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Wet oxidation of phenol, cresols and nitrophenols catalyzed by activated carbon in acid and basic media
Applied Catalysis B: Environmental 65 (2006) 269–281 www.elsevier.com/locate/apcatb
Wet oxidation of phenol, cresols and nitrophenols catalyzed by activated carbon in acid and basic media Aurora Santos *, Pedro Yustos, Sergio Rodriguez, Felix Garcia-Ochoa Dpt. Ingenierı´a Quı´mica, Facultad CC, Quı´micas, Universidad Complutense Madrid, 28040 Madrid, Spain Received 30 November 2005; received in revised form 7 February 2006; accepted 9 February 2006 Available online 20 March 2006
Abstract A commercial activated carbon, Industrial React FE01606A, was used as catalyst in the wet oxidation, in both acid and basic media, of phenolic pollutants, such as phenol, cresols and nitrophenols, currently found in industrial wastewaters. Reaction runs were carried out in a fixed-bed reactor (FBR) with concurrent upflow by feeding a 1000 mg L1 aqueous solution on each pollutant concurrently with a gas oxygen flow. Temperature and oxygen pressure of the reactor were set to 160 8C and 16 bar, respectively. The basic medium was maintained by using 500 ppm sodium bicarbonate as buffer reagent to keep the pH in the range 7–8. The initial pH 3.5 was set by adding sulphuric acid. Oxidation intermediates were identified and their distribution with respect to the pollutant oxidation progress was measured. Utilizing these results, oxidation routes for each phenolic compound were deduced. The intermediates produced were diverse in acid and basic media and their composition explains the evolution of the corresponding toxicity measured at the reactor effluent. While under acidic conditions hydroxybenzoic acids, dihydroxyl benzenes and quinones were obtained as primary products, these last two compounds (more toxic than the original pollutant) were not detected in basic conditions, and with such media lower toxicities at the reactor exit were obtained. Moreover, the catalyst was found to be stable in the time range studied (300 h). # 2006 Elsevier B.V. All rights reserved. Keywords: Catalytic wet oxidation; Activated carbon; Substituted phenols; pH; Toxicity
1. Introduction Organic pollution in industrial wastewaters is mainly due to the presence of substances such as PAH’s, dyes, halogenated hydrocarbons, phenolic compounds, etc. Among these substances, phenol is a pollutant typically produced in the petrochemical, chemical and pharmaceutical industries which is toxic to the aquatic environment and non-biodegradable. Typical concentration values of phenols in effluents are in the range 10–17,000 mg L1 [1]. Phenols are toxic, carcinogenic, mutagenic and teratogenic, and are classified as priority pollutants in the UESEPA list with limits of discharge below 0.5 mg L1 currently imposed. Typically, the phenol concentration in wastewater makes recovery non-profitable and alternatives for phenol removal must be considered. Treatment technologies available for handling phenolic waste are physical, chemical, biological and
* Corresponding author. Tel.: +34 913 94 41 71; fax: +34 913 94 41 71. E-mail address: [email protected] (A. Santos). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.005
electrochemical processes [2–4]. The model pollutant used most frequently in past studies has been simple phenol. However, the term phenol encompasses not only simple phenol but the mixture of phenolic compounds in wastewater. Cresols and nitrophenols are commonly used in the manufacture of wood preservation agents, pesticides, dyes and phenolic resins [5,6]. For example, the COD of wastewaters of coal conversion processes and coke ovens [7,8] contain on average 60% phenol and 30% cresols [9]; the creosote oil used for wood preservation is produced as a by-product in coal-tar distillation and contains 45% phenols – phenol and cresols – [10]. Because not only phenol but cresols are also found in high concentrations in several industrial wastewaters, studies about degradation of these last compounds are of interest. Only a few works in the literature deal with the abatement of cresols and nitrophenols in industrial wastewaters. Different methods have been considered: physical [11], biological [10,12,13], enzymatic [14], oxidation in supercritical water [15,16], photocatalytic oxidation [17], electrochemical oxidation [18], non-catalytic (WAO) and catalytic wet oxidation (WCO) [19–23]. Among these, the catalytic wet oxidation
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Nomenclature Ci C0 EC50 PO 2 QG QL Sg BET T tos TU W X
concentrations (mg L1) of individual pollutants in the mixture initial concentration of the pollutant (mg L1) effective nominal concentration (mg L1) oxygen partial pressure (bar) gas flow rate (mL min1 STP) liquid flow rate (mL min1) BET area of the catalyst (m2 g1) temperature (8C) time on stream (h) toxicity units catalyst weight in the reactor (g) conversion
looks like a promising method for removal of toxic organic compounds in wastewaters. The use of a heterogeneous catalyst in WAO allows oxidizing these refractory pollutants towards CO2 and short chain acids [24], which at relatively low temperature and pressure are more amenable to biodegradation than is phenol. Under typical operational conditions, the main problem to solve is that of catalyst stability [25]. The catalysts most frequently employed in CWO are based on noble metals [26–33], metallic oxides [34–48] and metallic salts [49]. Catalysts based on noble metals are high cost and are subject to fast deactivation by fouling [31–33,50]. Metallic oxides (mainly copper oxide) are deactivated by leaching of the active phase due to the acidification of the medium as oxidation progresses [41,51–53]. In light of these drawbacks, a solid with notable activity in the catalytic wet oxidation of organic compounds and without impregnation of some active phase would be highly attractive for CWO implementation. Recently, activated carbon (AC) has been successfully used as a catalyst for liquid phase reactions [49]. Wet oxidation of phenol with impregnation of different metals [26,55] or even as the catalyst itself [56–61] has been studied. Matsumura et al. [56] oxidized phenol using activated carbon as catalyst in supercritical water (673 K, 250 bar). Because of the severe operation conditions the activated carbon itself was burned, but sufficiently slow for its catalytic effect on phenol oxidation to be observed. Tukac and Hanika [57] studied the oxidation of amino, carboxy and sulfo substituents on the course of hydroxyl-aromatics oxidation in a temperature range from 120 to 160 8C and an oxygen partial pressure from 20 to 50 bar. They found that the activated carbon was active for the phenol conversion but it was not so effective for aromatic acids. This fact was also noticed by Santos et al. [59], studying the oxidation route of phenol at acid pH. A commercial, activated carbon was employed in the catalytic wet oxidation of phenol at 160 8C and 16 bar of oxygen pressure. A three phase reactor with concurrent upflow of gas and liquid phases was used [59], and negligible catalyst weight losses (under 5%) were found for times on stream of 200 h. On the other hand, notable reductions of BET area and micropore volume were noticed. On the contrary, Fortuny et al. [58] found a remarkable mass change of
the activated carbon bed by oxidation at 140 8C and 9 bar of oxygen pressure in the catalytic wet oxidation of phenol. They used a different activated carbon than employed by Santos et al. [59], and the reactor was operating as a trickle bed. Generalized [60] and complex [61] kinetic models have been previously discriminated [60] for catalytic wet oxidation of phenol by using the commercial AC elsewhere [59]. In these last two works, it was found that higher mineralization of phenol and lower concentrations of the toxic intermediates (mainly cathecol, hydroquinone and benzoquinone) were obtained by using activated carbon than those values achieved with copper oxides catalysts elsewhere [62]. Catalytic properties of the activated carbon in oxidation reactions have been attributed by Gallezot and co-workers [54,63] to its different oxygen surface complexes [64,65]. AC has a low cost and is stable in both acidic and alkaline environments. To avoid burning off the solid, the temperature and oxygen pressure must be fixed in an adequate operational range. The aim of this work is to study the viability of using activated carbon as a catalyst in the abatement of phenol, cresols (ortho and para) and nitrophenols (ortho and para) by using a commercial activated carbon as the catalyst. Because the medium pH can notably influence the interaction between the pollutant-activated carbon surface and oxygen, the influence of this variable has been analysed. Although these pollutants are not directly oxidized to CO2, some organic oxidation intermediates are produced, and for this reason the pH could have a remarkable effect on the oxidation route, conversion and mineralization of the phenolic compounds. The knowledge of the corresponding oxidation routes at each pH condition is useful for designing a process that will allow obtaining an aqueous effluent of toxicity lower than that of the original aqueous stream [53,62]. The toxicity of the initial solution fed to the reactor and the effluent from the reactor are quantified by means of the standard Microtox1 assay. These values are compared to the toxicities expected from the composition of the media. The commercial AC used as catalyst was selected elsewhere [59]. 2. Experimental 2.1. Oxidation runs A commercial active carbon Industrial React FE01606A, kindly supplied by Chemviron Carbon, has been used as the catalyst. Runs were carried out in an integral fixed-bed reactor (FBR) with co-current up-flow of gas and liquid phases at a temperature of 160 8C and oxygen pressure of 16 bar. The gas flow rate (oxygen) was 90 mL min1 (STP conditions), that is, it was higher than 40 times the stoichiometric value required for total mineralization of the pollutant. The FBR reactor is made of a stainless steel tube 0.75 cm in internal diameter and 25 cm in length. The scheme of the experimental set up is given elsewhere [60]. The initial pH were set to 3.5 and 8 by means of sulphuric acid or sodium bicarbonate (500 mg L1), respectively. The catalyst weight to liquid flow rate ratio (W/QL) was changed from 0.15 to 10.5 g min mL1.
