Secondary Mineralization of Ferrihydrite Under Reducing Redox Conditions: An Electrochemical Study

Open Access
Author:
Griffin, Aron M
Area of Honors:
Engineering Science
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Christopher Aaron Gorski, Thesis Supervisor
  • Joseph Paul Cusumano, Honors Advisor
  • Judith A Todd, Faculty Reader
Keywords:
  • iron oxides
  • iron
  • groundwater
  • remediation
  • electrochemistry
  • redox
  • aquatic chemistry
  • environmental engineering
Abstract:
Groundwater contamination due to improper waste disposal and storage is a significant source of water pollution both nationally and globally. In a search for alternatives to current groundwater treatment strategies, which are often prohibitively expensive and ineffective, researchers are investigating the potential use of naturally existing iron minerals to mitigate contaminants through reduction reactions. We investigated the reduction of ferrihydrite, a common ferric iron mineral, in an effort to determine the effects of solution and redox conditions on secondary mineralization product formation. Using an electrochemical batch reactor, we created an experimental method that mimics microbial reduction. We varied of ferrihydrite concentration, electrochemical mediator concentration, and applied potential to determine the effect on the transformation of ferrihydrite. Using this method, we collected data for mediator concentrations of 20, 100, and 200 μM with iron additions of 10 and 20 mg at EH=-0.16V vs SHE. We then chose the combination of 100μM mediator and 10mg Fe and collected data at EH= -0.21V and -0.26V vs SHE. In all cases only a fraction of structural Fe3+ in ferrihydrite added to the cell was reduced, and only a fraction of this reduction resulted in Fe2+ ions in solution. As we increased mediator concentration, electrons transferred increased, and Fe2+ in solution initially increased then decreased. Ferrihydrite and Fe3+ oxide in the residual solids decreased with increasing mediator concentration, and increasing amounts of magnetite was seen for the 10mg Fe trials while magnetite and goethite was found in the 20mg Fe trials. As we decreased electric potential we found an increase in electrons transferred and Fe2+ in solution. Also, the solids contained less ferrihydrite and Fe3+ oxide and increasing magnetite as potential decreased. This experimental model can be used for further studies on ferrihydrite, other iron oxides, and other mineral reductions.