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[Display omitted] • The highest adsorbed amounts of methylene blue and indigo carmine are obtained in less functionalized rGOs. • DFT calculations show that the most favorable rGO-dye interaction geometry is parallel via π-stacking interactions. • MLFM indicates that the elimination of functional groups increases the fraction of molecules that adsorb a parallel orientation. • MLFM revealed that adsorption sites on rGO surface are capable of orienting up to two adsorbate molecules. • The combination of DFT and MLFM fitting provides a comprehensive explanation for the adsorption experimental data. In this work, an alternative theoretical–experimental approach was combined to investigate the influence of the degree of functionalization of reduced graphene oxides (rGO) on the capacity and adsorption mechanisms of two model organic dyes. The experimental adsorption equilibrium data for methylene blue (MB) and indigo carmine (IC) were adjusted and studied using an efficient Multi-Layer Finite Model (MLFM) based on statistical mechanics. Additionally, density functional calculations (DFTB and DFT) were carried out to study the molecular interaction geometry and adsorption energies. With this approach, a greater amount of information about the adsorption process can be obtained in relation to commonly used methodologies for adsorption at a solid–liquid interface. Herein, kinetic and adsorption equilibrium experiments were conducted on rGOs prepared via thermal reduction at different temperatures (300, 700, and 1000 °C) in order to obtain very different contents of oxygenated functional groups. The highest adsorbed amounts are obtained for rGO1000, reaching 208 and 320 mg g−1 for MB and IC, respectively. The MLFM results revealed that the rGO-dye interaction occurs preferentially via π-stacking, and with the formation of up to two adsorption layers. The elimination of functional groups from the surface of rGOs is also revealed to be favorable, decreasing the adsorption energy and increasing the amount and fraction of molecules adsorbed in parallel orientation. [ABSTRACT FROM AUTHOR]

[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]