REMOVAL OF ACRYLONITRILE AND ACRYLIC ACID FROM AQUEOUS SOLUTION USING ADSORPTION

Abstract

Release of pollutants into the environment from industrial practices is a matter of global concern. Acrylonitrile (AN) and acrylic acid (AA) are emitted from industrial plants in the form of vapors in the air atmosphere and in aqueous effluents. They are widely used in several industries like petro-chemicals, paints, chemical fibers, adhesives, paper, oil additives, detergents, etc. AA and AN cause serious damage to the environment when they released into waterbodies due to their high toxicity to aquatic organisms. EPA recommends that total concentration of AA and AN should be limited to 0.058 ppb in the lakes and streams to prevent possible health problem from drinking water. Various treatment processes used for the removal and/or recovery of AN and AA include adsorption, microbial degradation, wet air oxidation, etc. For high strength and low volumes of wastewater, AN and AA removal by adsorption technique may be a good proposition. Granular/powdered activated carbon (GAC/PAC) is the most widely used adsorbent, as it has a good capacity for the adsorption of various adsorbates. However, high cost of activated carbon and 10-15 % loss during its regeneration poses an impediment in the utilization of (AC) in the developing countries. This has led to a search for cheaper alternative materials as adsorbents such as lignin, bagasse pith, peat, saw dust, coal fly ash, rice husk ash and bagasse fly ash (BFA), etc. The present study aims to investigate the suitability of PAC, GAC and BFA as the adsorbents for the removal of AN and AA from aqueous solution. BFA has been used as procured from the nearby industrial unit, after sieving. Commercial grade PAC and GAC have been used as procured. The physico-chemical characterization of the adsorbents has been carried out using standard methods e.g. sieving, scanning electron 11 Abstract microscopy, X-ray diffraction, FTIR spectroscopy, etc. Pore size distribution and pore area/volume have been determined by using a surface/pore area analyzer. The X-ray spectra of the adsorbents reflected the presence of various types of oxides in all the adsorbents alongwith some characteristic components. Various types of functional groups e.g. free and hydrogen bonded OH group, the silanol groups (Si-OH), CO group stretching from aldehydes and ketones on the surface of adsorbents have been found. The presence of polar groups on the surface gives considerable cation exchange capacity to the adsorbents as confirmed by the dissipation of some of these groups in the AN and AA loaded adsorbents. Thermo-gravimetric analysis exhibited the thermal stability of the adsorbents up to 300 °C temperature. The effect of the adsorbent dosage (w) on the uptake of AN and AA by the adsorbents was also studied. Optimum PAC and GAC dosage was found to be 20 g/1 for C0=100 mg/1 of acrylonitrile (AN) or acrylic acid (AA); whereas optimum BFAdosage was 4 g/1 for AN removal using univariate procedure. The study of the effect of contact time on the removal of AN and AA showed that the equilibrium sorption time is very low, ~ 5 min and 60 min onto PAC, whereas, it is -150 min for the GAC. However, the equilibrium sorption time is very low, ~ 5 min for AN adsorption onto BFA. The rate of AN or AA removal is found to be very rapid during the initial 15 min, and, thereafter, the rate of AN or AA removal decreases. During the initial stage of sorption, a large number of vacant surface sites are available for adsorption. After a lapse of some time, the solute molecules found it difficult to attach on to the remaining vacant surface sites due to the repulsive forces between the solute molecules on the solid surface and the bulk liquid phase. Besides, the AN or AA are adsorbed into the mesopores that get almost saturated with adsorbates during the initial stage of adsorption. Thereafter, the AN or AA have to traverse farther and deeper into the pores encountering much larger resistance. This results in the slowing down of the adsorption during the later period of adsorption. in Abstract Various kinetic models, viz. pseudo-first-order, pseudo-second-order, and intra- particle diffusion models have been used to study the kinetics of adsorption of AN or AA onto adsorbents. The pseudo-second-order kinetics represented the adsorption data well. The adsorption processes could be described satisfactorily by a two-stage diffusion model. An increase in temperature induces a positive effect on the sorption process. Equilibrium adsorption data were analyzed by applying different equilibrium isotherm models using non-linear regression technique. Redlich-Peterson and Freundlich isotherms generally well represented the equilibrium sorption of AN or AA onto PAC, GAC and BFA. The heat of adsorption (AH0) and change in entropy (AS0) for AN or AA adsorption onto PAC, GAC and BFA were found to be negative. The high negative value of change in Gibbs free energy (AG0) indicated the feasibility and spontaneity of adsorption of AN or AA onto the adsorbents. The results showed that the PAC, GAC and BFA possessed heterogeneous surface with sorption sites of different activities. Batch experiments for the removal of AN or AA from the aqueous solution by PAC, GAC and BFA were optimized by using Box-Behnken design methodology. Such factors as temperature, adsorbent dose and contact time could be optimized with the bigger-is-better as quality character with 17 sets of experiments only. The exhausted lowcost adsorbents alongwith the sorbed AN or AA can be separated from the solution (by filtration), dried and used as such or as fire briquettes to recover their energy value. The resulting bottom ash blended with cementatious mixture can be used for making building blocks or it may be used to make fire bricks, thus disposing of toxic compounds through chemical fixation. This approach of adsorbent disposal entails energy recovery and the safe disposal of the adsorbed toxic adsorbates.