MULTICOMPONENT ADSORPTION OF PHENOLIC COMPOUNDS FROM AQUEOUS SOLUTION

Abstract

Phenol and its derivatives (Ps) and its associated compounds, like Aniline (AN), Phenol (P), 4-chlorophenol (CP), 4-nitrophenol (NP), catechol (C), resorcinol (R) and Hydroquinone (HQ) are emitted from industrial plants in the form of vapors in the air atmosphere and in aqueous effluents. P and its derivatives can also originate from diffuse emissions, e.g. from the tar coatings of the roads and pipes and from the use of pesticides including their transformation products, e.g. herbicides, fungicides. P and its derivatives are very important organic intermediates, used in the manufacture of many products in such units as drugs, rubber, pesticides, varnishes and also, dyestuffs, chemicals, petrochemicals, paper, wood, metallurgy and coking plants. Phenols (Ps) constitute the 11th ofthe 126 chemicals, which have been designated as priority pollutants by the Environmental Protection Agency of USA [USEPA, 1987; Rodriguez et al., 2000]. USEPA has set a discharge limit of 0.1 mg/1 of P in wastewater. The World Health Organization (WHO) has a 0.001 mg/1 as the limit of P concentration in potable water [WHO, 1984].These compounds impart objectionable taste and odor to drinking water at concentrations as low as 0.005 mg/1. The Ministry of Environmental and Forests, Govt, of India has set a discharge limit of 1 mg/1 for phenols in waste water. Adsorption as a wastewater treatment process has aroused considerable interest during recent years. Granular activated carbon (GAC) is generally used as an effective adsorbent for controlling various organic and inorganic pollutants. Of all the methods proposed for the removal of P and its derivatives and associated compounds, adsorption appears to offer best prospects for overall treatment, especially for the effluents with moderate and low concentrations. Unconventional adsorbents like bagasse fly ash, rice husk ash, brick kiln ask, carbon slurry, sewage sludge, kaolin, soil and clay, silica, peat, lignite, bagasse pith, wood, saw dust, molecular sieves, resins, montmorillonite, etc. have attracted the attention of several investigators [Viraraghavan and Alfaro. 1998; Rai, et al., 1999; Ng et al., 2002; Ravikumar et al., 2005; Chakraborty et al., 2006; Srivastava et al., 2006a; Shakir et al., 2008; Maiti et al., 2009; Rallapalli et al., 2010]. Various physico-chemical end biological treatment techniques are suggested for the treatment of P and its derivatives from iii wastewaters, which include concentration followed by adsorption [Mall et al., 1996; Viraraghavan and Alfaro, 1998; Srivastava et al., 2006b], biodegradation [O'Neill et al., 2000; Kumar et al., 2005; Subramanyam and Mishra, 2008], catalytic oxidation [Garg et al., 2010], etc. Much of the work on the adsorption of Ps by GAC has focused on the uptake of single component. Since industrial effluents can contain a number of components, it is necessary to study the simultaneous sorption of two or more compounds and also to quantify the interference of one compound with the sorption of the others. Thus, the studies on adsorption of Ps from binary and ternary systems are of prime importance. The equilibrium adsorption isotherm equations proposed for single component adsorption have been extended and modified to represent the binary and multi-component adsorption equilibria. The literature concerning multicomponent phenolic compounds adsorption from aqueous solutions on to GAC is scarce. Taguchi*s method for optimal design of experiments is a very powerful method for studying the effect of various parameters and their interactions on the waste water treatment optimization [Roy, 1990]. Very little work has been done using the method for adsorption studies. No work is reported using Taguchi's method for the adsorption of Ps and their associated compounds from wastewater using batch and column systems. The present study has been undertaken with the following objectives: 1. To characterize GAC, and to study its use in batch and column adsorption of P and its associated compounds. 2. To study the effects of various parameters viz. adsorbent dose (m), initial pH ipHn), initial concentration (C0 in the range 50-1000 mg/1), contact time (/) and temperature (T) (288-318 K) on the adsorption capacity in batch and column processes. 3. To study the kinetics, equilibrium adsorption and thermodynamics of the adsorption process. 4. To study multicomponent adsorption isotherm behavior of the competitive adsorption equilibria of P and its derivatives and associated compounds in binary and ternary systems. IV 5. The application of Taguchi's design of experimental methodology for the single, binary and ternary adsorption systems for P and its derivatives and associated compounds in batch and column system. 