REMOVAL OF SOME INORGANIC AND ORGANIC TOXIC SUBSTANCES USING FERTILIZER AND BLAST FURNACE WASTE MATERIALS

Abstract

A rapid pace of industrialization, coupled with uncontrolled exploitation of nature, has caused dumping ofindustrial by-products, hazardous chemicals and nuclear waste, deforestation, and the pollution of river basins, lakes and seas in the recent past. Several toxic substances are found in polluted waters, the most dangerous amongst these are phenols, dyes, metal ions, detergents and polynuclear hydrocarbons. Toxic effects of these pollutants are quite well documented. Besides this, some foremost industries like Fertilizer, Steel and Paper & Pulp, are throwing out solid or semi solid wastes for which, to-date, no treatment methodology has been developed. Except for being used as filler or additive in some industry no proper utilization or disposal technique has, so far, been worked out for such materials. The removal of refractory organics from water is a difficult task because of the diverse number of compounds typically present and the extremely low concentration normally encountered. Among the various methods of wastewater treatment adsorption appears to offer the best overall results especially for those which cannot be removed by other techniques and, so far, activated carbon is found to be an universal adsorbent for the process. The drawbacks associated with this adsorbent are also quite well known. As such it becomes imperative to look for other economically feasible substituents. Objectives for undertaking the present investigations are two fold. Developing some suitable methodology for the treatment of waste material (solid) being thrown out by National Fertilizer and Steel Plants and explore the possibility of using the treated material as a low cost adsorbent. Heavy metal ions are toxic pollutants. Some of these are cumulative poisons capable of being assimilated, stored and concentrated by organisms that are exposed to low concentrations of these substances lor long periods or repeatedly. Eventually, the built up of the metal in tissues is sufficient to cause noticeable physiological effects. The incidence of Minimata and Itai-Itai diseases have focussed the attention ofscientific community on the dangers of metal pollution through the food chain. The two waste materials, under consideration, are tried as scavengers of a number of metal ions. Some promising results were obtained in the case of lead and chromium. Phenol, its derivatives and dyes occupy a prominent position on the priority list as organic pollutants. Phenolic compounds besides being a potential health hazard also impart bad smell and taste to water even when present in traces. The removal of dyes (ii) is necessary from aesthetic view point ifthe treated water is to be recycled. Dyes studied are basic red and malachite green while the phenol chosen for study is 2-aminophenol. In India Fertilizer Plants produce a waste, which is generated after liquid fuel combustion and washed away from the scrubbers after ferrous sulphate treatment in a slurry form. This material causes adisposal problem and is currently being used as filler in synthetic rubber plants. Efforts are made to convert the waste slurry into a carbonaceous adsorbent similar to activated carbon. The waste slurry was in the form ofsmall, spherical, black, greasy granules. This was first of all treated with hydrogen peroxide to oxidise the adhering organic material and then heated at 200° Ctill the evolution of black soot stopped. The heated product was cooled and then activated in air. Activation was performed in an ordinary furnace at 450° C for one hour. Besides this, all Steel Plants also produce granular blast furnace slag as a by product when the iron ore is subjected to caustic refining process and this material also causes a disposal problem. This waste was first washed with double distilled water and then dried at 200" C. The heated product was cooled and activated in air at 600" C for one hour. In both cases the procedure of converting the waste material into suitable adsorbent was optimized by observing the surface properties of the finished product. Activated carbon and activated slag, thus obtained, have been characterized by elemental analysis, I.R., X-ray diffraction and scanning electron microscopic studies. The carbonaceous adsorbent may be termed as "L" carbon. The extent of adsorption at equilibrium is dependent on the physical and chemical characteristics of the adsorbate, adsorbent and other experimental conditions. Laboratory scale batch studies have been performed to establish optimum conditions for the removal of 2-aminophenol, dyes, and heavy metal ions. The parameters evaluated include hydronium ion concentration, temperature and adsorbent dose etc. The effect of surfactants and competitive influence of other metals plus some salts has also been studied. Adsorption isotherms of the above mentioned adsorbates on the two adsorbents indicate a faster removal and the extent of uptake is seventy to hundred percent at low adsorbate concentrations. At higher concentrations the removal is comparatively slow and the amount of uptake is twenty five to eighty percent. The adsorption of heavy metal ions on these two adsorbents does not follow any set pattern. The uptake on activated slag is an endothermic process and on activated carbon it is an exothermic process. The optimum pHof uptake varies for various metal ions on the two adsorbents and it is found that the removal is maximum at a pH which precedes hydrolysis. Competitive adsorption from a mixture of metal ions plus some (iii) salts has also been studied. The adsorption of Pb2+ and Cr6+ significantly decreases in presence of other ions. Addition of an anionic surfactant does not affect the uptake of metal ions significantly. It is found that the uptake of dyes and 2-aminophenol on activated carbon and activated slag is an endothermic process and the removal increases with increase in temperature. 