Hydrological Sciences-Joumal-des Sciences Hydrologiques, 42(5) October 1997
7j 3
Adsorption of metal ions on bed sediments
C. K. JAIN & D. RAM
National Institute of Hydrology, Roorkee 247 667, India
Abstract The adsorption of lead and zinc ions on bed sediments of the River Kali in
western Uttar Pradesh, India, has been studied. The role of the coarser sediment
fraction (210-250 jtm) in controlling metal pollution has been elucidated and
compared to those of the clay and silt fractions. The parameters controlling metal
uptake, viz., solution pH, sediment dose, contact time, and particle size have been
evaluated. The optimum contact time needed to reach equilibrium is of the order of
45 min for both the metal ions. The extent of adsorption increases with an increase
of pH. Furthermore, the adsorption of the metal ions increases with increasing
adsorbent doses and decreases with adsorbent particle size. The two geochemical
phases of iron and manganese oxide act as the active support material for the
adsorption of the metal ions.
Adsorption d'ions métalliques sur les sédiments de fond
Résumé Nous avons étudié 1'adsorption des ions plomb et zinc dans les sédiments du
lit du fleuve Kali dans l'Uttar Pradesh occidental (Inde). Le rôle fondamental de la
fraction grossière des sédiments (210-250 fim) dans la pollution métallique a pu être
élucidé et comparé à ceux des fractions argileuses et silteuses. Les paramètres
commandant l'adsorption des métaux, à savoir le pH, la quantité de sédiment, le
temps de contact et la granulométrie du sédiment ont été estimés. Le temps de
contact optimal permettant d'atteindre l'équilibre est de l'ordre de 45 min pour les
deux ions métalliques étudiés. L'adsorption augmente avec le pH. De plus,
l'adsorption augmente avec la quantité de sédiment et diminue avec la taille des
particules adsorbantes. Les deux phases géochimiques que sont les oxydes de fer et
de manganèse constituent un matériel adsorbant très efficace vis à vis des ions
métalliques.
INTRODUCTION
Suspended and bed sediments play an important role in the transport of metals in
aquatic systems. The important components of the suspended load for geochemical
transport are silt, clay, hydrous iron and manganese oxides and organic matter. In
most publications, the clay and silt fractions of the sediments and organic matter
have been considered as the main adsorbing agents, while the contribution of the
coarser fraction of the sediments has been somewhat ignored.
Adsorption of metal ions by soils and sediments involves the distribution of the
ions among the surfaces of the component solid particles. Jenne (1976) reported that
the most significant role of clay-size mineral in trace element sorption by soils and
sediments is that of a mechanical substrate for the precipitation and flocculation of
organics and secondary minerals. The adsorption interactions between trace metals
on suspended clay minerals have been studied by Gagnon et al. (1992). Soong (1974)
reported that lead has a special affinity for clay minerals. Palheiros et al. (1989) have
studied the adsorption of cadmium onto a riverbed sediment and reported that pH is
Open for discussion until 1 April 1998
714
C. K. Jain & D. Ram
the most important controlling parameter. Bajracharya & Barry (1996) studied the
effect of zinc and ammonium ions on the adsorption of cadmium on sand and soil and
reported that both ions suppress the adsorption capacity significantly.
The adsorption behaviour of metals on oxides has been studied extensively
(Anderson & Benjamin, 1990; Benjamin & Bloom, 1981; Benjamin & Leckie, 1980;
Dempsey & Singer, 1980; Loganathan & Burau, 1973). These oxides constitute
significant sinks of heavy metals in aquatic systems. Fôrstner and different coauthors (Fôrstner & Millier, 1973; Fôrstner, 1977; Fôrstner & Wittman, 1983)
compiled the associations of heavy metals in particulate form in sediments and
described the active constituents with the nature of association. Recently Meng &
Letterman (1993a,b, 1996) studied the effect of component oxide interaction on the
adsorption properties of mixed oxides.
Although clay and silt components adsorb metal ions much better than sediment
fractions of higher size, one should take into account that most river sediments
contain 90-95% of sand and only 0-10% of clay and silt. Therefore, in river
sediments with a high sand percentage and a low clay and silt content, the overall
contribution of the sand content to adsorption of metal ions could be comparable to
or even higher than that of the clay and silt fraction.
