CC BY-NC-ND
0Arabian Journal of Chemistry (2015) xxx, xxx-xxx
King Saud University Arabian Journal of Chemistry
www.ksu.edu.sa www.sciencedirect.com
ORIGINAL ARTICLE
Feasibility of electrocoagulation/flotation treatment of waste offset printing developer based on the response surface analysis
Savka Adamovic a,% Miljana Prica a, Bozo Dalmacija b, Sanja Rapajic Dragoljub Novakovic a, Zivko Pavlovic a, Snezana Maletic b
a University of Novi Sad, Faculty of Technical Sciences, Department of Graphic Engineering and Design, Trg Dositeja Obradovica 6, 21000 Novi Sad, Serbia
b University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovica 3, 21000 Novi Sad, Serbia
c University of Novi Sad, Faculty of Sciences, Department of Mathematics and Informatics, Trg Dositeja Obradovica 4, 21000 Novi Sad, Serbia
Received 6 October 2014; accepted 30 March 2015
KEYWORDS
Electrocoagulation/flotation process;
Offset printing developer;
Response surface;
Copper;
Turbidity;
Organic substances
Abstract In the printing plate developing process, the offset printing developer undergoes changes, as well as enrichment by the various chemicals, i.e. metals, organic binders and photosensitive compounds. The objective of this study was to investigate the electrocoagulation/flotation (ECF) treatment efficiency for the removal of copper, turbidity and organic substances from the waste offset printing developer (WOPD). The effect of operational parameters, such as electrode materials, current density, interelectrode distance and operating time, was studied. Also, the response surface analysis was applied to evaluate the effect of main operational variables and to get a balanced removal efficiency of investigated WOPD parameters by ECF treatment. The removal efficiency increases significantly with the increasing of operating time and mainly increases with the increasing of current density. The obtained results show that the interelectrode distance and combinations of electrodes determine the removal efficiency of copper, turbidity and organic substances. Based on the obtained results, the optimized parameters for the ECF treatment removal of investigated WOPD parameters were identified as: Al(— )/Fe( + ) electrode combination with interelectrode
* Corresponding author. Tel.: +381 21 485 2634. E-mail address: adamovicsavka@uns.ac.rs (S. Adamovic). Peer review under responsibility of King Saud University.
http://dx.doi.org/10.1016/j.arabjc.2015.03.018
1878-5352 © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
distance of 1.0 cm, operating time of 5 min and current density of 8 mA cm~2. This study confirms the practical feasibility of ECF method for treating real printing industrial effluent under optimum conditions.
© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
The electrocoagulation/flotation process includes the in-situ generation of coagulants via the electro-dissolution of a sacrificial anode, which usually consists of iron or aluminum. Only a very few reports on the combined use of both aluminum and iron electrodes in the same cell were published (Jewel et al., 2007; Katal and Pahlavanzadeh, 2011; Linares-Hernandez et al., 2009). The use of combination electrodes of dissimilar metals may provide an alternative method for the efficient removal of heavy metals, turbidity and organic substances from the wastewater (Linares-Hernandez et al., 2009) such as WOPD.
The interaction between the coagulant and the pollutant is the most complicated aspect of the ECF process (El-Shazly et al., 2011). The ECF process combines three main interdependent processes, operating synergistically to remove pollutants: electrochemistry, coagulation and hydrodynamics (Bazrafshan, 2008; Nouri et al., 2010). This process may be summarized as follows (Adhoum et al., 2004; Phalakornkule et al., 2010):
• Compression of the diffuse double layer around the charged species by the interaction of ions generated by oxidation of the sacrificial anode.
• Charge neutralization of the ionic species present in wastewater takes place due to the counter-ion produced by the electrochemical dissolution of the sacrificial anode. These counter-ions reduce the electrostatic interparticle repulsion to the extent that the Van der Waals attraction predominates, thus causing coagulation.
• The flock formed as a result of coagulation creates a sludge blanket that entraps and bridges colloidal particles still remaining in the aqueous medium.
