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Phosphate adsorption and desorption in a tropical estuary (Maracaibo system)
Marine EnvironmentalResearch4 (1980-81)153-163
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY (MARACAIBO SYSTEM)
D. LOPEZ-HERNANDEZ, T. HERRERA Rt F. ROTONDO Facultad de Ciencias, Laboratorio de Estudio$ Ambientales, I.Z.T., University Central de Venezuela, Venezuela (Received: 1 February, 1980)
ABSTRACT
The Maracaibo Estuary is of particular interest because of its location in the tropics, its large size (over 12,000km2), the economic importance of its underlying hydrocarbon deposits and its increasing rate of polhttion. Sediments f r o m this estuary were studied in order to characterise their capacity to adsorb and desorb phosphorus. Considerable variability in capacity f o r phosphate sorption was found among the seventeen samples studied. This variability can be attributed first to the differences among the samples in total P content and, secondly, to their differences in free iron content. Also, it appears that P retention contributes to the control of P concentration in the water and therefore the sediments serve as a b,tffer in controlling P eutrophication in the estuary.
INTRODUCTION
Soil and sediments are able to remove phosphate from solution. The amount of P removed depends on both environmental and biotic conditions. The following factors, influencing the adsorption-desorption process, have been reported: (1) pH (Muljadi et al., 1966a; L6pez-Hernfindez & Burnham, 1974a; L6pez-Hernfindez et al., 1977b), (2) temperature (Carrit & Goodgal, 1954; Muljadi et al., 1966b; Gardner & Preston, 1973; L6pez-Hernfindez et al., 1977a), (3) 'active' forms of iron, aluminium, calcium and manganese (Williams, 1959; Hatter, 1969; L6pezHernfindez & Burnham, 1973, 1974b,c; Upchurch et al., 1974), (4) time of reaction (Larsen, 1967; Larsen et al., 1965), (5) ionic environment (Wiklander, 1950; Upchurch et al., 1974), (6) redox condition (Khalid et al., 1977) and (7) biological activity, including micro-organism activity and root exudates (Nagarajah et al., 1968; L6pez-Hernfindez et al., 1977b). 153 Marine Environ. Res. 0141-1136/80/0004-0153/$02.50 © Applied Science Publishers Ltd, England, 1980 Printed in Great Britain
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D. LOPEZ-HERN.~NDEZ, T. HERRERA, F. ROTONDO
Soil chemists have discussed at length the role of adsorption and precipitation reactions for P retention in soils. Evidence has been provided for both processes. Currently, the adsorption theory is favoured by most workers. Soils, lake sediments and estuary sediments are similar in many respects, especially in their chemical composition---e.g, abundance of phyllosilicates, iron and aluminium oxides and organic matter (humified or not). The main difference between these systems is in the more anoxic character of lake and estuary sediments, owing to their condition of permanent waterlogging. It appears that phosphate retention in lake sediments is controlled by similar mechanisms to those proposed for P retention in soils (Harter, 1968; Kuo & Lotse, 1974; Li et al., 1972; NorveU, 1974). We have extended this investigation to a tropical estuary, Maracaibo 'lake'. Apart from its tropical setting, the Maracaibo Estuary is of particular interest because of the economic importance of the petroleum deposits under and near it, its large size (12,274 km 2) and its increasing rate of pollution (Battelle Institute, 1974). The principal objective of this research was to characterise the phosphorus adsorption-desorption reactions occurring in the sediments of the Maracaibo Estuary. MATERIALS AND METHODS
Samples of surficial sediments were collected with an Eckmann dredge from seventeen different localities within Maracaibo lake (Fig. 1). The sampling distribution is reasonably representative of Maracaibo lake as a whole. The sediments were collected in June, 1975 and kept in sealed polyethylene bags in a frozen condition. Methods of determining the chemical properties included: pH in a 1:5 soil solution ratio, organic carbon by the Walkley & Black (1934) method, loss on ignition in a platinum crucible according to Peck (1964), total aluminium and phosphorus by Na2CO 3 fusion and colorimetric determination, total iron and manganese by Na2CO 3 fusion and atomic absorption analysis, 'active iron' extracted by dithionite by the method described by Coffin (1963) and with oxalate (MacKeague & Day, 1966). 'Active' manganese and aluminium were also determined in the oxalate extraction solutions.
Phosphate adsorption indices and isotherms Wet sediment equivalent to 5-0 g of dry material was shaken with 100 ml of a solution of 0"025 M KH2PO 4 and 0-02 M KCI for a period of 18 h. The temperature was maintained constant at 20 +_ 1 °C and two drops of chloroform were added to inhibit microbial activity. After the shaking period, the suspension was filtered and centrifuged and the final phosphorus concentration was determined using the Fogg & Wilkinson (1958) method.
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY
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156
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Phosphate adsorption isotherms were obtained for six of the samples following the method used by L6pez-Hern/mdez & Burnham (1974b), with corrections applied for the presence ofwater. For each isotherm, the equilibration time was determined.
Phosphorus desorption indices These were determined by shaking for 20 h a wet equivalent of 1.0 g air-dried nonphosphate-enriched sediment with 100 ml of distilled water and 5-0 g of a chloride resin in an Erlenmeyer flask. At the end of the shaking period the suspensions were filtered through a cloth sieve. The resin was regenerated by treatment with 50 m i - NaCI for 3 h, and the phosphate in the eluate was determined by using the blue colorimetric method described by Jackson (1970).
RESULTS AND DISCUSSION
General characteristics of the sediments analysed Physical and chemical properties of the sediment samples are given in Table 1. The pH had a mean value of 6.5, with a range of 5.0-8-5. Most of the samples with a pH above 7-0 were from the western zone of the lake (Fig. 1), where the sediments could be affected by a flux of saline water. Those samples registering lower pH values came from places near the mouths ofinflowing rivers. Loss on ignition and organic carbon were lowest for sample UDCZ-90, classified as a sand. In general the silt and clay sediment samples contained appreciable organic carbon. In total iron concentration, most of the samples clustered around the mean value TABLE 1 GENERALCHARACTERISTICSOF THE SEDIMENTSANALYSED
Sample
pH
Loss in
%C
% Fe
°//oAI
6.5 4.3 2.3 1-3 0.5 2.5 9.1 7-2 8-2 5-3 4.2 8.3 5.7 5-2 10-0 6.5 4-3
6.5 6.4 6-7 17.5 !'4 5.2 5.1 6.6 4.6 4.6 5.0 5-0 5.2 5-3 6-3 6.5 9.5
3.9 4.0 2.0 1.2 1.8 2.0 4.4 4.4 4.8 9.6 18.6 16.8 5.0 15-5 5-5 5.0 4-4
ignition M2 M4 M6 M8 UDCZ-90 La Caleta MI2 M9 MI3 AOI AO2 Bobures MI6 MI7 AB3 AB8 Lladna-114
6'9 5-4 7. i 7"0 7.4 8.5 5.7 7.5 7.7 5.3 5.8 5.0 6.2 6.2 7-6 5.1 6.6
12.7 16'8 9-0 9-0 1"8 13.1 18-2 10-3 17-2 12.4 14.6 18-2 21.2 19.4 18.3 15.9 15"3
Mn (ppm)
P (ppm)
% Clay
6 3 7 19 1 3 15 7 19 2 3 48 16 8 10 4 4
820 670 2650 2320 710 I110 670 1210 980 690 1060 680 840 550 690 2030 2170
23 29 9 0 0 35 35 50 35 19 53 43 39 44 40 38 30
PHOSPHATE ADSORPTIONAND DESORPTIONIN A TROPICALESTUARY
157
TABLE 2 AMOUNTS OF Fe, AI AND MB EXTRACTED WITH THE DIFFERENT REAGENTS AND P SORPTION AND P DF.SORPTION PARAMETERS
Sample
M2 M4 M6 M8 UDCZ-90 La Caleta Ml2 M9 Ml3 AOI AO2 Bobures Ml6 M17 AB3 AB8 Lladna-114
Dit.
