Environmental Engineering Reference
In-Depth Information
Fig. 7.3
Chemical structure of alginate [42]
on the sources (Fig. 7.3). Alginate is commercially available as a sodium salt of
alginate.
Gelation of alginate is a cross-linking of alginate with divalent cations, such as
Ca
2+
,Ba
2+
, and Sr
2+
. Calcium is the most widely used cross-linker and the CA
entrapment is simple, quick, and inexpensive. Normally, CA gel is prepared by the
droplet gelation method. When the sodium alginate solution is in contact with the
Ca
2+
solution, a semi-solid structure is formed immediately in its outer layer. The
Ca
2+
solution then passes through the outer layer to form the gel structure for the
entire alginate bead. The chemical structure of alginate affects its properties, stabil-
ity, and biodegradability [42]. Alginate containing high G content, especially with
a long GG structure, provides high gel strength and low shrinkage. This is because
the GG block favors more cation bindings, which consequently lead to higher gel
stability.
The CA gel is stable in broad ranges of pH (pH of 3-10) and temperature (up to
85
◦
C) [41]. The drawbacks of the CA gel are gel abrasion and swelling under some
conditions [4, 40, 41, 43]. The CA gel beads are demolished in the environment
containing high concentrations of divalent cations (except Ca
2+
), phosphate, and
chelating agents and swell in the presence of monovalent cations.
Procedures of Calcium Alginate Cell Entrapment and De-Entrapment
The CA cell entrapment procedures are similar in most previous studies. The fol-
lowing procedure is one of the successful methods which was used in several
environmental applications [20, 44, 45]. Sodium alginate powder is dissolved in
deionized water (DI) at 2% (w/v). To prevent agglomeration, the powder is slowly
added into stirred DI. The solution is stirred until all the powder is totally dis-
solved, which could take up to 12 h. A liquid medium containing microbial cells
is centrifuged at 7000 rpm for 10 min to obtain concentrated cells, which are then
uniformly mixed with the sodium alginate solution. The mixture is dropped into a
calcium chloride solution of 3.5% (w/v) using a peristaltic pump at a flow rate of
3 mL/min (bead size of 2.0-5.0 mm depending on the pump head). The droplets
