Ozonation of 1,3,6-naphthalenetrisulphonic acid catalysed by activated carbon in aqueous phase
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Applied Catalysis B: Environmental 39 (2002) 319–329
Ozonation of 1,3,6-naphthalenetrisulphonic acid catalysed by
activated carbon in aqueous phase
J. Rivera-Utrilla∗ , M. Sánchez-Polo
Departamento de Quı́mica Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
Received 9 February 2002; received in revised form 20 May 2002; accepted 20 May 2002
Abstract
This paper presents experimental results of the ozonation of a model aromatic sulphonic compound, 1,3,6-naphthalenetrisul-
phonic acid (NTS), in the presence of different activated carbons with different physical and chemical surface properties.
Carbons used were commercial activated carbons (Ceca AC40, Norit, Merck, Witco, Ceca GAC, Filtrasorb 400, Sorbo) with
or without demineralisation pre-treatment. Carbon samples were texturally and chemically characterised using N2 adsorption
isotherms, mercury porosimetry, pHPZC , selective neutralisation and elemental analysis. Results show that NTS was degraded
by ozone at a faster rate in the presence of activated carbon, especially in the case of Sorbo, Ceca GAC and Norit carbons,
which display catalytic activity, probably by enhancing ozone decomposition in aqueous phase in highly oxidative species.
These catalytic properties seem to be favoured by both the basicity of the carbon surface and the higher macropore volume.
Dissolved total organic carbon from the NTS degradation compounds was removed in the presence of activated carbon through
both the catalytic activity of activated carbon to mineralise organic matter and the adsorption of these organic compounds on
activated carbon.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ozone; Activated carbon; Catalysis; 1,3,6-Naphthalenetrisulphonic acid
1. Introduction Indeed, advanced oxidation processes (AOP) [1] have
attracted considerable attention due to the destruc-
Toxic organic contaminants, such as heterocyclic tive power of highly reactive free radicals involved
and phenolic compounds, present serious environmen- in these systems. Organic molecules are readily de-
tal risks and should be eliminated before discharge stroyed by OH• radicals, with typical rate constants
into natural water bodies. Unfortunately, conventional in the range 106 –109 M−1 s−1 .
biological effluent treatment systems cannot meet the In all cases, the aim of AOP is to generate OH•
abatement requirements set by the regulations be- radicals that react with organic molecules; never-
cause these compounds are difficult to metabolise or theless, due to their high reactivity and poor se-
may even inhibit microbial activity. In this context, lectivity, these free radicals may be consumed in
more sophisticated treatment alternatives should be chemical reactions with other compounds usually
implemented to deal with this type of contaminant. present in wastewater, such as butanol, methanol,
carbonates and bicarbonates, which act as free rad-
∗ Corresponding author. Tel.: +34-958-248523; ical scavengers, stopping propagation reactions in-
fax: +34-958-248526. volved in the oxidation of aromatic compounds
E-mail address: jrivera@ugr.es (J. Rivera-Utrilla). [2,3].
