Evaluating phenanthrene sorption on various wood chars

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Water Research 39 (2005) 549–558

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Evaluating phenanthrene sorption on various wood chars

Gavin Jamesa, David A. Sabatinia, Cary T. Chioub, David Rutherfordb,Andrew C. Scottc, Hrissi K. Karapanagiotid,�

aSchool of Civil Engineering and Environmental Science, University of Oklahoma, Norman, OK 73019, USAbUS Geological Survey, Denver, CO 80225, USA

cGeology Department, Royal Holloway University of London, Egham, Surrey, TW20 OEX, UKdMarine Sciences Department, University of the Aegean, Mytilene 81100, Greece

Received 21 April 2004; received in revised form 2 October 2004; accepted 22 October 2004

Abstract

A certain amount of wood char or soot in a soil or sediment sample may cause the sorption of organic compounds to

deviate significantly from the linear partitioning commonly observed with soil organic matter (SOM). Laboratory

produced and field wood chars have been obtained and analyzed for their sorption isotherms of a model solute

(phenanthrene) from water solution. The uptake capacities and nonlinear sorption effects with the laboratory wood

chars are similar to those with the field wood chars. For phenanthrene aqueous concentrations of 1 mgl�1, the organic

carbon-normalized sorption coefficients (log Koc) ranging from 5.0 to 6.4 for field chars and 5.4–7.3 for laboratory

wood chars, which is consistent with literature values (5.6–7.1). Data with artificial chars suggest that the variation in

sorption potential can be attributed to heating temperature and starting material, and both the quantity and

heterogeneity of surface-area impacts the sorption capacity. These results thus help to corroborate and explain the

range of logKoc values reported in previous research for aquifer materials containing wood chars.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Wood char; Sorption; Koc; Phenanthrene; Nonlinear isotherms; Surface area

1. Introduction

Chiou, (1995) and Gustafsson et al. (1997) suggested

that small amounts of high surface-area carbonaceous

material (HSACM) (e.g., wood chars or soot) may

significantly change the sorption behavior of soils or

sediments for organic contaminants. In such cases, the

sorption isotherms usually exhibit a significant concave-

downward shape at low concentrations (Ce) relative to

water solubilities (Sw), but a practically linear shape at

e front matter r 2004 Elsevier Ltd. All rights reserve

atres.2004.10.015

ing author. Tel.: +30225 103 6835;

3 6809.

ess: [email protected]

agioti).

moderate-to-high CeSw�1 (Gustafsson et al., 1997; Xing

et al., 1996; Huang et al., 1997; Chiou and Kile, 1998;

Xia, 1998; Kleineidam et al., 1999; White and Pignatello,

1999; Xia and Ball, 1999, 2000; Karapanagioti and

Sabatini, 2000; Chiou et al., 2000). The nonlinear

sorption at low CeSw�1 reflects the dominance of solute

adsorption with HSACM, while the linear sorption at

the moderate-to-high CeSw�1 reflects the dominance of

solute partitioning into the soil organic matter (SOM)

(Chiou and Kile, 1998; Allen-King et al., 2002;

Kleineidam et al., 2002).

The presence of HSACM in small amounts in some

soils and sediments has been corroborated by organic

petrographic methods applied to samples of different

origins (Kleineidam et al., 1999; Karapanagioti and

d.

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ARTICLE IN PRESSG. James et al. / Water Research 39 (2005) 549–558550

Sabatini, 2000; Karapanagioti et al., 2000, 2001; Ghosh

et al., 2000). Using this identification method, the char-

like particles were visually observed and characterized.

Recent work by Karapanagioti and Sabatini (2000) not

only identified opaque particles in an aquifer material

but also quantified the content of wood char particles in

the sample.

