Silicic Acid Binding Cd(â¡) and Its

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Effect of Silicified Degree on Silica/Silicic Acid Binding Cd(#) and Its Mechanism Jiewei Zheng, Quan Chen, Jianchang Xu, Liyang Wen, Fangbai Li, and Lijuan Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00823 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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The Journal of Physical Chemistry

Effect of Silicified Degree on Silica/Silicic Acid Binding Cd(Ⅱ) and Its Mechanism Jiewei Zheng, a Quan Chen, a Jianchang Xu, a Liyang Wen, a Fangbai Li, b Lijuan Zhang*, a

a

Guangdong Provincial Key Lab of Green Chemical Product Technology, School of

Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, P R China. E-mail: [email protected]. Telephone/Fax: +86-20-87112046. b

Guangdong Public Laboratory of Environmental Science and Technology,

Guangdong Institute of Eco-Environment and Soil Science, Guangzhou, 510650, China.

Abstract: Heavy metal pollution in farmland soil reduces crop yield and quality, and also potentially causes the crisis to human health. Formerly, the fact that silicon fertilizer could effectively reduce the residual concentration of heavy metal in crops have been identified at the tissue level. In this paper, molecular dynamics simulation was employed to investigate the effects of the silicified degree of silicic acids [namely the molar ratio of Si(OH)4 and SiO2] on the Cd(Ⅱ) bound in the aspect of radial distribution functions and mean square displacements. The results demonstrated that Si(OH)4 attracted Cd(Ⅱ) through the coordination, while SiO2 attracted Cd(Ⅱ) by the adsorption. In particular, when the silicified degree was 0, both the bound Cd(Ⅱ) amount and strength were the maximum value, indicating that the silicon fertilizer had the best efficiency of Cd(Ⅱ) bound as Si(OH)4. By comparing the adsorption energy and electronic transfer of Cd(Ⅱ) and Si(OH)4 adsorption onto the SiO2 surface through the quantum chemical simulation, we concluded that Cd(II) adsorption onto 1

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SiO2 surface was chemisorption, while the Si(OH)4 adsorption onto SiO2 surface was physisorption. Consequently, the adsorption capacity of Cd(Ⅱ) on the SiO2 surface was higher than that of Si(OH)4 adsorption on the SiO2 surface. Moreover, the compact hydration layers around Cd(Ⅱ) prevented the process of Cd(II) adsorption on the SiO2 surface, even so, the counter ion Cl- in the system promoted the adsorption process. The mechanism of silicon fertilizer binding heavy metal Cd(II) was investigated and revealed at the molecular and electronic level. This work has expanded the possibility of theoretical guidance for the design of silicon fertilizer.

1. Introduction The pollution of heavy metals in soils damages crops severely, especially Cd(Ⅱ) pollution in paddy soils 1. The rice grown in Cd-contaminated paddy soil causes not only lower production a potential, but also a threat for human health. Related studies indicated Cd(Ⅱ) in human body was hardly able to metabolize and excrete 2, resulting in extremely long half-life and severe damages to various human organs especially kidney, itai-itai disease was a typical and serious clinical disease decades ago 3. Therefore, it is urgent to mitigate Cd(Ⅱ) toxicity in rice planting 4. Silicon (Si), one of the essential elements for plant growth, could improve the plant’s resistance to heavy metal stress, disease, radiation, and drought, and then to increase the crop yields

5,6

. Si is also able to modulate the soil pH and bind heavy

metals in the soil and could stimulate the synthesis of antioxidant enzymes and chelate, precipitate and coordinate heavy metals after entering into the plants. Moreover, Si could change the occurrence form of heavy metals and regulate the expression of heavy-metal-transport genes to inhibit the heavy metal migration from roots to stems 7. Silicon was not only able to increase production of crops 8, 9, but also to mitigate toxic effects of heavy metals

10

. Hence, silicon was manufactured as

fertilizer in wheat, rice, corn, bamboo planting to avoid secondary pollution in 2


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agriculture 5, which was intensively used in crops planting and routinely applied to rice in South East Asia, the Indian subcontinent, northern, central and southern America 11. Silicon fertilizer used in soil was mainly in the form of monosilicic acid (Si(OH)4) solution with the concentrations from 3-17mg/L

12

, and will undergo

silicification to form SiO2 in cell walls of plants. Studies have proved that both of them have binding effects of Cd(Ⅱ) 13. Previously, several pieces of research confirmed that Si could bind heavy metal in the plants at the tissue level. For instance, Naeem et. al.

