Decolourization and Removal of Phenol Compounds from Olive
Mill Wastewater by O3/UV/NaBO3 and Pre-treatment
M. Uğurlu*, S. Avunduk**, A.J Chaudhary***, A.I. Vaizoğullar*, M. H. Karaoğlu* and S. Baştan*
* Department of Chemistry, Faculty of Science, Muğla University, 48000 Muğla, Turkey
**Muğla University, Vocat Sch Helth Care, Med Lab Programme, Muğla, Turkey
***Institute for the Environment, Brunel University, London, UB8 3PH, UK
Abstract
Olive mill wastewater (OMW) generated by the olive oil extracting industry is a major pollutant, because of its high organic load and phytotoxic and antibacterial phenolic compounds which resist biological degradation. The aim of this study was to evaluate the feasibility of decolourization and removal of phenol and lignin in OMW by O3/UV/NaBO3 and Pre-treatment. In pre-treatment experimentals, chemical coagulation experiments with lime and alum have been carried out. In the photolytic degradation experimentals, the effect of NaBO3 dosage, times, pH, O3, temperature and OMW concentration were determined to find the suitable operating conditions for the best removal. At the end of this procedure, it has been observed decreasing colour change intensity from 10.41 to 1.71, the phenol concentration from 300 mgL-1 to 111 mgL-1, the lignine concentration from 10.60 gL-1 to 1.61 gL-1, the value of COD from 61000gL-1 to 9.76gL-1. From experimental results, Optimum values for the degradation of phenol and lignin were favorable at pH 9.0, colour degradation was observed in acidic conditions (pH5.0 and pH3.0). The optimum time and temperature for removal of colour, phenol and lignin were found to be 10h and 308K, respectively. In addition, the pseudo-first order model was applied and r2 values were noted from 0.90 to 0.99. From these results, it can be said that this study proves the effectiveness of photolytic removal for highly concentrated organic pollutants present in OMW. Moreover, there is no study reported in the literature related to the use of O3/UV/NaBO3 in OMW treatment and O3/UV/NaBO3 may constitute an important step for further purification processes such as adsorption, membrane processes, etc.
Keywords: Olive mill wastewater (OMW); colour; phenols; lignin, ultraviolet (UV), sodium perborate (NaBO3), O3
1. Introduction
Olive mill wastewater (OMW) generated by the olive oil extracting industry is a major pollutant because of its high organic load and the phytotoxic and antibacterial phenolic compounds which resist biological degradation. Mediterranean countries are mostly affected by this serious environmental problem, since they are responsible for 95% of the worldwide olive-oil production [1-3]. There are many methods used for OMW treatment, such as that proposed by Kestioğlu et al. [4] who studied the phsico-chemical treatment and advanced oxidation processes by means of the ozone or Fenton’s reagent in the presence and absence of UV radiation. They showed that the same COD and total phenol removal efficiencies (99% removal for both COD and total phenol) were found to have been given by both H2O2/UV and O3/UV combinations. Another method was recently applied to the treatment of OMW and consists of the application of an integrated centrifugation-ultra filtration system [5] allowing an efficient reduction of pollution and a selective separation of some useful product. Traditional physical and chemical techniques, such as flocculation, coagulation, filtration, lagoons of evaporation, the electrochemical treatment of OMW and burning systems also solve the problem, but only partially [1,6,7]. In addition, Oukılı et al. [8] have investigated activated clay as adsorbents for removal of organic compounds from OMW, the removal of phenolic compounds have also been investigated effectively by using lime. Curi et al. ([9] have tested the treatment of OMW with a mixture of aluminium sulfate and ferric chloride, calcium hydroxide solution and also acidifying of the waste with hydrochloric acid solution. They have determined the clarifying percent of the wastewater. Calcium hydroxide and aluminium sulphate have also been used besides magnesium sulphate by Tsonis et al. [10]. They have reported that COD value dropped to 20–30% with calcium hydroxide, when it was added until the pH of the waste reached 11. Several biological studies have also been conducted to eliminate the pollution effect of OMW [11] and the organic content of OMW was oxidized using monopersulfuric acid [12]. Lallai et al. [3] have investigated the biodegradation of phenolic compounds by using aerobic microbial cultures. As a result, there is no such economical and easy solution for removal organic compounds from OMW.