%Fe~ Oxal.
0"35 0"86 5.00 5.60 0-64 1.10 0-25 1.10 0.38 0.59 2.00 0"47 3.10 0'15 0'15 2.90 4"90
0.80 1.30 2.70 1.90 0'77 0.61 0.49 1-20 0.84 0.69 1.80 0-66 0.29 0.50 0.44 5.90 5.20
% AP Oxal.
% Mnb Oxal.
1.8 3.7 4.5 1.9 1-5 4.7 1.9 3.3 2.6 0.9 0.8 0'6 !'6 0-6 1.7 2.6 3.2
3-0 21"0 1I'0 6.0 40'0 4-6 9.8 20-0 9-5 29-0 27.0 21.0 9.5 16.0 15-0 28-0 23.0
P sorption" P sorption" P Dried Feet samples desorption samples (ppm) 5-7 12-4 21"8 22-3 5-3 12-4 10.6 17-9 8.5 14.0 35.9 13.3 6.4 7.6 6-0 43.9 46-2
6.2 9.2 37.3 54.7 7.4 14.5 7.4 24.1 10.6 27.8 > 120.0 21-0 8-3 6.4 3.2 68.8 > 120.0
61 9 114 70 27 59 7 136 78 42 40 26 27 30 31 57 79
" ~ Fe extracted from the dried sample. b % AI and Mn extracted with oxalate from AI and Mn total. ' P sorption -- X/logC, where X is amount of P adsorbed, mg P/100g of soil, and C is P cone. in/amol P/litre. (6"3 ~). Only one (UDCZ-90) was low (1.4 ~ ) and two, both located in the northern part of the lake, were high in iron (ca. 9 ~). The samples with the highest value for total aluminium (above 15 ~ ) were those taken from the southern area of the lake. Total manganese was variable, with the lowest values corresponding to those sediments sampled near the shores. The samples were also variable in total phosphorus, ranging from 550 to 2650 ppm. The sediment samples with the highest P content were from the northern part of the estuary, where the shores have their highest human population density. Adsorption o f phosphate in wet and dry sediments Phosphate sorption by sediments showed great variability in both the wet and dry samples (Table 2). For the wet samples, the sorption indices ranged between 3.2 and 120 with a mean of 32.2. For the dry sediments, sorption ranged from 5.6 to 46-2, with a mean corresponding to 17-1. Phosphate adsorption variability could also be ascribed to the great variation amongst the chemical characteristics of the samples, especially those pertaining to the presence of'active' forms of iron (Table 2). Contrasting capacities for adsorption were observed when analysing the whole isotherm. Figure 2 presents the adsorption isotherms (Langmuir representation) for two selected samples. As expected, the
158
D, L6PEZoHERNANDEZ, T. HERRERA, F. ROTONDO 6.0"
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% N 3.0 ol X
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Fig. 2. Phosphateadsorption isotherms using Langmuir representation. isotherms follow the linear model of Langmuir at the lowest P doses but become curvilinear at the highest P equilibrium concentrations (above 150 gg P/ml). In general, when comparing adsorption values between wet and dry sediments, a lower capacity for P retention is indicated for the latter (Table 2). The sediments in the wet--and consequently more reduced and anaerobic condition--displayed a higher P fixing capacity. Similar results have been found by Khalid et al. (1977) when studying flooded soils. However, they studied the phenomenon in a more strict anaerobic condition by not allowing the samples to be exposed to atmospheric 0 2 or to oxygenated solutions throughout their study. In the experiments reported here, although the samples were not in a totally anaerobic condition, they were kept wet at temperatures below freezing to minimise microbial activity. These conditions impeded further oxidation of the 'active' iron and manganese forms. Khalid et al. (1977) found that considerably more amorphous Fe was released by ammonium oxalate solution when conditions were reducing instead of oxidising. This observation was attributed to an increase in surface active sites through the transformation of ferric hydroxide, Fe(OH)3, to ferrous hydroxide, Fe(OH) 2, under reducing conditions. However, L6pez-Hernfindez & Burnham (1974c), in their study of P sorption in organic British soils, found that P sorption capacity was augmented by the higher oxidation forms of iron and manganese and when the samples were dry instead of wet.
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY
159
In the case of flooded soils and lake sediments, drying appears to lower P sorption by blocking the accessibility and thereby reducing the numbers of adsorption sites. Harter (1968) has emphasised this for some temperate lakes. Another mechanism which could be involved in explaining the difference in P retention under aerobic and anaerobic conditions is the transformation of reactive compounds from the amorphous to the crystalline form. Such changes could be accelerated by drying. Amorphous forms are known to retain P in a higher proportion than their crystalline counterparts (Gorbunov et al., 1961; L6pez-Hern~indez & Burnham, 1974a; L6pez-Hernfindez, 1977). Comparison o f P sorption indices
Phosphate adsorption capacities of different materials--e.g, soils, hydrous oxides, clays, sediments, etc.--can be compared by means of their phosphate adsorption isotherms (Fig. 2). However, this technique requires a considerable amount of analytical work. Bache & Williams (1971) have suggested that a single point on the isotherm can be used as a simple phosphate sorption index. The index has been extensively used by L6pez-Hern~mdez and Burnham in order to discriminate P sorption capacities in organic soils and mineral soils from tropical as well as temperate regions (L6pez-Hernfindez & Burnham, 1973, 1974a, c). We now want to extend the use of this method to submerged sediments. L6pez-Hern~ndez & Burnham (1974a, c) found that peaty soils possess a greater variability in P retention capacity (0-120) than mineral soils. An even greater variability has been found for the sediments of Maracaibo lake (2- > 120). It may be noted here that a value of 120 is considerably higher than any value measured in normal agricultural soils (Bache& Williams, 1971 ; L6pez-Hermindez & Burnham, 1974a). The variability in P retention for the sediments analysed may be a consequence of the presence of free iron and could operate as a buffer in controlling P water level. Similar results have been found by Upchurch et al. (1974) in sediments from Pamlico Estuary and by Williams et al. (1971) when working with the sediments of fourteen Wisconsin lakes. The strong association between inorganic P and the iron extracted either by oxalate or citrate-dithionite-bicarbonate suggested the existence of an ~Fe-inorganic P complex'. The correlations reported by Upchurch et al. (1974) for Pamlico Estuary, although high, were below the values given by Williams et al. (1971). The effectiveness of retention and its importance in pollution depend, as noted by Pomeroy et al. (1965), upon 'the exchange capacity of the sediments, the exchange rate between water and sediments, the flushing time of the estuary, and the rapidity of vertical mixing in the water column'. Unfortunately, we did not analyse P water level in the estuary; however, if we make use of the information given by the Battelle Institute (1974) it will be found that the zone located between 9°30'-10°00 ' N and 71 ° 10'-71 °35' W, which is presented in the Battelle report as the area of maximum P
160
D. L O P E Z - H E R N / ~ N D E Z , T. H E R R E R A , F. ROTONDO
solubility, corresponds approximately with the zone (Fig. 1) containing sediments with the lowest P sorption capacities. This last fact emphasises the probable role of sediments in the partial control of P water level in the estuary.
