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  Kinetics of Activated Carbon Promoted Ozonation of Succinic Acid in Water Fernando J. Beltra ´ n,* Juan F. Garcı ´ a-Araya, Ine ´ s Gira ´ ldez, and Francisco J. Masa  Departamento de Ingenierı ´ a Quı ´ mica y Energe ´ tica, Uni V  ersidad de Extremadura, 06071 Badajoz, Spain The ozonation of succinic acid in water has been carried out in the presence of four different activated carbons.The influence of some variables, carbon type, particle size, gas flow rate, agitation speed, etc., has beenstudied for kinetic purposes. Succinic acid has been observed to be a promoter of ozone decomposition inwater. Reactivity of ozone with succinic acid is significantly increased in the presence of activated carbon,regardless of its nature, although basic activated carbons present the highest activity. Also, the presence of activated carbon allows the highest mineralization rates. Mass transfer and chemical reactions (bulk waterand surface reactions) have been considered for the kinetic study and rate constants of both reactions havebeen determined at different temperatures. Linear correlations have been obtained between surface reactionrate constants and the pH pzc  of activated carbons. Introduction Ozonation and activated carbon filtration are well-knownprocesses in water treatment mainly applied to remove con-taminants and improve biological processes. 1 So far, both ozoneand activated carbon are used in practice, separately, but recentresearch has shown that their simultaneous application can leadto mineralization of water. 2 Adsorption of ozone on activatedcarbon is thought to be a source of intermediate oxidant entitiessuch as hydrogen peroxide and, eventually, hydroxyl freeradicals. 3 The presence of hydroxyl radicals makes the activatedcarbon ozonation an advanced oxidation process. 4,5 It is alsoknown that hydroxyl radicals are the main oxidant species usedin water treatment able to mineralize the organic content of wateras has been reported in the case of oxalic acid. 6 Phenols, aromatic hydrocarbons, many pesticides, etc. con-stitute groups of priority pollutants of water that can beconveniently remove with ozone through dipolar cycloadditionand electrophilic substitution reactions. 7,8 These ozonations,however, give rise to significant amounts of saturated organiccompounds (aldehydes, ketones, carboxylic acids) as endproducts that mainly accumulate in water since they hardly reactwith ozone alone. However, a very small fraction of these acidscan be likely removed through hydroxyl radicals that come fromthe decomposition of ozone. Oxidation of ozonation endproducts (i.e., saturated carboxylic acids) is aimed to achievethe main goal of these processes, that is, the mineralization of water. However, a high degree of mineralization cannot bereached with ozonation alone, at the pH of natural waters,because this process leads to low concentrations of hydroxylradicals. Therefore, new processes are needed for this purpose.One of these possible processes can be activated carbonozonation. Thus, in this work, a very ozone resistant compound,succinic acid (butanedioic acid), has been chosen as a modelof an ozonation end product to study the synergism effect of ozone and activated carbon on the oxidation of recalcitrantsubstances. The objectives of the work are then to observe therates of mineralization and to study the kinetics of the process. Experimental Section Succinic acid was obtained from Merck and used as received.Ozone was produced from pure oxygen in a 301.7 Sanderlaboratory ozone generator able to generate a maximum of 75g of ozone per 1 m 3 of oxygen. Aqueous solutions of succinicacid were prepared in ultrapure water (MilliQ Millipore system).Phosphoric acid and sodium hydroxide (Panreac, Spain) wereused to achieve buffering conditions (pH 7) in the water treated.Four different activated carbons were used and supplied byHydraffin, Darco, Chenviron, and Norit. The first three wereof granular form (mainly spherical), and the last one was of pellet form. Table 1 gives some of their textural characteristics.The BET surface area was determined with a Quantachromeautosorb gas adsorption system. The total pore volume wasobtained from apparent and true densities that were determinedfrom mercury and helium porosimetries by using a stereopic-nometer and autoscan-60 Quantachrome equipments, respec-tively. The method of Noh and Schwarz 9 was applied tocalculate the pH pzc . Values of this parameter allow activatedcarbons be classified as basic (Hydraffin and Norit), neutral(Darco), or acid (Chenviron) (see Table 1).Ozonation reactions were carried out in a 750 mL slurry