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.
4.7 Conclusion and Future Aspects
Being the largest freshwater sources, groundwaters not only provide adequate supply for domestic, agricultural, and industrial activities but also ensure sustainability of the ecohydrological phenomenon. Their continuous depletion (due to overexploitation) and deterioration (due to pollution) are causing grave environmental and health consequences. Particularly, over the past few decades, release of unchecked industrial wastewaters into streams of heavy metal ions into groundwater resources has been found to be catastrophic and the devastating impacts of the use of heavy metal‐contaminated groundwater can be seen across the globe despite strong and rigid legislations. Therefore, the major challenge now is to maintain the sustainability of groundwater resources in conjunction with socioeconomic‐industrial development. The development of efficient methodologies that can be used to monitor groundwater sources in real time from the perspective of heavy metal pollution are highly desirable. Further, technologically feasible and commercial viable treatment processes need to be designed and developed for the pretreatment of industrial waste before its discharge to groundwater sources and other water bodies. Moreover, attention needs to be paid to the sustainability of new‐generation advanced treatment technologies like membrane filtration and adsorption so that treatment of contaminated groundwater can be achieved on a bulk scale at the point of source prior to its consumption. These technologies not only offer purified water from contaminated water but also provide an opportunity for recovery of heavy metals as resource materials from polluted waters. Therefore, in a nutshell, a long‐term planning solution is required to optimize groundwater resources by considering the future projections of industrial expansions, irrigation demands, and drinking water supplies.
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