Removal of Boron from Produced Water by Electrocoagulation

Advances in Environment, Computational Chemistry and Bioscience

Removal of Boron from Produced Water by Electrocoagulation EZERIE HENRY EZECHI, MOHAMED HASNAIN ISA, SHAMSUL RAHMAN MOHAMED KUTTY Civil Engineering Department Universiti Teknologi PETRONAS Bandar Seri Iskandar, 31750 Tronoh, Perak Darul Ridzuan, MALAYSIA [email protected], [email protected], [email protected] Abstract: This study investigated the removal of boron from produced water by electrocoagulation using iron plate electrode. Different operating parameters were selected. pH 3-11, current density 6.25,12.5 and 18.75, treatment time 15-90 minutes, inter-electrode spacing 0.5, 1.0 and 1.5 cm. The study was first conducted with synthetic wastewater prepared with boric acid with varying concentrations of 10, 20 and 30 mg/l to obtain the optimal operating parameters. The results show an upward trend in removal efficiency as pH was increased from 3-8 and a downward trend from pH 9-11. The removal efficiency increased and treatment time reduced with increase in current density. Increase in inter-electrode spacing reduced removal efficiency. Increase in boron concentration also reduced removal efficiency. Removal efficiencies of 98%, 81% and 72.7% were obtained for initial boron concentration of 10 mg/l, 20 mg/l and 30 mg/l respectively at pH 7, current density 12.5 mA/cm2, treatment time 90 minutes and inter-electrode spacing 0.5cm. The obtained optimal parameters were applied to produced water with boron concentration of 15 mg/l and a removal efficiency of 96.7% was observed at current density of 12.5 mA/cm2, pH 7.84, treatment time of 90 minutes, and inter-electrode distance of 0.5cm. The kinetics of the adsorption process obeyed a second order kinetic law. Electrocoagulation using iron plate electrode can be used to remove boron from produced water effectively. Key-Words: - Boron, Electrocoagulation, Produced water, Adsorption kinetics World Health Organization (WHO) has set the maximum boron level in drinking water at 0.5 mg/l [1]. Many plants are sensitive to high boron concentrations. Walnut, plum and pear are sensitive to boron concentration above 1 mg/l, sunflower, potato and cotton are sensitive to boron concentration above 2 mg/l, and asparagus, datepalm and sugar beet are sensitive to boron concentration above 4 mg/l [4]. Several techniques have been applied for boron removal from wastewater; including membrane processes, ion exchange, electrodialysis, biological methods, adsorption, and coagulation-flocculation. Membrane processes, often the preferred treatment method for drinking water, have not been able to reduce boron to the permissible limit of 0.5 mg/l because boric acid can diffuse through the membranes in a non-ionic way; similar to that of carbonic acid or water at low pH. Membrane fouling also affects the efficiency of the process [5]. Ion exchange using selective ion exchange resin has shown good removal efficiency [6] but resin regeneration and sludge disposal is a major setback to the process. Electrodialysis has been reported to remove only about 42-75% of boron [7]. Biological

1 Introduction Boron is one of the minor elements dissolved in natural water and one of the seven essential micronutrients required for the normal growth of most plants [1]. Boron appears mostly as boric acid and borax in nature while it exists primarily as undissociated boric acid and borate ions in aquatic systems [2]. Boron is widely used in industrial applications such as the manufacture of glass, cosmetics, flame retardants, detergents, mild antiseptics, fertilizers and dyestuff production [3]. Boron is an essential micronutrient for plant development and plays important roles in the metabolism of plant carbohydrates, sugar translocation, hormonal action, functioning of the apical meristem, biological membrane structure and function [4]. However, boron is associated with certain health risk hazards. Studies conducted with rats, mice and rabbits show that boron presents impediments to male reproduction as well as show several developmental deformities. On the other hand, boron deficiency in plant may result in reduced growth, yield loss, and even death of plant depending on the severity of the deficiency [4]. The

ISBN: 978-1-61804-147-0

87


treatment is typically not suitable and removes little amount of boron from wastewater [8]. Adsorption is also unsuitable due to adsorbent loss during the process and the need for backwashing [9]. Produced water is water found in underground formations during oil and gas production. Produced water is the largest waste-stream of oil and gas exploration. Global produced water generation is estimated at about 250 million barrels per day compared with about 80 million barrels per day of oil [10]. Produced water has distinctive characteristics which differentiates it from other wastewaters [11]. Produced water is increasingly being considered as a means to supplement limited freshwater resources in many parts of the world especially in arid areas for irrigation purposes. Electrochemical treatment processes have received significant attention in wastewater purification, as a result of their effectiveness and ease of operation [12,13]. Electrocoagulation is solely based on the basic scientific principle of water particles responding to strong electric field in an oxidation/reduction process. Electrocoagulation generally involves three mechanisms viz., (i) formulation of coagulants by electrolytic oxidation of sacrificial electrodes, (ii) destabilization of the contaminants, and (iii) aggregation of the destabilized phases to form a floc [4,14]. Iron oxidation in an electrolytic system produces iron hydroxide, Fe(OH)n where n = 2 or 3. Two mechanisms for the production of metal hydroxide are shown below [15]:

