**3.4. Effect of contact time on sorption**

the highest percentage of lead (II) ions was adsorbed by phosphatic clay at a pH of 5.0 [30]. Similar results were reported by Matheickal et al. [26] when they studied the biosorption of lead by marine alga *Ecklonia radiata*. Benhima et al. [31] observed that there was an increase in lead (II) ion uptake by inert organic matter (IOM) as the pH increased from 2.0 to 6.0. This is

in agreement with the observed results for lead (II) ions.

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**Figure 3.** Effect of pH on adsorption of lead, cadmium and copper ions—A, B and C, respectively.

The dependence of the adsorption process on the residence time of the adsorbate at the solid solution interphase was studied using batch sorption experiments for both the modified and unmodified algae. The solutions were set at optimal pH values of each metal, and the results of time-dependent adsorption obtained are shown in **Figure 4**.

It was observed that the general uptake rate was fast as over 90% of the adsorption which took place within the first 30 min for all the three metals after which a steady adsorption rate was realized (**Figure 3**). Keskinkan et al. [36] reported similar findings while studying the biosorption of lead (II) ions using aquatic *Ceratophyllum demersum*. Also, Yang and Volesky [37] made similar observations while studying the biosorption of cadmium (II) ions using dead brown *Sargassum fluitans*. The initial rapid uptake may be due to the physical adsorption or ion exchange at the cell surface and the subsequent slower phase due to the other mechanisms such as complexation, micro-precipitation or saturation at the binding sites [38]. Generally, there was a decrease in adsorption for the modified sorbent with the decrease more pronounced for cadmium (II) ions. Different functional groups with different affinities for the metal ions are usually present on the biomass surface; these are significantly altered by modification. New groups are also introduced which affect the binding ability of the sorbent. An example of a functional group that was affected is the –NH group which appeared at 3400.3 cm−1 and after modification shifted to a lower value of 3394.5 cm−1.

**Figure 4.** Effect of contact time on sorption of lead, cadmium and copper—A, B and C, respectively.

### **3.5. Effect of sorbent dosage on sorption**

An example of a functional group that was affected is the –NH group which appeared at 3400.3

cm−1 and after modification shifted to a lower value of 3394.5 cm−1.

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**Figure 4.** Effect of contact time on sorption of lead, cadmium and copper—A, B and C, respectively.

Results presented in **Figure 5** show the effect of varying the mass of the sorbent on the adsorption of the metal ions. The experiment was performed while the solutions were buffered at optimal pH value of each respective metal ion.

**Figure 5.** Effect of sorbent dose on sorption of lead, cadmium and copper—A, B and C, respectively.

The general observation is that the amount adsorbed metal ions increased with increase in sorbate dose. This can be attributed to the fact that as the concentration of the adsorbent increases, more adsorption sites are available due to increased surface area. More metal ions can therefore occupy those available active sites [39]. It was also noted that for both the unmodified adsorbates, copper was adsorbed more than both lead and cadmium. This could be attributed to the fact that the affinity to the binding sites is related to the ionic sizes of the respective metals, hence the polarizing power of copper being responsible for the observation. Maximum adsorption by the unmodified sorbate occurred at a mass of about 0.2 g for both lead and cadmium. Mwangi and Ngila [29] reported a similar observation while studying the biosorption of lead and cadmium using green seaweed, *Caulerpa serrulata*. Higher metal ion uptake at low sorbate mass concentrations has been attributed to an increased metal to biosorbent ratio which decreases upon an increase in sorbate mass concentration [40]. The modified sorbate showed maximum adsorption at a mass of 0.7 g for these two metals. For copper, maximum adsorption occurred at a sorbate mass of about 0.7 g for both modified and unmodified algae *Pediastrum boryanum*. A plateau is reached after a certain mass for all the metal ions. The unmodified sorbent material removed more metal ions than the modified material. This decrease in the adsorption capacity could be attributed by the fact that the functional groups in the parent material responsible for binding the metals had higher complexation ability than the anchored group [41]. A similar observation was reported by Drake and co-workers as they studied the sorption of chromium by *Datura inoxia* biomaterial [42].

### **3.6. Effect of initial ion concentration for determination of adsorption capacity**

In order to investigate the adsorption capacity of both modified and unmodified algae, 50 ml solutions of concentrations ranging from 10 μg l−1 to 100 μg l−1 were added to 0.2 g of the biosorbent. **Figure 5** shows the results that were obtained.

A linear pattern of metal uptake was observed followed by some plateau for all metals. This may be attributed to the saturation of the binding sites as the concentration of the ion increases resulting to a steady state. The saturation is more pronounced at very high concentrations [31]. This is attributed to the fact that concentration is the driving force for metal ions to occupy the available binding sites [38]. It was observed that there was no significant difference in the sorption of copper and cadmium by the parent and modified material (see **Figure 6**).

The experimental data above was fitted into the Langmuir and Freundlich adsorption isotherm represented by equations (3) and (4) in Chapter 3 to determine the adsorption capacity for the metal ions using both modified and unmodified algal materials. **Table 1** is a summary of the results that were obtained.

