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Available online 2 December 2016. Keywords: Zinc plating wastewater. Alkaline non-cyanide. Alkaline cyanide. Acidic zinc. Electrocoagulation. Operating cost.
Process Safety and Environmental Protection 1 0 5 ( 2 0 1 7 ) 373–385

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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep

Treatments of alkaline non-cyanide, alkaline cyanide and acidic zinc electroplating wastewaters by electrocoagulation M. Kobya a , E. Demirbas b,∗ , F. Ozyonar c , G. Sirtbas a , E. Gengec d a

Department of Environmental Engineering, Gebze Technical University, 41400, Gebze, Turkey Department of Chemistry, Gebze Technical University, 41400, Gebze, Turkey c Department of Environmental Engineering, Cumhuriyet University, Sivas, Turkey d Department of Environmental Protection, Kocaeli University, Kocaeli, Turkey b

a r t i c l e

i n f o

a b s t r a c t

Article history:

Treatments of alkaline non-cyanide, alkaline cyanide and acidic zinc electroplating rinse

Received 23 August 2016

wastewaters were investigated in an electrocoagulation (EC) reactor using Fe plate elec-

Received in revised form 1

trodes. This is the first study involved with removals of zinc and cyanide together from

November 2016

three different wastewaters in the literature. The effects of the operating parameters namely,

Accepted 26 November 2016

initial pH (pHi ), current density and operating time on the removal efficiencies were eval-

Available online 2 December 2016

uated. The removal efficiencies and operating costs were determined as 99.8% for Zn and 0.74 D /m3 at a pH of 7, 80 A/m2 and 60 min for alkaline non-cyanide, 99.9% for Zn, 99.9%

Keywords:

for CN and 1.72 D /m3 at a pH of 9.5, 60 A/m2 and 60 min for alkaline cyanide, and 99.9% for

Zinc plating wastewater

Zn and 2.26 D /m3 at a pH of 8, 80 A/m2 and 60 min for acidic zinc electroplating wastew-

Alkaline non-cyanide

aters, respectively. Moreover, toxicity test was conducted to obtain information about the

Alkaline cyanide

toxic effect of the raw and treated wastewaters. The toxicity results indicated that all the

Acidic zinc

raw wastewaters contained hardly toxic effect (EC50 for acidic, alkaline cyanide and alka-

Electrocoagulation

line non-cyanide were 0.62, 5.25 and 3.38). On the other hand, the treated wastewater was

Operating cost

non-toxic. This study revealed that the EC process with Fe electrode was very effective for

Iron electrode

removal of zinc and cyanide ions from different zinc electroplating rinse wastewaters. © 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

Metal plating is one of the major chemical processes that discard large

during rinsing processes. Additionally, batch dumping spent acid and cleaning solutions contribute to the complexity of waste treatment. The industrial use of zinc has increased because of the extensive

amounts of harmful and toxic wastewaters. Plating wastewaters contain heavy metals (Cd, Co, Cr, Cu, Ni, Pb, and Zn, etc.), cyanide, oil, grease and suspended solids at levels that might be considered hazardous to

use of zinc and zinc salts in the electroplating and metal finishing industries. Over the years, different processes have been developed for applying zinc coatings (Hosseini et al., 2016). In particular, electrode-

the environment and could pose risks to public health (Hosseini et al., 2016; Bojic et al., 2009). Heavy metals and cyanide, in particular, are of great concern due to their toxicity. Because of the high toxicity and corrosiveness of plating waste streams, plating facilities are required to treat wastewater prior to discharge. The metal plating process typically

posited zinc is used extensively in automotive and other industries as a protective coating for large quantities of steel wires, strips, sheets,

involves alkaline cleaning, acid pickling, plating, and rinsing. Copious amounts of wastewater are generated through these steps, especially

cyanide zinc, alkaline cyanide zinc and acidic zinc or chloride zinc (Schlesinger and Paunovic, 2010; Naik and Venkatesha, 2002, 2003). Alkaline non-cyanide zinc plating baths compose of zinc (sodium zincate, 6–23 g/L), caustic soda, water conditioner, and organic additives. Alkaline cyanide zinc plating baths consist of zinc cyanide (7.5–34 g/L),

tubes and other fabricated ferrous metal parts. Zinc deposits offer good protection and decorative appeal at low cost (Schlesinger and Paunovic, 2010). The most widely used zinc plating is categorized as alkaline non-

sodium cyanide and proprietary additives. Acidic zinc baths contain ∗

zinc chloride (15–38 g/L), potassium chloride, ammonium chloride or Corresponding author. Fax: +90 262 6053005. boric acid and proprietary additives. In the zinc electroplating proE-mail address: [email protected] (E. Demirbas). http://dx.doi.org/10.1016/j.psep.2016.11.020 0957-5820/© 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Process Safety and Environmental Protection 1 0 5 ( 2 0 1 7 ) 373–385

cess, town water is generally used in the cleansing rinse of solvent, alkaline and acid while deionized water is employed for the plating rinse and final rinse (Schlesinger and Paunovic, 2010; Naik and Venkatesha, 2002, 2003). The water in the rinsing baths gets contaminated during the cleaning or plating process due to the ‘drag-out’ from previous process baths. Generally, zinc concentrations vary from 0.112 to 252 mg/L in electroplating rinse effluents (EPA, 1985; Zouboulis et al., 2005). On the other hand, effluents from individual operations at electroplating and metal finishing plants generally contain from 1 to 3% (0.005–150,000 mg/L) of cyanide (EPA, 1985; Dash et al., 2009). The electroplating effluents usually contain metal-ion concentrations such as Zn, Fe, and Ni and anions like cyanide much higher than the permissible levels. As expected in zinc electroplating rinse wastewater, zinc and cyanide concentrations are higher than others. High concentrations of zinc can cause eminent health problems, such as stomach cramps, skin irritations, vomiting, nausea and anemia (Fu and Wang, 2011). Short-term cyanide exposure causes rapid breathing, irritation and sores on skin, tremors, and other neurological effects and longterm exposure to cyanide leads to weight loss, thyroid effects, nerve damage and death (Adhoum and Monser, 2002). The maximum permitted zinc and cyanide concentrations in industrial effluents after the treatment according to the industrial discharge standards and the receiving media are 0.5 and 3 mg/L for Zn and 0.01 and 0.2 mg/L for total cyanide, respectively (The Turkish Water Pollution Control Regulation, 1988). Various techniques have been applied for the treatment of heavy metals from wastewater, such as chemical precipitation (Blais et al., 2008), adsorption (Zhang et al., 2016), ion-exchange (Silva et al., 2008), electrodialysis (Mahmoud and Hoadley, 2012), reverse osmosis (Petrinic et al., 2015) and electrocoagulation (Espinoza-Quinones et al., 2012; Kobya et al., 2010a,b). The rest of treatment techniques except for electrocoagulation (EC) has a number of disadvantages such as huge space requirements, long operating time, high operating cost, membrane fouling and scaling, regeneration and performance reduction, highly sensitive to the solution pH, additive chemical reagents causing serious secondary pollution (Hosseini et al., 2016; Sancey et al., 2011; Malakootian et al., 2010; Mahvi et al., 2009). Recent research has shown that EC can offer a good opportunity to prevent and remedy pollution problems (Vasudevan and Oturan, 2014; Chen, 2004). EC consists of an in situ generation of coagulants by an electrical dissolution of Fe or Al electrodes. The generation of metallic cations takes place at the anode, whereas a H2 gas production occurs together with hydroxyl ions releasing at the cathode. Ferric or aluminum ions generated by electrochemical oxidation of Fe or Al electrode may form monomeric and polymeric hydroxyl metallic complexes depending on the pH of the aqueous medium, which has strong affinity or dispersed particles as well as counter ions to cause coagulation (Kobya et al., 2015; Chen, 2004). In conclusion, the formation of metal hydroxide flocs proceeds according to a complex mechanism. Formed amorphous such as Fe(OH)3(s) occurs “sweep flocs” having large surface areas. These flocs

real alkaline non-cyanide, alkaline cyanide and acidic zinc electroplating rinse wastewaters by the EC process and provide comparisons of the results with the literature based on the optimum operating conditions and the operational cost. Hence, the effects of experimental operating parameters such as initial pH, current density, and operating time on the zinc and cyanide removal efficiencies were evaluated to determine the optimum operating conditions. The operating costs of the treatment process according to the electrode and energy consumptions were also calculated for each wastewater. The toxicity levels were measured. The sludge remained after the EC process was characterized by scanning electron microscope (SEM), energy-dispersive analysis of Xrays (EDAX), X-ray diffractometer (XRD) and Fourier transform infrared spectroscopy (FTIR).

