Hydrological Sciences-Journal-des Sciences Hydrologiques, 46(3) June 2001 419

Adsorption of zinc onto bed sediments of the River Ganga: adsorption models and kinetics

C. K. JAIN National Institute of Hydrology, Jal Vigyan Bhawan, Roorkee 247 667, Uttar Pradesh, India e-mail: cki@nih.emet,in

Abstract A laboratory study was performed to study the effects of various operating factors, viz. initial metal ion concentration, solution pH, amount of sediment, contact time, particle size and temperature on the adsorption of zinc ions onto the bed sediments of the River Ganga (India). The equilibrium time was found to be of the order of 60 min. The adsorption curves are smooth and continuous leading to saturation, suggesting the possible monolayer coverage of zinc ions on the surface of the adsorbent. The extent of adsorption increases with an increase of pH. Furthermore the adsorption of zinc increases with increasing amount of adsorbent and decreases with adsorbent particle size. The important geochemical phases, iron and manganese oxide act as the active support material for the adsorption of zinc ions. The adsorption data have been analysed with the help of Langmuir and Freundlich adsorption models to determine the mechanistic parameters associated with the adsorption process. An attempt has also been made to determine thermodynamic parameters of the process, viz. free energy change, enthalpy change and entropy change. The negative values of free energy change (AG") indicated the spontaneous nature of the adsorption of zinc onto the bed sediments and positive values of enthalpy change (Aff°) suggest the endothermic nature of the adsorption process. The intraparticle diffusion of zinc in the adsorbent was found to be the main rate-limiting step.

Key words River Ganga; adsorption; Langmuir model; Freundlich model; kinetics; thermodynamic parameters

Adsorption du zinc par les sédiments du lit du Gange: modèles et cinétiques d'adsorption Résumé Les effets de plusieurs facteurs - la concentration ionique initiale, le pH de la solution, la quantité de sédiments, le temps de contact, la taille des particules et la température—sur l'adsorption des ions zinc par les sédiments du lit du Gange (Inde) ont été étudiés en laboratoire. Le temps d'équilibre identifié est de l'ordre de 60 min Les courbes d'adsorption sont régulières et continues jusqu'à saturation, suggérant une adsorption surfacique mono-couche des ions zinc sur les particules. Le degré d'adsorption augmente avec le pH. Il augmente également avec la quantité de sédiments, mais décroît avec la taille des particules. Les oxydes de fer et de manganèse s'avèrent être les supports actifs de l'adsorption des ions zinc. Les données d'adsorption ont été analysées avec les modèles de Langmuir et Freundlich, pour déterminer les paramètres mécaniques associés au processus d'adsorption. Une tentative a aussi été faite pour déterminer les paramètres thermodynamiques du processus, c'est-à-dire les variations d'énergie libre, d'enthalpie et d'entropie. Les valeurs négatives de la variation de l'énergie libre (AG°) mettent en évidence la spontanéité de l'adsorption du zinc par les sédiments du lit, tandis que les valeurs positives de la variation de l'enthalpie (Aff°) suggèrent un processus de nature endothermique. La diffusion intra-particulaire du zinc dans les sédiments s'avère être l'étape limitante.

Mots clefs RivièreGanga; adsorption; modèle de Langmuir; modèle de Freundlich; cinétique; paramètres thermodynamiques

INTRODUCTION

Adsorption is one of the important phenomena in water quality control, which may determine the fate and transport of pollutants in the aquatic environment. The tremen-

Openfor discussion until 1 December 2001

420 C. K. Jain

dous increase in the use of heavy metals over the past few decades has resulted in an increased concentration of metals in aquatic systems and has a great significance owing to their toxicity and adsorption behaviour. Thus, in the natural conditions of river water, suspended load and sediments have an important function of buffering higher metal concentrations of water particularly by adsorption or precipitation, Therefore, the study of sediments and their sorptive properties can provide valuable information relating to the tolerance of a system to increased heavy metal load and may determine the fate and transport of pollutants in the aquatic environment.

