influence of ph on phosphorus retention in oxidized lake ... · for lake okeechobee. the ph of lake...

Influence of pH on Phosphorus Retention in Oxidized Lake SedimentsO. G. Olila* and K. R. Reddy

ABSTRACTDiel pH changes in lake waters resulting from high photosynthetic

activity may regulate water-soluble P concentration (WSP) and Psorption by suspended sediments in shallow eutrophic lakes. Labora-tory studies were conducted to determine the pH effect on P fractionsand P sorption kinetics in oxidized sediment suspensions from twosubtropical lakes (Lake Apopka and Lake Okeechobee, Florida). TheP sorption rate was calculated for sediment suspensions adjusted tovarious pH levels: 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Apopka and6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Okeechobee. A decrease in pHincreased the WSP concentrations in Lake Apopka sediment suspen-sions but had no effect on WSP concentrations in Lake Okeechobeesediment suspensions. Lake Apopka sediment suspensions at pH 7.0(ambient) and below did not show net P uptake. Phosphorus uptakefor Lake Apopka occurred only when pH was increased to >8.5 andwhen P treatments were increased to > 27 mmol P kg"', which resultedin supersaturation with respect to octacalcium phosphate. Phosphatesolubility diagrams and mineral equilibria calculations suggest that Puptake by Lake Apopka sediment suspensions at pH >8.5 was dueto P coprecipitation with CaCO3 and probable formation of nonapatiticCa-P compounds. Phosphorus sorption on Lake Okeechobee sedimentsuspensions followed first-order kinetics for all pH levels studied, withrate constants (k) ranging from 0.003 to 0.75 h~'. High P uptake byLake Okeechobee sediment suspensions could be attributed to tworeactive components: (i) amorphous or poorly crystalline Fe and Aloxyhydroxides at pH <7.5, and (ii) Ca/Mg carbonates and otherminerals at pH >7.5.

PHOSPHORUS SORPTION and release by lake sedimentshas 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). Dependingon limnological conditions, sediments can act as sinksor sources of P. The influence of Eh on sediment Preactivity is mostly reported for noncalcareous systems(Wildung et al., 1977), whereas pH effects on P sorptionmay be observed hi both calcareous and noncalcareoussediments.

Noncalcareous sediments are efficient sorbents of Pdue to the presence of oxyhydroxides of Fe and Al. Ironoxyhydroxides, which have a high affinity for P, aresensitive to Eh and pH whereas Al oxyhydroxides areresponsive to pH. The P binding capacity of noncalcare-ous sediments increases with acidity due to protonationof surface Fe and Al functional groups in clays and hioxides and hydroxides of Fe and Al (Edzwald et al.,1976). Phosphorus sorption capacities of these compo-nents decrease with increasing pH as a result of competi-tion between the hydroxyl and phosphate ions (Lijklema,1980). Decreases in sediment pH (from 6 to 4.5) of anoncalcareous lake in Wisconsin increased P bindingand 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. JournalSeries 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 LakeOntario sediments, where P sorption maxima were asso-ciated with both pH and mineralogical properties ofsediments (Mayer and Kramer, 1986). The noncalcare-ous sediments have significantly greater P sorption capac-ities than calcareous sediments.

The influence of pH on P sorption by calcareoussediments is often associated with sorption by, and co-precipitation with, CaCOs. Phosphate sorption on a typi-cal 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 re-ported in a calcareous lake in Austria, where the pHincrease hi sestonic materials was accompanied by in-creased P sorption and formation of calcite (Gunatilaka,1982). The reaction of P with calcite involves a surfaceadsorption that consumes H+ ions (Avnimelech, 1980),followed by precipitation of CaHPO4-2H2O at higher Pconcentrations (Cole et al., 1953). Electron-probe micro-analysis and SEM confirmed adsorption of inorganic Pfrom dilute aqueous solutions hi equilibrium with calciteacross a temperature range of 5 to 35°C and a pH rangeof 7 to 9.5 (House and Donaldson, 1986). Phosphorussorption by, and coprecipitation with, calcite have beenconfirmed using SEM (House and Donaldson, 1986), lightscattering (Kleiner, 1988), electron-probe microanaly sis,and x-ray diffraction (Freeman and Rowell, 1981).

A high pH in a lake water column could result froman enhanced photosynthetic activity, withdrawing CO2from the water and shifting the COz-HCOf-COi" equi-librium that controls pH. The role of pH in P reactivityis important in eutrophic lakes, which undergo wide dielpH fluctuations due to photosynthesizing phytoplankton(Stabel, 1986). Excessive algal blooms can increase thepH of lakewater from 8.2 to 9.5 (Boers and Van Hesse,1988). During summer stratification, pH values of acalcareous lake can vary diurnally from 7.8 to 9.2 (Stabel,1986). Water columns of noncalcareous lakes can reachpH 11 during high photosynthetic productivity (Ander-sen, 1975; Sondergaard, 1989).

The bottom sediments of shallow lakes in Florida suchas 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 intothe water column during wind events, and thus contacthigh-pH, oxidized lake water. During wind events > 18km h"1, the total suspended solids were reported to be78 mg L"1 for Lake Apopka (Reddy and Graetz, 1991)and 64 to 100 mg L~' for Lake Okeechobee (Sheng etal., 1991, unpublished data; Olila and Reddy, 1993,unpublished data). The lake water pH for Lake Apopkafluctuates from «7.0 at night to 10.0 during the day(Reddy, 1981). We hypothesized that temporal changeshi pH may regulate WSP and affect P sorption characteris-tics of suspended sediments in shallow lakes; hence,wind-induced resuspension of sediments into the water

946

OLILA AND REDDY: PHOSPHORUS RETENTION IN OXIDIZED LAKE SEDIMENTS 947

column of variable pH can influence bioavailable P con-centration. To test these hypotheses, laboratory studieswere conducted with the following objectives: (i) deter-mine pH effects on P fractions of oxidized sedimentsuspensions; (ii) determine P sorption kinetics at variouspH levels; and (iii) evaluate the relationships betweenP sorption parameters and selected sediment properties.

MATERIALS AND METHODSSite Description

Lake ApopkaLake Apopka is located in central Florida, forming the

headwaters of the Oklawaha basin of lakes. The lake is hypereu-trophic and shallow (mean depth of 1.7 m) with a total surfacearea of 125 km2. Water column pH is regulated by the balancebetween photosynthesis and respiration and the buffering capac-ity of the system. From 0- to 35-cm depth, the sedimentconsists of an UCG, defined by Wetzel (1975) as a coprogenoussediment mixture of remains of all particulate organic matter,inorganic precipitation, and minerogenic matter.

