Abstract
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.
HIGHLIGHTS
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
INTRODUCTION
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.
MATERIALS AND METHODS
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).
RESULTS AND DISCUSSION
Characterization of composite
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.
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.
Pore structure parameter of S-nZVI@ATP
Sample . | S-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 |
Sample . | S-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 |
Effect of different sulfur to iron ratios on the adsorption of Cd2+ by S-nZVI@ATP
Adsorption capacity of cadmium by adsorbent reported in literatures
Sample . | Carrier . | C0 (mg/L) . | T(K) . | pH . | Dosage (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 |
Sample . | Carrier . | C0 (mg/L) . | T(K) . | pH . | Dosage (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 |
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.
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.
Adsorption kinetics
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)] . | R12 . | qm2/(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)] . | R12 . | qm2/(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.
Parameters of intra-particle diffusion kinetic for Cd2+ adsorption on S-nZVI@ATP
C0/(mg/L) . | kd1/[mg/(g•h1/2)] . | C1/(mmol/g) . | R2 . | kd2/[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) . | R2 . | kd2/[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.
Adsorption kinetic model fitting curves (a) and intra-particle diffusion model fitting curve (b) of S-nZVI@ATP for Cd2+.
Adsorption kinetic model fitting curves (a) and intra-particle diffusion model fitting curve (b) of S-nZVI@ATP for Cd2+.
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

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).
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).
Langmuir, Freundlich and Temkin adsorption isotherm parameters for Cd2+ on S-nZVI@ATP
T/K . | Langmuir . | Freundlich . | Temkin . | ||||||
---|---|---|---|---|---|---|---|---|---|
qm . | KL . | R2 . | KF . | nf . | R2 . | A . | Kt . | R2 . | |
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/K . | Langmuir . | Freundlich . | Temkin . | ||||||
---|---|---|---|---|---|---|---|---|---|
qm . | KL . | R2 . | KF . | nf . | R2 . | A . | Kt . | R2 . | |
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 |
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.
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.
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.
Thermodynamic parameters for Cd2+ on S-nZVI@ATP
Kd . | Co (mg/L) . | ΔHo (kJ/mol) . | ΔSo (J/(mol·K)) . | ΔGo(kJ/mol) . | ||
---|---|---|---|---|---|---|
298 K . | 308 K . | 318 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 |
Kd . | Co (mg/L) . | ΔHo (kJ/mol) . | ΔSo (J/(mol·K)) . | ΔGo(kJ/mol) . | ||
---|---|---|---|---|---|---|
298 K . | 308 K . | 318 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
Mechanism analysis of Cd2+ adsorption by ZVI sulfide-modified graptolite





(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.
(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.
CONCLUSIONS AND FUTURE DIRECTIONS
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.
AUTHOR CONTRIBUTIONS
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.
FUNDING
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).
AVAILABILITY OF DATA AND MATERIALS
The datasets used in the current study are available from the corresponding authors on reasonable request.
ETHICS APPROVAL AND CONSENT TO PARTICIPATE
Not applicable.
CONSENT FOR PUBLICATION
Not applicable.
CONSENT TO PARTICIPATE
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.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.