Competitive and synergistic effects of phosphate and cadmium coadsorbed on goethite

Soil acidification in southern China increases the environmental risk posed by heavy metals, especially cadmium (Cd), and has been accelerated by anthropic activities. Long-term fertilizer use increases the quantity of phosphate (P) in soils, which affects the biogeochemistry of heavy metals in soil environments. In this study, various analytical methods were used to investigate the critical mechanisms controlling P(V)-Cd(II) interactions with goethite (α-FeOOH, a typical iron oxide widely distributed in red soil areas) with added P(V)/Cd(II) in different molar ratios (to simulate the annual P fertilizer surplus). After excluding pH interference by using organic buffers, Cd(II) adsorption processes on α-FeOOH were found to be enhanced by the presence of P(V), whereas the Cd(II) adsorption type changed from single inner-sphere complexation to both inner- and outer-sphere complexation. The results of a zeta potential analysis, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) showed different dominant mechanisms underlying the promotion of Cd(II) adsorption by P(V) at different P(V)/Cd(II) molar ratios. Electrostatic adsorption and the formation of ternary complexes were proven to be the critical mechanisms controlling the reactivity of Cd(II), where coprecipitation occurs at high P(V)/Cd(II) molar ratios and considerably enhances the fixation of Cd(II) and P(V). The mechanism for Cd(II) and P(V) coadsorption onto α-FeOOH under stable pH conditions was found to include both competitive and synergistic effects, and the results in this study show that more attention should be given to the impact of anthropic activities on the reactivity of heavy metals in the soil environment, especially farmland.