Brazilian palygorskite as an alternative to commercial adsorbents for methylene blue: A discussion about composition, morphology and pore profile

Palygorskite has attracted particular attention as a specialized adsorbent due to its high specific surface area, porous texture and abundant surface groups, but its processing for commercial products makes the resulting material more expensive. In this scope, the aim of this work is to evaluate the potential of Brazilian naturally occurring palygorskite samples with low processing degree as an alternative to commercial palygorskites adsorbents for methylene blue, evaluating samples with different compositions, morphologies and microporous profiles. Materials were characterized using FTIR, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and surface area analysis (BET method). The Brazilian ones were processed in bar and disk mills and passed through a # 325 mesh sieve system. All samples showed common palygorskite X-ray diffraction patterns and specific surface area ranging from 100 to 147 m/g. The application of clay samples was investigated for the adsorption of methylene blue (MB) dye, and the influence time of adsorption, initial dye concentration and adsorbent dosage were evaluated. The methylene blue adsorption was best described by the pseudo-second order model. The adsorption kinetics of methylene blue by palygorskite conforms to the pseudo-second-order model (R = 0,9997). The maximum removal capacity for methylene blue was determined to be 694 (±36.53) mg/g. Additionally, the Freundlich model demonstrated the most favorable mathematical fit. Adsorption capacities varied between 49 and 150 mg/g for 300 mg/L MB solutions, with noteworthy performance exhibited by Brazilian samples, achieving up to twice the adsorption compared to their commercial counterparts, despite possessing lower specific surface areas. The presence of two distinct pore modes, centered at 25 and 45 Å, proved pivotal in the sample with the highest adsorption capacity, reaching adsorption equilibrium at 40 min.