Using the composite sorption model for single

contaminants (Karapanagioti and Sabatini, 2000),

Karapanagioti et al. (2001) found that phenanthrene-

char logKoc values ranged from 5.6 to 6.8 for a variety

of soils and geosolids, illustrating the range in adsorp-

tion power within the wood chars. However, the in-situ

effect of a char (or HSACM) on the sorption of a given

contaminant will depend critically on the char content,

the surface properties, and the competitive adsorption of

other coexisting contaminants (Chiou and Kile, 1998;

Cornelissen and Gustafsson, 2004).

The main goal of the current study is to present direct

evidence that wood chars from different origins do in

fact exhibit variations in their sorptive properties as has

been indirectly inferred from earlier research. A

secondary goal is to demonstrate potential reasons for

the observed variation in wood char sorption (e.g.,

variations in surface area based on starting material and

heating temperature). A tertiary goal is to demonstrate

that not only surface area but surface heterogeneity is

also responsible for the resulting sorption character-

istics. Toward this end, a series of char samples were

obtained from various forest fires and volcanic activity

sites and their sorptive properties were measured. To

gain further insight into the sorptive properties of the

field chars, the sorptive effects were also characterized

for a range of wood chars produced under controlled

laboratory conditions with different starting materials

and heating temperatures; while these variables have

been evaluated in the activated carbon literature

(Mattson and Mark, 1971; Puri, 1980), to our knowl-

edge they are studied here for the first time with

environmental wood chars. In this work, we refer to

the chars as wood chars instead of charcoals because the

term charcoal traditionally refers to materials produced

under highly specific conditions, which are not likely to

be found in nature.

2. Experimental methods

2.1. Char samples

The field wood char samples evaluated in this study

were obtained from sites of different climates with

different native plant species; details of the sampling

methods are documented elsewhere (Scott et al., 2000).

The plant species and locations of the wildfires-lava

flow from which these samples were taken are given in

Table 1. Each sample was collected by hand from the

surface and labeled according to the plant species in the

vicinity and the date of the wildfire or pyroclastic flow.

Artificial wood char samples were prepared using

methods outlined elsewhere (Jones et al., 1991). Other

researchers have also applied this method to produce

artificial chars that were then used to investigate natural

char properties (Bustin and Guo, 1999). Different plant

species of similar size and shape, all containing woody

tissue, were either buried under sand or charred in a

sealed clay pot in a thermostatically controlled furnace

at temperatures between 300 and 820 1C for 1 h. This

method restricts the oxygen supply to the woody tissue,

thereby allowing the material to be charred rather than

combusted. After firing, the samples were allowed to

cool before being removed to prevent further change.

The starting materials and heating temperatures for the

laboratory-produced samples are presented in Table 2.

It should be noted that other char-preparing methods

utilize continuous purging of the gas phase to help

prevent redeposition of volatilized materials (Nguyen et

al., 2003). Research has yet to demonstrate which of

these methods may best replicate a given environmental-

heating scenario. Nonetheless, the goal of this research is

to evaluate variations in sorption trends for a range of

wood chars rather than to exactly replicate a given

environmental-heating condition or produce materials

with specific characteristics.

2.2. Surface-area measurements

Char samples (about 0.2 g) were outgassed for roughly

16 h (overnight) at 90 1C under a flow of helium (about

10mlmin�1) before being used for surface-area analysis

using a GEMINI-2360 surface-area analyzer from

(Micromeritics). The surface areas were determined

from the BET plot of the N2 adsorption data at liquid

N2 temperature (77K) and relative pressures (PPo�1)

between 0.05 and 0.20. Five data points were used to

construct the plot to derive the monolayer adsorption

capacity, from which the surface area was calculated

using the N2 molecular area of 16.2E-20m2. The open

surface areas and the micropore volumes were deter-

mined from t-plots (de Boer et al., 1966) by use of the N2

adsorption data. Detailed methods for these analyses are

described by Rutherford et al. (1997).

