- Email: [email protected]
Treatment of wastewater containing EDTA-Cu(II) using the combined process of interior microelectrolysis and Fenton oxidation–coagulation
Separation and Purification Technology 89 (2012) 117–124
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Treatment of wastewater containing EDTA-Cu(II) using the combined process of interior microelectrolysis and Fenton oxidation–coagulation Shanhong Lan a,⇑, Feng Ju b,c, Xiuwen Wu a a
College of Chemical and Environmental Engineering, Dongguan University of Technology, Dongguan 523808, China Department of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China c The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters of Ministry of Education, Guangzhou 510006, China b
a r t i c l e
i n f o
Article history: Received 22 August 2011 Received in revised form 3 January 2012 Accepted 8 January 2012 Available online 14 January 2012 Keywords: Interior microelectrolysis Fenton oxidation Coagulation EDTA-Cu(II)
a b s t r a c t Wastewater containing EDTA-Cu(II) originating from the electroless copper-plating process was treated in a combined system of interior microelectrolysis (IM) and Fenton oxidation and coagulation (FOC). The intermittent operation of an IM reactor showed that the treatment efficiencies and Fe(II) yields were much higher under lower initial pH of the wastewater, and when the raw wastewater was treated by IM for 20 min, the Fe(II) yields and pH of the IM effluent were 336.1 mg/L and 4.6, respectively. The proper reaction conditions for the treatment of IM effluent by the FOC process were a [H2O2]/[COD] ratio of 2.0, [Fe(II)]/[H2O2] ratio of 0.2–0.3, initial pH of 2.0–5.0, and reaction time of 60–80 min; thus, the Fe(II)-rich effluent of IM was suitable for treatment during a subsequent Fenton oxidation (FO) process without Fe(II) addition or pH adjustment. Under the optimal operating parameters, 100% Cu(II) and 87.0% COD were removed by the IM–FOC process. The contributions of IM, FO and coagulation to Cu(II) removal were 97.5%, 0%, and 2.5%, respectively, and those to COD removal were 22.3%, 47.8%, and 10.9%, respectively. After treatment, the BOD5/COD ratio of wastewater was enhanced from 0 to 0.42, indicating that EDTA was effectively oxidized in the combined system. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Ethylenediaminetetraacetic acid (EDTA) is a strong complexing agent widely used in the electroless copper-plating process in the printed circuit boards (PCBs) industry. EDTA can form very stable complexes with cupric ions, greatly reducing the efficiency of Cu(II) removal by conventional chemical precipitation (e.g., OH, S2, CO2 3 ) and other processes [1]. The presence of Cu(II) ions in wastewater and drinking water exerts negative impacts on both potable bodies of water and human health. Furthermore, EDTACu(II) may escape treatment by wastewater treatment plants (WTP) and create great trouble during the treatment of industrial effluents and other contaminated waters due to its high toxicity and the notable affinity of EDTA for Cu(II) [2]. Indeed, EDTA is a nonbiodegradable compound whose environmental effects can be magnified by its persistence [3]. These problems have led to an urgent need for the development of effective treatment processes. Many technologies such as adsorption [4,5], Fe2+ replacement and precipitation [6,7], ion exchange [8], membrane filtration [9] and electro-coagulation and deposition [10,11] have been employed to treat wastewater containing metal-EDTA. However, ⇑ Corresponding author. Tel.: +86 0769 22861661. E-mail addresses: [email protected], [email protected] (S. Lan). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2012.01.009
these technologies have many limitations, such as poor treatment efficiency of EDTA, high cost, chemical toxicity, etc. Recently, interior microelectrolysis (IM), which uses inexpensive waste iron as a sacrificial anode and carbon as a cathodic catalyst, is becoming a particularly attractive option for the removal of both nonbiodegradable organic compounds and heavy metals from wastewater [12,13]. As documented in our previous study [14], IM can efficiently remove Cu(II) from EDTA-chelated solutions, but the removal efficiency of total organic carbon (TOC) is extremely low (approximately 20% if the contribution of activated carbon adsorption is not considered), indicating the poor treatment performance of EDTA species by IM. Therefore, efforts should be made to improve the decomposition of EDTA. In preliminary research, a considerable concentration of Fe(II) ions was detected in IM effluent. In particular, when the dissolved oxygen content and pH of the wastewater is low, the Fe(II) concentration can reach up to hundreds of mg/L. It is assumed that if hydrogen peroxide (H2O2) is added to the Fe(II)-rich effluent of IM, Fenton’s reaction should take place to generate highly oxidative hydroxyl radicals, which are well known for their capacity of oxidizing and mineralizing most organic molecules into CO2 and inorganic ions as final products, and it is most likely that the residual EDTA in IM effluent could be effectively oxidized by hydroxyl radicals. Additionally, the coagulation process, which occurs
118
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
immediately after the addition of alkali into Fenton’s effluent, may probably contribute to the further removal of EDTA species. Therefore, in this work, we attempted to treat wastewater containing EDTA-Cu(II) using a process combining IM and Fenton oxidation and coagulation (FOC), which, to our knowledge, has not been reported before. This paper mainly focuses on the following issues: (1) the treatment performance of the IM process under different pHs; (2) Fe(II) yields and pH variation during the operation of IM under different pHs; (3) the effects of H2O2 dosage, [Fe(II)]/[H2O2], initial pH and reaction time on the removal of COD from IM effluent by FOC; (4) the theoretical calculations of operating parameters of the IM–FOC process; (5) the efficiency of the IM–FOC process on COD and Cu(II) removal was evaluated in the treatment of real wastewater containing EDTA-Cu(II). 2. Materials and methods 2.1. Materials Wastewater containing EDTA-Cu(II) was obtained from a printed circuit board factory (Guangzhou, China). The composition of wastewater varied slightly in terms of concentrations of Cu(II) and the chelating agents used in the electroless copper-plating process. The characteristics of the chelated wastewater sampled from the workshop are provided in Table 1. Wastewater shows very low pH values and high concentrations of COD and Cu(II). The COD concentration reaches a high level of 998–1212 mg/L, while the BOD5/COD is 0, revealing the existence of large amounts of nonbiodegradable EDTA species in the wastewater. All chemicals were of analytical grade and used without further purification. Distilled water was used for the preparation and dilution of solutions. The pH of the wastewater was adjusted to the desired values by concentrated sulfuric acid (98%, w/w) and sodium hydroxide solids. Iron scraps were supplied by a metal-machining shop in Nanhai, China, with particle sizes ranging from 0.075 to 1.0 mm. Granular activated carbon (GAC) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China. 2.2. Analytical methods All samples were filtered through 0.45-lm nitrocellulose membrane filters before analysis. The pH measurements were performed using a pHS-3C meter. The Cu(II) concentration was determined by an atomic absorption spectrophotometer (AA6000, Techcomp). The concentration of Fe(II) was monitored by the o-phenanthroline method at k = 510 nm using UV759. In this work, chemical oxygen demand (COD) was used to indirectly determine the concentration levels of EDTA species in the wastewater. COD and BOD5 were monitored using standard methods [15]. The contribution of residual Fe(II) in samples was subtracted from all of the reported COD values (1 mg/L of Fe(II) ion contributes to a COD value of 1/7 mg/L). To prevent H2O2 from affecting the COD values, samples collected from the FO reactor were pretreated in a 60 °C water bath for 2 h to eliminate the residual H2O2 before determination. The removal percentage of Cu(II)/ COD from the chelated wastewater was calculated using the following equation: Table 1 Characteristics of the raw wastewater containing EDTA-Cu(II).
