Ecological Engineering 123 (2018) 65–73
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Phosphorus removal from domestic wastewater in electrocoagulation reactor using aluminium and iron plate hybrid anodes
Philip Isaac Omwenea, , Mehmet Kobyaa, Orhan Taner Canb a b
Department of Environmental Engineering, Gebze Technical University, Gebze, Turkey Department of Environmental Engineering, Bursa Technical University, Bursa, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphorus removal Electrocoagulation Hybrid electrode Domestic wastewater
The aim of this study was to investigate the eﬀects of initial pH (pHi = 4.0–7.0), current density (j = 10–40 A/ m2), initial phosphorus (P) concentration (Ci of 5.01–52.13 mg/L) and electrocoagulation (EC) time (tEC = 10–100 min) on phosphorus removal from domestic wastewater by a batch EC reactor using hybrid aluminium (Al)-iron (Fe) anodes and titanium cathode. Phosphorus removal from domestic wastewater containing 52.13 mg/L PO4-P was obtained to be 99.99% at optimum conditions (pHi = 4, j = 20 A/m2 and EC time = 80 min). The energy and electrode consumptions at optimum conditions were calculated as 3.422 kWh/ m3 and 0.328 kg/m3, respectively. The amount of removed P per electrochemically dissolved total metal electrode (qe) was calculated as 55.69 mg P/g, while the dissolved metal to removed phosphorus ratio (Me/P, mol/ mol) was 4.40 (Fe/P = 2.63 and Al/P = 1.77) at optimum conditions. It can be concluded that phosphorus removal by hybrid Al-Fe anodes is as eﬀective as using only Fe or Al anodes as per the results present in literature. In addition, the eﬄuent pH after EC treatment process at optimum conditions was 8.8, hence requiring no pH adjustment before discharge.
et al., 2016; Karageorgiou et al., 2006), biological processes (Zhang et al., 2013; Oehmen et al., 2007), chemical precipitation by adding aluminium, iron or calcium salts (Park et al., 2016; Yang et al., 2010; Georgantas and Grigoropoulou, 2007; Fytianos et al., 1998), ion-exchange (Choi et al., 2011) and membrane processes (Ensano et al., 2016; Wei et al., 2009). Chemical precipitation and adsorption are currently the most eﬀective and well-established methods. For low concentrations, biological treatment can remove up to 97% of the total phosphorus at a low-cost, but the variability in chemical composition, high phosphorus concentration and temperature of wastewater would make the implementation of this process impracticable for wastewater treatment. Furthermore, many of the above methods are disadvantaged by long operation time, removal ineﬃciencies and are expensive. Therefore, electrocoagulation (EC) for phosphorous removal has been attempted as an alternative process (to especially chemical precipitation) to overcome these disadvantages (Wysocka and Sokolowska, 2016). Moreover, this technique has advantages such as simple equipment, easy to operate, less retention time, reduction or absence of adding chemicals, rapid sedimentation of the electro-generated ﬂocs, less sludge production and environmental compatibility, when compared to conventional methods (Can et al., 2003; Chen et al., 2000). However, in any EC process, the electrode type is regarded as a
The release of phosphorus from wastewaters into watercourses can cause severe pollution problems, such as eutrophication (Roy, 2017; Park et al., 2016; Ramasahayam et al., 2014). Problems associated with eutrophication include profuse algal blooms, excessive growth of nuisance aquatic plants, negative aesthetic aspects, deoxygenation, and problems relating to water puriﬁcation for potable use (Dunne et al., 2015; de Haas et al., 2000). According to the European Environment Agency, the highest phosphorus concentration in European rivers is 0.91 mg/L, and this value was reported in Turkey (EEA, 2015). The critical concentrations for incipient eutrophication are about 0.1–0.2 mg/L PO4-P in running water and 0.005–0.01 mg/L PO4-P in still water. In view of the potential hazard to surface waters, EU Directive 91/271/EEC speciﬁes limit values for discharge of phosphate compounds into receiving waters. Depending on the size of the sewage treatment plant, these values are 2 mg/L Ptot for 10,000–100,000 PE (population equivalents) or 1 mg/L Ptot for > 100,000 PE (EEC, 1998). The need for eﬃcient removal of P from wastewaters before discharge into watercourses will help reduce eutrophication of receiving surface water bodies. Various technologies for phosphate removal from wastewater have been investigated, these include; adsorption (Paradelo
Corresponding author at: Department of Environmental Engineering, Gebze Technical University, 41400 Gebze-Kocaeli, Turkey. E-mail address: [email protected]
https://doi.org/10.1016/j.ecoleng.2018.08.025 Received 16 July 2017; Received in revised form 3 August 2018; Accepted 31 August 2018 0925-8574/ © 2018 Elsevier B.V. All rights reserved.
