Passivation Effect of Thiol-Modified Montmorillonite on Cadmium in Medium-Alkaline Farmland Soil in Northern China

ZHU Liwen; REN Chao; LI Jingtian; TIAN Peiyi; XIAO Jianhui; LI Ping

doi:10.15898/j.ykcs.202309010147

Rock and Mineral Analysis > 2024 > 43(1): 124-136. > DOI: 10.15898/j.ykcs.202309010147

ZHU Liwen，REN Chao，LI Jingtian，et al. Passivation Effect of Thiol-Modified Montmorillonite on Cadmium in Medium-Alkaline Farmland Soil in Northern China[J]. Rock and Mineral Analysis，2024，43（1）：124−136. DOI: 10.15898/j.ykcs.202309010147

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Passivation Effect of Thiol-Modified Montmorillonite on Cadmium in Medium-Alkaline Farmland Soil in Northern China

ZHU Liwen^{1, 2,},
REN Chao^{1, 2},
LI Jingtian^{1, 2},
TIAN Peiyi^{1, 2},
XIAO Jianhui^{1, 2},
LI Ping^{1, 2}

1.
Henan Province First Geology and Mineral Survey Institute Co., Ltd., Luoyang 471000, China
2.
Henan Provincial Engineering Research Center of Ecological Environment and Exploration Geochemistry, Luoyang 471000, China

More Information

Received Date: August 31, 2023
Revised Date: January 26, 2024
Accepted Date: February 03, 2024
Available Online: March 14, 2024

Abstract

HIGHLIGHTS
(1) The thiol-modified montmorillonite not only added C-H and S-H covalent bonds, but also enhanced the activity of −OH and C=O chemical bonds, and could adsorb Cd²⁺ with thiol and hydroxyl groups.

(2) The newly added thiol coordination adsorption made the form of Cd in soil change from ion exchange state to iron-manganese oxide binding state, and the enhanced hydroxyl coordination adsorption made it change into carbonate binding state.

(3) The passivation effect of montmorillonite on soil cadmium was significantly improved after thiol modification, and the decrease of available cadmium in soil increased with the increase of addition amount.

ABSTRACT

Passivation material is a key material for repairing heavy metal contaminated soil in farmland. To study and develop efficient soil heavy metal passivator, thiol-modified montmorillonite was studied. Montmorillonite was used as the raw material, and the thiol group was loaded on the surface or interlayer of montmorillonite to prepare thiol-modified montmorillonite. The characteristics of thiol-modified montmorillonite were characterized by XRD, SEM, TEM and FTIR, and the indoor soil culture test was carried out. The thiol-modified montmorillonite not only added C-H and S-H covalent bonds, but also enhanced the activity of -OH and C=O chemical bonds, and could adsorb with Cd²⁺ by thiol and hydroxyl coordination. As a result, the occurrence form of Cd in the soil was significantly changed, the ion exchange state was greatly reduced, and the available Cd in the soil absorbed by the crop roots was significantly reduced. After adding 1%, 3% and 5% thiol-modified montmorillonite, the available Cd in soil decreased by 21.92%, 69.11% and 82.90%, respectively. The passivation effect of montmorillonite on soil Cd was significantly improved after thiol modification, and the decrease of available Cd in soil tended to increase with the increase of thiol-modified montmorillonite. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202309010147.
BRIEF REPORT
Significance: Heavy metal pollution in farmland soil is the main factor affecting the environmental quality and safety of agricultural products in China. Passivation material is a key material for repairing heavy metal contaminated soil in farmland. Research and development of efficient soil heavy metal passivation materials is very important for repairing heavy metal contaminated farmland and ensuring the food safety of agricultural products. Passivation is one of the main technologies for the remediation of Cd-contaminated soil. There have been a lot of studies at home and abroad^[2-3], but most of the studies are aimed at acidic Cd-contaminated soil^[4-6]. Most of the northern and southern parts of China are alkaline soil^[7-9], and the passivation remediation materials for Cd-contaminated farmland still need to be studied.

　　The passivation effect of montmorillonite before and after modification on the alkaline Cd contaminated farmland soil in northern China needs to be further studied. Montmorillonite is a 2∶1 type aluminosilicate mineral^[10]. Due to its special layered structure, it has a large specific surface area and strong adsorption capacity for heavy metal ions in soil, and has been used to passivate and repair heavy metal pollution in soil^[11-13]. However, due to the strong hydrophilicity and weak bonding ability of the silicon-oxygen structure on the surface of natural montmorillonite, the adsorption process is reversible, the adsorption effect is limited, and the passivation effect is unstable^[14-15], which is limited in the application of adsorption and passivation of soil heavy metal Cd. Therefore, the current research focuses on the modification of natural montmorillonite before use, thereby enhancing the adsorption capacity^[16-18]. The thiol group has a strong complexing ability, strong adhesion, and is a weak base. Cd²⁺ is a weak acid, based on the theory of soft and hard acid-base, thiol group and Cd²⁺ can form a stable binding state, and can be a good adsorption of heavy metal ions^[19-20]. At present, studies have confirmed that thiol-modified montmorillonite has a passivation effect on acidic Cd-contaminated soil in the south. However, considering that the adsorption capacity of passivation materials for Cd²⁺ is greatly affected by soil pH^[22,25], and the passivation of montmorillonite before and after modification on alkaline Cd-contaminated farmland, soil in the north is rarely reported. Therefore, this study was carried out to analyze the changes in material characterization before and after montmorillonite modification, the passivation effect of montmorillonite before and after modification, and different amounts of montmorillonite on Cd in alkaline farmland in the north.

