Study On Removal Of An Anionic Dye RAWL Using Hydrotalcite And Its Calcined Product

Hydrotalcite (HT) is a type of layered anionic clay. It has received considerable attention as potential adsorbent in wastewater treatment in recent years due to high anion exchangeability capacity, high specific surface area, simple synthesis procedure, low cost etc. Calcined product (HTC) of hydrotalcite is also a highly effective adsorbent due to its special "memory effect", much greater anion sorption capacity and surface area than its parent material. As adsorbents, both HT and HTC have been used to remove inorganic and organic anionic pollutants in recent years. However, the studies of HT/HTC as adsorbents to treat dye-containing wastewaters are limited.In the present study, Mg-Al HTs with different Mg/Al molar ratio were synthesized using the co-precipitation method, and the calcined products of HTs were obtained by heating HTs at 500 ℃. The kinetic behaviors and sorption characters of a weak acid brilliant blue dye (RAWL) sorption by HTs/HTCs were investigated using batch equilibrium and kinetic methods. The main contents of our experiments included: the effects of contact time, temperature, initial pH, adsorbent dosage,inorganic anions (Cl-, SO42- and CO32-) and anionic surfactant dodecylsulfate (SDS) on RAWL removal by HT/HTC, and the recyclability of HT/HTC after used. Some main conclusions can be drawn as follow:1. Based on X-ray diffraction and infrared spectroscopy analysis, HTs were successfully synthesized using the co-precipitation method and structural reconstruction in an aquatic environment was confirmed after HTs were calcined under 500 ℃.2. Among the three HTCs with different molar ratio of Mg/Al (2, 3 and 4), HTC(Mg/Al=2) has the largest sorption capacity for RAWL, thus being the mosteffective adsorbent for RAWL.3. Both the sorption kinetic rate and sorption capacity of HTC are much higher than those of HT. The maximum sorption capacity of HTC is approximate eleven times larger than that of HT. Sorption data for dye RAWL sorption by HT and HTC are well fitted to the Langmuir isotherm. The kinetics of dye sorption by HTC is in accordance with the intraparticle diffusion model, but the mechanism is not the only rate-controlling step.4. Given the typical concentration range of dye in wastewaters (10-50 mg/L), both HT and HTC are highly efficient adsorbents for anionic dye RAWL. HT may be used to remove anionic dyes of low concentrations, whereas HTC could possibly be used to remove anionic dyes of relatively high concentrations.5. With increasing temperature, the sorption amount of dye obviously decreases. An increase in temperature would not favor sorption removal of anionic dyes by HTC. The effect of initial pH on RAWL sorption by HT/HTC is small. When the initial pH values are between 3.5 and 10.0, HT has high pH buffering capacity, whereas the buffering capacity disappeared when the initial pH is above 10.5. As for HTC, within a wide initial pH range (3.5~10.0), the pH values of equilibrium solutions could be stabilized in a narrow range (10.6-10.9), at pH>11.0, however, an obvious difference between the initial pH and equilibrium pH was not observed. The effect of the adsorbent dosage on dye sorption by HT/HTC is significant. Percentage RAWL absorbed increases almost linearly with increasing dosage of HTC /HT before a complete removal of RAWL. The use efficiency of HTC is clearly higher than that of HT.6. Generally, the effect of competitive inorganic anion (Cl-, SO42- and CO32-) on dye sorption by HTC is slight with the order of effect: Cl-