Adsorption behavior of inorganic- and organic-modified kaolinite for Congo red dye from water, kinetic modeling, and equilibrium studies

Raw kaolinite was used as a precursor for several types of modified kaolinite. The modification processes included modification by sodium hydroxide, sodium phosphate, sodium sulfate, CTAB, and sodium acetate. The structural, morphological, and chemical properties of raw kaolinite and the modified products were evaluated using XRD, SEM, TEM, and FT-IR analyses. The modified products were used as adsorbent materials for acidic Congo red dye from aqueous solutions. The adsorption processes were evaluated as a function of reaction time, initial dye concentration, and adsorbent masses. Phosphate-modified kaolinite achieved the best removal results followed by sulfate-modified kaolinite and kaolinite sample modified by CTAB. Kinetic studies indicated that the adsorption equilibrium was obtained after 360 min for the samples, which were modified by NaOH and CTAB, whereas the modified samples that were treated by phosphate, sulfate and, acetate achieve the equilibrium after 240 min. The adsorption by all the products is of chemical nature occurs through energetically heterogeneous surfaces and fitted well with pseudo-second order kinetic model. The equilibrium studies revealed that the adsorption using kaolinite modified by sodium hydroxide, sodium phosphate, and sodium sulfate occurs in monolayer form and represented well by Langmuir model. The estimated q max values are 136.98, 149.25, and 135.13 mg/g for the three products in order. The uptake using modified kaolinite by CTAB and sodium acetate shows more fitting with Tamkin and Freundlich isotherm models rather than with Langmuir model. Kaolinite sample was modified by organic and inorganic salts to enhance its adsorption properties. Modified kaolinite samples exhibit changes in the structural and morphological features. The modified samples showed high adsorption capacity than raw kaolinite. Highlights Kaolinite was modified by several inorganic and organic salts. Effect of modification and the structural and morphological features was investigated. The adsorption behavior of the modified products for Congo red dye was addressed. The change in the adsorption behavior was studied through kinetic and isotherm studies.