Pollution from industrial and agricultural sources is responsible to a great extent for high concentration of zinc in river water. Zinc may also be contributed by the water distribution system due to leaching of zinc from galvanized pipes. Zinc is an essential element for both animals and human beings and is necessary for the functioning of various systems.

The suspended and riverbed sediments play a major role in pollution studies due to their specific adsorption capacity. Heavy metals added to a river system by natural and manmade sources during their transport are distributed between the aqueous phase, suspended and bed sediments. The fraction in the sediment is expected not to have direct adverse effects, if the metal ions are tightly bound to it and subsequently get settled at the bottom in course of time. This state of affairs is maintained until there is remobilization from the sediment due to changing conditions in the system. The important components of the suspended load for geochemical transport are silt, clay, hydrous iron and manganese oxides and organic matter. Generally, the adsorption characteristics for sediment less than 50 um in size have been studied most extensively, while those for sediment larger than 75 |im have been studied in less detail because of its lesser adsorption activity. Palheiros et al. (1989) and Christensen (1984) reported the importance of pH in the control of cadmium adsorption. Bajracharya et al. (1996) examined the effect of zinc and ammonium ions on the adsorption of cadmium onto sand and soil and reported that both the ions suppress the adsorption capacity significantly. Gagnon et al. (1992) studied the sorption interactions between trace metals and phenolic substances on suspended clay minerals. Fu & Allen (1992) used a multi-site binding model for adsorption of cadmium by oxic sediments. Namasivayam & Ranganathan (1995) have used a waste product from fertilizer industry for removal of cadmium from waste water. The adsorption characteristics of bed sediments of River Kali for the uptake of lead and zinc ions have been reported in an earlier paper (Jain & Ram, 1997a).

The study of equilibrium adsorption on sediments in their natural state of occurrence has received much attention during recent years. However, the subject of adsorption kinetics and thermodynamics has not been studied in detail. Smith (1981) described importance of different processes in establishing the kinetics of adsorption process on porous adsorbent particles. Jackman & Ng (1986) studied the influence of external film diffusion and internal pore diffusion in controlling the kinetics of ion-exchange adsorption on natural sediments. Bencala et al. (1983) used film-diffusion-limiting kinetic models to describe adsorption kinetics.

Despite the apparent wealth of information on adsorption processes, little is known about quantitatively describing adsorption process by coarser sediment. Although the clay and silt component adsorb metal ions much better than the coarser fraction of sediment, one should take into account that most riverbed sediments contain 90-95%

Adsorption of zinc onto bed sediments of the River Ganga 421

sand and only 0-10% clay and silt. Therefore, in river systems having high sand and low clay and silt contents, 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 (Jain & Ram, 1997a,b; Jain & Ali, 2000).

In the present paper, adsorption characteristics of bed sediments of the River Ganga have been studied with a view to demonstrate the role of bed sediments in con­trolling metal pollution. Adsorption data have been analysed with the help of adsorption models to determine the mechanistic parameters associated with the adsorp­tion process and isotherms have been used to determine thermodynamic parameters.

THE RIVER GANGA

The River Ganga is a perennial river formed by the confluence of two smaller rivers at Devprayag: the Bhagirathi originating at Gaumukh in the Gangotri Glacier, 3129ma.s.l., and the Alaknanda with its origin in the Sapta Tal Glacier. After covering a distance of about 220 km in the Himalayas the Ganga enters the plains at Hardwar, meanders over a distance of about 2290 km in the plains in the states of Uttar Pradesh, Bihar and West Bengal, and joins the Bay of Bengal through a large number of branches flowing in India and Bangladesh (Fig. 1).

During the last few decades, the water of the river has become polluted and has developed dangerous levels of toxicity in certain stretches (Jain, 1998).

