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Adsorption-desorption of phosphorus by lake sediments under anaerobic conditions
Wat. Res. Vol. 23, No. 6, pp. 677-683, 1989 Printed in Great Britain.All rights reserved
0043-1354/89 $3.00+ 0.00 Copyright © 1989MaxwellPergamonMacmillanpie
ADSORPTION-DESORPTION OF PHOSPHORUS BY LAKE SEDIMENTS U N D E R ANAEROBIC CONDITIONS H. FURUMAI*@ and S. OHGAKI@ Department of Urban Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan (First received December 1987; accepted in revised form January 1989) Abstraet--Adsorption~iesorption reactions of phosphorus by lake sediments were studied using a radioisotope 32p inorganic phosphate. The sediments were collected at Lake Kasumigaura, Japan. The amount of isotopically exchangeable phosphorus in the sediments was measured in anaerobic sediments-water batch systems under various pH and phosphorus concentrations. The steady state condition practically prevailed with respect to 32p within 30min and the amount of exchangeable phosphorus in the sediments was calculated based on the steady state data. The results were compared with data previously obtained under aerobic conditions. The anaerobic conditions caused a large phosphorus release and an increase in exchangeable phosphorus especiallybelow pH 8. The exchangeable phosphorus under anaerobic conditions ranged from 27 to 49% of the total phosphorus in the sediments of Lake Kasumigaura, respectively, for pH in the range of 5-10. The relationship between the phosphorus concentration in the liquid phase and the amount of exchangeable phosphorus in the solid phase under anaerobic conditions can be explained by the Langmuir equilibrium model as well as under aerobic conditions. The bonding energy constant and adsorption maximum proved to be useful parameters for evaluating the effect of redox level and pH on the exchange reaction. The bonding energy constants under aerobic and anaerobic conditions ranged from 4.6 to 13.0, and from 1.2 to 2.3 (1mg -~ ) for pH in the range of 6-7, respectively.The adsorption maxima were higher under anaerobic conditions than aerobic conditions. For pH between 5.7 and 6.6, the increase in pH caused an increase in the bonding energy constant, but had only a slight influenceon the adsorption maximum under anaerobic conditions. Key words--phosphorus exchange, adsorption~desorption, release, lake sediments, radioisotope phosphate, exchangeable phosphorus, Langmuir model, anaerobic conditions, redox level
INTRODUCTION
Sediments play a big role in eutrophication in closed water-bodies such as lakes and bays. Lennox (1984) reported that it required a long period of time to improve the eutrophic status of Lough Ennell because of internal loading of phosphorus from sediments following the control of external loadings. Hosomi and Sudo (1987) pointed out the importance of phosphorus flux from sediments using their multicomponent dynamic model of phosphorus in the sediments-water system of Lake Kasumigaura. Phosphorus has accumulated on the surface of the sediments in the eutrophic Lake Kasumigaura and its main phosphorus fraction is NaOH-extratahle ortho-phosphorus (Furumai and Ohgaki, 1982). The phosphorus exchange between sediments and water is important for quantitatively evaluating the potential for influencing the phosphorus concentration in the overlying water. The exchangeable phosphorus, especially that on the surface of the sediments, is the easily-released-fraction and has an important effect *Present address: Department of Civil Engineering Hydraulics, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812, Japan.
on the phosphorus concentration during the short term such as hours or a day. Many researchers have investigated phosphorus adsorption by sediments and soils under aerobic conditions and phosphorus release from sediments under anaerobic conditions. The phosphorus behavior depends on phosphorus concentration, pH and the redox level in the overlying and interstitial water. Adsorption and release are not irreversible reactions and should not be considered separately either. With respect to phosphorus adsorption, a few reserchers (Pomery et al., 1965; Li et al., 1972; Ku et al., 1978; Gunatilaka, 1982) have studied the exchange reaction using radioisotope 32p under aerobic conditions. Ku et al. (1978), based on studies of two lake sediments, indicated that the adsorption capacity was dependent on the redox potential. However, their experimental results were obtained at a low pH of 4.8 in a reduced system. Further study in a region of higher pH is needed for evaluating the effect of redox level quantitatively. In the field of soil science, some information on phosphorus adsorption kinetics under anaerobic conditions (Patrick and Khalid, 1974; Holford and Patrick, 1979) was reported. However the information on soil is not necessarily relevant to lake sediments because of the differences in the character677
678
H, FURU~tAI and S. OHGAKI Table 1. Characteristics of lake sediments Date 2 August 1983 Water content* 77.6 (%) TotaI-P 1.70 [mg-Pg (dry) ~] Ignition loss 18.5 (%) Temp. in sediments 25 ('C) Fet 33 [mg g (dry)-i ] AI$ 50 [mg g (dry) t ] Ca~" 9.4 [mg g (dry) ]] *Data of centrifuged sediments. tData of sediments collected on 5 Nov. 1982.
istics between soils a n d sediments a n d in the high c o n c e n t r a t i o n range in the liquid phase. The purpose o f this study is (i) to investigate the effect o f p H o n the a m o u n t o f exchangeable phosp h o r u s which was determined using a radiochemical m e t h o d u n d e r a n a e r o b i c conditions; a n d (ii) to quantify the effect o f redox level on the p h o s p h o r u s exchange between sediments a n d water, by comparing the relationships between the a m o u n t o f exchangeable p h o s p h o r u s in the solid phase a n d p h o s p h o r u s c o n c e n t r a t i o n in the liquid phase u n d e r aerobic a n d a n a e r o b i c conditions. MATERIALS AND METHODS
Materials Sediment samples were taken from Lake Kasumigaura, Japan, using a core sampler (2 August 1983). The pretreatment of the collected sediments for experiments was described by Furumai et al. (1989). The characteristics of the sediments are shown in Table I. Figure 1 shows the phosphorus concentration in the interstitial water and water content at the sampling point. The water content in the top l cm layer was 90% (water content ratio = 900%) and sediments were slurry-like. The phosphorus concentration in the interstitial water ranged from 0.3 to 0.5 mg-P l-L This concentration corresponds to 0.003-0.005 mg-P g(dry)-I of sediment phosphorus. There was a gradual increase between
Water Content (%)
0
0
70
80
90 100
Phosphorus (mg-PL -1 ) 0 0.2 0.4 0.6 0.8
Phosphorus (mg-Pgdry-I ) 0.5
1.0
1.5
I~I¢~i~iiii
~/IIIIIJ'IIIIIIIA '
P'/////////~
~////////~ ~ b: NaOH-o-P
il}iiil
e- ,,o.-(,-oI-P d: HCl-o-P
20
e: HCI-(T-o)-P
~
"~ii!ill
f:
Residual-P
August 2, 1983
Fig. 2. Phosphorus fraction profiles of sediments by depth.