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 1 7 - 0320 J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329 In the case of ozone-based processes, the presence tronic density, leading to a decrease in its reactivity of such free radical scavengers has a significant ef- towards ozone. fect on performance, so that higher ozone consump- The present paper aimed to study the ozonation of tion is required to meet discharge standards [4], and NTS, as a model contaminant, in the presence of differ- may even lead to the formation of mutagenic organic ent commercial activated carbons to enhance its degra- by-products [5]. dation and removal. The catalytic effect of these acti- In order to overcome those shortcomings, catalysts vated carbons and the parameters affecting this ozona- have been introduced to increase ozonation perfor- tion process were investigated. mance [6]. Recently, Hewes et al. [7] reported that ozonation of phenolic compounds was more effec- 2. Experimental tive when conducted in the presence of Fe(II), Mn(II), Ni(II) or Co(II) sulphates. Moreover, Abdo et al. [8] 2.1. Methods showed that Zn(II) and Cu(II) sulphates, Ag(I) nitrate and Cr(III) oxide presented catalytic effects during Ozone was produced from pure oxygen using ozone decolouration of textile effluents. Similar re- an Ozokav ozone generator rated at 76 mg O3 /min. sults were reported by Gracia et al. [9] in the case A 2 l temperature-controlled stirred reactor system of ozonation of humic compounds in the presence of was used. Experiments were conducted at 25 ◦ C and Mn(II) and Ag(I), and by Andreozzi et al. [10] during 260 rpm. Analytical grade H3 PO4 and NaOH were ozonation of oxalic acid with Mn(II) at low pH. used to adjust the pH. One litre of NTS-free solution There are few published experimental studies at the set pH was poured into the reactor and ozone on heterogeneous catalysed ozonation of aromatic gas was continuously fed for 35 min to achieve satu- contaminants. In this respect, the combined use of ration. Then, 1.8 ml NTS (25 g/l stock solution) was ozone and activated carbon has been identified as injected into the reactor to obtain a NTS concentra- an interesting alternative to destroy toxic and poorly tion of around 45 mg/l. At the same time, 0.5 g acti- biodegradable organic molecules [11,12]. Unfortu- vated carbon (original or demineralised) was added nately, fundamental studies on this system are still into the reactor. Samples were taken regularly for required to unveil the physical and chemical mech- chemical assay; NaNO2 was used to stop the ozona- anisms involved. Jans and Hoigné [13] showed that tion reaction. NTS concentration, dissolved ozone, ozone reactions in aqueous phase were catalysed by total organic carbon (TOC) and dissolved inorganic carbon black and activated carbon. Indeed, radical carbon (DIC) were determined after different time chain reactions seem to be initiated by the action of periods. functional groups present on the carbon surface, thus Commercial activated carbons Filtrasorb 400, accelerating ozone decomposition in aqueous phase. Merck, Norit, Ceca AC40, Ceca GAC, Sorbo and Zaror et al. [14] reported that ozone stability in aque- Witco were used. NTS was obtained from Fluka; all ous solutions is drastically reduced by the presence other reagents were purchased from Merck. of activated carbon, probably due to a combination of surface catalysed ozone decomposition reactions 2.2. Analytical methods and chemical reactions with carbon surface functional groups. Gas phase ozone concentration was determined by In previous papers [15,16], we studied the ozona- spectrophotometry using Spectronic Genesis 5 equip- tion of three naphthalenesulphonic acids (mono, di ment connected to a flow cell. Dissolved ozone con- and trisulphonic acids) and found that the ozona- centration in aqueous solution was determined by the tion rate depended on the number of sulphonic Karman–Indigo method [17]. groups in the aromatic rings. The degradation of NTS concentration was measured by HPLC using a 1,3,6-naphthalenetrisulphonic acid (NTS) was much Merck–Hitachi with UV detector and a 250 mm long slower than that of 1-naphthalenesulphonic acid, be- RP-18 (5 m) LiChrosphere 100 column. A 35/65 cause of the presence of three sulphonic groups in the methanol–water solution was used as mobile phase, aromatic rings of NTS, which reduce the ring elec- containing 5 mM TBABr as ion exchanger and 10 mM
J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329 321
NaH2 PO4 (Merck) as pH regulator; flow rate was set Table 2
at 1.3 ml/min. Elemental analysis (%) of the activated carbons
Dissolved TOC and DIC were measured using a Activated carbon C H N S O (by
Shimadzu TOC-5000A unit with an experimental error difference)
of ±5%. Filtrasorb 400 87.50 0.18 0.80 0.65 10.87
Demineralisation of activated carbons with HCl and Sorbo 88.61 0.27 0.61 0.37 10.14
Merck 89.00 0.33 0.72 0.94 9.01
HF was carried out using procedures proposed by Mor-
Ceca GAC 78.85 0.36 0.73 0.43 19.63
gan et al. [18]. Ceca AC40 89.00 0.37 0.53 0.00 10.10
The surface area of carbon samples was determined Norit 88.98 0.29 0.70 0.00 10.03
from the BET equation applied to the N2 adsorption Witco 92.55 0.17 0.00 3.04 4.24
isotherms at 77 K, which were obtained using a Mi-
cromeritics Gemini 2370 adsorption unit. The volumes
of macropores (V3 ) and wider mesopores (V2 ) were cial activated carbons used in this study. Greater sur-
determined by mercury porosimetry using a Quan- face areas, in the range 1200–1300 m2 /g, were shown
tachrome Autoscan 60 apparatus. by Sorbo, Merck and Ceca AC40 activated carbons,
The pH of the point of zero charge (pHPZC ) of car- whereas Witco carbon presented the smallest surface
bons was determined following the pH drift tests re- area (808 m2 /g). According to the data presented in
ported elsewhere [19]. The determination of acid and Table 1, the Sorbo and Norit carbons had the most
basic groups was carried out following the method pro- marked macroporosity, while Ceca GAC carbon had
posed by Boehm [20]. Elemental analysis of the car- the greatest mesoporosity (V2 = 0.13 cm3 /g). Witco
bons used was performed with a Fison’s Instruments carbon had the lowest V2 and V3 values.
