Ferrihydrite in natural environment is commonly produced in the presence of significant amounts of Al and low molar mass organic impurities, which affect the surface reactivity of ferrihydrite depending on the impurity levels. Here, we investigated the effect of citrate on the species and levels of Al impurities in ferrihydrite. Al-bearing ferrihydrite was synthesized at different citrate/(Fe + Al) and Al/(Fe + Al) molar ratios. The samples were then characterized by chemical analysis, TEM, XRD, XPS and FTIR. The results showed that citrate promoted the formation of stable colloidal ferrihydrite particles, and induced Al accumulation in colloidal fraction at the citrate/(Fe + Al) ratio of 25% as well as in the surface layer of ferrihydrite at the citrate/(Fe + Al) ratios of 25 and 50 mol%. DFT analysis showed that Al-for-Fe substitution was closely related to the particular sites in ferrihydrite structure. We also estimated the mass of citrate (or aluminum citrate complexes) required to occupy the reactive sites on ferrihydrite nanoparticles. The results revealed that with increasing citrate content, the surface reactive sites favorable for Al-substitution of ferrihydrite became occupied by adsorbed citrate, which partially inhibited the substitution of Fe by Al but increased the adsorption of Al through Al-citrate surface complexation.
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