DEVELOPMENT OF SENSORS FOR HEAVY METALS AND THEIR REMOVAL BY SLAG - A BLAST FURNACE WASTE MATERIAL

Abstract

Increasing deterioration in the quality of surface water due to toxic pollutants is a serious problem. Among the various organic and inorganic pollutants the discharge of heavy metal bearing waste is a subject of great concern. These toxic metals are introduced to the waterbodies through wastewaters from various industries like fertilizer, metal plating, battery, distillary, cement, mining, pigment etc. Therefore, a systematic study of the removal of heavy metals from wastewater is of considerable significance from an environmental point of view. A number of technologies have been developed for the removal of heavy metal ions from wastewater. Out of these, adsorption is an important means for controlling the extent of pollution. Although, activated carbon is a known adsorbent and is quite widely used for the removal of heavy metals, the high cost of this product and the problems associated with the regeneration of spent columns impose limitatioi s on its large scale use. As such it becomes imperative to look for new economically feasible subsiitutes, and a good deal of efforts are being made in this direction. Some important national industries generate solid waste material, which do not find any use as such and cause disposal problems. The investigations, under consideration, were initiated to achieve the twin objectives of controlling the eiiv.ronmental pollution as well as putting to reuse the solid waste material. In India, steel plants produce granular blast furnace slag as a by-product in the iron ore with caustic lime refining process and this material causes a disposal problem. This waste was first washed with deionised water and then dried at 200°C. The iieated product was cooled and activated in air at 600°C for one hour. The method of converting the waste material into suitable adsorbent was optimized by observing the surface properties of the finished product. Activated slag, thus obtained ha: been characterized by elemental analysis,I.R., X-Ray diffraction and scanning electron microscope slud.es. Batch studies on the uptake of heavy metals like copper, zinc, cadmium and nickel (i) on the developed adsorbent material under varying physical conditions have been studied. The removal is fast at lower concentrations while at higher concentrations, the uptake is comparatively slow. The effect of temperature, pH, adsorbate and adsorbent concentration and competitive influence of other metals and surfactant has also been observed, to have an idea of the selective behaviour of the adsorbent material. The optimum pH varies for various metal ions and it is found that the removal is maximum at a pH which precedes hydrolysis. The uptake of various metal ions goes down significantly in presence of other metals as well as by the addition of cationic surfactant. Adsorption isotherms show positive adsorption for each metal ion and isotherms are regular and concave to equilibrium concentration axes. The equilibrium data could be described well by the Langmuir and Freundlich isotherm equations. Adsorption isotherms are used to obtain the thermodynamic parameters of the process. The negative Gibbs free energy (AG) values indicate the spontaneous nature of the process. The positive AH values for copper, zinc and nickel indicate the process to be endothermic while the same is negative for cadmium thereby indicating the exothermic process. The positive entropy of adsorption (AS) also supports the easibility of the process. Kinetic studies were undertaken to determine the rate of uptake of copper, zinc, cadmium and nickel on slag to have a quantitative understanding of ;ome mechanistic aspects. These show an initial rapid uptake followed by a slow approach to equilibrium. It was found that the Bt versus time plots for metal ions pass through the origin at higher concentrations while at lower concentrations, the curve deviates from linear behaviour, showing a particle diffusion controlled process at higher concentrations and film diffusion process at lower concentrations. Various parameters viz., effective diffusion coefficient (Di), energy of activation (Ea), pre-exponential coefficient^ ) o and entropy of activation (AS*) have been evaluated from rate expressions. Negative AS* values for the process show that the uptake of metal ions do not cause any structural change in the sorbent mate.ial during diffusion. Mass transfer coe. icients anu late nj constants have also been evaluated and the values are found to be consistent with the adsorption data. Some column studies have also been performed. The breakthrough curves are obtained under different conditions and the total time (t) needed for the primary adsorption zone to establish itself, move down the length of the column of slag and out of the bed; the time (tg) involved for the movement of the zone down its own length in the column after it has been established ; the fractional capacity (0, of the slag in adsorption zone at breakpoint to continue to remove solute from solutions; the length of the adsorption zone (5) and the percentage saturation at breakpoint have been evaluated for adsorbent columns for the uptake of metal ions under investigation. Regeneration of the spent adsorbent column has also been studied. In thermal regeneration, some adsorbent is lost during each cycle and recovery of adsorbate is not possible. Chemical regeneration by a suitable solvent (1% HNO) is a definite alternative to thermal regeneration and has been studied for the metal ions under study. It is also essential to have an idea of the amount of various ingredients present in an effluent, before subjecting it to the treatment process. This would be of special importance to small scale entreprenuers who are discharging heavy metal ions in wastewater. For this purpose one needs some quick, handy and cheap device. Ion selective electrodes are quite well known for the purpose. The best ion selective electrode is undoubtedly the pH glass electrode, which has been in general use for over sixty years. This is followed by the fluoride electrode which is by far the most successful ISE for anion. Thereafter a range of cationselective membrane electrodes based on neutral carriers, polymeric resins and inorganic gels have been reported, but very few have been commercialized so far. The development of highly selective membrane sensors based on r.eutral carriers has been a major breakthrough in the field of ion-selective electrodes. Th?se compounds are used as electroactive phase for the fabrication of membrane sensors for the estimation of various species. ii i Among the various neutral carriers, macrocyclic compounds are the matrix of choice and crown ethers are the most sought after substances for the fabrication of ISEs. Crown ethers, in addition to their use as potential extractants for various species, have also been widely exploited as suitable ionophores in ISEs. These compounds generally function on the basis of 'hole size concept' i.e. a strong complexation is expected between crown and metals when both are of matching size. Based on this concept, some plasticized PVC membranes of 12-crown-4 and monoaza-18-crown-6 have been prepared and investigated for the estimation of Zn2+ and Cd2+, respectively. The amounts of various membrane ingredients were optimized first so that the resulting membrane demonstrates best electroanalytical performance. The time and concentration of equilibrating solution was also ascertained, in order, the membrane develops reproducible, noise free and stable potentials. These investigations were performed with both the membranes. The membrane sensor based on 12-crown-4 can estimate Zn + in the concentration range 1.41xl0"5 M to 1.00x10"' M between pH 4.0 to 7.0 with a slope of 20.5 mV per decade of Zn2+ concentration. The response time of the sensor is < 10s and the standard deviation of 20 identical measurements being ± 1.0 mV. A membrane can be used continuously for a period of two months. The electrode can also be used in partially non-aqueous solution and it is found that solutions having a maximum of 25% (v/v) methanol or ethanol do not disturb the functioning of the proposed sensor. The electrode has successfully been used as an indicator in the potentiometric titration involving r-, 2+ • Zn ions. Monoaza-18-crown-6 is found to be a suitable material, in a PVC based membrane, for the estimation of Cd2+. It can estimate Cd2+ in the concentration range 1.00x10s Mto 1.00x10" M. The slope of the plot is Nernstian with a value of 29.0 mV per decade of Cd concentration. Stable potentials are recorded in 10s and are reproducible within an error of ± 1.2 mV. A membrane can be used for more than two months between the pH range 5.0 to 7.7. Selectivity coefficients for various interfering ions were also calculated iv, by the fixed interference method. This electrode can also be used in a solvent having 25% (v/v) non-aqueous content. In addition, the sensor can also be employed for the potentiometric titration of Cd2+ ions.