Removal Of Chlorinated Organic Contaminants By Nanoscale Zero Valent Iron Supported On Organobentonite

Zero valent iron (ZVI) has been suggested as a promising candidate for reductive dechlorination of chlorinated organic compounds. Nanoscale zero valent iron (NZVI) offering larger specific surface area and higher surface energy, can improve the removal efficiency. However, NZVI tends to aggregate together due to its high surface energy and intrinsic magnetic interaction,which results in an adverse effect on removal efficiency. Besides, the removal efficiency of hydrophobic organic contaminants is low because of the poor hydrophobicity on NZVI surface. So it is interesting to develop a supporting material to enhance the activity and reusability of NZVI.In this paper, organobentonite (CTMA-Bent) was prepared by intercalation with cetyltrimethylammonium bromide (CTMA). It was employed as the carrier of nanoscale zero valent iron (NZVI) due to its low-cost and the excellent ability on adsorption. The composite of nanoscale zero valent iron supported on organobentonite (NZVI/CTMA-Bent) was characterized by X-Ray Diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). The removal efficiency was tested by atrazine and various chlorophenols including 2-chlorophenol, 2, 4-dichlorophenol, 2, 4, 6-trichlorophenol and pentachlorophenol. The synergetic effect between adsorption by CTMA-Bent and reduction by NZVI was discussed, which was related to the advanced removal efficiency by NZVI/CTMA-Bent. The kinetics of chlorophenols removal was investigated. Considering with adsorption isotherm, the enhancement of removal chlorophenols by CTMA-Bent was suggested relating to octanol-water partition coefficients (K_ow). The effects of initial pH, initial concentration and iron dosage were investigated, and the recycling of NZVI/CTMA-Bent was evaluated by repetitive experiments. The roles of CTMA-Bent as the carrier of NZVI were discussed. The details are as follows:(1) As a carrier, CTMA-Bent effectually prevented the NZVI particles aggregating together.and helped the NZVI/CTMA-Bent composite exhibit long-term stability and reusability. In addition, the solution pH was buffered by the silanol groups in the bentonite.(2) It was found that the efficiency on removal of atrazine or chlorophenols by NZVI/CTMA-Bent on was much higher than by NZVI. It was also much higher than the simple summation of reduction by NZVI and adsorption by CTMA-Bent. This enhanced removal was attributed to the synergetic effect between CTMA-Bent and NZVI in the NZVI/CTMA-Bent composite. It was the adsorption by CTMA-Bent that increased the local concentration of contaminants in the vicinity of iron surface, thereby the mass transfer was enhanced, and at last the removal efficiency was enhanced.(3) It was found that the removal efficiency by NZVI and NZVI/CTMA-Bent was positively related to the K_ow of chlorophenols. The reaction rate (k) also showed the same tendency. In addition, the removal rate ratio of NZVI/CTMA-Bent to NZVI system was also found increasing with the K_ow of chlorophenols increased. This suggested that the enhancement of removal attributed to the synergetic effect between CTMA-Bent and NZVI was positively related to the hydrophobicity of chlorophenols.