Biomedical Engineering Reference
In-Depth Information
especially useful to control pH, since phosphorus is required as nutrient and they buffer the
solution at around neutral pH, where microorganisms like microalgae grow optimally. In this
work, the culture medium was buffered with hydrogen / di-hydrogen phosphates.
3.3. Cr(VI) Solutions
Hexavalent chromium solution was prepared by dissolving K
2
Cr
2
O
7
in distilled and
demineralised water. Test cultures were spiked with pre-defined volumes of this solution in
order to get different Cr(VI) concentrations.
3.4. Analytical Techniques
Light intensity
was measured using a luxmeter (Luxmeter HD 8366 Delta OHM).
The cell concentration was determined by optical density measurements at 680 nm, as
used by Schelenz [28]. Optical densities were measured with a UV-VIS Jenway 6405
spectrophotometer, using the sterile
medium culture as blank. In the range of optical densities
measured, cell concentration is proportional to optical density. A calibration curve was
prepared by plotting dry biomass weight against optical density. In order to eliminate the
interference of cell sedimentation in optical density values, samples were previously stirred in
a Vortex Mixtub, Raypa. The pH was daily measured with a 540 GLP/ WTW pH meter. Cells
were counted in an improved Neubauer hemacyclometer.
Total organic carbon (TOC) and dissolved organic carbon (DOC) concentrations were
measured using a Rosemount Analytical Dohrman DC-190 analyser. Prior to DOC
measurements, samples were centrifuged at 5470 rpm, in an Alresa Mod Digicen centrifuge,
for 15 minutes and then filtered through 0.45 μm porosity membranes (Gellman Sciences).
3.5. Experimental Procedures
Four 250-ml Erlenmeyer flasks containing 250 ml of medium were inoculated with the
above referred microalgae species to obtain an initial cell concentration around 100 000 cells
ml
-1
, which has given the best results in preliminary experiments. A flask containing only
culture medium was also used as blank for optical density measurements, sterilization control
and pH variation. Experiments were carried out at temperature approximately constant.
Mixing was accomplished and some CO
2
was provided by bubbling sterile air into the culture
medium. As source light, special Osram L36W/72 Biolux lamps, giving a spectrum similar to
sunlight, were used. Experiments were performed under a 24 h light photoperiod. All glass
material was sterilized at 180ºC for 1 h 30 min.
In the first set of experiments, room temperature was controlled at 24.6
±
1.0ºC
(mean
±
standard deviation). Daily pH measurements were carried out but no pH corrections
were performed.
Mixing was maintained approximately uniform in all flasks by adjusting the diffused air
flowrate. Besides, flasks were also manually stirred several times a day, to minimize the
formation of a biofilm onto the flask wall.