14

cultivated wheat in the

soil with 10 mg/kg of Cd(Ⅱ). The results showed that the Cd(Ⅱ) concentration in all the wheats’ shoots decreased apparently after adding 150 mg/kg of silicic acid, and none of the wheat cultivars exhibited the symptoms of Cd(II) toxicity or the growth retardation against Cd(Ⅱ) stress. Among the wheat varieties, the Cd(Ⅱ) content in the dry mass of Lasani-2008 cultivars reduced 27.4%. Liu et. al. 15 investigated the effects of silicic acid on the flow rate of Cd(Ⅱ) in the rice protoplast through the scanning ion-selective electrode technique (SIET). The flow rate of Cd(II) decreased from 1.39 to 0.76 pmol/cm2/s after adding silicic acid, suggesting that silicic acid could reduce the migration activity of Cd(Ⅱ). In addition, the effect of silicon on the Cd(II) migration in the rice from the biological point of view was explored by Shi’s group 16. It was found that silicon could deposit Cd(II) in the endodermis of rice and hinder the transport of Cd(II) to stems, and thus reduce the concentration of Cd(II) in the grains. However, the above studies mainly focused the effect of silicon fertilizer on the heavy metals, removal efficiency by different types of plants, the comparison of heavy metal content in different tissues of plants before and after the addition of silicon fertilizer, the concentration of different heavy metals, and the environmental factors affecting silicon fertilizer mitigating the toxicity of heavy metals 17. In fact, it should be noted that very little optimization work has been conducted on the microscopic mechanism. To shed light on the mechanism, we are asked for understanding the transformation process of the silicon fertilizer occurrence form in the plants. After 3



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consulting the reference, we found the process was as followed: the monosilicic acid was absorbed through the roots, then was transported into the xylem through the endodermis, and finally was gathered in the stems and leaves

18

. After the

transformation, the silicic acid was polymerized to amorphous silica (SiO2. nH2O) under the influence of specialized cells or cell walls, and the formed amorphous silica was also known as plant opal or phytolith

19

. Moreover, the polymerized process of

silicic acid was defined as silicification, and the silicified degree decided the molar ratio of Si(OH)4 and SiO2 in the plants. Therefore, it’s very important and full of significance to explore the relationship between the heavy metal associated and silicified degree. Previously, Orange et. al. 20 demonstrated that Fe(III) ion binding to the cell wall components is of fundamental importance for successful silicification, and especially for the excellent preservation of the cell wall, confirmed the positive effect of Fe(III) ion on the silicification. Benning et. al.

21

pointed out that most

studies of metal uptake by microbial cells measured bulk changes in the experimental fluid. And they assumed that the metals in the solution were associated onto the cell surface, but silicification was not considered in the systems. So the bulk measurements of the experimental fluid would overestimate the amount of metal associated by the microorganisms, thus it is of importance to figure out the bound proportions by silica and silicic acid. Both the silica and silicic acid exhibited certain ability for Cd(Ⅱ) affinity 22, but the mechanism and the effects of silicified degree on the binding efficiency are still unclear enough. Therefore, there is an urgent requirement but it is still a challenge to investigate the impacts of silicified degree on Cd(Ⅱ) bound, as well as the mechanism in the micro-perspective, what really matters in regulating the silicified degree of plants and promoting the Cd(Ⅱ) attracted ability. Recently, with the rapid development of computational simulation technology, the simulated system is getting more consistent with the actual situation. Amongst, molecular dynamics and quantum chemical simulation were widely concerned due to their special advantages in visualizing the dynamic behavior of molecules and 4


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analyzing the electronic transfer during the reaction process

23,24

. Zhang et. al.

25

carried out systematic first-principles molecular dynamics (FPMD) simulations to investigate the Ni(Ⅱ) complexing mechanism on the clay surfaces. Three representative complexes (monodentate, bidentate and tetradentate) were studied by computing the free energy values and the amounts of transferred electrons. The results indicated that the tetradentate complex is much more stable than the other two forms. This study formed a basis for understanding the heavy metal adsorption mechanism on clay surface at the electronic level. Hou et. al.

26

utilized molecular dynamics

simulation to investigate the Na+ transport mechanism in nanometer calcium silicate channel. His research provided more insights into the mechanism of water and ionic transportation in the nanometer pore. In this paper, molecular dynamics simulations are utilized to investigate the Cd(Ⅱ)-bound mechanism onto Si(OH)4/SiO2 mixtures and to analyze the influences of silicified degree on the Cd(Ⅱ) bound efficiency. Quantum chemical simulations are employed to illustrate the electronic transfer and adsorption energy, and the effects of medium (H2O) and counter ion (Cl-) on the Cd(Ⅱ) adsorption by the SiO2. The mechanism of Cd(Ⅱ) bound by Si at molecule-electron level is studied by the combination of the molecular dynamics simulation and quantum chemical simulation, and to provide ideas and theoretical guidance for improving the formulation of silicon fertilizer and enhancing the heavy metal bound efficiency of the plants.

2. Calculation details 2.1 Molecular dynamics (MD) simulation methods The models of the Cd(Ⅱ) bound by SiO2/Si(OH)4 with different silicified degrees (PSiO2) were constructed to simulate the silicified process. As displayed in Figure 1A, the intermediate layer is an amorphous SiO2 layer, and the upper and lower layers are both aqueous solutions of Cd(Ⅱ) and Si(OH)4. The counter ions (Cl-) were added into 5



the system to keep electrically neutral. The (1, 0, 0) surface of hydroxylated high-temperature cristobalite was used to approximately express the amorphous SiO2 surface, because the (1, 0, 0) surface of cristobalite not only has the similar body phase density, reflection index to that of amorphous silica surface, but also has “geminal” silanol groups (two hydroxyls attached to the same surface Si), which share the same types of silanol groups identified experimentally on the amorphous silica surface

27

. A large number of experimental and simulation studies have proved that

hydroxylated (1, 0, 0) surfaces of cristobalite has the similar structure of amorphous silica

28, 29

, hence cristobalite may work as a suitable platform to help systematic

sampling of Cd(Ⅱ) adsorption property of amorphous silica. The discussions in Sections 3.1 and 3.2 are based on these models.

