Biomedical Engineering Reference
In-Depth Information
Light is also of fundamental importance and, in algal cultures, light rapidly becomes
limiting due to absorption by the algal biomass. Light intensity also interacts strongly with
temperature. Algae growth rate increases with temperature until an optimal value is reached.
Further increases in temperature usually lead to a rapid decline in growth rate. At
temperatures close to the optimum, algae are also better able to tolerate much higher light
intensity before photoinhibition sets in [9].
In a general way, microalgae growth is complex, revealing non linear behaviours in
response to the alteration of several environmental parameters such as pH, temperature, light
intensity and nutrients, and the interactions between these factors are not well established [8,
10].
Heavy metals removal by the green algae has been widely recognized [11-15]. According
to Avery et al. [16], the accumulation of heavy metals comprises a rapid adsorption phase,
reversible and metabolism independent and a slower one, metabolism dependent, often
irreversible. Batch cultures of
Chlorella fusca
were used for testing the toxicity of Pb(II),
Cr(III), Cr(VI) and Cd(II) [17].
Tam et al. [18] investigated the influence of the concentration of C
hlorella vulgaris
dried
cells on copper removal from aqueous solution. The evolution of nitrogen and phosphorus
uptake in presence and absence of Pb (10
-6
M), at pH 7 and 4, was studied by Capelo et al.
[19] using the microalgae
Selenastrum Capricornutum
. Batch cultures of
Chlorella fusca
were used for testing the toxicity of Cu(II) and Zn(II) [20]
and also Pb(II), Cr(III), Cr(VI) and
Cd(II) [17].
Lam et al
.
[13] evaluated the effect of Cd and Cu on
Chlorella vulgaris
growth.
The direct toxic effect due to copper addition and the indirect impact arising from a decrease
in pH following an increase in the metal ions concentration were investigated. Light intensity
was in the range 85-90 μE s
-1
m
-2
. Lupi et al. [21] studied the inhibition of
Chlorella vulgaris
by Cu, at 27 ºC and 100 or 150 W m
-2
light intensity. The effect of the contact time on the
binding capacity of five toxic metals (Cd, Cu, Ni, Pb and Zn) in solution by
Chlorella
vulgaris
at pH 8.5 and constant temperature was also evaluated [14].
For an efficient biomass separation, good flocculation must occur [4, 22]. Algae
autoflocculation may result from a pH rise (>8), promoting an initial nucleation of calcium
phosphate crystals. In the presence of Ca
2+
, the calcium phosphate, positively charged,
precipitates, adsorbing the negatively charged cells, agglomerating them and promoting their
flocculation [8]. Under these conditions, polyvalent cations of Cr, Cu, Fe, Mn, Pb, Sr and Zn,
together with Ca and Mg, tend to become incorporated into the flocs, which settle down and
promote water clarification [23].
This work aims to describe the growth kinetics of pure cultures of four microalgae,
Chlorella fusca
ACOI 621,
Chlorella vulgaris
ACOI 879,
Scenedesmus acutus
ACOI 538
and
Scenedesmus obliquus
ACOI 550. Results are compared with those obtained in the
presence of Cr(VI). The effect of temperature and initial pH upon algal growth was also
investigated.
2. Modelling
Growth of unicellular organisms like microalgae is often considered as a first-order
autocatalytic reaction [24-26]. So, the increase in biomass concentration,
n
(mg l
-1
), as a
function of time,
t
(h), can be expressed as:
