Capture Mechanism of Cadmium in Agricultural Soil Via Iron-Modiﬁed Graphene

: Cadmium (Cd) contamination in agricultural soils has caused extensive concern to re-searchers. Biochar with iron-compound modiﬁcations could give rise to the synergistic effect for Cd restriction. However, the related capture mechanism based on physicochemical properties is unclear. In this study, ﬁrst principles calculations are proposed to explore the adsorption ability and potential mechanism of the ferric hydroxide modiﬁed graphene (Fe@G) for capturing CdCl 2 . The simulation results show that the adsorption energy to CdCl 2 could enhance to − 1.60 eV when Fe(OH) 3 is introduced on graphene. Subsequently, analyses of electronic properties demonstrated a signiﬁcant electron transfer between Cd s-orbital and O p-orbital, thereby leading to strong adsorption energy. This theoretical study not only identiﬁes a powerful adsorption material for Cd reduction in agricultural soils and reveals the capture mechanism of Fe@G for Cd but also provides a foundation and strategy for Cd reduction in agricultural soils.


Introduction
With recent rapid developments and increased industrial emissions, cadmium (Cd) contamination is becoming a serious concern in east and south Asia, especially in China, India, and Thailand [1][2][3][4]. Here, until 2005, approximately 1.3 × 10 5 ha of agricultural soils in China were reported to be contaminated by Cd [5]. It is well known that Cd is a nonessential substance for plants; however, it is easily accumulated in agricultural crops [6][7][8][9]. For example, paddy rice can uptake Cd easily, and straw is the main accumulation region for Cd. A wide number of rice straws have been reportedly polluted by Cd every year in China [6,10]. In 2016, more than 10% of rice samples from rice markets exceeded the China National Standard for food contamination of Cd (<0.2 mg kg −1 ) [11]. Furthermore, due to the 30 year biological half-life of Cd, it has the potential to cause serious medical conditions such as cancer and Itai-Itai disease [12][13][14]. Thus, it is necessary to reduce the availability of Cd in agricultural crops and consequently to the human system.
To date, in situ metal stabilization has raised considerable concern due to its high effect on reducing the bioavailability and toxicity of heavy metals in short periods [15][16][17]. Among them, biochar from pyrolysis with limited oxygen has an irregular aromatic structure, multi-layered accumulation forms, and unique physicochemical properties, which make it a great candidate for soil amendments to inhibit Cd in soils [18][19][20][21]. For instance, Luo et al. demonstrated that corncob biochar could significantly increase the total nitrogen and organic matter content of the soil while stabilizing the availability of arsenic (As) and Cd in soil [22]. Furthermore, iron-based materials could reduce the efficiency of Cd by changing the physicochemical property of Cd [23,24]. Compared with pure biochar treatment, hybrid soil amendments of biochar and inorganic materials containing iron are better for reducing the Cd contamination of agricultural soils [18,25,26]. Qiao et al. reported that zero-valent iron (ZVI)-biochar could reduce Cd contamination in agricultural soils effectively [27]. Yin et al. reported that Fe-modified biochar reduced the accumulation of As and Cd in rice by reducing soil acidification [28]. However, the capture mechanism of Fe-modified biochar on Cd physicochemical systems has rarely been reported. Therefore, clarifying the capture mechanism of Fe-modified biochar on Cd is of paramount importance.
In this study, graphene (G) was used as the substrate to emulate the biochar. Graphene and its ferric hydroxide modification (Fe@G) were taken as the research objects to explore the capture mechanism with CdCl 2 by first principles calculations. First, the structural and electronic property differences between G and Fe@G were checked. Then, the adsorption ability of G and Fe@G with CdCl 2 were identified. Moreover, inherent changes in electronic properties and charge transferability were demonstrated. These findings show that there is a significant charge transfer between Fe@G and CdCl 2 , which could be the main reason that Fe@G enhances the reduction of Cd. The calculation results will help to explain the capture mechanism of Fe-modified biochar and guide materials design for Cd reduction in agricultural soils.