Model 1108 CHS elemental analyser. Regarding the surface chemistry of the activated
Ash content in carbons was determined by incin- carbons, Sorbo (pHPZC = 9.42) and Norit (pHPZC =
eration at 850 ◦ C and ash chemical composition was 9.18) carbons showed the greatest concentration
estimated by X-ray fluorescence. The procedure was of surface basic groups (1713 and 2050 eq/g, re-
described in detail in a previous publication [21]. spectively), whereas Ceca AC40 carbon (pHPZC =
5.29) had the greatest concentration of surface acid
3. Results and discussion groups.
Table 2 lists the results of the elemental analysis
3.1. Activated carbon characterisation of each activated carbon. The percentage of oxygen
ranged from 19.63% for Ceca GAC carbon to 4.24%
Table 1 summarises the results obtained from the for Witco carbon. All remaining carbons presented a
textural and chemical characterisation of the commer- very similar percentage of oxygen of around 10%.
Table 1
Characterisation of activated carbons
Activated carbon SN2 (m2 /g) V2 (cm3 /g)a V3 (cm3 /g)b pHPZC Acid groups Basic groups Ash (%)
(eq/g)c (eq/g)d
Filtrasorb 400 1075 0.11 0.26 7.91 234 570 6.6
Sorbo 1295 0.06 0.37 9.42 88 1713 5.9
Merck 1301 0.09 0.26 7.89 114 582 5.2
Ceca GAC 966 0.13 0.16 6.83 323 99 12.0
Ceca AC40 1201 0.07 0.32 5.29 438 102 8.3
Norit 968 0.10 0.42 9.18 139 2050 4.8
Witco 808 0.04 0.05 6.85 183 253 0.3
a Volume of pores with diameter of 50–6.6 nm.
b Volume of pores with diameter above 50 nm.
c Determined by NaOH (0.1N) neutralisation.
d Determined by HCl (0.02N) neutralisation.322 J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329
Table 3
Chemical composition of activated carbon ashes (weight, %)
Activated carbon Si Ti Al Fe Mn Mg Ca Na K P
Filtrasorb 400 20.19 0.82 8.41 6.32 0.02 0.49 1.79 0.31 0.55 0.02
Sorbo 15.19 0.20 1.24 1.66 0.13 9.18 9.41 1.53 0.83 0.41
Merck 21.29 0.87 8.66 4.37 0.03 0.48 0.99 0.40 1.21 0.06
Ceca GAC 17.14 0.31 4.38 1.19 0.08 2.48 8.86 0.00 0.08 4.11
Norit 12.22 0.19 1.28 1.47 0.25 9.88 10.46 1.58 0.92 0.45
With regard to the mineral matter, Ceca GAC car- 6.72 M−1 s−1 , whereas the indirect reaction (free rad-
bon featured the highest ash content (12%) and Witco ical reaction) constant was 3.7 × 109 M−1 s−1 . Thus,
carbon the lowest (0.3%) (Table 1). Filtrasorb 400 ash the radical reaction proved more efficacious in oxidiz-
presented a high Fe (6.32%) and Al (8.41%) content, ing NTS. The main compounds obtained in the degra-
whereas Norit and Sorbo had a low content of Fe and dation of NTS by ozone are: oxalic acid, formic acid,
Al and a significant fraction of Mg and Ca (around and sulphate ions [15,16].