6. To study desorption of adsorbed species and disposal and management of spent GAC. All the batch experiments were carried out at 30±1 °C. For each experimental run, 0.05 1of adsorbates solution of known Cg,pHo (2 - 12) and m, taken in a 0.25 1 stoppered conical flask, was agitated in a temperature-controlled orbital shaker at a constant speed of 150 ± 5 rpm. Samples were withdrawn at appropriate time intervals and centrifuged using a research centrifuge. The residual P and its derivatives and its associated concentration (Cr) of the centrifuged supernatant were then determined. Physico-chemical characterization including surface area, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR) of the GAC before and after adsorption have been done to understand the adsorption mechanism. The initial pH (pHo) of the solution strongly affects the chemistry of P and its derivatives and its associated compounds and GAC in an aqueous solution. The point of zero charge (pH?zc) of GAC lies at a pH0 value of 10.3. Natural pH of 6.9, 6.0, 5.8, 6.0, 5.8, 5.6 and 6.1 were found to be optimum for the removal of AN, P, CP, NP, C, R and HQ, respectively. Optimum GAC dosages were found to be 10 g/1, for Co =500 mg/1 of P and its derivatives and its associated compounds. The pseudo-second-order kinetics represented the adsorption process well for all the adsorbates (single component). Equilibrium isotherms were analyzed by using different isotherm models viz. The maximum removal of the materials from the synthetic wastewaters was found in the range of 60 to 99% at higher and lower concentrations, respectively. Redlich-Peterson and Temkin isotherm models generally well represent the equilibrium adsorption of P and its derivatives and its associated compounds onto GAC. The heat of adsorption (A//) and change in entropy (AS0) for adsorption on GAC were found to be in the range of 18-52 kJ/mol and 121-243kJ/mol K. The negative value of change in Gibbs free energy (AG0) indicated the feasibility and spontaneity of adsorption on by GAC. The isosteric heat of adsorption calculated from the equilibrium adsorption data using the Clausius-Clapeyron equation was quantitatively correlated with the fractional loading of compounds onto GAC. The surface energetic heterogeneity patterns of the GAC were described as functions of isosteric enthalpy. The higher heats of adsorption at low coverages were likely related to the presence of surface defects. Batch studies for the removal of various components in binary and ternary systems by GAC were carried out using Taguchi's orthogonal array (OA) experimental design (DOE) methodology. Significant parameters, viz., concentration, temperature, adsorbent dose and contact time at three levels with a OA layout of L2j (313) were selected for the proposed experimental design for batch study. L9 (34) OA was used column study. In all, 27 sets of experiments were conducted for the adsorption in binary and ternary systems. The removal efficiency was found to be in the range of-90-95%. The adsorption of P and its derivatives and associated compounds from the binary and ternary solutions onto GAC is generally found to be antagonistic in nature. Equilibrium isotherms for the binary and ternary adsorption have been analyzed by using non-modified Langmuir, modified Langmuir, extended-Langmuir, extended-Freundlich and Sheindorf-Rebuhn-Sheintuch (SRS) models. The competitive extended-Freundlich and SRS models fit the equilibrium data satisfactorily and adequately. For the desorption experiments, several solvents (acids and alcohol, water) have been used. Among the various solvents, only NaOH was found to be better solvent for the desorption of P, CP, NP, C, R, HQ, while HNO3 was found to be better solvent for the desorption of AN. Thermal desorption at 623 K was found to better as compared to solvent desorption. GAC worked well for at least five adsorption-desorption cycle, with continuous decrease in adsorption efficiency after each thermal desorption. It is necessary to properly dispose of the spent-GAC and/or utilize it for some beneficial purpose, if possible. The dried spent-GAC can be used directly or by making fire-briquettes in the furnace combustors/incinerators to recover its energy value. Blank GAC has a heating value of about 8.26 MJ/kg. Thus, the GAC along-with the adsorbed P and other compounds can be dried and used as a fuel in the boiler furnaces/incinerators, or can be used for the production of fuel-briquettes. The bottom ash may be blended with clay to make fire bricks, or with cement-concrete mixture to make colored building blocks thus disposing of P and its derivatives and its associated compounds through chemical and physical fixation. Thus, spent-GAC could not only be safely disposed but also its energy value can be recovered.