1-Scm depth. The interstitial water was obtained by centrifuging immediately after slicing the sediment core. The profile of the phosphorus fraction in sediments over the depth was obtained by a phosphorus fractionation method (Furumai and Ohgaki, 1982). The results are shown in Fig. 2. They indicate that the surface layer contained more phosphorus and the largest fraction was NaOH-o-P, which corresponds to phosphorus associated with iron (Furumai and Ohgaki, 1982). The HCl-o-fraction, which corresponds to phosphorus associated with calcium (Furumai and Ohgaki, 1982), was less than 10% of total phosphorus,
Experimental procedure Phosphorus adsorption~lesorption under anaerobic conditions was investigated using glass vials of 120 ml volume (Fig. 3). The experiments were conducted in two stages. In the first stage, phosphorus release from sediments was investigated by putting 50 mg glucose and a 100 ml mixture of sediments and distilled water into bottles. The mixture contained sediments of 1 g wet wt (0.244gs dry wt). The addition of glucose (500 mg 1- L) simulated anaerobiosis and decreased the oxidation-reduction potential (ORP). Then the mixture was bubbled with N 2 gas to purge air in the bottles. The bottles were sealed with rubber caps and aluminum seals. The bottles were then incubated in a temperature-controlled vessel at 25 _+ I°C. The mixture was sampled with a syringe and anaerobically filtered through a 8.0 ym membrane filter (Fig. 3) and ortho-phosphate concentration in the filtrate was determined by the ascorbic acid Glass Vial
I
~ W.C.
OIAume 120mL luminum Seal Silicon Cap
f
!
ort
I f ! ! i ! !
10
l,
Press
Holder~
Total -P
S
ZO Fig. 1. Water content and phosphorus concentration in interstitial water.
Membrane ~qTI-
Filter~
Filtrate
Fig. 3. Experimental apparatus and procedure. Gas phase in a filter holder was replaced with N 2 gas before filtration.
Adsorption~iesorption of phosphorus by sediments method (APHA, 1975). The reasons why a 8.0k~m membrane filter was used for filtration were that it made filtration time much shorter than a 0.45 pm filter and that the differences in ortho-phosphate concentrations of several filtrates through a filter between 8.0 and 0.45 #m were negligible. In the second stage, the phosphorus exchange reaction was investigated using a carrier-free 32p radioisotope. After the phosphate concentration in the liquid phase of the phosphorus release experiments reached equilibrium, the pH of the mixture was adjusted by the addition of 1N NaOH. The added amounts were 0, 0.25, 0.5, 0.75 and 1.0ml per 100ml mixture. Phosphorus adsorptiondesorption equilibrations were developed at different pH values for measurement of exchangeable phosphorus in the sediments. The range of pH at equilibrium was from 4.7 to 9.8. In addition to the pH adjustment, a small volume of the concentrated phosphate (KH2PO4) solution was added to some of the bottles in order to develop new equilibrations at various phosphate concentrations to investigateexchange kinetics. After the ortho-phosphate concentration reached equilibrium at a new pH, about 1.85 x 10 Bq ml ~radioisotope 32p (0.1qL2ml as volume, 1.75 × 10-9 mgl ~ as concentration) were added to the bottles. Methods of measuring 32p concentration and of estimating the amount of exchangeable phosphorus are described by Furumai and Ohgaki (1989). R E S U L T S AND D I S C U S S I O N
679
'7
g l.O
Anaerobic •
•
~0.1
*
=
Aerobic i
i
i
pH
i
(-)
Fig. 5. Relationship between the equilibrium phosphorus concentration and pH. Data under aerobic conditions are quoted from a paper by Furumai and Ohgaki (1988).
vessels by Furumai et al. (1989) and Furumai and Ohgaki (1988). The concentrations under anaerobic conditions were about 5-10 times higher than those under aerobic conditions. An increase in pH accelerated dissolution of the phosphorus compounds in the sediments. The amount of released phosphorus ranged from 11.4 to 14.5% of total phosphorus in the sediments for pHs from 4.7 to 7.4.
Phosphorus concentration at equilibrium
Amount of exchangeable phosphorus
Figure 4 shows the changes of phosphorus concentrations, pH and ORP with time in a representative experiment before pH adjustment. ORP dropped rapidly and phosphorus was released from the sediments with time. The addition of glucose brought a drop of pH and induced anaerobiosis. After 15 days of incubation the closed system almost reached its equilibrium with respect to pH, ORP and
After addition of radioisotope 32p to the sediments-water batch system, the changes in the ratio of *C to the initial value *Co with time and orthophosphate concentration profile are shown in Fig. 6. The rapid transfer of 32p from solution to the sediments which occurred was observed within 1 min. There was a decrease in the ortho-phosphate concentration, which had been constant before the first sampling time except in the experiment where the pH was held at 9.8. Numerous samplings were conducted so as to ascertain when the 32p count reached a constant level. Thus, an air contamination, caused by numerous samplings with injection of N2 gas, possibly disturbed the steady state of the phosphate concentration. At a high pH such as 9.8, however, the air contamination might hardly affect phosphorus exchange. Judging from the results of the pH = 9.8 experiment, the 32p exchange reaction reaches a steady state within 30min under anaerobic conditions. Both physicochemical adsorption~lesorption of phosphorus and biological uptake-release might take part in the phosphorus exchange in this system where no inhibitor for bacteria was added. However, the 32p count rapidly reached the steady state within 30 min, and after that no decrease in the 32p count was observed. This indicated that the phosphorus exchange was mainly caused by a physicochemical reaction. Similar experiments were conducted for measuring constant 32p level with just two samplings (30 and 60 min) in order to avoid possible air contamination. The results are shown in Table 2. The phosphate concentrations at 30 and 60 min after 32p addition
ortho-phosphate. The pH of the mixture was adjusted by addition of
1N NaOH, resulting in the phosphorus concentration and pH reaching another constant level within 1 day. Figure 5 shows the relationship between the equilibrium of the phosphorus concentrations and pH. The data obtained under aerobic conditions, are also plotted alongside for comparison. The data under aerobic conditions were obtained using open glass
pH ortho-P
ORP
ORP
+200 +_0
5 - 0.2
-200
• llo 2*0 Time (day)
Fig. 4. Change of phosphorus concentration with time in the phosphorus release experiment before pH adjustment. WR 23/6--B
680
H. FURUMAIand S. OHGAK1
*C/*
ortho-P c / c o mgL- I )l O0 pH = 5.7
C0
100{ (%)
4
ortho-P (mgL- I ) pH = 6.6
(%11
0.4
i 50 0.2
,2
0
I
!
