Adsorption and desorption of SO2, NO and chlorobenzene on activated carbon

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Abstract

Activated carbon (AC) is very effective for multi-pollutant removal; however, the complicated components in flue gas can influence each other's adsorption. A series of adsorption experiments for multicomponents, including SO2, NO, chlorobenzene and H2O, on AC were performed in a fixed-bed reactor. For single-component adsorption, the adsorption amount for chlorobenzene was larger than for SO2 and NO on the AC. In the multi-component atmosphere, the adsorption amount decreased by 27.6% for chlorobenzene and decreased by 95.6% for NO, whereas it increased by a factor of two for SO2, demonstrating that a complex atmosphere is unfavorable for chlorobenzene adsorption and inhibits NO adsorption. In contrast, it is very beneficial for SO2 adsorption. The temperature-programmed desorption (TPD) results indicated that the binding strength between the gas adsorbates and the AC follows the order of SO2 > chlorobenzene > NO. The adsorption amount is independent of the binding strength. The presence of H2O enhanced the component effects, while it weakened the binding force between the gas adsorbates and the AC. AC oxygen functional groups were analyzed using TPD and X-ray photoelectron spectroscopy (XPS) measurements. The results reveal the reason why the chlorobenzene adsorption is less affected by the presence of other components. Lactone groups partly transform into carbonyl and quinone groups after chlorobenzene desorption. The chlorobenzene adsorption increases the number of C = O groups, which explains the positive effect of chlorobenzene on SO2 adsorption and the strong NO adsorption.

Graphical abstract

Oxygen surface functional groups on AC and the combine mechanism between SO2/NO/Chlorobenzne with carbon

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Introduction

The constant levels of huge energy consumption in China have produced serious levels of air pollution. There is an extremely urgent need for multi-pollutant control technologies with much stricter air pollutant emission standards for application to industrial kilns and furnaces. For the existing iron and steel industry, the effluent concentration of used sintering flue gas is restricted to 200 mg/m3 for SO2, 300 mg/m3 for NO, and 0.5 ng-TEQ/m3 for dioxin in GB 28662–2012. Compared with traditional limestone-gypsum wet flue gas desulfurization (WFGD) and selective catalytic reduction (SCR) denitration with ammonia technologies, the activated carbon (AC) adsorption method of simultaneously capturing SO2, NOx, dioxin, mercury and other substances is more economical, consuming much less water and energy (Guo et al., 2013, Li et al., 2014, Liu and Liu, 2013, Shahkarami et al., 2015).

The components of flue gas are very complicated, including large amounts of SO2 and NOx, with H2O, volatile organic compounds (VOCs) and other substances as well. Thus, one component may affect the adsorption of another component over AC. Previous studies have revealed that SO2 and NOx affect each other's adsorption. The presence of SO2 inhibits NO adsorption due to its higher permanent dipole moment and polarizability (Yi et al., 2012), whereas NO promotes SO2 adsorption through the formation of intermediate species [(NO2)(SO3)] (Tang et al., 2005). The co-adsorption of butane (C4H10) and NO2 or SO2 onto AC reduces the adsorption capacity for oxide components, but the butane adsorption is not influenced by SO2 or NO2 (Ahnert and Heschel, 2002). SO2 and NO removal on AC is a complicated process that involves adsorption and catalysis with various adsorption sites occupied, whereas the adsorption of polychlorinated biphenyl over AC is mainly physical adsorption (Kawashima et al., 2011). Thus, co-adsorption can limit the adsorption amount for certain components, and the adsorption behavior differs because of the gas properties.

Water vapor is frequently present at high concentrations in flue gases and may influence the AC adsorption behaviors. Water adsorption isotherms in carbons are type V in the IUPAC (International Union of Pure and Applied Chemistry) classification, and the sharp rise in the water isotherm has been ascribed to the coalescence of clusters of hydrogen-bonded water molecules (Striolo et al., 2005). SO2 adsorption on AC is marginally increased at above 30 vol.% of moisture in the atmosphere, as the oxidation of SO2 to SO3 is followed by hydration to H2SO4, and the presence of moisture can promote the release of vacant sites (Gaur et al., 2006). Water vapor has a partial inhibition effect on NO adsorption (Klose and Rincón, 2007). The adsorption capacity for trichloroethylene is almost equal under dry and moisture atmospheric conditions, but water vapor has a negative influence on n-butane adsorption (Marbán and Fuertes, 2004, Lee et al., 2005). The effects of moisture on single components have been widely discussed. Moreover, the influence of water on multi–components in situations involving their simultaneous presence is worthy of investigation.

In this work, a series of adsorption experiments with SO2, NO, H2O and chlorobenzene, as a volatile organic compound, on AC under various gas components were performed in a fixed-bed reactor to investigate the influence of the complicated flue gas components on each component's adsorption. Temperature-programmed desorption coupled with mass spectrometry (TPD-MS) measurements were adopted to analyze the adsorption sites of each component and the carbon surface functional groups, and then to reveal the interaction mechanisms between pairs of components.

Section snippets

Activated carbon sample

The commercial coconut-shell AC used in the experiment was from the Gongyi activated carbon plant in Henan province. The AC with particle sizes of 38–62 μm was dried at 393 K for 10 hr before the experiment. The specific surface area was 980.7 m2/g, as calculated from the N2 adsorption isotherms using the BET (Brunauer–Emmett–Teller) equation. N2 adsorption was performed at 77 K in an automatic surface area and porosity analyzer (AutosorbiQ, Quantachrome, USA). The total pore volume was 0.480 mL/g,

Single component adsorption

Fig. 1 shows the SO2, NO and chlorobenzene adsorption isotherms over the AC. The SO2 and NO adsorption amounts increase as the adsorbate concentration increases, with the line still climbing even as the concentrations increase over 0.5 mol%. The amount of chlorobenzene adsorption rapidly increases at chlorobenzene concentrations below 0.1 mol%, whereas it slows over 0.1 mol%. The adsorption amount at the highest concentration is 178.3 mg/g for chlorobenzene, 75.8 mg/g for SO2, and 38.1 mg/g for NO.

Conclusions

The SO2 and NO adsorption can be fit with Freundlich isotherms, indicating that the adsorption sites are heterogeneous. Chlorobenzene adsorption, described by a Langmuir isotherm, belongs to micropore adsorption. In a single-component atmosphere, the adsorption amount is 65.2 mg/g for chlorobenzene, 30.9 mg/g for SO2, and 11.3 mg/g for NO. The binding strength between the gas molecules and AC follows the order of SO2 > chlorobenzene > NO. The adsorption amount is independent of the binding strength.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21177129, 21207132) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB05050502).

References (28)

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