Lake OkeechobeeLake Okeechobee (south Florida) is a eutrophic lake with

a mean depth of 2.7 m and total surface area of 1800 km2.Due to its long fetch and shallow waters, the lake is easily mixedduring wind events (Pollman, 1983). The bottom sediments ofthe lake were categorized by Reddy et al. (1991, unpublisheddata) into four major zones: mud, littoral, peat, and marl-sand. The mud sediments, which constitute about 44% of thetotal lake surface area, may undergo resuspension during windevents.

Sediment SamplingAn Ekman dredge was used to obtain sediment samples

from the central station of Lake Apopka and the mud zone ofLake Okeechobee. The samples were mixed thoroughly, sealedin polycarbonate containers, and stored under ambient labora-tory conditions (26 ± 2°C) prior to use in this study. Thesamples were occasionally purged with N2 gas to maintain anEh = 100 to -150 mV, the redox status of 0- to 10-cm depthof sediment under field conditions.

Experimental SetupLarge flasks (2.8-L capacity), equipped with pH and Pt

electrodes, calomel half cells, and gas (CO2 and air) lines wereset up in the laboratory design, slightly modified from Patricket al. (1973). Each flask was filled with 2.5 L of fresh sedimentdiluted with filtered lake water (0.45-nm filter membrane) inwet sediment/lake water ratios of 25:100 (w/v) for Apopkaand 7:100 for Okeechobee. These ratios correspond to 1:100(w/v) based on the precalculated dry weight of each sediment.The sediment suspensions were adjusted to the desired pHlevels: 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Apopka, and6.5,7.0,7.5,8.5,9.5, and 10.5 for Lake Okeechobee. Ambient(pCO2 of 1.52) pH values were 7.0 for Lake Apopka and 7.5for Lake Okeechobee. The pH of Lake Okeechobee sedimentswas lowered to 7.0 by bubbling with 3% CO2 (pCO2 = 0.477)balanced with air. An artificially low pH of 6.5 was attainedin both lake sediments by adding 0.1 M HC1 to the CO2-purgedsuspensions. Decreasing the pH values lower than ambient

would simulate an acidic condition caused by addition of metalsalts such as A12(SO4)3, FeCl3, and others for lake restoration(Funk and Gibbons, 1979). Lake restoration techniques thatuse A12(SO4)3 or Na2Al2O4 require a pH reduction to =6.0,a pH favorable for forming insoluble Al hydroxides and forassuring that dissolved Al remains below potential toxic con-centrations (Cooke and Kennedy, 1981).

Suspension pH values were increased to 8.5 and 9.5 byaerating with CO2-free air (CO2 stripping). The highest pH(10.5) was attained by dropwise addition of 0.1 M NaOH tothe CO2-stripped suspensions. The sediment suspensions werecontinuously stirred throughout the experiment and maintainedunder oxic conditions (Eh >400 mV). The pH values wereclosely monitored using a pH/Eh controller (Model 5997-20,Cole Farmer, Chicago, IL) which was set to automaticallyadd either CO2 or CO2-free air at a tolerance level of ±0.1for pH and ±4 mV for Eh.

The sediment suspensions were allowed to equilibrate inthe dark under laboratory conditions (26 ± 2°C), purgingwith CO2 or CO2-free air. A system in equilibrium was definedas that condition where changes in pH were ±0.1 unit fromthe desired pH level. An equilibrium condition was attainedfor all reactors after 12 d of incubation. Separate sets ofduplicate samples were then withdrawn from each flask, usingsyringes. The samples were analyzed for pore water P, 1 MNFLCl extractable P (N^Cl-P), 0.1 M NaOH extractable P(NaOH-P), and 0.5 M HC1 extractable P (HC1-P) (Olila etal., 1995), a chemical fractionation scheme slightly modifiedfrom Hieltjes and Lijklema (1980). The slight modificationincludes the determination of both dissolved reactive P (Ameri-can Public Health Association, 1992) and total P in NaOHextracts and the calculation of the moderately resistant organic Pfraction by difference (van Eck, 1982). The NHiCl-P representsloosely bound P and labile organic P (van Eck, 1982). TheNaOH-P represents Fe- and Al-bound P and moderately resis-tant organic P, whereas HC1-P represents Ca- and Mg-bound P.

Phosphorus Sorption KineticsAfter the desired pH levels were attained, the sediment

suspensions were spiked with incremental amounts of standardP solution (as an aqueous solution of KH2PO4) at 24-h intervals.The desired P levels (1.6,3.2,6.2,16.0, and33.0mmolPkg-')were precalculated based on the total volume of suspension andsolid/solution ratio. Aliquots for WSP analysis were withdrawnimmediately preceding each P addition. After each P treatment,duplicate 10-mL suspensions were sampled at the followingtime intervals: 0.3, 0.6, 1, 2, 4, 8, 12, and 24 h. The aliquotswere taken using syringes and immediately filtered through a0.45-|im filter membrane. The filtrates were acidified to =2pH with one drop of concentrated H2SO4 (Baker ultrapure)and stored at4°C until assayed for P in an autoanalyzer. Filteredlake water was included as blanks in each P determination andwas found to contain undetectable P concentration (<0.03uAfP).

The initial P concentration (C0) was defined as the sum ofadded P (in millimoles) and the water-soluble P prior to eachP treatment. Phosphorus that disappeared from solution aftertime t was considered sorbed. Sorbed P (Si) was calculatedfrom the equation

5, = (Vlm)(C0 - Q [1]where C, (mM P) is the P concentration at sampling time (t),V is the volume of suspension (L), and m is the mass of drysediment (kg). The native sorbed P (S)0 was estimated using

948 SOIL SCI. SOC. AM. J., VOL. 59, MAY-JUNE 1995

the least squares fit of the linear plot of S\ against equilibriumP concentration (C24); i.e., at 24 h after P addition:

5, = S0 + bC24 [2]where b is the slope of the line. The S0 value (y intercept)was added to Si to obtain total P sorbed as proposed by Fitterand Sutton (1975).

