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United States Department of Agriculture

Agricultural Research Service

Title: Role of Eh and Ph on Phosphorus Geochemistry in Sediments of Lake Okeechobee, Flordia

Authors
item Moore, Philip
item Reddy, K. - UNIV OF FLORDIA

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 1994
Publication Date: N/A

Interpretive Summary: Increases in phosphorus levels in Lake Okeechobee, Florida, have given rise to concern over eutrophication. The objective of this study was to evaluate the effects of redox potential and pH on the solubility of P in lake sediments. Bulk sediment samples were obtained from the mud zone of Lake Okeechobee and were equilibrated under controlled conditions at fixed Eh and pH levels. The pH levels evaluated were 5.5, 6.5, 7.5 and 8.5; the Eh levels studied were 500, 250, 0 and -250 mV. The results of this study indicated that redox reactions are very important in the regulation of P reactivity in Lake Okeechobee sediments. Under oxidized conditions, SRP concentrations are low (<0.1 mg P L-1), whereas under reducing conditions SRP increases to over 1 mg P L-1. Water soluble and exchangeable Fe behaved in a similar fashion, implicating it as a possible agent governing P behavior. The results of the P fractionation scheme showed that NaOH extractable P (Fe and Al bound) increased with increases in Eh, which would indicate Fe phosphate precipitation or adsorption of P by Fe oxides or hydroxides. Calcium bound P was higher under reducing conditions. Therefore, we suggest that Fe most likely controls the behavior of P in Okeechobee sediments under oxidizing conditions, whereas Ca phosphate mineral precipitation probably governs P solubility under reducing conditions.

Technical Abstract: Increases in total P levels in Lake Okeechobee, Florida, have given rise to concern over eutrophication. The objective of this study was to evaluate the effects of redox potential and pH on the solubility of P in lake sediments. Bulk sediment samples were obtained from the mud zone of Lake Okeechobee and were equilibrated under controlled conditions at fixed Eh and pH levels. The pH levels evaluated were 5.5, 6.5, 7.5 and 8.5; the Eh levels studied were 500, 250, 0 and -250 mV. Redox reactions were very important in the regulation of P in Lake Okeechobee sediments. Under oxidized conditions, soluble reactive P (SRP) concentrations were low (0.1 mg P L-1), whereas under reducing conditions SRP increased to over 1 mg P L-1. Soluble reactive P was extremely high (18 mg P L-1) under acidic (pH 5.5), reducing (Eh < 0 mV) conditions. Water soluble Fe was highly correlated to water soluble P, implicating it as a possible agent governing P behavior. Sodium hydroxide extractable P (Fe and Al bound) increased with increases in Eh, which indicated Fe phosphate precipitation or adsorption of P by Fe oxides or hydroxides. This was supported by mineral equilibria calculations, which showed porewaters were supersaturated with respect to strengite under oxidized conditions. Calcium bound P was higher under reducing conditions. The results suggest that Fe phosphate controls the behavior of P in sediments under oxidizing conditions, whereas Ca phosphate mineral precipitation governs P solubility under reducing conditions. Results also suggest that large fluxes of P from the sediment could occur if the lake water column were to experience low dissolved O2 levels, due to the reduction and subsequent solubilization of ferric phosphate minerals in surficial sediments.

Last Modified: 7/12/2014
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