Hydrogen production from natural organic matter via cascading oxic-anoxic photocatalytic processes: An energy recovering water purification technology.

Photocatalysis provides a "green" strategy to produce the clean energy of H. However, the realization of efficient H production is usually accomplished by the consumption of electron donors, which are costly energy carriers themselves. Here, we attempted to utilize the naturally abundant humic acid (HA), a representative natural organic matter (NOM), as the source of electron donor in a cascading oxic-anoxic photocatalytic system. Results showed that degradation of HA and remarkable H yield (1660.9 μmol g h at optimal condition) were obtained successively, whereas the anoxic photocatalytic treatment of pristine HA did not improve H yield but substantially eliminated the H production and HA degradation efficiency. These phenomena suggested the preoxidation process played a vital role in counteracting the detrimental effect of HA on photocatalytic H production. Electrochemical measurement indicated that the preoxidized HA harbored more redox-active moieties than the untreated HA and thus leading to a higher photo-induced charge carrier separation efficiency. A variety of advanced spectroscopic analyses revealed that the photocatalytic oxic pre-treatment resulted in breakdown of chemically inert, electron mediating and chromophoric aromatic macrostructure of HA to form smaller sized oxygenated organic intermediates. These intermediates were more nucleophilic than the pristine HA and acted as sacrificial reagent in the subsequent anoxic process for boosting H production. This study showcases an energy recovering water remediation process and paves the way for the design of novel photocatalytic technologies for environmental application.