Efficient microwave regeneration of iron-modified Na-based montmorillonite and Na-based attapulgite for enhanced adsorption of tetracycline in water

A novel iron-modified Na-based montmorillonite (Fe-MT) or iron-modified Na-based attapulgite (Fe-AT) adsorbent is successfully prepared using ion exchange method and applied to remove tetracycline (TC) in water, and the spent adsorbents saturated with organic pollutants are regenerated under microwave (MW) irradiation. The influences of the ratio of Fe3+ and cation exchange capacity, adsorption time, adsorbent dosage, MW power, and irradiation time are investigated. The possible adsorption and MW regeneration mechanism is also proposed. The results indicate that Fe-MT and Fe-AT have enhanced interlayer spacing and surface area after the Na-based montmorillonite (Na-MT) or Na-based attapulgite (Na-AT) are modified using Fe3+. 97.70% and 95.04% adsorption are obtained using Fe-MT and Fe-AT within 30.0 min at 298 K, respectively, while 78.54% and 61.92% adsorption are obtained using Na-MT and Na-AT, respectively. Also, the adsorption processes fit well with pseudo-second-order reaction kinetic and Freundlich model, signifying chemical and multilayered adsorption. The adsorption of TC using the above four adsorbents are spontaneous and endothermic. Moreover, MW regeneration can enhance the adsorption capacity of Fe-MT or Fe-AT, and the regeneration ratios can reach 98.82% and 93.14% under 800 W MW within 5.0 min, respectively. By comparison, the adsorption capacity, regeneration performance and reusability of Fe-MT with larger interlayer spacing and surface area are superior to that of Fe-AT. Therefore, this work provides new perspectives on the adsorption technique using modified clay minerals as adsorbents for dealing antibiotics in water and wastewater, and the MW regeneration technology for regenerating spent adsorbents saturated with organic pollutants.