Evaluating Untreated Clay’s Adsorptive Capacity to Remove an Anionic Dye from Aqueous Solution

Adsorption study is conducted in batch mode, using natural clay to remove the titan yellow dye (DY9) from aqueous solution. Physical and chemical characteristics of clay are determined by XRF, XRD, FTIR, TGA and BET analyses. It is a smectite–illite clay. The Doehlert design was selected, and response surface methodology is applied to model and optimize operating parameters. The parameters studied are the initial concentration of DY9, adsorbent mass (m_ad), pH of the solution, temperature (T) and stirring speed (v). A quadratic regression model including the significant parameters is suggested. When the pH, m_ad and T varied, respectively, in the range of 3.4–6.0, 30–35 g L⁻¹ and 20–30 °C, the removal yield varied between 93.6 and 97.6%. The kinetic modeling of adsorption shows that data follow closely pseudo-second-order and intraparticle diffusion model. The equilibrium adsorption is modeled according to the two-parameter (Langmuir, Freundlich, Temkin and Dubinin–Radushkevich) and three-parameter (Sips, Redlich-Peterson and Toth) models. Nonlinear regression method is used to fit the data. The data fitted very well with Temkin and Sips models, and all adjusted regression coefficients values exceed 0.90, except Dubinin–Radushkevich model. These models give a maximum adsorption capacity of 5.8–6.8 mg g⁻¹. The Langmuir separation factor value (0 < 1) indicates that the DY9 adsorption is favorable. Thermodynamic parameters such as ∆G° (< 0), ∆H° (− 17.169 kJ mol⁻¹) and ∆S° (− 0.011 kJ mol⁻¹ K⁻¹) suggest that the DY9 adsorption on untreated clay is a spontaneous, exothermic and physical process with an increase in randomness at the solid/solution interface.