Influence of Particle Size and Micronization on the Adsorption Efficiency of Aflatoxin B1 by Bentonite in Animal Feed Applications

Aflatoxin B1 (AFB1) is one of the most toxic mycotoxins contaminating animal feed, and bentonite clays are widely used as adsorbents to reduce its bioavailability. This study introduces and characterizes a new, previously unexplored bentonite deposit from Bijelo Polje (Bar, Montenegro), with ~55% montmorillonite in the raw material. Size fractions (<0.200 mm, <0.037 mm, <0.005 mm) were obtained by sieving and centrifugation and characterized by laser diffraction, chemical composition, BET, CEC, and quantitative XRD (Rietveld). In vitro AFB1 adsorption (2–50 ppm, pH 3.0, 0.02% w/v adsorbent) simulated monogastric gastrointestinal conditions. Progressive size reduction increased smectite content (from ~55% to 91%), CEC (44–70 meq/100 g), purity, and BET specific surface area (26.5–50.8 m2/g), while reducing impurities and heavy metals to undetectable levels. The finest fraction (<0.005 mm) achieved the highest maximum adsorption capacity (qmax ≈ 240 mg/g), attributed to enhanced surface homogeneity and site accessibility, significantly outperforming coarser fractions and most unmodified natural bentonites. Only the <0.005 mm fraction fully complies with EU regulatory requirements (Commission Implementing Regulation (EU) No 1060/2013) for AFB1-binding feed additives (≥70% dioctahedral smectite, >90% binding, low quartz/calcite). These results demonstrate that simple mechanical fractionation can yield exceptional performance in a novel natural raw material, offering a cost-effective, sustainable alternative to chemical modification for mycotoxin mitigation.