Industrial growth is of utmost importance to mankind but the environmental pollution due to it is never desired. Heavy metal contamination exists in aqueous wastes of many industries and these usually contain metal-ion concentrations much higher than the permissible levels and do not degrade easily into harmless products (Narayanan and Ganesan, 2009). Separation techniques of heavy metals, such as chromium, cadmium, copper, zinc and nickel, from industrial wastewater include precipitation, ion exchange, adsorption, electro-dialysis and filtration, but these techniques have limitations (Nouri et al., 2010). Ion exchange, for example, while highly effective in removal of certain charged contaminants, requires resin regeneration or replacement at a high cost (Escobar et al., 2006). The costs of adsorption, ultrafiltration, reverse osmosis and ozonation exceed that of chemical coagulation. While chemical precipitation is a simple process, it does generate a high volume of sludge. When chemical coagulation is used to treat wastewater, the pollution may be caused by a chemical
substance added at a high concentration. Excessive coagulant material can be avoided by ECF process (Merzouk et al., 2009). The ECF process has been successfully employed in the removal of cadmium (Mahvi et al., 2010; Khaled et al., 2015), zinc (Nouri et al., 2010), copper (Adhoum et al., 2004; Akbal and Camci, 2010; Escobar et al., 2006; Hunsom et al., 2005), nickel (Dermentzis et al., 2011; Mouedhen et al., 2008), chromium (Bazrafshan, 2008; Zongo et al., 2009a), silver (Hangeidman and Wolfg, 2008) and arsenic (Oehmen et al., 2011; Pan et al., 2010) from a variety of liquid wastes. It has been shown that ECF process is able to eliminate tannin and organic dyes (Nandi and Patel, 2013; SanromÉm et al., 2004; Trujillo-Ortega et al., 2013), phenolic compounds (El-Ashtoukhy et al., 2013), benzoquinone (Can and Bayramoglu, 2010), natural organic matter (Mohora et al., 2012; Vepsalainen et al., 2012) and different organic substances from various types of liquid by using sacrificial aluminum or iron electrodes. The literature publications show that the ECF process has been proposed as an effective method of treating various effluents such as textile wastewater, paper mill wastewater, baker's yeast wastewater, restaurant wastewater, urban wastewater, laundry wastewater, nitrate and phosphate bearing wastewater, electroplating wastewater, and chemical mechanical polishing wastewater (Can et al., 2006; Kobya and Delipinar, 2008; Narayanan and Ganesan, 2009; Nouri et al., 2010). On the other hand, the ECF process has not yet been taken into consideration for the treatment of WOPD.
The critical physicochemical phenomenon, during the developing processes in offset printing, is the generation of the non-image area on the printing plate surface by using an aqueous solution known as the offset printing developer. The printing plate is introduced in the developer bath, in order to make the image areas visible. The image areas become ink-receptive thanks to a chemical change on the previously coated printing plate surface. The non-image areas stay water-receptive (Andrade et al., 2012). After semi-automatic offset plate insertion into the platesetter, rollers accept and pass the plate to the offset printing developer tank. In the developer tank, a roller brush makes the offset plate clean. After the developing process, the offset plates are washed with water, preserved and dried. In the preservation process, plate is covered with a thin solution of "gum arabic'' or similar chemical, which gives the non-image areas storage-resistant hydrophilic properties (Kipphan, 2001). The waste offset printing developer is expected to contain residual ingredients and products present in the offset plate surface such as organic binders and photosensitive compounds (Vengris et al., 2004). All these processes resulted in a high amount of metals (part of the offset plate), organic substances (originated from chemicals) and turbidity in WOPD. Therefore, the offset printing sites should apply measures that would be focused on monitoring, prevention and then on preparation for re-use of the WOPD before being discharged into water and soil recipients.
The goal of this study was to investigate the feasibility and efficiency of ECF process in the removal of copper, turbidity and organic substances from WOPD under different operational conditions (electrode materials and combinations, current density, interelectrode distance and operating time). The analysis based on the response surfaces was used to obtain the best parameters for the optimum ECF process design with the least number of experiments and highest removal efficiency of investigated parameters from WOPD.
2. Materials and methods
2.1. Offset printing developer
The widely distributed offset printing developer, trade-named Fujifilm LP-DS developer, is described by the manufacturer as an aqueous, alkaline solution containing inorganic salts. The main characteristics of the initial (IOPD) and waste offset printing developer (WOPD) used in this research are presented in Table 1.
