Effect of phosphate on the adsorption of cadmium on iron and manganese oxides: a comparative study of goethite, ferrihydrite, and potassium manganese materials

Iron and manganese oxides participate in a variety of geochemical processes, controlling element cycling and soil quality. The co-adsorption properties of heavy metal cations and oxyanions on iron (manganese) oxides contribute to our understanding of these processes and the ions’ distribution and transformation in geological environments. In this study, ferrihydrite (Fh), goethite (Gt), and manganese oxide (K-MnO 2 ) were prepared by X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) Surface analysis were used to characterize the minerals. The Zeta (ζ) potential and zero potential point (pH IEP ) were measured by a potential analyzer, and cadmium ions and oxygen anions (phosphate) were tested for ferrihydrite (Fh) and goethite (Gt). The adsorption behavior of manganese oxide (K-MnO 2 ) was studied, and the pH change characteristics of the system before and after adsorption were analyzed. The results show that the maximum adsorption rate of K-MnO 2 to Cd 2+ (pH = 4, Cd 2+ 20 mg·L − 1 ) is approximately 95%. After adding phosphate, the adsorption rate of K-MnO2 to Cd 2+ increases significantly; the adsorption rate was close to 98% when pH = 4, and phosphate improved the process of K-MnO 2 adsorption to Cd 2+ reaching equilibrium. Goethite and ferrihydrite have similar adsorption trends for Cd 2+ after adding phosphate. When pH = 2-3.5, the adsorption rate for Cd 2+ is approximately 10–30%. When original pH is 3.5-5, the pH value of the reaction system is almost higher than 6 when the adsorption end point is reached, and the adsorption rate is already close to more than 95% at pH 7. The addition of phosphate enhanced the equilibrium process of Cd 2+ adsorption on goethite ( or ferrihydrite ).