| In the present study, white mud was acidified with dilute hydrochloric acid to prepare a leaching solution of white mud with plenty of metal ions such as Ca2+, Mg2+, Fe3+ and Al3+. The removal of anion dyes, reactive brilliant red K-2BP, reactive light yellow K-6G, acid orangeâ…¡and direct yellow R, from aqueous solution was investigated in batch mode using freshly formed hydroxide precipitates, which were generated in alkaline solution by the leaching solution. The chemical composition of the white mud was determined by X-ray fluorescence and the hydroxide precipitates were characterized by scanning electron microscopy, dynamic light scattering and Fourier-transform infrared spectroscopy. Batch coprecipitation/adsorption experiments were conducted to determine the effect of contact time, dosage, pH and temperature. Langmuir and Freundlich isotherms were used to analyze the dye adsorption by hydroxide precipitates. The disposal mechanism was discussed. The experimental results showed that:(1) The main metallic elements of white mud are calcium and magnesium, followed by iron and aluminum. So CaCl2, MgCl2, FeCl3 and AICl3 are the main constituents in the leaching solution. In alkaline conditions, the metal ions contained in the leaching solution could combine with OH- to form sparingly soluble hydroxide precipitates such as Ca(OH)2, Mg(OH)2, Fe(OH)3, Al(OH)3, which have a high surface area. The average particle size of the freshly formed hydroxide precipitates was 6 nm. When grow up, the hydroxide particles present branched structure, and there are a large amount of irregular holes with different pore sizes in the hydroxide particles.(2) Effects of reaction time, dose of the leaching solution and pH were studied. The results showed that the leaching solution was rapid and effective in removing anion dyes. The removal rate reached maximum at 90 s. The removal rate of four anion dyes increased with increasing amount of leaching solution of white mud, appropriate dosage were 2.0 g/L when the concentration of dyes were 100 mg/L. The removal rate of dyes depended significantly on the pH of the dye wastes. The optimum pH of reactive brilliant red K-2BP and reactive light yellow K-6G was 12.5, and acid orange â…¡, direct yellow R was 12.0.(3) The isotherm data was consistent with both the Freundlich and Langmuir models, and the correlation coefficients suggested that a better fit was obtained using the Freundlich model than the Langmuir model. The adsorption process was spontaneous with negative values ofâ–³G. The negative value of AH confirms the exothermic nature of adsorption, so adsorption capacity of freshly formed hydroxides decreased with temperature. The negative AS indicates that the degrees of freedom decreased at the solid-liquid interface. Comparison of the adsorbent capacities of various types of adsorbents for the adsorption of reactive dyes, the adsorption capacities of the freshly formed hydroxide precipitates used in this work has a relatively high uptake capacity of reactive dyes.(4) The 100 mg/L initiative solution of reactive brilliant red K-2BP has characteristic peaks in the wavelength range from 300 to 650 nm. After treatment all characteristic peaks disappeared, meaning that the dye in wastewater was transferred from the solution onto the precipitates by adsorption and no colored by-product was formed in the reaction. The precipitates with adsorbed dye were acidified to neutral pH with HCl solution to form the desorption solution. The spectrum of the desorption solution was the same as the original dye water, suggesting that the molecular structure of the dye was not changed in the adsorption process.(5) The FT-IR showed that reactive brilliant red K-2BP, reactive light yellow K-6G, acid orangeâ…¡and direct yellow R were adsorbed by freshly formed hydroxides.
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