Effect of partial oxidation by ozonation on the photocatalytic degradation of humic acids

In this study humic acids, which are known to be a heterogeneous group of organic macromolecules found in natural waters, were oxidized using ozonation and photocatalysis in a sequential system. Ozonation was employed for achieving partial oxidation of humic acids prior to photocatalytic oxidation. Degradation of humic acid was explained by using pseudo first order reaction rate model based on UV-vis measurements. An improvement was achieved in the photocatalytic degradation rates with respect to the degree of pre-oxidation by ozonation. Due to the surface oriented nature of photocatalysis, adsorption characteristics of partially oxidized humic acid samples on TiO2 photocatalyst were evaluated by the application of the Freundlich adsorption model. The photocatalytic degradation rates did not correlate well with the dark adsorption characteristics of the pre-ozonated as well as untreated humic acid samples.


INTRODUCTION
Removal of humic substances has long been of concern in water treatment due to their diverse reactivity and abundance in natural waters.The acid insoluble humic acid fraction of humic substances in water is the main precursor of Disinfection by Products (DBP's) especially Trihalomethanes (THM's) forming upon chlorination.Presence of humic acids in the distribution system favors the bacterial regrowth in the network, which may lead to serious sanitary problems in drinking water quality.Humic acids also act as natural photosensitizers in heterogeneous as well as homogeneous solutions yielding active oxygen species such as 1 O 2 , HO • , HO 2

•
and O 2 •− [1,2].They impart yellow color, exhibit acidic character, are chemically complex in nature and behave like polyelectrolytes in aqueous medium, therefore they may initiate detrimental reactions.Consequently, the removal of humic acids constitutes one of the prevailing tasks in drinking water treatment.Naturally occurring humic acids (HA's) possess partially aromatic structure with significant amounts of unsaturated sp 2 hybridized carbon (e.g.aromatic carbon).They also contain volatile organic compounds that have become entrapped in polymeric network.Therefore, humic acids are considered to be polydisperse and heterogeneous with respect to size and shape as well as charge.
Ozone treatment reduces or eliminates the need for chlorination and decreases formation of DBP's as well as THM's [3].Ozonation of humic acids leads to a fast color removal and a steady decrease in UVabsorbance [4].An improvement in biodegradability is also observed upon ozonation of natural waters due to the cleavage of aromatic rings [5].Although the opportunity of enhancing further treatments such as coagulation-flocculation or activated carbon adsorption is offered by ozonation, one major disadvantage of this process is the observed inefficiency in abatement of the total organic matter TOC [6].
It has been shown previously that heterogeneous photocatalysis is a promising route for the degradation of humic acids under various experimental conditions simulating natural water systems [7][8][9].Photocatalytic inactivation of E. coli and the reactivity of humic acid with hypochlorite ion via photolytic and photocatalytic pathways were evaluated in relation to the elimination of THM's surrogates and for the assessment of the disinfection capacity of TiO 2 [10,11].Recently, the photocatalytic efficiencies of TiO 2 powders were also investigated on the decolorization of humic acids [12].
Due to the inefficacy of ozonation in attaining complete TOC abatement and the advantage of photocatalysis in achieving total mineralization, a sequential oxidation system was considered.This research has addressed the effect of partial oxidation by ozonation on the photocatalytic degradation of humic acids.

Materials and methods.
Humic acid was supplied from Aldrich and working humic acid solutions (20 mgL −1 ) were prepared upon dilution of a 1000 mgL −1 stock solution that was prepared by using distilled-deionized water.Selected properties of humic acid used in this study: Carbon content, wt % 34.4.
Parameters studied: Humic acid degradation was followed by spectroscopic measurements in UV-vis region by using Shimadzu 160A spectrophotometer.All absorbance measurements were performed at wavelengths of 254 nm (UV 254 ), 280 nm (UV 280 ) and 436 nm (Color 436 ) using a 1 cm path length quartz cell.
Orion pH meter was used to follow the changes in pH.
Ozonation experiments were carried out according to the procedure outlined previously by Kerc, et al., 2000 [13].Photocatalytic degradation and batch adsorption experiments were performed according to Bekbolet, et al., 2002 [12].