In the present paper, the adsorption of lead and zinc ions on different fractions of
the sediment of a highly polluted river in western Uttar Pradesh, India, has been
studied. The objective was to determine the relative contribution of the coarser
fraction of the sediments in controlling metal pollution. Both metal ions are toxic and
relatively widespread in the environment (Fôrstner, 1979). The role of important
geochemical phases has also been investigated.
THE RIVER SYSTEM
The Kali River in western Uttar Pradesh, India, is a small perennial river having a
basin area of about 750 km2, and lies between latitude 29°33'N to 29°21'N and
longitude 77°43'E to 77°39'E in the Muzaffarnagar district of Uttar Pradesh (Fig. 1).
The climate in this region is moderate subtropical monsoonal. The average annual
rainfall in the area is about 1000 mm, a major part of which is received during the
monsoon period. The major land use is agriculture and there is no effective forest
cover. The soils of the area are loam to silty loam and are normally free of
carbonates.
The river basin contains towns and villages surrounded by agricultural areas.
The quantity and quality of the river water is affected by the discharge from
municipal and industrial areas as well as runoff from agricultural areas. The River
Kali is a typical water course containing municipal and industrial effluents. All those
who have access to the river use it for bathing, laundry and even for defecating and
are very difficult to regulate. The main pollution sources include Muzaffarnagar city,
a variety of industries (steel, rubber, ceramic, chemical, plastic, dairy, pulp and
paper, and laundries), and the Mansurpur sugar mill and distillery. The composite
waste from a variety of industries is transferred through Muzaffarnagar main drain
Adsorption of metal ions on bed sediments
715
77-45
29-35
29-30
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SHAHPUR
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Fig. 1 The study area. (I: upstream section; II: downstream section; and ad: wastewater outfalls).
into the river. The chemical characteristics of this river have been reported earlier
with special reference to the disposal of municipal and industrial wastes (Jain, 1992).
In another publication a mass balance approach has been utilized to discriminate
between point and non-point sources of pollution to the river (Jain, 1996).
EXPERIMENTAL METHODOLOGY
Sampling and storage
For the adsorption studies of heavy metal ions (Pb2+ and Zn2+) freshly deposited
sediments in shallow water near the river bank were collected in polyethylene bags
from the upstream section of the river and brought to the laboratory. Samples were
taken from the upper 5 cm of the sediments at places where flow rates were low and
sedimentation was assumed to occur. Sakai et al. (1986) and Subramanian et al.
(1987) adopted the same method in their work.
The size distribution of the sediment samples was found by the dry sieving
method using ASTM standard sieves to obtain various fractions, viz., 0-75, 75-150,
150-210, 210-250, 250-325 and 325-425 /mi. Sieved sediments were oven-dried
overnight at 105°C and, after cooling in a desiccator, stored in air-tight containers.
716
C, K. Jain & D. Ram
Materials and reagents
All chemicals used in the study were of analytical grade. Aqueous solutions of lead
and zinc were prepared from lead nitrate and zinc sulphate respectively. Deionized
water was used throughout the study. All glassware and other containers were
thoroughly cleaned by soaking in detergent followed by soaking in 10% nitric acid
for 48 h and finally rinsed with deionized water several times prior to use.
Adsorption experiments
Adsorption experiments were conducted in Erlenmeyer flasks with ground-glass
stoppers. Fifty ml of metal ion solution were transferred to 100 ml flasks together
with the desired adsorbent doses (Ws in g l"1), and placed in a water bath shaker
maintained at 25°C. Adsorption tests were terminated after the attainment of
equilibrium conditions. Aliquots were retrieved periodically, filtered through
0.45 /xm membrane filters and analysed for the residual concentration of the
respective metal ions.
Metal ion analysis
Metal ion concentrations were determined by flame atomic absorption spectrometry
using a Perkin-Elmer Atomic Absorption Spectrometer (Model 3110) with an airacetylene flame. The detection limits for the two metal ions studied are 0.03 and
0.002 mg l"1 for lead and zinc, respectively. Operational conditions were adjusted to
yield optimal determinations. Quantification of the metals was based upon calibration
curves of standard solutions for the respective metals. These calibration curves were
determined several times during the period of analysis.
