odts

chemical activationapplication to air and water treatments

wastewater and gas treatments have been determined using response surfaces methodology. In order to have a

is widely used for water and gaseous emissions puri- oxygen groups (Bansal et al., 1988). Activated carbons

are commonly obtained from various organic precursors

such as peat, wood, bituminous coal, coconut shells,

polymers, etc. (Bansal et al., 1988). In the physical

activation process, the precursor is rst carbonized at

600800 C under an inert atmosphere and activated

*Corresponding authors. Tel.: +33-2-5185-8295 (S. Rio),

tel.: +33-2-51-85-82-94; fax: +33-2-51-85-82-99 (C. Faur-

Chemosphere 58 (2005)Brasquet).

E-mail addresses: sebastien.rio@emn.fr (S. Rio), cather-high mass yield and to minimize the energetic cost of the process, the following optimal conditions, 1.5 g of H2SO4 g1

of sludge, 700 C and 145 min are more appropriate for use of activated carbon from sludge in water and gas treat-ments.

2004 Elsevier Ltd. All rights reserved.

Keywords: Sewage sludge; Valorization; Activated carbon; Chemical activation; Porosity; Adsorption

1. Introduction

Activated carbon is a highly eective adsorbent that

cation. This is due to their extended specic surface area

between 500 and 2000 m2 g1, their high pore volumeand the presence of surface functional groups, especiallySebastien Rio a,*, Catherine Faur-Brasquet a,*, Laurence Le Coq a,Philippe Courcoux b, Pierre Le Cloirec a

a Ecole des Mines de Nantes, GEPEA UMR CNRS 6144, 4 rue Alfred Kastler, BP 20722, 44307 Nantes Cedex 3, Franceb ENITIAA, SMAD Department, rue de la Geraudiere, BP 82225, 44322 Nantes Cedex 3, France

Received 24 March 2004

Abstract

The objective of this study is to optimize experimental conditions of sorbent preparation from sewage sludge

using experimental design methodology. Series of carbonaceous sorbents have been prepared by chemical activation

with sulfuric acid. The sorbents produced were characterized, and their properties (surface chemistry, porous and

adsorptive properties) were analyzed as a function of the experimental conditions (impregnation ratio, activation

temperature and time). Carbonaceous sorbents developed from sludge allow copper ion, phenol and dyes (Acid Red 18

and Basic Violet 4) to be removed from aqueous solution as well as VOC from gas phase. Indeed, according to

experimental conditions, copper adsorption capacity varies from 77 to 83 mg g1, phenol adsorption capacity variesbetween 41 and 53 mg g1 and VOC adsorption capacities (acetone and toluene) range from 12 to 54 mg g1. Eachresponse has been described by a second-order model that was found to be appropriate to predict most of the responses

in every experimental region. The most inuential factors on each experimental design response have been identied.

Regions in which optimum values of each factor were achieved for preparation of activated carbons suitable for use inExperimental design methof carbonaceous sorbenine.faur@emn.fr (C. Faur-Brasquet).

0045-6535/$ - see front matter 2004 Elsevier Ltd. All rights reservdoi:10.1016/j.chemosphere.2004.06.003ology for the preparationfrom sewage sludge by

423437

www.elsevier.com/locate/chemosphereusing steam or carbon dioxide at upper temperature.

ed.

and elemental composition of sludge samples are assessed

C) and activation time (60180 min) are analyzed usingexperimental design methodology. The regions of

424 S. Rio et al. / Chemosphere 58 (2005) 423437When activated carbons are prepared by chemical

activation, the precursor is mixed with chemical com-

pounds such as phosphoric acid or sulfuric acid and

carbonized-activated in a same step at various temper-

atures (Bagreev et al., 2001a). In recent years, a grow-

ing research interest consists in the production of

carbon-based adsorbents from a range of low-cost

precursors, mainly industrial or agricultural by-prod-

ucts. Polymers (Yue et al., 2002), wastes tires (San

Miguel et al., 2003) and numerous agricultural by-

products including wood (Tseng et al., 2003), almond

shells (Toles et al., 2000), apple pulp (Suarez-Garcia

et al., 2001), sugarcane bagasse (Mohan and Singh,

2002) or pecan shells (Bansode et al., 2003) have been

successfully converted into activated carbons on labo-

ratory scale.

On searching for economical and available starting

material, sewage sludge, the by-product generated dur-

ing wastewater treatment processes, has been identied

as an attractive precursor for activated carbon produc-

tion (Chiang and You, 1987; Lu et al., 1995; Bagreev

et al., 2001b; Tay et al., 2001; Calvo et al., 2001). In

France, sewage sludge production is about 950 000 tons

of dried matter per year. The traditional ways of sludge

valorization include farmland application, landll and

incineration. But, with some traditional disposal ways

coming under pressure like farmland applications and

others being phased out like landll, it is necessary to

seek cost-eective and innovative solutions to sewage

sludge disposal problem. Activated carbon preparation

from sewage sludge can be considered as an attractive

method in reducing sludge volume, and at the same

time, it produces reusable by-products. But, recent

studies which have shown the feasibility of this conver-

sion rarely determine the eects of activation conditions

on physico-chemical properties of sorbents produced as

well as on adsorption capacities of standard industrial

wastewater pollutants and volatile organic compounds

(VOC) in gas phase.

