Beyond the boundaries of organics biosorption: Reusable and cost-effective millet-straw-based carboxylate-functionalized mesoporous MCM-41

Abstract

In this study, we prepared highly porous citric acid-functionalized mesoporous MCM-41 (C-MCM-41) molecular sieve adsorbent from millet straw agricultural residue, which was used for the effective remediation of wastewater contaminated with methylene blue (MB) and ciprofloxacin (CIP). The two-step synthesis process involved extracting highly pure silica from millet straw as a precursor for the ordered mesoporous silica MCM-41. The MCM-41 was synthesized via surfactant-mediated pore design before calcination, followed by citric acid grafting to improve surface functionality and enhance adsorption efficiency. Characterization techniques including XRD, SEM, HRTEM, FTIR, and zeta potential analysis confirmed the mesophase structure, honeycomb pore architecture, functional groups incorporation, and a more electronegative surface, respectively, due to the incorporated carboxyl moieties. Nitrogen adsorption-desorption studies showed a high surface area, which reduced slightly after functionalization. Adsorption of MB and CIP on the C-MCM-41 was superior to the unmodified MCM-41, and the Langmuir isotherm and pseudo-second-order models fitted the equilibrium and kinetic data, respectively. Reuse and cost analysis revealed that C-MCM-41 maintained over 90 % efficiency after four adsorption-desorption cycles and offers a sustainable and cost-effective alternative to conventional adsorbents, demonstrating higher efficiency in treating real wastewater. Sustainable and cost-effective silica was sourced from biomass and employed for the synthesis of reusable citric acid-functionalized ordered mesoporous silica which was used for the effective treatment of antibiotics contaminated wastewater • Highly pure silica was sourced from millet straw biomass waste. • MCM-41 adsorbent was synthesized from this silica which was citric acid functionalized. • The adsorbent was effective for aqueous methylene blue and ciprofloxacin adsorption. • The adsorbent is reusable losing only 5 % of its potential after 4-reuse cycles. • The adsorbent is economical and sustainable.