Kinetic CalculationsSolution P concentrations (Ca) at time zero (/„) was denned

as the sum of added P (C\) and P in solution immediatelypreceding each P treatment. Solution P concentrations weremonitored by withdrawing aliquots of sediment suspensionsat 0, 0.3, 0.6, 1, 2, 8, 12, and 24 h after each P treatment.Target P treatments were based on precalculated P additionsto account for the volume of subsamples removed from theflasks. The sediment/solution ratio was at 1:100 (w/v) through-out the experiment, confirming a well-mixed system. Thesolution P concentrations were plotted against t and fitted tovarious kinetic equations: (i) zero-, first-, and second-orderkinetics (Pavlatou and Polyzopoulos, 1988); (ii) the Elovichequation (Ungarish and Aharoni, 1981); and (iii) a two-constantparameter equation (Kuo and Lotse, 1972). Kinetic calculationswere based on the assumptions that: (i) P desorption waszero in all cases, and (ii) the sediment suspensions were atequilibrium before each P treatment.

Analytical MethodsPhosphorus

Phosphorus in all extracts, except that of 1 M NHtCl (pH7.0), was analyzed according to the ascorbic acid method(Murphy and Riley, 1962) using a Technicon A A II autoana-lyzer (Technicon Industrial Systems, Tarrytown, NY). Deter-mination of P in the NHtCl extracts was slightly modified byheating the sample-reagent mixture to 60°C for 5 min on ahotplate. The mixture was allowed to cool to room temperatureand the intensity of the blue color was measured at 880 nmusing a Shimadzu (UV-160, Shirnadzu Scientific Instruments,Columbia, MD) spectrophotometer. Total P in sediment sus-pension was analyzed using the ignition method (Saunders andWilliams, 1955) followed by analysis on an autoanalyzer.

Metals and Other AnalysesPrior to P spiking, subsamples were withdrawn and analyzed

for: CuCl2-extractable Al (Juo and Kamprath, 1979); ammo-nium oxalate extractable Fe and Al (McKeague and Day,1966); CDB-extractable Fe and Al (Mehra and Jackson, 1960);water-soluble Ca and Mg (American Public Health Association,1992); 1 M KC1 extractable Ca and Mg; and 1 M HC1 extract-able Fe, Al, Ca, and Mg. The 0.5 M CuCl2 extractable Alrepresents organically bound Al (Hargrove and Thomas, 1981),which may actively participate in P sorption (Bloom, 1981).Ammonium oxalate extracts the poorly crystalline Fe andAl (McKeague and Day, 1966), whereas CDB extracts bothamorphous and crystalline forms (Mehra and Jackson, 1960).

Prior to the above extractions, pore water was removed bycentrifugation at 3620 X G for 15 min. The pore water extractwas analyzed for Fe, Al, Ca, and Mg (American Public HealthAssociation, 1992). Metals were analyzed using atomic absorp-tion spectrophotometry (Perkin Elmer 2380, Norwalk, CT).Alkalinity of pore water was determined using standard meth-ods (American Public Health Association, 1992) and electricalconductivity was measured using a conductivity meter (YSIModel 31, Yellow Springs Instruments, Yellow Springs, OH).

Inorganic C in sediments was analyzed by coulometry (Huff-man, 1977), whereas total C was measured using a Carlo-ErbaCNS analyzer (Carlo-Erba, Strumentazione, Rodano, Italy).

Statistical MethodsStatistical analyses were performed using SAS (SAS Insti-

tute, 1985). The WSP data were analyzed with ANOVA, one-waylayout. The data for P sorption and sediment properties weretested for correlation using Pearson correlation coefficients.

Mineral Equilibria CalculationsChemical speciation of P in the liquid phase was predicted

using SOILCHEM (Sposito and Coves, 1991). The computerprogram was used to calculate ion activities in the sedimentsuspension. Input consisted of pH andpCO2; measured concen-trations of Al, Ca, Fe, Mg, and P; and an estimate of Cl, Na,and NH* concentrations. Solubility diagrams were correctedfor ion pairs and complex ions such as CaHPOS, CaCO§,CaHCO3

+, CaOH+, CaCl+, and others. Solubility product con-stants (pK) were obtained from the data compiled and providedby Stumm and Morgan (1981) and Lindsay (1979).

RESULTS AND DISCUSSIONPhysicochemical Properties of Sediments

AtO-to 10-cm depth, Lake Apopka and Lake Okeechobeesediments had ambient pH values of 7.0 (±0.1) and7.5 (±0.1), respectively (Table 1). The lake sedimentsdiffered widely in bulk density, total organic and inor-ganic C contents, and 1 M HC1 extractable Fe and AJ.The soft sediments of Lake Apopka, which have highorganic C (290 ± 20 g C kg"1) and low bulk density(0.03 g dry cm"3), are primarily composed of paniculateorganic matter and partially decomposed algae that havesettled to the bottom of the lake, whereas organic sedi-ments in the central mud zone of Lake Okeechobee(organic C = 160 ± 30 g C kg"1) are reported to have

Table 1. Selected physical and chemical properties of LakeApopka unconsolidated gyttja (UCG) (n = 8) and Lake Okee-chobee mud (n = 6) sediments. All mass units are on dry-weightbasis.

Parameters

SedimentPHBulk density, g cm-3

Total P, mmol P kg"1

Total organic C, g kg"1

Total inorganic C, g kg"1

1 M HCl-extractableCa, mmol Cakg"1

Mg, mmol Mg kg"1

Fe, mmol Fe kg"1

Al, mmol Al kg"1

Pore waterWater-soluble P, |iM PAlkalinity, mM CaCO3Electrical conductivity, uS cm"1

Ca, mMMg, mMFe, fiM

Lake Apopka(UCG)

7.0 (O.l)t0.03 (0.01)

42(7)290 (20)12(7)

83 (0.5)9 (0.2)

12 (0.3)18 (0.1)

42(6)4.3 (1)802 (150)2.7 (0.5)1.2 (0.1)

trt

Lake Okeechobee(mud)

7.5 (0.1)0.15 (0.06)

39(3)160 (30)27(8)

70 (1.5)14 (0.1)66(2)31(3)

9(6)2.4 (1)538 (33)1.5 (0.2)0.8 (0.3)1.3 (0.5)

t Mean with standard deviation in parentheses.i tr = trace or undetectable (detection limits = 0.2 (iM for Fe and 0.3

uM for Al).


been accumulated elsewhere and transported to the siteof deposition (Gleason and Stone, 1975).

