Synthetic dyes like malachite green (MG) and rhodamine B (RhB) are mainstays in the textile industry, but their release into wastewater poses serious environmental and health risks, as they are toxic and potentially carcinogenic. While photocatalysis—using light to activate a catalyst that breaks down pollutants—is a promising cleanup method, most research focuses on single dyes. Real industrial effluent, however, is often a complex cocktail of multiple dyes Addressing this challenge, Peter A. Ajibade and Thandi B. Mbuyazi from the University of KwaZulu-Natal, in a study published in Environmental Science and Pollution Research, have developed highly efficient nanocomposites that excel at degrading both MG and RhB, even when mixed. Their approach cleverly combines low-cost biochar with magnetic iron oxide nanoparticles.

The researchers started with a sustainable and rather unique biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More source: the leaves of Portulacaria afra (commonly known as spekboom or elephant bush), a plant known for its high carbon dioxide absorption capacity. They converted these leaves into biochar— at three different temperatures: 200°C, 400°C, and 600°C⁶. Biochar itself is a good adsorbent due to its porous structure, but the team enhanced its properties by using it to “cap” tiny magnetic iron oxide (Fe3O4) nanoparticles synthesized via co-precipitation\ This created three distinct nanocomposites, labelled Fe3O4@BC-1 (200°C biochar), Fe3O4@BC-2 (400°C biochar), andFe3O4@BC-3 (600°C biochar). Characterization showed these composites had tiny particle sizes (11-13 nm) and energy band gaps suitable for activation by visible light (1.79–1.97 eV)

When tested under visible light, all three nanocomposites effectively degraded both MG and RhB in single-dye solutions. Fe3O4@BC-3 showed the best performance for individual dyes, removing 94.91% of MG and 80.01% of RhB after 180 minutes. The researchers attributed this to its larger pore volume and mesopores, allowing better transport of the dye molecules. However, the standout performer emerged when tackling the more challenging binary mixture of MG and RhB. Here, Fe3O4@BC-2 (made with biochar carbonized at 400°C) achieved remarkable degradation efficiencies of 99.74% for MG and 98.89% for RhB within 180 minutes. This superior performance in the mixture was linked to Fe3O4@BC-2 having the highest specific surface area (91.5 m²/g), offering more active sites for the photocatalytic reactions to occur. The biochar component also plays a crucial role by acting as an electron sink, preventing the rapid recombination of light-generated electrons and holes, thus prolonging the activity of the reactive species that destroy the dyes.

Further experiments revealed that the degradation process worked best under basic conditions (pH 10). Scavenger tests confirmed that hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻) were the primary agents responsible for breaking down the dye molecules. Importantly, the nanocomposites proved to be robust and reusable; even after five consecutive cycles, Fe3O4@BC-2 maintained high degradation efficiency (over 97% for MG and 91% for RhB in the mixture). The magnetic nature of the iron oxide core also allows for easy separation and recovery of the catalyst using a simple magnet, adding to the practicality of the system. This study successfully demonstrates that biochar derived from Portulacaria afra, when combined with iron oxide nanoparticles, creates a potent, stable, and reusable photocatalyst capable of efficiently tackling complex mixtures of textile dyes in wastewater, offering a promising, low-cost, and eco-friendly treatment solution.

Source: Ajibade, P. A., & Mbuyazi, T. B. (2025). Photocatalytic degradation of single and binary mixture of malachite green and rhodamine B dyes by biochar-capped iron oxide nanocomposites. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-025-37025-8