2.3. Total organic-carbon fraction measurements

The total organic-carbon fraction (foc) of char

samples was analyzed by measuring the amount of

CO2 produced by sample combustion using a total

organic-carbon analyzer (TOC 5050A, Shimadzu, Ja-

pan). Calibration was achieved with a 144 g l�1 standard

glucose solution (Fisher Biotech, New Jersey). Analysis

was carried out with an oven temperature of 900 1C on

ARTICLE IN PRESS

Table

1

Info

rmation

ofnatu

ralch

ars

Sample

Startingmaterial

Samplingplace

f oca

Totalsu

rface

areab

(m2g�1)

Open

csu

rface

area

(m2g�1)

Totalpore

volumeb

(cm

3g�1)

Micro

pore

cvolume

(cm

3g�1)

MVO-456

Angiosp

erm

from

pyro

clastic

flow

Montserra

t,Caribbea

nIslands

0.81

19

1.4

0.0128

0.0096

MVO-457

Angiosp

erm

from

pyro

clastic

flow

Montserra

t,Caribbea

nIslands

0.81

3.8

3.2

0.0075

0.0003

OF

Mixed

wood

char

Wildfiresin

Western

Canada

0.58

3.1

2.2

0.0054

0.0005

PP

Pop

ulu

ssp

p.

Wildfiresin

Western

Canada

0.54

2.4

1.6

0.0044

0.0004

1989

Pop

ulu

ssp

p.

Fro

m1989Canadian

Forest

Fire

0.68

1.7

0.3

0.0022

0.0007

1982

Pop

ulu

ssp

p.

Fro

m1982Canadian

Forest

Fire

0.83

85

6.2

0.0502

0.0399

af o

c:org

anic-carb

on

fractionalco

ntent.

bErrors

on

surface

area,pore

volume,

open

surface

area,and

micro

pore

are

allwithin

5%

(plusand

minus).

cDetermined

from

t-plots

(deBoer

etal.,1966).

G. James et al. / Water Research 39 (2005) 549–558 551

the solid sample module. Since this analytical method

could not distinguish between carbons from carbonized

material (such as char) and normal organic matter, the

determined carbon fractions of char samples are

operationally termed the total organic-carbon fractions

(foc).

2.4. Isotherm tests

Phenanthrene solutions for sorption by wood chars

were prepared in synthetic ground water (deionized

water with 44mg l�1 CaCl2 H2O, 14mg l�1 CaSO4, and

17mg l�1 NaHCO3 added). Sodium azide was added at a

concentration of 200mg l�1 to prevent phenanthrene

biodegradation. Batch isotherm experiments were con-

ducted in triplicate in 100ml crimp-top glass vials. To

expedite equilibrium, char samples were pulverized by

hand before being passed through a 200-mesh sieve.

Others have demonstrated that pulverization of acti-

vated carbon (Randtke and Snoeyink, 1983) or sedi-

mentary samples containing natural organic matter

(Rugner et al., 1999) did not significantly alter the

sorption capacity. Selected samples, including wood

chars with low and high surface areas, were monitored

for sorption kinetics. Samples with pulverized material

reached sorption equilibrium within 7 days. A range of

phenanthrene concentrations [0.04–0.77 of Sw where

Sw ¼ 1.3mg l�1(Kleineidam et al., 2002)] were prepared

and added to the char samples (approximately 3mg).

Samples were kept in the dark at 20 1C, and shaken once

each day for 7 days. Phenanthrene concentrations were

analyzed using a Shimadzu RF-551 PC Spectrofluoro-

metric Detector in cuvette mode.

For each batch experiment triplicate blank samples

(containing only phenanthrene) were prepared and

monitored. These blank samples did not indicate any

significant phenanthrene degradation or sorptive losses

on the glassware for the duration of the experiment.

Previous studies have described these methods in

additional detail (Kleineidam et al., 1999; Karapanagioti

and Sabatini, 2000; Karapanagioti et al., 2000; James,

2001).