In this work, we focus on the preparation of acti-

vated carbon from sewage sludge. This is carried out by

chemical activation process using sulfuric acid. The

preparation of activated carbon is inuenced by many

factors. For this reason, a preliminary study on the eect

of these factors on activated carbon production has

been carried out in order to determine the most impor-

tant ones and their regions of interest (Rio et al., 2003).

In this study, the most inuential factors were found to

be impregnation ratio, activation temperature and acti-

vation time. Dierent characteristics of the activated

carbon produced are analyzed as a function of these

three factors. The main advantages of experimental de-

signs are: (i) the ability to evaluate the eects of inter-

actions between tested examined parameters; (ii) the

gain in reducing the number of experiments to beinterest of these factors have been detected in previous

study (Rio et al., 2003). This study, factor by factor,

was carried out by varying activation temperature be-

tween 400 and 900 C, activation duration between 1and 3 h and H2SO4 concentration from 1 to 3 M. And,

the inuence of these operating conditions on porosity

development have been studied. Notably, higher acti-

vating agent concentration is not used because pre-

liminary experiments have shown that, when sulfuric

acid concentration is higher than 3 M, destruction of the

material structure occurs during the thermal treatment

(Rio et al., 2003).by French standard methods (AFNOR NF U 44-171,

1982).

2.2. Activated carbon preparation

Chemical activation with sulfuric acid is carried out

using an impregnation method. Dried viscous liquid

sludge is mixed with H2SO4 solutions (13 M) for 6 h.

The resulting mixture is ltered and dried at 105 C.Then, the impregnated sludge is introduced in a 2 l

horizontal furnace and carbonized under nitrogen

atmosphere at 20 C min1. Nitrogen ow rate is equalto 2.5 l min1. The eects of impregnation ratio (0.51.5g H2SO4 g

1 sludge), activation temperature (600800undertaken, in comparison to a classical approach for

the same number of estimated parameters (Box and

Draper, 1987). This methodology is widely used in

chemical engineering notably to optimize adsorption

processes (Ricou et al., 2001; Annadurai et al., 2002) but

also to optimize experimental conditions for activated

carbon preparation from waste (Bacaoui et al., 2001).An experimental design is used to represent the re-

sponses of the factors in all the experimental regions

studied and to optimize the experimental conditions

for carbonaceous sorbents preparation from sewage

sludge.

2. Materials and methods

2.1. Raw material

Viscous liquid sludge collected from the municipal

wastewater treatment plant of Nantes-Tougas, in France,

is used as precursor in this study. In this domestic

wastewater treatment plant, wastewater undergoes a

biological treatment by activated sludges. Viscous liquid

sludge corresponds to aerobically digested sludge dewa-

tered by centrifugation. Dried matter content, ash content

S. Rio et al. / Chemosphere 58 (2005) 423437 4252.3. Activated carbon characterization

2.3.1. Porosity measurements

Nitrogen adsorption/desorption isotherms are mea-

sured at 77 K with a MICROMERITICS ASAP 2010

surface area analyzer. Samples are outgassed between

100 and 350 C, depending on the materials activated ornot, in a vacuum during 3 d before analysis. The specic

surface area (SBET) is calculated by BET equation(Brunauer et al., 1938). The mesopore (250 nm) and

micropore (

3. Results and discussion

pollutants, the following responses are also studied:

426 S. Rio et al. / Chemosphere 58 (2005) 4234373.1. Characterization of raw sewage sludge

The results of physico-chemical analysis of viscousmethodology allows experimental responses behavior to

be described as precisely as possible as a function of

factor variation and optimal conditions of the factors to

be determined for each experimental response.

Table 1

Experimental design matrix

No. Exp. Temperature

(C)X1

Time

(min)

X2

Impregnation

ratio

X3

1 600 60 0.5

2 600 60 1.5

3 600 180 0.5

4 600 180 1.5

5 800 60 0.5

6 800 60 1.5

7 800 180 0.5

8 800 180 1.5

Central run 1 700 120 1

Central run 2 700 120 1

Central run 3 700 120 1

Star point 1 532 120 1

Star point 2 868 120 1

Star point 3 700 20 1

Star point 4 700 220 1

Star point 5 700 120 0.15

Star point 6 700 120 1.85liquid sludge are presented in Table 2. First, dried

matter content of viscous liquid sludge is equal to 19.2%.

The relatively high quantity of carbon within viscous

liquid sludge (39.4 wt%) shows that it has the potential

to be converted in carbonaceous sorbent. Viscous liquid

sludge develops low SBET of 2.9 m2 g1, and the porestructure of the sludge is mainly macroporous. Finally,

the sludge contains heavy metals like lead or mercury

but their concentration are lower than the values xed

by French regulation. To evaluate the stability of these

toxic metallic compounds, the raw material and the

resulting sorbents have been submitted to leaching test

adapted from the French standard AFNOR X31-210

(2000). The results notably show that these heavy metals

are very stable within the raw material and the resulting

sorbents. The leaching percentage of these metallic

compounds was very low and did not exceed 2% in

demineralized water acidied at pH 5.

