State Key Laboratory of Environmental Aquatic Chemistry, Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, People's Republic of China.
University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China.
Environ Sci Technol. 2023 Aug 22;57(33):12489-12500. doi: 10.1021/acs.est.3c03463. Epub 2023 Aug 8.
In situ Fe(III) coprecipitation from Fe oxidation is a widespread phenomenon in natural environments and water treatment processes. Studies have shown the superiority of in situ Fe(III) (formed by in situ oxidation of a Fe(II) coagulant) over ex situ Fe(III) (using a Fe(III) coagulant directly) in coagulation, but the reasons remain unclear due to the uncertain nature of amorphous structures. Here, we utilized an in situ Fe(III) coagulation process, oxidizing the Fe(II) coagulant by potassium permanganate (KMnO), to treat phosphate-containing surface water and analyzed differences between in situ and ex situ Fe(III) coagulation in phosphate removal, dissolved organic matter (DOM) removal, and floc growth. Compared to ex situ Fe(III), flocs formed by the natural oxidizing Fe coagulant exhibited more effective phosphate removal. Furthermore, in situ Fe(III) formed through accelerated oxidation by KMnO demonstrated improved flocculation behavior and enhanced removal of specific types of DOM by forming a more stable structure while still maintaining effective phosphate removal. Fe K-edge extended X-ray absorption fine structure spectra (EXAFS) of the flocs explained their differences. A short-range ordered strengite-like structure (corner-linked PO tetrahedra to FeO octahedra) was the key to more effective phosphorus removal of in situ Fe(III) than ex situ Fe(III) and was well preserved when KMnO accelerated in situ Fe(III) formation. Conversely, KMnO significantly inhibited the edge and corner coordination between FeO octahedra and altered the floc-chain-forming behavior by accelerating hydrolysis, resulting in a more dispersed monomeric structure than ex situ Fe(III). This research provides an explanation for the superiority of in situ Fe(III) in phosphorus removal and highlights the importance of atomic-level structural differences between ex situ and in situ Fe(III) coprecipitates in water treatment.
在自然环境和水处理过程中，从铁氧化中就地共沉淀 Fe(III) 是一种广泛存在的现象。研究表明，与原位 Fe(III)（通过原位氧化 Fe(II) 混凝剂形成）相比，原位 Fe(III)（直接使用 Fe(III) 混凝剂）在混凝过程中具有优越性，但由于非晶态结构的不确定性，其原因仍不清楚。在这里，我们利用原位 Fe(III) 混凝过程，用高锰酸钾（KMnO）氧化 Fe(II) 混凝剂，处理含磷酸盐的地表水，并分析了原位和异位 Fe(III) 在除磷、去除溶解有机物（DOM）和絮体生长方面的差异。与异位 Fe(III)相比，由天然氧化 Fe 混凝剂形成的絮体表现出更有效的除磷效果。此外，通过 KMnO 加速氧化形成的原位 Fe(III) 通过形成更稳定的结构表现出更好的絮凝行为，并增强了对特定类型 DOM 的去除，同时仍保持有效的除磷效果。絮体的 Fe K 边扩展 X 射线吸收精细结构谱（EXAFS）解释了它们的差异。短程有序的菱铁矿样结构（角相连的 PO 四面体到 FeO 八面体）是原位 Fe(III)比异位 Fe(III)更有效去除磷的关键，并且当 KMnO 加速原位 Fe(III)形成时，该结构得到很好的保留。相反，KMnO 通过加速水解，显著抑制了 FeO 八面体的边缘和角配位，并改变了絮体链形成行为，导致形成比异位 Fe(III)更分散的单体结构。这项研究为原位 Fe(III)在除磷方面的优越性提供了解释，并强调了原子级结构差异在水处理中对异位和原位 Fe(III)共沉淀的重要性。
