HEAVY METALS REMOVAL FROM AQUEOUS SOLUTION USING LOW-COST ADSORBENTS

Abstract

Release of heavy metals into the environment from industrial practices is a matter ofglobal concern. Metal ions like Cadmium (Cd(II)), Nickel (Ni(II)) and Zinc (Zn(II)) are toxic, and are frequently encountered together in industrial waste waters. The Ministry of Environment and Forests (MOEF), Government of India has set Minimal National Standards (MINAS) of 1.0, 3.0, 5.0 mg/1, respectively, for Cd(II), Ni(II) and Zn(II) for safe discharge ofthe industrial effluents containing these metal ions into surface waters. Adsorption as a wastewater treatment process has attracted considerable interest in recent years. The activated carbon (AC) is regarded as the most effective adsorbent for controlling the organic load. However, its high cost and about 10-15 %loss during regeneration inhibits its usage. Therefore, the search for unconventional and cheaper adsorbents like bagasse fly ash (BFA), rice husk ash (RHA), silica, peat, lignite, bagasse pith, wood, saw dust, etc. for the removal of various pollutants from industrial effluents have attracted the attention of several investigators. The RHA and the BFA are available in plenty as they are collected from the particulate collection devices attached to the flue gas line ofthe combustory boilers/furnaces using rice husk or sugar cane bagasse as a fuel. BFA has been found to be a very potent and very effective adsorbent in the removal of COD and colour from paper mill effluents [Srivastava et al., 2005], dyes [Mall et al., 2005a,b; 2006], phenol [Srivastava et al., 2006a] and metal ions [Srivastava et al., 2006b,d,e,f] from aqueous solutions. RHA has also been used as an adsorbent for the removal of metal ions from aqueous solution [Srivastava et al., 2006c]. Since industrial effluents can contain several metals, it is necessary to study the simultaneous sorption of two or more metal ions as also to quantify the interference of one metal ion with the sorption of the other metal ion. Thus, the studies on the adsorption of heavy metal ions from the binary and ternary metal systems in aqueous solution are of prime importance. Multi-component (metal ions) adsorption from aqueous solutions on to low-cost adsorbents such as BFA and RHA has not been reported in the literature. The literature concerning the adsorption multi-component metal ions by AC is also scarce. The Taguchi's method for the designing and conducting optimal number ofexperiment at runs for the multi-component adsorption ofheavy metals from aqueous solutions has not been reported in the literature. Aims and Objectives : The present study aims to investigate the suitability of BFA and RHA as the low-cost adsorbents as the substitutes of the conventional activated carbons for the removal of Cd(II), Ni(II) and Zn(II) metal ions from aqueous solution. Commercial activated carbon (ACC) has been used as the standard adsorbent for the comparative assessment of the adsorptive capacities of BFA and RHA. BFA and RHA have been used as procured from the nearby industrial units, after sieving. The physico-chemical characterization of the adsorbents has been done using standard methods e.g. sieving, scanning electron 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 ofthe adsorbents reflected the presence of various types of oxides in all the adsorbents along-with some characteristic components. The BFA and RHA showed a mesoporous nature. FTIR spectra of the adsorbents indicated the presence of 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. 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 metal loaded in adsorbents. Thermo-gravimetric analysis exhibited the thermal stability of the adsorbents upto 300 °C temperature. The initial pH (pH0) ofthe metal ion solution strongly affects the chemistry of both the metal ions and the adsorbents in an aqueous solution. Speciation diagram showed that Cd(II), Ni(II) and Zn(II) are the only ions present in the metal solution at pH < 6. The precipitation of metal ions takes place at pH > 7.5. The point of zero charge (pHPZC) of the BFA, RHA and AC was found to be 9.0, 8.3 and 8.5, respectively. The pH0~6.Q is found to be the optimum for the adsorptive removal of Cd(II), Ni(II) and Zn(II) ions individually by the adsorbents. The system pH, however, increases during the initial period of about 20 min of the sorption process and, thereafter, remains constant. This increase in pH during the sorption process was due to the simultaneous and, perhaps, competitive adsorption of the metal cations and H+ ions onto the adsorbents. The effect ofthe adsorbent dosage (m) on the uptake ofCd(II), Ni(II) and Zn(II) ions by the adsorbents was also studied. Optimum BFA, RHA and ACC dosages were found to be 10, 10 and 20 g/1, for C0=100 mg/1 ofCd(II), Ni(II) and