Electrocoagulation in water treatment has been employed for more than 100 years as the first electrocoagulation treatment plant was built in London in 1889. In 1909, in the United States, J.T. Harries received a patent for wastewater treatment by electrolysis with sacrificial aluminium and iron anodes. An “Electronic Coagulator”, which electrochemically dissolved aluminium ions into solution and reacted them with the hydroxyl ions formed at the cathode to form aluminium hydroxides, was used in the 1940s for water treatment. Electrocoagulation (EC) is a process where reagents/coagulants are generated in situ by an electrical dissolution of electrodes. The metallic cation formation occurs at the anode, while hydrogen gas (H2) is typically formed at the cathode (Hansen, et al., 2007).
The chemistry in arsenic removal by electrocoagulation is similar to in arsenic removal by chemical precipitation. The same principles apply: the chemical reagent (ferric chloride or aluminium sulphate) is mixed with the arsenic-contaminated water. After a specific time, the precipitated particles need to be removed from the solution by settling, filtration, and other processes. In electrocoagulation, the chemical reagent is formed with the help of the applied electric current and the sacrificial anodes (iron or aluminium) (Ucar, et al., 2013).
During the electrocoagulation process with iron electrodes, many iron oxide and oxyhydroxide species are formed like green rust (layered Fe(II)-Fe(III) hydroxide), Hematite (Fe2O3), Maghemite (γ-Fe2O3), Magnetite (Fe3O4), Lepidocrocite (γ-FeOOH), Ferrihydrite (hydrous ferric oxide, Fe2O3*0.5H2O) and Goethite (α-FeOOH) (Moreno, et al., 2009). Strong adsorption properties of these iron oxyhydroxides and oxides have been observed in electrocoagulation-related studies (Emamjomeh & Sivakumar, 2009).
Layered Fe(II)-Fe(III) hydroxides have an internal surface area, and therefore, these particles have both reactivity and a specific surface for interacting with aqueous arsenic species (Moreno, et al., 2009). Besides ferric hydroxides, Ferrihydrite and Goethite have also been strong arsenic adsorbing agents (Jain, Raven & Loeppert, 1999, Kumar et al.
2004).
Lakshmipathiraj et al. (2010) proposed that the following interactions between iron oxy-hydroxide and aqueous arsenic species take place in variable conditions:
FeOOH + H2AsO4- FeOHAsO4- + H2O (5.9)
5 Arsenic removal methods According to Lakshmipathiraj et al. (2010), the surface of iron oxy-hydroxide (FeOOH) is negatively charged above pH 7.8 in aqueous solutions. This value is consistent with the isoelectric point of other mineral phases such as Magnetite (Fe3O4) and Lepidocrocite (γ-FeOOH). The presence of arsenic, either as a trivalent or pentavalent ion, shifts the isoelectric point (IEP) of iron oxy-hydroxide towards the acidic region. This indicates the interaction of both arsenic species arsenite and arsenate with the iron oxide / oxy-hydroxide precipitate. Kumar et al. (2004) also reported that the electrocoagulation-based arsenic removal process had not only very high As(V) removal efficiency but also high As(III) removal efficiency. They concluded that the reason for this is in the electrochemical mechanism; in the EC process occurs the simultaneous oxidation of As(III) to As(V) and removal of oxidized species by adsorption/complexation with metal hydroxides generated in the same process. These are significant findings as separate As(III) oxidizing step could be avoided when utilizing electrocoagulation for arsenic removal.
Arsenic removal by electrocoagulation has been studied, e.g. from groundwaters, drinking waters, copper refining industry effluents, and abandoned mining area waters.
Arsenic has been present in both As(III) and As(V) forms in these solutions, and it can be reduced at least by 99% from its initial concentrations of 5 to 100 ppm depending on the treatment time and current density (Lakshmipathiraj, et al., 2010). These results are supported by Hansen & Ottosen (2010) and Ali et al. (2012), who observed arsenic removal rates above 99% in their studies from the initial arsenic concentrations of 5 ppm.
The removal rate depends not only on operational parameters but also on arsenic concentration in the feed solution.
Martikainen et al. (2016) studied continuous arsenic removal by electrocoagulation using industrial gypsum saturated wastewater (calcium 749 mg/L and sulphate 1832 mg/L) using sacrificial iron electrodes. Arsenic removal was tested with three different current densities and three different retention times. The wastewater had an initial total As concentration of 8.53 mg/L and pH7.8. Arsenic was in the form of As(III). Results of the continuous EC testes are shown in Figure 5.8. As the retention time of the wastewater in the electrocoagulation cell increases, the residual arsenic concentration decreases in all current densities. The effect of the retention time is diminished as the current density increases.
99
Figure 5.8. Arsenic removal from the gypsum saturated wastewater with continuous electrocoagulation. The initial arsenic concentration was 8.53 mg/L in all of the test cases.
(Martikainen, et al., 2016).
Martikainen et al. also studied arsenic removal from mineral processing wastewater to be recycled back to process. The arsenic content of the process water increases due to constant reuse of the water. Arsenic removal by electrocoagulation from recycled process water containing 1.61 mg/L total arsenic was tested with the continuous process having a retention time of 43s, and the current density was varied between 37 A/m2 and 54 A/m2 (Figure 5.9). With the highest current density, the residual arsenic concentration was below the drinking water limit of 10 µg/L. Even with the lower current density of 45 A/m2, the water quality was good enough to be recycled back to the process. Thus, it is evident that with electrocoagulation, one can treat even low concentration arsenic waters and reach very low residual arsenic levels if needed.
5 Arsenic removal methods 100
Figure 5.9. Arsenic removal from recycled water from mineral processing with continuous electrocoagulation. Residual arsenic as a function of current density. Initial arsenic concentration was 1.61 mg/L in all test cases (Martikainen, et al., 2016).
Lakshmipathiraj et al. (2010) tested the solid waste from the electrocoagulation process according to the USEPA TCLP test. Tests confirmed that the solid waste generated during the arsenic removal with the electrocoagulation process is not hazardous to the environment and can de be disposed to landfill. Lakshmipathiraj et al. also noted that the EC process forms less solid waste than conventional chemical precipitation processes (i.e., aluminium or ferric salts). A similar conclusion was made by Barrera-Díaz et al.(2018); One of the main drawbacks of using chemical coagulation is the large amount of solid waste coming from the precipitation process. Electrocoagulation produces about half the solid waste compared to chemical coagulation.