STUDIES ON THE UTILIZATION OF SOME INDUSTRIAL WASTES AS ADSORBENTS

Abstract

Water is one of the basic necessities required for the sustenance and continuation of the life. It is therefore, important that supply of good quality water is available for various activities. However, this is becoming increasingly difficult in view of large scale pollution caused by industrial, agricultural and domestic activities. These activities generate wastewater which contains both inorganic and organic pollutants. Some of the common pollutants are phenols, dyes, detergents, polynuclear hydrocarbons, insecticides, pesticides and heavy metals. The nature of pollutants in wastewater depends on the source of generation and varies from place to place. These pollutants are often toxic and cause adverse effects on human and animal life if present above certain concentration levels. In order to avoid pollution of natural water bodies, it is essential to treat wastewater for the removal of pollutants before being discharged into them. A number of methods such as coagulation, ozonation, membrane process, adsorption, dialysis, foam flotation, osmosis, photocatalytic degradation, biological methods have been generally used for the removal of toxic materials from wastewater. The type of the process to be employed may depend on nature of pollutant. However, adsorption process is considered the best as it can remove in general, both inorganic and organic pollutants and the operation of the process is convenient. Activated carbon has been found to be a very good adsorbent for effluent treatment and is commonly used for the removal of various pollutants. However, its widespread use in wastewater treatment is sometimes restricted due to its higher cost. As such, for quite sometime, efforts are being made to prepare cheaper adsorbents, generally from agricultural and industrial wastes which are available almost free of cost. The various materials investigated so far for this purpose include plum kernels, corncob wastes, coir pith, almond husk, banana and orange peels, palm seed coat, fly ash, red mud, slag, bagasse fly ash etc. These (i) materials have not shown promising adsorption characteristics in comparison to activated carbon. Further, comparative data on adsorption efficiency of different adsorbents is not available. It is, therefore, still necessary to develop better low cost alternatives to activated carbon which may exhibit good adsorption efficiency. We have, therefore, investigated industrial wastes of both inorganic and organic nature. The wastes studied are blast furnace (BF) sludge, dust and slag from steel plant and carbon slurry from National Fertilizer Plant. The wastes were procured, processed and activated. These were then studied as adsorbents. All the four adsorbents prepared were analyzed and it was found that BF slag and dust are basically inorganic in nature i.e. they contain mainly inorganic constituents like silica (32.7 and 15.8%), calcium oxide (31.7 and 4.7%). magnesium oxide (6.8 and 4.2%) and R203 (22.8 and 44.9%) whereas BF sludge contains besides inorganic constituents, appreciable quantity of carbon (35.0%). On the other hand, carbonaceous adsorbent prepared from carbon slurry contains negligible quantity of inorganic constituents and the main constituent is carbon (89.8%). In order to assess the adsorption characteristics of these four adsorbents, their surface area was determined by N2-gas adsorption and found to be 380. 28. 13 and 4 m2g"' for carbonaceous adsorbent, BF sludge, BF dust and BF slag, respectively. The surface area increases as carbon content increases from BF slag to carbonaceous adsorbent indicating that the porosity of carbon is responsible for higher surface area. To confirm it, SEM photographs of the four adsorbents were taken which clearly showed that BF sludge, dust and slag possess very poor porosity whereas carbonaceous adsorbent exhibits significant porous structure giving rise to comparatively high surface area. The adsorption characteristics of the adsorbents were further evaluated by determining their methylene blue and iodine numbers. These values were found to be appreciable for carbonaceous adsorbent as compared to other three adsorbents. These values thus show that carbonaceous adsorbent is much better for the adsorption of organic substances apparently due to its larger surface area as compared to other three adsorbents (ii) which have poor surface area and consequently show very low values of methylene blue and iodine numbers. The adsorption characteristics, in terms of