Adsorption Kinetics, Thermodynamics And Mechanisms Of Organic Dyes By Mesoporous Hybrid Gels

Without using any templating agents mesoporous hybrid xerogels were prepared using the mixtures of tetraethoxysilane (TEOS) with n-propyltriethoxysilane (PTES), or bis(trimethoxysilyl)hexane (TSH), or bis(trimethoxysilylpropyl)amine (TSPA) as precursors (The corresponding products are Xerogels 1, 2, and 3, respectively). FTIR, N₂ adsorption/desorption, TG, point of zero charge (PZC), and water vapor adsorption measurements were used to characterize the xerogels. The adsorption of methyl orange (MO), methyl red (MR), bromocresol purple (BP), phenol red (PR), neutral red (NR), and brilliant blue FCF (BBF) by the xerogels in both acidic and basic solutions were compared comprehensively. Xerogels 1, 2, and 3 have BET surface areas of 387, 695, and 233 m2·g^-1, pore sizes of 17.7, 3.74, and 18.8 nm, PZCs of 6.28, 6.20, and 6.88, respectively. FTIR and TG analyses conform the existence of the organic groups in the xerogels. Xerogel 2 has the highest hydrophobicity, while Xerogels 1 and 3 have a similar hydrophobicity. In general Xerogel 2 shows the highest dye adsorption among all the xerogels in both acidic and basic solutions. All the dyes except NR have much lower adsorption in basic solutions than in acidic solutions. In acidic solutions Xerogels 1 and 2 have similar adsorption trends for the dyes except for BP, while Xerogel 3 presents a different trend from Xerogels 1 and 2. In basic solutions the order of dye adsorption by all the xerogels is shown to follow the sequence of NR>>MR～BBF>MO>BP～PR.The adsorption kinetics and thermodynamics of NR and BBF by Xerogel 2 in aqueous solutions were investigated comprehensively. Kinetic studies show that the kinetic data are well described by the pseudo second-order kinetic model. Initial adsorption rate increases with the increase in temperature. Adsorption activation energy of NR and BBF is found to be 42.7～51.1 kJ·mol^-1 and 45.0～61.0 kJ·mol^-1, respectively. Internal diffusion appears to be the rate-limiting step for the whole adsorption process. The adsorption isotherms of NR and BBF can be described with both Langmuir and Freundlich models with Freundlich model giving a better fitting. For both NR and BBF, their adsorption thermodynamic parameters ofΔG^o,ΔH^o andΔS^o are all negative, indicating that the adsorption is spontaneous and exothermic with the order of the adsorption systems increasing.Four wet gels were prepared using TSPA, or the mixture of TSPA with TSH, or PTES, or TEOS as precursors (The corresponding products are Wet Gels 1, 2, 3, and 4, respectively). The adsorption behavior of BBF on the wet gels in aqueous solutions was investigated comprehensively. The effects of temperature, pH, and ionic strength on the adsorption and the linear shrinkage of the wet gels were studied. Under different adsorption conditions, the kinetic data are well described by the pseudo second-order kinetic model. Initial adsorption rate increases with the increase in temperature, but decreases with the increase in pH and ionic strength. Internal diffusion appears to be the rate-limiting step for the whole adsorption process. After adsorption for seven days, temperature does not have an obvious effect on BBF adsorption amount. While the BBF adsorption amount decreases with the increase in pH and ionic strength. pH and ionic strength do not have an obvious effect on the linear shrinkage of the wet gels. While the linear shrinkage of the wet gels increases with the increase in temperature. Under different experimental conditions, Wet Gels 3 and 4 show a larger linear shrinkage than Wet Gels 1 and 2.