Uptake of azaarenic 2-Methylpyridine by pre-cooled carboxyl functionalized graphene nanocomposite: Detection, sorption and optimization
Graphical abstract
Introduction
Industrial solvents are chemical compounds used for dissolution and dilution of components used in the manufacture of goods. Azaarenes like 2-Methylpyridine are nitroaromatics found in textile and pharmaceutical wastewater, the two largescale manufacturing industries. Wastewaters from animal husbandry farms including poultry have also shown the presence of 2Mp since it is used as a solvent in coccidiostats manufacture [1]. 2Mp is used as a solvent in dyeing and rinsing baths of cotton. Compounds with 2Mp pass completely unaltered through the conventional activated sludge processes of biological treatment due to non-mineralization of refractory organics and poor BOD removal [2]. While one time exposure to low concentrations of 2Mp (0–5 mg L−1) does not bio-magnify, repeated exposure to 2Mp contaminated waters can lead to the bio-accumulation of the neurotoxin, in turn leading to endocrine disrupting activity and multiple organ failure in humans [3]. Acute exposure can lead to suppression of central nervous system, severe depression and physical exhaustion [4]. Thus, there arises the need to curb the presence of 2Mp from waterbodies.
Methylpyridine concentration of 12.26 mg L−1 has been found in coking wastewaters, although a concentration lesser than 1 mg L−1 is recommended by the industry [5]. Wastewater from tar plants was recorded to compose of a high concentration of 2Mp in the range of 54–140 mg L-1 [6]. For industrial workers, a threshold limit value of 15 mg m-3 has been set for 2Mp exposure on a daily basis. 2Mp is poorly degraded in the natural environment, and its removal is made harder because it bears no net charges [7,8]. It is pertinent to restrict the discharge of 2Mp bearing wastewaters into subsurface water sources. A concentration limit of 0.316 mg L-1 and 1 mg L−1 has been set in this regard by the US-EPA and the UK standards for heterocyclic bases. Another finding showed that even 0.5 mg L−1 of 2Mp in waterbodies is enough to make fishes unsuitable for human consumption [3]. The Indian Central Pollution Control Board (CPCB, India) addresses this point by labelling pyridinic pollutants as dangerous odorous nuisances in water classes SW-I and SW-II (regarding marine and coastal outfalls). It also advises against presence of these pollutants in ecologically sensitive zones and bath waters [9].
Literature on 2Mp removal focuses on removal of high concentration (100−500 mg L−1) of 2Mp from aqueous solutions [7,8]. Such low volume, high concentration (LV-HC) discharge of 2Mp is observed from industries where 2Mp is used as a primary chemical component in manufacture, for e.g. pharmaceuticals. For textile industries utilizing 2Mp as a solvent with a high volume, low concentration discharge (HV-LC), designing a LV-HC system with a high safety coefficient is unnecessary, resulting in over utilization of a large volume of adsorbent [8,10]. This in turn leads to production of a large footprint of spent adsorbent, drastically increasing hazards associated with disposal. Prior to conceptualization of this study, HPLC analysis of freshly discharged treated dye house effluent from a textile industry detected the presence of 2Mp. It was found to contain 2Mp in concentrations greater than the recommended limit. This finding is of concern because in developing and under developed countries, textile plants of capacities greater than 1000 m3 per day consistently discharge effluents into storm drains that lead them directly to local water bodies. Government policies focus primarily on LV-HC 2Mp concentrations whilst not considering HV-LC systems, thus creating an exploitable loophole. This study involving removal of HV-LC 2Mp concentrations from simulated synthetic solutions is thus of importance.
Researchers have attempted to remove azaarenic alkylpyridines by the application of adsorptive methods using coconut shells, amorphous silicon dioxide, Jordanian zeolites and kaolin [7,8,10]. All of these materials require high adsorbent doses for the treatment of 2Mp, which in turn leads to production of a large volume of spent adsorbent [11]. The large footprint of the spent adsorbent is hazardous to dispose of into landfills because of the greater surface area available for leaching of previously adsorbed species. Activated carbon is also structurally weak, further increasing risk of structural disintegration and fouling [12]. Thus, the need for development of an improved adsorbent with low dosage, better performance and structural stability was felt. New age sorbents like graphene nanocomposites have shown great promise in this field and their utilization in uptake of refractory industrial solvents has been lacking in literature [13]. High adsorptive capacities have been observed in graphene based nanomaterials which contain sp2 hybridized carbon atoms as building blocks of honey-comb like structures (similar to fullerenes and carbon nanotubes) [[14], [15], [16], [17]]. Applications for similar products have been observed in electrochemical sensors with high selectivity towards a given species [[18], [19], [20], [21]], electronics, medicine and contamination curtailment [[22], [23], [24], [25]]. As an adsorbent, graphene oxide has shown great potential in removal of emerging organic contaminants like nitroaromatics, tetracyclines, antibiotics and dyes [[26], [27], [28]]. Apart from having a hydrophobic effect on organic pollutants, the presence of a π−π interaction between the graphene oxide structure and an organic toxicant molecule results in their removal from solution [29,30]. Graphene nanoparticles have shown an ability to exhibit high theoretical exposed surface area (2620 m2 g−1), although a loss in observed surface area to 300−1000 m2 g−1 has been reported [[31], [32], [33], [34], [35]]. To counteract the effect of accretion in graphene oxide, the process of exfoliation is used to separate the layers by mechanical, chemical or thermal means [36].