Results and Discussion
Firstly, the structural stability and electronic properties of graphene were considered with modified ferric hydroxide (Fe(OH) 3 ) (Figures 1 and S1). The optimization structures of G and Fe@G were compared. There was a long distance from Fe(OH) 3 to G, which meant that a weak interaction existed between Fe(OH) 3 and G (Figure 1a,b). Furthermore, the electron localization function (ELF) diagram proved that although the electron property of G changed with the Fe(OH) 3 modification, there was no distinct electron exchange region between Fe(OH) 3 and G that would demonstrate that a physical adsorption effect exists (Figure 1c,d). Meanwhile, these results implied that G would not have a negative influence on Fe(OH) 3 to adsorb CdCl 2 . senic (As) and Cd in soil [22]. Furthermore, iron-based materials could redu ciency of Cd by changing the physicochemical property of Cd [23,24]. Comp pure biochar treatment, hybrid soil amendments of biochar and inorganic ma taining iron are better for reducing the Cd contamination of agricultural soils Qiao et al. reported that zero-valent iron (ZVI)-biochar could reduce Cd contam agricultural soils effectively [27]. Yin et al. reported that Fe-modified biochar r accumulation of As and Cd in rice by reducing soil acidification [28]. Howeve ture mechanism of Fe-modified biochar on Cd physicochemical systems has r reported. Therefore, clarifying the capture mechanism of Fe-modified biochar paramount importance.
In this study, graphene (G) was used as the substrate to emulate the bio phene and its ferric hydroxide modification (Fe@G) were taken as the research explore the capture mechanism with CdCl2 by first principles calculations. First tural and electronic property differences between G and Fe@G were checked adsorption ability of G and Fe@G with CdCl2 were identified. Moreover, inhere in electronic properties and charge transferability were demonstrated. Thes show that there is a significant charge transfer between Fe@G and CdCl2, whic the main reason that Fe@G enhances the reduction of Cd. The calculation resul to explain the capture mechanism of Fe-modified biochar and guide materials Cd reduction in agricultural soils.

Results and Discussion
Firstly, the structural stability and electronic properties of graphene were with modified ferric hydroxide (Fe(OH)3) (Figures 1 and S1). The optimization of G and Fe@G were compared. There was a long distance from Fe(OH)3 to meant that a weak interaction existed between Fe(OH)3 and G (Figure 1a,b). Fu the electron localization function (ELF) diagram proved that although the elec erty of G changed with the Fe(OH)3 modification, there was no distinct electron region between Fe(OH)3 and G that would demonstrate that a physical adsorp exists (Figure 1c,d). Meanwhile, these results implied that G would not have influence on Fe(OH)3 to adsorb CdCl2.  To further explore the adsorption ability of CdCl 2 on G and Fe@G, the bond length (Cd-Cl), bond angle (Cl-Cd-Cl), and adsorption energy were considered. In previous studies, the long bond length and the large bond angle could facilitate the decomposition of the compound. Meanwhile, the weak adsorption energy could lead to the adsorption being unstable [29][30][31]. Compared with the pure CdCl 2 (179.56 • ) and that adsorbed on G (171.03 • ), the CdCl 2 showed the lowest bond angle (125.01 • ) when adsorbed on Fe@G (Figure 2a-c). Furthermore, in contrast to the pure CdCl 2 and adsorbed on G, the Fe@G possessed a longer bond length (2.95Å, Cd-Cl) near the Fe(OH) 3 , whereas it had a shorter bond length (2.33Å, Cd-Cl) at the distance from Fe(OH) 3. This was substantial evidence that CdCl 2 would not decompose easily on Fe@G. Moreover, adsorption energies of G and Fe@G with CdCl 2 were taken for comparison. According to the computation formula of adsorption energy, the negative value implied a stronger restriction effect of CdCl 2 adsorbed on substrates [32]. Due to the weak physisorption of G and CdCl 2 , G showed a poor adsorption ability (−0.42 eV), while Fe@G showed more negative values (−1.60 eV) when CdCl 2 was adsorbed (Figure 2d). The above results demonstrated that with the introduction of Fe(OH) 3 , the Fe@G showed a considerable effect in restricting the migration of CdCl 2 .