Maximum Minimum
Cu (mg/L)
CODCr (mg/L)
BOD5 (mg/L)
pH
BOD/COD5
254 196
1212 998
0 0
1.6 1.0
0 0
RCu=COD ¼
C0 Ct 100% C0
ð1Þ
where C0 (mg/L) and Ct (mg/L) represent the initial Cu(II)/COD concentration and the residual Cu(II)/COD concentration at time t (min), respectively. 2.3. Experimental procedures 2.3.1. Pretreatment of the IM fillings A series of pretreatment procedures was conducted to clean the iron scraps and GAC prior to use. The iron scraps were washed with industrial alkali and diluted hydrochloric acid (2%, v/v) to remove the grease and rust on the surfaces of the scraps. Then, the particles were washed with tap water and distilled water before they were dried, sieved and saved. Similarly, GAC was washed with tap water and distilled water to eliminate the impurities on the surface and then dried. Finally, the iron scraps and activated carbon were thoroughly mixed in a mass ratio of 3:1 before use. 2.3.2. IM reactor and its intermittent operation The IM reactor was a cylindrical fluidized bed with a 30-L working volume and 7.5-L IM fillings. The fluidized bed was equipped with an intermittently aerated device and a reflux pump at the bottom. The wastewater reflux (1 time per 60 s, last for 1 s) and intermittent aeration (1 time per 15 min, last for 1 s) conditions in the fluidized bed guarantee a fluidized state of the iron and carbon particles and thus overcome the critical weaknesses of traditional IM reactors, such as reactor clogging and iron deactivation. The IM reactor was operated intermittently, and the fillings were reused. In this work, the primary role of the IM process was to remove Cu(II) and partial EDTA from wastewater; another important role played by the process was to produce Fe(II)-rich effluent with a proper pH to facilitate the subsequent Fenton oxidation (FO) process. Therefore, the IM process was operated intermittently to investigate the treatment efficiency, Fe(II) yields, and pH variation during the treatment of wastewater containing EDTA-Cu(II) at pH levels ranging from 1.39 to 4.53. Samples were collected to examine the concentrations of Cu(II), COD, Fe(II) and pH at reaction times of 0, 5, 10, 20, 30, 40 and 60 min. 2.3.3. Batch experiment of FOC Batch experiments of FOC were performed to determine the influence of important operating parameters such as H2O2 dosage, [Fe(II)]/[H2O2], initial pH of the wastewater, and the reaction time on the removal of COD from IM effluent by FOC. The desired Fe(II) dosage was achieved by adding the necessary amount of solid FeSO47H2O if the Fe(II) concentration in the IM effluent was insufficient. The optimum values of the parameters obtained from the batch experiment were used as references in the pilot-scale experiment. The batch experiment procedure can be divided into three steps. First, raw wastewater was pumped into the IM reactor, and the effluent was collected after 30 min of reaction. Second, 200 mL of the IM effluent was poured into a 500-mL conical reactor, and the FO process was initiated immediately after the addition of the proper concentration of H2O2 and FeSO4 (if necessary). Samples were collected to detect the COD content after the completion of FO. Finally, the pH of the Fenton effluent was adjusted to 9.0 by adding NaOH solid under mild stirring. Samples were collected and tested after coagulation and precipitation for 30 min. 2.3.4. Pilot-scale experiment of the IM–FOC process 2.3.4.1. The setup of the reactors. The combined process of IM–FOC was applied to treat wastewater containing EDTA-Cu(II) in a
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
pilot-scale test. Fig. 1 is a schematic diagram of the IM–FOC system. The IM reactor was described in Section 2.3.2. Reactor (II), the FO reactor and the coagulation reactor each had a working volume of 120 L. Five layers of orifice plates were installed in reactor (II) and the coagulation reactor to create a crisscross water-flow pattern and strengthen the mixing effect, and the three-phase flow design of the FO reactor guaranteed a sufficiently long retention time and effective stirring of the wastewater during reaction. The reflux pump between the IM reactor and reactor (II) was set up to pump the wastewater in reactor (II) back into the IM reactor every 60 s (last for 1 s) such that the fillings could be kept in a fluidized state. The wastewater entered from the bottom of each reactor and came out from the top. 2.3.4.2. The operational procedures. The IM–FOC process was operated continuously; the treatment procedures are described as follows. First, the raw wastewater was pumped into the IM reactor; after treatment by the IM process, the Fe(II)-rich effluent of the IM was pumped into a subsequent FO reactor. Second, H2O2 solution (50%, w/w) was continually supplied to the FO reactor using a peristaltic pump. After Fenton’s oxidation, the effluent of the FO reactor entered into the coagulation reactor in which NaOH solution (50%, w/w) and PAM (white powder with a molecular weight of 12 million) solution were continually supplied from the bottom of the reactor using peristaltic pumps. Finally, samples were collected for analysis after coagulation at the proper pH level. Overall, the treatment efficiency of Cu(II) and COD by the IM–FOC process was investigated, and the contributions of IM, FO and coagulation were separately determined and evaluated. 3. Results and discussions 3.1. Intermittent operation of IM 3.1.1. Mechanisms of IM The IM process, which employs iron and carbon as the anodic and the cathodic materials, operates on principles very similar to those of electrochemistry methods [16]. However, the electron flows are provided by a galvanic cell instead of an external power supply. The pertinent half-cell reactions are as follows [17]: Anode (iron):
FeðsÞ 2e ! Fe2þ ðaqÞ; Eh ðFe2þ =FeÞ ¼ 0:44 V
ð2Þ
Fe2þ ðaqÞ e ! Fe3þ ðaqÞ; Eh ðFe3þ =Fe2þ Þ ¼ þ0:77 V
119
ð3Þ
Cathode (carbon):
2Hþ ðaqÞ þ 2e ! 2½H ! H2 ðgÞ; Eh ðHþ =H2 Þ ¼ 0 V
ð4Þ
In the presence of oxygen:
O2 ðgÞ þ 4Hþ ðaqÞ þ 4e ! 2H2 O;Eh ðO2 =H2 OÞ ¼ þ1:23 V
ð5Þ
O2 ðgÞ þ 2Hþ ðaqÞ þ 2e ! H2 O2 ; Eh ðO2 =H2 O2 Þ ¼ þ0:68 V
ð6Þ
O2 ðgÞ þ 2H2 O þ 4e ! 4OH ðaqÞ; Eh ðO2 =OH Þ ¼ þ0:40 V
ð7Þ
According to our previous study [14,18], EDTA-chelated Cu(II) can be removed from wastewater by IM through the following mechanisms: (1) Fe2+-based replacement and precipitation. Fe0 is oxidized to Fe(II) ion (Eq. (2)), and Fe(II) ion is oxidized to Fe(III) ion by oxygen (Eq. (3)); then, EDTA-Cu(II) is first replaced by Fe(III) ion, and chelated Cu(II) is released to the solution in the free state; Finally, free Cu(II) ions are gradually precipitated by OH- produced in the cathodic reaction (Eq. (7)). (2) Fe2+-based electro-coagulation: the generated Fe(II) and Fe(III) ions can serve as coagulant precursors and form active Fe(OH)2 and Fe(OH)3 to remove EDTA-Cu(II) from wastewater through adsorption and coprecipitation. (3) Electro-electrophoresis and redox: Fe(II) and atomic hydrogen released from the galvanic cell reactions (Eqs. (2), (3)) and hydroxyl radicals generated by Fenton’s reaction between Fe(II) and H2O2 (Eqs. (2), (6)) exhibit high activities to reduce Cu(II) and decompose organic contaminants. Meanwhile, as the electron flow creates the electric field in the galvanic cell reaction, free and chelated Cu(II) ions could move to the surface of the electrodes by the motivation of electrophoresis and then be reduced by Fe0 and atomic hydrogen. 3.1.2. Intermittent operation of the IM process under different pHs The IM reactor was operated intermittently at different pH levels to investigate its performance on the treatment of wastewater containing EDTA-Cu(II). Fig. 2 displays the removal efficiency of Cu(II) (a) and COD (b) and by IM at initial pH levels of 1.39 (without pH adjustment), 3.23, 3.65 and 4.53, respectively.
Fig. 1. Schematic diagram of the IM–FOC system.
120
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
Fig. 2. Removal efficiency of Cu(II) (a) and COD (b) in wastewater by IM at initial pH values of 1.39, 3.23, 3.65 and 4.53.
From Fig. 2a, it can be observed that IM had a high Cu(II) removal efficiency in the pH range of 1.39–4.53, although Cu(II) was removed much faster at lower pH values. For instance, the Cu(II) concentrations at pH = 4.53, 3.65, 3.23 and 1.39 were rapidly reduced from 225.3, 230.6, 225.2 and 200.5 mg/L at 0 min to 3.8, 3.7, 5.0 and 1.6 mg/L at 40 min, respectively. All of the Cu(II) removal efficiencies at 20 min were greater than 93.3%. Fig. 2b shows that the removal efficiencies of COD by IM were very low, especially at higher initial pH values. For example, the COD concentrations at pH = 4.53, 3.65, 3.23 and 1.39 were only reduced from 1008.5, 1023.0, 1065.0 and 1199.6 mg/L at 0 min to 786.5, 785.13, 794.7, 838.7 mg/L at 60 min, respectively. All of the COD removal efficiencies at 60 min were less than 30.0%. In summary, the maximum removal efficiencies of Cu(II) and COD by the IM process were achieved at an initial pH of 1.39, indicating that the raw wastewater containing EDTA-Cu(II) is quite suitable for direct treatment in the IM reactor without pH adjustment. 3.1.3. Fe(II) yields and pH variation in the IM reactor The initial pH of wastewater is a significant parameter that affects iron corrosion and thus determines the Fe(II) yields and the final pH in the IM effluent. In this work, we struggled to obtain an IM effluent with high Fe(II) yields and proper pH values to facilitate the operation of the subsequent FO process. Fig. 3a illustrates Fe(II) yields over time at initial pH values of 1.39 (without pH adjustment), 2.05, 3.65 and 4.53. It can be observed that the Fe(II) yields decreased sharply with the increase in initial pH. The maximum Fe(II) yields corresponding to initial pH
values of 1.39, 2.05, 3.65 and 4.53 were 338.0 mg/L (30 min), 160.3 mg/L (30 min), 43.69 (20 min) and 14.2 mg/L (10 min), respectively. As shown, Fe(II) yields could achieve 336.1 mg/L when the IM process was operated at the initial pH of 1.39 for merely 20 min. Therefore, the optimum pH value of wastewater required to achieve the maximum Fe(II) yield was 1.39, which indicates that the raw wastewater was quite suitable for direct treatment in the IM reactor without pH adjustment. Fig. 3b shows the pH variation with time at initial pH values of 1.39 (without pH adjustment), 2.05, 3.65 and 4.53. As shown in the figure, the pH of the wastewater increased with time regardless of the initial pH level. This increase in pH in acidic wastewater during treatment by the IM process is in accordance with observations reported in previous studies [12,19]. Moreover, it was observed that the pH values of the IM effluent at 20 min remained between 4.6 and 5.68, which may be a suitable pH range for the operation of FO. Considering both the Fe(II) yields and the final pH of the IM effluent, it was concluded that the IM process should operate for a retention time of 20 min without adjusting the pH of the raw wastewater. Under such conditions, the Fe(II) yield was 338.0 mg/L, and the pH of the IM effluent was approximately 4.6. 3.2. Batch experiment of FOC 3.2.1. Effect of H2O2 dosage on the removal of COD H2O2 plays a very significant role as a source of OH generated during Fenton’s reaction. The effect of H2O2 dosage on the removal
Fig. 3. Fe(II) yields (a) and pH variation (b) over time in the IM reactor at different initial pH values.