3.2. Responses analysis and interpretation

The experimental responses studied include: activa-

tion mass yield Y1, porous structure, surface chemistryand adsorption capacities at equilibrium. Porosity

development is characterized by SBET Y2, mesoporevolume Y3 and micropore volume Y4. Surface chem-istry evolution during activation is determined by mea-

surements of surface pH Y5, acidic surface functionalgroups Y6 and basic surface functional groups Y7.These characteristics are chosen because of their strong

inuence on activated carbon adsorption capacity

(Boehm, 1966). In order to estimate adsorption e-

ciency of sorbents produced towards standard industrial

Table 2

Physico-chemical characteristics of raw sewage sludge

Property/element Sewage sludge

SBET (m2 g1) 2.9Macroporosity (vol.%) 97.0

Dried matter (wt%) 19.2

Ash content (wt% of dried matter) 22.0

Carbon (wt%) 39.4

Oxygen (wt%) 19.8

Hydrogen (wt%) 5.6

Nitrogen (wt%) 6.4

Sulfur (wt%) 0.9

Phosphorus (wt%) 1.9

Calcium (wt%) 2.9

Potassium (wt%) 0.7

Magnesium (wt%) 0.5

Iron (wt%) 0.8

Copper (ppm) 306

Nickel (ppm) 76

Lead (ppm) 64

Zinc (ppm) 634

Mercury (ppb)

Table 3

Experimental design responses

No. Exp. Yield Porosity development Chemical prop

Y1 Y2 Y3 Y4 Y5 Y6

1 44.3 157 0.061 0.084 5.8 0.89

2 44.9 259 0.048 0.131 5.4 0.92

3 44.1 177 0.042 0.094 6.1 0.77

4 45.2 353 0.244 0.129 5.9 0.79

5 36.4 77 0.048 0.032 5.9 0.71

6 37.1 322 0.217 0.125 5.0 0.80

7 36.4 58 0.046 0.023 6.5 0.64

8 36.6 327 0.239 0.125 6.0 0.75

Central run 1 38.4 248 0.046 0.128 6.4 0.83

Central run 2 38.6 251 0.045 0.13 6.3 0.79

.81

.84

.65

.85

.76

.70

.81

.967

1); Ymeqg

on of

betwe

S. Rio et al. / Chemosphere 58 (2005) 423437 427Central run 3 38.9 255 0.047 0.131 6.5 0

Star point 1 49.6 69 0.042 0.029 6.7 0

Star point 2 33.3 61 0.047 0.024 7.1 0

Star point 3 37.5 200 0.046 0.104 5.9 0

Star point 4 35.1 252 0.048 0.125 5.6 0

Star point 5 38.7 231 0.23 0.085 6.9 0

Star point 6 37.6 352 0.231 0.148 5.9 0

R2 0.941 0.895 0.806 0.929 0.672 0Lack-of-t test 0.03

Y1, mass yield (%); Y2, SBET (m2 g1); Y3, mesopore volume (cm3 gfunctional groups (meq g1); Y7, basic surface functional groups ((mg g1); Y10, adsorption of Basic Violet 4 (mg g1); Y11, adsorptiadsorption of toluene (mg g1); R2, coecients of determinationlack-of-t statistical test.statistical model, and experimental responses. The ef-

fects of the three factors on mass yield Y1 are presentedin Fig. 1. Activation temperature X1 has a highlynegative eect on yield Y1, i.e., the increase of tem-perature from 600 to 800 C leads to a decrease of massyield, compared with others factors that have a very

weak eect on this response. Indeed, the increase in

temperature quickens the gasication reactions and,

therefore, the removal of amorphous components which

obstruct the pores, leading to a decrease in mass yield

(Bacaoui et al., 2001). Then, no signicant interactionsbetween the factors have been observed.

Fig. 1. Main eect plot for mass yield (%).erties Adsorptive properties

Y7 Y8 Y9 Y10 Y11 Y12 Y13

0.34 78 47 69 50 26 29

0.29 81 52 77 49 49 51

0.27 79 49 70 48 24 31

0.24 83 54 89 54 48 54

0.22 77 41 63 46 15 18

0.18 82 53 88 53 48 53

0.20 77 41 62 48 12 14

0.16 82 53 88 53 48 54

0.23 81 42 76 49 49 52

0.24 81 43 77 50 50 52

0.24 81 43 77 49 51 52

0.39 77 51 63 46 14 15

0.14 77 47 64 46 12 13

0.20 80 54 76 47 32 40

0.20 81 51 77 47 49 51

0.36 81 52 75 53 30 39

0.20 83 54 88 53 51 54

0.914 0.907 0.748 0.895 0.901 0.899 0.887

0.11

4, micropore volume (cm3 g1); Y5, surface pH; Y6, acidic surface

1); Y8, adsorption of copper (mg g1); Y9, adsorption of phenolAcid Red 18 (mgg1); Y12, adsorption of acetone (mg g1); Y13,en observed values and predicted values, Lack-of-t, p-value of3.2.2. Porosity development

The activation process allows to create and develop

the porous surface and volume giving adsorbent quali-

ties to resulting activated carbon. The nal porous

structure of the activated carbon is governed by the

precursor structure and by activation conditions (Bansal

et al., 1988). Generally, porosity of commercial activated

carbons is mainly constituted of micropores and even

mesopores.

