Remediation of Perfluorooctane Sulfonate in ... - Springer Link

Water Air Soil Pollut (2013) 224:1714 DOI 10.1007/s11270-013-1714-y

Remediation of Perfluorooctane Sulfonate in Contaminated Soils by Modified Clay Adsorbent—a Risk-Based Approach Piw Das & Victor A. Arias E. & Venkata Kambala & Megharaj Mallavarapu & Ravi Naidu

Received: 15 January 2013 / Accepted: 1 May 2013 / Published online: 19 November 2013 # Springer Science+Business Media Dordrecht 2013

Abstract Perfluorooctane sulfonate (PFOS), which has numerous uses besides being an ingredient in the formulation of aqueous film-forming foams, is considered as an emerging pollutant of increasing public health and environmental concern due to recent reports of its worldwide distribution, environmental persistence and bioaccumulation potential. In an attempt to recommend a ‘risk-based’ remediation strategy, this study investigates the removal of PFOS from impacted waters and fixation of PFOS in impacted soils using a novel modified clay adsorbent (MatCARE™, patent number 2009905953). Batch adsorption tests demonstrated a much faster adsorption kinetics (only 60 min to reach equilibrium) and remarkably higher PFOS adsorption capacity (0.09 mmol g−1) of the MatCARE™ compared to a commercial activated carbon (0.07 mmol g−1). Treatability studies, performed by treating the PFOS-

contaminated soils with the MatCARE™ (10 % w/w) and then incubating at 25 and 37 °C temperatures maintaining 60 % of the maximum water holding capacity of the soils for a period of a year, demonstrated a negligible release (water extractable) of the contaminant (only 0.5 to 0.6 %). The fixation of PFOS in soils by the new adsorbent was exothermic in nature. Soils with higher clay and organic matter content, but lower pH values, retained PFOS to a much greater extent. A cost analyses confirmed that the MatCARETM could be an economically viable option for the ‘risk-based’ remediation of PFOS in contaminated waters and soils.

Guest Editors: R Naidu, Euan Smith, MH Wong, Megharaj Mallavarapu, Nanthi Bolan, Albert Juhasz, and Enzo Lombi

1 Introduction

This article is part of the Topical Collection on Remediation of Site Contamination

Over the last half century, the use of perfluorinated chemicals (PFCs) has seen a surge both in the industry and as components of consumer products such as surfactants and coatings (Busch et al., 2010; Ma and Shih, 2010). In addition, PFCs were widely and routinely used for decades in the formulation of aqueous fire fighting foams (AFFF) commonly used for combating fires caused by hydrocarbon burning (Chen et al., 2009; Mak et al., 2009). They are still being used for fire training purposes in many countries. The effluents from AFFF fire-fighting activities were neither impounded nor pre-treated prior to discharge in the environment. As a

P. Das : V. A. Arias E. : V. Kambala : M. Mallavarapu : R. Naidu CERAR—Centre for Environmental Risk Assessment and Remediation, Building X, University of South Australia, Mawson Lakes, SA 5095, Australia P. Das : V. A. Arias E. : V. Kambala : M. Mallavarapu : R. Naidu (*) CRC CARE—Cooperative Research Centre for Contamination Assessment and Remediation of the Environment, P.O. Box 486, Salisbury, SA 5106, Australia e-mail: [email protected]

Keywords Perfluorooctane sulfonate (PFOS) . MatCARE™ . Adsorption . PFOS-contaminated soil . Remediation