glasscylindrical reactor operated in semibatch way. A 500 mL portionof a buffered (pH 7) succinic acid aqueous solution and knownamounts of activated carbon, when needed, were charged intothe reactor. The reactor was submerged in a thermostatic bathto keep the temperature constant within  ( 0.1  ° C. Ozone - oxygen gas mixtures were fed to the reactor through a porousplate situated at the reactor bottom with varying gas flow rates(20 - 60 L h - 1 ). Mechanical agitation (100 - 300 rpm) wasprovided to ensure perfect mixing conditions in the water, gas,and solid phases. Liquid phase perfect mixing was checkedthrough tracer experiments while those of the gas and solidphases were assumed to hold.During any experiment samples were steadily withdrawn fromthe reactor to analyze succinic acid and ozone concentrationsin water. For this purpose, HPLC and colorimetric methods wereused, respectively. Thus, a 1100 Hewlett-Packard apparatus witha C-110H Supelcogel column (30 cm long, 7.8 mm i.d.) wasused. The mobile phase was ultrapure water with 0.1%phosphoric acid at a rate of 1 mL min - 1 . Detection was madeat 200 nm. The indigo method 10 was used to determine thedissolved ozone concentration. Also, in the gas, the ozoneconcentration was determined with an Anseros Ozomat ozoneanalyzer. The total organic carbon was measured with an I/OAnalytic 1010R carbon analyzer based on the persulfateoxidation method. * To whom correspondence should be addressed. E-mail: fbeltran@unex.es. Fax no.: 34-924-289385. Phone no.: 34-924-289387. 3015  Ind. Eng. Chem. Res.  2006,  45,  3015 - 3021 10.1021/ie060096r CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 03/30/2006  Results and Discussion Succinic acid removal by ozonation alone, in the presenceof the different activated carbons used, and by activated carbonadsorption were first compared (see Figure 1). Hydraffin resultedin being the most active carbon for the removal of succinic acidlikely due to its basic properties. 11 However, as shown in Figure1, removal of succinic acid by adsorption alone hardly reached10% after 3 h of treatment. At pH 7, ozonation alone of succinicacid did improve the removal rate to about 65% in 3 h. This islikely due to the action of hydroxyl radicals of ozone decom-position since at pH 5 negligible reaction between ozone andsuccinic acid was noticed (see also Figure 1). These observationsare also valid for the removal of the total organic carbon (TOC).Then, it is concluded that a higher hydroxyl radical concentrationshould be needed to improve the degree of oxidation - mineralization. In Figure 1, results of the dimensionless succinicacid concentration with time corresponding to ozonation in thepresence of the different activated carbons are also shown. Ascan be observed, regardless of the nature of the activated carbon,oxidation rates are improved compared to ozonation alone.However, Hydraffin AC seems to be the most active amongthe activated carbons used since after 150 min total oxidationof succinic acid is achieved with this carbon. As far as theremoval of TOC is concerned, about 82% mineralization isachieved after this period of reaction. After 180 min of reaction,remaining TOC in the presence of Hydraffin AC was only dueto byproducts of ozonation. However, data corresponding to theother activated carbons showed that the remaining TOC wasdue to both unreacted succinic acid and byproducts formed.Remaining TOC only from succinic acid represented about 2.3,9, and 4.5% when Darco, Chenviron, and Norit activated carbonswere used, respectively. The positive effect of activated carbonis undoubtedly due to the formation of hydrogen peroxide andits reaction with ozone to yield hydroxyl radicals. Formationof hydrogen peroxide develops from ozone decomposition bothin bulk water (ozonation alone) and on the carbon surface. 2,3 Subsequent reaction between ozone and hydrogen peroxide canalso take place both in bulk and on the carbon surface. Figure2 presents the evolution of dissolved ozone concentration withtime during the ozonation of succinic acid in the presence andabsence of Hydraffin activated carbon. As can be seen, in thepresence of activated carbon and succinic acid, dissolved ozonefirst accumulates in water and then it decomposes. In the absenceof activated carbon and presence of succinic acid, a similar trendis observed. These can be attributed to a promoting characterof succinic acid to decompose ozone through reactions 6 and 4of the following mechanism: 12 where P is a byproduct. From reactions 1 to 7, it is seen thatthere is a competition between ozone and succinic acid forhydroxyl radicals (reactions 6 and 7). However, rate constantsof these reactions are of similar magnitude 13 but the concentra-tion of succinic acid is much higher than that of dissolved ozone(see Figures 1 and 2 for concentrations values); as a conse-quence, most of the hydroxyl radicals react with succinic acid.If this compound is a promoter of ozone decomposition, reaction6 will develop by yielding hydroperoxide radicals (HO 2 ã ) and,then, the superoxide ion radical (O 2 ã- ), through equilibrium 3which is shifted to the right at pH 7.5. In a following step, theseradicals immediately react with ozone through reaction 4. Notethat succinic acid does not react at all with the superoxide ionradical. 