Influence of the pore structure and surface chemical properties of activated carbon on the adsorption of mercury from aqueous solutions

https://doi.org/10.1016/j.marpolbul.2013.11.007Get rights and content

Highlights

  • Activated carbons with different pore structure and surface chemical properties were prepared by modification process.

  • HgCl2 as a pollution target to evaluate the adsorption performance.

  • Influence of pore structure and surface chemical properties of activated carbon on adsorption of mercury was investigated.

Abstract

Reactivation and chemical modification were used to obtain modified activated carbons with different pore structure and surface chemical properties. The samples were characterized by nitrogen absorption–desorption, Fourier transform infrared spectroscopy and the Bothem method. Using mercury chloride as the target pollutant, the Hg2+ adsorption ability of samples was investigated. The results show that the Hg2+ adsorption capacity of samples increased significantly with increases in micropores and acidic functional groups and that the adsorption process was exothermic. Different models and thermodynamic parameters were evaluated to establish the mechanisms. It was concluded that the adsorption occurred through a monolayer mechanism by a two-speed process involving both rapid adsorption and slow adsorption. The adsorption rate was determined by chemical reaction.

Introduction

Mercury and its related compounds discharged into industrial waste water and natural water bodies are serious concerns for all living organisms and the environment due to their high toxicity, volatility and long persistence time (Pvalish et al., 2004, Pacyna et al., 1995, Park et al., 2008). Although the use of mercury has decreased in industry, its overall emission is still a health hazard. The toxicological effects of mercury depend upon the type and amount of the mercury compound and its mode of entry into the body, which can cause emotional and other mental deterioration, blindness, loss of consciousness, involuntary mobilization, etc. The World Health Organization recommends a maximum intake of 1 μg/L and 0.3 mg/week as the maximum acceptable concentration in drinking water (Forster and Wase, 1997). Therefore, mercury-control techniques for aqueous solutions have become a research focus. These techniques include precipitation, ion exchange, reduction, reverse osmosis, adsorption, etc. Among these, adsorption is one of the most reliable technologies for removing mercury from water (Mohan et al., 2001, Granite et al., 2007, Miretzky and Cirelli, 2009, Feng et al., 2004).

Activated carbons have been proven to be effective adsorbents due to their developed internal pore structure, huge surface area and the presence of surface functional groups (Saha et al., 2001, Lu et al., 2012, Liu et al., 2007). Treatments such as thermal treatment and chemical treatment can modify the pore structure and the chemical nature of the activated carbon. The modification of activated carbon is an attractive route towards the novel application of these materials as both liquid-phase and gas-phase adsorbents, as well as catalyst supports (Han et al., 2003, Abdel-Nasser, 2003, Lu et al., 2011, Lin et al., 2006). In recent years, activated carbon has been used as an effective adsorbent to remove mercury. In particular, there are a great number of studies regarding the mercury-adsorption performance of activated carbon impregnated with sulfur, halogens, etc. (Graydon et al., 2009, Wenguo et al., 2006, Lee et al., 2004, Hu et al., 2009, Liu, 2008). In spite of the great number of experiments on this subject, the application of activated carbon to control mercury contamination is limited by a lack of understanding of the effect of pore structure and surface chemical properties on mercury-adsorption performance.

The aim of this work is to investigate the influence of pore structure and chemical properties on the adsorption performance of activated carbon for mercury and also to gain a better understanding of the adsorption mechanism. To this aim, a comprehensive experimental analysis of mercury adsorption onto activated carbons was carried out to identify a potential correlation between adsorption capacity and the properties of activated carbon. Adsorption models were also used to investigate the adsorption mechanism.

Section snippets

Materials

Coconut activated carbon (SBET 797 m2/g, Vtot 0.389 cm3/g) was used as the starting material, which was produced by MU LIN SEN Activated Carbon Co., Ltd. Before the modification processes, activated carbon was washed with deionized water, dried at 110 °C for 5 h and marked as AC.

Regulation of pore structure

AC was heated in a nitrogen atmosphere to an activated temperature of 850 °C at a heating rate of 10 °C/min. After reaching the activation temperature, nitrogen gas was stopped and water vapor was used (1.20 g/min) to

Adsorption–desorption isotherms

The analysis of the nitrogen adsorption–desorption isotherms of the samples is shown in Fig. 1. As shown in Fig. 1, all the nitrogen adsorption–desorption isotherms of the samples belong to type IV. In the low-pressure region where p/p0  0.1, the nitrogen uptake was significant and the adsorption capacity increased rapidly with increasing pressure. Adsorption saturation was reached quickly, which indicated that samples contained micropores. In the high-pressure region, a hysteresis loop was

Conclusions

In this paper, experiments on the adsorption of mercury onto activated carbons were carried out to analyze the influence of the pore structure and surface chemical properties on adsorption performance and to understand the adsorption mechanism. The results obtained show that the specific surface area was not a key factor for mercury adsorption but that the ratio of micropores and acidic surface functional groups influenced the adsorption capacity. The kinetics studies presented here show that

Acknowledgements

This research is financially supported by a Central-level Scientific Research Institutions basic funding grant (CAFINT2013C02) and the Natural Science Foundation of Jiangsu province (BK2012514).

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