To further explore the adsorption ability of CdCl2 on G and Fe@G, (Cd-Cl), bond angle (Cl-Cd-Cl), and adsorption energy were considered. ies, the long bond length and the large bond angle could facilitate the d the compound. Meanwhile, the weak adsorption energy could lead to th ing unstable [29][30][31]. Compared with the pure CdCl2 (179.56°) and tha (171.03°), the CdCl2 showed the lowest bond angle (125.01°) when adsorb ure 2a-c). Furthermore, in contrast to the pure CdCl2 and adsorbed on G sessed a longer bond length (2.95Å, Cd-Cl) near the Fe(OH)3, whereas bond length (2.33Å, Cd-Cl) at the distance from Fe(OH)3. This was sub that CdCl2 would not decompose easily on Fe@G. Moreover, adsorption e Fe@G with CdCl2 were taken for comparison. According to the comput adsorption energy, the negative value implied a stronger restriction eff sorbed on substrates [32]. Due to the weak physisorption of G and Cd poor adsorption ability (−0.42 eV), while Fe@G showed more negative v when CdCl2 was adsorbed (Figure 2d). The above results demonstrated troduction of Fe(OH)3, the Fe@G showed a considerable effect in restricti of CdCl2. Next, the electronic properties of CdCl2 on G and Fe@G were invest understanding of the inhibition mechanism of CdCl2 on G and Fe@G. Init charge density difference of CdCl2 was drawn on G and Fe@G to probe t of charge. There was a clear charge depletion region between Cd and O tinct charge accumulation region between Fe and Cl atoms, which indica icant electron transfer occurred between the CdCl2 and Fe@G (Figure 3b,d phenomenon was only expressed between the CdCl2 and Fe@G interface configuration of CdCl2 on G was no evidence of an obvious charge tran or Cl and G, which may be the main reason for the weak adsorption abil (Figure 3a,c). Next, the electronic properties of CdCl 2 on G and Fe@G were investigated to gain an understanding of the inhibition mechanism of CdCl 2 on G and Fe@G. Initially, the electron charge density difference of CdCl 2 was drawn on G and Fe@G to probe the redistribution of charge. There was a clear charge depletion region between Cd and O atoms and a distinct charge accumulation region between Fe and Cl atoms, which indicated that a significant electron transfer occurred between the CdCl 2 and Fe@G (Figure 3b,d). However, this phenomenon was only expressed between the CdCl 2 and Fe@G interface. The adsorption configuration of CdCl 2 on G was no evidence of an obvious charge transfer between Cd or Cl and G, which may be the main reason for the weak adsorption ability of CdCl 2 on G (Figure 3a,c).  The above results are also observed in the ELF diagram ( Figure 4). The C configuration had no distinct electron exchange between the CdCl2 and G ( However, in contrast to G, the CdCl2 on Fe@G configuration had a greater elec tribution than the ELF diagram of pure Fe@G, where the localization degree of t for the -OH group near the Cd atom was reduced (Figure 4b,d). This phenom firmed that CdCl2 had influenced the electronic properties of Fe(OH)3 in Fe@G, implied that a significant charge exchange existed between Fe(OH)3 and CdCl Bader charge analysis method was used to gain a deeper insight into the charges transferred between substrates and CdCl2. Generally, more charges t expresses the higher restriction ability of the substrates to the molecules. Comp pure CdCl2, the Cd site of the CdCl2 on Fe@G configuration had an apparent ch fer (0.141 e) that was higher than the CdCl2 on G configuration (0.008 e) (Fig  addition, as shown in Table S1, the two Cl atoms in the CdCl2 on Fe@G configu showed a charge exchange that was 0.05 e and 0.1 e greater than that of a Cl at CdCl2. The reason for the significant difference in charge number between the oms was ascribed to the different distances from Fe(OH)3. Meanwhile, the to number of CdCl2 in the CdCl2 on Fe@G configuration showed a higher char than in other situations, and the results were consistent with Figures 3 and 4. The above results are also observed in the ELF diagram ( Figure 4). The CdCl 2 on G configuration had no distinct electron exchange between the CdCl 2 and G (Figure 4a). However, in contrast to G, the CdCl 2 on Fe@G configuration had a greater electron redistribution than the ELF diagram of pure Fe@G, where the localization