Electrosorption of inorganic ions from aqueous by carbon electrode

Chen Zhao-Lin, Zhang Hong-Tao, Wu Chun-Xu, Gao Lei, Wang Yu-Shuang, Xue Fang-Qin

School of Environment Tsinghua University

Beijing, 100084, P.R. China zhaolinbest@126.com

Sun Mo-Han College of Resources and Environmental Sciences

China Agricultural University Beijing 100193, P.R. China

Abstract—In this study, effects of ionic radius, charge, and mass

on the electrosorption desalination performance of activated

carbon electrodes were investigated. The sorption capacity of

carbon electrodes was approximately 0.08-0.18 mmol/g carbon.

The electrosorption capacity of monovalent ions was higher than

multivalent ions. Covering area of cation on electrode is smaller

than anion, when considering Covering area of hydrated ions,

the cation and anion is almost equal. Because of the relatively

small pore diameter (2 nm) of the carbon electrode, only 15.62 to

37.90 m2/g carbon surface area was available to ions

eletrosorption.

Keywords-electrosorption; desalination; electric double layers

I. INTRODUCTION

Water shortage is one of the important problems all over world. Brackish water desalination is a very available solution to this problem. Many desalination technologies can make pure water from salty water. Conventional technologies, such as thermal distillation, ion exchange, electric dialysis, reverse osmosis etc [1,2], have many problems in maintenance, complex pretreatment and high energy consumption [3,4].

Electrosorption is a potential induced adsorption of ions on the surface of charged electrodes. When an electrical field is applied to a pair of electrodes, ions move to oppositely electrode surface and formed the electric double layers (EDLs). When the circuit is shorted, the electro-sorbed ions are quickly diffused back to solution [5]. Compared to other

conventional desalination process, electrosorption is an energy-efficient desalination technology due to it operates at lower electrode voltage (1–2V) at which no obvious electrolysis reactions exist [6]. And this technology is environmentally friendly because it requires no acids or alkali for regeneration [7]. The materials, such as activated carbon, carbon nanofibers, carbon aerogel and carbon nanotubes are used as electrosorption electrode materials [8–10]. Although carbon nano-fibers, carbon nano-tubes and carbon aerogel can be used for good electrosorption electrode materials, the manufacturing process of these materials is complicate and its production is very expensive. Hence, these carbon materials are difficult for large scale application. While activated carbon also has large specific surface area, and it’s cheap and easily available as raw material. Therefore, it is the most suitable electrode material for large scale application [11].

To better understand the mechanism of electrosorption, the electrosorption performance of various ionic radius, charge, and mass on the adsorb capacity has been investigated.

II. EXPERIMENTAL

Electrodes were supplied by Beijing EST research institute. Electrosorption experiments were conducted with a flow-through system was shown in Fig. 1. The electrosorption system consisted of an electro-sorb module, a peristaltic pump, a rectifier, a water tank and a conductivity meter. The electro-sorb module consisted of a pair of carbon electrodes separated by Teflon net. Each carbon electrode has an area of

2012 International Conference on Biomedical Engineering and Biotechnology

978-0-7695-4706-0/12 $26.00 © 2012 IEEE

DOI 10.1109/iCBEB.2012.194

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60×70 mm2, the thickness of carbon electrodes is 1.3 mm, and the distance between two carbon electrodes is 1.5 mm. The rate of peristaltic pump is 10 mL/min, the volume of solution is 700 mL. Electrosorption experiment was carried out on constant voltage conditions. A rectifier maintained a constant potential (1.2V) between a pair of carbon electrodes during electrosorption.

The morphology of carbon electrode was observed by scanning electron microscope (SEM) (Hitachi S-3400N). The BET specific surface area of carbon electrode materials was detected by high purity nitrogen adsorption and desorption at 77 K by a surface analyzer (Micromeritics ASAP 2020). Before the nitrogen adsorption, the carbon materials were degassed under vacuum at 423 K for 6 h.

Figure 1. Schematic of adsorption experiments.

A series of 12 experiments was carried out to evaluate the effects of charge, ionic radius and mass, on the electrosorption capacity of activated carbon electrodes. Ions selected for evaluation were sodium (Na+), magnesium (Mg2+), potassium

(K+), nitrate (NO3-), chloride (Cl-), fluoride (F-), and sulfate

(SO42-). Table 1 shows the design test matrix.

