REMOVAL OF CADMIUM AND LEAD FROM EFFLUENTS BY ADSORPTION IN A MOVING MEDIA REACTOR

Abstract

The present investigation represents new experimental data related to the performance of a given moving media reactor for the removal of cadmium-, and lead-ions from their respective influents by the Dalbergia Sisso sawdust particles, a forest residue locally available at low cost. The reactor consists of the reactor column of 152 mm diameter, and 2100 mm height, the sawdust feeding conveyor with electronic speed regulator, the rotameter and other accessories. The Dalbergia Sisso sawdust was selected out of the many other indigenously available forest residues from the sawmills after ascertaining their metal removal capacities in batch reactors. The Ficus benghaensis(Bud) was found to be the best and the Dalbergia Sisso ranked after Ficus benghaensls (Bud). But the Ficus benghaensis was not available in adequate quantities for the experimental runs. Therefore the Dalbergia Sisso sawdust was considered the best choice. It is important to underline that the sawdust offers advantages over the commercially used adsorbents namely hydrous oxides of aluminium, and iron, activated carbon, etc. which, as a matter of fact, are costly and need regeneration.In fact,the cost of these adsorbents and their regeneration cost add substantilly to the operating costs of the reactors. For the case of the sawdust adsorbent, it is important to mention that it is a low cost forest residue which is locally available. Due to this it does not necessitate regeneration. The present investigation has succeeded in obtaining the adsorptive response of the sawdust particles for cadmium-, and lead-ions to the changes in the sawdust mass input rate, and the influent flow rate, its initial concentration,pH,and temperature.The sawdust mass input rate ranges from 11.0 g/min to 23.5 g/min,the influent flow rate from 0.250 1/min to 0.625 1/min; the initial concentration of the influent from 1 mg/1 to 50 mg/1; pH of the influent from 3 to 11, and the influent temperature from 10°C to 40°C. In the present moving media reactor, 74 to 96 % removal of cadmium-, and lead-ions from their effluents takes place depending upon the sawdust mass input rate, Ws, the influent flow rate,Q,and the initial concentration of the influent, Co. There is an increase of amount of cadmium-, and lead-ions adsorbed on the sawdust particles proportional to the increase in the sawdust mass input rate. It suggests that this range of the sawdust mass input rate is effective for the adsorption of cadmium-, and lead-ions, suggesting that there is no overcrowding of the sawdust particles and thereby the surface areas of the individual particles do not overlap. n The experimental data obtained in the present investigation have been successfully correlated by the dimensional end dimensionless equations as follows: Based on the experimental data for cadmium-, and lead-influents,the generalised correlatios relating removal capacity of sawdust,qe,and the exit concentration, C to the sawdust mass input rate, Ws, the influent flow rate, Q and the initial concentration of cadmium-, and lead-ions, Co, at pH = 7 are : qe = 655.3 Ws-0-908 Q0.896 CoO.968 C = 1.030 WS" 0-7 21 Q0.6 50 Co*-*" The above correlation for removal capacity of sawdust, qe, is a best fit for 160 data points within a maximum deviation of ±10 % whereas for exit concentration,C, about 71 % of the experimental data are within a maximum deviation of ±30 % . It is also established that the removal efficiency of the metal ions by the sawdust declines with increase in pH of the influent. At a value of pH of the influent of 3, the removal efficiency is the best, thereafter it decreases continuously. The following generalised correlations are obtained for cadmium-, and lead-ions influents for a given set of value of the influent flow rate, Q ( =0.625 1/min ) and its initial concentration ( - 1 mg/1 ) : qe = 515.03 WS" 0.908 pH-0.128 in Almost all the experimental data points are correlated within a maximum deviation of ±10 % . C = 0.2789 Ws-o-721 pHO.727 About 65 % of the experimental data lies along the line dsscribing the correlation within a maximum deviation of ±30 % . It is established that the adsorbate removal by the given amount of the sawdust particles decreases marginally with the increase in influent temperature,implying that the adsorption of these heavy metals is slightly exothermic. A generalised dimensionless correlation for the proportionate exit concentration, C/Co in terms of dimensionless numbers, accounting for the column diameter, D, the velocity of the influent in the reactor, U,(= Q /tcD2 ) the sawdust mass input rate, Ws, T ' and the initial concentration of influent, Co , the density, f and viscosity of the influent (/*) diameter of the sawdust particle, dp, and the effective diffusivity of cadmium-, and lead-ions in the pores of the sawdust particles, De, is recommended as follows: C/Co=gD U f/ jj.)0.108 (Ws/W)-0-72i (U dp/ Do)"0.17tl«•**»» This correlation provides a procedure for the calculation of exit concentration of cadmium-, and lead-ions in their respective effluents leaving the reactor within a maximum deviation of ± 40 % . IV