Effects of pH on the simultaneous removal of hydrogen sulfide and ammonia in a combined absorption and electro-oxidation system.

Biogas commonly contains both H2S and NH3, and these impurities need to be removed before use. In this study, a combined system consisting of an absorption column and an electro-oxidation reactor was developed to simultaneously treat H2S and NH3. In particular, the effect of the pH (6, 8, and 10) on the system performance was investigated. The mass transfer rate of H2S from the gas to liquid phases was sensitive to pH because of its relatively low solubility at low pHs, while more than 99% of the introduced NH3 was steadily absorbed. Therefore, a pH higher than 8 was favorable for the simultaneous removal of both gases. In the electro-oxidation reactor, H2S was primarily oxidized, while the NH3 oxidation started after H2S was completely eliminated. Furthermore, the oxidation rate and current efficiency of both H2S and NH3 increased with decreasing pH value. The results showed that a low pH was advantageous for the electro-oxidation. In conclusion, the mass transfer rate and oxidation kinetics should be balanced to increase the simultaneous removal of H2S and NH3. Therefore, among the tested pH values, the best performance in the combined system was achieved using a pH of 8.