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環 境 工 学 研 究 論 文 集 ・第44巻 ・2007(Environmental Engineering Research. Vol. 44, 2007)
(23) REMOVAL OF PERFLUOROCHEMICALS FROM
WASTEWATER BY GRANULAR ACTIVATED
CARBON ADSORPTION
Yong QIU1, Shigeo FUJII2* and Shuhei TANAKA1
1Research Center for Environmental Quality Management, Kyoto University(Yumihama 1-2, Otsu, Shiga, 520-0811, Japan)2
Graduate School of Global Environmental Studies, Kyoto University(Yoshida-honmachi, Sakyo-ku, Kyoto, 606-8501, Japan)
* Email:fujii@eden.env.kyoto-u.ac.jp
Perfluorochemicals (PFCs) are widely consumed as surfactants, additives, repellent, insecticides, andso on. Recently some of them have been suspected as persistent, bioaccumulated and toxic chemicals.Reports on PFCs in surface and drinking water indicated ineffective removal in current water treatmentfacilities including granular activated carbon (GAC) filtration. This study aims to understandcharacteristics of GAC adsorption to remove PFCs. Freundlich equation and the homogenous surfacediffusion model (HSDM) were successfully applied to interpret experimental data. GAC showedincreasing adsorption capacities and velocities for PFCs with longer carbon chain length. Carbon foulingeffect reduced GAC adsorption capacities of PFCs more intensively than those of background organics.Coexisting organics and bulk pH did not significantly affect adsorption kinetics of PFCs. Coal-basedGAC performed better to remove PFCs than nutshell-based GAC. Preliminary experiments by the raidsmall scale column test (RSSCT) showed effective removal of some PFCs, including highly concernedperfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA).
Key Words: perfluorochemicals, wastewater, GAC adsorption, HSDM, RSSCT
1. INTRODUCTION
Pzerfluorochemicals (PFCs) are widely producedand consumed as surfactants, repellents, additives,fire fighting foams, polymer emulsifiers, insecticide,and so on1). At present, some of them aresuspected as persistent, bioaccumulated and toxic,such as perfluorooctanoate (PFOA) and perfluoro-octane sulfonate (PFOS). Global distribution ofPFCs has been recognized in surface water, ground-water, soil, air, biota, food and human body2, 3).There are two major categories of PFCs, which areperfluorocarboxylic acids (PFCAs) andperfluoroalkyl sulfonates (PFASs). PFOA andPFOS are typical compounds of PFCAs and PFASs,respectively, and are usually dominant inenvironment.
Reports4,5) of environmental monitoring programsrevealed positive correlations between PFCs insurface water and those in drinking water. Surfacewater and drinking water in Japan, such as Yodo
River6) and Tama River basins7), were found to becontaminated by PFOA. High concentrations ofPFOS and PFOA were also detected in humanblood samples of residents in Kinki area8,9).Control of these chemicals in highly polluted areasis considered important and urgent for the sake ofhuman health and environmental safety.
Granular activated carbon (GAC) filtration,which is often used in water supply plants as anadvanced treatment process to control odor andcolor, is effective to remove trace organics inwastewater. However, natural organic matter
(NOM) may reduce removal of trace organics bycompetition on surface sites and pore blockage10).Improper operation will make GAC filtrationexpensive and ineffective.
GAC filtration showed diverse performance toremove PFCs. GAC removed several mg/L ofPFOS and PFOA effectively11), and powderactivated carbon (PAC) did hundreds mg/L offluorinated surfactants in a laboratory1). GAC
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columns in a wastewater treatment plant (WWTP)effectively removed 99% of 2 mg/L PFOA withoutbreaking through for more than one year12).However, some drinking water treatment facilitiesin Germany including GAC filtration wereconsidered ineffective to remove PFOS and PFOAin hundreds of ng/L5). Investigation in a WWTP inJapan also indicated ineffective removal of PFOSand PFOA in tens of ng/L by ozone and GACfiltration13). Therefore, studies on adsorptioncharacteristics of GAC to remove PFCs are not onlynecessary to understand these debatable facts, butalso helpful to improve operations of GACfiltration.
