Marine Macrophytes: Biosorbents 24.3 Biosorption 605
Part D | 24.4
that is composed of protein or polysaccharides or both
(Table 24.1) [24.60]. In the leaves of seagrasses, carboxylic groups present on the cutin layer are probably
responsible for the binding of heavy metal ions [24.24].
The biosorption ability of different chemical groups
depends on many factors: the number of sites on the
adsorbent material, accessibility of the site, their chemical status (i. e., availability), and the affinity between
site and metal ions (i. e., bond strength) [24.25, 58, 75].
So it is important to understand what kind of interaction takes place between the functional groups
present on the adsorbent material and ionic metal speciation in solutions; MEDUSA software can be used for
this [24.76].
24.3.4 Chemical and Physical Mechanisms
Mechanisms involved in the biosorption process have
a chemical-physical nature. More specifically, heavy
metal chelation depends on the constituents of the
outer layers of macroalgae and seagrasses. The biosorption capacity of different chemical groups depends on
several factors: the number of sites on the adsorbent
material, accessibility, their chemical status (availability), and the affinity between site and metal (bond
strength) [24.77]. In particular:
1) Key functional groups (Table 24.1)
2) Ion-exchange processes, and
3) Ionic speciation of metals
have a fundamental role in biosorption studies. The carboxylic groups are abundant in brown algae, due to
the presence of alginic acid, and in the cutin of seagrasses. The adsorption capacity of the macrophytes
is directly associated with the presence of these sites
on cell walls and on the cuticles of the nonliving
biomass. Ionic speciation of metals in solution is important for metal uptake and frequently depends on
the pH of the sorption system. The MEDUSA software [24.76] creates theoretical predictions of ionic
dissociation of metals as a function of pH. The most
common metals investigated, such as Pb
2C , Cd
2C ,
Cu
2C , Zn
2C and Fe
2C , show a maximal or nearly
maximal sequestration at pH near the apparent dissociation constant of carboxylic acids (RCOOH), which
indicates adsorption on carboxyl groups. The role of
carboxylic groups in the adsorption process has been
widely demonstrated through the adsorption capacity of
lead and other metals cations by dried biomass of Cystoseira, Scytosiphon, Sargassum, Ascophyllum (brown
algae), Posidonia, and Cymodocea (seagrasses) following partial or complete esterification of the carboxylic
sites [24.24, 45, 75, 78–81]. The other acidic functional
group that is important in biosorption activity is the sulfonate group of fucoidans (brown algae) and sulfated
polysaccharides (red algae) (Table 24.1). Sulfonic acid
groups have an important role when metal binding occurs at low pH values. Hydroxyl groups present in all
macrophytes studied except brown algae (Table 24.1)
become negatively charged at high pH values (about
pH > 10), and consequently have a minor role at low
pH [24.25, 75, 80]. Concerning heavy metals that have
negative ionic speciation in solution, such as As(V), recent studies have demonstrated that red algae (phylum
Rhodophyta), which are known to be bad binders of
cationic metals, showed very good arsenic sorption performance at pH values higher than 7 [24.38].
Ion-exchange is an essential mechanism at the base
of biosorption. It is the process in which ions are exchanged between a solution and an insoluble solid,
usually a resin. During the biosorption activity, dried
macrophytes can be viewed as natural ion-exchange
materials that contain weakly acidic and basic groups
(Table 24.1). In the pH range 2:55, the binding of
heavy-metal cations is determined primarily by the state
of dissociation of the weakly acidic groups (acid–base
equilibrium theory) [24.25, 41]. In studies of biosorption, maintaining a low pH (around pH 4:55) in the
sorption system, microprecipitation of metals and their
contribution to the uptake are avoided. pH control in the
system is important both for its effects on the configuration of the active ion-exchange sites and for the ionic
state of the metals in the solution. In solution, at low
pH the concentration of protons is high, and the ionexchange sites, therefore, become solidly protonated.
This allows displacing the metals sequestered on the
biosorbent by a simple acidic wash. The regeneration of
the biosorbent biomaterial allows its multiple recycling,
further increasing the economy of its use [24.82, 83].
Consequently, the use of a biosorbent such as dried
algae and seagrasses in wastewater treatment often depends not only on the biosorptive capacity, but also
on how well the biosorbent can be regenerated and
recycled.
