2.4. Sorptive surface area of cells
The surface area of cells was measured using a
method adopted from the surface area determination
of activated carbon (Scheffler, 1966). In the method,
methylene blue was employed as the sorptive agent. It
was considered that 1g of methylene blue is sorbed by
20.94m2 of surface area of microalgae. The adequacy
of the method described above was confirmed by gaseous
nitrogen sorption using BET method.
The method was carried out according to the following
procedure: 0.15ml of 11.1gl1 filtered solution of
methylene blue was added to 50ml of culture. After
30min, the absorbance of filtered sample was measured
at wavelength 665nm and was related to the absorbance
of methylene blue solutions at different initial concentrations.
The decrease in the absorbance was used to calculate
the specific surface area of cells that was related to
physical adsorption of methylene blue.
2.5. Potentiometric titrations
Potentiometric titrations were performed in order to
determine the deprotonation constants and absolute
concentrations of the specific functional groups present
on the algal cell walls. The cells were removed from
the medium by centrifugation and prewashed three times
with deionized water. The known quantity of cells was
suspended in deionized water, which had been bubbled
with argon for 3h in order to purge it of dissolved
CO2. The titration vessel was sealed immediately. Afterwards,
the cells were titrated with 0.1M HCl and reversely
with 0.1M NaOH. The pH of the algal suspension
was recorded after each addition of titrant, after stabilization
of the record.