REMOVAL OF SOME INORGANIC AND ORGANIC TOXIC SUBSTANCES FROM WATER USING INDUSTRIAL WASTE MATERIALS

Abstract

With the growing industrialization there is a continuous increase in the discharge of toxic organic and inorganic substances in surface and inland waters as well as in air. Besides many by-products of various industries are wastes and their disposal is always a challenging task for the industries as well as to the environmentalists. Safe and effective disposal of solid wastes produced by the aluminium industries and sugar industries is a major problem especially in India. Alumina is produced from bauxite through the bayer process and the major by-product is converted to a waste slurry called red mud. It contains very fine particles and is difficult to settle in water or compact on land. At present no methodology exist for the safe disposal of this waste. The combustion of bagasse in sugar industries generate substantial amount of solid waste called as bagasse fly ash. This material also causes a disposal problem. Numerous techniques exist to remove toxic metal ions and phenols from aqueous solutions. The process of adsorption is by far the most versatile and widely used method especially in less developed countries and activated carbon has been the adsorbent of the choice for this purpose since long. Inspite of the huge use of activated carbon in cleaning processes it has been highly expensive and therefore continuous efforts in the recent times are being made to develop low cost alternatives of activated carbon especially by using industrial wastes. In an endeavour to prepare efficient and low cost adsorbents, efforts have been made to convert the red mud and bagasse fly ash into suitable adsorbents, 00 by treating them chemically and physically, and utilize for the removal of metal ions and phenols. Preliminary studies provided interesting results and therefore, detailed studies were taken up for the uptake of four metal ions viz. lead, zinc, cadmium and chromium on red mud and two metal ions (lead and zinc) and two phenols (phenol and p-nitrophenol) on bagasse fly ash. The waste material red mud was obtained from Hindustan Aluminium Company (HINDALCO), Renukoot, Distt-Mirzapur, India. This material was treated with hydrogen peroxide at room temperature for 24 h to oxidise the adhering organic matter and washed with doubly distilled water. The resulting product was dried at 100°C. The product was cooled and again activated in presence of air at 500°C for 3 h. This material with a surface area of 108 m2 g1 gave the best surface properties. The material was crushed and sieved to different particle size. The temperature and time were optimized by observing the surface properties of the product under different conditions of activation. The material was characterized by its chemical composition and xray spectra. The uptake of metal ion on the activated red mud was investigated as a function of metal ion concentration, adsorbent dose, pH, particle size and temperature. The effect of cationic and anionic detergents was also studied under optimum conditions of adsorption. The results indicated that the maximum removal for lead and cadmium took place at a PH of 4.0 while for zinc at pH 5.0 and for chromium at a pH of 2.0. Lead is 100% removed at lower concentrations (< 1.45 x10 *M) and the adsorption decreases with increase in concentration. The trend iis same (iii) in the case of other metal ions also but the limits of 100% removal are different i.e. the 100% removal of zinc takes place at concentrations < 6.12 x 10 4M, cadmium at concentrations < 3.56 x 10"4 Mand that of chromium at concentrations < 9.60 x 10"4 M. The uptake decreases with increasing temperature for all the metal ions suggesting the exothermic nature of the removal process. The presence of other metal ions as well as surfactants affect the removal and it goes down by different percentage depending upon the nature of the interferent. The ionic interactions have been interpreted using the ratio of sorption capacity of the primary ion in presence of the other metal ions (qj to that in a single component (qo). The values of qT1/q0 in the present studies are <1 and suggest the suppression of the adsorption by the presence of other metal ions/surfactants. The adsorption data have been fitted to Freundlich and Langmuir models. The results on Freundlich model indicated that the 'KF' values decreases in the following order Cr6+ > Pb2+ > Zn2+ > Cd2+ The Langmuir constants indicated that the adsorption capacity 'Q0- is maximum for Cr6+ and least for Cd2+ and decreases with increasing temperature. The dimensionless constant separation factor RL for all the metal ions is «1 indicating the highly favourable nature of adsorption process. The thermodynamic parameters also confirm the affinity of the material towards the metal ions studied. The kinetic studies revealed that the process is quite rapid and 23-45% of the ultimate adsorption occurs within the first hour of contact. The saturation is reached in about 8 h. 10 g L1 of the adsorbent dose was quite sufficient for the optimal removal of all the metal ions, while 150-200 mesh size is fit (iv) for the studies. The values of mass transfer coefficient have been calculated and the linear plots between log (qp-q) versus time are used to calculate the rate constant of adsorption. The linearity test of Bt versus time plots are employed to distinguish between the film diffusion and particle diffusion controlled rates of adsorption in the present studies. It was observed that under conditions of lower concentrations film diffusion operates while at higher concentrations the particle diffusion becomes the rate determining process. These observations are further verified by drawing the McKay plots. The column studies have been performed by using a glass column (40 x 0.5 cm) filled with a known amount of adsorbent (mesh size 200-250) on glass wool support for different metal ions. Investigations were carried out by feeding the column with solutions of Pb2+(3.38 x 10"3 M), Zn2+(1.84 x 10~3 M), Cd2+(8.89 x lO"4 M) and Cr6+(5.77 x 10"3 M) at the desired flow rate, by adjusting it with a clip at the bottom of the column. The breakthrough curves were used to determine the capacity at the complete exhaustion and other parameters required for designing the fixed bed adsorbers. It was noticed that the column capacities of all the metal ions are greater than their corresponding batch capacities. The recovery of the adsorbate as well as regeneration of the exhausted column are quite necessary, therefore, the same have been achieved by using 1% HN03. 