Magnesium-Boosted Spinel Catalyst Zaps Antibiotics in Wastewater Under UV Light

Pharmaceutical residues have become one of the most stubborn categories of water pollution of our time. Even after passing through conventional wastewater treatment plants, drugs such as antibiotics can be detected in surface water, groundwater and even drinking water at concentrations on the order of nanograms per litre. Because these compounds are built around complex aromatic ring structures, they resist ordinary biological degradation, and their persistence raises growing concerns about ecological damage and the spread of antibiotic resistance. A new open-access study published in the journal Results in Chemistry by Youssef El Jabbar, Hind Lakhlifi, Said Ait Talhajt and Rachida El Ouatib reports a promising answer drawn from the toolbox of materials chemistry: a magnesium-substituted zinc aluminate spinel that can break down the antibiotic tetracycline hydrochloride under ultraviolet light.

The material at the heart of the study belongs to a well-known family of compounds with the general formula AB2O4, known as spinels. These mixed metal oxides are stable semiconductors that have repeatedly demonstrated strong photocatalytic efficiency in destroying organic pollutants. In a photocatalytic process, the semiconductor absorbs photons, generating reactive species such as hydroxyl radicals and superoxide radicals that attack pollutant molecules and tear them apart. How well an oxide performs this job depends on several parameters, including its band gap energy, crystallinity, specific surface area and morphology. The Moroccan team set out to tune those parameters by replacing half of the zinc ions in zinc aluminate with magnesium, producing a compound written as Mg0.5Zn0.5Al2O4, and then testing it against tetracycline hydrochloride, a widely used antibiotic chosen as a representative pharmaceutical pollutant.

The preparation method is one of the most appealing aspects of the work, because it is deliberately simple and inexpensive. The researchers dissolved analytical grade zinc nitrate, aluminium nitrate and magnesium nitrate in water at a metal ion concentration of 0.2 molar, keeping the stoichiometric molar ratio of trivalent to divalent cations at 0.5. Urea was added in excess to act simultaneously as a chelating agent and as a fuel. The solution was evaporated at 80 degrees Celsius until a viscous gel formed, then dried at 120 degrees Celsius for 24 hours to yield a xerogel. That xerogel was pre-calcined at 300 degrees Celsius for 12 hours in air to burn off the organic matter through pyrolysis, and the resulting precursor was finally heat-treated at 700 degrees Celsius for 5 hours. This soft chemistry route avoids the high temperatures and elaborate equipment often required to make high-quality spinel ceramics.

Characterisation of the powders followed established analytical protocols. Thermogravimetry and differential thermal analysis tracked the crystallisation of the xerogel, revealing a total mass loss of about 78 percent between 30 and 550 degrees Celsius. Those mass losses appeared as thermal effects attributable to dehydration, the elimination of ammonium nitrate species, decomposition of the complexing agent and the eventual crystallisation of the oxide phase. X-ray diffraction, performed with a Bruker D8 Advance diffractometer using copper K-alpha radiation, then mapped the crystalline structure of samples taken before and after the high-temperature treatment. The pre-calcined sample showed broad, weak diffraction peaks signalling the very beginning of crystallisation, while the sample annealed at 700 degrees Celsius displayed sharper, more intense reflections, a clear sign of improved crystallinity.

The diffraction pattern of the annealed powder told a satisfying story. Peaks appeared at 2-theta values of 31.2, 36.8, 44.8, 49.1, 55.8, 59.4, 65.3, 74.1 and 77.5 degrees, and these could be indexed to the (220), (311), (400), (331), (422), (511), (440), (620) and (533) crystallographic planes of a cubic spinel structure belonging to the Fd-3m space group. Crucially, no additional diffraction peaks were detected, indicating that a single-phase spinel had formed rather than a mixture of unwanted secondary phases. Scanning electron microscopy with a JEOL JSM-6400 instrument showed that the calcined powder consists of submicron-sized agglomerates, a morphology typical of powders produced by gel-based combustion and chelation routes. The authors note that the study was limited to a single magnesium-to-zinc ratio of x equal to 0.5, and that a systematic survey of different ratios would be needed to fully map how composition controls phase formation, crystallinity, optical behaviour and catalytic performance.