Boy of Bengal

• Sediment sampling site

Fig. 1 The Ganga basin showing location of sediment sampling site.

422 C. K. Jain

EXPERIMENTAL METHODOLOGY

Freshly deposited sediments from shallow water near the bank of the River Ganga at Hardwar were collected in polyethylene bags 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 ah, 1986; Subramanian et al, 1987).

The size distribution of the sediment samples was determined using nylon sieves to obtain various fractions. Textural features of the sediments were observed and a preliminary classification made according to grain size and distinctive geochemical features. The important geochemical phases for the adsorption process are organic matter, manganese oxides, iron oxides and clays. The contents of manganese oxide and iron oxide were measured as total manganese and total iron, respectively, and extracted from the sediment samples using an acid digestion mixture (HF + HCIO3 + HN03) in an open system. Organic matter was determined by oxidation with hydrogen peroxide.

Adsorption experiments were conducted in a series of Erlenmeyer flasks of 100 ml capacity covered with a Teflon sheet to prevent introduction of any foreign particles. Fifty millilitres of zinc ion solution (200-2000 ug l"1) were transferred into each flask together with the desired amount of adsorbent (Ws in g F1), and the flasks were placed in a water bath shaker maintained at desired temperature. A pH of 6.5 ± 0 . 1 was maintained throughout the experiment using dilute HNO3 and NaOH solutions. Aliquots were retrieved periodically and filtered through 0.45 u.m cellulose nitrate membrane filters. The filters were soaked in dilute (1% v/v) HNO3 for 1 h and thoroughly rinsed with deionized water prior to use.

The concentration of zinc ions was determined by flame atomic absorption spectrometry, using a Perkin-Elmer Atomic Absorption Spectrometer (Model 3110) with air-acetylene flame. The detection limit for the zinc ion was 0.002 mg l"1.

RESULTS AND DISCUSSION

The sediment under study has a rather coarse texture, being composed of more than 90% sediment of size >75 um and <10% silt and clay. The organic content of the sediment was of the order of 0-1%. The background zinc level in the various fractions of the sediments was negligible (below detection limits) in the unpolluted zone, compared to the amount of adsorbate added for the adsorption tests. This observation confirms the absence of any zinc attached to the sediment particles.

The content of two important geochemical phases (iron and manganese) in different particle size fractions along with weight percentages 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 potential of the two geochemical phases to act as the active support material for the adsorption of zinc ions. However, the relative contribution of individual components could not be obtained from the present studies because, in natural systems, type and composition of sediment mineral and organic fractions vary simultaneously and the effect of indi­vidual constituents cannot be isolated. The content of iron in the sediment fractions is relatively higher and indicates the possibility of presence of iron minerals other than hydroxides. However, this observation needs to be confirmed by further investigations. It is further evident from Table 1 that the sediment fraction of the 150-210 (im particle

Adsorption of zinc onto bed sediments of the River Ganga 423

Table 1 Characteristics of sediments.

Sediment fraction (\lm) Weight (%) Total Mn (mg g )

1.570 1.420 1.199 0.844 0.830 0.823 0.824

Total Fe (mg g" )

68.2 62.0 58.0 55.1 52.1 52.0 52.0

<75 75-150 150-210 210-250 250-300 300-^25 425-600

9.8 13.5 68.6

6.6 0.7 0.6 0.2

size constitutes 68.6% of the total sediment load. Therefore, it was considered appro­priate to study the adsorption of zinc ions onto this fraction (150-210 (xm) and make comparisons with the clay and silt fraction (<75 um particle size) in order to demon­strate the relative importance of the coarser fraction in controlling metal pollution.