Although the two sediments had comparable amountsof total P, the pore water P concentration in Lake Apopkasediments (42 \iM P) was about five times greater thanthat of Lake Okeechobee (Table 1). Pore water is definedas the water removed from fresh sediment after centrifu-gation at 3620 x g for 15 min. Electrical conductivity,alkalinity, and pore water Ca and Mg concentrationswere also higher in Lake Apopka than in Lake Okeechobeesediments. Lake Apopka sediment suspensions had unde-tectable amounts of pore water Fe, whereas Lake Okeecho-bee sediment suspensions had 1.3 uM Fe in the pore water.

Both sediments exhibited high pH buffering capacity.Continuous bubbling with 3% CC-2 decreased the pH ofsediment suspensions by only about 0.2 units, i.e., frompH 7.0 to 6.8 for Lake Apopka and from pH 7.5 to 7.3for Lake Okeechobee (Fig. la). Lake Apopka sediment

ent s

u

•sQ.

10

9.E

9

8.6

a

7.5

7

6.6

ambient pH,Okeechobee

00

i UteApopla Late OkMohobM° »

ambient pH, Apopka

1150) (100) (SO) 0 50 100 150 200 260 300•4- CO, purging | CO, tU&na _«».

Time (min)

_g

«w

"0e»^

OTa

1 1

10

9

8

7

6

B

tl

- « ^.

ambient pH, Okeechobee^^^

Jh

• • • • • * o VQ O Q ambient pH, Apopka~ o

0°

1 1 1 1 1 1 1

2) (10) (8) (6) (4) (2) 0 2 4^ HOtddHhn 1 IMWaddMon _

HCI/NaOH (mM)Fig. 1. (a) Effect of CO2 addition (3% CO2 balanced with air) and CO2

stripping on pH of Lake Apopka and Lake Okeechobee sedimentsuspensions, (b) Changes in pH of sediment suspensions in responseto dropwise addition of either 0.1 M HC1 or 0.1 M NaOH. TheHCI-treated suspensions were continuously purged with 3% CO2whereas those treated with NaOH were constantly aerated withCOj-free air.

suspensions (pH 7.0, ambient) needed 0.5 mM acid (0.1M HC1 added dropwise) to lower the pH to 6.5 whereasLake Okeechobee sediment suspensions (pH 7.5, ambi-ent) required 0.5 and 3 mM HC1 to lower the pH to 7.0and 6.5, respectively (Fig. Ib). The pHs of both sedimentsuspensions, however, were easily increased to 8.5 and9.5 by CO2 stripping alone. The maximum pH obtainedby CO2 removal was 9.8, which was attainable after 4 h.This confirms the ease at which pH of these lakes mayfluctuate due to high photosynthetic activity (Reddy, 1981).

Lake Okeechobee sediment suspensions were highlybuffered at pH 6.4 and resisted further decrease in pHdespite increasing additions of 0.1 M HC1 (Fig. Ib).To attain the pH 10.5 level, Lake Apopka and LakeOkeechobee sediment suspensions needed about 1 and2 mmol of 0.1 MNaOH, respectively, while continuouslypurging with COi-free air.

Solubility of P FractionsLowering the pH of Lake Apopka sediment suspension

from 7.0 (ambient) to 6.5 increased the WSP concentra-tion from 5 to 28 u,M P (Fig. 2). This was accompaniedby an increase in water-soluble Ca and Mg concentrations(Table 2), suggesting the dissolution of calcite and dolo-mite and, possibly, Ca-P compounds. This observationhas an important implication for the use of acid-formingcompounds such as Al2(SO4)3, FeCls, and others in restor-ing the lake. Application of acid-forming amendmentsto Lake Apopka could result in P release due to: (i) thedissolution of carbonate-associated P and Ca- and Mg-Pcompounds, and (ii) the dissolution of P-retaining sur-

35

30

25

P< 20U

feI15

10

Lake Apopka Lake Okeechobee

7 8 9 10pH of sediment suspension

u

Fig. 2. Effect of pH on water-soluble P (WSP) in Lake Apopka andLake Okeechobee sediments.


Table 2. Selected extractable cations for Lake Apopka and Lake Okeechobee sediment suspensions at different pH levels.

Water-solubleSediment

Apopka

Okeechobee

pH

6.578.59.5

10.56.577.58.59.5

10.5

Ca

788360228206137864709153887943

Mg

179146129123

8715513467575130

Fe.. TUT

trjtrtrtr

2351

113

298

Al

trtrtrtr16trtrtrtrtr

490

1

Ca

35839597

103204070808187

M HCl-extractable

Mg

479

10124

1314151516

Fe

151813139

656666636757

Al

1012101012272931333223

Oxalate-extractable

Fe

1820161512666569667160

Al.„-!

1919191919242427272417

CDBt-extractable

Fe

141312108

686150504544

Al

4.70.00.90.90.0

13.612.46.45.20.00.0

CuClz-

Al

98663854432

t CDB = citrate-dithionite-bicarbonate.t tr = trace or undetectable (detection limits = 0.2 jiM for Fe; 0.3 (iAf for Al).

faces as proposed by Barrow (1984). The Ca- and Mg-associated P fraction accounts for 28 to 40% of total Pin Lake Apopka sediments (Olila et al., 1995) and maybe released by dissolution at low pH. Earlier x-raydiffraction studies on Lake Apopka sediments have identi-fied considerable amounts of calcite and dolomite (Olilaet al., 1995) which, on dissociation, may decrease the

10 ll

NaOH-P

10 11

20181614121086

V HC1-P•mbicutpH

10 11

pH of sediment suspensionFig. 3. Various P fractions of Lake Apopka and Lake Okeechobee

sediments as influenced by pH. NH,C1-P is loosely bound P, NaOH-Prepresents Fe-Al-bound P and hydrolyzable organic P, and HC1-Pis Ca-Mg-bound P.

P-binding capacity of sediments and enhance P release(Amer et al., 1985). Increases hi pH from 7.0 (ambient)to 8.5, 9.5, and 10.5 had no effect on water-soluble Pconcentrations in Lake Apopka sediments (Fig. 2). Thissuggests that P sorption and solubility at this pH rangewas probably controlled by Ca-P compounds.

Decreases in pH of Lake Okeechobee sediment suspen-sions from 7.5 (ambient) to 7.0 and 6.5 caused a slight,insignificant (P < 0.05) increase in WSP concentrations(Fig. 2). We speculated that acidification of the sedimentsuspension did release some P associated with carbonateand Ca and Mg compounds but due to the presence ofhigh Fe and Al concentrations (HC1, oxalate, and CDBextractable) in Lake Okeechobee (Table 2), the releasedP was readsorbed onto oxyhydroxides of Fe and Al.Lake Okeechobee sediment suspensions had greater con-centrations of extractable Fe and Al than those of LakeApopka, although the suspensions had similar levels oforganically bound Al (CuCl2-extractable Al).

