3.3. Effect of solution of pH on metal ion uptake
Fig. 4 shows the metal ions uptake in the solutions of lead nitrate,
cadmiumnitrate and cobalt nitrate, as a function of pH. In the experimental
system (maximum concentration 2.0 mg/l), Pb2+ remains
the dominant species below pH 5.5. Beyond this pH, solid lead
hydroxide is thermodynamically the most stable phase. In addition,
the effect of sodium and nitrate ions (present from pH adjustment
during the sorption experiments) on the metal speciation was negligible.
The same methods were applied to determine the metal
speciation for the other 2 metal ion solutions at a concentration of
2.0 mg/l. For instance, it was found that Cd remains as the dominant
species in the copper nitrate system if the pH is maintained below
5.5. In the systems with cobalt nitrate, the metallic ions are the
dominant species below pH 6.0. It is well documented that solution
pH is an important parameter that affects the biosorption of heavy
metal ions [20,21]. Fig. 4 shows the biosorption of lead, cadmium
and cobalt by un-extracted dyes mangosteen shell and extracted
dyes mangosteen shell. It can be seen that higher pH led to higher
metal uptake. Un-extracted dyes mangosteen shell showed better
biosorption performance than extracted dyes in the removal of all
three metal ions.