ADSORPTION AND STABILIZATION OF HEAVY METALS ON FLY ASH

Abstract

Removal of heavy metals from wastewater is one of the most challenging environmental problems many researchers face worldwide. Unlike other organic pollutants, heavy metals' degradation rate is very slow; thus, these metals become a part of the food chain and accumulate in a living organism's body. Pb(II) and Cu(II) containing wastewater are discharged into the environment by various industrial activities. Pb(II) is widely used as a raw material for manufacturing batteries, cable sheaths, pigments, and explosives. Pb(II) consumption may cause diseases like hepatitis, anemia, encephalopathy, and nephritic syndrome even at very low concentrations. Cu(II) is widely used in various industrial processes, such as electroplating, acid mine drainage, copper smelting, printed circuit board, wood and paper, fertilizer industry, etc. Though Cu(II) is a vital trace nutrient for plants and animals, it can be poisonous to human beings and other living organisms when its concentration is high. That's why it is essential to treat this heavy metal-containing wastewater before it is discharged. This has led to the formation of rigorous regulations. For example, as per WHO guidelines, Pb(II) and Cu(II) concentrations in mining and electroplating wastewater should be less than 0.01 and 2.0 mg/L. USEPA also set concentration limits of 0.015 mg/L and 1.3 mg/L, respectively. According to the Central Pollution Control Board, India, the discharge concentration limit of Cu(II) in the effluent is limited to 3.0 mg/L. The discharge concentration limit for Pb(II) is 0.10 mg/L. Fly ash, one of the most abundant waste materials obtained during the combustion of coal in thermal power plants, is a potential material for removing heavy metals present in wastewater. The safe disposal of fly ash is a matter of great concern as it is not environmental-friendly and requires a large area of land to dispose of it in landfills. Fly ash used in the present experiment was collected from a coal-based power plant operated by the Hindalco industry located at Renukoot (Uttar Pradesh, India). The results specify the fly ash used in this study was class F-type, as the combined weight percentage of SiO2 and Al2O3 is more than 70%, and lime content was less than 10%, according to ASTM C 618. The XRD patterns revealed that the fly ash mainly contains quartz, a moderate amount of mullite, and a meager amount of rutile and hematite. pHZPC of this fly ash was found to be 2.9. i The initial Pb(II) concentration, pH, and fly ash dosage significantly influence the removal efficiency. It was found that the model obtained by BBD showed a high correlation between the experimental value and the model-predicted value. According to the ANOVA table, the predicted model had an F-value of 213.32. The adequate precision value of the developed model was 47.95 and indicated an appropriate signal-to-noise ratio. The proposed model possesses a high correlation coefficient (R2) 0.9964, which shows all the terms depicted in this model were very significant. At optimum condition, about 97.6% of Pb(II) removal efficiency was achieved at the Pb(II) concentration of 10 mg/L, pH 5.3, and fly ash dosage 18 g/L. The equilibrium data of the sorption process obeyed Freundlich isotherm and followed the pseudo-second-order kinetic model. Thermodynamic study reveals that ∆G's value for all temperatures was negative, suggesting that Pb(II) adsorption onto fly ash was spontaneous. The value of ∆H was negative, and it indicated the adsorption was exothermic. The initial Cu(II) concentration, pH, and fly ash dosage significantly influence the Cu(II) removal efficiency. It was found that the model obtained by CCD shows a high correlation between the experimental value and model-predicted value. The R2 value of the model was 0.995, indicating that this model could not explain only 0.5% of the total variation. The optimum level of 93.8% removal efficiency of copper was achieved at an initial copper concentration of 43 mg/L, pH 6, and fly ash dosage 63 g/L. The equilibrium data of the sorption process obeyed Freundlich isotherm and followed the pseudo-second-order kinetic model. The maximum adsorption capacity of this fly ash for Pb(II) and Cu(II) were 2.1 and 0.83 mg/g. SEM and EDX analysis of fly ash after adsorption of Pb(II) and Cu(II) showed metal in the form of a metal silanol complex. After adsorption, approximately 1.32% Pb(II) and 1.39% of Cu(II) were present on the fly ash surface. FTIR analysis of metal adsorbed fly ash showed that the different peaks' transmittance was changed due to the adsorption of the metal ion. XRD analysis of fly ash after Pb(II) and Cu(II) adsorption revealed the existence of Pb(II) and Cu(II) in the form of metal silicate hydrate. This Pb(II) and Cu(II) laden fly ash in different proportions (10, 20, and 30%) was mixed with cement and sand to prepare mortar samples for use as building materials. The addition of Pb(II) and Cu(II) laden fly ash retards the setting times of cement paste, and retardation increases with an increase in the replacement level of metal-loaded fly ash. For 56 days cured control, 10%, 20%, and 30% fly ash mortar, the compressive strength was 15.7, 16.4, 15.6, and 15 MPa, respectively. The compressive strength of the stabilized product decreases with increasing the replacement level of ii Pb(II) and Cu(II) laden fly ash in mortar. The maximum compressive strength achieved for 56 days cured 10%, 20%, and 30% Pb(II) laden fly ash mortar was 11.5, 10.8, and 8.1 MPa, respectively, and for Cu(II) laden fly ash, it was 12.4, 11.9 and 9.2 MPa respectively. However, the compressive strength of the metal-laden fly ash mortar meets the minimum strength requirement of 7.5 MPa as per IS 2250:1981. With the inclusion of Pb(II) and Cu(II) laden fly ash in the mortar matrix, almost all flexural strength values decreased significantly. SEM and EDX analysis of metal-laden mortar showed the interconnected honeycombed structure of fibrous CSH gel break down and form tiny calcium silicate hydrate crystals (CSH). It was also found that the micrograph contains a higher amount of hexagonal plate of portlandite. A spherical and smooth surface of unreacted fly ash particles was also detected, the indication of incomplete pozzolanic reaction. From the EDX analysis, it was observed that adsorbed metal was efficiently immobilized in the cement matrix. The XRD pattern of Pb(II) and Cu(II) laden fly ash mortar revealed that the metal-laden fly ash mortar sample contains peaks of calcium lead oxide hydroxide and calcium copper silicate hydrate, respectively. This is due to the incorporation of adsorbed copper in the cement matrix. From TG/DTG analysis, it was observed that portlandite (%) in the Cu(II) and Pb(II) laden fly ash mortar was higher than fly ash mortar. The higher CH (%) of Pb(II) and Cu(II) laden fly ash mortars explained the reduction in pozzolanic reactions causing lesser consumption of CH to form secondary CSH. The quantification of CSH gives 4.14% and 3.86% of CSH for Pb(II) laden and Cu(II) laden fly ash mortars, respectively, which was lower than that of fly ash cement mortar (4.77%). The leaching properties of 28 and 56 days cured solidified fly ash mortar and Pb(II) and Cu(II) laden fly ash mortar were investigated in two different mediums, acetic acid fluid, and water medium. The TCLP study conducted on 28 and 56 days cured metal-laden fly ash mortar shows Pb(II) and Cu(II) concentration in the leachate does not exceed the discharge standards. The development of a more dense interconnecting honeycombed structure of fibrous CSH gel reduces Pb(II) and Cu(II) concentration as the curing period increases from 28 to 56 days. Setting time, compressive strength, micrograph analysis, and leaching study suggested that Pb(II) and Cu(II) laden fly ash could be considered for use in construction materials. The proposed integrated adsorption-solidification/stabilization process for fixing and immobilizing the Pb(II) and Cu(II) ions provides a two-fold aim: wastewater treatment and solid waste management.