Redox Chemistry of Six Anaerobic Swine Lagoons in Eastern North Carolina

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Title: Redox Chemistry of Six Anaerobic Swine Lagoons in Eastern North Carolina
Author: Miller, William Jerome
Advisors: Wayne Robarge, Committee Chair
Abstract: Emissions of ammonia (NH&#8323;) from swine lagoons have been well documented, but recently the emission of di-nitrogen gas (N&#8322;) has also been reported. Formation of di-nitrogen gas affords a possible mechanism to lessen the impact of swine lagoons on the environment. Currently there are no known mechanisms for di-nitrogen production in swine lagoon systems as used for swine production in North Carolina. The production of di-nitrogen from either organic nitrogen or ammonium (NH&#8324;&#8314;) must involve a change in the redox state of nitrogen. There are few studies of swine lagoons as redox systems, and this research was concerned with detailing the redox processes active in swine lagoons with the goal of identifying a possible mechanism for di-nitrogen formation. Five lagoons in the eastern North Carolina were monitored weekly from August 2nd, 2000 until February 1st, 2001, and one lagoon from June 7th, 2000 until February 1st, 2001. Three measurements were carried out in the field: Eh, pH, and lagoon temperature. Lagoon fluid samples were collected while field measurements were taken. Fluid samples were analyzed for total and total dissolved (<0.4 micron ) concentrations of various elements including manganese, iron, copper, zinc, phosphorous, sulfur, calcium, magnesium, sodium, and potassium. Measured redox potential (Eh) values ranged from &#8211;27 to &#8211;220 mV, with a range of mean values for individual lagoons of &#8211;87 to &#8211;154 mV. Measured pH values ranged from 7.11 to 8.34. Total iron concentrations ranged from 0.3 to 5.3 mg L&#8315;&#185. Dissolved iron concentrations ranged from 0.1 to 1.0 mg L&#8315;&#185;, with a range in mean values for individual lagoons of 0.4 to 0.7 mg L&#8315;&#185. Ammonium concentrations ranged from 135 to 815 mg NH&#8324;-N L&#8315;&#815;. Overall there was no significant stratification of measured lagoon parameters within the lagoon water column. Observation of the data as a function of time suggests that there are temporal trends to the data which seem to occur across all the lagoons studied, implying that changes in weather affect the chemical processes occuring within the lagoons. Plotting measured Eh and pH values on pe/pH phase diagrams constructed using redox couples assumed to be present in the swine lagoons suggest the presence of an active iron (Fe&#178;&#8314;/Fe&#179;&#8314;) redox couple. In addition, using chemical speciation software (MinteqA2) and analytical data, calculated equilibrium Eh values compared favorably to measured Eh values when the iron redox couple was considered the potential determining reaction. Ferric iron (Fe&#179;&#8314;) represents an electron acceptor that can be used to drive the oxidation of ammonium to nitrate/nitrite, ultimately resulting in the production of di-nitrogen gas. Ferric iron, produced by the oxidation of dissolved ferrous iron (Fe&#178;&#8314;) by oxygen diffusing through the lagoon surface, could exist as a separate solid phase in the lagoon effluent or possibly bound to a microbial surface Thermodynamically the conversion of ammonium to nitrite via oxidation by ferric iron could occur according to the following reaction: Fe(OH)&#8323; + 1/6 NH&#8324;&#8314; + 5/3H&#8314; = Fe&#178;&#8314; + 1/6NO&#8322;&#8331; + 8/3H&#8322;0 The equilibrium constant for this reaction is 10[superscript 0.7], which although small results in a negative Gibbs free energy of reaction of 1.6 kcal/mol. Alternatively ammonium could be oxidized to form nitrate with hydroxylamine formed as in an intermediate according to the two following reactions: NH&#8324;&#8314; + 2Fe(OH)&#8323; + 3H&#8314; = 2Fe&#178;&#8314; + NH&#8322;OH + 5H&#8322;O 1/6NH&#8322;OH + Fe(OH)&#8323; + 11/6H&#8314; = 1/6NO&#8323;&#8315; + Fe&#178;&#8314; + 8/3H&#8322;O The equilibrium constant for the oxidation of ammonium is 10[superscript -5.2], while the equilibrium constant for the oxidation of hydroxylamine is 10[superscript 4.5. Nitrate/nitrite produced by these two processes could be used to oxidize organic matter while generating di-nitrogen gas, referred to as the denitrification process, liberating a significant amount of energy for microbial growth.
Date: 2003-04-27
Degree: MS
Discipline: Soil Science

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