Figure 1. Model diagram (A) and the models at the PSiO2 of 0 (B), 25% (C), 50% (D), 75% (E), and 100% (F). The SiO2 surface of PSiO2= 100% was obtained by optimizing the high-temperature cristobalite supercell 4×4×1. The number of silicon atoms in the model was 288, and the other silicified degree models (PSiO2 = 75%, 50%, 25%) were maintained the numbers of silicon atoms with 288 by removing the silicon on the 6


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optimized SiO2 4×4×1 supercell surface and increasing the number of corresponding Si(OH)4. In Figure 1 (B), (C), (D), (E) and (F), the number of Si(OH)4 was 288, 216, 144, 72 and 0, respectively, and the corresponded number of SiO2 was 0, 72, 144, 216 and 288, the results for PSiO2 = 25% may have some minor contributions due to structural variances at sharp edges. In the upper and lower of SiO2 regions, the number of Cd(Ⅱ), Cl- and water molecules was 50, 100 and 2585, respectively. The box size was 43.0 Å×43.0 Å×105 Å in all models, and the water molecules were hidden for clearer illustration. Periodic boundary condition was applied for above models. Molecular dynamics simulation was carried out using the Forcite module in Materials Studio 2017 R2

30

. The parameters for dynamics simulation were as

followed: the universal force field 31 was selected, and the temperature was set at 298 K with Nose thermostat, 4 ns NVT ensemble dynamic relaxation was set to equilibrate above models. The methods of electrostatic interaction and van der Waals were both Ewald with the cutoff radius of 18.5 Å. Binding energy was calculated based on the equation as followed: Ebin = ESi+Cd(Ⅱ) - ESi - ECd(Ⅱ)

(1)

Where ESi+Cd(Ⅱ), ESi, and ECd(Ⅱ) mean energy of total Si and Cd(Ⅱ), Si including SiO2 and Si(OH)4, and Cd(Ⅱ), respectively. 2.2 Density function theory (DFT) calculation methods The density functional theory (DFT) with Grimme dispersion correction was applied to calculate the adsorption energy of Cd(Ⅱ), Si(OH)4, [Cd (H2O)6]2+ and [Cd (H2O)5Cl]+ on SiO2 (1, 0, 0) surface

32

, and the models were shown in Figure 2.

Moreover, adsorption mechanisms of the above molecules or ions on the surface of SiO2 were explored at the electronic level. The optimized SiO2 with 2×2×1 supercell was used as the substrate of the adsorbent. Periodic boundary condition was applied for above models. The DFT calculations were performed by using the Dmol3 package 33,34

in the Materials Studio 2017 R2. The atomic center grid was used as the atomic 7



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basis function. The electronic basis set was chosen to be dual numerical polarization (DNP4.4), which was similar to Gaussian function type 6-31G (d, p) basis set, but was more accurate than that. This high-precision numerical basis set can reduce the base group overlap effect, so the system was described accurately. The energy convergence tolerance of self-consistent field was set as 1.0×10-6 Hartree. The “DFT semi-core-pseudopots (DSPPs)” method, which introduced some degree of relativistic correction into the core, was adopted as the core treatment. The dispersion was corrected by the Grimme scheme to consider the van der Walls interactions, which could offset the shortage of calculating some weak interactions by DFT. The conductor like screening model (COSMO) with the permittivity of 78.5 (water) was used to describe the aqueous solvation effect, and the temperature was set at 298 K. The exchange-correlation interaction between electrons was described using generalized gradient correction (GGA) and expressed by the scheme of Perdew-Bruke-Ernzerhof (PBE) function

35

. The adsorption energy of different

adsorbents (Cd(Ⅱ), Si(OH)4, [Cd (H2O)6]2+and [Cd (H2O)5Cl]+) on SiO2 was calculated by the following equation. Eads = Etotal - Eadsorbate - ESiO2

(2)

Where Etotal, Eadsorbate and ESiO2 were the energy of the total system, adsorbates and SiO2, respectively.