First, the SBET Y2 of the sorbents varies from 59 to352 m2 g1 according to the experimental conditions(Table 3). These values are in agreement with literature

datas. Lu and Lau (1996) have obtained, during sewage

sludge chemical activation with 3 MH2SO4 at 650 C, anSBET of 220 m2 g1. Identical experiments have also beencarried out by Rozada et al. (2003) and they have ob-

tained adsorbents developing 390 m2 g1 at 625 C. But,SBET values obtained in this paper remain lower thanthese of commercial activated carbons which usually

vary between 500 and 2000 m2 g1 (Bansal et al., 1988).Note that commercial activated carbon consists of vari-

ous mineral compounds (ash) in quantities lower than

10% (Hassler, 1974) where as sorbents produced from

sludge in this study contains between 35% and 40% of

ash (Table 4). That ash content limits the porosity

development occurring within organic matter and be-

haves only as an inert material which does not contribute

to the porosity (Linares-Solano et al., 2000). By this way,

porosity development could be recalculated, taking only

organic matter content into account, and reaches about

500 m2 g1 of organic matter.A correct correlation between observed responses

and predicted responses is obtained by the statistical

model, with coecient of determination of 0.895 (Table

3). The eects and interaction of the factors on SBET Y2are presented in Fig. 2. On one hand, response analysis

shows that activation temperature X1 has slightlynegative eect on this response, but, the important

estimated variable as a function of pairs of factors. In

each plot, the SBET evolves versus the rst written factorin the X-axis and is parameterized according to the

second written factor. The + curves stand for the

maximum values of the parameter and the ) curvesstand for the minimum values. So, important interac-

tions exist between activation temperature and impreg-

nation ratio X1X3 (positive) which is more importantthan the eects of X1. Indeed, Fig. 2 shows that when theimpregnation ratio is xed to the lower value (0.5 g

H2SO4 g1 sludge), the increase of temperature leads to

Table 4

Ash and carbon, hydrogen, oxygen, nitrogen and sulfur contents (wt%)

X1 X2 X3 Ash content Carbon Hydrogen Oxygen Nitrogen Sulfur

600 60 0.5 35.0 42.2 1.2 11.0 4.4 3.1

600 60 1.5 34.5 40.8 1.2 11.6 3.7 4.1

600 180 0.5 38.1 37.3 1.5 10.4 4.2 4.3

600 180 1.5 37.4 35.9 1.5 10.7 3.8 4.4

800 60 0.5 40.3 44.5 1.5 10.1 3.0 3.9

800 60 1.5 40.3 39.9 1.6 10.3 2.6 5.0

800 180 0.5 41.7 42.5 1.2 8.6 2.7 4.1

800 180 1.5 41.2 37.0 1.3 9.1 2.5 5.0

700 120 1 35.9 43.1 1.6 8.6 3.6 5.0

428 S. Rio et al. / Chemosphere 58 (2005) 423437quadratic eect observed suggests that an optimal SBETvalue is reach between 600 and 800 C. On the otherhand, the increase in activation time X2 and impreg-nation ratio X3 allows SBET Y2 to be developed.Interaction plot is also presented in Fig. 2, it shows theFig. 2. Main eect plot and interaction plot for SBa dramatic decrease of SBET, while, when experiments arecarried out with high impregnation ratio (1.5 g H2SO4g1 sludge), the increase in temperature allows SBET to beincreased. Such interaction is observed between activa-

tion time and impregnation ratio X2X3.ET (m2 g1) and mesopore volume (cm3 g1).

For micropore volume response Y4 which resultsare not presented in this paper, exactly the same trends

for SBET response in terms of eects and interactions ofthe factors are obtained. In addition, these two experi-

mental responses are very dependent, the coecient of

determination between SBET Y2 and micropore volumeY4 is equal to 0.937 (Table 5). The increase ofimpregnation ratio X3 and activation time X2 im-proves the micropore volume Y4 of the sorbents ob-tained. The increase of activation time X2 would causethe opening of closed or non-accessible micropores. The

increase of impregnation agent concentration improves

SBET and micropore volume because the role of theimpregnation agent is to minimize the formation of tars

and any other liquids that could block the pores and

inhibit the porous structure development within the

sorbent (Guo and Lua, 1999).

The value of coecient of determination (Table 3)

between calculated and observed mesopore volume

impregnation ratio X1X3 and between activation timeand impregnation ratio X2X3 because the increase oftemperature X1 or time X2 improve mesopore volumeY3 if impregnation ratio X3 is xed to the value 0.5while, this response decreases with increasing tempera-

ture or time when impregnation ratio is xed to the

value 1.5.

Finally, the observation of pore size distributions of

the resulting sorbents activated at 600 and 800 C, withan impregnation ratio of 1.5, conrms the positive eect

of temperature on mesopore volume Y3 (between 2 and50 nm) and the positive eect of activation time (be-

tween 1 and 3 h) on mesopore volume (as well as on

micropore volume (

of acidic surface functional groups as shown in Table 3.