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Fe3 + + 3H2 O→ Fe(OH)3(s) + 3H+
signiﬁcant factor aﬀecting the process performance. Aluminium and iron are the most common sacriﬁcial electrodes used in the EC process for phosphorus removal, and up to 51–100% removals have been reported in the literature (Nguyen et al., 2016; Inan and Alaydın, 2014; Attour et al., 2014; Bouamra et al., 2012; Gharibi et al., 2010; Irdemez et al., 2006a, 2006b; Vasudevan et al., 2008, 2009; Zheng et al., 2009). Aluminium plate anode has been well documented to demonstrate higher performance compared to iron plate anodes. This is ascribed the lower co-precipitation and adsorption capacity of hydrous ferric oxides for phosphate ions compared to hydrous aluminium oxide (Behbahani et al., 2011; Irdemez et al., 2006b; Bektas et al., 2004). Phosphorus removal studies by EC process have investigated the inﬂuence of the operational parameters such as initial pH, type of supporting electrolyte and its concentration, and current density. However, just as for the case of chemical coagulation, phosphate removal eﬃciency is greatly affected by the molar ratios of dissolved metal coagulant (Fe or Al) to the removed phosphorus during EC (Staﬀord et al., 2014). In addition, appropriate selection of the electrode pair (Al and Fe hybrid electrodes) at diﬀerent operating parameters (i.e., initial pH, current density, reaction time and initial phosphorus concentration) is important. Investigations on phosphorus removal using hybrid electrode in view of the present literature is still inadequate (Kuokkanen et al., 2015; Chen et al., 2014; Gao et al., 2012). Most of the existing studies focused on analyses of metal to phosphorous (Me/P) ratio of a single electrode. In the present study the Me/P ratio is provided as combination of Al and Fe hybrid electrodes and the results discussed in relation to those available in the literature. In this regard, we present phosphorous removal from synthetic domestic wastewater using hybrid Al and Fe plate electrodes in a batch EC reactor. To optimize the conditions, diﬀerent parameters like eﬀect of the initial pH, eﬀect of current density, and eﬀect of initial phosphate concentration were studied. We hope that the ﬁndings of this study shall be of interest to many researchers involved in the development of electrochemical treatment technologies for domestic wastewater.
The rate of the oxidation of Fe dissolved oxygen (Chen et al., 2000).
3Fe2 + + 2PO34− → Fe3 (PO4 )2(s)
Anode and cathodes reactions for Fe electrodes:
Anode: Fe2 +
Fe → Fe2 + + 2e− (E0 = 0.44 V) Fe3 +
Cathode:⋮2H2 O +
(E0=−0.771 V) 2e−
→ H2(g) +
= −0.828 V)
rFe3 + + PO34− + (3r−3)OH− → Fer PO4 (OH)3r − 3(s)
→ Alr PO4 (OH)3r − 3(s)
In general, two major mechanisms have been linked to the coagulation of PO34− with Al/Fe(III) salts; (i) Formation of Al/Fe-hydroxylphosphate complexes; Me (OH )3 − x (PO4 )x (s) . These complexes either adsorb onto positively charged Al/Fe(III) hydrolysis species or act as centres of precipitation for Al/Fe(III) hydrolysis products. (ii) Adsorption of PO34− ions on to the Al/Fe(III) hydrolysis species. Such hydroxides are partially transformed into hydroxyl-complexes depending on the pH of the solution, but these can remove phosphate by adsorption (Nguyen et al., 2016; Lacasa et al., 2011; Zhang et al., 2010; Szabo et al., 2008; Thistleton et al., 2002; Jiang and Graham, 1998). FePO4(s) (strengite), AlPO4(s) (variscite) and [Al6(OH15)]PO4(s) are the stable solid phases if phosphate is precipitated in the pH range of 5.0–7.0 (Ensano et al., 2016). Minimum solubility of AlPO4(s) occurs at pH 6.0 which is one unit higher than that of FePO4(s). The stoichiometric molar mass ratios (r) of Fe/P for FePO4(s) and Fe3(PO4)2(s) are 1.8/1 and 2.7/1, respectively. The solubility products of some precipitant phosphate compounds such as AlPO4(s), Al1.4PO4(OH)1.2(s), FePO4(s) (for r = 1), Fe3(PO4)2(s), and Fe2.5PO4(OH)4.5(s) (for r = 2.5) at the pH > 9 at 25 °C are 6.3 × 10−19, 1.3 × 10−22, 1.3 × 10-30, and 1 × 10−97, respectively (Zheng et al., 2009; Fytianos et al., 1998). Iron salt in either ferrous or ferric form is commonly used in phosphorus removal from municipal wastewater, and the inﬂuent with total
In the case of Al electrodes, Al3+ and ions generated by anode and cathode reactions (1) and (2) form various monomeric and polymeric species, which transform ﬁnally into Al(OH)3(s) according to complex precipitation kinetics. At 25 °C, the solubility product of Al(OH)3(s) is 5 × 10−33 (mol/L).
