Exploiting rice industry wastewater for more sustainable sunlight-driven photocatalytic hydrogen production using a graphitic carbon nitride polymorph†

This paper shows the results collected in lab-scale experiments for photocatalytic H₂ evolution from rice industry wastewater, by using a cheap and eco-friendly graphitic carbon nitride catalyst, one-pot prepared by the sacrificial template method. The final effluent from the production of “rice milk” beverage proved to be really rewarding compared to pure water, highlighting the possibility of taking advantage of a sugar-rich matrix to boost H₂ formation. After preliminary experiments in glucose aqueous solution, yielding a maximum gas evolution above 1000 μmoles g⁻¹ h⁻¹, the study moved on to wastewater and the operational conditions were optimized through designed experiments, under simulated solar light. Production of about 150 μmoles g⁻¹ h⁻¹, at least 380-fold greater than production from neat water, was achieved by working with just 0.5 g L⁻¹ of catalyst directly in the raw effluent, thus limiting the amount of metal co-catalyst and avoiding sample dilution. The reproducibility of the process was good, with relative standard deviations below 12% (n = 3). The production was also verified under natural sunlight, obtaining a mean production of nearby 115 μmoles g⁻¹ h⁻¹. The sustainability of this photocatalytic setup is strengthened by the recyclability of the catalyst, which maintains its photoactivity for at least four treatments.