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Under the operational conditions employed, the steady state for the outlet composition of the reactor was achieved in the first 40 h of operation when fresh catalyst is placed in the reactor. For consecutive runs the time required to reach the steady state is about 10–20 h. Liquid samples were periodically drawn and analysed.
Sigma–Aldrich and the micro-organisms were Microtox1 Acute Reagent supplied by I.O. Analytical.
2.2. Analytical methods
Figs. 1 and 2, respectively, show conversion and mineralization values at the steady state achieved in the catalytic wet oxidation of p-cresol ( p-C), o-cresol (o-C), p-nitrophenol ( pNP) and o-nitrophenol (o-NP) versus the ratio between catalyst weight (W) and liquid flow rate (QL). The corresponding conversion and mineralization data obtained elsewhere [60] for phenol (PhOH) as pollutant under the same reaction conditions is also shown in Figs. 1 and 2. As can be seen in Fig. 1, the oxidation rate of the substituted phenols with the methyl group is slightly higher than that obtained for simple phenol, and the oxidation rate of nitrophenols is slower than that obtained for phenol and cresols. These differences can be attributed to:
Phenol and organic compounds were identified and quantified by HPLC (Hewlett–Packard, mod. 1100) using a Diode Array detector (HP G1315A); a Chromolith Performance column (monolithic silica in rod form, RP-18e 100–4.6 mm) was used as stationary phase; a mixture of acetonitrile, water and a solution of 3.6 mM H2SO4 in the ratio 5/90/5 (v/v/v) was used as mobile phase. Flow rate of the mobile phase was 1 mL min1 and a UV detector was used at wavelengths of 192, 210 and 244 nm. Organic acids were analysed by ionic chromatography (Metrohm, mod. 761 Compact IC) using a conductivity detector; a column of anion suppression Metrosep ASUPP5 (25 cm long, 4 mm diameter) was used as the stationary phase and an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 as the mobile phase, at a constant flow rate of 0.7 mL min1. Total organic carbon (TOC) values in the liquid phase were determined with a Shimadzu TOC-V CSH analyzer by oxidative combustion at 680 8C, using an infrared detector. Catalyst samples were taken from the reactor at different times on stream and characterized. The BET area, micropore and mesopore (2–8 nm in size) pore volume was measured by N2 adsorption at 77 K with a Quantachrome Autosorb-1. The mesopore pore volume (8–50 nm in size) was measured by mercury porosimetry in a Carlo Erba Porosimeter 4000.
3. Results and discussion 3.1. Acid medium
the electron donating properties of the extra methyl group of the benzene ring, which produce a slight increase on the activation of the ring towards electrophilic addition of the oxidant and
2.3. Toxicity measurements The toxicity of the liquid samples at the reactor outlet obtained at different oxidation conversions of the pollutants was determined by means of a bioassay following the standard Microtox test procedure (ISO 11348-3, 1998) (based on the decrease of light emission by Photobacterium phosphoreum resulting from its exposure to a toxicant), using a Microtox1 M500 Analyzer (Azur Environmental). The inhibition of the light emitted by the bacteria was measured after 15 min contact time. The EC50 is defined as the effective nominal concentration of toxicant (mg L1) that reduces the intensity of light emission by 50%. The parameter IC50 is defined as the percentage of the initial volume of the sample to the volume of the sample yielding, after the required dilution, a 50% reduction of the light emitted by the micro-organisms. The toxicity units of the wastewater are calculated as: TUs ¼
100 IC
Fig. 1. Conversion obtained for p-cresol ( p-C), o-cresol (o-C), p-nitrophenol ( p-NP), o-nitrophenol (o-NP) and phenol (PhOH) in acid media. PO2 = 16 bar, T = 160 8C.
(1)
Before measuring the toxicity, the pH values of all the samples were re-adjusted to between 6 and 7, in order to prevent the pH effect. All the chemicals used were purchased from
Fig. 2. Mineralization obtained for p-cresol, o-cresol, p-nitrophenol, o-nitrophenol and phenol in acid media. PO2 = 16 bar, T = 160 8C.
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the electron withdrawing effect of the nitro group on the benzene, which decreases the electron density in the ring and decreases the reactivity of the benzene ring towards electrophilic attack. Due to these facts, p-C and o-C and phenol are more reactive towards the oxidant medium than are o-NP and p-NP themselves. As is shown in Fig. 1, the oxidation rates for p-C and o-C are approximately the same. This is to be expected since all the ring substituents are ortho- and para-directing towards electrophilic addition and the number of possible addition sites for the OH radical is the same for both compounds. However, the oxidative rate of o-NP is higher than that of p-NP itself. The nitro group reduces the negative electron density of the ring by extracting it from the ring by resonance; this effect is slightly lower in o-NP because its geometry allows introduction of negative electron density by an intramolecular hydrogen bond between the hydroxyl and nitro groups. Such an intramolecular hydrogen bond is not possible in p-NP. For a set of given operational conditions, the steady state was achieved in the first 40 h if a fresh catalyst is placed in the reactor. After this time, the p-C or o-C conversion kept constant with time on stream (tos); thus, the pollutant adsorption and oxidation on the carbon surface occurred at similar rates. Some of the catalysts employed in Fig. 1 were taken from the bed reactor and characterized. Morphology and BET surface area were determined by SEM (at magnification of 500) and nitrogen adsorption at 77 K, respectively. Results obtained are shown in Fig. 3 and Table 1. The fresh catalyst before CWO is named Sample 1; Sample 2 corresponds to the catalyst after 2 h under the reaction conditions (T = 160 8C, PO2 = 16 bar) and feeding to the reactor acid distillated water, rather than pollutant solution. Samples 3 and 4 are related to the AC obtained after 80 and 200 h, respectively, in the CWO of a solution 1000 mg L1 in PhOH at a temperature of 160 8C and oxygen pressure of 16 bar. Samples 5–8 correspond to the catalyst obtained after 120 h of time on stream (tos) when aqueous solutions 1000 mg L1 in p-C, o-C; p-NP and o-NP, respectively, were fed to the reactor at acid conditions. Samples 9–13 correspond to the
catalyst obtained after 120 h of tos at basic conditions when PhOH, p-C, o-C, p-NP and o-NP, respectively, are fed to the reactor. As can be seen in Fig. 3, a texture change is observed for the AC catalyst after it has been under reaction conditions. However, as similar images are obtained from samples at acid and basic conditions and this change could be attributed to the oxygen pressure and/or temperature (the external catalyst surface can be oxidized) and not to the CWO of the pollutant. In addition, the variations in texture among samples shown in Fig. 3 do not correlate with the changes in BET areas summarized in Table 1. Indeed, Sample 1 has even lower BET area than Sample 2, while Samples 4, 9, 5 and 10 show a BET area notably lower than that of Sample 1. The increase in the BET area for Sample 2 could be due to the cleaning or opening of closed pores as found in literature [66]. The decrease of the BET area observed after the oxidation reaction is almost independent of the tos (in the studied interval 80–200 h) and the pollutant employed. However, it seems that if p-nitrophenol is fed into the reactor the decrease is smaller than that obtained for the rest of phenols. Therefore, the BET area reduction could be due to the formation of organic deposits on the catalyst surface during the oxidation of the pollutant. It was found that the micropore volume was drastically reduced after reaction (i.e., 0.322 cm3 g1 for Sample 1, 0.477 for Sample 2 and 0.07 cm3 g1 for Sample 4). On the other hand, the external surface due to mesopores (At) was almost constant or slightly increased after reaction (63 m2 g1 for Sample 1, 76 m2 g1 for Sample 2 and 110 m2 g1 for Sample 4). After a certain tos value, the formation and oxidation of these organic deposits can take place simultaneously on the catalyst surface. A notable increases of the carbonyl/quinone and carboxylic groups after reaction in both acid and alkaline media was found by TPD measurements. This increase was markedly higher if not only water but also the pollutant was fed to the reactor. Therefore, this increase could be due to the functional groups of phenol and organic oxidation intermediates as well as polymerization compounds remaining in the catalyst surface. The change in the functional groups of the carbon surface after reaction is probably related to the molecular aspects of the interactions among organic pollutants and carbon surface as a function of the pH. These different
Table 1 BET area of the activated carbon IndReact FE01606A employed as catalyst Sample
tos (h)
T (8C)
PO2 bar
Reactor feeding (liquid phase)
pHo
Sg BET (m2 g1)
1 2 3 4 5 6 7 8 9 10 11 12 13
0 2 80 200 120 120 120 120 120 120 120 120 120
– 160 160 160 160 160 160 160 160 160 160 160 160
0 16 16 16 16 16 16 16 16 16 16 16 16
– Water 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1
– 3.5 3.5 3.5 3.5 3.5 3.5 3.5 8 8 8 8 8
745 1048 320 245 251 327 430 260 180 264 250 400 238
PhOH PhOH p-C o-C p-NP o-NP PhOH p-C o-C p-NP o-NP
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Fig. 3. SEM images of AC fresh and after reactor (samples in Table 1). Magnification of 500. Sample 1: fresh AC; Sample 2: water + O2, Sample 4: phenol, tos = 200 h, pH 3.5; Sample 9: phenol, tos = 120 h, pH 8; Sample 5: p-cresol, tos = 120, pH 3.5; Sample 10: p-cresol, tos = 120, pH 8; Sample 7: p-nitrophenol, tos = 120 h, pH 3.5; Sample 13: o-nitrophenol, tos = 120 h, pH 8.