OPERATING VARIABLES

Equilibrium time (t)

In order to determine the equilibrium time for the adsorption process, adsorption experiments were performed for the uptake of zinc ions for different duration of contact times at a fixed amount of sediment (0.5 g F1), initial metal ion concentration of 1000 jig l"1 and a pH value of 6.5 for two two-particle size of adsorbent (0-75 and 150-210 urn) (Fig. 2). The solution pH for the experiments was chosen to be as close as possible to that encountered in the river water. These plots indicate that the remaining concentration of zinc ions becomes asymptotic to the time axis such that there is no appreciable change in the remaining concentration after 60 min for both the fractions. This time is presumed to represent the equilibrium time at which an equi­librium concentration is assumed to have been attained.

According to Weber & Morris (1963), for most adsorption processes, the uptake varies almost proportionately with tm rather than with the contact time. Therefore,

-0-75 micron -150-21 Omicron -0-75 micron -150-21 Omicron

(%)

c

Ads

orpt

ic

80-

75 -

70

m -

^ ~

r • •

/ ^

-o-o-o

m • •

60 90 120 150 180 10 15

f (min) tvi (min)

Fig. 2 Effect of contact time (t and / ! / 2) on percent adsorption of zinc ions.

424 C. K. Jain

plots of zinc adsorbed, Ct vs tm, are presented for the two particle sizes of adsorbent in Fig. 2. It is clearly evident that adsorption of zinc ions onto the bed sediments follows three phases: an instantaneously extremely fast uptake, a transition phase and an almost flat plateau section. Phase I, is attributed to the instantaneous utilization of the most readily available adsorbing sites on the adsorbent surface (bulk diffusion). Phase II, exhibiting additional removal, is attributed to the diffusion of the adsorbate from the surface film into the macro-pores of the adsorbent (pore diffusion or intraparticle diffusion), stimulating further migration of adsorbate from the liquid phase onto the adsorbent surface. Phase III, the plateau section shows equilibrium state. The visual observation indicates the minor importance of any precipitation from ion exchange.

Adsorption isotherm

The adsorption isotherms for the zinc adsorption onto the bed sediments are shown in Fig. 3 with a fixed adsorbent dose of 0.5 g l"1 at a pH of 6.5 ±0.1. The adsorption data indicate linear distribution in the initial concentration range of 0-1000 fig l"1. It is evident that, for the same equilibration time, the zinc adsorbed is higher for greater values of initial concentration of zinc ions, or the percentage adsorption is more for lower concentration of zinc and decreases with increasing initial concentration. This is obvious because 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 evident that the affinity of zinc is more for the <75 urn fraction, i.e. clay and silt, as compared to the coarser fraction. It is also evident from Table 1 that the <75 urn fraction contains more iron and manganese than the 150-210 urn fraction, indicating the possibility of association of these substrates with clay and silt particles. These findings illustrate the possible role of clay and silt com­ponents as substrate for zinc adsorption. However, due to the paucity of data, correlation statistics could not be attempted to confirm this statement. The clay and silt fraction constitute <10% of the total sediment load; therefore, comparing the weight percentage of the two fractions and their corresponding adsorption capacities for zinc ions, it is clear that the contribution of the coarser sediment is important in controlling

00-75 micron • 150-210 micron

CD

E

2500"

2000

1500

S. 1000

500

O 0-75 micron • 150-21 Omicron

200 400 600 800 1000 200 400 600 800

Equil. concn. (mg 1-1 ) Equif. concn. (mg 1-1 )

Fig. 3 Adsorption of zinc ions on bed sediments at different concentrations.

1000

Adsorption of zinc onto bed sediments of the River Ganga 425

zinc pollution as compared to the clay and silt component. Similar behaviour was also observed in the case of lead and zinc ions for the River Kali (Jain & Ram, 1997a,b).