R E L A T I O N S H I P BETWEEN PHOSPHATE A D S O R P T I O N BY S E D I M E N T S - - B O T H W E T A N D DRYwAND
O T H E R PROPERTIES
In studying the relationship between the amount of P sorbed at the Bache and Williams index (1550ppm P) and other soil properties, such as 'active' iron, aluminium and manganese, the simple correlation technique was used. The results presented in Table 3 indicate that the adsorption of added P by the wet TABLE 3 CORRELATIONS BETWEEN P SORPT1ON AND P DESORPTION WITH OTHER SEDIMENT PROPERTIES
pH
C ~/0 clay ~o P F¢ D i t h i o n a t e Fe Oxalate AI O x a l a t e Mn Oxalate P Desorption
P sorption Wet samples
P sorption Dried samples
P desorption
- 0.25 as - 0 . 2 8 as - 0-27 ns 0.82**** 0.71 **** 0.86**** 0.18 as 0-14 ns 0.41 ns
- 0.29 as nd 0.12 ns 0.69*** 0'63*** 0.91 **** 0.20 ns nd nd
0.48* - 0 . 1 5 ns 0-08 as 0.64*** 0.44* 0.35 ns 0-53* -0.18 -
ns = N o t significant. nd= Not determined.
estuary sediments is very closely related to the oxalate iron. This is also true for dithionite extracted iron. Contrary to the findings of several previous studies (Williams et al., 1958; Saini & McLean, 1965; Kaila, 1959; Syers et al., 1971 ; L6pezHernfindez & Burnham, 1973, 1974a, c; Bortleson & Lee, 1974)'active' forms of aluminium and manganese do not contribute appreciably to the sorption processes in the estuary sediments studied. The total P content, on the other hand, is of considerable importance. Williams et ai. (1971), in their study of samples from Wisconsin lakes, also found no clear association between A1 and Mn parameters and extractable inorganic P. However, they reported a strong association between total P and the different iron forms. When analysing the various factors responsible for P adsorption in the dried samples, iron is again the most important in explaining P retention. Aluminium extracted with oxalate does not appear to control P sorption in dried--and
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY
161
consequently more oxidised--sediments. As reported previously, total P is associated with the P sorption characteristic. The experimental results indicate that the property most related to the adsorption process in the estuarine sediments of the Maracaibo System is the oxalateextractable iron. This reagent is known to dissolve amorphous and poorly crystalline oxides of Fe, the forms most responsible for P sorption (MacKeague & Day, 1966). Although Coffin (1963) suggested that dithionite removes "free" iron only from the amorphous fraction in the soil, other authors (Bascomb, 1968: MacKeague & Day, 1966) believe that this reagent extracts iron from the crystalline fraction as well. The results presented in this study (Table 2) demonstrate that the amounts of 'active' iron removed by both reagents are similar. The mean value for iron extracted with dithionite is slightly higher than for Fe-oxalate. However, the LSD values did not reach statistical significance (Herrera, 1977).
Desorption of phosphate and its relationship with other sediment properties The amount of phosphate desorbed with resin (Amberlite-C1) was used as an index for P desorption. This parameter displays considerable variability, ranging between 7 and 136ppm (Table 2). In a previous report for soils L6pez-Hern~ndez & Burnham (1978) found a wide distribution in desorption values for both tropical and temperate soils. A statistical analysis indicated the importance of free iron and organic matter in controlling the desorption process (L6pez-Hern~ndez & Burnham, 1978). For estuary sediments the parameter most associated with the desorption index was found to be total P (0-64***). Although free iron and aluminium extracted with ammonium oxalate also affect P desorption, they are of less importance. The absence of a clear correlation between P desorption and soil properties such as pH, organic matter content and the active forms of both Fe and AI can be attributed to the unsaturated state, with respect to P, of the sediment samples. When sediment samples are previously treated with soluble P prior to extraction, P desorption values correlate well with the other properties mentioned (Caldwell et al., 1971; L6pez-Hern~indez & Burnham, 1978). Phillips (1972) emphasises that estuarine muds are a reservoir of phosphorus but the importance of phosphate exchange across the sediment/water interface in buffering the water column overlying the sediments has only come to be appreciated slowly.
ACKNOWLEDGEMENT
We are indebted to Dr E. Marcucci (Instituto de Canalizaciones) for information, assistance and facilities for sediment sampling. The work has been supported by grants from CONICIT (Proyecto 0460) and Consejo de Desarrollo Cientifico y Humanfstico. Appreciation is expressed to Dr C. P. Burnham (Wye College,
162
D. LOPEZ-HERN.~.NDEZ, T. HERRERA, F. ROTONDO
University of London) for reading the manuscript and offering helpful suggestions. We are also grateful to Mr F. Ramos (Instituto Quimica, U.C.V.) for technical assistance.