A digital calibrated pH-meter (EC30 pH meter) and a conductivity-meter (Cond 3210 conductometer) were used to measure the pH value and the electrical conductivity of the IOPD and WOPD according to the standard EPA 150.1 and EPA 120.1 methods, respectively.
Copper analyses have been carried out by the Atomic Absorption Spectroscopy (PerkinElmer Aanalyst 700) according to the standard method EPA 7000B. The concentration levels of other metals (cadmium, chromium (total), nickel, zinc and lead) were below the method detection limit (MDL).
Turbidity has been determined by HI 93703 microprocessor turbidity meter (HANNA Instruments) according to the standard method EPA 180.1.
The UV326 absorbance measurements of organic substances have been performed before and after the ECF treatment in accordance with standard methods (AWWA-APHA-WEF, 1998) by UV-1800 SHIMADZU spectrophotometer at a wavelength of 326 nm with a 1 cm quartz cell. Samples were diluted with distilled water in a 1:20 ratio.
2.2. Electrocoagulation/flotation procedures in batch mode
The ECF experiments have been performed in a batch cell with four plate electrodes connected in parallel (bipolar) mode. Also, four sets of experiments have been performed with different electrode combinations: (1) four iron electrodes (Fe(—)/ Fe( + )), (2) four aluminum electrodes (Al(—)/Al( + )), (з) two aluminum (one was anode) and two iron electrodes (Al(—)/
Fe( + )) and (4) two iron (one was anode) and two aluminum electrodes (Fe(—)/Al( + )). Only the outer electrodes have been connected to the DC power supply (DF 1730LCD), and anodic and cathodic reactions occurred on each surface of the inner electrode when the current passed through the electrodes. The experiments have been conducted in a cell, capacity of 250 mL, which was made out of borosilicate glass. The electrodes with the same dimensions of 10 cm x 5 cm x 0.1cm and total area of 100 cm2 have been used and placed vertically in the cell. After being dipped in the WOPD, the effective area of each electrode used for electrolysis was 40 cm2.
In each batch ECF experiment, 220 mL aliquots of WOPD (collected from a waste tank in the platesetter) added with the same amount of potassium chloride (0.50 g) have been stirred at 450 rpm by a magnetic stirrer (IKA color squid). The addition of halide salts will: (1) avoid excessive ohmic drop, (2) limit the formation of the passivation layer on aluminum or iron electrodes, (3) decrease the energy consumption, and (4) limit the temperature variations, due to the Joule effect (Adhoum et al., 2004).
To follow the progress of the ECF process, samples of 15 mL were periodically taken from the electrocoagulation cell at certain operating times (1, 5, 10, 20, 40 and 60 min). Upon the completion of the process, the test samples were cen-trifuged (CentrifugeTehtnica Zelezniki) at 2000 rpm for 15 min and the supernatant was then used for the analyses.
The ECF processes have been investigated at current densities of 2, 4 and 8 mA cm—2 (corresponding to 0.08, 0.16 and 0.32 A, respectively) for the interelectrode distances of 0.5, 1.0 and 1.5 cm, respectively.
The electrodes were prepared in an appropriate way in order to ensure electrodes surface reproducibility. Before each run, the electrode surface was first mechanically polished with abrasive paper (Yadav et al., 2012), rinsed with distilled water, immersed for 10 min in a 5 M solution of hydrochloric acid (35%) (Dermentzis et al., 2011), then washed again with distilled water, and dried (Mouedhen et al., 2008).
2.3. Determination of removal efficiency of copper, turbidity and organic substances from WOPD
The removal efficiencies of copper, of turbidity and of organic substances based on UV326 absorbance, were evaluated by the following universal equation (Hunsom et al., 2005; Mohora et al., 2012):
Removal efficiency (%) = X° — Xt ■ 100 (1)
where Xo - the initial values of copper concentration or of turbidity or of content of organic substances in WOPD and Xt -values of copper concentration or of turbidity or of the organic substances in WOPD after a certain ECF electrolysis time (t).
