Marine Macrophytes: Biosorbents 24.3 Biosorption 603
Part D | 24.3
to 70% of the dry weight of the wall and consists of
mucilaginous material that may be extracted with hot
water. It is constituted by several types of highly hydrophilic sulfated polygalactans, polymers of ˇ-(1 !
4) galactose, and ˛-(1 ! 3) linked 3,6 anhydrogalactose, which are mostly combined to form two types of
polysaccharides: agars and carrageenans. The presence
of D-galactose and anhydro-D-galactose distinguishes
the more highly sulfated carrageenans from the less
highly sulfated agars (anhydro-L-galactose) [24.60].
Agar and carrageenans have colloidal properties and
for this reason are widely used in many industrial applications [24.61]. Carrageenans consist of a repeating
disaccharide backbone: ˇ-(1 ! 4)-D-galactopyranosyl˛-(1 ! 3)-D-galactopyranosyl. There are 17 different
types of carrageenans, which occur in different species
and can also vary within a species. Agars are formed
by alternating 3-O-linked ˇ-D-galactopyranose and 4O-linked 3,6 anhydro-˛-L-galactopyranose, but the repetition can be interrupted by blocks of repeated units of
either one of the two constituents [24.60].
In all species of the order Corallinales and some
species belonging to other orders (Nemaliales, Peyssonneliales), the cell wall is encrusted by calcium carbonate. In the Corallinales this compound is present
primarily as crystals of calcite (rhomboid crystals),
whereas the calcified members of other orders deposit
it mainly as crystals of aragonite (orthorhombic crystals) [24.59]. In other red algal genera (e.g., Mazzaella)
the surface of the alga is covered by a continuous layer
of proteinaceous material [24.60]. This cuticle is neither homologous or biochemically similar to the cuticle
of vascular plants.
Phaeophyceae (Brown Algae)
In this group the fibrillar portion of the wall is formed
by a felt-like network of cellulose, which is probably
strengthened by insoluble alginates [24.6]. The amorphous portion constitutes most of the wall mass and is
composed of alginates and fucans. Alginates are salts
of alginic acid, a polymer formed by monomers of
two acid sugars (D-mannuronic acid and L-guluronic
acid) connected by ˇ-1,4 bonds. The residues can form
salts with various cations, such as Ca
CC , Mg
CC , Na
C
(this is the predominant form of alginates linked with
the cellulose fibrils in the fibrillar portion). Fucans
(or fucoidans, or ascophyllans) are sulfated polysaccharides containing, in addition to the monosaccharide
L-fucose, varying proportions of galactose, mannose,
xylose, and glucuronic acid [24.6]. The amorphous portion is responsible for the elasticity of the cell walls and
contributes to prevent desiccation of the thallus during
emersion.
Alginates confer flexibility upon the thallus,
help prevent desiccation, and function in ion exchange [24.60]. They are used extensively in industry
because of their colloidal properties. In alginates, the
relative proportion of mannuronic acid and guluronic
acid vary, depending on species, age of the thallus,
tissue, time of the year, and geographic location. The
sequence and the relative amounts of these two sugars
produce different chemical structures, and, therefore,
different physical properties of these polysaccharides.
Haug showed that the affinity of some metal cations
(Pb
2C , Cu
2C , Cd
2C , Zn
2C
/ for alginates differs in
relation to the relative amounts of the two sugars, increasing with the content of guluronic acid. The high
specificity of alginates containing high amounts of guluronic acid for bivalent cations is explained by their
zig-zag structure. In the presence of these ions, the
alginates develop a network-like arrangement by dimerization of polyguluronic sequences, which allows the
pairing of two chains and the formation of spaces suitable to accommodate Ca
CC and other bivalent cations
similar to Ca
CC in size. These spaces are produced by
the grouping of polyguluronic chains determined by the
alignment of carboxylates and other atoms of oxygens
of the guluronic units. This structure is known as the
egg-box model.
Alginates contain three different functional groups
to which metal cations may bind, carboxyl (COO),
ether (COC), and hydroxyl (OH). Carboxyl
groups are usually the most abundant acidic groups in
brown algae, and most metals (Cd, Co, Cu, Fe, Ni, Pb)
are adsorbed at pH close to the pH of dissociation of
carboxyl groups in these algae (pK 0 close to 5) [24.62].
The second most abundant acidic group is the sulfonic
acid of fucoidans [24.25]; this compound occurs both
in the amorphous matrix and in close association with
the cellulose microfibrils in the innermost portion of
the wall [24.63–65]. Sulfonic acid plays a secondary
role in the bond with metals, except when this happens at low pH. This is also the case for the hydroxyl
groups present in all polysaccharides, which are negatively charged only at pH > 10 [24.25].
Proteins represent a minor component in the cell
wall of brown algae [24.66]. This implies that the carboxyl groups of the alginates are more abundant than
the carboxyl groups and the amine groups of proteins
and represent the majority of the binding sites [24.58].