Amount of acidic surface groups Y6 is always at leasttwo times more important than basic surface groups

Y7. The quantities of acidic surface groups Y6 andbasic surface groups Y7 can be very correctly describedby a second order model, R2 values are equal to 0.967and 0.914, respectively (Table 3). In the case of surface

pH response Y5, the second order model is not adap-ted. A lack-of-t test is performed to test the adequacy

of the model. The p-value of lack-of-t statistical test isless than 0.05 and equal to 0.03 (Table 3). So, there is

statistically signicant lack-of-t at the 95% condence

level. This means that the model as tted does not

adequately represent the data. In addition, during the

analysis of variance of surface pH response, the test of

statistical signicance of each factor is also performed. It

is carried out by comparing the mean square against an

estimate of the experimental error. The results show that

only two eects, activation duration X2 and impreg-nation ratio X3, have p-values less than 0.05, indicatingthat they are signicantly dierent from zero at 95% of

430 S. Rio et al. / Chemosphere 58 (2005) 423437First, every sorbents produced are acidic, surface pH

varying between 5 and 7, due to the important quantity

Fig. 3. Pore size distributions of sorbent produced with

impregnation ratio of 1.5.

Fig. 4. Micrographs of dried sludge (a) andcondence level. The statistical analysis of this response

is not carried out in this paper.

Then, Fig. 5 clearly shows that activation temperature

X1 and time X2 have a negative eect on amounts ofacidic surface groups Y6 and basic surface groups Y7.As indicated in literature, surface groups decompose at

elevated temperature and their decomposition increases

with increasing activation time (Boehm, 1966). The in-

crease of impregnation ratio X3 leads to increase inacidic character of the sorbents produced because this

factor has a strong positive impact on amount of acidic

surface groups Y6 and a negative eect on amount ofbasic surface groups Y7 (Fig. 5).chemically activated sludge (b)(d).

he variability of the response (Table 3). These values are

n agreement with copper adsorption capacities obtained

ith commercial activated carbons (Faur-Brasquet

t al., 2002) and activated carbon prepared from agri-

ultural by-products (Toles et al., 1999). Main eect plot

hows a negative eect of activation temperature X1,hile, activation time X2 and impregnation ratio X3ave a positive eect (Fig. 6). But, the important qua-

ratic eect of activation temperature X1 reveals thatptimal value of this factor can be achieved. The eects

f the three factors are similar to those obtained for SBETY2 (Fig. 2) and micropore volume Y4. This observa-ion is conrmed by the important coecient of deter-

ination between copper adsorption capacity Y8 andBET Y2, and between, copper adsorption capacity Y8

S. Rio et al. / Chemosphere 58 (2005) 423437 431The elemental analysis carried out on the resulting

activated carbon shows that oxygen contents decrease

when activation temperature X1 and time X2 increase(Table 4). It would conrm that amounts of acidic and

Fig. 5. Main eect plots for acidic and basic surface functional

groups (meq g1).basic surface functional groups decrease with the in-

crease of these experimental design factors (Boehm,

1966). Sulfur content varies from 3.1% to 5.0% accord-

ing to the experimental conditions, the highest values

(between 4% and 5%) being found for the highest

impregnation ratio of 1.5 (Table 4). Sulfur content

within dried sludge being equal to 0.9%, important

quantity of sulfur incorporated into the materials during

impregnation step, is not removed during thermal

treatment. Table 4 also shows that both oxygen and

sulfur contents increase with increasing impregnation

ratio because of the oxydizing eect of H2SO4 (Daley

et al., 1996). Carbon content varies from 36% to 45%

according to activation conditions and carbon content

seems to decrease with increasing impregnation ratio

due to dilution eect. Finally, ash content varies from

45.0% to 51.7% and it increases with increasing tem-

perature and time. Ash content seems to slightly de-

crease with increasing impregnation ratio because the

activating agent limits the formation of tars (Martin

et al., 1996).

3.2.4. Adsorption tests in aqueous solution

First, copper adsorption capacity Y8 varies from 77to 83 mg g1 and this response is correctly modeled bythe statistic model because it explains more than 90% ofand micropore volume Y4 (Table 5). So, these porouscharacteristics seem to partly control the copper removal

eciency by the sorbents produced. During copper

adsorption experiments, the evolution of calcium con-

centration in solution has been followed. The evolution

of Cu2 removed and Ca2 exchanged during kineticexperiment with chemically activated sludge at 600 Cfor 180 min and with impregnation ratio of 1.5 are

presented in Fig. 7. The simultaneous desorption of

calcium in solution would indicate that ion exchange

mechanism would also take place in the sorption phe-

nomenon of metallic ion.

Experimental design results show that phenol

adsorption capacity varies from 41 to 54 mg g1

according to activation conditions and the second order

model as tted explains only 75% of the variability of

this response (Table 3). So, a lack-of-t statistical test is

realized to test the adequacy of this model. The p-value

of the lack-of-t is greater than 0.05 (Table 3), the model

appears to be adequate for the observed data at the 95%

condence level. The values of phenol adsorption

capacities remain lower than phenol adsorption capaci-

ties obtained with commercial activated carbon (Nevs-

kaia et al., 1999) but they are similar to those obtained

using activated carbon from sludge (Martin et al., 1996;

Fig. 6. Main eect plot for copper adsorption capacity1t

i

w

e

c

s

w

h

d

o

o

t

m

S(mg g ).

volume Y4 responses. So, when the quantity of sulfuricacid is low during impregnation, the increase of activa-

tion temperature would lead to a micropore widening

(or destruction) and a decrease of phenol adsorption.

