Adsorption Mechanism of Congo Red on Mg–Al-layered Double Hydroxide Nanocompound

In this work, congo red (CR) was removed by applying carbonate intercalated Mg-Al-layered double hydroxide (Mg-Al-LDH) nanocompound as an adsorbent. Batch adsorption experiments performed under various temperatures, ionic strengths, initial CR concentrations, alkalinities and shaking rates. The maximum adsorption capacities of Mg-Al-LDH for CR were 100, 105 and 86.8 mg g -1 at 308, 318 and 328 K, respectively. Adsorption sites of Mg-Al-LDH for CR were –OH groups attached to Al atoms of adsorbent layers. Adsorption isotherms of the process were studied by the ARIAN model and analysis of obtained data showed that there were two kinds of adsorption sites on the surface of Mg-Al-LDH. Results of instrumental analysis showed that these adsorption sites were –OH groups located on the surface of mesopores and micropores of adsorbent and were named MP and 003 sites, respectively. The kinetic data were studied by the KASRA model and ISO and intraparticle diffusion (pore-diffusion) equations which showed that CR molecules were adsorbed at first on the MP sites. Also, during the adsorption of CR on MP sites the interaction of CR with adsorbent surface was rate-controlling step. Furthermore, during CR adsorption on 003 sites, adsorption kinetics was diffusion-controlled.