The different binding mechanisms of arsenic with metal oxides, hydroxides, or oxyhydroxides are referred to as ‘adsorption’. However, ligand exchange, chemisorption and surface precipitation could technically be more precise terms (Johnston & Heijnen, 2001).
Ligand exchange
In arsenic adsorption, the surface complexation is the primary mechanism. In this mechanism, the surface hydroxyl groups are exchanged with arsenic ions. This exchange is strongly dependent on pH. The surface complex formed between the metal and the arsenate anion is strong and not easily reversed. However, reversal reaction happens at high pH due to the increased number of hydroxide ions in the solution (Stumm, 1992).
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Figure 5.10 shows the formation of a binuclear bidentate surface complex on a metal oxyhydroxide surface when the arsenate (H2AsO4–) ligand replaces two surface hydroxide ligands. When the adsorption of arsenic anion takes place on the surface, simultaneously happens the release of OH-ions. Therefore adsorption is more favourable at lower pH values where excess hydrogen ions consume the released hydroxide ions (Lakshmanan, et al., 2008).
Figure 5.10. Mechanism of arsenate ligand exchange on the surface of the metal oxyhydroxides (Lakshmanan, et al., 2008).
Besides the binuclear bidentate surface complex, the arsenate ion can also form a mononuclear bidentate surface complex and the unstable monodentate one, as presented in the figure below (Song & Li, 2020).
Figure 5.11. Arsenate surface complexes (Song & Li, 2020).
Goldberg & Johnston (2001) studied arsenic adsorption to amorphous iron and aluminium oxides. They concluded that the arsenate (As(V)) forms innersphere surface complexes on amorphous Al and Fe oxides. Arsenite (As(III)) forms both inner-and outer-sphere surface complexes on amorphous Fe oxide and outer-sphere surface complexes on amorphous Al oxide (see Figure 5.13). The inner-sphere complexes form through ligand exchange, while the outer-sphere complexes form through electrostatic interaction. The Figure 5.12 presents the outer-sphere complex and inner-sphere complex.
5 Arsenic removal methods 102
a) b)
Figure 5.12. a) Surface complex formation of an ion on the hydrous oxide surface. The ion may form an inner-sphere complex ("chemical bond"), an outer-sphere complex (“ion pair”), or be in the diffuse swarm of the electric double layer. b) shows a schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups "s", inner-sphere complexes "a", outer-sphere complexes “β” and the diffuse ion swarm "d" (Stumm, 1992).
Figure 5.13. Arsenite surface complexes a)inner-sphere complex and b)outer-sphere complex (Song & Li, 2020).
Surface precipitation
Adsorption of arsenic through surface precipitation mechanism follows three steps. First happens the previously mentioned formation of the arsenic iron surface complex. After this, Fe3+ dissolves from the surface, followed by the re-adsorption of dissolved Fe3+ on the surface complex. After that, arsenic will adsorb on the cations again. This repeating process will result in a slow build-up of a surface precipitate. The mechanism is presented in the Figure 5.14.
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Figure 5.14. Schematic diagram of surface complexation and surface precipitation (Xiuli, et al., 2015).
The adsorption for arsenic removal is an attractive option due to the simple and easy to operate process and minimal sludge production. Several adsorptive media have been reported to remove arsenic from drinking water:
Activated alumina
Clay minerals
Zeolites
Activated bauxite
Activated carbon
Iron and manganese coated sand
Iron oxides, oxyhydroxides, and hydroxides
Titanium hydroxides and oxides
Many natural oxide and hydroxide minerals
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Iron, aluminium, and titanium-based adsorption materials are among the most effective adsorbents for their strong affinity to arsenic. Iron-based adsorbents have been intensively studied over the years for arsenic removal from aqueous solutions. There are more than sixteen common iron oxides, and hydroxides and almost all of them have been investigated in arsenic removal. The most common iron oxides and hydroxides in these studies include Ferrihydrite (Fe2O3•0.5H2O), Goethite (α–FeOOH), Akaganeite (β–
FeOOH), Lepidocrocite (γ–FeOOH) and Hematite (α-Fe2O3). (Song & Li, 2020). Due to intensive scientific studies, iron compounds (both oxides and hydroxides) are nowadays widely used adsorbents for arsenic removal, and they have higher removal efficiency at a lower cost than many other adsorbents. In addition, iron-based adsorbents have a strong affinity for arsenic under natural pH conditions (Mohan & Pittman Jr., 2007).