Physical-chemical data for each of the above ions are shown in Table 2. These experiments were conducted using 1L of 5 mmol/L salt solutions made up in deionized water.

TABLE 2 CHEMICAL DATA OF TEST IONS[12,13]

Ion Mass

(amu)

Charge Radius

(nm)

Hydrated raius

(nm)

Na+ 22.990 +1 0.116 0.358

K+ 39.098 +1 0.152 0.331

Mg2+ 24.305 +2 0.086 0.428

Cl- 35.453 -1 0.167 0.331

NO3- 62.005 -1 0.165 0.335

SO42- 96.064 -2 0.244 0.379

F- 18.998 -1 0.133 0.352

III. RESULTS AND DISCUSSION

A. Characteristics of carbon electrodes

The SEM photo of carbon electrode was shown in Fig. 2. Fig. 3 shows the pore diameter distribution of carbon electrode, the total surface area was 1153 m2/g, the micropore area was 458 m2/g, and the average pore diameter was 2.01 nm. It was showed that the pore diameter were mainly mesopores. According to the previous literature, mesopores are beneficial to the electrosorption of ions from solution, but not micropores due to the electric double layer will overlap in it [12]. Hence, the activated carbon electrode used in this paper is ideal to investigate the electrosorption experiments.

TABLE 1 STATISTICAL DESIGN MATRICES TO EVALUATE THE EFFECT OF ION PROPERTIES ON ELECTROSORPTION

Ion Charge Ionic radius Ion mass

Test Cation Anion Test Cation Anion Test Cation Anion

1 Na+ Cl- 5 Na+ Cl- 9 Na+ Cl-

2 Na+ SO42- 6 Na+ F- 10 Na+ NO3

-

3 Mg2+ Cl- 7 K+ Cl- 11 K+ Cl-

4 Mg2+ SO42- 8 K+ F- 12 K+ NO3

-

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Figure 2. SEM image of carbon electrode.

10 100 10000.00

0.05

0.10

0.15

0.20

0.25

Incr

emen

tal p

ore

volu

me

(cm

3 /g)

Pore diameter ( )

Figure 3. Pore diameter distribution of carbon electrode.

B. Effects of Ion Properties on Electrosorption Capacity

The performances of ion charge, size, and mass on electrosorption were researched in several series experiments containing representative ions of varying properties. Cations (Na+, K+, and Mg2+) and anions (Cl-, F-, NO3

-, and SO42-) were selected based on their presence in

natural water. Each of the test matrices in Table 1 was designed to isolate a test variable (e.g., ion charge) to the greatest extent possible. For example, to evaluate the effects of charge on sorption potential, test ions were selected that had similar ionic radius and atomic mass. Electrosorption experiments were conducted with 5 mmol/L solution using an electro-sorb module. The experiments were operated for 3 h at 1.2V until saturation, as indicated by the constant effluent conductivity measurements. The maximum (equilibrium) ion electrosorption capacity was achieved in these experiments based on constant effluent conductivity at the end of each experimental operation.

The effects of ion charge in terms of mmol/g carbon electrode are shown in Fig. 4. Here, variance on both cations (Na+ and Mg2+) and anions (Cl- and SO4

2-) was elucidated in

four series of experiments (tests 1-4, Table 1). Complete saturation of the carbon electrode was achieved in each experiment. The capacities for monovalent cation (Na+) and divalent cation (Mg2+) were 0.158~0.169 and 0.078~0.079 mmol/g, the capacity of monovalent cation is bigger than divalent cation. The capacities for monovalent anion (Cl-) and divalent anion (SO4

2-) were 0.172~0.183 and 0.084~0.086 mmol/g, the capacity of monovalent anion is bigger than divalent anion. However, consider to equivalent capacity (mmol × valence/g carbon) of adsorption ions, monovalent ions and divalent ions is approximately. Compare to test 1, the capacity of Na+ in test 2 and the capacity of Cl- in test 3 increased a little, due to initial concentration of solution increased. Another important phenomenon is that equivalent capacity of cations and anions is discrepancy. At this point, authors do not understand this phenomenon, and will research in future.

