Abstract: Chromium electroplating waste water contains toxic Cr6+ ions which is normally treated by reductive precipitation. However, in Indonesian small-medium scale electroplating industries, these processes are often ineffective since they are rather complicated to apply. Therefore, a batch-wise UV/TiO2 photocatalysis potentially offers a more effective and easy-to-use process. In this study, waste water containing Cr6+ similar to real industrial effluents was treated by UV/TiO2 photocatalysis, in which photogenerated electrons could reduce highly toxic Cr6+ to relatively harmless Cr3+. The photoreactor used in this investigation was a 3-L bubble column photoreactor equipped with a 64 W low pressure UV amalgam lamp (20 W at 254 nm). Preliminary experiments showed 150 min irradiation was required per batch and 3 L min-1aeration was found to be optimum, resulting approximately 90% removal of hexavalent chromium. The photoreductive treatment was not possible without a hole scavenger (such as EDTA, oxalic acid, citric acid) presence due to the fast recombination of the photogenerated electrons and holes. This investigation, in particular, focused on the two main factors affecting the effectiveness of Cr6+ photoreductive process which were photocatalyst concentration (varied from 0.5 to 2 g L-1) and pH (in the range of 1-3). Under the experimental conditions, the removal Cr(VI) was between 76.5-94.2%. The most cost effective conditions for the photoreduction of Cr6+ were found at pH 1 and photocatalyst concentration of 0.5 g L-1 TiO2.
INTRODUCTION
Many studies have been conducted to investigate the potential use of photocatalysts in many different areas of interest. For example, the application of thin film titanium oxide for self-cleaning surfaces has been successfully used in Japan (Fujishima et al., 1999). Extensive studies on hydrogen production through water splitting reaction using semiconductor photocatalysts are promising as an alternative energy in the future (Anpo and Takeuchi, 2003). Another major area of photocatalysis study is the use of photocatalysts to oxidize various types of toxic organic pollutants, either in water purification or wastewater treatment processes.
Photocatalysis involves redox reactions of organic or inorganic molecules (Teichner and Formenti, 1985), induced by suitable light irradiation upon a semiconductor particle. Thus, photocatalytic reaction steps can be described as follows (Schiavello, 1988):
| • | Photogeneration of electron-hole pairs by exciting a semiconductor with radiation of equal or higher energy than the semiconductor band-gap energy |
| • | Separation of electron-hole pairs by surface traps |
| • | Redox reaction between the separated electrons and holes with adsorbed substrates |
In other words, some key events of kinetic processes at the surface of the photoexcited semiconductor are: photoexcitation, migration and trapping of the photogenerated electrons and holes, surface charge transfer reaction between trapped electron or hole and adsorbed species and unfavorable electron-hole recombination.
Among several possible semiconductor (ZnO, CdS, WO3, SrTiO3, ZnS, TiO2, SnO2 and Fe2O3) photocatalysts, TiO2 or TiO2-based photocatalysts are the most widely used for several reasons. These photocatalysts are non-toxic, inexpensive and have an excellent photoactivity as well as a good chemical stability/durability. In the application of organic pollutant photodestruction, TiO2 photocatalysts have generally been the most active compared to other semiconductors (Cunningham et al., 1999). The TiO2 photocatalytic reduction process has been reported as one of the effective methods for the removal of various hazardous metal ions, such as Hg(II) (Chen and Ray, 2001), As(V)/As(III) (Yang et al., 1999; Lee and Choi, 2002) and Cr(VI) (Ku and Jung, 2001; Testa et al., 2001; Mohapatra et al., 2005; Xu et al., 2006; Rengaraj et al., 2007).
The absorption of photon or light energy at or near the surface of a semiconductor particle can excite the solid semiconductor. Semiconductors are characterized by energetically non-overlapping bands (Pitchat and Fox, 1988). Thus, this photo-excitation means a band-to-band electronic transition, moving an electron from the topmost filled band (valence band) to the closest empty band (conduction band). The difference between energy level of these bands is known as band-gap energy. TiO2 semiconductor in anatase form has a relatively high energy band gap (3.2 eV), corresponding to irradiation threshold of λ below 387 nm (Pareek and Adesina, 2003). The rutile form of TiO2 has a slightly smaller energy band gap (3.02 eV) and hence the threshold is 410 nm (Li et al., 2000).