The WSP in Lake Okeechobee sediment suspensionswere not affected by pH increases from 7.0 (ambient)to 8.5 and 9.5 (by CO2 stripping) (Fig. 2). Furtherincrease in pH to 10.5 by addition of dilute NaOH(0.1 M), however, resulted hi a 20-fold increase in WSPover the ambient concentration. This was accompaniedby a large increase hi water-soluble Fe and Al (Table2). Results suggest the dissolution of Fe- and Al-Pcompounds after the NaOH treatment. Phosphorus re-lease could be due to an artifact caused by elevatedalkalinity, in agreement with the results reported byBoers (1991), who found an enhanced P release fromthe NaOH-treated sediment suspension from Lake Vel-uwe (the Netherlands).

The NH4C1-P concentrations (representing looselybound and labile organic P) for Lake Apopka sedimentsuspensions ranged from 1.6to4.8mmolPkg"1 and wereconsistently greater than the values for Lake Okeechobeesediment suspensions (Fig. 3). The NrL.Cl-P concentra-tions for Lake Apopka sediment suspensions increasedwhen pH levels were raised from 7.0 (ambient) to 8.5,9.5, and 10.5, suggesting an increase in carbonate-boundP pools (van Eck, 1982) with pH. We speculate that pHincreases in Lake Apopka sediment suspensions resultedhi CaCO3-MgCO3 precipitation, which, hi turn, favoredP coprecipitation as proposed by Otsuki and Wetzel


0.035

0.03

0.025 -

10 15 20 25 30 10 15 20 25 30

Time (hours)Fig. 4. Solution P concentrations in Lake Apopka and Lake Okeechobee sediment suspensions with time after addition of 1.6 mmol P kg-' at

various pH levels. Ambient pHs were 7.0 for Lake Apopka and 7.5 for Lake Okeechobee. Lake Apopka sediments were not adjusted to pH7.5, hence no data are available.

(1972). Changes in pH did not affect the NH^Cl-P fractionin Lake Okeechobee sediment suspensions.

At ambient pHs, the NaOH-extractable P concentrationfor Lake Apopka sediment suspension (pH 7.0) was4.8 mmol P kg"1, whereas that of Lake Okeechobeesediments (pH 7.5) was 2.3 mmol P kg"1 (Fig. 3).Lowering the pH levels below the ambient increased theNaOH-P concentrations in both sediment suspensions.Raising the pH levels above ambient, however, had noeffect on the NaOH-P fraction (Fig. 3). The HC1-P

fraction, which is the dominant P pool in both sediments,decreased as pH decreased below pH 7.0, suggestingthe dissolution of Ca-P components.

Phosphorus SorptionLake Apopka sediment suspensions showed no net P

uptake within 24 h after the first P treatment (1.6 mmolP kg'1), maintaining solution P concentrations of 0.03mM P at pH 6.5 and < 0.02 mM P at pHs > 7.0 through-


0.8

0.6

0.4

0.2

pH6.560 P

60 100 140 200 300 400

pH 7.0, ambient0.8

0.6

0.4

0.2

pH8.5

60 P

27P

60 100 140 200 300 400 60 100 140 200 300 400

pH9.5 pH 10.5

60 100 140 200 300 400

Time (hours)60 100 140 200 300 400

Time (hours)Fig. 5. Effect of successive P additions on solution P concentrations in Lake Apopka sediment suspensions with time at various pH levels. Numbers

on top of data points represent cumulative P level added (nunol P kg"1).

out (Fig. 4). Minimal P uptake for these sediment suspen-sions could be attributed to high EPC0 (Olila and Reddy,1993), which acts as a buffer to P sorption (Froelich,1988). Lake Apopka sediments have high concentrationsof pore water P (42 \iM P) and NRtCl-P that buffers Puptake.

Lake Apopka sediments showed no net P uptake atpH 6.5 even in high-P treatments (60 mmol P kg"1)(Fig. 5). At pH 7.0 (ambient), P uptake was not observeduntil the P treatment was increased to >27 mmol P kg"1

(Table 3), which showed first-order kinetics (k = 0.008-

0.034 h"1). This is consistent with the results of earlierstudies (Olila and Reddy, 1993), which showed a linearP sorption isotherm for Lake Apopka sediments only athigh equilibrium P concentrations (between 50 and 650\iM P). Solubility product calculations, based on porewater data at pH 7.0 (ambient) and low-P treatments(0-1.6 mmol P kg"1), indicate undersaturation with re-spect to nonapatitic P compounds (Fig. 6). At high-Ptreatments (60 mmol P kg"1), the equilibrium P concen-tration in sediment suspensions at pH 7.0 (ambient)reached super saturation levels with respect to OCP (Fig.


Table 3. Phosphorus uptake rate constants (first-order, k) for Lake Apopka and Lake Okeechobee sediment suspensions at various pHlevels. Ambient pHs are 7.0 for Lake Apopka and 7.5 for Lake Okeechobee.

Lake Apopka sediment

PH

6.5

7.0

8.5

9.5

10.5

P added(cumulative)

minol P kg"'1.64.8

112760

1.64.8

112760

1.64.8

1127601.64.8

1127601.64.8

112760

k

h-1

ndtndndndndndndnd

0.0080.034

ndndnd

0.0700.244

nd0.0340.0680.0980.056

ndndnd

0.0300.068

r2

_—__———_

0.600.71

__—

0.880.65

—0.880.750.890.80

—_—

0.770.99

n

——__———_55

___55_6556———66

Lake Okeechobee mud sedimentP added

pH (cumulative)

mmol Pkg-'6.5 1.6

4.8112760

7.0 1.64.8

112760

7.5 1.64.8

112760

8.5 1.64.8

112760

9.5 1.64.8

112760

10.5 1.64.8

112760

k

h-'0.700.380.240.020.0030.270.160.120.110.200.460.020.020.040.060.220.020.070.030.750.250.080.080.090.440.0040.010.010.050.27

r2

0.630.550.670.770.600.710.580.610.890.750.550.520.560.660.580.650.370.560.810.840.720.560.720.890.850.780.770.720.590.77

n

555555555558776585765675576666

t nd = no net decrease in solution P concentration within 24 h after P addition.

