Elucidating Phosphate\nand Cadmium Cosorption Mechanisms\non Mineral Surfaces with Direct Spectroscopic and Modeling Evidence

The simultaneous sorption of cations and anions at the\nmineral–water\ninterface can substantially alter their individual sorption characteristics;\nhowever, this phenomenon lacks a mechanistic understanding. Our study\nprovides direct spectroscopic and modeling evidence of the molecular\ncosorption mechanisms of the cadmium ion (Cd<sup>2+</sup>) and phosphate\n(P) on goethite and layered manganese (Mn) oxide of birnessite, through\nin situ attenuated total reflection Fourier-transform infrared (ATR-FTIR),\nP <i>K</i>-edge X-ray absorption near-edge structure (XANES)\nspectroscopy, and surface complexation modeling. Phosphate synergistically\ncosorbed with Cd on goethite predominantly through P-bridged ternary\ncomplexes (Fe–P–Cd) and electrostatic interactions\nat wide pH conditions. Likewise, P and Cd exhibited synergistic cosorption\non birnessite by forming P-bridged ternary complexes (Mn–P–Cd)\nand weak competitive sorption at the layer edge sites. As pH and Cd\nloading increased, the surface P species transitioned from a binary\ncomplex to a ternary complex and/or Cd<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> precipitate for both goethite and birnessite. Compared to\nthat in solution at pH 8, the formation of Cd<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> was inhibited by the presence of goethite and birnessite,\nascribed to the specific adsorption of P and Cd, more pronounced in\nbirnessite due to the stronger sorption of Cd at its vacant sites.\nThe discovered cosorption mechanisms of P and Cd have important implications\nfor understanding and predicting their mobility and availability in\nCd-contaminated settings.