Environ. Sci. Technol. 1991, 25,403-409
(36) Ramamoorthy, S.; Manning, P. G. J . Inorg. Nucl. Chem. 1974, 36, 695. (37) Ramamoorthy, S.; Manning, P. G. J . Inorg. Nucl. Chem. 1975, 37, 363.
Received for review December 7, 1989. Revised manuscript
received August 21,1990. Accepted October 12,1990. This work was supported in part by a cooperative agreement with the US. EPA (Duluth Environmental Laboratory),J. G. Eaton, project officer. I t has not been reviewed by the EPA for technical merit or policy implications, and no official endorsement should be inferred.
Phosphorus Sorption by Sediments from a Soft-Water Seepage Lake. 2. Effects of pH and Sediment Composition Naomi E. Detenbeck" and Patrick L. Breronik Department
of
Civil and Mineral Engineering, University of Minnesota, Minneapolis, Minnesota 55455
The effects of pH and sediment composition on phosphorus sorption by sandy littoral and organic-rich pelagic sediments from Little Rock Lake (Vilas County, WI) were evaluated in laboratory experiments. About two-thirds of total sedimentary P is in organic forms. Concentrations of inorganic P (Pi)are low in littoral sediments (0.66-1.71 pmol/g). In pelagic sediments Pi concentrations are approximately 10 times higher (16.1-19.4 pmol/g). Most of the Pi (-70%) is readily exchangeable. Variability in Pi is related to surface area and aluminum oxyhydroxide content but not to iron oxyhydroxides. Potential effects of pH on sorption are large; predicted equilibrium phosphate concentrations (EPC) decrease by 88-91 % between pH 6.0 and 4.5, as P binding by the sediments increases. Diffusive fluxes of Pi out of the sediments could decrease by as much as 90% if surficial sediments in Little Rock Lake become acidified. Introduction
The importance of lake sediments in regulating phosphorus cycling has been known for many years. Exchangeable phosphorus content and sorption capacities have been correlated with sediment compositional characteristics such as iron, aluminum, and organic matter content (I-3),and in turn, sediment composition has been related (qualitatively) with lake chemistry, i.e., with hardness, alkalinity, and pH (4). Laboratory studies on soils and pure minerals have shown that phosphorus sorption varies with pH ( I , 5 ) , but little information is available on the direct effects of p H on phosphorus sorption by lake sediment, especially within the pH range of interest for sediments from low-alkalinity (acid-sensitive) lakes, i.e., pH 4.5-6.0. These effects are of interest with regard to the consequences of acidification for phosphorus cycling in lakes, and this issue bears on the hypothesis that acidification induces lake oligotrophication (6). The effects of pH on lacustrine phosphorus cycling depend in part on the direct, short-term effects of pH on sorption equilibria and kinetics, as well as on the long-term effects of acidification on organic mineralization rates and on sediment composition. Phosphorus sorption by sediment from a clear-water acidic lake in Florida was found to increase with decreasing solution pH over the range 6.0-3.5, and the increase in sorption was greatest in the range 5.5-4.5 (7). However, results of this study are insufficient to determine sorption isotherm parameters. More recently, Mayer and Kramer
* Present address: Natural Resources Research Institute, University of Minnesota, Duluth, MN 55811. 00 13-936X/9 1/0925-0403$02.50/0
(8) compared sorption isotherms for calcareous and noncalcareous lake sediments as a function of pH. Although sorption increased as pH decreased for a given sediment, the authors concluded that sediment composition was more significant than pH in affecting phosphorus sorption. In order to predict the effects of lake acidification on phosphorus cycling, however, the factors controlling phosphorus sorption within a given lake and the effects of pH on sorption to a given sediment are more important than differences among lakes. Our study was designed to model the short-term effects of pH on phosphorus sorption by sandy and organic sediments from a dilute, low-alkalinity lake, Little Rock Lake (Vilas County, WI), as well as to describe the factors controlling the variability of exchangeable phosphorus in the lake's sediments. Methods
Study Site. Little Rock Lake, a small (18 ha), two-basin seepage lake in north-central Wisconsin, is the site of an ongoing whole-basin acidification experiment (9). Its watershed is wholly forested, and surrounding soils are highly permeable glacial till. The lake water is highly dilute (conductivity 11pS/cm, alkalinity 25 pequiv/L, pH = 6.0). Physical and chemical characteristics of the lake's sediments were measured before acidification of the north basin began ( I O , I I ) , and few interbasin differences were found, except for those related to water depth. Sandy littoral sediments occupy -30% of the lake area. They contain 1-25% organic matter, and their inorganic fraction is mostly made up of sand-sized particles (88-98%). The transition between littoral sediments (organic matter content