9–10% each) (Table 3). Unlike other samples, ashes The carbons that most enhanced the NTS ozonation
from Ceca GAC showed a high P (4.11%) content. rate were those with greatest pHPZC values and high-
A substantial concentration of Ti (∼
=1%) was detected est concentrations of surface basic groups (Table 1).
in Filtrasorb 400 and Merck ash samples, while Mn However, no clear relationship was observed between
contents were significant in Norit and Sorbo ashes the NTS ozonation rate and the SN2 of the activated
(0.25 and 0.13%, respectively). These metals are fre- carbon. Thus, Merck carbon had the largest surface
quently used as catalysts in oxidation processes. In- area (SN2 = 1301 m2 /g) but did not show the highest
deed, Ti is favoured as catalyst in photocatalysis [22] rate of NTS oxidation. These results indicate that the
and Bhat and Gurol [23] recently used low concentra- process of ozonation catalysed by activated carbon is
tions of MnO2 to enhance chlorobenzene degradation not affected by its microporosity. On the other hand,
by ozone. the carbons that most favoured the removal of NTS
from the medium (Sorbo and Norit) were those with
3.2. Ozonation of NTS in the presence of activated greatest macropore volumes (Table 1). These pores act
carbons as transport pores, facilitating the access of ozone to
the carbon surface and reducing diffusion problems.
3.2.1. Influence of chemical and textural Thus, the low catalytic activity showed by Witco car-
characteristics of activated carbon on the NTS bon could be related, in part, to its small V2 and V3
oxidation rate values. However, there was no close relationship be-
Fig. 1 shows experimental results of NTS ozona- tween macropore volume and catalytic activity in the
tion in the presence of the different activated carbons. remaining carbons under study.
All of the carbons increased the ozonation rate. Sorbo, Although activated carbon is a heterogeneous mate-
Norit, and Ceca GAC carbons greatly enhanced NTS rial with a large number of surface groups and different
degradation rates, whereas Witco activated carbon had pore size distributions, the above results suggest that
a lower effect on the NTS degradation rate. These re- the catalytic activity of these activated carbons in NTS
sults were obtained at pH 2.3, at which there is poor ozonation is mainly a function of the carbon basicity.
NTS reactivity towards ozone in the absence of ac- Thus, the catalytic activity seems to be enhanced by
tivated carbon [15]. The greater rate of NTS ozona- increased carbon basicity.
tion in the presence of these carbons could, therefore, The basicity of an activated carbon is due to
be explained by an increase in free radical hydroxyl the presence of basic oxygen-containing functional
concentration. In previous investigations [15,16] of groups (e.g. pyrones or chromenes) and/or graphene
the ozonation of NTS in the absence of activated car- layers acting as Lewis bases and forming elec-
bon, we found that the direct reaction constant was tron donor–acceptor (EDA) complexes with H2 OJ. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329 323
Fig. 1. NTS ozonation in the presence of commercial activated carbons. pH 2.3, T 298 K. (䉫) Without carbon; (䊊) Filtrasorb 400; (䉭)
Merck; (䊐) Ceca GAC; ( ) Ceca AC40; (×) Norit; (+) Sorbo; ( ) Witco.
molecules. These latter basic sites are located at 3.2.2. TOC removal during ozonation treatment
electron-rich regions within the basal planes of car- The TOC present in solution is an important pa-
bon crystallites away from the crystallite edges [24]. rameter to evaluate the efficacy of a given water treat-
This delocalised electron system can act as a Lewis ment system. Fig. 2 shows the TOC concentration as a
base in aqueous solution: function of NTS ozonation time in the presence of the
−C + 2H2 O C–H3 O+ + OH− (1) activated carbons. All activated carbons reduced the
TOC during ozonation time and Norit, Sorbo and Ceca
The delocalised electron system of basic carbons GAC carbons had the greatest effect on TOC removal.
and oxygenated basic groups (chromene and pyrone) The increased TOC concentration of Norit, Merck,
would, therefore, act as catalytic centres of reaction, Sorbo and Filtrasorb 400 activated carbons (Fig. 2)
reducing the ozone molecules to hydroxyl ion and hy- after 10 min ozonation may be due to electrophilic at-
drogen peroxide following the reactions: tack on surface aromatic rings of the activated carbon,
O3 + H2 O + 2e− O2 + 2OH− (2) generating soluble organic by-products.
