OI

Phosphorus

and calcium carbonate

solubilities

in

Lake Kinneret

Yoram Avnimelech

Technion-Israel Institute of Technology, Tcchnion City, Haifa 32000

Ahs truct

Phosphate and calcium carbonate solubilities were calculated from data obtained for ncar- bottom water samples from Lake Kinneret during the years 1975-1977. A potential diagram technique was used to determine which solid phase is in equilibrium with the solution.

Both phosphorus and calcium carbonate solubilities arc controlled, during the January- August period, through equilibrium with calcium-carbonate-phosphate, Ca3(HCO:,):3P0,. The calcium carbonate ionic product changes, depending on the phosphoric acid potential, during this period. The solubility product, (Ca2+)3 x (HC03-)3 x (POd3-) is 10-2s.“. Calcium carbonate

activity is constant ( 10-8*56) during the last months of the stratified period (September-De- cembcr).

These findings support the hypothesis that the first and most reactive product of the reac- tions occurring between CaCO, and phosphorus is a CaCO,H,,PO, surface complex.

Phosphorus is a major nutrient regulat-

ing algal growth in lakes (Vollenweider

1968) and specifically in Lake Kinneret

(Serruya and Berman 1975). The supply

of phosphorus to water bodies depends

to a large extent on the solubility of phos-

phorus found in sediments and in sus-

pcndcd inorganic particles. However, no

clear agreement between a given solu-

bility product and soluble phosphorus

concentration has been found, probably

due to the complex system involved.

Syers et al. (1973, p. 3) concluded that “It

is difficult, if not impossible, however, to

take the finding obtained with pure sys-

tems in the laboratory and solubility pre-

dictions based on thermodynamics and to

extrapolate these to the extremely com-

plex environment in lake sediments .”

Most attempts to study the solubility of

phosphorus in lakes or lake sediments

have been based on the comparison of

ionic products in the lake with those of

well defined crystalline phosphatic

species (Sycrs et al. 1973; Emerson and

Widmer 1978). However, mixed crystals

or surface complexes of phosphorus arc

probably common in lakes. For example,

it has been suggested that mixed ferric-

hydroxo-phosphates precipitate in many

natural lake waters (Morgan and Stumm

1965), and that phosphorus is adsorbed

on hydrous ferric oxide colloids (Lee

1970) and is possibly chcmisorbed on

CaCO,,, forming a surface complex that is

later transformed into crystalline apatite

(Stumm and Leckie 1971). Such com-

pounds are usually poorly crystallized OI

present as surf&e complexes. Crystallo-

graphic study of them is difficult and their

presence may be elucidated mainly

through solubility functions.

Avnimelech (1980) showed that the re-

action between dilute orthophosphate so-

lutions and CaCO, cannot be attributed

to the formation of any distinct calcium

phosphate but that a surface complex

WHCQM04 is formed. This complex,

having a defined solubility behavior, is

probably kinetically favored over apatites

and thus could be of importance in con-

trolling phosphorus concentration in lake

waters.

Lake Kinneret is essentially a &CO,

system, The lake sediments contain up to 50% CaCO, (S erruya 1971) and the waters

are hard (4.5-5.2 meym liter-l as Ca2+, 225-

260 mg. liter-l as CaCO,) and alkaline (2-

2.4 meq. liter--l, loo-120 mg. liter-l as

CaCO,) (Scrruya 1978). Interaction of

phosphorus with CaCO, seems to be the

dominant factor controlling phosphorus

concentration under such conditions. I

used data available for the water com-

position in Lake Kinneret to compute the

appropriate solubility parameters and to

test the hypothesis that phosphorus con-

centrations in the bottom water are de-

641

termined by the solubility product of the

calcium-carbonate-phosphate or other

solid phases. I thank C. Serruya and T. P.

Murphy for comments and M. Gophen for

making the data available.

Muterials and methods

The data used here were provided by

the Lake Kinneret Limnological Labora-

tory. Data for the period January 1975 till

December 1977 at the deepest water

sampling depth in station A (42 m) are

used. Samples were taken weekly 1.5 m

above the bottom (Serruya et al. 1974).