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Fig. 4. Toxicity units at the reactor effluent for the CWO of phenol, o-cresol, pcresol, p-nitrophenol and o-nitrophenol in acid media. PO2 = 16 bar, T = 160 8C. Experimental results as symbols. Predicted values by Eq. (2) as lines.
interactions should also be responsible for the diverse oxidation routes, giving diverse adsorbed and polymerized compounds as a function of the pH. A detailed analysis will be accomplished in further works to clarify these aspects. The toxicity units of the liquid samples at the reactor outlet obtained in the CWO of p-C, o-C, p-NP and o-NP are plotted as symbols in Fig. 4; the corresponding values previously obtained for CWO of PhOH [62] are also shown. As can be
seen in Fig. 4, at the initial stages of the o-C, and p-NP oxidation under acidic conditions a remarkable increase of toxicity occurs in addition to what was noticed for PhOH oxidation while only a slight increment of the toxicity units is produced in the CWO of o-NP. On the contrary, the toxicity units of the liquid samples with p-C as pollutant always decrease with p-C conversion increase. As shown in Fig. 4, a remarkable detoxification at W/QL of 20 g min mL1 is achieved for all the studied pollutants, with the exception of p-NP, because this pollutant is also the most refractory to the oxidation and mineralization (Figs. 1 and 2). The TUs profile can be explained by the intermediates produced in the CWO of these pollutants at acidic conditions. When p-C was used as pollutant, 4-hydroxybenzyl alcohol (4-BZOL), 4-hydroxybenzaldehyde (4-BZAL), 4-methylcatechol (4-MeCTL), p-hydroxybenzoic acid (4-HBZO), and the short chain organic acids acetic (ACE), formic (FOR), maleic (MAL) and oxalic acid (OXAL) were identified and quantified. For o-C oxidation, the intermediates measured were methylhydroquinone (MeHQ), methylbenzoquinone (MeBQ), 3methylcatechol (3-MeCTL), and the short chain organic acids ACE, FOR, MAL, malonic (MLO) and OXAL. For p-NP oxidation, the intermediates measured were hydroquinone (HQ), 4-nitrocatechol (4-NCTL) in trace amounts, and the short chain organic acids ACE, FOR, MAL and OXAL. For
Fig. 5. Intermediates distribution in the CWO of (a) p-cresol, (b) o-cresol, (c) p-nitrophenol and (d) phenol. PO2 = 16 bar, T = 160 8C. C0 = 1000 mg L1 of the pollutant.
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Table 2 Data from individual toxicity tests for the intermediates detected in the catalytic wet oxidation of o-C, p-C, PhOH, p-NP and o-NP Compound
Acronym
EC50
Reference
p-Cresol 4-Hydroxybenzyl alcohol 4-Hydroxybenzaldehyde 4-Methylcatechol o-Cresol Methylhydroquinone Methylbenzoquinone 3-Methylcatechol Phenol Hydroquinone Catechol p-Benzoquinone p-Nitrophenol 4-Nitrocatechol o-Nitrophenol
p-C 4-BZOL 4-BZAL 4-MeCTL o-C MeHQ MeBQ 3-MeCTL PhOH HQ CTL BQ p-NP 4N-CTL o-NP
1.3, (1.30–2.37) (5.06) (8.85) 0.43, (0.43) 27, (15.3–31.2) 0.43, (0.43) 0.41 0.41 16, (16.7), (21.1–42) (0.041), (0.0382–0.0798) (8.32), (29.6–31.8) (0.1), (0.022–0.08) 14.7, (9.85–13.6) (7.77) 55.6, (34.9)
This work, [67] [67] This work, This work, This work, This work, This work This work, [62,67] [62,67] [62,67] This work, [67] This work,
[67]
[67] [67] [67] [67] [62,67]
[67] [67]
Data at 15 min. Values in parenthesis are data from literature Refs. [62,67].
o-NP oxidation, the intermediates measured were 4-nitrohydroquinone (4-NHQ) in trace amounts, and the short chain organic acids ACE, FOR, MAL and OXAL. As was described in previous studies [59,61], the primary products of the catalytic PhOH oxidation are HQ, pbenzoquinone (BQ), 4-HBZO and catechol (CTL), this last as traces, these compounds being further oxidized to the short chain acids previously cited and to CO2. Product distributions obtained in the CWO of p-C, o-C and p-NP with AC are shown in Fig. 5a–c. The distribution previously obtained [61] for CWO of PhOH is also plotted in Fig. 5d. The measured and/or literature values [62,67] of EC50 for the original pollutants and intermediates detected in the CWO of p-C, o-C, p-NP and PhOH, are summarized in Table 2. The results in Table 2 show that intermediates containing hydroquinone or p-benzoquinone structures, such as those obtained in the o-C and p-NP oxidation, are more toxic than the original pollutant. This fact was also found in the CWO of PhOH with HQ and BQ, these being the most toxic oxidation intermediates obtained. On the contrary, in the p-C oxidation, the intermediates produced are less toxic than the original pollutant. Thus, the results in Fig. 5 are able to explain the experimental TUs evolution of the reactor effluent given in Fig. 4 for acidic conditions. From the results in Fig. 5a, it can be surmised that p-C is oxidized at acidic conditions through two different routes. One is the oxidation of the methyl group of p-C to produce 4BZAL proceeding via 4-BZOL; the aldehyde is further oxidized to the corresponding 4-HBZO. This is in agreement with previous data reported in the literature in which 4-BZAL was obtained by oxidation of p-C with a different catalyst [68,69]. On the other hand, the intermediate 4-MeCTL is produced via oxidation at the ortho position of the aromatic ring with respect to the hydroxyl group of the p-C. The mechanism proposed for the oxidation of phenolic derivatives is based on the electrophilic addition to the aromatic ring of a phenol derivative. The oxidation agent attacks the positions activated by the OH phenolic group. Oxidative attack
fractions on the ipso, ortho, meta and para positions of PhOH are 0.08, 0.48, 0.088 and 0.36, respectively [70]. Since the attack on the para position is not possible in p-C (the para position of the ring lacks the hydrogen atom necessary for forming a hydroxyl group in an aromatic system), and also the reaction substituting a methyl group for a hydroxy group is not easy, only the attack on the ortho position of the aromatic ring is produced yielding 4-MeCTL. Likewise, an electrophilic attack is produced over the hydrogen atoms of the methyl group of p-C. The sequential oxidation of the methyl group produces the formation of 4-BZOL, 4-BZAL and then 4-HBZO. Taking all of this into account leads to the oxidation route proposed for the CWO of p-C and presented in Fig. 6a. With respect to o-C oxidation routes, the primary products formed are the methyldihydroxylbenzenes, such as MeHQ, and traces of 3-MeCTL, as is shown in Fig. 5b. MeHQ is oxidized to MeBQ that is further oxidized to short chain acids. The oxidation of 3-MeCTL does not yield 3-methyl-obenzoquinone but 3-MeCTL is further oxidized to short chain acids. The MeBQ is slowly oxidized under the operational conditions used; this low reactivity of MeBQ is apparently due to a strong deactivating effect of the two carbonyl groups on electrophilic addition to the ring. The oxidation route proposed for the CWO of o-C by using AC is given in Fig. 6b. Moreover, the oxidation pathway of the o-C is quite similar to that previously found for PhOH oxidation by using this AC as catalyst [59], as is shown in Fig. 6e. As can be seen in Fig. 5a, b and e, the amounts of CTL obtained in the CWO of PhOH are lower than those obtained for 4-MeCTL and 3-MeCTL in the CWO of p-C and o-C, respectively. This could be due to the negative steric impediment effect of the methyl group over the adsorption of the methylcatechols on the catalyst surface. With respect to the oxidation routes of the nitrophenols under acidic conditions, the electrophilic addition to the aromatic ring is impeded for the withdrawing effect of nitro group over ortho and para positions of the ring (with respect to the nitro group itself). This occurs because the electron density
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in the ring diminishes preferentially at the ortho and para positions leaving the meta position relatively more negative. With respect to p-NP oxidation routes, the primary products formed are the dihydroxylbenzenes, such as HQ and traces of 4-NCTL, as is shown in Fig. 5c. HQ is formed via nucleophilic substitution of the nitro group by a hydroxyl group [71]. Since in the p-NP the favored ortho position with respect to the hydroxyl group is the meta position with respect to the nitro group, the attack on the ortho position of the aromatic ring (with respect to the hydroxyl group) is produced to form 4-NCTL. The oxidation of HQ does not yield BQ but HQ is further oxidized to short chain acids. 4-NCTL is oxidized to short chain acids (Fig. 6c). The oxidation route proposed for the CWO of o-NP in acid media by using AC is given in Fig. 6d. Only traces of 4-nitrohydroquinone (4-NHQ) are detected. 4-NHQ is oxidized to short chain acids.