3. Data analysis

Since the primary interest of this study is the sorption

capacity and isotherm nonlinearity associated with a

wood char, the Freundlich equation was used to evalute

these effects. The Freundlich equation relates the mass

of chemical sorbed per unit mass of solid (qe) (mg kg�1)

to the equilibrium solute concentration (Ce) (mg l�1) as

follows:

qe ¼ K frCNe ; (1)

ARTICLE IN PRESS

1,E+05

1,E+06

1,E+07

1,E+08

1 10 100 1000

Ce (µg l-1)

qe

(µg

kg

-1)

f oc-1

MVO 456MVO 457OFPP19891982

Fig. 1. Phenanthrene sorption isotherm data for natural chars.

Table 2

Information of artificial wood chars

Sample Starting

material

Heating

temperature (1C)

foca Total surface

areab (m2 g�1)

Openc surface

area (m2 g�1)

Total pore volumeb

(cm3 g�1)

Microporec

volume (cm3 g�1)

PS-500 P. sylvestris 500 0.75 320 24 0.186 0.16

PS-480 P. sylvestris 480 0.65 240 46 0.144 0.10

PS-450 P. sylvestris 450 0.68 3.6 2 0.0060 0.0009

PS-380 P. sylvestris 380 0.69 1.3 o0.1 0.0016 0.0007

PS-300 P. sylvestris 300 0.61 1.0 1 0.0017 0.0000

B-820 B. pendula 820 0.81 6670.017 61 0.05970.001 0.002

B-700 B. pendula 700 0.78 430 82 0.253 0.18

B-600 B. pendula 600 0.67 5.6 2.1 0.0073 0.0018

B-500 B. pendula 500 0.81 6.5 1.8 0.0068 0.0024

B-400 B. pendula 400 0.81 3.0 0.7 0.0036 0.0011

B-300 B. pendula 300 0.76 2.3 1.9 0.0035 0.0001

AA-450 Araucaria

araucana

450 0.79 39772 34 0.226370.00 0.19

afoc: organic-carbon fractional content.bErrors on surface area, pore volume, open surface area, and micropore are all within 5% (plus and minus).cDetermined from t-plots (de Boer et al., 1966).

G. James et al. / Water Research 39 (2005) 549–558552

where Kfr is the Freundlich constant and N is the

Freundlich exponent.

To further elucidate the sorption processes, selected

data were fitted with the Polanyi–Dubinin–Manes model

as described by Kleineidam et al. (2002)

qe ¼ Voro exp½�RTðlnðSw=CeÞ=E�2; (2)

where Vo, ro, R, T, and E are the maximum volume of

sorbed chemical per unit mass of sorbent, the compound

density, the ideal gas constant, the temperature, and the

characteristic free energy of adsorption of a compound

compared to that of a reference compound, respectively.

In the present work, Vo and E were determined by

applying the model to the data.

The legend indicates the sample names as they appear in

Table 1.
4. Results
4.1. Sorption characteristics

4.1.1. Natural wood chars

Fig. 1 presents phenanthrene sorption isotherms with

the field char samples. These isotherms, along with the

Freundlich parameters (Table 3), demonstrate that the

natural char samples exhibit varying sorption coeffi-

cients (Kfr) and isotherm curvature (N). The range of log

Koc values for the field chars spans more than one order

of magnitude (5.0–6.4), which is similar to the range

inferred from prior research findings (Table 4).