1714, Page 2 of 14

result, they found huge exposure into the groundwater, soils and sediments around a large number of fire training areas (Moody and Field, 1999; Moody et al., 2000). Understandably, street runoff, surface runoff, runoff from municipal and wastewater treatment plants and also runoff from fire-fighting activities are the probable direct sources of PFCs into the aqueous and soil environments (Moody and Field, 1999; Moody et al., 2000; 2002; Kim and Kannan, 2007; Becker et al., 2008; Ahrens et al., 2009; Murakami et al., 2009). Recent investigations by our group have demonstrated the presence of the eight carbon chained perfluorooctane sulfonate (PFOS; aqueous solubility 1.14 mmol L−1) in soils, groundwater and in plants growing on the edges of the disposal pond in the fire training areas of the Australian RAAF bases. Historically, PFCs were considered to be inert and thus safe, but they could actually release during certain industrial applications and during the lifetime of commercial products containing them (Wang et al., 2009). Among PFCs, the PFOS have received most attention in recent years due to its persistence, bioaccumulation and toxicity (Taniyasu et al., 2005; Nakata et al., 2006; Higgins et al., 2007; Hongjian, 2009; Murakami et al., 2009). In addition to the direct sources, about 50 precursor chemicals were named to contribute PFOS in the environment (Pelley, 2004). The PFOS, proven to cause liver and thyroid cancer in rats (Key et al., 1997), was detected in the biota of those areas which were even remote from its source of production or usage, such as the Arctic, suggesting the potential for long-range transport and global distribution of this compound and its precursors (Martin et al., 2004; Stock et al., 2007; Shoeib et al., 2010). Through the environmental pathways and also by being in contact with PFOS or its precursors containing consumer products such as carpets, packaging and apparels, the compound has found its way even in human blood. Pooled serum data from the general Australian population showed mean concentrations of PFOS at 15 ng mL−1 (Toms et al., 2009), comparable to those observed internationally (Calafat et al., 2007). Pharmatokinetic modelling of this concentration has even indicated that an average Australian is exposed to approximately 100±37 ng day−1 PFOS (Toms et al., 2009). Therefore, the importance of developing suitable remediation technology for treating PFOS-contaminated soils and water is paramount. But many conventional treatment technologies such as hydrolysis, biodegradation, enzymatic degradation, advanced oxidation and reduction are proven ineffective (Tang et al., 2010; Zhou

Water Air Soil Pollut (2013) 224:1714

et al., 2010) because of the extreme stability of this compound accountable to the presence of strong carbon–fluorine (C-F) bonds (Mak et al., 2009). The strategies investigated so far include membrane processes (Tang et al., 2010), sonochemical treatment (Moriwaki et al., 2005), photocatalysis with various catalysts (Hori et al., 2004) and reduction with zero-valent iron (Hori et al., 2006). However, these treatments have limitations mainly due to their high cost, energy requirement and/or interference from other compounds present. Adsorption of PFOS to a suitable inexpensive adsorbent could be considered as an alternative costeffective method for removing this compound from highly contaminated wastewaters or fixing it in contaminated soils. So, this study aims to assess the adsorption of aqueous PFOS by a novel material (MatCARETM, patent number 2009905953) developed by Naidu and his co-researchers from natural clay mineral. The material was also tested for its ability to remediate PFOS in contaminated soils. The adsorption of PFOS by MatCARETM in contaminated soils can prove to be an outstanding approach for the ‘risk-based remediation’ of this pollutant.

2 Experimental 2.1 Materials The adsorbent used in this study is a modified clay material developed by our group (MatCARE TM ; a palygorskite-based material modified with oleylamine; Australian provisional patent number 2009905953). The adsorption ability was also compared with a commercially available activated carbon (Hydraffin CC8*30). The physico-chemical properties of these adsorbents determined by standard procedures are shown in Table 1. The porosity, particle size distribution and grain density of these materials were determined by Mercury Injection Capillary Pressure (MICP) Analysis (Micromeritics Autopore-III porosimeter). The surface area was measured using the BET method by adsorbing N2 into the adsorbents in a Micromeritics Gemini V surface area analyser. 2.2 Adsorption Experiments The adsorption kinetic experiment was conducted by equilibrating 1 g of the adsorbent with 100 mL of 0.6 mmol L−1 PFOS solution for various time intervals

Water Air Soil Pollut (2013) 224:1714 Table 1 Summary of the adsorbent properties

Page 3 of 14, 1714 MatCARETM

Property

Hydraffin CC8*30

Bulk density (kg m−3)