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
121
efficiency of COD (RCOD) was examined by varying the initial values of [H2O2]/[COD] from 0.5 to 6.0; the results are shown in Fig. 4. From the figure, it can be observed that with the increase in [H2O2]/[COD] from 0.5 to 2.0, the RCOD by FO was markedly increased from 35.9% to 87.2%, and the RCOD by FOC was enhanced from 73.6% to 89.4%. However, with a further increase in [H2O2]/ [COD] from 2.0 to 6.0, the increase in the RCOD by FO and FOC was negligible. Considering the dosage of H2O2, attention should be paid not only to the improvement of the treatment efficiency but also to the economic cost. Therefore, the efficiency of H2O2 was evaluated, and the results are shown in the inset of Fig. 4. The relationship between Q (mg H2O2/mg COD) and [H2O2]/ [COD] can be expressed by the following equation:
Q ¼ 1:08062
½H2 O2 ½COD
ð8Þ
where Q represents the amount of H2O2 that was consumed in removing 1 mg COD, mg H2O2/mg COD; R2 = 0.99897. From this equation, it can be calculated that when the value of [H2O2]/[COD] was increased by 1.0, 1.08062 mg of extra H2O2 was needed for the removal of 1 mg COD. Therefore, the lower the [H2O2]/[COD] values are, the lower the Q values become and thus the higher the efficiency of H2O2 becomes. As shown in Fig. 4, when the [H2O2]/[COD] increased from 2.0 to 6.0, the removal efficiency of COD by FO and FOC increased by merely 3.8%-4.4%, but the Q value sharply increased from 2.17204 to 6.51612 mg H2O2/mg COD. Taking the treatment efficiency as well as the economic cost into full consideration, the optimum [H2O2]/[COD] was determined to be 2.0 for the FO and FOC processes. 3.2.2. Effect of [Fe(II)]/[H2O2]on the removal of COD The removal efficiency of COD (RCOD) by the Fenton process is influenced by the concentration of Fe(II) ions, which catalyze H2O2 decomposition, resulting in OH production and consequently the degradation of organic molecules. Fig. 5 shows the effect of [Fe(II)]/[H2O2] on RCOD during the treatment of IM effluent by FOC at different initial [Fe(II)]/[H2O2] values ranging from 0.2 to 1.0. It can be observed that RCOD increased when [Fe(II)]/[H2O2] was increased from 0.2 to 0.3. The fact that higher RCOD values were achieved at higher Fe(II) dosages is mainly attributed to the production of higher amounts of OH with more Fe(II) as a catalyst in Fenton’s reaction (Eq.(9)). However, RCOD gradually decreased
Fig. 4. Effect of [H2O2]/[COD] on the removal of COD during the treatment of IM effluent by FO and FOC, respectively. Experimental conditions: [COD] of the IM effluent = 780.0 mg/L; [Fe(II)]/H2O2 = 2.0; reaction time = 90 min; initial pH = 3.0. The inset illustrates the efficiency of H2O2.
Fig. 5. Effect of [Fe(II)]/[H2O2] on the removal of COD during the treatment of IM effluent by FOC. Experimental conditions: COD of IM effluent = 828 mg/L; [Fe(II)] of IM effluent = 310.5 mg/L; [H2O2] = 1656 mg/L; reaction time = 90 min; pH = 3.0. The inset compares the removal efficiency of COD by FO and FOC at initial [Fe(II)]/[H2O2] values of 0.3 and 1.0.
with the further increase in [Fe(II)]/[H2O2] from 0.3 to 0.6. This result can be attributed to the scavenging effect of Fe(II) ions on the hydroxyl radicals (Eq.(10)).
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH Fe2þ þ HO ! Fe3þ þ OH
ð9Þ ð10Þ
In addition, it was observed that when [Fe(II)]/[H2O2] increased from 0.6 to 1.0, RCOD was enhanced from 82.2% to 88.1%, which may be attributed to the strengthening effect of coagulation at higher Fe(II) dosages; moreover, the rise in RCOD due to coagulation at higher Fe(II) dosages outweighed the drop in RCOD by FO, resulting in the improvement of the total RCOD. To distinguish the respective contributions of oxidation and coagulation to the removal of total COD, the RCOD by FO and FOC at initial [Fe(II)]/[H2O2] ratios of 0.3 and 1.0 were compared; the results are shown in the inset of Fig. 5. As shown, RCOD by coagulation at a [Fe(II)]/[H2O2] ratio of 0.3 was (87.2%-65.0%)=22.2%, while RCOD by coagulation at a [Fe(II)]/[H2O2] ratio of 1.0 was (88.1%-50%)=38.1%, indicating that COD removal by FO was reduced and COD removal by coagulation was increased with the increase in initial [Fe(II)]/[H2O2]. Due to the higher RCOD achieved at lower Fe(II) dosages, [Fe(II)]/[H2O2] values in the range of 0.2 and 0.3 were chosen as the optimal values for further experiments. 3.2.3. Effect of initial pH on the removal of COD FO is known as a highly pH-dependent process because pH plays an important role in the mechanism of OH production in Fenton’s reaction [20]. The effect of initial pH on the removal efficiency of COD (RCOD) during the treatment of IM effluent by FOC is illustrated in Fig. 6. It can been observed that RCOD by FOC remained high at pH values ranging from 2.0 and 4.0 and later suffered a slight decline when the initial pH was further increased from 4.0 to 7.0. At initial pH values of 8.0 and 10.0, RCOD by FOC sharply decreased, which was primarily due to the formation of Fe(II)/Fe(III) hydroxide complexes. This led to the deactivation of the Fe(II) catalyst, resulting in a rapid decrease in OH production [21]. It should also be mentioned that at the initial pH values of 8.0 and 10.0, Fe(II) was seriously coagulated when FO was negligible; thus, COD was principally removed by coagulation. In summary, the optimum initial pH value was 3.0, and a suitable initial
122
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
[H2O2]/[COD] ratio of 2.0, [Fe(II)]/[H2O2] ratio of 0.2–0.3, pH value of 2.0–5.0 and reaction time of 60–80 min. In our pilot-scale investigation, all of the chemicals, including the H2O2 solution (50%, w/ w), NaOH solution (50%, w/w) and PAM solution (100 mg/L), were continually supplied to the corresponding reactor using peristaltic pumps. 3.3.1.1. The calculation of influent quantity. The influent quantity of wastewater can be calculated using the following equation:
q¼
Fig. 6. Effect of initial pH on the removal of COD during the treatment of IM effluent by FOC. Experimental conditions: COD of IM effluent = 740.2 mg/L; H2O2 = 1480.4 mg/L; [Fe(II)]/H2O2 = 2.0; reaction time = 90 min.
pH range for the removal of COD by FOC was determined to be 2.0– 5.0. 3.2.4. Effect of reaction time on the removal of COD and Cu(II) Tests were also carried out to evaluate the effect of reaction time on the removal of COD and Cu(II) during the treatment of IM effluent by FOC. To this end, experimental samples were taken at preselected time intervals and immediately neutralized with sodium hydroxide to stop the reaction; the results are shown in Fig. 7. The figure shows that 89.6% of COD and 100% of Cu(II) were removed within 80 min, and COD and Cu(II) were reduced from 810.2 mg/L and 2.3 mg/L to 84.3 mg/L and 0 mg/L, respectively. After 80 min, the change in the concentrations of residual COD and Cu(II) were insignificant. Therefore, a reaction time of 60– 80 min would be enough for the treatment of both COD and Cu(II) in IM effluent by the FOC process. 3.3. Pilot-scale experiment 3.3.1. Theoretical calculations of the optimal operating parameters for the IM–FOC process As determined by the batch experiments, the IM process should operate at a retention time of 20 min without adjusting the pH of the raw wastewater, and the appropriate conditions for FOC are a
Fig. 7. COD and Cu(II) removal as a function of time during the treatment of IM effluent by the FOC process. Experimental conditions: COD of IM effluent = 810.2 mg/L; Cu(II) of IM effluent = 2.3 mg/L; H2O2 = 1620.4 mg/L; [Fe(II)]/ H2O2 = 0.2; pH = 3.0.
60V T
ð11Þ
where q represents the influent quantity (L/h); V is the working volume of the reactor (L); T is the retention time (min). In this work, the IM reactor, with a working volume of 30 L, was operated for a retention time of 20 min. Thus, it can be calculated from Eq. (11) that q = (60 30)/20 = 90 L/h. In the pilot-scale experiment, the working volumes of the reactors of FO and coagulation were both 120 L. When the q was set to 90 L/h, the retention time of FO and coagulation was determined to be 80 min, which is sufficiently long for Fenton’s reaction to occur to completion. 3.3.1.2. The calculation of the flow rate of H2O2. The optimum value of the [H2O2]/[COD] ratio was determined to be 2.0. After measuring the COD in IM effluent, the flow rate of H2O2 (q1, mL/min) can be calculated as follows:
q1 ¼
qCn 60 w 1000 q
ð12Þ
where C represents the COD concentration of the IM effluent, mg/L; n represents the value of [H2O2]/[COD]; w is the mass fraction of H2O2 solution; q is the density of H2O2 solution, g/cm3. The COD concentration of the raw wastewater was between 998 and 1212 mg/L (Fig. 1). As indicated in Section 2.1, approximately 24.9% of COD was removed after treatment by IM for 20 min; therefore, it is estimated that the COD concentration of the IM effluent was between 749.5 and 910.2 mg/L. In other words, 749.5 < C < 910.2 mg/L. In this work, q = 90 L/h; n = 2.0; w = 50%; q = 1.4067 g/cm3. Therefore, it can be calculated from Eq. (12) that q1 was between 3.20 and 3.88 mL/min. 3.3.1.3. The calculation of the flow rate of the NaOH solution. The optimum pH for coagulation must be determined before the determination of the NaOH flow rate. Five groups of 200-mL effluent
Fig. 8. Effect of coagulation pH on the removal of COD in the FO effluent by coagulation.