Degradation kinetics. The global degradation
of humic acids by ozone application or by photocatalysis can be represented by the following simple reaction: where Ox implies oxidizing species and HA ox denotes oxidized humic acid.
The humic acid concentration versus reaction time curves suggest that the degradation reaction can be described by pseudo-first-order kinetic model with respect to humic acid concentration.According to this kinetic approach; the rate equation can be given as: where, k represents the apparent pseudo-first-order rate constant for the degradation process.
Upon integration with the boundary conditions; [HA] = [HA] 0 for t = 0, The above given equation transforms into; Thus, a curve of ln [HA]/[HA] 0 versus t should give straight line with a slope equal to apparent reaction rate constant, k (time −1 ).The half-life, t 1/2 (time) of a reactant is the time required for its concentration to reach 50% of its initial value.Then, t 1/2 is expressed as;

RESULTS AND DISCUSSION
The use of ultraviolet absorbance at 254 nm (UV 254 ) is widely accepted to represent the concentration of humic acid in aqueous solutions [14].The parameters represented by UV 254 and UV 280 exhibit the potential of reactions of aromatic functional groups.Therefore, the changes in UV 254 and UV 280 as well as the changes in visible absorbance values measured at 436 nm (Color 436 ) indicate preferably the structural changes due to degradation [15].

Partial oxidation of humic acid by preozonation.
Ozonation was applied to humic acid so- lutions under the specified experimental conditions to achieve partially oxidized humic acid samples prior to photocatalytic oxidation.The applied ozone dosages were presented in Table 1 both as input dosage form (mgL −1 ) and as volume corrected normalized to the organic carbon content form.The extent of oxidative degradation was determined by the changes in the parameters (Table 1) and presented in the normalized forms as [HA]/[HA] 0 based on UV 254 and Color 436 and percent UV 280 removal (Figure 1).
The applied ozone doses for the preparation of partially oxidized humic acid samples could be considered as equivalent to pre-ozonation ozone dosages for an effective Color 436 and UV 254 removal [5].
Ozonation time, min The samples as represented by S1-S4 exhibited changes in Color 436 as 20%-80% and in UV 254 as 10%-55% in relation to the properties of humic acid solution.
Based on the previous findings, dissolved ozone was rapidly consumed by humic acid during the experiments resulting in partially oxidized humic acid fragments [13].The rate of reaction between ozone and organic carbon compounds is known to be a function of carbon bonding, functional group content with aromatic compounds or compounds with e − donating functional groups [16].Therefore the effect of preozonation degree could be ascribed to the changes in UV 254 and Color 436 as well as changes in UV 280 (Figure 1).UV 280 removal rates were found to be ∼ 8% faster than UV 254 and ∼ 16% slower than the Color 436 removal rates for samples 2-4.For the slightly oxidized sample S1, the comparative results showed merely 5% changes for both of the parameters, UV 254 and Color 436 .
Ozone is widely accepted as a powerful pre-oxidant (E = 2.07 V) that is capable of reducing humic acid content of a water sample to a certain extent.The pH of the reaction medium determines the differential effects between the highly specific-direct molecular O 3 attack and the rapid/nonspecific-secondary free radical species attack (e.g., • OH).Since all of the ozonation experiments are carried out under neutral pH the direct action of O 3 on humic acid is assumed to be the main route of oxidation.
Ozone action on humic acid can be presented by the following reaction scheme; where MW denotes molecular weight.
Attacking as an electrophile, ozone preferentially reacts with electron rich humic acid moieties.The major source of electron rich moieties is sp 2 -hybridized carbon, such as those contained in unsaturated carbon structures, eg: aromatic, olefinic etc. Functional groups, such as hydroxyls (−OH) and amines (−NH 2 ) are electron donating and enhance the reactivity of adjacent carbon bonds.On the other hand; functional groups such as carboxyl groups (−COOH) are electron withdrawing and may have tended to decrease the reactivity of adjacent bonded carbon.Therefore, the initial carbon structure and functionality have affected the reaction rate of ozone with humic acid [16,17].
In practice, oxidation by pre-ozonation leads to rapid elimination of color and this step partly causes the degradation of the aromatic structure.For an efficient removal of natural organic matter as well as humic acids, intermediate ozonation is recommended [5].Ozonation of humic acid solutions leads to a quick decolorization and a substantial decrease in UV absorbance as presented in Table 1. in terms of UV spectroscopic properties as UV 280 and UV 254 .This is mainly attributed to a loss of aromaticity by decyclization of the humic acid macromolecule.Under extended ozone application conditions, the resultant products are basically aldehydes and carboxylic acids that accumulate in the medium due to their resistance towards O 3 reactivity [18].
The changes in Color 436 reflected the removal of color forming chromophoric moieties of humic acid.The electrophilic attack of O 3 on phenolic groups of humic acid results in the formation of pseudo-quinoic groups and at higher O 3 doses a decrease of the quinoic groups corresponding to the break down of the aromatic cycle leads to the generation of carboxylic acids.The main contributing factor to the explanation of this complex mechanism was reported as O 3 /humic acid ratio [5,16].It is also reported that both the aromaticity and size of the organic acids are reduced after ozonation and as a consequence although the addition of acidic functional groups increases the polarity of the partially oxidized molecules, the charge density of the compounds remains relatively low and the solubility of the compounds is not increased substantially [19].The acid formation should be the major indication of the release of the acidic products and this effect on pH could be followed during sample preparation due to the differences in ozonation dosages.The recorded pH changes in between the samples can be accepted as insignificant under the specified experimental conditions.The highly oxidized humic acid sample exhibited only 9.5 × 10 −7 moles H + L −1 (Table 1).