2.4. Response surface analysis
The response surfaces are used to show the relationship between three operational variables and three responses on four different electrode combinations. As in similar investigations (Amani-Ghadima et al., 2013; Behbahani et al., 2011; Bhatti et al., 2009; Chung et al., 2014; Elksibi et al., 2014; Fakhri, 2015; Khataee et al., 2010; Kumar et al., 2009;
Table 1 The characteristics of initial (IOPD) and waste offset
printing developer (WOPD).
Parameters IOPD WOPD
t (°C) 25 ± 1 25 ± 1
pH 12.44 11.81
Electrical conductivity (mS cm ') 0.65 0.77
Electrical conductivity (mS cm—') with NaCl - 16.27
Turbidity (NTU) 184 2860
Organic substances based on UV326 absorbance 24.44 79.76
Copper (mg L—') MDL 23.95
Thirugnanasambandham et al., 2013, 2014; Wang et al., 2007), the quadratic and cubic models were chosen to model the effects of the independent variables (interelectrode distance, operating time and current density). The statistical significance of models was analyzed by analysis of variance (ANOVA) using software package Mathematica 8.0.
3. Results and discussion
The values of temperature (from 25.4 to 25.6 0C), pH (from 11.81 to 11.83) and electrical conductivity (from 16.32 to 16.34 mScm-1) after the each ECF experiment on WOPD show that this treatment does not lead to changes in these parameters. In alkaline medium (pH > 8), the final pH does not vary significantly and a slight drop has been recorded. This result is in accord with previously published works and suggests that electrocoagulation can act as a pH buffer (Nouri et al., 2010). Therefore, pH adjustment before the treatment is not required in practical applications. The electrical conductivity was controlled by adding solid potassium chloride, which prevented the passivation of the aluminum or iron electrodes.
3.1. The influence of current density on the ECF treatment efficiency
With an increase in the current density from 2 to 8 mA cm~2, there is a substantial increase in the removal efficiency of turbidity in the ECF process (Fig. 1).
The current density is expected to exhibit a strong effect on removal efficiency (Can et al., 2006; Chen, 2004; Holt et al., 2005; Mollah et al., 2004, 2001), especially on the kinetics of turbidity removal: higher the current, shorter the ECF treatment. This is ascribed to the fact that at high current density, the extent of anodic dissolution of electrodes increases, resulting in a greater amount of precipitate for the removal of pollutants (Merzouk et al., 2009).
The results (Figs. 2 and 3) indicate that at the beginnings of the experiments, the copper and organic substances removals are greater at higher values of current density. This expected behavior can easily be explained by the increase of coagulant and bubbles generation rate, resulting in a more efficient and faster removal, when the current density is increased (Adhoum et al., 2004; Hunsom et al., 2005; Nouri et al., 2010; Zongo et al., 2009a,b). The removal efficiencies of
Figure 1 The removal efficiency of turbidity from the WOPD at current densities of (a) 2 mA cm 2, (b) 4 mA cm 2 and (c) 8 mA cm 2 (four electrode combinations and interelectrode distances of 0.5, 1.0 and 1.5 cm).
Figure 2 The removal efficiency of copper from the WOPD at current densities of (a) 2 mA cm 2, (b) 4 mA cm 2 and (c) 8 mA cm 2 (four electrode combinations and interelectrode distances of 0.5, 1.0 and 1.5 cm).
copper from the WOPD after 60 min at current densities of 8mAcm—2 with Al(—)/Al( + ), Al(—)/Fe( + ), Fe(—)/Fe( + ) and Fe(—)/Al( + ) electrode combinations (r = 0.5 cm) were 99.0, 97.0, 96.7 and 92.8, respectively. The highest current (8 mA cm—2) produced the quickest removal rate, with a 22.5%, 35.6%, 42.9% and 52.6% reduction of organic substances occurring after 60 min using Fe(—)/Fe(+), Al(—)/ Al(+), Al(—)/Fe(+) and Fe(—)/Al(+) electrode combinations (r = 0.5 cm), respectively.