(3)
It is widely known [25–28] that both hydroxyl ion
and hydrogen peroxide act as initiators of the ozone This attack was experimentally observed by ozonat-
decomposition process in aqueous phase. ing activated carbons in the absence of NTS (Fig. 3).
Thus, the higher degradation rate of NTS in the pres- All of the activated carbons were attacked by ozone
ence of Sorbo (pHPZC = 9.42) and Norit (pHPZC = to a greater or lesser extent, yielding soluble organic
9.18) carbons is because these carbons have greater by-products. Under these experimental conditions, ba-
reducing properties, favouring reactions 2 and 3 and, sic carbons present a greater increase in dissolved
therefore, increasing the extent of ozone decomposi- TOC after the first 10 min of ozonation. The reduc-
tion into highly oxidative radicals. tion in dissolved TOC with the increase in ozonation324 J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329 Fig. 2. TOC removal during NTS ozonation in the presence of activated carbon. pH 2.3, T 298 K. (䉫) Without carbon; (䊊) Filtrasorb 400; (䉭) Merck; (䊐) Ceca GAC; ( ) Ceca AC40; (×) Norit; (+) Sorbo; ( ) Witco. (Fig. 3) may be due to: (i) mineralisation of organic organic matter to CO2 by highly reactive species catal- matter by highly reactive species and (ii) reduction in ysed by activated carbon and (ii) the adsorption of ox- activated carbon reactivity by the generation of oxy- idised by-products from NTS on activated carbon. genated functional groups that reduce electronic den- In order to determine the contributions of adsorption sity on the surface. The comparison of results shown and catalysis to the overall process of TOC removal, in Fig. 2 with those in Fig. 3 indicates that TOC re- experimental studies were undertaken using reactor moval is much faster in the presence of NTS than in in discontinuous mode. A solution of NTS (45 mg/l) its absence. was treated with ozone for 25 min and then degasified The reduction in dissolved TOC during NTS ozona- for 5 min, in order to remove the dissolved ozone. tion (Fig. 2) may be due to: (i) mineralisation of The concentration of dissolved ozone was followed Fig. 3. Dissolved TOC during ozonation of commercial activated carbon. pH 2.3, T 298 K. (䊊) Filtrasorb 400; (䉭) Merck; (䊐) Ceca GAC; ( ) Ceca AC40; (×) Norit; (+) Sorbo; ( ) Witco.
J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329 325
Table 4 sation of the organic matter present. This carbon pro-
Contribution of catalytic and adsorptive mechanisms to the global duced the catalytic removal of 2.47 mg/l of dissolved
process
organic matter after 30 min of treatment (Table 4). This
Activated Adsorption Catalysis DIC greater catalytic capacity is due to a larger amount of
carbon (mg/l) (mg/l) (mg/l)a free radicals in solution. Sorbo carbon is of basic na-
Filtrasorb 400 2.14 1.55 1.53 ture (pHPZC = 9.42) and thus, as commented above,
Sorbo 3.14 2.47 2.37
has a greater reducing capacity, producing the decom-
Merck 2.12 1.75 1.75
Ceca GAC 2.85 1.89 1.80 position of the ozone in aqueous phase.
Ceca AC40 4.05 0.68 0.60 On the other hand, although Ceca GAC is consid-
Norit 3.54 1.34 1.54 ered of neutral nature (pHPZC = 6.83), it showed a ma-
Witco 3.44 0.45 0.41 jor catalytic contribution to the removal of dissolved
a Increase in dissolved inorganic carbon due to the presence organic material. This may have resulted from the
of activated carbon in the system after 30 min of ozonation. presence of a large amount of mineral matter (Table 1).