14 This free radical only reacts with ozone. Results of the absorption of ozone in succinic acid free water in thepresence of activated carbon (see also Figure 2) confirm thisconclusion. Thus, from Figure 2, it is seen that in the absenceof succinic acid dissolved ozone concentration increases and Table 1. Textural Characteristics of Activated Carbons Used in This Work type form particle size (mm) BET surface (m 2 g - 1 ) total pore volume (cm 3 g - 1 ) % ash pH pzc Hydraffin P110 granular 0.6 - 2 967 0.723 6.3 10.4Darco granular 0.6 - 2 703 0.946 16.3 7.0Chemviron, SS4P granular 0.6 - 2 1193 0.765 0.6 1.8Norit cylindrical pellet  D p ) 0.4,  L ) 5 924 1.050 4.5 8.2 Figure 1.  Evolution of the dimensionless concentration of succinic acidwith time. The conditions were as follows: ( T  ) 25  ° C; (pH) 7.0; (gas flowrate) 60 L h - 1 ; ( C  O3g ) 40 mg L - 1 ; (w) 10 g L - 1 ; (agitation speed) 200 rpm;(particle size: 1.0 - 1.6 mm. The various experiments correspond to graphicsymbols as follows: ( 0 ) adsorption; ( 4 ) ozonation alone at pH 5;( ] ) ozonation alone at pH 7; ( 9 ) Hydraffin activated carbon ozonation;( 2 ) Darco activated carbon ozonation; ( [ ) Chemviron activated carbonozonation; ( b ) Norit activated carbon ozonation. Figure 2.  Evolution of the dissolved ozone concentration with timecorresponding to different ozonation experiments. The conditions were thesame as those listed in Figure 1. The various experiments correspond tographic symbols as follows: ( 9 ) in the presence of Hydraffin activatedcarbon and succinic acid; ( 0 ) in the presence of succinic acid withoutactivated carbon; ( 4 ) in the presence of activated carbon without succinicacid. O 3 + OH - f  HO 2 - + O 2  (1)O 3 + HO 2 - f  HO 2 ã + O 3 ã- (2)HO 2 ã u  pK ) 4.8 O 2 ã- + H + (3)O 3 + O 2 ã- f  O 3 ã- + O 2  (4)O 3 ã- + H + f  HO 3 ã f  HO ã + O 2  (5)S + HO ã f  S ã + H 2 O f  P + HO 2 ã (6)O 3 + HO ã f  HO 2 ã + O 2  (7) 3016  Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006  reaches a stationary value. Then, no decrease in ozone concen-tration, in this case, is observed likely due to the lack of formation of hydroperoxide/superoxide ion radicals throughreaction 6. In the presence of succinic acid and activated carbon,the dissolved ozone concentration follows a similar trend as inthe absence of activated carbon and presence of succinic acidalthough the concentration is much lower likely due to ozonereactions on the carbon surface, with hydrogen peroxide(generated on the carbon surface when ozone decomposes 11 )and with the superoxide ion radical coming from reaction 6.Formation of hydrogen peroxide was detected in the experi-ments, but its concentration remained very low and was notquantified. The reason for this was likely due to reaction 2through which hydrogen peroxide is consumed by reacting withozone. Higher concentrations of hydrogen peroxide have alreadybeen reported in a previous work during the activated carbonozonation of gallic acid 2 at pH 5. At this pH value, hydrogenperoxide is less dissociated and hardly reacts with ozone. Itshould also be noted that hydrogen peroxide is not formed inexperiments of succinic acid activated carbon adsorption so asits formation is mainly due to ozone decomposition on thecarbon surface. Similar results were obtained when ozonationwas carried out in the presence of the other three activatedcarbons used. Influence of Variables.  