2. MATERIALS AND METHODS
(1) GACs
Filtra sorb 400 (F400) from Calgon Co., which is
popularly used in wastewater treatment, was applied
to major experiments of this study. Three other
kinds of GACs were also studied for comparison
with F400, which were Diasorb W10-30 (DW) from
Mitsubishi Corp., PK1-3 (PK) from Norit Corp. and
laboratory-used GAC (Wako) from Wako Pure
Chemical Industries, Ltd. GACs F400, PK and
Wako are made from bituminous coal, peat coal and
char coal, respectively, while DW is made from
coconut shell. Their surface areas are in range of
875•`1000 m2/g, according to the suppliers.
GACs were firstly boiled in pure water for one
hour to remove fine particles and preloaded
organics. Floating grease and particles were
readily removed from water surface during boiling.
Abundant pure water was flushed on boiled GACs
for cooling. After that, GACs were submerged
into pure water and stored in room temperature for
one day to equilibrate surface properties. Then the
GACs were dried up in an oven at 105 •Ž for two
days to eliminate moisture on inside of GACs.
Dried GACs were finally stocked in polypropylene
(PP) or stainless steel bottles with airtight covers to
prevent exposure to atmosphere.
In order to obtain GACs in different diameters,
original GAC was pulverized in a mortar by a pestle
and then separated by a series of standard sieves.
Since fine particles were generated during pulveriz-
ation and attached on granules of sieved GACs, the
GACs were repeatedly washed by pure water to
remove fine particles until the water above GAC
granules became clean and clear. The washed
GAC was dried and stored in the same method as
described before. Geometric averages of mesh
sizes were used as diameters of sieved GACs.
(2) PFCs and NOM
Table 1 shows 11 kinds of PFCs examined in this
study. PFC standards were obtained from Wako
Pure Chemical Industries, Ltd. and Tokyo Chemical
Industry Co. with purities of 95•`98%. Each PFC
standard was dissolved into pure methanol to
prepare 10 g/L single component solution. PFC
concentrations were given with the total mass of salt
or acid without any correction by their purities. A
multi component solution was prepared by diluting
all single component solutions together into 50%
acetonitrile/water solvent. 10mM phosphate
buffer at pH 7 was prepared and diluted 10 times in
bulk solution to control pH. Before experiments,
the PFC multi component solution was spiked into
pure water with pH buffer or wastewater to prepare
PFC dilute solution, and then the diluted solution
was contacted with GAC to proceed adsorption.
Table 1 Perfluorochemicals used in this study
Humic acid from Wako Pure Chemical Industries,
Ltd. (WHA) and wastewater effluent (WWE) from a
municipal WWTP were used as NOM in this study.
Firstly, WHA was boiled in pure water to increase
its solubility. Then the mixed liquor was filtrated
by 1 •¬m glass fiber paper (Millipore) to obtain
filtrate for experiments. Effluents from secondary
clarifier and biological activated carbon filtration
were sampled from one municipal WWTP and used
for experiments after filtration (GF/B, Whatmann).
(3) Experimental methods
Table 2 shows experimental conditions of batch
and continuous experiments. "Bottle-Point" method
was applied for Runs I and K to obtain adsorption
isotherms and kinetics. Before experiments,
GACs were precisely weighed and transferred into
60 mL PP bottles. A small amount of pure water
was added inside and vacuum state was given for
half an hour to eliminate small bubbles inside
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GACs. This step can accelerate pre-wetting of
GACs which may last for 24 hours14). After
vacuuming, the supernatant was discarded by a
pipette. Then, PFC dilute solution was added into
the bottles to start GAC adsorption experiments.
