REMOVAL OF SOME TOXIC SUBSTANCES FROM WASTEWATER USING INEXPENSIVE ALTERNATIVES TO CARBON

Abstract

Water is the most precious natural resource that covers 70% part of the globe. It is the main and essential component of earth ecosystem without which life can not exist. Unfortunately, the quality of water is deteriorating worldwide due to its contamination continuously, through point and non-point sources. The main water polluting sources are industrial, domestic, agricultural activities and other environmental and global changes. The surface and ground waters, at some places of the world are not fit for drinking purpose. Moreover, about 14,000 people are dying daily, due to various water born diseases. The global population is supposed to reach up to 7.9 billion by 2020 and because of this, the world may be under great fresh water scarcity. Therefore, scientists, academicians, governments and other agencies are very much concerned, about the availability of fresh water in future. Our water resources are limited and under such circumstances water recycling and treatment technologies are the only alternatives to fulfill water demand in coming decades. Hence, there is an emergent need for the development of suitable, inexpensive and rapid wastewater treatment and recycling methods in the present scenario. The pollutants can be categorized as organic, inorganic and biological in nature. Organic pollutants are mainly dyes, phenols, pesticides, PCBs (polychlorinated biphenyls), detergents, aryl hydrocarbons, organohalide compounds etc. Inorganic pollutants mainly include heavy metals; caused by mining industrial discharge, acidity; including acid mine drainage, fertilizers (nitrates and phosphates) etc. Whereas, biological pollutants are pollen grains, bacteria, protozoans, worms, viruses etc. These pollutants are often toxic and cause different adverse effects and diseases in human beings and animals. In order to provide healthy lives and vital environment to human beings and other animals, it is essential to treat wastewater, for the removal of pollutants before being discharged into natural water bodies. A number of methods, such as coagulation, ozonation, membrane process, adsorption, dialysis, foam floatation, osmosis, photocatalytic degradation, biological methods are being used, for the removal of toxic pollutants from wastewater. Adsorption process is considered better as compared to other methods, because of its convenience, easy operation and inexpensiveness. Additionally, this process can (i) remove/minimize different kinds of pollutants and, thus, has a wider applicability in water treatment. Most popular adsorbents, used for pollution abetment; are silica gel, activated alumina, zeolite, ion exchange resins and activated carbon. Among them, the activated carbon has been found to be a very good adsorbent, for effluent treatment and is generally used for the removal of various pollutants. Though, its widespread use in wastewater treatment is occasionally restricted due to its high cost and necessity of regeneration. As such, for quite sometime, efforts are being made to prepare inexpensive adsorbents worldwide. In general, agricultural and industrial wastes, which are available almost free of cost, are being used for the preparation of low cost adsorbents. In the same direction, attempts have been made to develop better inexpensive alternatives to activated carbon for the removal of some toxic pollutants. Therefore, we have developed three different adsorbents viz. red mud, MSWI bottom ash and carbon slurry and have used these for the removal of phenols, dyes, pesticides and fluoride from wastewater. Red mud, a waste material, was obtained from Hindustan Aluminium Company (HINDALCO), Renukoot, India. The raw material was treated with hydrogen peroxide at room temperature for 24 h to oxidise adhering organic matter and washed repeatedly with double distilled water. The resulting material was dried at 100°C, cooled and again activated in a muffle furnace at 500°C for 3 h. It was found to contain 38.80% Fe203, 18.80% Ti02, 9.64% Si02, 17.28% A1203 and 6.86% Na20 (w/w). The loss on ignition was found to be 7.34%. The density and porosity were found to be 2.0 gem"3 and 0.45% fractions respectively. The surface area of the adsorbent was calculated by the Brunauer-Emmett-Teller (BET) method and was 108 m g" . The presence of