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| Nanoscale sequestration mechanism of Cd during anaerobic transformation of ferrihydrite |
| Received:February 25, 2025 Revised:April 23, 2025 |
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| KeyWord:Cd;ferrihydrite coprecipitates;Fe oxides transformation;Cs-STEM;microscopic sequestration mechanism |
| Author Name | Affiliation | E-mail | | LI Wei | School of Environment and Energy, South China University of Technology, Guangzhou 510006, China | | | LI Rong | School of Environment and Energy, South China University of Technology, Guangzhou 510006, China | | | LU Yang | South China Institute of Environmental Sciences, Ministry of Ecology and Environment(MEE), Guangzhou 510645, China | superloo@163.com | | SHI Zhenqing | School of Environment and Energy, South China University of Technology, Guangzhou 510006, China | | | WU Wencheng | South China Institute of Environmental Sciences, Ministry of Ecology and Environment(MEE), Guangzhou 510645, China | |
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| Abstract: |
| This study aims to elucidate the immobilization and release behavior of Cd during the anaerobic transformation of iron oxides, as well as its microscopic interaction mechanisms with iron oxides. Ferrihydrite, a widely distributed iron oxide in the environment, was selected, and Fe（Ⅱ）induced ferrihydrite- Cd（Ⅱ）coprecipitates transformation were conducted. Batch extraction experiments were used to explore the sequestration and release characteristics of Cd at various aging times of iron oxides. Advanced characterization techniques, including X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, and spherical aberration-corrected scanning transmission electron microscopy（Cs-STEM）were conducted to elucidate the influence of Cd on the transformation process of iron oxides and the sequestration mechanisms of Cd on iron oxide nanoparticles. Results revealed that goethite（80.9%）and magnetite（19.1%）were the final products of the Fe（Ⅱ）-induced ferrihydrite-Cd（Ⅱ）coprecipitation transformation system, with lepidocrocite serving as an intermediate phase. Batch extraction experiments indicated that 34.1% of Cd（Ⅱ）was ultimately immobilized by iron oxides in a non-extractable form. Elemental analysis of minerals using Cs-STEM demonstrated that the Cd（Ⅱ）content on goethite（1.9%）was significantly higher than that on magnetite（0.8%）and lepidocrocite（0.3%）at 8 h. Sub-nanoscale high-resolution imaging and interplanar spacing analysis of individual goethite particles revealed distinct bright spots corresponding to Cd（Ⅱ）on the characteristic crystal planes of goethite. The most significant change in interplanar spacing was observed for the G（111）plane, which exhibited an increase of 0.043 8 Å compared to Cdfree pure goethite. XPS results demonstrated a notable decrease in the metal-oxygen bond ratio from 45.0%（8 h）to 4.8%（168 h）during the transformation process, accompanied by a reduction in surface-bound Cd. Integrating chemical extraction experiments, Cs-STEM, and XPS results, this study confirms that Cd（Ⅱ）can be immobilized within the goethite crystal structure through isomorphous substitution during the anaerobic transformation of iron oxides, thereby reducing its mobility. |
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