Figure 2. The configuration diagrams of Cd(Ⅱ) (A), Si(OH)4 (B), [Cd (H2O)6]2+ (C) and [Cd (H2O)5Cl]+ (D) adsorption on the SiO2 surface. 3. Results and Discussions 3.1 Interactions between Si(OH)4/SiO2 mixtures and Cd(Ⅱ) 8


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Radial distribution function (RDF) is the most informative way in describing the distributive orders among atoms in the molecular dynamics simulation where g(r) function means the probability of presence of an atom at given distance r, and it could reflect the mutual aﬃnity between the atoms. The higher peak of RDF means the more intense of mutual aﬃnity between the atoms

36

. In this section, RDF was

introduced to analyze the interactions between Cd(Ⅱ) and Si(OH)4 under different silicified degrees. The higher peak of RDF indicated the stronger coordination between Si and Cd(Ⅱ), and the peak position corresponded to the coordinating distance. The RDF curves of Cd(Ⅱ) and Si in Si(OH)4 under varied silicified degree were shown in Figure 3 A. The peak positions were close to each other under different silicified degrees, and multiple peaks emerged in all the curves, illustrating that there were multiple coordinating forms of Cd(Ⅱ) and Si(OH)4

37

. When the

distance between Si and Cd(Ⅱ) reached at 3.00 Å, the values of g(r) first increased from zero, indicating that the minimum coordinating distance between Si and Cd(Ⅱ) was 3.00 Å. With the increase of the distance between Si and Cd(Ⅱ), the values of g(r) increased continuously until the first and highest peak appeared at about 6.00 Å, which corresponded to the strongest coordinating distance. Eventually, several peaks emerged at 9.00 Å, 12.0 Å and 15.0 Å with lower peak height, owing to the gradual decrease of coordination. When the distance was larger than 15.0 Å, the values of g(r) fluctuated at 1.00, indicating that there was no coordination between Si and Cd(Ⅱ) at this distance. Although the peak positions were very close to each other, it was still able to figure out the difference of peak height at various silicified degrees. For instance, when the silicified degree was 0, the concentration of Si(OH)4 and the peak of RDF were the highest, demonstrating that the coordination between Si(OH)4 and Cd(Ⅱ) was the most intense at this silicified degree. The coordination between Si(OH)4 and Cd(Ⅱ) without the presence of SiO2 (PSiO2 = 0) was more intense than those of the presence of SiO2 (PSiO2 = 75%, 50%, 25%), demonstrating that the 9



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presence of silica surface could weaken the coordination between Si(OH)4 and Cd(Ⅱ). In order to further investigate the effects of silica surface on coordination between Cd(Ⅱ) and Si(OH)4, different methods of sampling were adopted as shown in Supporting Information. Based on the samplings of Cd(Ⅱ) and Si(OH)4 in central and upper region of aqueous phase, the RDF results (Figure 3B) showed relatively higher peaks in central region of aqueous phase compared to those of upper side at the corresponding distance, confirming the presence of silica surface could impair the coordination between Cd(Ⅱ) and Si(OH)4.

Figure 3. RDF curves of Cd(Ⅱ) and Si(OH)4 at different silicified degrees in overall region (A) and in different regions (B) for central region of aqueous phase (solid lines) and upper side of aqueous phase (dash lines). Mean square displacements (MSD) was employed to analyze the diffusive behavior 38 of Cd(Ⅱ) on the SiO2 surface and to study the Cd(Ⅱ) adsorption behavior on SiO2. To exclude the effects of Si(OH)4 on the Cd(Ⅱ) diffusion and adsorption, the model with PSiO2=100% was selected because all silicon existed as SiO2 at this silicified degree. MSD can be obtained by the equations as follows: MSD =

(3)

= 6Dt

(4)

Where R(t) and R(0) represent the atomic displacement at time t and 0, respectively, and D represents the diffusion coefficient of the molecules. The steeper slope of the 10


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curve (6D) means the faster diffusion velocity of the atoms, conversely corresponds to the weaker Cd(Ⅱ) adsorption ability by SiO2. Plotted MSD with time t, and the results were shown in Figure 4. The Cd(Ⅱ) diffusion rate was the slowest in the range of 0-10 Å from the surface of SiO2. And the plateau appeared after 200 ps because the Cd(Ⅱ) diffusion was restricted heavily, indicating that most of the Cd(Ⅱ) in this range of distance were adsorbed by SiO2. With the increase of the distance between Cd(Ⅱ)and SiO2, the Cd(Ⅱ) diffusion rate increased at the same time. That was to say, the adsorption of Cd(Ⅱ) by SiO2 was weakened with the increasing distance. It was because the adsorption was mainly driven by the electrostatic force and Van der Waals force, which decreased with the increasing interaction distance. In addition, the curves in the distance range of 20-30 Å and 30-40 Å were almost coincident at the first 100 ps, due to the SiO2 surface had little impact on the Cd(Ⅱ) adsorption at these distances. The above analysis confirmed that silicon fertilizer had adsorption and binding effect on Cd(Ⅱ) as SiO2, and then inhibited the diffusion of Cd(Ⅱ) 39.