Analyses were carried out according to

standard methods (Am. Public Health As-

soc. 1971). Ionic distribution in the so-

lution was computed from constants for

the orthophosphoric and carbonic acids

and the stability constants for the ionic

pairs CaHPOdO, Ca( H2P04)+ (Gregory et

al. 1970), CaCO,O, Ca( HCO,)+ (Nakayama

1968), and CaOH+ (Gimblett and Monk

1959). Activity coefficients were calculat-

ed with the extended Debye-Huckel

equation, using an estimate for the ionic

strength from the measured electrical

conductivity of the water (Griffin and Jur-

inak 1973). Phosphoric acid, calcium hy-

droxide, and calcium carbonate poten-

tials are taken as the negative logarithms

of (H+)” x (POd3-), ( Ca2+) x (OH-)2, and

(Ca2+) x (C0,2-), where values in paren-

theses represent activities.

Avnimelech (1980) assumed the for-

mation of a calcium-carbonate-phosphate

surface complex, Caz(HCO,),PO,, in

CaCO,-H,PO,-H,O systems. The rcac-

tion between the reacting electroneutral

components can be formulated as

3 CaCO,(l) + H,PO,(l)

+ Ca,( HCO&PO,( s). ₍₁₎

The two reacting components, CaCO, and

HtsP04, are taken as reacting electroneu-

tral species in the solution. The selection

of electroneutral species rather than that

of the reacting ionic species is conven-

tional (e.g. Lindsay 1979) and is advan-

tageous because of the avoidance of extra

thermodynamic functions,

At equilibrium, the chemical potentials

T&k 1. Soluble phosphorus and related param- etcrs in water overlying the sediment (monthly av- eragcs).

Psol Gl HCO;- 1o-‘~nlol ’ IfF~mol ’ 1W~rnol ’ _PII

Dee 75 1.6.61 1.46 1.83 7.33

J an 76 0.753 1.24 1.23 8.01 Feb 76 0.887 1.27 1.23 8.06 Mar 76 1.421 1.26 1.25 8.15 Apr 76 1.532 1.30 1.28 7.65 May 76 0.968 1.22 1.38 7.60 Jun 76 2.044 1.37 1.45 7.58 Jul 76 1.290 1.48 1.50 7.62 Aug 76 3.226 1.46 1.54 7.42 Sep 76 7.985 1.51 1.62 7.44 Ott 76 13.87 1.56 1.63 7.36 Nov 76 16.05 1.60 1.73 7.36 Dee 76 16.85 1.57 1.79 7.36

J an 77 2.984 1.38 1.41 7.92 Feb 77 0.806 1.29 1.28 8.08

of the reacting components (pi) can be

related through Eq. 2:

3pCaC0, + pII,P04 = F” solid (2)

or

3 ln(CaCO,,) + ln(H,PO,) = constant (3)

(where the values in parentheses repre-

sent the activities of the given compo-

nents). If the solution composition is

determined by the solubility of the above-

mentioned complex, logarithms of H,PO,

activities plotted against those of C&O,

activities should follow a straight line with

a slope of -3. Similarly, if hydroxyapatite

is the phase controlling the phosphorus

concentration, a plot of log(H,,PO,) vs.

log[Ca(OH),] should yield a straight line with a slope of 1.67.

Results

Monthly average values for phos-

phorus, calcium, alkalinity, and pII in the

filtered water samples for the period De-

cember 1975-February 1977 are given in

Table 1. A rapid decrease in soluble P,

Ca, and alkalinity, and an increase in pH

occur after turnover (January). When the

lake stratifies again (March), levels of

these components are built up slowly,

reaching a peak during fall (October-De-

36.

B In35

27 -i

34

33,

O’\O 0 -January -August I+

I! cF

- -lag CaCQ = 8.56 o\: 0

OP” l 0 b $8

.

A

I 1 I

a a.5 0 9

-log taco,

Fig. 1. H,PO, potentials as a function of CaCO, potentials in Lake Kinneret bottom water samples.

iere (Serruya

scribed in detail elsewl

1978).

The data in Table 1 can he grouped in

three periods: the mixed period, Janu-

ary-March, when P, Ca, and HCO, are

low; a transition period (April-August)

when P, Ca, and HC09 increase and pH

decreases slowly; and the completely

stratified period (September-December)

when P, Ca, and alkalinity are high and

pH is lowest. A similar grouping can be

seen in Fig. 1, where phosphoric acid po-

tentials are plotted against calcium car-

bonate potentials. Data from the first

group-the mixed period points-define

a diagonal line, showing a dependence

between phosphoric acid and calcium

carbonate potentials. Data from the com-

pletely stratified period are grouped along

a vertical line indicating no dependence

between CaCO, and HsP04 potentials and

a constant CaCO, potential (8.563 +

0,081). The data of the transition period

are located mostly at the intersection of

these two lines, The relationship be-

tween the HRP04 and &CO, potentials

for the mixed period is given by

-log H,PO, = A - B(-log CaCO,). (4)

The parameters of this equation, tested

for different periods during the 3 years of

study are given in Table 2. The slope of

the logarithmic dependence is very close

to the expected value, -3, all along the

period from January to August. The

regression coefficients are high and the

intercept, A, which is a function of the

solubility constant, is fairly uniform.