Methods: Montmorillonite was used as the raw material, and the thiol group was loaded on the surface or interlayer of montmorillonite to prepare thiol-modified montmorillonite. The characteristics of thiol-modified montmorillonite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). At the same time, indoor soil experiments were carried out to study the passivation effect of thiol-modified montmorillonite on Cd in northern alkaline farmland soil with different addition amounts, and explore the effects of montmorillonite modification and different addition amounts on the passivation and repair effect of Cd in northern alkaline farmland soil. Finally, combined with material characterization, the passivation mechanism of thiol-modified montmorillonite on Cd was analyzed in order to provide a theoretical basis for the remediation of Cd-contaminated farmland soil in northern China.

　　A total of 7 treatments were set up. Blank control treatment CK; montmorillonite powder treatments M1, M3 and M5 were added with 1%, 3% and 5% (percentage of soil quality, added to 100g contaminated soil per share). GM1, GM3 and GM5 were treated with 1%, 3% and 5% thiol-modified montmorillonite powder. Each treatment was set up with 3 replicates. The passivation material was added to the Cd-contaminated soil in proportion and fully mixed in the No.5 self-sealing bag. The soil samples treated above were accurately moved to the corresponding glass culture dish (the soil depth of the simulation experiment was 15mm), and the soil was replenished with deionized water every 2 days. The soil moisture content was controlled by the weighing method to maintain 70% of the field water holding capacity, and the culture dish was covered and placed in a constant temperature incubator at 25±2℃. At the same time, the soil samples were stirred and mixed with a plastic spoon to ensure multiple full mixing. Samples were taken 5 times at 7, 15, 30, 50 and 70 days, and 20g soil samples were weighed and dried in natural state through 20 mesh sieves. The pH, total Cd content, available Cd content and Cd content of each form were determined. The specific detection and analysis methods and basis are shown in Table 1.

Data and Results: The crystal structure, microstructure, particle morphology and functional group information were analyzed by material characterization techniques. The change of crystal structure was analyzed by XRD. The XRD patterns of montmorillonite and thiol-modified montmorillonite (Fig.1) were basically the same, indicating that this modification method did not destroy the original mineral structure. The thiol group may be loaded between the layers of the material, or it may be wrapped outside the material to complete the modification. According to the results of SEM (Fig.2), TEM (Fig.3) and BET, the particle size of montmorillonite increased after modification, the surface structure changed from flat and regular to curly and loose, and the interlayer spacing decreased. The thiol group had been effectively grafted onto the surface or interlayer of montmorillonite, forming a stable thiol-modified montmorillonite. Combined with FTIR analysis (Fig.4), it was found that the modified montmorillonite not only added C-H and S-H covalent bonds, but also enhanced the activity of -OH and C=O chemical bonds.

　　The passivation effect of thiol-modified montmorillonite on Cd in soil was significant. After the addition of thiol-modified montmorillonite to the test soil, the ion-exchanged Cd in the soil was converted into an iron-manganese oxide-bound state due to the thiol coordination adsorption, and the enhanced hydroxyl coordination adsorption converted it into a carbonate-bound state. As a result, the occurrence form of Cd in the soil was significantly changed, the ion exchange state was greatly reduced, and the available Cd in the soil absorbed by the crop roots was significantly reduced. After adding 1%, 3% and 5% thiol-modified montmorillonite, the available Cd in soil decreased by 21.92%, 69.11% and 82.90%, respectively (Fig.5). As the control group, the addition of 1%, 3%, and 5% montmorillonite only decreased by 3.37%, 1.80%, and 6.71%, respectively. The passivation effect of montmorillonite on soil Cd was significantly improved after thiol modification, and the decrease of available Cd in soil tended to increase with the increase of thiol-modified montmorillonite. When 5% thiol-modified montmorillonite was applied, the decrease of available Cd in soil could be increased to 82.90% on the basis of 6.71% in the control group. The passivation effect of thiol-modified montmorillonite on Cd-contaminated farmland soil in northern China is significant, and it can be used as a passivation material for safe utilization of Cd-contaminated farmland soil in Northern China.

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References (39)

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