Previous sorption studies have quantified phenan-

threne logKoc values (at 1 mg l�1) with various sediments

containing char or coal particles. Table 4 provides a

comparison of these literature values and the logKoc

values measured in this research. The previous logKoc

data include phenanthrene sorption parameters derived

from: (a) activated carbon (Snoeyink, 1990; Grathwohl

and Peschik, 1997; Gustafsson et al., 1997), (b) charcoal

(Kleineidam et al., 2002), and (c) sedimentary rocks and

fluvial sands where char was the majority of the organic

matter (Kleineidam et al., 1999; Karapanagioti et al.,

2000), including values derived from an additive model

evaluating the cumulative logKoc values (Karapanagioti

and Sabatini, 2000; Karapanagioti et al., 2001). The

field char samples produced a range in logKoc

values (5.0–6.4), similar to that reported by others

(5.6–7.1). Thus, the variability in phenanthrene sorp-

tion with the ‘‘whole’’ char samples in this study is

comparable with that inferred from previous research

ARTICLE IN PRESS

Table 3

Isotherm results with phenanthrene and chars collected in nature

Samplea Kfrb (mg kg�1) (lm g�1)N Nc log (Kfr

bfoc�1d) ¼ logKoc

e at Cef¼ 1mg l�1, 95% CIg

MVO-456 1.1 E 571.3 E 4 0.5370.04 5.0–5.2

MVO-457 1.2 E 571.4 E 4 0.4070.03 5.1–5.3

OF 3.9 E 578.2 E 4 0.4670.05 5.6–6.1

PP 4.0 E 576.2 E 4 0.4970.04 5.7–6.0

1989 1.7 E 575.3 E 4 0.6070.07 5.0–5.8

1982 1.3 E 672.9 E 5 0.2470.05 5.9–6.4

aSample names are explained in Table 1.bKfr: Freundlich sorption constant.cN: Freundlich exponent.dfoc: organic-carbon fractional content.eKoc: organic-carbon fractional content normalized sorption coefficient.fCe: equilibrium concentration of the contaminant in solution (mg l�1).gCI: confidence interval based on triplicate samples.

Table 4

Phenanthrene logKoca values at equilibrium concentration ¼ 1mg l�1 for pyrogenic materials from previous studies and the present

work

Source Material logKoca N b

(Snoeyink, 1990)c Activated carbon 7.1 0.44

(Grathwohl and Peschik, 1997)c Activated carbon 5.8–6.0 0.56–0.70

(Gustafsson et al., 1997) Soot 7.1

Activated carbon 6.5–7.1d

(Kleineidam et al., 2002) Charcoal 6.4e

(Kleineidam et al., 1999) Opaque particles, char, coal 6.3–6.8

(Karapanagioti et al., 2000) Char or coal-dominated particulate organic matter 5.6–6.4 0.55–0.57

(Karapanagioti and Sabatini, 2000) Opaque particles 5.8

(Karapanagioti et al., 2001) Opaque particles 5.6–6.8

Present research Natural chars 5.0–6.4 0.24–0.60

Artificial wood chars 5.4–7.3 0.13–0.42

aKoc: organic-carbon fractional content normalized sorption coefficient.bN: Freundlich exponent.cCalculated based on isotherm results and assuming activated carbon organic carbon content fraction foc ¼ 0.8.dCalculated by present work, reference (Gustafsson et al., 1997) from other references.eCalculated from (Kleineidam et al., 2002) using the same data analysis and units as in the present work, reference (Kleineidam et al.,

2002) reports values of 5.6 and 5.0 calculated from a combined partitioning and Polanyi potential theory based model, on the bases of

subcooled liquid solubilities and water solubilities, respectively, at Ce ¼ 1mg l�1.


with char-containing aquifer materials (Karapanagioti

and Sabatini, 2000; Karapanagioti et al., 2001).

4.2. Artificial wood chars

Given that the field chars show a high degree of

variability in their surface properties and behavior, it is

worthwhile to explore the contributing factors using

chars produced under controlled conditions. Artificial

wood chars were thus prepared with different starting

materials and heating temperature; while in the activated

carbon literature these factors are well known to affect

the sorptive properties of the resulting material (Matt-

son and Mark, 1971; Puri, 1980), to our knowledge this

is the first time they have been evaluated for subsurface

wood chars. Given that it is impossible to exactly

reproduce the specific field charring conditions, our

intent here is to evaluate whether starting material and

heating temperature can help explain the observed range

in sorption characteristics for field wood chars.