RESULTS AND DISCUSSION
The sediment under study was composed of more than 99% sand and less than 1%
silt and clay. It was therefore felt worth studying the adsorption of metal ions on the
sand fraction compared to the clay and silt fraction ( < 75 jtm particle size) in order
to evaluate the importance of the coarser fraction in controlling metal pollution. The
organic content of the sediment was of the order of 0-1%. The important
geochemical constituents in the different particle size fractions, along with the weight
percentages of the different fractions, are given in Table 1. It is evident from the data
that the manganese and iron contents in the various fractions of the sediment decrease
with increasing particle size. This indicates the possibility of the two geochemical
phases acting as the active support material for the adsorption of lead and zinc ions.
However, the relative contribution of individual components cannot be obtained from
the present studies because in natural systems, the type and composition of the
Adsorption of metal ions on bed sediments
111
Table 1 Characteristics of sediments.
Particle fraction
(/an)
<75
75-450
150-210
210-250
250-300
300-425
425-600
Weight
(%)
0.6
3.9
19.1
35.6
15.0
24.3
1.5
Total Mn
(mg g"1)
1.32
1.03
0.39
0.23
0.15
0.12
Total Fe
(mg g"1)
32.41
21.48
10.75
7.88
7.87
7.05
mineral and organic fractions of sediments vary simultaneously and the effect of
individual constituents cannot be isolated.
Contact time (t)
The adsorption data for the uptake of metal ions vs contact time for a fixed adsorbent
dose of 5 g l"1 and with an initial metal ion concentration of 10 mg l"1 are shown in
Fig. 2 for the two particle sizes of adsorbent (0-75 and 210-250 /an) at pH 5.0.
These plots indicate that the remaining concentration of metal ions becomes
asymptotic to the time axis such that there is no appreciable change in the remaining
metal ion concentration after 45 min in the case of the 210-250 /u,m fraction and
30 min in the case of the 0^75 fxm fraction. These times represent the equilibrium
times at which an equilibrium metal ion concentration (Ce) is presumed to have been
attained.
An empirically found functional relationship, common to most adsorption
processes, is that the uptake varies almost proportionally with tU2 rather than with the
30
60
90
120
150
Co n t a c t T i me ( m i n )
30
60
90
120
Co n t o c t T I me (
Fig. 2 Plots of contact time vs percent adsorption of lead and zinc ions on bed
sediments: (a) 0-75 fim fraction; and (b) 210-250 fim fraction.
C. K. Jain & D. Ram
718
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Lead
QOPDO
Zinc
35 TT
A
AA&AA
Leod
nnaaa
Zinc
B
a25i
-a—o—o—a
-O—H—B
4
8
12
16
0
4
6
12
16
t1"2
t1'2
Fig. 3 Plot of tm vs percent adsorption of lead and zinc ions on bed sediments:
(a) 0-75 iim fraction; and (b) 210-250 /tm fraction.
contact time, t (Weber & Morris, 1963). Therefore, in Fig. 3, plots of metal ions
adsorbed, C, vs tm are presented for both the metal ions under consideration for the
two particle size fraction. It is evident that the adsorption of the two metal ions on
the bed sediments follows two phases, a linear phase of adsorption and then an
almost flat plateau section. This may be attributed to the instantaneous utilization of
the most readily available adsorbing sites on the adsorbent surface. Visual
observation clearly indicates the minor importance of any precipitation from ion
exchange.
Initial metal ion concentration (C0)
To evaluate the effect of the initial metal ion concentration (C0) on the adsorption
behaviour of lead and zinc on the bed sediments of River Kali, studies were
conducted with initial metal ion concentrations ranging from 2 to 25 mg l 1 with a
fixed adsorbent dose of 5 g l"1 at a pH value of 5.0 ± 0.1. Plots were prepared
between the metal ion adsorbed ( Q versus the equilibrium metal ion concentration at
particle sizes of 0-75 and 210-250 jim (Fig. 4). It is inferred that for the same
equilibrium time, the metal ion adsorbed is higher for greater values of initial metal
ion concentration or the percentage adsorption is more for lower concentration of
metal ions and decreases with increasing initial metal ion concentration. This is
obvious because a more efficient utilization of the adsorptive capacities of the
adsorbent is expected due to a greater driving force (by a higher concentration
gradient pressure). Comparing the two plots it is clearly evident that the affinity of
the two metal ions is more for the <75 /mi fraction, i.e. clay and silt. It is also
evident from Table 1 that the <75 /mi fraction contains more iron and manganese
content than that of the 210-250 /xm fraction. This is responsible for the higher
adsorption of the two metal ions on the clay and silt fraction. However, the clay and
Adsorption of metal ions on bed sediments
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6
Ib r Iu m
12
14
( mg/L)
5
10
15
Eq u 11 I b r l u m Co n e n
25
mg/L)
Fig. 4 Percent adsorption of lead and zinc ions on bed sediments: (a) 0-75 ixm
fraction; and (b) 210-250 /im fraction.