Effect of pH

The adsorption of zinc onto the bed sediments was studied over the pH range 2-7 for a fixed initial concentration of zinc (C,- = 1000 ug F1) and adsorbent dose of 0.5 g 1"1 at particle sizes 0-75 and 150-210 um (Fig. 4). The pH of the solution was adjusted using dilute hydrochloric acid and sodium hydroxide solutions. The pH was measured before and after the solution had been in contact with the sediment, the difference between the two values being generally less than 0.1 pH unit. A general increase in adsorption with increasing pH of solution was observed up to the pH value of 5.0 for both the fractions of the sediment. From the results it is evident that the pH for maximum uptake of zinc ion is 5.0. The adsorption of zinc rises from 0.5% at pH 2.0 to 82% at pH 5.0 in the case of the clay and silt fraction (0-75 um) and from 0.4% at pH 2.0 to 75% at pH 5.0 in the case of the coarser sediment fraction (150-210 um). A similar trend was reported by Palheiros et al. (1989) for the adsorption of cadmium on riverbed sediment.

0-75 micron —•—150-210 micron

100 T

80-

& c 60-.o o. o 40 -

0 4 0.0 2.0 4.0 6.0 8.0

pH

Fig. 4 Effect of pH on percent adsorption of zinc ions.

Amount of adsorbent (Ws)

The effects of amount of adsorbent on the adsorption properties of bed sediments are shown in Fig. 5 for a fixed initial zinc concentration of 1000 ug l"1 at pH 6.5 with different amount of adsorbent varying from 0.5 to 2.5 g 1"1. The experiments were conducted in a water bath shaker to disperse the sediment particles in the aqueous media. It is observed that, for a fixed initial concentration of zinc, the adsorption of zinc per unit weight of adsorbent decreases with increasing adsorbent load. On the other hand, percentage adsorption increases from 82.5 to 91.3% for the 0-75 um fraction with increasing amount of adsorbent from 0.5 to 2.5 g F1. The adsorption of zinc was higher for the 0-75 urn fraction than for the 150-210 um fraction. This is

426 C. K. Jain

O 0-75 micron 1150-210 micron

100

I I 200 300 400 500 600

Adsorbent size (mm) Amount of adsorbent (g 1-1 )

Fig. 5 Effect of amount of adsorbent and adsorbent size on percentage adsorption of zinc ions.

because of the higher content of iron and manganese in the 0-75 urn fraction, which is the main driving force for the adsorption of zinc ions.

Particle size (dp)

The effects of particle size of the adsorbent on zinc adsorption are also shown in Fig. 5 for a fixed initial concentration of zinc (C; = 1000 ug l"1), an adsorbent dose of 0.5 g F1

and pH 6.5. These plots reveal that, for a fixed adsorbent dose, the zinc adsorbed is higher for smaller sediment particle size. Further, it is observed that the percentage of zinc adsorbed decreases from 82.5% for the 0-75 um fraction to 68% for the 425-600 um fraction, with the increasing geometric mean of particle size. This is because, adsorption being a surface phenomenon, the smaller particle sizes offered comparatively larger surface area and hence higher adsorption occurs at equilibrium. The higher content of iron and manganese in the 0-75 urn fraction also accounts for higher adsorption of zinc in this fraction as compared with higher fractions of the sediment.

ADSORPTION MODELS

Adsorption information for a wide range of adsorbate concentrations are most fre­quently described by adsorption models, such as the Langmuir or Freundlich isotherm, which relate adsorption density qe (metal uptake per unit weight of adsorbent) to equilibrium adsorbate concentration in the bulk fluid phase, Ce. The adsorption information of zinc onto the bed sediments of the River Ganga have been analysed with the help of Langmuir and Freundlich models to evaluate the mechanistic parameters associated with the adsorption process.

Langmuir model

Langmuir's isotherm model is valid for monolayer adsorption onto a surface con­taining a finite number of identical sites (Langmuir, 1918). The Langmuir treatment is

Adsorption of zinc onto bed sediments of the River Ganga 427

Table 2 Langmuir parameters fc

Sediment fraction (Jim)

0-75 150-210

IT adsorption of zinc ions.