Part D | 24.4
that is composed of protein or polysaccharides or both
(Table 24.1) [24.60]. In the leaves of seagrasses, carboxylic groups present on the cutin layer are probably
responsible for the binding of heavy metal ions [24.24].
The biosorption ability of different chemical groups
depends on many factors: the number of sites on the
adsorbent material, accessibility of the site, their chemical status (i. e., availability), and the affinity between
site and metal ions (i. e., bond strength) [24.25, 58, 75].
So it is important to understand what kind of interaction takes place between the functional groups
present on the adsorbent material and ionic metal speciation in solutions; MEDUSA software can be used for
this [24.76].
24.3.4 Chemical and Physical Mechanisms
Mechanisms involved in the biosorption process have
a chemical-physical nature. More specifically, heavy
metal chelation depends on the constituents of the
outer layers of macroalgae and seagrasses. The biosorption capacity of different chemical groups depends on
several factors: the number of sites on the adsorbent
material, accessibility, their chemical status (availability), and the affinity between site and metal (bond
strength) [24.77]. In particular:
1) Key functional groups (Table 24.1)
2) Ion-exchange processes, and
3) Ionic speciation of metals
have a fundamental role in biosorption studies. The carboxylic groups are abundant in brown algae, due to
the presence of alginic acid, and in the cutin of seagrasses. The adsorption capacity of the macrophytes
is directly associated with the presence of these sites
on cell walls and on the cuticles of the nonliving
biomass. Ionic speciation of metals in solution is important for metal uptake and frequently depends on
the pH of the sorption system. The MEDUSA software [24.76] creates theoretical predictions of ionic
dissociation of metals as a function of pH. The most
common metals investigated, such as Pb
2C , Cd
2C ,
Cu
2C , Zn
2C and Fe
2C , show a maximal or nearly
maximal sequestration at pH near the apparent dissociation constant of carboxylic acids (RCOOH), which
indicates adsorption on carboxyl groups. The role of
carboxylic groups in the adsorption process has been
widely demonstrated through the adsorption capacity of
lead and other metals cations by dried biomass of Cystoseira, Scytosiphon, Sargassum, Ascophyllum (brown
algae), Posidonia, and Cymodocea (seagrasses) following partial or complete esterification of the carboxylic
sites [24.24, 45, 75, 78–81]. The other acidic functional
group that is important in biosorption activity is the sulfonate group of fucoidans (brown algae) and sulfated
polysaccharides (red algae) (Table 24.1). Sulfonic acid
groups have an important role when metal binding occurs at low pH values. Hydroxyl groups present in all
macrophytes studied except brown algae (Table 24.1)
become negatively charged at high pH values (about
pH > 10), and consequently have a minor role at low
pH [24.25, 75, 80]. Concerning heavy metals that have
negative ionic speciation in solution, such as As(V), recent studies have demonstrated that red algae (phylum
Rhodophyta), which are known to be bad binders of
cationic metals, showed very good arsenic sorption performance at pH values higher than 7 [24.38].
Ion-exchange is an essential mechanism at the base
of biosorption. It is the process in which ions are exchanged between a solution and an insoluble solid,
usually a resin. During the biosorption activity, dried
macrophytes can be viewed as natural ion-exchange
materials that contain weakly acidic and basic groups
(Table 24.1). In the pH range 2:55, the binding of
heavy-metal cations is determined primarily by the state
of dissociation of the weakly acidic groups (acid–base
equilibrium theory) [24.25, 41]. In studies of biosorption, maintaining a low pH (around pH 4:55) in the
sorption system, microprecipitation of metals and their
contribution to the uptake are avoided. pH control in the
system is important both for its effects on the configuration of the active ion-exchange sites and for the ionic
state of the metals in the solution. In solution, at low
pH the concentration of protons is high, and the ionexchange sites, therefore, become solidly protonated.
This allows displacing the metals sequestered on the
biosorbent by a simple acidic wash. The regeneration of
the biosorbent biomaterial allows its multiple recycling,
further increasing the economy of its use [24.82, 83].
Consequently, the use of a biosorbent such as dried
algae and seagrasses in wastewater treatment often depends not only on the biosorptive capacity, but also
on how well the biosorbent can be regenerated and
recycled.