LITERATURE REVIEW
2.3 Review of Removal Techniques
2.3.5 Adsorption Processes
Various adsorbents have been discussed for arsenic, iron, nitrate and fluoride removal in the literature. The criteria for selection of a suitable adsorbent media for contaminant removal from drinking water includes: medium cost, initial arsenic concentrations, adsorption capacity, ease of operation and maintenance, optimization of adsorbent dose, potential for regeneration and reuse, other elements and their
concentration in water etc. (Mohan & Pittman Jr, 2007). Mohan and Pittman Jr (2007) listed all potential adsorbents for arsenic removal and suggested that iron Based adsorbents are most widely used adsorbents. Aluminium, iron, titanium and low magnesium based substance have been shown very high arsenic removal efficiency.
Activated alumina, granular ferric hydroxide and granular TiO2 in the form of metal, metal oxides and/or hydroxides are commercially available arsenic adsorbents. In recent years, cerium (Ce) and zirconium (Zr), added adsorbents such as granular Fe-Ce oxide (Zhang et al., 2010), Ce-Ti oxide (Deng et al., 2010), Fe-Zr binaryoxides (Ren et al., 2011), Zr(IV)-loaded ligand exchange fiber (Awual et al., 2012) etc. have been used to enhanced As adsorption performance because of their increased surface areas, surface hydroxyl group, and pore accessibility. Very recently, Hassan et al. (2014) used potassium hydroxide activated carbon based apricot stone (C), calcium alginate beads (G) and calcium alginate/activated carbon composite beads (GC) for adsorption of arsenic.
The adsorption capacity was found to be 27.0, 42.4 and 66.7 mg/g (at 30◦C) for C, G and GC respectively for an initial arsenic dose of 75mg/L. Türk et al. (2009) reported arsenic adsorption to below 5.0 µg/L from 300 µg/L at optimum pH 7.1 using commercially available nanomagnetite. Bibi et al. (2015) studied industry based adsorbents such as hydrated cement, bricks powder and marble powder and reported removal >90% for arsenic and >75% for fluoride from an initial concentration of 1000 µg/L of arsenic and 30 mg/L of fluoride at pH 7.0 and 8.0, contact time of 60 min and a dose of 30 g/L. The major disadvantage of using adsorption process for drinking water is the disposal of both the spent media and the wastewater produced during regeneration/cleaning of the column.
Several adsorbents such as carbon and clay based, naturally occurring, chitosan, zeolites, double layered hydroxides, agricultural wastes, industrial wastes, bio sorbents and other synthetic organic and inorganic compounds have been used by previous researchers for nitrate removal from water (Bhatnagar & Sillanpaa, 2011; Loganathan et al., 2013a). Bhatnagar and Sillanpaa (2011) and Loganathan et al. (2013a) reviewed most of the nitrate adsorbents and their characteristics thoroughly and suggested that double layered hydroxide type compounds and modified chitosan have shown better adsorption compared to other conventional adsorbents. Surface modified agricultural wastes and other modified adsorbents have also been shown considerable potential for nitrate adsorption with only disadvantage of cost modification. Modified adsorbents have been
shown 4-11 times higher nitrate adsorption capacity than unmodified adsorbents thus they can be applied in remediation of high nitrate containing waters and where ultra-pure waters are required (Loganathan et al., 2013a). These reviewers have shown that most of the nitrate adsorption experiments were done in batch mode and most of the adsorbents were satisfactorily following the Langmuir equilibrium model and pseudo-second-order kinetic model. In most cases, the maximum Langmuir capacities were 1.7-92.1 mg/g and 125-363 mg/g for unmodified and modified adsorbents respectively.
A diverse range of adsorbents have been used so far to remove fluoride from water and wastewater. Most of them includes multivalent metal oxides and hydroxides, clay and soils, synthetic resins, layered double hydroxides (LDHs), zeolites, carbon materials, calcium materials, biopolymers, natural industrial by products, and organic wastes (Bhatnagar et al., 2011; Loganathan et al., 2013b; Mohapatra et al., 2009). Loganathan et al. (2013b) suggested multivalent metal oxides and hydroxides and layered double hydroxides as most promising because of their high fluoride adsorption capacities (1.08- 28.0 mg/g). The WHO and USEPA classified activated alumina adsorption as one of the best available technologies for fluoride removal. The carbon-based adsorbents have been found less efficient however after some modifications they have been shown improved water defluoridation. Similarly natural materials such as different types of clays and bio sorbents have been found less efficient under high fluoride concentrations and also difficult to regenerate (Singh et al., 2016). However, the adsorption capacities of these substances could be increased by modifying the surface by incorporating organic functional groups or multivalent metallic cations. In spite of having low adsorption capacities, some waste materials (e.g., red mud, slag, and sludge), natural and industrial by products could be used for defluoridation in rural areas, especially in developing countries because they are low-cost alternative adsorbents.
Recently, Yu et al. (2015) developed highly efficient Fe-Mg-La metal composite for fluoride adsorption. The adsorption capacity was found to be 270 mg/g, which is much higher than most reported adsorbents. Nath and Bhattacharyya (2015) studied adsorption of arsenite and fluoride on untreated and treated bamboo dust and found acid activation of bamboo dust increases adsorption capacity. Ippolito et al. (2011) reviewed the use of water treatment residuals (WTR) and suggested WTR successfully removes potential environmental contaminants such as arsenic, selenium, perchlorate and mercury etc. WTR
adsorbs a variety of anions due to their porosity, amorphous nature and the presence of Al and Fe (hydr)oxides. Chiang et al. (2012) studied WTR as alternative sorbents for multi- heavy metal removal from synthetic solutions, contaminated sediments, and surface waters. The WTR surpassed the adsorption capacity of commercially available goethite by 100-400% for single contaminant tests and by 240% for total sorption in multi contaminant tests.