During the ECF process, the electrochemical reactions taking place at the sacrificial aluminum and iron anode produce the metallic cations (Al37 and Fe27). For aluminum electrodes, Al37 cations interact with OH2212 to form a monomeric (Al(OH)27, Al(OH)2 + , Al2(OH)2 + , Al(OH)4—) and polymeric hydroxides (Al6(OH)?7, Al7(OH)i7+, Al8(OH)47, Al13O4(OH)74+, Al13(OH)57) (Dermentzis et al., 2011; Mollah et al., 2004; Khaled et al., 2015), which have high adsorption properties thus bonding with the pollutants from the WOPD in the ECF process. In the case of the iron electrode, beside Fe(OH)2, other monohydroxy, polyhydroxy and aquahydroxy complexes (Fe(OH)27, Fe(OH)°,
Fe2(OH)4 +, Fe(OH)4, Fe(H2O)2+, Fe(H2O)5OH2 + , Fe(H2O)4(OH)2+, Fe(H2O)s(OH)22, Fe2(H2O)6(OH)4+) may also be present in the ECF system (Mollah et al., 2004; Narayanan and Ganesan, 2009). These hydroxides, polyhydroxides, polyhydroxymetallic complexes are responsible for the trapping of colloidal particles, copper and organic substances from the WOPD. The suspended aluminum and iron hydroxides can remove pollutants from the WOPD solution by adsorption, co-precipitation or electrostatic attraction, followed by coagulation and flotation (Chen, 2004; Parga et al., 2005).
3.2. The influence of the electrode material and combinations on the ECF treatment efficiency
Choice of electrode material is one of the main steps to ensure maximum efficiency of the ECF process. Aluminum and iron are suitable electrode materials for the treatment of various wastewaters by ECF due to easy availability, low cost, and better dissolution than PbO2, graphite, Ti/PbO2 or Ti/SiO2, IrOx and Ti/IrOx-Ta2O5 anodes (Sahu et al., 2014). Analyzing
Time (min) 2 ^КЛ л «л Л -2 .
Figure 3 The removal efficiency of organic substances from the WOPD at current densities of (a) 2 mA cm 2, (b) 4 mA cm 2 and (c) 8 mA cm-2 (four electrode combinations and interelectrode distances of 0.5, 1.0 and 1.5 cm).
literature data, some authors point to the benefits of iron electrodes, while others point to the advantages of aluminum electrodes. According to some authors, the advantages of the use of iron electrodes are:
• Iron is relatively cheaper (Vasudevan and Oturan, 2014).
• Iron more heavy than aluminum induces formation of higher flocks size which offers best solid phase (flocks) to the coagulated pollutants (Bellebia et al., 2012).
• Iron is a 3d block transition metal that has better complex-ing properties with inorganic/organic pollutants (Chopra and Sharma, 2013).
However, Katal and Pahlavanzadeh (2011) reported that Al/Al electrode combinations were effective for color removal, Fe/Fe electrode for COD and phenol removal, while Al/Fe and Fe/Al electrode combinations were effective for color, COD and phenol removal from paper mill wastewater.
Chopra and Sharma (2013) concluded that Al/Fe electrode combination proved to be more effective in comparison with Fe/Al electrode combination for removal of turbidity, chemical oxygen demand and biochemical oxygen demand
from secondarily treated sewage water by ECF treatment. The adsorption of Al3+ ion with colloidal pollutants results in coagulation. Resulting coagulants can be more efficiently removed by settling, surface complexation and electrostatic attraction in comparison with Fe2+ ions (Chopra and Sharma, 2013; Sahu et al., 2014). From the results obtained for ECF treatment of the textile wastewater, one Fe atom complexes around 9 carbon atoms, whereas Al allows com-plexation of 3 carbon atoms only. The above estimated "coordination" numbers of Fe or Al largely depend on the wastewater to be treated since one Al atom is involved in the complexation of much more organic matter (Zongo et al., 2009b). Therefore, type of wastewater and contaminants may be a deciding factor in the selection of anodic sacrificial electrode and further in ECF treatment efficiency (Chopra and Sharma, 2013).