Chromium occurs in two common oxidation states in nature, Cr(III) and Cr(VI). Cr(VI) is highly toxic to most organisms, carcinogenic in animals and causes irritation and corrosion of the skin in humans. It is highly soluble in water and forms the mono- and divalent oxyanions, chromate (HCrO4¯/CrO42¯) and dichromate (Cr2O72¯), respectively, depending on its concentration and pH. Cr(VI) toxicity is one hundred times higher than that of Cr(III) (Costa, 2003; Yoon et al., 2005). Therefore, the reduction of Cr(VI) to Cr(III) is highly desirable in order to minimize the toxicity level of the waste water. After Cr(VI) reduction, Cr(III) can be separated by precipitation in alkaline solution as Cr(OH)3 (Loyaux-Lawniczak et al., 2001; Schlautman and Han, 2001).
Cr(VI) is a frequent contaminant in industrial waste water, arising from some industrial processes such as electroplating, leather tanning and paint-making and it has been regulated stringently due to its acute toxicity, carcinogenic action and high mobility in water. In Indonesia, small-medium scale electroplating plants are normally run by small businesses with limited knowledge and resources. Therefore, conventional chemical reductive treatment of Cr(VI) followed by alkaline precipitation is proven not very effective (Sugiharto et al., 2003), yet rather complicated to apply. Since the UV/TiO2 appears to be an effective technology for heavy metal ions removal from waste water, its application especially in a batch mode in such small scale electroplating business offers a promising outcome. Batch mode operation is very simple to follow and an established SOP (Standard Operating Procedure) can be developed to ensure the success removal of toxic Cr(VI) from the effluent.
Yoon et al. (2009) conducted a series of experiments in an attempt for reducing Cr(VI) by UV/TiO2 photocatalysis. In their experiments, the pH of the solution ranged from 3-9. Their experimental results showed that the photocatalytic Cr(VI) reduction was favorable in acidic conditions, with around 98% of the Cr(VI) could be reduced within 2 h at pH 3. However, the photoreduction efficiencies decreased with increasing pH, ranged only 5-15% at pH 5-9.
Kajitvichyanukul and Changul (2005) reported that the photoreduction of total chromium, including hexavalent and trivalent chromium, was improved with increasing doses of TiO2 photocatalyst. The 300 ppm chrome waste water used in that study was adjusted to pH 3.
From the photocatalysis point of view, those two parameters are quite important and determining the overall photoreduction and photooxidation rates occuring at the photocatalyst surface. Hence, the aim of this study was to determine the optimal combination of pH (in acidic condition with pH = 3) and TiO2 photocatalyst concentration which had not been explored in previous investigations using UV/TiO2 photocatalytic system. However, since those optimal conditions will be used for treating Cr6+ waste water from typical Indonesian small-medium scale electroplating plants, the process must be as cost-effective as possible.
MATERIALS AND METHODS
Materials: A synthetic waste solution of 50 ppm Cr(VI) was made by dissolving potassium dichromate K2Cr2O7, resembling to chromium containing waste water from a medium scale electroplating plant in Bandung, Indonesia. The photocatalyst used was Merck TiO2, consists of 99% anatase TiO2. Citric acid as a hole scavenger was added to the solution, in which the mole ratio of hole scavenger and Cr(VI) used was 1.25:1 (Meichtry et al., 2007). The solution pH was varied between 1-3, meanwhile the TiO2 concentration ranged between 0.5-2 g L-1.
Photoreactor: Experiments were conducted in a 3 L laboratory scale bubble column photoreactor (Fig. 1). The reactor was equipped with a cooling water jacket to maintain the reaction temperature. The light source used was a 64 W Heraeus Low Pressure UV amalgam lamp and placed in a quartz cylinder. Prior to illumination, the mixture was aerated under dark condition for 30 min to reach adsorption equilibrium. Reaction time was 150 min and 25 mL sample was withdrawn for every 30 min.
