Best of both worlds: Updating the chemical process to remove organic pollutants from water

In 2021, pure unpolluted water for consumption and use is rapidly becoming a distant memory. Most water bodies in the world are contaminated, and organic pollutants form a major part of this contamination. Organic pollutants can not only endanger aquatic life but can also affect human health. Phenols, in particular, are a robust category of toxic organic pollutants that accumulate over time in soil and groundwater and are not easily biodegradable.

Over the years, scientists have discovered many chemical techniques for removing phenol from water. One of the most energy-efficient and flexible approaches for this is the “Fenton oxidation” process, in which a reaction between hydrogen peroxide (H₂O₂) and Fe²⁺ ions yields highly oxidizing hydroxyl ions (^.OH) that, in turn, degrade phenols into carbon dioxide and water. However, despite being highly feasible and repeatable, the Fenton process is very slow, consumes an excessive amount of H₂O₂ during oxidation, and has lower catalytic activity compared to semiconductor-assisted photocatalysts.

Semiconductor-based photocatalysts, when exposed to light, generate pairs of electrons and “holes”(the absence of electrons in atomic sites). These pairs produce ^.OH radicals (similar to the Fenton process) along with ^.O₂ radicals, which help with the degradation of phenol. Despite their better catalytic activity, however, photocatalytic reactions are often bottlenecked by weak light activity and inadequate electron-hole separation.

Now, scientists seem to have found the perfect strategy—coupling the Fenton reaction and photocatalysis to extract the best of both worlds. In a recent study published in the Journal of Industrial and Engineering Chemistry, researchers from India and Saudi Arabia have combined an Ag₃PO₄/CdS/Fe-g-C₃N₄ (AP/CdS/FeCN) photocatalyst with H₂O₂ to develop a heterogenous photo-Fenton system wherein the photocatalyst mimics the natural process of photosynthesis and harvests light to undergo oxidation/reduction reactions. Prof. Pardeep Singh from Shoolini University, India, who was part of the study, explains, “We opted for a dual Z-scheme approach since it is emerging as an effective way to enhance light absorption ability, photostability and separation of photogenerated electron-hole pair in Ag3PO4 photocatalyst.”

Scientists subjected the fabricated system to phenol removal tests and found that the new coupled system showed higher degradation rates than those from individual approaches. Furthermore, the photo-Fenton system was highly photostable and reusable, maintaining its catalytic activity even after five successive cycles of usage. Scientists attributed the enhancement in phenol degradation rate to the generation of more Fe²⁺ ions and ^.OH radicals in the reacting mixture due to the coupling.

The new photocatalytic system shows promise for practical applications in wastewater purification. In addition, the findings of this study could provide insights on how to design a new generation of catalytic systems for large-scale water treatment. “Apart from its practical implications, the novelty of this technique could invoke interest in researchers to explore and discover newer methods for environmental remediation and pave a path towards sustainable development,” comments Associate Professor Pankaj Raizada of Shoolini University, who was also part of the research team.

One thing’s for sure: this work adds one more stepping stone towards a greener planet with cleaner water.

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Reference

Authors: Aftab Aslam Parwaz Khan¹, Pardeep Singh², Pankaj Raizada², Abdullah M Asiri^1,3

DOI: https://doi.org/10.1016/j.jiec.2021.04.007

Affiliations:

¹Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia

² School of Advanced Chemical Sciences Shoolini University, Solan, Himachal Pradesh, 173212, India

³Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia

About the authors

As a Professor and Head of the School of Advanced Chemical Sciences, Shoolini University, Pardeep Singh is in the top 2% scientists’ list ranked by Stanford University. His current research focuses on a new generation of graphene-based photocatalytic materials and processes to combat water pollution. He has authored 99 papers and 7 book chapters and filed 22 patents.

As an associate professor at Shoolini University, Dr. Pankaj Raizada’s research focuses on the design and synthesis of advanced photocatalytic materials for energy and environmental remediation. She has published over 102 research papers and 7 book chapters and has received 21 patents.