Song Li, Zhang Weiguo, Liu Quan, Zhang Xiao, Liu Fuqiang, Li Aimin
State Key Laboratory of Pollution Control and Resource Reuse, School of Environment, Nanjing University, Nanjing, 210023, PR China; College of Environment and Ecology, Jiangsu Open University, Nanjing, 210036, PR China.
State Key Laboratory of Pollution Control and Resource Reuse, School of Environment, Nanjing University, Nanjing, 210023, PR China.
Environ Res. 2025 Aug 21;285(Pt 5):122664. doi: 10.1016/j.envres.2025.122664.
Various organic acids commonly coexist with Cr(VI) in wastewater. However, direct electron transfer between Cr(VI) and electron-rich organic acids is typically slow. Current Fe-based mediators primarily operate through the aqueous Fe(III)/Fe(II) cycle, which faces challenges in achieving efficient Fe recycling. To address the efficient removal of Cr(VI) and sodium citrate (Cit) composite pollutants (Cr(VI)-Cit), iron-loaded carbon nitride composites (Fe/CN) were synthesized via a combined loading-calcination approach. Crucially, coordination between carboxylate groups of Cit and undercoordinated Fe sites on Fe/CN formed photoactive complexes, effectively stimulating ligand-to-metal charge transfer (LMCT). This significantly enhanced the solid-phase Fe(III)/Fe(II) cycling efficiency, leading to a markedly increased Cr(VI) reduction rate constant (k) of 0.213 mg/(L·min). Furthermore, coordination with the N-rich matrix effectively inhibited Fe leaching, resulting in a post-reaction total Fe concentration below 0.5 mg/L. The formation of a Cr(VI)-Fe-Cit ternary cyclic complex effectively lowered the activation energy for Cr(VI) reduction by 81.3 % compared to pristine CN. In the dark, intramolecular electron transfer within this complex generated Fe(II)-Cit species with a low electrode potential. Under irradiation, the photoactive Fe(III)-Cit complex generated Fe(II) via LMCT to reduce Cr(VI); concurrently, photogenerated electrons (e) and superoxide radicals (·O) also contributed to Cr(VI) reduction. Reusability tests showed Fe/CN retained ∼90 % Cr(VI) removal efficiency from actual wastewater over five cycles, demonstrating good stability under realistic conditions. This study provides valuable insights for the cost-effective and efficient remediation of Cr(VI)-organic acid complexes in real wastewater.
废水中各种有机酸通常与六价铬（Cr(VI)）共存。然而，Cr(VI)与富电子有机酸之间的直接电子转移通常较慢。目前基于铁的介质主要通过水相铁（III）/铁（II）循环起作用，这在实现高效铁循环方面面临挑战。为了有效去除Cr(VI)和柠檬酸钠（Cit）复合污染物（Cr(VI)-Cit），采用负载-煅烧相结合的方法合成了负载铁的氮化碳复合材料（Fe/CN）。至关重要的是，Cit的羧酸根基团与Fe/CN上配位不足的铁位点之间的配位形成了光活性配合物，有效促进了配体到金属的电荷转移（LMCT）。这显著提高了固相铁（III）/铁（II）的循环效率，使Cr(VI)还原速率常数（k）显著增加至0.213 mg/(L·min)。此外，与富含氮的基质配位有效抑制了铁的浸出，反应后总铁浓度低于0.5 mg/L。与原始CN相比，Cr(VI)-Fe-Cit三元环状配合物的形成有效降低了Cr(VI)还原的活化能81.3%。在黑暗中，该配合物内的分子内电子转移产生了电极电位较低的Fe(II)-Cit物种。在光照下，光活性Fe(III)-Cit配合物通过LMCT产生Fe(II)以还原Cr(VI)；同时，光生电子（e）和超氧自由基（·O）也有助于Cr(VI)的还原。可重复使用性测试表明，Fe/CN在五个循环中对实际废水的Cr(VI)去除效率保持在约90%，表明在实际条件下具有良好的稳定性。本研究为实际废水中Cr(VI)-有机酸络合物的经济高效修复提供了有价值的见解。