The effect of the different variables(gas flow rate, agitation speed, particle size, etc.) was checkedby using Hydraffin activated carbon. The main reason for thevariable effect study was to get the most appropriate experi-mental conditions for a kinetic study of the process.A first series of activated carbon ozonation experiments atdifferent agitation speeds (between 100 and 300 rpm) werecarried out. From the results obtained (not shown), it isconcluded that this variable does not exert any influence onthe process rate. Then, 200 rpm was taken as the standard valuefor the rest of the experiments.In a new series of experiments, the influence of the gas flowrate was checked. Figure 3 shows the evolution of thedimensionless concentration of succinic acid with time corre-sponding to ozonation experiments in the presence and absenceof Hydraffin activated carbon at different gas flow rates. Asobserved from Figure 3, regardless of the presence of activatedcarbon, the process rate increases with the increase of gas flowrate. It is likely that there is a gas flow rate higher than 60 Lh - 1 for which the process rate would become independent of the gas flow rate and, hence, independent of gas - liquid externaldiffusion limitations. However, gas flow rates higher than 60 Lh - 1 could not be checked because of experimental reactorlimitations.In a following step, ozonation experiments of succinic acidwere carried out in the presence of activated carbons of differentparticle size (between 0.6 and 2 mm). It was observed (notshown) that particle size does not affected the process rate, sothat internal diffusion limitations can be considered to benegligible with respect to controlling the process rate. A particlesize between 1 and 1.6 mm was taken for the activated carbonused in the rest of the experiments.The influence of the ozone concentration in the gas fed tothe reactor was then checked. Figure 4 presents the resultsobtained in the ozonation of succinic acid in the presence andabsence of Hydraffin activated carbon at different ozone gasconcentrations. It is seen that this variable exerts a positive effecton the removal rate of succinic acid regardless of the presenceand absence of activated carbon.Finally, some other experiments were carried out at differentactivated carbon and initial succinic acid concentrations (seeFigures 5 and 6). As observed from these figures, both variablesexert positive effects on the process rate. Kinetic Study.  It is evident that the hydroxyl radical is themain oxidant species responsible for succinic acid removal fromwater in ozonation processes. The attack of hydroxyl radicals,however, can develop through four different paths because of the heterogeneous nature of the process. Thus, both hydroxylradicals and succinic acid can react in the bulk water or asadsorbed species. These reactions are the consequence of different mechanisms that come from the decomposition of dissolved ozone both on bulk water and on the carbon surface. 2,12 According to this, the concentration of hydroxyl radicals (bothin bulk water and on activated carbon) can be consideredproportional to the concentration of ozone. For kinetic study Figure 3.  Evolution of the dimensionless concentration of succinic acidwith time corresponding to ozonation experiments in the presence andabsence of Hydraffin activated carbon at different gas flow rates. All otherconditions were the same as those listed in Figure 1except the gas flowrate: ( 9  and  0 ) 20, ( 2  and  4 ) 36, ( [  and  ] ) 60 L h - 1 (white symbolscorrespond to experiments in the absence of activated carbon). Figure 4.  Evolution of the dimensionless concentration of succinic acidwith time corresponding to ozonation experiments in the presence andabsence of Hydraffin activated carbon at different inlet ozone gas concentra-tions. All other conditions were the same as those listed in Figure 1 unlessindicated: (gas flow rate) 20 L h - 1 ; ( C  O3g ) ( 9  and  0 ) 12, ( 2  and  4 ) 40,( [  and  ] ) 69 mg L - 1 (white symbols correspond to experiments in theabsence of activated carbon). Figure 5.  Evolution of the dimensionless concentration of succinic acidwith time corresponding to ozonation experiments in the presence andabsence of Hydraffin activated carbon with different activated carbonconcentrations. All other conditions were the same as those listed in Figure1 unless indicated: (gas flow rate) 20 L h - 1 ; (activated carbon concentration, w ) ( 9 ) 10, ( 2 ) 20 g L - 1 . Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006  3017  purposes, then, the disappearance of succinic acid in activatedcarbon ozonation processes can be assumed to be the result of two general reactions between ozone and succinic acid devel-oped in bulk water and on the carbon surface. These reactionscan also be assumed to be of second order as most of thereactions ozone undergoes in water are. 15 Thus, the chemicalrate of these reactions will be the following:In bulk waterOn the carbon surfaceWith this assumption, the kinetics of the ozonation process canbe simplified but still it remains complex because of the possiblelimitations of mass transfer rates. Thus, the total reaction rateof succinic acid in activated carbon promoted ozonation is notthe sum of rate eqs 8 and 9 but that calculated from theabsorption kinetics of ozone,  N  O3 . The ozone