Adsorption behavior and mechanism of methyl green on synthesized MoS₂ nanosheets

Two-dimensional molybdenum disulfide (MoS₂) is a promising adsorbent for wastewater treatment because of its layered structure, tunable surface chemistry, and accessible edge sites. Here, commercial MoS₂ (MoS₂-C) and hydrothermally synthesized MoS₂ nanosheet assemblies (MoS₂-S) were comparatively evaluated for methyl green (MG) adsorption. MoS₂-S exhibited improved colloidal stability, near-complete MG removal (> 98%) across pH 2-12, and an operational equilibrium time of approximately 15 min. The kinetic data were best described by the pseudo-first-order model for MoS₂-C and by the pseudo-second-order model for MoS₂-S; these fits were treated as empirical descriptions rather than direct mechanistic evidence. The MoS₂-S equilibrium data were most parsimoniously described by the Langmuir model, with a maximum adsorption capacity of 7.1 × 10² mg g^-1. Charge-corrected XPS revealed preservation of the dominant Mo⁴⁺ component after adsorption, together with the appearance of Mo⁵⁺, an increased Mo⁶⁺ fraction, modest changes in oxygen- and sulfur-related components, and MG-derived C-N signals. These results support a mixed adsorption mechanism, with predominantly non-covalent uptake on MoS₂-C and a stronger defect-associated, site-specific contribution on MoS₂-S. Box-Behnken optimization produced a significant model (R² = 0.925; adjusted R² = 0.884) with non-significant lack of fit and identified adsorbent dose as the principal operational factor.