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 from contaminated fine grain soils (Chen and Zhang 2012; Hussein and Alatabe 2019). The ER process contains three phenomena: electroosmosis (EO), electromigration (EM) and electrophoresis (EP). The ER process works on low‐level direct current like a “cleaning agent” including several transport mechanisms as EO, EM, EP, and electrochemical reactions (electrolysis and electrodeposition). It is an easily controllable method both in‐situ and ex‐situ. This method is applicable for low permeability soil treatment and removal of organic and inorganic pollutants from soil also. The EO process occurs due to fluid flow from anode to cathode. On the other hand, the EP process exhibits the movement of the charged particle under applied electric field. The last phenomenon in the ER process is EM, where cations move towards the negatively charged cathode and anions moves towards the anode under an applied electric field. The electrokinetics method has been implemented to remove various types of contaminants like polychlorinated biphenyls, pesticides, and heavy metals. The major disadvantage of this method is that it is not appropriate for removing nonpolar pollutants and this method takes more time to complete the remediation process.
4.6.7 Adsorption Method
The adsorption method is considered to be the best method in comparison to the other methods discussed here because of their limitations like low efficiency, high cost, etc. The key advantages of the adsorption method are low cost, sludge free, high efficiency, easy recovery of metals and possibility to reuse the adsorbent (Sharma et al. 2015; Agrawal et al. 2018; Agrawal et al. 2019). Generally, adsorption is a mass transfer process, in which a substance transfers from the liquid phase to the solid phase and bounds by physical and chemical interactions (Babel and Kurniawan 2003). The adsorption process can be accomplished either by electrostatic interaction (ionic interaction between positively charged metal ions and negatively charged matrices) or by chelation (donation of the lone‐pair electrons of the matrices to metal ions to form coordinate bonds). More effective adsorption of toxic metals depends on the properties of the heavy metal solutions such as temperature, pH of the solution, and specification of heavy metal; for example, at neutral or lower pH, As(V) shows better adsorption efficiency than As(III). The use of activated carbon (Li et al. 2018; Hashim et al. 2019), activated alumina (Szatyłowicz and Skoczko 2018), carbon nanotubes (CNTs) (Zhu et al. 2019), porous carbon (Agrawal et al. 2019), graphene (Fausey et al. 2019; Dai et al. 2020), and ferric oxide particles (Majumder et al. 2019) have generated much interest, and novel metal modified adsorbents have demonstrated superior performance towards heavy metal decontamination. Carbonaceous materials have comparatively good efficiency of adsorption, large surface area, tuneable pores, high porosity, and good sorption sites for adsorbate, and are easy to synthesize and cost effective.
