Biochar-Based Materials from Plant Waste: Physicochemical and Structural Characteristics, Zearalenone Binding in Vitro

Contamination of feed with zearalenone (ZEN) poses a threat to animal and human health, which drives interest in low-cost sorbents derived from plant waste. For non-activated biochars, the properties that govern ZEN binding remain not fully studied. A hypothesis was proposed stating that binding efficiency depends primarily on the amount and type of the mineral fraction rather than on porosity or degree of carbonization. Four feedstocks were pyrolyzed under identical conditions (550 °C, 10 °C/min, N2): Amaranthus cruentus (AC) and Amaranthus retroflexus (AR) biomass (Ca- and K-rich ash; 28–34 wt%), rice husk (RH; Si-rich ash; 44 wt%), and buckwheat husk (BH; 7 wt% ash). The resulting biochars were characterized by proximate analysis, X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), low-temperature nitrogen adsorption (BET/BJH), and iodine adsorption. ZEN-binding capacity was assessed in vitro (adsorption at a nominal pH of 2.0, desorption at pH 8.0; 37 °C; toxin-to-sorbent mass ratio of 1:1000). All biochars had a very low BET specific surface area (0.45–2.30 m2/g). ZEN removal efficiency decreased in the order ACB > ARB > RHB > BHB (90.5 > 77.3 > 65.8 > 10.6%), and desorption did not exceed 1.5% of the bound toxin. No statistically significant correlation between any property and ZEN removal efficiency was found for the four samples; the observed order is consistent with the hypothesis that calcite-containing Ca/K ash plays an important role, but does not conclusively prove it. All biochars were non-toxic to the ciliate Stylonychia mytilus (survival > 70%). ACB merits further investigation, including adsorption isotherms, kinetic studies, experiments at different pH values, and in vivo trials.