2.

Materials and methods

2.1. Characterizations of zinc electroplating rinse wastewaters Three different zinc electroplating rinse wastewaters were collected from a local electroplating factory located in Istanbul. Acidic zinc electroplating bath consisted of 70 g/L ZnCl2 , 170 g/L NH4 Cl, proprietary additive agents (10%), brightening agent (3%), and Zn metal anode (purity >99.9%). Non-cyanide alkaline zinc electroplating bath contained of 8–10 g/L zinc metal anode, 140–170 g/L water conditioning. The alkaline cyanide zinc plating bath included 45 g/L ZnO, 70 g/L NaOH, 90 g/L NaCN, 0.004 g/L brightening agent, and 0.004 g/L As2 S3 . After these baths were used in the factory, the characteristics of the electroplating processes wastewaters were determined as COD of 410 mg/L, TOC of 158.4 mg/L, iron of 0.643 mg/L, a pH of 12.3, conductivity of 43.6 mS/cm, and total Zn of 381 mg/L for alkaline non-cyanide zinc electroplating, COD of 570 mg/L, TOC of 107.2 mg/L, iron of 0.682 mg/L, a pH of 9.5, conductivity of 7.6 mS/cm, total cyanide of 135 mg/L and total Zn of 175 mg/L for alkaline cyanide zinc electroplating, and COD of 483 mg/L, TOC of 161 mg/L, iron of 1.282 mg/L, a pH of 6.5, conductivity of 20.4 mS/cm and total Zn of 1477 mg/L for acidic zinc electroplating wastewaters, respectively.

2.2.

Experimental set-up and procedure

The EC experiments were conducted in a batch process using 1 L capacity of an EC reactor made from Plexiglas with dimensions of 12 × 11 × 11 cm (Fig. 1). Iron (Fe) plates (purity >99.5%)

are active in rapid adsorption of soluble organic and inorganic compounds and trapping of colloidal particles and are easily separated from aqueous medium by sedimentation or H2 flotation. Moreover, this process is characterized by simple equipment and easy operation, short operating time, reduced or no required for addition of chemicals, and decreased amount of sludge (Chen, 2004). EC process using iron and aluminum electrodes showed the successful removals of free cyanide (Moussavi et al., 2011), Cd and Ni-cyanide (Kobya et al., 2010a,b), Zn-cyanide (Senturk, 2013), and heavy metals such as Cr, Cu, Mn, Ni, Pb, and Zn (Al-Shannag et al., 2015; Kobya et al., 2015; Espinoza-Quinones et al., 2012; Mansoorian et al., 2012; Hanay and Hasar, 2011; Dermentzis et al., 2011; Akbal and Camci, 2011; Heidmann and Calmano, 2010; Huang et al., 2009; Arroyo et al., 2009; Heidmann and Calmano, 2008; Kabdasli et al., 2009; Adhoum et al., 2004) from synthetic and industrial wastewaters. Although there is considerable success for treatment of various types of wastewater containing cyanide and heavy metals in the literature with the EC process, its application for the treatments of different zinc electroplating wastewaters (alkaline non-cyanide, alkaline cyanide and acidic zinc electroplating) using iron plate electrodes has not been reported yet. The novel finding of this study was to treat

Fig. 1 – A schematic diagram of the experimental set up.

Process Safety and Environmental Protection 1 0 5 ( 2 0 1 7 ) 373–385

with dimensions of 5.0 × 7.3 × 0.3 cm were used as the sacrificial electrodes. In each batch, four Fe plate electrodes (two anodes and two cathodes) spaced by 0.5 cm were placed vertically in the EC reactor and connected at monopolar parallel mode. The electrodes were dipped in the electroplating wastewater to a depth of 8 cm yielding a total effective electrode surface area of 219 cm2 . All chemicals used in the EC experiments were of analytical grade. All runs were performed with 0.85 L of wastewater at a constant temperature and 300 rpm (Heidolp MR 3000D). The solution was constantly stirred to reduce the mass transport over potential of the EC reactor. The electrodes were connected to a digital DC power supply (Agilent 6675A model; 30 V, 6 A) operated at galvanostatic mode. Before each run, the impurities on the electrode surfaces were removed by mixing hydrochloric acidhexamethylenetetramine aqueous solution (Kobya et al., 2015). The current was held constant at desired values for each run and the experiment was started. The samples taken from the EC reactor at the different operating times were filtered using 0.45-␮m pore size filter and then, total zinc and cyanide concentrations were determined. At the end of the run, the electrodes were washed thoroughly with water to remove any solid residues on the surfaces and dried. The lost amount of anode weight was determined by subtracting the weight of the electrodes before and after the experiment. In addition, sludge generated after the EC experiment was dried in an oven (Memmert, Germany) at 105 ◦ C for 24 h.

2.3.

Analytical procedure

Zinc and cyanide analyses were conducted by the procedures described in Standard Methods (APHA, 1998). Cyanide concentration in the sample was determined by pyridine–barbutiric acid method using a UV spectrophotometer (PerkinElmer Lambda 35 UV/vis spectrophotometer, USA), zinc concentration was measured with an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Perkin Elmer Optima 7000 DV). The accuracy of these measured values for cyanide, zinc, COD and TOC was estimated around 1%. pH and conductivity of the samples before and after the EC process was measured by a pH meter and a conductivity meter (Hach Lange HQ40). The experiments were repeated three times and the average data was reported. The morphologies of the sludge were characterized by the SEM (Philips XL30S-FEG). Crystal phases of the precipitates were characterized using the XRD (Rigaku 2000 D/max with CuK␣ -radiation,  = 0.154 nm at 40 kV and 40 mA). The FTIR spectrum of the sludge was collected in the range of 4000–650 cm−1 on a Bio Rad FTS 175 C spectrophotometer.

2.4.

Cost analysis of EC

In this preliminary investigation, the operating cost (OC) of the treated electroplating rinse wastewater (OC, D /m3 ) can be calculated by considering three parameters as major cost items (Kobya et al., 2015) namely, the amounts of energy (Cenergy , kWh/m3 ) and electrode material (Celectrode , kg Fe/m3 ) consumptions, and chemicals consumed (Cchemicals , kg/m3 ) in the EC process. The following equations are used to calculate Cenergy and Celectrode from the Faraday’s law Cenergy =

i × U × tEC v

(1)

Celectrode =

i × tEC × Mw z×F×v

375

(2)

where U is cell voltage (V), i is current (A), tEC is the operating time (min) and v is the volume (m3 ) of wastewater (0.85 L), Mw is molecular mass of iron (56.8 g/mol), z is the number of electron transferred (2) and F is Faraday’s constant (96,487 C/mol). According to the Turkish market in March 2016, prices for electrical energy were 0.095 D /kWh, and price for Fe electrode was 0.85 D /kg, respectively. Prices of chemicals used for adjustment of a desired pH were 0.73 D /kg for NaOH, 0.29 D /kg for H2 SO4 . The operating cost for the EC process was calculated with the following equation: OC = ˛ × Cenergy + ˇ × Celectrode +  × Cchemicals

2.5.

(3)

Toxicity tests

Toxicity tests were performed according to the International Standard Method (ISO) 21338 water quality-kinetic determination of the inhibitory effects of sediment, other solids and colored samples on the light emission of Vibrio fischeri (kinetic luminescent bacteria test) (ISO, 2010). Luminescence was measured using a high performance Sirius luminometer and light output was recorded automatically by FB12 software (Berthold detection systems, Germany). Prior to measurement, freezedried V. fischeri were re-hydrated in reagent diluent (2% NaCl) at 4 ◦ C for at least 30 min and then stabilized at 15 ◦ C for approximately 1 h in a dry cooling block. NaCl content and pH of samples were adjusted to 2% and 7.0 ± 0.2. The samples were subsequently diluted with 2% NaCl solution to obtain a dilution series (1:2, 1:4, 1:8, 1:16, 1:32 and 1:64). Toxicity measurements were performed by initially placing 300 ␮L diluted sample into luminometer cuvettes (Sarstedt 55.476) and incubating at 15 ◦ C for 10 min. Following introduction into the Sirius luminometer, 300 ␮L bacterial suspensions were automatically injected into the sample, and bioluminescence was measured, and then repeated after 30 min so that the relationship between end point toxicity and peak toxicity could be elucidated. A correction factor was applied based on the response obtained from the non-toxic reference sample (2% NaCl). The inhibition percentage, EC50 (the concentration that causes 50% reduction of bacteria relative to control) and EC20 values were calculated according to the ISO standard method (11348-3, 1998).