REFERENCES BAcHE, B. W. & WILLIAMS,E. O. (1971). A phosphate sorption index for soils. J. SoilSci.. 22, 289-301. BASCOtedLC. L. (1968). Distribution of pyrophosphate extractable iron and organic carbon in soils of various groups. J. Soil Sci., 19, 251-68. BATTELLEINSTITUTE(1974). Study of effects of oil discharges and domestic and industrial wastewaters on the fisheries of Lake Maracaibo, Venezuela. Report: Battelle Pacific Northwest Laboratory, Richland, Washington, 99352. BORTLESON,G. C. & LEE,F. (1974). Phosphorus, iron and manganese distribution in sediment cores of six Wisconsin lakes. Limnol. and Oceanogr., 19, 794-801. CALDWELL,T. H., JONES,J. L. & COOPER,B. J. ( 1971). The influenceof'drumminess' on the availabilityof phosphate in Fenland peaty soils. Tech. Bull. Ministr. Agric. Fish. Fd. London., 20, 101-13. CARRIT,D. E. & GOODGAL,S. (i 954). Sorption reactions and some ecological implication. Deep-Sea Res., 1,224-43. COmN, D. E. (1963). A method for the determination of free iron in soils and clays. Canad. J. SoilSci., 3, %17. FOGG,D. N. R, WILKINSON,N. T. (1958). The colorimetric determination of phosphorus. Analyst, 83, 406-14. GARDNER,B. R. & PRESTON,J. (1973). The effect of temperature on phosphate sorption and phosphate desorption. Comm. Soil Sci. and Plant Anal., 4, 83-93. GORSUNOV, N. I., DEY~a~EVXC8,G. S. & TUNIK, B. M. (1961). Methods of determining non-silicate amorphous and crystalline sesquioxides in soils and clays. Soviet Soil Sci. for 1961, 1252-59. HARTER,R. (1968). Adsorption of phosphorus by lake sediments. Soil Sci. Soc. Amer. Proc., 32, 514- ! 8. HART~R,R. (1969). Phosphorus adsorption sites in soils, Soil Sci. Soc. Amer. Proc., 33, 630-32. HERREaa, T. (1977). Adsorci6n y desorei6n de fosfato en sedimentos del sistema Maracaibo. I. Caracterizaci6n Fisicoquimica. Trabajo Especial de Grado. Esc. de Quimica Fac. Ciencias Universidad Central de Venezuela. JACKSON,M. L. (1970). Analisis Quimico de Suelos. Edit Omega. p. 199. KAILA,A. (1959). Retention of phosphate by peat samples. J. Sci. Agric. Soc. Fin., 31,215-25. KHALID,R. A., PATRICK,W. H. JR. & DELXUNE,R. D. (1977). Phosphorus sorption characteristic of flooded soils, Soil Sci. Soc. Amer. Proc., 41, 305-10. Kuo, S. & LOTSE,E. G. (1974). Kinetics of phosphate adsorption and desorption by lake sediments, Soil Sci. Soc. Amer. Proc., 38, 50-4. LARSEN,S. (1967). Soil phosphorus. Adv. Agron., 19, 191-210. LARSEN,S., GUNARY,D. & SUTTON,C. D. (1965). The rate of immobilisation of applied phosphate in relation to soil properties, J. Soil Sci., 16, 141-48. Ll, W. C., ARMSTRONG,D. E., WILLIAMS,J. D., HxRPdS,R. F. & SYERS,J. K. (1972). Rate and extent of inorganic phosphate exchange in lake sediments. Soil. Sci. Soc. Amer. Proc., 36, 279-85. LOP~z-HERNANDEZ,D. (1977). La qulmica delfdsforo en suelos f~eidos. Edic. Biblioteca Univ. Central Venezuela. 123 pp. LOPEZ-HEaN^NDEZ,D. & BURNHAM,C. P. (1973). Extraction methods for aluminiumand iron in relation to phosphate adsorption, Soil Sci. and Plant Anal., 4, 9-16. LOPEZ-HERNXNDEZ,D. & BURNHAM,C. P. (1974a). The covariance of phosphate sorption with other soil properties in some British and tropical soils. J. Soil. Sci., 25, 196-205. LOPEZ-HERNXNDEZ,D. & BURNHAM,C. P. (1974b). The effect of pH on phosphate sorption in soils. J. Soil Sci., 25, 207-16. LOPEz-HERNANDEZ& BtnL'qHXM,C. P. (1974c). Phosphate adsorption by organic soils in Britain. X. Congr. Intern. Soil Sci. Moscow., 2, 73-80. LOPEz-HERNANDEZ,D. & BtmNHAM,C. P. (1978). The covariance of phosphate availability with other soil properties in some British and tropical soils. Agrochimica, 22, 150-9.
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LOPEZ-HERNANDEZ, D., ALVAREZ, L. & POLANCO, M. (1977a). Phosphate requirement for Vigna unguiculata L. var. Tuy. growing in two contrasting Venezuelan soils. Proc. Intern. Seminar on Soil Env. and Fert. Manag. in Agric. Inten. Tokyo, 517-24. LOPEZ-HERNANDEZ,D., RODRmUEZ,J. V., SmGERT,G. & FLO~S AGUILAR,D. (1977b). La desorci6n del fosfato retenido en sueios ~cidos mediante diferentes formas i6nicas de los aniones: Oxalato, malato y citrato. Acta Ciem. Vene:., 28, 138-41. MACKEAGUE, J. & DAY, J. H. (1966). Dithionite and oxalate-extractable Fe and At as aids in differentiating various classes of soil. Canad. J. Soil. Sci., 46, 13-33. MUUAm, D. A., POSNER,A. M. & QUIRK,J. P. (1966a). The mechanism of phosphate adsorption by kaolinite, gibbsite and pseudobochmite. I. The isotherm and the effect of pH on adsorption. J. Soil. Sci., 17, 212-29. MULJAm, D., POSNER,A. M. & QUIRK, J. P. (1966b). The mechanism of phosphate adsorption by kaolinite, gibbsite and pseudobochmite, I I. The effect of temperature on adsorption. J. Soil. Sci., 17, 238-47. NAGARAJAH,S., POSNER,A. M. & QUmK, J. P. (1968). Desorption of phosphate from kaolinite by citrate and bicarbonate. Soil Sci. Amer. Proc., 32, 507-10. NORVELL,E. A. (1974). Insolubilization of inorganic phosphate by anoxic lake sediments, SoiISci. A mer. Proc., 38, 441-45. PECK, L. (1964). Systematic analysis of silicates, Geo. Surv. Bull. 1170. PHILLIPS, J. (1972). Chemical processes in estuaries. The estuarine environment, (Barnes, R. S. K. & Green, J.) (Eds.)), Applied Science Publishers Lid, London, 35--50. POMEROY, L. R., SMXTH,E. E. & GRANT, C. M. (1965). The exchange of phosphate between estuarine water and sediments. Limnol. Oceanogr., 10, 167-72. SAXNI,G. R. & McLEAN,A. A. (1965). Phosphorus retention capacities of some New Brunswick soils and their relationship with soil properties. Canad. J. Soil Sci., 49, 89-94. SVERS,J. K., EVA.'~S,T. D., WILLIAMS,J. D. H. & MURDOCK,I. T. ( 1971). Phosphate sorption parameters of representative soils from Rio Grande Do Sul, Brazil, Soil Sci. !12, 267-75. UPCHURCH, J. B., EDZWALD,J. K. & O'MEUA, Ca. R. (1974). Phosphates in sediments of Pamlico Estuary Environ. Sci. Technol., 8, 56-58. WALKLE'¢,A. & BLACK,I. A. (I 934). An estimation of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci., 37, 29-38. WIKLANDER,L. (I 950). Kinetics of phosphate exchange in soils. Ann. Roy. Agr. Coll. Sweden, 17, 407-24. WILLIAMS, E. G. (1959). Influences of parent material and drainage conditions on soil phosphorus relationships. Agrochimica, 3, 279-309. WILLIAMS,E. G., SCOTT,N. M. & MACDONALD,N. J. (1958). Soil properties and phosphate sorption. J. Sci. Fd. Agric., 9, 551-9. WILLIAMS,J. D., SYEnS, J. K., SHUKLA,S. S. • HARMS, R. F. (1971). Levels of inorganic and total phosphorus in lake sediments as related to other sediment parameters. Environ. Sci. Technol., 5, 111 3-20.
