Partitioning and transformation behavior of Cd(II) and As(V) during As(V)-Cd(II)-Fe(III) coprecipitation: Effect of aging under aerobic conditions at pH 5 and 11 relevant to tailings and acid waste

Aerobic coprecipitation and subsequent aging of the As(V)-Cd(II)-Fe(III) system are ubiquitous processes in engineered settings, natural shallow water, surficial soils, and mine tailings. Herein, we have investigated the partitioning of As(V) and Cd(II) coupled with the solid evolution during As(V)-Cd(II)-Fe(III) coprecipitation and subsequent aging process under aerobic conditions at different pH (5 and 11) and Fe(III)/As(V) ratios (10 and 40) with various Cd(II) concentrations at 90 °C. X-ray diffraction and transmission electron microscopy results reveal that ferrihydrite is the dominant phase in the initial coprecipitates, regardless of pH and the coexisting As(V) and Cd(II). Low pH and high Fe(III)/As(V) ratios favor the transformation of initial ferrihydrite to hematite during the aging process, which can further promote the incorporation of ferrihydrite-bound As(V) and Cd(II) into stable hematite. Chemical analyses indicate that the partitioning fractions of adsorbed/extractable and solid-incorporated As(V) and Cd(II) are controlled by pH, Fe(III)/As(V) ratio, and aging time during ferrihydrite transformation. High Fe(III)/As(V) ratios facilitate the incorporation of the extractable Cd(II) into the aging intermediates, whereas the As(V) partitioning is less affected by the initial Cd(II) concentration. The crystallization rates of ferrihydrite at high Fe(III)/As(V) ratios or low pH are higher than those at low Fe(III)/As(V) ratios or high pH. Furthermore, increased Cd(II) concentrations significantly reduce the crystallization rates of the initial ferrihydrite. These findings have shed new light on the mobility and transformation behavior of toxic As(V) and Cd(II) in the As(V)-Cd(II)-Fe(III) coexisting environment under aerobic conditions and are useful for developing novel As(V)-Cd(II)-compound contaminant management strategies for metallurgical waste residues and water treatment sludges. •Hematite is the dominant aging product of As(V)-Cd(II)-ferrihydrite under aerobic conditions.•Increasing As(V) and Cd(II) concentrations retards the transformation of ferrihydrite to hematite.•Ferrihydrite-hematite transformation is important to the immobilization of As(V) and Cd(II) in surficial environments and hydrometallurgical operations.