Graphene has displayed affinity towards functionalization [37]. When functionalized, it has shown remarkable capacity for sorption of toxic pollutants [38]. Carboxylic functionalization of exfoliated graphene oxide nanocomposite results in a mono layer of highly oxidized structure bearing carboxylic and hydroxylic functional group in the basal plane. This has been known to increase efficiency of organic pollutant adsorption. It is also reported to improve alignment of GON sheets in crystallites that helps in control of crystallite size. Better thermal control in GON synthesis also preserves structural integrity of basal planes of GON layers. Thermal control also helps to drastically make the synthesis process of graphene oxide safe with minimization of exothermicity of the reaction and SO2 emission. The use of carboxyl graphene has been very limited in industrial effluent extraction [39].
Optimal utilization of the sorbent is of prime importance for its conservation, reuse and disposal. In real life situations, impact of factor interactions on removal efficiency of the sorbent hold greater importance than single factor batch studies. These objectives are realized by mathematical modelling of sorption by the process of design of experiments (DOE), analyzing the impact of each controllable factor affecting adsorption [40,41]. Full factorial method lays excessive stress on each individual factor interaction resulting in large number of experiments. Fractional factorial on the other hand is unable to gauge factor interactions with precision [42]. Another hindrance is the usage of a fixed number of levels per factor, even though each factor affects adsorption to a different degree. Experimental data optimization by non-linear Doehlert design was thus deemed appropriate since it combines fewer experimental runs with relatively low error in output prediction and employs varying number of levels for each factor depending upon their impact on adsorption [43]. In this study, 2Mp removal by GON was optimized by using Doehlert design matrix formulation which utilizes uniform distribution of experimental points chosen from the predetermined four factor DD matrix [44]. The predicted results from the developed mathematical model were corroborated by results of batch studies. Response surfaces and contours were plotted for visualization of multi-factor effects on removal. These studies establish GON as an effective adsorptive media with great potential in industrial solvent treatment.
This study was conducted to, (i) detect the solvent 2Mp in treated industrial effluent samples, (ii) synthesize the advanced adsorbent, exfoliated carboxyl graphene oxide nanocomposite and achieve thermal control in graphene oxide synthesis, (iii) compare the sorption capacity, adsorption mechanism and structural stability of GON with activated carbon SAC for uptake of 2Mp at low concentrations (2.5–20 mg L−1), and (iv) optimize the adsorption process by 4-factor Doehlert design matrix for understanding single factor and interaction effects.
Section snippets
Materials
GON was synthesized in the laboratory along with all associated reagents reported elsewhere [45]. 2-Methylpyridine was procured from Sigma-Aldrich Chemical Co., Missouri, USA. SAC was obtained from Central Drug House, India. Ammonium hydroxide (NH4OH), graphite powder, potassium permanganate (KMnO4), concentrated sulfuric acid (H2SO4), orthophosphoric acid (H3PO4), hydrogen peroxide (H2O2) and chloroacetic acid (C2H3ClO2) were obtained from Merck, Germany. Textile dye house wastewater
Characterization of adsorbing materials
The physico-chemical properties of average particle size, proximate analysis parameters and pHzc for GON and SAC are listed in Table 2. GON showed higher fixed carbon content, along with lower composition of other entities. The pHzc were found to lie in the acidic region, with pHzc(GON) < pHzc(SAC). Upon analysing other recorded studies of pHzc of graphene oxide [51,52], while one study reported a similar pHzc value, another reported a lower pHzc. This difference in results could be attributed
Conclusion
This study concludes, based on batch studies and mathematical optimization that GON can be used to replace traditional adsorbent (SAC) in treatment of industrial solvent 2Mp in the concentration range of 2.5–20 mg L−1. A safe thermally controlled process for synthesis of exfoliated carboxyl graphene oxide nanocomposite was suggested. Characterization of GON was used to enunciate the structure, crystallite/particle size and mechanism of 2Mp sorption. The exfoliation, carboxyl functionalization
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank IIEST Shibpur along with MHRD India and TEQIP-III for providing facilities for carrying out conducive research.
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