6). When pH levels were raised to pH 8.5, Lake Apopkasuspensions took up P at high solution P treatments (> 27mmol P kg"1) (Fig. 5), giving k values of 0.07 and0.244 h"1 for 27 and 60 mmol P kg"1, respectively.Sediment suspensions at pH 9.5 showed P uptake at alower P treatment (4.8 mmol P kg"1), suggesting theformation of a new compound or sorbent. Sedimentsuspensions at pH 10.5 showed a similar P uptake patternto those at pH 8.5 and 9.5 but at a lower capacity (Fig.5). Phosphorus uptake in Lake Apopka was probablydue to P coprecipitation with CaCOs. These results arein agreement with the reports obtained in an aqueouscalcitic limestone suspension where Ca-P formation oc-curred when the pH was increased from 7.0 to 8.4(Brown, 1980).

The slow initial P sorption by Lake Apopka sedimentsat high pH (>8.5) and at P concentrations >27 mmolP kg"' suggests P adsorption onto Ca and Mg carbonates,similar to the findings reported by Amer et al., 1985.Initial P sorption was possibly followed by more rapidCa-P precipitation at P levels >27 mmol P kg"1. Phos-phorus sorption capacity in Lake Apopka sediments wasassociated with HCl-extractable Ca and Mg (P < 0.01).Based on solubility product principles, the sedimentstreated with 27 mmol P kg"1 at pH 8.5 were supersatu-rated with respect to OCP, whereas the sediment suspen-sions treated with a similar P level at pH 9.5 weresupersaturated with respect to DCP (Fig. 6). The rapidoccurrence of P sorption atpH 9.5 coincided with super-saturation with respect to DCP. Thus, we speculate that

DCP, known to form more rapidly than OCP (Freemanand Rowell, 1981), could be forming in Lake Apopkasediments at pH 9.5 and high P concentrations (60 mmolP kg"1). Precipitation of DCP dihydrate in the presenceof humic and fulvic acids has been reported by Grossland Inskeep (1991). The DCP can transform slowly intoOCP, as shown in x-ray diffraction studies (Freemanand Rowell, 1981).

Lake Okeechobee sediment suspensions adjusted topH 6.5, 7.0, 7.5, 8.5, and 9.5 showed rapid P uptakeafter the first P treatment (1.6 mmol P kg"1) (Fig. 4).Sediment suspensions that received <11 mmol P kg"1

tended to approach equilibrium after 12 h, whereas thosethat received higher P treatments (>27 mmol P kg"1)continued to take up P even after 12 h (Fig. 7). Thehighest P treatments (60 mmol P kg"1) did not approachequilibrium until after 72 h. Phosphorus uptake by sedi-ment suspension adjusted to 10.5, however, was slowand reached equilibrium after 4 h (Fig. 4). This may beattributed to an alkalinity effect and dissociation of or-ganic ligands due to NaOH addition (Boers, 1991).

Phosphorus uptake by Lake Okeechobee sediment sus-pensions during the first 4-h reaction followed first-orderkinetics. The k obtained for Lake Okeechobee sedimentsuspensions during the first P treatments (1.6 mmol Pkg"1) ranged from 0.004 to 0.74 h"1 (Table 3). Sedimentsuspensions at pH 7.5 (ambient) sorbed P at a rate of0.46h"1, which decreased to <0.06h"1 with subsequentP additions. The rapid P sorption during the first Ptreatment indicates the presence of highly reactive sur-


26

Q. 24

I22

20

APA'

pHKU

13 14 152pH - PCa2+

16 17

0-1.6P 4.8-11 P 27P COPO D A •

Fig. 6. Calculated solubility diagrams of Ca phosphate minerals andchanges in the solubility of Ca phosphates in Lake Apopka sedimentsat various P additions and pH levels. All calculations for activitiesof HiPOi", HOPj", and Ca2+ were corrected for ion pairs suchas CaHPOS, CaCOS, CaHCO3

+, CaOH+, CaCl + , and others basedon SOILCHEM (Sposito and Coves, 1991).

faces and suggests monolayer coverage. The rate of Psorption decreased with surface coverage, as observedin the succeeding P treatments (Table 3).

The overall it obtained for Lake Okeechobee (0.003-0.75 h"1) sediments are in agreement with the resultsobtained for a-FeOOH (Atkinson et al., 1972) systems(Table 4). The k values obtained in this study, however,were much lower than those reported for noncalcareoussediments (Rippey, 1977; Furumai and Ohgaki, 1988)and higher than those reported for soils (Munns andFox, 1976), kaolinite, and a-Al2O3 (Chen et al., 1973)systems. The differences in k values among the sediments(Table 4) could be explained by the differences in method-ology and laboratory techniques (i.e., either batch orcore experiments) and by differences in chemical andmineralogical composition between calcareous and non-calcareous systems. Noncalcareous sediments containinglarge amounts of Fe and Al have a stronger affinity forP (Green et al., 1978), as well as higher P sorptioncapacities, than calcareous sediments (Mayer andKramer, 1986). In addition, the k values for soils (Munnsand Fox, 1976), kaolinite, and a-AhOs (Chen et al.,1973) listed in Table 4 were obtained during the slowreaction step, i.e., after the rapid, initial reaction, Thek values reported in this study were calculated withinthe first 4 h, which represented the initial, rapid reaction.

Lake Okeechobee sediments sorbed P at all pH levelsand exhibited a multiphasic pattern: (i) a rapid initial Psorption (Fig. 4), followed by (ii) a slow P reaction, and

(iii) a rapid P sorption at high P concentrations (>27mmol P kg"1) (Fig. 7). Phosphorus uptake at high P levels(>27 mmol P kg"1) was most favored at pH 8.5 and 9.5.Based on calculations, a 0.5 pH unit decrease from 7.5(ambient) increased P uptake by 40%, whereas a 1-unitdecrease increased P uptake by =60%. At pH 7.5, thesediments sorbed =50% of added P. The disappearanceof solution P, added at 60 mmol P kg"1, was highest atpH 8.5 and 9.5 where equilibrium P concentration wasmaintained at about 0.2 mAf P (Fig. 7).