Adsorption maxima, Q° (mg g'1)

5.0 5.0

Binding energy constant, b (mg I"1)"1

2.35 1.55

based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate in the plane of the surface. The linear form of Langmuir isotherm equation is represented by:

1c

1 1

Q° bQ°Ce (1)

where qe is the amount adsorbed at equilibrium (|ig g"1), Ce is the equilibrium concen­tration of the adsorbate ions (tig T1), and Q° and b are Langmuir constants related to maximum adsorption capacity (monolayer capacity) and energy of adsorption, respec­tively. Langmuir isotherm plot is shown in Fig. 6(a) and Langmuir parameters are given in Table 2. These values may be used for comparison and correlation of the sorptive properties of the sediments.

Freundlich model

The Freundlich equation has been widely used for isothermal adsorption (Freundlich, 1926). This is a special case for heterogeneous surface energies in which the energy term, b, in the Lagmuir equation varies as a function of surface coverage, qe, strictly due to variations in heat of adsorption (Adamson, 1967). The Freundlich equation has the general form:

qe = KFa (2)

00-75 micron 1150-210 micron

0.004

(a)

0.01 0.02 0.03 0.04

1/Ce

2 2.0

(b)

1.0 1.5 2.0 2.5

log Ce

Fig. 6 Graphical representation of adsorption isotherms: (a) Langmuir isotherm, and (b) Freundlich isotherm.

428 C. K. Jain

Table 3 Freundlich parameters for adsorption of zinc ions.

Sediment fraction Adsorption capacity, KF (M.m) (mg g"1)

0-75 0.069 150-210 0.035

Adsorption intensity, lln

0.5565 0.6428

The logarithmic form of the equation is:

logc7e=logii:F+-logCe n

(3)

where qe is the amount adsorbed (u,g g4), Ce is the equilibrium concentration of the adsorbate ions (u.g l"1), and KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The Freundlich isotherm plot is shown in Fig. 6(b). The intercept of the line is roughly an indicator of the adsorption capacity, and the slope is an indication of adsorption intensity (Weber, 1972). The Freundlich parameters are given in Table 3.

The Freundlich type adsorption isotherm is an indication of surface heterogeneity of the adsorbent while the Langmuir type isotherm corresponds to surface homo­geneity of the adsorbent. This leads to the conclusion that the surface of the bed sediments of the River Ganga is made up of small heterogeneous adsorption patches which are very much similar to each other in respect of adsorption phenomena (Ajmal etal., 1998).

ADSORPTION KINETICS

Effect of temperature

In order to study the effect of temperature on adsorption phenomena, experiments were performed for the uptake of zinc ions at different temperatures for different duration of contact times with an initial metal ion concentration of 1000 u.g T1 at pH 6.5 (Fig. 7). It

-20C -30 C -40 C -20C - 30C -40C

1800

Ï, 1700 o>

ë- 1600 c o 'g. 1500 -o y)

< 1400

1300

p ^ o o o o

(a)

I

30 60 90 120 150 180 0 30 60 90 120 150 180

Contact time (min) (b) Contact time (min)

Fig. 7 Effect of contact time and temperature on adsorption of zinc ions: (a) sediment fraction 0-75 Jim, and (b) sediment fraction 210-250 um.

Adsorption of zinc onto bed sediments of the River Ganga 429

is evident from the plots that adsorption of zinc increases with an increase in temperature.

Thermodynamic parameters

Thermodynamic parameters such as free energy change (AG°), enthalpy change (Aff°) and entropy change (AS0) were determined using the following equation (Singh et al., 1988; Catena & Bright, 1989; Frajii et al, 1992),

à.G"=-RT\aKr

AG" = AH0-TAS0

(4)

(5)

where AG° is the change in free energy (kJ moF1), AH° is the change in enthalpy (kJ mol"1), AS0 is the change in entropy (J moF1 IC1), 7 is the absolute temperature (K), R is the gas constant (= 8.314 x 10"3) and Kc is the equilibrium constant, which may be defined as:

r

C. (6)

where CA€ and Ce are the equilibrium concentrations (|a,g 1" ) of the metal ion on the adsorbent and in the solution respectively.