Optical measurements provided the key numbers that connect the material to its photocatalytic role. Ultraviolet-visible absorption spectroscopy revealed a broad absorption band extending into the visible region. Using Tauc analysis, in which the absorption coefficient raised to an exponent appropriate for direct transitions is plotted against photon energy, the team identified two quasi-linear regions corresponding to apparent transition energies of approximately 3.14 and 4.65 electronvolts. The authors are careful to point out that these values should not necessarily be read as two independent fundamental band gaps. The lower energy of 3.14 electronvolts is treated as the more relevant optical absorption edge for discussing photocatalytic behaviour under ultraviolet irradiation, while the higher-energy feature is regarded as an apparent transition whose precise origin cannot be pinned down from the available diffraction and spectroscopy data alone.

With the catalyst characterised, the team turned to the practical test: degrading tetracycline hydrochloride in water. The experiments took place in a cylindrical reactor built around a 250-millilitre double-walled Pyrex beaker held at room temperature. The light source combined two UVA lamps and two UVB lamps, each rated at 10 watts and positioned roughly 12 centimetres from the beaker, plus a 15-watt submersible UVC lamp placed directly in the reaction solution. The whole apparatus sat inside an opaque box to exclude ambient sunlight, and no optical filters were used. The pollutant solution started at a concentration of 30 milligrams per litre, and its absorbance maximum at 357 nanometres allowed concentration to be tracked with a linear calibration curve obeying the Beer-Lambert law.

The results unfolded in two stages. First, 50 milligrams of the Mg0.5Zn0.5Al2O4 powder were dispersed in 200 millilitres of the antibiotic solution and stirred in the dark. Adsorption-desorption equilibrium was reached within 30 minutes, during which 17 percent of the tetracycline disappeared from solution, an initial removal attributed mainly to adsorption onto the catalyst surface. Then the lamps were switched on. Samples collected every 30 minutes showed a steady decline in absorbance, and by the end of two hours of ultraviolet irradiation a total of 76 percent of the pollutant had been eliminated. The removal process followed pseudo-first-order kinetics, with an apparent rate constant of 0.020 per minute. As a benchmark, the authors cite a recent study reporting 66.9 percent tetracycline degradation with the spinel ZnFe2O4 alone, although they caution that differences in catalyst composition, pollutant concentration, catalyst dosage and irradiation conditions make direct comparisons between spinel materials imperfect.

The authors are equally candid about what the experiments cannot yet prove. Because no light-only photolysis control and no radical-scavenger tests were performed, the specific contributions of direct photolysis and of reactive oxygen species such as hydroxyl and superoxide radicals cannot be experimentally separated; their involvement is discussed only on the basis of the generally accepted photocatalytic mechanism. The team also notes that further control experiments using undoped ZnAl2O4 would be needed to quantify exactly how much the magnesium substitution enhances performance. Even with those caveats, the study demonstrates that a light-induced process makes a significant contribution to tetracycline removal and that a cheap, urea-assisted gel route carried out at just 700 degrees Celsius can deliver a phase-pure, crystalline spinel photocatalyst capable of attacking one of the most worrying classes of aquatic micropollutants. For an environmental challenge in which pharmaceutical effluents, particularly wash water from mixing tanks in drug and cosmetics manufacturing, can be highly concentrated and variable throughout the year, robust and adaptable treatment technologies are urgently needed, especially in developing countries where resources and advanced infrastructure remain limited. Alongside established approaches such as dense-membrane filtration, activated carbon adsorption and other advanced oxidation processes, this magnesium-substituted spinel now offers a low-cost candidate worth refining, one whose next iterations will need to probe different magnesium-to-zinc ratios, sunlight-driven operation and recyclability if it is to move from the laboratory beaker to real treatment trains.

Subject of Research: Magnesium-substituted zinc aluminate spinel photocatalysts for degrading tetracycline antibiotics in wastewater

Article Title: Facile preparation, characterization and photocatalytic activity of magnesium-substituted zinc aluminate

Article References: El Jabbar, Y., Lakhlifi, H., Talhajt, S. A., & El Ouatib, R. (2026). Facile preparation, characterization and photocatalytic activity of magnesium-substituted zinc aluminate. Results in Chemistry, 31, Article 103980. https://doi.org/10.1016/j.rechem.2026.103980

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103980

Keywords: photocatalysis, zinc aluminate, spinel, tetracycline, water treatment, pharmaceutical pollution, magnesium substitution, urea-assisted gel route, semiconductor, antibiotics, UV irradiation, wastewater