Kinetic Equilibrium and Adsorption Thermodynamics of Direct Dyes by Ca/CTS/FA Composite

Abstract

A novel composite material, calcium/chitosan/fly ash (Ca/CTS/FA) adsorbent prepared by adding a small amount of chitosan (CTS) and calcium ion(Ca) on fly ash (FA) was applied to the adsorption of three direct dyes including direct orange S (DO-S), direct green BE (DG-BE) and direct bordeaux BE (DB-BE). Adsorption equilibrium, isotherms and adsorption thermodynamics were studied . The results show that the adsorption rates of three direct dyes to Ca/CTS/FA are high and the adsorption equilibrium can be reached within 60 minutes. isothermal adsorption data agree with Langmuir model, and the saturated adsorption capacities are 1667mg/g for DO-S, 1111mg/g for DG-BE and 1111mg/g for DB-BE respectively at 298K. Dubinin-Radushkevich (D-R) model is used to investigate the adsorption behavior of direct dyes, and theaverage adsorption energy is in the range of 17.2 ~ 23.6kJ/mol at different temperatures. the thermodynamic parameters indicate that the adsorption processes of direct dyes to Ca/CTS/FA are spontaneous and endothermic from 298K to 328K. Characterization results by FT-IR indicate that Ca/CTS/FA composite is successfully prepared. As a low-value composite adsorbent, Ca/CTS/FA has very high adsorption capacity for direct dyes.