REMOVAL OF NITROGENOUS HETEROCYCLIC COMPOUNDS FROM AQUEOUS SOLUTION

Abstract

Nitrogenous heterocyclic compounds are one of the most important classes of chemicals. Nitrogenous compounds containing wastewater are generated in industries like petroleum, pharmaceutical, textile, chemicals, etc. Nitrogen containing hetero-aromatic compounds like pyridine, quinoline, indole, pyrrole, etc. has received immense attention recently because of their presence in the environment and their toxic and carcinogenic potential. Heterocycles are also produced in large quantities as a result of industrial activity. Gas oil fractions derived from sand and coals liquid have higher nitrogen concentration than those of petroleum fractions. Crude oil spills contaminate ground water with polycyclic aromatic hydrocarbons and heterocyclic compounds. Their heterocyclic structure makes them more soluble than their homocyclic analogues, and therefore they can get easily transported through the soil and contaminate ground water. Due to their toxicity, mutagenicity and carcinogenicity, they constitute a danger for environment and odour potential. Because of their properties, pyrrole and indole are considered as non-basic nitrogenous compounds whereas pyridine and quinoline are considered as basic nitrogenous compounds. The different physico-chemical methods available for treatment of wastewaters containing nitrogenous heterocyclic compounds include methods like thermal catalytic incineration, deep well injection, soil percolation, ultra filtration, chemical coagulation and adsorption on various materials. In this context, researchers are exploring the feasibility of using various alternative processes such as ultrafiltration, advanced oxidation process, membrane separation, pervaporation, etc. Physicochemical methods such as adsorption utilizing activated carbon and other adsorbents including low cost adsorbents have generated much interest among researchers and practitioners of environmental engineering and science. Adsorption has been proven to be one of the most efficient, promising and widely used technique in removal of wide variety of compounds from wastewater. Low cost adsorbents such as bagasse fly ash (BFA) have also been by a number of investigators. Conventional biological methods are believed to be the most economical treatment options for these heterocyclic compounds. However, anaerobic degradation of organic compounds are found to be slow, and therefore, less attractive for full scale application. Major areas in electrochemical treatment are electro-coagulation, electro-flotation and electro-oxidation. In electrochemical treatment, organic pollutants present in the wastewater are oxidized to give carbon dioxide, water and other oxides. In this process, adsorbed hydroxyl radical or chemisorbed active oxygen is responsible for oxidation of organic pollutants. The Taguchi’s method was developed by Genichi Taguchi to optimize the experimental variables as it minimizes time as well as cost of experiment. Optimization of parameters is generally carried out so as to obtain one factor at time. Taguchi method is Abstract iv popular and powerful approach used in optimization of process. This methodology has been extensively used in chemical and environmental engineering field. Analysis of variance (ANOVA) utilizes the experimental information providing information regarding statistically significant variables for particular operation. Taguchi’s methodology was applied previously for separation of copper ions by electrodialysis, for the removal of metal from ternary system. A review of the literature shows that only few studies are reported on adsorption of pyrrole and indole from aqueous media. In these studies, most important aspects of adsorption studies such as adsorption process control mechanism, adsorption kinetic and thermodynamic aspects were not discussed. Studies on simultaneous adsorption of heterocyclic nitrogenous compounds like pyrrole, indole, etc. from aqueous solutions are scarcely reported. Study on simultaneous adsorption of pyrrole and indole from aqueous solution is very necessary for understanding the effect of adsorption of one compound on other. However, no study is reported on simultaneous adsorptive removal of pyrrole and indole from binary aqueous mixture. Electrochemical treatment including electro-oxidation of pyrrole and indole from aqueous solution by platinum coated titanium plate electrode (Pt/Ti) has been very scarcely reported. In the present study, individual and simultaneous adsorptive removal of pyrrole and indole was studied by granular activated carbon (GAC) and BFA separately. Physico-chemical characterization including textural analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) analyses of the GAC and BFA before and after adsorption were carried out to understand the adsorption mechanism. Effect of various parameters such as pH, adsorbent dose (m), contact time (t), initial concentration (Co) and temperature (T) on the adsorption of pyrrole and indole by GAC and BFA were studied. Maximum removal of pyrrole and indole was observed at their natural pH without any pH adjustment for both GAC and BFA. Equilibrium contact time for adsorption of both pyrrole and indole onto GAC and BFA was same i.e. 8 h. Optimum value (mopt) for pyrrole and indole removal by GAC was same i.e. 20 g.l-1. However, with BFA, mopt value pyrrole and indole removal was found to be 15 and 7 g.l-1, respectively. For Co=500 mg.l-1, T=303 K and t=8 h, maximum 84% pyrrole removal and 95% indole removal was obtained with GAC at mopt=20 g.l-1. Maximum pyrrole and indole removal of 93% and 95%, respectively, was observed with BFA using mopt values for Co=500 mg.l-1, T=303 K and t=8 h. Adsorption equilibrium, kinetic and thermodynamic study for individual adsorption were carried out with both the adsorbents. The pseudo-second-order kinetics best represented the adsorptive removal of pyrrole and indole by GAC and BFA individually. Intra-particle diffusion study showed that the pore diffusion was the rate limiting step. Redlich–Peterson isotherm model was found to best-represent the individual