Recovery of sulfuric acid from piranha solution over a dimensionally stable anode (DSA) Ti-RuO₂ electrode and beyond

Piranha solution is a highly acidic mixture generated by combining sulfuric acid and hydrogen peroxide solutions. The microelectronics industry uses Piranha solution to clean organics off silicon wafer surfaces and create hydroxyl groups on the surface, which renders Piranha solution, a strong acidic oxidative agent, hydrophilic. More recently, the mixture has been used to functionalize carbon nanotube surfaces through oxidation, lowering the oxygen evolution overpotential. Generally, after its use, Piranha solution is left to attenuate as the hydrogen peroxide slowly degrades; the solution is later neutralized by adding a base or a buffer. After this process, the no longer acidic solution is disposed. This process only partially degrades hydrogen peroxide and it relies on the addition of chemicals (base/buffer) in order to dispose it. This project relies on electrochemical methods to recover the sulfuric acid from the solution while fully degrading hydrogen peroxide for the reuse of sulfuric acid without unnecessary disposal of chemicals to the environment.
The electrochemical degradation of hydrogen peroxide can undergo both oxidation and reduction. On the one hand, reduction of hydrogen peroxide utilizes protons present in the solution to generate water. This process consumes some protons, making the solution less acidic. On the other hand, oxidation generates oxygen gas, protons and electrons, which replenishes the solution acidity. The present study uses a dimensionally stable anode (DSA) made of titanium metal electrode coated with Ruthenium Dioxide (RuO2). Results show that hydrogen peroxide in Piranha solution can be fully degraded in 5 hr at a constant current of 2 A. Results also show that anodic hydrogen peroxide degradation is twice faster than cathodic hydrogen peroxide reduction. The hydrogen oxidation rate is zero-order controlled at applied current of <1.5 A; a surface reaction control of first-order reaction when the applied current is > 1.5 A. The extreme conditions in which the DSA performs in Piranha solution allows its applications in other, less extreme, environments.