608

410

Particle density (kg m−3)

1,677

–

Porosity (%)

40

–

–

7.87×10−4 0.6–2.36 mm

−3

Pore volume (kg m ) Particle size

77.4 % between 2,000 and 1,180 μm

Surface area (m2 g−1)

31.91

1,000

Reversible swelling (%)

2.5

–

Moisture holding capacity (%)

50.28

40.5

(0 to 120 h). The adsorption isotherms were developed at PFOS concentrations ranging from 0 to 0.4 mmol L−1. Typically, 100 mL of aqueous solutions containing varying amounts of PFOS were added to 1 g of the adsorbent in 150-mL glass bottle and shaken for 24 h in an endover-end shaker, revolving at 10 revolutions per minute at room temperature (23 °C). Following equilibration, the samples were filtered through a 0.45-μm filter. The control experiments without adding any adsorbent material showed no adsorption of PFOS onto the laboratory ware. All the experiments were carried out in duplicate.

2.3 Analysis of PFOS PFOS was analysed on an Agilent high-performance liquid chromatography (HPLC) system (Model G1316A) equipped with an auto-sampler, binary pump, a mass selective detector (Model G1946D) with negative ionisation mode of atomic pressure ionization-electro spray and employing Chemstation software for the data integration. Chromatographic separation of PFOS was made using a ZORBAX Eclipse XDB-C18 150×4.6 mm, 5 μm column operated at 25 °C temperature. The mobile phase consisted of a combination of two solvents—solvent A (2 mM ammonium acetate) and solvent B (methanol) maintaining a flow rate of 0.8 mL min−1. The total runtime was 17 min which included a post run period of 2 min. The ratio of solvents A and B varied with linear gradient program (0.1 min 30 % B, 0.1–7 min 30–100 % B, 7–15 min 100 % B). Blanks were continuously run in between the samples to assure that the column was clean and traces of analyte were not carried over between samples. The PFOS concentrations were determined from the linear calibration

curve (R2>0.99) in the range of 0.01 to 2 μmol L−1. The limit of detection was determined to be 0.14 nmol L−1. 2.4 X-Ray Diffraction Analysis The MatCARETM before and after the adsorption of PFOS was characterised by X-ray diffraction (XRD) using CuKα radiation (λ=1.5418 Å) on a PANalytical, Empyrean X-ray diffractometer (PANalytical, Australia) operating at 40 kVand 40 mA between 2.0 and 90° (2θ) at a step size of 0.013°. The PFOS-loaded sample was obtained by equilibrating the MatCARETM in PFOS solution (0.2 mmol L−1) at pH 6.3 for 24 h. After drying, the finely powdered samples were pressed into the backpacked sample holders and the XRD patterns were recorded. The basal spacing (d) was calculated from the 2θ value using Bragg’s equation. 2.5 Fourier Transformed Infrared Analysis Infrared (IR) spectra of the MatCARETM before and after adsorbing PFOS were obtained using an Agilent Cary 600 Series Fourier transformed infrared (FTIR) spectrometer (Agilent Technologies, USA) equipped with DRIFT (Diffuse Reflectance Infra-red Fourier Transform) accessories. Spectra over the 4,000– 400 cm−1 range were obtained by the co-addition of 64 scans with a resolution of 4 cm−1. The band component was analysed using PeakFit v4.12 software package (Hearne Scientific Software). 2.6 Soil Treatability Studies The soils (code: A, B, C and D) for the treatability study were collected from the fire training areas of Royal

