In situ Adsorption Technology for Remediation of Cr(Ⅵ) Contaminated Soil

(1) ATP/PPy composite materials with strong adsorption affinity for Cr(Ⅵ) were prepared. ATP/PPy can effectively remove Cr(Ⅵ) from aqueous solutions and soil. The adsorption mechanisms include electrostatic attraction, chelation, reduction, and ion exchange.

(2) The adsorption of Cr(Ⅵ) by ATP/PPy follows the pseudo-second-order kinetic model, indicating that the adsorption process is mainly chemical adsorption. The adsorption of Cr(Ⅵ) by ATP/PPy follows the Langmuir isotherm model, indicating that the removal process of Cr(Ⅵ) by ATP/PPy is monolayer adsorption, with a maximum adsorption capacity of 185.19mg/g.

(3) The remediation experimental results of ATP/PPy on Cr(Ⅵ) contaminated soil under different conditions through in situ adsorption technology show that after 30 days of remediation under optimal conditions, Cr(Ⅵ) in the soil filtrate decreased by 58.51%, and the Cr(Ⅵ) content in the soil decreased to 2.97mg/kg.

Significance: Cr is widely used in industrial and agricultural activities, such as leather tanning, metallurgy, electroplating, ceramic glaze, and wood anti-corrosion. It is listed by the international community as one of the eight most competitive resource-based raw material products^[1-3]. However, with the development of industry, a large amount of Cr enters the environment along with the discharge of wastewater and waste residue, leaving behind a large number of Cr pollution sites^[4-6]. The main forms of Cr in soil are Cr(Ⅲ) and Cr(Ⅵ). Due to the negative charge of the soil, Cr(Ⅲ) is easily adsorbed or precipitated by soil colloids, with low activity and less toxicity to organisms. However, Cr(Ⅵ) has weak adsorption and strong migration with soil colloids, making it easy to accumulate and enter the human body through the food chain. After Cr(Ⅵ) enters the human body, its anionic compound (CrO₄²⁻) has a chemical structure similar to SO₄²⁻ and HPO₄²⁻ in the intracellular fluid. It can easily enter the cell through protein channels on the cell membrane, react with reducing substances such as ascorbic acid, and damage the DNA in the nucleus, causing diseases such as lung cancer, birth defects, and decreased reproductive ability^[7]. Due to the enormous harm of Cr(Ⅵ), Cr(Ⅵ) and its compounds are listed as one of the 17 highly hazardous toxic substances by the US Environmental Protection Agency (EPA)^[8-10]. Therefore, research on the remediation and treatment of Cr(Ⅵ) contaminated sites has received widespread attention from environmental workers in various countries.

　　According to reports, 11% of contaminated sites in the United States, and 14% of contaminated sites in Japan are Cr(Ⅵ) contaminated^[11]. The area of Cr contaminated soil in China accounts for more than 5% of the total polluted soil area^[12]. In recent years, researchers have developed various in situ and ex situ remediation technologies to accommodate, clean, or restore Cr contaminated soil, such as guest soil method, soil washing, electric extraction, chemical reduction, and phytoremediation^[13-16]. These remediation methods have different working mechanisms and demonstrate specific application advantages and limitations. In addition, due to differences in physical and chemical soil properties, texture, pollution situation, etc., the effectiveness and cost of these technologies in on site practice vary greatly. For example, the cost of guest soil method and soil washing technology is extremely high, and they cause significant disturbance to the surrounding environment. They are only suitable for small-scale, high concentration Cr(Ⅵ) polluted sites; chemical reduction is currently a common method for remediation of Cr(Ⅵ) contaminated sites, but the reduced Cr(Ⅲ) poses a risk of being re-oxidized to Cr(Ⅵ); the electric extraction method has high energy consumption and the actual site restoration effect is not ideal; the efficiency of plant restoration is low and the cycle is long. Therefore, based on the concept of green and sustainable development, developing a new technology, method, and material for economically efficient and environmentally friendly remediation of Cr(Ⅵ) contaminated sites is a key issue that urgently needs to be addressed.

Methods: ATP/PPy adsorbent materials were prepared with PPy as the “shell” and ATP as the “core” by loading polypyrrole onto the surface of attapulgite through in situ polymerization (Fig.1). ATP/PPy has high adsorption affinity and specificity for Cr(Ⅵ), and can remove Cr(Ⅵ) through electrostatic attraction, chelation, reduction, and ion exchange. Meanwhile, ATP/PPy is embedded into the soil through adsorption plates, relying on the vertical migration and concentration difference infiltration of Cr(Ⅵ) in the soil to achieve in situ adsorption of Cr(Ⅵ) in the soil, and complete the recovery and regeneration of ATP/PPy after adsorption saturation. The effects of four factors, namely rainfall, soil pH, soil bulk density, and soil organic matter content, on the removal of Cr(Ⅵ) from soil by ATP/PPy were investigated to understand the feasibility of adsorption technology for the remediation of Cr(Ⅵ) contaminated soil.

