Enhancement of phosphate adsorption during mineral transformation of natural siderite induced by humic acid: Mechanism and application

The interfacial interactions between siderite, humic acid (HA), and phosphate, is critical to understand the transport of HA and phosphate in the environment. In this study, the influence of HA on the adsorption of phosphate by siderite not only includes competitive adsorption, but also includes indirectly facilitated adsorption. It was attributed to that humic acid as a representative natural organic matter possesses abundant carboxylic groups, which facilitate chelation between minerals and HA, and then the phosphate adsorption performance is affected. The experimental results concluded that three stages of the transformation from siderite to goethite under the action of HA can be divided. Firstly, chelation can be formed between HA and the ferrous ions from siderite, and it is favorable for the oxidation of Fe(II) by the O2 in water to form amorphous ferrihydrite; and then the ferrihydrite nano-particles were attached to the surface of goethite crystals; finally, the goethite crystal structure was formed by the structural rearrangement of ferrihydrite nano-particles. Meanwhile, the surface complex species of phosphate was formed on precipitate. It is revealed that the diprotonated bidentate binuclear complex is the main phosphate species when the HA concentration was 0.5 mg/L; as the HA concentration rose to 10 mg/L, the main phosphate species change to diprotonated monodentate mononuclear complex, which is attributed to the competitive adsorption of humic acid. We believe that these results are beneficial to guide the impact of HA for the biogeochemical cycling of phosphate, iron-bearing mineral transformation, as well as environmental remediation.