123
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124 Table 2 COD and Cu(II) removal from EDTA-Cu(II) wastewater by the IM–FOC process and the contributions of IM, FO and coagulation process (CP). Samples from
Cu(II) con. (mg/L) COD con. (mg/L) BOD5/COD
The contribution rate to the total removal (%)
Influent
IM
FO
CP
IM
FO
CP
Total
225.3 1096.6 0
5.6 851.5 0.09
5.6 262.1 –
0 142.6 0.42
97.5 22.3 –
0 47.8 –
2.5 10.9 –
100.0 87.0 –
samples taken during the FO process were adjusted to the desired pH by solid NaOH under mild stirring for 40 min, and the results are shown in Fig. 8. The figure shows that the optimum coagulation pH was 9.0. The flow rate of the NaOH solution (q2, mL/min) can be calculated by the following equation:
q2 ¼
q V1 6
ð13Þ
where V1 is the volume of NaOH solution (50%, w/w) that was required to adjust the pH of the 100-mL FO effluent to 9.0. In this work, q = 90 L/h; V1 = 1.0–2.0 mL. Thus, it can be calculated from Eq. (13) that q2 = 15–30 mL/min. 3.3.1.4. The calculation of the flow rate of PAM. In the pilot-scale tests, PAM was added to facilitate the coagulation and precipitation of iron and copper hydroxides. The flow rate of PAM (q3, mL/ min) can be calculated using the following equation:
q3 ¼
q C 1 1000 C 2 60
ð14Þ
where C1 is the dosage of PAM, mg/L; C2 is the concentration of the PAM solution, mg/L. In this work, q = 90 L/h; C1 = 1.0 mg/L, and C2 = 100 mg/L; thus, it can be calculated from Eq. (14) that q3 = (90 1.0 50)/ (100 3)=15 mL/min. 3.3.2. The operation of the IM–FOC process for the treatment of wastewater containing EDTA-Cu(II) The IM–FOC process was operated continuously to treat raw wastewater containing EDTA-Cu(II) under the optimized conditions determined by the batch experiments and theoretical calculations discussed above: an influent quantity of 90 L/h without pH adjustment, a retention time of 20 min for IM reactor, a H2O2 (50%, w/w) flow rate of 3.63 mL/min, a retention time of 80 min for the FO reactor, a NaOH solution (50%, w/w) flow rate of 24.0 mL/min, a PAM solution flow rate of 15 mL/min and a coagulation pH of 9.0 for 80 min. Under these conditions, the treatment efficiency of EDTA-Cu(II) wastewater by the IM–FOC process and the contribution of each process were investigated; the results are shown in Table 2. Table 2 shows that Cu(II) and COD decreased from 225.3 mg/L and 1096.6 mg/L to 0 mg/L and 142.6 mg/L, respectively, after being treated by the IM–FOC process, and the removal efficiencies of Cu(II) and COD reached 100.0% and 87.0%. Of the 100.0% of Cu(II) removed, 97.5% was removed by the IM process, while only 2.5% was removed by coagulation, indicating that the IM process was responsible for the removal of the majority Cu(II) ions from the wastewater. However, the COD removed by the IM process was relatively low: only 22.3%. Most of the COD (47.8%) was degraded by the FO process, and approximately 10.9% of the COD was removed by coagulation and precipitation with iron hydroxides. In this work, samples were tested with respect to their BOD5/COD values to evaluate the variation in the biodegradability of wastewater during treatment. The BOD5/COD value of the raw wastewater was 0, indicating that EDTA-Cu(II) wastewater is totally nonbiodegradable. The BOD5/COD of the IM effluent and coagulation effluent were determined to be 0.09 and 0.42, respectively. The poor
treatment performance of COD by IM and the low BOD5/COD of IM effluent indicated that EDTA species cannot be effectively decomposed into small biodegradable organic molecules by the IM process. In contrast, the efficient removal of COD by FOC and the high BOD5/COD value of the coagulation effluent suggest that EDTA can be efficiently oxidized into small organic molecules or mineralized into CO2 by the highly oxidative hydroxyl radicals generated during Fenton’s reaction. The results of this study suggest that the combined process of IM–FOC is efficient with respect to the decontamination of EDTACu(II) wastewater originating from the electroless copper-plating process. Additionally, the combination of the IM and FO processes for the treatment of chelated wastewater is potentially of low cost. First, the IM process does not require a chemical coagulant or highpower consumption, as in the cases of chemical coagulation and electrolysis, respectively. Only low-cost iron scraps are consumed in the IM process, and little external power is required (for wastewater reflux). Then, the problems of iron deactivation and reactor clogging are eliminated due to the fluidized-bed design of the IM reactor and the fluidized state of the IM fillings, which reduces the cost of concentrated acid required for iron activation. Most importantly, apart from removing Cu(II) and COD, the IM process can produce Fe(II)-rich effluent with the proper pH, which can be directly used for FO without Fe(II) addition or pH adjustment, largely reducing the cost of the Fenton process. Overall, the IM–FOC process is highly efficient and potentially inexpensive with respect to the decontamination and detoxification of the EDTA-Cu(II) wastewater originating from the printed circuit boards industry. 4. Conclusions The efficient treatment of wastewater containing EDTA-Cu(II) was achieved by the combined process of IM–FOC. The results demonstrated that the treatment efficiencies and Fe(II) yields of the IM process were much higher at lower initial pH levels of the wastewater. The appropriate conditions for the treatment of IM effluent by FOC were as follows: [H2O2]/[COD] of 2.0, [Fe(II)]/ [H2O2] of 0.2–0.3, initial pH of 2.0–5.0, and reaction time of 60– 80 min; thus, the Fe(II)-rich effluent of IM was suitable for direct treatment in a subsequent FO process without Fe(II) addition or pH adjustment. Under the optimal operating parameters, 100% Cu(II) and 87.0% COD were removed by the IM–FOC process. The contributions of IM, FO and coagulation were 97.5%, 0% and 2.5%, respectively, for Cu(II) removal and were 22.3%, 47.8%, and 10.9%, respectively, for COD removal. After treatment, the BOD5/COD ratio of wastewater was enhanced from 0 to 0.42, indicating that EDTA was effectively oxidized in the combined system. From these results, it was concluded that the combined process of IM–FOC is a highly efficient and potentially low-cost method for the detoxification of nonbiodegradable wastewater containing EDTA-Cu(II). Acknowledgements This research was supported by the National Key Project of Water Pollution Control and Treatment (Grant No.: 2009ZX07211005-02), the Natural Science Foundation of Guangdong Province (Grant No.: 9151042001000007), Science and Technology Plan
124
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
Projects Fund of Dongguan City (Grant No.: 2011108102012) Science and Technology Plan Projects Fund of Dongguan City (Grant No.: 2011108102005). References [1] R.S. Juang, S.W. Wang, Electrolytic recovery of binary metals and EDTA from strong complexed solutions, Water Res. 12 (2000) 3179–3185. [2] U. Aydin, Y. Buluta, C. Yerlikayab, Removal of copper (II) from aqueous solution by adsorption onto low-cost adsorbents, J. Environ. Manage. 87 (2008) 37–45. [3] A.P. Davis, D.L. Green, Photocatalytic oxidation of cadmium-EDTA with titanium dioxide, Environ. Sci. Technol. 33 (1999) 609–617. [4] P.X. Wu, J.B. Zhou, X.R. Wang, Y.P. Dai, Z. Dang, N.W. Zhu, P. Li, J.H. Wu, Adsorption of Cu-EDTA complexes from aqueous solutions by polymeric Fe/Zr pillared montmorillonite: behaviors and mechanisms, Desalination 277 (2011) 288–295. [5] G. Issabayeva, M.K. Aroua, N.M. Sulaiman, Study on palm shell activated carbon adsorption capacity to remove copper ions from aqueous solutions, Desalination 262 (2010) 94–98. [6] I. Licsko, I. Takacs, Heavy metal removal in the presence of colloid-stabilizing organic material and complexing agents, Water Sci. Technol. 18 (1986) 18–19. [7] S.X. Jiang, J.X. Qu, Y. Xiong, Removal of chelated copper from wastewaters by Fe2+-based replacement-precipitation, Environ. Chem. Lett. 29 (2009) 597– 599. [8] R.S. Juang, L.C. Lin, Electrochemical treatment of copper from aqueous citrate solutions using a cation-selective membrane, Sep. Purif. Technol. 22–23 (2001) 627–635. [9] G. Borbely, E. Nagy, Removal of zinc and nickel ions by complexationmembrane filtration process from industrial wastewater, Desalination 240 (2009) 218–226.
[10] I. Kabdasli, T. Arslan, T.Ö. Hancıa, I.A. Alatona, O. Tünay, Complexing agent and heavy metal removals from metal plating effluent by electrocoagulation with stainless steel electrodes, J. Hazard. Mater. 165 (2009) 838–845. [11] Y. Oztekin, Z. Yazicigil, Recovery of metals from complexed solutions by electrodeposition, Desalination 190 (2006) 79–88. [12] Y.Z. Jin, Y.F. Zhang, W. Li, Microelectrolysis technology for industrial wastewater treatment, J. Environ. Sci. 15 (2003) 334–338. [13] X.Y. Yang, Interior microelectrolysis oxidation of polyester wastewater and its treatment technology, J. Hazard. Mater. 169 (2009) 480–485. [14] F. Ju, Y.Y. Hu, Removal of EDTA-chelated copper from aqueous solution by interior microelectrolysis, Sep. Purif. Technol. 78 (2011) 33–41. [15] APHA, Standard Methods—for the Examination of Water and Wastewater, 20th ed., American Public Health Association, American Water Works Association, Water Pollution Control Federation, Washington, DC, 1999. [16] X.Y. Yang, Y. Xue, W.N. Wang, Mechanism, kinetics and application studies on enhanced activated sludge by interior microelectrolysis, Bioresour. Technol. 100 (2009) 649–653. [17] X.C. Ruan, M.Y. Liu, Q.F. Zeng, Y.H. Ding, Degradation and decolorization of reactive red X-3B aqueous solution by ozone integrated with internal microelectrolysis, Sep. Purif. Technol. 74 (2010) 195–201. [18] F. Ju, Y.Y. Hu, J.H. Cheng, Removal of chelated Cu(II) from aqueous solution by adsorption-coprecipitation with iron hydroxides prepared from microelectrolysis process, Desalination 274 (2011) 130–135. [19] J.H. Fan, L.M. Ma, The pretreatment by the Fe–Cu process for enhancing biological degradability of the mixed wastewater, J. Hazard. Mater. 164 (2009) 1392–1397. [20] M. Perez, F. Torrades, X. Domenech, J. Peral, Fenton and photo-Fenton oxidation of textile effluents, Water Res. 36 (2002) 2703–2710. [21] Y. Deng, J.D. Englehardt, Treatment of landfill leachate by the Fenton process, Water Res. 40 (2006) 3683–3694.
