95
4.2 Heavy Metal Removal by Clay Minerals
Heavy metals have serious adverse effects on ecology, genetics, nutrition, and environment. Heavy metal cations discharge from many industrial processes as a waste
and accumulate in soil and/or water basin and mobilize by the aid of groundwater
stream. Thus, heavy metals may be consumed by humans feeding by plant and animal. The most commonly found heavy metals in wastewater are arsenic, cadmium,
chromium, copper, lead, nickel, zinc, and other heavy metals of the periodic table
(Jaishankar et al. 2014). The usage of As(III, V), Sb(III, V), Pb(II), Cd(II), Hg(I, II),
and Cr(VI) metals is restricted by The European Union Restriction of Hazardous
Substances Directive and The European Union End of Life Vehicles Directive
(European Union Directive 2000/53/EC; European Union Directive 2002/95/EC).
Many water treatment methods have been applied for removal of heavy metals
from wastewater effluents. But, the adsorption process stands out due to costeffectiveness and easy applicability of adsorption. Especially, the adsorption onto
clay is widely used for the purpose of heavy metal removal and environmental
remediation. Heavy metal adsorptions are performed onto natural or modified clay,
or clay nanocomposite. Clays are economically and technically convenient to be an
adsorbent, and additionally, clays are good and effective sorbents for radionuclides.
Sr
2+
, Cs
+
, and UO 2
2+
are readily adsorbed by clay minerals as illite, kaolinite, and
montmorillonite (Arda et al. 2006; Abdel Karim et al. 2016; Siroux et al. 2017).
The studies, dealt with inspection of the types of surface sites having different
functional groups on clay minerals as kaolinite and montmorillonite and the individual and competitive heavy metal adsorption on those surface sites, showed that
three types of surface site are mentioned considering surface acidic functional
groups of adsorbent: (i) silanol sites and/or carboxyl group having close pKa values
to that of silanol, (ii) aluminol sites and/or phenolics having compatible pKa values
with that of aluminol, and (iii) permanent negatively charged sites. The electrostatic
interaction between heavy metal ions and permanent negatively charged surface of
clay minerals has indirect pH dependency and is dominant at low pH because of
positively charged silica and alumina groups (Hızal and Apak 2006a, b; Hızal et al.
2009, 2016).
While pH of solution increases, at first, silica groups start to lose H
+
ions and
become negatively charged upon zero acidity constant of silica groups. In this pH
interval, heavy metal cations mostly bind to negatively charged silica surface.
Because alumina groups have more basic character than silica groups, alumina
groups lose their proton at higher pHs (Tombacz and Szekeres 2004). Between two
pH points, corresponding to the surface acidity constant of silica and alumina surfaces, silica sites have lost releasable hydrogen, and alumina sites are still in protonated form. Thus, heavy metal–silica surface complexation is dominant. At higher
pHs, alumina surfaces become activated and then metal–alumina surface complexation gets dominant (Hızal and Apak 2006a, b).
The clay minerals prefer to behave as if they were chelating ion-exchange sorbent for heavy metals in the presence of strong Lewis base such as humic acid by
4 Montmorillonite Clay Composite for Heavy Metal Removal from Water
4.2 Heavy Metal Removal by Clay Minerals
Heavy metals have serious adverse effects on ecology, genetics, nutrition, and environment. Heavy metal cations discharge from many industrial processes as a waste
and accumulate in soil and/or water basin and mobilize by the aid of groundwater
stream. Thus, heavy metals may be consumed by humans feeding by plant and animal. The most commonly found heavy metals in wastewater are arsenic, cadmium,
chromium, copper, lead, nickel, zinc, and other heavy metals of the periodic table
(Jaishankar et al. 2014). The usage of As(III, V), Sb(III, V), Pb(II), Cd(II), Hg(I, II),
and Cr(VI) metals is restricted by The European Union Restriction of Hazardous
Substances Directive and The European Union End of Life Vehicles Directive
(European Union Directive 2000/53/EC; European Union Directive 2002/95/EC).
Many water treatment methods have been applied for removal of heavy metals
from wastewater effluents. But, the adsorption process stands out due to costeffectiveness and easy applicability of adsorption. Especially, the adsorption onto
clay is widely used for the purpose of heavy metal removal and environmental
remediation. Heavy metal adsorptions are performed onto natural or modified clay,
or clay nanocomposite. Clays are economically and technically convenient to be an
adsorbent, and additionally, clays are good and effective sorbents for radionuclides.
Sr
2+
, Cs
+
, and UO 2
2+
are readily adsorbed by clay minerals as illite, kaolinite, and
montmorillonite (Arda et al. 2006; Abdel Karim et al. 2016; Siroux et al. 2017).
The studies, dealt with inspection of the types of surface sites having different
functional groups on clay minerals as kaolinite and montmorillonite and the individual and competitive heavy metal adsorption on those surface sites, showed that
three types of surface site are mentioned considering surface acidic functional
groups of adsorbent: (i) silanol sites and/or carboxyl group having close pKa values
to that of silanol, (ii) aluminol sites and/or phenolics having compatible pKa values
with that of aluminol, and (iii) permanent negatively charged sites. The electrostatic
interaction between heavy metal ions and permanent negatively charged surface of
clay minerals has indirect pH dependency and is dominant at low pH because of
positively charged silica and alumina groups (Hızal and Apak 2006a, b; Hızal et al.
2009, 2016).
While pH of solution increases, at first, silica groups start to lose H
+
ions and
become negatively charged upon zero acidity constant of silica groups. In this pH
interval, heavy metal cations mostly bind to negatively charged silica surface.
Because alumina groups have more basic character than silica groups, alumina
groups lose their proton at higher pHs (Tombacz and Szekeres 2004). Between two
pH points, corresponding to the surface acidity constant of silica and alumina surfaces, silica sites have lost releasable hydrogen, and alumina sites are still in protonated form. Thus, heavy metal–silica surface complexation is dominant. At higher
pHs, alumina surfaces become activated and then metal–alumina surface complexation gets dominant (Hızal and Apak 2006a, b).
The clay minerals prefer to behave as if they were chelating ion-exchange sorbent for heavy metals in the presence of strong Lewis base such as humic acid by
4 Montmorillonite Clay Composite for Heavy Metal Removal from Water