Preparation Of Organically Modified Montmorillonite And Adsorption Of Methyl Orange

Wastewater from the dyeing industry with a large amount of highly colored and toxic organic pollutants has become an important source of water pollution, which is harmful to human body and disturbs the biological processes in water environment. Therefore, the removal of hazardous organic substances before the disposal of dyestuffs in wastewater effluents is of great necessity. Adsorption by solid adsorbents is effective to remove the organic pollutants and decrease the toxicity of wastewater.Montmorillonite, a natural clay mineral, having high cationic exchange capacity and swelling capacity, has been widely used as an adsorbent for wastewater treatment. However, because of the hydration of inorganic ions on the exchange sites, the raw montmorillonite is hydrophilic, which make the natural clay ineffectively adsorptive for organic pollutants.In the present thesis, the natural montmorillonite was modified by cationic surfactants or organosilane to make its surface from hydrophilic to hydrophobic. The adsorption capability of these organoclays for methyl orange was measured under different conditions. The effect of adsorption time, adsorption temperature, initial concentration and pH value of methyl orange solution on the adsorptive capacity of organoclays was investigated. The kinetic data, equilibrium data of adsorption studies were processed, and the thermodynamic parameters were calculated to understand the adsorption mechanism of methyl orange molecules onto organoclays. In addition, the regeneration ways of spent adsorbents were searched.On the basis of the experimental data, the following conclusions have been drawn.(1) The basal spacing of the organoclays is largely higher than that of raw montmorillonite. When the amount of surfactant is the same, the adsorptive capacity of clay modified with long-alkyl-chain cationic surfactant is higher than that of one modified with short-alkyl-chain cationic surfactant. The adsorptive capacity of the cationic surfactant-modified clays was influenced by the pH value of adsorbate solution and reduced with the increase in pH value.(2) As for the clays modified with two cationic surfactants, when the sum of the two modifiers is fixed, the adsorptive capacity of the organoclays is correlated with the ratio of the long-alkyl-chain cationic surfactant to the short-alkyl-chain surfactant. The adsorption of organoclays for methyl orange includes partition and adsorption actions, and the partition action became dominant for the adsorbent with long-alkyl-chain cationic surfactant. The adsorptive capacity is related with the organic matter content and thus the best adsorbent with the highest adsorptive capacity for methyl orange has the highest content of organic matters. The adsorptive action of cationic surfactant-modified clays fits well with the pseudo-second-order rate equation and Langmuir isotherm model.(3) In general, the adsorption of cationic surfactant-modified clays is a spontaneously and exothermically physical process, the adsorption of clay modified with organosilane is also an exothermic physisorption but spontaneously at low temperature and unspontaneously at relatively high temperature, while the interaction of activated carbon with methyl orange is spontaneous and endothermic physisorption. The adsorptive capacity of cationic surfactant-modified clays is higher than the clay modified with organosilane, and slightly lower than that of activated carbon.(4) The spent cationic surfactant-modified clays could be regenerated by heat-treatment in nitrogen or NaOH addition, but the adsorptive capacity of revived organoclay was obviously lower than that of the fresh sample. The heat treatment under inert gas is an effective method for the regeneration of spent activated carbon, whose adsorption capacity can restore to 70% of the fresh adsorbent.