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Modified Ferrihydrite for Enhanced Removal of Arsenic from Mine Wastewater
Primary Issue Addressed: Trace Metal Removal
Secondary Issues Addressed: Acid Drainage/Water Treatment
Project Site: Not applicable
Collaborating Entities: Montana Tech
Cost Share: None
Project Description
The main objective of this project is to investigate the adsorption characteristics of ferrihydrite and aluminum modified ferrihydrite for removing both arsenate and arsenite species as well as investigating the relative stability of the products that will be most effective in removing arsenic from mine wastewaters.
Arsenic removal by adsorption on a ferrihydrite surface is used throughout the world for removing arsenic from wastewaters; however, long term stability of ferrihydrite, especially in outdoor storage ponds is questionable. This has been shown by many studies that have demonstrated amorphous ferrihydrite propensity to convert to crystalline goethite (FeOOH) or hematite (Fe2O3) with a drastic decrease in solid surface area in a matter of months. Pathways for these crystallizations have been studied in great detail but have not yet been completely defined. The adsorptive properties of the crystalline goethite or hematite solids for arsenic have been shown to be much less than the ferrihydrite amorphous solid. Previous investigations have demonstrated that aluminum modification of the precipitated ferrihydrite changes the adsorptive properties of the solid so that it is much more effective for adsorbing arsenic than is unmodified ferrihydrite. The rationale behind the concept of using a modified-ferrihydrite structure for the adsorption of arsenic from aqueous solution is based on the idea that by incorporating other element ions into the ferrihydrite structure the modified structure should adsorb arsenic ions and heavy metal ions more effectively due to better matching of ion and adsorbent atom to atom distances.
Status
Most of the laboratory testing to be included in the graduate student’s thesis has been completed for this project, and the final report is expected to be finished in FY05. Six separate full factorial two-level design matrix tests were conducted. Five of these tests used modified ferrihydrite as the adsorption surface, and one used ferrihydrite as the adsorption surface. Each design matrix test consisted of ten experiments (i.e., eight required by the design matrix and two midpoint tests) to investigate the influence of ferrihydrite:arsenic and modified ferrihydrite:arsenic mole ratio, initial arsenic concentration, and pH on removing arsenic. Preliminary analysis of the elevated temperature aging study on the sulfate bearing water revealed that arsenic is desorbed in appreciable quantities with time from ferrihydrite but much less is released from modified ferrihydrite. Further testing revealed that the selection of reagent anion (chloride, nitrate, or sulfate) affects arsenate removal. A more in-depth evaluation and analysis of the data will be provided in the final report, which is expected to be completed in FY05.