Commonly applied method for removal of COD, colour, phenol and organic compounds from industrial effluents is Advanced Oxidation Processes (AOPs). AOPs are related to the formation of OH radicals, which will accelerate the oxidative degradation of numerous organic compounds dissolved in wastewater. It has been found that AOPs include several processes such as ultraviolet/ozone (UV/O3), ultraviolet/hydrogen peroxide (UV/H2O2), and ozone/hydrogen peroxide (O3/H2O2) [4,14].
Hydroxyl radicals produced in either way described above may attack organic molecules by abstracting a hydrogen atom from the molecule [4, 15]. In addition, sodium perborate (NaBO3) can be used as oxidizing substance instead of O3 or H2O2 to produce OH radical due to its low cost.
In the present study, it was aimed to investigate the decolourization and removal of some organic compounds (phenol, lignin from OMW by using O3/UV/NaBO3). In addition, there is no study reported in the literature related to the use of O3, UV and NaBO3 together in the OMW treatment.
2. Experimental Methods
2.1 Characterization of OMW
OMW samples were obtained from an olive-oil producing plant (Muğla area of Turkey) which uses a modern production process. No chemical additives are used during the olive oil production.
2.2 The pre-treatment experimentals
Pre-refining process was carried out to increase the more removal ratio of OMW. In this process, the chemical coagulaton technique and the mixture of lime and alum (Aluminium sulfate) in certain proportions were used. In this step, 1g of lime and 4g of alum were put in 1L of crude OMW and stirred for 15 minutes at 100rpm/min then for 30 minutes at 30 rpm/min slowly. The mixture was set aside for 24 hours to formation of flocks and precipitation. After filtration, 10g of NaBO3.4H2O (sodium tetra borate) was added to 1L of OMW. Considering the studies on the significant amount of peroxide formation of NaBO4 between 60 oC and 70oC and provide important contribution to further oxidation, it was heated for 30 minutes. In the second pre-refining process significant amount of solid particles were formed and it was set aside for 24 hours to reprecipitation. Then this mixture was filtered and the chemical analysis of some compounds in the mixture was carried out according to waste-water removal standarts (Table 1).
Table1: After pre-refining, the changes of OMW composition.
Parametre / Colour (abs) / Phenol (mgL-1) / Lignin (gL-1) / COD (mgL-1)OMW / 10.41 / 300 / 10.60 / 61000
Lime-Alum / 4.59 / 160 / 3.46 / 22480
NaBO3 / 1.71 / 111 / 1.61 / 9760
2.3 Photolytic experiments
In photolytic experiments, the effects of reaction temperature, NaBO3 amount, OMW concentration, solution pH and O3 were investigated. In all the experiments, colour, phenol and lignin concentration changes taking place in OMW were analyzed through spectroscopic methods. OMW samples carried out the pre-treatment were directly treated by taking them into UV reactor. For oxidation, specially designed UV reactor was used. This reactor consists of a closed system having an UV lamp, properties of fixed mixing and cooling and oxygen entry (Fig 1). In the experiments of pH, the pH solution was adjusted by using diluted HCI and NaOH solutions.
Fig 1: Appearance schematic of UV reactor used at the experimental study
2.4 Determination of colour changes
OMW was put into the Dr. Lange spectro-photometer and maximum wave length in the visible area and absorbance intensity were determined to be 420 nm and 4.0, respectively. Then, colour changes were investigated at this wavelength and the colour removal (%) was calculated using the following expression.
Colour removal (%):
2.5 Lignin measurement
APHA Standard Methods were used for the measurements of lignin in OMW [26]. The concentration of lignin and lignin degradation compounds were determined by analysis of the developed colour resulting from the reaction of phenol with 4-aminoantipyrine and from the reaction of lignin with folin phenol reagent (tungstophosphoric and molybdophosphoric acid) at λmax 700 nm, respectively.
2.6 Phenol measurement
A spectrophotometric method for the determination of phenol in wastewater was used. The method is based on the oxidative coupling of phenols with 4-aminoantipyrine (4AAP) in alkaline solution in the presence of potassium ferricyanide [26]. The optimum determination wavelength is at 500 nm.
In addition, the rate expression of phenol and lignin was simplified as a pseudo-first order kinetic model as follows.
(1)
where kobs denotes a pseudo-first order kinetic constant, C0 is concentration before irradiation and Ct is concentration after irradiation for time t. All experiments were run at least twice.
