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Dissolution of soil heavy metal contaminants as affected by pH and redox potential

by 1974- Bang, JiSu

Abstract (Summary)
BANG, JISU. Dissolution of Soil Heavy Metal Contaminants as Affected by pH and Redox Potential. (Under the direction of Dean L. Hesterberg) The solubility of heavy metal (trace metal) contaminants in soils depends on metal concentration, chemical speciation, and conditions such as pH, redox potential, and ionic strength of the soil solution. The objective of this study was to determine the dissolution (potential mobilization) of metal contaminants in response to induced changes in pH and redox potential in soils surrounding abandoned incinerators at two outlying US Marine Corps air fields: MCALF-Bogue and MCOLF-Atlantic. Concentrations of heavy metals measured in 17 soil samples ranged from 1 to 101 mg Zn kg -1, 2 to 45 mg Cu kg -1, 3 to 105 mg Pb kg -1, 0.3 to 12 mg Cr kg -1, <0.01 to 0.6 mg Cd kg -1, < 0.1 to 0.6 mg Se kg -1, 0. 5 to 81 mg Ba kg -1, and <0.1 to 10 mg As kg -1. In some samples, metal concentrations were greater than those found in reference samples collected at location > 500 m from the incinerators. Silver was not detected in any of the17 samples. To assess the impacts of acidification on metal dissolution, acidified calcium chloride solutions were flowed through 1 cm long by 3 cm diameter soil columns for approximately 300 h to decrease effluent pH to about pH 4. With decreasing pH, dissolved Cu, Pb, and Zn concentrations in the column effluent solutions increased to maximum concentrations of 0.317 mg Cu L -1, 1.17 mg Pb L -1, and 1.3 mg Zn L -1 between pH 3.8 and 4.1. Aqueous concentrations of Cr, As, Cd, Se, and Ag in selected column effluent solutions remained below our analytical detection limits. Results indicate that the soils at the MCALF-Bogue and MCOLF-Atlantic sites should be maintained at pH > 5 to minimize the mobility of Cu, Pb, and Zn in soil at each site. Synchrotron X-ray absorption near edge structure (XANES) analysis showed that less toxic and less mobile Cr(III) and As(V) were dominant in soil at each site, and that Cr(III) was not oxidized to Cr(VI) by soil acidification in column flow experiments. During 30 day redox incubation studies, dissolved Zn in aqueous suspensions reached a maximum concentration of 0.04 mg Zn L -1 as the redox potential decreased from 455 to as low as 250 mV (pH 7.7 ± 0.2). Aqueous concentrations of Cu, Pb, and Cr remained below our analytical detection limits. The results of laboratory mobility studies, along with the XANES results, indicated that trace metals in soils at the incinerator sites would remain immobile if the soil pH is maintained at pH > 5. Decreasing redox potential (Eh) if soil samples from at the MCALF-Bogue site to 250 mV caused minimal dissolution of Cu, Zn, Pb, and Cr.
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School:North Carolina State University

School Location:USA - North Carolina

Source Type:Master's Thesis

Keywords:north carolina state university

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