0.00

0.04

0.08

0.12

0.16

0.20

MgSO4MgCl2Na2SO4

Elec

troso

ptio

n ca

paci

ty (m

mol

/g)

Cations Anions

NaCl

Figure 4. Effect of ion charge on electrosorption

The effects of ionic radius are illustrated in Fig. 5 (tests 5-8, Table 1). Monovalent cations (Na+ and K+) and anions (F- and Cl-) with large ion radius variance were investigated in deionization experiments. The capacity of Na+ and K+ is approximately. The capacity of Cl- is larger than F-.

0.00

0.04

0.08

0.12

0.16

0.20

KFKClNaF

Elec

troso

ptio

n ca

paci

ty (m

mol

/g)

Cations Anions

NaCl Figure. 5. Effect of ionic radius on electrosorption

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The effects of ion mass were investigated in several series of experiments (tests 9-12, Table 1), illustrated in Fig. 6. Monovalent cations (Na+ and K+) and anions (Cl- and NO3

-) with differences in atomic mass were selected. The capacity of Cl- and NO3

- is approximately.

0.00

0.04

0.08

0.12

0.16

0.20

KNO3KClNaNO3

Elec

troso

ptio

n ca

paci

ty (

mm

ol/g

)

Cations Anions

NaCl Figure 6. Effect of ion mass on electrosorption

The data for the three series of experiments clearly show that ion charge had the single greatest effect on electrosorption behavior. The ion mass and radius had no observable effect on these experimental conditions. There was evidence that the counter ion charge also played an distinguished role in an individual ion’s electrosorption capacity. The hydrated radius of an ion is a function of charge and ionic radius (i.e., charge-to-radius ratio). Therefore, as with ion the exchange and membrane separation processes, ion hydrated radius dictated removal phenomena. However, unlike reverse osmosis and ion exchange, electrosorption with carbon electrode showed better removal characteristic of smaller, monovalent ions as opposed to larger, divalent ions.

C. Saturation of Carbon Surface

To better understand the mechanism of carbon sorption, the saturated equilibrium adsorption capacities were calculated for each compound based on the amount of carbon surface area covered by the sorbed species, using both the ionic area and hydraulic area of each ion (Table 3). Complete saturation of the carbon was assumed for each experimental condition.

Table 3 presents the carbon surface area coverage using both ionic area and hydrated area. These data show that approximately 1.02-3.32 m2/g carbon surface was covered by cations and 4.35-9.65 m2/g carbon surface was covered by anions. Using ionic area, the anion sorption capacity was significantly greater than for cations. When the hydrated

radius was used to calculate the amount of surface area occupied by each ion, the surface coverage greatly increased (Table 3). As the “hydration sphere radius” increased, the planar surface area increased by the square of the radius. For example, the saturation adsorption capacity for potassium from the potassium chloride experiments increased from 3.32 to 15.73 m2/g carbon. Based on these data, the surface coverage of the carbon surface ranged 15.62 to 37.90 m2/g carbon. Covering area of cation on electrode is smaller than anion, when considering Covering area of hydrated ions, the cation and anion is almost.

TABLE 3 EFFECT OF ION HYDRATION ON ELECTROSORPTION

CAPACITY OF CARBON ELECTRODE

Variable Solute

Ionic Area

(m2·g-1 carbon)

Hydrated Area

(m2·g-1 carbon)

Cation Anion Cation Anion

charge NaCl 2.14 9.08 20.40 35.68

Na2SO4 2.48 9.57 23.58 23.10

MgCl2 1.03 9.65 25.63 37.90

MgSO4 1.02 9.34 25.29 22.53

radius

NaCl 2.14 9.08 20.40 35.68

NaF 2.25 4.68 21.42 32.78

KCl 3.32 8.86 15.73 34.80

KF 3.29 4.35 15.62 30.46

mass

NaCl 2.14 9.08 20.40 35.68

NaNO3 2.04 8.60 19.44 35.47

KCl 3.32 9.08 15.73 35.67

KNO3 3.31 8.59 15.69 35.41

IV. CONCLUSIONS

The impacts of ionic radius, charge, and mass on the electrosorption performance of activated carbon electrodes were investigated. The sorption capacity of carbon electrodes was approximately 0.08-0.18 mmol/g carbon. The electrosorption capacity of monovalent ions with smaller hydrated radii was higher than multivalent ions on molar basis. Because of the relatively small average pore diameter (2.01 nm) of the carbon material, only 15.62 to 37.90 m2/g carbon surface area was useful for ion electrosorption.

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