Competitive coprecipitation of divalent and trivalent metals and REE with ferric iron as a function of pH was studied with titration experiment at ambient temperature and pressure. X-ray diffraction patterns of the Fe precipitates indicated two broad peaks at 0.153 and 0.256 nm, suggesting that the dominant phase is 2-line ferrihydrite. High-resolution transmission electron microscopy analysis showed that Fe precipitates were single crystallites with 2-5 nm in size. No segregated phases were observed in the clusters or at their edges. Solution chemistry analysis demonstrated that Fe3+ precipitated at pH ~ 4. The uptake of Cr3+, Al3+, Ga3+, and In3+ took place over a narrow pH range (0.1-0.3 pH unit) near pH 4. The sorption edges for Pb2+, Cu2+, Zn2+, Ni2+, Co2+ , and Mn2+ occurred in a wider pH range of 4.0-7.5, with a preferential partitioning order of Pb2+ &gt; Cu2+ &gt; Zn2+ &gt; Ni2+ &gt; Co2+ &gt; Mn2+, which basically follows the Irving-Williams sequence. The uptake of alkaline earth metals (Mg2+, Sr2+, and Ba2+) occurred at pH &gt; 7. No obvious sorption onto ferrihydrite was found for alkali metals (Li+ and Na+). The competitive sorption effects were examined for both low and high sorbate/sorbent scenarios (Fe : metals = 560 : 1 and 56 : 1, respectively). The sorption edges for trivalent metals (Cr3+, Al3+, Ga3+, and In3+) and Pb2+ were less influenced by the competing metals than other divalent metals. In addition, for the examined transition metals (Cu2+, Zn2+, Ni2+, Co2+, and Mn2+), the competitive coprecipitation effects are more evident for the metals low in the Irving-Williams series. The coprecipitation process was simulated with the generalized diffuse layer model. However, higher surface site densities and larger complexation constants for surface species than those retrieved in adsorption experiments were required to fit the coprecipitation data, indicating a higher efficiency of the trace metal uptake with coprecipitation process. log K values for REE adsorption onto ferrihydrite generally increase with increasing atomic number across the REE series, indicating that the affinity for REE adsorption generally increase from LREEs to HREEs.

Competitive metal and REE coprecipitation with ferrihydrite nano-particles

256th National Meeting (2018)

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