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Treatment of wastewater containing EDTA-Cu(II) using the combined process of interior microelectrolysis and Fenton oxidation–coagulation Shanhong Lan a,⇑, Feng Ju b,c, Xiuwen Wu a a
College of Chemical and Environmental Engineering, Dongguan University of Technology, Dongguan 523808, China Department of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, China c The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters of Ministry of Education, Guangzhou 510006, China b
a r t i c l e
i n f o
Article history: Received 22 August 2011 Received in revised form 3 January 2012 Accepted 8 January 2012 Available online 14 January 2012 Keywords: Interior microelectrolysis Fenton oxidation Coagulation EDTA-Cu(II)
a b s t r a c t Wastewater containing EDTA-Cu(II) originating from the electroless copper-plating process was treated in a combined system of interior microelectrolysis (IM) and Fenton oxidation and coagulation (FOC). The intermittent operation of an IM reactor showed that the treatment efficiencies and Fe(II) yields were much higher under lower initial pH of the wastewater, and when the raw wastewater was treated by IM for 20 min, the Fe(II) yields and pH of the IM effluent were 336.1 mg/L and 4.6, respectively. The proper reaction conditions for the treatment of IM effluent by the FOC process were a [H2O2]/[COD] ratio of 2.0, [Fe(II)]/[H2O2] ratio of 0.2–0.3, initial pH of 2.0–5.0, and reaction time of 60–80 min; thus, the Fe(II)-rich effluent of IM was suitable for treatment during a subsequent Fenton oxidation (FO) process without Fe(II) addition or pH adjustment. Under the optimal operating parameters, 100% Cu(II) and 87.0% COD were removed by the IM–FOC process. The contributions of IM, FO and coagulation to Cu(II) removal were 97.5%, 0%, and 2.5%, respectively, and those to COD removal were 22.3%, 47.8%, and 10.9%, respectively. After treatment, the BOD5/COD ratio of wastewater was enhanced from 0 to 0.42, indicating that EDTA was effectively oxidized in the combined system. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Ethylenediaminetetraacetic acid (EDTA) is a strong complexing agent widely used in the electroless copper-plating process in the printed circuit boards (PCBs) industry. EDTA can form very stable complexes with cupric ions, greatly reducing the efficiency of Cu(II) removal by conventional chemical precipitation (e.g., OH, S2, CO2 3 ) and other processes [1]. The presence of Cu(II) ions in wastewater and drinking water exerts negative impacts on both potable bodies of water and human health. Furthermore, EDTACu(II) may escape treatment by wastewater treatment plants (WTP) and create great trouble during the treatment of industrial effluents and other contaminated waters due to its high toxicity and the notable affinity of EDTA for Cu(II) [2]. Indeed, EDTA is a nonbiodegradable compound whose environmental effects can be magnified by its persistence [3]. These problems have led to an urgent need for the development of effective treatment processes. Many technologies such as adsorption [4,5], Fe2+ replacement and precipitation [6,7], ion exchange [8], membrane filtration [9] and electro-coagulation and deposition [10,11] have been employed to treat wastewater containing metal-EDTA. However, ⇑ Corresponding author. Tel.: +86 0769 22861661. E-mail addresses: [email protected], [email protected] (S. Lan). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2012.01.009
these technologies have many limitations, such as poor treatment efficiency of EDTA, high cost, chemical toxicity, etc. Recently, interior microelectrolysis (IM), which uses inexpensive waste iron as a sacrificial anode and carbon as a cathodic catalyst, is becoming a particularly attractive option for the removal of both nonbiodegradable organic compounds and heavy metals from wastewater [12,13]. As documented in our previous study [14], IM can efficiently remove Cu(II) from EDTA-chelated solutions, but the removal efficiency of total organic carbon (TOC) is extremely low (approximately 20% if the contribution of activated carbon adsorption is not considered), indicating the poor treatment performance of EDTA species by IM. Therefore, efforts should be made to improve the decomposition of EDTA. In preliminary research, a considerable concentration of Fe(II) ions was detected in IM effluent. In particular, when the dissolved oxygen content and pH of the wastewater is low, the Fe(II) concentration can reach up to hundreds of mg/L. It is assumed that if hydrogen peroxide (H2O2) is added to the Fe(II)-rich effluent of IM, Fenton’s reaction should take place to generate highly oxidative hydroxyl radicals, which are well known for their capacity of oxidizing and mineralizing most organic molecules into CO2 and inorganic ions as final products, and it is most likely that the residual EDTA in IM effluent could be effectively oxidized by hydroxyl radicals. Additionally, the coagulation process, which occurs
118
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
immediately after the addition of alkali into Fenton’s effluent, may probably contribute to the further removal of EDTA species. Therefore, in this work, we attempted to treat wastewater containing EDTA-Cu(II) using a process combining IM and Fenton oxidation and coagulation (FOC), which, to our knowledge, has not been reported before. This paper mainly focuses on the following issues: (1) the treatment performance of the IM process under different pHs; (2) Fe(II) yields and pH variation during the operation of IM under different pHs; (3) the effects of H2O2 dosage, [Fe(II)]/[H2O2], initial pH and reaction time on the removal of COD from IM effluent by FOC; (4) the theoretical calculations of operating parameters of the IM–FOC process; (5) the efficiency of the IM–FOC process on COD and Cu(II) removal was evaluated in the treatment of real wastewater containing EDTA-Cu(II). 2. Materials and methods 2.1. Materials Wastewater containing EDTA-Cu(II) was obtained from a printed circuit board factory (Guangzhou, China). The composition of wastewater varied slightly in terms of concentrations of Cu(II) and the chelating agents used in the electroless copper-plating process. The characteristics of the chelated wastewater sampled from the workshop are provided in Table 1. Wastewater shows very low pH values and high concentrations of COD and Cu(II). The COD concentration reaches a high level of 998–1212 mg/L, while the BOD5/COD is 0, revealing the existence of large amounts of nonbiodegradable EDTA species in the wastewater. All chemicals were of analytical grade and used without further purification. Distilled water was used for the preparation and dilution of solutions. The pH of the wastewater was adjusted to the desired values by concentrated sulfuric acid (98%, w/w) and sodium hydroxide solids. Iron scraps were supplied by a metal-machining shop in Nanhai, China, with particle sizes ranging from 0.075 to 1.0 mm. Granular activated carbon (GAC) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd., China. 2.2. Analytical methods All samples were filtered through 0.45-lm nitrocellulose membrane filters before analysis. The pH measurements were performed using a pHS-3C meter. The Cu(II) concentration was determined by an atomic absorption spectrophotometer (AA6000, Techcomp). The concentration of Fe(II) was monitored by the o-phenanthroline method at k = 510 nm using UV759. In this work, chemical oxygen demand (COD) was used to indirectly determine the concentration levels of EDTA species in the wastewater. COD and BOD5 were monitored using standard methods [15]. The contribution of residual Fe(II) in samples was subtracted from all of the reported COD values (1 mg/L of Fe(II) ion contributes to a COD value of 1/7 mg/L). To prevent H2O2 from affecting the COD values, samples collected from the FO reactor were pretreated in a 60 °C water bath for 2 h to eliminate the residual H2O2 before determination. The removal percentage of Cu(II)/ COD from the chelated wastewater was calculated using the following equation: Table 1 Characteristics of the raw wastewater containing EDTA-Cu(II).
