Removal of manganese from water by electrocoagulation: Adsorption ...

Removal of Manganese from Water by Electrocoagulation: Adsorption, Kinetics and Thermodynamic Studies Pandian Ganesan, Jothinathan Lakshmi, Ganapathy Sozhan and Subramanyan Vasudevan* CSIR-Central Electrochemical Research Institute, Karaikudi 630 006, Tamil Nadu, India

The present study provides an electrocoagulation process for the removal of manganese (Mn) from water using magnesium as anode and galvanised iron as cathode. The various operating parameters like effect of initial pH, current density, electrode configuration, inter-electrode distance, coexisting ions and temperature on the removal efficiency of Mn were studied. The results showed that the maximum removal efficiency of 97.2% at a pH of 7.0 was achieved at a current density 0.05 A/dm2 with an energy consumption of 1.151 kWhr/m3 . Thermodynamic parameters, including the Gibbs free energy, enthalpy and entropy, indicated that the Mn adsorption of water on magnesium hydroxides was feasible, spontaneous and endothermic. The experimental data were fitted with several adsorption isotherm models to describe the electrocoagulation process. The adsorption of Mn preferably fitting the Langmuir adsorption isotherm suggests monolayer coverage of adsorbed molecules. In addition, the adsorption kinetic studies showed that the electrocoagulation process was best described using the second-order kinetic model at the various current densities. Keywords: electrocoagulation, manganese removal, adsorption kinetics, isotherms

INTRODUCTION

M

anganese (Mn) is the second most abundant metal in nature (Dabeka et al., 2002). It is an essential metal for the human system and many enzymes are activated by Mn. Mn has a variety of applications in ceramics, primary cells (dry battery) and electrical coils. Mn is also used in alloying element of many alloys. It is present in the atmosphere as suspended particulates resulting from industrial emission, soil erosion, volcanic emissions and the burning of MMT-containing petrol (WHO, 2004). The Mn contaminant in ground water affects the intelligent quotient (IQ) of children (Dabeka et al., 2002). The neurotoxic disease like Parkinsonism is caused by the Mn over intake (Erikson et al., 2007). The prolonged over intake is potentially affect the central nervous system and lungs. These Mn contaminants in the ground water cause a disease, called manganism and also causes diseases of disturbed speech called prognosis, also cause bronchitis and pneumonia (USEPA report). The World Health Organisation (WHO) prescribed the permissible limit for the Mn in the ground water is 0.05 mg/L (WHO, 2004). The common methods for removing toxic metals from water include electrodialysis, chemical coagulation, reverse osmosis, co-precipitation, complexation, solvent extraction, ion exchange

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THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING

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and adsorption (Friberg, 1983; Gupta et al., 1997b; Nogawa et al., 2004; Sekar et al., 2004; Youssef et al., 2004; Gupta et al., 2006a, 2008, 2009; Ali and Gupta, 2007; Ali and Gupta, 2008; Bedow et al., 2008; Gupta and Rastogi, 2008a; Gupta and Suhas, 2009; Vasudevan et al., 2010). Physical methods like ion exchange, reverse osmosis and electrodialysis have proven to be either too expensive or inefficient to remove Mn from water. At present, chemical treatments are not used due to disadvantages like high costs of maintenance, problems of sludge handling and its disposal and neutralisation of effluent (Melnik et al., 1999; Mollah et al., 2001; Mouedhen et al., 2008). Recent research has demonstrated that electrocoagulation offers an attractive alternative to above-mentioned traditional methods for treating water (Mollah et al., 2001). In this process, anodic dissolution of metal electrode takes place with the evolution of hydrogen gas at the cathode

∗ Author to whom correspondence may be addressed. E-mail address: [email protected] Can. J. Chem. Eng. 91:448–458, 2013 © 2012 Canadian Society for Chemical Engineering DOI 10.1002/cjce.21709 Published online 4 June 2012 in Wiley Online Library (wileyonlinelibrary.com).

