Environment
5.1 Introduction
Anthropogenic, black particulate matters generally dubbed as black carbon (BC) is the most significant, aerosolic light-absorbing material and defined in the literature as the most concerned noxious waste next to the carbon dioxide [1–9]. BC emitted globally via the combustion of fossil, biofuel, and waste biomasses in both outdoor and indoor atmosphere [5, 6, 10]. Outdoor BC constitutes a major portion of diesel particulate matters (DPMs) [11] and are very compelling because of its enormous adverse impacts [12], in terms of concerning global warming, promoting air pollution, and several health issues [8, 13, 14]. Few examples of the unusual behavior occurred due to outdoor BC are the frequent observation of sudden warming and cooling the environment [15]. Such as summer in south China, drought in north China, dust storms in India [16], and high-density fog formation in winter season particularly in urban area [17]. The rapid increase in industrialization and the use of automobile transportation with continuous advances in technology increases the density of BC in the outdoor atmosphere. Figure 5.1a and b demonstrates a comparison of clean and polluted environment of USA and Canada border in the months of September 2005 and January 2006, respectively. In order to analyze the structural identities and shapes of BC particulates, Murr et al. [9] collected a variety of BC (both from indoor and outdoor atmospheres) directly on transmission electron microscope (TEM) grid. This BC was characterized by various microscopic techniques. Such as TEM and scanning electron microscopy (SEM) as illustrated in Figures 5.1c, d, and g and 5.1e and h, respectively. TEM image along with its SAED pattern, shows the presence of prominent graphite reflection rings for the sample collected from diesel engine exhaust is shown in Figure 5.1c. Figure 5.1e and g demonstrated the SEM, TEM characterization of soot collected from candle and wood burning, respectively. Elementally, BC is carbonaceous in nature having the mixture of different degree of graphitic/amorphous carbon depending upon the quality and composition of fuel, with its burning conditions. Not only the outdoor BC, anthropogenically generated indoor BC (in confined areas), such as burning candles, incenses are significantly affecting the indoor air-quality parameters, causing several respiratory problems [18], importantly associated with the generation of
reactive oxygen species (ROS) [5, 7, 9].
Figure 5.1 Early morning view (looking southwest) of particulate/smoke inversion on the El Paso, TX, USA/Juarez, Mexico border region (just north of the city centers); (a) Clean day view for reference in September 2005 and (b) inversion in January 2006 in same area. Atmospheric (outdoor) BC
collections; (c) TEM image of soot (with SAED pattern), aggregate collected at highway truck stop and presumed to be diesel BC; (d) soot BC and
fragments of chrysotile asbestos collected along interstate highway
traversing; (e) SEM image of candle soot collected on glass fiber filter; (g) corresponding TEM image with its SAED pattern in (f) of the same candle soot, as in (e); (h) SEM image of carbon dots (CDs) derived from wood soot collected on a glass fiber filter [9].
(Reprinted with permission)
Atmospheric, particulate matters are generally divided into three categories based upon their particle size, PM10 (diameter < 10 μm), PM25 (diameter <
2.5 μm) and PM0.1 (diameter < 0.1 μm) [19]. Out of these BC falls under the
category of PM2.5 and are smaller [5]. Due to its smaller size and high volume-to-surface ratio, ultra-small-sized particulates (PM0.1) are much more hazardous than others [13]. As they are showing an active response to the rapid adsorption of potentially toxic polycyclic aromatic hydrocarbons, which can be easily and deeply penetrates inside the alveolar system of our lungs [19] and increases health hazards, such as related cardiovascular dysfunction, lung cancer [8, 9], chromosomal and DNA damage [12].
