Creating precise Cd(II)-targeted site through bioinspired tannic acid-modified coal gangue-derived zeolite for enhancing Cd(II) immobilization from aqueous solutions

Coal gangue (CG) is a mixture of abundant SiO 2 and Al 2 O 3 , and greatly suitable for the synthesis of zeolites. In this work, a cost-effective and eco-friendly tannic acid modified zeolite (zeolite@TA) was successfully developed via alkali melting-hydrothermal synthesis of zeolite from CG and followed by the modification of tannic acid onto zeolite surface using the dip-coating method. Organic additive-free fabrication process rendered zeolite@TA synthesis eco-friendly and cost-effective process. Various characterization results revealed the formation of zeolite structure, and tannic acid coating on zeolite. The results demonstrated that the zeolite@TA displayed a notable enhancement for Cd(Ⅱ) adsorption capacity, outperforming the zeolite derived from coal gangue, and zeolite@TA with a maximum theoretical Cd(Ⅱ) adsorption capacity of 184.5   mg/g exceeded zeolite (66.09   mg/g) by approximately 2.8-fold. From adsorption kinetic tests, the kinetic analysis confirmed pseudo-second-order model dominance. Among coexisting ions (Na⁺, K⁺, Mg²⁺ and NH₄⁺), Ca 2+ exhibited the strongest inhibitory effect on Cd(Ⅱ) adsorption. Thermodynamic parameters (△G°＜0, △H°＞0,△S°＞0) revealed the nature of spontaneous, endothermic and entropy-increasing process for zeolite@TA. Overall, Strategic utilization of coal gangue as a primary resource will establish a foundation for new waste-free technological solutions with dual economic-environmental benefits, and is equipped a prospective development space on wastewater cleanup applications.