Al3 + + 3H2 O → Al(OH)3(s) + 3H+
− 3) Al3 + or Fe3 + + Hn PO(n → FePO4(s) or AlPO4(s) 4
r Al3 +
Cathode :3H2 O + 3e− → 3/2H2(g) + 3OH− (E0=−0.828 V)
Al3+, Fe3+ and Fe2+ ions also react with hydroxide to form amorphous aluminium and iron hydroxide ﬂocs. Consequently, the removal mechanism of pollutants from wastewaters with both electrodes is related to forming of Fe(OH)3(s), Al(OH)3(s), monomeric and polymeric iron and aluminium species due to coagulation, precipitation, coprecipitation, and electro-oxidation (Chen et al., 2000). Furthermore, freshly formed amorphous Al(OH)3(s) and Fe(OH)3(s) “sweep ﬂocs” in the EC process have large surface areas which are beneﬁcial for a rapid adsorption of soluble organic compounds such as phosphate ions and trapping of colloidal particles. Phosphate ion has strong aﬃnity for metal ions like Al3+ and Fe3+, Oxides of Al and Fe in aqueous medium will have surface OH groups. Phosphate ions in solution undergo ligand exchange with the OH−. Adsorption of phosphate ions on aluminium and iron oxide surfaces leads to inner surface complexation. When H2O molecules are present between the adsorbed phosphate ions and oxide surface, it’s referred as outer surface complexation. The electrolysis of the electrode produces not only precipitates such as ferrous, ferric, and aluminium phosphate or hydroxyl-phosphate: Al1.4PO4(OH)1.2(s), Fe1.6H2PO4(OH)3.8(s), and Fe2.5PO4(OH)4.5(s), as shown in Equations (9), (10), and (11), but also hydroxides such as Fe(OH)2(s), Fe(OH)3(s), FeOOH(s) and Al(OH)3(s) (Zheng et al., 2009; Karageorgiou et al., 2006; Fytianos et al., 1998). It was found that when iron and aluminium are present in the water, FePO4(s) and AlPO4(s) forms at low pH range (< 6.5) and at higher pH range (> 6.5) iron and aluminium increasingly convert to oxides and hydroxides. FePO4(s) has minimum solubility within pH range of 4.5–5.5, but solubility of the FePO4(s) increases with increasing pH (Nguyen et al., 2016; Zhang et al., 2010; Irdemez et al., 2006a). Moreover, the optimum pH for phosphate precipitation with ferric iron is the range of pH 4.0 to 5.0, while that for ferrous iron is close to pH 8.0.
EC involves the generation of coagulants in situ by electrochemically dissolving either aluminium or iron ions from the respective electrodes. The metal ions generation takes place at the anode, whereas hydrogen gas is simultaneously released from the cathode. The generated hydrogen gas also helps in ﬂoatation of the ﬂocculated particles out of the water (Moussa et al., 2017; Can et al., 2003). Anode and cathodes reactions for Al electrodes:
Al → Al3 + + 3e− (E0=1.66 V)
depends on the availability of
O2(g) + 42 + + 2H2 O→ 4Fe3 + + 4OH−
2. Mechanism of phosphate removal by EC
Ferric ions (Fe ) generated by electrochemical oxidation of Fe electrodes used in EC process may form monomeric and polymeric species depending on the pH of the aqueous medium, and which transform ﬁnally into Fe(OH)3(s). At 25 °C, the solubility product of Fe (OH)3(s) is 4 × 10-38 (mol/L). 66
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Fig. 1. Eﬀect of current density on phosphorus removal; (a) Phosphorous removal eﬃciency at diﬀerent current densities, (b) Variation pf phosphorus adsorption capacity (mg P/g Me) with time.
3.3. Analytical methods
phosphorus concentrations of 10 mg/L are consistently reduced to efﬂuent concentrations of 0.03–0.04 mg/L at Fe/P (both ferric and ferrous) molar ratio of 2.0 (Zhang et al. 2015).
Wastewater samples collected from the reactor at speciﬁed intervals of time were ﬁltered by 0.45 µm cellulose acetate membrane ﬁlter before analysis. The analysis of phosphate was carried out using the Vanadomolybdophosphoric Acid Method by a single beam spectrophotometer (PerkinElmer Lambda 35) according to the Standard Methods for Examination of Water and Wastewater (APHA, 1998). The pH and conductivity of samples before and after the EC process was measured by a pH and a conductivity meter (Hach Lange HQ40). The experiments were repeated three times and the average data was reported. All chemicals used in the EC experiments were of analytical grade.