1714, Page 4 of 14

Australian Air Force bases located in Darwin and Tindal, Northern Territory, Australia. The PFOS-contaminated soil samples were collected in high-density polypropylene buckets, transported to the laboratory and then airdried, homogenized and passed through a stainless steel 2-mm sieve to remove pebbles and debris. The physicochemical properties of the soils including pH, organic carbon content and textural classes were determined following standard procedures. Table 3 shows the physicochemical properties and the PFOS contents of the soils. The pH of the soils varied in the range of 4.8 (A) to 8.1 (C) while the total organic carbon (TOC) contents differed from 0.29 % (C) to 2.03 % (D). The clay content of the soils ranged from between 16 % for C and 35 % for B. The PFOS contents of the soils were 3.66, 148.72, 32.33, and 18.52 nmol g−1 for A, B, C and D soils, respectively. For the treatability study, 1 kg of each soil was taken in individual glass jars and adjusted to 60 % of maximum water holding capacity of the soils. Then the MatCARETM was added to these soils at an application rate of 10 g per 100 g soil. To simulate any PFOS-contaminated hot spot at the actual site, an additional treatment containing PFOSspiked soils (0.2 mmol kg−1) was also included in the present study. The soils (1 kg) were spiked by sprinkling 10 ml of PFOS stock solution (20 mmol L−1 in HPLC grade acetone) followed by homogeneous mixing in an end-over-end shaker for 24 h. The residual solvent (acetone) was evaporated out by keeping the opened soil containing jars under fume hood for a week. Occasional turnings of the soils in the jars were done to facilitate the evaporation. The control samples without any MatCARETM addition were also maintained. The untreated and treated soils were incubated at 25 and 37 °C temperatures to comply with the seasonal temperatures of the soil collection sites. The moisture content of the soils throughout the experimental duration was maintained at 60 % of the maximum water holding capacity of the soils by occasional addition of deionised water. A sub-sample of 10 g was taken quarterly from the jars over 1 year time to determine the water extractable fraction of PFOS present. The experiment was conducted in duplicate. The water extractable fraction of PFOS released from the soils was determined by extracting the samples with deionised water at 1:4 soil/water ratios in 50mL polypropylene tubes over a period of 24 h in an end-over-end shaker, revolving at 10 revolutions per minute. The extraction tubes were then centrifuged at 4,000 rpm for 20 min. The supernatant was filtered


through a 0.45-μm filter and then analysed by HPLC to quantify the amount of PFOS released over the study period. The aqueous extraction was carried out in order to portray the real situation that might happen in the environment. The water extractable fraction would represent the quantity of PFOS which might seep down through the soil profile after a torrential downpour.

3 Results and Discussion 3.1 Adsorption Studies 3.1.1 Adsorption Kinetics Figure 1 shows the adsorption kinetics of PFOS onto the MatCARE™ and the activated carbon. The adsorption onto the MatCARE™ occurred mostly during the first 20 min of the experiment and the equilibrium attained after only 60 min, involving a fast kinetics (Fig. 1a). On the contrary, the activated carbon demonstrated a much slower kinetics reaching the equilibrium after 3 days (72 h; Fig. 1b). The adsorption process onto porous adsorbents involves four main steps: diffusion in the liquid phase, external mass transfer to the particle surface, internal diffusion inside of the adsorbent and attachment onto the adsorption sites. The experimental kinetic data in this study were fitted to the intraparticle diffusion model, considering surface diffusion as the rate-controlling step, which is normally used to describe diffusion in most of the porous adsorbents (Do, 1998). The goodness of the fit is shown in Fig. 1 (R2=0.96 for the MatCARE™ and R2=0.91 for the activated carbon). The governing equations of this model for a spherical particle are (Ruthven, 1984): 2 ∂q ∂ q 2 ∂q ¼ Ds þ ð1Þ ∂t ∂r2 r ∂r with the initial condition: ∀r; t ¼ 0⇒q ¼ 0