Maximum Minimum
Cu (mg/L)
CODCr (mg/L)
BOD5 (mg/L)
pH
BOD/COD5
254 196
1212 998
0 0
1.6 1.0
0 0
RCu=COD ¼
C0 Ct 100% C0
ð1Þ
where C0 (mg/L) and Ct (mg/L) represent the initial Cu(II)/COD concentration and the residual Cu(II)/COD concentration at time t (min), respectively. 2.3. Experimental procedures 2.3.1. Pretreatment of the IM fillings A series of pretreatment procedures was conducted to clean the iron scraps and GAC prior to use. The iron scraps were washed with industrial alkali and diluted hydrochloric acid (2%, v/v) to remove the grease and rust on the surfaces of the scraps. Then, the particles were washed with tap water and distilled water before they were dried, sieved and saved. Similarly, GAC was washed with tap water and distilled water to eliminate the impurities on the surface and then dried. Finally, the iron scraps and activated carbon were thoroughly mixed in a mass ratio of 3:1 before use. 2.3.2. IM reactor and its intermittent operation The IM reactor was a cylindrical fluidized bed with a 30-L working volume and 7.5-L IM fillings. The fluidized bed was equipped with an intermittently aerated device and a reflux pump at the bottom. The wastewater reflux (1 time per 60 s, last for 1 s) and intermittent aeration (1 time per 15 min, last for 1 s) conditions in the fluidized bed guarantee a fluidized state of the iron and carbon particles and thus overcome the critical weaknesses of traditional IM reactors, such as reactor clogging and iron deactivation. The IM reactor was operated intermittently, and the fillings were reused. In this work, the primary role of the IM process was to remove Cu(II) and partial EDTA from wastewater; another important role played by the process was to produce Fe(II)-rich effluent with a proper pH to facilitate the subsequent Fenton oxidation (FO) process. Therefore, the IM process was operated intermittently to investigate the treatment efficiency, Fe(II) yields, and pH variation during the treatment of wastewater containing EDTA-Cu(II) at pH levels ranging from 1.39 to 4.53. Samples were collected to examine the concentrations of Cu(II), COD, Fe(II) and pH at reaction times of 0, 5, 10, 20, 30, 40 and 60 min. 2.3.3. Batch experiment of FOC Batch experiments of FOC were performed to determine the influence of important operating parameters such as H2O2 dosage, [Fe(II)]/[H2O2], initial pH of the wastewater, and the reaction time on the removal of COD from IM effluent by FOC. The desired Fe(II) dosage was achieved by adding the necessary amount of solid FeSO47H2O if the Fe(II) concentration in the IM effluent was insufficient. The optimum values of the parameters obtained from the batch experiment were used as references in the pilot-scale experiment. The batch experiment procedure can be divided into three steps. First, raw wastewater was pumped into the IM reactor, and the effluent was collected after 30 min of reaction. Second, 200 mL of the IM effluent was poured into a 500-mL conical reactor, and the FO process was initiated immediately after the addition of the proper concentration of H2O2 and FeSO4 (if necessary). Samples were collected to detect the COD content after the completion of FO. Finally, the pH of the Fenton effluent was adjusted to 9.0 by adding NaOH solid under mild stirring. Samples were collected and tested after coagulation and precipitation for 30 min. 2.3.4. Pilot-scale experiment of the IM–FOC process 2.3.4.1. The setup of the reactors. The combined process of IM–FOC was applied to treat wastewater containing EDTA-Cu(II) in a
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
pilot-scale test. Fig. 1 is a schematic diagram of the IM–FOC system. The IM reactor was described in Section 2.3.2. Reactor (II), the FO reactor and the coagulation reactor each had a working volume of 120 L. Five layers of orifice plates were installed in reactor (II) and the coagulation reactor to create a crisscross water-flow pattern and strengthen the mixing effect, and the three-phase flow design of the FO reactor guaranteed a sufficiently long retention time and effective stirring of the wastewater during reaction. The reflux pump between the IM reactor and reactor (II) was set up to pump the wastewater in reactor (II) back into the IM reactor every 60 s (last for 1 s) such that the fillings could be kept in a fluidized state. The wastewater entered from the bottom of each reactor and came out from the top. 2.3.4.2. The operational procedures. The IM–FOC process was operated continuously; the treatment procedures are described as follows. First, the raw wastewater was pumped into the IM reactor; after treatment by the IM process, the Fe(II)-rich effluent of the IM was pumped into a subsequent FO reactor. Second, H2O2 solution (50%, w/w) was continually supplied to the FO reactor using a peristaltic pump. After Fenton’s oxidation, the effluent of the FO reactor entered into the coagulation reactor in which NaOH solution (50%, w/w) and PAM (white powder with a molecular weight of 12 million) solution were continually supplied from the bottom of the reactor using peristaltic pumps. Finally, samples were collected for analysis after coagulation at the proper pH level. Overall, the treatment efficiency of Cu(II) and COD by the IM–FOC process was investigated, and the contributions of IM, FO and coagulation were separately determined and evaluated. 3. Results and discussions 3.1. Intermittent operation of IM 3.1.1. Mechanisms of IM The IM process, which employs iron and carbon as the anodic and the cathodic materials, operates on principles very similar to those of electrochemistry methods [16]. However, the electron flows are provided by a galvanic cell instead of an external power supply. The pertinent half-cell reactions are as follows [17]: Anode (iron):
FeðsÞ 2e ! Fe2þ ðaqÞ; Eh ðFe2þ =FeÞ ¼ 0:44 V
ð2Þ
Fe2þ ðaqÞ e ! Fe3þ ðaqÞ; Eh ðFe3þ =Fe2þ Þ ¼ þ0:77 V
119
ð3Þ
Cathode (carbon):
2Hþ ðaqÞ þ 2e ! 2½H ! H2 ðgÞ; Eh ðHþ =H2 Þ ¼ 0 V
ð4Þ
In the presence of oxygen:
O2 ðgÞ þ 4Hþ ðaqÞ þ 4e ! 2H2 O;Eh ðO2 =H2 OÞ ¼ þ1:23 V
ð5Þ
O2 ðgÞ þ 2Hþ ðaqÞ þ 2e ! H2 O2 ; Eh ðO2 =H2 O2 Þ ¼ þ0:68 V
ð6Þ
O2 ðgÞ þ 2H2 O þ 4e ! 4OH ðaqÞ; Eh ðO2 =OH Þ ¼ þ0:40 V
ð7Þ
According to our previous study [14,18], EDTA-chelated Cu(II) can be removed from wastewater by IM through the following mechanisms: (1) Fe2+-based replacement and precipitation. Fe0 is oxidized to Fe(II) ion (Eq. (2)), and Fe(II) ion is oxidized to Fe(III) ion by oxygen (Eq. (3)); then, EDTA-Cu(II) is first replaced by Fe(III) ion, and chelated Cu(II) is released to the solution in the free state; Finally, free Cu(II) ions are gradually precipitated by OH- produced in the cathodic reaction (Eq. (7)). (2) Fe2+-based electro-coagulation: the generated Fe(II) and Fe(III) ions can serve as coagulant precursors and form active Fe(OH)2 and Fe(OH)3 to remove EDTA-Cu(II) from wastewater through adsorption and coprecipitation. (3) Electro-electrophoresis and redox: Fe(II) and atomic hydrogen released from the galvanic cell reactions (Eqs. (2), (3)) and hydroxyl radicals generated by Fenton’s reaction between Fe(II) and H2O2 (Eqs. (2), (6)) exhibit high activities to reduce Cu(II) and decompose organic contaminants. Meanwhile, as the electron flow creates the electric field in the galvanic cell reaction, free and chelated Cu(II) ions could move to the surface of the electrodes by the motivation of electrophoresis and then be reduced by Fe0 and atomic hydrogen. 3.1.2. Intermittent operation of the IM process under different pHs The IM reactor was operated intermittently at different pH levels to investigate its performance on the treatment of wastewater containing EDTA-Cu(II). Fig. 2 displays the removal efficiency of Cu(II) (a) and COD (b) and by IM at initial pH levels of 1.39 (without pH adjustment), 3.23, 3.65 and 4.53, respectively.
Fig. 1. Schematic diagram of the IM–FOC system.
120
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
Fig. 2. Removal efficiency of Cu(II) (a) and COD (b) in wastewater by IM at initial pH values of 1.39, 3.23, 3.65 and 4.53.
From Fig. 2a, it can be observed that IM had a high Cu(II) removal efficiency in the pH range of 1.39–4.53, although Cu(II) was removed much faster at lower pH values. For instance, the Cu(II) concentrations at pH = 4.53, 3.65, 3.23 and 1.39 were rapidly reduced from 225.3, 230.6, 225.2 and 200.5 mg/L at 0 min to 3.8, 3.7, 5.0 and 1.6 mg/L at 40 min, respectively. All of the Cu(II) removal efficiencies at 20 min were greater than 93.3%. Fig. 2b shows that the removal efficiencies of COD by IM were very low, especially at higher initial pH values. For example, the COD concentrations at pH = 4.53, 3.65, 3.23 and 1.39 were only reduced from 1008.5, 1023.0, 1065.0 and 1199.6 mg/L at 0 min to 786.5, 785.13, 794.7, 838.7 mg/L at 60 min, respectively. All of the COD removal efficiencies at 60 min were less than 30.0%. In summary, the maximum removal efficiencies of Cu(II) and COD by the IM process were achieved at an initial pH of 1.39, indicating that the raw wastewater containing EDTA-Cu(II) is quite suitable for direct treatment in the IM reactor without pH adjustment. 3.1.3. Fe(II) yields and pH variation in the IM reactor The initial pH of wastewater is a significant parameter that affects iron corrosion and thus determines the Fe(II) yields and the final pH in the IM effluent. In this work, we struggled to obtain an IM effluent with high Fe(II) yields and proper pH values to facilitate the operation of the subsequent FO process. Fig. 3a illustrates Fe(II) yields over time at initial pH values of 1.39 (without pH adjustment), 2.05, 3.65 and 4.53. It can be observed that the Fe(II) yields decreased sharply with the increase in initial pH. The maximum Fe(II) yields corresponding to initial pH
values of 1.39, 2.05, 3.65 and 4.53 were 338.0 mg/L (30 min), 160.3 mg/L (30 min), 43.69 (20 min) and 14.2 mg/L (10 min), respectively. As shown, Fe(II) yields could achieve 336.1 mg/L when the IM process was operated at the initial pH of 1.39 for merely 20 min. Therefore, the optimum pH value of wastewater required to achieve the maximum Fe(II) yield was 1.39, which indicates that the raw wastewater was quite suitable for direct treatment in the IM reactor without pH adjustment. Fig. 3b shows the pH variation with time at initial pH values of 1.39 (without pH adjustment), 2.05, 3.65 and 4.53. As shown in the figure, the pH of the wastewater increased with time regardless of the initial pH level. This increase in pH in acidic wastewater during treatment by the IM process is in accordance with observations reported in previous studies [12,19]. Moreover, it was observed that the pH values of the IM effluent at 20 min remained between 4.6 and 5.68, which may be a suitable pH range for the operation of FO. Considering both the Fe(II) yields and the final pH of the IM effluent, it was concluded that the IM process should operate for a retention time of 20 min without adjusting the pH of the raw wastewater. Under such conditions, the Fe(II) yield was 338.0 mg/L, and the pH of the IM effluent was approximately 4.6. 3.2. Batch experiment of FOC 3.2.1. Effect of H2O2 dosage on the removal of COD H2O2 plays a very significant role as a source of OH generated during Fenton’s reaction. The effect of H2O2 dosage on the removal
Fig. 3. Fe(II) yields (a) and pH variation (b) over time in the IM reactor at different initial pH values.