methylene blue and iodine numbers, were also compared with standard sample of activated carbon (E. Merck) and it was found that the prepared carbonaceous adsorbent is about half as efficient as standard activated carbon. Thus on the basis of surface area, methylene blue number and iodine number, carbonaceous adsorbent is likely to be a good material for the removal of toxic organic substances as compared to other three adsorbents prepared. Among the various organic pollutants present in wastewaters, phenols and dyes are the important ones whose removal is desired. The adsorption studies of phenols and dyes on the prepared adsorbents were, therefore, taken up with a view to remove them. Two classes of phenols viz. methyl-phenols and halogenatedphenols are studied. The four methyl-phenols chosen for this purpose are 2- methylphenol, 4-methylphenol, 2,4-dimethylphenol and 2,4,6-trimethylphenol and their adsorption was studied by batch method. The adsorption isotherms obtained revealed that the maximum adsorption of methyl-phenols on carbonaceous adsorbent, BF sludge and dust was found to be 37.3. 12.9 and 7.6 mgg"1, respectively for 2-methylphenol ; 40.5, 16.2 and 12.9 mg g"1 , respectively for 4- methylphenol ; 65.9. 21.9 and 18.3 mgg"1 , respectively for 2,4-dimethylphenol and 88.5. 30.6 and 25.2 mgg'1 . respectively for 2,4,6-trimethylphenol. Negligible adsorption was observed on BF slag. Thus, the extent of adsorption of phenols on different adsorbents is found to be in the order; carbonaceous adsorbent > BF sludge > BF dust > BF slag. This order is parallel to the porosity and surface area of the adsorbents. Carbonaceous adsorbent having the maximum surface area adsorbs maximum whereas slag with minimum surface area adsorbs the least. Of all the four adsorbents studied, only carbonaceous adsorbent adsorbs phenols to a sufficient extent whereas the other three are poorer materials for the removal of phenols. The carbonaceous adsorbent is, therefore, a potential adsorbent for the removal of phenols. As such, all further detailed studies were undertaken only (iii) with carbonaceous adsorbent. In order to have insight into the adsorption process, the adsorption of phenols was studied as a function of contact time, concentration, particle size and temperature. The effect of contact time on adsorption shows that it took about 8 h for equilibrium adsorption to be attained. The analysis of adsorption vs. contact time data further revealed that 50% of ultimate adsorption, in case of all the phenols, is achieved within 2 h. It was also seen that time required for obtaining a definite fraction of maximum equilibrium adsorption is independent of the initial concentration indicating the process to be first order. The adsorbent particle size effect revealed that, of the three particle sizes viz. 100-150. 150-200 and 200-250 BSS mesh studied, the adsorption capacity was found to be slightly higher for smaller particles. The adsorption of phenols on carbonaceous adsorbent was also investigated as a function of temperature and it was observed that the adsorption increases with rise in temperature indicating the phenols adsorption is endothermic. Thermodynamic parameters were calculated. The adsorption isotherms were also analysed and were found to conform to Langmuir model with good correlation coefficients varying from 0.995 to 0.999. From Langmuir plots, the monolayer adsorption capacity was calculated and the values for all phenols were found nearly equal to the maximum adsorption observed. The Lagergrems equation was applied to the kinetic data of adsorption and found applicable. This shows that the adsorption of methyl-phenols on carbonaceous adsorbent is a first order process. The first order rate constants were determined to be 4.54x10"', 4.66x10"'. 4.96x10"' and 5.09x10"' h"1 for the adsorption of 2-methylphenol, 4-methylphenol, 2,4-dimethylphenol and 2.4,6- trimethylphenol, respectively. Finally, the applicability of Bangham's equation to the adsorption data indicated that adsorption of phenols under consideration is pore diffusion controlled. The adsorption behaviour of another class of phenols viz. halogenatedphenols was also studied. The halogenated-phenols chosen for this purpose are 2- bromophenol, 4-bromophenol, 2,4-dibromophenol, 2-fluorophenol and 2- (iv) iodophenol. Their adsorption was investigated on all the prepared adsorbents in the same way as that of methyl-phenols. In this case too, carbonaceous adsorbent was found to be the potential adsorbent and other three adsorbents to be poorer materials. The maximum adsorption on carbonaceous adsorbent was found to be 35.3. 