STUDY ON REMOVAL OF CADMIUM FROM WATER ENVIRONMENT BY ADSORPTION ON GAC, BAC AND BIOFILTER
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Cadmium is introduced in to bodies of water from smelting, metal plating, Cadmium-Nickel batteries, phosphate fertilizer, mining, pigments, stabilizers, alloy industries and sewage sludge. The harmful effects of Cadmium include a number of acute and chronic disorders, such as “itai-itai” disease, renal damage, emphysema, hypertension, and testicular atrophy (1).
The drinking water guideline value recommended by World Health Organization (WHO), is 0.005 mg Cd/L. Low concentration (less than 5 mg/L) of Cadmium is difficult to treat economically using chemical precipitation methodologies. Ion exchange and reverse Osmosis while can guarantee the metal concentration limits required by regulatory standards, have high operation and maintenance costs (2).
MATERIALS AND METHODS
The granular activated carbon used in this study was Darco 12-20 mesh supplied by Aldrich. Carbon was washed with double distilled water and dried in an oven at 120 C for 24 hours. All the Cadmium solutions were prepared using Cd(NO3)2.4H2O and the solution pH was adjusted with HNO3 and NaOH 0.01N. Experimental data for the adsorption isotherms were obtained as follows. A predetermined mass of plain GAC and Biofilm/GAC were contacted with a fixed volume of a Cadmium solution of known initial concentration. The Cadmium solution remained in contact with adsorbent untill equilibrium was reached. Batch sorption studies were performed at an ionic strengh of 0.01 (added as NaCl) at different temperature
(5C , 15C , 25C) and at different pH (5 , 7 and 8.5 ). The contact time were selected on the basis of preliminary experiments that demonstrated that the equlibrium were stablished in 4 hours for GAC and Biofilm and 1.5 hours for Biofilm/GAC
For isotherm studies , a series of 250 mL Erlenmeyer flask were employed
RESULTS AND DISCUSSION
Calculated values of correlation coefficients (R2) at different pH value are given in table 1. According to Langmuir model, reasonable straight –line correlations (R2) were achieved for Cd(II) adsorption by GAC and Biofilm , because R2 for Langmuir isotherm were greater than for the Freundlich isotherm. For adsorption of Cd(II) by GAC/Biofilm, the correlation coefficients showed that in general the Freundlich model fitted the results better than the Langmuir model.
Granular Activated carbon (GAC) is well known as an excellent adsorber of organic pollutants from contaminated water streams. GAC by itself is not in general, however, an effective adsorbent for heavy metals. Whereas, it has been shown that with a biofilm attached to the GAC surface, the uptake rate and quantity of metal ions extracted from solutions can be significantly increased. As a consequence, by immobilizing bacterial biofilms, metal removal can be combined with the adsorption of other contaminants such as organic residues.
Biosorption has the potential to provide economic metal decontamination of low concentration waste streams, but leaves the problem of metal-laden biosorbent disposal. There are, however, significant industrial and environmental process opportunities from metal impregnated over GAC surfaces, as they can usefully enhance surfacr activity. It is shown that it is possible to distribute metals over GAC by biosorption, through using attached biofilms. If the intention is to remove metals from contaminated streams, then ideally these biofilms should have a structure open enough not to negate the adsorption characteristics of the carbon surface for other contaminants, such as organic residues.