Particles that interact with the insoluble metal hydroxides of iron are removed by surface complexation or electrostatic attraction [15]. The electric field enables electrophoretic migration of particles and increases the tendency of charges in suspension to interact with each other. The objective of this paper is to determine the efficiency of boron removal from produced water with electrocoagulation using iron plate electrode and study its adsorption kinetics.

2 Experimental 2.1 Source of wastewater and characteristics This study was conducted in two phases. Firstly, to obtain the optimal parameters, the wastewater was synthetically prepared with appropriate amount of boric acid (H3BO3) to yield boron concentrations of 10 mg/l, 20 mg/l and 30 mg/l. Secondly, produced water collected from a local crude oil terminal in Malaysia was treated with the optimal parameters obtained from the synthetic study. The characteristics of the produced water were analyzed using atomic adsorption spectrometry (AAS) and ion chromatography (IC). The characteristics of produced water include boron 15 mg/l, TSS 136 mg/l, pH 7.84, conductivity 30,000 µs/cm, turbidity 72 NTU, TDS 15,829 mg/l, iron 1.66 mg/l, chloride 7,546.5 mg/l, sodium 3,952 mg/l, calcium 357 mg/l, magnesium 600 mg/l, sulphate 168 mg/l, and potassium 284 mg/l.

2.2

Mechanism I

Anode:

Fe(s) → Fe2+(aq) + 2eFe 2+(aq) + 2OH-(aq) → Fe(OH)2(s)

(1) (2)

Cathode: 2H2O(I) + 2e- → 2OH-(aq) + H2(g)

(3)

Overall:

Fe(s) + 2H2O(I) → Fe(OH)2(s) + H2(g)

(4)

Mechanism II Anode: 4 Fe(s) → 4Fe2+(aq) + 8e-

(5)

Cathode: 8H+(aq) + 8e- →4H2(g)

(7)

Overall: 4Fe(s)+10H2O(I)+O2(g) → 4Fe(OH)3(s) + 4H2(g)

(8)

4 Fe2+(aq) + 10H2O(I) + O2(g) → 4Fe(OH)3(s) + 8H+(aq) (6)

ISBN: 978-1-61804-147-0

Electrochemical setup

The batch study was conducted with 500 ml beaker. Two groups of alternating electrodes, cathodes and anodes (three of each type), made of iron plates were arranged vertically. The net spacing between the iron electrodes was 0.5 cm. The electrodes were connected to a digital DC power supply characterized by the ranges of 0-3A for current and 0-30V for voltage. The size of the electrodes was 10 cm × 1 cm × 0.3 cm. A digital ammeter and voltmeter was used to measure the current and voltage respectively. A pH meter (Hach Sension2 pH meter) was used to measure pH of the samples. The conductivity of the samples was measured using a conductivity meter (Myron L conductivity meter) while the turbidity of the samples was measured using a turbidity meter (2100P portable turbidity meter). 1M NaOH and H2SO4 were used to control the pH of the samples. The treatment time for the samples was varied from 15-90 minutes. After each run of the experiment, the supernatant was collected and analyzed. After each experiment,

88


the iron electrodes were dipped in dilute HCl for 5 minutes. It was then washed with acetone and rinsed with deionized water to remove surface grease and impurities and dried at 105ºC for 10 minutes before being reused. Boron concentration after treatment was analyzed with carmine method using DR 2800 spectrophotometer. All chemicals and reagents used for this study were analytical grade (Merck).

