Sequential adsorption enabled by dye-saturated biogenic vaterite: A waste-to-material strategy for enhanced methylene blue removal.

The coexistence of multiple dyes in industrial wastewater poses significant challenges for conventional adsorption treatment due to competitive and synergistic interactions. Herein, we present a sequential adsorption strategy using biogenic vaterite (BV), a microbially precipitated calcium carbonate, to treat mixed-dye wastewater. BV was first saturated with Congo red (CR) to form BV-CR, which was then used for methylene blue (MB) removal. CR pre-loading significantly enhanced MB adsorption capacity from 452.47 to 836.57 mg/g, with equilibrium reached within 10 min. Mechanistic analyses (XPS, FTIR, zeta potential) revealed synergistic non-covalent interactions, including electrostatic attraction, hydrogen bonding, and π-π stacking. BV-CR exhibited robust performance across pH value = 2-11, in real water matrices, and under coexisting ions and organic interferents, while also enabling co-removal of heavy metals (Cd²⁺, Zn²⁺, Pb²⁺). In binary dye systems, BV-CR achieved near 100% MB removal with high selectivity over other cationic (rhodamine B, RB) and anionic (acid orange, AO; methyl orange, MO) dyes. Fixed-bed column tests confirmed scalability and regenerability over 10 cycles. Life cycle assessment showed that the BV-CR route reduces global warming potential by ∼70% (5.16 kg CO₂ eq) compared to direct MB adsorption on BV (17.33 kg CO₂ eq), with energy and nutrient inputs identified as key hotspots. The end-of-life management of the spent adsorbent is also discussed, with regeneration, thermal treatment, and cement stabilization identified as viable pathways for minimizing secondary pollution and closing the material loop. This work establishes BV-CR as a scalable, selective, and sustainable adsorbent platform for mixed-dye wastewater treatment, aligning with green chemistry and circular economy principles.