Based on the earlier discussed issues, ecological awareness for the stringent control over the emissions of BC and alternatively its second life use into valuable carbon nanostructures [7] have created a great interest. Till now, except few reports [20], most of the scientific groups have focused on the chemical, structural characterization [15, 21–23] of BC soot generated via diesel engine exhaust and other anthropogenic sources. Knauer et al. used Raman, high-resolution TEM (HRTEM), temperature-programmed oxidation, and electron energy loss spectroscopy (EELS) techniques to investigate the effect of oxidation condition at different temperature with the structural variation and reactivity of soot particles [24]. Müller et al.
investigated the impact of microstructures upon the oxidation behavior of soot obtained from different exhausts of the diesel engine by using HRTEM and thermogravimetric analysis [25]. In his another report, they analyzed structural and electrical characterization of soot collected from different sources using HRTEM, EELS, and XPS (X-ray photoelectron spectroscopy) [26]. They did a correlation study between microstructure and electronic structure of soot particles and concluded that increased “surface defects” [27, 28] help in the incorporation of heteroatom such as oxygen and hydrogen [26]. Roden et al. did real-time field measurements and studied the optical properties from traditional wood burning cook stoves [29]. They proposed that variables such as temperature, moisture, time, fuel size, and design of stoves are significantly affected the emission factor and change in the chemical composition of emitted BC.
Recently, few groups were utilized this pollutant soot BC as “carbon precursor” for the synthesis of nano-carbons, which displays the potential for a wide range of multifunctional applications [20, 30–33], especially for outdoor collected BC. Such as for imaging [30], heavy metal ion sensing [31], biosensors, and wastewater treatment [34–37]. Among nano-carbons,
conversion of BC to graphitic CDs, graphene quantum dots (GQDs) [38], and carbon quantum dots exhibiting tunable multicolored emissions [39–41] with novel physicochemical and optical properties [42] could be the simplest approach. As mentioned in the literature, CDs were synthesized by a variety of existing methods including both top-down and bottom-up approaches [27, 43–48]. More significantly, the use of different kinds and sometimes unusual variety of precursor materials and diversity in synthetic techniques confirms the reproducible nature of CDs with gram scale yield [41, 49–51]. Regarding the potential properties of fluorescent CDs [41, 52], these are reasonably competitive with conventional metal-based QDs (in terms of high quantum yield value) [53–55], along with its bio-compatibility [56 57]. Besides imaging [39, 40, 58, 59], these also exhibiting several other potential applications, such as in electronic devices [60], batteries [61, 62], super- capacitors [63], photo-catalysis [41, 64], sensors and actuators [49], drug delivery [65], cancer treatment [66], and acting as plant growth promoter [44]. As well used for sensing and separation of a variety of organic and inorganic contaminants from water and could play a significant role in water purification [36, 38, 51].
Uchida et al. used diesel soot [20] as “carbon precursor” for the synthesis of SWCNTs using laser vaporization technique [67]. Wang et al. synthesized fluorescent water-soluble carbon nanoparticles (wsCNPs) from the diesel soot [31] and used these for metal sensing. Sarkar and coworkers used diesel engine exhaust waste soot as DPM for the synthesis of water-soluble fluorescent CDs (wsCDs) with near infrared (NIR) emissions [30]. Their approach toward the use of waste soot is very simple, efficient, realistic, and scalable for quantitative yield synthesis (yield ~90%) of soluble CDs, and further applied for imaging Escherichia coli cells and sensing cholesterol [30]. In-shed of earlier discussed reports; importantly, we do not need to synthesize CDs. These were automatically formed during the combustion process inside the engine and exhausted globally in atmosphere as BC.
Depending upon the burning conditions and nature of oil, these can be varying in their composition and crystallinity, but this could be optimized easily by varying few parameters. Our work could be simple and straight forward, collect the soot, oxidize it and separate it based on degree of functionalization and degree of crystallinity.
Not only the air pollution another most challenging global issue for the humankind is to avoid and secure water pollution. As worldwide ~1.8 million child lost their life yearly due to diseases arises from the consumption of contaminated water [68]. Addressing the issues associated with wastewater treatment had a transformative impact, and being of great significance, if BC soot can be utilized to purify water. Currently, there is a significant demand of requirement for the robust of simple and cost-effective technique to disinfect and decontaminate water, especially in developing countries [35].
Many allotropic forms of nano-carbons are currently being used for wastewater management, such as CNTs [69, 70], graphene [71], activated carbons (ACs) [72, 73], and CDs-based nanocomposites [37]. But the wastewater treatment using CDs still needs to be addressed in detail. Till now, only few researches applied CDs in water treatment technology. Sarkar and coworkers report an effective technique for water purification by using low-cost nano-carbons and their composites [37, 70].