3. Results and Discussion
3.1. Photolytic Experiments
The temperatures, the amount of NaBO3, the concentration of OMW, pH and the amount of O3 have been studied as parameters in the photolytic experiments.
3.2. The effect of Temperature
As considered that the temperature affects the rate of reaction in chemical reactions, the photolytic reactions were carried out at 298K, 308K and 318K. It was shown that the colour, phenol and lignine removals depending on the temperature respectively in figure 2.
/ (a)/ (b)
/ ( c)
Fig 2. The changes of color (a), phenol (b) and lignin (c) due to temperature and times (pH:12, NaBO3.H2O.3H2O: 10gL-1, O3:1,5Lmin-1 and UV:17 Watt)
When it has been researched the colour changes of OMW due to the temperature and time, it was observed increasing at 298K and 308K then fixing due to the time (Fig 2a). When the changes of phenol and lignine concentrations in the same conditions were examined, it was observed rising in the phenol removal with increasing temperature and maximum removal was 75% (Fig 2b). When the changes of lignine concentrations were analysed, it was observed that lignine removal increased with increasing temperature. It was seen maximum lignine removal was estimated 80% at 303K (Fig 2c).
In the literature survey, peroxydiborate salts hydrolyzed rapidly and produced tetrahydroxy borate anions, great amount of hydrogen peroxyde, it has been expressed that there can be an interaction with H2O2 directly and OH. radicals indirectly in the organic material removals [14]. In another study, it has been reported during the photolytic processes, O3 and UV are used together the radicalic products can increase at significant proportion and organic compounds can oxidize obviously [13]. In our study, phenol and lignine removal being higher than room temperature confirms literature studies. Additionally, during both temperature and the other experimental studies, as measuring the pH changes continously, it was observed that the value of pH moved away initial value (pH=12) and decreased 5 and 6 units. This has given an impression at the end of photolytic reasctions, organic compounds in OMW can more oxidize to carboxylic acids and ketons and especially inorganic forms can be dominant and they can oxidize to CO2 and H2O which more stable compounds after a while.
3.3. The effect of sodium perborate tetrahydrate (NaBO3.H2O.3H2O)
It has been studied the colour, phenol and lignine removals as sodium perborate tetrahydrate is used different proportions due to its’bleaching effect. The results were plotted in figure 3 respectively.
/ (a)/ (b)
/ ( c)
Fig 3: the changes of colour (a), phenol (b), and lignine (c) removals in OMW depending on the amount of perborate and time (pH:12, O3:1,5Lmin-1, 308K and UV intensity:17 Watt).
As figüre 3(a) is examined, it was observed that the colour removals generally increased with rising perborate amounts after a while. For all amounts, while a rapid increasing occurs until 120 minutes, after that point a slow down in removals occurs due to the time. Besides, the best colour removal occured at 5gL-1and % 90. When 10gL-1 was used, it was seen 85% removal was provided. Considering NaBO3.H2O.3H2O has an important oxygen provider, aqueus peroxydiborate salts hydrolysed rapidly then produced tetrahydroxy borate anions great amount of H2O2, the related reaction scheme can be shown as below.
As it can be seen from reactions above, H2O2 is a good radical producer. Therefore, its’concentration will increase depending on increasing perborate amount and this will affect the degradation of organic compounds. In our study, although the amount of perborate increased, adequate decreasing didn’t occur in the colour removal, borate compounds occured a turbitdity and this has been appeared to a negative effect in the colour removal. When the experimental results about the phenol removal from OMW depending on NaBO3.H2O.3H2O amount were analysed, it was observed the same situation for phenol (şekil 3b). As figüre 3b was examined while 80% removal occured for 5gL-1 and 10gL-1 NaBO3.H2O.3H2O at the end of 5 hours. It was observed more removal can be provided with 10 gL-1 at the end of 6 hours. However, when 20gL-1 was used, it was seen very low phenol removal can ocur. This situation has been appeared to phenolic compounds dimerizated in the presence of O3, UV, high perborate and O3 has a great contribution on dimerization.
When the lignine removals from OMW depending on the amount of NaBO3.H2O.3H2O was examined, it was seen generally estimated 60 % removal occured for 5 gL-1 ve 10 gL-1 at the end of 3 hours, after that point the more removal occured for 10gL-1 and maximum ratio was 80%. However, for 20gL-1 lower removal occuring can be related to increasing turbitiy and dimerization forming.
3.4. The effect of the concentration of OMW