**2.2 Particle size of adsorbent**

Smaller particle sizes reduce internal diffusion and mass transfer limitation to penetrate of the adsorbate inside the adsorbent (i.e., equilibrium is more easily achieved and nearly full adsorption capability can be attained). Figure 2 represents the removal efficiency Fe3+ ions by natural zeolite through three different particle sizes (45, 125 and 250 m). It can be observed that the maximum adsorption efficiency is achieved with particle size 45 m. This is due to the most of the internal surface of such particles might be utilized for the adsorption. The smaller particle size gives higher adsorption rates, in which the Fe3+ ion has short path to transfer inside zeolite pores structure of the small particle size [19].

Fig. 2. Percent removal of Fe3+ ions (1000 ppm) *vs.* natural zeolite particle size: 1 g adsorbent/ 50 ml Fe3+ ion solution, initial pH of 1% HNO3, and 300 rpm.

### **2.3 Contact time or residence time**

The longer residence time means the more complete the adsorption will be. Therefore, the required contact time for sorption to be completed is important to give insight into a sorption process. This also provides an information on the minimum time required for considerable adsorption to take place, and also the possible diffusion control mechanism

Thermodynamics Approach in the Adsorption of Heavy Metals 741

Large molecules may be too large to enter small pores. This may reduce adsorption

More highly ionized molecules are adsorbed to a smaller degree than neutral molecules.

The degree of ionization of a species is affected by the pH (e.g., a weak acid or a weak basis). This, in turn, affects adsorption. For example, the precipitation of Fe3+ ions occurred at pH greater than 4.5 (see Figure 4). The decrease in the Fe3+ ions removal capacity at pH > 4.5 may have been caused by the complexing Fe3+ ions with hydroxide. Therefore, the removal efficiency increases with increasing initial pH. For example, at low pH, the concentration of proton is high. Therefore, the positively charged of the Fe3+ ions and the protons compete for binding on the adsorbent sites in Zeolite, Bentonite, Quartz, olive cake, Tripoli in which, this process decrease the uptake of iron ions. The concentration of proton in the solution decrease as pH gradually increases in the ranges from 2 to 4.5. In this case, little protons have the chance to compete with Fe3+ ions on the adsorption sites of the olive cake. Thus, higher pH in the acidic media is facilitated the greater uptake of Fe3+ ions. Above pH 4.5, the removal efficiency decreases as pH increases, this is inferred to be attributable to the

> 13579 pH

Fig. 4. Effect of initial pH on the removal efficiency, %, of Fe3+ ions at different temperatures. (Initial concentration of Fe3+ ions: 100 ppm, Agitation speed: 100 rpm, Mass of olive cake: 1

At high-level concentrations, the available sites of adsorption become fewer. This behaviour is connected with the competitive diffusion process of the Fe3+ ions through the micro-

**2.6 Size of the molecule with respect to size of the pores** 

**2.7 Degree of ionization of the adsorbate molecule** 

independently of other causes.

**2.8 pH** 

hydrolysis [19-22].

20

g, Dose: 5 g/l, Contact time: 24 hr).

**2.9 Effect of initial concentration** 

30

40

50

60

The removal efficiency of Fe (III),

70

80

28 C 35 C 45 C

90

between the adsorbate, for example Fe3+ ions, as it moves from the bulk solution towards the adsorbent surface, for example natural zeolite [19].

For example, the effect of contact time on sorption of Fe3+ ions is shown in Figure 3. At the initial stage, the rate of removal of Fe3+ ions using natural quartz (NQ) and natural bentonite (NB) is higher with uncontrolled rate. The initial faster rate may be due to the availability of the uncovered surface area of the adsorbent such as NQ and NB initially [20]. This is because the adsorption kinetics depends on: (i) the surface area of the adsorbent, (ii) the nature and concentration of the surface groups (active sites), which are responsible for interaction with the Fe3+ ions. Therefore, the adsorption mechanism on both adsorbent has uncontrolled rate during the first 10 minutes, where the maximum adsorption is achieved. Afterward, at the later stages, there is no influence for increasing the contact time. This is due to the decreased or lesser number of active sites. Similar results have been shown in our results using zeolite and olive cake as well as other reported in literatures for the removal of dyes, organic acids and metal ions by various adsorbents [19, 21].