30 Time (min)
60
0
0
30 Time (min)
ortho-P *C/ *C 0 [mgL-1 )1(~0
* C / * C0
1009 _ (%) ~ V ~
ortho-P [mgL-! )
pH = 9.8
pH = 7.5
k
).4
ortho-P>
~
60
~_~
m~r-
m
1.5
~
ortho-P
5 -
50 ~
n
1.0
0.2 0.5
o I 0
m 30
l'I
Time
I
0
150
(min)
0
0
I 30 Time (min)
I 60
0
Fig. 6. Adsorption of 32p from the liquid phase to the solid phase with time at various pHs. *C = cpm of 32p in the liquid phase; *CO= cpm of 32p in the liquid phase at t = 0.
were almost the same. There was little likelihood of air contamination and there is less than 4% variation in the ortho-phosphate concentrations and 32p counts in the two samples. The average was used for estimation of exchangeable phosphorus. The results are previously shown in Table 2. The total exchangeable phosphorus at various pH levels under both aerobic and anaerobic conditions are shown in Fig. 7. The exchangeable phosphorus under anaerobic conditions ranged from
27 to 49% of the total phosphorus in the sediments for pH in the range of 4.7-9.8. In the wide pH range the amounts of exchangeable phosphorus were much higher under anaerobic conditions than under aerobic conditions. The difference between them was more noticeable at a pH less than 8. This indicates that the anaerobic conditions cause a large increase in exchangeable phosphorus especially at a pH less than 8. The amount of exchangeable phosphorus plays a major role in phosphorus release to overlying water in lakes in response to changes in both pH and redox level.
Exchangeable-P [%) 1O0
Total-P
0
50 I
/
4.7 L\\\\\\\\\\~
Aer o b I c
5.0 ---1
I J Anaerobic k\\\\\\\\\\~
5.7 ~\\\\\\\\\~1
Increase in exchangeable phosphorus by phosphate addition. Experiments with phosphorus addition were
6.0----7 "7" 6 . 6 x\\\\\\\\\\\'q
7.0-1
~- 7.5 ~',\\\\\\\\\\\'~ 8.0-q 9.0 - - ] 9.8
~\\\\\\\\\\\\
\ \\\\\\x]
10.0
I i
i
Exchange kinetics under anaerobic conditions
i
0.5 1.0 1.5 Sediments Phosphorus (mg-Pgdry -1)
Fig. 7. Exchangeable phosphorus in sediments at various pHs. Data under aerobic conditions are quoted from a paper by Furumai et al. (1989).
conducted to examine the phosphorus exchange kinetics. Various amounts of phosphorus were added to the sediments-water system and then the pH of the sYstem was adjusted. Each steady state at each pH level was attained with respect to ortho-phosphate within 1 day. Then 3~p was added and its concentration was measured 30 and 60 min after addition. The equilibrium of the ortho-phosphate concentration and 32P in the liquid phase at pHs 5.5 and 6.6 is shown in Table 3. The amount of exchangeable phosphorus in the solid phase increased with increasing equilibrium of the ortho-phosphate concentration at both pH levels. Unfortunately, data with phosphorus concentrations lower than 0.4 mg 1- ~were not
A d s o r p t i o n - d e s o r p t i o n o f p h o s p h o r u s by sediments
681
Table 2. Exchangeable phosphorus in sediments without phosphorus addition
ortho-P [mg l- ']
*C /*Co [%]
pH [--]
After 30 rain
After 60 rain
After 30 rain
After 60 rain
4.7
0,520
0.523
53.5
45.2
5.7
0.541
0.564
46.5
47.8
6.6
0.430
0.441
36.3
32.9
7.4
0.514
0.536
44.0
41.0
9.8
1.365
1.349
71.8
73.4
Cc/SS [mg g ' ] (%)
X, [mg g- t ] (%)
Total [mg g- ' ] (%)
0.233 (13.7) 0.246 (14.5) 0.194 (11.4) 0.234 (13.8) 0.605 (35.6)
0.242 (14.2) 0.276 (16.2) 0.369 (21.7) 0.318 (18.7) 0.229 (13.5)
0.474 (27.9) 0.522 (30.7) 0.563 (33.1) 0.552 (32.5) 0.834 (49.0)
Values in parentheses indicate the percentage of total sediments phosphorus. Table 3. Increase in exchangeable-P by phosphorus addition under anaerobic conditions
co
*Cl*Co
(mg I -t) pH
(--) 5.7
6.6
Ac ffi ( c o -
(%)
AC + AX
P added (mgl i)
After 30min
After 60rain
After 30rain
After 60rain
X¢ (mgg -I)
AC + AX (mgg i)
No 0.5 1.25 1.5
0.541 0.855 1.274 1.330
0.564 0.849 1.251 1,318
38.0 53.7 55.4 62.0
39.0 51.5 55.5 57.2
0.276 0.356 0.447 0,439
--
--
0,212 0.482 0.504
95.1 86.5 75.3
No 0.5 1.0 1.5
0.430 0.626 0.978 1.285
0.441 0.687 0.939 1,240
36.3 38.7 46.8 52.8
32.9 38.2 41.5 51.0
0.369 0.469 0.543 0.530
-0.198 0.407 0.520
-88.8 91.2 77.7
AP (%)
c®)/ss [ms s(dry)-'].
SS = sediments content = 2.245 [g(dry)l-I]. ~,x = xo - x=o [mgs(dry)-'].
AP = added P/SS [rag g(dry)- J]. Coo at pH = 5.7 = (0.541 + 0.564)/2 = 0.553 [rag 1-~]; pH = 6.6 = (0.430 + 0.441)/2 = 0.436 [mg I - ~]. Xe0 at pH = 5.7 = 0.276 [rag g- ~]; pH = 6.6 = 0.369 [mg g-'].
obtained by this procedure because the phosphorus concentrations in the control experiments were higher than 0.4 mg 1-1. Comparing the data obtained at different pHs of 5.7 and 6.6, pH increase causes an increase in exchangeable phosphorus in the solid phase under anaerobic conditions. Discussion on the inactivation of exchangeable phosphorus will be based on the data in the last two
columns of Table 3. (AC + AX) refers to the sum of the exchangeable phosphorus in the solid phase and the phosphorus in the liquid phase exceeding those in the control experiment. The last column indicates the recovery of phosphorus added as the sum of exchangeable phosphorus in the solid phase and phosphorus in the liquid phase. The recovery decreased with increasing phosphorus as shown in
pH = 5.7
~
pH = 6.6
~ ,~'" 1.0
/
/'
Inactivated
inactivated
/"
,/
.
ed
o
II 0 O.S 1.251.5 Added Phosphorus
(mg-PL -1 )
entS
il
0 0.5 1.0 1.5 Added Phosphorus 1
(mg-PL")
Fig. 8. Increase in exchangeable phosphorus by phosphorus addition.