The bottles were immediately capped and placed on
a thermo-stat shaker (Eyela NTS-1300s) at 25•Ž
with shaking speed of 120 rpm. In Run I-5, GAC
F400 was, in advance, reacted with 30 mgDOC/L
WHA solution for one week, and then filtrated and
dried in oven at 105•Ž to produce preloaded GAC.
The differential column batch reactor (DCBR)
was applied in Runs D to estimate diffusion
coefficient (Ds) in GAC adsorption, which was
successfully used to estimate GAC adsorption
kinetics for pesticides15). About 100 mg pre-
wetted GAC was packed into a small PP tube jointer
with an inner diameter of 3mm. The jointer was
connected with PP tubes and linked to a 125mL PP
bottle. 100mL PFC dilute solution was circulated
in the GAC package with 10mL/min by a
peristaltic pump.
The rapid small scale column test (RSSCT) is
useful to predict performance of fixed bed
adsorption process in short time by the hydraulic
scaling method16). Preliminary RSSCT was
applied in Runs R to understand general PFCs
behavior in fixed GAC bed. Pre-wetted GAC in 0.2
mm diameter was packed into a chromatograph
column. Spaces beneath and above GAC package
were filled with 5cm depth of high density
polyethylene (HDPE) balls with 1mm diameter.The layers of HDPE can homogenize hydraulic
conditions in the plug flow column. PFCs dilute
solution was passed through the column
continuously in 10mL/min by a peristaltic pump.
(4) Sampling and analysis
PFCs in aqueous phase were measured by
LC-ESI-MS. Reverse phase columns, Zorbax
XDB-C18 (2.1•~150mm) with guard column XDB-C8
(2.1•~10mm), were applied as stationary phase in LC
separation. Pure water with 10 mM CH3COONH4
(solvent A) and pure acetonitrile (solvent B) wereused as mobile phase. Gradient flow was applied to
accelerate PFCs elution and to clean up the column,
by increasing solvent B from 50% to 100% in 1•`7
minutes. Negative ions of PFCs were detected in
selected ion mode at m/z shown in Table 1. Linear
relationships (R2>0.99) were observed between PFC
concentrations in 1-100 •¬g/L and corresponding
peak areas at specific m/z in mass chromatograms..
In isothermal experiments (Runs I), 5 days was
assumed as equilibrium time for GACs with 0.1mm
diameter. After 5 days, mixed liquor was filtrated
by 1 •¬m filter paper, and 1 mL of filtrate was
sampled for LC-MS analysis. In Runs I-2&3,
solid phase extraction (SPE) by Presep-C Agri
cartridge (220mg, Waters) was applied to
concentrate analytes in 100 times. Detailed
information about SPE process was shown in our
previous papers13,17) In kinetics experiments (Runs
K and D), about 0.5 mL solution was sampled into
PP vials by a pipette at several sampling times and
then analyzed by LC-MS.
Table 2 Experimental conditions
Note: a preloaded by WHA in 30mgDOC/L. b numbers in brackets (3.5) mean matrix DOC concentrations, mg/L.
c geometric average diameters of sieved GACs, equal to 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 1.1mm
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(5) Isothermal modelsLangmuir equation and Freundlich equation, as
shown in Eq. 1a and Eq. 1b respectively, wereapplied to interpret isotherms in this study.
(1a)
(1b)
where q is adsorbate loading on GAC in unit of
•¬g/g, qm, is maximum adsorbate loading on GAC, C
is bulk concentration at equilibrium state in unit of
•¬g/L, k, K and n are constants in unit of (•¬g/L)-1,
(•¬g/g)/(•¬g/L)n and dimensionless respectively.
