adsorption of Cd2+ at different temperatures were all greater than 0.95, and the Langmuir model had the worst fitting results, with R2 less than 0.72. In comparison, the experimental process of S-nZVI@ATP adsorption of Cd2+ was more consistent with Freundlich isothermal model. The results show that the adsorption behavior on S-nZVI@ATP is actually heterogeneous multilayer chemisorption. At the three temperatures, 1/n ranges from 0.0667 to 0.0777, indicating that S-nZVI@ATP has a high adsorption strength for Cd2+ and the reaction is easy to proceed.
Table 5

Langmuir, Freundlich and Temkin adsorption isotherm parameters for Cd2+ on S-nZVI@ATP

T/KLangmuir
Freundlich
Temkin
qmKLR2KFnfR2AKtR2
298 147 9.62 0.695 106 0.0667 0.969 8.05 68,300 0.945 
308 152 12.4 0.716 109 0.0677 0.978 8.36 628,000 0.957 
318 159 13.6 0.639 109 0.0777 0.953 9.53 146,000 0.915 
T/KLangmuir
Freundlich
Temkin
qmKLR2KFnfR2AKtR2
298 147 9.62 0.695 106 0.0667 0.969 8.05 68,300 0.945 
308 152 12.4 0.716 109 0.0677 0.978 8.36 628,000 0.957 
318 159 13.6 0.639 109 0.0777 0.953 9.53 146,000 0.915 
Figure 7

Adsorption isotherms and fitted models of S-nZVI@ATP for the removal of Cd2+: (a) 298 K, (b) 308 K, (c) 318 K, and (d) Vant’ Hoff curve.

Figure 7

Adsorption isotherms and fitted models of S-nZVI@ATP for the removal of Cd2+: (a) 298 K, (b) 308 K, (c) 318 K, and (d) Vant’ Hoff curve.

Close modal

In order to further understand the thermodynamic behavior and mechanism of S-nZVI@ATP for Cd2+ adsorption process, the corresponding thermodynamic parameters of S-nZVI@ATP for Cd2+ adsorption at different initial concentrations at 298, 308 and 318 K were calculated, and the results are shown in Figure 7.

The thermodynamic parameters of S-nZVI@ATP for Cd2+ at the adsorption temperatures of 298, 308 and 318 K are shown in Table 6. ΔG0 is negative at low concentration and positive at high concentration, indicating that the adsorption process of S-nZVI@ATP for Cd2+ can proceed spontaneously at low concentration. When the concentration increases to a certain extent, namely ΔG0 > 0, the adsorption reaction could not proceed spontaneously, so the initial concentration of Cd2+ should be avoided as much as possible during the experiment. At the same initial concentration, ΔG0 decreases with the increase of adsorption temperature, indicating that temperature increase will promote the adsorption reaction. The positive values of ΔH0 and ΔS0 during the whole process indicate that the adsorption process endothermic and S-nZVI@ATP adsorption of Cd2+ increases the disorder of the solid–liquid interface.

Table 6

Thermodynamic parameters for Cd2+ on S-nZVI@ATP

KdCo (mg/L)ΔHo (kJ/mol)ΔSo (J/(mol·K))ΔGo(kJ/mol)
298 K308 K318 K
Kd=qe/Ce (L/g) 100 23.3 129.1 −15.2 −16.5 −17.8 
150 10.4 48.1 −3.95 −4.43 −4.91 
200 4.59 22.5 −2.10 −2.33 −2.55 
250 6.74 24.5 −0.71 −0.959 −1.21 
300 5.34 17.7 0.0560 −0.121 −0.298 
350 6.63 19.8 0.731 0.533 0.355 
400 6.80 18.9 1.16 0.970 0.781 
450 6.31 15.9 1.58 1.42 1.26 
500 6.01 13.7 1.92 1.78 1.64 
KdCo (mg/L)ΔHo (kJ/mol)ΔSo (J/(mol·K))ΔGo(kJ/mol)
298 K308 K318 K
Kd=qe/Ce (L/g) 100 23.3 129.1 −15.2 −16.5 −17.8 
150 10.4 48.1 −3.95 −4.43 −4.91 
200 4.59 22.5 −2.10 −2.33 −2.55 
250 6.74 24.5 −0.71 −0.959 −1.21 
300 5.34 17.7 0.0560 −0.121 −0.298 
350 6.63 19.8 0.731 0.533 0.355 
400 6.80 18.9 1.16 0.970 0.781 
450 6.31 15.9 1.58 1.42 1.26 
500 6.01 13.7 1.92 1.78 1.64 