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(Mouedhen et al., 2008). Electrochemically generated metallic ions from the anode can undergo hydrolysis to produce a series of activated intermediated that are able to destabilise and adsorb (Gupta et al., 1997a, 2006b, 2007; Gupta and Rastogi, 2008b) the finely dispersed particles present in the water to be treated. The adsorbed/destabilised particles then aggregate to form flocks as outlined below: (i) When iron/aluminum is used as the electrode, the reactions are as follows: At the cathode: 2 H2 O + 2 e− → H2 (g) + 2 OH−

(1)

At the anode (Iron): 4 Fe + 10 H2 O + O2 → Fe(OH)3 ↓ + 4 H2 (2) At the anode (Aluminum): Al3+ (aq) + 3 H2 O → Al(OH)3 ↓ + 3 H+

(3)

(ii) In the present study, magnesium is used as anode and the reactions are as follows: At the cathode: 2 H2 O + 2 e− → H2 (g) + 2 OH−

(4)

At the anode: Mg → Mg2+ + 2 e−

(5)

In the solution: Mg2+ (aq) + 2 H2 O → Mg(OH)2 ↓ +2 H+

(6)

The advantages of electrocoagulaton include high particulate removal efficiency, a compact treatment facility, relatively low cost and the possibility of complete automation. This method is characterised by reduced sludge production, a minimum requirement of chemicals and ease of operation (Agrawal and Sahu, 2006). Besides, the main disadvantage in case of aluminum electrode is the residual aluminum (The USEPA guidelines suggest maximum contamination is 0.05–0.2 mg/L) present in the treated water due to cathodic dissolution. This will lead to health problems like cancer. There is no such health problem in the case of magnesium electrode, because the USEPA guidelines suggest that the maximum contamination level of magnesium in water is 30 mg/L. Although, there are numerous reports dealt with electrocoagulation as a means of removal of many pollutants from water and wastewater, but there are few studies on the removal of Mn by electrocoagulation method. This study presents the results of the laboratory scale studies on the removal of Mn using magnesium and stainless steel as anode and cathode respectively by electrocoagulation process. To optimise the maximum removal efficiency of Mn, different reaction conditions like effect of current density, co-existing ions, effect of temperature and pH were studied. In doing so, the equilibrium adsorption behaviour is analysed by fitting the isotherm models of Langmuir and Freundlich. Adsorption kinetics of electrocoagulants is analysed using first- and second-order kinetic models. Activation energy was evaluated to study the nature of adsorption.

EXPERIMENTAL SECTIONS Experimental Apparatus and Procedures Figure 1 shows the electrolytic cell consisted of a 1.0-L Plexiglas vessel that was fitted with a polycarbonate cell cover with slots to

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Figure 1. Laboratory scale cell assembly, (1) DC power supply, (2) pH meter, (3) electrochemical cell, (4) cathodes, (5) anode, (6) electrolyte, (7) outer jacket, (8) thermostat, (9) inlet for thermostatic water, (10) outlet for thermostatic water, (11) PVC cover, (12) pH sensor and (13) magnetic stirrer.

introduce the anode, cathode, pH sensor, a thermometer and electrolytes. Magnesium sheet (Alfa Aesar, Lancashire, UK) of surface area (0.017 m2 ) acted as the anode. The cathodes were stainless steel (commercial grade) sheets of the same size as the anode is placed at an inter-electrode distance of 0.005 m. The temperature of the electrolyte was controlled to the desired value with a variation of ±2 K by adjusting the rate of flow of thermostatically controlled water through an external glass-cooling spiral. A regulated direct current (DC) was supplied from a rectifier (10 A, 0–25 V; Aplab model). Manganese nitrate (Mn(NO3 )2 ; MERCK, Darmstadt, Germany, AR Grade) was dissolved in distilled water for the required concentration. In all the experiments 2 mg/L of Mn was used. The solution of 0.90 L was used for each experiment as the electrolyte. The pH of the electrolyte was adjusted, if required, with HCl (MERCK; AR Grade) or NaOH (MERCK; AR Grade) solutions before adsorption experiments. To study the effect of co-existing ions, in the removal of Mn, sodium salts (Analar Grade) of phosphate (5–50 mg/L), silicate (5–15 mg/L), carbonate (5–250 mg/L) and arsenic (0.2–5 mg/L) was added to the electrolyte.

Analytical Methods The concentration of Mn was determined using UV–Visible spectrophotometer with manganese kits (Pharo 300; MERCK). The SEM and EDAX of Mn-adsorbed magnesium hydroxide coagulant were analysed with a scanning electron microscope (SEM) made by Hitachi (Chiyoda-ku, Tokyo, Japan; model s-3000h). The Fourier transform infrared spectrum of magnesium hydroxide was obtained using Nexus 670 FT-IR spectrometer made by Thermo Electron Corporation (Waltham, MA). The XRD for magnesium hydroxide coagulant was analysed by X-ray diffractmeter made by JEOL X-ray diffractmeter (Type—JEOL, Tachikawa, Tokyo, Japan). The concentration of carbonate, silicate, arsenic and phosphate were determined using UV–Visible spectrophotometer with respective standard ion kits supplied by MERCK (Pharo 300).