And, when, impregnation ratio and activation temper-

ature increase, an important micropore development

would occur, favoring phenol adsorption.

Finally, similar adsorption experiments have been

carried out to estimate the anity of the sorbents pre-

pared to remove Basic Violet 4, a cationic dye, and Acid

Red 18, an anionic dye, for aqueous solution. The

experimental data can be correctly tted by second order

model, the coecients of determination between experi-

mental and predicted adsorption capacities of Basic

same trend is observed for Acid Red 18 adsorption in

Fig. 7. Evolution of Cu2 removed (mM) and Ca2 exchanged

432 S. Rio et al. / Chemosphere 58 (2005) 423437Chen et al., 2002). The increase of activation tempera-

ture X1 and time X2 lead to the lightly decrease ofphenol adsorption capacity Y9 when the increase ofimpregnation ratio has a positive eect on this response

(Fig. 8). Even no strong dependence between this re-

sponse and porosity development, notably micropore

development Y4, has been identied, this increase ofphenol adsorption with increasing impregnation ratio is

attributed to the important microporosity development

(Martin et al., 1996). Experimental results show that

highest phenol adsorption capacities are observed when

important microporosity development occurs during

activation that corresponds to experiments (nos. 2, 4, 6,

8) carried out using impregnation ratio of 1.5 (Table 3).

Finally, an important interaction exists between activa-

tion temperature and impregnation ratio X1X3. Asshown in Fig. 8, when impregnation ratio X3 is xed tothe lower level (0.5 g H2SO4 g

1 sludge), the increase ofactivation temperature X3 leads to important decreaseof phenol adsorption capacity Y9, and, when impreg-nation ratio X3 is xed to the value +1 (1.5 g H2SO4

(mM) during kinetic experiment with chemically activated

sludge at 600 C for 180 min and with impregnation ratio of 1.5.g1 sludge), phenol adsorption capacity Y9 slightly in-crease with increasing activation temperature X3. Thesame trend was observed for SBET Y2 and micropore

Fig. 8. Main eect plot and interaction plot fterms of eects of the experimental design factors. First,

activation temperature X1 seems to have no eect ondye adsorption capacity but important quadratic eect of

this factor allows an optimal value to be determined. The

increase of activation time X2 and impregnation ratioX3 allow dye adsorption capacity to be increased (Fig.9). The factors eects are identical to those obtained for

mesopore volume Y3 (Fig. 2). Indeed, a strong depen-dence would exist between dye adsorption capacities

(Basic Violet 4 and Acid Red 18) and SBET Y2, mesoporevolume Y3 and micropore volume Y4 (Table 5). Due todye molecule size (23 nm) both micropore and meso-

pore development are important for the adsorption

process of these organic pollutants. Because of anionic

character of Acid Red 18 molecule, an important corre-

lation also exists between Acid Red 18 adsorption

capacity Y11 and the amount of acidic surface groupsY6 that would participate to adsorption mechanism ofthis dye, negatively charged in aqueous solution.

3.2.5. Adsorption tests in gas phase

Adsorption experiments in gas phase are carried out

to estimate the eciency of the sorbents produced to

1Violet 4 and Acid Red 18 are equal to 0.895 and 0.901,

respectively. Fig. 9 presents the statistical results ob-

tained for Basic Violet 4 adsorption because exactly theor phenol adsorption capacity (mg g ).

and 800 C due to quadratic eect of this factor, andpositive eects of activation time and impregnation ratio

(Fig. 10). The eects of these factors on VOC adsorption

capacities are identical to those obtained for SBET andmicropore volumes responses (Fig. 2). This observation

is conrmed by values of correlation coecients

revealing a strong dependence between these responses

(Table 5). This dependence between VOC adsorption

and microporosity development have been observed in

numerous research works (Subrenat, 1999; Chiang et al.,

2001; Lillo-Rodenas et al., 2002).

3.3. Optimization step

S. Rio et al. / Chemosphere 58 (2005) 423437 433remove two dierent VOC: acetone and toluene.

Fig. 9. Main eect plot for Basic Violet 4 and acetone

adsorption capacities (mg g1).Experimental design results show that adsorption

capacities of acetone Y12 and toluene Y13 vary be-tween 1251 mg g1 and 1354 mg g1, respectively.Adsorption capacities of acetone and toluene onto

commercial activated carbons are, in the same experi-

mental conditions, at least two times higher than values

obtained in this study (Subrenat, 1999). These data are

correctly tted by a second order model (Table 3). Solely

statistical results of acetone adsorption capacity Y12are presented in Fig. 10 because the same trends are

observed for toluene adsorption capacity Y13. First,main eect plot shows negative eect of activation

temperature, even an optimal is observed between 600

Fig. 10. Estimated response surfaces for SBET (m2 g1) and mesoporeconditions.volume (cm3 g1) with impregnation ratio of 1.5; (+) optimalTable 6 summarizes the eects of experimental design

factors on porous structure and adsorption capacities of

the sorbents produced. It shows that optimization of all

the responses under the same experimental conditions is

impossible because the inuence of the factors is dier-

ent.

