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Received April 12, 2024
Accepted August 7, 2024
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This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/bync/3.0) which permits
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Compost as Green Adsorbent for the Azo Dyes: Structural Characterization and Dye Removal Mechanism
Abstract
The study aimed to determine the feasibility of using compost as a ‘green adsorbent’ for the removal of fi ve anionic azo dyes
belonging to the monoazo, disazo and trisazo classes: Direct Red 81 (DR-81), Direct Blue 74 (DB-74), Reactive Blue 81
(RB-81), Reactive Red 198 (RR-198) and Acid Black 194 (ABk-194) from aqueous solutions. The adsorption capacity of the
compost was determined using a batch method with initial dye concentrations ranging from 1 to 1000 mg/L. The kinetics of
dye removal followed a pseudo-second-order model, indicating chemisorption as the rate-limiting step. The monoazo dyes
RB-81, RR-198 and ABk-194 with the smaller molecule size were adsorbed the fastest. The Langmuir and Sips models best
fi t the adsorption system with maximum adsorption capacities in the range of 12.64 mg/g (RR-198)—20.92 mg/g (ABk-194)
and 12.57 mg/g (RR-198)—25.43 mg/g (ABk-194), respectively. The adsorption depended on the dye structure, especially on
the ratio of the numbers of proton donors to proton acceptor locations in functional groups. The diff erences in the adsorption
mechanism could be explained by thermodynamic properties such as dipole moments, HOMO–LUMO energy gap, polarizability,
electron affi nity, ionization potential, electronegativity and chemical hardness obtained by Density Functional Theory.
