Influence of pH on Phosphorus Retention in Oxidized Lake Sediments

Diel pH changes in lake waters resulting from high photosynthetic activity may regulate water-soluble P concentration (WSP) and P sorption by suspended sediments in shallow eutrophic lakes. Laboratory studies were conducted to determine the pH effect on P fractions and P sorption kinetics in oxidized sediment suspensions from two subtropical lakes (Lake Apopka and Lake Okeechobee, Florida). The P sorption rate was calculated for sediment suspensions adjusted to various pH levels: 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Apopka and 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Okeechobee. A decrease in pH increased the WSP concentrations in Lake Apopka sediment suspensions but had no effect on WSP concentrations in Lake Okeechobee sediment suspensions. Lake Apopka sediment suspensions at pH 7.0 (ambient) and below did not show net P uptake. Phosphorus uptake for Lake Apopka occurred only when pH was increased to >8.5 and when P treatments were increased to > 27 mmol P kg"', which resulted in supersaturation with respect to octacalcium phosphate. Phosphate solubility diagrams and mineral equilibria calculations suggest that P uptake by Lake Apopka sediment suspensions at pH >8.5 was due to P coprecipitation with CaCO3 and probable formation of nonapatitic Ca-P compounds. Phosphorus sorption on Lake Okeechobee sediment suspensions followed first-order kinetics for all pH levels studied, with rate constants (k) ranging from 0.003 to 0.75 h~'. High P uptake by Lake Okeechobee sediment suspensions could be attributed to two reactive components: (i) amorphous or poorly crystalline Fe and Al oxyhydroxides at pH <7.5, and (ii) Ca/Mg carbonates and other minerals at pH >7.5. P SORPTION and release by lake sediments has been associated with physicochemical factors such as pH (Ku et al., 1978), Eh (Bostrom and Pettersson, 1982), bioturbation and mixing (Holdren and Armstrong, 1980), and temperature (Sondergaard, 1989). Depending on limnological conditions, sediments can act as sinks or sources of P. The influence of Eh on sediment P reactivity is mostly reported for noncalcareous systems (Wildung et al., 1977), whereas pH effects on P sorption may be observed hi both calcareous and noncalcareous sediments. Noncalcareous sediments are efficient sorbents of P due to the presence of oxyhydroxides of Fe and Al. Iron oxyhydroxides, which have a high affinity for P, are sensitive to Eh and pH whereas Al oxyhydroxides are responsive to pH. The P binding capacity of noncalcareous sediments increases with acidity due to protonation of surface Fe and Al functional groups in clays and hi oxides and hydroxides of Fe and Al (Edzwald et al., 1976). Phosphorus sorption capacities of these components decrease with increasing pH as a result of competition between the hydroxyl and phosphate ions (Lijklema, 1980). Decreases in sediment pH (from 6 to 4.5) of a noncalcareous lake in Wisconsin increased P binding and decreased EPC0 and diffusive P flux (Detenbeck and Soil and Water Science Dep., Inst. of Food and Agricultural Sciences, Univ. of Florida, Gainesville, FL 23611. Florida Agric. Exp. Stn. Journal Series no. R-04302. Received 21 June 1993. *Corresponding author (IN% "[email protected]). Published in Soil Sci. Soc. Am. J. 59:946-959 (1995). Brezonik, 1991). Similar results were reported for Lake Ontario sediments, where P sorption maxima were associated with both pH and mineralogical properties of sediments (Mayer and Kramer, 1986). The noncalcareous sediments have significantly greater P sorption capacities than calcareous sediments. The influence of pH on P sorption by calcareous sediments is often associated with sorption by, and coprecipitation with, CaCOs. Phosphate sorption on a typical marl lake in southern Michigan increased with pH (pH 8-9.5) due to coprecipitation of P with Ca carbonates (Otsuki and Wetzel, 1972). Similar findings were reported in a calcareous lake in Austria, where the pH increase hi sestonic materials was accompanied by increased P sorption and formation of calcite (Gunatilaka, 1982). The reaction of P with calcite involves a surface adsorption that consumes H ions (Avnimelech, 1980), followed by precipitation of CaHPO4-2H2O at higher P concentrations (Cole et al., 1953). Electron-probe microanalysis and SEM confirmed adsorption of inorganic P from dilute aqueous solutions hi equilibrium with calcite across a temperature range of 5 to 35°C and a pH range of 7 to 9.5 (House and Donaldson, 1986). Phosphorus sorption by, and coprecipitation with, calcite have been confirmed using SEM (House and Donaldson, 1986), light scattering (Kleiner, 1988), electron-probe microanaly sis, and x-ray diffraction (Freeman and Rowell, 1981). A high pH in a lake water column could result from an enhanced photosynthetic activity, withdrawing CO2 from the water and shifting the COz-HCOf-COi" equilibrium that controls pH. The role of pH in P reactivity is important in eutrophic lakes, which undergo wide diel pH fluctuations due to photosynthesizing phytoplankton (Stabel, 1986). Excessive algal blooms can increase the pH of lakewater from 8.2 to 9.5 (Boers and Van Hesse, 1988). During summer stratification, pH values of a calcareous lake can vary diurnally from 7.8 to 9.2 (Stabel, 1986). Water columns of noncalcareous lakes can reach pH 11 during high photosynthetic productivity (Andersen, 1975; Sondergaard, 1989). The bottom sediments of shallow lakes in Florida such as Lake Apopka (located near Orlando, mean depth = 1.7 m) and Lake Okeechobee (located in south Florida, mean depth = 2.7 m) may undergo resuspension into the water column during wind events, and thus contact high-pH, oxidized lake water. During wind events > 18 km h", the total suspended solids were reported to be 78 mg L" for Lake Apopka (Reddy and Graetz, 1991) and 64 to 100 mg L~' for Lake Okeechobee (Sheng et al., 1991, unpublished data; Olila and Reddy, 1993, unpublished data). The lake water pH for Lake Apopka fluctuates from «7.0 at night to 10.0 during the day (Reddy, 1981). We hypothesized that temporal changes hi pH may regulate WSP and affect P sorption characteristics of suspended sediments in shallow lakes; hence, wind-induced resuspension of sediments into the water