hematite, cancrinite, geothite, rutile, anataze and quartz was confirmed with the X-ray diffraction. The cost of prepared adsorbent was estimated about US $ 0.03 per kg, including its purchase, transport, chemicals and physical treatment. The activated red mud was then used for the adsorption of phenol, 2-chlorophenol 4-chlorophenol, and 2,4-dichlorophenol from aqueous solutions. In order to optimize adsorption process for maximum uptake, the batch adsorption of phenols was studied as a function of contact time, pH, adsorbent dose, concentration and temperature. From the effect of contact time, it was observed that the equilibration time for maximum uptake of phenol was 4 h contrary to the 3 h for each chlorophenol. The study of pH effect, shows that the maximum adsorption of phenol and 2- chlorphenol occurred at pH 6.0, while the maximum adsorption of 4-chlorphenol and 2,4- 00 dichlorphenol is achieved at pH 5.0 and 4.0 respectively. The adsorption of phenols on red mud was also investigated as a function of temperature, which shows, that the adsorption increases with the rise in temperature indicating endothermic adsorption. The maximum adsorption capacity of red mud within optimum condition for phenol, 2-chlorophenol, 4-chlorophenol, and 2,4-dichlorophenol were achieved 0.47, 1.04, 1.20 and 1.58 mgg"1 respectively. The adsorption isotherms were analyzed with Langmuir and Freundlich models. The constants, evaluated from adsorption isotherms were further used to calculate thermodynamic parameters, such as free energy, enthalpy and entropy of adsorption. In kinetic studies, pseudo-first-order and pseudosecond- order equations were applied to the kinetic data, out of which, pseudo-first-order equation was found suitable. The applicability of Bangham's equation, to the adsorption data was investigated, the plots show that adsorption of phenols under investigation was pore diffusion controlled. The competitive adsorption of phenols, over red mud was carried out using industrial wastewater. The removal of phenols and its derivatives was also carried out by column experiments at the flow rate of 0.5 mLmin"1. The order of maximum removal with column experiments was 2,4-Dichlorophenol > 4-chlorophenol > 2-chlorophenol > phenol. The prepared adsorbent was compared with other low cost adsorbents reported previously for phenols removal in terms of adsorption capacity. Likewise, a waste material was obtained from a municipal solid waste incinerator (MSWI), where solid waste is incinerated at elevated temperature and bottom ash is expelled out in large quantities. Arepresentative sample of the bottom ash was collected from a local MSWI from Belgium. The raw bottom ash was dried at 105°C for 24 h and further activated at 500°C in furnace for 6 h. The chemical analysis of bottom ash showed that it has 27.8% Si02, 3 9.9% A1203, 2.0% Ti02, 3.3% Na20, 1.8% K20, 4.0% Fe203, 25.9% CaO, 6.9% P205, 3.3% MgO and 0.2% MnO (w/w). The loss on ignition was found about 9.53%. The density, porosity and surface area were measured, 1.22 gem"3, 0.3% and 14.10 m2g"' respectively. XRD-Spectrum of bottom ash shows that it consists of oxides, silicates and aluminosilicates of calcium, iron, and other bases. The cost of prepared adsorbent was estimated as about US $ 0.02 per kg. The activated bottom ash was used for the removal of alizarin yellow, fast green and methyl violet from aqueous solution. Their adsorption was investigated on bottom ash in the same way as that of phenols on the red mud. The contact time to reach the equilibrium condition for all the three dyes was found 4 h while the maximum adsorption was observed at 5.5, 5.0 and 8.0 pH for (iii) alizarin yellow, fast green and methyl violet, respectively. The adsorption of alizarin yellow and fast green was found to decrease with the increase in temperature, showing exothermic nature of adsorption for both dyes, while adsorption of methyl violet was endothermic in nature. The maximum uptake on bottom ash for alizarin yellow, fast green and methyl violet were 25.66, 50.14 and 18.91 mgg"', respectively. The adsorption isotherms were analyzed by using Langmuir and Freundlich models. Thermodynamic parameters such as free energy, enthalpy and entropy of adsorption for the adsorption systems were evaluated. Kinetics of adsorption was fitted well in pseudo-first-order equation. Adsorption processes were found film diffusion-controlled for all the