Spectrophotometric adsorption isotherms of paraquat and methylene blue on montmorillonite: Unveiling adsorption modes without filtration or centrifugation

Abstract

Measuring adsorption isotherms in a single experimental run is important for designing environmentally friendly adsorption experiments, reducing resources usage and contamination risks. In this study, one-pot adsorption isotherms of the divalent cation paraquat (PQ) and the monovalent cation methylene blue (MB) on montmorillonite were directly measured by UV-Vis spectrophotometry, with no need for separation of the solid phase from the aqueous phase by filtration or centrifugation. PQ adsorption resulted in a bathochromic shift (red shift) in the absorption band position, moving from 258 nm to 277 nm, and only one adsorbed species was detected. MB adsorption induced a hypsochromic shift (blue shift) from 664 nm to lower wavelengths, and at least three different adsorbed species were identified. In the case of PQ, the spectrophotometric isotherm coincided with a regular isotherm, saturating at the cation exchange capacity (CEC) of the solid, 9.1×10−4 Eq/g. In the case of MB, the spectrophotometric isotherm also saturated at the CEC, but the regular isotherm saturated at a higher adsorption extent, 1.2×10−3 Eq/g. In both cases, the presence of negative structural charges of the clay and the planar environment of the interlayer space were crucial for inducing the observed spectral changes. PQ neutralizes the charges of montmorillonite adsorbing as a monomer and undergoing rigidification and planarization in the clay interlayer. On the contrary, MB is forced to form dimers to accomplish the same charge neutralization.