Photocatalytic degradation of pre-ozonated humic acid solutions.
The pre-ozonated samples were subjected to photocatalytic degradation and the removal efficiencies were presented in terms of UV 254 .The degradation profiles of the pre-ozonated samples are given in Figure 2. The data were fitted to the pseudo first order reaction rate model and the kinetic parameters are shown in the Table 2. From a general point of view, the pseudo first order rate constants differed by 50% in relation to a preoxidation of 55% for UV 254 and 80% for Color 436 .One striking feature of the degradation data is that no significant effect (< 5%) is observed in photocatalytic rate constants for pre-ozonation removal degrees up to 35% for UV 254 and 55% for Color 436 .Further changes in humic acid structure due to the ozonation significantly altered the photocatalytic rate constants by 27% and 50% for the samples 3 and 4, respectively.
The most pronounced effect was observed in 30 minutes of irradiation time, at which 55% of untreated humic acid was degraded whereas 70% removal was calculated for sample 4 that was already oxidized to a high level.For 60 minutes of irradiation time, the degradation rates increased by approximately 45% as presented in Figure 3.The kinetics of the photocatalytic oxidation of the pre-ozonated humic acid samples revealed halflife values < 30 min for all of the samples.For longer irradiation periods no distinct behavioral difference was observed between the samples towards OH radical/h + induced pre-oxidation of humic acid.oxidizing species namely • OH radical possessing a high oxidation potential.When TiO 2 is irradiated with photons of less than 385 nm, the band-gap energy (E bg ) is exceeded and an electron is promoted from the valance band to the conduction band.The resultant electron-hole pair (e CB − /h VB + ) has a lifetime in the space-charge region that enables its participation in chemical reactions.The most widely postulated reactions are shown below: Initiation reaction:

Oxidation reactions induced by h VB
Reduction reactions induced by e CB − : The OH radical bound to the semiconductor surface is a chemical equivalent to the surface trapped hole.The hydroxyl radical is able to react with almost all organic molecules thus initiate the oxidative degradation.On the other hand, since the photocatalytic oxidation reactions take place on the surface of the photocatalyst, the elementary reaction steps of humic acid (HA) on the TiO 2 /water interface can be represented by the following equations: where, subscripts aq and ads denote aqueous and adsorbed species, respectively.In case of irradiation with light in visible region: Hydroxyl radical is an extremely powerful oxizing agent with a redox potential of + 2.8 V.The attack of the produced oxidants as • OH/ • HO 2 radicals may result in structural changes via hydroxylation, decarboxylation and decyclization in humic acid molecule producing less hydrophobic and less adsorbing aromatic moieties.
• OH radical reactions: Therefore, upon direct action of the oxidants, humic acid is expected to be degraded by far to the formation of CO 2 and H 2 O.For prolonged irradiation periods (t > 3 h) photocatalysis transforms to photomineralization as followed for sample 4 revealing almost 95% TOC removal.