Figure 4. Mean square displacements of Cd(Ⅱ) at different distances on SiO2 surface. 11



To analyze the synergic effect of Si(OH)4 and SiO2 to Cd(Ⅱ), the relative concentration distribution along Z-axis (Figure 5A) of each component on the SiO2 surface was utilized. The model in Figure 5B is isotropic, the models in Figure 5C, E are of anisotropy in X, Y, Z directions, and the models in Figure 5D, F are of anisotropy along Z-axis. The difference of this distribution was mainly reflected along Z-axis, so we adopted Z-axis for analysis. If the peak positions of two components in the relative concentration distribution curve were adjacent, it reflected the comparatively strong interaction between the two components. When the silicified degree was 0 (Figure 5B), the Cd(Ⅱ) peaks were accompanied by the peak of Si(OH)4 (the positions of the two peaks are close), indicating that Si(OH)4 had a strong coordination ability for Cd(Ⅱ). And the results also could be identified by the above RDF analysis. Similarly, when the silicified degree was 100% (Figure 5F), the peak of Cd(Ⅱ) appeared near the peak of SiO2. That was to say, the Cd(Ⅱ) peaks had a relatively high concentration distribution around SiO2, suggesting that SiO2 also had strong adsorption ability for Cd(Ⅱ), which was also confirmed by the previous MSD analysis. As shown in Figure 5D and 5E, the concentration distribution curves at the two silicified degrees (50% and 75%) showed the same pattern. Firstly, the peaks of Si(OH)4 appeared around SiO2 surface earlier than that of Cd(Ⅱ), showing that Si(OH)4 was adsorbed by SiO2 preferentially

40

. Secondly, the peak positions of

Si(OH)4 were among the peak positions of SiO2 and Cd(Ⅱ), illustrating that Cd(Ⅱ) was adsorbed by SiO2 and coordinated by Si(OH)4 simultaneously. There was a competitive relationship between Cd(Ⅱ) and Si(OH)4, and this competitive relationship had a negative impact on both the SiO2 adsorption and Si(OH)4 coordination. On the other hand, there were also some differences among the three silicified degrees (25%, 50%, 75%). For instance, with the rising of silicified degree, the concentration of Cd(Ⅱ) and Si(OH)4 on the SiO2 surface increased, owing to the increasing of SiO2 adsorption. Above all, it could be concluded that Cd(Ⅱ) was simultaneously absorbed by SiO2 and coordinated by Si(OH)4. And Si(OH)4 was 12


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preferentially adsorbed by SiO2, which had negative impacts on both the SiO2 adsorption and Si(OH)4 coordination.

Figure 5. Directions of Z-axis (A), relative concentration distribution curves of each component at the silicified degree of 0 (B), 25% (C), 50% (D), 75% (E) and 100% (F). 3.2 Effects of silicified degree on Cd(Ⅱ) bound amount and strength 3.2.1 Effect of silicified degree on Cd(Ⅱ) bound amount If Cd(Ⅱ) was within the SiO2 critical adsorption distance 12.0 Å (the distance between the Cd(Ⅱ) peak of and SiO2 peak in Figure 5), it was considered that Cd(Ⅱ) was absorbed by SiO2, and the adsorbed amounts were defined as the adsorption amount (Nads). Similarly, if Cd(Ⅱ) was within the Si(OH)4 critical coordination distance 6.00 Å (the distance of RDF highest peak in Figure 3), it was reputed that Cd(Ⅱ) wascoordinated by Si(OH)4, and the coordinated amounts were equated as the coordination amount (Ncoo). What’s more, if Cd(Ⅱ) was within both the SiO2 critical adsorption distance and the Si(OH)4 critical coordination distance, it was thought that Cd(Ⅱ) was bound by the collaboration of SiO2 adsorption and Si(OH)4 coordination, and its amounts were defined as the synergistic bound amount (Nsyn). Hence, the Cd(Ⅱ) bound amount (NCd2+) could be calculated by the following equation: 13



NCd2+

Nads

=

+

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Ncoo

-

Nsyn

(5) The Nsyn was subtracted to avoid the double counting of the bound amounts of Cd(II). Consequently, in order to identify the Nsyn, the coordinates of the adsorbed or coordinated Cd(Ⅱ) were exported by using Script (See Figure S2 and S3) respectively. And if the coordinates of the adsorbed or coordinated Cd(Ⅱ) were coincident, meaning that Cd(Ⅱ) were adsorbed and coordinated concurrently. As can be seen from Figure 6, with the increase of silicified degree, the Ncoo decreased while the Nads increased, and then the NCd2+ increased firstly and then decreased. The order of the bound amount was: 0>25%>75%>100% >50%. When the PSiO2=0, all the silicon was distributed dispersedly in aqueous solution as Si(OH)4, and could capture the free Cd(Ⅱ) through the coordination, so NCd2+ was the largest. When the PSiO2=25%, the content of Si(OH)4 decreased, the increasing amount of the adsorbed Cd(Ⅱ) by SiO2 was not enough to make up the decrease amount of coordinated Cd(Ⅱ) by Si(OH)4, resulting in the decrease of NCd2+. When the PSiO2=75%, NCd2+ was mainly dominated by the Nads, even the Nads was relatively large, but the Ncoo was comparatively small, so the NCd2+ further decreased. When the PSiO2=100%, all the bound Cd(II) were contributed to the SiO2 adsorption. It was because that the adsorption interaction scope was limited in the vicinity of the interface, unlike the Si(OH)4 dispersing in solution and coordinating free Cd(Ⅱ), therefore the NCd2+ was relatively small. When the PSiO2=50%, both the Nads and Ncoo were relatively small, so the NCd2+ was the smallest.

