## 4 Arsenic Removal Processes
### Adsorptive Processes
#### 4.4.2 Application of Nanoparticles for Arsenic Removal from Water  
Various nanomaterials have been employed for water treatment [136]. Their reactivities and high surface areas have attracted a lot of interest in their consideration as novel adsorbents for removal of heavy metals and arsenic [137]. [PERSON] et al. [138] have reviewed the use of nanomaterials for arsenic removal from water and some of the key nanomaterial technologies are summarized in this section. The implementation of these technologies has been limited by the costs of production and the difficulty in recovery and recycling of the nanoparticles. Of these, iron- and titanium-based nanoparticles are most commonly used for removal of arsenic [139; 140; 141; 142; 143]. The nano-adsorbents that have been considered for arsenic removal include nanoparticles of zero-valent iron (nZVI) [144; 145], iron oxide nanoparticles \(\propto\)-Fe\({}_{2}\)O\({}_{3}\), Fe \(\gamma\)O\({}_{4}\) and \(\propto\)-FeOOH [139; 146; 147]. The arsenic removal capacity achieved with the iron oxide nanoparticles is influenced by the oxidation state of iron [143]. The oxidation of As(III) to As(V) is supported by the metal core and a thin layer of amorphous iron (oxy)hydroxide present in nanoparticles of nZVI [148]. Although nZVI is a useful adsorbent for arsenic removal, the production of toxic solid wastes from its synthesis is a main disadvantage [149]. Due to their high surface-to-volume ratios [75], iron oxide nanoparticles are 5-10 times more effective for arsenic removal than with the use of micron-sized particles [139]. Ultrafine hematite \(\propto\)-Fe\({}_{2}\)O\({}_{3}\) nanoparticles were successfully synthesized and used for treatment of laboratory-prepared and arsenic contaminated natural water [150]. Rapid removal of As(III) and As(V) was achieved, enabling removal of 74% of As(III) within 30 min. The BET (Brunauer-Emmett-Teller) specific surface area was 162 m\({}^{2}\)/g and the average particle diameter was 5.0 nm. The achieved adsorption capacity for As(III) was 95 mg/g, while for As(V) it was much lower at 47 mg/g. The performance of magnetite nanoparticles which have a BET surface area of 69.4 m\({}^{2}\)/g and a mean particle diameter of 20 nm has also been investigated for arsenic removal [151]. At pH > 7, the adsorption of As(V) rapidly decreases, while a more consistent adsorption was achieved for As(III) at pH 2-9. The achieved adsorption capacity for As(III) was 8.0 mg/g and was 8.8 mg/g for As(V). It was found that the presence of phosphate interfered with arsenic removal [151] and its removal prior to the treatment is recommended. The removal of As(V) from water was investigated with a nanocomposite of silica and goethite [146]. The synthesized silica nanoparticles had particle sizes ranging from 150 to 250 nm. The achieved adsorption capacity for the goethite/silica nanocomposite was 17.64 mg/g at pH 3.0. The adsorption of As(V) on the nanocomposite was rapid, reaching equilibrium within 120 min [146]. Arsenic removal and photocatalytic oxidation of As(III) have been investigated in a number of studies with nanocrystalline titanium dioxide (TiO\({}_{2}\)) [152; 153; 154; 155]. The adsorption of As(III) and As(V) reached equilibrium on this nano-adsorbent within four hours and 80% or more of both species were adsorbed [152]. The photocatalytic efficiency of nanocrystalline TiO\({}_{2}\) in oxidizing As(III) was also demonstrated, achieving full conversion in the presence of sunlight and dissolved oxygen to As(V) within 25 min [152]. Another titanium-based nano-adsorbent for As(III) which does not require oxidation to As(V) is hydrous titanium dioxide nanoparticles (TiO\({}_{2}\)\(\times\) H\({}_{2}\)O) [156]. High maximum adsorption capacities have been achieved with TiO\({}_{2}\) nanoparticles for As(III). An adsorption capacity of 83 mg/g was achieved at a near neutral pH, while at pH 9 it was higher at 96 mg/g [157].