Ultra-High Adsorption\nCapacity of Calcium–Iron\nLayered Double Hydroxides for HEDP Removal through Phase Transition\nProcesses

Antiscalant disposal in reverse osmosis concentrate (ROC)\ntreatment\nis a significant obstacle in desalination. This study investigated\nthe adsorption performance of LDHs for removing 1-hydroxyethylidene-1,1-diphosphonic\nacid (HEDP). CaFe-LDH presented a specific adsorption behavior and\nultrahigh adsorption capacity for HEDP, with a maximum adsorption\ncapacity of 335.7 mg P/g (1116.5 mg HEDP/g) at pH 7.0. X-ray diffraction\n(XRD) demonstrated that HEDP adsorption induced a structural transformation\nof CaFe-LDH from a layered configuration to a highly ordered structure,\nleading to a noticeable phase transition. Time-of-flight secondary\nion mass spectrometry (ToF-SIMS) and Raman spectroscopy further confirmed\nthat two distinct binding modes of HEDP, relating to chelation with\nCa<sup>2+</sup> and adsorption on Fe<sup>3+</sup> simultaneously,\nare connected by phosphonic acid groups (−C–PO(OH)<sub>2</sub>), forming the CaFe-HEDP complex. X-ray fluorescence (XRF)\nand X-ray photoelectron spectroscopy (XPS) analyses revealed that\nthe CaFe-HEDP ternary complex exhibits a highly ordered arrangement\nin an oxygen-bridged framework. The construction of an oxygen-coordinated\nframework contributes to the incorporation of more HEDP into CaFe-LDH,\nleading to a well-aligned lattice in the new phase. These findings\nprovide valuable insights into developing novel LDH-based adsorbents\nfor removing phosphorus-containing antiscalants, establishing a sustainable\napproach to ROC management, and potential environmental risk reduction.