3. Material and methods 3.1. Material Typical composition of untreated domestic wastewater is pH of 7.0–8.5 (∼7.7), conductivity of 0.403–1.284 mS/cm (∼0.85 mS/cm), chemical oxygen demand (COD) of 250–800 mg/L (∼430 mg/L), total phosphorous of 4–12 mg/L (∼7 mg/L), chloride of 30–90 mg/L (∼50 mg/L), and sulphate of 20–50 mg/L (∼30 mg/L) (Tchobanoglous et al., 2004). According to these, synthetic domestic wastewater for this study was prepared from KH2PO4 with distilled water to obtain required concentration (5–52 mg/L PO4-P). Also, typical chloride, sulphate and alkalinity concentrations found in domestic wastewater were obtained by addition of NaCl, Na2SO4 and Na2CO3 of compounds. The characteristics of synthetic domestic wastewater used in this study are; pH of 7.3, conductivity of 810 µS/cm, COD of 520 mg/L, chloride of ∼38.2 mg/L, sulphate of ∼33.2 mg/L, and bicarbonate alkalinity of ∼148.5 mg/L CaCO3.
4. Results and discussion 4.1. Eﬀect of current density on phosphorus removal Current density (j = i/Aelectrode, A/m2) is important parameter for controlling the reaction rate in the EC process. Operating current density is critical in EC process as it is the only operational parameter that can be controlled directly (Irdemez et al., 2006b; Chen et al., 2000). Operating current determines the coagulant dosage, bubble production rate, ﬂoc size and growth; these inﬂuence the treatment eﬃciency of the EC (Vasudevan et al., 2009; Zheng et al., 2009). The eﬀect of current intensity on the orthophosphate removal at 10, 20, 30 and 40 A/m2 were studied at pHi = 4.0 and Ci = 52.13 mg/L. Fig. 1(a) illustrates the eﬀect of the applied current on the residual phosphate concentration as a function of EC time. Phosphorus removal eﬃciencies for 10, 20, 30 and 40 A/m2 were 96.96%, 99.99%, 99.99%, and 99.99%, respectively. As seen in the Fig. 1(a), the required EC time to reach > 99% phosphorus removal by EC process were determined to be 100 min at 10 A/ m2, 80 min at 20 and 30 A/m2, and 50 min at 40 A/m2. On the other hand, the phosphate removal depended on the amount of coagulant generated in the EC process since the applied charge and EC time was directly proportional to the amount of coagulant generated. Electrochemical oxidation of sacriﬁcial Fe and Al anodes produces hydrous or hydrated ferric and aluminium oxides (ferric and aluminium precipitates) which are highly insoluble solids and oﬀer strong adsorption for phosphate ions. The amount of P removed for current
3.2. Experimental setup and procedure The electrocoagulation unit consisted of a rectangular Plexiglas reactor of 0.50 L capacity, operated in a batch mode. The cathode was of two titanium plate electrodes while the anodes were of aluminium and iron plate electrodes. All the electrodes were of equal dimension (4.4 cm × 4.7 cm) with eﬀective area of the electrode pair as 62 cm2. The electrodes were connected in a monopolar parallel mode to a digital DC power supply (Agilent 6675A mode, 0–120 V and 0–20 A). All experimental runs were conducted with 0.35 L of wastewater and stirred at 300 rpm by a magnetic stirrer (Heidolp MR 3000D). During the experiment, wastewater samples were withdrawn from the reactor at speciﬁed time intervals and ﬁltered by 0.45 µm microspore membrane ﬁlter before analyses. Furthermore, to reduce the eﬀect of electrode passivation the electrodes were rinsed in diluted HCl solution before starting each experiment, dried and re-weighed. 67
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Fig. 2. (a) Variation in Me/P ratio with time and (b) Variation of phosphorous removal eﬃciency and Me/P ratio mole ratio at diﬀerent current densities.
densities of 10, 20, 30 and 40 A/m2 and Ci of 52.13 mg/L were 50.54 mg at 100 min, 52.129 mg at 100 min, 52.129 mg at 80 min, and 52.13 mg at 60 min, respectively, all current densities apart from 10 A/ m2 had over 99.99% removal eﬃciency. Amount of experimentally dissolved total electrode from hybrid anodes were 0.1673 g at 10 A/m2, 0.3276 g at 20 A/m2, 0.3819 g at 30 A/m2 and 0.3867 g at 40 A/m2. Experimentally obtained amount of total dissolved electrode (Me,t) from Eq. (13), were higher than those anticipated theoretically. This could be due to high current eﬃciencies, since experimental current eﬃciencies were 107%, 105%, 102%, and 104% for 10, 20, 30 and 40 A/m2.