4.3. Sorption properties and heating temperatures

Fig. 2 shows the phenanthrene uptake isotherms with

the artificial wood char samples from the species Pinus

sylvestris and Betula pendula. The isotherm data clearly

demonstrates that phenanthrene sorption (qe) increases

for materials exposed to higher temperatures, showing a

sharp jump at specific heating temperatures (450–480 1C;

Fig. 2a). Also, for samples from the species B. pendula, a

sorption jump occurs for samples heated at 600–700 1C

(Fig. 2b). While the specific temperature at which this

jump occurs may depend on the method used to produce

the char samples, these results show that the heating

temperature can be at least partly responsible for the

observed variation in wood char adsorptivity. These

sorption jumps also coincide with dramatic increases in

surface area (Table 2). Additionally, isotherms with the

artificial chars become increasingly nonlinear as the

heating temperature increases. The N values range from

0.42, which is moderately nonlinear to 0.13, which is

highly nonlinear (Table 5 presents the Kfr and N values

of the isotherms shown in Figs. 2a and b). This

1,E+05

1,E+06

1,E+07

1,E+08

1 10 100 1000

Ce (µg l-1)

qe

(µg

kg

-1)

f oc-1

PS 500 PS 480PS 450PS 380PS 300

1,E+05

1,E+06

1,E+07

1,E+08

1 10 100 1000

Ce (µg l-1)

q e

(µg

kg

-1)

f oc-1

B 820

B 700

B 600

B 500

B 400

B 300

(a)

(b)

Fig. 2. Phenanthrene sorption isotherm data for artificial chars

from the species (a) P. sylvestris and (b) B. pendula. The legend

indicates the temperature of heating.

increasing nonlinearity reflects an increasing heteroge-

neity of the char surface and thus its affinity for

nonpolar solutes.

4.4. Variation with source material

The surface properties of a wood char produced at a

given heating temperature also depend on the starting

material. The phenanthrene uptake isotherms with

450 1C P. sylvestris and 500 1C B. pendula samples are

very similar (Fig. 3, Kfr and N values in Table 5).

However, the isotherms for P. sylvestris and Araucaria

araucana samples are different (Fig. 3), even though

their heating temperatures are the same (450 1C). Thus,

at a given heating temperature, wood chars from varying

starting materials may exhibit differing sorptive capa-

cities even if they are heated under similar conditions.

For comparison purposes, Table 5 presents the

normalized sorption capacities (Kfrfoc�1) of phenanthrene

on wood chars at Ce ¼ 1 mg l�1 (logKoc). The phenan-

threne logKoc values are observed to range from 5.4 to

7.3 (Table 5). These values are consistent with the range

of logKoc values (5.6–6.8) that Karapanagioti et al.

(2001) found from samples containing opaque (char-

like) particles. The pattern of increasing logKoc values

with increasing heating temperature is observed for all

the artificially produced wood chars (Table 5). Thus, the

present results demonstrate that heating temperature

and starting material are potential factors that can help

explain the sorptive variability of natural chars,

concepts well known in the activated carbon literature

but to our knowledge demonstrated here for the first

time with natural wood chars.

4.5. Surface area and related properties of chars

Tables 1 and 2 present data on total surface area,

open surface area, total porosity, and microporosity for

the field and artificial wood char samples. The term open

surface area refers to the area associated with the

nonporous structure of the solid whereas the total

surface area includes the surface area from the porous

structure of the solid. Total porosity refers to the total

volume of all micro-, meso-, and macro-pores and

microporosity refers only to the volume of micropores

that have a diameter less than 20 A (Gregg and Sing,

1982; Chiou, 2002).

For the field chars, total surface areas range from 1.7

to 85m2 g�1. Samples with high total surface areas also

exhibit high micropore volumes (e.g., MVO-456, 1982).