The toxicity of the samples in Fig. 4 can be predicted based on the EC50 values corresponding to the individual components in Table 2. Toxicity values, as TUs, are calculated according to the following expression, which is based on the concept of concentration addition [72,73]: TUs ¼
X Ci EC50i
(2)
Here Ci is the concentration in the mixture, in mg L1, of the individual pollutant i. Predicted values of toxicity units for the composition of the reactor effluent are calculated by using Eq. (2) and are plotted as lines in Fig. 4. Notice that the predicted curves have a similar trend than the experimental ones. However, the TU values predicted from the intermediates identified for phenol and o-cresol oxidation runs do not fit completely the ones that were experimentally measured. Main
Fig. 6. Oxidation pathway for the CWO of (a) p-cresol, (b) o-cresol, (c) p-nitrophenol, (d) o-nitrophenol and (e) phenol by using AC as catalyst in acid media.
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Fig. 6. (Continued ).
differences between experimental and predicted TU values appear at (each temperature) at the W/QL values when maximum concentration of hydroquinone and benzoquinone type intermediates are detected. Therefore, the discrepancy could be due to synergistic effects between these compounds with negative influence on the toxicity. As the concentration of those compounds decrease the experimental and predicted values of the toxicity units become very similar. Moreover, the predicted TOC and experimental values are closer for all the substituted phenols (deviations of about 5%, which is the experimental error). Only if p-cresol is fed to the reactor, about 15% of the TOC remains unidentified but this unidentified TOC does not seem to be appreciably toxic. 3.2. Basic medium Conversion and mineralization values in basic medium at the steady state achieved in the CWO of PhOH, p-C, o-C, p-NP
Fig. 7. Results obtained for p-cresol, o-cresol, p-nitrophenol, o-nitrophenol and phenol conversion in basic media. PO2 = 16 bar, T = 160 8C.
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Fig. 8. Results obtained for p-cresol, o-cresol, p-nitrophenol, o-nitrophenol and phenol mineralization in basic media. PO2 = 16 bar, T = 160 8C.
Fig. 9. Toxicity units at the reactor effluent for the CWO of phenol, o-cresol, pcresol, p-nitrophenol and o-nitrophenol in basic media. PO2 = 16 bar, T = 160 8C. Experimental results as symbols. Predicted values as lines.
Fig. 10. Oxidation Pathway for the CWO of (a) p-cresol, (b) o-cresol, (c) p-nitrophenol, (d) o-nitrophenol and (e) phenol by using AC as catalyst in basic media.
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Fig. 10. (Continued ).
and o-NP versus the ratio between catalyst weight (W) and liquid flow rate (QL) is shown in Figs. 7 and 8, respectively. As can be seen in Fig. 7, the oxidation rates of phenols with weakly activating groups are higher than are the oxidation rates of phenols with strongly deactivating groups, as was also noticed in the oxidation process in acid medium. Therefore, p-C, o-C and phenol are more reactive towards the oxidant medium than are o-NP and p-NP themselves. The toxicity units of the liquid samples at the reactor outlet obtained in the CWO of PhOH, p-C, o-C, p-NP and o-NP are plotted as symbols in Fig. 9. As can be seen in Fig. 9, at the initial stages of PhOH, o-C, o-NP and p-NP oxidations the remarkable toxicity increase that was observed for oxidation in acid medium (Fig. 4) is not detected here. Only a slight increase is noticed at the initial stages of PhOH, o-NP and p-NP oxidation runs and the toxicity units decrease for most of phenol derivatives in basic conditions. The TUs profile must be explained by the intermediates produced in the CWO of p-C, o-C, p-NP, o-NP and PhOH, as is shown in Fig. 9. In the CWO of phenolic compounds in basic medium by using AC only benzoic acid derivatives and the short chains acids acetic (ACE), formic (FOR), maleic (MAL) and oxalic acid (OXAL) were identified and quantified (Fig. 10a, b and e). From these results it can be surmised that PhOH, o-C and p-C react through two different routes. One route is the carboxylation of the aromatic ring at the ortho or para positions to obtain the corresponding derivative benzoic acid that is further oxidized to short chain acids. This is in agreement with previous data reported in the literature in which benzoic acid was one of the products obtained by oxidation of PhOH with different catalyst [74]. On the other hand, the direct oxidation of phenol and cresols to short chain acids is also produced. With respect to the CWO of nitrophenols over AC in basic media, only the short chains acids acetic (ACE), formic (FOR), maleic (MAL) and oxalic acid (OXAL) were identified and quantified (Fig. 10c and d). In all cases no catechol, hydroquinone or benzoquinone derivatives
Fig. 11. Mineralization vs. pollutant conversion at acid (A) conditions (pHo 3.5 by adding H2SO4) or basic (B) medium (pHo 8 by adding 500 mg L1 of sodium bicarbonate).
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were found in basic medium. These results are adequate to explain the TUs change in Fig. 9. The toxicity of the samples (Fig. 9) can be predicted based on the EC50 values corresponding to the individual components in Table 2. Toxicity values, as TUs, are calculated according to Eq. (2). Calculated values of toxicity units are plotted as lines in Fig. 9. 3.3. Comparison between acid and basic media The mineralization and toxicity units of the reactor effluent versus the pollutant conversion are plotted in Figs. 11 and 12, respectively, for both acid (A) and bicarbonate media (B). As can be seen in Fig. 11, the mineralization versus conversion profiles achieved for phenol and o-NP are quite similar when acid (pHo 3.5 by adding H2SO4) or basic media (pHo 8 by adding 500 mg L1 of sodium bicarbonate) are used. Slightly higher values of XTOC versus XPollutant are obtained in acid media for p-cresol. On the contrary if o-C or p-NP are fed to the reactor higher mineralizations are obtained in basic media for the same value of the pollutant conversion. As can be seen in Fig. 12 the detoxifications obtained for o-cresol or phenol solutions improve remarkably if basic media were to be used: no toxic intermediates are obtained at basic conditions and the maximum observed for the toxicity of the reactor effluent at acid conditions is not detected with basic media. Moreover, the remarkable toxicity increase noticed at initial stages of the p-NP oxidation in acid media is not found when bicarbonate media is used. Finally, little influence of the reaction media on the toxicity values was observed for p-C and o-NP catalytic oxidation while these pollutants did not produce toxic intermediates in acid and basic media. 4. Conclusions
Fig. 12. Toxicity units of the effluent vs. pollutant conversion at acid (A) conditions (pHo 3.5 by adding H2SO4) or basic (B) medium (pHo 8 by adding 500 mg L1 of sodium bicarbonate) by using the Microtox1 assay.
The CWO of Phenol, o-cresol, p-cresol, o-nitrophenol and pnitrophenol by using Activated Carbon as catalyst in acid and basic media has been found to be a promising technology for the elimination of these pollutants. Almost total pollutant conversions have been observed at the mild temperatures and pressures used in this work. The catalyst shows changes in both its initial texture (probably due to the oxidation of the external surface) and its original physicochemical properties (a remarkable BET area reduction has been found after reaction). However, from the results obtained, it can be deduced that after a certain time on stream the catalyst surface achieves a steady state. It was found that after a tos of about 10–40 h, depending mainly if the reactor is filled with fresh catalyst or not, the pollutant conversion and mineralization values keep stable in the time range studied (200 h). A remarkable increase of the toxicity of the liquid samples at the reactor outlet has been measured in the CWO in acid medium of o-cresol, o-nitrophenol and p-nitrophenol; similar results for the CWO of phenol have been reported in a previous study [57]. This fact can be explained by the similar oxidation intermediates obtained in these cases: hydroquinone and benzoquinone derivative compounds. The toxicity decreases
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when these intermediates are further oxidized. These substances are not produced neither in the CWO of p-cresol in acid medium, nor in the CWO of phenols in basic medium, a decrease in the toxicity of the reactor effluent is always found with the phenol derivative conversion increasing. In all the cases, a remarkable detoxification of the inlet wastewater can be achieved by selecting the proper W/QL value.