100

Removal efficiency (%)

90 80 70 60 50 40 30 20 10 0 2

3 Results and Discussion

3

4

5

6

7 pH

8

9

10

11

12

Fig. 1 Effect of pH on boron removal, treatment time 90 minutes, concentration 10 mg/l, current density 12.5 mA/cm2, inter-electrode spacing 0.5cm

3.1 Effect of pH The pH of the medium has been reported to change during electrochemical process [16,17]. However this change depends on the type of electrode material and also the initial pH [18]. To investigate the effect of pH in the removal of boron, 0.2 ml (w/w 63%) HNO3- was added to the sample to maintain the pH [19]. Different pH values ranging from 3 to 11 were studied. After series of experiment with different boron concentration and current density, the results show that the removal efficiency increased when the pH was increased from 3 to 8 and decreased from pH 9 to 11. The removal efficiency of boron at pH 7 and 8 was 97% and 98% respectively after 90 minutes as shown in Fig. 1. The lowest removal efficiency of 41% was observed at pH 11. At low pH, boron exist in water as boric acid (H3BO3) and from pH 10, the borate anion [B(OH)4−] predominates. From about pH 11, highly water soluble polyborate ions such as B3O3(OH)4−, B4O5(OH)4− and B5O6(OH)4− are formed [2]. However, the solubility state of the released coagulants from the electrodes is controlled by the pH. From pH 4-9, iron electrode releases insoluble coagulants which interact with particles in suspension through surface complexation or electrostatic attraction. Above pH 9, highly soluble monomeric Fe(OH)4- anion concentration increases at the expense of Fe(OH)3 [20]. From this result, it can be concluded that the highest amount of coagulants was released between pH 7 and 8. Therefore, the optimal pH for this study was considered to be pH 7.

where W quantity of electrode material dissolved (g), I current (A), T time (s), M molar mass of electrode, Z number of electron, F Faraday’s constant (c/mol). Fig 2 shows a removal efficiency of 59,81 and 87% at 6.25, 12.5 and 18.75 mA/cm2, respectively, after 90 minutes treatment time with an initial boron concentration of 20 mg/l at pH 7. The water sample was initially green in colour, then changed to yellow and finally changed to turbid. The green colour suggests the formation of Fe2+ ions and the yellow colour is due to the formation of Fe3+. Fe2+ ions are formed at the dissolved electrodes and are easily oxidized to Fe3+ by dissolved oxygen in water. Considering the energy consumption between the two current densities of 12.5 mA/cm2 and 18.75 mA/cm2, it can be concluded that 12.5 mA/cm2 is more feasible for boron removal in this study.

3.2 Effect of current density

3.3 Effect of initial boron concentration

density is increased, removal efficiency also increases and treatment time reduces correspondingly. This can be explained since an increase in current density increases the amount of metal ions released at the sacrificial anode. The amount of metal ions released from the electrode can be calculated using the Faraday equation [22].

W =

(9)

The effect of initial boron concentration on its removal was investigated with solutions of boron concentrations 10, 20 and 30 mg/l. It was observed (figure not shown) that as initial boron concentration increases, removal efficiency decreases. The reason being, although the same amount of Fe3+ was released into the solution at the

Current density directly affects process performance and operating cost as well as determines the amount of ions released from the respective electrodes [21]. To investigate the effect of current density on boron removal, three different current densities of 6.25 mA/cm2, 12.5 mA/cm2 and 18.75 mA/cm2 were studied. The results show that when current

ISBN: 978-1-61804-147-0

ITM ZF

89



90

3.5 Produced water To investigate the removal efficiency of boron from produced water, the optimum parameters obtained from the synthetic study were kept constant while inter-electrode spacing was varied from 0.5 cm, 1.0 cm and 1.5 cm to investigate the effect of interelectrode spacing. The pH of the sample was used without any adjustment since it falls within the optimal pH obtained from the synthetic study. From Fig. 4, the results show a removal efficiency of 96.7% at 0.5cm inter-electrode spacing, 89.3% at 1.0 cm and 82.7% at 1.5 cm inter-electrode spacing. This can be explained that when the electrodes were kept close to each other, the electric field strength increased and coagulant generation was accelerated. In addition, the electric resistance is low when electrodes are close to each other which results in increased rate of electrode oxidation.

80

70

60

50 0

6.25

12.5 18.75 Current density (mA/cm2)

25

Fig. 2 Effect of current density on boron removal, treatment time 90 minutes, concentration 20 mg/l, pH 7, inter-electrode spacing 0.5cm same current density for all boron concentrations, these metal ions were insufficient for solutions of higher boron concentration. Therefore, when boron concentration was increased, removal efficiency was observed to decrease. At current density of 12.5 mA/cm2, boron removal efficiency of 97% was observed for initial boron concentration of 10 mg/l. When the boron concentration was increased to 20 mg/l, removal efficiency decreased to 81% and subsequently to 72.7% when initial boron concentration was increased to 30 mg/l.