Aluminium, similar to iron, has a strong affinity to arsenic. Therefore, activated alumina (Al2O3) is classified among the best available technologies (BAT) for arsenic removal from water by the United Nations Environmental Program agency (UNEP) (Mohan &
Pittman Jr., 2007).
Titanium dioxide is an interesting alternative for arsenic adsorption as it has a strong photocatalytic behaviour and therefore is capable of oxidising As(III) to As(V). The potential for oxidising As(III) is essential as removal of As(III) species is typically more complex than As(V). Besides the oxidation potential, TiO2 has shown a high arsenic adsorption capacity (Song & Li, 2020).
Table 5.3 below shows the adsorption capacities of different oxide and hydroxide adsorbents were composed of the data presented by Banerjee et al. (2008), Hao et al.
(2018), Moahn & Pittman (2007), David & Mmereki (2018).
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Table 5.3: Adsorption capacity of different metal oxide and hydroxide adsorbents studied for arsenic removal.
Adsorbent pH Adsorption
capacity [mg/g]
As(III)
Adsorption capacity [mg/g]
As(V)
Ferrihydrite Fe2O3•0.5H2O 3 - 146.9
Ferrihydrite Fe2O3•0.5H2O 6.5 125.5 67.7
Ferrihydrite Fe2O3•0.5H2O 7 - 68.8
Goethite α-FeOOH 5.5 12.5 -
Goethite α-FeOOH 6.5 7.4 2.9
Goethite α-FeOOH 9 26.8 -
Goethite α-FeOOH nanoparticles 3 - 76
Akagenite β-FeOOH 7 32.6 29.1
Hematite α-Fe2O3 6.5 5.3 2.9
Jarosite (H3O)Fe3(SO4)2(OH)6 - 21
Magnetite Fe3O4 - - 3.6-5.0
Magnetite Fe3O4 7 92.9
Magnetite Fe3O4 nanoparticles 5 16.6 46.1
Fe3O4-RGO-MnO2 composite 7 14.0 12.2
Granular Ferric hydroxide Fe(OH)3 6.5-7.5 - 1.1
Iron-modified activated carbon 6 38.8 51.3
Nano scale zero-valent iron 7 2.5 -
Ultrafine α-Fe2O3 7 - 95
Maghemite (Fe2O3, γ-Fe2O3) 3 50.0
Muscovite (KF)₂(Al₂O₃)₃(SiO₂)₆ 4.2-5.5 2.9 -
Activated Alumina Al2O3 6.9 3.48 -
Activated Alumina Al2O3 5.2 - 15.9
Activated Alumina Al2O3 6-8 - 25
unmodified Alumina Al2O3 - 0.9 0.6
Granular activated Alumina Al2O3 - 3.5 15.9
Granular TiO2 7 32.4 41.4
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6 Materials and methods
6.1 Chemicals
In experiments simulating drinking water treatment, the phosphate and arsenic test solutions were prepared by adding a commercial standard solution to ion-exchanged water or the City of Oulu drinking water. The chemicals used are presented in Table 6.1.
When needed, the pH was controlled with 1 M NaOH or HCl, diluted from p.a. grades.
Table 6.1: Chemicals used.
Element Chemical PO43- KH2PO4 (s)
As(V) Arsenic AAS standard solution 1000 mg/l As(V) in 1 M HNO3
As(III) Arsenic AAS standard solution 1000 mg/l As(III) in 1 M HCl