In accordance with these findings, the metallic centres
up using indigo. After the ozone was removed, 0.5 g present in mineral matter would behave as active cen-
of activated carbon was added and the solution was tres in the ozone decomposition process in aqueous
agitated for 30 min. phase. This aspect will be discussed later. Witco car-
The reduction in TOC observed in the above exper- bon showed the smallest catalytic contribution, due to
iment may be considered exclusively due to the ad- its low basicity and insignificant ash content (0.3%).
sorption of NTS ozonation products on the activated In order to enhance the dissolving of the CO2 gen-
carbon. Knowledge of the contribution of adsorption erated and thereby determine the increase in miner-
to TOC removal allows the catalytic contribution of alisation of organic matter caused by the presence of
the carbon to be determined, simply subtracting the the activated carbon, NTS ozonation experiments were
contribution of adsorption from the difference between run at pH 7. As an example, Fig. 4 shows the evolution
the initial TOC and the TOC after ozone/activated car- of the TOC in the presence and absence of Ceca GAC
bon treatment of the NTS for 30 min. activated carbon during NTS ozonation at pH 2 and 7.
Table 4 lists the values of the catalytic and adsorp- At pH 7, a decrease in TOC was observed in both the
tive contributions to TOC removal in the carbons under absence and the presence of activated carbon. Thus,
study. Sorbo carbon showed the greatest catalytic con- the difference between the dissolved inorganic car-
tribution, indicating that it caused a greater minerali- bon (from the mineralisation of organic matter) deter-
Fig. 4. Influence of pH on TOC removal during NTS ozonation. T 298 K. (䉬) pH 2.3 without carbon; (䉫) pH 2.3 with carbon; (䊏) pH
7 with carbon; (䊐) pH 7 without carbon.326 J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329
mined in the presence and absence of activated car- ash content), with the demineralised sample showing
bon yields a measurement of the catalytic effect of similar behaviour to that of the original Witco car-
the activated carbon on TOC removal (DIC). In fact, bon. Fig. 5 depicts, as an illustration, the results of
Table 4, which includes DIC values, shows that the NTS ozonation in the presence of both demineralised
increase in inorganic carbon due to the presence of and original Ceca GAC carbon samples. These find-
activated carbon is very similar to the experimentally ings indicate that the mineral matter in activated car-
determined value of the catalytic contribution of the bon must contribute positively to the catalytic activity
carbon to the overall removal of organic matter. These of the carbons in NTS ozonation.
results confirm the goodness of the method used to In a previous study [33], we discovered that car-
determine the contribution of the catalytic process to bon demineralisation treatment using HCl and HF did
the overall removal of organic matter. not affect the concentration of oxygen surface groups.
Moreover, the pHPZC values of our demineralised ac-
3.2.3. Influence of mineral matter present in carbon tivated carbons, determined in the present study, are
on its catalytic activity similar to those of the original carbons. Thus, the lower
degradation rate of NTS in the presence of deminer-
3.2.3.1. Effect of carbon demineralisation treatment alised carbon would be mainly due to a reduction in
on NTS oxidation rate. Al Hayek et al. [29] reported its mineral matter content.
that phenol ozonation was enhanced in the presence of The increase in the extent of ozonation reactions
Fe(III) supported on alumina. Karpel Vel Letimer et with heterogeneous catalysis is an incipient method-
al. [30] found that ozonation of salicylic acid, peptides ology and there is still considerable uncertainty about
and dissolved humic substances was enhanced when the mechanism by which the metals produce decom-
supported metals were present. Moreover, the catalytic position of the ozone in aqueous phase. Among the
activity of MnO2 and TiO2 to decompose ozone in metals, Mn has been the most widely studied. Ma and
aqueous phase is widely known [31,32]. Graham [34] reported that MnO2 , formed in situ by
In order to quantify the extent to which mineral mat- ozonation of atrazine in the presence of small amounts
ter present in activated carbon affects ozonation, ex- of Mn(II), leads to a much greater degree of atrazine
periments were conducted using demineralised carbon oxidation by ozone. The authors ascribed these re-
samples. In all carbons, the rate of NTS ozonation was sults to the generation of highly oxidative intermediate
reduced when the activated carbon used was deminer- species, although they did not propose the mechanism
alised. The only exception was Witco carbon (0.3% involved. Andreozzi et al. [31] described a significant
Fig. 5. Effect of Ceca GAC activated carbon demineralisation on NTS ozonation. pH 2.3, T 298 K. (䊐) Untreated; (䊏) demineralised.J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329 327
Fig. 6. Effect of Ceca GAC activated carbon demineralisation on TOC removal during NTS ozonation. pH 2.3, T 298 K. (䊐) Untreated;
(䊏) demineralised.
