Metal-Free Photocatalysts Show Promise for Scrubbing Drug Pollution from Water

Pharmaceutical pollution has quietly become one of the most pervasive contamination problems of the modern era. Antibiotics, painkillers, hormones, and anticancer drugs are now routinely detected in rivers, lakes, groundwater, and even drinking water supplies around the world. These compounds are designed to be biologically active at very low concentrations, which is precisely what makes their environmental presence so troubling. Conventional wastewater treatment plants were never engineered to capture them, and biological treatment steps often leave them largely intact. A new review published in Environmental Geochemistry and Health by Do Thi Minh Hanh and Pham Thi Thu Hoai of the University of Economics-Technology for Industries in Hanoi, Vietnam, takes a comprehensive look at one of the most promising technological responses to this challenge: hybrid photocatalysts built around graphitic carbon nitride, a remarkable metal-free material that can harness sunlight to destroy drug molecules in water.

Graphitic carbon nitride, known chemically as g-C3N4, is a polymeric semiconductor composed of carbon and nitrogen arranged in layered sheets reminiscent of graphite. It has attracted intense scientific interest because it is chemically stable, non-toxic, inexpensive, and remarkably easy to synthesize from common nitrogen-rich precursors. Crucially, it absorbs visible light, meaning it can in principle be driven by ordinary sunlight rather than expensive ultraviolet lamps. When photons strike the material, they excite electrons from the valence band to the conduction band, leaving behind positively charged holes. These photogenerated charge carriers can then react with water, oxygen, and dissolved species to produce highly reactive oxidizing agents capable of shredding organic pollutant molecules into harmless fragments.

Yet pristine g-C3N4 suffers from well-documented weaknesses that have kept it out of real-world deployment. Photogenerated electrons and holes recombine rapidly, releasing energy as heat before they can do useful chemical work. The material’s surface is comparatively inert, limiting its ability to adsorb and react with target pollutants, and its visible-light absorption, while real, does not extend deeply enough into the solar spectrum. The Hanoi review systematically examines how researchers have overcome these limitations by constructing hybrid photocatalysts, or CN-HPs, in which g-C3N4 is coupled with other materials to form sophisticated junctions that separate charges, extend light harvesting, and boost surface reactivity.

The strategies catalogued in the review are strikingly diverse. Heterojunctions pair g-C3N4 with metal oxides such as ZnO, TiO2, WO3, and BiVO4, or with more exotic partners including MXenes, layered double hydroxides, perovskites, and metal-organic frameworks. Z-scheme and S-scheme architectures are particularly elegant: they mimic natural photosynthesis by selectively retaining the most energetic electrons and holes on different components while allowing the useless carriers to recombine, thereby preserving strong oxidation and reduction power simultaneously. Doping with elements such as sulfur or phosphorus tunes the electronic structure, while carbon quantum dots and noble metal nanoparticles act as electron sinks and co-catalysts. Magnetic components such as Fe3O4 even allow the catalyst particles to be retrieved from treated water with a simple magnet, addressing one of the classic headaches of nanoparticle-based treatment.

The performance figures reported across the literature are impressive, at least under laboratory conditions. CN-HPs have achieved high removal efficiencies for a wide range of pharmaceutical pollutants, including tetracycline, ciprofloxacin, levofloxacin, oxytetracycline, amoxicillin, sulfamethoxazole, ibuprofen, naproxen, acetaminophen, and the anticancer drug 5-fluorouracil. Many systems operate under visible light or even natural sunlight, and some have been tested in continuous-flow configurations and real hospital or mariculture wastewater matrices. The review emphasizes that these materials also retain their activity over repeated catalytic cycles, a critical requirement for any technology hoping to move from the bench to a treatment plant.

Beneath the headline efficiency numbers lies a more nuanced chemical story. The review details how degradation proceeds through the concerted action of three principal reactive species: superoxide radicals, hydroxyl radicals, and photogenerated holes. Each attacks drug molecules at characteristic bonds, cleaving rings and side chains to produce a cascade of smaller intermediate compounds. Ideally, these intermediates are then progressively mineralized into carbon dioxide, water, and inorganic ions. However, the authors highlight an important caveat that recurs throughout the literature: total organic carbon reduction and complete mineralization typically require substantially more time than the initial disappearance of the parent pollutant. In other words, a water sample may test clean for the original drug while still carrying a cocktail of partially oxidized fragments.

That gap between degradation and detoxification is where the review delivers its most consequential warning. Toxicity assessments show that the by-products formed during photocatalytic degradation can carry toxicity profiles different from, and in some cases potentially greater than, those of the original compounds. High removal efficiency alone, the authors argue, does not demonstrate environmental safety. A treatment system that rapidly converts an antibiotic into unidentified fragments without verifying their ecological impact may simply be trading one hazard for another. The review therefore calls for toxicity evaluation to become a standard, non-negotiable component of photocatalysis research, rather than an optional add-on performed in only a minority of studies.

The authors are equally candid about the obstacles standing between laboratory success and field deployment. Most published experiments use purified water, single pollutants at concentrations far above environmental levels, and idealized light sources. Real wastewater presents a hostile environment: competing organic matter scavenges radicals, suspended solids scatter light, mixtures of drugs interact unpredictably, and pH fluctuates continuously. Long-term environmental safety of the nanomaterials themselves, including potential release of catalyst components into treated water, remains insufficiently characterized. Energy efficiency and the engineering challenge of scaling thin-film or slurry reactors to the volumes handled by municipal plants are also unresolved. The review frames these not as reasons for pessimism but as a well-defined research agenda.

One of the most forward-looking threads in the review concerns artificial intelligence. The authors highlight the potential of combining experimental photocatalysis data with machine learning models to optimize treatment conditions and predict how CN-HP systems will perform against specific target pollutants in the field. Given the enormous design space of possible hybrid materials, dopants, junction types, and operating parameters, data-driven approaches could dramatically accelerate the search for formulations that balance efficiency, stability, cost, and safety. Similar AI-assisted strategies are already gaining traction across membrane design and pollution monitoring, and photocatalysis appears poised to follow the same trajectory.

What emerges from the Hanoi analysis is a technology at an inflection point. The fundamental chemistry works: metal-free, sunlight-driven catalysts can genuinely dismantle the pharmaceutical residues that conventional treatment leaves behind, and they can do so repeatedly and, in many configurations, using abundant and benign raw materials. The remaining task is to prove that this chemistry survives contact with the messy, variable, and economically constrained reality of full-scale water treatment, and that the end products of degradation are demonstrably safe for the ecosystems that receive them. If researchers can close the gap between breaking molecules and proving detoxification, graphitic carbon nitride hybrids may become a cornerstone of the next generation of water purification infrastructure.

Subject of Research: g-C3N4-based hybrid photocatalysts for photocatalytic degradation of emerging pharmaceutical pollutants in water and wastewater

Article Title: g-C3N4-based hybrid photocatalysts for removal of emerging pharmaceutical pollutants from water: recent advances, degradation pathways, toxicity assessment,and future perspectives

Article References: Hanh, D. T. M., & Hoai, P. T. T. (2026). g-C3N4-based hybrid photocatalysts for removal of emerging pharmaceutical pollutants from water: recent advances, degradation pathways, toxicity assessment,and future perspectives. Environmental Geochemistry and Health, 48(16), Article 625. https://doi.org/10.1007/s10653-026-03530-z

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03530-z

Keywords: graphitic carbon nitride, photocatalysis, pharmaceutical pollutants, water treatment, heterojunctions, advanced oxidation processes, degradation pathways, toxicity assessment, antibiotics, mineralization, artificial intelligence, wastewater remediation