Data and Results: Scanning electron microscopy analysis shows that the unmodified ATP was a blocky structure formed by rod crystal stacking (Fig.2a), and ATP/PPy was coated with PPy microspheres on the surface of ATP (Fig.2b). At the same time, after adsorption of Cr(Ⅵ), the surface morphology of ATP/PPy remained almost unchanged (Fig.2c). The FTIR specta (Fig.2d) show that the characteristic peaks of ATP at 3560cm⁻¹ and 3419cm⁻¹ correspond to the −OH stretching vibration caused by mineral layer and surface adsorbed water, while the peak at 1665cm⁻¹ corresponds to the −OH stretching vibration of H₂O. In addition, the peaks at 1037cm⁻¹ and 791cm⁻¹ are attributed to Si−O−Si stretching vibration and bending vibration, respectively. The characteristic peak of PPy at 1555cm⁻¹ corresponds to the C−C stretching vibration of the pyrrole ring, the characteristic peak at 1160cm⁻¹ corresponds to the C−N stretching vibration, and the characteristic peaks at 1030cm⁻¹ and 772cm⁻¹ correspond to the in-plane stretching vibration peak and the out of plane bending vibration peak of the C−H ring, respectively. The FTIR spectrum of ATP/PPy clearly showed the characteristic absorption peak of PPy, indicating that PPy has been successfully loaded onto the surface of ATP^[25-26]. In addition, after adsorbing Cr(Ⅵ), the infrared spectra of ATP/PPy-Cr shift towards higher values, such as 1565cm⁻¹, 1390cm⁻¹, 1210cm⁻¹, 1045cm⁻¹, and 912cm⁻¹. This is due to the presence of different forms of Cr in ATP/PPy, which leads to the destruction of the conjugated structure of PPy and the limitation of the degree of charge delocalization in PPy^[22]. The zero potential display shows that due to the introduction of PPy, the zero potential of ATP/PPy increases from 2.2 to 6.4 (Fig.2e), which is beneficial for ATP/PPy to remove negatively charged Cr(Ⅵ) through electrostatic attraction^[27].

　　The adsorption kinetics of ATP/PPy for Cr(Ⅵ) in water were shown in Fig.3a. The experimental data shows that ATP/PPy had a fast adsorption rate for Cr(Ⅵ), reaching an equilibrium adsorption capacity of 95% within 30min. In addition, the pseudo-second-order kinetic model perfectly fitted the adsorption data of ATP/PPy for Cr(Ⅵ), with a correlation coefficient R²>0.9999, indicating that the adsorption process was mainly chemical adsorption^[28-29]. In order to explore the maximum adsorption capacity of ATP/PPy for Cr(Ⅵ), adsorption isotherms were studied (Fig.3b). The results show that the adsorption capacity of ATP/PPy for Cr(Ⅵ) increased with the initial concentration of Cr(Ⅵ). The Langmuir model can better fit the adsorption data of ATP/PPy for Cr(Ⅵ), with a correlation coefficient R²>0.9996, revealing that the removal process of Cr(Ⅵ) by ATP/PPy was monolayer adsorption^[30-31]. At 25℃, the maximum adsorption capacity of ATP/PPy for Cr(Ⅵ) was 185.19mg/g, indicating that ATP/PPy had strong removal ability for Cr(Ⅵ).

　　In order to study the removal mechanism of Cr(Ⅵ) by ATP/PPy, XPS was used to analyze the adsorption of Cr(Ⅵ) by ATP/PPy before and after. Compared with ATP/PPy, Cr energy bands were clearly observed in ATP/PPy-Cr (Fig.4a), indicating the successful loading of Cr on the surface of ATP/PPy. The core energy level spectrum of Cr 2p shows that Cr(Ⅲ) and Cr(Ⅵ) coexist on the surface of ATP/PPy (Fig.4b), indicating that the portion of Cr(Ⅵ) adsorbed on the surface of ATP/PPy was reduced to less toxic Cr(Ⅲ) by electron rich PPy^[38]. The core energy level spectrum of N 1s shows that compared with ATP/PPy, the peak area ratio of NH− and −NH− groups in ATP/PPy-Cr decreased (Fig.4c), but the percentage of −NH⁺-groups increased, indicating that the N groups on the PPy surface participate in the adsorption of Cr(Ⅵ) through adsorption and reduction^[22]. In addition, after the adsorption of Cr(Ⅵ) by ATP/PPy, the peak area of Cl 2p significantly decreased (Fig.4d), indicating ion exchange between Cl⁻ doped in ATP/PPy and Cr(Ⅵ)^[39-41]. Based on XPS analysis, four mechanisms for removing Cr(Ⅵ) by ATP/Ppy were identified (Fig.1): (1) The positively charged N in ATP/PPy removes Cr(Ⅵ) through electrostatic attraction; (2) Cr(Ⅵ) is reduced to Cr(Ⅲ) by the action of nitrogen groups in PPy; (3) Cr(Ⅲ) is chelated onto deprotonated pyrrole N; (4) Cr(Ⅵ) replaces Cl⁻ doped in ATP/PPy through ion exchange.

　　ATP/PPy was embedded into the soil through adsorption plates and Cr(Ⅵ) was removed from the soil through in situ adsorption technology (Fig.5a). We investigated the effects of different factors namely rainfall, soil pH, soil bulk density, and soil organic matter content on the removal efficiency of Cr(Ⅵ) in soil. The experimental results show that when the concentration of Cr(Ⅵ) in the tested soil was 30mg/kg, the simulated rainfall was 6mL/day, the soil bulk organic matter content was 7.6g/kg, the soil bulk density was 1.22g/cm³, and the soil pH was 5.86, the removal rate of Cr(Ⅵ) in the soil filtrate was 58.51% (Fig.5b). The Cr(Ⅵ) content in the soil decreased to 2.97mg/kg (Fig.5c, Fig.5d, Fig.5e, Fig.5f), which was lowered to below the screening value of 5.7mg/kg for the second category of development land in the Soil Environmental Quality: Risk Control Standard for Soil Contamination of Development Land (GB 36600—2018).