Article pubs.acs.org/est
Mineralization Behavior of Fluorine in Perfluorooctanesulfonate (PFOS) during Thermal Treatment of Lime-Conditioned Sludge Fei Wang,† Kaimin Shih,†,* Xingwen Lu,† and Chengshuai Liu†,‡ †
Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong SAR, China Guangdong Institute of Eco-Environmental and Soil Sciences, Guangzhou 510650, China
‡
S Supporting Information *
ABSTRACT: The fate and transport of the fluorine in perfluorooctanesulfonate (PFOS) during the thermal treatment of lime-conditioned sludge were observed using both qualitative and quantitative X-ray diffraction techniques. Two main fluorine mineralization mechanisms leading to the substantial formation of CaF2 and Ca5(PO4)3F phases were observed. They had a close relationship with the thermal treatment condition and the PFOS content of the sludge. At low temperatures (300−600 °C), CaF2 dominated in the product and increases in treatment time and temperature generally enhanced the fluorine transformation. However, at higher temperatures (700−900 °C), increases in treatment time and temperature had a negative effect on the overall efficiency of the fluorine crystallization. The results suggest that in the high temperature environment there were greater losses of gaseous products such as HF and SiF4 in the transformation of CaF2 to Ca5(PO4)3F, the hydrolysis of CaF2, and the reaction with SiO2. The quantitative analysis also showed that when treating sludge with low PFOS content at high temperatures, the formation of Ca5(PO4)3F may be the primary mechanism for the mineralization of the fluorine in PFOS. The overall results clearly indicate the variations in the fate and transport of fluorine in PFOS when the sludge is subject to different PFOS contents and treatment types, such as heat drying or incineration.
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Germany9 and Sweden10 has revealed that PFCs in leachates can reach as high as 12 000 ng/L and 30 000 ng/L, respectively. Furthermore, it has been reported that the groundwater around landfill sites holding PFC-containing waste can be contaminated by PFCs.11,12 The environmental risks of PFCs derived from the land application of municipal wastewater sludge (biosolids) are also not well studied. However, one recent study13 has shown the high transport potential of PFCs in soils amended with municipal biosolids, and the possible contamination of water and food is a primary concern for human health. In addition to landfill and land application, thermal treatment (such as incineration) is another strategy for solid waste management, which is very effective in breaking the robust halogen-carbon bonds in waste compounds.14,15 The thermal chemistry of perfluorochemicals has been reported16−18 in previous studies. Fabes and Swaddle16 introduced the high temperature chemistry of trifluoromethanesulfonate acid and its salts in aquatic conditions. Krusic and Roe17 investigated the thermal decomposition kinetics of ammonium perfluorooctanoate using gas-phase NMR technique. A study by Yamada and
INTRODUCTION Perfluorochemicals (PFCs) are a type of pollutant with highenergy carbon−fluorine (C−F) bonds that render them persistent in the environment.1 One of the most common PFCs is perfluorooctanesulfonate (PFOS), which was listed as a persistent organic pollutant (POP) in Annex B of the Stockholm Convention in 2009.2 It is estimated that between 1972 and 2002, 122 500 tons of perfluorooctylsulfonyl fluoride (POSF, a major precursor of PFOS) were produced.3 Direct emission from POSF-derived products such as stain repellent treated carpets, waterproof apparel, and aqueous fire-fighting foamsresulting in the release of 450−2700 tons of PFOS into wastewater streamswas the major source of the environmental pollutants. Due to the persistent and wide distribution of PFCs in wastewater streams, the fate and transport of PFCs (such as PFOS) in wastewater treatment plants has recently attracted attention. Many studies4−8 have shown that wastewater treatment sludge is a sink for environmental PFCs. Therefore, the process used for waste sludge treatment plays a crucial role in determining the subsequent fate and transport of PFC pollutants in the environment. Like most types of solid waste, waste sludge is commonly disposed of in landfills. However, many recent studies have confirmed the significant environmental impact of PFC-bearing waste in landfills. The analysis of landfill leachate from © 2013 American Chemical Society
Received: June 1, 2012 Accepted: January 28, 2013 Published: January 29, 2013 2621
dx.doi.org/10.1021/es305352p | Environ. Sci. Technol. 2013, 47, 2621−2627
Environmental Science & Technology
Article