H. FURUMAIand S. OHGAKI
682
liquid phase and exchangeable phosphorus in the solid phase, respectively. From non-linear regression of equation (l) with respect to X e and Ce, the two parameters, bonding energy (b) and adsorption maximum (Xm), were obtained at each pH under anaerobic conditions. The corresponding results are shown in Table 4. The Langmuir regression curves are given in Fig. 9. The bonding energy constants under aerobic and anaerobic conditions were from 4.6 to 13.0 and from Fig. 8. When 1.5mgl I phosphorus were added, 1.2 to 2.3 (l mg -1 ) in the range of slightly acidic and about 25% phosphorus was not recovered as phos- neutral conditions, respectively, as shown in Table 4. phorus corresponding to exchangeable phosphorus at The values were much smaller under anaerobic conboth pH levels. This fact implies that some part of the ditions than aerobic conditions. The adsorption adsorbed phosphorus is converted to inactive phos- maxima were higher under anaerobic conditions. phorus and this amount is found to increase with This result is rather controversial because the adsorption reaction has been assumed to slacken under increasing addition of phosphorus. anaerobic conditions. Application o f the Langmuir equilibrium model. The increase in adsorption maxima could probably Ku et al. (1978) showed the adequacy of the Langmuir model in describing phosphate equilibria in two be explained by the greater surface area of gel-like lake sediments. The adsorbed phosphorus was, how- reduced ferrous compounds which result in a greater ever, calculated by adding the newly adsorbed phos- adsorption capacity, as reported with respect to phorus to the native exchangeable phosphate anaerobic soils (Patrick and Khalid, 1974). When the (Surface-P) as measured by 32p. It seems that the bonding energy is decreased, even as the adsorption adsorbed phosphorus is an overestimation of the maximum is increased under anaerobic conditions, phosphorus in the solid phase which is exchangeable more phosphorus in the solid phase is released to the to phosphate in the liquid phase for the reason liquid phase under anaerobic conditions than under mentioned above. In this paper, the relationship aerobic conditions as shown in Fig. 9. The increase in pH from 5.7 to 6.6 caused an between the equilibrium phosphorus concentration and the amount of "exchangeable" phosphorus in the increase in the bonding energy constant but this had solid phase is described by the Langmuir equilibrium only a small influence on the adsorption maximum under anaerobic conditions. model. The relationship between the phosphorus concenThe Langmuir model is given by the following tration in the liquid phase and the amounts of equation: exchangeable phoshorus in the solid phase are XmbCe described well by the Langmuir equilibrium model. Xe = - (1) l+bCe The two parameters prove to be useful for evaluating the effects of redox level and pH on the exchange Xe = adsorbed phosphorus (mg-P g - l ) reaction. The results show that these parameters are X m = adsorption maximum (mg-P g - l) well suited for estimation of phosphorus concenb = bonding energy (1 mg-P- l ) tration changes near the sediments surface caused by C, = equilibrium concentration (mg-P 1- l ). pH change under anaerobic conditions. Here Ce and Xe correspond to dissolved-P in the Acknowledgements--We thank Dr Morioka and Miss Yuko Tanaka, Isotope Center, The University of Tokyo, for their advice and help. We would also like to thank the FreshWater Fishery Research Institute, Ibaragi Prefecture, Japan, ........ Aerobic for assisting in sampling. Anaerobic Table 4. Parametersof the Langmuirmodel Anaerobic Aerobic pH 5.7 6.6 5 6 7 (--) Xm 0.813 0.716 0.611 0.397 0.409 (mgg-I) b 0.924 2.67 8.33 13.0 4.61 (ling-1) Data under aerobicconditionsare quoted from a paper by Furumai et al. (1989).
REFERENCES
.5 ~ ° ' 6 I
I
to. l
i ol.
i
|
I
1.0 1.5 Phosphorus Cone. (mg-pL-1 ) 0.5
Fig. 9. Regression curves of phosphorus adsorption isotherms by the Langmuir model. Data under aerobic conditions are quoted from a paper by Furumai et al. (1989).
APHA (1975) Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C. Furumai H. and Ohgaki S. (1982) Fractional composition of phosphorus forms in sediments related to release. War. Sci. Technol. 14, 215-226. Furumai H. and Ohgaki S. (1988) Radiochemical analysis of phosphorus exchange kinetics between sediments and water under aerobic conditions. J. envir. Qual. 17, 205-212. Furumai H., Kondo T. and Ohgaki S. (1989) Phosphorus exchange kinetics and exchangeable phosphorus forms in sediments. Wat. Res. 23, 685-691.
Adsorption--desorption of phosphorus by sediments Gunatilaka A. (1982) Phosphate adsorption kinetics of resuspended sediments in a shallow lake, Heusiedlersee, Austria. Hydrobiologia 91, 293-298. Holford I. C. R. and Patrick W. H. Jr (1979) Effects of reduction and pH changes on phosphate sorption and mobility in acid soil. Soil Sci. Soc. Am. J. 43, 292-297. Hosomi M. and Sudo R. (1987) A model of phosphorus release from lake sediments. Proc. envir, sanit. Engng Res. 23, 15-28 (in Japanese). Ku A. C., DiGianol F. A. and Feng T. H. (1978) Factors affecting phosphate adsorption equilibria in lake sediments. Wat. Res. 12, 1069-1074. Lennox L. J. (1984) Sediment-water exchange in Lough
683
Ennell with particular reference to phosphorus. Wat. Res. 18, 1483-1485. Li W. C., Armstrong D. E., Williams J. D. H., Harris R. F. and Syers J. K. (1972) Rate and extent of inorganic phosphate exchange in lake sediments. Soil Sci. Soc. Am. Proc. 36, 279-285. Patrick W. H. Jr and Khalid R. A. (1974) Phosphate release and sorption by soils and sediments: Effect of aerobic and anaerobic conditions. Science 186, 53-55. Pomeroy L. R., Smith E. E. and Grant C. M. (1965) The exchange of phosphate between estuarine water and sediments. LimnoL Oceanogr. 10, 167-172.
