DEVELOPMENT OF LOW COST ADSORBENTS FROM INDUSTRIAL WASTES FOR THE REMOVAL OF TOXIC SUBSTANCES

Abstract

As a result of widespread industrial, agricultural and domestic activities, water is getting polluted by inorganic and organic materials and biological agents. Some of the common pollutants are phenols, dyes, detergents, polynuclear hydrocarbons, insecticides, pesticides and heavy metals. The exact nature of pollutants in a wastewater depends on the source of generation and varies from place to place. These pollutants are often toxic and cause harmful effects to human and animal life if present above certain concentration levels. It is, therefore, important that wastewater is treated for the removal of toxic materials before being discharged into natural water bodies. Various 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. The type of process to be used may depend on nature of pollutant. However, adsorption process has emerged as the best as it can in general, remove both the inorganic and organic pollutants of different types. Activated carbon has been found to be a very good adsorbent and is used for the removal of various toxic materials from wastewater. Inspite of its good efficiency for the pollutant removal, its use sometimes is restricted due to its high cost. As such, for quite sometime, efforts are being made to prepare cheaper adsorbents, generally from industrial and agricultural wastes, which are available almost free of cost. The various materials investigated so far for this purpose include eucalyptus bark, plum kernels, resin, fly ash, red mud, slag, bagasse flyash, etc. These materials have not shown promising adsorption characteristics in comparison to activated carbon. Further, comparative data on (i) adsorption efficiency of different adsorbents is not available. It is, therefore, still necessary to develop 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 analysed and it was found that the BF dust and slag are basically inorganic in nature i.e. they contain mainly inorganic contents like silica (15.8 and 32.7%), calcium oxides (4.7 and 31.7%), magnesium oxide (4.2 and 6.8%) and iron oxide (44.9 and 22.8%) whereas BF sludge contains besides inorganic constituents, appreciable quantity of carbon (~ 40%). On the other hand, carbonaceous adsorbent prepared from carbon slurry contains negligible quantity of inorganic constituents and the main constituent is carbon. 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 m2 g"1 for carbonaceous adsorbent, BF sludge, BF dust and BF slag, respectively. The surface area increases as carbon content increases from blast furnace 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 dust, sludge 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 00 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 large surface area as compared to other three adsorbents 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 number and iodine number, were also compared with standard samples of activated carbon (E. Merck) and it was found that carbonaceous adsorbent is half as efficient as compared to 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. 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 and the results obtained form the subject matter of the present thesis. The adsorption of both the cationic and anionic dyes was studied on the prepared adsorbents. The four cationic dyes chosen for this purpose are meldola blue, crystal violet, chrysoidine G and methylene blue. In order to assess the adsorption properties of the four adsorbents, the adsorption of cationic dyes was studied by batch method for adsorbents of particle size 200-250 BSS mesh. The adsorption isotherms revealed that the maximum adsorption of dyes on carbonaceous adsorbent, BF sludge, BF dust and BF foamed slag was found to be 170, 67.3, 34.2 and 3.7 mg g" , respectively for meldola blue; 161, 25.2, 11.1 and 3.0 mg g'1 for crystal violet; 75.1, 10.1, 5.4 and 1.9 mg g"1 for chrysoidine Gand 92.2, 6.4, 3.3 and 2.1 mg g"1 for methylene blue. Thus the extent of (iii) adsorption of the four dyes on different adsorbents is found to be in the order, carbonaceous adsorbent > BF sludge > BF dust > BF foamed 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. Ofall the four adsorbents studied, only carbonaceous adsorbent adsorbs dyes to a sufficient extent whereas the other three are poorer material for the removal of dyes. The carbonaceous adsorbent is, therefore, a potential adsorbent for the removal of dyes. As such, all further studies were undertaken only with carbonaceous adsorbent. In order to have insight into the adsorption process, the adsorption ofdyes was studied as a function of contact time, particle size, concentration and temperature. The adsorbent particle size effect revealed that, of the three particle sizes viz. 100-150, 150-200 and 200-250 mesh studied, the adsorption capacity was found to be slightly higher for smaller particles. The effect ofcontact time on adsorption shows that it took about 25 minutes 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 dyes is achieved in less than 3.5 minutes. 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 adsorption of dyes on carbonaceous adsorbent was also investigated as a function of temperature and it was observed that adsorption decreases with rise in temperature indicating the process to be exothermic. Thermodynamic parameters were calculated and enthalpy of adsorption was found to be small and negative, pointing out the physical nature of adsorption. The adsorption isotherms were also analysed and were (iv) found to conform to Langmuir equation. From Langmuir plots, the monolayer adsorption capacity was calculated and the values for all dyes were found similar to the maximum adsorption observed. The Lagergren's equation was applied to the kinetic data of adsorption and found applicable. The applicability of Lagergren's equation shows that adsorption of cationic dyes on carbonaceous adsorbent is a first order process. The rate constants as calculated were found to be 0.258, 0.227, 0.209 and 0.177 min"1 for meldola blue, crystal violet, chrysoidine G and methylene blue, respectively. Finally the applicability of Bangham's equation to the adsorption data indicates that adsorption of dyes under consideration is pore diffusion controlled. The adsorption of three anionic dyes viz. ethyl orange, metanil yellow and acid blue 113 was also studied on all the adsorbents in the same way as that of cationic dyes. In this case too, carbonaceous adsorbent was found to be the potential material and other three adsorbents to be poor adsorbents. The detailed study on adsorption of anionic dyes on carbonaceous adsorbent revealed that the adsorption (i) conforms to Langmuir equation (ii) is exothermic in nature (iii) is a first order process and pore diffusion controlled. The dyes adsorption studies have shown that carbonaceous adsorbent possess appreciable adsorption capacity for organics. In order to test this inference further, the adsorption of another class of organic compounds, viz. phenols which are also important pollutants, was investigated. For this purpose, the phenols chosen are phenol. 2- chlorophenol, 4-chlorophenol and 2,4-dichlorophenol. The adsorption was studied as a function of contact time, adsorbent particle size, concentration and temperature. It was found that about 8 h was required for equilibrium adsorption to be attained. Further, the

