Preparation And Modification Of Vermiculite-based Adsorbents For The Separation Of Organic Pollutants From Aqueous Solution

The industry wastewater, such as dying, coking and pharmaceutical wastewater, contain many toxic and refractory organic pollutants, which may not only prolong the hydraulic retention time (HRT), some persistent pollutants may also be discharged into the natural water due to their low concentration. Adsorption is an important cooperative technology for biological treatment, which is suitable for the treatment of refractory and micro pollutants. Moreover, adsorption technology has a high separation speed and creates favorable conditions for the decomposition and detoxification of organic pollutants.Vermiculite is a kind of natural clay minerals, which has been used in the treatment of heavy metal wastewater due to its cost advantage, but the application in organic wastewater is seldom because of its crystal and surface-charge properties. The aim of this study is to develop vermiculite-based adsorbents for the removal of organic pollutants from water. The adsorption characteristics of different organic pollutants onto vermiculite-based adsorbents were studied, and a novel modification method was developed for the property of vermiculite. The main results are as follows:(1) The adsorption capacity of natural vermiculite had a close relationship with the charge of organic compounds. For cationic dyes, they are adsorbed in the interlayer by the ion exchange and the measured maximum uptake of methylene blue and crystal violet were 24.4 mg/g and 25.2 mg/g, respectively. The adsorption reaction tended to the balance after 2 h. The molecule of methylene blue has a 10 ? molecular width, which might occupy more adsorption sites on vermiculite surface. Ruled by Fick law, this phenomenon induced the decrease of uptake and the deflexion of isotherms when the initial dye concentration exceeded 80 mg/L. Because of the lack of effective adsorption sites and the hydrophilic surface, natural vermiculite had weak affinity for anionic dyes and phenol compounds.(2) Using natural vermiculite as the inorganic carrier, organovermiculites were prepared by hexadecyl trimethylammonium bromide by intercalation method and were characterized by XRD?FTIR and SEM. Its adsorption capacity for congo red was increase from 5.4 mg/g to 175.4 mg/g, which is 60 percents higher than powder activated carbon. Anionic exchange was the main mechanism. The adsorption capacity of phenol and o-chlorophenol were enhanced from 2.5 and 3.5 mg/g to 49.3 and 22.1 mg/g, hydrophobic adsorption was the main mechanism.(3) The liquid-solid adsorption of organovermiculite-pollutant system was simulated by quantum calculation of MP2/6-31g(d) method. The adsorption energy between phenol and different alkyl (C2-C6) were -62.64, -66.12, -67.79, -68.47 and -70.06 kJ/mol, the increment of alkyl length could increase the London force between adsorbate and adsorbent but the total organic loading was the key factor for adsorption capacity. The adsorption energies of aniline, phenol, benzene, toluene and chlorobenzene were -70.28, -68.47, -61.00, -74.97 and -70.07 kJ/mol, but the solvation energy were -40.13?-46.77?-17.29?-15.41 and -19.11 kJ/mol, the calculated partition energy were -46.17, -37.27, -57.92, -77.04 and -67.36 kJ/mol. Hydrophilicity was the key factor for adsorption capacity.(4) For resolving the shortage of low specific surface area (SSA) of intercalation method, natural vermiculite was activated by HCl for improving SSA. After treated with 1, 2, 4, 6 mol/L hydrochloric acid, the SSA were increased from 14.4 m²/g to 201.0, 500.0, 417.6 and 375.2 m²/g. Moderate leaching of octahedral cations was the key factor for improving SSA. The largest SSA of acid bentonite and palygorskite were only 124.3 and 251.6 m²/g, high Fe content of vermiculite played an important role in increasing SSA. Trimethylsilane group was grafted covalently on the surface of acid vermiculite through the condensation reaction with SiOH groups. The largest SSA of Trimethylchlorosilane modified acid vermiculites (TMAVs) was 355.4 m²/g, which was 50 times higher than that of organovermiculite. TMAVs had a good adsorption capacity for diethyl phthalate (DEP), a kind of hydrophobic pollutant, and the adsorption controlled by partition mechanism between liquid-solid phases. The largest partioning parameter (K_d) of TMAVs was 3.1 times higher than that of organovermiculite. The adsorption capacity of TMAVs for coking wastewater was 18 mg TOC·g^-1, which was higher than that of organovermiculite (12 mg TOC·g^-1).