Me,t = me,Al + me,Fe
It has been noted in chemical and electrocoagulation studies that phosphate removal eﬃciency is generally aﬀected by iron or aluminium to phosphorus molar ratios (Fe/P or Al/P) and initial phosphate concentration (Staﬀord et al., 2014). Me/P ratios and phosphate removal eﬃciencies according to EC time for diﬀerent current densities in this study are given in Fig. 2(a and b). Me/P ratio for current densities of 10, 20, 30 and 40 A/m2 was obtained as 2.32, 4.40, 5.14 and 5.18, respectively, for > 99% phosphorus removal eﬃciency. This trend could be explained by the higher dissolution rate of the anode at high applied current. Chemical coagulation results in literature indicated an increase of P removal ratio with increase in Fe/P and initial phosphate concentration (Huang et al., 2017; Zhang et al., 2010; Szabo et al., 2008; Georgantas and Grigoropoulou, 2007; Thistleton et al., 2002; Caravelli et al., 2001; De Haas et al., 2000). In a study by Szabo et al., 2008, optimum metal to phosphorus ratio in the precipitate using diﬀerent coagulants such as aluminium sulphate, poly-Al-chloride and Fe-sulphate varied from < 1–10 at initial phosphorus concentration of 0.5–12.5 mg/L and initial pH of 3–10. They also noted the Me/P ratio as Fe/P = 2.8 and Al/ P = 3.2 at pH = 4.5–7.5 and Ci = 3.6–4.0 mg/L and Cf = 0.10 mg/L. Fytianos et al., (1998) obtained the optimum phosphate removal ratio as Fe/P = 1/1 at a pH of 4.5. When Fe3+ ions are added at Fe/P molar ratio of 1:1, ferric phosphate (FePO4(s)) precipitation may be formed, whereas at Fe/P molar ratio of > 1/1, Fe2.5PO4(OH)4.5(s) precipitates are formed. Caravelli et al. (2001) obtained PO4-P removals higher than 97% at pH above 6.2 using Fe/P = 1.9. Thistleton et al. (2002) determined the required Fe/P molar ratios for 80% (initial phosphate concentration of 5 mg/L) total phosphorus removal as 1.48 for
where Me,t is total amount of electrochemically dissolved aluminium (me,Al) and iron (me,Fe). The phosphorus removal capacities (qe) for 10, 20, 30, and 40 A/m2 at optimum EC times were calculated as 105.70, 55.69, 47.77, and 47.18 mg/g, respectively (Fig. 2b).
(Ci−Cf ) × V me , t
where Ci and Cf are the initial and eﬄuent P concentrations (mg/L) of EC process, V is solution volume in the EC reactor. As seen in Fig. 1(b), higher EC time led to more PO4-P removal eﬃciencies. However, the phosphorus removal capacity decreased signiﬁcantly because of low residual P concentrations and increase of electrochemically generated coagulant dosage with increase of charge loading (q = it tEC). This behaviour was likely due to a decrease in adsorption capacity of iron and aluminium oxy(hydroxides) at low PO4-P concentrations (Table 1).
Table 1 Obtained EC results at current desities of 10, 20, 30 and 40 A/m2 at EC time of 100 min. j (A/m2)
10 20 30 40
1.59 0.001 0.001 0.001
0.044 0.000 0.000 0.000
96.95 99.99 99.99 99.99
100 100 80 60
1.074 3.422 6.064 6.101
0.1673 0.328 0.382 0.387
1.41 2.63 3.07 3.13
0.91 1.77 2.07 2.05
2.32 4.4 5.14 5.18
105.7 55.69 47.77 47.18
Cf is reported as the mean of three experimental trials. Std is the standard deviation. 68
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4.2. Eﬀect of initial pH on phosphorus removal
FeCl3, > 4.8 for Fe(OH)3(s), and 1.86 for FeCl3/Fe(OH)3(s). Narasiah et al. (1991) found that a ratio of Fe/P = 2.9 was needed to achieve 90% removal of phosphorus from municipal wastewater. Similarly, Huang et al. (2017) reported that Fe(III)/P and Al/P molar ratios by chemical coagulation were higher than 1.3, whereas Fe(II)/P was > 1.6. Furthermore, the PO4-P in the wastewater was almost completely removed at experimental conditions of pHi 6.5 and EC time = 30 min for Fe(II), pHi = 4.5 and EC time = 30 min for Fe(III), pHi = 5.0 and EC time = 30 min for Al(III). They also noted molar ratio of Fe/P in the ﬁltered sludge anaerobic supernatant (Ci = 148 ± 6.6 mg/L) during the electrolysis with Fe electrode to be much higher than that during EC process using Al electrode (Fe/P = 0.1–4.5 and Al/P = 0.1–2.5 at j = 1.25–6.25 A/m2). Generally, an increase in current density favors anodic oxidation, which in turn enhances formation of adequate amorphous metal hydroxides species in the vicinity of the electrodes as well as in the bulk solution. Current density increases the eﬃciency of ion production on the anode and cathode according to Faraday’s law (Eq. (15)). Energy (Cen) and electrode (Cel) consumptions are calculated from the following equations:
