Modified Ferrihydrite for Enhanced Removal of Arsenic from Mine Wastewater
Principal Investigator: Dr. Larry Twidwellltwidwell@mtech.edu
Arsenic removal by adsorption on a ferri-oxyhydroxide (ferrihydrite) surface is EPA's Best Demonstrated Available Technology (BDAT). In 2005, the EPA lowered the MCL for arsenic in drinking water from 50 μg/L to 10 μg/L. This may affect discharge requirements for treated wastewater. Should it be necessary to decrease arsenic concentrations in waste streams, facilities using ferrihydrite may need to increase ferric iron addition, which would in turn increase the amount of sludge produced. This study investigated both the adsorption behavior and long-term stability of an aluminum modified ferrihydrite (AMF) co-precipitated product. The objectives were to investigate adsorption characteristics of ferrihydrite and AMF for removing both arsenate and arsenite species and to investigate the long-term stability of AMF compared to ferrihyrdrite. Study variables for the adsorption portion of the study were Fe/As mole ratio, [Al+Fe]/As mole ratio, initial arsenic concentration, arsenic valence (III or V), and pH.
Study results showed that AMF is comparable to ferrihydrite for arsenate removal at pH values up to six. At pH values greater than six, the AMF was superior to ferrihydrite for arsenate removal and removal was achieved at metal loading/arsenate ratios lower than those required with ferridhyrite. For initial arsenic concentrations between 100 and 10, 000 μg/L arsenic was removed to less than 10 μg/L over a wide range of pH values and [Al+Fe] doping values. In arsenite systems, AMF does not perform as well as ferrihydrite as a function of pH.
Ferrihydrite and AMF were held at pH 7, and aged for up to 500 days at both room temperature and 70°C. The conversion rates for ferrihydrite and AMF were similar, however, the presence of As(V) in the solids significantly decreased the conversion rate, especially for AMF/As. Under the conditions of this study, AMF/As was significantly more stable to conversion than was ferrihydrite/As. However, a significant release of As(V) back to the solution phase was observed for both ferrihydrite/As and AMF/As.
MWTP Activity IV, Project 31
Final Report
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