2-aminophenol is strongly adsorbed at pH below its dissociation constant value (pK^). The removal ofbasic red and malachite green on activated carbon is higher at low pH while the same on activated slag increases with the increase in hydronium ion concentration. The presence of an anionic surfactant slightly affects the adsorption of dyes and 2-aminophenol on the two adsorbents. The results show a positive adsorption in all adsorption isotherms. These are regular andconcave to equilibrium concentration axis. The removal of metal ions/dyes/ 2-aminophenol occurs in two phases. In the first phase, the uptake of solute is fast while in the second phase, rate of removal becomes quite slow and subsequent removal of solute continues over longer periods of time. The data for the adsorption of heavy metal ions, dyes and 2-aminophenol on the carbonaceous material and activated slag fit well into Freundlich and Langmuir models. The Freundlich constants signify favourable adsorbate-adsorbent systems and a considerable degree of non-linearity between adsorbate concentration in solution and the extent of adsorption. Langmuir parameter Q° (the amount of adsorbate necessary for forming a complete monolayer) is higher than the adsorption maxima observed experimentally for all the systems. This may be attributed to the fact that in batch process a complete utilization of the sorption capacity is never achieved. Further suitability of the process is obtained from theshape of isotherms [235] in terms of a dimensionless constant separation factor (r). Thermodynamic parameters of the process have also been evaluated. The Gibbs free energy (AG) values for all the adsorbate and adsorbent systems are negative thereby indicating the spontaneous nature of the process. The positive AH values indicate the process to be endothermic except for metal ions-activated carbon system where a negative AH indicates the process to be exothermic. The positive entropy of adsorption (AS) also supports the feasibility of the process under investigation. In order to explore some mechanisitic aspects, kinetic studies were undertaken on the rate of uptake of Pb2+; Cr6+ ; basic red; malachite green; and 2-aminophenol on carbon and slag. These indicate an initial rapid uptake followed by a slow approach to equilibrium. The Bt versus time plots for metal ions, dyes and 2-aminophenol on the two adsorbents pass through the origin at higher concentrations while at low concen( iv) trations, the curves deviate from linear behaviour indicating a particle diffusion controlled process at higher concentrations whereas at low concentrations, the film diffusion also affects the uptake of adsorbates on both adsorbents. Adsorption rate is also found to be dependent on the amount and particle size of adsorbent, temperature and initial adsorbate concentration. The values of effective diffusion coefficient (D.) are in the same order in which these substances are adsorbed on different adsorbents. The energy of activation (Ea) has also been evaluated from the linear behaviour of log D. vs 1/T plots and Do values have been used to calculate AS", the entropy of activation. Anegative AS* for all the adsorbates indicate that these do not cause any structural change in the sorbent material during diffusion. Mass transfer coefficients and rate constants for all the systems have also been evaluated and the values are quite consistent to the sorption data. Column studies were also undertaken to obtain the relevant parameters necessary for designing a fixed bed adsorber. The totaltime (tx) involvedfor the primary zone to establish itself, move down the length of the column of carbon and slag and out of the bed, the time (ts) required for the movement of the zone down its own length in the column after it has been established; the fractional capacity (f) for both the adsorbents in adsorption zone at break- point to continue to remove solute from solution; the length of the adsorption zone (5); and the percentage saturation at break-point have been evaluated for the columns of different adsorbents and the said adsorbates. The bed-depth-service-time (BDST) model has also been applied only in the case of leadslag system. Elution studies have been carried out for the desorption of Cr6+ by 3M NH4 OH; Pb2+ by3MNaNO,; malachite green and 2-aminophenol by acetone from the columns of carbonaceous material. The desorption ofPb24 , and malachite green/2-aminophenol from the columns of slag has also been observed with the help of 1% HN03 and acetone, respectively. After desorption, the columns were again loaded and the cycles were repeated for four to six times. A significant loss in the sorption capacity of the column is recorded after the first cycle. However, a treatment with 1MHN03 solution at a flow rate of 0.5 ml min"1 , restores, to some extent, the column capacity of the carbonaceous material. In the end, the investigations were extended to explore the efficacy of the developed material for the treatment of actual waste systems. It is found that the two adsorbents are able to reduce the COD (chemical oxygen demand) ofreclaimed rubber factory effluents by 50 to 65 percent.