Transformation of alunite residuals into layered double hydroxides and oxides for adsorption of acid red G dye

For efficient utilization of alum sludge residuals, transformation of them into magnesium-aluminum layered double hydroxides (Mg-Al-LDHs) by the coprecipitation method was studied. The adsorption of Acid Red G (ARG) in aqueous solution onto the calcined LDHs was then investigated. The structure of synthesized materials was characterized by using X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), thermal analysis (TG-DTA) and scanning electron microscopy (SEM). The results showed that high crystalline hexagonal LDHs were synthesized from alum sludge. The particle size of LDHs synthesized from the direct use of alum sludge was much larger than that of LDHs synthesized from the use of alum sludge pretreated by acid or alkali. There existed the silicon in the LDHs synthesized from alum sludge pretreated by alkali. The LDO obtained by calcination of LDHs synthesized from alum sludge pretreated by acid was purer than that of LDO synthesized from alum sludge pretreated by alkali. The results of adsorption experiments indicated that the removal percentage of ARG with an adsorbent dose of 0.05 g was about 93.1% at pH = 10 in room temperature for 150 min. The regenerated LDO for reuse was feasible within two cycles and ARG might be mainly absorbed on the surface of LDO.