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[Display omitted] • Ferrihydrite (Fh) shows a superior capacity for amoxicillin-AMX uptake (75 mg g−1). • The Al-for-Fe substitution increases surface OH and decreases AMX adsorption. • Bidentate-binuclear complexation is the main AMX adsorption mechanism on Fh. • Inner-sphere complexation characterizes strong sorption, less mobile adsorbate. • Strong sorption on soils can minimize superficial and groundwater contamination. The fixation of amoxicillin (AMX) by soils is a strategy to minimize the contamination of plants and groundwater by this antibiotic. Ferrihydrite (Fh) is a mineral largely found in the soil, and capable of removing emerging contaminants, such as AMX. To better understand the role of this mineral on AMX retention, nanoparticles containing 0, 10, and 20 mol% of Al, a common substitute for iron in Fh, were synthesized. Ferrihydrites showed AMX adsorption capacity superior (75 mg g−1) to other common soil constituents. The optimum pH for the AMX adsorption by the studied materials (Fh-0 %Al, Fh-10 %Al, and Fh-20 %Al) was pH 4. This condition favors electrostatic attraction (AMX-carboxyl/Fh), though experiments demonstrated this is not the main mechanism of adsorption. Raman and Fourier transform infrared spectroscopies combined with a theoretical approach using the density functional tight-binding method showed the bidentate binuclear complexation of the AMX - COO– group with the Fh particle as the main adsorption mechanism. The Al substitution decreases AMX uptake. The effect was attributed to the increase of surface OH, thus avoiding CH 3 interaction with the Fh surface. The interaction mechanism suggests that a strong AMX fixation on Fh-rich natural soil will occur, preventing water contamination and allowing for further biotic degradation. [ABSTRACT FROM AUTHOR]