Integrated Experimental and DFT Investigations of a Bivalvia Shell Waste-Derived Microcomposite for Sustainable Congo Red Dye Removal via Adsorption and Dye-Sensitized Photocatalysis

Utilization of bio-waste in environmental remediation can be a sustainable model for improving the effectiveness of cleanup efforts. Here, we have successfully investigated the phase evolution of an adsorbent material derived from Bivalvia shells via thermal treatment and implemented it for waste water remediation using a model dye. The structural investigation unveiled a polymorphic and compositional transformation of aragonite (85.8%)/calcite (14.2%) to calcite (100%), and aragonite (5.5%)/calcite (18.5%)/calcium hydroxide Ca(OH)₂ (76.0%) upon thermal treatment at 300 °C (WP-3), 600 °C (WP-6), and 900 °C (WP-9), respectively. The electron microscopic images showed a beautiful sea urchin-like morphology after heating at 900 °C and with a surface area of 3.95 m²/g via BET analysis. The remediation studies showed maximum adsorption capacities of 170.11 mg/g and 183.98 mg/g calculated using the Sips and Langmuir isotherm models, respectively, for the WP-9 material. A mechanistic model for the interaction of the WP material and Congo red dye during adsorption is also presented from DFT studies. The study also show that the WP-9 material can act as a dye-sensitized photocatalyst to efficiently degrade the Congo red dye. Quantitively, 250 mg/L WP-9 degraded 98.24% of CR dye at a rate of 0.09192 min⁻¹, which is almost twice that of commercial Degussa P25 under sunlight. The products of degradation were also studied using gas chromatography.