Studies on the utilization of red mud for environmental application

Abstract

Red mud is a waste by product generated from alumina processing industries which creates a number of environmental problems. Reports of the utilization of the red mud for the treatment of natural, industrial and domestic waste water are available in literature. Clean water and a clean environment are the today’s requirements. The use of one waste for the removal of another waste is a challenging job. The present dissertation is an embodiment of the investigations for developing simple inexpensive adsorbents by the modification of red mud for waste water purification.
In laboratory scale, neutralization of highly alkaline red mud is carried out by acid and
CO2 gas treatment and modified by calcination. In another method, red mud adsorbent is prepared by surface modification with the impregnation of an anionic surfactant SDS. After modification, the red mud is called as Activated red mud (ARM) which is used as an adsorbent. The adsorbents (ARM) are used for the removal of Pb(II), Cd(II) ions and organic safranin-O dye separately from aqueous solutions in batch mode. The adsorbents, before and after treatment are characterized by XRD, SEM, EDX, TGA-DSC, FTIR, UV-Visible, BET surface area .The AAS analytical techniques is used to measure the residual metal ion concentration in treated water.
The activated red mud used for the removal of hazardous Pb(II) possess the rounded shape aggregate particles with surface area 67.10 m2/g and particle size in the range of 0.1–150 µm . The maximum adsorption capacity as calculated from Langmuir isotherm model is found to be 6.0273 mg/g at pH 4. The pseudo-second-order kinetics describes the adsorption process. The adsorption process is described by ion exchange mechanism.
The maximum adsorption capacities of Cd(II) on activated red mud (ARM) are found to be 12.046 and 12.548 mg/g at temperature 293 and 303 K, respectively. The endothermic, spontaneous and feasible nature of adsorption is known from the thermodynamic parameters. The external mass transfer coefficient (kf ) is found to be 0.084 ×10 -3, 0.012 ×10 -3 at temperature 293 K and 9.9 ×10 -3, 11.5 ×10 -3 at 303 K which are evaluated by the McKay et al: and Weber–Mathews equation respectively. The desorption efficiency of Cd(II) is found
to be 91.29% with 0.2 mol/L HCl.
The BET surface area of the SDS/RM is found to be 67.10 m2/g. The maximum adsorption capacity of modified red mud (SDS/RM) is found to be 8.94 mg/g at temperature
308 K and pH 4 obtained from Langmuir isotherm model. The external mass transfer
coefficient (kf ) value as obtained from McKay et al: equation is found to be 3.49 ×10¬¬-4 , 4.61 ×10 -4 at temperature 308 K and 2.13 ×10 -4, 3.11 ×10 -4 at 328 K obtained from Weber–Mathews equation .These values indicate the faster adsorption of safranin-O on the surface of ARM at lower temperature.
The response surface methodology (RSM) is applied to examine the efficiency of the removal of safranin-O dye from aqueous solution by the activated red mud neutralized by CO2 gas. A 24 full factorial central composite design (CCD) method is used to evaluate the effects of adsorption parameters. The operating parameters for maximum uptake capacity of 9.768 mg/g is; adsorbent dose (0.62 g), temperature (29.06 ℃), pH (8.3) and initial safranin-O concentration (37.3 mg/L). At this optimum condition, the adsorption of safranin-O from aqueous solution is found to be 94.5%.