Title

Characterization of kinetics and mechanisms of electrochemical nitrate reduction using surface-modified carbon fiber electrode

Date of Award

1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Interdepartmental Studies

First Committee Member

James D. Englehardt, Committee Chair

Second Committee Member

Peter P. Tarjan, Committee Member

Abstract

Methemoglobinemia and gastric cancer are common concerns due to the increase of nitrates in rural domestic and public water sources. Besides health effects, there are also several impacts on environment at high nitrate concentration. Electrochemical reduction of nitrate is one of the methods used to separate nitrate from water. In this dissertation, noble metal deposited carbon fiber electrode, which has a high nitrate reduction current density and a high effective surface area, was used as an electrode to study nitrate reduction, and to understand the complicated nitrate reduction mechanisms and kinetics.Cyclic voltammetry (CV), chronoamperometry and adsorption pseudocapacitance were used to investigate the mechanisms of nitrate and nitrite reductions. The CV studies were performed at different nitrate and nitrite concentrations, pH and scan rates. Adsorption controlled the reduction reaction. Tafel slopes were obtained for nitrate and nitrite reductions from the chronoamperometry studies. The mechanisms for nitrate and nitrite reductions were established from the Frumkin/Temkin adsorption isotherm. Nitrate and nitrite reduction products were analyzed quantitatively and qualitatively using spectroscopy, ion chromatography and gas chromatography techniques. The first step of the mechanistic paths, i.e., the one electron transfer reaction of nitrate or nitrite reduction and adsorption of resulting product on the electrode surface, was the rate determining step.The water replacement model also supported the first step of nitrate and nitrite reduction as the slowest step, where three to four water molecules were replaced during the reactions.Kinetic studies in a batch reactor suggested that the nitrate and nitrite reductions were first order inhibition reactions and supported the observations of cyclic voltammetry and chronoamperometry studies. The amount of nitrate reduction was more at --950 mV than at --850 mV although the ratio of current efficiency for nitrate reduction to hydrogen evolution was less at higher potential. Nitrite reduction at two potentials was almost the same. Nitrite oxidized to nitrate very fast in a divided cell. Therefore, nitrate reduction was less in a divided cell than an undivided cell.A model was developed for batch recycling and series-parallel reactors to understand the reduction process in the electrochemical reactor, and its effectiveness for nitrate reduction on a large scale operation which helps to design the reactor. The model was validated only for the batch recycling system, and it was in good agreement with the experimental results.

Keywords

Engineering, Chemical; Engineering, Environmental

Link to Full Text

https://iiiprxy.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9961241