i × tEC × Mw z×F×v
U × i × tEC v
The initial pH (pHi) of the electrolyte is one of the important factor aﬀecting performance of electrochemical process particularly EC (Kuokkanen et al., 2015; Irdemez et al., 2006a). The eﬀect of initial pHi on the removal of phosphate was explored within the pH range of 4–7 at current density of 20 A/m2 (i = 0.136 A) and an operating time of 0–100 min. Fig. 4(a) shows the removal eﬃciency of phosphate as a function of the EC time at diﬀerent pHi. Phosphate removal was observed to increase with EC time, but decreased with increasing initial pHi. In this case, the treated eﬄuent phosphorus concentrations of 0.001 mg/L (∼100%), 0.50 mg/L (99.04%), 0.55 mg/L (98.95%), and 3.83 mg/L (92.65%) were obtained for pHi of 4, 5, 6 and 7, respectively at EC time of 100 min. Also, pHf of treated water after EC process was measured as 8.9, 9.15, 9.25 and 9.55 for corresponding pHi of 4, 5, 6 and 7. The increase of pH was associated with the formation of hydrogen gas and hydroxyl ions produced by the anode and cathode reactions according to Eqs. (1)–(6) in EC process. On the other hand, according to solution chemistry of the orthophosphate ion, phosphate species are dominant as H2 PO4− and HPO42 − between pH 2 and 12. Under very acidic conditions H3PO4 is more prevalent whereas PO3− is dominant in extremely basic conditions. The concentration of H2 PO4− is higher for pH below 7 whilst HPO42 − species prevail between pH 7 and 10 (Georgantas and Grigoropoulou, 2007; Karageorgiou et al., 2006; Fytianos et al., 1998). Both Al and Fe phosphate or hydroxy-phosphate precipitate in solution according to Eqs. (9)–(12), and Fe(OH)3(s)/Al(OH)3(s) are also precipitated according to Eqs. (3) and (7). Al/Fe phosphate or hydroxylhydroxy-phosphate forms at pH ≤ 6.5 and Fe/Al hydroxides form at pH ≥ 6.5 (Irdemez et al., 2006a). Therefore, at low EC times (0–20 min) and initial pHi < 6.5 (i.e 4, 5 and 6), phosphate from domestic wastewater precipitated as Al/Fe phosphates, then as EC time increased (> 20 min), and at pH > 6.5 precipitation occurred as Al-Fe hydroxides. In this case, the main mechanism for phosphate removal was considered as co-precipitation at low EC times and pH and adsorption at high EC times and pH. The Me/P for pHi of 4, 5, 6 and 7 were calculated as 4.43 (Fe/P = 2.63 and Al/P = 1.80), 4.40 (Fe/P = 2.68 and Al/P = 1.72), 4.37 (Fe/P = 2.61 and Al/P = 1.57), 4.48 (Fe/ P = 2.60 and Al/P = 1.88), respectively. The electrode and energy consumptions were 3.422 kWh/m3 and 0.937 kg/m3 at pHi = 4,
where Cen (kWh/m3) and Cel (kg/m3) are energy and electrode consumptions, i is current (A), U is cell voltage (V), tEC is the operating time (s or hour), v is the volume m3 of wastewater, Mw is molecular mass of metal electrode (MFe = 55.85 g/mol and MAl = 27 g/mol), z is number of electrons transferred (zFe = 2 and zAl = 3) and F is Faraday’s constant (96487C/mol). To obtain eﬄuent P concentration of 0.10 mg/L, energy consumptions at 10, 20, 30 and 40 A/m2 were calculated as 1.074 kWh/m3 (i = 0.0678 A and U = 3.325 V), 3.422 kWh/m3 (i = 0.1356 A and U = 530 V), 6.064 kWh/m3 (i = 0.2034 A, U = 7.83 V, and 80 min), and 6.101 kWh/m3 (i = 0.2712 A and U = 7.87 V), respectively, (Fig. 3a). The electrode consumptions at optimum conditions were calculated as 0.1673 kg/m3 for 10 A/m2, 0.328 kg/m3 for 20 A/m2, 0.382 kg/m3 for 30 A/m2, and 0.387 kg/m3 for 40 A/m2 (Fig. 3b).
Fig. 3. (a) Energy consumptions (kWh/m3) at current densities of 10, 20, 30 and 40 A/m2, (b) Electrode consumption (kg/m3) at 10, 20, 30 and 40 A/m2. 69
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Fig. 4. . (a) Eﬀect of initial pH on phosphorous removal from domestic waste water: (b) Energy consumption (kWh/m3) at diﬀerent initial pH values. Table 2 Results of phosphorous removal by EC at diﬀerent at initial pH values. pHi (–)
1 Cf (mg/L)
4 5 6 7
0.001 0.50 0.55 3.83
0.00 0.02 0.03 0.18
99.99 99.04 98.94 92.65
100 100 100 100
3.422 2.967 4.076 3.051
0.328 0.327 0.321 0.3053
2.63 2.68 2.61 2.6
1.77 1.72 1.76 1.88
4.4 4.4 4.37 4.48
55.69 55.27 56.17 55.38
Cf is reported as the mean of three experimental trials. Std is the standard deviation.