improvement in oxalic acid ozonation at acidic pH in- As above, experiments were performed at a pH 7
duced by the presence of MnO2 , although they did not to observe the increase in DIC produced by the pres-
account for these findings. ence of demineralised activated carbon in the system
All of the metals that showed catalytic activity in (Table 5), which was slightly lower than that produced
ozonation processes in organic compounds are part by the presence of the original carbons (Table 4). The
of the mineral matter of the activated carbons used present results indicate that the generation of highly
(Table 3). However, it is difficult to ascertain the role oxidative species that participate in TOC removal is
of each metal in the catalyzed ozonation of NTS. reduced when demineralised carbon is used. This con-
The present results showed that some components firms that the mineral matter present in activated car-
of the mineral matter of carbons behave as active bon intervenes in the ozone decomposition into highly
centres in the decomposition of ozone in aqueous oxidative species.
phase, enhancing NTS ozonation. Nevertheless, fur- Interestingly, the ozonation of activated carbon in
ther research is required to identify the mechanism by the absence of NTS was little affected by deminer-
which this matter operates in the process of catalytic alisation pre-treatment, as shown in Fig. 7. Indeed,
ozonation. ozonation of pre-treated and virgin Ceca GAC car-
bon samples yielded similar dissolved TOC from
3.2.3.2. Influence of demineralisation treatment on the ozone attack to carbon, indicating that metallic
TOC removal. In all cases, the efficacy of the sites may not be involved in the generation of solu-
ozone/activated carbon system to remove dissolved
organic matter deriving from NTS degradation was Table 5
reduced when demineralised carbons were used. Dissolved inorganic carbon increase in the presence of deminer-
Fig. 6 shows, as an example, results obtained for alised activated carbon
NTS ozonation in the presence of both untreated and Activated carbon DIC (mg/l)
demineralised Ceca GAC activated carbon samples.
Filtrasorb 400 0.75
There was a lesser mineralisation of the dissolved Sorbo 1.85
organic matter as a result of the absence of mineral Merck 0.95
matter in the carbons. The absence of metallic sites Ceca GAC 0.44
on the carbon surface leads to a reduction in the con- Ceca AC40 0.00
Norit 0.34
centration of highly oxidative species that mineralise
Witco 0.00
organic matter.328 J. Rivera-Utrilla, M. Sánchez-Polo / Applied Catalysis B: Environmental 39 (2002) 319–329
Fig. 7. Effect of activated carbon demineralisation on TOC evolution during Ceca GAC ozonation. pH 2.3, T 298 K. (䊐) Untreated; (䊏)
demineralised.
ble by-products. However, the removal of this TOC The mineral matter present in activated carbons en-
from solution is also enhanced in the case of original hanced their catalytic activity, increasing the rate of
activated carbon, which, again, points out the role NTS degradation and enhancing TOC removal, con-
of the mineral matter in the removal of TOC from tributing to the mineralisation of the organic material
solution. present in solution.
4. Conclusions Acknowledgements
Basic activated carbons have greatest catalytic The authors thank the financial support pro-
activity in the ozonation process. The basal plane vided by the MCT-DGI and FEDER (Project:
electrons and oxygenated surface groups of basic PPQ2001-3246-C02-01).
nature (chromene and pyrone) in activated carbons
are mostly responsible for ozone decomposition in References
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that initiated the decomposition of ozone in aqueous 53 (1999) 51.
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