0043-1354/89 $3.00+ 0.00 Copyright © 1989MaxwellPergamonMacmillanpie
ADSORPTION-DESORPTION OF PHOSPHORUS BY LAKE SEDIMENTS U N D E R ANAEROBIC CONDITIONS H. FURUMAI*@ and S. OHGAKI@ Department of Urban Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan (First received December 1987; accepted in revised form January 1989) Abstraet--Adsorption~iesorption reactions of phosphorus by lake sediments were studied using a radioisotope 32p inorganic phosphate. The sediments were collected at Lake Kasumigaura, Japan. The amount of isotopically exchangeable phosphorus in the sediments was measured in anaerobic sediments-water batch systems under various pH and phosphorus concentrations. The steady state condition practically prevailed with respect to 32p within 30min and the amount of exchangeable phosphorus in the sediments was calculated based on the steady state data. The results were compared with data previously obtained under aerobic conditions. The anaerobic conditions caused a large phosphorus release and an increase in exchangeable phosphorus especiallybelow pH 8. The exchangeable phosphorus under anaerobic conditions ranged from 27 to 49% of the total phosphorus in the sediments of Lake Kasumigaura, respectively, for pH in the range of 5-10. The relationship between the phosphorus concentration in the liquid phase and the amount of exchangeable phosphorus in the solid phase under anaerobic conditions can be explained by the Langmuir equilibrium model as well as under aerobic conditions. The bonding energy constant and adsorption maximum proved to be useful parameters for evaluating the effect of redox level and pH on the exchange reaction. The bonding energy constants under aerobic and anaerobic conditions ranged from 4.6 to 13.0, and from 1.2 to 2.3 (1mg -~ ) for pH in the range of 6-7, respectively.The adsorption maxima were higher under anaerobic conditions than aerobic conditions. For pH between 5.7 and 6.6, the increase in pH caused an increase in the bonding energy constant, but had only a slight influenceon the adsorption maximum under anaerobic conditions. Key words--phosphorus exchange, adsorption~desorption, release, lake sediments, radioisotope phosphate, exchangeable phosphorus, Langmuir model, anaerobic conditions, redox level
INTRODUCTION
Sediments play a big role in eutrophication in closed water-bodies such as lakes and bays. Lennox (1984) reported that it required a long period of time to improve the eutrophic status of Lough Ennell because of internal loading of phosphorus from sediments following the control of external loadings. Hosomi and Sudo (1987) pointed out the importance of phosphorus flux from sediments using their multicomponent dynamic model of phosphorus in the sediments-water system of Lake Kasumigaura. Phosphorus has accumulated on the surface of the sediments in the eutrophic Lake Kasumigaura and its main phosphorus fraction is NaOH-extratahle ortho-phosphorus (Furumai and Ohgaki, 1982). The phosphorus exchange between sediments and water is important for quantitatively evaluating the potential for influencing the phosphorus concentration in the overlying water. The exchangeable phosphorus, especially that on the surface of the sediments, is the easily-released-fraction and has an important effect *Present address: Department of Civil Engineering Hydraulics, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812, Japan.
on the phosphorus concentration during the short term such as hours or a day. Many researchers have investigated phosphorus adsorption by sediments and soils under aerobic conditions and phosphorus release from sediments under anaerobic conditions. The phosphorus behavior depends on phosphorus concentration, pH and the redox level in the overlying and interstitial water. Adsorption and release are not irreversible reactions and should not be considered separately either. With respect to phosphorus adsorption, a few reserchers (Pomery et al., 1965; Li et al., 1972; Ku et al., 1978; Gunatilaka, 1982) have studied the exchange reaction using radioisotope 32p under aerobic conditions. Ku et al. (1978), based on studies of two lake sediments, indicated that the adsorption capacity was dependent on the redox potential. However, their experimental results were obtained at a low pH of 4.8 in a reduced system. Further study in a region of higher pH is needed for evaluating the effect of redox level quantitatively. In the field of soil science, some information on phosphorus adsorption kinetics under anaerobic conditions (Patrick and Khalid, 1974; Holford and Patrick, 1979) was reported. However the information on soil is not necessarily relevant to lake sediments because of the differences in the character677
678
H, FURU~tAI and S. OHGAKI Table 1. Characteristics of lake sediments Date 2 August 1983 Water content* 77.6 (%) TotaI-P 1.70 [mg-Pg (dry) ~] Ignition loss 18.5 (%) Temp. in sediments 25 ('C) Fet 33 [mg g (dry)-i ] AI$ 50 [mg g (dry) t ] Ca~" 9.4 [mg g (dry) ]] *Data of centrifuged sediments. tData of sediments collected on 5 Nov. 1982.
istics between soils a n d sediments a n d in the high c o n c e n t r a t i o n range in the liquid phase. The purpose o f this study is (i) to investigate the effect o f p H o n the a m o u n t o f exchangeable phosp h o r u s which was determined using a radiochemical m e t h o d u n d e r a n a e r o b i c conditions; a n d (ii) to quantify the effect o f redox level on the p h o s p h o r u s exchange between sediments a n d water, by comparing the relationships between the a m o u n t o f exchangeable p h o s p h o r u s in the solid phase a n d p h o s p h o r u s c o n c e n t r a t i o n in the liquid phase u n d e r aerobic a n d a n a e r o b i c conditions. MATERIALS AND METHODS
Materials Sediment samples were taken from Lake Kasumigaura, Japan, using a core sampler (2 August 1983). The pretreatment of the collected sediments for experiments was described by Furumai et al. (1989). The characteristics of the sediments are shown in Table I. Figure 1 shows the phosphorus concentration in the interstitial water and water content at the sampling point. The water content in the top l cm layer was 90% (water content ratio = 900%) and sediments were slurry-like. The phosphorus concentration in the interstitial water ranged from 0.3 to 0.5 mg-P l-L This concentration corresponds to 0.003-0.005 mg-P g(dry)-I of sediment phosphorus. There was a gradual increase between
Water Content (%)
0
0
70
80
90 100
Phosphorus (mg-PL -1 ) 0 0.2 0.4 0.6 0.8
Phosphorus (mg-Pgdry-I ) 0.5
1.0
1.5
I~I¢~i~iiii
~/IIIIIJ'IIIIIIIA '
P'/////////~
~////////~ ~ b: NaOH-o-P
il}iiil
e- ,,o.-(,-oI-P d: HCl-o-P
20
e: HCI-(T-o)-P
~
"~ii!ill
f:
Residual-P
August 2, 1983
Fig. 2. Phosphorus fraction profiles of sediments by depth.
