Organo-vermiculites modified by zwitterionic gemini surfactants as efficient adsorbents for Congo red

Zwitterionic gemini surfactants, 1,2-bis[N-methyl-N-(3-sulfopropyl)-alkylammonium] ethane betaines (alkyl, n = 12, 14, 16), are synthesized and used to modify Na-vermiculite (Na-Vt) into organo-vermiculites (OVts) serving as adsorbents for Congo red (CR). FT-IR, XRD, EA, TG-DTG, SEM and BET are utilized for character-ization of OVts. Adsorption factors such as surfactant dosage, contact time, pH, initial concentration and tem-perature are adopted to batch experiments, which all fit well with pseudo-second kinetic model and Freundlich isotherm model. The results show that the OVts have resistance to pH change, but are greatly affected by temperature. 12-2-12Vt is exothermic sorption process which shows higher efficient CR removal of 807.55 mg g-1 (C0 = 1000 mg L-1) at 298 K. On the contrary, 14-2-14Vt and 16-2-16Vt are endothermic sorption pro-cesses exhibiting superior uptake amounts as 1682.76 mg g-1 and 1644.78 mg g-1 (C0 = 2000 mg L-1) at 328 K, respectively. Combining the results of thermodynamic data, logKom values, frontier orbital simulations and ECT (electrophilicity charge transfer) values as well as characterization of spent OVts, it can be concluded that (i) The functional groups (SO3-) of modifiers which are the HOMO orbitals serve as electron-donor and promote the CR adsorption. (ii) The two long alkyl chains form hydrophobic interaction and favor the CR removal. The OVts show efficient adsorptive capacities implying that zwitterionic gemini surfactants are well desirable for modi-fication of Na-Vt for removal of CR from wastewater.

Automated summary

Zwitterionic gemini surfactants were synthesized and used to modify Na-vermiculite, resulting in efficient adsorbents for the removal of Congo red. The modified organo-vermiculites showed resistance to pH change but were greatly affected by temperature. The adsorption process was found to fit well with pseudo-second kinetic model and Freundlich isotherm model. The functional groups and long alkyl chains in the surfactants played important roles in promoting the adsorption of Congo red. This study provides insights into the mechanism and application of zwitterionic gemini surfactants in wastewater treatment.