Tannic acid modified microscale zero valent iron (TA-mZVI) with enhanced anti-passivation capability for Cr(VI) removal

The removal of Cr(VI) from aqueous solutions using microscale zerovalent iron (mZVI) shows promising potential. However, the surface passivation of mZVI particles hinders its widespread application. In this study, we prepared tannic acid (TA) modified mZVI composite (TA-mZVI) by a simple sonication method. The introduction of TA allowing TA-mZVI composite to adsorb Cr(VI) rapidly under electrostatic forces attraction, guarantying TA-mZVI exhibited remarkable Cr(VI) removal capacity with a maximum adsorption capacity of 106.1 mg⋅g−1. At an initial pH of 3, it achieved a rapid removal efficiency of 96.2% within just 5 min, which was 7.7 times higher than that of mZVI. Various characterizations, including XPS and CV analysis, indicated that the formation of TA-Fe complexes accelerates electron transfer. In addition, TA endows functional groups to TA-mZVI, raising the dispersion and stability and serves as a protective layer hindering passivation. Further mechanistic analysis revealed that Cr(VI) removal by TA-mZVI followed an adsorption-reduction-precipitation mechanism, with TA mitigating the surface passivation of mZVI and facilitating the reduction of most Cr(VI) to Cr(III). Batch cyclic experiments revealed that TA-mZVI exhibited satisfactory performance, maintaining over 85% Cr(VI) removal even after five cycles and minimally affected by various coexisting ions. With notable advantages in cost-effectiveness, ease-synthesis and recovery, this work provides a great promise for developing efficient reactive adsorbent for addressing Cr(VI) contamination in aqueous solutions.