The effective electrode combinations for the removal efficiencies of turbidity and copper in the WOPD by ECF treatment go in the following order: Al(—)/Al( + ), Al(—)/ Fe( + ), Fe(-)/Fe( + ) and Fe(-)/Al( + ). As can be seen (Fig. 1b), the efficiency of turbidity removal (>90%) in using Al(-)/Al( + ), Al(-)/Fe( + ), Fe(-)/Fe( + ) and Fe(-)/Al( + ) at
a current density of 4 mA cm—2 and interelectrode distance of 1 cm has been achieved after 1, 10, 10 and 20 min, respectively. Also, as can be seen from Fig. 1, the removal efficiency of turbidity was higher than 80% for Al(—)/Al( + ) electrode combination after 1 min at all current densities and interelec-trode distances. Solak et al. (2009) studied the removal of turbidity from marble processing wastewaters by electrocoag-ulation process and by using aluminum and iron electrodes. They preferred to use aluminum electrode (the removal efficiency of turbidity was higher then 99% after 1 min) because iron electrode caused an additional color formation in the effluent due to the chemical features of iron electrode. A relatively clean and stable effluent could be achieved by using aluminum electrode combinations in the ECF process. Here, use of iron electrode combinations resulted in a greenish effluent, color of which changed into yellow along the ECF process, emphasizing that the electrode provides an extra turbidity loading into the effluent. The forming of greenish and yellow effluent after the process might be originated due to the Fe2 + and/or Fe3 + ions dissoluted from the surface of the electrode (Solak et al., 2009; Chen et al., 2000; Kobya et al., 2006; Ozyonar and Karagozoglu, 2012). The copper removal efficiency increased from 29.4% to 100% for the Al(—)/Al( + ), from 17.2% to 99.3% for the Al(—)/Fe( + ), from 13.0% to 96.5% for the Fe(—)/Fe( + ), and from 8.5% to 94.7% for the Fe(—)/Al( + ) at a current density of 4 mA cm—2 and interelectrode distance of 1 cm (Fig. 2b).
Fe(—)/Al( + ) electrode combinations have higher removal efficiency than Al(—)/Fe( + ), Al(—)/Al( + ), and Fe(—)/Fe( + ) electrode combinations in the ECF deposition of organic substances from WOPD. At a current density of 4 mA cm—2 and interelectrode distance of 0.5 cm the values of removal efficiency of Al(—)/Al( + ) (35.6%) and Fe(—)/Fe( + ) (22.5%) electrode combinations are almost 1.5 and 2 times lower than the removal efficiency values of Fe(—)/Al( + ) (52.6%) and Al(—)/Fe( + ) (42.9%), respectively.
3.3. The effect of the interelectrode distance on the ECF treatment efficiency
The shorter interelectrode distance is desirable, because the electrical resistance (IR drop) increases with the interelectrode distance increase (Mohora et al., 2012). In accordance with that, when the current density and interelectrode distance were increased from 1 to 3 cm, it was observed that the turbidity removal efficiency decreases (Merzouk et al., 2009). Therefore, the highest removal efficiency of all pollutants from the WOPD was expected at 0.5 cm.
The highest removal efficiency of organic substances has been achieved with the interelectrode distance of 0.5 cm (Fig. 3) for all electrode combinations, whereas the highest removal efficiency of turbidity (Fig. 1) and of copper (Fig. 2) unexpectedly has been achieved with the interelec-trode distance of 1.0 cm for all electrode combinations. The results can be explained by the flotation of hydrogen bubbles produced at the cathode, which along with the suspended particles and copper create more stabile flocks when the inter-electrode distance was 1.0 cm. The interelectrode distances of 0.5 cm and 1.5 cm obstruct the adequate mass transport in the ECF cell reducing its turbidity and copper removal efficiency rates.
3.4. The effect of the operating time on the ECF treatment efficiency
The removal efficiency increases with the increase of electrolysis time for all electrode combinations, which is in accordance with the results of other authors (Mouedhen et al., 2008; Solak et al., 2009). According to the results, the highest copper removal efficiency (>92.8%) was obtained after 5 min at current density of 8 mA cm—2, interelectrode distance of 1.0 cm and Al(—)/Al( + ) electrode combination (Fig. 2c).
The turbidity removal efficiency (92.9%) was obtained after 1 min by using Al(—)/Al( + ) electrode combination, interelectrode distance of 1.0 cm and current density of 8 mA cm—2. In the case of organic substances, the removal efficiency (52.6%) with Fe(—)/Al( + ) electrode combination at 8 mA cm—2 for 0.5 cm was achieved after 60 min (Fig. 3c).