Zn(II), respectively. Aquasi-equilibrium adsorption was observed at a contact time of 5h. The rate ofmetal removal is found to be very rapid during the initial 15 minutes, and thereafter, the rate of metal ions removal decreases. During the initial stage of sorption, a large number ofvacant 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 metal ions are adsorbed into the mesopores that get almost saturated with metal ions during the initial stage ofadsorption. Thereafter, the metal ions have to traverse farther and deeper into the pores encountering much larger resistance. This results in the slowing down ofthe adsorption during the later period of adsorption. IV Various kinetic models, viz. pseudo-first-order, pseudo-second-order, and intraparticle diffusion models have been used to study the kinetics of adsorption of Cd(II), Ni(II) and Zn(II) 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. The effective diffusion coefficient for the metal ions into the adsorbents is found to be of the order of 10'13 m2/s. An increase in temperature induces a positive effect on the sorption process. Equilibrium adsorption data were analyzed by applying different equilibrium isotherm models using linear regression technique. Freundlich and Redlich-Peterson isotherms generally well represented the equilibrium sorption of metal ions onto BFA, whereas, Temkin and Langmuir isotherms best represented the equilibrium adsorption data of RHA. The heat of adsorption (AH0) and change in entropy (AS0) for metal adsorption onto BFA and RHA were found to be in the range of 22-53 kJ/mol and 125-253 MJ/mol K, respectively. The high negative value of change in Gibbs free energy (AG0) indicated the feasibility and spontaneity of adsorption of metal ions onto the adsorbents. The isosteric heat of adsorption calculated from the equilibrium adsorption data using the Clausius-Clapeyron equation was quantitatively correlated with the fractional loading of metal ions onto the adsorbents. The results showed that the BFA and RHA possessed heterogeneous surface with sorption sites of different activities. The adsorption capacity of Zn(II) was found to be higher than that for either Ni(II) or Cd(II) in the binary- and ternary-metal solutions. This is in agreement with the single-component adsorption data for all the metal ions. The equilibrium metal removal decreased with an increase in the concentration of the other metal ions. The adsorption of Cd(II), Ni(II) and Zn(II) ions from the binary and ternary solutions onto adsorbents was generally found to be antagonistic in nature. Equilibrium isotherms for the binary and ternary adsorption of metal ions have been analyzed by using non-modified Langmuir, modified Langmuir, extended-Langmuir, extended-Freundlich, Sheindorf- Rebuhn-Sheintuch (SRS), using non-modified R-P and modified R-P models. The competitive extended-Freundlich and SRS models fitted the experimental equilibrium data satisfactorily and adequately. Batch experiments for the simultaneous removal of Cd(II), Ni(II) and Zn(II) metal ions from the aqueous solution by BFA, RHA and ACC were optimized by using Taguchi orthogonal array (OA) experimental design (DOE) methodology. This approach facilitated the understanding of the interactive effects of a large number of variables spanned by factors and their settings with a small number of experiments leading to considerable saving in time and cost for the process optimization. The metal ion concentration, temperature, pH0, adsorbent dose and contact time at three levels with an OA layout of L27 (313) were selected for the proposed experimental design. In all, 27 sets of experiments were conducted for the adsorption of metal ions. The experiments were analyzed with the bigger is better as the quality character. To understand the stability of the spent adsorbents and the leaching of the sorbed metal ions, desorption studies were conducted. Several solvents (acids, alkalies and water) have been used as the eluting agents. Mineral acids, viz. HC1, H2S04, and HN03 showed similar recovery efficiencies (= 65 %) for BFA. Deionized water showed very poor elution characteristics. For all the eluents, the desorption of Cd(II) is greater than that for Ni(II) and Zn(II) from all the adsorbents. The BFA and RHA are available in abundance as a waste from various industries at almost no cost, except for the handling charges for the collection and transportation of the material. Therefore, the cost of these adsorbents is insignificant than that of the activated carbon. Hence, the exhausted low cost adsorbents along-with the sorbed metal ions can be separated from the solution (by filtration), dried and used as such or as fire briquettes to recover its energy value. The resulting bottom ash blended with cementitious 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 metal ions.