degree of the electron for the -OH group near the Cd atom was reduced (Figure 4b,d). This phenomenon confirmed that CdCl 2 had influenced the electronic properties of Fe(OH) 3 in Fe@G, which also implied that a significant charge exchange existed between Fe(OH) 3 and CdCl 2 . Next, the Bader charge analysis method was used to gain a deeper insight into the number of charges transferred between substrates and CdCl 2 . Generally, more charges transferred expresses the higher restriction ability of the substrates to the molecules. Compared to the pure CdCl 2 , the Cd site of the CdCl 2 on Fe@G configuration had an apparent charge transfer (0.141 e) that was higher than the CdCl 2 on G configuration (0.008 e) (Figure 4c). In addition, as shown in Table S1, the two Cl atoms in the CdCl 2 on Fe@G configuration also showed a charge exchange that was 0.05 e and 0.1 e greater than that of a Cl atom in pure CdCl 2 . The reason for the significant difference in charge number between the two Cl atoms was ascribed to the different distances from Fe(OH) 3 . Meanwhile, the total charge number of CdCl 2 in the CdCl 2 on Fe@G configuration showed a higher charge transfer than in other situations, and the results were consistent with Figures 3 and 4.
The above data show that the formation of the Cd-O and Fe-Cl bonds benefitted from charges transferred between CdCl 2 and the sorbent. However, in-depth studies must explore the reaction mechanism of CdCl 2 adsorption on Fe@G. Thus, the total density of states (DOS) and partial density of states (PDOS) of CdCl 2 on G and Fe@G configurations were investigated to gain a perspective on the interval states of energy and electron transfer ( Figure 5). Differing from the CdCl 2 on G configuration (Figure 5a), there were prominent energy shifts that could be observed in the DOS of the CdCl 2 on Fe@G configuration, which means that the electron transfer had occurred under the adsorption situation as displayed in Figure 5b. Furthermore, the PDOS of the O and Cd atoms in the CdCl 2 on Fe@G configuration before and after adsorption were also considered. Due to the distinct charge donation of the Cd atom, the s-orbital of Cd exhibited a dramatic energy upshift after adsorption on Fe@G (Figure 5d). In contrast, the downshift of the p-orbital of O was evident, which was attributed to the O gain electrons when Fe@G interacted with CdCl 2 . Meanwhile, these phenomena were also present in the Fe s-orbital and Cl p-orbital of the CdCl 2 on Fe@G configuration ( Figure S2). However, there was no significant energy shift between Inorganics 2022, 10, 150 5 of 8 the C p-orbital and Cd s-orbital of the CdCl 2 on the G configuration (Figure 5c), which was consistent with the results of ELF, charge density difference, and Bader charge analysis. The above data show that the formation of the Cd-O and Fe-Cl bonds benefitted from charges transferred between CdCl2 and the sorbent. However, in-depth studies must explore the reaction mechanism of CdCl2 adsorption on Fe@G. Thus, the total density of states (DOS) and partial density of states (PDOS) of CdCl2 on G and Fe@G configurations were investigated to gain a perspective on the interval states of energy and electron transfer ( Figure 5). Differing from the CdCl2 on G configuration (Figure 5a), there were prominent energy shifts that could be observed in the DOS of the CdCl2 on Fe@G configuration, which means that the electron transfer had occurred under the adsorption situation as displayed in Figure 5b. Furthermore, the PDOS of the O and Cd atoms in the CdCl2 on Fe@G configuration before and after adsorption were also considered. Due to the distinct charge donation of the Cd atom, the s-orbital of Cd exhibited a dramatic energy upshift after adsorption on Fe@G (Figure 5d). In contrast, the downshift of the p-orbital of O was evident, which was attributed to the O gain electrons when Fe@G interacted with CdCl2. Meanwhile, these phenomena were also present in the Fe s-orbital and Cl p-orbital of the CdCl2 on Fe@G configuration ( Figure S2). However, there was no significant energy shift between the C p-orbital and Cd s-orbital of the CdCl2 on the G configuration (Figure 5c), which was consistent with the results of ELF, charge density difference, and Bader charge analysis.

