*3.6. The Difference between Alkaline Leaching Process and Electrochemical Advanced Oxidation Leaching Process*

The purpose of alkaline leaching of arsenic-containing waste is to transfer arsenic from solid waste to the leachate. However, the alkaline leaching process has disadvantages such as a large amount of waste liquid, easy generation of H2S gas, and low arsenic leaching efficiency. The oxidation treatment of the waste residue containing low valent arsenic is beneficial to transforming the arsenic from a stable form to an unstable form (e.g., a water-soluble state, an exchange state, and a carbonate bond state), thereby increasing the leaching rate of arsenic. The electrochemical advanced oxidation method uses reactive oxygen groups with strong oxidation properties generated in situ to oxidize low arsenic in the ARCD to the leaching solution, which improves the removal rate of arsenic. The alkaline leaching process has the problem of incomplete removal of arsenic. The use of an electrochemical advanced oxidation process can increase the removal rate of arsenic without secondary pollution. The alkaline leaching process cannot oxidize the As(III). The advanced oxidation technology is used to oxidize the low valence arsenic in the solid waste and transfer it into the leaching solution, increasing the arsenic removal rate. As shown in Figure 8d, the removal rate of arsenic is only 80.69%, but it reaches 98.04% through advanced electrochemical oxidation.

#### **4. Conclusions**

The use of an electrochemical advanced oxidation leaching process can effectively remove arsenic from arsenic-containing solid waste. By adding carbon black and PTFE to improve the electrochemical performance of the carbon felt, the specific surface area of the carbon felt, the electrochemical performance, and the content of in suit generated H2O<sup>2</sup> are all increased. The arsenic in the ARCD can be transferred to the solution by the electrochemical AOPs; therefore, the high-efficiency detoxification of the ARCD is realized. Under the condition of the voltage of 1.0 V, the electrolysis duration of 90 min, the oxygen flow rate of 0.4 L/min, and the CF of 1:5, the removal rate of arsenic reaches 98.4%. In addition, the electrochemical oxidation leaching method avoids secondary pollution and can be considered a clean and efficient arsenic removal method.

**Author Contributions:** Data curation, B.L.; writing—original draft preparation, M.L.; writing review and editing, J.Y.; supervision, D.D. and H.D.; project administration, Y.C.; funding acquisition, G.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by [the National Natural Science Foundation of China] (Grant No. 52004252), [the National Key Research and Development Program of China] (Grant No. 2018YFC1901601), [the Postdoctoral Research Grant in Henan Province] (Grant No. 201902016), [Henan Province Key R&D and Promotion Special (Scientific and Technical) Project] (Grant No. 212102310600) and [the Key Research Project of Henan Province Colleges and Universities] (Grant No. 20A440011).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was financially supported by the National Natural Science Foundation of China (Grant No. 52004252), the National Key Research and Development Program of China (Grant No. 2018YFC1901601), the Postdoctoral Research Grant in Henan Province (Grant No. 201902016), Henan Province Key R&D and Promotion Special (Scientific and Technical) Project (Grant No. 212102310600) and the Key Research Project of Henan Province Colleges and Universities (Grant No. 20A440011).

**Conflicts of Interest:** The authors declare no conflict of interest.