However, no simple correlation exists between these

properties and the sampling site, the cause/extent of

charring, or the starting material. Samples MVO-456

and MVO-457, both produced from pyroclastic flow and

collected at the same site, exhibit total surface areas of

19 and 3.8m2 g�1, respectively. Also, samples 1989 and

ARTICLE IN PRESS

Table 5

Isotherm results with phenanthrene and artificial wood chars

Samplea Kfrb (mg kg�1)

(lm g�1)NNc qe

d SA�1 (mgm�2) at

Cee¼ 300mg l�1

Vof (cm3 kg�1) log (Kfr

bfoc�1g) ¼ logKoc

h at

Cee¼ 1mg l�1 (95% CIi)

PS-500 1.1 E 771.1 E 6 0.1370.03 78 0.022 7.1–7.3

PS-480 5.6 E 677.1 E 5 0.1570.03 58 0.014 6.8–7.1

PS-450 9,3 E 577.3 E 4 0.2570.02 1500 0.0037 6.1–6.2

PS-380 3.3 E 574.3 E 4 0.3670.04 2600 0.0026 5.5–5.9

PS-300 2.5 E 574.2 E 4 0.4270.04 3400 0.0034 5.4–5.8

B-820 1.0 E 777.9 E 5 0.2170.02 470 0.028 7.0–7.2

B-700 1.2 E 771.8 E 6 0.1670.04 62 0.024 7.0–7.3

B-600 4.3 E 571.1 E 5 0.4270.07 700 0.0073 5.5–6.1

B-500 6.4 E 571.2 E 5 0.3670.04 740 0.0050 5.7–6.1

B-400 6.6 E 576.8 E 4 0.3270.03 1600 0.0041 5.8–6.0

B-300 7.5 E 571.5 E 5 0.2770.05 3400 0.0030 5.8–6.2

AA-450 3.4 E 674.0 E 5 0.3470.04 56 0.019 6.5–6.7

aSample names are explained in Table 2.bKfr: Freundlich sorption constant.cN: Freundlich exponent.dqe: mass of chemical sorbed per unit mass of soil.eCe: equilibrium concentration of the contaminant in solution (mg l�1).fVo: maximum volume of sorbed chemical per unit mass of sorbent.gfoc: organic-carbon fractional content.hKoc: organic-carbon fractional content normalized sorption coefficient.iCI: confidence interval; mean7standard deviation.

1,E+06

1,E+07

1,E+08

1 10 100 1000

Ce (µg l-1)

qe

(µg

kg

-1)

f oc-1

Araucaria araucana-450Pinus sylvetris-450Birch pendula-500

Fig. 3. Comparison of the phenanthrene sorption isotherms for

artificial chars from three different species given similar heating

temperatures.


1982, both from the same starting material, Populus spp.,

have total surface areas of 1.7 and 85m2 g�1, respec-

tively.

For the artificial wood char samples, a dramatic

increase in surface area occurs at a certain heating

temperature. For example, for the wood char from the

B. pendula series (B-300 to B-820), the total surface area

is less than 6.5m2 g�1 for all samples heated between 300

and 600 1C, but increases dramatically to 430m2 g�1

when heated at 700 1C. Likewise, for the P. sylvestris

series the total surface area increases from less than

3.6m2 g�1 when heated below 450 1C to greater than

240m2 g�1 when heated at higher than 480 1C. Similar

trends are also observed for the other three properties

(i.e. open surface area, total porosity and microporos-

ity).

From Table 2, the fractions of the open surface area

to the total surface area decrease for samples with high

total surface areas. For example, 38% of the total

surface area (66m2 g�1) for sample B-600 comes from

the open surface area, whereas for B-700 only 19% of

the total surface area (430m2 g�1) comes from the open

surface. This suggests that the heating process produces

a significant amount of micropores, which agrees with

the measured micropore volumes in Table 2 (e.g. B-600:

0.0018 cm3 g�1 vs. B-700: 0.18 cm3 g�1).

Samples with low total surface area generally have

larger fractions of open surface area. It appears that low

temperatures do not activate the charred wood well

enough to produce micropores. In some cases, however,

the high-temperature heating may also produce small

surface areas. For the B. pendula series with the heating

temperature increasing from 700 to 820 1C, the total

surface area decreased from 430 to 66m2 g�1; a similar

decrease occurred for the other three properties (i.e.

open surface area, total porosity, and microporosity). At

high heating temperatures (i.e. higher than 820 1C) the

pores created do not contribute to surface area and

adsorption of N2 and our solute (Van Krevelen, 1993).

