(Marg 2011; Talabi and Kayode 2019).
4.6 Remediation Methods of Heavy Metals
To solve this problem, over the last decades various types of methods are used for the elimination of heavy metals from water (see Figure 4.2). The literature reveals that some methods, such as membrane filtration, oxidation, ion exchange, coagulation‐flocculation, phytoremediation, electro‐kinetics, and adsorption are carried out to eliminate the heavy metals from contaminated water (Barakat 2011; Agarwal and Singh 2017; Azimi et al. 2017). All these methods with their advantages and limitation are explained in brief.
4.6.1 Oxidation
The oxidation method includes the use of an oxidizing agent (Cl2, ClO2, O3, H2O2, NH2Cl, MnO4−, FeO42−, etc.) to eliminate heavy metals from water (Sharma et al. 2007; Mondal et al. 2013; Bora and Dutta 2019). Investigation of the photochemical and photocatalytic oxidation process includes the oxidation of heavy metals using UV radiation, O2, etc. for the removal of dissolved pollutants. UV/solar radiation assists to develop the hydroxyl radicals during the photolysis of iron and iron hydroxide (fe(OH)3), hydroxyl radicals and O2 oxidizes the toxic metals like As(III) to As(V). The presence of these radicals, the oxidation reaction becomes faster (Yoon and Lee 2005; Sharma et al. 2007). On the other hand, the photocatalytic reaction is carried out in the presence of TiO2 for oxidation of the heavy metals (Miller et al. 2011).
Figure 4.2 Schematic of conventional different conventional methods to decontaminate water from heavy metals.
4.6.2 Coagulation‐Flocculation
The coagulation and flocculation method has been investigated to eliminate the toxic metal pollution from contaminated water (Abouri et al. 2019; Bora and Dutta 2019). In this process, the coagulant is incorporated into the contaminated water and then forms the floc, which has the potential to eliminate heavy metals from the water. The positively charged coagulants such as aluminium and iron salts, which are widely used for heavy metal like As removal, help to decrease the negative charge of colloids. The larger particles (floc) form due to agglomeration of the particles, which settle down in water due to the influence of gravity (Choong et al. 2007). The formed floc helps to precipitate out the soluble heavy metals from the water and the solution can be filtered rapidly. Several reported coagulants like aluminium, ferrous sulphate, and ferric chloride result in the formation of amorphous metal hydroxide precipitates that are more suitable to eliminate toxic metals from contaminated water (Pallier et al. 2010). At lower pH, the colloidal substances with negative charges can be coagulated, but the cationic ion cannot be eliminated and at higher pH, the turbidity removal decreases and the cationic removal is favoured.
4.6.3 Phytoremediation
The phytoremediation process is based on the use of plants for the removal of toxic metals from the environment and it is even used to clean up contaminated air, soil, and water. It consists of several steps like phytoextraction, phytoming, phytostabilisation, rhizofiltration, and phytovolatilization (Dickinson et al. 2009; Behera 2014). Over the last decades, several researchers have been using various types of plants like Pteris Vittata, Vetiver grass, for remediation of toxic metals through the phytoremediation process (Hosamane 2012). The disadvantage of this process is that the plants adsorb high levels of toxic metals, which contaminate food crops and require more time to eliminate heavy metals from soil and water.
4.6.4 Membrane Filtration
This technique shows great potential for decontamination of heavy metals for their high efficiency and easy operation (Hao et al. 2018; Khulbe and Matsuura 2018). The membrane process has been explored with various types of techniques like microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), and electrodialysis. In the UF technique, a low‐pressure driven membrane process including pore size (10–10 000 Å) is used for the removal of dissolved and colloidal materials. High removal efficiency of metal ions could be achieved by using micellar‐enhanced ultrafiltration (MEUF) and polymer‐enhanced ultrafiltration (PEUF). The micelles containing membrane are having smaller pore size than a UF retaining contaminants, where the untapped species readily pass through the UF membrane. An anionic surfactant, sodium do‐decyl sulphate (SDS), is often selected for the effective elimination of toxic metal ions in MEUF. PEUF uses a water‐soluble polymer to complex metallic ions and form a macromolecule which will be retained, when pumped through UF membrane. Thereafter, the retained material can be recycled in order to recover metallic ions and to reuse the polymeric agent. In PEUF, the main complexing agents like polyacrylic acid (PAA) (Labanda et al. 2009) and polyethylene imine (PEI) (Aroua et al. 2007) used to achieve selective separation and recovery of heavy metals with low energy requirements.
The electrodialysis (ED) process carries out the separation of ions across charged membranes from one solution to another solution following an electric field as the driving force. Mostly, ion‐exchange membranes (cationic and anionic) are used in ED processes. It has been widely used for the production of drinking water, process water from brackish water and seawater, treatment of industrial effluents, recovery of useful materials from effluents, salt production, and for heavy metal wastewater treatment.
According to the literature, the RO (reverse osmosis) process is a very old and famous method thought of as the best method to remove arsenic from water. The RO membrane has extremely small pores (<0.001 μm). The RO membrane process can achieve a high rejection of low molecular mass compound and ions. The cellulose acetate RO membrane was been investigated for the first time in the 1980s, and has a 90% As removal efficiency with the RO system with 400 psi high pressure. However, Akin et al. have investigated the RO operational parameters. A semi‐permeable membrane is used, which allows the fluid that is being purified to pass through it, while rejecting the contaminants. It accounts for more than 20% of the world's desalination capacity (Shahalam et al. 2002). RO is an alternative option for wastewater treatment in chemical and environmental engineering. The major problem with RO is the high power consumption due to the pumping pressures, and the restoration of the membranes.
4.6.5 Ion Exchange
The ion exchange method has been employed to eliminate both cationic and anionic impurities from water. It has many advantages, such as high treatment capacity, high removal efficiency, and fast kinetics (Rahimizadeh and Liaghatb 2015). The resin used may be either synthetic or natural solid resin (synthetic resins are comparatively more effective) (Alyüz and Veli 2009). As the solution containing heavy metal passes through the cations column, metal ions are exchanged for the hydrogen ions on the resin. The uptake of heavy metal ions by ion‐exchange resins is affected by certain variables such as pH, temperature, initial metal concentration, and contact time (Gode and Pehlivan 2006). Ionic charge also plays an important role in the ion‐exchange process. In contrast to resin, natural zeolites also exhibit good cation exchange capacities for heavy metal ions under different experimental conditions. Clinoptilolite is one of the most frequently studied natural zeolites that has received extensive attention due to its selectivity for heavy metals (Doula 2009). Although there are many reports on the use of zeolites and montmorillonites as ion‐exchange resin to decontaminate heavy metal, they are limited at present compared with the synthetic resins and the application of zeolites is on the laboratory experimental scale.
4.6.6 Electrokinetics Remediation
Electrokinetics remediation (ER) has been evaluated for heavy metal removal