ð2Þ

and boundary conditions: ∀t; r ¼ 0⇒

∂q ¼0 ∂r

∀t; r ¼ rc ⇒q ¼ f ðcÞ; equilibrium

ð3Þ

ð4Þ


Page 5 of 14, 1714

Fig. 1 Adsorption kinetics of PFOS onto the a MatCARE™ and b activated carbon (Hydraffin CC8*30)

where Ds is the surface diffusivity which is assumed to be constant, r is the radial coordinate and rc is the particle radius. If the quantity of the adsorbate adsorbed is not negligible compared with the quantity introduced from the ambient fluid phase, such as in batch systems, the adsorbate concentration in the fluid will change after the initial step, resulting in a time-dependent boundary condition at the surface of the adsorbent particle. The solution for the uptake curve is (Crank, 1956): . ∞ 6αðα þ 1Þexp −Ds p2 t r2 X n c q−q0 F¼ ¼ 1− ð5Þ 2 α2 9 þ 9α þ p q∞ −q0 n n¼1 where F is the fractional uptake, q is the average concentration through the adsorbent, q0 is the initial adsorbate concentration of the adsorbent, q∞ is the final adsorbate concentration of the adsorbent (q∞ is in equilibrium with c∞) and pn the non-zero roots of: tanpn ¼

3pn 3 þ αp2n

ð6Þ

where α is the ratio of the volumes of solution (V) and sphere and is defined by: 3V ð7Þ α¼ 4πr3c To determine the diffusivity, the experimental fractional uptake values were matched to the transient solution of the diffusion Eq. 5. The diffusivity obtained for the MatCARE™ and activated carbon are 4.21×10−11 and 1.75×10−14 m2 s−1, respectively. These values fall within the range expected for liquids (Garcia et al., 2002). These results confirmed that surface diffusion was the rate-controlling step for the adsorption of PFOS onto these adsorbents.

3.1.2 Adsorption Isotherm The isothermal data indicated that the MatCARE™ had higher affinity to PFOS and thus higher adsorption

1714, Page 6 of 14 Fig. 2 Freundlich fittings for the adsorption of PFOS onto the a MatCARETM and b activated carbon (Hydraffin CC8*30)

Fig. 3 XRD patterns of the palygorskite, MatCARE™ and PFOS-loaded MatCARE™



Page 7 of 14, 1714

capacity than the activated carbon. Two commonly used isothermal models, namely the Langmuir and Freundlich models, which are expressed respectively as the following equations, were tested to explain the experimental adsorption data. qe ¼

bqm C e 1 þ bC e

qe ¼ KC βe

ð8Þ

ð9Þ −1

where qe is the equilibrium adsorption amount (mmol g ), Ce represents the equilibrium concentration (mmol L−1) of PFOS in solution, qm is the maximum adsorption capacity (mmol g−1), b is the adsorption equilibrium constant (L mmol−1), K is a constant representing the adsorption capacity (mmol1−β Lβ g−1) and β is a constant depicting the adsorption intensity. The Freundlich model best fitted the PFOS adsorption data (R2=0.98 for the MatCARE™ and R2=0.95 for the activated carbon; Fig. 2a and b). The values of K for the MatCARE™ and activated carbon are 0.05 and 0.03 mg1 −β β −1 L g , respectively and the values of β are 0.235 and 0.2642, respectively. The monolayer coverage in terms of adsorption capacity was also derived from the Langmuir model. This value is 0.093 mmol g−1for the MatCARE™ and 0.07 mmol g−1 for the activated carbon. Therefore, the

model results confirmed the higher PFOS adsorption capacity of the MatCARE™ compared to the activated carbon. It was noted that the monolayer adsorption capacities of these adsorbents were not dependent on their BET surface areas (Table 1). This is attributable to the heterogeneous potential of the MatCARE™ surface (Sarkar et al., 2010). It also indicated partitioning of PFOS molecules at the MatCARE™ surface during the adsorption process. 3.1.3 Adsorption Mechanism In order to better understand the adsorption mechanisms of PFOS onto the modified clay, the MatCARE™ samples were analysed by XRD and FTIR before and after the adsorption process. The XRD patterns revealed that the palygorskite used to prepare MatCARE™ contained impurities like quartz, kaolinite, amorphous materials, dolomite and traces of sodium chloride (Fig. 3). Neither after the modification of the palygorskite with oleylamine nor after the adsorption of PFOS onto the modified clay mineral, a significant change of the position of the d001 peak occurred (Fig. 3). It suggested that (a) the attachment of oleylamine to palygorskite occurred though surface interaction and (b) PFOS got adsorbed on the MatCARETM surface only, not in the interlayer. It is attributed to the nonswelling nature of palygorskite used to prepare the