3.

Results and discussion

3.1. Treatment of alkaline non-cyanide zinc rinse wastewater Initial pH (pHi ) on the treatment process influences the surface charge of organic and inorganic pollutants from the wastewater. The species from different pollutants can be formed as a result of this and pHi can modify the species of coagulant and stability of various hydroxide species (Heidmann and Calmano, 2008; Kobya et al., 2010a,b). In this study, the effect of pHi (5–8) on the zinc removal from the alkaline noncyanide rinse wastewater was investigated at a current density of 80 A/m2 and an operating time of 60 min. Fig. 2 shows the initial and residual zinc concentrations as a function of operating time at different pHi values. Zinc concentrations reduced from 345.1 to 5.55 mg/L (98.4%) at a pHi of 5, from 329.5 to 2.38 mg/L (99.3%) at a pHi of 6, from 215.7 to 0.40 mg/L (99.8%)

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200

Initial pHi

(a)

300

5.0 6.0 7.0 8.0

250 200 150 100 50 0

j (A/m) 20 40 60 80 2

(b) Residual zinc concentration (mg/L)

Residual zinc concentration (mg/L)

350

150

100

50

0

0

5 10 15 20 25 30 35 40 45 50 55 60 65

0

10

20

30

40

50

60

70

Operating time (tEC, min)

Operating time (tEC , min)

Fig. 2 – Effects of (a) initial pHi and (b) current density on the treatment of non-cyanide Zn electroplating rinse wastewater. at a pHi of 7, and from 195.5 to 0.26 mg/L (99.9%) at a pHi of 8.0 (Fig. 2(a)). The effect of operating time can be explained by the Faraday’s law (Eq. (2)), the increasing current and operation time caused an increase in the amount of dissolved coagulant from the anode. The amount of electro-generated iron species increased with the increase in the operating time. It also resulted in increase in the amount of flocs which was made up of insoluble monomeric species, Fe(OH)3 + and polymeric hydroxyl complexes namely, FeOH2+ , Fe(OH)2 , 4+ − + 2+ 3+ Fe2 (OH)2 , Fe(OH)4 , Fe(H2 O)2 , Fe(H2 O)6 , Fe(H2 O)5 (OH) , + 4+ 2+ Fe(H2 O)4 (OH)2 , Fe2 (H2 O)8 (OH)2 and Fe2 (H2 O)6 (OH)4 depending on redox conditions and pH of the aqueous medium. The species of metallic iron depending on the final pH (pHf ) of EC process in turn precipitate as Fe(OH)2 , a variety of Fe(II/III)(oxy)(hydro)oxides and Fe(OH)3 (Kobya et al., 2010a,b). These iron (oxy)(hydro)oxides have strong affinity for dispersed particles as well as counter ions to cause coagulation. Contaminants (Ox) in the solution are removed via adsorption and co-precipitation by the HFO species produced in the EC process (Eqs. (4) and (5)) (Noubactep and Schoner, 2010): FeOOH(s) + Ox(aq) → [FeOOH − Ox] (3x−y)

nFex (OH)y

+ Ox(aq) →



(4)

(adsorption) (3x−y)

Fex (OH)y

n · Ox

 (s)

(5)

(co-precipitation) The precipitation reactions could promote both coprecipitation of zinc ions with iron and the sorption of Zn2+ onto iron-corroded surfaces which contribute to the removal of metal ions from the solution. The operating time at a current density proceeded sufficient formation of iron(oxy) hydroxides (HFOs, also known as amorphous iron oxyhydroxide, amorphous ferric hydroxide, or ferrihydrite) and zinc ions are adsorbed on and these species are co-precipitated. HFO is formed by rapid hydrolysis of Fe3+ solution. Amorphous iron oxide will transform into more stable oxide forms such as goethite (˛-FeOOH), lepidocrocite (-FeOOH), or hematite (˛-Fe2 O3 ). The removal efficiency remained almost the same after the operating time of 60 min since the insoluble monomeric and polymeric iron oxyhydroxides sufficiently adsorbed almost all Zn2+ in the bulk solution, or precipitate as insoluble zinc hydroxide. The residual zinc concentrations in the solution

were expected to decrease (i.e., low pH does not favor hydroxides and hydroxyl ions formation and consequently inhibits the EC procedure). In addition, the dominant zinc species could affect the removal efficiencies at various initial pHs in Fig. 3 (Reichle et al., 1975). Thus, final pH (pHf ) values after the EC process were measured as 6.2 for a pHi of 5, 6.9 for a pH of 6, 8.8 for a pHi of 7, and 9.2 for a pHi of 8. The pH of the treated samples increased when the pHi was low due to the excess of hydroxyl ions produced at the cathode (Eq. (6)). In the alkaline medium (pH > 8), the final pH did not change markedly because the generated hydroxyl ions at the cathode were consumed by Fe3+ ions generated at the anode forming Fe(OH)3(s) flocs. Moreover, Taqvi et al. (2007) reported that the dominant species were Zn2+ (∼90%), Zn(OH)+ (∼5%) and Zn(OH)2 (∼5%) at a pH of 8; Zn(OH)2 (∼78%), Zn(OH)+ (∼9%) and Zn2+ (∼13%) at a pH of 9; Zn(OH)2 (∼93%) and Zn(OH)− 3 (∼5%) at a pH of 10. Furthermore, hydroxyl ions could also partially combine with Zn2+ ions to form the insoluble Zn(OH)2(s) at a pH of 9.2. Therefore, the positive zinc ions at a pH of 99.9% and 1.565 kWh/m3 , respectively. The current efficiency (CE, %) or the Faradic yield is defined as the ratio of the experimental or actual electrode consumption (Cexp ) to the theoretical value (Ctheory ). It is also an

140 120 100 80 60

2

j (A/m ) 20 40 60 80

120 100 80 60 40 20 0 0

10

40

20

30

40

50

60

70

Operating time (tEC, min)

20 0 0

10

20

30

40

50

60

70

Operating time (tEC, min) Fig. 4 – Effect of (a) pHi and (b) current density on treatment of the alkaline cyanide zinc electroplating rinse wastewater.

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Process Safety and Environmental Protection 1 0 5 ( 2 0 1 7 ) 373–385

Table 1 – Results for the treatment of alkaline non-cyanide zinc plating rinse wastewater. Parameter

i (A)

U (V)

tEC (min)

Cenergy (kWh/m3 )

Celectrode (kg/m3 )

Wsludge (kg/m3 )

pHf (−)

OC (D /m3 )

pHi = 5.0 6.0 7.0 8.0 j = 20 A/m2 40 60 80

1.656 1.656 1.656 1.656 0.414 0.828 1.242 1.656

1.64 1.95 2.20 2.41 1.11 1.54 2.20 2.41

60 60 40 20 60 60 40 20

3.195 3.799 2.857 1.565 0.521 1.501 2.143 1.565

2.010 2.010 1.332 0.666 0.520 1.031 1.030 0.666

2.258 2.783 1.981 1.563 2.125 2.343 1.971 1.563

6.21 6.91 7.82 8.19 7.56 8.12 7.62 8.19

2.06 2.11 1.44 0.74 0.49 1.02 1.08 0.74

important parameter for the EC process because it affects the lifetime of the electrodes. The theoretical mass of Fe (Ctheory ) was obtained by using the Faraday equation and the experimental mass (Cexp ) was obtained by the electrode mass difference before and after the experiment. However, the actual electrode consumption may be reduced or increased from this theoretical value depending upon the wastewater characteristics and operational condition due to the electrochemical side and chemical reactions in the solution. CE was calculated as 99.8% for a pHi of 5, 99.6% for a pHi of 6, 100.1% for a pHi of 7 and 100.2% for a pHi of 8. Ctheory for a pHi of 8 was calculated as 0.666 kg/m3 at 1.656 A and 20 min. The experimental electrode consumptions at different pHi for the wastewater were calculated as 2.010 kg/m3 for a pHi of 5, 2.010 kg/m3 for a pHi of 6 at 60 min, and 1.332 kg/m3 for a pHi of 7 (tEC = 40 min), respectively (Table 1). Charge loading (q) is defined as the charges transferred in the electrochemical reactions for a given amount of water treated. It is calculated with the following equation (Kobya et al., 2016): q (C/L) =