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THEORY Adsorption Kinetic Modelling The amount of generated coagulant (metal hydroxides) can be stoichiometrically determined according to Faraday’s law. Since the amount of coagulant can be estimated for a given time, the pollutant removal can be modelled using an adsorption phenomenon. In order to investigate the mechanisms of the adsorption process, two different kinetic models, the first- and second-order Lagergren models were applied to describe the kinetics of the Mn adsorption onto magnesium hydroxides. The best-fit model was selected according to the linear regression correlation coefficient values, R2 (Johnson, 1990).

First-Order Lagergren Model The first-order Lagergren model is one of the most widely used expressions describing the adsorption of solute from a solution. The first-order Lagergren model is generally expressed as follows: dqt = k1 (qe − qt ) dt

(7)

where qe (mg/g) and qt (mg/g) are the amounts of Mn adsorbed on the adsorbent at equilibrium and at any time t (min), respectively, and k1 (min−1 ) is the rate constant of the first-order model. The integrated form of the above equation with the boundary conditions t = 0 to >0 (q = 0 to >0) is rearranged to obtain the following time-dependence function: log(qe − qt ) = log(qe ) −

k1 t 2.303

(8)

The first-order model considers the rate of occupation of the adsorption sites proportional to the number of unoccupied sites.

Second-Order Lagergren Model The Lagergren second-order kinetic model is expressed as (Izanloo and Nassir, 2005): dqt = k2 (qe − qt )2 dt

(9)

where k2 is the rate constant of second-order adsorption. The integrated form of Equation (9) with the boundary condition t = 0 to >0 (q = 0 to >0) is: 1 1 = + k2 t qe − qt qe

(10)

Equation (10) can be rearranged and linearised as: 1 t t = + qt k2 qe2 qe

(11)

where qe and qt are the amount of Mn adsorbed on Mg(OH)2 (mg/g) at equilibrium and at time t (min), respectively, and k2 is the rate constant for the second-order kinetic model.

Adsorption Isotherms The pollutants are generally adsorbed at the surface of the metal hydroxides generated during the electrocoagulation process. In order to identify the mechanism of the adsorption process, it is important to establish the most appropriate correlation for the

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equilibrium curves. In this study, two adsorption isotherms, viz., Langmuir and Freundlich were applied to establish the relationship between the amounts of Mn adsorbed onto the magnesium hydroxides and its equilibrium concentration in the electrolyte containing Mn ions (Hamadi et al., 2001).

Freundlich Isotherm The Freundlich adsorption isotherm model includes considerations of surface heterogeneity and exponential distribution of the active sites and their energies. The isotherm is adopted to describe reversible adsorption and is not restricted to monolayer formation. This isotherm typically fits the experimental data over a wide range of concentrations. The linearised and logarithmic expression of the Freundlich model is (Agrawal and Sahu, 2006): log qe = log kf + n log Ce

(12)

where kf (mg/g) and n (dimensionless) are constants that account for all factors affecting the adsorption process, such as the adsorption capacity and intensity. The Freundlich constants kf and n are determined from the intercept and slope, respectively, of the linear plot of log qe versus log Ce .

Langmuir Isotherm The Langmuir isotherm model was developed to represent chemisorption at a set of well-defined localised adsorption sites with the same adsorption energy, independent of the surface coverage and with no interaction between adsorbed molecules. This model assumes a monolayer deposition on a surface with a finite number of identical sites. The Langmuir equation is valid for a homogeneous surface. The linearised form of Langmuir adsorption isotherm model is (Sohn and Kim, 2005; Demiral et al., 2008): Ce Ce 1 + = qe qm b Qm

(13)

where qe (mg/g) is the amount adsorbed at equilibrium, Ce (mg/L) is the equilibrium concentration, qm is the Langmuir constant representing maximum monolayer adsorption capacity and b is the Langmuir constant related to energy of adsorption. The essential characteristics of the Langmuir isotherm can be expressed as the dimensionless constant RL (Oguz, 2004): RL =

1 1+bCo

(14)

where RL is the equilibrium constant it indicates the type of adsorption, b is the Langmuir constant and Co is the various concentration of Mn solution. The RL values between 0 and 1 indicate the favourable adsorption.