The high P retention on Lake Okeechobee sedimentscan be attributed to sorption on noncrystalline Fe-Aloxyhydroxides and minerals such as calcite, dolomite,and, possibly, sepiolite, which was confirmed (by x-raydiffraction) to be present in Lake Okeechobee sediments(Olila et al., 1995). Phosphorus uptake by Lake Okeecho-bee sediment suspensions was believed to occur in threestages: (i) rapid P sorption at low P concentration, whichis unaffected by pH, (ii) slow P sorption at moderate Pconcentrations, which decreases with increasing pH, and(h'i) P sorption at supersaturated conditions, which in-creases with increasing pH. The first stage is character-ized by the lack of relationship between pH and P sorptionat the low-P treatment (1.6 mmol P kg"1). This couldbe explained by the concept that the first phosphate ionsare sorbed mainly at high-affinity sites (Muljadi et al.,1966), possibly by binuclear adsorption (Atkinson et al.,1972), with little or no repulsive forces from the adjacentphosphate ions (Helyar et al., 1976). The sorbed P atlow P concentrations may not be easily replaced by theincreasing concentrations of hydroxyl ions at high pH(Eze and Loganathan, 1990). This may explain the uni-form P sorption hi the first P treatment (1.6 mmol Pkg"1) within the pH range (6.5-10.5) studied.

The second stage shows the decline in P sorption withincreasing pH at moderate P levels (4.8-11 mmol Pkg"1). This suggests a typical adsorption mechanismcontrolled by Fe-Al oxides (Goldberg and Sposito, 1984)and clay minerals (Rippey, 1977). Pure samples of Fe-Al oxides have been reported to have a PZC at =8.0(Barrow, 1984). Iron and Al oxyhydroxides have netpositive charges at pH <7.5, which provides high-affinity sites for P sorption. The P sorbed at the low-Plevel (1.6 mmol P kg"1) was correlated with oxalate-extractable Al (P < 0.01), CuCl2-extractable Al (P <0.01), and CDB-extractable Fe-Al (P < 0.05).

Decreases in P sorption with increasing pH in LakeOkeechobee sediments at intermediate P additions (4.8-11 mmol P kg"1) can be attributed to: (i) solubilizationof Fe-Al-P at high pH (Lindsay and Moreno, 1960),and (ii) increased competition between hydroxyl andphosphate ions (Lijklema, 1977). Various Fe-Al-P com-pounds dissolve at high pH to form the more stablegoethite-gibbsite, accompanied by P release (Lindsayand Moreno, 1960). Increasing pH can also change theelectrostatic potential (Kingston et al., 1967) and createmore negative charges on sorbing surfaces that wouldrepel phosphate ions (Eze and Loganathan, 1990). Inaddition, greater amounts of hydroxyl ions at high pHfavor ligand exchange reactions (Lijklema, 1977).

The third stage is characterized by supersaturated con-

OLILA AND REDDY: PHOSPHORUS RETENTION IN OXIDIZED LAKE SEDIMENTS

pH 6.5 pH 7.0

955

U.B

0.6

0.4

0.2

60 P(

27P

IIP f^

S

I

1 1 * — --.° 60 100 140 200 300 40

VJ.O

0.6

0.4

0.2

n

6011

27P

i-T

pi

y 1 1>-— -.

pH 7.5, ambient

J

.9

u.o

0.6

0

27(

• . ,

60 P4<

P

»<H

>*i

I

Si ——— .

60 100 140 200 300 40

0.8

0.6

0.4

0.2

pH8.5

27

IIPrrH

60 P

•I\° 60 100 140 200 300 40

pH9.5 pH 10.5

60 100 140 200 300 400

Time (hours)60 100 140 200 300 400

Time (hours)Fig. 7. Effect of succesive P additions on solution P in Lake Okeechobee mud sediment suspensions with time at various pH levels. Numbers on

top of data points represent cumulative P level added (mmol P kg"1).

ditions at the high-P (60 mmol P kg"1) concentration.As added P levels increase, P sorption slowly shifts to

a precipitation-like process (Eze and Loganathan, 1990),particularly at pH 8.5 and 9.5. The large increase in P

Table 4. Phosphorus uptake rate constants (first-order, k) found in literature and in this study.System

Soils, HawaiiPure systems

Kaolinitea-AM),K-A1-PO4a-FeOOHK-Fe-PO,

SedimentsLough Neagh, Northern IrelandLake Kasumigaura, JapanLake Apopka, Florida

pH 6.5 (0.1 M HCl-treated)pH 7.0 (ambient)pH 8.5-9.5 (COz stripped)pH 10.5 (0.1 M NaOH treated)

Lake Okeechobee, FloridapH 6.5-7.0 (0.1 Af HCl-treated)pH 7.5 (ambient)pH 8.5-9.5 (COz stripped)pH 10.5 (0.1 MNaOH treated)

t b = batch; c = core.J Observed only in P treatments a 27 mmol P kg"1.§ Observed only in P treatments >4.8 mmol P kg-'.

kh-'

0.00003-0.0005

0.0001-0.00230.0003-0.0013

0.37-4.150.061-1.650.005-0.54

4.320.95-14.9

No net P uptake0.008-0.034*0.03-0.244§0.03-0.068$

0.003-0.700.02-0.460.02-0.75

0.004-0.27

Techniquet

b

bbbbb

cbb

b

References

Munns and Fox, 1976

Chen et al., 1973Chen et al., 1973Kirn et al., 1983aAtkinson et al., 1972Kirn et al., 1983b

Rippey, 1977Furumai and Ohgaki, 1988This study

This study


sorption with increasing pH at the highest P treatment(60 mmol P kg"1) is consistent with precipitation ofCa-P (White and Taylor, 1977) following P sorptiononto calcite or carbonates (Freeman and Rowell, 1981).This phase of P sorption is in agreement with the resultsobtained in sestonic materials from Lake Neusiedlersee(Austria), where P sorption increased with increasingpH and was highest at pH 9.0 (Gunatilaka, 1982).

Formation of apatite under natural freshwater condi-tions has been reported in some studies (Andersen, 1975;Lofgren and Ryding, 1985). This is not expected tooccur in Lake Okeechobee because the sediments havehigh amounts of Mg and humic-fulvic acids, whichinterfere with apatite formation (Martens and Harriss,1970; Inskeep and Silvertooth, 1988). The most likelyP solid phase forming in Lake Okeechobee mud sedi-ments at P treatments >27 mmol P kg"1 and pH 9.5 isOCP (Fig. 8).

The solubility product diagram for Lake Okeechobeesediments at pH 6.5 (a decrease of 1 pH unit from theambient level) suggests undersaturation with respect toapatite (Fig. 8). This unstable thermodynamic conditionwould favor dissolution of apatite and may explain theobserved decrease in HC1-P at pH 6.5. The decrease hiHC1-P concentration was accompanied by increases inNaOH-P, suggesting a transformation of Ca-bound P intoFe-Al-bound P. This observation supports the contention

Q.