When data from the completely stratified

period (September-December) are in-

cluded in the regression, variability in-

creases and the slope is rising, up to

-4.34. These last data, treated separate- ly, do not agree with the model tested.

The potential diagram relating the po-

tentials of H3P04 to those of Ca(OH), is

given in Fig. 2. The seasonal effect is less clear here, yet it is obvious that the data

of the completely stratified period de-

viate from the line defined by the rest of

the points. The relations between the

H,PO, and the Ca(OH), potentials are

given in the last three columns of Table

2. Examination of this table results in

conclusions somewhat similar to those ar-

rived at for the dependence with C&O,

potential. The slope of the logarithmic

dependence between the phosphoric acid

and the Ca(OH), potentials is fairly con-

stant, about - 1.3, for the period January-

August. Inclusion of the late summer data

raises the slope up to -1.78, yet, if the

latter data are treated separately, a slope

of -1.23-1.25 is obtained.

Discussion

The solubility behavior of phosphorus

in the water overlying the sediment in

Lake Kinneret was considered using two

possible mechanisms: control of solubil-

ity by a calcium-carbonate-phosphate sol-

id phase and control of solubility by a cal-

cium phosphate solid phase. Solubility

considerations are used here to test these

mechanisms.

The agreement between the data and

the first mechanism is good for the period

P und CaCO, solubility

Table 2. Linear regression matrixes for the equations: pHSPO, -k A - B pCaCO,, and pIIsPO, + A - B pCa(OH),.

Period

No. of ,snmple.s

.II:,PO, vs. CaCO, II,P04 VS. Ca(OH),

, A -H r A -13

I Jan-Mar 25 0.796 62.57 3.27 0.8 14 57.44 1.43

Jan-Apr 33 0.770 61.48 3.14 0.832 54.59 1.24

Jan-May 47 0.822 59.65 2.91 0.817 53.63 1.18

Jan- Jun 57 0.787 60.69 3.04 0.805 54.32 1.22 Jan- Jul 69 0.806 61.95 3.20 0.840 55.75 1.32 Jan-hug 77 0.8 13 62.57 3.28 0.857 56.54 1.37 Jan-&p 89 0.793 65.29 3.62 0.861 58.78 1.52

Jan-Ott 99 0.791 68.18 3.97 0.869 60.76 1.65

Jan-Nov 112 0.793 71.18 4.34 0.882 62.57 1.76 Jan-Dee 123 0.750 67.95 3.96 0.879 62.75 1.78

Scp-Ott 44 0.352 42.67 1.07 0.719 53.47 1.23

Ott-Dee 32 0.263 40.19 0.79 0.695 53.85 1.25

lishment of complete stratification. The

slope of the relationship between the po-

tentials of phosphoric acid and those of

calcium carbonate is close to -3, in

agreement with the hypothesis that the

solid phase controlling the solubility is

Ca,(HCO,,),PO,. The solubility of this

phase is somewhat higher than that

found in laboratory experiments (Avni-

melech 1980). The ionic product

( Ca2+)“( HC03-)3( POd3-) was lo-” l** in the

laboratory experiments and 10-2R*5 for the

lake water (calculated for the period Jan-

uary-August). This differcncc could be

due to poorer crystallization in the lake

than in the laboratory equilibration ex-

periments. The variation of the &CO,

activity as a function of the HtjP04 activ-

ity is in agreement with this first hypoth-

esis. Calcium carbonate activity [( Ca2+).

(CO,“-)] is equivalent to the ionic prod-

uct of calcium carbonate and is usually

considered to be constant [p7c, +8.31 (Na-

kayama 1968) to 8.42 (Lindsay 1979)].

This was not the case during most of the

year. Calcium carbonate activity was not

constant and depended on the phospho-

ric acid potential. The product of the

&CO, and H,,PO, activities rather than

the CaCO, activity tended to be constant,

as expected for systems controlled

through the solubility of Ca,(HCO,),PO,.