Equations (4) and (5) can be rewritten as:

log*; = AS0 AH0

2.3037? 2303RT (7)

When log Kc is plotted against HT, a straight line with slope A//72.303/? and intercept AS72.303/? is obtained (Fig. 8). The values of AH° and AS° were obtained from the slope and intercept of the van't Hoff plots of log Kc vs IIT (Fig. 8). The thermodynamic parameters for the adsorption process are given in Table 4.

Positive values of AH° suggest the endothermic nature of the adsorption. The negative values of AG0 indicate the spontaneous nature of the adsorption process.

0.0 4

3.1 3.2 3.3 3.4

(1/T)x1000(1/K)

3.5

(b) 3.2 3.3 3.4

(1/T)x1000x(1/K)

3.5

Fig. 8 Van't Hoff plot for adsorption of zinc ions: (a) sediment fraction 0-75 (Am, and (b) sediment fraction 210-250 um.

430 C. K. Jain

Table 4 Thermodynamic parameters for the adsorption of zinc ions.

Sediment fraction (Urn)

0-75

210-250

Temperature (°Q

20 30 40 20 30 40

Kc

3.505 4.780 7.000 2.472 3.202 4.376

AG0

(kJ mol4)

-3.055 -3.942 -5.065 -2.206 -2.930 -3.842

Aff° (kJ moF1)

26.337

21.745

A5° (J mol 'V)

100.19

81.65

However, the negative value of AG° decreased with an increase in temperature, indicating that the spontaneous nature of adsorption is inversely proportional to the temperature. The positive values of A5° show the increased randomness at the solid/solution interface during the adsorption process. The adsorbed water molecules, which are displaced by the adsorbate species, gain more translational energy than is lost by the adsorbate ions, thus allowing the prevalence of randomness in the system. Enhancement of adsorption capacity at higher temperatures may be attributed to the enlargement of pore size and/or activation of the adsorbent surface (Vishwakarma et al, 1989).

ADSORPTION DYNAMICS

The rate constant of adsorption is determined using Lagergren first order rate expression (Singh et al, 1989; Namasivayam & Ranganathan, 1995; Zhang et al, 1998):

\og(.qt-q) = \ogqt-^Xt (8)

where q and qe are amounts of metal adsorbed (jxg g"1) at time, t (min) and at equilibrium, respectively, and kad is the Lagergren rate constant for adsorption (min-1). The straight line plots of log(ge - q) vs t for different concentrations and temperatures (Fig. 9) indicate the applicability of the above equation. Values of kad were calculated from the slope of the linear plots and are presented in Table 5 and 6 for different concentrations and temperatures, respectively. The rate constant was higher at higher temperature.

Intraparticle diffusion

The rate constant for intraparticle diffusion (kid) is given by Weber & Morris (1962):

q = kJA~ (9)

where q is the amount adsorbed (iig gl) at time, t (min). Plots of q vs tm are shown in Fig. 10 for different initial concentrations and temperatures. All the plots have the same general features of an initial curved portion followed by a linear portion and a plateau. The initial curved portion is attributed to the bulk diffusion, the linear portion

Adsorption of zinc onto bed sediments of the River Ganga 431

3.0-1

2.5-

2.0

1.5

1.0

0.5

0.0 4

0

A 0.5 mg/L D 1.0 mg/L

y = -0.0283x + 2.27 5

y = -0.0297x +2.1557

10 20 30 Time (min)

• 20C

40 50

D30C A40C

3.0

2.5-

2.0-

1.5

1.0

0.5

0.0

A 0.5 mg/L • 1.0 mg/L

y = -0.029X + 2.368

y = -0.0248X + 2.0803

• 20 C I30C

0 10 20 30 40

(b) Time (min)

50

A40C

(C)

20 30

Time (min) (d)

-0.0313x +2.3792

20 30 Time (min)

50

Fig. 9 Lagergren plots at different initial concentrations: (a) sediment fraction 0-75 |im, and (b) sediment fraction 210-250 |im; and at different temperatures: (c) sediment fraction 0-75 jirn, and (d) sediment fraction 210-250 \xm.