+%

12.512

11.511

10.510

9.59

26

24

22

20

18

pH6.5 pH7.0

9.5 10 10.5 11

- pH7.5(ambient pH)

pH8.511 12 13

2pH - pCa2

14 15 16

1.6 P 4.8-11 P 27 P 60 P

O D A •

Fig. 8. Calculated solubility diagrams of Ca phosphate minerals andchanges in the solubility of Ca phosphates in Lake Okeechobeemud sediments at various P additions and pH levels. All calculationsfor activities of H2PO4~, HOPS", and Ca2+ were corrected for ionpairs such as CaHPOJ,CaCOS,CaHCO3

+,CaOH+,CaCl + , andothers based on SOILCHEM (Sposito and Coves, 1991).

that P sorption by Lake Okeechobee sediments is regu-lated by both Fe-Al and Ca-Mg compounds.

The trend for k values to decrease with increasing Padditions in Lake Okeechobee sediments at pH 6.5 sug-gests high reactivity of poorly crystalline Fe and Aloxyhydroxides. The k values were correlated (P < 0.01)with CDB-extractable Fe-Al and CuCl2-extractable Al,indicating that the rate of P sorption was regulated byboth crystalline and noncrystalline Fe-Al and organicallybound Al. Conversely, the trend for k values to increasewith increasing P additions hi the highly alkaline sedi-ments (pH 10.5) suggests a slow, initial P reaction withCaCOs, which men developed into a rapid precipitationprocess similar to that described for Lake Apopka sedi-ments.

The rate of P sorption by Lake Okeechobee sedimentsat intermediate pH levels (7.0-9.5) could be describedby two processes: (i) a rapid, initial P sorption thatdecreased with P addition, and (ii) increased P sorptionat high P concentrations. This observation is in agreementwith the results obtained for a gibbsite system (vanRiemsdijk and de Haan, 1981), where the reaction ratedecreased with the amount of P sorbed. The initial, rapidreaction has a high energy of adsorption (Kuo and Lotse,1974) at low surface saturation. The succeeding slowreaction rate, at intermediate P levels (between 4.8 and27 mmol P kg"1), was probably a result of the followingmechanisms: (i) increased surface negative charge arisingfrom specific anion adsorption (Hingston et al., 1967),(ii) increased interaction between phosphate ions, and(iii) decreased adsorption energy (Kuo and Lotse, 1974).The increase in P sorption at the highest P level (60mmol P kg"1) suggests the occurrence of precipitationreactions similar to the results obtained for Lake Apopkasediments.

The amount of P sorbed by sediment suspensions wascalculated based on P loss 24 h after each P treatment.Phosphorus uptake by Lake Apopka and Lake Okeecho-bee sediment suspensions after each P addition is shownin Fig. 9. AtpH 6.5 and 7.0, Lake Okeechobee sedimentsuspensions showed greater P sorption capacity thanthose from Lake Apopka. Phosphorus sorption, however,was almost identical for both sediments at pH 8.5 and9.5, suggesting that P uptake was controlled by thesame mechanism, i.e., probable coprecipitation of P withCaCOs. At pH 10.5, Lake Apopka sediment suspensioncontinued to assimilate P at high-P treatments whereasLake Okeechobee sediment suspension decreased its Psorption capacity (Fig. 9). Lake Okeechobee sedimentshave high amounts of extractable Fe and Al (Table 2),hence this decrease in P uptake could be attributed to Prelease from dissolution of Fe- and Al-P compoundsdue to NaOH addition, in agreement with the resultsreported by Boers (1991).

CONCLUSIONSLake Apopka sediment suspensions were highly re-

sponsive to pH decrease, which caused a significantincrease in WSP. Acidification increases the WSP,NRtCl-P, and NaOH-P fractions of sediment suspensions


J3P

|

8PL,

100

80

60

40

20

pH7.5

J

pH8.5

pH9.5

0 0.2 0.4 0.6 0 0.2 0.4 0.6

Equilibrium solution P concentration (mM P)Fig. 9. Phosphorus sorbed by Lake Apopka and Lake Okeechobee

sediment suspensions at various pH levels 24 h after each P treat-ment. Each point represents 24-h equilibration with added P. Ambi-ent pHs were 7.0 for Lake Apopka and 7.5 for Lake Okeechobee.Lake Apopka sediments were not adjusted to pH 7.5.

from this hypereutrophic lake. Lake restoration tech-niques that use acid-forming compounds such as^2(804)3, FeCla, and others, therefore, may not beadvisable for Lake Apopka. A decrease in pH from 7.0(ambient) to 6.5 would release P due to dissolution ofCa-P compounds and dissociation of carbonate mineralsthat served as P-binding materials. Increases in pH byCO2 stripping (which simulates high photosynthetic activ-ity), however, does not affect the ambient WSP concen-tration. Lake Apopka sediments have minimal P sorptionat ambient pH and low P treatments. For P uptake tooccur, Lake Apopka sediments require an elevated pH(>8.5) and high P (>27 mmol P kg"1) treatments. Wespeculate that the pH increase favors precipitation ofCaCO3, which then serves as P-binding material. Dueto high concentrations of WSP in its pore water andloosely bound P, Lake Apopka sediments serve as sourcesof P to the overlying water column.

Changes in pH had minimal effect on WSP in LakeOkeechobee sediment suspensions except at pH 10.5,which was treated with dilute 0.1 M NaOH. SignificantP release at pH 10.5 was an artifact due to increasedalkalinity resulting from NaOH addition. The high P

sorption at both low (6.5) and high (10.5) pH levelssuggests that the mud sediments can function as sinksfor P across a wide pH range. Phosphorus uptake bythe sediments followed first-order kinetics, with k rangingfrom 0.003 to 0.75 h"1, which is comparable to thosepublished in the literature. Lake Okeechobee sedimentshave high pH buffering capacity and hence, may havesome tolerance for acid-forming amendments and acidicprecipitation.

ACKNOWLEDGMENTSWe are grateful to Dr. Mark Brenner and Matt Fisher for

helping us during field sampling. This work was supported inpart by the South Florida Water Management District and theSt. Johns River Water Management District. We thank Dr.G. A. O'Connor and Dr. C.T. Johnston for their comprehensivereviews of the manuscript.

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