three dyes. The competitive adsorption of dyes over bottom ash was examined using urban wastewater. During column studies, the order of maximum removal was found, fast green > alizarin yellow > methyl violet. The prepared adsorbent was compared with other low cost adsorbents reported in literature for the removal of dyes. In succeeding study, development of low cost adsorbent from carbon slurry, which was obtained from a fuel oil based fertilizer plant (generated due to partial combustion of liquid fuel). It was treated with hydrogen peroxide and then heated to 200°C in air until the emission of black soot stopped. Then the material was treated with 1.0 M HC1 solution to remove ash content, after it the activation was performed by heating the sample for 1 h in a muffle furnace at 450°C in the presence of air. The chemical analysis of carbon slurry indicates that it has carbon and ash content 89.8% and 0.9% respectively. The density, porosity and surface area were measured, 0.52 gem"3, 10.31% and 380 m2g"', respectively. XRD-spectrum of carbon slurry did not show any peak thereby, indicating its amorphous nature. However, IR spectrum of the sample of adsorbent indicates the presence of two prominent bands lying at 1605 and 3340 cm"1 which may be assigned due to the presence of carbonyl group and hydroxy group respectively. The cost of prepared adsorbent was estimated to be 0.20 US $ per kg (including transportation, processing and chemical treatment). The adsorbent was used for the removal of pesticides and fluoride from wastewater. The adsorptions of carbofuran, 2,4-dichlorophenoxyacetic acid (2,4-D), methoxychlor and endosulfan were studied. The minimum time required to achieve maximum adsorption is 90 min for carbofuran and 2,4-D, however it is 2 h for methoxychlor and endosulfan. All the four pesticides have shown maximum adsorption at lower solution pH. The adsorption of each pesticide was found to increase with decrease in temperature, thereby showing exothermic nature (iv) of adsorption. The maximum uptake of carbofuran, 2,4-D, methoxychlor and endosulfan by carbon slurry was found 208.21, 212.09, 36.06 and 34.13 mgg"1 respectively. The adsorption isotherms were analyzed with Langmuir and Freundlich models and the related constants were calculated for each pesticide. The thermodynamic parameters were calculated for each adsorption system. The kinetic data for each pesticide's adsorption was found to fit well in pseudo-second-order equation. The rate controlling step during the adsorption of each pesticide on carbon slurry was also calculated with the help of Bangham's equation. Column experiments at a flow rate of 1.5 mLmin"1 show that the order of maximum removal was 2,4-D > carbofuran> methoxychlor > endosulfan on carbon slurry. The efficiency of present adsorbent was compared with the other reported adsorbent, used for the removal of pesticides from wastewater. Due to enthusiastic results of carbon slurry for pesticides, attempts have been made to remove fluoride from water, which is a big threat to human being. During optimization the contact time and pH for maximum fluoride uptake were found 1 h and 7.58 respectively. Maximum adsorption capacity of fluoride on carbon slurry was found, 4.861 mgg" . The experimental isotherm data were analyzed by using Langmuir and Freundlich models. The adsorption was found endothermic and follow second-ordermechanism. The rate controlling step of the adsorption was found pore diffusion controlled. The column experiments were carried out at a flow rate of 1.5 mLmin"1. The adsorbent has been used to remove fluoride from groundwater and wastewater. The performance of carbon slurry was compared with many other reported adsorbents for the removal of fluoride. Briefly, the developed adsorbents are inexpensive (0.02 to 0.20 US $ per kg) in comparison to many other reported adsorbents. The adsorption capacities of these adsorbents are quite good indicating their applicability at large scale. The column experiments also suggested the potential use of these adsorbents on industrial level. The developed adsorption methodologies were tested to remove the reported pollutants from wastewater. Therefore the evolved adsorption methods are inexpensive, fast, effective and efficient for the removal of studied pollutants. Furthermore, these adsorbents may be used for the removal of other pollutants present in the water.