Adsorbate loading, q can be calculated by mass
balance from PFC equilibrium concentrations, C
and GAC amount. Then, parameters of qm, k, K
and n can be estimated from experimental data, C—q
by using the following as regression equations:
(2a)
(2b)
(6) Kinetics modelIt is widely accepted that adsorption process
contains four steps; (1) diffusion in liquid phase, (2)external mass transfer to particle surface, (3)intraparticle diffusion including porous diffusionand surface diffusion inside particle, and (4)attachment onto the sites18). Surface diffusion wasfound dominant in intraparticle diffusion 19). Inorder to simplify simulation, adsorption kinetics wasusually assumed to be governed only by externalmass transfer and surface diffusion15). Therefore,the homogenous surface diffusion model (HSDM)was applied to interpret kinetics data in this study.HSDM contains six assumptions as follows 18):
1) Mass is balanced in a contactor.2) The particle is spherical and homogenous.3) External mass transfer is governed by linear
driving force.4) Internal mass transfer is only governed by
surface diffusion.5) Flux of external mass transfer is equal to
surface diffusion at interface (sphere surface).6) Concentrations at interface are in equilibrium
between solid and liquid phase.Based on linear driving force and the Fick's
second law, seven equations shown in Table 3 couldbe derived from above the assumptions,.
Numerical methods are available to solve
partially differential equations in HSDM, e.g. threepoints orthogonal collocation method 20) which wasadopted in this study. Given Kf and Ds, bulk
concentration (Csim) can be calculated by HSDM atany sampling time. With observed concentrationsCexp at N sampling times in kinetics experiment, Kfand Ds can be determined by minimizing a criterionshown in Eq. 3.
(3)
Table 3 Description of HSDM
<Variables> V: bulk volume, M: GAC mass, Cb: PFC
concentration in bulk, rp: granular radius, q(r,t):
adsorbate loading at time t and at radius r inside GAC,q
: average of q(r,t) along radius, Cs: PFC
concentration at interface, qs: PFC loading on GAC at
interface, •¬p: apparent density of GAC, Ds: surface
diffusion coefficient, Kf: mass transfer coefficient.
3. RESULTS AND DISCUSSION
(1) Isothermal and kinetics models
Fig. 1 shows original data in Runs I-1•`3, and
regression results by Langmiur equation and
Freundlich equation. In a wide range of
concentrations, Freundlich equation can fit
isotherms of PFOS and PFOA very well. However
Langmiur model failed in estimating isotherms in
trace level concentrations which were important for
wastewater treatment. Therefore, Freundlich
equation will be more applicable to estimation of
isotherms and kinetics. Although PFOS and
PFOA in Runs I-1•`3 were present in different ways,
which were single solution, mixture of both, and
mixture of 11 kind PFCs, their isotherms did not
show obvious difference among them. This result
indicated that competition among PFCs in trace
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(a) PFOA
(b) PFOS
Fig. 1 PFC isotherms interpreted by Langmuir equation and
Freundlich equations
(a) Bottle-Point (PFOA, GAC)
(b) DCBR (PFOA, 1g/L GAC)
Fig. 2 HSDM estimation of PFOA adsorption
levels did not affect their isotherms greatly.
Therefore, isotherms obtained in mixture of 11 kind
PFCs were assumed as the same as isotherms in
single solution and applied in HSDM to estimate
adsorption kinetics.
Fig. 2 shows HSDM estimation for PFOA
adsorption in Runs K-1 and D-3. As shown in the
figure, both of bottle-point and DCBR experiments
can be fitted well by HSDM. Surface diffusion
coefficients for PFOA were in magnitude of 10-10
cm2/s, similar for atrazine and bromoxynil15).
(2) Influence of PFC chain lengths on isotherms
Table 4 shows estimation results of PFC
isotherms in different matrices, which were pure
water in Run I-3, biological activated carbon
filtration effluent (BAC) and activated sludge
process effluent (ASP) from the WWTP in Run I-4,
and preloaded GAC in Run I-5. PFC
concentrations at equilibrium state were in a range
of 0.05•`5 •¬g/L. In Freundlich equation, K implies
maximum adsorption capacities or the position of
isotherm, and n determines the shape of isotherm.