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
121
efficiency of COD (RCOD) was examined by varying the initial values of [H2O2]/[COD] from 0.5 to 6.0; the results are shown in Fig. 4. From the figure, it can be observed that with the increase in [H2O2]/[COD] from 0.5 to 2.0, the RCOD by FO was markedly increased from 35.9% to 87.2%, and the RCOD by FOC was enhanced from 73.6% to 89.4%. However, with a further increase in [H2O2]/ [COD] from 2.0 to 6.0, the increase in the RCOD by FO and FOC was negligible. Considering the dosage of H2O2, attention should be paid not only to the improvement of the treatment efficiency but also to the economic cost. Therefore, the efficiency of H2O2 was evaluated, and the results are shown in the inset of Fig. 4. The relationship between Q (mg H2O2/mg COD) and [H2O2]/ [COD] can be expressed by the following equation:
Q ¼ 1:08062
½H2 O2 ½COD
ð8Þ
where Q represents the amount of H2O2 that was consumed in removing 1 mg COD, mg H2O2/mg COD; R2 = 0.99897. From this equation, it can be calculated that when the value of [H2O2]/[COD] was increased by 1.0, 1.08062 mg of extra H2O2 was needed for the removal of 1 mg COD. Therefore, the lower the [H2O2]/[COD] values are, the lower the Q values become and thus the higher the efficiency of H2O2 becomes. As shown in Fig. 4, when the [H2O2]/[COD] increased from 2.0 to 6.0, the removal efficiency of COD by FO and FOC increased by merely 3.8%-4.4%, but the Q value sharply increased from 2.17204 to 6.51612 mg H2O2/mg COD. Taking the treatment efficiency as well as the economic cost into full consideration, the optimum [H2O2]/[COD] was determined to be 2.0 for the FO and FOC processes. 3.2.2. Effect of [Fe(II)]/[H2O2]on the removal of COD The removal efficiency of COD (RCOD) by the Fenton process is influenced by the concentration of Fe(II) ions, which catalyze H2O2 decomposition, resulting in OH production and consequently the degradation of organic molecules. Fig. 5 shows the effect of [Fe(II)]/[H2O2] on RCOD during the treatment of IM effluent by FOC at different initial [Fe(II)]/[H2O2] values ranging from 0.2 to 1.0. It can be observed that RCOD increased when [Fe(II)]/[H2O2] was increased from 0.2 to 0.3. The fact that higher RCOD values were achieved at higher Fe(II) dosages is mainly attributed to the production of higher amounts of OH with more Fe(II) as a catalyst in Fenton’s reaction (Eq.(9)). However, RCOD gradually decreased
Fig. 4. Effect of [H2O2]/[COD] on the removal of COD during the treatment of IM effluent by FO and FOC, respectively. Experimental conditions: [COD] of the IM effluent = 780.0 mg/L; [Fe(II)]/H2O2 = 2.0; reaction time = 90 min; initial pH = 3.0. The inset illustrates the efficiency of H2O2.
Fig. 5. Effect of [Fe(II)]/[H2O2] on the removal of COD during the treatment of IM effluent by FOC. Experimental conditions: COD of IM effluent = 828 mg/L; [Fe(II)] of IM effluent = 310.5 mg/L; [H2O2] = 1656 mg/L; reaction time = 90 min; pH = 3.0. The inset compares the removal efficiency of COD by FO and FOC at initial [Fe(II)]/[H2O2] values of 0.3 and 1.0.
with the further increase in [Fe(II)]/[H2O2] from 0.3 to 0.6. This result can be attributed to the scavenging effect of Fe(II) ions on the hydroxyl radicals (Eq.(10)).
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH Fe2þ þ HO ! Fe3þ þ OH
ð9Þ ð10Þ
In addition, it was observed that when [Fe(II)]/[H2O2] increased from 0.6 to 1.0, RCOD was enhanced from 82.2% to 88.1%, which may be attributed to the strengthening effect of coagulation at higher Fe(II) dosages; moreover, the rise in RCOD due to coagulation at higher Fe(II) dosages outweighed the drop in RCOD by FO, resulting in the improvement of the total RCOD. To distinguish the respective contributions of oxidation and coagulation to the removal of total COD, the RCOD by FO and FOC at initial [Fe(II)]/[H2O2] ratios of 0.3 and 1.0 were compared; the results are shown in the inset of Fig. 5. As shown, RCOD by coagulation at a [Fe(II)]/[H2O2] ratio of 0.3 was (87.2%-65.0%)=22.2%, while RCOD by coagulation at a [Fe(II)]/[H2O2] ratio of 1.0 was (88.1%-50%)=38.1%, indicating that COD removal by FO was reduced and COD removal by coagulation was increased with the increase in initial [Fe(II)]/[H2O2]. Due to the higher RCOD achieved at lower Fe(II) dosages, [Fe(II)]/[H2O2] values in the range of 0.2 and 0.3 were chosen as the optimal values for further experiments. 3.2.3. Effect of initial pH on the removal of COD FO is known as a highly pH-dependent process because pH plays an important role in the mechanism of OH production in Fenton’s reaction [20]. The effect of initial pH on the removal efficiency of COD (RCOD) during the treatment of IM effluent by FOC is illustrated in Fig. 6. It can been observed that RCOD by FOC remained high at pH values ranging from 2.0 and 4.0 and later suffered a slight decline when the initial pH was further increased from 4.0 to 7.0. At initial pH values of 8.0 and 10.0, RCOD by FOC sharply decreased, which was primarily due to the formation of Fe(II)/Fe(III) hydroxide complexes. This led to the deactivation of the Fe(II) catalyst, resulting in a rapid decrease in OH production [21]. It should also be mentioned that at the initial pH values of 8.0 and 10.0, Fe(II) was seriously coagulated when FO was negligible; thus, COD was principally removed by coagulation. In summary, the optimum initial pH value was 3.0, and a suitable initial
122
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
[H2O2]/[COD] ratio of 2.0, [Fe(II)]/[H2O2] ratio of 0.2–0.3, pH value of 2.0–5.0 and reaction time of 60–80 min. In our pilot-scale investigation, all of the chemicals, including the H2O2 solution (50%, w/ w), NaOH solution (50%, w/w) and PAM solution (100 mg/L), were continually supplied to the corresponding reactor using peristaltic pumps. 3.3.1.1. The calculation of influent quantity. The influent quantity of wastewater can be calculated using the following equation:
q¼
Fig. 6. Effect of initial pH on the removal of COD during the treatment of IM effluent by FOC. Experimental conditions: COD of IM effluent = 740.2 mg/L; H2O2 = 1480.4 mg/L; [Fe(II)]/H2O2 = 2.0; reaction time = 90 min.
pH range for the removal of COD by FOC was determined to be 2.0– 5.0. 3.2.4. Effect of reaction time on the removal of COD and Cu(II) Tests were also carried out to evaluate the effect of reaction time on the removal of COD and Cu(II) during the treatment of IM effluent by FOC. To this end, experimental samples were taken at preselected time intervals and immediately neutralized with sodium hydroxide to stop the reaction; the results are shown in Fig. 7. The figure shows that 89.6% of COD and 100% of Cu(II) were removed within 80 min, and COD and Cu(II) were reduced from 810.2 mg/L and 2.3 mg/L to 84.3 mg/L and 0 mg/L, respectively. After 80 min, the change in the concentrations of residual COD and Cu(II) were insignificant. Therefore, a reaction time of 60– 80 min would be enough for the treatment of both COD and Cu(II) in IM effluent by the FOC process. 3.3. Pilot-scale experiment 3.3.1. Theoretical calculations of the optimal operating parameters for the IM–FOC process As determined by the batch experiments, the IM process should operate at a retention time of 20 min without adjusting the pH of the raw wastewater, and the appropriate conditions for FOC are a
Fig. 7. COD and Cu(II) removal as a function of time during the treatment of IM effluent by the FOC process. Experimental conditions: COD of IM effluent = 810.2 mg/L; Cu(II) of IM effluent = 2.3 mg/L; H2O2 = 1620.4 mg/L; [Fe(II)]/ H2O2 = 0.2; pH = 3.0.
60V T
ð11Þ
where q represents the influent quantity (L/h); V is the working volume of the reactor (L); T is the retention time (min). In this work, the IM reactor, with a working volume of 30 L, was operated for a retention time of 20 min. Thus, it can be calculated from Eq. (11) that q = (60 30)/20 = 90 L/h. In the pilot-scale experiment, the working volumes of the reactors of FO and coagulation were both 120 L. When the q was set to 90 L/h, the retention time of FO and coagulation was determined to be 80 min, which is sufficiently long for Fenton’s reaction to occur to completion. 3.3.1.2. The calculation of the flow rate of H2O2. The optimum value of the [H2O2]/[COD] ratio was determined to be 2.0. After measuring the COD in IM effluent, the flow rate of H2O2 (q1, mL/min) can be calculated as follows:
q1 ¼
qCn 60 w 1000 q
ð12Þ
where C represents the COD concentration of the IM effluent, mg/L; n represents the value of [H2O2]/[COD]; w is the mass fraction of H2O2 solution; q is the density of H2O2 solution, g/cm3. The COD concentration of the raw wastewater was between 998 and 1212 mg/L (Fig. 1). As indicated in Section 2.1, approximately 24.9% of COD was removed after treatment by IM for 20 min; therefore, it is estimated that the COD concentration of the IM effluent was between 749.5 and 910.2 mg/L. In other words, 749.5 < C < 910.2 mg/L. In this work, q = 90 L/h; n = 2.0; w = 50%; q = 1.4067 g/cm3. Therefore, it can be calculated from Eq. (12) that q1 was between 3.20 and 3.88 mL/min. 3.3.1.3. The calculation of the flow rate of the NaOH solution. The optimum pH for coagulation must be determined before the determination of the NaOH flow rate. Five groups of 200-mL effluent
Fig. 8. Effect of coagulation pH on the removal of COD in the FO effluent by coagulation.
