Solid–liquid partitioning and speciation of Pb(II) and Cd(II) on goethite under high pH conditions, as examined by subnanomolar heavy metal analysis, X-ray absorption spectroscopy, and surface complexation modeling

• Published in: Chemosphere (Oxford) 2024-09, Vol.363, p.142766, Article 142766
• Main Authors: Takeda, Natsumi, Fukushi, Keisuke, Okuyama, Akihiro, Takahashi, Yoshio
• Format: Article
• Language: English
• Subjects: Adsorption; Cadmium; Cadmium - analysis; Cadmium - chemistry; Goethite; heavy metals; High pH; Hydrogen-Ion Concentration; Iron Compounds - chemistry; Lead; Lead - chemistry; Metals, Heavy - analysis; Metals, Heavy - chemistry; Minerals - chemistry; Models, Chemical; Surface completion modeling; toxicity; X-Ray Absorption Spectroscopy; XAFS
• ISSN: 0045-6535; 1879-1298; 1879-1298
• DOI: 10.1016/j.chemosphere.2024.142766

The adsorption of heavy metals on iron oxides generally increases with pH and is almost complete at neutral to slightly alkaline pH. However, almost complete adsorption on a linear scale does not imply sufficient removal of the heavy metals in terms of their toxicity. Here, we elucidated the chemical reactions that determine the solid–liquid partitioning of Pb(II) and Cd(II) on goethite at high pH. While the removal of both heavy metals was almost complete on a linear scale above pH 7 for Pb(II) and pH 9 for Cd(II), the dissolved metal concentrations decreased on a logarithmic scale with pH, reaching minima at around pH 10 for Pb(II) and pH 10–11 for Cd(II), and then they increased with pH thereafter. The XAFS spectra of Pb(II)- or Cd(II)-adsorbed goethite prepared at pH > 11 were almost the same as those at neutral pH, suggesting that removal of the heavy metals from solution was achieved by a single adsorption reaction over the entire pH range. Based on the observed macroscopic and microscopic adsorption behaviors at high pH, a robust surface complexation model was developed to predict the solid–liquid partitioning of divalent heavy metals over the entire pH range. [Display omitted] •The adsorption of Pb and Cd on goethite at high pH was studied experimentally.•The adsorption reached to maxima at pH 10 for Pb and pH 10–11 for Cd.•The speciation of Pb and Cd on goethite did not change with pH.•A single surface complexation reaction could reproduce the adsorption at high pH.•A robust model was developed to predict any divalent heavy metals on goethite.