i × tEC V

or q (F/m3 ) =

i × tEC F×V

(6)

where q is the charge loading (C/L or F/m3 ). Iron dosages were determined by charge loading in the EC process. Theoretically, whenever 1 Faraday of charge passes through the circuit, 28 g of iron is dissolved in the EC process. When the charge loading was low, the iron dosages were not sufficient to remove all zinc ions from the wastewater, and thus the zinc removal efficiency was not high. The zinc removal efficiency increased with increase in both charge loading and current density. However, the high charge loading resulted in high energy and electrode consumptions, a prime concern of the operating cost. Therefore, the charge loading was needed to be optimized. The minimum charge dosage required for the zinc removal efficiency of >98.5% was >1753.4 C/L at 20 A/m2 (0.414 A and 60 min), 3506.8 C/L or 26.26 F/m3 at 40 A/m2 (0.828 A and 60 min), 3506.8 C/L or 26.26 F/m3 at 60 A/m2 (1.242 A and 40 min), and 2337.9 C/L or 17.51 F/m3 at 80 A/m2 (1.656 A and 20 min). On the other hand, the zinc removal capacity was calculated as the zinc removed per charge loading (Coulomb) or mg Fe (electrochemically dissolved iron concentration) at different current densities. The removal capacity was 0.055 mg Fe/C (0.0971 mg Fe/C L) or 7.36 g/Faraday at a current density of 80 A/m2 and an operating time of 20 min. The zinc removal efficiency and effluent zinc concentration were determined as 98.7% and 2.64 mg/L. Moreover, zinc removal efficiency of 98.6% was obtained as 0.055 mg/C (0.065 mg Fe/C L) or 7.34 g/Faraday at 60 A/m2 and 40 min. According to the results, the calculated energy consumptions for the treatment of the wastewater increased with

the increase in the current densities as expected. The operating costs were calculated as 0.49 D /m3 for 20 A/m2 , 1.02 D /m3 for 40 A/m2 , 1.08 D /m3 for 60 A/m2 and 0.74 D /m3 for 80 A/m2 . The minimum operating costs for removal efficiencies of zinc over 98.5% and 99.8% were 0.74 D /m3 at 80 A/m2 , 20 min, 17.51 Faraday/m3 or 2337.9 C/L and 1.353 D /m3 at 60 A/m2 , 50 min, 26.26 Faraday/m3 or 3506.8 C/L. The amounts of sludge produced in the EC process at 20–80 A/m2 varied from 2.125 to 1.563 kg/m3 . The optimum conditions for the wastewater were a pHi of 8, 80 A/m2 and 20 min. Effluent zinc concentration, removal efficiency and operating cost for the treated wastewater at the optimum operating conditions were 2.64 mg/L, 98.6% and 0.74 D /m3 , respectively. The amounts of the sludge per treated wastewater in m3 with respect to pHi after the removal were 2.258 kg/m3 for a pHi of 5, 2.783 kg/m3 for a pHi of 6, 1.981 kg/m3 for a pHi of 7, and 1.563 kg/m3 for a pHi of 8 (Table 1). The operating cost of the treatment process is an important criterion to evaluate its applicability on an industrial scale. Operating costs were calculated as 2.06–0.74 D /m3 at a pH range of 5–8. The results indicated that the OCs increased along with energy and electrode consumptions in the EC process.

3.2. Treatment of the alkaline cyanide zinc electroplating wastewater The residual concentrations after the EC process in the wastewater at pHi of 8.0, 8.5, 9.0, 9.5 and 60 A/m2 were determined as 1.60 (99.1%), 0.92 (99.5%), 0.35 (99.8%) and 0.25 (99.9%) mg/L for zinc and 0.90 (99.3%), 0.30 (99.8%), 0.20 (99.9%) and 0.12 (99.9%) mg/L for cyanide, respectively (Fig. 4(a)). The results were met with the discharge standards for zinc and cyanide and the corresponding removal efficiencies were over 99.5% and 99.8%, at an operating time of 60 min and a pH of >8, respectively. Cyanide and zinc removals from the alkaline cyanide zinc rinse wastewater during the EC process were depended on produced iron oxy(hydroxyl) species and pHi . As the pHi increased from 8.0 to 9.5, then the residual concentrations decreased significantly. Values of final pHs after the EC process increased to 8.6, 8.9, 9.4, and 9.6, respectively. There are two metal sources; Zn from wastewater and iron ions produced during the EC, which can react with cyanide to form cyanide complexes, when Mez+ and cyanide are mainly 2−n in the solution as Me(CN)n (n = 0–4) in the pH range of 5–10. Zinc-cyanide interaction at low pH occurs according to the following equation (where n varies from 2 to 4): −(n−2)

Zn2+ + n CN− → Zn(CN)n

(7)

The main mechanism involved in the removal of zinc and cyanide from the wastewater might be Fe oxidation into fer-

Process Safety and Environmental Protection 1 0 5 ( 2 0 1 7 ) 373–385

Fig. 5 – Speciation of Zn(II) in the Zn(II)/CN− /OH− system as a function of pH (cyanide = 5 × 10−4 M). rous ions (Eq. (8)) and concurrent water electrolysis on the surface of anode, resulting in the generation of oxygen (Eq. (9)); (b) oxidation of ferrous ions into ferric ones through reaction with oxygen molecules and subsequent formation of iron hydroxide/polyhydroxide/polyhydroxyoxide precipitates (Eqs. (10) and (11)) depending on the solution pH; and (c) interaction of zinc and cyanide ions with iron precipitates (Eq. (12)). In summary, the surface complexation of zinc and cyanide ions with iron precipitates formed might be the predominant mechanisms of zinc and cyanide removals in the EC (Moussavi et al., 2011). Fes → Fe2+ + 2e− (at the anode) (aq)

(8)

H2 O + 2e− → 2H4 + 1⁄2O2 + 2e−

(9)

2Fe2+ + 3⁄2O2 + 3H2 O → 2Fe(OH)3(s) (aq)

(6 < pH < 10)

nFe(OH)3 → Fen (OH)3n

(10) (11)

CN− + Fen (OH)3n floc → CN − Fe (aq)

precipitate complex (12)

The affinity of cyanide for metals in the solution resulted in the formation of different Zn(II)-cyanide complexes (Fig. 5). Yngard et al. (2007) indicated that the predominant species in Zn-CN-OH− system were Zn(CN)2− 4 , − 2− Zn(CN)− , Zn(CN) OH , and Zn(CN) OH at pH 9.0–11. On 2 3 3 the other hand, monomeric and/or polymeric iron hydroxides led to significant improvement in zinc and cyanide removals mainly due to co-precipitation and adsorption like Fe(OH)3(s) , Fe(OH)(2−z) (O)z (Zn)(s) , and Fe(OH)2(s) ∗ Zn(CN)2−n n  (s)

(Latkowska and Figa, 2007). The Zn(II)-cyanide complexes via co-precipitation and/or adsorption mechanism were presented in Eqs. (13) and (14):



Fe(OH)2(s) + Zn(CN)2−n → Fe(OH)2 ∗ Zn(CN)2−n n n



→ Fe(OH)3 ∗ Zn(CN)2−n Fe(OH)3(s) + Zn(CN)2−n n n

 (s)

 (s)

(13) (14)

The theoretical electrode consumption at pHi 8.0–9.5, 60 min and 1.242 A was calculated as 1.548 kg/m3 . The practical electrode consumptions at pHi of 8.0, 8.5, 9.0 and 9.5 were 1.554, 1.602, 1.628 and 1.658 kg/m3 , respectively. In this case, the current efficiencies at pHi 8.0, 8.5, 9.0, and 9.5 were 100.4%, 104.2%, 107.1% and 105.8%, respectively. In addition, the cur-

