Fossil fuels are the major contributors to the emission of anthropogenic carbon dioxide (CO 2 ) to the atmosphere, rendering global warming a challenging issue to the researchers and industries. Although natural gas has been recommended as a clean fuel compared to other fossil fuels, geological sources of natural gas are not free of impurities. Economical commercialization of natural gas with high sour gas contents as well as facilitating the geosequestration of sour gases for enhanced oil recovery (EOR) need several environmentally sound and cost-effective gas separation methods. Moreover, stringent restrictions should be drawn to mitigate the unfettered greenhouse gas emissions to the atmosphere. In the present study, existing low-temperature conventional CO 2 capture methods, namely, cryogenic distillation process along with emerging nonconventional and hybrid methods, have been demonstrated. Also, the limitations and operational conditions during the application of these processes have been mentioned. The future prospects of the emerging technologies have been compared with conventional methods. Hybrid cryogenic distillation networks for multiproduct industrial production of different hydrocarbons and CO 2 products at higher pressures of 40 bar and above showed promising potentials. A concise classification and summary of innovative emerging technologies along with conventional methods has been presented in this paper for possible future commercial exploitation.

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High fluoride content in the natural water sources is a serious matter of concern and adsorption is recommended as one of the most convenient, affordable and widely applied defluorination technologies. In this study, a novel composite was synthesized by impregnating magnesium (Mg), manganese (Mn) and zirconium (Zr) on powdered activated carbon (AC) for effective fluoride adsorption and the synthesis was made using sonochemical method. The characterization of the prepared adsorbent AC-Mg-Mn-Zr along with individual metal composites AC-Zr, AC-Mg and AC-Mn were done by SEM, EDX, FTIR, XRD and BET analysis to understand the major functional bonds, and changes in surface chemistry after adsorption. The mechanism of the process was discussed through major reactions involved for individual metals. Due to high point of zero charge (pH = 11.9), the adsorbent was able to remove more than 96% of fluoride consistently with only 1 g/L of optimum adsorbent dosage for a wide pH range (2 to 10). The maximum adsorption capacity obtained was 26.27 mg/g within an equilibrium time of 3 h. More than 96% energy saving was achieved in the sonochemical synthesis route compared to conventional precipitation method of synthesis.

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Environmental concern associated with the side effects of high fluoride content in ground water and surface water has prompted the researchers to look for an efficient, convenient and easy method. Considering the potential of a good adsorbent, present study reports the synthesis of a composite by impregnating zirconium on powdered activated carbon (AC) using ultrasound as the tool for synthesis and applying it for fluoride adsorption from water. The nature of the composite was determined through characterization by scanning electron microscopy (SEM), energy dispersive Xray (EDX), Xray diffraction (XRD), N adsorption analysis (BET) and Fourier Transform Infrared Spectroscopy (FTIR) analysis. The pH (point of zero charge) of the adsorbent was found to be 5.03; with the optimum pH obtained at 4 for adsorption of strong electronegative fluoride ions. The initial fluoride concentration was varied from 2.5 up to 20 mg.L and the maximum adsorption capacity of 5 mg.g was obtained. A maximum fluoride removal of 94.4% was obtained for an initial concentration of 2.5 mg.L within an equilibrium time of 180 min. The adsorption isotherm followed the Langmuir isotherm model indicating a monolayer adsorption process and the adsorption kinetics followed pseudo second order model. The effects of various coexisting ions (HCO, NO, SO, Cl) commonly present in the water were found to have negligible impact on the process performance. Conducting the adsorption-desorption studies for five consecutive cycles for an initial fluoride concentration of 10 mg.L, the removal efficiency reduced from 86.2 to 32.6%. The ultrasonic method provided an easy route to synthesize the composite in less time and significantly reduced energy consumption by more than 96% compared to the conventional method.