Adsorption Study Of Anions Onto Surfactant Modified Palygorskite

Palygorskite is a kind of low-cost and widely reserved inorganic nano-material which has the characteristics of chemical stability and strong adsorption. An extensive study on the preparation of surfactant (octodecyltrimethylammonium chloride, OTMAC) modified acid-activated palygorskite (SMP) by the method of ultrasound was performed in order to adsorb sulphate ions (SO₄^2-). The results showed that the optimum preparation conditions were: OTMAC addition/ acid-activated palygorskite 35 mmol/ 100 g; pH of OTMAC solution 8; reaction time 11 min; electric current 0.6 A. The adsorption amount per gram of SO₄^2- was the most under the condition, and its nitrogen content reached 3.01 At % according to CHN element analyzer.In this paper, we adopted a series of characterization analysis methods, such as Fourier Transform Infrared Spectrophotometer (FTIR), Thermo gravimetric Analysis, N2-adsorption/desorption, Scanning Electron Microscope (SEM), X Ray Electron Spectrum (XPS), to analyze the structure of natural, acid-activated palygorskite and SMP. The specific surface area of SMP decreased while the average pore diameter increased, for the reason that OTMAC was too large to enter the hole of SMP, so OTMAC molecules covered on the surface and reduced its surface area. From SEM image we can see that the fibrous group got larger after modification; FTIR spectra showed that C-H stretches and C-N stretches which belong to OTMAC exist in SMP. All the analysis results showed that OTMAC had successfully grafted onto acid-activated palygorskite. Based on the molecular characteristic of OTMAC and palygorskite, the modification mechanism was that interaction between negatively charged palygorskite and positive charged end of OTMAC, and then, another OTMAC connected to it and formed bilayer structure through hydrophobic bond.The optimum adsorption conditions were received by factorial test: pH of SO₄^2- solution 4; SMP addition 0.4 g; adsorption time 60 min. SO₄^2- adsorption amount could reach 3.5 mg/g.The adsorption capacity of SO₄^2- onto SMP was investigated through adsorption isotherm equations. Compared to Dubinin-Radushkevich and Langmuir equation, the adsorption process was fit to Freundlich isotherm more, which indicated that adsorption took place at heterogeneous sites. The thermodynamics data showed that the process was an endothermic, entropy increase and spontaneous reaction. Results of kinetic experiments showed that this reaction was best described by second-order pseudo kinetics model. It was also fitted to inter-particle diffusion model when it divided to three parts.The competition adsorption effect was investigated by contrast among SO₄^2-, NO₃^- and H₂PO₄^- adsorption capacity and hybrid solution system of SO₄^2-/NO₃^- and H₂PO₄^-/SO₄^2-. The single adsorption isotherms showed the orders of adsorption capacity were as follows: H₂PO₄^->SO₄^2->NO₃^-. SO₄^2- and NO₃^- adsorption onto SMP was weak while H₂PO₄^- was strong from the magnitude of adsorption capacity. The difference between SO₄^2- and NO₃^- was due to their electric charge. The hybrid system adsorption also illustrated the competition of anions in the order of H₂PO₄^->SO₄^2->NO₃^-.