[33] [34] [35] [36] [37] [38] [39] [40]
Acknowledgements [41]
The authors acknowledge financial support for this research from the Spanish MCYT (Contract No. PPQ2003-01452) and CAM (GR/AMB/0605/2004). The authors wish also to thank Chemviron Carbon for kindly supplying the catalyst used in this work. References
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Wet oxidation of phenol, cresols and nitrophenols catalyzed by activated carbon in acid and basic media Aurora Santos *, Pedro Yustos, Sergio Rodriguez, Felix Garcia-Ochoa Dpt. Ingenierı´a Quı´mica, Facultad CC, Quı´micas, Universidad Complutense Madrid, 28040 Madrid, Spain Received 30 November 2005; received in revised form 7 February 2006; accepted 9 February 2006 Available online 20 March 2006
Abstract A commercial activated carbon, Industrial React FE01606A, was used as catalyst in the wet oxidation, in both acid and basic media, of phenolic pollutants, such as phenol, cresols and nitrophenols, currently found in industrial wastewaters. Reaction runs were carried out in a fixed-bed reactor (FBR) with concurrent upflow by feeding a 1000 mg L1 aqueous solution on each pollutant concurrently with a gas oxygen flow. Temperature and oxygen pressure of the reactor were set to 160 8C and 16 bar, respectively. The basic medium was maintained by using 500 ppm sodium bicarbonate as buffer reagent to keep the pH in the range 7–8. The initial pH 3.5 was set by adding sulphuric acid. Oxidation intermediates were identified and their distribution with respect to the pollutant oxidation progress was measured. Utilizing these results, oxidation routes for each phenolic compound were deduced. The intermediates produced were diverse in acid and basic media and their composition explains the evolution of the corresponding toxicity measured at the reactor effluent. While under acidic conditions hydroxybenzoic acids, dihydroxyl benzenes and quinones were obtained as primary products, these last two compounds (more toxic than the original pollutant) were not detected in basic conditions, and with such media lower toxicities at the reactor exit were obtained. Moreover, the catalyst was found to be stable in the time range studied (300 h). # 2006 Elsevier B.V. All rights reserved. Keywords: Catalytic wet oxidation; Activated carbon; Substituted phenols; pH; Toxicity
1. Introduction Organic pollution in industrial wastewaters is mainly due to the presence of substances such as PAH’s, dyes, halogenated hydrocarbons, phenolic compounds, etc. Among these substances, phenol is a pollutant typically produced in the petrochemical, chemical and pharmaceutical industries which is toxic to the aquatic environment and non-biodegradable. Typical concentration values of phenols in effluents are in the range 10–17,000 mg L1 [1]. Phenols are toxic, carcinogenic, mutagenic and teratogenic, and are classified as priority pollutants in the UESEPA list with limits of discharge below 0.5 mg L1 currently imposed. Typically, the phenol concentration in wastewater makes recovery non-profitable and alternatives for phenol removal must be considered. Treatment technologies available for handling phenolic waste are physical, chemical, biological and
* Corresponding author. Tel.: +34 913 94 41 71; fax: +34 913 94 41 71. E-mail address: [email protected] (A. Santos). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.005
electrochemical processes [2–4]. The model pollutant used most frequently in past studies has been simple phenol. However, the term phenol encompasses not only simple phenol but the mixture of phenolic compounds in wastewater. Cresols and nitrophenols are commonly used in the manufacture of wood preservation agents, pesticides, dyes and phenolic resins [5,6]. For example, the COD of wastewaters of coal conversion processes and coke ovens [7,8] contain on average 60% phenol and 30% cresols [9]; the creosote oil used for wood preservation is produced as a by-product in coal-tar distillation and contains 45% phenols – phenol and cresols – [10]. Because not only phenol but cresols are also found in high concentrations in several industrial wastewaters, studies about degradation of these last compounds are of interest. Only a few works in the literature deal with the abatement of cresols and nitrophenols in industrial wastewaters. Different methods have been considered: physical [11], biological [10,12,13], enzymatic [14], oxidation in supercritical water [15,16], photocatalytic oxidation [17], electrochemical oxidation [18], non-catalytic (WAO) and catalytic wet oxidation (WCO) [19–23]. Among these, the catalytic wet oxidation
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Nomenclature Ci C0 EC50 PO 2 QG QL Sg BET T tos TU W X
concentrations (mg L1) of individual pollutants in the mixture initial concentration of the pollutant (mg L1) effective nominal concentration (mg L1) oxygen partial pressure (bar) gas flow rate (mL min1 STP) liquid flow rate (mL min1) BET area of the catalyst (m2 g1) temperature (8C) time on stream (h) toxicity units catalyst weight in the reactor (g) conversion
looks like a promising method for removal of toxic organic compounds in wastewaters. The use of a heterogeneous catalyst in WAO allows oxidizing these refractory pollutants towards CO2 and short chain acids [24], which at relatively low temperature and pressure are more amenable to biodegradation than is phenol. Under typical operational conditions, the main problem to solve is that of catalyst stability [25]. The catalysts most frequently employed in CWO are based on noble metals [26–33], metallic oxides [34–48] and metallic salts [49]. Catalysts based on noble metals are high cost and are subject to fast deactivation by fouling [31–33,50]. Metallic oxides (mainly copper oxide) are deactivated by leaching of the active phase due to the acidification of the medium as oxidation progresses [41,51–53]. In light of these drawbacks, a solid with notable activity in the catalytic wet oxidation of organic compounds and without impregnation of some active phase would be highly attractive for CWO implementation. Recently, activated carbon (AC) has been successfully used as a catalyst for liquid phase reactions [49]. Wet oxidation of phenol with impregnation of different metals [26,55] or even as the catalyst itself [56–61] has been studied. Matsumura et al. [56] oxidized phenol using activated carbon as catalyst in supercritical water (673 K, 250 bar). Because of the severe operation conditions the activated carbon itself was burned, but sufficiently slow for its catalytic effect on phenol oxidation to be observed. Tukac and Hanika [57] studied the oxidation of amino, carboxy and sulfo substituents on the course of hydroxyl-aromatics oxidation in a temperature range from 120 to 160 8C and an oxygen partial pressure from 20 to 50 bar. They found that the activated carbon was active for the phenol conversion but it was not so effective for aromatic acids. This fact was also noticed by Santos et al. [59], studying the oxidation route of phenol at acid pH. A commercial, activated carbon was employed in the catalytic wet oxidation of phenol at 160 8C and 16 bar of oxygen pressure. A three phase reactor with concurrent upflow of gas and liquid phases was used [59], and negligible catalyst weight losses (under 5%) were found for times on stream of 200 h. On the other hand, notable reductions of BET area and micropore volume were noticed. On the contrary, Fortuny et al. [58] found a remarkable mass change of
the activated carbon bed by oxidation at 140 8C and 9 bar of oxygen pressure in the catalytic wet oxidation of phenol. They used a different activated carbon than employed by Santos et al. [59], and the reactor was operating as a trickle bed. Generalized [60] and complex [61] kinetic models have been previously discriminated [60] for catalytic wet oxidation of phenol by using the commercial AC elsewhere [59]. In these last two works, it was found that higher mineralization of phenol and lower concentrations of the toxic intermediates (mainly cathecol, hydroquinone and benzoquinone) were obtained by using activated carbon than those values achieved with copper oxides catalysts elsewhere [62]. Catalytic properties of the activated carbon in oxidation reactions have been attributed by Gallezot and co-workers [54,63] to its different oxygen surface complexes [64,65]. AC has a low cost and is stable in both acidic and alkaline environments. To avoid burning off the solid, the temperature and oxygen pressure must be fixed in an adequate operational range. The aim of this work is to study the viability of using activated carbon as a catalyst in the abatement of phenol, cresols (ortho and para) and nitrophenols (ortho and para) by using a commercial activated carbon as the catalyst. Because the medium pH can notably influence the interaction between the pollutant-activated carbon surface and oxygen, the influence of this variable has been analysed. Although these pollutants are not directly oxidized to CO2, some organic oxidation intermediates are produced, and for this reason the pH could have a remarkable effect on the oxidation route, conversion and mineralization of the phenolic compounds. The knowledge of the corresponding oxidation routes at each pH condition is useful for designing a process that will allow obtaining an aqueous effluent of toxicity lower than that of the original aqueous stream [53,62]. The toxicity of the initial solution fed to the reactor and the effluent from the reactor are quantified by means of the standard Microtox1 assay. These values are compared to the toxicities expected from the composition of the media. The commercial AC used as catalyst was selected elsewhere [59]. 2. Experimental 2.1. Oxidation runs A commercial active carbon Industrial React FE01606A, kindly supplied by Chemviron Carbon, has been used as the catalyst. Runs were carried out in an integral fixed-bed reactor (FBR) with co-current up-flow of gas and liquid phases at a temperature of 160 8C and oxygen pressure of 16 bar. The gas flow rate (oxygen) was 90 mL min1 (STP conditions), that is, it was higher than 40 times the stoichiometric value required for total mineralization of the pollutant. The FBR reactor is made of a stainless steel tube 0.75 cm in internal diameter and 25 cm in length. The scheme of the experimental set up is given elsewhere [60]. The initial pH were set to 3.5 and 8 by means of sulphuric acid or sodium bicarbonate (500 mg L1), respectively. The catalyst weight to liquid flow rate ratio (W/QL) was changed from 0.15 to 10.5 g min mL1.