Cr(VI) analysis: The concentration of Cr(VI) was followed colorimetrically by spectrophotometry analysis using LW Scientific UV-VIS Spectrophotometer (model UV-200 RS).
| Fig. 1: | Schematic diagram of bubble column photoreactor |
In this spectrophotometric method, each sample was prepared using the Diphenylcarbazide (DPC) solution. DPC solution was made from 250 mg of 1.5-diphenylcarbazide dissolved in 50 mL of acetone (APHA/AWWA/WEF, 1998) and 0.2 N H2SO4 and H3PO4 were used for pH adjustment.
RESULTS AND DISCUSSION
The experimental results of Cr(VI) reduction with photocatalysis UV/ TiO2 showed that there was a color changing in the waste solution from yellow to transparent. Initial yellow color indicated the high concentration of hexavalent chromium still remaining in the waste solution, meanwhile the fading yellow color was due to the concentration of hexavalent chromium was gradually reduced. The quantitative result was analyzed by the value of k (pseudo 1st order reduction rate constant) as well as the% removal of Cr(VI).
Preliminary experiments: In a bubble column photoreactor, the mixing of reaction mixture is achieved by using aeration. The photocatalysis is a heterogeneous process in which the photo reduction of Cr(VI) occurs at the surface of the TiO2 photocatalyst. The aeration rate in one hand affects the rate of mass transfer from liquid bulk phase to catalyst surface significantly. An excellent mixing of mixture will enhance the photocatalysis process since increasing aeration rate minimizes mass transfer resistance. However, on the other hand the rate of aeration must not be too excessive, because oxygen (an electron scavenger) will compete for the photogenerated electrons which are meant to reduce hexavalent chromium, hence the reduction reaction will be hampered. Therefore, the optimum rate of aeration must be determined in the UV/TiO2 system beforehand. Figure 2 shows the reduction rate constants in different aeration rates. Initially, higher aeration rate results in greater Cr(VI) reduction rate constant. It means that the reduction process are faster due to more intimate mass transfer at high aeration rate. However, experimental results show that the Cr(VI) reduction rate constant decreases at aeration rate 4 L min-1 and hence the optimum aeration is found at 3 L min-1.
In some preliminary experiments, the photoreduction of Cr(VI) was not possible without the presence of an additive agent, as shown in Fig. 3. The additive agent is named as hole scavenger (such as EDTA, citric acid and oxalic acid). As the photogenerated electrons and holes experience a series reaction with other adsorbed species on photocatalyst surface, at the same time they can also recombine because of the instability of electron and hole which have opposite charge. Electron-hole recombination may greatly reduce the Cr(VI) reduction rate, because it is thermodynamically favorable. Hole scavengers contribute to minimize the recombination reaction through the binding of produced holes (Meichtry et al., 2007), so that, the photogenerated electrons may be utilized more effectively for reductive purposes.
The measured Cr(VI) reduction rate constants using each hole scavenger are shown in Table 1.
| Fig. 2: | Cr(VI) reduction rate constants at various aeration rate |
| Fig. 3: | Photoreduction process of Cr(VI) with and without the assistance of hole scavengers |
| Table 1: | Reduction rate constants for several types of hole scavenger |
EDTA and citric acid resulted in a much higher reduction rate constant than that of oxalic acid. Since citric acid price is around one third of EDTA, citric acid was then chosen as a hole scavenger throughout the experiments.
Effect of photocalyst concentration and pH on the reduction rates: In the following experiments, the effects of photocatalyst concentration and pH on the Cr6+ reduction effectiveness were determined. Solution pH was varied between 1 to 3, meanwhile TiO2 concentration ranged from between 0.5 to 2 g L-1. Those variations were chosen in accordance with the previous experiment done by Yoon et al. (2009), in which the photocatalytic Cr(VI) reduction was favorable in acidic environment. Moreover, similar results were also found by Slamet et al. (2003).