40.7. 170.4, 190.2 and 235.5 mg g"' for 2-fluorophenol, 4-bromophenol, 2- bromophenol. 2,4-dibromophenol and 2-iodophenol, respectively. The detailed study of adsorption of halogenated-phenols on carbonaceous adsorbent revealed that the adsorption (i) conforms to Langmuir equation (ii) is endothermic in nature and (iii) is a first order process and pore diffusion controlled. The results of phenols adsorption have shown that carbonaceous adsorbent possesses appreciable adsorption capacity for them. As the phenols are organic compounds and the adsorption appears due to substantial surface area of the adsorbent, it may be inferred that this adsorbent would show good adsorption capacity for organics in general. However, in order to test this inference further, the adsorption of another class of organic compounds, viz. dyes, which are important pollutants, was also investigated. The adsorption of both anionic and cationic dyes was studied. The two anionic dyes chosen for this purpose are methyl orange and brilliant blue G. The adsorption was studied as a function of contact time, adsorbent particle size, concentration and temperature. It was found that 45 min are required for equilibrium adsorption to be attained. Further, the maximum adsorption on carbonaceous adsorbent was found to be 202.3 mg g" for methyl orange and 212.3 mg g"' for brilliant blue G, respectively. The adsorption on other three adsorbents was found to be relatively much smaller. Adsorption of dyes on carbonaceous adsorbent conforms to Langmuir equation and the analysis of data gave the monolayer capacity, which was nearly equal to maximum adsorption obtained. The adsorption was found to decrease with rise in temperature indicating that the dyes adsorption is exothermic. Thermodynamic parameters were calculated and enthalpy of adsorption was found to be small and negative, pointing out the physical nature of adsorption. Further, the applicability (v) ofLagergren's and Bangham's equations to the kinetic data ofadsorption revealed that dyes adsorption is a first order process and pore diffusion controlled. The adsorption behaviour of cationic dyes was also studied. The cationic dyes chosen for this purpose are bismarck brown R and rhodamine B. Their adsorption was investigated on all the prepared adsorbents and carbonaceous adsorbent was found to be the potential material and other three adsorbents to be the poor adsorbents. The maximum adsorption of dyes on carbonaceous adsorbent observed was 71.5 and 82.8 mg g"' for bismarck brown R and rhodamine B, respectively. The detailed study of adsorption of cationic dyes on carbonaceous adsorbent revealed that the adsorption (i) conforms to Langmuir equation (ii) is exothermic in nature and (iii) is a first order process and pore diffusion controlled. The studies carried out have shown that carbonaceous adsorbent is a potential material for the adsorption and, therefore, removal of dyes and phenols (organic compounds) from solution. In order to test the utility of carbonaceous adsorbent for the removal of these compounds, column operations were also carried out. It was found that columns can remove phenols and dyes from feed water solution. The breakthrough capacity was determined and is slightly lesser than maximum adsorption capacity (batch method) whereas exhaustion capacity was more than maximum capacity. The columns of carbonaceous adsorbent could be used successfully to treat effluent from a local dyeing unit and also paper and pulp mill effluent. Thus, the results of present investigations have shown that carbonaceous adsorbent is a potential material for the removal of dyes and phenols from wastewater. As both the pollutants are organics with different structures and characteristics, it is reasonable to infer that carbonaceous adsorbent can remove organics in general. In order to assess the adsorption efficiency of carbonaceous adsorbent, the results were compared with those on standard activated carbon sample and the efficiency of carbonaceous adsorbent was found to be 40-45% and 80-90% to that of activated carbon in removing phenols and dyes, respectively. Therefore, it is reasonable to conclude that prepared carbonaceous adsorbent is a (vi) suitable low cost alternative to activated carbons and can be used for the removal of phenols and dyes in particular and organics in general from wastewaters. In view of low cost (US $ 0.1 kg'1), the spent carbonaceous adsorbent need not be regenerated and can be disposed of by burning.