379

rent efficiency at higher pHi values was found to be higher than that of lower pHi . This mass over consumption may be due to the chemical hydrolysis of the cathode, but it can also be explained by the “corrosion pitting” phenomenon which caused holes and led practically to a metallic metal loss on the electrode surface. These findings were reported earlier (Kobya et al., 2011). The amounts of produced sludge after the EC process were 2.118 kg/m3 at a pHi of 8.0, 2.281 kg/m3 at a pHi of 8.5, 2.673 kg/m3 at a pHi of 9.0, and 2.845 kg/m3 at a pHi of 9.5. Voltages in the EC reactor at pHi of 8.0, 8.5, 9.0, and 9.5 were measured as 1.27, 1.75, 1.96 and 2.23 V, respectively. In this case, the energy consumptions were calculated as 1.856 kWh/m3 at a pHi of 8.0, 2.557 kWh/m3 at a pHi of 8.5, 2.864 kWh/m3 at a pHi of 9.0, and 3.258 kWh/m3 at a pHi of 9.5. Operating costs at pHi 8.0, 8.5, 9.0, and 9.5 were calculated as 1.497, 1.605, 1.656, and 1.719 D /m3 , respectively. On the other hand, iron dosage and adsorption capacity at an operating time of 60 min were 1.380 g/L and 125.6 mg Zn/g (114.3 mg CN/g) at a pHi of 8.0, 1.551 g/L and 112.3 mg Zn/g Fe (102.2 mg CN/g Fe) at a pHi of 8.5, 1.586 g/L and 110.2 mg Zn/g Fe (100.1 mg CN/g Fe) at a pHi of 9.0, and 1.660 g/L and 105.4 mg Zn/g Fe (95.7 mg CN/g Fe) at a pHi of 9.5. The minimum charge loading pHi of 8.0–9.5 according to the discharge standards of the cyanide–zinc plating wastewater was 5260 C/L or 39.39 F/m3 at 60 min and 60 A/m2 . The effect of current density at 20–80 A/m2 and initial pHi 9.5 on treatment of the alkaline cyanide Zn electroplating wastewater is shown in Fig. 4(b). The residual zinc and cyanide concentrations after 60 min were 4.80 mg/L (97.3%) and 2.20 mg/L (98.4%) at 20 A/m2 , 0.85 mg/L (99.5%) and 1.30 mg/L (99%) at 40 A/m2 , 0.25 mg/L (99.9%) and 0.10 mg/L (99.9%) at 60 A/m2 and 0.16 mg/L (99.9%) mg/L and 0.10 mg/L (99.9%) at 80 A/m2 . Cyanide and zinc removal efficiencies increased with the increase in the current density because more iron ions passed into the wastewater at a higher current density and the formation rate of iron hydroxides increased. The effluent pHs and conductivity values in the EC were determined to be 9.6 and 5.0 mS/cm at 20 A/m2 , 9.6 and 5.2 mS/cm at 40 A/m2 , 9.7 and 5.4 mS/cm at 60 A/m2 , and 9.8 and 5.6 mS/cm at 80 A/m2 . The highest cyanide and zinc removal efficiencies were obtained at 80 A/m2 because the residual Zn concentration decreased more with the increase in operating time and current density. The amounts of produced sludge after the EC process were 1.013 kg/m3 at 20 A/m2 , 1.932 kg/m3 at 40 A/m2 , 2.845 kg/m3 at 60 A/m2 , and 2.135 kg/m3 at 80 A/m2 . Values of the other experimental parameters at 20, 40, 60 and 80 A/m2 were 0.477 kWh/m3 (0.414 A and 0.98 V), 1.393 kWh/m3 (0.828 A and 1.43 V), 3.258 kWh/m3 (1.242 A and 2.23 V), and 4.714 kWh/m3 (1.656 A and 2.42 V) for energy, 0.524 kg/m3 , 1.075 kg/m3 , 1.658 kg/m3 , and 2.185 kg/m3 for electrode consumptions, 101.4%, 104.2%, 107.1%, and 105.9% for current efficiencies and 0.49, 1.05, 1.72 and 2.31 D /m3 for operating costs. On the other hand, iron dosages at 20, 40, 60, and 80 A/m2 in the EC process were 0.503, 1.085, 1.523 and 2.135 g Fe/L, respectively. The adsorption capacities for cyanide and zinc removals from the cyanide zinc plating rinse wastewater were calculated as 310.4 mg CN/g (397.8 mg Zn/g) at 20 A/m2 , 145.1 mg CN/g (188.8 mg Zn/g) at 40 A/m2 , 104.2 mg CN/g (135.1 mg Zn/g) at 60 A/m2 , and 74.4 mg CN/g (96.34 mg Zn/g) at 80 A/m2 . Cyanide and zinc removals per Coulomb (C) or Faraday’s (F) were obtained as 0.208 mg CN/C and 0.267 mg Zn/C (23638 mg CN/F and 30295 mg Zn/F) at 20 A/m2 , 0.049 mg CN/C and 0.063 mg Zn/C (5521 mg CN/F and 7191 mg Zn/F)

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1600

(a)

1400 1200

(b)

900

5 6 7 8

Residual zinc concentration (mg/L)

Residual zinc concentration (mg/L)

1000

Initial pHi

1000 800 600 400 200 0

2

j (A/m) 20 40 60 80

800 700 600 500 400 300 200 100 0

0

10

20

30

40

50

60

70

Operating time (tEC, min)

0

10

20

30

40

50

60

70

Operating time (t EC, min)

Fig. 6 – Effect of (a) pHi and (b) current density on the treatment of acidic zinc electroplating rinse wastewater. at 40 A/m2 , 0.0233 mg CN/C and 0.0302 mg Zn/C (2646 mg CN/F and 3428 mg Zn/F) at 60 A/m2 , and 0.0125 mg CN/C and 0.0162 mg Zn/C (1415 mg CN/F and 1834 mg Zn/F) at 80 A/m2 . The required Faraday’s per m3 treated wastewater at 20, 40, 60, 80 A/m2 and an operating time of 60 min were 13.13, 26,26, 39.39, and 52.52 F/m3 , respectively. The optimum conditions for the treatment of the cyanide zinc rinse wastewater in the EC process were an operating time of 60 min, a pHi of 9.5 (original pH of the wastewater), and a current density of 60 A/m2 .

3.3.

Treatment of acidic zinc rinse wastewater

Fig. 6(a) shows the residual zinc concentrations at 0–60 min, pHi of 5–8 and 80 A/m2 for the acidic zinc rinse wastewater. The residual zinc concentrations at 60 min reduced from 1476.8 to 495.2 mg/L Zn (66.47%) for a pHi of 5, from 1266.3 to 329.8 mg/L (73.96%) for a pHi of 6, from 1012.1 to 1.6 mg/L (99.84%) for a pHi of 7, and from 985 to 0.80 mg/L (99.92%) for a pHi of 8. The residual zinc concentrations decreased with the increase in pHi values. Higher zinc removal efficiency was achieved at higher current density and operating time in the EC process (Fig. 6). Over 99.5% of Zn removal efficiency was obtained at a pHi of 7. Values of conductivity (initial value was 20.4 mS/cm) and final pHs after the EC process were 13.6 mS/cm and 6.3 at a pHi of 5, 13.2 mS/cm and 6.9 at a pHi of 6, 11.8 mS/cm and 7.8 at a pHi of 7, 10.4 mS/cm and 8.8 at a pHi of 8. Experimentally dissolved iron dosages at pHi of 5–8 and 60 min in the EC process were 1.998, 2.023, 2.124 and 2.116 g Fe/L, respectively. In this case, the amounts of adsorption capacity were 491.3 mg/g or 18691 mg/Faraday (0.141 mg/C) at a pHi of 5, 463.1 mg/g or 17832 mg/Faraday (0.134 mg/C) at a pHi of 6, 475.8 mg/g or 19241 mg/Faraday (0.144 mg/C) at a pHi of 7, and 465.1 mg/g or 18740 mg/Faraday (0.14 mg/C) at a pHi of 8. Energy and electrode consumptions were calculated as 4.384 kWh/m3 and 1.998 kg/m3 at a pHi of 5, 4.773 kWh/m3 and 2.023 kg/m3 at a pHi of 6, 4.988 kWh/m3 and 2.124 kg/m3 at a pHi of 7, and 5.112 kWh/m3 and 2.116 kg/m3 at a pHi of 8, respectively. The cell voltage values at pHi of 5–8 and an applied current of 1.656 A (80 A/m2 ) were measured as 2.250, 2.450, 2.560 and 2.624 V, respectively. According to the Faraday law, the theoretical electrode consumption at pHi of 5–8 was 2.0644 kg/m3 (1.656 A and 60 min). In this case, the current efficiencies for pHi of 5–8 were calculated as 96.78%, 97.99%, 102.89% and 102.50%, respectively. The amounts of sludge after the EC process were 2.152 kg/m3 for a pHi of 5, 2.690 kg/m3