Taylor18 has demonstrated that PFOS can be thermally decomposed at temperatures above 600 °C, and that the final products are tetrafluoromethane (CF4) and hexafluoroethane (C2F6). Unfortunately, the generated CF4 and C2F6 compounds are potent greenhouse gases. The global warming effects of CF4 and C2F6 are 6500 and 9200 times higher than that of CO2, and their atmospheric lifespans are 50 000 years and 10 000 years, respectively.19 Therefore, a more sustainable treatment method for PFC-containing sludge that can effectively decompose the PFC compounds and also reduce the subsequent emission of potent CF4 and C2F6 greenhouse gases is required. Previous mechanistic studies of the chemical interaction between PFOS and Ca(OH)2 under thermal conditions have shown that the mineralization of PFOS produces CaF2 when assisted by Ca(OH)2.20 During sludge conditioning and stabilization, a large amount of lime is often added to the sludge to raise its pH or to reduce the levels of odor-causing microorganisms and pathogens. In addition, Ca(OH)2 is usually injected into the furnace chamber or postcombustion zone of an incinerator system to reduce the pollution of acidic gases such as SO2 and HCl.21 Therefore, the fluorine transformation of PFC-containing sludge initiated by Ca(OH)2 is promising, but the mineralization behavior of PFC compounds in a sludge matrix has not been investigated. In this study, the wastewater sludge collected from the wastewater treatment plant (WWTP) in Hong Kong was spiked with PFOS to observe its interaction with Ca(OH)2 in a sludge matrix under thermal conditions. A quantitative X-ray diffraction (XRD) analysis based on the Rietveld refinement method with the addition of the corundum (α-Al2O3) internal standard (NIST SRM 676a) was used to observe the efficiency of the fluorine mineralization. The operational parameters of the thermal treatment process, such as the retention time, temperature, and Ca/F molar ratio, were systematically optimized from observations of their influences on product phase compositions. The gas phase products after thermally treating PFOS-containing samples were analyzed by GC/MS. Furthermore, this study identified the fluorine mineralization pathways that were established during the thermal treatment of PFOS in lime-conditioned sludge, providing a better understanding of PFC mineralization behavior in sludge thermal treatment.
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Figure 1. The XRD results of (a) dry municipal WWTP sludge (ST dry sludge), sludge + Ca(OH)2, and sludge + Ca(OH)2+ PFOS samples, heated at (b) 400 °C and (c) 900 °C for 15 min. The mineralization of fluorine into crystalline phases was observed in the end products of the thermal treatment.
To analyze the gas products after PFOS thermal treatment, four PFOS-containing samples (PFOS, PFOS + Ca(OH)2, PFOS + Sludge, and PFOS + Ca(OH)2 + Sludge) were placed in glass columns and heated in a muffle furnace. With 15 min of thermal treatment, the 0.5 L Tedlar plastic bag (Plastic Film Enterprises, MI) was used to collect the gas products emitted from the sample column for GC/MS analysis. To determine the PFOS residue in solid, 10 mg solid was extracted by 10 mL methanol in a 60 °C sonication bath for 30 min. The solution was then centrifuged for 5 min with its supernatant analyzed by LC/MS/MS. XRD Analysis. The phase transformation after the heat treatment was determined using the powder XRD technique. XRD patterns were recorded using a D8 Advance Diffractometer (Bruker AXS) equipped with a Cu X-ray tube and a LynxEye detector. The diffractometer was operated at 40 kV and 40 mA, and the 2θ scan range was from 10° to 80°, with a step width of 0.02° and a scan speed of 0.3 s/step. The system was calibrated for the line position using the Standard Reference Material (SRM) 660a (lanthanum hexaboride, LaB6) from the U.S. National Institute of Standard and Technology (NIST). Qualitative phase identification was performed with EVA XRD Pattern Processing software (Bruker Co. Ltd.); powder XRD patterns were matched with those retrieved from the standard powder diffraction database of the
EXPERIMENTAL SECTION
Chemicals and Sample Preparation. PFOS and Ca(OH)2 were purchased from Sigma-Aldrich, and the mineral phase of Ca(OH)2 was confirmed with the XRD method (Supporting Information (SI) Figure S1). A sludge sample collected from the secondary wastewater treatment plant in Hong Kong, was dried at 105 °C for 24 h, and then ground to homogeneous fine particles by ball milling. The dried sludge sample was examined by XRD to determine its initial crystal phases (Figure 1(a)) and analyzed by X-ray fluorescence (XRF) (JEOL JSX-3201Z) for elemental information (SI Figure S2). To observe the thermal mineralization behavior of PFOS in lime-conditioned sludge, 0.70 g fine sludge particles and 0.30 g Ca(OH)2 with different PFOS contents (0.016−0.256 g) were mixed and combusted in a muffle furnace that was preheated to targeted temperatures (300−900 °C). The period of thermal treatment ranged from 1 to 30 min, and the samples were then air-quenched, weighted, and ground (to particle size