Effect of Temperature on Thermodynamic Adsorption Parameters To understand the effect of temperature on adsorption process, thermodynamic parameters should be determined at various temperatures. The energy of activation for adsorption of Mn can be determined by the second-order rate constant is expressed in Arrhenius form (Yang and Al-Duri, 2001): log k2 = log k0 − 2.303 E/RT

(15)

where ko is the constant of the equation (g/mg min), E is the energy of activation (J/mol), R is the gas constant (8.314 J/mol K)

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and T is the temperature (K). The free energy change is obtained using the following relationship: G = −RT ln Kc

(16)

where G is the free energy (kJ/mol), Kc is the equilibrium constant, R is the universal gas constant and T is the temperature (K). The relationship between G, H and S can be expressed by the following equation: G = H − T S

(17)

Combining Equations (16) and (17) leads to: ln Kc =

H S − R RT

(18)

Enhancement of adsorption capacity of electrocoagulant (magnesium hydroxide) at higher temperatures may be attributed to the enlargement of pore size and or activation of the adsorbent surface.

Pore Diffusion Coefficient (D) The diffusion coefficient (D) for intraparticle transport of Mn species into the adsorbent particles has been calculated at different temperature by: t1/2 = 0.03 ×

r02 D

(19)

where t½ is the time of half adsorption (s), ro is the radius of the adsorbent particle (cm) and D is the diffusion coefficient (cm2 /s; Golder et al., 2006).

Figure 2. Effect of inter-electrode distance on removal efficiency and energy consumption of manganese. Conditions—concentration: 2 mg/L; pH of the electrolyte: 7.0; temperature: 305 K.

removal efficiency for the removal of Mn is 97.2% at pH 7 and the minimum efficiency is 82.4% at pH 3. The decrease of removal efficiency at more acidic and alkaline pH was observed by many investigators and was attributed to an amphoteric behaviour of Al(OH)3 which leads to soluble Al3+ cations (at acidic pH) and to monomeric anions Al(OH)4− (at alkaline pH). When the initial pH was kept in neutral, all the aluminum produced at the anode formed polymeric species (Al13 O4 (OH)7+ 24 ) and precipitated Al(OH)3 leading to more removal efficiency. In the present study, the electrolyte pH was maintained in neutral, so the formation of Mg(OH)2 is more predominant (like aluminium), leading to greater removal efficiency.

RESULTS AND DISCUSSION Effect of Inter-Electrode Distance

Effect of Electrode Configurations

To determine the effect various inter-electrode distances between anode and cathode were kept at different spacing of 0.003, 0.005, 0.007, 0.009 and 0.011 m using 2 mg/L Mn containing solutions at a current density of 0.05 A/dm2 . The results of inter-electrode distance on removal efficiency and energy consumption are presented in Figure 2. While decreasing the inter-electrode distance shows decrease in energy consumption and increase in removal efficiency. Lower distance between anodes and cathodes require low electrical energy for motion of ions. This is due to shorter travel path that reduce the resistance of motion of ions and the situation is reverse for the case of large distance between each electrode. In maintaining the inter-electrode distance of 0.003 m was practically difficult. So, further experiments were carried out at inter-electrode distance of 0.005 m. Inter-electrode spacing of 0.005 m had the low energy consumption and removal efficiency.

The percentage removal of Mn from Mg(OH)2 along with the experimental conditions utilised at various current densities from 0.025 to 0.2 A/dm2 were analysed for monopolar and bipolar configuration with concentration 2 mg/L. The data were presented in Table 1. From Table 1, it was observed that, for a given current densities, the removal efficiency of bipolar electrolytic cell was slightly higher than the monopolar electrode configurations. And the energy consumption was higher for monopolar electrode than the bipolar electrode.

Table 1. Effect of electrode configuration at various current densities with concentration 2 mg/L Monopolar electrodes

Effect of Electrolyte pH It is believed that the initial pH is an important operating factor influencing the performance of electrochemical process. To explain this effect, a series of experiments were carried out using 2 mg/L of Mn containing solutions, by adjusting the initial pH in the interval from 3 to 11. The removal efficiency of Mn was increased with increasing the pH up to 7. When the pH is above 7, removal efficiency was decreased. It is found that the maximum

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Current density (A/dm2 ) 0.025 0.05 0.10 0.15 0.20

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Bipolar electrodes

Removal efficiency (%)

Energy consumption kWhr/kL)

Removal efficiency (%)

Energy consumption (kWhr/kL)

84.8 97.2 98.5 98.6 98.8

1.056 1.151 1.224 1.298 1.365

85.0 97.3 98.6 98.7 99.0

0.994 1.002 1.121 1.186 1.239


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Figure 3. Effect of time on the amount of manganese-adsorbed qe at various current density. Conditions—concentration: 2 mg/L; pH of the electrolyte: 7.0; temperature: 305 K.