1-Scm depth. The interstitial water was obtained by centrifuging immediately after slicing the sediment core. The profile of the phosphorus fraction in sediments over the depth was obtained by a phosphorus fractionation method (Furumai and Ohgaki, 1982). The results are shown in Fig. 2. They indicate that the surface layer contained more phosphorus and the largest fraction was NaOH-o-P, which corresponds to phosphorus associated with iron (Furumai and Ohgaki, 1982). The HCl-o-fraction, which corresponds to phosphorus associated with calcium (Furumai and Ohgaki, 1982), was less than 10% of total phosphorus,
Experimental procedure Phosphorus adsorption~lesorption under anaerobic conditions was investigated using glass vials of 120 ml volume (Fig. 3). The experiments were conducted in two stages. In the first stage, phosphorus release from sediments was investigated by putting 50 mg glucose and a 100 ml mixture of sediments and distilled water into bottles. The mixture contained sediments of 1 g wet wt (0.244gs dry wt). The addition of glucose (500 mg 1- L) simulated anaerobiosis and decreased the oxidation-reduction potential (ORP). Then the mixture was bubbled with N 2 gas to purge air in the bottles. The bottles were sealed with rubber caps and aluminum seals. The bottles were then incubated in a temperature-controlled vessel at 25 _+ I°C. The mixture was sampled with a syringe and anaerobically filtered through a 8.0 ym membrane filter (Fig. 3) and ortho-phosphate concentration in the filtrate was determined by the ascorbic acid Glass Vial
I
~ W.C.
OIAume 120mL luminum Seal Silicon Cap
f
!
ort
I f ! ! i ! !
10
l,
Press
Holder~
Total -P
S
ZO Fig. 1. Water content and phosphorus concentration in interstitial water.
Membrane ~qTI-
Filter~
Filtrate
Fig. 3. Experimental apparatus and procedure. Gas phase in a filter holder was replaced with N 2 gas before filtration.
Adsorption~iesorption of phosphorus by sediments method (APHA, 1975). The reasons why a 8.0k~m membrane filter was used for filtration were that it made filtration time much shorter than a 0.45 pm filter and that the differences in ortho-phosphate concentrations of several filtrates through a filter between 8.0 and 0.45 #m were negligible. In the second stage, the phosphorus exchange reaction was investigated using a carrier-free 32p radioisotope. After the phosphate concentration in the liquid phase of the phosphorus release experiments reached equilibrium, the pH of the mixture was adjusted by the addition of 1N NaOH. The added amounts were 0, 0.25, 0.5, 0.75 and 1.0ml per 100ml mixture. Phosphorus adsorptiondesorption equilibrations were developed at different pH values for measurement of exchangeable phosphorus in the sediments. The range of pH at equilibrium was from 4.7 to 9.8. In addition to the pH adjustment, a small volume of the concentrated phosphate (KH2PO4) solution was added to some of the bottles in order to develop new equilibrations at various phosphate concentrations to investigateexchange kinetics. After the ortho-phosphate concentration reached equilibrium at a new pH, about 1.85 x 10 Bq ml ~radioisotope 32p (0.1qL2ml as volume, 1.75 × 10-9 mgl ~ as concentration) were added to the bottles. Methods of measuring 32p concentration and of estimating the amount of exchangeable phosphorus are described by Furumai and Ohgaki (1989). R E S U L T S AND D I S C U S S I O N
679
'7
g l.O
Anaerobic •
•
~0.1
*
=
Aerobic i
i
i
pH
i
(-)
Fig. 5. Relationship between the equilibrium phosphorus concentration and pH. Data under aerobic conditions are quoted from a paper by Furumai and Ohgaki (1988).
vessels by Furumai et al. (1989) and Furumai and Ohgaki (1988). The concentrations under anaerobic conditions were about 5-10 times higher than those under aerobic conditions. An increase in pH accelerated dissolution of the phosphorus compounds in the sediments. The amount of released phosphorus ranged from 11.4 to 14.5% of total phosphorus in the sediments for pHs from 4.7 to 7.4.
Phosphorus concentration at equilibrium
Amount of exchangeable phosphorus
Figure 4 shows the changes of phosphorus concentrations, pH and ORP with time in a representative experiment before pH adjustment. ORP dropped rapidly and phosphorus was released from the sediments with time. The addition of glucose brought a drop of pH and induced anaerobiosis. After 15 days of incubation the closed system almost reached its equilibrium with respect to pH, ORP and
After addition of radioisotope 32p to the sediments-water batch system, the changes in the ratio of *C to the initial value *Co with time and orthophosphate concentration profile are shown in Fig. 6. The rapid transfer of 32p from solution to the sediments which occurred was observed within 1 min. There was a decrease in the ortho-phosphate concentration, which had been constant before the first sampling time except in the experiment where the pH was held at 9.8. Numerous samplings were conducted so as to ascertain when the 32p count reached a constant level. Thus, an air contamination, caused by numerous samplings with injection of N2 gas, possibly disturbed the steady state of the phosphate concentration. At a high pH such as 9.8, however, the air contamination might hardly affect phosphorus exchange. Judging from the results of the pH = 9.8 experiment, the 32p exchange reaction reaches a steady state within 30min under anaerobic conditions. Both physicochemical adsorption~lesorption of phosphorus and biological uptake-release might take part in the phosphorus exchange in this system where no inhibitor for bacteria was added. However, the 32p count rapidly reached the steady state within 30 min, and after that no decrease in the 32p count was observed. This indicated that the phosphorus exchange was mainly caused by a physicochemical reaction. Similar experiments were conducted for measuring constant 32p level with just two samplings (30 and 60 min) in order to avoid possible air contamination. The results are shown in Table 2. The phosphate concentrations at 30 and 60 min after 32p addition
ortho-phosphate. The pH of the mixture was adjusted by addition of
1N NaOH, resulting in the phosphorus concentration and pH reaching another constant level within 1 day. Figure 5 shows the relationship between the equilibrium of the phosphorus concentrations and pH. The data obtained under aerobic conditions, are also plotted alongside for comparison. The data under aerobic conditions were obtained using open glass
pH ortho-P
ORP
ORP
+200 +_0
5 - 0.2
-200
• llo 2*0 Time (day)
Fig. 4. Change of phosphorus concentration with time in the phosphorus release experiment before pH adjustment. WR 23/6--B
680
H. FURUMAIand S. OHGAK1
*C/*
ortho-P c / c o mgL- I )l O0 pH = 5.7
C0
100{ (%)
4
ortho-P (mgL- I ) pH = 6.6
(%11
0.4
i 50 0.2
,2
0
I
!