Table 5 Rate constant at different concentrations.

Sediment fraction Concentration of Zn Lagergren rate constant, kad Intraparticle rate constant, k, (Urn) (Hg 1") (min" ) (Hg1 g"1 min"2)

0-75

210-250

500 1000

500 1000

6.84 x 10"'

6.52 x 10"3

5.71 x 10"3

6.68 x 10"3

10.857

17.333

9.230

13.270

Table 6 Rate constant at different temperatures.

Sediment fraction Temperature (Uin) (°C)

Lagergren rate constant, km

(min"1) Intraparticle rate constant, kid

(Hg1 g"1 min"2)

0-75

210-250

20 30 40

20 30 40

5.83 x 10~2

6.52 x 10"2

10.76 x 10"3

7.21 x 10"2

6.68 x 10"2

10.76 X10"3

20.37 20.55 20.86 23.80 25.10 29.38

432 C. K. Jain

-0.5mg/L •1.0mg/L -0.5mg/L •1.0mg/L

1800

5 10

t1/2 (min)1/2

O20C D30C A40C

1800

1700

1600-

1500

1400-

1300

(C) 5 10 t1/2 (min)1/2

ÇT

o

o

<

1600-

1400-

1200-

1000-

800

600-I X**

m • •

- A - A * A A A A

(b) 5 10

t1/2(min)1/2

15

• 20 C I30C A40C

E

<

1600-

1500^

1400-

1300

ionn -

M —m • • • ' - •

Sr~* / ^

15 (d)

5 10 t1/2(min)1/2

15

Fig. 10 Intraparticle diffusion plots at different initial concentrations: (a) sediment fraction 0-75 pxa, and (b) sediment fraction 210-250 \im; and at different temperatures: (c) sediment fraction 0-75 \im, and (d) sediment fraction 210-250 |J,m.

to the intraparticle diffusion and the plateau to the equilibrium. This indicates that transport of zinc ions from the solution through the particle solution interface, into the pores of the particle as well as the adsorption on the available surface of sediment are both responsible for the uptake of zinc ions. The deviation of the curves from the origin also indicates that intraparticle transport is not the only rate-limiting step. The values of rate constants (kid) were obtained from the slope of the linear portion of the curves for each concentration of metal ions (Table 5) and temperatures (Table 6). The value of intraparticle rate constant (kid) was higher at higher concentration. Increasing temperature slightly increased the kid, but did not have any significant effect.

CONCLUSION

The study has shown the potential of freshly deposited sediments to adsorb zinc ions. Although the zinc ions have more affinity for the clay and silt fraction of the sediment, the overall contribution of coarser fraction to adsorption is high. The adsorption data suggest that the pH is the most important parameter in the control of zinc adsorption.

Adsorption of zinc onto bed sediments of the River Ganga 433

The percentage adsorption increases with increasing adsorbent doses, and as such removal increases with decreasing size of the adsorbent particle. The two important geochemical phases, iron and manganese oxide, also play an important role in the adsorption process. The results of this experimental study are highly useful and may be extended to other rivers with coarser sediments. The relative contribution of individual components could not be obtained from the present studies because in natural systems, type and composition of sediment mineral and organic fractions vary simultaneously and the effect of individual constituents cannot be isolated. The equilibrium data describe the Langmuir and Freundlich isotherm models satisfactorily. The kinetic data suggest that the adsorption of zinc onto bed sediments is an endothermic process, which is spontaneous at low temperature. The uptake of zinc is controlled by both bulk as well as intraparticle diffusion mechanisms.

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Received 31 May 2000; accepted 7 December 2000