Fig. 3. Adsorption of Fe3+ ions onto olive cake. Variation of the Fe3+ ions concentration with time. (Initial concentration of Fe3+ ions: 100 ppm, Agitation speed: 100 rpm, pH: 4.5, temperature 28 ºC).

#### **2.4 Solubility of adsorbent/ heavy metals in wastewater/ water**

The slightly soluble metal ions in water will be more easily removed from water(i.e., adsorbed) than substances with high solubility. Also, non-polar substances will be more easily removed than polar substances since the latter have a greater affinity for adsorption.

#### **2.5 Affinity of the solute for the adsorbent**

If the surface of adsorbent is slightly polar, the non-polar substances will be more easily picked up by the adsorbent than polar ones (the opposite is correct).

### **2.6 Size of the molecule with respect to size of the pores**

Large molecules may be too large to enter small pores. This may reduce adsorption independently of other causes.

#### **2.7 Degree of ionization of the adsorbate molecule**

More highly ionized molecules are adsorbed to a smaller degree than neutral molecules.

#### **2.8 pH**

740 Thermodynamics – Interaction Studies – Solids, Liquids and Gases

between the adsorbate, for example Fe3+ ions, as it moves from the bulk solution towards the

For example, the effect of contact time on sorption of Fe3+ ions is shown in Figure 3. At the initial stage, the rate of removal of Fe3+ ions using natural quartz (NQ) and natural bentonite (NB) is higher with uncontrolled rate. The initial faster rate may be due to the availability of the uncovered surface area of the adsorbent such as NQ and NB initially [20]. This is because the adsorption kinetics depends on: (i) the surface area of the adsorbent, (ii) the nature and concentration of the surface groups (active sites), which are responsible for interaction with the Fe3+ ions. Therefore, the adsorption mechanism on both adsorbent has uncontrolled rate during the first 10 minutes, where the maximum adsorption is achieved. Afterward, at the later stages, there is no influence for increasing the contact time. This is due to the decreased or lesser number of active sites. Similar results have been shown in our results using zeolite and olive cake as well as other reported in literatures for the removal of

> 0 20 40 60 80 100 120 140 160 t, min.

Fig. 3. Adsorption of Fe3+ ions onto olive cake. Variation of the Fe3+ ions concentration with time. (Initial concentration of Fe3+ ions: 100 ppm, Agitation speed: 100 rpm, pH: 4.5,

The slightly soluble metal ions in water will be more easily removed from water(i.e., adsorbed) than substances with high solubility. Also, non-polar substances will be more easily removed than polar substances since the latter have a greater affinity for adsorption.

If the surface of adsorbent is slightly polar, the non-polar substances will be more easily

**2.4 Solubility of adsorbent/ heavy metals in wastewater/ water** 

picked up by the adsorbent than polar ones (the opposite is correct).

**2.5 Affinity of the solute for the adsorbent** 

adsorbent surface, for example natural zeolite [19].

0

20

40

60

Ct

temperature 28 ºC).

80

100

120

dyes, organic acids and metal ions by various adsorbents [19, 21].

The degree of ionization of a species is affected by the pH (e.g., a weak acid or a weak basis). This, in turn, affects adsorption. For example, the precipitation of Fe3+ ions occurred at pH greater than 4.5 (see Figure 4). The decrease in the Fe3+ ions removal capacity at pH > 4.5 may have been caused by the complexing Fe3+ ions with hydroxide. Therefore, the removal efficiency increases with increasing initial pH. For example, at low pH, the concentration of proton is high. Therefore, the positively charged of the Fe3+ ions and the protons compete for binding on the adsorbent sites in Zeolite, Bentonite, Quartz, olive cake, Tripoli in which, this process decrease the uptake of iron ions. The concentration of proton in the solution decrease as pH gradually increases in the ranges from 2 to 4.5. In this case, little protons have the chance to compete with Fe3+ ions on the adsorption sites of the olive cake. Thus, higher pH in the acidic media is facilitated the greater uptake of Fe3+ ions. Above pH 4.5, the removal efficiency decreases as pH increases, this is inferred to be attributable to the hydrolysis [19-22].

