Adsorption-Deposition Behavior of Typical Minerals on Antimony in Soil

Significance: Antimony (Sb) is a non-essential metal, primarily introduced into the environment through anthropogenic activities, such as mineral extraction. It can enter the human body through respiratory tract, skin contact, and food chain, causing antimony poisoning. In soil, it may undergo adsorption, desorption, or redox reactions. Under natural conditions, it can form stable secondary minerals through adsorption or co-precipitation with metal hydroxides. Conversely, antimony present in these minerals may also dissolve and diffuse into the surrounding environment, including groundwater, contributing to environmental contamination. Exploring the adsorption deposition behavior of typical soil minerals on antimony is of great significance for predicting its environmental fate and preventing pollution.

　　The active components that adsorb antimony in soil are mainly iron, aluminum, manganese oxides, and clay minerals. The fixed proportion of antimony in soil iron oxides such as goethite, hematite, and ferrihydrite can reach 40% to 75%^[15]. Studies have shown that the adsorption and precipitation behavior of antimony on the surface of iron oxides have a complex dependence on the exposure and concentration of mineral crystal planes^[16], and can form various coordination configurations of inner sphere complexes on the surface of iron oxides^[17]. Research has indicated that manganese oxide plays a vital role in mitigating the toxicity of antimony and influencing its migration behavior via oxidation and adsorption mechanisms.^[19]. The adsorption behavior of antimony varies on different exposed crystal faces of manganese oxide, such as Sb(Ⅲ) preferentially adsorbing on the {310} crystal face ofα-MnO₂, {131} crystal face of γ-MnO₂ (ramsdellite), and {111} crystal face of δ-MnO₂^[21]. Although aluminum oxide and clay materials have lower adsorption capacity for antimony, their relative contents may be higher, and their impact on cannot be ignored^[22]. Research has found that Sb(Ⅴ) can form exosphere complexes on γ-Al₂O₃^[23], and the maximum adsorption capacities of bentonite for Sb(Ⅲ) and Sb(Ⅴ) are 370−555μg/g and 270−500μg/g, respectively^[24]. Although existing literature has focused on the adsorption and desorption behavior of natural minerals towards antimony^[26-27], there have been no reports on the in situ characterization of antimony morphology at natural mineral interfaces and its correlation with antimony concentration.

　　In this study, the adsorption performance and mechanism of antimony on six selected oxides and minerals were determined. Generally, the adsorption of Sb(Ⅴ) and Sb(Ⅲ) was predominantly governed by chemisorption, with electrostatic adsorption also playing a notable role in the adsorption of Sb(Ⅴ). Results from high-concentration adsorption isotherm experiments and Raman spectroscopic analysis confirmed that at elevated equilibrium concentrations, Sb(Ⅴ) primarily undergoes chemisorption, whereas Sb(Ⅲ) tends to deposit on the oxide and mineral surfaces to form α-Sb₂O₃. This deposition behavior may result in distinct environmental mobility for Sb(Ⅲ) compared to its chemisorbed state. These findings provide valuable insights for the risk assessment and management of antimony contamination in soils.

Methods: Hematite, ramsdellite, kaolinite, goethite and aluminum oxide were purchased from a reagent company, while ferrihydrite was prepared as follows^[28]: concentrated ammonia solution was added to 0.2mol/L Fe(NO₃)₃ solution dropwise under stirring, until the solution pH reached 7.0. After further reaction for 30min, the product was centrifuged, ultrasonically washed 3 times, and vacuum dried.

　　(1) Adsorption experiments. The initial pH of 10mL of Sb(Ⅲ) and Sb(Ⅴ) solutions were adjusted to 7.0 using 0.1mol/L hydrochloric acid or 0.1mol/L sodium hydroxide, and potassium nitrate was added to control the ion strength to 10mmol/L. A predetermined amount of minerals were added to the solution and oscillated for adsorption at 25℃ for 24h. The upper clear liquid was removed and ICP-OES used to detect the concentration of antimony. The adsorption amount q_e(mg/g) was then calculated. During the experiment, each group of samples was performed in triplicate and the average value was taken for analysis.

　　(2) Raman experiment. Mineral dispersion (10μL) after adsorption was dropped onto a glass slide for Raman testing. The following parameters were used: laser wavelength of 780nm, laser power of 100mW, integration time of 10s, integration times of 10, and a spectral resolution of 2cm⁻¹.

Data and Results: The XRD spectra of 6 minerals were compared with the standard card. Except for ferrihydrite, which had no obvious crystal structure, the XRD spectra of the other 5 minerals were in good agreement with the mineral standard spectra, and there were no other impurities (Fig.1). Ferrihydrite is an amorphous mineral, and its XRD spectrum is consistent with the results reported in the literature^[29]. The specific surface areas of hematite, goethite, ramsdellite, alumina oxide, kaolinite, and ferrihydrite (Fig.2) were 6.6, 18.9, 32.3, 6.1, 10.5, and 335.6m²/g, respectively. The adsorption rates of Sb(Ⅲ) and Sb(Ⅴ) were relatively fast within the first 2h. As the adsorption time increased, the active adsorption sites on the mineral surface gradually became saturated, and the adsorption rate gradually decreased within 2−6h. Except for the adsorption of Sb(Ⅲ) by ramsdellite, all other materials reached adsorption equilibrium around 24h (Fig.3). The adsorption of Sb(Ⅲ) by ramsdellite rapidly reached a high adsorption capacity in the initial stage (about 5min). The adsorption capacity of the same mineral material for Sb(Ⅲ) was greater than that for Sb(Ⅴ). The order of adsorption capacities (mg/g) of six soil minerals for Sb(Ⅲ)/Sb(Ⅴ) were as follows: ferrihydrite (101.4, 55.9)>ramsdellite (16.52,7.58)>goethite (13.30, 5.67)>hematite (5.13, 3.70)>aluminum oxide (1.66, 1.69)>kaolinite (0.27, 0.51) (Fig.4). The adsorption capacities of mineral materials for Sb(Ⅲ) did not change significantly (0.3%−14%) under different pH conditions, while with the increase of pH value, the adsorption capacity of mineral materials for Sb(Ⅴ) decreased (24%−78%) (Fig.5). After exceeding the saturation concentration, Sb(Ⅲ) deposited on the mineral surface (Fig.6). The mineral material adsorbed with high concentration of Sb(Ⅴ) did not obtain the characteristic peak of Sb₂O₅ (Fig.7), indicating that there was no obvious deposition of Sb(Ⅴ). After the deposition of Sb(Ⅲ), it might exist in the form of Sb₂O₃, and Raman detection results showed obvious Sb₂O₃ characteristic peaks. The Raman peaks at 190 and 452cm⁻¹ can be attributed to the bending vibration between Sb—O—Sb, while the Raman peaks at 256, 357, 374, and 715cm⁻¹ can be attributed to the stretching vibration between Sb—O—Sb (Fig.8). Raman spectroscopy can be used as a convenient method to monitor the adsorption and deposition of antimony on minerals.