According a paper published in ACS of DOI: 10.1021/acs.est.6b04803 , H2o2 can also be formed in the absence of light. It suggested dark formation of H2O2 and claimed it is more likely to occur in transitional redox environments where reduced elements (e.g., reduced metals and NOM) meet oxygen, such as oxic-anoxic interfaces. Just wanted to know the ORP value preferring its formation.