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Biosorption studies on waste cotton seed for cationic dyes sequestration: equilibrium and thermodynamics

Sivarajasekar, N., Baskar, R., Ragu, T., Sarika, K., Preethi, N., Radhika, T.
Appl Water Sci 2017 v.7 no.4 pp. 1987-1995
Gossypium hirsutum, acetic acid, adsorbents, adsorption, agricultural wastes, aqueous solutions, biosorption, cottonseed, dyes, endothermy, models, pH, sorption isotherms, sulfuric acid, surface area, temperature, thermodynamics
The immature Gossypium hirsutum seeds—an agricultural waste was converted into a novel adsorbent and its effectiveness for cationic dyes removal was discussed in this study. Characterization revealed that sulfuric acid activated waste Gossypium hirsutum seed (WGSAB) contains surface area 496 m² g⁻¹. The ability of WGSAB to adsorb basic red 2 (BR2) and basic violet 3 (BV3) from aqueous solutions has been studied. Batch adsorption studies were carried out at different initial dye concentrations (100–300 mg l⁻¹), contact time (1–5 h), pH (2–12) and temperature (293–323 K) to understand the adsorption mechanism. Adsorption data were modeled using Langmuir, Freundlich and Toth adsorption isotherms. Equilibrium data of the adsorption process fitted very well to the Toth model for both dyes. The Langmuir maximum adsorption capacity was 66.69 mg g⁻¹ for BV3 and 50.11 mg g⁻¹ for BR2 at optimum conditions. The near unity value of Toth isotherm constant (BR2: 0.999 and BV3: 1.0) indicates that WGSAB surface is heterogeneous in nature. The maximum adsorption capacity predicted by Toth isotherm of BV3 (66.699 mg g⁻¹) is higher than BR2 (50.310 mg g⁻¹). The kinetic investigation revealed that the BR2 and BV3 were chemisorbed on WGSAB surface following Avrami fractional order kinetics. Further, the fractional order and rate constant values are almost similar for every concentration in both the dyes. The thermodynamic parameters such as ΔH ⁰, ΔS ⁰ and ΔG ⁰ were evaluated. The dye adsorption process was found to be spontaneous and endothermic for the two dyes. Regeneration of WGSAB exhausted by the two dyes could be possible via acetic acid as elutant.