't was observed that 100 ml of 1% HNO:, is appropriate for desorbing all the metal ions. After desorption the column was washed with 10 ml fractions of hot distilled water at a fixed flow rate and was again loaded with various metal ions to check the sorption efficiency of the material during subsequent cycles. It was noticed that the adsorption efficiency is regained to (v) almost 100% if the column after elution is treated with 50 ml of 1 MHNO. solution and washed with 100 ml of distilled water. The bagasse fly ash, obtained from sugar industry at Bijnor (U.P.), was treated with excess hydrogen peroxide (100 volume) at 60°C for 24 h, till the evolution of bubbles stopped. The product so obtained was cooled and washed with doubly distilled water and dried at 100°C for 24 h. The surface area of the activated product was 440 m2 g1. The material was crushed and sieved to different particle sizes. This product was characterized by its chemical analysis and x-ray diffraction pattern. In this case also the removal was studied as a function of adsorbate concentration, adsorbent dose, pH, particle size and temperature. The effect of anionic surfactant was also studied under the optimum conditions of adsorption. The results show that the maximum adsorption of lead and zinc took place at pH 3.0 and 4.0 respectively. Removal of lead was found to be 100% at the lower concentration < 9.7 x 10"4 Mand the uptake decreases with increase in concentration. Similarly zinc was adsorbed 100% at lower concentration < 6.12 x 104 Mand the adsorption decreases with increase in concentration. The uptake of metal ions decreases with a rise in temperature, indicating the process to be exothermic in nature. The presence of other metal ions as well as anionic surfactant suppressed the adsorption. The adsorption data have been correlated with Freundlich and Langmuir models. The results on Freundlich model indicated that 'KF' value for Pb2+ is greatar than Zn2+. The Q° value is quite high for Pb2+ and RL values are «1, which indicate highly favourable adsorption of lead and zinc on bagasse fly (vi) ash. The thermodynamic parameters also confirm the affinity of the material towards the adsorption of lead and zinc. The kinetic studies indicated the process to be quite rapid. Typically 50 to 65% of the ultimate adsorption occurs within first hour of contact and the saturation reached in 6-8 hours time. It was found that 10 g L1 of the adsorbent was quite sufficient for the optimal removal of lead and zinc, while 150-200 mesh size is suitable for the studies. The adsorption was found to be film diffusion at lower concentrations and particle diffusion at higher adsorbate concentrations. These finding were substantiated by drawing McKay plots. The removal of Pb2+ from an actual wastewater sample from ametal finishing plant has been successfully achieved at pH 3.2 with 4.0 g L1 of the adsorbent. The column studies indicated that the column capacities for lead and zinc are greater than their corresponding batch capacities. The recovery of the adsorbates in this case also has been achieved by using 100 ml of 1% HNOa for desorbing the metal ions. After Resorption the column was washed with 10 ml fractions of distilled water at afixed flow rate and was again loaded with various metal ions to check the sorption efficiency of the material during subsequent cycles. It was found that the adsorption efficiency was almost 100% regained if the column is treated with 50 ml of 1MHNO,, solution and washed with 100 ml of distilled water. Only 10% loss was recorded in the second cycle and no further decrease was observed. The removal of phenol and p-nitrophenol was also studied on bagasse fly ash under optimum conditions (i.e. pH 4.0, temperature 30°C, adsorbent dose 10 gL1 and particle size 150 -200 mesh for both the adsorbates). The removal was 95% and 100% respectively at lower concentration < 1.0 x (vii) 10 M. Further the adsorption decreases with increase in concentration. The uptake of both the adsorbate decreases with increasing temperature indicating the process to be exothermic in nature. The effect of an anionic surfactant (manoxol IB) on the adsorption of phenol and p-nitrophenol was studied and a decrease in scavenging efficiency of the adsorbent in the presence of the detergent was observed. The adsorption processes follow both Freundlich and Langmuir models. The findings on Freundlich model show that the 'KF' value for p-nitrophenol is greater than phenol. Similarly the adsorption capacity Q° was found to be greater for p-nitrophenol. The RL values are « 1 suggesting the adsorption of phenols to be favourable on the adsorbent. The thermodynamic parameters were evaluated and are in confirmation of the affinity of adsorbent material towards phenols. The kinetic studies revealed that the 50% of the ultimate adsorption occurs within the first hour of contact and the equilibrium is reached in about 6-8 hours time. All the parameters like mass transfer coefficient and rate constant of adsorption were calculated. The linearity test, of Bt versus time plots are employed and it was found that adsorption of phenols take place via film diffusion mechanism at lower concentrations and through particle diffusion at higher concentrations. These results are further verified by drawing McKay plots. The column studies, as discussed above, were carried out by feeding 5.0 x 10" Mof the solutions of both phenol and p-nitrophenol at the desired flow rate. The column capacity for both phenols are greater than their batch capacities. Desorption was tried with a number of eluting agents but almost (viii) complete (~ 98%) desorption of phenols could be achieve with 8% NaOH. It was further observed that the column loses about 2-8% of its adsorption efficiency, after the first run and atreatment with 1MHN03 solution restores the lost sorption capacity to almost the original value. No loss is however observed in subsequent runs and a column can be used for 8-10 cycles without further regeneration . Wastewater obtained from a coal gasification plant was successfully treated on the columns of this adsorbent material. Almost complete removal of phenol from 50 ml of wastewater at pH 4.0 was possible with the column of bagasse fly ash (particle size 200 - 250 mesh, adsorbent dose 4.0 g, flow rate 0.4 cm min" ). The data may be helpful in designing a fixed bed adsorber for the treatment of adsorbate of known concentrations if applied on a large scale. The cost of bagasse fly ash was estimated considering all the economic factors and the cost of finished product was ~ US$ 12 ton1 in comparison to the cheapest variety of commercially available carbon (in India) i.e. US$285 ton"1. Hence, the developed adsorbent would be a good replacement for expensive commercially available carbons due to its low cost and good efficiency.