maximum adsorption on carbonaceous adsorbent was found to be 132.5, 57.4, 50.3 and 17.2 mg g"1 for 2,4-dichlorophenol, 4-chlorophenol, 2-chlorophenol and phenol, respectively. The decrease in adsorption from 2,4-dichlorophenol to phenol is parallel to increase in the solubility of these phenols in water. This indicates that lesser soluble phenols are adsorbed more. The adsorption on other three adsorbents was found to be relatively much smaller. Adsorption of phenols 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 also found to increase with rise in temperature indicating that the phenol adsorption is endothermic. The positive value of enthalpy apparently could be attributed to a pre-adsorption step involving breaking of hydrogen bonds. Further, the applicability of Lagergren and Bangham's equations to the kinetic data of adsorption revealed that phenols adsorption is also first order process and pore diffusion controlled. In view of good adsorptive properties of carbonaceous adsorbent its performance has been evaluated with respect to standard sample of activated charcoal. The comparison has shown that carbonaceous adsorbent is 50 to 70% as efficient as standard activated charcoal in removing dyes and phenols from aqueous solutions. Thus the batch studies have indicated that the carbonaceous adsorbent has good adsorption capacity for dyes and phenols (organics). However, it was thought desirable to test the practical feasibility of the carbonaceous adsorbent for the removal of these compounds, by carrying out column operations. It was found that columns can remove phenols and dyes from influent/feed water solution and effluent was found to contain negligible amount of phenols and dyes. The breakthrough capacities were determined and are slightly lesser than maximum

adsorption capacity (batch method) whereas exhaustion capacity was more than maximum adsorption. A real sample was taken and column of carbonaceous adsorbent could remove dye pollutants. Thus the results of present investigations have shown that carbonaceous adsorbent is a potential material for removal of the 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 view of low cost and high adsorption capacity, it is reasonable to conclude that the carbonaceous adsorbent is a good alternative to activated carbons for the removal of pollutants through adsorption process.