The obtained results show that the type of pollutants in WOPD determines the operating time which would generate the highest removal efficiency.
3.5. Response surface and data analysis
The experimental data are fitted with the quadratic and cubic models in order to obtain regression equations. The values of coefficient of determination R2 and adjusted R2 of quadratic and cubic models for copper, turbidity and organic substances removal efficiency with four electrode combinations are presented in Table 2. As it can be seen from Table 2, the values of R2 and adjusted R2 for quadratic model are lower in comparison with these values obtained from cubic model, showing that the ECF treatment was most properly demonstrated with a cubic model. Therefore, the cubic model is chosen to describe the effects of operational variables on the removal efficiency of cooper, turbidity and organic substances, so experimental data are fitted with the third order polynomial functions.
Tables 2 and 3 show high values of R2 and adjusted R2, while the P values indicate the statistical significance of the regression models. Diagnostic plots confirmed good agreement between actual and predicted values, but are omitted for sake of brevity. The P values (P < 0.05) in responses for regression
Table 2 The values of R2 and adjusted R2 of quadratic and cubic models for copper, turbidity and organic substances removal efficiency with four electrode combinations.
Quadratic R2 R2 adj Cubic R2 R2 adj
Fe(- -)/Fe(- Copper 0.8968 0.8757 0.9585 0.9372
Turbidity 0.8756 0.8502 0.9575 0.9356
Organic substances 0.9146 0.8972 0.9756 0.9631
Al(- -)/Al( + ) Copper 0.8122 0.7738 0.9432 0.9139
Turbidity 0.8626 0.8345 0.9627 0.9435
Organic substances 0.9531 0.9434 0.9883 0.9823
Al(- -)/Fe(- ) Copper 0.8634 0.8354 0.9498 0.9240
Turbidity 0.8642 0.8364 0.9642 0.9459
Organic substances 0.9557 0.9467 0.9828 0.9740
Fe(- -)/Al(- ) Copper 0.9023 0.8823 0.9487 0.9223
Turbidity 0.9362 0.9231 0.9907 0.9859
Organic substances 0.9585 0.9499 0.9886 0.9827
model equations imply that third-order polynomial models fitted to the experimental results well. Additionally, high R2 values of 0.9432-0.9907 for removal efficiency of copper, turbidity and organic substances express a high correlation between the observed and predicted values. This is in accordance with the literature data (Bhatti et al., 2009; Thirugnanasambandham et al., 2013).
In order to study the interactive effects of process variables on the responses, 3D response surface plots constructed from three models on every of four different combinations of electrodes are used. Fig. 4 indicates that the removal efficiency of cooper, turbidity and organic substances is sensitive to alterations of interelectrode distance and current density. It can be seen that on every electrode combination the removal efficiency of organic substances decreases with increasing inter-electrode distance. Besides, increasing the current density causes increasing the removal efficiency of organic substances on Al(-)/Al( + ) and Fe(-)/Al( + ) electrode combinations. Interelectrode distance and current density have the minor effect on the removal efficiency of turbidity on Al(-)/Al(+)
electrode combination, and also the minor effect on removal efficiency of organic substances on Fe(-)/Fe(+) electrode combination. The most influence of interelectrode distance and current density on removal efficiency of cooper is noticed on Fe(-)/Al(+) electrode combination. From the obtained results it can be concluded that interelectrode distance and current density are very important parameters which influence the removal efficiency of copper, turbidity and organic substances from WOPD.
The objective of the optimization was to determine the operating conditions that gave the maximum efficiency of copper, turbidity and organic substances removal from WOPD by ECF treatment. Based on the analysis, the optimum operating conditions for the removal of copper, turbidity and organic substances from WOPD by ECF treatment were predicted to be: Al(-)/Fe(+) electrode with interelectrode distance of 1.0 cm, operating time of 5 min and current density of 8 mA cm-2 that resulted in high efficiencies of copper (91.0%), turbidity (93.9%) and lower organic substances (21.5%) removal.
Table 3 Analysis of variance (ANOVA, cubic model) for copper, turbidity and organic substances removal efficiency with four
electrode combinations.
