Combining the highest degradation efficiency with the lowest environmental impact in zinc oxide based photocatalytic systems

Heterogeneous photocatalysis may diversify and augment access to unconventional water sources like wastewater. However, the energy embodied in water treatment and supply in photocatalytic systems make process efficiency considerations and environmental impacts assessment of primary relevance towards the setting of sustainable strategies. In this work, we evaluated the degradation rate of phenol in water by using different photocatalysts based on zinc oxide doped with rare earth elements. Aiming at identifying the best operating conditions that couple the highest degradation efficiency with the lowest environmental burdens, the results of an experimental design face centred model were combined to life cycle assessment. Based on previous works of the authors, the concentration of photocatalyst, type of rare earths used as dopant, its precursor and dopant concentration in the photocatalyst constituted the set of explanatory variables. Cumulative energy demand and IPCC 2013 GWP 100y were used as life cycle impact assessment methods. The results show that the highest degradation efficiency is obtained with 1700 mgL−1 of cerium-doped zinc oxide from nitrate precursor. However, setting the photocatalyst concentration at 800 mgL−1 may be preferable to reduce the overall environmental impact, mainly associated with the input of electrical energy and the synthesis of the material. Advanced oxidation processes like the heterogeneous photocatalytic process investigated in this study are promising ways to achieve high standards of wastewater purification and environmental protection. To this aim, the chance of exploiting the solar light to promote photocalysis and the possibility to hold the catalyst on supports that may facilitate its recovery for regeneration are of great interest to scale this technique up to real systems at lower economic and environmental cost.