positive surface charge resulting in a lower electrostatic repulsion
between the surface and metal ions [14]. Further increase
in biosorption was insignificant as the optimum biosorption
for all the three metals was reached at pH 5 (97.3% copper,
95.3% cadmium, and 66.6% zinc). All the three metals
showed a trend of declining sorption when the pH was increased
to 6. This may be attributed to the decreased solubility
of metals at high pH. For this reason, metal ion biosorption
was not investigated at pH above 6.