DF Sum of square Mean square F Value P Value
Fe(-)/Fe( + ) Copper Model 18 50406.70 2800.37 44.97 0.00
Error 35 2179.73 62.28
Total 53 52586.40
Turbidity Model 18 42747.60 2374.87 43.80 0.00
Error 35 1897.56 54.22
Total 53 44645.20
Organic substances Model 18 2070.37 115.02 77.80 0.00
Error 35 51.74 1.48
Total 53 2122.11
Al(-)/Al( + ) Copper Model 18 41110.60 2283.92 32.26 1.11 x 10~16
Error 35 2477.77 70.79
Total 53 43588.40
Turbidity Model 18 1617.30 89.85 50.17 0.00
Error 35 62.68 1.79
Total 53 1679.99
Organic substances Model 18 3608.95 200.50 164.02 0.00
Error 35 42.79 1.22
Total 53 3651.74
Al(-)/Fe( + ) Copper Model 18 48676.50 2704.25 36.81 0.00
Error 35 2571.28 73.47
Total 53 51247.70
Turbidity Model 18 23045.50 1280.30 52.49 0.00
Error 35 853.75 24.39
Total 53 23899.20
Organic substances Model 18 5340.44 296.69 111.35 0.00
Error 35 93.26 2.66
Total 53 5433.70
Fe(-)/Al( + ) Copper Model 18 56954.10 3164.12 35.95 0.00
Error 35 3080.62 88.02
Total 53 60034.80
Turbidity Model 18 51959.60 2886.65 206.56 0.00
Error 35 489.11 13.97
Total 53 52448.70
Organic substances Model 18 6284.07 349.12 168.43 0.00
Error 35 72.55 2.07
Total 53 6356.62
Figure 4 Response surface 3D plots for the effects of interelectrode distance and current density on copper, turbidity and organic substances removal efficiency at operating time 60min, on (a) Fe(-)/Fe( + ), (b) Al(-)/Al( + ), (c) Al(-)/Fe( + ), and (d) Fe(-)/Al( + ) electrode combinations.
4. Conclusion
The results obtained in this study provide an opportunity for the application of the ECF process as a purification technology of the WOPD. The conclusion, that the treatment of the WOPD by ECF is effective, can be drawn from the following:
• The application of the ECF process on the WOPD showed higher removal efficiencies (>90%) of turbidity and of copper at a current density of 8 mA cm-2 and interelectrode distance of 1.0 cm, after 1 and 5 min, respectively. The effective electrode combinations for the removal efficiency of turbidity and of copper in the WOPD in the ECF treatment are in the following order: Al(-)/Al(+), Al(-)/ Fe(+), Fe(-)/Fe(+) and Fe(-)/Al(+).
• The application of the ECF process on the WOPD showed average removal efficiency (>50%) of organic substances at a current density of 8 mA cm-2 and interelectrode distance of 0.5 cm, after 60 min. The effective electrode combinations for the removal efficiency of organic substances in the WOPD in the ECF treatment are in the following order: Fe(-)/Al(+), Al(-)/Fe(+), Al(-)/Al(+) and Fe(-)/Fe(+).
• The electrocoagulants produced by the electrodissolution of sacrificial aluminum and iron anodes showed high adsorption capacity in the removal of copper and turbidity.
• Based on the results obtained for the effective removal of copper and turbidity, ECF proves to be a very promising technology for the removal of a wide range of pollutants which can be present in the WOPD.
The ECF process has been investigated in this study predominantly to point out the treatment's removal efficiency, and not so much with respect to its fundamental aspects. The ECF is still an empirically optimized process that requires more fundamental studies to be conducted and fully exploited in the case of the WOPD treatment.
Overall, the optimum operating conditions for metal removal from waste fountain solution by ECF were predicted to be: Al(—)/Fe( + ) electrode with interelectrode distance of 1.0 cm, operating time of 5 min and current density of 8 mA cm—2 which resulted in high copper and turbidity removal efficiency (>90%).
In the future, a combination of the ECF process with other purification techniques, such as adsorption, could achieve a removal efficiency of organic substances higher than the results obtained in this study (52.6%).
Acknowledgment
The authors acknowledge the financial support of the Ministry of Education, Science and Technological Development, Republic of Serbia (Grant No. 43005).
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