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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