3.422 kWh/m3 and 0.936 kg/m3 at pHi = 5, 4.076 kWh/m3 and 0.918 kg/m3 at pHi = 6, and 3.051 kWh/m3 and 0.872 kg/m3 at pHi = 7. Current eﬃciency values were determined to be 106%, 103%, 104% and 103% for pHi of 4, 5, 6 and 7 respectively. Noteworthy is that the Me/P for the pHi of 4–7 remained fairly constant. This agrees with the results obtained by Zhang et al., (2010) on phosphate removal using ferric chloride at pH range of 4.0–7.0 for initial phosphate concentration of 20 mg/L, in which the Fe/P molar ratio showed slight variation of 1.5–2.0. However, the higher Me/P reported in this study could be ascribed to excess dissolution of metal electrode as a result of higher applied current (Table 2).
of the process resulted in a slower reaction rate. For all initial phosphate concentrations, phosphate adsorption capacity was initially high followed by a gradual decrease (Fig. 6). The P removal capacity for Ci = 5, 10, 25 and 52.13 mg/L were calculated as 87.55, 53.66, 44.06 and 55.69 mg/g, respectively. The respective Me/P mole ratios were obtained as 2.78 (Fe/P = 1.70 and Al/P = 1.08), 4.56 (Fe/P = 2.75 and Al/P = 1.81), 5.57 (Fe/P = 3.33 and Al/P = 2.24), and 4.40 (Fe/P = 2.63 and Al/P = 1.77) (Table 3). Thus, phosphorus removal capacity was observed to decrease with increase in initial phosphorus concentration. Metallic hydroxide ﬂocs easily adsorbed phosphate at higher phosphate concentrations, but their adsorption capacity was limited. Hence, adsorption capacity at initial phosphate concentration of > 10 mg/L did not change signiﬁcantly. To obtain eﬄuent P concentration of < 0.10 mg/L, electrode and energy consumptions were 1.165 kWh/m3 and 0.020 kg/m3 for Ci = 5 mg/L, 2.406 kWh/m3 and 0.065 kg/m3 for Ci = 10 mg/L, 3.076 kWh/m3 and 0.199 kg/m3 for Ci = 25 mg/L, and 3.422 kWh/m3 and 0.328 kg/m3 for Ci = 52.13 mg/L (Table 3). Nevertheless, the electrode and energy consumptions increased with increase of Ci due to the longer requirement in EC time.
4.3. Eﬀect of initial phosphorus concentration on phosphorus removal Eﬀect of initial P concentration (Ci) was studied at 5, 10, 25, and 52.13 mg/L, current density of 20 A/m2, pHi of 4 and EC time of 0–100 min. Variation of the phosphorus removal eﬃciency at diﬀerent initial P concentrations are shown in Fig. 5(a). As can be seen from Fig. 5(a) and Table 3, 99.78% P removal was obtained at 6 min for Ci = 5 mg/L, 99.99% at 20 min for Ci = 10 mg/L, 99.99% at 60 min for Ci = 25 mg/L, and 99.99% at 100 min for Ci = 52.13 mg/L. The phosphorus removal eﬃciency increased with increase in EC time especially for Ci of 10 mg/L and above. At high initial phosphorus concentration, more treatment time was required to achieve > 99.99% removal. From these results, the phosphorus removal by EC process was observed to be greatly dependent on EC time and the initial phosphate concentration. This clearly suggested that the removal of phosphate ions depended on the generation of coagulant dosage from anode and the subsequent formation of aluminium and ferric hydroxides complexes. Moreover, the phosphate ions were more abundant at the beginning of the EC process; hence the rate of reaction was high while on the other hand, the reduced concentration of phosphate ions at the end
4.4. Comparison with other of P removal results by EC process using hybrid electrodes There are limited studies available in the literature about phosphorus removal from wastewater by the EC process using hybrid electrodes. In this section, these studies are summarized and compared with the present study. Phosphorus removal by batch laboratory-scale EC reactor from synthetic wastewater solutions containing 30 mg/L PO4-P was found as 96% at operating conditions of; anode/cathode = Al/Fe and Fe/Al, initial pHi = 5, EC time = 15 min, j = 100 A/m2, supporting electrolyte = 1 g/L NaCl, and electrode gap = 0.7 cm. Whereas, P 70
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Fig. 5. (a) Eﬀect of initial phosphorous concentrations on phosphorous removal from domestic waste water and (b) Variation of total dissolved metal to phosphorous ratio at initial diﬀerent phosphorous concentrations.
