The motivation behind the study lies in a growing environmental problem. Pharmaceuticals and personal care products, or PPCPs, are rich in organic groups and accumulate in rivers, lakes, and groundwater as production rises with population growth and improving living standards. Their documented hazards include toxic effects on plants and animals, growth inhibition, immune system damage, organ damage, and reproductive harm, with additional synergistic effects possible when multiple PPCPs coexist in the environment. Removing these compounds from aquatic systems is therefore a pressing priority for environmental and human health.

Among the advanced oxidation processes available for degrading persistent pollutants, photocatalysis has attracted particular attention because it can operate under different light conditions. The researchers focused on layered double hydroxides, or LDHs, a class of inorganic materials consisting of positively charged metal hydroxide layers separated by negatively charged anions and water. LDHs possess interlayer spaces and large surface areas that make them attractive for photocatalytic degradation, but they suffer from serious defects: excessive accumulation, or agglomeration, of particles and high metal leaching rates that limit their practical efficiency.

To overcome these weaknesses, the team turned to biomass charcoal as a substrate. Corn straw was cleaned, split, crushed, sieved, and pyrolyzed in a tube furnace for six hours at 500 degrees Celsius under oxygen-limited conditions to produce biochar. Mixing this biochar with urea at a 1:2 mass ratio followed by a second pyrolysis step yielded nitrogen-modified biochar, designated N-BC. The large specific surface area, easy accessibility, and low cost of the biochar allow it to disperse agglomerating LDH particles uniformly across its surface, exposing far more active reaction sites than the hydroxide could offer alone.

The synthesis involved several sequential hydrothermal steps. Bismuth vanadate was first prepared from sodium vanadate and bismuth nitrate in aqueous glycerol at 180 degrees Celsius for 12 hours, then combined with N-BC in ethanol at a 1:4 mass ratio. Separately, CoAl-LDH was grown hydrothermally at 135 degrees Celsius from cobalt nitrate, aluminium nitrate, urea, and ammonium fluoride, then etched in a 0.2 mol/L sodium sulfide solution. The etching step transformed the flower-like LDH spheres into hollow structures and, crucially, generated oxygen vacancies. Finally, the etched LDH and the BiVO4-NBC support were reacted together hydrothermally to form the finished composite.

An extensive characterization campaign confirmed the architecture of the material. X-ray diffraction detected all characteristic peaks of both BiVO4-NBC and CoAl-LDH@CoSx in the final composite, while X-ray photoelectron spectroscopy verified the presence of Co2+, Bi3+, carbon, and oxygen. High-resolution transmission electron microscopy revealed distinct phase interfaces between BiVO4 and the LDH, with lattice spacings of 0.309 nanometres corresponding to the (121) plane of BiVO4 and 0.2 nanometres matching the (012) plane of CoAl-LDH@CoSx. Nitrogen adsorption measurements showed the composite achieved a Brunauer-Emmett-Teller specific surface area of 166.84 square metres per gram, exceeding that of the etched LDH alone at 154.57 square metres per gram.

Optical and electrochemical tests demonstrated why the composite outperforms its individual components. Ultraviolet-visible diffuse reflectance spectroscopy showed a red-shift in light absorption, and the band gap narrowed from about 3.90 electronvolts for pristine CoAl-LDH to 3.23 electronvolts for the composite, broadening the usable portion of the spectrum. Photoluminescence measurements indicated reduced electron-hole recombination, while electrochemical impedance spectroscopy showed a smaller impedance radius and transient photocurrent tests revealed a larger photocurrent, both confirming more efficient separation and migration of photogenerated charge carriers.

The heart of the enhancement, according to density functional theory calculations, is a built-in electric field formed at the interfaces between the three components. The calculated work functions of BiVO4, CoAl-LDH@CoSx, and NBC are 5.27, 6.25, and 4.53 electronvolts respectively, with corresponding Fermi energy levels of -2.54, -2.18, and -2.02 electronvolts. Because these levels differ, electrons flow from NBC through CoAl-LDH@CoSx toward BiVO4 when the materials are brought into contact, creating an internal electric field directed from NBC to CoAl-LDH@CoSx to BiVO4. Under illumination, this field drives photogenerated electrons toward the NBC and holes toward BiVO4, suppressing recombination and boosting catalytic activity.

Radical trapping and electron spin resonance experiments identified the reactive species doing the destructive work. When p-benzoquinone was added to capture superoxide radicals, degradation collapsed to just 4.9 percent, identifying the superoxide radical anion as the dominant active species, with holes, hydroxyl radicals, and singlet oxygen also contributing. Liquid chromatography-mass spectrometry mapped several degradation pathways, including hydroxylation of PABA, decarboxylation to aniline, deamination to benzoic acid, and esterification to methyl 4-aminobenzoate. Toxicity estimation software predicted that the oral lethal dose for rats increases for most intermediates relative to PABA, whose own LD50 of 2935.67 milligrams per kilogram classifies it as hazardous, though some intermediates showed elevated bioconcentration factors, suggesting longer degradation times help reduce residual toxicity risk.

The study also probed practical operating conditions and durability. Degradation favoured neutral pH, and performance first improved with catalyst dosage and pollutant concentration before declining when excessive catalyst clouded the solution or too much PABA overloaded the system. Dissolved salts such as sodium sulfate, sodium carbonate, sodium chloride, and potassium chloride all inhibited the reaction, with sulfate halving degradation efficiency. In recycling tests, the composite degraded 93 percent of PABA in the first cycle, though removal efficiency fell by more than 35 percent by the tenth cycle, a decline attributed to structural changes, reduced visible light absorption, and differential leaching of metal ions, with cobalt leaching fastest and aluminium slowest. Even so, X-ray diffraction after repeated runs showed characteristic peaks remained clearly discernible, indicating structural stability conducive to longer-term use. The researchers say the approach provides a novel pathway for recycling waste biomass into new photocatalysts and opens avenues for targeting other PPCP-like contaminants in real wastewater.

Subject of Research: A biochar-supported CoAl-LDH@CoSx-BiVO4 composite photocatalyst with a built-in electric field for degrading persistent PPCP pollutants in water

Article Title: CoAl-LDH@CoSx loaded BiVO4-NBC composites: constructing a novel in-built electric field to enhance photocatalytic performance for persistent pollutants in water

Article References: Wang, W., Zhao, X., Ren, X., Duan, X., & Sarmah, A. K. (2026). CoAl-LDH@CoSx loaded BiVO4-NBC composites: constructing a novel in-built electric field to enhance photocatalytic performance for persistent pollutants in water. Journal of Saudi Chemical Society, 30(3), Article 35. https://doi.org/10.1007/s44442-026-00088-4

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00088-4

Keywords: photocatalysis, para-aminobenzoic acid, layered double hydroxide, bismuth vanadate, biochar, corn stover, oxygen vacancies, built-in electric field, PPCPs, water treatment, density functional theory, reactive oxygen species