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY (MARACAIBO SYSTEM)
D. LOPEZ-HERNANDEZ, T. HERRERA Rt F. ROTONDO Facultad de Ciencias, Laboratorio de Estudio$ Ambientales, I.Z.T., University Central de Venezuela, Venezuela (Received: 1 February, 1980)
ABSTRACT
The Maracaibo Estuary is of particular interest because of its location in the tropics, its large size (over 12,000km2), the economic importance of its underlying hydrocarbon deposits and its increasing rate of polhttion. Sediments f r o m this estuary were studied in order to characterise their capacity to adsorb and desorb phosphorus. Considerable variability in capacity f o r phosphate sorption was found among the seventeen samples studied. This variability can be attributed first to the differences among the samples in total P content and, secondly, to their differences in free iron content. Also, it appears that P retention contributes to the control of P concentration in the water and therefore the sediments serve as a b,tffer in controlling P eutrophication in the estuary.
INTRODUCTION
Soil and sediments are able to remove phosphate from solution. The amount of P removed depends on both environmental and biotic conditions. The following factors, influencing the adsorption-desorption process, have been reported: (1) pH (Muljadi et al., 1966a; L6pez-Hernfindez & Burnham, 1974a; L6pez-Hernfindez et al., 1977b), (2) temperature (Carrit & Goodgal, 1954; Muljadi et al., 1966b; Gardner & Preston, 1973; L6pez-Hernfindez et al., 1977a), (3) 'active' forms of iron, aluminium, calcium and manganese (Williams, 1959; Hatter, 1969; L6pezHernfindez & Burnham, 1973, 1974b,c; Upchurch et al., 1974), (4) time of reaction (Larsen, 1967; Larsen et al., 1965), (5) ionic environment (Wiklander, 1950; Upchurch et al., 1974), (6) redox condition (Khalid et al., 1977) and (7) biological activity, including micro-organism activity and root exudates (Nagarajah et al., 1968; L6pez-Hernfindez et al., 1977b). 153 Marine Environ. Res. 0141-1136/80/0004-0153/$02.50 © Applied Science Publishers Ltd, England, 1980 Printed in Great Britain
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D. LOPEZ-HERN.~NDEZ, T. HERRERA, F. ROTONDO
Soil chemists have discussed at length the role of adsorption and precipitation reactions for P retention in soils. Evidence has been provided for both processes. Currently, the adsorption theory is favoured by most workers. Soils, lake sediments and estuary sediments are similar in many respects, especially in their chemical composition---e.g, abundance of phyllosilicates, iron and aluminium oxides and organic matter (humified or not). The main difference between these systems is in the more anoxic character of lake and estuary sediments, owing to their condition of permanent waterlogging. It appears that phosphate retention in lake sediments is controlled by similar mechanisms to those proposed for P retention in soils (Harter, 1968; Kuo & Lotse, 1974; Li et al., 1972; NorveU, 1974). We have extended this investigation to a tropical estuary, Maracaibo 'lake'. Apart from its tropical setting, the Maracaibo Estuary is of particular interest because of the economic importance of the petroleum deposits under and near it, its large size (12,274 km 2) and its increasing rate of pollution (Battelle Institute, 1974). The principal objective of this research was to characterise the phosphorus adsorption-desorption reactions occurring in the sediments of the Maracaibo Estuary. MATERIALS AND METHODS
Samples of surficial sediments were collected with an Eckmann dredge from seventeen different localities within Maracaibo lake (Fig. 1). The sampling distribution is reasonably representative of Maracaibo lake as a whole. The sediments were collected in June, 1975 and kept in sealed polyethylene bags in a frozen condition. Methods of determining the chemical properties included: pH in a 1:5 soil solution ratio, organic carbon by the Walkley & Black (1934) method, loss on ignition in a platinum crucible according to Peck (1964), total aluminium and phosphorus by Na2CO 3 fusion and colorimetric determination, total iron and manganese by Na2CO 3 fusion and atomic absorption analysis, 'active iron' extracted by dithionite by the method described by Coffin (1963) and with oxalate (MacKeague & Day, 1966). 'Active' manganese and aluminium were also determined in the oxalate extraction solutions.
Phosphate adsorption indices and isotherms Wet sediment equivalent to 5-0 g of dry material was shaken with 100 ml of a solution of 0"025 M KH2PO 4 and 0-02 M KCI for a period of 18 h. The temperature was maintained constant at 20 +_ 1 °C and two drops of chloroform were added to inhibit microbial activity. After the shaking period, the suspension was filtered and centrifuged and the final phosphorus concentration was determined using the Fogg & Wilkinson (1958) method.
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY
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156
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Phosphate adsorption isotherms were obtained for six of the samples following the method used by L6pez-Hern/mdez & Burnham (1974b), with corrections applied for the presence ofwater. For each isotherm, the equilibration time was determined.
Phosphorus desorption indices These were determined by shaking for 20 h a wet equivalent of 1.0 g air-dried nonphosphate-enriched sediment with 100 ml of distilled water and 5-0 g of a chloride resin in an Erlenmeyer flask. At the end of the shaking period the suspensions were filtered through a cloth sieve. The resin was regenerated by treatment with 50 m i - NaCI for 3 h, and the phosphate in the eluate was determined by using the blue colorimetric method described by Jackson (1970).
RESULTS AND DISCUSSION
General characteristics of the sediments analysed Physical and chemical properties of the sediment samples are given in Table 1. The pH had a mean value of 6.5, with a range of 5.0-8-5. Most of the samples with a pH above 7-0 were from the western zone of the lake (Fig. 1), where the sediments could be affected by a flux of saline water. Those samples registering lower pH values came from places near the mouths ofinflowing rivers. Loss on ignition and organic carbon were lowest for sample UDCZ-90, classified as a sand. In general the silt and clay sediment samples contained appreciable organic carbon. In total iron concentration, most of the samples clustered around the mean value TABLE 1 GENERALCHARACTERISTICSOF THE SEDIMENTSANALYSED
Sample
pH
Loss in
%C
% Fe
°//oAI
6.5 4.3 2.3 1-3 0.5 2.5 9.1 7-2 8-2 5-3 4.2 8.3 5.7 5-2 10-0 6.5 4-3
6.5 6.4 6-7 17.5 !'4 5.2 5.1 6.6 4.6 4.6 5.0 5-0 5.2 5-3 6-3 6.5 9.5
3.9 4.0 2.0 1.2 1.8 2.0 4.4 4.4 4.8 9.6 18.6 16.8 5.0 15-5 5-5 5.0 4-4
ignition M2 M4 M6 M8 UDCZ-90 La Caleta MI2 M9 MI3 AOI AO2 Bobures MI6 MI7 AB3 AB8 Lladna-114
6'9 5-4 7. i 7"0 7.4 8.5 5.7 7.5 7.7 5.3 5.8 5.0 6.2 6.2 7-6 5.1 6.6
12.7 16'8 9-0 9-0 1"8 13.1 18-2 10-3 17-2 12.4 14.6 18-2 21.2 19.4 18.3 15.9 15"3
Mn (ppm)
P (ppm)
% Clay
6 3 7 19 1 3 15 7 19 2 3 48 16 8 10 4 4
820 670 2650 2320 710 I110 670 1210 980 690 1060 680 840 550 690 2030 2170
23 29 9 0 0 35 35 50 35 19 53 43 39 44 40 38 30
PHOSPHATE ADSORPTIONAND DESORPTIONIN A TROPICALESTUARY
157
TABLE 2 AMOUNTS OF Fe, AI AND MB EXTRACTED WITH THE DIFFERENT REAGENTS AND P SORPTION AND P DF.SORPTION PARAMETERS
Sample
M2 M4 M6 M8 UDCZ-90 La Caleta Ml2 M9 Ml3 AOI AO2 Bobures Ml6 M17 AB3 AB8 Lladna-114
Dit.