As shown in the table, both K and n increased with
ascendant carbon chain lengths for PFCA (4•`10) in
four cases. This tendency implied that PFCs with
longer chains were easier to be adsorbed by GAC
F400, which can be explained by their increasing
hydrophobic properties. Generally hydrophobic
interactions are the dominant mechanism for
removal of organic compounds by activated carbon
adsorption, although ion exchange interactions are
also important for removal of polar solutes21,22).
(3) Influence of NOM on isotherms
Competition effects between PFCs and organics
in WWE may appear in Runs I-3 and 4, as shown in
Table 4. Values of K were decreased from pure
water to BAC effluent, and further decreased by
more extent to ASP effluent, which implied that
PFCs adsorption capacities were reduced by the
increasing DOC concentrations in wastewater
effluent. Values of n were not changed obviously
between pure water and wastewater effluent, which
indicated parallel isotherms as shown in Fig. 3a.
PFC molecules have very small and straight shape,
so that they might mainly occupy micropores.
NOM has diverse size distribution, and only its
small fraction may compete with PFC molecules in
micropores.
Carbon fouling (=preloading) effect on PFC
adsorption was examined in Run I-5 and also
shown in Table 4. The value of K for each PFC
were decreased in much greater extent than that
under competition with organics in wastewater
effluent, which may mean that carbon fouling can
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Table 4 Influence of competition and preloading on isotherms
Table 5 Influence of pH and wastewater effluent matrix on kinetics
Note: a=BAC effluent, b=ASP effluent, numbers with a, b are DOC concentration in wastewater effluent, mg/L
reduce PFCs adsorption capacities more intensivelythan competition. Values of n for PFCs with 4-10carbons had no obvious differences between freshand preloaded GAC, indicating parallel isotherms asshown in Fig. 3a. This behavior was similar tocarbon fouling effect on GAC adsorption forpesticides such as atrazine10).
Long-chained PFC as PFHxDA(16) seemed not tobe adsorbed onto preloaded GAC, because both Kand n values were quite smaller than those of freshGAC, as shown in Fig. 3b. Although long-chainedPFC molecules had higher hydrophobicity, they alsohave large molecular weights and sizes to beintensively influenced by pore blockage. Thedecreased n value indicated that adsorption ofPFHxDA was not only affected by pore blockage,but also suffered from other mechanisms likesurface properties modification by preloadedorganics.
All of these results might mean that access tomicropores was very important for both PFCsadsorption capacity and velocity.
(4) Influence of pH, coexisting organics andGAC diameters on PFC kinetics
Table 5 shows estimation results by HSDM forRuns D-1-2. Only surface diffusion coefficientswere shown in the table. pH of bulk solution wasadjusted by 0.1N HC1 and NaOH. According to
(a) PFDA
(b) PFHxDA
Fig. 3 Influence of NOM on PFCs isotherms
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Table 6 Influences of GAC diameters on kinetics
the results, pH of bulk solution showed small effect
on the diffusion coefficients. By applying
different wastewater effluent as bulk solution,
coexisting organics also showed only small influence
on diffusion coefficients in fresh GAC.
Table 6 shows estimation results by HSDM for
Run D-3, which applied sieved GACs in DCBR
experiments. Equilibrium concentrations of PFCs
in isotherms from Runs I-3 and I-6 were in range of
0.05-100 •¬g/L, and they might be suitable for
HSDM estimation in Run D-3, as shown in Table 4.
In most of PFCs, diffusion coefficients were
generally increased by ascending diameters of GAC,
which meant that smaller size GAC resulted in
faster adsorption. This result is similar to
adsorption kinetics of pesticides23,24). For GAC in
same size, diffusion coefficients were generally
decreased by ascending carbon chain lengths of
PFC molecules. This result indicated that
adsorption on GAC was faster in PFC molecules
with longer carbon chains.