Figure 4. Second-order kinetic model plot of different current density of manganese. Conditions—concentration: 2 mg/L; temperature: 305 K; pH of the electrolyte: 7.

Effect of Current Density

Kinetic Modelling

Among the various operating variables, current density is an important factor which strongly influences the performance of electrocoagulation. The amount of Mn removal depends upon the quantity of adsorbent [Mg(OH)2 ] generated, which is related to the time and current density (Mohammad et al., 2004). The amount of adsorbent [Mg(OH)2 ] was determined from the using Faraday law. With the increase in current density the amount of magnesium hydroxide generation also increases. To investigate the effect of current density on the Mn removal, a series of experiments were carried out by solutions containing a constant pollutants loading of 2 mg/L, at a pH 7.0, with current density being varied from 0.025 to 0.2 A/dm2 . Figure 3 shows the plot between amounts of Mn adsorbed with respect to time at different current densities. From the figure it was well seen that the uptake of Mn (mg/g) increased with increase in current density and remained nearly constant after equilibrium time. The equilibrium time was found to be 45 min for all concentration studied. After 45 min the amount of Mn adsorbed (qe ) increased from1.115 to 1.981 mg/g as the current density increased from 0.025 to 0.2 mg/L. The figure also shows that the adsorption is the rapid in the initial stages and remains almost constant with the progress of the adsorption. The plots are single, smooth and continuous curves leading to saturation, suggesting the possible monolayer coverage to Mn on the surface of the adsorbent (Jiang et al., 2002; Malkoc and Nuhoglu, 2007; Vasudevan et al., 2008; Vasudevan and Lakshmi, 2011; Vasudevan et al., 2011a,b,c).

In the present study, kinetic models, namely, first- and secondorder models were tested with the Mn concentration of 2 mg/L at various current densities from 0.025 to 0.2 A/dm2 .

First-Order Lagergren Model The experimental data were analysed initially with first-order Lagergren model. The plot of log (qe − qt ) versus t should give the linear relationship, from which k1 and qe can be determined by the slope and intercept respectively from Equation (8). The computed results are presented in Table 2. The results show that the theoretical qe (cal) value does not agree to the experimental qe (exp) values at all current densities studied with poor correlation coefficient. So, further the experimental data were fitted with second-order Lagergren model.

Second-Order Lagergren Model The kinetic data were fitted to the second-order Lagergren model using Equation (11). The equilibrium adsorption capacity, qe (cal) and k2 were determined from the slope and intercept of plot of t/qt versus t (Figure 4) and are compiled in Table 2. The plots were found to be linear with good correlation coefficients. The theoretical qe (cal) values agree well to the experimental qe (exp) values at all current density studied. This implies that the secondorder model is in good agreement with experimental data and can be used to favourably explain the Mn adsorption on Mg(OH)2 .

Table 2. Comparison of experimental and calculated qe values at different current densities for first- and second-order adsorption kinetics of manganese with concentration 2 mg/L at 305 K First-order adsorption Current density (A/dm2 ) 0.025 0.05 0.1 0.15 0.2

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Second-order adsorption

qe (exp)

qe (Cal)

k1 × 104 (min/mg)

R2

qe (cal)

k2 × 104 (min/mg)

R2

1.115 1.456 1.704 1.894 1.981

25.33 26.67 27.82 28.64 29.01

−0.0066 −0.0075 −0.0081 −0.0086 −0.0091

0.7664 0.7856 0.8024 0.7645 0.7981

1.099 1.367 1.691 1.808 1.944

0.0961 0.0845 0.0714 0.0641 0.0523

0.9985 0.9989 0.9996 0.9956 0.9997


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Table 2 depicts the computed results obtained from first- and second-order models. From the results, it is observed that the correlation coefficients for the first-order kinetic model were relatively lower than those obtained for the second-order kinetic model for the different current densities. These results indicate that the second-order kinetic model can be applied suitably to predict the Mn adsorption process onto magnesium hydroxides.