This dependence did not occur during the

completely stratified period, when the

CaCO, activity was almost constant and

very close to the solubility product of

CaCO,. The different behavior of the

water overlying the sediment during the

completely stratified period could be due

to the specific composition of the water

at that time, i.e. lack of oxygen, low pH,

high iron (Serruya 1978), or to a lack of

equilibrium with the sediment during that

period, Serruya et al. (1974) found that

the sediment does not mix with the over-

lying water during this period, although

it does during the rest of the year. It seems

that the water sampled 1.5 m above the

bottom is not in equilibrium with the

&CO,-rich sediment or that P solubility

is controlled by Fe during the stratified

period. The significant dependence of the

phosphoric acid potentials on the Ca( OH),

potentials does indicate that the solution

composition is also controlled by a solu-

bility product of calcium phosphate solid

phase, the second mechanism tested here.

Avnimelech (1980) interpreted the de-

pendence of H3P0, potential on both

CaCO, and Ca(OH), potentials as im-

posed on the system when pC0, is con-

stant. Under such conditions, CaCO, and

Ca(OH), activities are linearly related.

This explanation does not hold in the

present study, where pC0, is certainly

not constant, being relatively low in the

mixed period and high in the stratified period.

Stumm and Leckie (1971) suggested

t-1

‘\ \

\ \

h \

7---

---- -

T --

Period o January-March

l April -August

A September-December

l .

. l

I .--.---A

15 16 17

-log Ca(OH),

Fig. 2. As Fig. 1, but of Ca(OH), potentials. Dashed line-hydroxyapatitc solubility isotherm.

sorbed on calcium carbonate particles and

recrystallized to form hydroxyapatite

crystals. It is plausible according to phase

rule considerations that the solution will

be controlled by both solid phases: cal-

cium-carbonate-phosphate and calcium

phosphate. Four components-H,O,

HsP04, H2COn, and Ca(OH),-and two

solid phases in equilibrium with the

aqueous phase are involved in this sys-

tem. If we assume that pressure and tem- perature are constant, we still have 1 df

enabling a change along the solubility

line. The two solid phases that seemingly

are in equilibrium with the water are cal-

cium-carbonate-phosphate and calcium

phosphate during the winter and spring

periods, while calcium carbonate and

calcium phosphate seem to control the

system during late summer. There seem

to be two alternatives for the exact nature

of the calcium phosphate in equilibrium

with the water. One is hydroxyapatite, as

suggested by Stumm and Leckie (1971)

and the other is octacalcium phosphate.

The data points plotted in the H,P04-

Ca(OH), potential diagram (Fig. 2) are

very close to the line defined by the hy-

droxyapatite solubility product (Avnime-

lech et al. 1973). The octacalcium phos-

phate has a higher solubility in this range,

However, the slope of the H,PO,-Ca(OH),

potential (Fig. 2 and Table 2), being close

to 1.3, is in agreement with the existence

of octacalcium phosphate, (OCP),

C~&WU~, rather than hydroxyapatite.

Octacalcium phosphate is more readily

crystallized and thus was found to be an

important intermediate calcium phos-

phate in solutions, soils, and biological

systems (Brown et al. 1962). It is thus

plausible that it would also play an im-

portant role in lakes. The nature of the

calcium phosphate phases as well as the

properties of the calcium-carbonate-

phosphate have to be studied further,

possibly by more direct crystallographic

methods.

Conclusions

The application of potential diagrams

and the consideration of complex, or

mixed, phosphates improve our ability to

understand the equilibrium between the

solid phase and aqueous orthophos-

phates in lakes. The evidence presented

here indicates that in Lake Kinneret this

equilibrium is controlled mainly by cal-

cium phosphate crystals formed in con-

junction with surface complex calcium-

carbonate-phosphate. The exact nature of

the calcium phosphate (hydroxyapatite or

octacalcium phosphate) and the effects of

environmental conditions on this equi-

librium deserve further study.

A solubility equilibrium will be effec-

tive in the water layer in contact with the

sediment. Phosphorus concentration in

the bulk of the water column will be af-

fected by other mechanisms such as

transfer from the lake bottom upward, up- take by algae, bacteria, and other organ-

isms, release of orthophosphate by bio-

logical activity and external supplies.

Nevertheless, understanding the prcdic-

tive ability of the equilibrium mecha-

nism is a prerequisite of any basic ap-

P and CaCO, solubility

References

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Submitted: 21 December 1981