Hydroxide and oxide based precipitates, especially iron and aluminum oxides and hydroxides, often dictate the fate of contaminants in natural and engineered systems due to their high reactivity. As a result, numerous studies have examined the formation and reactivity of these solid phases and highlight the impact of background water chemistry on morphology, composition, and reactivity of the precipitates. In this study, the effect of various contaminants (anions such as F-, SO42-, and Cl-; heavy metals such as As(III), As(V), Cr(III), and Cr(VI); and natural organic matter (NOM)) on nucleation, crystal growth and contaminant removal are explored by comparing morphology (e.g., size and volume), composition (SEM-EDS, XPS, FTIR) and removal mechanism. Batch reactor experiments (jar tests) were performed by simultaneously adding Al or Fe salt and one or more of the selected ligands to the reactor, adjusting the pH to promote precipitation, and allowing amorphous precipitates to form over short periods of time (i.e., 30 minutes). For some cases, longer periods of time (i.e., 90 minutes) were also applied to understand slower precipitation kinetics in multi-solute systems. Physical size and volume of precipitates were measured by Coulter Counter and compared to X-ray photoelectron spectroscopy (XPS) results to correlate the ion-oxide bonding environment and physical characteristic changes. In some cases, the presence of contaminant ions in the system led to the formation of less crystalline precipitates. Mechanistic differences between aluminum and iron hydroxide precipitates were evident for a number of contaminants including fluoride and arsenic. For example, F- was actively incorporated into aluminum hydroxide precipitates and the presence of fluoride led to a decrease in the average particle diameter after 30 minutes. For iron hydroxide precipitation, minimal fluoride removal was observed, but the presence of fluoride also led to a reduction in particle diameter after 30 minutes. Similar differences in the effects of ligands on floc diameter of aluminum and iron hydroxide precipitates were observed for As(III) and NOM. These results indicate that the presence of contaminant ions can affect the rate of precipitation, but these effects are dependent on the mechanism of interaction of the ligands and precipitating phases.

Physicochemical relationships between metal-based (Fe3+, Al3+) precipitates and inorganic/organic contaminants

255th National Meeting (2018)

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