silt constitute only 0.6% of the total sediment load and therefore, comparing the two
weight percentage of the two fractions and their corresponding adsorption capacities
for the metal ions, it is clear that the contribution of the sediment fraction of size
210-250 jum is more in controlling the metal pollution as compared to the clay and
silt content. Among the two ions, lead and zinc, the affinity is more pronounced in
the case of lead as compared to zinc ions. A special affinity of lead ions on clay
mineral structures was also reported by Soong (1974).
Effect of pH
The adsorption of lead and zinc ions on the bed sediments of the River Kali was
studied over a pH range of 2-6 for a fixed adsorbent dose of 5 g 1"' at particle sizes
of 0-75 and 210-250 yum (Fig. 5). The pH was adjusted using dilute nitric acid and
sodium hydroxide solutions. The pH was measured before and after the attainment of
equilibrium, the difference between the two values being generally less than 0.1 unit
in the case of sediment of size 210-250 ptm and 0.2 unit in the case of sediment of
size < 75 jxm. A general increase in adsorption with increasing pH of solution was
observed for both fractions and for both the metal ions up to a pH value of 6.0.
Beyond this pH the determination could not be performed due to the low solubility of
the metal ions. A similar behaviour has been reported by many authors (Christensen,
1984; Farrah & Pickering, 1976a,b, 1979; Netzer & Wilkinson, 1976; Benjamin &
Leckie, 1980) for the uptake of metal ions on various adsorbents. It is evident from
the results that the pH for maximum uptake (or removal) of lead and zinc ions is 6.0
for the coarser sediment fraction with the corresponding adsorption values being 530
and 206 ngAg for lead and zinc ions respectively. At this pH the concentration of
both the ions in solution reduced appreciably due to pH adjustments.
Further, it was found that the adsorption of lead rises from 13.8% at a pH of 4.0
C. K. Jain & D. Ram
720
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Ztnc
B
50-
40^
/
3 0 -_
//
20 \
/
/
/
/
Q_
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0 - Tirrrrrr^m
4
PH
Fig. 5 Plot of pH vs percent adsorption of lead and zinc ions on bed sediments:
(a) 0-75 fjm fraction; and (b) 210-250 ^m fraction.
to 42% at a pH of 6.0 for the coarser fraction of the sediments which reveals the
sediment's capacity for adsorbing lead ions even in acidic media. The adsorption of
zinc rose from about 2% at a pH of 3.0 to 15% at a pH of 6.0 for the same fraction
of the sediments. The percentage adsorption was found to be more for the <75 /xm
fraction as compared to the 210-250 (am fraction.
Adsorbent dose (Ws)
The effect of adsorbent dose on the adsorption properties of the bed sediments of the
River Kali was studied at a pH of 5.0 with different adsorbent doses varying from 2
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Lead
Zinc
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Zinc
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901
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701
60 \
Q_
50 \
40 i T m m
m|1 , i " "
i
2
4
6
8
Ads o r b e n t dose
i illinium
10
g/L)
12
0
2
4
6
A d s o r b e n t
8
dose
10
12
( g / L )
Fig. 6 Plot of adsorbent dose vs percent adsorption of lead and zinc ions on bed
sediments; (a) 0-75 fim fraction; and (b) 210-250 fim fraction.
Adsorption of metal ions on bed sediments
721
to 10 g l"1 and at a fixed initial metal ion concentration of 10 mg 1 ' (Fig. 6). It may
be observed that for a fixed initial metal ion concentration (C0 = 10 mg l"1), the
adsorption of metal ions per unit weight of adsorbent decreased with increasing
adsorbent dose. The adsorption increased from 15% to 31% in the case of lead and
from 6% to 15% in the case of zinc for the 210-250 /xm fraction with an increasing
adsorbent dose for both the metal ions under study. However, the percent adsorption
(or removal) of metal ions was about 3-5 times more for the 0-75 /xm fraction as
compared to the 210-250 pxa. fraction. This is because of the higher content of iron
and manganese in the 0-75 /xm fraction, which act as the active support material for
the adsorption of metal ions.