123
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124 Table 2 COD and Cu(II) removal from EDTA-Cu(II) wastewater by the IM–FOC process and the contributions of IM, FO and coagulation process (CP). Samples from
Cu(II) con. (mg/L) COD con. (mg/L) BOD5/COD
The contribution rate to the total removal (%)
Influent
IM
FO
CP
IM
FO
CP
Total
225.3 1096.6 0
5.6 851.5 0.09
5.6 262.1 –
0 142.6 0.42
97.5 22.3 –
0 47.8 –
2.5 10.9 –
100.0 87.0 –
samples taken during the FO process were adjusted to the desired pH by solid NaOH under mild stirring for 40 min, and the results are shown in Fig. 8. The figure shows that the optimum coagulation pH was 9.0. The flow rate of the NaOH solution (q2, mL/min) can be calculated by the following equation:
q2 ¼
q V1 6
ð13Þ
where V1 is the volume of NaOH solution (50%, w/w) that was required to adjust the pH of the 100-mL FO effluent to 9.0. In this work, q = 90 L/h; V1 = 1.0–2.0 mL. Thus, it can be calculated from Eq. (13) that q2 = 15–30 mL/min. 3.3.1.4. The calculation of the flow rate of PAM. In the pilot-scale tests, PAM was added to facilitate the coagulation and precipitation of iron and copper hydroxides. The flow rate of PAM (q3, mL/ min) can be calculated using the following equation:
q3 ¼
q C 1 1000 C 2 60
ð14Þ
where C1 is the dosage of PAM, mg/L; C2 is the concentration of the PAM solution, mg/L. In this work, q = 90 L/h; C1 = 1.0 mg/L, and C2 = 100 mg/L; thus, it can be calculated from Eq. (14) that q3 = (90 1.0 50)/ (100 3)=15 mL/min. 3.3.2. The operation of the IM–FOC process for the treatment of wastewater containing EDTA-Cu(II) The IM–FOC process was operated continuously to treat raw wastewater containing EDTA-Cu(II) under the optimized conditions determined by the batch experiments and theoretical calculations discussed above: an influent quantity of 90 L/h without pH adjustment, a retention time of 20 min for IM reactor, a H2O2 (50%, w/w) flow rate of 3.63 mL/min, a retention time of 80 min for the FO reactor, a NaOH solution (50%, w/w) flow rate of 24.0 mL/min, a PAM solution flow rate of 15 mL/min and a coagulation pH of 9.0 for 80 min. Under these conditions, the treatment efficiency of EDTA-Cu(II) wastewater by the IM–FOC process and the contribution of each process were investigated; the results are shown in Table 2. Table 2 shows that Cu(II) and COD decreased from 225.3 mg/L and 1096.6 mg/L to 0 mg/L and 142.6 mg/L, respectively, after being treated by the IM–FOC process, and the removal efficiencies of Cu(II) and COD reached 100.0% and 87.0%. Of the 100.0% of Cu(II) removed, 97.5% was removed by the IM process, while only 2.5% was removed by coagulation, indicating that the IM process was responsible for the removal of the majority Cu(II) ions from the wastewater. However, the COD removed by the IM process was relatively low: only 22.3%. Most of the COD (47.8%) was degraded by the FO process, and approximately 10.9% of the COD was removed by coagulation and precipitation with iron hydroxides. In this work, samples were tested with respect to their BOD5/COD values to evaluate the variation in the biodegradability of wastewater during treatment. The BOD5/COD value of the raw wastewater was 0, indicating that EDTA-Cu(II) wastewater is totally nonbiodegradable. The BOD5/COD of the IM effluent and coagulation effluent were determined to be 0.09 and 0.42, respectively. The poor
treatment performance of COD by IM and the low BOD5/COD of IM effluent indicated that EDTA species cannot be effectively decomposed into small biodegradable organic molecules by the IM process. In contrast, the efficient removal of COD by FOC and the high BOD5/COD value of the coagulation effluent suggest that EDTA can be efficiently oxidized into small organic molecules or mineralized into CO2 by the highly oxidative hydroxyl radicals generated during Fenton’s reaction. The results of this study suggest that the combined process of IM–FOC is efficient with respect to the decontamination of EDTACu(II) wastewater originating from the electroless copper-plating process. Additionally, the combination of the IM and FO processes for the treatment of chelated wastewater is potentially of low cost. First, the IM process does not require a chemical coagulant or highpower consumption, as in the cases of chemical coagulation and electrolysis, respectively. Only low-cost iron scraps are consumed in the IM process, and little external power is required (for wastewater reflux). Then, the problems of iron deactivation and reactor clogging are eliminated due to the fluidized-bed design of the IM reactor and the fluidized state of the IM fillings, which reduces the cost of concentrated acid required for iron activation. Most importantly, apart from removing Cu(II) and COD, the IM process can produce Fe(II)-rich effluent with the proper pH, which can be directly used for FO without Fe(II) addition or pH adjustment, largely reducing the cost of the Fenton process. Overall, the IM–FOC process is highly efficient and potentially inexpensive with respect to the decontamination and detoxification of the EDTA-Cu(II) wastewater originating from the printed circuit boards industry. 4. Conclusions The efficient treatment of wastewater containing EDTA-Cu(II) was achieved by the combined process of IM–FOC. The results demonstrated that the treatment efficiencies and Fe(II) yields of the IM process were much higher at lower initial pH levels of the wastewater. The appropriate conditions for the treatment of IM effluent by FOC were as follows: [H2O2]/[COD] of 2.0, [Fe(II)]/ [H2O2] of 0.2–0.3, initial pH of 2.0–5.0, and reaction time of 60– 80 min; thus, the Fe(II)-rich effluent of IM was suitable for direct treatment in a subsequent FO process without Fe(II) addition or pH adjustment. Under the optimal operating parameters, 100% Cu(II) and 87.0% COD were removed by the IM–FOC process. The contributions of IM, FO and coagulation were 97.5%, 0% and 2.5%, respectively, for Cu(II) removal and were 22.3%, 47.8%, and 10.9%, respectively, for COD removal. After treatment, the BOD5/COD ratio of wastewater was enhanced from 0 to 0.42, indicating that EDTA was effectively oxidized in the combined system. From these results, it was concluded that the combined process of IM–FOC is a highly efficient and potentially low-cost method for the detoxification of nonbiodegradable wastewater containing EDTA-Cu(II). Acknowledgements This research was supported by the National Key Project of Water Pollution Control and Treatment (Grant No.: 2009ZX07211005-02), the Natural Science Foundation of Guangdong Province (Grant No.: 9151042001000007), Science and Technology Plan
124
S. Lan et al. / Separation and Purification Technology 89 (2012) 117–124
Projects Fund of Dongguan City (Grant No.: 2011108102012) Science and Technology Plan Projects Fund of Dongguan City (Grant No.: 2011108102005). References [1] R.S. Juang, S.W. Wang, Electrolytic recovery of binary metals and EDTA from strong complexed solutions, Water Res. 12 (2000) 3179–3185. [2] U. Aydin, Y. Buluta, C. Yerlikayab, Removal of copper (II) from aqueous solution by adsorption onto low-cost adsorbents, J. Environ. Manage. 87 (2008) 37–45. [3] A.P. Davis, D.L. Green, Photocatalytic oxidation of cadmium-EDTA with titanium dioxide, Environ. Sci. Technol. 33 (1999) 609–617. [4] P.X. Wu, J.B. Zhou, X.R. Wang, Y.P. Dai, Z. Dang, N.W. Zhu, P. Li, J.H. Wu, Adsorption of Cu-EDTA complexes from aqueous solutions by polymeric Fe/Zr pillared montmorillonite: behaviors and mechanisms, Desalination 277 (2011) 288–295. [5] G. Issabayeva, M.K. Aroua, N.M. Sulaiman, Study on palm shell activated carbon adsorption capacity to remove copper ions from aqueous solutions, Desalination 262 (2010) 94–98. [6] I. Licsko, I. Takacs, Heavy metal removal in the presence of colloid-stabilizing organic material and complexing agents, Water Sci. Technol. 18 (1986) 18–19. [7] S.X. Jiang, J.X. Qu, Y. Xiong, Removal of chelated copper from wastewaters by Fe2+-based replacement-precipitation, Environ. Chem. Lett. 29 (2009) 597– 599. [8] R.S. Juang, L.C. Lin, Electrochemical treatment of copper from aqueous citrate solutions using a cation-selective membrane, Sep. Purif. Technol. 22–23 (2001) 627–635. [9] G. Borbely, E. Nagy, Removal of zinc and nickel ions by complexationmembrane filtration process from industrial wastewater, Desalination 240 (2009) 218–226.
[10] I. Kabdasli, T. Arslan, T.Ö. Hancıa, I.A. Alatona, O. Tünay, Complexing agent and heavy metal removals from metal plating effluent by electrocoagulation with stainless steel electrodes, J. Hazard. Mater. 165 (2009) 838–845. [11] Y. Oztekin, Z. Yazicigil, Recovery of metals from complexed solutions by electrodeposition, Desalination 190 (2006) 79–88. [12] Y.Z. Jin, Y.F. Zhang, W. Li, Microelectrolysis technology for industrial wastewater treatment, J. Environ. Sci. 15 (2003) 334–338. [13] X.Y. Yang, Interior microelectrolysis oxidation of polyester wastewater and its treatment technology, J. Hazard. Mater. 169 (2009) 480–485. [14] F. Ju, Y.Y. Hu, Removal of EDTA-chelated copper from aqueous solution by interior microelectrolysis, Sep. Purif. Technol. 78 (2011) 33–41. [15] APHA, Standard Methods—for the Examination of Water and Wastewater, 20th ed., American Public Health Association, American Water Works Association, Water Pollution Control Federation, Washington, DC, 1999. [16] X.Y. Yang, Y. Xue, W.N. Wang, Mechanism, kinetics and application studies on enhanced activated sludge by interior microelectrolysis, Bioresour. Technol. 100 (2009) 649–653. [17] X.C. Ruan, M.Y. Liu, Q.F. Zeng, Y.H. Ding, Degradation and decolorization of reactive red X-3B aqueous solution by ozone integrated with internal microelectrolysis, Sep. Purif. Technol. 74 (2010) 195–201. [18] F. Ju, Y.Y. Hu, J.H. Cheng, Removal of chelated Cu(II) from aqueous solution by adsorption-coprecipitation with iron hydroxides prepared from microelectrolysis process, Desalination 274 (2011) 130–135. [19] J.H. Fan, L.M. Ma, The pretreatment by the Fe–Cu process for enhancing biological degradability of the mixed wastewater, J. Hazard. Mater. 164 (2009) 1392–1397. [20] M. Perez, F. Torrades, X. Domenech, J. Peral, Fenton and photo-Fenton oxidation of textile effluents, Water Res. 36 (2002) 2703–2710. [21] Y. Deng, J.D. Englehardt, Treatment of landfill leachate by the Fenton process, Water Res. 40 (2006) 3683–3694.