TREATMENT OF HEAVY METAL BEARING WASTE WATER

Abstract

Among various organic and inorganic pollutants, heavy metal ions are very toxic and carcinogenic in nature. The presence of heavy metals in the aquatic environment has been of great concern because of their toxicity at lower concentrations. These are found to be present in the effluent of a number of industries such as metal plating, mining operation, tanneries smelter plants, pulp and paper, textile, caustic, chlorine and agriculture industries etc. Various treatment processes used for the removal of and/or recovery of heavy metal ions include precipitation, oxidation, ultra filtration, electrodialysis, solvent extraction, ion exchange, adsorption etc. To attain maximum removal efficiency at low concentration of heavy metal ions, membrane filtration, ion exchange methods are recommended. But use of these treatment techniques is expensive and sophisticated. For high strength and low volumes of wastewater, heavy metal removal by adsorption technique may be a good proposition. Granular/ Powdered activated carbon is the most widely used adsorbent, as it has a good capacity for the adsorption of carcinogenic metals. However, high cost of activated carbon and 10-15 % loss during the regeneration has deterrents in the utilization of activated carbon in the developing countries. This has led to search for cheaper alternative materials as adsorbents such as lignin, bagasse pith, peat, saw dust, coal fly ash and bagasse fly ash. The present study has been undertaken with the objective to investigate the suitability of bagasse fly ash as a low cost adsorbent for the removal of heavy metals as a replacement of conventional activated carbon. Adsorption studies have been carried out for evaluating the suitability of bagasse fly ash as a low cost adsorbent. The effects of various parameters like pH, adsorbent dose, initial concentration of heavy metals, contact time and temperature on the removal of heavy metals was studied using bagasse fly ash (BFA). The detailed characterization pertaining to physico-chemical, structural and morphological properties of bagasse fly ash have been carried out. The straight-line plot of log (Qe-Q,) versus time (t) for adsorption of heavy metals shows the validity of the lagergern equation. Various steps involved in the adsorbate transport from the solution to the surface of the adsorbent particles have been Ri dealt with by using the Weber-Morris plot, IQ, versus tu for the bagasse fly ash. The rate controlling parameters - kid,' and kid.2 have been determined and it is found that macro pore diffusion rate is much larger than the micro pore diffusion rate. Equilibrium data obtained from the experiments was analyzed for the various isotherms such as, Freundlich, Langmuir, Redlich-Peterson and Tempkin. All these isotherms have been found fitted with the equilibrium data and based on percent relative absolute error it is concluded that the Redlich-Peterson equation shows best fit followed by Langmuir isotherm for bagasse fly ah. Characterization of electroplating effluents containing cadmium and nickel, which include pH, total, solids-suspended and dissolved, hardness, turbidity and initial metal concentration was done. Their removal of heavy metals with bagasse fly ash (BFA) was also studied. Adsorption column studies for cadmium and nickel removal on bagasse fly ash (BFA) was also studied. The effect of bed height and flow velocity on the breakthrough for bagasse fly ash (BFA) was also studied. Bohan Adams model is used for the predication of model specific parameters. Column studies showed that the breakthrough point depends upon the bed height and the flow rate. The removal rate decreases with increase in flow rate. It is also found that the removal rate increases with increase in bed height.