关键词: 蒙脱石, 有机改性, 零价纳米铁, 四溴双酚A, 土壤修复

Abstract:

Tetrabromobisphenol A (TBBPA) is one of the most widely used brominated flame retardants (BFRs). It is predominantly used as a reactive additive in the fabrication of printed circuit boards and finds extensive applications in industries such as the electronics, electrical appliances, plastics, textiles, and building materials. Owing to its lipophilic nature, TBBPA tends to accumulate persistently in soil. Long-term accumulation in soil poses potential risks to human health through food chain transmission, thus rendering the remediation of TBBPA-contaminated soil an issue of critical importance. Current methods for TBBPA removal include physical, biological, and chemical technologies, yet each suffers from inherent limitations: Physical methods are often constrained by low adsorption and degradation efficiency, while biological and chemical technologies are constrained by their stringent reaction conditions, hindering large-scale industrial application. Against this backdrop, the integration of multiple treatment technologies has emerged as a major research focus. Montmorillonite (MT), owing to its layered structure and expandable interlayers, is commonly exploited as an adsorbent or catalyst support in environmental remediation. Nanoscale transition metals are widely utilized as catalytic active sites in various catalytic systems, such as Advanced Oxidation Processes (AOPs). When immobilized on catalyst surfaces, nanoscale transition metals can generate hydroxyl radicals to degrade pollutants; however, their poor dispersibility often impedes degradation efficiency. The multilayered structure of MT not only serves as an ideal carrier for nanoscale transition metals but also, as studies have shown, organically modified MT can effectively enhance the dispersion of these metals, thereby improving their pollutant degradation efficiency. To achieve efficient remediation of TBBPA-contaminated soil, this study prepared a series of MT-based materials. By comparing the TBBPA removal efficiencies of different materials, the optimal target remediation material was screened. Initially, organically modified montmorillonite (CMT) was synthesized via an ion exchange method. Subsequently, several materials were prepared, including zero-valent iron nanoparticle-modified organomontmorillonite (nZVI@CMT). The TBBPA removal rate was adopted as the primary indicator to evaluate the performance of both pristine and unmodified MT materials. High-performance liquid chromatography (HPLC) was used to determine TBBPA concentrations. Moreover, soil enzyme activity assays, earthworm epidermal damage tests, and seed germination experiments were conducted to comprehensively assess the ecological remediation efficacy of nZVI@CMT on TBBPA-contaminated soil. Results from comparative studies demonstrated that nZVI@CMT exhibited the best TBBPA removal performance, achieving a removal rate of 98.28% within 60 minutes. Accordingly, nZVI@CMT was identified as the target remediation material. The material was characterized using SEM, TEM, XRD, FTIR, and XPS to analyze morphological and structural changes before and after modification. SEM images revealed that zero-valent iron nanoparticles were uniformly distributed on the montmorillonite surface in a granular form, effectively mitigating their agglomeration tendency. TEM results further corroborated these observations. XRD analysis indicated that the structure of the modified nano-zero-valent iron montmorillonite was similar to that of unmodified montmorillonite, confirming that the layered molecular structure remained largely unchanged after organic modification and nZVI loading. However, a significant increase in the interlayer spacing after organic modification confirmed the intercalation of modifiers into the montmorillonite layers. The diffraction peaks of zero-valent iron also verified the successful immobilization of nZVI on CMT. XPS and FTIR were employed to investigate changes in elemental valence states and functional groups. The XPS spectrum of nZVI@CMT exhibited distinct Fe2p characteristic peaks. FTIR analysis showed weakened absorption peaks corresponding to H-O stretching vibrations of bound water, H-O stretching vibrations of interlayer free water, and Si-O and Al-O stretching vibrations compared with pristine montmorillonite. These findings confirm that organic modification combined with zero-valent iron nanoparticle modification effectively reduced the mineralization degree of montmorillonite, thereby enhancing its adsorption and degradation capacity for TBBPA. The morphological and structural changes in nZVI@CMT not only verify the success of the modification but also provide critical evidence for mechanistic analysis based on surface structure and functional group alterations before and after reactions. The remediation effectiveness of nZVI@CMT was systematically evaluated by monitoring changes in TBBPA concentration, soil enzyme activity, and biological indicators (e.g., earthworm damage and seed germination). Additionally, LC-MS was used to identify reaction intermediates and deduce the potential TBBPA removal pathway by nZVI@CMT from soil. The results demonstrated that under natural aging conditions, the TBBPA removal rate in the contaminated soil group without nZVI@CMT addition was only 19.90% on day 42, whereas that in the group treated with nZVI@CMT showed a significant increase to 73.15% on day 42, indicating that nZVI@CMT effectively accelerated the TBBPA removal process, leading to a substantial reduction in TBBPA concentration in soil within a relatively short period. Moreover, nZVI@CMT was found to positively regulate soil enzyme activity, particularly enhancing urease activity, which is closely associated with the increased bacterial diversity and abundance facilitated by nZVI@CMT. Ecotoxicological tests showed that nZVI@CMT also effectively alleviated TBBPA-induced damage to earthworms. Compared to the contaminated group, earthworms in the nZVI@CMT-treated soil exhibited only minor effects such as blurred setae and slight dehydration in some segments, while the contaminated group displayed severe damage including body swelling, segment distortion, and epidermal cracking. Furthermore, nZVI@CMT improved seed germination rates and trends in contaminated soil. After treatment, the seed germination index increased from 2.95 to 4.33, and the seedling vigor index rose from 6.9 to 8.33, effectively mitigating the toxic effects of TBBPA on seedlings and demonstrating favorable remediation performance. Additionally, LC-MS analysis confirmed that the primary degradation pathways of TBBPA in soil included β-bond cleavage, demethylation, and benzene ring opening reactions. By extending the reaction time, the intermediate products could be further oxidized into small molecules such as carbon dioxide and water. In conclusion, this study systematically compared the TBBPA removal efficiencies of various montmorillonite-based materials, including MT, CMT, and nZVI@CMT. It confirmed that nZVI@CMT exhibits highly efficient and stable TBBPA removal. MT effectively addresses the agglomeration issue of nZVI and enhances the stability of nZVI particles. nZVI@CMT significantly accelerates TBBPA removal in soil. The primary degradation pathways of TBBPA involve β-bond cleavage, demethylation, and benzene ring opening, with complete mineralization achievable through extended reaction time or increased catalyst dosage. The addition of nZVI@CMT positively regulates soil physicochemical properties and enzyme activity, aiding in soil ecological restoration. nZVI@CMT reduces the toxicity of TBBPA to soil fauna, thereby maintaining ecological stability. Furthermore, nZVI@CMT improves the seed germination rate and germination potential in contaminated soil, alleviates the inhibitory effects of TBBPA on seed germination and seedling growth, and provides a foundation for vegetation restoration.

Key words: montmorillonite, organic modification, zero-valent iron nanoparticles (nZVI), tetrabromobisphenol A (TBBPA), soil remediation