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

Biogas commonly contains both HS and NH, 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 HS and NH. In particular, the effect of the pH (6, 8, and 10) on the system performance was investigated. The mass transfer rate of HS 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 NH was steadily absorbed. Therefore, a pH higher than 8 was favorable for the simultaneous removal of both gases. In the electro-oxidation reactor, HS was primarily oxidized, while the NH oxidation started after HS was completely eliminated. Furthermore, the oxidation rate and current efficiency of both HS and NH 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 HS and NH. Therefore, among the tested pH values, the best performance in the combined system was achieved using a pH of 8.