Insight into Adsorption Properties and Mechanism of Geopolymer Adsorbents with Inherent Alkali Release for Tetracycline

The crux to practical application of geopolymer adsorbents for antibiotics that are easily protonated and deprotonated is to clarify the dynamic adsorption mechanism of geopolymer with intrinsic alkaline release. The adsorption properties, adsorption mechanism, adsorption-desorption cycle and waste adsorbent regeneration based on the geopolymer adsorbent for adsorbing tetracycline under the spontaneous dynamic pH conditions were systematically investigated by experiment, characterization and density functional theory. The results confirmed that the structures of tetracycline depended on dynamic pHs of adsorption systems changed by geopolymer adsorbent. The electrostatic interaction assisted by hydrogen-bonding and n-pi interaction dominated the adsorption process when the dynamic pHs were less than 7. And the hydrogen-bonding and n-pi interaction provided the possibility of weak adsorption effect in highly alkaline environments. The geopolymer powder exhibited outstanding adsorption efficiency (similar to 95.62%, 30 min) due to the strong electrostatic adsorption between electronegative skeleton of geopolymer and zwitterions species tetracycline under dynamic pH range of 4-6.3 and fitted the pseudo-second-order model. The outstanding competitive adsorption properties in NaCl solution, adsorption (4 times)-desorption (3 times) cycle properties and regenerative property of waste adsorbent were exhibited. The mechanism study based on the dynamic pHs which is closer to actual conditions can provide important reference for practical application of easy preparation and low cost geopolymer adsorbents.