%Fe~ Oxal.
0"35 0"86 5.00 5.60 0-64 1.10 0-25 1.10 0.38 0.59 2.00 0"47 3.10 0'15 0'15 2.90 4"90
0.80 1.30 2.70 1.90 0'77 0.61 0.49 1-20 0.84 0.69 1.80 0-66 0.29 0.50 0.44 5.90 5.20
% AP Oxal.
% Mnb Oxal.
1.8 3.7 4.5 1.9 1-5 4.7 1.9 3.3 2.6 0.9 0.8 0'6 !'6 0-6 1.7 2.6 3.2
3-0 21"0 1I'0 6.0 40'0 4-6 9.8 20-0 9-5 29-0 27.0 21.0 9.5 16.0 15-0 28-0 23.0
P sorption" P sorption" P Dried Feet samples desorption samples (ppm) 5-7 12-4 21"8 22-3 5-3 12-4 10.6 17-9 8.5 14.0 35.9 13.3 6.4 7.6 6-0 43.9 46-2
6.2 9.2 37.3 54.7 7.4 14.5 7.4 24.1 10.6 27.8 > 120.0 21-0 8-3 6.4 3.2 68.8 > 120.0
61 9 114 70 27 59 7 136 78 42 40 26 27 30 31 57 79
" ~ Fe extracted from the dried sample. b % AI and Mn extracted with oxalate from AI and Mn total. ' P sorption -- X/logC, where X is amount of P adsorbed, mg P/100g of soil, and C is P cone. in/amol P/litre. (6"3 ~). Only one (UDCZ-90) was low (1.4 ~ ) and two, both located in the northern part of the lake, were high in iron (ca. 9 ~). The samples with the highest value for total aluminium (above 15 ~ ) were those taken from the southern area of the lake. Total manganese was variable, with the lowest values corresponding to those sediments sampled near the shores. The samples were also variable in total phosphorus, ranging from 550 to 2650 ppm. The sediment samples with the highest P content were from the northern part of the estuary, where the shores have their highest human population density. Adsorption o f phosphate in wet and dry sediments Phosphate sorption by sediments showed great variability in both the wet and dry samples (Table 2). For the wet samples, the sorption indices ranged between 3.2 and 120 with a mean of 32.2. For the dry sediments, sorption ranged from 5.6 to 46-2, with a mean corresponding to 17-1. Phosphate adsorption variability could also be ascribed to the great variation amongst the chemical characteristics of the samples, especially those pertaining to the presence of'active' forms of iron (Table 2). Contrasting capacities for adsorption were observed when analysing the whole isotherm. Figure 2 presents the adsorption isotherms (Langmuir representation) for two selected samples. As expected, the
158
D, L6PEZoHERNANDEZ, T. HERRERA, F. ROTONDO 6.0"
-
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Fig. 2. Phosphateadsorption isotherms using Langmuir representation. isotherms follow the linear model of Langmuir at the lowest P doses but become curvilinear at the highest P equilibrium concentrations (above 150 gg P/ml). In general, when comparing adsorption values between wet and dry sediments, a lower capacity for P retention is indicated for the latter (Table 2). The sediments in the wet--and consequently more reduced and anaerobic condition--displayed a higher P fixing capacity. Similar results have been found by Khalid et al. (1977) when studying flooded soils. However, they studied the phenomenon in a more strict anaerobic condition by not allowing the samples to be exposed to atmospheric 0 2 or to oxygenated solutions throughout their study. In the experiments reported here, although the samples were not in a totally anaerobic condition, they were kept wet at temperatures below freezing to minimise microbial activity. These conditions impeded further oxidation of the 'active' iron and manganese forms. Khalid et al. (1977) found that considerably more amorphous Fe was released by ammonium oxalate solution when conditions were reducing instead of oxidising. This observation was attributed to an increase in surface active sites through the transformation of ferric hydroxide, Fe(OH)3, to ferrous hydroxide, Fe(OH) 2, under reducing conditions. However, L6pez-Hernfindez & Burnham (1974c), in their study of P sorption in organic British soils, found that P sorption capacity was augmented by the higher oxidation forms of iron and manganese and when the samples were dry instead of wet.
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY
159
In the case of flooded soils and lake sediments, drying appears to lower P sorption by blocking the accessibility and thereby reducing the numbers of adsorption sites. Harter (1968) has emphasised this for some temperate lakes. Another mechanism which could be involved in explaining the difference in P retention under aerobic and anaerobic conditions is the transformation of reactive compounds from the amorphous to the crystalline form. Such changes could be accelerated by drying. Amorphous forms are known to retain P in a higher proportion than their crystalline counterparts (Gorbunov et al., 1961; L6pez-Hern~indez & Burnham, 1974a; L6pez-Hernfindez, 1977). Comparison o f P sorption indices
Phosphate adsorption capacities of different materials--e.g, soils, hydrous oxides, clays, sediments, etc.--can be compared by means of their phosphate adsorption isotherms (Fig. 2). However, this technique requires a considerable amount of analytical work. Bache & Williams (1971) have suggested that a single point on the isotherm can be used as a simple phosphate sorption index. The index has been extensively used by L6pez-Hern~mdez and Burnham in order to discriminate P sorption capacities in organic soils and mineral soils from tropical as well as temperate regions (L6pez-Hernfindez & Burnham, 1973, 1974a, c). We now want to extend the use of this method to submerged sediments. L6pez-Hern~ndez & Burnham (1974a, c) found that peaty soils possess a greater variability in P retention capacity (0-120) than mineral soils. An even greater variability has been found for the sediments of Maracaibo lake (2- > 120). It may be noted here that a value of 120 is considerably higher than any value measured in normal agricultural soils (Bache& Williams, 1971 ; L6pez-Hermindez & Burnham, 1974a). The variability in P retention for the sediments analysed may be a consequence of the presence of free iron and could operate as a buffer in controlling P water level. Similar results have been found by Upchurch et al. (1974) in sediments from Pamlico Estuary and by Williams et al. (1971) when working with the sediments of fourteen Wisconsin lakes. The strong association between inorganic P and the iron extracted either by oxalate or citrate-dithionite-bicarbonate suggested the existence of an ~Fe-inorganic P complex'. The correlations reported by Upchurch et al. (1974) for Pamlico Estuary, although high, were below the values given by Williams et al. (1971). The effectiveness of retention and its importance in pollution depend, as noted by Pomeroy et al. (1965), upon 'the exchange capacity of the sediments, the exchange rate between water and sediments, the flushing time of the estuary, and the rapidity of vertical mixing in the water column'. Unfortunately, we did not analyse P water level in the estuary; however, if we make use of the information given by the Battelle Institute (1974) it will be found that the zone located between 9°30'-10°00 ' N and 71 ° 10'-71 °35' W, which is presented in the Battelle report as the area of maximum P
160
D. L O P E Z - H E R N / ~ N D E Z , T. H E R R E R A , F. ROTONDO
solubility, corresponds approximately with the zone (Fig. 1) containing sediments with the lowest P sorption capacities. This last fact emphasises the probable role of sediments in the partial control of P water level in the estuary.
