BIOSORPTION OF Pb2+, Cd2+, & Ni2+ FROM WATERS BY ...

different pHs while keeping the other parameters constant. Solution temperature,
solution volume, biosorbent amount, initial metal ion concentration, and shaking time
were 25ºC, 10.0 mL, 10.0 mg, 10.0 mg/L, and 60 min, respectively. For Pb 2+ , pH of the
solution was varied from 2.0 to 6.0, while for Cd 2+ and Ni 2+ the range was 2.0 to 12.0.
Three of the algae, namely Dunaliella salina, Oocystis sp. and Scenedesmus
protuberans have demonstrated a very similar sorption characteristics towards the metal
ions investigated. The “sorption % vs. pH” plots have reached a plateau around pH=4.0
(Figure 3.7). After this pH, the change in the sorption percentage was not very
significant. On the other hand, the maximum sorption with Porphyridium cruentum
could only be obtained after a pH of 10.0 for both Cd ve Ni. As with the other metal
ions, even Porphyridium cruentum exhibited a very efficient sorption for Pb 2+ at
pH=4.0. Another critical point with Pb 2+ ions was that the pH values greater than 6.0
was not applied in the sorption experiments due to the possibility of formation of Pb-
hydroxides. Actually, during the initial stages of optimization for Pb sorption, when the
pH of the solution was adjusted to 8.0, an immediate formation of a white cloudiness,
possibly due to the precipitation of Pb(OH)2, was observed.
Although it has been shown that any pH greater than 4.0 could be used for
sorption, pH of 6.0 was selected as a compromise. As explained before, higher pHs
were not applied due to the possibility of formation of metal hydroxides. It can be said
that if the objective is the removal of toxic metal ions from the solutions, it does not
matter whether it is the biosorption of the metal ions or the precipitation of metal
hydroxides causing the removal. However, in a study like this, the aim is to enlighten
the actual mechanism responsible for the elimination of the pollutants. Therefore, the
subsequent studies were focused on the understanding of the characteristics of sorption
by the biomasses. A possible mechanism might be the electrostatic attraction between
the metal ions in the solution and the functional groups of the biomass. It has been
reported that the isoelectronic point (IEP) of algae lies between 3.0–4.0, and at pH
values greater than the IEP of that specific alga, functional groups on its surface will be
negatively charged (Christ, et al. 1981, Forster 1997). This leads to electrostatic
attraction between the metal cations and the negatively charged functional groups on the
biomass. The speciation diagrams of Pb, Cd, and Ni given in Figure 3.8 indicate the pH
dependent forms of these metals in the solution. As can be seen from the diagrams, the
predominant forms of the metals at pH 6.0 are +2 oxidation state; namely, Pb 2+ , Cd 2+ ,
and Ni 2+ . These diagrams, together with the “% sorption vs. pH graph” have proven that
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