for a pHi of 6, 3.122 kg/m3 for a pHi of 7, and 3.922 kg/m3 for a pHi of 8. Operating costs at pHi of 5–8 were 2.120, 2.170, 2.279, and 2.284 D /m3 , respectively. The effect of current density on the treatment of the acidic zinc electroplating wastewater at a pHi of 8 is shown in Fig. 6(b). Initial acidic zinc concentration in the wastewater was 985 mg/L. The residual zinc concentrations after 60 min were 98.5 mg/L (90%) at 20 A/m2 , 65.1 mg/L (93.4%) at 40 A/m2 , 40.8 mg/L (95.9%) at 60 A/m2 and 0.8 mg/L (99.9%) at 80 A/m2 . The highest removal efficiency was obtained at 80 A/m2 and an operating time of 60 min because the residual zinc concentration decreased with the increase in the operating time and current density after the EC process. Experimentally dissolved iron dosages for 20, 40, 60, and 80 A/m2 at 60 min were 0.501, 1.036, 1.429 and 2.116 g Fe/L, respectively. The amounts of adsorption capacity were calculated as 1769.5 mg/g or 67519 mg/Faraday (0.595 mg/C) for 20 A/m2 , 887.9 mg/g or 35031 mg/Faraday (0.309 mg/C) for 40 A/m2 , 660.8 mg/g or 23972 mg/Faraday (0.211 mg/C) for 60 A/m2 , and 465.1 mg/g or 18740 mg/Faraday (0.14 mg/C) for 80 A/m2 . The current efficiencies changed from 97.07% at 20 A/m2 to 102.50% at 80 A/m2 . The average cell voltages for 20, 40, 60, and 80 A/m2 were determined as 1.124, 1.427, 1.681, and 2.624 V, respectively. The values of conductivity, operating costs and amounts of sludge were 13.2 mS/cm, 0.49 D /m3 and 2.144 kg/m3 at 20 A/m2 , 12.6 mS/cm, 1.04 D /m3 and 2.825 kg/m3 at 40 A/m2 , 11.7 mS/cm, 1.56 D /m3 and 3.351 kg/m3 at 60 A/m2 , 10.4 mS/cm, 2.26 D /m3 and 3.922 kg/m3 at 80 A/m2 . As seen in the results, the optimum operating conditions for treatment of the acidic zinc electroplating wastewater were a pHi of 8, 80 A/m2 and 60 min. The zinc removal efficiency and effluent zinc concentration after the EC process were 99.9% and 0.80 mg/L.

3.4. The effect of EC on the toxicity levels of wastewater The toxicity levels of wastewater were measured by the kinetic luminescent bacteria test. The light emission was measured and recorded from the moment of dispensing of the bacterial suspension to the sample until the maximum value was reached and after a contact time of 30 min. The inhibition by a sample was expressed as the effective concentrations (EC20 and EC50 ) which resulted in 20% and 50% of light reductions compared to the control sample. The ratios of inhibitory effect and effective concentration values were calculated by means

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(b)

(a)

(c)

Fig. 7 – The inhibition ratios of (a) gamma values, (b) raw wastewater and (c) treated wastewater at different dilution ratios. of dilution series (Fig. 7). The gamma values ( , ratio of light lost) were calculated using Eq. (15) for each dilution level of the test sample after 30 min to evaluate the relationship between concentration and inhibitory effect by using a linear regression technique

30 =

Ht (100 − Ht )

(15)

where Ht is the inhibitory effect of a sample after a contact time of 30 min. EC50 for raw acidic, alkaline cyanide and alkaline non-cyanide were calculated as 0.62, 5.25 and 3.38 by means of the gamma values, respectively. Results for EC50 showed that all the raw wastewaters were hardly toxic. EC50 value of cyanide was reported as 1.91 mg/L (Marugán et al., 2012) which was lower than alkaline cyanide zinc and alkaline non-cyanide zinc electroplating wastewaters. EC50 value for a metal plating wastewater by Choi and Meier (2001) was measured as 0.7 mg/L. These observations were consistent to other studies in the literature. Moreover, negative inhibition values were measured for the treated wastewater. This phenomenon is called as induction. There are two possible ways for induction. It happens when there are nutrients for the bacteria that induce the light output by giving better conditions for living and, second the con-

centration of the toxic compound is small and the bacteria are using it as nutrient (mainly small molecular organic compounds). If the induction was >30%, it means that there were better conditions for the bacteria than the control sample. These results showed that the EC process presented a vigorous alternative for the treatment of toxic wastewater.

3.5.

Analyses of sludge

Scanning electron microscope (SEM) was used to analyze the sludge generated under the selected conditions to determine the surface characterization in this study. The SEM image of the EC sludge was composed of agglomerated fine powder-like particles at the micron scale and a heterogeneous morphology (Fig. 8(a–c)). Energy-dispersive analysis of X-rays (EDAX) was used to analyze the elemental constituents of the sludge shown in Fig. 8(d). The EDAX analysis provided that the sludge was composed mainly 27% of zinc, 46% of iron, 16% of oxygen, 3% of potassium and 8% of chloride. It also confirmed that Fe and Zn were precipitated as sludge (Fig. 8(d)). The infrared spectrum of the sludge also analyzed at wavenumbers between 4.000 and 650 cm−1 is depicted in Fig. 9(a) The strong peaks at 3435–3401 cm−1 were assigned to stretching vibration of hydroxyl groups indicating that hydroxide species were the prominent in the precipitates. A

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Fig. 8 – SEM image of sludge for (a) acidic Zn, (b) alkaline cyanide Zn, (c) alkaline non cyanide Zn electroplating rinse wastewaters and (d) EDAX surface analysis of the sludge. sharp band was observed at 2066 cm−1 related to the complex of CN–Fe precipitates. Therefore, the cyanide was removed through complexation/interaction with the iron hydroxide precipitates. The other peaks were assigned to stretching frequency of Zn–O at 1436–1269 cm−1 and Fe–O at 744 cm−1 . The sludge analysis indicated that the removals of Zn and cyanide

could be due to complexation with the iron hydroxide precipitates. XRD patterns of EC sludge obtained from alkaline non cyanide, alkaline cyanide and acidic zinc plating wastewaters indicated the presence of the solid product. Sludge produced at the optimum conditions showed peaks con-

Process Safety and Environmental Protection 1 0 5 ( 2 0 1 7 ) 373–385

383

Fig. 9 – (a) FTIR and (b) X-ray diffraction spectrum of the sludge generated in the EC process. firming the presences of KCl at 40.58◦ , 50.21◦ and 58.74◦ , ZnFe2 O4 and ZnO at 29.91◦ , 62.21◦ , 66.16◦ and 73.20◦ , FeOOH at 56.56◦ , Fe2 O3 at 35.22◦ (Fig. 9(b)). The peak observed at 28.38◦ was due to the presence of Fe(OH)3 . The surface areas of iron(oxy) hydroxides (HFO) such as FeOOH and Fe2 O3 were in the range of 159–7200 m2 /g (Lee and Anderson, 2005). It was clear that, the observed iron hydroxide had very good adsorption/co-precipitation properties for removing of metal ions. The presence of potassium and chloride in the sludge was also determined from the EDS spectra (Fig. 8).