ISOTHERM MODELLING Freundlich Isotherm In testing the isotherm, the Mn concentration used was 0.5–2.5 mg/L with various current densities from 0.025 to 0.2 A/dm2 and at an initial pH 7. The adsorption data are plotted as log qe versus log Ce by Equation (12) should result in a straight line with slope n and intercept kf . The intercept and the slope are indicators of adsorption capacity and adsorption intensity, respectively. The value of n falling in the range of 1–10 indicates favourable sorption. The kf and n values were listed in Table 3 for each concentration and current density. It has been reported that values of n lying between 0 and 10 indicate favourable adsorption. From the analysis of the results it is found that the Freundlich plots fit satisfactorily with the experimental data obtained in the present study.

Langmuir Isotherm Langmuir isotherm was tested from Equation (13). The plots of 1/qe as a function of 1/Ce for the adsorption of Mn on Mg(OH)2 are shown in Figure 5. The plots were found linear with good correlation coefficients (>0.99) indicating the applicability of Langmuir model in the present study. The values of monolayer capacity (qm ) and Langmuir constant (b) is given in Table 3. The values of qm calculated by the Langmuir isotherm were all close to experimental values at given experimental conditions. These facts suggest that Mn is adsorbed in the form of monolayer coverage on the surface of the adsorbent. The applicability of the two isotherm equations was compared using the correlation coefficient (R2 ). The correlation coefficient values of Freundlich and Langmuir isotherm models are presented in Table 3. The values of correlation coefficient (R2 ) are found to be >0.9 for both isotherms. However, based on the R2 values, the Langmuir isotherm model provided a better fit compared to Freundlich isotherm model. This suggests the adsorption of Mn by magnesium hydroxides is apparently with monolayer coverage of adsorbed molecules. The dimensionless constant RL were calcu-

Table 3. Constant parameters and correlation coefficient for different adsorption isotherm models for manganese adsorption at 0.5–2.5 mg/L at various current densities of 0.025–0.2 A/dm2 Current density (A/dm2 ) Isotherm

Parameters

0.025

0.05

0.1

0.15

0.2

Freundlich

kf (mg/g) n (L/mg) R2 qm (mg/g) b (L/mg) R2 RL

1.7125 1.0026 0.9885 1.7361 5.1892 0.9995 0.5646

1.5959 1.0065 0.9856 2.2261 4.9912 0.9999 0.6125

1.4321 1.1061 0.9823 2.8616 4.8321 0.9989 0.5946

1.3782 1.0996 0.9881 2.9465 4.5641 0.9991 0.6864

1.2566 1.0664 0.9876 3.0155 4.3981 0.9988 0.6355

Langmuir

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Figure 5. Langmuir plot (1/qe vs. 1/Ce ) for adsorption of manganese. Conditions—pH of the electrolyte: 7.0; current density: 0.025–0.2 A/dm2 ; temperature: 303 K; concentration: 0.5–2.5 mg/L.

lated from Equation (14). The RL values were found to be between 0 and 1 for all the concentration of Mn studied.

Thermodynamic Parameters Figure 6 shows that the rate constants vary with temperature according to Equation (15). The activation energy (0.665 kJ/mol) is calculated from the slope of the fitted equation. The free energy change is obtained from Equation (16). The Kc and G values are presented in Table 4. From the Table, it is found that the negative value of G indicates the spontaneous nature of adsorption. The enthalpy change (H = 3.063 kJ/mol) and entropy change (S = 0.6975 J/mol K) were obtained from the slope and intercept of the van’t Hoff linear plots of ln kc versus 1/T (Figure 7, Equation 17). Positive value of enthalpy change (H) indicates that the adsorption process is endothermic in nature, and the negative value of change in internal energy (G) show the spontaneous adsorption of Mn on the adsorbent. Positive values of entropy change show the increased randomness of the solution interface during the adsorption of Mn on the adsorbent (Table 4). Enhancement of adsorption capacity of electrocoagulant (magnesium hydroxide) at higher temperatures may be attributed to the enlargement of pore size and/or activation of the adsorbent surface. Using Lagergren rate equation, second-order rate constants and correlation coefficient were calculated for different temperatures (305–343 K). The calculated ‘‘qe ’’ values obtained from the second-order kinetics agrees with the experimental ‘‘qe ’’ values better than the first-order kinetics model, indicating adsorption following second-order kinetics. Table 5 depicts the computed results obtained from first- and second-order kinetic models.