Fig. 4. Effect of initial pH on the removal efficiency, %, of Fe3+ ions at different temperatures. (Initial concentration of Fe3+ ions: 100 ppm, Agitation speed: 100 rpm, Mass of olive cake: 1 g, Dose: 5 g/l, Contact time: 24 hr).

### **2.9 Effect of initial concentration**

At high-level concentrations, the available sites of adsorption become fewer. This behaviour is connected with the competitive diffusion process of the Fe3+ ions through the micro-

Thermodynamics Approach in the Adsorption of Heavy Metals 743

that the removal efficiency of adsorbents generally improved with increasing amount of JNZ. This is expected because the higher dose of adsorbent in the solution, the greater availability of exchangeable sites for the ions, *i.e.* more active sites are available for binding of Fe3+ ions. Moreover, our recent studies using olive cake, natural quartz and natural bentonite and tripoli [19-22] are qualitatively in a good agreement with each other and with

> 0 10 20 30 40 50 **dose,g/l**

Fig. 6. The adsorbent dose of JNZ vs. Percent removal of Fe3+ ions: 1 g adsorbent/ 50 ml Fe3+ ions solution, 30 C, initial pH of 1% HNO3, 300 rpm, and constant initial concentration

Adsorption from solution is usually conducted using either the column or the batch operation. It should be possible to characterize the solution - adsorbent system by both technique operations and arrive at the same result. This is due to the physical and/or chemical forces applicable in each case must be identical. Furthermore, the results obtained from the batch experiment should be somewhat more reliable. Among the most serious objections of the column experiments are: (1)the inherent difficulties associated to maintain a constant flow rate; (2) the difficulty of ensuring a constant temperature throughout the column; (3) the appreciable probability of presence the channels within the packed column; and (4) the relatively large expenditure both in time and manpower required for a column experiment.

In a batch operation, fixed amount of adsorbent is mixed all at once with specific volume of adsorbate (with the range of initial concentration). Afterwards, the system kept in agitation for a convenient period of time. Separation of the resultant solution is accomplished by filtering, centrifuging, or decanting. The optimum pH, contact time, agitation speed and optimum temperature are fixed and used in this technique. For instance, the contact time study, the experiment are carried out at constant initial concentration, agitation speed, pH, and temperature. During the adsorption progress, the mixture container must be covered by

those found in the literatures [26].

**The percent rem**

**3. Adsorption operation** 

**3.1 Batch operation** 

(1000 ppm).

**oval of Fe (III), %**

channel and pores in NB [20]. This competitive will lock the inlet of channel on the surface and prevents the metal ions to pass deeply inside the NB, *i.e.* the adsorption occurs on the surface only. These results indicate that energetically less favorable sites by increasing metal concentration in aqueous solution. This results are found matching with our recently studies using natural zeolite [19] and olive cake [21], in addition to other reported by Rao *et al.* [23] and Karthikeyan *et al.* [24]. The removal efficiency of Fe3+ ions on NQ and NB as well as zeolite at different initial concentrations (50, 100, 200, 300 and 400 ppm) is shown in the Figures 5-6. It is evident from the figure that the percentage removal of Fe3+ ions on NQ is slightly depended on the initial concentration. While the removal efficiency of Fe3+ ions using NB decreases as the initial concentration of Fe3+ ions increases. For example, the percentage removal is calculated 98 % using the initial concentration of 50 ppm, while it is found 28 % using high-level of 400 ppm [20].

On the other hand, it is clear from Figure 6 that the removal efficiency of Fe3+ ions using NQ is less affected by the initial concentration. For instance, the percentage removal using 50 ppm of the initial concentration is found 35 %, while is found 34.9 % using high-level concentrations (400 ppm). This means that the high concentration of Fe3+ ions will create and activate of some new activation sites on the adsorbent surface [20, 25].

