Enhanced adsorption of Pb(II), Cd(II), and Zn(II) by tannic acid-modified magnetic fly ash-based tobermorite

Despite the high adsorption capacity of fly ash-based tobermorite (TFA) for Pb(II), its performance for Cd(II) and Zn(II) was poor, and it proved difficult to separate and recover as a powdered adsorbent. Based on a multifunctional design strategy, this study proposes the innovative use of a tannic acid-modified fly ash-based magnetic tobermorite composite material (TFA-Fe₃O₄/TA). Characterization revealed that TFA-Fe₃O₄/TA possessed superparamagnetic properties and enriched hydroxyl groups. The adsorption performance of TFA-Fe₃O₄/TA for heavy metals was significantly improved. In the single-metal system, the adsorption capacities of TFA-Fe₃O₄/TA for Pb(II), Cd(II), and Zn(II) were 344.0 mg/g, 108.1 mg/g, and 99.16 mg/g, respectively, representing an increase of 14.97 %, 72.13 %, and 73.53 %, respectively, compared to unmodified TFA. The adsorption procedure followed the Langmuir model and the pseudo-second-order kinetic model, signifying monolayer chemisorption. Moreover, in the multi-metal system, TFA-Fe₃O₄/TA maintained adsorption capacities of 112.7, 58.81, and 65.56 mg/g for Pb(II), Cd(II), and Zn(II), respectively, although these values decreased compared to the single-metal system. The primary adsorption mechanisms involved coordination, ion exchange, metal-π interaction, electrostatic interaction and pore adsorption. TFA-Fe₃O₄/TA demonstrated exceptional integrated functions of "adsorption-separation-recovery," making it an ideal adsorbent material for removing Pb(II), Cd(II), and Zn(II) from contaminated water.