**Table 1** shows that the adsorption for both cadmium and lead fitted well with the Freundlich model, while copper fitted well with the Langmuir model. Similar results were obtained by Ng and co-workers [22] as they studied the adsorption of metals on cross-linked seaweed. This indicates that the adsorption process for lead and cadmium is a multi-site or physical sorption as a result of weak Van der Waals forces.

A Comparative Study of Modified and Unmodified Algae (*Pediastrum boryanum*) for Removal of Lead, Cadmium... http://dx.doi.org/10.5772/65745 259

The general observation is that the amount adsorbed metal ions increased with increase in sorbate dose. This can be attributed to the fact that as the concentration of the adsorbent increases, more adsorption sites are available due to increased surface area. More metal ions can therefore occupy those available active sites [39]. It was also noted that for both the unmodified adsorbates, copper was adsorbed more than both lead and cadmium. This could be attributed to the fact that the affinity to the binding sites is related to the ionic sizes of the respective metals, hence the polarizing power of copper being responsible for the observation. Maximum adsorption by the unmodified sorbate occurred at a mass of about 0.2 g for both lead and cadmium. Mwangi and Ngila [29] reported a similar observation while studying the biosorption of lead and cadmium using green seaweed, *Caulerpa serrulata*. Higher metal ion uptake at low sorbate mass concentrations has been attributed to an increased metal to biosorbent ratio which decreases upon an increase in sorbate mass concentration [40]. The modified sorbate showed maximum adsorption at a mass of 0.7 g for these two metals. For copper, maximum adsorption occurred at a sorbate mass of about 0.7 g for both modified and unmodified algae *Pediastrum boryanum*. A plateau is reached after a certain mass for all the metal ions. The unmodified sorbent material removed more metal ions than the modified material. This decrease in the adsorption capacity could be attributed by the fact that the functional groups in the parent material responsible for binding the metals had higher complexation ability than the anchored group [41]. A similar observation was reported by Drake and co-workers as they studied the sorption of chromium by *Datura inoxia* biomaterial

**3.6. Effect of initial ion concentration for determination of adsorption capacity**

biosorbent. **Figure 5** shows the results that were obtained.

In order to investigate the adsorption capacity of both modified and unmodified algae, 50 ml solutions of concentrations ranging from 10 μg l−1 to 100 μg l−1 were added to 0.2 g of the

A linear pattern of metal uptake was observed followed by some plateau for all metals. This may be attributed to the saturation of the binding sites as the concentration of the ion increases resulting to a steady state. The saturation is more pronounced at very high concentrations [31]. This is attributed to the fact that concentration is the driving force for metal ions to occupy the available binding sites [38]. It was observed that there was no significant difference in the sorption of copper and cadmium by the parent and modified

The experimental data above was fitted into the Langmuir and Freundlich adsorption isotherm represented by equations (3) and (4) in Chapter 3 to determine the adsorption capacity for the metal ions using both modified and unmodified algal materials. **Table 1** is a summary of the

**Table 1** shows that the adsorption for both cadmium and lead fitted well with the Freundlich model, while copper fitted well with the Langmuir model. Similar results were obtained by Ng and co-workers [22] as they studied the adsorption of metals on cross-linked seaweed. This indicates that the adsorption process for lead and cadmium is a multi-site or physical sorption

[42].

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material (see **Figure 6**).

results that were obtained.

as a result of weak Van der Waals forces.

**Figure 6.** Effect of initial ion concentration on sorption of lead, cadmium and copper ions—A, B and C, respectively.


**Table 1.** Results for Langmuir and Freundlich models.

Copper on the other hand fitted well with the Langmuir model with *R*<sup>2</sup> values of 0.974 for the unmodified and 0.512 for the modified algal material. These values show that the data has a strong correlation and therefore the sorption mechanism can be prescribed to the Langmuir model. The adsorption capacities were found to be 0.059 and 0.001 mg g−1 for copper, by the unmodified and modified materials, respectively. Sorption of cadmium and lead prescribed to the Freundlich model with a sorption of 1.137 and 2.843 and 0.791 and 1.695, respectively, by the unmodified and modified adsorbent materials in the same order.

### **3.7. Kinetics of adsorption of the metal ions**

Lagergren's first-order and Ho's second-order kinetics were applied to the data that was obtained [17, 18]. This was used to determine the molecularity of the adsorption and the rate controlling step. A summary of the results that were obtained for all the three metals is shown in **Table 2**.


**Table 2.** The Lagergren first-order and Ho et al. second-order data.

For all metals the adsorption rates are very fast initially and become almost constant as time increases showing that an equilibrium has been reached.

For cadmium and copper, the adsorption followed pseudo-first-order kinetics showing that only one molecule was involved in the rate determining step [43]. The adsorption of lead followed pseudo-second-order kinetics for both the ethylenediamine modified and the unmodified algal material. Similar results were obtained by Mwangi and Ngila [29] when studying the removal of heavy metals from wastewater using ethylenediamine-modified seaweed.