absorption rate ina reacting system where there are two main paths to ozoneconsumption, represented by rate eqs 8 (in bulk water) and 9(on the carbon surface) is given by eq 10: 16 where  P O3  is the ozone partial pressure, He is the Henryequilibrium constant of the ozone - water system,  k  g a,  k  l a, and k  c a p  are the volumetric mass transfer coefficients of ozonethrough gas and water films closed to interfacial surfaces (gas - liquid and liquid - solid, respectively),  w  is the concentrationof activated carbon, and  η  is the internal effectiveness factorthat accounts for possible diffusion limitations inside the poresalso called internal diffusion. Thus, the terms of the denominatorof eq 10 represent the resistances of external and internaldiffusion steps and chemical reactions. By considering now themass balance of succinic acid in water that, in this case,corresponds to a batch reactor design equation, once stoichi-ometry is accounted for, the following expression is obtained:where  z  represents the stoichiometric ratio of the reactionsbetween ozone and succinic acid. Substitution of eq 10 in eq11 gives rise to one first order differential equation that oncesolved allows the succinic acid concentration profile with timeto be known. However, rate constants,  k  bw  and  k  cs  first need tobe determined. For so doing, it is advisable to simplify eq 10.Likely, in aqueous ozone reacting systems, some of the masstransfer resistances can be simplified when experiments arecarried out at appropriate experimental conditions. First, the gasfilm mass transfer resistance (1/He k  g a) can be considerednegligible because ozone is a sparingly soluble gas in water. 17 Second, from experimental results, it was concluded that internaldiffusion limitations are also negligible since no effect of particlesize on the succinic acid removal rate was observed ascommented above. Then, the internal effectiveness factor canbe taken as unity in eq 10. 18 The other two mass transferresistances (1/  k  l a and 1/  k  c a p ) cannot be, however, eliminatedbecause of the influence of the gas flow rate on the processrate (see Figure 3). Then, these terms must be considered in eq10. Finally, with these simplifications, eq 11 becomes Succinic Acid Ozonation in the Absence of ActivatedCarbon.  Experimental results in the absence of activated carbonallow a further simplification of eq 12. Since no solid is present,the removal of succinic acid is in bulk water so that onlychemical rate eq 9 and the liquid film resistance to mass transfer,1/  k  l a, apply. Then, eq 12 reduces toIn eq 13,  P O3  is the ozone partial pressure at the reactor outletbecause of perfect mixing conditions. Experimental resultsshowed that  P O3  rapidly increases during the first 5 min to reacha stationary value. Given the fact that the reaction time was 3h, it can be accepted that  P O3  is a constant in eq 13. Therefore,integration of eq 13 after variable separation yieldsEquation 14 can be rearranged to get a straight line equation:According to eq 15, a plot of its left side against ln( C  B  /  C  B0 )should lead to a straight line of slope 1/  k  bw . Application of thisprocedure requires the knowledge of the mass transfer coef-ficient  k  l a and the equilibrium constant, He, that were obtainedfrom existing correlation data. 16,19 Values of   k  bw  were thenobtained from the inverse of the slopes deduced by least-squaresanalysis of straight lines obtained from plots indicated (notshown). Table 2 show the values of the rate constant  k  bw  atdifferent temperatures. As observed from Table 2, values of   k  bw are very low which is in accordance with what would be Figure 6.  Evolution of the dimensionless concentration of succinic acidwith time corresponding to ozonation experiments in the presence of Hydraffin activated carbon with different initial succinic acid concentrations.All other conditions were the same as those listed in Figure 1 unlessindicated: (gas flow rate) 20 L h - 1 ; (initial succinic acid concentration)( 9 ) 4  ×  10 - 3 , ( 2 ) 8  ×  10 - 3 M. - r  bw ) k  bw C  O3 C  B  (8) - r  CS ) k  cs C  O3 C  B  (9)  N  O3 ) P O3  /He1He k  g a +  1 k  l a +  1 k  bw C  B +  11 w [  1 k  c a p +  1 η k  cs C  B ] (10) - d C  B d t   )  zN  O3  (11) - d C  B d t   )  zP O3  /He1 k  l a +  1 k  bw C  B +  11 w [  1 k  c a p +  1 k  cs C  B ] (12) - d C  B d t   )  zP O3  /He1 k  l a +  1 k  bw C  B (13) C  B - C  B0 k  l a  +  1 k  bw ln C  B C  B0 )-  zP O3 He t   (14) C  B0 - C  B k  l a  -  zP O3 He t  )  1 k  bw ln C  B C  B0 (15) 3018  Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006
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