The role of the carboxyl groups in adsorption processes
has been clearly demonstrated by the uptake of Cd and
Part D | 24.3
to 70% of the dry weight of the wall and consists of
mucilaginous material that may be extracted with hot
water. It is constituted by several types of highly hydrophilic sulfated polygalactans, polymers of ˇ-(1 !
4) galactose, and ˛-(1 ! 3) linked 3,6 anhydrogalactose, which are mostly combined to form two types of
polysaccharides: agars and carrageenans. The presence
of D-galactose and anhydro-D-galactose distinguishes
the more highly sulfated carrageenans from the less
highly sulfated agars (anhydro-L-galactose) [24.60].
Agar and carrageenans have colloidal properties and
for this reason are widely used in many industrial applications [24.61]. Carrageenans consist of a repeating
disaccharide backbone: ˇ-(1 ! 4)-D-galactopyranosyl˛-(1 ! 3)-D-galactopyranosyl. There are 17 different
types of carrageenans, which occur in different species
and can also vary within a species. Agars are formed
by alternating 3-O-linked ˇ-D-galactopyranose and 4O-linked 3,6 anhydro-˛-L-galactopyranose, but the repetition can be interrupted by blocks of repeated units of
either one of the two constituents [24.60].
In all species of the order Corallinales and some
species belonging to other orders (Nemaliales, Peyssonneliales), the cell wall is encrusted by calcium carbonate. In the Corallinales this compound is present
primarily as crystals of calcite (rhomboid crystals),
whereas the calcified members of other orders deposit
it mainly as crystals of aragonite (orthorhombic crystals) [24.59]. In other red algal genera (e.g., Mazzaella)
the surface of the alga is covered by a continuous layer
of proteinaceous material [24.60]. This cuticle is neither homologous or biochemically similar to the cuticle
of vascular plants.
Phaeophyceae (Brown Algae)
In this group the fibrillar portion of the wall is formed
by a felt-like network of cellulose, which is probably
strengthened by insoluble alginates [24.6]. The amorphous portion constitutes most of the wall mass and is
composed of alginates and fucans. Alginates are salts
of alginic acid, a polymer formed by monomers of
two acid sugars (D-mannuronic acid and L-guluronic
acid) connected by ˇ-1,4 bonds. The residues can form
salts with various cations, such as Ca
CC , Mg
CC , Na
C
(this is the predominant form of alginates linked with
the cellulose fibrils in the fibrillar portion). Fucans
(or fucoidans, or ascophyllans) are sulfated polysaccharides containing, in addition to the monosaccharide
L-fucose, varying proportions of galactose, mannose,
xylose, and glucuronic acid [24.6]. The amorphous portion is responsible for the elasticity of the cell walls and
contributes to prevent desiccation of the thallus during
emersion.
Alginates confer flexibility upon the thallus,
help prevent desiccation, and function in ion exchange [24.60]. They are used extensively in industry
because of their colloidal properties. In alginates, the
relative proportion of mannuronic acid and guluronic
acid vary, depending on species, age of the thallus,
tissue, time of the year, and geographic location. The
sequence and the relative amounts of these two sugars
produce different chemical structures, and, therefore,
different physical properties of these polysaccharides.
Haug showed that the affinity of some metal cations
(Pb
2C , Cu
2C , Cd
2C , Zn
2C
/ for alginates differs in
relation to the relative amounts of the two sugars, increasing with the content of guluronic acid. The high
specificity of alginates containing high amounts of guluronic acid for bivalent cations is explained by their
zig-zag structure. In the presence of these ions, the
alginates develop a network-like arrangement by dimerization of polyguluronic sequences, which allows the
pairing of two chains and the formation of spaces suitable to accommodate Ca
CC and other bivalent cations
similar to Ca
CC in size. These spaces are produced by
the grouping of polyguluronic chains determined by the
alignment of carboxylates and other atoms of oxygens
of the guluronic units. This structure is known as the
egg-box model.
Alginates contain three different functional groups
to which metal cations may bind, carboxyl (COO),
ether (COC), and hydroxyl (OH). Carboxyl
groups are usually the most abundant acidic groups in
brown algae, and most metals (Cd, Co, Cu, Fe, Ni, Pb)
are adsorbed at pH close to the pH of dissociation of
carboxyl groups in these algae (pK 0 close to 5) [24.62].
The second most abundant acidic group is the sulfonic
acid of fucoidans [24.25]; this compound occurs both
in the amorphous matrix and in close association with
the cellulose microfibrils in the innermost portion of
the wall [24.63–65]. Sulfonic acid plays a secondary
role in the bond with metals, except when this happens at low pH. This is also the case for the hydroxyl
groups present in all polysaccharides, which are negatively charged only at pH > 10 [24.25].
Proteins represent a minor component in the cell
wall of brown algae [24.66]. This implies that the carboxyl groups of the alginates are more abundant than
the carboxyl groups and the amine groups of proteins
and represent the majority of the binding sites [24.58].
The role of the carboxyl groups in adsorption processes
has been clearly demonstrated by the uptake of Cd and