Effect of solution pH

The isoelectric point of S-nZVI@ATP in solution is 7.43, such that the surface of S-nZVI@ATP is positively charged when the solution pH is <7.43 and negatively charged when the solution pH is >7.43. Figure 8 shows the adsorption performance of S-nZVI@ATP at an initial concentration of 200 mg/L Cd2+ in the pH range of 2–6. The adsorption capacity of S-nZVI@ATP on Cd2+ culminated in a solution pH of 4, reaching 143.88 mg/g. At pH 2, the adsorption capacity of the solution was minimal at 130.35 mg/g, which is less than 10% different from the optimal adsorption capacity. For S-nZVI@ATP, the isoelectric point is 7.43, the surface of S-nZVI@ATP is positively charged at a low pH, and the presence of electrostatic repulsion does not favor the adsorption of Cd2+. However, the actual adsorption effect shows that the adsorption of Cd2+ at a low pH is insignificantly affected, mainly because after sulfidation, an FeS shell is generated on the surface of nZVI. Moreover, even under acidic conditions, the internal Fe0 core is not easily corroded, and the electronegativity of FeS (5.02 eV) exceeds that of Fe0 (4.04 eV), which can increase the rate of electron transfer from Fe0 to Cd2+. Furthermore, Cd2+ can be removed by generating CdS precipitation with FeS. Figure 6 shows that the change in the solution pH slightly affects Cd2+ adsorption of Cd2+ by S-nZVI@ATP, indicating that S-nZVI@ATP has a good removal effect on Cd2+ in a wide pH.
Figure 8

Adsorption of Cd2+ by S-nZVI@ATP with different pH.

Figure 8

Adsorption of Cd2+ by S-nZVI@ATP with different pH.

Close modal

Mechanism analysis of Cd2+ adsorption by ZVI sulfide-modified graptolite

Figure 9 shows the XRD patterns before and after the reaction of S-nZVI@ATP with Cd2+. After the reaction, the diffraction peaks diffuse, reflecting the deterioration of the crystalline shape of the material, and the intensity of the corresponding diffraction peak of Fe0 is significantly weakened. In contrast, the characteristic peak of FeOOH (▽) at 2θ = 26.58° is stronger. Further, at 2θ = 36.66° and 47.1°, there are weaker characteristic diffraction peaks of FeOOH (Liang et al. 2021). Additionally, the diffraction peaks at 2θ = 37.1° and 53.41° are attributed to the appearance of Fe3O4 (▪) (Lv et al. 2012), indicating that Fe0 is oxidized during the adsorption of Cd2+ by S-nZVI@ATP. After the oxidation of Fe0 by H2O, Fe2+ and OH are formed, and under certain conditions, they react to form Fe(OH)2 and corrode continuously to form Fe3O4, which is further converted to FeOOH when dissolved oxygen is present in the solution. The CdS diffraction peak is not found in the spectrum, which may be because of the poor crystallinity of the generated CdS.
Figure 9

XRD spectra of S-nZVI@ATP before (a) and after (b) adsorption.

Figure 9

XRD spectra of S-nZVI@ATP before (a) and after (b) adsorption.

Close modal
Figure 10 shows the FTIR plots of S-nZVI@ATP before and after the adsorption of Cd2+. The Fe–O vibrational peaks in Fe3O4/Fe2O3 located around 665 and 467 cm−1 in spectrum (a) are weakened after the reaction, indicating the S-nZVI@ATP in which the iron oxide of nZVI is involved in the adsorption of Cd2+, and the complexation reaction between it and Cd2+ weakens the characteristic peak. Further, the vibrational absorption bands of –OH around 3,404 and 1,630 cm−1 are enhanced, mainly because Fe0 in S-nZVI@ATP generates Fe–O–OH in contact with water or oxygen during the reaction, which promotes Cd2+ adsorption.
Figure 10

FTIR spectra of S-nZVI@ATP before (a) and after (b) adsorption.