Fig. 4 FTIR spectra of the palygorskite, MatCARE™ and PFOS-loaded MatCARE™

1714, Page 8 of 14

MatCARETM (Xi et al., 2010; Sarkar et al., 2011b). The FTIR study further revealed the mechanism of PFOS adsorption onto the adsorbent. The signature regions (wave number 2,950–2,850 cm−1) for symmetric and asymmetric stretching vibration of the ‘–CH2’ groups present in the organic molecules attached on the palygorskite surface


were prominent in the FTIR spectra (Fig. 4). These peaks were absent in case of unmodified palygorskite (Fig. 4). The ‘–CH2’ symmetric stretching vibration appeared at 2,854 cm−1 wave number for both the PFOS loaded and unloaded MatCARETM samples. However, the ‘–CH2’ asymmetric stretching vibration for MatCARE TM

Fig. 5 Release behaviour of PFOS from contaminated soils A, B, C and D at a 25 °C and b 37 °C temperature


appeared at 2,927 cm−1 wave number, whereas that for PFOS-loaded MatCARETM appeared at 2,923 cm−1. So, there was a shift for the asymmetric stretching bands towards the lower frequency. It was indicative of an ordered conformation of the PFOS molecules on the MatCARETM surface (Sarkar et al., 2011a; 2012). Because PFOS has a hydrophobic subunit (C8F17) present

Page 9 of 14, 1714

in its molecule, its adsorption occurs mainly through the hydrophobic interactions (partitioning) with the long chains of the surfactant molecules attached to the modified clay (Boyd et al., 1988). The fitting of the adsorption isotherm data to the Freundlich model also supported the hydrophobic nature of the interactions between the MatCARETM and PFOS molecules.

Fig. 6 Release behaviour of PFOS from soils A, B, C and D spiked with 0.2 mmol kg−1 PFOS at a 25 °C and b 37 °C temperature

1714, Page 10 of 14


Table 2 Physico-chemical properties of the soils Soils

pH

TOC (%)

PFOS (nmol g−1)

Texture

Solvent extracted

Water extracted

Sand (%)

Silt (%)

Clay (%)

Textural class Sandy clay loam

A

4.8

0.96

3.66

0.52

52.63

25.62

21.74

B

4.9

1.97

148.72

21.13

43.21

21.42

35.37

Clay loam

C

8.1

0.29

32.33

4.72

75.15

9.11

15.74

Sandy loam

D

6.5

2.03

18.52

1.86

57.04

10.93

32.03

Sandy clay loam

3.2 Treatability Studies 3.2.1 PFOS Release from Treated and Untreated Soils

Fig. 7 Relationship between the total organic carbon (TOC) contents in soils and the partition coefficients (K) for PFOS

K = (Total PFOS)/(PFOS in Water)(Total Clay)

The release pattern of PFOS from four different MatCARETM-treated and control soils at 25 and 37 °C temperatures over a period of 1 year are shown in Fig. 5. The average PFOS release from untreated/control soils A, B, C and D over the 1-year period were 9.32, 7.19, 10.83 and 5.25 % respectively at 25 °C temperature (Fig. 5a). The same average values at 37 °C temperature were 11.49, 9.10, 11.62 and 5.70 %, respectively (Fig. 5b). In case of the treated soils no release of PFOS occurred at 25 °C temperature during the entire 1 year incubation period. However, at 37 °C temperature, the average release values were 0.15, 0.05, 0.31 and 0.08 %, respectively, for treated soils A, B, C and D. The treatability study was also conducted with PFOS-spiked soils (0.2 mmol kg−1) to simulate a contamination hot spot under the field conditions. The pattern of PFOS release from treated and untreated (control) spiked soils during the 1 year study period