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Under the operational conditions employed, the steady state for the outlet composition of the reactor was achieved in the first 40 h of operation when fresh catalyst is placed in the reactor. For consecutive runs the time required to reach the steady state is about 10–20 h. Liquid samples were periodically drawn and analysed.
Sigma–Aldrich and the micro-organisms were Microtox1 Acute Reagent supplied by I.O. Analytical.
2.2. Analytical methods
Figs. 1 and 2, respectively, show conversion and mineralization values at the steady state achieved in the catalytic wet oxidation of p-cresol ( p-C), o-cresol (o-C), p-nitrophenol ( pNP) and o-nitrophenol (o-NP) versus the ratio between catalyst weight (W) and liquid flow rate (QL). The corresponding conversion and mineralization data obtained elsewhere [60] for phenol (PhOH) as pollutant under the same reaction conditions is also shown in Figs. 1 and 2. As can be seen in Fig. 1, the oxidation rate of the substituted phenols with the methyl group is slightly higher than that obtained for simple phenol, and the oxidation rate of nitrophenols is slower than that obtained for phenol and cresols. These differences can be attributed to:
Phenol and organic compounds were identified and quantified by HPLC (Hewlett–Packard, mod. 1100) using a Diode Array detector (HP G1315A); a Chromolith Performance column (monolithic silica in rod form, RP-18e 100–4.6 mm) was used as stationary phase; a mixture of acetonitrile, water and a solution of 3.6 mM H2SO4 in the ratio 5/90/5 (v/v/v) was used as mobile phase. Flow rate of the mobile phase was 1 mL min1 and a UV detector was used at wavelengths of 192, 210 and 244 nm. Organic acids were analysed by ionic chromatography (Metrohm, mod. 761 Compact IC) using a conductivity detector; a column of anion suppression Metrosep ASUPP5 (25 cm long, 4 mm diameter) was used as the stationary phase and an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 as the mobile phase, at a constant flow rate of 0.7 mL min1. Total organic carbon (TOC) values in the liquid phase were determined with a Shimadzu TOC-V CSH analyzer by oxidative combustion at 680 8C, using an infrared detector. Catalyst samples were taken from the reactor at different times on stream and characterized. The BET area, micropore and mesopore (2–8 nm in size) pore volume was measured by N2 adsorption at 77 K with a Quantachrome Autosorb-1. The mesopore pore volume (8–50 nm in size) was measured by mercury porosimetry in a Carlo Erba Porosimeter 4000.
3. Results and discussion 3.1. Acid medium
the electron donating properties of the extra methyl group of the benzene ring, which produce a slight increase on the activation of the ring towards electrophilic addition of the oxidant and
2.3. Toxicity measurements The toxicity of the liquid samples at the reactor outlet obtained at different oxidation conversions of the pollutants was determined by means of a bioassay following the standard Microtox test procedure (ISO 11348-3, 1998) (based on the decrease of light emission by Photobacterium phosphoreum resulting from its exposure to a toxicant), using a Microtox1 M500 Analyzer (Azur Environmental). The inhibition of the light emitted by the bacteria was measured after 15 min contact time. The EC50 is defined as the effective nominal concentration of toxicant (mg L1) that reduces the intensity of light emission by 50%. The parameter IC50 is defined as the percentage of the initial volume of the sample to the volume of the sample yielding, after the required dilution, a 50% reduction of the light emitted by the micro-organisms. The toxicity units of the wastewater are calculated as: TUs ¼
100 IC
Fig. 1. Conversion obtained for p-cresol ( p-C), o-cresol (o-C), p-nitrophenol ( p-NP), o-nitrophenol (o-NP) and phenol (PhOH) in acid media. PO2 = 16 bar, T = 160 8C.
(1)
Before measuring the toxicity, the pH values of all the samples were re-adjusted to between 6 and 7, in order to prevent the pH effect. All the chemicals used were purchased from
Fig. 2. Mineralization obtained for p-cresol, o-cresol, p-nitrophenol, o-nitrophenol and phenol in acid media. PO2 = 16 bar, T = 160 8C.
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the electron withdrawing effect of the nitro group on the benzene, which decreases the electron density in the ring and decreases the reactivity of the benzene ring towards electrophilic attack. Due to these facts, p-C and o-C and phenol are more reactive towards the oxidant medium than are o-NP and p-NP themselves. As is shown in Fig. 1, the oxidation rates for p-C and o-C are approximately the same. This is to be expected since all the ring substituents are ortho- and para-directing towards electrophilic addition and the number of possible addition sites for the OH radical is the same for both compounds. However, the oxidative rate of o-NP is higher than that of p-NP itself. The nitro group reduces the negative electron density of the ring by extracting it from the ring by resonance; this effect is slightly lower in o-NP because its geometry allows introduction of negative electron density by an intramolecular hydrogen bond between the hydroxyl and nitro groups. Such an intramolecular hydrogen bond is not possible in p-NP. For a set of given operational conditions, the steady state was achieved in the first 40 h if a fresh catalyst is placed in the reactor. After this time, the p-C or o-C conversion kept constant with time on stream (tos); thus, the pollutant adsorption and oxidation on the carbon surface occurred at similar rates. Some of the catalysts employed in Fig. 1 were taken from the bed reactor and characterized. Morphology and BET surface area were determined by SEM (at magnification of 500) and nitrogen adsorption at 77 K, respectively. Results obtained are shown in Fig. 3 and Table 1. The fresh catalyst before CWO is named Sample 1; Sample 2 corresponds to the catalyst after 2 h under the reaction conditions (T = 160 8C, PO2 = 16 bar) and feeding to the reactor acid distillated water, rather than pollutant solution. Samples 3 and 4 are related to the AC obtained after 80 and 200 h, respectively, in the CWO of a solution 1000 mg L1 in PhOH at a temperature of 160 8C and oxygen pressure of 16 bar. Samples 5–8 correspond to the catalyst obtained after 120 h of time on stream (tos) when aqueous solutions 1000 mg L1 in p-C, o-C; p-NP and o-NP, respectively, were fed to the reactor at acid conditions. Samples 9–13 correspond to the
catalyst obtained after 120 h of tos at basic conditions when PhOH, p-C, o-C, p-NP and o-NP, respectively, are fed to the reactor. As can be seen in Fig. 3, a texture change is observed for the AC catalyst after it has been under reaction conditions. However, as similar images are obtained from samples at acid and basic conditions and this change could be attributed to the oxygen pressure and/or temperature (the external catalyst surface can be oxidized) and not to the CWO of the pollutant. In addition, the variations in texture among samples shown in Fig. 3 do not correlate with the changes in BET areas summarized in Table 1. Indeed, Sample 1 has even lower BET area than Sample 2, while Samples 4, 9, 5 and 10 show a BET area notably lower than that of Sample 1. The increase in the BET area for Sample 2 could be due to the cleaning or opening of closed pores as found in literature [66]. The decrease of the BET area observed after the oxidation reaction is almost independent of the tos (in the studied interval 80–200 h) and the pollutant employed. However, it seems that if p-nitrophenol is fed into the reactor the decrease is smaller than that obtained for the rest of phenols. Therefore, the BET area reduction could be due to the formation of organic deposits on the catalyst surface during the oxidation of the pollutant. It was found that the micropore volume was drastically reduced after reaction (i.e., 0.322 cm3 g1 for Sample 1, 0.477 for Sample 2 and 0.07 cm3 g1 for Sample 4). On the other hand, the external surface due to mesopores (At) was almost constant or slightly increased after reaction (63 m2 g1 for Sample 1, 76 m2 g1 for Sample 2 and 110 m2 g1 for Sample 4). After a certain tos value, the formation and oxidation of these organic deposits can take place simultaneously on the catalyst surface. A notable increases of the carbonyl/quinone and carboxylic groups after reaction in both acid and alkaline media was found by TPD measurements. This increase was markedly higher if not only water but also the pollutant was fed to the reactor. Therefore, this increase could be due to the functional groups of phenol and organic oxidation intermediates as well as polymerization compounds remaining in the catalyst surface. The change in the functional groups of the carbon surface after reaction is probably related to the molecular aspects of the interactions among organic pollutants and carbon surface as a function of the pH. These different
Table 1 BET area of the activated carbon IndReact FE01606A employed as catalyst Sample
tos (h)
T (8C)
PO2 bar
Reactor feeding (liquid phase)
pHo
Sg BET (m2 g1)
1 2 3 4 5 6 7 8 9 10 11 12 13
0 2 80 200 120 120 120 120 120 120 120 120 120
– 160 160 160 160 160 160 160 160 160 160 160 160
0 16 16 16 16 16 16 16 16 16 16 16 16
– Water 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1 1000 mg L1
– 3.5 3.5 3.5 3.5 3.5 3.5 3.5 8 8 8 8 8
745 1048 320 245 251 327 430 260 180 264 250 400 238
PhOH PhOH p-C o-C p-NP o-NP PhOH p-C o-C p-NP o-NP
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273
Fig. 3. SEM images of AC fresh and after reactor (samples in Table 1). Magnification of 500. Sample 1: fresh AC; Sample 2: water + O2, Sample 4: phenol, tos = 200 h, pH 3.5; Sample 9: phenol, tos = 120 h, pH 8; Sample 5: p-cresol, tos = 120, pH 3.5; Sample 10: p-cresol, tos = 120, pH 8; Sample 7: p-nitrophenol, tos = 120 h, pH 3.5; Sample 13: o-nitrophenol, tos = 120 h, pH 8.