3.6. Comparisons of Zn and cyanide removal efficiencies in the EC process Supplementary Table 2 presents the removal of zinc-cyanide, cyanide and zinc from industrial wastewaters by EC process. The main parameters chosen for the EC process by researches in the literature were initial pH, current density, operating time, electrolyte concentration, distance between electrodes,

initial cyanide and zinc concentrations, etc. There was a single work on the treatment of alkaline cyanide-zinc plating rinse wastewater by the EC process in the literature (Senturk, 2013). In this study, removal efficiencies of 99.97% for cyanide and 100% for zinc were obtained using Fe plate electrodes at the optimum conditions (an initial pH of 9.5, 30 A/m2 and 40 min). The removal efficiency and operating cost for the treatment of real alkaline cyanide zinc wastewater in this study were 99.8% for zinc, 99.9% for cyanide and 1.62 D /m3 at the optimum conditions (a pHi of 9.5, a current density of 60 A/m2 and an operating time of 60 min). 85% of cyanide removal from mining wastewater (Parga et al., 2012), 99.4% of Cd and 99.7% of CN removals from Cd–CN plating rinse wastewater with operating cost of 1.05 D /m3 (Kobya et al., 2010a,b) were achieved. Removals of metal plating wastewaters containing zinc in the EC process are listed in Supplementary Table 3. 99% of Zn (Al-Shannag et al., 2015) and 100% of Zn (Akbal and Camci, 2011) in metal plating wastewaters, 99% of Zn from galvanic wastewaters (Kobya et al., 2015; Espinoza-Quinones et al.,

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2012), 99.9% of Zn from electroplating wastewater (Bhagawan et al., 2014), 95.5 of Zn (Fe electrode) and 92.5% of Zn (stainless steel electrode) from battery building industrial wastewater (Mansoorian et al., 2014), 99.96% of Zn (batch EC) and 99.94% of Zn (continuous EC) from billet plant pickling wastewater (Petsriprasit et al., 2010), 96.7% of Zn (batch EC using Al electrode), 97.8% of Zn (batch EC using Al electrode), 99.8% of Zn (continuous EC using Al electrode) and 99.3% of Zn (continuous EC using Fe electrode) from Zn phosphating rinse wastewater (Kobya et al., 2010a,b) were removed in the EC process. The EC process was indeed very effective for the treatment of wastewater containing free CN and metals and water from the treatment could be used in the process.

3.7. Comparisons of the conventional treatment systems and the EC process The conventional treatment systems for alkaline cyanide zinc electroplating processes generally involve three steps; cyanide oxidation, precipitation and flocculation. On the other hand, treatment of acid/alkali zinc electroplating wastewater occurs through neutralization, precipitation and flocculation (Supplementary file). The cost of conventional treatment systems for the electroplating wastewater depends on the volume of wastewater and the concentration of pollutants in wastewater. The cost of conventional treatment for the alkaline cyanide, alkaline non-cyanide, and acidic zinc electroplating wastewater (Supplementary file) were 3.76 D /m3 , 0.78 D /m3 , and 2.35 D /m3 , respectively. When the cost of conventional treatment system was compared to cost from the EC (1.72 D /m3 for alkaline cyanide, 0.74 D /m3 for alkaline non-cyanide, and 2.26 D /m3 for acidic zinc electroplating wastewater), the EC system was clearly seen as more economical including the higher removal efficiencies.

4.

Conclusions

The residual zinc and cyanide concentrations were reduced with the increase in the current density, initial pH and operating time. The removal efficiencies, operating costs and amounts of sludge at the optimum operating conditions were determined as 99.8%, 1.62 D /m3 and 2.971 kg/m3 at pHi 7, 60 A/m2 and 60 min for alkaline non-cyanide zinc wastewater, 99.9%, 2.258 D /m3 , 2.845 kg/m3 at pHi 9.5, 60 A/m2 and 60 min for alkaline cyanide zinc wastewater, 99.9%, 2.258 D /m3 and 3.922 kg/m3 at pHi 8, 80 A/m2 and 60 min for the acidic zinc electroplating wastewater, respectively. The final effluent pH at the optimum operating conditions in the EC process fell into the limit range values set by the Turkish Water Pollution Control Regulation. Toxicity tests revealed that the treated wastewaters were non-toxic and act as a nutrient for bacteria. The results indicated that the EC was a very effective treatment process for the removals of zinc and cyanide ions from the alkaline non-cyanide, alkaline cyanide and acidic zinc electroplating wastewaters.

Acknowledgement The authors thank to Gebze Technical University for their financial support of this project under contract of BAP-2013A18.

Appendix A. Supplementary data Supplementary data associated with cle can be found, in the online http://dx.doi.org/10.1016/j.psep.2016.11.020.

this artiversion, at

References Adhoum, N., Monser, L., 2002. Removal of cyanide from aqueous solution using impregnated activated carbon. Chem. Eng. Process. 41, 17–21. Adhoum, N., Monser, L., Bellakhal, N., Belgaied, J.E., 2004. Treatment of electroplating wastewater containing Cu2+ , Zn2+ and Cr(VI) by electrocoagulation. J. Hazard. Mater. 112, 207–213. Akbal, F., Camci, S., 2011. Copper, chromium and nickel removal from metal plating wastewater by electrocoagulation. Desalination 269, 214–222. Al-Shannag, M., Al-Qodah, Z., Bani-Melhem, K., Qtaishat, M.R., Alkasrawi, M., 2015. Heavy metal ions removal from metal plating wastewater using electrocoagulation: kinetic study and process performance. Chem. Eng. J. 260, 749–756. APHA (American Public Health Association), 1998. Standard Methods for the Examination of Water and Wastewater, 18th ed, Washington DC. Arroyo, M.G., Perez-Herranz, V., Montanes, M.T., Garcia-Anton, J., Gunion, J.L., 2009. Effect of pH and chloride concentration on the removal of hexavalent chromium in a batch electrocoagulation reactor. J. Hazard. Mater. 169, 1127–1133. Bhagawan, D., Poodari, S., Pothuraju, T., Srinivasulu, D., Shankaraiah, G., Rani, M.Y., Himabindu, V., Vidyavathi, S., 2014. Effect of operational parameters on heavy metal removal by electrocoagulation. Environ. Sci. Pollut. Res. 21, 14166–14173. Blais, J.F., Djedidi, Z., Cheikh, R.B., Tyagi, R.D., Mercier, G., 2008. Metals precipitation from effluents: review. Pract. Period. Hazard. Toxic Radioact. Waste Manag. 12, 135–140. Bojic, A.L., Bojic, D., Andjelkovic, T., 2009. Removal of Cu2+ and Zn2+ from model wastewaters by spontaneous reduction-coagulation process in flow conditions. J. Hazard. Mater. 168, 813–819. Chen, G., 2004. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 38, 11–41. Choi, K., Meier, P.G., 2001. Toxicity evaluation of metal plating ® wastewater employing the Microtox assay: a comparison with cladocerans and fish. Environ. Toxicol. 16, 136–141. Dash, R.R., Gaur, A., Balomajumder, C., 2009. Cyanide in industrial wastewaters and its removal: a review on biotreatment. J. Hazard. Mater. 163, 1–11. Deliyanni, E.A., Peleka, E.N., Matis, K.A., 2007. Removal of zinc ion from water by sorption onto iron-based nanoadsorbent. J. Hazard. Mater. 141, 176–184. Dermentzis, K.K., Christoforidis, A., Valsamidou, E., 2011. Removal of nickel, copper, zinc and chromium from synthetic and industrial wastewater by electrocoagulation. Int. J. Environ. Sci. 1, 697–710. Dyer, J.A., Trivedi, P., Scrivner, N.C., Sparks, D.L., 2004. Surface complexation modeling of zinc sorption onto ferrihydrite. J. Colloid Interface Sci. 270, 56–65. EPA (United States Environmental Protection Agency), 1985. Reducing Water Pollution Control Costs in the Electroplating Industry, EPA/625/5-85/016, Washington DC. Escobar, C., Soto-Salazar, C., Toral, M.I., 2006. Optimization of the electrocoagulation process for the removal of copper, lead and cadmium in natural waters and simulated wastewater. J. Environ. Manag. 81, 384–391. Espinoza-Quinones, F.R., Modenes, A.N., Theodoro, P.S., Palacio, S.M., Trigueros, D.E.G., Borba, C.E., Abugderah, M.M., Kroumov, A.D., 2012. Optimization of the iron electro-coagulation process of Cr, Ni, Cu, and Zn galvanization by-products by

