145
regulations. For this reason, thousands of wastewater treatment plants are established annually around the world, and physical, chemical, or biological processes
including solvent extraction, chemical precipitation, coagulation/flocculation, electrochemical treatment, ion exchange, membrane filtration, and adsorption (Azimi
et al. 2017; Duruibe et al. 2007; Carolin et al. 2017; Gunatilake 2015; Barakat 2011;
Hashim et al. 2011; Hegazi 2013) are applied to remove these pollutants. However,
since each heavy metal has its unique nature, usually a sequence of several removal
processes should be performed for effective treatment (Fu and Wang 2011).
Furthermore, conventional techniques such as electrochemical treatments require
capital investments and power input that make them less desired. On the other side,
chemical processes such as precipitation, coagulation, and ion exchange include
usage of large amount of chemicals that can cause pollution in the processed water
(Azimi et al. 2017). Furthermore, biological processes still require further research
and are time-consuming (Carolin et al. 2017). Table 6.2 summarizes the advantages
and disadvantages of different methods used for the recovery of heavy metals from
contaminated waters.
Recent methods of heavy metal removal have shifted into adsorption-based processes as they satisfy flexibility in design and operation (Qdais and Moussa 2004).
These processes are known as reversible processes, in which the adsorbents can be
efficiently used multiple times, as they can be regenerated by suitable desorption
processes (Hua et al. 2012). Adsorption-based studies mainly focus on the economy, environment, and efficiency of the processes; therefore, developing low-cost
adsorbent with high affinity and regeneration capability under mild conditions is a
challenge since it can meet alone all the requirements above. Recently, technical
feasibilities of several low-cost adsorbents obtained from natural materials, agricultural and biological wastes, industrial by-products, biopolymers, and hydrogels
have been explored for the treatment of heavy metal-containing wastewater and
extensively reviewed in the literature (Khulbe and Matsuura 2018). On the other
hand, among the existing ones, metal oxide nanoparticles, such as titanium oxides,
Table 6.2 Advantages/disadvantages of several processes available for the treatment of heavy
metal-containing wastewater (Carolin et al. 2017)
Methods
Advantages
Disadvantages
Coagulation
Simple and cost-effective
Toxic sludge
Adsorption
Easy to operate, reusable, and low-cost
adsorbent
Desorption
Membrane
filtration
High rejection, lower space
requirement, no chemical usage
High investment and operational
cost, membrane fouling
Electrodialysis
High segregation of metals
Clogging, energy loss
Ion exchange
High transformation of components
Affinity to limited metal ions
Photocatalysis
Eliminates both metal ions and organic
contaminants simultaneously
Requires prolonged time
Biological
treatment
Beneficial
Stability problem under variable
conditions
Oxidation
Does not need electricity
Rusting
6 Recovery of Heavy Metals by Membrane Adsorbers
regulations. For this reason, thousands of wastewater treatment plants are established annually around the world, and physical, chemical, or biological processes
including solvent extraction, chemical precipitation, coagulation/flocculation, electrochemical treatment, ion exchange, membrane filtration, and adsorption (Azimi
et al. 2017; Duruibe et al. 2007; Carolin et al. 2017; Gunatilake 2015; Barakat 2011;
Hashim et al. 2011; Hegazi 2013) are applied to remove these pollutants. However,
since each heavy metal has its unique nature, usually a sequence of several removal
processes should be performed for effective treatment (Fu and Wang 2011).
Furthermore, conventional techniques such as electrochemical treatments require
capital investments and power input that make them less desired. On the other side,
chemical processes such as precipitation, coagulation, and ion exchange include
usage of large amount of chemicals that can cause pollution in the processed water
(Azimi et al. 2017). Furthermore, biological processes still require further research
and are time-consuming (Carolin et al. 2017). Table 6.2 summarizes the advantages
and disadvantages of different methods used for the recovery of heavy metals from
contaminated waters.
Recent methods of heavy metal removal have shifted into adsorption-based processes as they satisfy flexibility in design and operation (Qdais and Moussa 2004).
These processes are known as reversible processes, in which the adsorbents can be
efficiently used multiple times, as they can be regenerated by suitable desorption
processes (Hua et al. 2012). Adsorption-based studies mainly focus on the economy, environment, and efficiency of the processes; therefore, developing low-cost
adsorbent with high affinity and regeneration capability under mild conditions is a
challenge since it can meet alone all the requirements above. Recently, technical
feasibilities of several low-cost adsorbents obtained from natural materials, agricultural and biological wastes, industrial by-products, biopolymers, and hydrogels
have been explored for the treatment of heavy metal-containing wastewater and
extensively reviewed in the literature (Khulbe and Matsuura 2018). On the other
hand, among the existing ones, metal oxide nanoparticles, such as titanium oxides,
Table 6.2 Advantages/disadvantages of several processes available for the treatment of heavy
metal-containing wastewater (Carolin et al. 2017)
Methods
Advantages
Disadvantages
Coagulation
Simple and cost-effective
Toxic sludge
Adsorption
Easy to operate, reusable, and low-cost
adsorbent
Desorption
Membrane
filtration
High rejection, lower space
requirement, no chemical usage
High investment and operational
cost, membrane fouling
Electrodialysis
High segregation of metals
Clogging, energy loss
Ion exchange
High transformation of components
Affinity to limited metal ions
Photocatalysis
Eliminates both metal ions and organic
contaminants simultaneously
Requires prolonged time
Biological
treatment
Beneficial
Stability problem under variable
conditions
Oxidation
Does not need electricity
Rusting
6 Recovery of Heavy Metals by Membrane Adsorbers