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[Display omitted] • Synthesis of Mg-Al-CO 3 LDH. • Effective and environmental desilication method. • Not Cr(VI)-containing compounds formation. • Synergistic effect between ion exchange of Si(IV) and zeolite formation. The Mg-Al-CO 3 LDH was synthesized as a desilication agent to effectively and environmentally remove Si(IV) from Na 2 CrO 4 -NaAlO 2 -NaVO 3 -Na 2 SiO 3 -H 2 O solutions. It was observed that the changes of reaction parameters including temperature, NaOH concentration, time, the dosage of Mg-Al-CO 3 LDH and NaAlO 2 concentration significantly affected the desilication efficiency, while the effect of Cr(Ⅵ) and V(Ⅴ) concentration on the desilication was negligible. The results showed that 91.5% Si(IV) could be removed after desilication at 90 °C for 6 h with the additive of 4 g/L Mg-Al-CO 3 LDH, without the formation of hazardous Cr(VI)-containing compounds. It could be concluded that the desilication mechanism was dominated by the synergistic effect between the ion exchange of Si(IV) and the formation of zeolite compounds. The method proposed in this study provides a new insight for Si(IV) removal from the highly concentrated Cr(VI) solutions in the chromate clean production. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ion-adsorption type rare earths desorption residues as adsorbents. • The maximum Y(III) adsorption capacity of 4.45 mg·g−1. • Y(III) adsorbed onto the adsorbents surface possibly via Sii-Oi-Y bond. • Adsorption process governed by chemical adsorption. Ion-adsorption type rare earths ore, as the primary source for middle-heavy rare earth elements, was extracted by various electrolytes via the adsorption–desorption effect. In this work, adsorption behavior and mechanism for yttrium ions onto ion-adsorption type rare earths ore as adsorbents were investigated systematically and comprehensively. The isotherms and kinetics suggested the adsorption of yttrium ions was consistent to Langmuir and pseudo-second-order model, revealing that the adsorption process was homogeneous and primarily governed by chemical adsorption. Maximum adsorption capacity could attain as 4.45 mg·g−1 in pH of 3 with 10 g·L−1 yttrium ions. Moreover, thermodynamics analysis indicated the adsorption process was spontaneous, and random. ATR-FTIR, XPS and DFT results indicated that Y(III) adsorbed onto ion-adsorption type rare earths ore surface possibly via Si-O-Y bond. [ABSTRACT FROM AUTHOR]

[Display omitted] • Efficient removal of V(V) and Cr(VI) of γ-AlOOH-Cr-V to 0.0656% and 0.0018%, respectively. • A method for improving the peptization index of γ-AlOOH to 98.1%. • Synergistic effect between the improvement of peptization performance and the removal of V(V) and Cr(VI). • Green process of utilizing aluminum waste in chromate industry. Cr(VI)-containing aluminum residues are inevitable industrial hazardous waste in the chromate industry. To reduce and utilize these residues, a combined desorption and peptization technology for disposing of Cr(VI)-V(V)-containing boehmite hazardous waste (γ-AlOOH-Cr-V) with NaOH solutions was proposed, which realized the removal of V(V) and Cr(VI) effectively and the improvement of peptization index (PI) of obtained pseudo-boehmite (PB) synergistically. The results showed that PB with high PI (98.1%) and low content of V(V) (0.0656%) and Cr(VI) (0.0018%) could be achieved by desorbing the γ-AlOOH-Cr-V, while the surface area and pore volume would be decreased during desorption with the increase of NaOH concentration. Besides the leaching toxicity of Cr(VI) of obtained PB of 0.09 mg/L was extremely lower than that of the standard of hazardous waste. It was noted that the evolution of structural hydroxyl groups governed the improvement of peptization performance. DFT results showed that proper interlayer water content was beneficial to the adsorption of hydroxyl ions on the structural hydroxyl groups of PB. This work not only reduced the emission of Cr(VI)-V(V)containing hazardous waste, but also paved the way for the high-valued utilization of co-associated aluminum and vanadium resources in chromite. [ABSTRACT FROM AUTHOR]