Figure 10

FTIR spectra of S-nZVI@ATP before (a) and after (b) adsorption.

Close modal
Through XPS, the mechanism was further explored by analyzing the valence state information and the elemental composition of the surface elements before and after the S-nZVI@ATP reaction. In Figure 11(a), the characteristic peak of S2p appeared before the S-nZVI@ATP reaction, indicating sulfide formation, while that of Cd3d appeared after the reaction, showing that Cd2+ was successfully adsorbed on the material surface. Figures 11(b) and 11(c) show the characteristic peaks of O1s before and after the reaction. Four typical peaks can be observed in the figure, namely 529.5 eV Fe3O4, 530.4 eV Fe–O, 531.2 eV C–O/FeOOH, and 532.0 eV C = O (Liu et al. 2012). Fe and O were the main elements on the S-nZVI@ATP surface, appearing mainly in the form of Fe3O4, Fe2O3, FeOOH, and FeO (Xu et al. 2019a). After the reaction, the Fe–O peak intensity slightly decreased, revealing that the iron oxide of nZVI in S-nZVI@ATP was involved in the Cd2+ adsorption, and the complexation reaction occurred with Cd2+, which was consistent with the FTIR analysis results. Artwork (d) illustrates the Fe2p spectra of S-NZVI before and after the S-nZVI@ATP reaction with Cd2+. Accordingly, 710.46, 712.9, and 706.9 eV corresponded to Fe(II), Fe(III), and Fe0, respectively, while 723.21 and 727.51 eV corresponded to Fe(II) and Fe(III) of Fe2p1/2, respectively (Wei et al. 2021). The apparent iron (hydrogen) oxide formation on the nZVI surface may have been caused by the reactions between water and Fe0 during preparation or accidental oxidation during storage and sample transfer. Artwork (e) depicts the S2p spectrum with the three characteristic peaks of S2− (161.5 eV), (163.2 eV), and (168 eV) (Lv et al. 2018; Jiang et al. 2022; Yuan et al. 2022). The S-nZVI@ATP surface S was mainly composed of S2−, , and , whereas the nZVI surface came from FeSO4 synthesized by nZVI (Kim et al. 2011; Rajajayavel & Ghoshal 2015). The peak intensity of S2− slightly increased after the reaction. This result was attributed to the CdS formation after the reaction. Figure 11(f) shows the Cd3d spectrum. Cd3d3/2 and Cd3d5/2 exhibited two strong peaks at 411.2 and 404.5 eV, respectively, with a 6.7 eV spin orbit separation. This result was consistent with that for Cd(II) reported in the literature (Wu et al. 2006; Lv et al. 2018). The characteristic spectrum of Cd3d3/2 appeared at the 411.2 eV binding energy, corresponding to CdS and indicating that S2− reacted with Cd2+ in the cured material. In addition, the standard reduction potential of Cd2+ was very close to that of Fe2+; hence, no Cd2+ reduction would occur during the reaction (Boparai et al. 2013).
Figure 11

(a) XPS survey spectra of S-nZVI@ATP before and after adsorption. (b) High-resolution XPS scan spectra over O1s of S-nZVI@ATP before reaction. (c) High-resolution XPS scan spectra over O1s of S-nZVI@ATP after reaction. (d) High-resolution XPS scan spectra over Fe2p of S-nZVI@ATP before and after adsorption. (e) High-resolution XPS scan spectra S2p of nZVI@ATP before and after adsorption. (f) High-resolution XPS scan spectra over Cd3d of S-nZVI@ATP. Note:* B.E. stands for electron binding energy or kinetic energy, and a.u. stands for relative photoelectron flow intensity.