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Fig. 4. Toxicity units at the reactor effluent for the CWO of phenol, o-cresol, pcresol, p-nitrophenol and o-nitrophenol in acid media. PO2 = 16 bar, T = 160 8C. Experimental results as symbols. Predicted values by Eq. (2) as lines.
interactions should also be responsible for the diverse oxidation routes, giving diverse adsorbed and polymerized compounds as a function of the pH. A detailed analysis will be accomplished in further works to clarify these aspects. The toxicity units of the liquid samples at the reactor outlet obtained in the CWO of p-C, o-C, p-NP and o-NP are plotted as symbols in Fig. 4; the corresponding values previously obtained for CWO of PhOH [62] are also shown. As can be
seen in Fig. 4, at the initial stages of the o-C, and p-NP oxidation under acidic conditions a remarkable increase of toxicity occurs in addition to what was noticed for PhOH oxidation while only a slight increment of the toxicity units is produced in the CWO of o-NP. On the contrary, the toxicity units of the liquid samples with p-C as pollutant always decrease with p-C conversion increase. As shown in Fig. 4, a remarkable detoxification at W/QL of 20 g min mL1 is achieved for all the studied pollutants, with the exception of p-NP, because this pollutant is also the most refractory to the oxidation and mineralization (Figs. 1 and 2). The TUs profile can be explained by the intermediates produced in the CWO of these pollutants at acidic conditions. When p-C was used as pollutant, 4-hydroxybenzyl alcohol (4-BZOL), 4-hydroxybenzaldehyde (4-BZAL), 4-methylcatechol (4-MeCTL), p-hydroxybenzoic acid (4-HBZO), and the short chain organic acids acetic (ACE), formic (FOR), maleic (MAL) and oxalic acid (OXAL) were identified and quantified. For o-C oxidation, the intermediates measured were methylhydroquinone (MeHQ), methylbenzoquinone (MeBQ), 3methylcatechol (3-MeCTL), and the short chain organic acids ACE, FOR, MAL, malonic (MLO) and OXAL. For p-NP oxidation, the intermediates measured were hydroquinone (HQ), 4-nitrocatechol (4-NCTL) in trace amounts, and the short chain organic acids ACE, FOR, MAL and OXAL. For
Fig. 5. Intermediates distribution in the CWO of (a) p-cresol, (b) o-cresol, (c) p-nitrophenol and (d) phenol. PO2 = 16 bar, T = 160 8C. C0 = 1000 mg L1 of the pollutant.
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275
Table 2 Data from individual toxicity tests for the intermediates detected in the catalytic wet oxidation of o-C, p-C, PhOH, p-NP and o-NP Compound
Acronym
EC50
Reference
p-Cresol 4-Hydroxybenzyl alcohol 4-Hydroxybenzaldehyde 4-Methylcatechol o-Cresol Methylhydroquinone Methylbenzoquinone 3-Methylcatechol Phenol Hydroquinone Catechol p-Benzoquinone p-Nitrophenol 4-Nitrocatechol o-Nitrophenol
p-C 4-BZOL 4-BZAL 4-MeCTL o-C MeHQ MeBQ 3-MeCTL PhOH HQ CTL BQ p-NP 4N-CTL o-NP
1.3, (1.30–2.37) (5.06) (8.85) 0.43, (0.43) 27, (15.3–31.2) 0.43, (0.43) 0.41 0.41 16, (16.7), (21.1–42) (0.041), (0.0382–0.0798) (8.32), (29.6–31.8) (0.1), (0.022–0.08) 14.7, (9.85–13.6) (7.77) 55.6, (34.9)
This work, [67] [67] This work, This work, This work, This work, This work This work, [62,67] [62,67] [62,67] This work, [67] This work,
[67]
[67] [67] [67] [67] [62,67]
[67] [67]
Data at 15 min. Values in parenthesis are data from literature Refs. [62,67].
o-NP oxidation, the intermediates measured were 4-nitrohydroquinone (4-NHQ) in trace amounts, and the short chain organic acids ACE, FOR, MAL and OXAL. As was described in previous studies [59,61], the primary products of the catalytic PhOH oxidation are HQ, pbenzoquinone (BQ), 4-HBZO and catechol (CTL), this last as traces, these compounds being further oxidized to the short chain acids previously cited and to CO2. Product distributions obtained in the CWO of p-C, o-C and p-NP with AC are shown in Fig. 5a–c. The distribution previously obtained [61] for CWO of PhOH is also plotted in Fig. 5d. The measured and/or literature values [62,67] of EC50 for the original pollutants and intermediates detected in the CWO of p-C, o-C, p-NP and PhOH, are summarized in Table 2. The results in Table 2 show that intermediates containing hydroquinone or p-benzoquinone structures, such as those obtained in the o-C and p-NP oxidation, are more toxic than the original pollutant. This fact was also found in the CWO of PhOH with HQ and BQ, these being the most toxic oxidation intermediates obtained. On the contrary, in the p-C oxidation, the intermediates produced are less toxic than the original pollutant. Thus, the results in Fig. 5 are able to explain the experimental TUs evolution of the reactor effluent given in Fig. 4 for acidic conditions. From the results in Fig. 5a, it can be surmised that p-C is oxidized at acidic conditions through two different routes. One is the oxidation of the methyl group of p-C to produce 4BZAL proceeding via 4-BZOL; the aldehyde is further oxidized to the corresponding 4-HBZO. This is in agreement with previous data reported in the literature in which 4-BZAL was obtained by oxidation of p-C with a different catalyst [68,69]. On the other hand, the intermediate 4-MeCTL is produced via oxidation at the ortho position of the aromatic ring with respect to the hydroxyl group of the p-C. The mechanism proposed for the oxidation of phenolic derivatives is based on the electrophilic addition to the aromatic ring of a phenol derivative. The oxidation agent attacks the positions activated by the OH phenolic group. Oxidative attack
fractions on the ipso, ortho, meta and para positions of PhOH are 0.08, 0.48, 0.088 and 0.36, respectively [70]. Since the attack on the para position is not possible in p-C (the para position of the ring lacks the hydrogen atom necessary for forming a hydroxyl group in an aromatic system), and also the reaction substituting a methyl group for a hydroxy group is not easy, only the attack on the ortho position of the aromatic ring is produced yielding 4-MeCTL. Likewise, an electrophilic attack is produced over the hydrogen atoms of the methyl group of p-C. The sequential oxidation of the methyl group produces the formation of 4-BZOL, 4-BZAL and then 4-HBZO. Taking all of this into account leads to the oxidation route proposed for the CWO of p-C and presented in Fig. 6a. With respect to o-C oxidation routes, the primary products formed are the methyldihydroxylbenzenes, such as MeHQ, and traces of 3-MeCTL, as is shown in Fig. 5b. MeHQ is oxidized to MeBQ that is further oxidized to short chain acids. The oxidation of 3-MeCTL does not yield 3-methyl-obenzoquinone but 3-MeCTL is further oxidized to short chain acids. The MeBQ is slowly oxidized under the operational conditions used; this low reactivity of MeBQ is apparently due to a strong deactivating effect of the two carbonyl groups on electrophilic addition to the ring. The oxidation route proposed for the CWO of o-C by using AC is given in Fig. 6b. Moreover, the oxidation pathway of the o-C is quite similar to that previously found for PhOH oxidation by using this AC as catalyst [59], as is shown in Fig. 6e. As can be seen in Fig. 5a, b and e, the amounts of CTL obtained in the CWO of PhOH are lower than those obtained for 4-MeCTL and 3-MeCTL in the CWO of p-C and o-C, respectively. This could be due to the negative steric impediment effect of the methyl group over the adsorption of the methylcatechols on the catalyst surface. With respect to the oxidation routes of the nitrophenols under acidic conditions, the electrophilic addition to the aromatic ring is impeded for the withdrawing effect of nitro group over ortho and para positions of the ring (with respect to the nitro group itself). This occurs because the electron density
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in the ring diminishes preferentially at the ortho and para positions leaving the meta position relatively more negative. With respect to p-NP oxidation routes, the primary products formed are the dihydroxylbenzenes, such as HQ and traces of 4-NCTL, as is shown in Fig. 5c. HQ is formed via nucleophilic substitution of the nitro group by a hydroxyl group [71]. Since in the p-NP the favored ortho position with respect to the hydroxyl group is the meta position with respect to the nitro group, the attack on the ortho position of the aromatic ring (with respect to the hydroxyl group) is produced to form 4-NCTL. The oxidation of HQ does not yield BQ but HQ is further oxidized to short chain acids. 4-NCTL is oxidized to short chain acids (Fig. 6c). The oxidation route proposed for the CWO of o-NP in acid media by using AC is given in Fig. 6d. Only traces of 4-nitrohydroquinone (4-NHQ) are detected. 4-NHQ is oxidized to short chain acids.