Particle size (dp)
The adsorption data for the two metal ions with a fixed adsorbent dose of 5 g 1"1 and
at a pH value of 5.0 are shown in Fig. 7 for different sizes of adsorbent. The plots
between percent adsorption of metal ions at equilibrium vs adsorbent size reveals that
for a fixed adsorbent dose (Ws = 5 g 11), the metal ion adsorbed was higher for
smaller adsorbent size. Further, it may be observed that the percent metal ion
adsorbed decreased from 90% to 20% in the case of lead and from 50% to 8% in the
case of zinc with the increasing geometric mean of adsorbent size. This is because,
adsorption being a surface phenomenon, the smaller adsorption sizes offered
comparatively larger surface area and hence higher adsorption occurs at equilibrium.
In addition to the particle size of adsorbent, the geochemical phases, namely iron and
manganese oxides, also act as the active support material for the adsorption of the
metal ions under study.
AAAAA
QÛQÛP
0
Lead
Zinc
40 80 120 160 200 240 280 320 360
Ge o me L r I c me an s i z e
(micron)
Fig. 7 Plot of adsorbent size on percent adsorption of lead and zinc ions on bed
sediments.
722
C. K. Jain & D. Ram
CONCLUSION
The present study has shown the potentiality of freshly deposited sediments to
control metal pollution which may enter a river system through the disposal of
municipal and industrial effluents. The lead and zinc metal ions have more affinity
for the clay and silt fraction of the sediment, but the overall contribution of the
coarser fraction (210-250 ^im) to adsorption is more as compared to clay and silt
fraction. The adsorption data further suggest that the pH of the solution is the most
important parameter controlling adsorption of metal ions on sediments. The percent
metal ion removal increases with increasing adsorbent dose, and such removal
increases with decreasing size of the adsorbent material. Among the two ions
studied, the bed sediments of the River Kali have greater affinity for lead as
compared to zinc ions. The two important geochemical phases, namely iron and
manganese oxide, may act as the active support material for the adsorption of the two
metal ions in addition to the particle size of the sediments. However, in natural river
systems, salinity and pH highly affect the speciation of the metal ions and greatly
interfere with their fixation on clay minerals. Adsorption/desorption equilibria and
complexation with fulvic and humic acids also play an important role on speciation
and further studies are needed to better understand processes affecting the sorption of
metal ions in natural systems.
REFERENCES
Anderson, P. R. & Benjamin, M. M. (1990) Constant capacitance surface complexation model: Adsorption in silica-iron
binary suspensions. In: Chemical Modelling of Aqueous Systems II, ed. by D. C. Melchior & R. L. Bassett, 272281. ACS Symposium Series 416, American Chemical Society, Washington, DC.
Bajracharya, K. & Barry, D. A. (1996) Heavy metal adsorption in soils: comparison of bisolute adsorption models and
laboratory experiments. In: Water Quality Hydrology, ed. by V. P. Singh & B. Kumar, 19-26. Kluwer Academic
Publishers, The Netherlands.
Benjamin, M. M. & Bloom, N. S. (1981) Effect of strong binding of anionic adsorbates on adsorption of trace metals on
amorphous iron oxyhydroxide. In: Adsorption from Aqueous Solutions, ed. by P. H. Tewari, 41-60. Plenum Press,
New York.
Benjamin, M. M. & Leckie, J. O. (1980) Adsorption of metals at oxide surface: Effects of the concentrations of
adsorbate and competing metals. In: Contaminants and Sediments, ed. by R. A. Baker, vol. 2, 305-322. Ann
Arbor Science, Ann Arbor, Michigan, USA.
Christensen, T. H. (1984) Cadmium soil sorption at low concentrations: I. Effect of time, cadmium load, pH, and
calcium. Water, Air Soil Poll. 21, 105-114.
Dempsey, B. A. & Singer, P. C. (1980) The effect of calcium on the adsorption of zinc by Mn02 and Fe(OH)3. In:
Contaminants and Sediments, ed. by R. A. Baker, vol. 2, 333-353. Ann Arbor Science, Ann Arbor, Michigan,
USA.