Figure 11

(a) XPS survey spectra of S-nZVI@ATP before and after adsorption. (b) High-resolution XPS scan spectra over O1s of S-nZVI@ATP before reaction. (c) High-resolution XPS scan spectra over O1s of S-nZVI@ATP after reaction. (d) High-resolution XPS scan spectra over Fe2p of S-nZVI@ATP before and after adsorption. (e) High-resolution XPS scan spectra S2p of nZVI@ATP before and after adsorption. (f) High-resolution XPS scan spectra over Cd3d of S-nZVI@ATP. Note:* B.E. stands for electron binding energy or kinetic energy, and a.u. stands for relative photoelectron flow intensity.

Close modal

The adsorption performance of S-nZVI@ATP for Cd2+ was significantly improved compared with that of nZVI@ATP, and the best loading ratio was achieved at an S-to-Fe molar ratio of 0.75. The removal rate of Cd2+ by S-nZVI@ATP was 100% under the following conditions: 1 g/L adsorbent addition, 30 mL solution, adsorption was performed at 25 °C for 24 h, and the 100 mg/L initial concentration of Cd2+. Characterization analysis revealed that the surface roughness of ATP increased after the loading of ZVI sulfide nanoparticles, and irregular particles wrapped in flocculants appeared. The successful loading of ZVI sulfide nanoparticles on ATP was confirmed via EDS analysis. The crystallinity of Fe0 in S-nZVI@ATP worsened after sulfidation modification compared with nZVI@ATP. Moreover, Fe0 in S-nZVI@ATP was oxidized to some extent, and iron oxides were present on its surface. The oxidation of S-nZVI@ATP was less severe than that of nZVI@ATP, indicating that the sulfide modification reduced the occurrence of nZVI oxidation. The adsorption process of Cd2+ by S-nZVI@ATP follows the pseudo-second-order kinetic model, indicating that the transfer and sharing of electrons occur in the adsorption process and that the rate-controlling step was accomplished by both the liquid membrane and intraparticle diffusion. The adsorption capacity of S-nZVI@ATP for Cd2+ exhibited similar characteristics at different pH values. The adsorption capacity of S-nZVI@ATP increased sharply with increasing pH in the range of 2–2.5. However, the adsorption capacity stabilized with increasing pH above 2.5. S-nZVI@ATP showed a stable adsorption effect on Cd2+ over a wide pH range. There are various mechanisms for the removal of Cd2+ by S-nZVI@ATP: (1) the complexation between the iron (hydrogen) oxide shell of S-nZVI and Cd2+, (2) Cd2+ forms CdS and Cd(OH)2 precipitates with S2− and OH in solution, respectively, (3) co-precipitation with Fe2+/Fe3+ produced by corrosion of nZVI (CdxFe(1−x)(OH)2 precipitation). This study demonstrates that S-nZVI@ATP is a promising adsorbent for heavy metals in water (e.g., groundwater or industrial wastewater remediation). However, the influence of typical ions (e.g., Ca2+, Mg2+, and ) in the natural environment in water on adsorption was not studied in this work, and the regeneration experiment of the adsorbent was not performed. These are relevant research directions for the future.

G.M. and W.Z. performed the research and wrote the paper. J.R. and L.D. designed the research study and managed the project. L.T. and K.M. contributed to the writing-review. Y.Z. and C.L. performed the research. X.T., H.W., and Y.H. analyzed the data.

The work was financially supported by the Foundation of Key Laboratory of Yellow River Environment of Gansu Province (20JR2RA002, 21YRWEK007, 21YRWEG003), industrial Support Program of Education Department of Gansu Province (2021CYZC-31), the Lanzhou Talent Innovation and Entrepreneurship Project (2021-RC-41), the Natural Science Foundation of Ningxia province (2022AAC03310), the National Training Programs of Innovation and Entrepreneurship for Undergraduates (202210753012), and the “Innovative Star” Project for Outstanding Graduate Students in Gansu Province (2022CXZX-514).

The datasets used in the current study are available from the corresponding authors on reasonable request.

Not applicable.

Not applicable.

All authors agreed with the content and all gave explicit consent to submit and they obtained consent from the responsible authorities at the institute where the work was carried out, before the work was submitted.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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