are elucidated in Fig. 6. The average releases from the untreated spiked soils incubated at 25 °C temperature were 18.21, 13.20, 19.33 and 14.76 %, respectively, for soils A, B, C and D (Fig. 6a). On the contrary, the average values for treated soils incubated at 25 °C were 0.57, 0.07, 0.14 and 0.27 %, respectively (Fig. 6a). Similar results were obtained at 37 °C temperature. The average release values recorded for untreated soils at 37 °C temperature were 18.89, 11.41, 23.84 and 20.68 %, respectively for soils A, B, C and D, while the same values recorded for treated soils were 0.17, 0.52, 0.31 and 0.53 %, respectively (Fig. 6b). The percent releases of PFOS from the treated soils were remarkably less in comparison to the untreated soils at both the studied temperatures. 3.2.2 Effect of Temperature It was noted that the incubation temperatures had an effect on the release of PFOS from the treated and untreated soils. In case of both the naturally contaminated soils and PFOS-spiked soils, more PFOS release occurred

0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

0.5

1

1.5 TOC (%)

2

2.5


at 37 °C than at 25 °C temperature. In fact, no release took place from the naturally contaminated soils at 25 °C. Such observation indicated that the adsorption of PFOS by the MatCARETM is an exothermic process (Sarkar et al., 2010; 2012). It has tremendous implication while applying this adsorbent material for the fixation of PFOS in contaminated soils in tropical regions where the summer temperature frequently rises above 37 °C. Where such a situation might prevail, the PFOS release patterns thus should regularly be monitored after the MatCARETM addition. 3.2.3 Periodic Release of PFOS from Treated and Untreated Soils The periodic trend of PFOS release observed in this study was a decrease in the release value over time (Figs. 5 and 6). Not surprisingly, the trend was more pronounced in case of PFOS release from spiked soils simulating contamination hot spots. In case of naturally contaminated soils, no release occurred throughout the experiment at 25 °C (Fig. 5a), while in the PFOS-spiked soils the average release was small but significant (0.6 %) (Fig. 6a). The average release was the highest during the first quarter of sampling, but gradually decreased in the subsequent three sampling occasions (Fig. 6a). Such observation proved the pronounced influence of ageing for the release of PFOS from PFOScontaminated soils. Ageing refers to the increasing contact time between a chemical and soil, which may allow a compound to become more strongly associated with soil components over time (Hatzinger and Alexander, 1995). It may promote the formation of covalent bond between PFOS and organic carbon (Yao et al., 2008) along with diffusion of the contaminant in macro and

Table 3 Cost comparison for different adsorbents for PFOS available in the market

Adsorbent

Page 11 of 14, 1714

micropores in minerals’ crystal lattice. In this study, a difference in the aging process was observed for each of the four different soils. The different physical and chemical properties of the soils used could be held accountable for such a difference. 3.2.4 Soil Parameters Affecting PFOS Release As can be seen from the results, soil C with the lowest TOC had the highest PFOS release both at 25 °C (10.83 %) and 37 °C (11.62 %) temperatures. Similarly, in case of the spiked soils the highest PFOS release occurred for soil C at both 25 °C (19.33 %) and 37 °C (23.84 %) temperatures. The least value of percent PFOS release was found to occur for soil D at 25 °C (5.25 %) and 37 °C (5.70 %) temperatures. However, following spiking the soils, the least release occurred from soil B both at 25 °C (13.20 %) and 37 °C (11.41 %) temperatures. These release values from each of the soils could well be interpreted in terms of their respective textural classes (sandy loam for soil C, while clay loam and sandy clay loam for soils B and D, respectively; Table 2). Soil C having the least clay content (15.74 %) and the lowest TOC content (0.29 %) have the highest release. On the other hand, soils D and B, which have the highest clay contents (32.03 and 35.37 %) and the highest TOC content (1.97 and 2.03 %), possesses the lowest PFOS release, suggesting that the PFOS release is governed by both the carbon and clay contents of soils. Figure 7 shows that the partition coefficient (K) is dependent on the TOC content present in the soils. Similar result was reported by Hongjian (2009) who found lower extractability of organic compounds in soil with higher clay content. The soil organic matter could provide more sites for adsorption or partition, thereby reducing the extractability of the