The toxicity of the samples in Fig. 4 can be predicted based on the EC50 values corresponding to the individual components in Table 2. Toxicity values, as TUs, are calculated according to the following expression, which is based on the concept of concentration addition [72,73]: TUs ¼
X Ci EC50i
(2)
Here Ci is the concentration in the mixture, in mg L1, of the individual pollutant i. Predicted values of toxicity units for the composition of the reactor effluent are calculated by using Eq. (2) and are plotted as lines in Fig. 4. Notice that the predicted curves have a similar trend than the experimental ones. However, the TU values predicted from the intermediates identified for phenol and o-cresol oxidation runs do not fit completely the ones that were experimentally measured. Main
Fig. 6. Oxidation pathway for the CWO of (a) p-cresol, (b) o-cresol, (c) p-nitrophenol, (d) o-nitrophenol and (e) phenol by using AC as catalyst in acid media.
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277
Fig. 6. (Continued ).
differences between experimental and predicted TU values appear at (each temperature) at the W/QL values when maximum concentration of hydroquinone and benzoquinone type intermediates are detected. Therefore, the discrepancy could be due to synergistic effects between these compounds with negative influence on the toxicity. As the concentration of those compounds decrease the experimental and predicted values of the toxicity units become very similar. Moreover, the predicted TOC and experimental values are closer for all the substituted phenols (deviations of about 5%, which is the experimental error). Only if p-cresol is fed to the reactor, about 15% of the TOC remains unidentified but this unidentified TOC does not seem to be appreciably toxic. 3.2. Basic medium Conversion and mineralization values in basic medium at the steady state achieved in the CWO of PhOH, p-C, o-C, p-NP
Fig. 7. Results obtained for p-cresol, o-cresol, p-nitrophenol, o-nitrophenol and phenol conversion in basic media. PO2 = 16 bar, T = 160 8C.
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Fig. 8. Results obtained for p-cresol, o-cresol, p-nitrophenol, o-nitrophenol and phenol mineralization in basic media. PO2 = 16 bar, T = 160 8C.
Fig. 9. Toxicity units at the reactor effluent for the CWO of phenol, o-cresol, pcresol, p-nitrophenol and o-nitrophenol in basic media. PO2 = 16 bar, T = 160 8C. Experimental results as symbols. Predicted values as lines.
Fig. 10. Oxidation Pathway for the CWO of (a) p-cresol, (b) o-cresol, (c) p-nitrophenol, (d) o-nitrophenol and (e) phenol by using AC as catalyst in basic media.
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279
Fig. 10. (Continued ).
and o-NP versus the ratio between catalyst weight (W) and liquid flow rate (QL) is shown in Figs. 7 and 8, respectively. As can be seen in Fig. 7, the oxidation rates of phenols with weakly activating groups are higher than are the oxidation rates of phenols with strongly deactivating groups, as was also noticed in the oxidation process in acid medium. Therefore, p-C, o-C and phenol are more reactive towards the oxidant medium than are o-NP and p-NP themselves. The toxicity units of the liquid samples at the reactor outlet obtained in the CWO of PhOH, p-C, o-C, p-NP and o-NP are plotted as symbols in Fig. 9. As can be seen in Fig. 9, at the initial stages of PhOH, o-C, o-NP and p-NP oxidations the remarkable toxicity increase that was observed for oxidation in acid medium (Fig. 4) is not detected here. Only a slight increase is noticed at the initial stages of PhOH, o-NP and p-NP oxidation runs and the toxicity units decrease for most of phenol derivatives in basic conditions. The TUs profile must be explained by the intermediates produced in the CWO of p-C, o-C, p-NP, o-NP and PhOH, as is shown in Fig. 9. In the CWO of phenolic compounds in basic medium by using AC only benzoic acid derivatives and the short chains acids acetic (ACE), formic (FOR), maleic (MAL) and oxalic acid (OXAL) were identified and quantified (Fig. 10a, b and e). From these results it can be surmised that PhOH, o-C and p-C react through two different routes. One route is the carboxylation of the aromatic ring at the ortho or para positions to obtain the corresponding derivative benzoic acid that is further oxidized to short chain acids. This is in agreement with previous data reported in the literature in which benzoic acid was one of the products obtained by oxidation of PhOH with different catalyst [74]. On the other hand, the direct oxidation of phenol and cresols to short chain acids is also produced. With respect to the CWO of nitrophenols over AC in basic media, only the short chains acids acetic (ACE), formic (FOR), maleic (MAL) and oxalic acid (OXAL) were identified and quantified (Fig. 10c and d). In all cases no catechol, hydroquinone or benzoquinone derivatives
Fig. 11. Mineralization vs. pollutant conversion at acid (A) conditions (pHo 3.5 by adding H2SO4) or basic (B) medium (pHo 8 by adding 500 mg L1 of sodium bicarbonate).
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were found in basic medium. These results are adequate to explain the TUs change in Fig. 9. The toxicity of the samples (Fig. 9) can be predicted based on the EC50 values corresponding to the individual components in Table 2. Toxicity values, as TUs, are calculated according to Eq. (2). Calculated values of toxicity units are plotted as lines in Fig. 9. 3.3. Comparison between acid and basic media The mineralization and toxicity units of the reactor effluent versus the pollutant conversion are plotted in Figs. 11 and 12, respectively, for both acid (A) and bicarbonate media (B). As can be seen in Fig. 11, the mineralization versus conversion profiles achieved for phenol and o-NP are quite similar when acid (pHo 3.5 by adding H2SO4) or basic media (pHo 8 by adding 500 mg L1 of sodium bicarbonate) are used. Slightly higher values of XTOC versus XPollutant are obtained in acid media for p-cresol. On the contrary if o-C or p-NP are fed to the reactor higher mineralizations are obtained in basic media for the same value of the pollutant conversion. As can be seen in Fig. 12 the detoxifications obtained for o-cresol or phenol solutions improve remarkably if basic media were to be used: no toxic intermediates are obtained at basic conditions and the maximum observed for the toxicity of the reactor effluent at acid conditions is not detected with basic media. Moreover, the remarkable toxicity increase noticed at initial stages of the p-NP oxidation in acid media is not found when bicarbonate media is used. Finally, little influence of the reaction media on the toxicity values was observed for p-C and o-NP catalytic oxidation while these pollutants did not produce toxic intermediates in acid and basic media. 4. Conclusions
Fig. 12. Toxicity units of the effluent vs. pollutant conversion at acid (A) conditions (pHo 3.5 by adding H2SO4) or basic (B) medium (pHo 8 by adding 500 mg L1 of sodium bicarbonate) by using the Microtox1 assay.
The CWO of Phenol, o-cresol, p-cresol, o-nitrophenol and pnitrophenol by using Activated Carbon as catalyst in acid and basic media has been found to be a promising technology for the elimination of these pollutants. Almost total pollutant conversions have been observed at the mild temperatures and pressures used in this work. The catalyst shows changes in both its initial texture (probably due to the oxidation of the external surface) and its original physicochemical properties (a remarkable BET area reduction has been found after reaction). However, from the results obtained, it can be deduced that after a certain time on stream the catalyst surface achieves a steady state. It was found that after a tos of about 10–40 h, depending mainly if the reactor is filled with fresh catalyst or not, the pollutant conversion and mineralization values keep stable in the time range studied (200 h). A remarkable increase of the toxicity of the liquid samples at the reactor outlet has been measured in the CWO in acid medium of o-cresol, o-nitrophenol and p-nitrophenol; similar results for the CWO of phenol have been reported in a previous study [57]. This fact can be explained by the similar oxidation intermediates obtained in these cases: hydroquinone and benzoquinone derivative compounds. The toxicity decreases
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when these intermediates are further oxidized. These substances are not produced neither in the CWO of p-cresol in acid medium, nor in the CWO of phenols in basic medium, a decrease in the toxicity of the reactor effluent is always found with the phenol derivative conversion increasing. In all the cases, a remarkable detoxification of the inlet wastewater can be achieved by selecting the proper W/QL value.
[33] [34] [35] [36] [37] [38] [39] [40]
Acknowledgements [41]
The authors acknowledge financial support for this research from the Spanish MCYT (Contract No. PPQ2003-01452) and CAM (GR/AMB/0605/2004). The authors wish also to thank Chemviron Carbon for kindly supplying the catalyst used in this work. References
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