Adsorption properties of pre-ozonated humic acid solutions onto TiO 2 . It has been shown exten-
sively that adsorption plays a prominent role in the course of photocatalytic degradation of organic pollutants.Therefore, the adsorption characteristics of untreated as well as treated humic acid samples on TiO 2 have been investigated.The data obtained by batch adsorption experiments were fitted to the Freundlich adsorption model as given below: q A = K F C e 1/n , q A : adsorbed amount of adsorbate per mass of adsorbent (m −1 g −1 ), C e : equilibrium concentration of the adsorbate (m −1 ), K F and 1/n: empirical constants.
The isotherms revealed perfect fits to the Freundlich model especially for the untreated humic acid molecule.Degree of pre-ozonation significantly altered the adsorption profile of humic acid on TiO 2 .As compiled in Table 3, K F values were in between 256 and 512 and 1/n values reflected an adsorption intensity trend towards linearity.On the other hand, for Sample 4, adsorption profile showed a cluster type accumulation in a C e region of 5.4-17.5 m −1 and a q A region of 484-3120 m −1 g −1 .Therefore for comparison purposes the q A values corresponding to 0.5 mgmL −1 photocatalytst loading that was used during photocatalytic degradation experiments were selected.The adsorption trend of q A calculated as 2664, 2368, 1969 and 880 m −1 g −1 for samples 1-4 respectively clearly explains the tendency of decreasing adsorption capacity of treated humic acid solutions on TiO 2 .These results may suggest that the preozonation transformed humic acid molecule into more desorbable compounds that might migrate to the bulk solution.Bearing in mind that the structure of humic acid molecule was strongly altered due to partial oxidation, the decreasing trend in q A can be explained by the changes in size and the degree of the functional groups taking part in surface reactions.
Untreated humic acid molecule contained more functional groups as deprotonated carboxylic groups leading to a strong attraction with the positively sites of TiO 2 through carboxylate linkages as well as hydrogen bonding [20].The effect of ozonation on humic acid structure is mainly expressed by the decrease in size and loss of aromaticity as well as increased hydrophilicity thereby increased charge attraction.Humic acid/TiO 2 binary system is mainly governed by the binding capability of humic acid onto TiO 2 primarily due to the presence of oxygenated functional groups.
Most of the formed new groups on ozonation are strongly acidic and hydrophilic and would undergo charge attraction when brought into close proximity with the positively charged sites of TiO 2 and repulsion due to the negatively charged centers.Therefore, pH dependent deprotonation of the functional groups of humic acid and the characteristics of the titanium dioxide surface should govern the adsorption characteristics of humic acid samples.
Since humic acid is considered as negatively charged at the pH of zero proton condition, pH zpc = 6.3 due to the deprotonation of the carboxylic groups, the electrostatic interaction between charged humic acid segments and TiO 2 should be attractive at pH < 6.3 and impulsive at pH > 6.3.
Besides Coulomb forces of attraction, favorable energetic compensation must exist; nonelectrostatic specific interactions between humic acid functional groups and TiO 2 surface should be involved for adsorption to occur.Contribution of the hydrophobic properties of humic acids may also be considered.
Under the presented experimental conditions, photocatalytic degradation rates revealed no distinct correlation with the adsorption characteristics of the partially oxidized humic acid samples.The reason may be explained by the complex nature of the reaction mechanism.Dark adsorption coefficients are not sufficiently indicative of the photo adsorption/photo desorption processes taking place during the early stages of irradiation.
For a better understanding of the kinetics of the photocatalytic oxidation of the pre-ozonated humic acid samples, the application of the Langmuir-Hinshelwood kinetic model is recommended.

CONCLUSIVE REMARKS AND RECOMMENDATIONS
Oxidation by pre-ozonation alters the spectroscopic properties of humic acid as explained by UV 254 , UV 280 and Color 436 .On the other hand, the complex nature of the ozonated humic acid medium could not be specifically expressed by only UV-vis properties.Photocatalytic oxidation rates as explained by pseudo-first order kinetic model, were significantly affected by the degree of pre-oxidation by ozonation.The initial rate of UV 254 removal differed by 35% in relation to the preozonation effect.On the other hand, preozonation followed by photocatalytic oxidation resulted in 80% color removal of humic acid.
No significant correlation could be attained between the photocatalytic oxidation rates and the dark adsorption properties of the ozonated as well as untreated humic acid samples.
Further experimental study on the sample characterization and the application of the Langmuir-Hinshelwood kinetic model is recommended to elucidate the sequential oxidation of humic acids via ozonation followed by photocatalytic oxidation as well as the application of the filtration technique to express the molecular size distribution of the samples.

Figure 2 .
Figure 2. Photocatalytic degradation profiles of the humic acid and the pre-ozonated humic acid samples.

TiO 2 Figure 3 .
Figure 3. Time-dependent comparison of photocatalytic degradation of humic acid and pre-ozonated humic acid samples.

Table 2 .
Kinetic parameters for UV 254 reduction by photocatalysis.

Table 3 .
Adsorption properties of humic acid onto TiO 2 .