Materials and Methods
The first principles calculations were carried out using the simulation software Vienna Ab initio Simulation Package (VASP) based on density functional theory (DFT) with

Materials and Methods
The first principles calculations were carried out using the simulation software Vienna Ab initio Simulation Package (VASP) based on density functional theory (DFT) with the projector augmented wave (PAW) method [33][34][35]. Exchange-correction interactions were parameterized by the Perdew, Burke, and Ernzerhof (PBE) type, and DFT-D3 was used to consider the van der Waals interaction [36,37]. The energy cutoff was set to 500 eV, and convergence criteria for energy and force were set to 10 −5 eV and 0.02 eV Å −1 per atom, respectively. A vacuum layer with a thickness of 15 Å was employed to avoid interactions between the adjacent periodic units. The k-point was meshed by the Monkhorst-Pack method with a 3 × 3 × 1 grid for geometry optimizations and 11 × 11 × 1 grid for DOS calculation [38,39]. The calculations were computed within the DFT + U formalism to describe the localized d electrons, and the U values of Fe and Cd were set to 4 eV and 2 eV, respectively [40,41]. The adsorption energy (E a ) formula was defined as where E Cd , E substrate , and E (Cd-substrate) represent the total energy of the CdCl 2 , the substrates of G or Fe@G, and the energy of the CdCl 2 adsorption on the different substrates. The differential charge density (∆ρ) that was used to describe the charge distribution for the adsorption system is defined with the following formula [42]: where ρ Cd-substrate , ρ substrate , and ρ Cd represent the charge density of the CdCl 2 adsorption on the different substrates, the substrates of G or Fe@G, and the CdCl 2 . The electron localization function (ELF) diagram was drawn by VESTA [43].

Conclusions
In summary, we have revealed the capture ability and mechanism of CdCl 2 with Fe(OH) 3 modified graphene via first principles calculations. The Fe(OH) 3 modified carbon did not have a negative influence on adsorption of CdCl 2 , and it could increase the chemisorption effect when the Fe(OH) 3 was introduced. This led to an adsorption energy improvement from −0.42 eV (G) to −1.60 eV. The adsorption behavior of Fe@G would then result in a large change in bond length (Cd-Cl) and bond angle (Cl-Cd-Cl) when CdCl 2 was adsorbed onto Fe@G. Furthermore, the electron charge density difference, ELF, and Bader charge analysis were used to confirm the charge transfer capacity. The Bader charge analysis displayed a 0.141 e charge transfer between Cd and Fe@G, which showed a greater amelioration over the CdCl 2 on G configuration. In addition, the electron transfer process was revealed by DOS and PDOS analysis. The PDOS proved that the Cd s-orbital and the O p-orbital exhibited a dramatic energy shift when the Fe@G interacted with CdCl 2 , which provided powerful evidence for the capture mechanism of CdCl 2 with Fe(OH) 3 modified carbon. This study identified a powerful adsorption material for Cd reduction in agricultural soils and revealed the capture mechanism of Fe@G for Cd addition, showing potential as a strategy for Cd reduction in agricultural soils.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/inorganics10100150/s1, Figure S1: ELF diagram of the top view of Fe@G; Figure S2: The partial density of states of CdCl 2 adsorbed on Fe@G; Table S1: The number of charge transfers of the pure CdCl 2 , and adsorbed on substrate.
Author Contributions: X.W., J.D. and Z.F.: conception and design of the study, approval of the version of the manuscript to be published; H.W. performed the study, analyzed the data, and wrote the manuscript; J.H., W.Z. and P.L. provided guidance on the data analysis; Y.W., F.Z. and X.M. revised the manuscript critically for important intellectual content. All authors have read and agreed to the published version of the manuscript.