30 Time (min)
60
0
0
30 Time (min)
ortho-P *C/ *C 0 [mgL-1 )1(~0
* C / * C0
1009 _ (%) ~ V ~
ortho-P [mgL-! )
pH = 9.8
pH = 7.5
k
).4
ortho-P>
~
60
~_~
m~r-
m
1.5
~
ortho-P
5 -
50 ~
n
1.0
0.2 0.5
o I 0
m 30
l'I
Time
I
0
150
(min)
0
0
I 30 Time (min)
I 60
0
Fig. 6. Adsorption of 32p from the liquid phase to the solid phase with time at various pHs. *C = cpm of 32p in the liquid phase; *CO= cpm of 32p in the liquid phase at t = 0.
were almost the same. There was little likelihood of air contamination and there is less than 4% variation in the ortho-phosphate concentrations and 32p counts in the two samples. The average was used for estimation of exchangeable phosphorus. The results are previously shown in Table 2. The total exchangeable phosphorus at various pH levels under both aerobic and anaerobic conditions are shown in Fig. 7. The exchangeable phosphorus under anaerobic conditions ranged from
27 to 49% of the total phosphorus in the sediments for pH in the range of 4.7-9.8. In the wide pH range the amounts of exchangeable phosphorus were much higher under anaerobic conditions than under aerobic conditions. The difference between them was more noticeable at a pH less than 8. This indicates that the anaerobic conditions cause a large increase in exchangeable phosphorus especially at a pH less than 8. The amount of exchangeable phosphorus plays a major role in phosphorus release to overlying water in lakes in response to changes in both pH and redox level.
Exchangeable-P [%) 1O0
Total-P
0
50 I
/
4.7 L\\\\\\\\\\~
Aer o b I c
5.0 ---1
I J Anaerobic k\\\\\\\\\\~
5.7 ~\\\\\\\\\~1
Increase in exchangeable phosphorus by phosphate addition. Experiments with phosphorus addition were
6.0----7 "7" 6 . 6 x\\\\\\\\\\\'q
7.0-1
~- 7.5 ~',\\\\\\\\\\\'~ 8.0-q 9.0 - - ] 9.8
~\\\\\\\\\\\\
\ \\\\\\x]
10.0
I i
i
Exchange kinetics under anaerobic conditions
i
0.5 1.0 1.5 Sediments Phosphorus (mg-Pgdry -1)
Fig. 7. Exchangeable phosphorus in sediments at various pHs. Data under aerobic conditions are quoted from a paper by Furumai et al. (1989).
conducted to examine the phosphorus exchange kinetics. Various amounts of phosphorus were added to the sediments-water system and then the pH of the sYstem was adjusted. Each steady state at each pH level was attained with respect to ortho-phosphate within 1 day. Then 3~p was added and its concentration was measured 30 and 60 min after addition. The equilibrium of the ortho-phosphate concentration and 32P in the liquid phase at pHs 5.5 and 6.6 is shown in Table 3. The amount of exchangeable phosphorus in the solid phase increased with increasing equilibrium of the ortho-phosphate concentration at both pH levels. Unfortunately, data with phosphorus concentrations lower than 0.4 mg 1- ~were not
A d s o r p t i o n - d e s o r p t i o n o f p h o s p h o r u s by sediments
681
Table 2. Exchangeable phosphorus in sediments without phosphorus addition
ortho-P [mg l- ']
*C /*Co [%]
pH [--]
After 30 rain
After 60 rain
After 30 rain
After 60 rain
4.7
0,520
0.523
53.5
45.2
5.7
0.541
0.564
46.5
47.8
6.6
0.430
0.441
36.3
32.9
7.4
0.514
0.536
44.0
41.0
9.8
1.365
1.349
71.8
73.4
Cc/SS [mg g ' ] (%)
X, [mg g- t ] (%)
Total [mg g- ' ] (%)
0.233 (13.7) 0.246 (14.5) 0.194 (11.4) 0.234 (13.8) 0.605 (35.6)
0.242 (14.2) 0.276 (16.2) 0.369 (21.7) 0.318 (18.7) 0.229 (13.5)
0.474 (27.9) 0.522 (30.7) 0.563 (33.1) 0.552 (32.5) 0.834 (49.0)
Values in parentheses indicate the percentage of total sediments phosphorus. Table 3. Increase in exchangeable-P by phosphorus addition under anaerobic conditions
co
*Cl*Co
(mg I -t) pH
(--) 5.7
6.6
Ac ffi ( c o -
(%)
AC + AX
P added (mgl i)
After 30min
After 60rain
After 30rain
After 60rain
X¢ (mgg -I)
AC + AX (mgg i)
No 0.5 1.25 1.5
0.541 0.855 1.274 1.330
0.564 0.849 1.251 1,318
38.0 53.7 55.4 62.0
39.0 51.5 55.5 57.2
0.276 0.356 0.447 0,439
--
--
0,212 0.482 0.504
95.1 86.5 75.3
No 0.5 1.0 1.5
0.430 0.626 0.978 1.285
0.441 0.687 0.939 1,240
36.3 38.7 46.8 52.8
32.9 38.2 41.5 51.0
0.369 0.469 0.543 0.530
-0.198 0.407 0.520
-88.8 91.2 77.7
AP (%)
c®)/ss [ms s(dry)-'].
SS = sediments content = 2.245 [g(dry)l-I]. ~,x = xo - x=o [mgs(dry)-'].
AP = added P/SS [rag g(dry)- J]. Coo at pH = 5.7 = (0.541 + 0.564)/2 = 0.553 [rag 1-~]; pH = 6.6 = (0.430 + 0.441)/2 = 0.436 [mg I - ~]. Xe0 at pH = 5.7 = 0.276 [rag g- ~]; pH = 6.6 = 0.369 [mg g-'].
obtained by this procedure because the phosphorus concentrations in the control experiments were higher than 0.4 mg 1-1. Comparing the data obtained at different pHs of 5.7 and 6.6, pH increase causes an increase in exchangeable phosphorus in the solid phase under anaerobic conditions. Discussion on the inactivation of exchangeable phosphorus will be based on the data in the last two
columns of Table 3. (AC + AX) refers to the sum of the exchangeable phosphorus in the solid phase and the phosphorus in the liquid phase exceeding those in the control experiment. The last column indicates the recovery of phosphorus added as the sum of exchangeable phosphorus in the solid phase and phosphorus in the liquid phase. The recovery decreased with increasing phosphorus as shown in
pH = 5.7
~
pH = 6.6
~ ,~'" 1.0
/
/'
Inactivated
inactivated
/"
,/
.
ed
o
II 0 O.S 1.251.5 Added Phosphorus
(mg-PL -1 )
entS
il
0 0.5 1.0 1.5 Added Phosphorus 1
(mg-PL")
Fig. 8. Increase in exchangeable phosphorus by phosphorus addition.