14


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Figure 6. Coordinated, adsorbed and bound amount of Cd(Ⅱ) at different silicified degrees. 3.2.2 Effect of silicified degree on Cd(Ⅱ) bound strength MSD was adopted to analyze Cd(Ⅱ) bound strength by Si(OH)4 and SiO2 under different silicified degrees, the smaller slope of MSD curves meant the stronger bound strength. As shown in Figure 7, the MSD reflects the total Cd(Ⅱ) diffusive behavior rather than the diffusive behavior at a certain distance from SiO2 (Figure 4). The slope of the curve was in the order of 25%>50%>75%>100%>0, that is the bound strength was in the order of 0>100%>75%>50%>25%. Additionally, the binding energy was in the order of 0 (Ebin = -129 kcal/mol) < 100% (Ebin = -101 kcal/mol) < 75% (Ebin = -90.3 kcal/mol < 50% (Ebin = -80.8 kcal/mol) < 25% (Ebin = -64.8 kcal/mol), which was coincident with the MSD analysis. When the PSiO2=0, the slope of the MSD curve was the smallest, indicating that the Cd(Ⅱ) diffusive velocity was the slowest and the bound strength was the strongest. The reason might be the 15



strongest Cd(Ⅱ) coordination by Si(OH)4. When the PSiO2=100%, the Cd(Ⅱ) adsorption by SiO2 was relatively strong, but the SiO2 only could adsorb Cd(Ⅱ) near the interface. Therefore, the Cd(Ⅱ) were weakly adsorbed far from the SiO2 interface, resulting in a relatively faster diffusive velocity. And the Cd(Ⅱ) bound strength under PSiO2=100% was not as intense as that under PSiO2=0. When the PSiO2=75%, the adsorption by SiO2 and the coordination by Si(OH)4 both were weakened, owing to the existence of competitive adsorption between Si(OH)4 and Cd(Ⅱ) on the SiO2 surface. The results led to the increase of the Cd(Ⅱ) diffusive velocity and the decrease of bound strength. When the PSiO2=50%, the Si(OH)4 content increased, the competitive adsorption between Si(OH)4 and Cd(Ⅱ) on SiO2 surface became more intense, which further impaired the bound strength. When the PSiO2=25%, the content of Si(OH)4 which aggregated on the SiO2 surface further increased. Then both the strength of the Cd(Ⅱ) coordination by Si(OH)4 and the adsorption by SiO2 decreased, so the diffusive velocity was the fastest and the bound strength was the weakest.

16


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Figure 7. Mean square displacements of Cd(Ⅱ) at different silicified degrees. 3.3 Analysis of adsorption behavior of Cd(II) and Si(OH)4 on SiO2 surface and their bound mechanism. According to the above descriptions, there was competitive adsorption between Si(OH)4 and Cd(Ⅱ) on the SiO2 surface, and SiO2 adsorbed Si(OH)4 preferentially, thus the silicification was detrimental to the Cd(Ⅱ) attraction by silicon fertilizer. In this section, DFT calculations were applied to study the adsorption difference between Si(OH)4 and Cd(Ⅱ) on the SiO2 surface, and then to investigate the effect of silicified degree on the Cd(Ⅱ) bound mechanism. Firstly, the adsorption energy of Cd(Ⅱ)/SiO2 and Si(OH)4/SiO2 was calculated through Equation (2), respectively. The results showed that the adsorption energy of Cd(Ⅱ) by SiO2 (-2.96 eV) was much lower than that of Si(OH)4 by SiO2 (-0.564 eV ), with the adsorption distance of Cd(Ⅱ) by SiO2 (2.28 Å) shorter than that of Si(OH)4 by SiO2 (2.89 Å). According to the value of adsorption energy and distance, the Cd(Ⅱ) adsorption by SiO2 could be regarded as the chemisorption

41

adsorption by SiO2 could be regarded as the physisorption

, while the Si(OH)4

42

. Additionally, the

electronic cloud overlapping was analyzed to study the electronic transfer behavior in the adsorption process. The greater overlapped degree of the electronic cloud meant the larger amount of transferred electrons, and more intense interaction between them 43

. As can be seen in Figure 8A and 8B, the electronic clouds only overlapped slightly

in the Si(OH)4 adsorption by SiO2, and Si(OH)4 obtained merely 0.0700 e- electrons from SiO2 based on the Mulliken charge analysis. However, electronic clouds of Cd(Ⅱ) and SiO2 overlapped with a higher degree in the Cd(Ⅱ) adsorption by SiO2, and Cd(Ⅱ) obtained 0.865 e- electrons from SiO2. Moreover, the partial density of states of Si(OH)4 hardly varied before and after adsorption (Figure 8C), indicating that there was no visible charge transfer during the adsorption process. The results also further confirmed that the adsorption process was mainly donated by the 17



physisorption 44. In contrast, as depicted in Figure 8D, the p-electrons density of SiO2 located at -7.50 eV and -2.50 eV decreased apparently and split into multiple peaks after Cd(Ⅱ) adsorption. The reason might be that the p electrons of O belonging to SiO2 surface were captured by the Cd(Ⅱ). Meanwhile, the d electrons of Cd(Ⅱ) fed back to the SiO2, so the d-electrons density of SiO2 located at -9.00 eV and -5.00 eV increased after adsorption. On the other hand, the energy of d electrons was among that of p electrons, indicating that the p-d electronic conjugations were formed between the Cd(Ⅱ) and SiO2

45,46

. The electronic state density of SiO2 changed

significantly before and after the adsorptio4n of Cd(Ⅱ), revealing that the migration of electrons was involved in the adsorption process. The results also further confirmed that the Cd(Ⅱ) adsorption by SiO2 was chemisorption.