105.70, 55.69, 47.77, and 47.18 mg/g, respectively. As seen in results available in literature and the present study, the use of hybrid electrodes in the EC process is a suitable electrode combination for phosphorus removal from wastewater.
removal from real industrial mining (Ci = 4.6–5.1 mg/L P) and dairy (Ci = 130–140 mg/L) wastewaters were reported as 79% in 30 min and pHi = 7, and 93% in 60 min and pHi = 6, respectively. Energy consumption and operating costs at optimum conditions were reported respectively as, 0.75 kWh/m3 and 0.17 €/m3 for the synthetic wastewater, 2.11 kWh/m3and 0.28 €/m3 for the mining wastewater, and 1.46 kWh/m3 and 0.31 €/m3 for the dairy wastewater (Kuokkanen et al (2015). In another study, Chen et al. (2014) indicated that continuous EC processes with Al-Fe hybrid electrodes (2 anodes and 2 cathodes, monopolar connection mode) obtained higher phosphate removal rate and lower energy consumption than those when only Al or Fe electrodes were used. The phosphorus removal eﬃciencies and energy consumptions at EC time of 60 min (current density of 80 A/m2, initial pHi of 7, distance between electrodes of 1 cm and wastewater ﬂow rate of 0.05 L/min) were 99% and 0.246 kWh/m3, 87% and 0.237 kWh/m3, 61% and 0.203 kWh/m3, and 62% and 0.180 kWh/m3 for initial P concentration of 10, 60, 110, and 160 mg/L, respectively. Besides, at Ci = 60 mg/L P, the removal rates and energy consumptions for 40, 80, 120, and 160 A/m2 were reported as 65% and 0.075 kWh/m3, 87% and 0.22 kWh/m3, 96% and 0.46 kWh/m3, and 98% and 0.76 kWh/m3. At initial pHi of > 8, the P removal eﬃciency was reported to decrease, and energy consumption increased. In this study, optimum conditions to attain phosphorus removal eﬃciency of 99.99% from synthetic domestic wastewater using Al-Fe hybrid electrode were determined as j = 20 A/m2, EC time = 100 min, pHi = 4 and Ci = 52.13 mg/L. In this case, metal to phosphorus (Me/P) ratio, energy and electrode consumptions were also found to be 4.40 (Fe/P = 2.63 and Al/P = 1.77), 3.422 kWh/m3 and 0.328 kg/m3 at 80 min of EC time, respectively. Phosphorus removal capacities (mg removed P per g amount of electrochemical coagulant for 10, 20, 30 and 40 A/m2 were calculated as
5. Conclusion Experiments were carried out to analyze the eﬀect of diﬀerent operating parameters on phosphorus removal from synthetic domestic wastewater using Al-Fe hybrid electrodes. The analyzed parameters were current density, initial phosphate concentration and initial pHi. At initial PO4-P concentration of 52.13 mg/L, current density of 20 A/m2, EC time of 100 min and pHi = 4, P removal eﬃciency of > 99.99% was achieved. Metal to phosphorus (Me/P) ratio, energy and electrode consumptions at optimum conditions were also found to be 4.40 (Fe/ P = 2.63 and Al/P = 1.77), 3.422 kWh/m3 and 0.328 kg/m3 respectively, at 80 min of EC time. Treatment time to obtain phosphate removal eﬃciency above 99.99% for initial phosphorus concentrations between 5.0 and 52.13 mg/L was found to increase from 6 min to 80 min. This result indicated that phosphorus removal at constant current density was mainly dependent on EC time and initial phosphorus concentration. Furthermore, phosphorus removal eﬃciency of > 99.99% (or eﬄuent P concentration of 0.10 mg/L) was achieved within EC time of 20 mins, for initial phosphate concentration of < 10 mg/L at current density of 20 A/m2 and pHi of 4.0. Phosphorus removal capacities (mg removed PO4-P per g amount of metal) for all the tested current densities at pHi = 4 and Ci = 52.13 mg/L P were calculated as 105.70 mg/g at 10 A/m2 (> 100 min), 55.69 mg/g at 20 A/m2 (100 min), 47.77 mg/g at 30 A/m2 (80 min), and 47.18 mg/g at 40 A/m2 (60 min). Phosphorus removal by generated Fe2+, Fe3+ and Al3+ hydroxy-species in EC process using hybrid Al-Fe electrodes was
Table 3 Results of phosphorous removal by EC at diﬀerent initial phosphorous concentrations of 5, 10, 25 and 52 mg/L PO4-P. Ci (mg/L)
5 10 25 52
0.01 0.001 0.001 0.001
0.001 0.000 0.000 0.000
99.78 99.99 99.99 99.99
6 20 60 > 100
1.165 2.406 3.076 3.422
0.02 0.065 0.199 0.328
1.7 2.75 3.33 2.63
1.08 1.81 2.24 1.77
2.78 4.56 5.57 4.4
87.55 53.66 44.09 55.69
Cf is reported as the mean of three experimental trials. Std is the standard deviation. 71
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Fig. 6. For pHi of 4 and current density of 20 A/m2, (a) Phosphorus adsorption capacity (mg P/g Me) and (b) Energy consumption (kWh/m3) for diﬀerent initial phosphorous concentrations.
attributed to formation of Al/Fe-hydroxy-phosphorus and adsorption of phosphate ions on to the Al/Fe(III) hydrolysis species. This study showed that the results for phosphate removal by EC process using hybrid Al-Fe plate electrodes from domestic wastewater is as eﬀective as those obtained by EC process using only Fe or Al plate electrodes.
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