Type

Adsorption capacity (mmol g−1)

Price ($ kg−1)

Hydraffin CC8*30

GAC

0.07

14.60

Natural modified clay

Clay

0.09

26.00

Filtrasorb 400

GAC

0.002

5.77

Amberlite IRA 400

Resin

0.42

88.00

U.S. Filter A-714

Acrylic resin

0.30

33.00

Dowex V-493

Macro porous resin

0.25

690.90

Dowex L-493

Macro porous resin

0.04

359.10

Amberlite XAD4

Resin

1.59

218.00

1714, Page 12 of 14

adsorbed compound. The organic matter could be the most dominating sediment parameter influencing adsorption of anionic perfluorinated surfactants (Moody et al., 2002; Higgins et al., 2005; Higgins and Luthy, 2006; Hongjian 2009). It was also suggested that adsorption to organic matter was more important for anionic perfluorinated surfactants than adsorption to mineral surfaces (Higgins and Luthy, 2006). Another soil parameter which considerably influenced the release of anionic surfactant (PFOS) was the soil pH. As might be expected from the electrostatic theory, adsorption of anionic surfactants would increase with decreasing pH. With the increase of pH, the negative charge increases in a variably charged soil (Naidu et al., 1990; 1994). Soil C having the highest pH thus had the least adsorption and highest percentage release of PFOS. 3.3 Cost Comparison of Adsorbent Materials Suitable for PFOS Removal Only a limited number of publications are available reporting the removal of PFOS using commercial adsorbents (Lampert et al., 2007; Ochoa-Herrera and Sierra-Alvarez, 2008; Yu et al., 2008; 2009). The costs of these adsorbents were analysed and summarised in Table 3. Although, activated carbons (Hydraffin CC8*30 and Filtrasorb 400) involve a lower cost ($14.6 kg−1 and $5.7 kg−1, respectively) than the MatCARETM ($26 kg−1), it has remarkably higher adsorption capacity (0.09 mmol g−1) in terms of monolayer coverage. On the other hand, elevated cost of ionexchange resins makes them an economically unviable option for PFOS remediation in contaminated soils. This cost analyses suggests that the modified clay studied in this report is an economically feasible option for PFOS remediation both in waste waters and contaminated soils.

4 Conclusions A new material (MatCARETM) was developed and assessed for its ability to remediate waters and soils impacted with PFOS derived from the injudicious disposal of AFFF formulations. The new material displayed a much faster kinetics (only 60 min) to reach adsorption equilibrium and remarkably higher PFOS adsorption capacity (0.093 mmol g−1) as compared to a commercial activated carbon. When applied to PFOS-contaminated


soils, the MatCARETM demonstrated high capability to fix PFOS in soils. A negligible release (0.5 to 0.6 %) of the contaminant occurred over the entire incubation period of 1 year. The fixation of PFOS in soils by the new adsorbent was exothermic in nature. The release pattern of PFOS from soils depended on their clay content, organic matter content and pH. Soils with higher clay and organic matter content, but lower pH values, retained PFOS to a much greater extent. A cost analyses confirmed that the MatCARETM could be an economically viable option for the remediation of PFOS in contaminated waters and soils. To the best of our knowledge, development of a clay-based adsorbent for the removal of PFOS from contaminated soils was never published earlier. Future research is needed to reveal the extent of bioavailability of PFOS fixed by the application of MatCARETM in the contaminated soils. Acknowledgments This research was conducted as part of the CRC CARE (Cooperative Research Centre for Contamination Assessment and Remediation of the Environment) project number 6-4-03-08/09 funded by the Department of Defence, Australian Government. The infrastructural support from the Centre for Environmental Risk Assessment and Remediation (CERAR), University of South Australia is gratefully acknowledged.

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