Efficient removal of Cd by diatom frustules self-modified in situ with intercellular organic components.

The organic modification of three-dimensional porous diatom frustules (biosilica) and their fossils (diatomite) is promising in heavy metal adsorption. However, the preparation of such materials involves complex processes, high costs, and environmental hazards. In this study, organic-biosilica composites based on in situ self-modification of diatoms were prepared by freeze-drying pretreatment. Freeze-drying resulted in the release of the intercellular organic components of diatoms, followed by loading on the surface of their diatom frustules. The bio-adsorbent exhibits outstanding Cd adsorption capacity (up to 220.3 mg/g). The adsorption isotherms fitted the Langmuir model and the maximum adsorption capacity was 4 times greater than that of diatom biosilica (54.1 mg/g). The adsorption kinetics of Cd was adequately described by a pseudo-second-order model and reached equilibrium within 30 min. By combining focused ion beam thinning with transmission electron microscopy-energy dispersive X-ray spectroscopy, the internal structure of the composite and the Cd distribution were investigated. The results showed that the organic matter of the composite adsorbed approximately 10 times more Cd than inorganic biosilica. The adsorption mechanism was dominated by complexation between the abundant organic functional groups (amide, carboxyl, and amino groups) on the surfaces of composite and Cd. The bio-adsorbent was demonstrated to have wide applicability in the presence of competitive cations (Na, K, Ca, and Mg) and under a wide range of pH (3-10) conditions. Thus, the self-modification of diatoms offers a promising organic-inorganic composite for heavy metal remediation.