Optimum conditions for Cr(VI) treatment can be determined by evaluating the pseudo 1st order rate constants (Wang et al., 2010) and the (%) removals (Table 2). The rate constants represent how fast the reduction occurs, meanwhile% removals show how much Cr(VI) can be converted to Cr(III).
Based on the experimental results in Table 2, the reduction of the Cr(VI) is significantly affected by the solution pH, because first of all, the pH of the solution affects the surface charge on TiO2 photocatalyst. The pH value for the zero ionic charge is called point of zero charge (pHZpC) (Hoffmann et al., 1995). For anatase TiO2, the point of zero charge is around 6.0-6.4 (Berube and de Bruyn, 1968). Below this point of zero charge, the surface charge of the TiO2 will be positive and conversely, above this point the surface charge will be negative. Considering the HCrO4¯, CrO42¯ and Cr2O72- as the predominant species at medium to low pH values (>2), the photocatalyst surface which have positive charge in the acidic solution will adsorb the Cr(VI) species strongly due to the opposite charge interaction (Yoon et al., 2009) at pH 3. At pH less than 2 the dominant species is neutral chromic acid molecule, H2CrO4 which is adsorbed less strongly than the negatively charged species at higher pH.
Interestingly, the reaction rate constants in Table 2 appear to be conflicting with the previously described phenomena. Therefore, there must be another effect of the pH on the complex system. In fact, the pH also significantly affects the Cr(VI) to Cr(III) reduction potential (E0Cr(VI)/Cr(III)) and lower pH increases the potential to be more positive (by 0.13 V/unit pH) than that of the conduction band of TiO2 (Xu et al., 2004; Cappelletti et al., 2008).
| Table 2: | Rate constants and % removals of Cr(VI) in various condition |
Consequently, at low pH, the reduction of Cr(VI) is thermodynamically favorable and hence, greater rate constant and Cr(VI)% removal is observed in the more acidic solution, overweigh the effect of adsorption strengths at various pH. The reduction of Cr(VI) by photogenerated electrons can be described as follows:
| (1) |
Heterogeneous photocatalytic reactions which take place on the catalyst surface, largely depend on physical characteristics of catalyst, such as particle size, surface area, surface defects, surface metal deposits and crystalline phase (Wang et al., 2010). However, the experimental results in Table 2 show that the variation of TiO2 concentration (at every pH level) does not cause any discernible effects on both rate constant and Cr(VI)% removal. The increasing catalyst concentration may enhace the reaction rates as the photocatalysis process took place at the catalyst surface. Nonetheless, the effect is not reflected from the result of this investigation due to the fact that the concentration of Cr(VI) in the waste solution is relatively very small of 50 ppm compared to a similar study (Kajitvichyanukul and Changul, 2005). In other words, the TiO2 concentration of 0.5 g L-1 provided sufficient surface area for Cr(VI) adsorption and reduction at that low concentration.
The experiment results in Table 2 show that the reduction of Cr(VI) reaches the maximum rate at pH 1 and 2 g L-1 TiO2. However, the% removal at pH 1 and 0.5 g L-1 L TiO2 is just 3% lower than that of 2 g L-1 experiment. Therefore, taking the cost of the photocatalyst into account, it can be concluded that the Cr(VI) reduction is best carried out at pH 1 and TiO2 concentration of 0.5 g L-1.
CONCLUSION
Preliminary experiments 3 L min-1 aeration was found to be optimum for hexavalent chromium photoreduction in a 3 L batch UV/TiO2 photoreactor system. The reductive treatment was not possible without a hole scavenger (such as EDTA, oxalic acid, citric acid) presence due to the fast recombination of electrons and holes. Under the experimental range, the Cr(VI) removal was achieved from 76.5-94.2%, under 150 min irradiation by a 64 W UV lamp. The most cost effective condition for the reduction of Cr(VI) was found at pH 1 and 0.5 g L-1 TiO2.
ACKNOWLEDGMENT
This study was funded by The Indonesian Ministry of Education (Higher Education Commission) under contract No: 062/SP2H/PP/DP2M/III/2010. The authors are immensely thankfull for the financial as well as technical support from the Ministry.