Fig. 5. The effect of initial concentration namely 50, 100, 200, 300 and 400 ppm of Fe3+ ions at constant contact time (2.5 hours), adsorbent dosage 4 g/L of natural NQ and NB, Temperature (30 ºC) and agitation speed (300 rpm).

#### **2.10 Dosage effect**

The removal efficiency is generally increased as the concentration dose increases over these temperature values. This can be explained by the fact that more mass available, more the contact surface offered to the adsorption. The effect of the Jordanian Natural Zeolite (JNZ) dosage on the removal of Fe3+ ions is shown in Figure 6 [19]. The adsorbent dosage is varied from 10 to 40 g/l. The initial Fe3+ ions concentration, stirrer speed, initial pH and temperature are 1000 ppm, 300 rpm, 1% HNO3, and 30 ºC, respectively. This figure shows that the maximum removal of 69.15 % is observed with the dosage of 40 g/l. We observed

channel and pores in NB [20]. This competitive will lock the inlet of channel on the surface and prevents the metal ions to pass deeply inside the NB, *i.e.* the adsorption occurs on the surface only. These results indicate that energetically less favorable sites by increasing metal concentration in aqueous solution. This results are found matching with our recently studies using natural zeolite [19] and olive cake [21], in addition to other reported by Rao *et al.* [23] and Karthikeyan *et al.* [24]. The removal efficiency of Fe3+ ions on NQ and NB as well as zeolite at different initial concentrations (50, 100, 200, 300 and 400 ppm) is shown in the Figures 5-6. It is evident from the figure that the percentage removal of Fe3+ ions on NQ is slightly depended on the initial concentration. While the removal efficiency of Fe3+ ions using NB decreases as the initial concentration of Fe3+ ions increases. For example, the percentage removal is calculated 98 % using the initial concentration of 50 ppm, while it is

On the other hand, it is clear from Figure 6 that the removal efficiency of Fe3+ ions using NQ is less affected by the initial concentration. For instance, the percentage removal using 50 ppm of the initial concentration is found 35 %, while is found 34.9 % using high-level concentrations (400 ppm). This means that the high concentration of Fe3+ ions will create and

> Fe-NB Fe-NQ

0 100 200 300 400 **Ci (ppm)**

Fig. 5. The effect of initial concentration namely 50, 100, 200, 300 and 400 ppm of Fe3+ ions at

The removal efficiency is generally increased as the concentration dose increases over these temperature values. This can be explained by the fact that more mass available, more the contact surface offered to the adsorption. The effect of the Jordanian Natural Zeolite (JNZ) dosage on the removal of Fe3+ ions is shown in Figure 6 [19]. The adsorbent dosage is varied from 10 to 40 g/l. The initial Fe3+ ions concentration, stirrer speed, initial pH and temperature are 1000 ppm, 300 rpm, 1% HNO3, and 30 ºC, respectively. This figure shows that the maximum removal of 69.15 % is observed with the dosage of 40 g/l. We observed

constant contact time (2.5 hours), adsorbent dosage 4 g/L of natural NQ and NB,

activate of some new activation sites on the adsorbent surface [20, 25].

found 28 % using high-level of 400 ppm [20].

0

Temperature (30 ºC) and agitation speed (300 rpm).

20

40

60

**% Removal of Fe(III)**

**2.10 Dosage effect** 

80

100

120

that the removal efficiency of adsorbents generally improved with increasing amount of JNZ. This is expected because the higher dose of adsorbent in the solution, the greater availability of exchangeable sites for the ions, *i.e.* more active sites are available for binding of Fe3+ ions. Moreover, our recent studies using olive cake, natural quartz and natural bentonite and tripoli [19-22] are qualitatively in a good agreement with each other and with those found in the literatures [26].

Fig. 6. The adsorbent dose of JNZ vs. Percent removal of Fe3+ ions: 1 g adsorbent/ 50 ml Fe3+ ions solution, 30 C, initial pH of 1% HNO3, 300 rpm, and constant initial concentration (1000 ppm).
