**3.3. Effect of pH on sorption**

ln ln ln *e FF e q KbC* = + (6)

where *qe* and *Ce* have the same meaning as in Eq. (3); the numerical value of *KF* presents adsorption capacity, and *bF* indicates the energetic heterogeneity of adsorption sites [24]. From the data, if a plot of **ln** *qe*versus **ln** *Ce* gave a straight line, it indicates that the adsorption

The algal material was modified with an amino compound to improve its thermal stability [9]. The resulting product obtained was a water-insoluble solid material. The raw and the modified products were characterized using FTIR to obtain the functional groups present; sorption parameters were established and then used for adsorption experiments in both synthetic

The modified and unmodified materials were characterized with FTIR, and the resulting

The results show the presence of many functional groups capable of metal sorption. The broad and strong band at 3400.3 cm−1 could be attributed to either ─OH or ─NH group [25]. The band at 2927.72 cm−1 was assigned to C─H stretches, while the band at 1651.0 cm−1 was assigned to stretching ─OH, C═O or N═C [25] .The band at 1380.9 cm−1 confirms the presence of an amide

prescribes to the Freundlich model.

**3. Results and discussion**

solutions and real water samples.

spectra are presented in **Figure 1**.

**Figure 1.** FTIR spectrum of unmodified algae.

**3.2. FTIR analysis of modified and unmodified algae**

**3.1. Introduction**

252 Water Quality

The adsorption of metal ions into the biosorbent is dependent on pH of the solution. pH affects the biosorbent surface charge and degree of ionization. The sorbent also has nitrogen atoms (with a lone pair of electrons) which can be influenced by the pH of the medium. The effect of pH on sorption of lead, cadmium and copper ions are represented in **Figure 3**.

The pH of the solution influences the chemistry of the metal binding sites and the behaviour of the metal itself in solution. The results show that the maximum adsorption for lead was found to occur at a pH of 3.5 by the unmodified sorbent and at a pH of 7.0 for this same metal by the modified adsorbent. There was an increase in the amount adsorbed as the pH increased from 3.5 to 7.0. Beyond this, as the pH increases, the amount adsorbed decreases. Similar results have been reported for other biosorbents [29]. Singh and co-workers (2006) also reported that 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.

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

At low pH, the biomass surface would be completely covered with hydrogen ions. H+ lead (II) ions cannot compete effectively for the adsorption sites. This can be attributed to the fact that protons are strongly competing due to their high concentration. Godhane et al. [32] reported that the minimal sorption obtained at low pH may be due to high mobility of protons and partly due to the fact that the solution pH influences the sorbent surface charge.

The unmodified biosorbent has a maximum adsorption for cadmium at a pH of 5.2, while the modified form was at a pH of 6.7. Similar results were obtained by Singh and co-workers [30] when they investigated the adsorption of cadmium using phosphatic clay. They observed maximum adsorption at a pH of 5.4.

Copper unlike the other metals has a maximum adsorption at a pH of 4.2 for both the modified and unmodified sorbents. This can be explained by the small size of copper giving it a high polarizing power on electrons of the adsorbent [33].

The sorbent has nitrogen atoms (with alone pair of electrons) as well as other functional groups, all of which may be influenced by pH. At low pH, the adsorbent is positively charged because the pH is lower than the isoelectric point or point of zero charge (PZC), i.e. pH < PZC. At such low pH range, adsorption is poor due to the charge on the adsorbent [34]. At high pH (pH > PZC), the adsorbent is negatively charged contributing to the high adsorption [26]. This arises from the fact that when the metal is in solution, it is positively charged and will be attracted to the surface of the negatively charged adsorbent at pH > PZC favouring adsorption. At pH > 7, there is metal hydrolysis leading to precipitation due to formation of hydroxyl metal ions [35].
