Investigation of the mechanisms controlling chromate and arsenate removal from water using zerovalent iron media

Persistent Link:
http://hdl.handle.net/10150/289806
Title:
Investigation of the mechanisms controlling chromate and arsenate removal from water using zerovalent iron media
Author:
Melitas, Nikos
Issue Date:
2002
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
This research investigated the mechanisms controlling chromate and arsenate removal by zerovalent iron media. The removal kinetics of aqueous Cr(VI) and As(V) were studied in batch experiments for initial concentrations ranging from 100 to 10,000 μg/L. Removal kinetics were also studied in columns packed with zerovalent iron filings over this same concentration range. Electrochemical analyses were used to investigate the electron transfer reactions occurring on the iron surface, and to determine the effect of chromate and arsenate on the iron corrosion behavior. The removal mechanism for chromate involved reduction to Cr(III) and the formation of hydroxide precipitates. Increasing chromate concentrations resulted in decreasing removal rates due to iron surface passivation. Even at low concentrations, chromate acts as a corrosion inhibitor and decreases iron corrosion rates. The condition of the iron surface prior to exposure to chromate determined the chromium removal kinetics. Air-formed oxides significantly inhibited chromate removal, whereas oxides formed in anaerobic, chromate-free water resulted in higher removal rates. Although direct reduction of chromate at cathodic sites on the iron surface was observed at early elapsed times, chromate removal eventually became limited by the rate at which Fe²⁺ could be generated at anodic sites. The removal mechanism for arsenate did not involve reduction and was due to the formation of inner-sphere, bidentate complexes with iron corrosion products. At low arsenate concentrations the rate of arsenate removal was limited by diffusion to adsorption sites. At high concentrations the rate of arsenate removal was limited by the rate of adsorption site generation resulting from iron corrosion. Adsorbed arsenate blocked electroactive sites on the iron surface and decreased iron corrosion rates. Arsenate is expected to remain as the principal adsorbed species in iron filter media because electrochemical reduction of As(V) to As(III) is not favorable under the conditions relevant to freely corroding iron.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Engineering, Environmental.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Chemical and Environmental Engineering
Degree Grantor:
University of Arizona
Advisor:
Farrell, James

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleInvestigation of the mechanisms controlling chromate and arsenate removal from water using zerovalent iron mediaen_US
dc.creatorMelitas, Nikosen_US
dc.contributor.authorMelitas, Nikosen_US
dc.date.issued2002en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractThis research investigated the mechanisms controlling chromate and arsenate removal by zerovalent iron media. The removal kinetics of aqueous Cr(VI) and As(V) were studied in batch experiments for initial concentrations ranging from 100 to 10,000 μg/L. Removal kinetics were also studied in columns packed with zerovalent iron filings over this same concentration range. Electrochemical analyses were used to investigate the electron transfer reactions occurring on the iron surface, and to determine the effect of chromate and arsenate on the iron corrosion behavior. The removal mechanism for chromate involved reduction to Cr(III) and the formation of hydroxide precipitates. Increasing chromate concentrations resulted in decreasing removal rates due to iron surface passivation. Even at low concentrations, chromate acts as a corrosion inhibitor and decreases iron corrosion rates. The condition of the iron surface prior to exposure to chromate determined the chromium removal kinetics. Air-formed oxides significantly inhibited chromate removal, whereas oxides formed in anaerobic, chromate-free water resulted in higher removal rates. Although direct reduction of chromate at cathodic sites on the iron surface was observed at early elapsed times, chromate removal eventually became limited by the rate at which Fe²⁺ could be generated at anodic sites. The removal mechanism for arsenate did not involve reduction and was due to the formation of inner-sphere, bidentate complexes with iron corrosion products. At low arsenate concentrations the rate of arsenate removal was limited by diffusion to adsorption sites. At high concentrations the rate of arsenate removal was limited by the rate of adsorption site generation resulting from iron corrosion. Adsorbed arsenate blocked electroactive sites on the iron surface and decreased iron corrosion rates. Arsenate is expected to remain as the principal adsorbed species in iron filter media because electrochemical reduction of As(V) to As(III) is not favorable under the conditions relevant to freely corroding iron.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectEngineering, Environmental.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineChemical and Environmental Engineeringen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorFarrell, Jamesen_US
dc.identifier.proquest3053889en_US
dc.identifier.bibrecord.b42813098en_US
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