R E L A T I O N S H I P BETWEEN PHOSPHATE A D S O R P T I O N BY S E D I M E N T S - - B O T H W E T A N D DRYwAND
O T H E R PROPERTIES
In studying the relationship between the amount of P sorbed at the Bache and Williams index (1550ppm P) and other soil properties, such as 'active' iron, aluminium and manganese, the simple correlation technique was used. The results presented in Table 3 indicate that the adsorption of added P by the wet TABLE 3 CORRELATIONS BETWEEN P SORPT1ON AND P DESORPTION WITH OTHER SEDIMENT PROPERTIES
pH
C ~/0 clay ~o P F¢ D i t h i o n a t e Fe Oxalate AI O x a l a t e Mn Oxalate P Desorption
P sorption Wet samples
P sorption Dried samples
P desorption
- 0.25 as - 0 . 2 8 as - 0-27 ns 0.82**** 0.71 **** 0.86**** 0.18 as 0-14 ns 0.41 ns
- 0.29 as nd 0.12 ns 0.69*** 0'63*** 0.91 **** 0.20 ns nd nd
0.48* - 0 . 1 5 ns 0-08 as 0.64*** 0.44* 0.35 ns 0-53* -0.18 -
ns = N o t significant. nd= Not determined.
estuary sediments is very closely related to the oxalate iron. This is also true for dithionite extracted iron. Contrary to the findings of several previous studies (Williams et al., 1958; Saini & McLean, 1965; Kaila, 1959; Syers et al., 1971 ; L6pezHernfindez & Burnham, 1973, 1974a, c; Bortleson & Lee, 1974)'active' forms of aluminium and manganese do not contribute appreciably to the sorption processes in the estuary sediments studied. The total P content, on the other hand, is of considerable importance. Williams et ai. (1971), in their study of samples from Wisconsin lakes, also found no clear association between A1 and Mn parameters and extractable inorganic P. However, they reported a strong association between total P and the different iron forms. When analysing the various factors responsible for P adsorption in the dried samples, iron is again the most important in explaining P retention. Aluminium extracted with oxalate does not appear to control P sorption in dried--and
PHOSPHATE ADSORPTION AND DESORPTION IN A TROPICAL ESTUARY
161
consequently more oxidised--sediments. As reported previously, total P is associated with the P sorption characteristic. The experimental results indicate that the property most related to the adsorption process in the estuarine sediments of the Maracaibo System is the oxalateextractable iron. This reagent is known to dissolve amorphous and poorly crystalline oxides of Fe, the forms most responsible for P sorption (MacKeague & Day, 1966). Although Coffin (1963) suggested that dithionite removes "free" iron only from the amorphous fraction in the soil, other authors (Bascomb, 1968: MacKeague & Day, 1966) believe that this reagent extracts iron from the crystalline fraction as well. The results presented in this study (Table 2) demonstrate that the amounts of 'active' iron removed by both reagents are similar. The mean value for iron extracted with dithionite is slightly higher than for Fe-oxalate. However, the LSD values did not reach statistical significance (Herrera, 1977).
Desorption of phosphate and its relationship with other sediment properties The amount of phosphate desorbed with resin (Amberlite-C1) was used as an index for P desorption. This parameter displays considerable variability, ranging between 7 and 136ppm (Table 2). In a previous report for soils L6pez-Hern~ndez & Burnham (1978) found a wide distribution in desorption values for both tropical and temperate soils. A statistical analysis indicated the importance of free iron and organic matter in controlling the desorption process (L6pez-Hern~ndez & Burnham, 1978). For estuary sediments the parameter most associated with the desorption index was found to be total P (0-64***). Although free iron and aluminium extracted with ammonium oxalate also affect P desorption, they are of less importance. The absence of a clear correlation between P desorption and soil properties such as pH, organic matter content and the active forms of both Fe and AI can be attributed to the unsaturated state, with respect to P, of the sediment samples. When sediment samples are previously treated with soluble P prior to extraction, P desorption values correlate well with the other properties mentioned (Caldwell et al., 1971; L6pez-Hern~indez & Burnham, 1978). Phillips (1972) emphasises that estuarine muds are a reservoir of phosphorus but the importance of phosphate exchange across the sediment/water interface in buffering the water column overlying the sediments has only come to be appreciated slowly.
ACKNOWLEDGEMENT
We are indebted to Dr E. Marcucci (Instituto de Canalizaciones) for information, assistance and facilities for sediment sampling. The work has been supported by grants from CONICIT (Proyecto 0460) and Consejo de Desarrollo Cientifico y Humanfstico. Appreciation is expressed to Dr C. P. Burnham (Wye College,
162
D. LOPEZ-HERN.~.NDEZ, T. HERRERA, F. ROTONDO
University of London) for reading the manuscript and offering helpful suggestions. We are also grateful to Mr F. Ramos (Instituto Quimica, U.C.V.) for technical assistance.
