1.3 Adsorbents
7
Their structures are composed of three-dimensional SiO 4 and AlO 4 tetrahedra frameworks, with an oppositely charged lattice and this charge is cations distributed in solutions. Natural zeolites are classified according to the ratio of Al to Si in their structures,
and the species and number of cations present within their structures. This adsorbent
comprises of a quantity of species; however, the very common of them is clinoptilolite
with chemical formula of Na 0.1 K 8.57 Ba 0.04 (Al 9.31 Si 26.83 O 72 ) · 19.56 H 2 O [54–56].
Clinoptilolite can be used in its natural also in its chemically modified/pretreated
forms to remove adsorbates, pretreatment aims to replace the exchangeable cations
present in clinoptilolite with cations that are eager to go through ion exchange
phenomenon, thus facilitating metal ion removal [57]. Gunay et al. [58] examined
the adsorption of lead from aqueous solution using natural and pretreated clinoptilolite. The optimum adsorption potential was observed to be 80.93 and 122.40 mg/L,
respectively. Dirmirkou and Doula [59] conducted an investigation on the adsorption
of zinc from drinking water by natural and modified clinoptilolite. The maximum
adsorption capacity obtained was found to be 71.3 and 94.8 mg/L, respectively.
Zeolite has been examined widely by authors owing to their capability in eradicating
small amount of heavy metal ions from aqueous solution by utilizing ion exchange
process [60]. Nascimento et al. [61] reported that the adsorption process of zeolite
depends largely on many effects including nature and concentration of the adsorbate,
the characteristic of the adsorbent and the adsorption variables which comprises of
pH, temperature and ratio of solid/liquid. Montolito et al. [62] reported that zeolite
contains about 10–25 wt% water in the channels within its structure. This water can
be driven out by heating in a vacuum under high temperatures. However, the amount
of water removed is therefore a measure of its adsorptive capacity. The adsorption
capacity of zeolite for heavy metal ions is shown in Table 1.3. One of the major limitations of using zeolite as an adsorbent is their low permeability; also, the adsorption
mechanism of zeolite is complex because of their porous structure and inner and
outer charged surfaces [63].
1.3.3 Peat
Peat is an adsorbent which is used to treat polluted water. The use of peat as an
adsorbent to extract a large variety of pollutants extends back to 1970. In Ireland
and Northern Europe, peat is mined and used for fuel and as a soil conditioner
in the USA [10]. Brown et al. [76] reported that what makes comparison of peat
results difficult are: different locations of origin, degrees decomposition and different
experimental conditions. The adsorption mechanism of peat onto heavy metal ions
is believed to be that of hydrogen ion exchanger. At pH less than 3, numerous
adsorbates can be leached from peat. However, favourable adsorption of metals by
peat occurs at pH range of 3–8.5. Moreover, when the pH of the solution is above
8.5, peat adsorbent becomes unstable [77]. Ringqvist and Oborn [78] also reported
that when polluted waters are treated with peat, a higher metal removal can be
expected at high pH than at low pH. Peat is a permeable adsorbent with a dense
7
Their structures are composed of three-dimensional SiO 4 and AlO 4 tetrahedra frameworks, with an oppositely charged lattice and this charge is cations distributed in solutions. Natural zeolites are classified according to the ratio of Al to Si in their structures,
and the species and number of cations present within their structures. This adsorbent
comprises of a quantity of species; however, the very common of them is clinoptilolite
with chemical formula of Na 0.1 K 8.57 Ba 0.04 (Al 9.31 Si 26.83 O 72 ) · 19.56 H 2 O [54–56].
Clinoptilolite can be used in its natural also in its chemically modified/pretreated
forms to remove adsorbates, pretreatment aims to replace the exchangeable cations
present in clinoptilolite with cations that are eager to go through ion exchange
phenomenon, thus facilitating metal ion removal [57]. Gunay et al. [58] examined
the adsorption of lead from aqueous solution using natural and pretreated clinoptilolite. The optimum adsorption potential was observed to be 80.93 and 122.40 mg/L,
respectively. Dirmirkou and Doula [59] conducted an investigation on the adsorption
of zinc from drinking water by natural and modified clinoptilolite. The maximum
adsorption capacity obtained was found to be 71.3 and 94.8 mg/L, respectively.
Zeolite has been examined widely by authors owing to their capability in eradicating
small amount of heavy metal ions from aqueous solution by utilizing ion exchange
process [60]. Nascimento et al. [61] reported that the adsorption process of zeolite
depends largely on many effects including nature and concentration of the adsorbate,
the characteristic of the adsorbent and the adsorption variables which comprises of
pH, temperature and ratio of solid/liquid. Montolito et al. [62] reported that zeolite
contains about 10–25 wt% water in the channels within its structure. This water can
be driven out by heating in a vacuum under high temperatures. However, the amount
of water removed is therefore a measure of its adsorptive capacity. The adsorption
capacity of zeolite for heavy metal ions is shown in Table 1.3. One of the major limitations of using zeolite as an adsorbent is their low permeability; also, the adsorption
mechanism of zeolite is complex because of their porous structure and inner and
outer charged surfaces [63].
1.3.3 Peat
Peat is an adsorbent which is used to treat polluted water. The use of peat as an
adsorbent to extract a large variety of pollutants extends back to 1970. In Ireland
and Northern Europe, peat is mined and used for fuel and as a soil conditioner
in the USA [10]. Brown et al. [76] reported that what makes comparison of peat
results difficult are: different locations of origin, degrees decomposition and different
experimental conditions. The adsorption mechanism of peat onto heavy metal ions
is believed to be that of hydrogen ion exchanger. At pH less than 3, numerous
adsorbates can be leached from peat. However, favourable adsorption of metals by
peat occurs at pH range of 3–8.5. Moreover, when the pH of the solution is above
8.5, peat adsorbent becomes unstable [77]. Ringqvist and Oborn [78] also reported
that when polluted waters are treated with peat, a higher metal removal can be
expected at high pH than at low pH. Peat is a permeable adsorbent with a dense