Response Surface Analysis of Manganese Removal Onto Sodium Dodecyl Sulfate-Modified Montmorillonite Clay

The adsorption of Mn²⁺ on montmorillonite and optimization by response surface methodology have been studied. The alteration of K10 and MMT-Na by sodium dodecyl sulfate (SDS) yields promising data on Mn²⁺ uptake. The SDS-induced embolism of K10 and MMT-Na montmorillonites opens lamellae and increases surface area, thereby increasing their adsorption capacity for Mn²⁺. The adsorption of Mn²⁺ increases slightly with increasing pH until pH = 8. From the Mn²⁺ species distribution, at pH < 8.5, Mn²⁺ is the only soluble form in solution, and no other species form before pH = 8. The best RSM model describing the adsorption of Mn²⁺ on K10 and K10-SDS was the linear degree model (adj. R² = 0.626, 0.668, respectively), while the best RSM model for MMT-Na and MMT-SDS (adj. R² = 0.897, 0.894, respectively) was the central composite design. In RSM modeling, C₀ (mg/L) was the highest effective parameter (p value for all clays < 0.0001) among C₀ (mg/L), CT (min), and T (°C). Practical and simulated adsorption isotherms show a best-fitting Langmuir isotherm, with an increase in the maximum adsorption capacity (q_m) (mg/g) from K10 (5.95) to K0-SDS (10.22) and from MMT-Na (19.63) to MMT-SDS (21.41). The best-fitting adsorption kinetic model was the pseudo-second-order (PSO) model for all studied clay forms. The adsorption mechanism for all clay forms involved the Boyd liquid-film diffusion step, followed by Boyd interparticle diffusion, both of which are rate-limiting. The adsorption of Mn²⁺ is thermodynamically endothermic, with higher ΔH° values (kJ/Mol) in the modified clays (+46.3 for K10-SDS and +62.5 for MMT-SDS) than in the unmodified clays (+39.8 and +23.2). Gibbs free energy ΔG° (KJ/Mol) becomes increasingly negative as the temperature of the adsorption medium rises for all examined clay forms. Wet analysis of total organic carbon (TOC) shows 71.3% of intercalation of SDS in K10-SDS and 77.5% for MMT-SDS. The intercalation increases the selectivity of Mn²⁺ among many cations (equivalent cation exchange capacity (CEC) = 31.3, 46.4, 74.74, and 80.3 meq/100 g for K10, K10-SDS, MMT-Na, and MMT-SDS, respectively). The presence of total manganese in the application removal increases with intercalation by SDS from groundwater (Removal%: 67.7, 84.2, 69.9, and 81.2 for K10, K10-SDS, MMT-Na, and MMT-SDS, respectively).