Farrah, H. & Pickering, W. F. (1976a) The sorption of copper species by clays, I. Kaolinite. Aust. J. Chem. 29, 11671176.
Farrah, H. & Pickering, W. F. (1976b) The sorption of copper species by clays, II. Illite and montmorillonite. Aust. J.
Chem. 29, 1167-1184.
Farrah, H. & Pickering, W. F. (1979) The sorption of zinc species by clays minerals. Chem. Geol. 25, 317-326.
Fôrstner, U. (1977) Metal concentration in freshwater sediments—natural background and cultural effects. In:
Interaction Between Sediments and Freshwater, ed. by H. L. Golterman, 94-103. Pudoc/Junk, Wageningen, The
Netherlands.
Fôrstner, U. (1979) Metal transfer between solid and aqueous phases. In: Metal Pollution in the Aquatic Environment,
ed. by U. Forstner & G. T. W. Wittmann, 197-270. Springer-Verlag, New York.
Fôrstner, U. & Miiller, G. (1973) Heavy metal accumulation in river sediments, a response to environmental pollution.
Geoforum 14, 53-62.
Fôrstner, U. & Wittman, G. T. W. (eds) (1983) Metal Pollution in the Aquatic Environment, 2nd ed., Springer-Verlag,
Germany.
Gagnon, C , Arnac, M. & Brindle, J. R. (1992) Sorption interactions between trace metals (Cd and Ni) and phenolic
substances on suspended clay minerals. Wat. Res. 26, 1067-1072.
Adsorption of metal ions on bed sediments
723
Jain, C. K. (1992) Effect of waste disposals on quality of water of river Kali (Uttar Pradesh), Tech. Report TR-148,
National Institute of Hydrology, Roorkee, India.
Jain, C. K. (1996) Application of chemical mass balance approach to upstream/downstream river monitoring data.
J. Hydrol. 182, 105-115.
Jenne, E. A. (1976) Trace element sorption by sediments and soils—sites and processes. In: Symposium on Molybdenum,
ed. by W. Chappel & K. Petersen, vol. 2, 425-553. Marcel Dekker, New York.
Loganathan, P. & Burau, R. G. (1973) Sorption of heavy metal ions by hydrous manganese oxide. Geochim Cosmochim
Acta 37, 1277.
Meng, X. G. & Letterman, R. D. (1993a) Effect of component oxide interaction on the adsorption properties of mixed
oxides. Environ. Sci. Technol. 27, 970-975.
Meng, X. G. & Letterman, R. D. (1993b) Modelling ion adsorption on aluminium hydroxide modified silica. Environ.
Sci. Technol. 27, 1924-1929.
Meng, X. G. & Letterman, R. D. (1996) Modelling cadmium and sulfate adsorption by Fe(OH)3/Si02. Wat. Res. 30(9),
2148-2154.
Netzer, A. & Wilkinson, P. (1976) Removal of heavy metal from waste water by adsorption on sands. Proc. 29th Waste
Conference, Purdue, 841-845.
Palheiros, B., Duarte, A. C , Oliveira, J. P. & Hall, A. (1989) The influence of pH, ionic strength and chloride
concentration of the adsorption of cadmium by a sediment. Wat. Sci. Technol. 21, 1873-1876.
Sakai, H., Kojima, Y. & Saito, K. (1986) Distribution of metals in water and sieved sediments in the Toyohira River.
Wat. Res. 20, 559-567.
Soong, K. L. (1974) Versuche zur adsorptiven Bindung von Schwermetall Ionen an ktmstlichen Tongemischen. Unpubl.
PhD Thesis, University of Heidelberg, Germany.
Subramanian, V., Grieken, R. V. & Vant, D. L. (1987) Heavy metal distribution in the sediments of Ganges and
Brahmaputra Rivers. Environ. Geol. Water Sci. 9(2), 93-103.
Weber, W. J. Jr. & Moris, J. C. (1963) /. Sanit. Engng. Div., ASCE, SA2, 31, cited in S. D. Faust & O. M. Aly.
(1987) Adsorption Processes for water treatment. 65-122, Butterworth, London.
Received 18 November 1996; accepted 4 February 1997