Figure 8. Charge density diagrams of SiO2 adsorbing Si(OH)4 (A) and Cd(Ⅱ) (B), partial density of state of SiO2 adsorbing Si(OH)4 (C) and Cd(Ⅱ) (D) (dotted line and solid line represent the PDOS before and after adsorption, respectively). As described above, Cd(Ⅱ) adsorption on the SiO2 surface was chemisorption, 18


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while Si(OH)4 adsorption on the SiO2 surface was physisorption. However, the adsorption capacity of Cd(Ⅱ) onto SiO2 was lower than that of Si(OH)4 onto SiO2. In order to clarify the reason of this phenomenon, we have done a series of studies, and finally speculated that the reason might be the difference of hydration between the Si(OH)4 and Cd(Ⅱ) 47. Therefore, the hydration of Si(OH)4 and Cd(Ⅱ) was discussed below to further shed light on the mechanism and explain the interesting behavior. RDF was employed to analyze the hydration of Si(OH)4 and Cd(Ⅱ). The curves of Cd(Ⅱ) and Si(OH)4 between Ow (O in H2O) at different distances from SiO2 surface were displayed in Figure 9A and 9B, respectively. The higher and narrower RDF peak represented the stronger interaction between the atom and water. It can be seen from Figure 9A, the highest RDF peak of Cd(Ⅱ)-Ow appeared at 0-10 Å of Cd(Ⅱ) away from the SiO2, and the hydrated distance was closer (about 2.40 Å, which was closed to experimental result of 2.37 Å

48

), indicating that hydration of

Cd(II) was stronger at SiO2 interface. This was because the presence of bound water around the SiO2 surface 49, and the proximal water molecules were easily captured by Cd(II) due to the strong hydrated ability of Cd(II). On the surface of SiO2, Cd(Ⅱ) was surrounded by bound water molecules, which made Cd(Ⅱ) more likely to form dense cage molecules (the inset of Figure 9A). The water molecules in the Cd(Ⅱ)-hydrate formed hydrogen bonds with SiO2 on the surface, and the negative charge of O in SiO2 was transferred to H of H2O, which weakened the Cd(Ⅱ) adhesion ability on the surface of SiO2. However, the lowest RDF peak of Si(OH)4-Ow appeared in the range of 0-10 Å from the SiO2 surface, and its hydration distance was relatively far (about 5.00 Å), suggesting that the hydration of Si(OH)4 on the SiO2 interface was much weaker, and the hydration layer was comparatively loose. The reason might be that the water molecules of Si(OH)4 hydration layer were easily captured by the SiO2, so the hydration layer affected the Si(OH)4 adsorption slightly. Therefore, even the adsorption capacity of non-hydrated Cd(Ⅱ) on the SiO2 surface was much stronger than that of Si(OH)4 on the SiO2 surface, but the Cd(Ⅱ) adsorption capacity would be 19



lowered at some extent after the hydration. To further explore this concept, the adsorption energy and charge transfer of hydrated Cd(Ⅱ) adsorption on the SiO2 surface were investigated. It was known that the non-hydrated Cd(Ⅱ) adsorption energy on the SiO2 surface was -2.96 eV. However, the [Cd (H2O)6]2+ (the hydrated Cd(Ⅱ) existed as the form of hexa-coordination with H2O)

50

adsorption energy on the SiO2 surface was 0.692 eV.

Moreover, 0.0710 e- electrons were transferred in the [Cd (H2O)6]2+ adsorption, and this value was obviously smaller than that in the Cd(Ⅱ) adsorption (0.865 e-), indicating that the hydrated layer hindered the charge transfer in the Cd(Ⅱ) adsorption process. Consequently, the Cd(Ⅱ) adsorption capacity by SiO2 was significantly lowered after the hydration, and the adsorption process was a non-spontaneous process. Although the process was non-spontaneous, the results of molecular dynamics simulation showed that the hydrated Cd(Ⅱ) remained a certain adsorption amount by SiO2. To gain more insight, we speculated that the reason was the diffusion distribution of counter ion Cl- on the SiO2 surface, which increased the negative charge of the surface and then enhanced the Cd(Ⅱ) adsorption capacity. As can be seen in the inset of Figure 9C, the adsorbed Cl- on SiO2 surface connected with Cd(Ⅱ) through the electrostatic interaction, and the RDF curves also confirmed that the Clwas adsorbed on SiO2 surface. Additionally, the highest peak of the RDF curve of Cd(Ⅱ) and Cl- appeared at about 2.50 Å in all distance ranges from SiO2 (Figure 9D), suggesting that the electrostatic interaction distance between Cd(Ⅱ) and Cl- was 2.50 Å 51. The distance was close to the hydrated distance of Cd(Ⅱ), and the reason might be that the water molecules of hydrate Cd(Ⅱ) were replaced by Cl- ions. Meanwhile, the electrostatic interaction between Cd(Ⅱ) and Cl- in the 0 to 10 Å away from the SiO2 surface was the weakest, illustrating that the adsorbed Cl- on the SiO2 surface led to partial electrons of Cl- being captured, and weakened the electrostatic interaction between Cd(Ⅱ) and Cl- consequently. 20


