Enhanced adsorption of perfluorooctane sulfonate and perfluorooctanoate by bamboo-derived granular activated carbon
Graphical abstract
Introduction
PFOS and PFOA are the typical perfluorinated compounds (PFCs), and they have been produced and used in many industries. Although PFOS was listed as one of the persistent organic pollutants in 2009 [1], it is still allowed to be used in some special industries such as chrome plating in China. PFOA is often used as an emulsifier in the production of fluoropolymers [2], [3], and it is also used in semiconductor industry. The wastewater from the photolitho process was estimated to have 1000 mg/L of PFOA [4]. It has been reported that the concentrations of PFOS and PFOA in surface water near industrial zone were up to 5.7 μg/L and 19.2 μg/L, respectively [5]. Therefore, it's very important to find some effective techniques to remove PFOS and PFOA from industrial wastewater and contaminated surface water.
Adsorption is an effective method to remove PFOS and PFOA from wastewater [6], [7], [8], and some efficient adsorbents such as activated carbon, resin [7], [9], chitosan [10], [11], hydrotalcite [5], alumina [12], and organo-montmorillonite [13] have been reported. Among these adsorbents, the anion-exchange resins and crosslinked chitosan have the highest sorption capacity for PFOS and PFOA, but the sorption velocity is not satisfactory due to the slow intraparticle diffusion in the porous materials [7], [9], and especially the two adsorbents are expensive. In consideration of actual application, the cheap activated carbon is preferred. The granular activated carbons (GAC) have the sorption capacity of 160–229 mg/g for PFOS and 112–161 mg/g for PFOA [6], [7], [14]. The powdered activated carbons (PAC) have the sorption capacity of 374–550 mg/g for PFOS and 175–524 mg/g for PFOA [5], [7], [15], [16]. It is obvious that the PAC has higher adsorption capacity for PFOS and PFOA than the GAC. Similarly, the PAC has faster adsorption velocity than the GAC. The reason is that these commercially available activated carbons mainly contain micropores, unfavorable for PFOS/PFOA diffusion and adsorption in the inner pores of the GAC [7]. The formed micelles or hemi-micelles of PFOS and PFOA may block the micropores of activated carbon, making some inner surfaces unavailable for adsorption [7]. Although the PAC has the relatively fast and high adsorption of PFOS and PFOA, it cannot be reused due to the difficult separation of powder. Therefore, it is a challenge to prepare GAC with fast and high adsorption of PFOS and PFOA. If the prepared GAC has bigger pores than the commercially available GAC, the adsorption of PFOS and PFOA may be enhanced.
When bamboo was used as the precursor to prepare the GAC, many micron-sized pores were formed in the adsorbent prepared by KOH activation [17], which should be favorable for PFOS and PFOA diffusion in the activated carbon. If the micropores in the bamboo-derived activated carbon are enlarged, they should facilitate the adsorption of PFOS and PFOA, resulting in the high adsorption. In this study, the bamboo-derived GAC was optimized to have high sorption capacity for PFOS and PFOA, and then the batch adsorption experiments including adsorption kinetics, isotherms and effect of solution pH were conducted to investigate their adsorption behavior on the optimum activated carbon. Finally, the regeneration of the spent activated carbon was also evaluated using different regeneration agents. The novelty of this study is to prepare the bamboo-derived activated carbon through KOH activation to achieve the fast and high adsorption of PFOS and PFOA, and the suitable regeneration agents were also found to successfully regenerate the spent activated carbon. Although the bamboo-derived activated carbons prepared by different activation methods have been studied before [17], [18], this activated carbon prepared via KOH activation for PFOS/PFOA adsorption was not reported.
Section snippets
Chemicals and materials
The bamboo was obtained from a local market in Beijing, and crushed and sieved into the particle size in the range of 20–30 mesh (0.85–0.60 mm). Perfluorooctane sulfonate (PFOS, potassium salt) and perfluorooctanoate (PFOA, sodium salt) were purchased from Tokyo Kasei Kogyo Co., Ltd. (Japan), and their properties are listed in Table 1. Other chemicals including KOH, HCl, NaCl, methanol and ethanol were purchased from Beijing Modern Eastern Fine Chemical Co., Ltd., and they were reagent grade.
Preparation of bamboo-derived GAC
In
Preparation of bamboo-derived activated carbon
The KOH/C mass ratio and activation temperature have significant effects on the pore size distribution of the bamboo-derived activated carbon, which is closely related to the adsorption of PFOS. The adsorbed amounts of PFOS on the activated carbons increased with increasing KOH/C mass ratio and activation temperature (Fig. 1). When the activated carbons were prepared at 800 °C and different KOH/C ratios of 1–6, the adsorbed amounts of PFOS increased rapidly at the KOH/C ratios below 4 and then
Conclusions
The bamboo-derived GAC prepared under the severe activation condition contained the enlarged pores, which facilitated the diffusion and adsorption of PFOS and PFOA in the granular adsorbent. The adsorption equilibrium of PFOS and PFOA on the bamboo-derived GAC was achieved within 24 h, faster than other granular adsorbents reported. This GAC had the adsorbed amounts of 2.32 mmol/g for PFOS and 1.15 mmol/g for PFOA at pH 5.0, much higher than other PAC and GAC reported. Increasing the size of
Acknowledgments
We thank the National High-Tech Research and Development Program of China (Project no. 2013AA06A3) for financial support. Additionally, the analytical work was supported by the Laboratory Fund of Tsinghua University (Project no. LF20135412).
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