adsorption equilibrium data for both the adsorbates onto GAC and BFA. Values for entropy change and heat of adsorption Abstract v for indole adsorption onto GAC were found as 101 kJ.mol-1.K-1 and 12.4 kJ.mol-1 whereas for BFA respective values were 198.88 kJ.mol-1.K-1 and 32.22 kJ.mol-1. Similarly, values for entropy change and heat of adsorption for pyrrole adsorption onto BFA were found as 47.6 kJ.mol-1.K-1 and -3.9 kJ.mol-1 whereas for BFA respective values were 74.3 kJ.mol-1.K-1 and 2.1 kJ.mol-1. The negative value of change in Gibbs free energy (ΔGo) indicated the feasibility and spontaneity of adsorption on by GAC. Reusability and adsorptive capacity of adsorbents, desorption study was carried out where adsorption capacity of pyrrole for GAC was found much less as compared to that of indole after successive thermal desorption. BFA showed very less desorption efficiency and that removal efficiencies using regenerated BFA was comparatively lower than that of GAC. Simultaneous adsorption of pyrrole and indole from aqueous solution was carried out with GAC and BFA separately. First, Taguchi’s method (L27 orthogonal array) was applied to optimize various parameters like Co, m, T and t for their simultaneous adsorption onto both the adsorbents. Thereafter, binary adsorption equilibrium data were generated and modelled by various multi-component isotherm models. During initial optimization of parameters by Taguchi’s methodology, 27 sets of experiments were conducted for the binary adsorption. Amount of adsorbate adsorbed per unit mass of adsorbent (q) was taken as the response. m and the interaction between initial concentrations Co’s were found to be the most significant factor. The values of qtot, qPy and qInd are found to be highly dependent on various parameters (Coi T, m and t). The adsorption of pyrrole and indole from the binary solutions onto GAC or BFA was found to be antagonistic in nature. Indole adsorption onto GAC was found to be higher than that of pyrrole. The predicted maximum value of qtot, qPy and qInd for GAC were 0.35, 0.15 and 0.20 mmol.g-1, respectively. For BFA, respective values were found to be 0.78, 0.32 and 0.46 mmol.g-1, respectively. Three confirmation experiments were conducted at selected optimal levels for the simultaneous removal of pyrrole and indole from binary solution by GAC. The calculated value of qtot, qPy and qInd are within 95% confidence interval. After optimization of operating parameters, binary adsorption equilibrium data were generated and modeled by various multi-component isotherm models such as non-modified Langmuir, modified Langmuir, extended-Langmuir, extended-Freundlich and Sheindorf–Rebuhn–Sheintuch (SRS) models. Extended-Langmuir or extended-Freundlich isotherm best-represented the isotherm data at 30 oC for simultaneous adsorption of pyrrole and indole from aqueous solution onto BFA and GAC. Electrochemical treatment of pyrrole and indole in aqueous solution was carried by Pt/ Ti electrode individually. Experiments were performed in circular glass batch reactor having 1 litre volume. Experiments were performed under controlled current condition using a direct current (D.C.) power supply. Pt/Ti electrodes were used having actual anodic dip area of 0.012 m2 in the aqueous solution containing pyrrole and indole individually with 1cm electrode gap. For binary system both solutions were mixed in varying concentration. Abstract vi Solutions were mixed with constant stirring speed of 600 rpm by magnetic stirrer. Conductivity of the solution was adjusted by adding NaCl. All experiments were conducted with controlled temperature of 30±2 oC. After desired treatment time, the treated solution was centrifuged and used for determining residual concentration in terms of chemical oxygen demand (COD). Full factorial central composite (CCD) design was used to study the effect of four key process parameters on the COD reduction. The parameters used in this study were: initial pH (pHo): 2.8 – 8.8; current density (j): 83.34–416.66 A.m-2; conductivity (k): 2.91–6.7 mS.cm-1 and electrolysis time (t): 30–150 min. Quadratic models were developed and further used for determining parametric condition for maximum COD and minimum specific energy consumption. An increase in j and t was found to increase the concentration of ions generated (as per Faraday’s law) causing adsorption of pollutants and formation of polymeric species in the solution which increased the treatment efficiencies. The optimum operational parameters during electrochemical treatment of pyrrole were found to be pH=8.76, j=175.19 A.m-2, k=2.94 mS.cm-1 and t=150 min. Under this optimized conditions predicted values of percent COD removal and specific energy consumption were found to be 69.30% , 99.25 kWh/kg of COD removed, respectively. The value of overall desirability (D) was found to be equal to 0.975. Similarly, optimum operational parameters during electrochemical treatment of indole were found to be pH=8.61, j=161.02 A.m-2, k=6.69 mS.cm-1 and t=150 min. The predicted values of percent COD removal and specific energy consumption were found to be 82.92%, 37.75 kWh/kg of COD removed, respectively. Parameters for electrochemical treatment of binary mixture of pyrrole and indole by Pt/Ti were optimized using Taguchi’s design of experiments (L16 orthogonal array). Optimum values for removal efficiency of pyrrole, indole and COD were found to be 46.1%, 62.4% and 61.4%, respectively. Scum generated has also been characterized by FTIR, SEM, energy dispersive X-ray (EDX) analysis and thermo-gravimetric analysis (TGA) so as to evaluate its disposal aspect. Mechanism of EC treatment was studied by carrying out UV-visible, UPLC, FTIR and cyclic voltammetric analysis of solution before and after treatment. It was found that the electrochemical treatment of pyrrole and indole occurred by a combination of electro-oxidation and electro-floatation process. Overall, adsorptive removal of pyrrole and indole was most economic process among the treatment methods studied. Though, considering the mineralization of pyrrole and indole, electrochemical treatment by Pt/Ti is a good option of treatment.