External mass transfer coefficient Kf can be
determined by hydraulic conditions and GAC
diameters, and is usually distributed in a range of
values rather than a constant15). Because Kf is not
useful for GAC filtration operation, the estimated
results were not shown.
(5) Influence of GAC materialsTable 7 shows estimation results of Run I-6,
which examined four kinds of GAC for PFCs
adsorption. Equilibrium concentrations for these
isotherms were in a range of 1-100 •¬g/L. In all
kinds of GACs, values of K and n were generally
increased with ascendant carbon chain length for
PFCs, or with their hydropobicity, which indicated
long-chained PFCs were more easily adsorbed by
different kinds of GACs. Among three kinds of
commercial GACs, F400 seemed better to remove
more kinds of PFCs effectively.
According to the results, GAC PK can not adsorb
short-chained PFCs, as K for PFCA(4•`6) was
almost zero. However, the GAC very well
performed adsorption of long-chained PFCs such as
PFNA and PFDA, which indicates better removal
for strong hydrophobic organics. GAC Wako was
specially produced for laboratory use and showed
best performance among the four carbons.
Carbon DW, made from coconut shell, has very
high surface area and large volume of micropores.
Compared with GAC F400, GAC DW showed
better performance in short-chained PFCs such as
PFCA(4•`6). This might be related with dominant
micropores in coconut shell based GAC. On the
contrary, DW showed the worst removal of
long-chained PFCs such as PFHxDA(16),
indicating that larger volume of micropores was not
helpful to remove such PFC molecules.
(6) Continuous experiments
Figure 4 shows results of the preliminary rapid
small scale column tester (RSSCT). After influent
had passed the small column 5000 times of bed
volume (Bed Vol.), influent of Run R-1 in pure
water and influent of R-2 in WWE matrix were
exchanged for a short period.
With coexisting organics in WWE matrix,
adsorption of short- chained PFCs were influenced
more intensively, such as PFPeA. Therefore
short-chained PFCs in WWE matrix broke through
GAC column (C/C0>5%) at much earlier stage
than other PFCs. Although organics in WWE also
showed adverse effects on adsorption of
long-chained PFCs, such as PFOS and PFOA, they
were removed effectively without breaking through
the GAC column. Total PFCs removal was also
shown in the figure, which clearly demonstrated
effect of coexisting organics.
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Table 7 Influence of GAC materials on PFCs isotherms
(a)NOMs
(c)PFOS(8) Cinf=88μg/L
(b)PFOA(8) Cinf=80μg/L
(d)sum of 11 PFCs Cinf=750μg/L
Fig. 4 Preliminary RSSCT results for PFCs
4. CONCLUSIONS
Adsorptive properties of PFCs by GAC wereinvestigated by a series of batch experiments andRSSCT. Freundlich equation and HSDM wereapplied to interpret experimental data.Conclusions are summarized as follows:
(1) Adsorptive properties of PFCs were affectedby carbon chain length of molecules. Withlonger carbon chains, PFCs had higheradsorption capacities and velocities.
(2) Coexisting organics reduced adsorptioncapacities of PFCs by mechanisms of carbon
fouling and competition. Carbon foulingaffected PFC adsorption capacities moreintensively than competition by NOM.
(3) PFC adsorption velocities on fresh GAC werenot significantly influenced by organics inwastewater effluent and pH of bulk solution.
(4) Adsorption capacities were related with GACmaterials. Coal based GAC showed
performance than nutshell based GAC toremove more kinds of PFCs.
(5) Preliminary RSSCT experiments showedeffective removal for some kinds of PFCs,including PFOS and PFOA.
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ACKNOWLEDGEMENTS:
This research is partially supported by Grant-in-Aid for Scientific Research (No. B(2)17360257)and Mitsubishi Foundation 2004.
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(Received May 25, 2007)
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