While this may well be an artifact of our experimental


procedure for producing wood chars, such a situation

might also occur in natural systems.

5. Discussion

Since the Koc values indicate that partitioning is not

the only sorption mechanism active, alternate mechan-

isms are explored. Fig. 4 presents logKoc values (at

Ce ¼ 1mg l�1) for all chars studied in the present work

versus their surface areas. As expected there is a general

trend of increasing logKoc with increasing surface area;

however, the increase is not proportional for all samples.

These results suggest that while increases in surface area

do result in increased sorption, the surface areas created

by the different chars exhibit varying sorption affinities.

This is also corroborated by the calculated qe SA�1 at

Cee¼ 300 mg l�1 (the maximum equilibrium concentra-

tion in our experiments) and Vo which were calculated

on the assumption of surface sorption and pore filling,

respectively (Table 5). The qe SA�1 results suggest that

not all samples have equal coverage; Vo in some cases

overestimates the sample microporosity suggesting that

not only pore filling takes place for samples of low

surface area.

Thus, it is observed that surface area alone is not

adequate to fully characterize char adsorption. A recent

study shows that different heating temperatures produce

different amounts of hydrophilic acidic/basic surface

groups (Chun et al., 2004). Such hydrophilic groups

strongly attract water and thus reduce the adsorption of

organic solutes from water by adsorptive competition.

Thus, the adsorption per unit SA may vary significantly

among char samples. Also, many low-temperature chars

(or those heated at high temperatures with short heating

times) may also contain a significant amount of organic

residue that could contribute to char uptake by partition.

Thus, on a unit SA basis, one may find a higher uptake

5

6

7

8

1 10 100 1000

SA (m2 g-1)

log

Ko

c

Fig. 4. logKoc versus surface area (SA) for the laboratory and

field char samples.

with these low-temperature chars (Table 5). Certainly the

involvement of both organic residue and surface acidic/

basic polar groups could significantly affect the solute

uptake on a unit SA basis. With this consideration, it

appears that the simple properties (e.g., the quantity and

surface area of char or black carbon) commonly used to

characterize other well-defined adsorbent solids (e.g.,

activated carbon) might not be sufficient for characteriz-

ing the behavior of natural wood chars. Similar results

have been observed with black carbon contents in natural

solids (Karapanagioti et al., 2004).

6. Conclusions

The main conclusions of this study are as follows:

�
The uptake capacities and nonlinear sorption effects
with the laboratory wood chars are similar to those

with the field wood chars and observed in previous

research for char-containing aquifer materials; the

sorptive properties of the wood chars in this research

are highly heterogeneous.

�
For phenanthrene aqueous concentrations at 1 mg l�1,
the organic carbon-normalized sorption coefficients,

logKoc, range from 5.0 to 6.4 for field chars and from

5.4 to 7.3 for laboratory wood chars, which is

consistent with variations inferred for wood chars in

previous studies (5.6–7.1).

�
The results with artificial chars suggest that there is a
critical heating temperature above which a given

starting material produces a wood char with the

highest adsorption capacity and surface area (i.e., both

starting material and heating temperature impact

sorptive characteristics). Thus, these processes, which

are well known to be important in the activated carbon

literature, also prove to be important for sorption with

naturally produced wood chars.

�
Surface area alone is not adequate for characterizing
sorptive characteristics; heterogeneous surface prop-

erties also contribute to sorptive properties of the

wood chars.

Acknowledgments

The authors would like to thank Dr. Mark Nanny

from the University of Oklahoma for assistance in

making foc measurements. Hrissi Karapanagioti was

partially funded by the Greek National Scholarship

Foundation while working on this project.

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Evaluating phenanthrene sorption on various wood chars