Abstract
Humic acid (HA) is particularly important in iron-bearing mineral transformations and erosion at the water-mineral boundary zone of the Earth. In this study, three stages of the possible pathway by which HA causes mineral transformation from siderite to goethite are identified. Firstly, a Fe(II)-HA complex is formed by chelation, which accelerates the dissolution and oxidation of Fe(II) from the surface of siderite. As the Fe(II)-HA complex retains Fe atoms in close proximity of each other, ferrihydrite is formed by the agglomeration and crystallization. Finally, the ferrihydrite structurally rearranges upon attachment to the surface of goethite crystals and merges with its structure. The influence of low concentrations of HA (0–2 mg/L) on phosphate adsorption is found to be beneficial by the inducing of new mineral phases. We believe that these results provide a greater understanding of the impact of HA in the biogeochemical cycle of phosphate, mineral transformation.
Highlights
Humic acid (HA) is important in iron-bearing mineral transformations and erosion at the water-mineral boundary zone of the Earth
Temperatures, and pressures, the free-energy barrier is relatively large[17,18,19,23], so the processes of nucleation and growth should be rare in the natural environment in those predicted by classical models[19,24]
To confirm the composition of the sphere, the lattice fringes of the polyhedral subunits were investigated by HRTEM (Fig. 1d), and its lattice fringes were calculated as 2.81 Å, which coincided with the (104) interplanar distance of siderite
Summary
Humic acid (HA) is important in iron-bearing mineral transformations and erosion at the water-mineral boundary zone of the Earth. To provide direct evidence of humic acid-induced mineral transformation from siderite to goethite, the product of siderite after reacting with HA (2 mg/L, DO: 4 mg/L) was carefully investigated by SEM, TEM, HRTEM, XRD and selected area electron diffraction (SAED).
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