100.0

96.0

92.0

88.0

84.0

3.4 Effect of treatment time The effect of treatment time was determined by conducting electrocoagulation from 15-90 min. Fig. 3 shows that when treatment time is increased,

80.0 0

0.5

1 Inter‐electrode spacing (cm)

1.5

2

Fig. 4 Effect of inter-electrode spacing on boron removal, pH 7.84, conc. 15 mg/l, treatment time 90 minutes, current density 12.5 mA/cm2.

removal efficiency also increases. It was observed that removal efficiency was rapid at the first 15 minutes and then followed a slower process. However, the removal efficiency at 75 minutes treatment time was 96% at 10 mg/l initial boron concentration which meets the minimum permissible limit outlined by WHO.

4.0 Process kinetics 4.1 Lagergren pseudo first order kinetics The linearized form of pseudo first order equation is represented as


100

90

80

70

Where qe and qt are the amount of boron adsorbed at equilibrium (mg/g) on Fe(OH)3 and at any time t respectively, k1 (min -1) is the calculated pseudo first order rate constant of adsorption . A plot of log (qe-qt) v. t should be linear; k1 and qe can be calculated from the slope and intercept respectively. Data fitted into the equation (figure not shown) was near linear with a good coefficient of determination but the calculated qe deviated from

60

50 0

15

30

45 60 75 Treatment time (Minutes)

90

105

Fig. 3 Effect of treatment time on boron removal, pH 7, Conc. 10 mg/l, current density 12.5 mA/cm2, inter-electrode spacing 0.5 cm2

ISBN: 978-1-61804-147-0

90


Electrocoagulation is effective; the materials are accessible and environmental friendly, and can be used to remove boron from wastewater with optimal parameter conditions.

the experimental qe. This implies that the adsorption did not follow the pseudo first order kinetics very well. The results of the plot of log (qe-qt) v. t are shown in Table 1.

4.2 Pseudo second order kinetics

12

The linearized form of the pseudo second order equation is represented as:

10

t/qt (mg/g)

10 mg/l 20 mg/l 30 mg/l

8 6 4 2 0

Where qe and qt are the amount of boron adsorbed at equilibrium (mg/g) on Fe(OH)3 and at any time t respectively, k2 is the calculated pseudo second order rate constant of adsorption. Data fitted into the equation was found to be linear in all the boron concentration studied. The equilibrium adsorption capacity (qe) and the second order rate constant k2 was calculated from the slope and intercept of the of t/qt v. t plot. The correlation coefficient was high and the equilibrium adsorption capacity (qe) corresponded well with the experimental qe. Therefore it is obvious that the adsorption of boron onto Fe(OH)3 followed a second order rate law as shown in Fig 5. The results of the t/qt v. t plot are compiled in Table 1. These findings suggest that the rate-limiting step of the adsorption process may be chemisorption.

0

15

30

45 Time (Minutes)

60

75

Fig. 5 Second order kinetic plot of different concentrations. Conditions: pH 7, current density 12.5 mA/cm2, inter-electrode spacing 0.5cm, temperature 308K.

Acknowledgment The financial assistance as graduate assistantship from University Teknologi PETRONAS (UTP) to the first is acknowledged. References [1] H. Liu, X. Ye, Q. Li, T. Kim, B. Qing, M. Guo, "Boron adsorption using a new boronselective hybrid gel and the commercial resin D564", Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 341, pp. 118-126, 2009. [2] M. Bryjak, J. Wolska, N. Kabay, "Removal of boron from seawater by adsorption– membrane hybrid process: implementation and challenges", Desalination, vol. 223, pp. 57-62, 2008. [3] Y. Cengeloglu, A. Tor, G. Arslan, M. Ersoz, S. Gezgin, "Removal of boron from aqueous solution by using neutralized red mud",

Conclusion Results obtained from this study have shown that electrocoagulation using iron plate electrode can be applied to remove boron from produced water. A removal efficiency of 96.7% was obtained at 15 mg/l concentration of boron in produced water, pH 7.84, current density of 12.5 mA/cm2 and interelectrode distance of 0.5 cm. The adsorption of boron on Fe(OH)3 followed second order kinetics.

Table 1 First order and second order adsorption kinetic constants Co (mg/l) 10 20 30

qe exp (mg.g-1) 7.536 13.458 20.302

ISBN: 978-1-61804-147-0

90

Pseudo-First Order k1 (min -1) qe cal. (mg/g) 0.0507 3.899 0.0276 7.328 0.0299 10.889