H. FURUMAIand S. OHGAKI
682
liquid phase and exchangeable phosphorus in the solid phase, respectively. From non-linear regression of equation (l) with respect to X e and Ce, the two parameters, bonding energy (b) and adsorption maximum (Xm), were obtained at each pH under anaerobic conditions. The corresponding results are shown in Table 4. The Langmuir regression curves are given in Fig. 9. The bonding energy constants under aerobic and anaerobic conditions were from 4.6 to 13.0 and from Fig. 8. When 1.5mgl I phosphorus were added, 1.2 to 2.3 (l mg -1 ) in the range of slightly acidic and about 25% phosphorus was not recovered as phos- neutral conditions, respectively, as shown in Table 4. phorus corresponding to exchangeable phosphorus at The values were much smaller under anaerobic conboth pH levels. This fact implies that some part of the ditions than aerobic conditions. The adsorption adsorbed phosphorus is converted to inactive phos- maxima were higher under anaerobic conditions. phorus and this amount is found to increase with This result is rather controversial because the adsorption reaction has been assumed to slacken under increasing addition of phosphorus. anaerobic conditions. Application o f the Langmuir equilibrium model. The increase in adsorption maxima could probably Ku et al. (1978) showed the adequacy of the Langmuir model in describing phosphate equilibria in two be explained by the greater surface area of gel-like lake sediments. The adsorbed phosphorus was, how- reduced ferrous compounds which result in a greater ever, calculated by adding the newly adsorbed phos- adsorption capacity, as reported with respect to phorus to the native exchangeable phosphate anaerobic soils (Patrick and Khalid, 1974). When the (Surface-P) as measured by 32p. It seems that the bonding energy is decreased, even as the adsorption adsorbed phosphorus is an overestimation of the maximum is increased under anaerobic conditions, phosphorus in the solid phase which is exchangeable more phosphorus in the solid phase is released to the to phosphate in the liquid phase for the reason liquid phase under anaerobic conditions than under mentioned above. In this paper, the relationship aerobic conditions as shown in Fig. 9. The increase in pH from 5.7 to 6.6 caused an between the equilibrium phosphorus concentration and the amount of "exchangeable" phosphorus in the increase in the bonding energy constant but this had solid phase is described by the Langmuir equilibrium only a small influence on the adsorption maximum under anaerobic conditions. model. The relationship between the phosphorus concenThe Langmuir model is given by the following tration in the liquid phase and the amounts of equation: exchangeable phoshorus in the solid phase are XmbCe described well by the Langmuir equilibrium model. Xe = - (1) l+bCe The two parameters prove to be useful for evaluating the effects of redox level and pH on the exchange Xe = adsorbed phosphorus (mg-P g - l ) reaction. The results show that these parameters are X m = adsorption maximum (mg-P g - l) well suited for estimation of phosphorus concenb = bonding energy (1 mg-P- l ) tration changes near the sediments surface caused by C, = equilibrium concentration (mg-P 1- l ). pH change under anaerobic conditions. Here Ce and Xe correspond to dissolved-P in the Acknowledgements--We thank Dr Morioka and Miss Yuko Tanaka, Isotope Center, The University of Tokyo, for their advice and help. We would also like to thank the FreshWater Fishery Research Institute, Ibaragi Prefecture, Japan, ........ Aerobic for assisting in sampling. Anaerobic Table 4. Parametersof the Langmuirmodel Anaerobic Aerobic pH 5.7 6.6 5 6 7 (--) Xm 0.813 0.716 0.611 0.397 0.409 (mgg-I) b 0.924 2.67 8.33 13.0 4.61 (ling-1) Data under aerobicconditionsare quoted from a paper by Furumai et al. (1989).
REFERENCES
.5 ~ ° ' 6 I
I
to. l
i ol.
i
|
I
1.0 1.5 Phosphorus Cone. (mg-pL-1 ) 0.5
Fig. 9. Regression curves of phosphorus adsorption isotherms by the Langmuir model. Data under aerobic conditions are quoted from a paper by Furumai et al. (1989).
APHA (1975) Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C. Furumai H. and Ohgaki S. (1982) Fractional composition of phosphorus forms in sediments related to release. War. Sci. Technol. 14, 215-226. Furumai H. and Ohgaki S. (1988) Radiochemical analysis of phosphorus exchange kinetics between sediments and water under aerobic conditions. J. envir. Qual. 17, 205-212. Furumai H., Kondo T. and Ohgaki S. (1989) Phosphorus exchange kinetics and exchangeable phosphorus forms in sediments. Wat. Res. 23, 685-691.
Adsorption--desorption of phosphorus by sediments Gunatilaka A. (1982) Phosphate adsorption kinetics of resuspended sediments in a shallow lake, Heusiedlersee, Austria. Hydrobiologia 91, 293-298. Holford I. C. R. and Patrick W. H. Jr (1979) Effects of reduction and pH changes on phosphate sorption and mobility in acid soil. Soil Sci. Soc. Am. J. 43, 292-297. Hosomi M. and Sudo R. (1987) A model of phosphorus release from lake sediments. Proc. envir, sanit. Engng Res. 23, 15-28 (in Japanese). Ku A. C., DiGianol F. A. and Feng T. H. (1978) Factors affecting phosphate adsorption equilibria in lake sediments. Wat. Res. 12, 1069-1074. Lennox L. J. (1984) Sediment-water exchange in Lough
683
Ennell with particular reference to phosphorus. Wat. Res. 18, 1483-1485. Li W. C., Armstrong D. E., Williams J. D. H., Harris R. F. and Syers J. K. (1972) Rate and extent of inorganic phosphate exchange in lake sediments. Soil Sci. Soc. Am. Proc. 36, 279-285. Patrick W. H. Jr and Khalid R. A. (1974) Phosphate release and sorption by soils and sediments: Effect of aerobic and anaerobic conditions. Science 186, 53-55. Pomeroy L. R., Smith E. E. and Grant C. M. (1965) The exchange of phosphate between estuarine water and sediments. LimnoL Oceanogr. 10, 167-172.