Effect of different mechanisms of heating of layered aluminosilicate on sorption processes Communication 1. Effect of preliminary thermal and electromagnetic (microwave) heating of montmorillonite on sorption of water

Abstract

Clay minerals of layered structure can be applied for production of highly effective sorbents due to variation of their liophilic, structural-sorption and structural-mechanic properties in modification or activation processes (acidic or basic activation, ion-exchange, hydrothermal or ultrasonic treatment, hydrophobization, etc.). Thermal treatment, directed from the surface to interior of the sample, is a conventional way of sorbents activation and regeneration. The present work considered two different mechanisms of heating (thermal heating and heating due transformation of electromagnetic waves of ultrahigh frequency into thermal energy of material) of the sorbent based on natural layered mineral montmorillonite with the purpose of activation of its sorption ability.

Samples of natural aluminosilicate (M95) containing 95% of montmorillonite and 5% of illite (deposit in Khakassiya, Russia) were preliminary thermally treated at earlier determined temperature 453K for 1 hour to acquire maximal adsorption capacity towards water vapours.

Electromagnetic treatment was carried out at 800 W power of electromagnetic field for 4 min in a microwave. Thermal analysis was used to determine thermal effects, range of thermal stability and mass loss by thermal treatment of the sorbent. Equilibrium and kinetic dependences of water vapours adsorption by the investigated samples of natural and activated sorbent were obtained applying gravimetric method at an equilibrium atmospheric pressure and temperature of 293 K.

Thermal activation of M95 sample increased its sorption capacity to water vapours in 3.3 times, whereas microwave treatment in 5.5 times, mainly, due to aluminosilicate dehydration and release of micropores.

According to derivatograms of natural sample M95 the endoeffect in the range 353-533K was caused by removal of physically bound water from the channels and pores of the sorbent. The loss of mass was 13.5%. Themal treamtnet of the sample led to disappearance of this edoeffect (mass loss of 5.1%), whereas subsequent adsorption of water vapours again demonstrates this endoeffect as a result of sorbent rehydration.

Applying the two binomial equation of theory of micropore volume filling (TMVF) derived for microporous sorbents, the adsorption of water by investigated M95 sample was calculated.

Kinetic dependences of water vapour adsorption on natural montmorillonite illustrated that adsorption equilibrium was set within 20 hours. The calculated values of internal and external diffusion was correspondingly 1.25·10^-11 and 112.0·10^-11 cm²/c that determine a stage of internal diffusion of water as limiting stage of adsorption.

Thus, UHF-heating of natural montmorillonite allowed increasing its adsorption capacity to water vapours in more than 5 time and reduce the time of treatment in 15 times comparing to conventional heating.