91

R2 0.985 0.983 0.980

Pseudo-Second Order k2 qe cal. R2 (g/mg.min) (mg.g) 0.0217 0.0656 0.0470

8.065 14.286 21.740

0.999 0.994 0.995


[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

Journal of Hazardous Materials, vol. 142, pp. 412-417, 2007. A. E. Yilmaz, R. Boncukcuoğlu, M. M. Kocakerim, M. T. Yilmaz, C. Paluluoğlu, "Boron removal from geothermal waters by electrocoagulation", Journal of Hazardous Materials, vol. 153, pp. 146-151, 2008. D. Hou, J. Wang, X. Sun, Z. Luan, C. Zhao, X. Ren, "Boron removal from aqueous solution by direct contact membrane distillation", Journal of Hazardous Materials, vol. 177, pp. 613-619, 2010. N. Kabay, I. Yılmaz, S. Yamac, S. Samatya, M. Yuksel, U. Yuksel, "Removal and recovery of boron from geothermal wastewater by selective ion exchange resins. I. Laboratory tests", Reactive and Functional Polymers, vol. 60, pp. 163-170, 2004. Z. Yazicigil, Y. Oztekin, "Boron removal by electrodialysis with anion-exchange membranes", Desalination, vol. 190, pp. 7178, 2006. S. Şahin, "A mathematical relationship for the explanation of ion exchange for boron adsorption", Desalination, vol. 143, pp. 3543, 2002. M. Malakootian N. Yousefi, "The efficiency of electrocoagulation process using aluminum electrodes in removal of hardness from water", Iranian Journal of Environmental Health Science & Engineering, vol. 6, 2009. A. Fakhru’l-Razi, A. Pendashteh, L. C. Abdullah, D. R. A. Biak, S. S. Madaeni, Z. Z. Abidin, "Review of technologies for oil and gas produced water treatment", Journal of Hazardous Materials, vol. 170, pp. 530551, 2009. E. H. Ezechi, M. H. Isa, S. R. B. M. Kutty, "Boron in Produced Water: Challenges and Improvements: A Comprehensive Review", Journal of Applied Sciences, vol. 12, pp. 402-415, 2012. S. Mohajeri, H. A. Aziz, M. H. Isa, M. A. Zahed, M. N. Adlan, “Statistical optimization of process parameters for landfill leachate treatment using electroFenton technique”, Journal of Hazardous Materials, vol. 176, pp. 749-758, 2010. S. Mohajeri, H. A. Aziz, M. H. Isa, M. A. Zahed, M. J. K. Bashir, M. N. Adlan, “Application of the central composite design for condition optimization for semiaerobic landfill leachate treatment using

ISBN: 978-1-61804-147-0

[14]

[15]

[16]

[17]

[18] [19]

[20]

[21]

[22]

92

electrochemical oxidation”, Water Science and Technology,vol. 61, pp. 1257-1266, 2010. R. R. Babu, N. Bhadrinarayana, K. M. M. S. Begum, N. Anantharaman, "Treatment of tannery wastewater by electrocoagulation", Journal of the University of Chemical Technology and Metallurgy, vol. 42, pp. 201-206, 2007. N. Daneshvar, A. Oladegaragoze, N. Djafarzadeh, "Decolorization of basic dye solutions by electrocoagulation: an investigation of the effect of operational parameters", Journal of Hazardous Materials, vol. 129, pp. 116-122, 2006. M. J. K. Bashir, M. H. Isa, S. R. M. Kutty, Z. B. Awang, H. A. Aziz, S. Mohajeri, leachate treatment by "Landfill electrochemical oxidation", Waste Management, vol. 29, pp. 2534-2541, 2009. M. H. Isa, T. K. Kee, A. A. Zinatizadeh, S. Mohajeri, H. Aziz, Y. T. Hung, "Electrochemical treatment of semi-aerobic landfill leachate using Response Surface Methodology (RSM)", International Journal of Environment and Pollution, vol. 43, pp. 324-338, 2010. S. H. Lin, M. L. Chen, "Treatment of textile wastewater by chemical methods for reuse", Water Research, vol. 31, pp. 868-876, 1997. A. E. Yılmaz, R. Boncukcuoğlu, E. Kocadağistan, "An empirical model for kinetics of boron removal from boroncontaining wastewaters by the electrocoagulation method in a batch reactor", Desalination, vol. 230, pp. 288297, 2008. F. Ozyonar, B. Karagozoglu, "Operating Cost Analysis and Treatment of Domestic Wastewater by Electrocoagulation Using Aluminum Electrodes", Polish Journal of Environmental Studies, vol. 20, pp. 173179, 2011. G. Chen, "Electrochemical technologies in wastewater treatment", Separation and Purification Technology, vol. 38, pp. 11-41, 2004. A. A. Bukhari, "Investigation of the electrocoagulation treatment process for the removal of total suspended solids and turbidity from municipal wastewater", Bioresource Technology, vol. 99, pp. 914921, 2008.