Process Safety and Environmental Protection 1 0 5 ( 2 0 1 7 ) 373–385

using response surface methodology. Sep. Sci. Technol. 47, 688–699. Fu, F., Wang, Q., 2011. Removal of heavy metal ions from wastewaters: a review. J. Environ. Manag. 92, 407–418. Hanay, O., Hasar, H., 2011. Effect of anions on removing Cu2+ , Mn2+ and Zn2+ in electrocoagulation process using aluminum electrodes. J. Hazard. Mater. 189, 572–576. Heidmann, I., Calmano, W., 2008. Removal of Zn(II), Cu(II) Ni(II), Ag(I) and Cr(VI) present in aqueous solutions by aluminium electrocoagulation. J. Hazard. Mater. 152, 934–941. Heidmann, I., Calmano, W., 2010. Removal of Ni, Cu and Cr from a galvanic wastewater in an electrocoagulation system with Feand Al-electrodes. Sep. Purif. Technol. 71, 308–314. Hosseini, S.S., Bringas, E., Tan, N.R., Ortiz, I., Ghahramani, M., Shahmirzadi, M.A.A., 2016. Recent progress in development of high performance polymeric membranes and materials for metal plating wastewater treatment: a review. J. Water Process Eng. 9, 78–110. Huang, C.H., Chen, L., Yang, C.L., 2009. Effect of anions on electrochemical coagulation for cadmium removal. Sep. Purif. Technol. 65, 137–146. International Standard Method 11348-3, 2010. Water quality — kinetic determination of the inhibitory effects of sediment, other solids and coloured samples on the light emission of Vibrio fischeri (kinetic luminescent bacteria test). Kabdasli, I., Arslan, T., Olmez-Hanci, T., Arslan-Alaton, I., Tunay, O., 2009. Complexing agent and heavy metal removals from metal plating effluent by electrocoagulation with stainless steel electrodes. J. Hazard. Mater. 165, 838–845. Kobya, M., Demirbas, E., Parlak, N.U., Yigit, S., 2010a. Treatment of cadmium and nickel electroplating rinse water by electrocoagulation. Environ. Technol. 31, 1471–1481. Kobya, M., Demirbas, E., Dedeli, A., Sensoy, M.T., 2010b. Treatment of rinse water from zinc phosphate coating by batch and continuous electrocoagulation processes. J. Hazard. Mater. 173, 326–334. Kobya, M., Ulu, F., Gebologlu, U., Demirbas, E., Oncel, M.S., 2011. Treatment of potable water containing low concentration of arsenic with electrocoagulation: different connection modes and Fe–Al electrodes. Sep. Purif. Technol. 77, 283–293. Kobya, M., Erdem, N., Demirbas, E., 2015. Treatment of Cr, Ni and Zn from galvanic rinsing wastewater by electrocoagulation process using iron electrodes. Desalin. Water Treat. 56, 1191–1201. Kobya, M., Demirbas, E., Ulu, F., 2016. Evaluation of operating parameters with respect to charge loading on the removal efficiency of arsenic from potable water by electrocoagulation. J. Environ. Chem. Eng. 4, 1484–1494. Latkowska, B., Figa, J., 2007. Cyanide removal from industrial wastewaters. J. Environ. Stud. 16, 748–752. Lee, S., Anderson, P.R., 2005. EXAFS study of Zn sorption mechanisms on hydrous ferric oxide over extended reaction time. J. Colloid Interface Sci. 286, 82–89. Mahmoud, A., Hoadley, A.F.A., 2012. An evaluation of a hybrid ion exchange electrodialysis process in the recovery of heavy metals from simulated dilute industrial wastewater. Water Res. 46, 3364–3376. Mahvi, A.H., Mansoorian, H.J., Rajabizadeh, A., 2009. Performance evaluation of electrocoagulation process for removal of sulphate from aqueous environments using plate aluminum electrodes. World Appl. Sci. J. 7, 1526–1533. Malakootian, M., Mansoorian, H.J., Moosazadeh, M., 2010. Performance evaluation of electrocoagulation process using iron-rod electrodes for removing hardness from drinking water. Desalination 255, 67–71. Mansoorian, H.J., Rajabizadeh, A., Bazrafshan, E., Mahvi, A.H., 2012. Practical assessment of electrocoagulation process in removing nickel metal from aqueous solutions using iron-rod electrodes. Desalin. Water Treat. 44, 29–35. Mansoorian, H.J., Mahvi, A.H., Jafari, A.J., 2014. Removal of lead and zinc from battery industry wastewater using electrocoagulation process: influence of direct and alternating

385

current by using iron and stainless steel rod electrodes. Sep. Purif. Technol. 135, 165–175. Marugán, J., Bru, D., Pablos, C., Catala, M., 2012. Comparative evaluation of acute toxicity by Vibrio fischeri and fern spore based bioassays in the follow-up of toxic chemicals degradation by photocatalysis. J. Hazard. Mater. 213, 117–122. Moussavi, G., Majidi, F., Farzadkia, M., 2011. The influence of operational parameters on elimination of cyanide from wastewater using the electrocoagulation process. Desalination 280, 127–133. Naik, Y.A., Venkatesha, T.V., 2002. Electrodeposition of zinc from chloride solution. Turk. J. Chem. 26, 725–733. Naik, Y.A., Venkatesha, T.V., 2003. A new brightener for zinc plating from non-cyanide alkaline bath. Indian J. Eng. Mater. Sci. 10, 318–323. Noubactep, C., Schoner, A., 2010. Metallic iron for environmental remediation: learning from electrocoagulation. J. Hazard. Mater. 175, 1075–1080. Nowack, B., Kari, F.G., Kruger, H.G., 2001. The remobılızatıon of metals from ıron oxıdes and sedıments by metal-edta complexes. Water Air Soil Pollut. 125, 243–257. Parga, J.R., Rodriguez, M., Vazquez, V., Valenzuela, J.L., Moreno, H., 2012. Recovery of silver and gold from cyanide solution by magnetic species formed in the electrocoagulation process. Miner. Process. Extr. Metall. Rev. 33, 363–373. Petrinic, I., Korenak, J., Povodnik, D., Helix-Nielsen, C., 2015. A feasibility study of ultrafiltration/reverse osmosis (UF/RO)-based wastewater treatment and reuse in the metal finishing industry. J. Clean. Product. 101, 292–300. Petsriprasit, C., Namboonmee, J., Hunsom, M., 2010. Application of the electrocoagulation technique for treating heavy metals containing wastewater from the pickling process of a billet plant. Korean J. Chem. Eng. 27, 854–861. Reichle, R.A., McCurdy, K.G., Hepler, L.G., 1975. Zinc hydroxide: solubility product and hydroxy-complex stability constants from 12.5-75 ◦ C. Can. J. Chem. 53, 3841–3845. Sancey, B., Trunfio, G., Charles, J., Minary, J.F., Gavoille, S., Badot, P.M., Crini, G., 2011. Heavy metal removal from industrial effluents by sorption on cross-linked starch: chemical study and impact on water toxicity. J. Environ. Manag. 92, 765–772. Schlesinger, M., Paunovic, M., 2010. Modern Electroplating, 5th edition. Wiley, New York. Senturk, E., 2013. The treatment of zinc-cyanide electroplating rinse water using an electrocoagulation process. Water Sci. Technol. 68, 2220–2227. Silva, R.M.P., Manso, J.P.H., Rodrigues, J.R.C., Lagoa, R.J.L., 2008. A comparative study of alginate beads and an ion-exchange resin for the removal of heavy metals from a metal plating effluent. J. Environ. Sci. Health A Tox. Hazard. Subst. Environ. Eng. 43, 1311–1317. Taqvi, S.I.H., Hasany, S.M., Bhanger, M.I., 2007. Zn(II) ions removal from aqueous solution by Karachi beach sand, a mixed crystal systems. Sep. Purif. Technol. 61, 153–160. Trivedi, P., Dyer, J.A., Sparks, D.L., Pandy, K., 2004. Mechanistic and thermodynamic interpretations of zinc sorption onto ferrihydrite. J. Colloid Interface Sci. 270, 77–85. Turkish Water Pollution Control Regulation (TWPCF). Official Gazette Numbered 19919 and dated 4 September 1988, Ankara, Turkey. Vasudevan, S., Oturan, M.A., 2014. Electrochemistry: as cause and cure in water pollution-an overview. Environ. Chem. Lett. 12, 97–108. Yngard, R., Damrongsiri, S., Osathaphan, K., Sharma, V.K., 2007. Ferrate(VI) oxidation of zinc-cyanide complex. Chemosphere 69, 729–735. Zhang, Y.L., Yang, J., Yu, X.J., Wang, Q.L., Huang, W., 2016. Adsorption technology and mechanism of Cu and CN− from cyanide waste water on modified peanut shell. Synth. React. Inorg. Metal-Org. Nano-Metal Chem. 46, 561–569. Zouboulis, A.I., Prochaska, C.A., Solozhenkin, P.M., 2005. Removal of zinc from dilute aqueous solutions by galvanochemical treatment. J. Chem. Technol. Biotechnol. 80, 553–564.