Using response surfaces methodology, the ranges of

each factor allowing the ideal activated carbon to be

obtained are determined. The rst step is to establish the

values of structural characteristics of an ideal activated

carbon. This sorbent should have a large SBET Y2 andimportant micropore Y4 and mesopore Y3 volumesallowing their use for removal of a large range of pol-

lutants. Experimental design analysis has shown that

impregnation ratio X3 must be xed to the value 1.5 toobtain well developed porous structure. The optimal

point for SBET indicated by the model is equal to 376m2 g1 corresponding to an activation temperature of691 C and an activation time of 220 min (Fig. 10). Thesame optimal temperature is determined to obtain a

maximum micropore volume Y4 of 0.155 cm3 g1 butfor an optimal activation time of 140 min. More devel-

oped mesopore volume Y3 is obtained when activationtemperature is equal to 868 C and an activation time of220 min (Fig. 10). But, as shown in Fig. 10, when acti-

vation temperature varies from 532 to 868 C for anactivation time of 220 min, mesopore volume remains

nearly unchanged (0.2250.240 cm3 g1). So, if activa-tion temperature is xed to the value 691 C in order to

163 min, respectively (Fig. 12). These values are very

close to those obtained during response surface esti-

Table 6

Summary of the eects of experimental design factors on sorbents properties

Response Activation temperature Activation time Impregnation ratio

Micropore volumeSBET ) + +Mesopore volume + + +

Copper adsorption ) + +Phenol adsorption ) ) +Dye adsorption + + +

VOC adsorption ) + +

434 S. Rio et al. / Chemosphere 58 (2005) 423437maximize SBET and micropore volume, the mesoporevolume obtained is 0.232 cm3 g1 which is very close tothe optimal one (0.240 cm3 g1).

Then, optimization of adsorption capacities of cop-

per Y8, phenol Y9, Basic Violet 4 Y10 and Acid Red18 Y11 have also been carried out. First, the optimalconditions to maximize copper adsorption capacity Y8are impregnation ratio of 1.5, activation temperature of

680 C and an activation time equal to 220 min (Fig. 11).Under these experimental conditions copper adsorption

capacity reaches 84 mg g1. The response surfaces ofBasic Violet 4 adsorption capacity Y10 is identical tothis of copper adsorption capacity and optimal value of

this response is 91 mg g1. It conrms that adsorptionmechanisms of these two positively charged molecules in

aqueous solution would be identical and favored under

the optimal conditions determined by response surfaces

methodology. The estimated response surfaces for Acid

Red 18 adsorption capacity Y11 is also presented in Fig.11 and reveals that this response is maximum when

impregnation ratio is equal to 1.5 with activation tem-

perature of 750 C and activation time of 180 min. But,the variation of these adsorption capacities weak in the

experimental conditions studied allowing a compromise

to be found.

In the case of acetone Y12 or toluene Y13 adsorp-tion capacities, The optimal conditions maximizing

these responses are impregnation ratio of 1.5, activation

temperature of 703 C and activation time of 145 andFig. 11. Estimated response surfaces for copper and Acid Red 18 ad

optimal conditions.mation of micropore volume Y4 (X1: 691 C, X2: 140min, X3: 1.5) because VOC adsorption eciency isstrongly dependent on microporosity development

(Table 4).

To optimize all the responses under the same exper-

imental conditions is dicult because regions of interest

of the factors are dierent. Therefore, a compromise

must be found. The cost of the process must be also

taken into account and this activated carbon should

have a relatively high mass yield (3540%).

The optimal experimental conditions already identi-

ed can be classied in two distinct regions. The rst

one: activation temperature of about 700 C, activationtime of 145160 min and impregnation ratio of 1.5,

correspond to optimal conditions of the following re-

sponses: micropore volume Y4, acetone Y12 and tol-uene Y13 adsorption capacities. The second region ofinterest, corresponding to optimal conditions for SBETY2 development, copper Y8 and Basic Violet 4 Y10adsorption capacities, is dened by an activation tem-

perature of about 700 C, an activation time of 220 minand impregnation ratio of 1.5. Results of response sur-

faces methodology allow the experimental design re-

sponses to be estimated in these two regions of interest

and compared. As shown in Table 7, on one hand, some

experimental responses, like micropore volume Y4,copper Y9, Acid Red 18 Y11 and toluene Y13adsorption capacities do not signicantly vary between

the two optimal conditions obtained due to the lowsorption capacities (mg g1) with impregnation ratio of 1.5; (+)

rptio

0

5

olum

1, ad

S. Rio et al. / Chemosphere 58 (2005) 423437 435eect of activation time X2 on these responses. On theother hand, response surfaces curves of SBET Y2 andmesopore volume Y3 show an increase, even weak, ofthese responses between the rst and the second region

of interest (Fig. 10). Finally, a slight decrease of VOC

adsorption capacities is observed between the rst and

the second region of interest (Fig. 12). It could be

Table 7

Predicted responses for optimal activated carbons

Optimal conditions X1, X2, X3 Y2 Y3 Y4700 C, 145160 min, 1.5 wt/wt 350 0.175 0.13700 C, 220 min, 1.5 wt/wt 377 0.232 0.12