REFERENCES BAcHE, B. W. & WILLIAMS,E. O. (1971). A phosphate sorption index for soils. J. SoilSci.. 22, 289-301. BASCOtedLC. L. (1968). Distribution of pyrophosphate extractable iron and organic carbon in soils of various groups. J. Soil Sci., 19, 251-68. BATTELLEINSTITUTE(1974). Study of effects of oil discharges and domestic and industrial wastewaters on the fisheries of Lake Maracaibo, Venezuela. Report: Battelle Pacific Northwest Laboratory, Richland, Washington, 99352. BORTLESON,G. C. & LEE,F. (1974). Phosphorus, iron and manganese distribution in sediment cores of six Wisconsin lakes. Limnol. and Oceanogr., 19, 794-801. CALDWELL,T. H., JONES,J. L. & COOPER,B. J. ( 1971). The influenceof'drumminess' on the availabilityof phosphate in Fenland peaty soils. Tech. Bull. Ministr. Agric. Fish. Fd. London., 20, 101-13. CARRIT,D. E. & GOODGAL,S. (i 954). Sorption reactions and some ecological implication. Deep-Sea Res., 1,224-43. COmN, D. E. (1963). A method for the determination of free iron in soils and clays. Canad. J. SoilSci., 3, %17. FOGG,D. N. R, WILKINSON,N. T. (1958). The colorimetric determination of phosphorus. Analyst, 83, 406-14. GARDNER,B. R. & PRESTON,J. (1973). The effect of temperature on phosphate sorption and phosphate desorption. Comm. Soil Sci. and Plant Anal., 4, 83-93. GORSUNOV, N. I., DEY~a~EVXC8,G. S. & TUNIK, B. M. (1961). Methods of determining non-silicate amorphous and crystalline sesquioxides in soils and clays. Soviet Soil Sci. for 1961, 1252-59. HARTER,R. (1968). Adsorption of phosphorus by lake sediments. Soil Sci. Soc. Amer. Proc., 32, 514- ! 8. HART~R,R. (1969). Phosphorus adsorption sites in soils, Soil Sci. Soc. Amer. Proc., 33, 630-32. HERREaa, T. (1977). Adsorci6n y desorei6n de fosfato en sedimentos del sistema Maracaibo. I. Caracterizaci6n Fisicoquimica. Trabajo Especial de Grado. Esc. de Quimica Fac. Ciencias Universidad Central de Venezuela. JACKSON,M. L. (1970). Analisis Quimico de Suelos. Edit Omega. p. 199. KAILA,A. (1959). Retention of phosphate by peat samples. J. Sci. Agric. Soc. Fin., 31,215-25. KHALID,R. A., PATRICK,W. H. JR. & DELXUNE,R. D. (1977). Phosphorus sorption characteristic of flooded soils, Soil Sci. Soc. Amer. Proc., 41, 305-10. Kuo, S. & LOTSE,E. G. (1974). Kinetics of phosphate adsorption and desorption by lake sediments, Soil Sci. Soc. Amer. Proc., 38, 50-4. LARSEN,S. (1967). Soil phosphorus. Adv. Agron., 19, 191-210. LARSEN,S., GUNARY,D. & SUTTON,C. D. (1965). The rate of immobilisation of applied phosphate in relation to soil properties, J. Soil Sci., 16, 141-48. Ll, W. C., ARMSTRONG,D. E., WILLIAMS,J. D., HxRPdS,R. F. & SYERS,J. K. (1972). Rate and extent of inorganic phosphate exchange in lake sediments. Soil. Sci. Soc. Amer. Proc., 36, 279-85. LOP~z-HERNANDEZ,D. (1977). La qulmica delfdsforo en suelos f~eidos. Edic. Biblioteca Univ. Central Venezuela. 123 pp. LOPEZ-HEaN^NDEZ,D. & BURNHAM,C. P. (1973). Extraction methods for aluminiumand iron in relation to phosphate adsorption, Soil Sci. and Plant Anal., 4, 9-16. LOPEZ-HERNXNDEZ,D. & BURNHAM,C. P. (1974a). The covariance of phosphate sorption with other soil properties in some British and tropical soils. J. Soil. Sci., 25, 196-205. LOPEZ-HERNXNDEZ,D. & BURNHAM,C. P. (1974b). The effect of pH on phosphate sorption in soils. J. Soil Sci., 25, 207-16. LOPEz-HERNANDEZ& BtnL'qHXM,C. P. (1974c). Phosphate adsorption by organic soils in Britain. X. Congr. Intern. Soil Sci. Moscow., 2, 73-80. LOPEz-HERNANDEZ,D. & BtmNHAM,C. P. (1978). The covariance of phosphate availability with other soil properties in some British and tropical soils. Agrochimica, 22, 150-9.
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LOPEZ-HERNANDEZ, D., ALVAREZ, L. & POLANCO, M. (1977a). Phosphate requirement for Vigna unguiculata L. var. Tuy. growing in two contrasting Venezuelan soils. Proc. Intern. Seminar on Soil Env. and Fert. Manag. in Agric. Inten. Tokyo, 517-24. LOPEZ-HERNANDEZ,D., RODRmUEZ,J. V., SmGERT,G. & FLO~S AGUILAR,D. (1977b). La desorci6n del fosfato retenido en sueios ~cidos mediante diferentes formas i6nicas de los aniones: Oxalato, malato y citrato. Acta Ciem. Vene:., 28, 138-41. MACKEAGUE, J. & DAY, J. H. (1966). Dithionite and oxalate-extractable Fe and At as aids in differentiating various classes of soil. Canad. J. Soil. Sci., 46, 13-33. MUUAm, D. A., POSNER,A. M. & QUIRK,J. P. (1966a). The mechanism of phosphate adsorption by kaolinite, gibbsite and pseudobochmite. I. The isotherm and the effect of pH on adsorption. J. Soil. Sci., 17, 212-29. MULJAm, D., POSNER,A. M. & QUIRK, J. P. (1966b). The mechanism of phosphate adsorption by kaolinite, gibbsite and pseudobochmite, I I. The effect of temperature on adsorption. J. Soil. Sci., 17, 238-47. NAGARAJAH,S., POSNER,A. M. & QUmK, J. P. (1968). Desorption of phosphate from kaolinite by citrate and bicarbonate. Soil Sci. Amer. Proc., 32, 507-10. NORVELL,E. A. (1974). Insolubilization of inorganic phosphate by anoxic lake sediments, SoiISci. A mer. Proc., 38, 441-45. PECK, L. (1964). Systematic analysis of silicates, Geo. Surv. Bull. 1170. PHILLIPS, J. (1972). Chemical processes in estuaries. The estuarine environment, (Barnes, R. S. K. & Green, J.) (Eds.)), Applied Science Publishers Lid, London, 35--50. POMEROY, L. R., SMXTH,E. E. & GRANT, C. M. (1965). The exchange of phosphate between estuarine water and sediments. Limnol. Oceanogr., 10, 167-72. SAXNI,G. R. & McLEAN,A. A. (1965). Phosphorus retention capacities of some New Brunswick soils and their relationship with soil properties. Canad. J. Soil Sci., 49, 89-94. SVERS,J. K., EVA.'~S,T. D., WILLIAMS,J. D. H. & MURDOCK,I. T. ( 1971). Phosphate sorption parameters of representative soils from Rio Grande Do Sul, Brazil, Soil Sci. !12, 267-75. UPCHURCH, J. B., EDZWALD,J. K. & O'MEUA, Ca. R. (1974). Phosphates in sediments of Pamlico Estuary Environ. Sci. Technol., 8, 56-58. WALKLE'¢,A. & BLACK,I. A. (I 934). An estimation of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Sci., 37, 29-38. WIKLANDER,L. (I 950). Kinetics of phosphate exchange in soils. Ann. Roy. Agr. Coll. Sweden, 17, 407-24. WILLIAMS, E. G. (1959). Influences of parent material and drainage conditions on soil phosphorus relationships. Agrochimica, 3, 279-309. WILLIAMS,E. G., SCOTT,N. M. & MACDONALD,N. J. (1958). Soil properties and phosphate sorption. J. Sci. Fd. Agric., 9, 551-9. WILLIAMS,J. D., SYEnS, J. K., SHUKLA,S. S. • HARMS, R. F. (1971). Levels of inorganic and total phosphorus in lake sediments as related to other sediment parameters. Environ. Sci. Technol., 5, 111 3-20.