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Figure 9. RDFs of Cd(Ⅱ)and H2O(Ow) (A) at different distances from SiO2 surface, and the inset shows the diagram of hydrated Cd(Ⅱ); RDFs of Si(OH)4 and H2O(Ow) (B); RDFs of SiO2 and Cl- (C) at different distances from SiO2, and the inset shows the distribution of Cl- on SiO2 surface. RDFs of Cd(Ⅱ) and Cl- (D). The above explanations showed that the adsorbed Cl- on the SiO2 surface could adsorb Cd(Ⅱ) through the electrostatic interaction. In this paragraph, the adsorption energy and charge transfer in the presence of Cl- were analyzed to deeply investigate the mechanism. Basically, a water molecule of [Cd(H2O)6]2+ was replaced by the Cl-and then formed a [Cd(5H2O)Cl]+ complex. The adsorption energy of [Cd(5H2O)Cl]+ on the SiO2 surface was -0.449 eV, which was much smaller than that of [Cd(6H2O)]2+ (0.692 eV), indicating that the existence of Cl- decreased the adsorption energy and promoted the Cd(Ⅱ) adsorption on the SiO2 surface. Furthermore, the adsorption process changed from non-spontaneous to spontaneous. On the other hand, the charge transfer of [Cd(5H2O)Cl]+ was 0.0850 e- in the 21



adsorption process, while that of [Cd(6H2O)]2+ was 0.0710 e-, showing that Cd(Ⅱ) was supplied with more electrons in the presence of Cl-. At last, differential charge density (Figure 10A) was utilized to analyze the role of Cl- in the charge transfer process

52,53

. As shown in the pictures, the interacted distance between Cl- and H on

the SiO2 surface was 2.08 Å, and the electrons of H were captured (red for accumulation regions, blue for depletion regions) by the Cl-, leading to the increase of electronegativity

54,55

. Meantime, Cl- attracted Cd(Ⅱ) through the electrostatic

interaction with the distance of 2.53 Å. The electrostatic interaction distance was close to the RDF result in Figure 9D (2.50 Å), and the Cl- acquired the electrons and the Cd(Ⅱ) lost the electrons, respectively. Conversely, Cd(Ⅱ) obtained the fed back electrons of Cl- at the same time (Figure 10B). Therefore, the direction of charge transfer was: H→Cl-↔Cd(Ⅱ), and formed the complex Si-O-H-Cl--Cd(Ⅱ) on the SiO2 surface. Thence, Cd(Ⅱ) could gain more electrons when complex [Cd(5H2O)Cl]+ was formed. That was to say, the counter ion Cl- could promote the Cd(Ⅱ) adsorption on SiO2 surface.

Figure 10. The differential charge diagram of synergistic adsorption of Cl- and Cd(Ⅱ) on the surface of SiO2 (A) and its sectional view of diagram (B). 4. Conclusions To sum up, Si(OH)4 forming amorphous SiO2 underwent the silicified process in plants. Si(OH)4 attracted Cd(Ⅱ) through the coordination, while SiO2 attracted Cd(Ⅱ) by the adsorption. On the basis of our findings we conclude that silicon fertilizer had the best Cd(Ⅱ) bound efficiency when the silicified degree was 0. The silicification process was detrimental to the Cd(Ⅱ) bound. During the silicification process, both 22


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SiO2 and Si(OH)4 existed in the system, the compact hydration layer of Cd(Ⅱ) formed hydrogen bonds with the hydroxyl on the SiO2 surface, which hindered the Cd(Ⅱ) adsorption on the SiO2 surface. The Si(OH)4 was adsorbed on the SiO2 surface preferentially owing to the loose hydration layer and occupied the Cd(Ⅱ) adsorption sites of SiO2, resulting in the weakening of both the adsorption and coordination of Cd(Ⅱ). Conversely, the counter ion Cl- in the system promoted the Cd(Ⅱ) adsorption on the SiO2 surface by enhancing the electronegativity of the SiO2 surface. Our results could provide some theoretical guidance for the design of silicon fertilizer, and will be helpful to enhance the Cd(II) bound of the plants.

Notes The authors declare no competing financial interest. All authors have given approval to the final version of the manuscript. The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgments This work was financially supported by the Science and Technology Planning Project (No. 2015B020237008; No. 2016B020242004) of Guangdong Province, China.

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