Table 4. Thermodynamics parameters for adsorption of manganese Temperature (K) 323 333 343

Kc 17.085 18.122 19.098

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G◦ (J/mol) H◦ (kJ/mol) S◦ (J/mol K) −222.32 −229.21 −236.17

3.063

0.6975


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Table 5. Comparison between the experimental and calculated qe values for the manganese concentration of 2 mg/L with 0.05 A/dm2 in first- and second-order adsorption kinetics First-order adsorption

Second-order adsorption

Concentration (mg/L)

qe (exp)

qe (Cal)

k1 × 104 (min/mg)

R2

qe (Cal)

k2 × 104 (min/mg)

R2

323 333 343

3.9031 4.0078 4.2155

24.33 28.34 29.61

−0.0061 −0.0068 −0.0071

0.8002 0.7661 0.6844

3.8641 3.9914 4.0022

0.0661 0.0665 0.0671

0.9963 0.9991 0.9994

Effect of Competing Ions Carbonate Effect of carbonate on Mn removal was evaluated by increasing the carbonate concentration from 0 to 250 mg/L in the electrolyte. The removal efficiencies are 97.2%, 95.3%, 72.8%, 50.7%, 38% and 19% for the carbonate concentration of 0, 2, 5, 65, 150 and 250 mg/L, respectively. From the results, it is found that the removal efficiency of the Mn is not much affected by the presence of carbonate below 2 mg/L. Significant reduction in removal efficiency was observed above 2 mg/L of carbonate concentration is due to the passivation of anode resulting the hindering of the dissolution process of anode.

Phosphate

Figure 6. Plot of log k2 and 1/T. Conditions—pH of 7.0; current density: 0.025–0.2 A/dm2 ; concentration: 0.5–2.5 mg/L.

Pore Diffusion Coefficient (D) For all chemisorptions system the diffusivity coefficient should be 10−5 to 10−13 cm2 /s. In the present study, D is found to be in the range of 10−10 cm2 /s. The D values for various temperatures and different initial concentrations of Mn are presented in Table 6.

The concentration of phosphate ion was increased from 0 to 50 mg/L, the contaminant range of phosphate in the ground water. The removal efficiency for Mn was 97.2%, 96.3%, 64.7%, 41.5% and 36.2% for 0, 2, 5, 25 and 50 mg/L of phosphate ion, respectively. There is a negligible change in removal efficiency of Mn below 2 mg/L of phosphate in the water was observed. At higher concentrations (at and above 2 mg/L) of phosphate, the removal efficiency decreases drastically. This is due to the preferential adsorption of phosphate over Mn as the concentration of phosphate increases.

Arsenic The concentration of arsenic was gradually increased from 0 to 5 mg/L. From the results it is found that the removal efficiency of Mn was decreased by increasing the concentration of arsenic. The removal efficiencies are 97.2%, 95.7%, 83.5%, 73.6% and 54.6% for 0, 0.2, 0.5, 2.5 and 5.0 mg/L of arsenic, respectively.

Table 6. Pore diffusion coefficients for the adsorption of manganese at temperature 305 K Pore diffusion constant D × 10−9 (cm2 /s)

Figure 7. Plot of ln Kc and 1/T. Conditions: pH of the electrolyte: 7.0; current density of 0.025–0.2 A/dm2 ; concentration: 2 mg/L.

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Concentration (mg/L) 0.5 1.0 1.5 2.0 2.5 Temperature (K) 323 333 343

1.5229 1.1221 0.7847 0.5123 0.4838 2.5528 2.6318 2.7109

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This is due to the preferential adsorption of arsenic over Mn as the concentration of arsenic increases. So, when arsenic ions are present in the water to be treated arsenic ions compete greatly with Mn ions for the binding sites.

Silicate From the results it is found that no significant change in Mn removal was observed, when the silicate concentration was increased from 0 to 2 mg/L. The respective efficiencies for 0, 2, 5, 10 and 15 mg/L of silicate are 97.2%, 87.2%, 82.4%, 59.6% and 51.8%. In addition to preferential adsorption, silicate can interact with magnesium hydroxide to form soluble and highly dispersed colloids that are not removed by normal filtration.

SURFACE MORPHOLOGY Characterisation of the Cathode Surface To investigate the possibility of direct deposition of Mn on the cathode surface, the EDAX analysis was carried out on the cathode surface. The results showed that the amount of deposited Mn on cathode surface is very low. The reason for this phenomenon may be due to the hydrogen evolution from aqueous solutions will compete with metal deposition on the cathode surface. Hence, the direct cathodic deposition has insignificant effect on the removal of Mn ions from aqueous solution.

