Page 8 - report

Sustainability 2022, 14, x FOR PEER REVIEW 8 of 37

indirect reactions, the EO technique has proved that it can convert PFASs into compounds that are safe 224

for the environment. Transfer of electrons from the PFAS molecule to the anode causes the direct 225
oxidation. At the same time, indirect methods include the electrochemical production of strong oxidants 226

known as radicals [50-52]. Long-chain PFAS treatment has shown removal rates of up to >99%. 227
However, short- chain PFASs are harder to breakdown using the EO technique, and because the 228

precursors are converted during treatment, the PFAS concentration may potentially rise as a result [53- 229

55]. The duration of PFAS treatment with EO is influenced by a variety of factors, including electrode 230
surface area and properties, starting PFAS concentration (co-contaminant present), efficiency target, 231

and voltage. EO is a superior oxidation technology since it can operate at low temperatures without 232
adding too many chemicals [56]. Some of the benefits of this method are its high PFAS oxidation 233

performance, low volume use, and minimal environmental effect when compared to other destruction 234
technologies [57]. Furthermore, hybrid PFAS destruction systems combining EO and IXR, as well as 235

concentrated retentate from nanofiltration and reverse osmosis procedures, have showed significant 236

PFAS destruction efficiency [58]. With high destruction rates, EO efficiently removes PFASs in 237
synthetically generated solutions as well as real polluted groundwater and wastewater [59]. 238

Furthermore, it permanently removes PFASs, does not require secondary waste removal, and does not 239

require any extra chemicals or pre-treatment processes [60]. The biggest obstacle for the full-scale 240
implementation of EO for PFAS treatment is scaling up this technology. This is due to low 241

concentration of PFAS, the high energy requirement electrolysis part, and high initial expenses due to 242
the high cost of electrode procurement. Boron-doped diamond (BDD) is the most often used electrode; 243
2
aside from being difficult to make, it costs around $7125/m [61]. As a result, research on PFAS 244
breakdown using EO is still restricted to the laboratory scale [46]. New factors such as electric field 245

amplitude and reaction chamber geometry might have a substantial impact on PFAS degrading 246

efficiency [62-64]. Another issue of this method is the creation of extremely corrosive hydrogen 247
fluoride vapour because of its interaction [65]. After PFAS destruction using the Ti/RuO2 anode [19] 248

and the BDD anode [66-68], the development of oxidation by products such as perchlorate and chlorate, 249
as well as the formation of bromine compounds [69, 70], has been described in the literature. The 250

generation of hazardous chlorine (Cl2) was reported in chloride-containing water, albeit the process is 251
yet unclear [56]. These hazardous substances might have come from electrodes used as anodes in EO 252

treatment, which often contain toxic heavy metals that could be discharged into the treated water. As a 253
result, more research into the development of harmful bioproducts is required. 254

3.5 Plasma Technology 255