Y2, SBET (m2 g1); Y3, mesopore volume (cm3 g1); Y4, micropore vphenol (mg g1); Y10, adsorption of Basic Violet 4 (mg g1); Y1(mg g1); Y13, adsorption of toluene (mg g1).Fig. 12. Estimated response surfaces for acetone and toluene adso

conditions.attributed to the decrease, even weak, of micropore

volume Y4, these responses being strongly correlated asshown on Table 4.

To conclude on this optimization step, in order to

limit the energetic cost of the process and to obtain a

satisfying mass yield, the following conditions: activa-

tion temperature of 700 C, activation time of 145 minand impregnation ratio of 1.5, allow to obtain a satis-

fying compromise between the dierent responses stud-

ied in this paper.

4. Conclusion

Carbonaceous materials with adsorptive properties

have been produced from sewage sludge by chemical

activation process using sulfuric acid. Experimental de-

sign methodology has been used in order to determine

the eects of activation temperature (600800 C), acti-vation time (13 h) and impregnation ratio (0.51.5 g of

H2SO4 g1 of sludge) on chemical properties, porous

structure and adsorption capacities of the sorbents pre-

pared. Response surfaces methodology allowed the

optimal conditions for activated carbon preparation to

be determined.First, the carbonaceous sorbents produced are acidic

in nature due to the important quantity of acidic surface

functional groups. This acidic character increases with

increasing impregnation ratio while the increase of

activation temperature and activation time lead to an

increase of surface pH of the activated carbons. The

analysis of the responses characterizing the porosity

n capacities (mg g1) with impregnation ratio of 1.5; (+) optimal

Y8 Y9 Y10 Y11 Y12 Y13

83 50 88 52 58 62

84 55 91 52 54 61

e (cm3 g1); Y8, adsorption of copper (mg g1); Y9, adsorption ofsorption of Acid Red 18 (mg g1); Y12, adsorption of acetonedevelopment shows, on one hand, a negative eect of

temperature on SBET and micropore volume and positiveeects of activation time and impregnation ratio on

these responses. And, on the other hand, the three fac-

tors seem to have a positive eect on mesopore volume

in the ranges studied.

The eects of the three factors of the experimental

design on VOC adsorption capacities in gas phase follow

the same trend as micropore volume development, i.e.,

positive eects of activation time and impregnation ratio

and negative eect of activation temperature. Anionic

and cationic dyes adsorption capacities in aqueous

solution are strongly dependent on micropore and

mesopore development. In the case of copper adsorption

capacity, this response depends on porous structure

development but, an ion exchange mechanism has been

identied, taking part in the adsorption process.

The optimization step, carried out using response

surfaces methodology, allows two dierent optimal

experimental conditions for activated carbon production

to be identied. The rst region of interest: activation

temperature of 700 C, activation time of 145 min andimpregnation ratio of 1.5, correspond to optimal con-

ditions for micropore volume and VOC adsorption

capacities. The second region of interest, corresponding

436 S. Rio et al. / Chemosphere 58 (2005) 423437AFNOR NF U 44-171. AFNOR Ed., Paris, 1982.

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Y. Gouriou (Ecole des Mines de Nantes) for the

experimental assistance.

Referencesto optimal conditions for SBET development, copper andBasic Violet 4 adsorption capacities, is dened by acti-

vation temperature of 700 C, activation time of 220 minand impregnation ratio of 1.5. In order to have a high

mass yield and to reduce the energetic cost of the pro-

cess, the following optimal conditions, 1.5 g of H2SO4g1 of sludge, 700 C and 145 min are more appropriate.

Further experiments should be carried out to increase

the porosity development and adsorptive properties of

activated carbons produced from sludge, like the intro-

duction of a preliminary carbonization step at low

temperature (300400 C) in the overall activation pro-cess. A pre-development of porosity could occur allow-

ing a more ecient impregnation step before activation.

Finally, in order to evaluate the economic feasibility

of sludge chemical activation process, a technico-eco-

nomic approach will be carried out and based on a mass

yield of approximately 38%. This cost analysis would

include equipment, sulfuric acid consumption, nitrogen

consumption, electricity usage. . .

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S. Rio et al. / Chemosphere 58 (2005) 423437 437

Experimental design methodology for the preparation of carbonaceous sorbents from sewage sludge by chemical activation--application to air and water treatmentsIntroductionMaterials and methodsRaw materialActivated carbon preparationActivated carbon characterizationPorosity measurementsChemical properties of materials

Adsorption testsAdsorption experiments in aqueous solutionAdsorption experiments in gas phase

Experimental design methodology

Results and discussionCharacterization of raw sewage sludgeResponses analysis and interpretationMass yield (Y1)Porosity developmentChemical propertiesAdsorption tests in aqueous solutionAdsorption tests in gas phase

Optimization step

ConclusionAcknowledgementsReferences