Characterisation of the Anode Surface A SEM image of magnesium electrode after electrocoagulation of Mn electrolyte was obtained (Figure 8). The electrode surface is found to be rough, with a number of dents of ca. 10 um. These dents are formed around the nucleus of the active sites where the electrode dissolution results in the production of magnesium

Figure 8. SEM image of the anode after treatment.

hydroxides. The formation of a large number of dents may be attributed to the anode material consumption at active sites due to the generation of oxygen at its surface. The elemental constituent of Mn-adsorbed magnesium hydroxide was shown in Figure 9. It shows that the presence of Mn, Mg and O appears in the spectrum. EDAX analysis provides direct evidence that Mn is adsorbed on magnesium hydroxide.

Figure 9. EDAX curve of the manganese-adsorbed anode plate.

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The following conclusions were drawn based on the presented information:

Figure 10. FT-IR spectrum of manganese-adsorbed magnesium hydroxide.

(1) The optimal removal efficiency of 97.2% was achieved at a current density 0.05 A/dm2 and at pH 7.0 using magnesium as anode and stainless steel as cathode. (2) The first- and second-order model was applied to identify the kinetics of Mn adsorption onto magnesium hydroxides. The kinetic results showed that the Mn adsorption on magnesium hydroxides was best described using the second-order kinetic model at the various current densities studied. (3) The electrocoagulation process was modelled using adsorption isotherm models, viz., Langmuir and Freundlich. The Mn adsorption was best fitted by the Langmuir adsorption isotherm, and the results were in good agreement with the experimental data. (4) The thermodynamic parameters like G, H and S were determined. Their values indicated that the adsorption process was favourable, spontaneous and endothermic in nature. As the temperature increased from 223 to 343 K, G became less negative, indicating a stronger driving force, resulting in a greater adsorption capacity at higher temperatures. The positive value of H confirmed that the process was endothermic. The positive value of S suggested that the increased randomness of the Mn adsorption from the water containing Mn onto magnesium hydroxides. (5) From the surface characterisation studies, it is confirmed that the magnesium hydroxide generated in the cell-adsorbed Mn present in the water than direct cathodic deposition.

NOMENCLATURE R2 qt qe

Figure 11. XRD spectrum of manganese-adsorbed electrocoagulant.

MATERIAL CHARACTERISATION FT-IR and XRD Studies Figure 10 presents the FT-IR spectrum of Mn–magnesium hydroxide. The sharp and strong peak at 3694.57 cm−1 is due to the (O–H) stretching vibration in the Mg(OH)2 structures. The 1476.6 cm−1 peak indicates the bent vibration of H–O–H. A broad absorption band at 3696.4 cm−1 implies the transformation from free protons into a proton conductive state in brucite. The strong peak at 439.2 cm−1 is assigned to the Mg–O stretching vibration (Golder et al., 2006; Vasudevan et al., 2010). The spectrum data are in good agreement with the reported data (Golder et al., 2006). Mg–Mn is observed in –OH stretching region. The Figure 11 shows the Xray diffraction of Mn-adsorbed magnesium hydroxide. From the figure it is found that electrocoagulation by product showed the well crystalline phase of magnesium hydroxide.

t k1 k2 kf n Ce Co qm b RL ko E R T G Kc H S t½ D r0

regression correlation coefficient values the amount of manganese adsorbate adsorbed on adsorbent at any time (mg/g) the amount of manganese adsorbate adsorbed on adsorbent at equilibrium time (mg/g) time (min) the rate constant of first-order model (min) the rate constant of second-order model (L/g min) adsorption capacity of the Frendlich isotherm (mg/g) adsorption intensity of the Frendlich isotherm the non-retained manganese (mg/L) different concentrations of manganese (mg/L) the maximum monolayer adsorption capacity (mg/g) energy of Langmuir adsorption Langmuir equilibrium constant thermodynamical Arrhenius constant (g/mg min) energy of activation (J/mol) gas constant which is 8.314 (J/mol K) Temperature (K) Gibbs free energy for adsorption (kJ/mol) equilibrium constant of adsorption process enthalpy of the adsorption (J/mol) entropy of the adsorption (J/mol) half time of adsorption (min) diffusion coeffficient (cm2 /s) radius of the particle (cm)

ACKNOWLEDGEMENTS CONCLUSIONS The results indicate that the electrocoagulation is a promising technology for the removal of Mn from drinking water.

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The authors wish to express their gratitude to the Director, Central Electrochemical Research Institute, Karaikudi to publish this article.

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Manuscript received December 2, 2011; revised manuscript received February 4, 2012; accepted for publication February 8, 2012.

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