Removal of Humic Acid from Groundwater by Electrocoagulation

Dec. 2007

Journal of China University of Mining & Technology

J China Univ Mining & Technol

Vol.17

No.4

2007, 17(4): 0513 – 0515

Removal of Humic Acid from Groundwater by Electrocoagulation FENG Qi-yan, LI Xiang-dong, CHENG Yu-jie, MENG Lei, MENG Qing-jun School of Environment and Spatial Informatics, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China Abstract: With this study, we investigated an approach of applying an electrocoagulation method for the removal of humic acid from groundwater. Aluminium electrodes were selected in the experiment. Some major experimental factors, such as electrode distance, current densities and pH values were explored. Results suggest that, given a small electrode interval and/or a high current density, a lower pH value leads to an improved removal rate of humic acid. Under acid conditions with a current density 47.6 A/m2, for instance, humic acid concentrations were reduced from 20 mg/L to 0.43 mg/L which resulted in the removal of 97.8% of the humic acid. This encouraging result demonstrated that our electrocoagulation method is effective in the removal of humic acid from groundwater. Key words: groundwater; humic acid; electrocoagulation; aluminium electrode CLC number: X 131.2

1

Introduction

Humic acid (HA) is a complex macromolecular product of the bio-chemical degradation of plants and animal residues. HA can be found in most surface or ground water. According to previous studies, the high HA concentration in drinking water has the potential for some serious diseases, such as stomach cancer. To maintain the safety of drinking water, some removal methods have been employed including adsorption, coagulation, microfiltration and so on[1–3]. The major characteristics of HA are their amorphous, hydrophilic, acidic and dispersive molecular weights. These generate some complicated chemical and physical processes. For example, various functions on the surface of HA become easily complex with metal ions in the water. Moreover, HA can be adsorbed on the surface of colloid granules to form a protective organic film, which greatly enhances the stability of colloid granules. These factors provide grounds to give the removal of HA from water high priority. The existing methods do not satisfactorially remove the dissolved and biodegradable resistant humic acid. Hence it is important to explore new treatment techniques. In the past decades electrocoagulation has been applied for the treatment of many kinds of wastewater, especially industrial wastewater[4–7]. In these applica-

tions, iron and aluminium are commonly used as electrodes. For aluminium electrodes, the electrolytic dissolution of the aluminium anode produces Al3+, which is transformed initially into Al(OH)3 and finally polymerized to Aln(OH)3n. The main reactions occurring in the anodic compartment are as follows[8–9]. Al→Al3++3e

(1)

3+

Al +3H2O→Al(OH)3+3H

+

(2)

nAl(OH)3→Aln(OH)3n

(3)

The main reaction in the cathodic compartment is: ˉ

2H2O+2e→H2+2OH

(4)

The main stages involved in the electrochemically assisted coagulation are shown in Fig. 1[10].

Received 12 March 2007; accepted 20 June 2007 Corresponding author. Tel: +86-516-83883169; E-mail address: [email protected]

Anode e

Pollutants

Alċ Flocs

e

Cathode

H2O

Flotation H2 O

H+ +O2

e

H2 +OH

Fig. 1

Main stages involved in electrocoagulation

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Journal of China University of Mining & Technology

Organic pollutants are generally removed by four reactions, i.e., electrocoagulation, adsorption, net rooling-sweeping and electro-oxidation[11]. A generalized scheme for direct and indirect electro-oxidation processes is demonstrated in Fig. 2. Anode

e

Anode

Product (s) Solution

2

Anode C

Pollutants

Fig. 2

e

C+

to maintain a homogeneous solution in the reactor. The temperature during the experiment was (20±2) ć.

e C

R

C ++RėO Solution

O Solution

Fig. 3

Schematic diagram of experimental set-up

1. Current power supply regulator; 2. Input of HA; 3. Peristaltic pump; 4. Electrodes; 5. Magnetic stirrer; 6. Scraper; 7. Sand filter column; 8. Water tank; 9. Effluent

Schematic representation of direct and indirect electro-oxidation process

Materials and Methods

Humic acid for this experiment was purchased from the Tianjin Chemical Industry Research Institute. The stock solution was prepared by dissolving the required amounts of humic acid in deionized water. The pH value of the solution was adjusted to 10 with 0.1 mg/L sodium hydroxide. The concentration of humic acid was analyzed by using an ultraviolet spectrophotometer (UV-3100). The humic acid concentration is perfectly correlated with absorbency at 254 nm (y=42.0xˉ0.083, R2=1), where y is concentration of HA, x is absorbance[12]. Different concentrations and pH values of humic acid solution can be prepared. We tested pH values of the solution with a WTW526 pH meter. The components of our experimental system are schematically shown in Fig. 3. The reaction container was made of plexiglass. Aluminium plates with dimensions of 70 mm × 120 mm × 3 mm were chosen as electrodes. The interval of the electrode plates is changeable. Magnetic stirring at 300 r/min was used

Water samples were pumped to the reactor by a peristaltic pump and flowed out over a given reaction period. The effluent solution filtrated by a 0.45 µm microfiltration membrane was analyzed by an ultraviolet spectrophotometer at a wave length of 254 nm. The removal rate of humic acid was calculated by the following formula:

η HA˙

A0 − A1 × 100% A0

(5)

where ηHA stands for the removal rate of HA, A0 is the absorbency of the solution before treatment and A1 the absorbency of the solution after treatment.

3

Results and Discussion

3.1 Effect of electrode distance With a current density of 20.5 A/m2 and an intial humic acid concentration of 10 mg/L, the removal efficiency of humic acid under different electrode distance is shown in Fig. 4.

100

20

80

d = 1.0 cm d = 2.0 cm d = 3.0 cm

60 40

15 10 5

20 0

Vol.17 No.4

10

20 30 40 50 Reaction time (min)

60

(a) Removal efficiency

Fig. 4

0

10

20 30 40 50 Reaction time (min)

60

(b) Concentration

Effect of inter-electrode distance on HA removal

The results indicate that the removal process was more efficient at smaller electrode distance. When the inter-electrode distance was 1.0 cm, the removal efficiency of humic acid reached 79% after a reaction time of 20 min, while the removal efficiency was only 56% when the distance was 3.0 cm. An explanation of this may be that over a small distance, the gas, was blown into the cathode compartment, and in-

duces the phenomenon of the formation of great floating complexes. At the beginning of the reaction, the removal efficiency of HA obviously changed with the reaction time. For short electrode distance the current becomes too high and probably causes a short circuit. The shortest electrode interval maintained in our experiment thereafter was 1.0 cm.

FENG Qi-yan et al

3.2

Removal of Humic Acid from Groundwater by Electrocoagulation

Effect of initial pH

The pH value of the solution is one of the more important factors affecting the performance of the electrochemical process[7]. The effect of pH was tested on the synthetic solution by adding acid (HCl) or an alkaline solution (NaOH). In our experiment,

pH values ranged from 3.0 to 9.5. The comparative results for different pH values (3.0, 4.5, 5.5, 7.5 and 9.5), with current density and initial humic acid concentration set at 20.5 A/m2 and 20 mg/L respectively, are shown in Fig. 5.

(a) Concentration

Fig. 5

(b) Removal efficiency

Result of humic acid removal under different pH values

negative charge. Therefore, the treatment efficiency will decrease under higher pH values. 3.3 Effect of current density Fig. 6 illustrates the comparative removal efficiency at different current densities (with the electrode interval at 1.0 cm and an initial humic acid concentration of 20 mg/L). For a given time, the removal efficiency increases significantly with an increase in current density. For example, over a period of 25 min, 94.6% of HA removal can be achieved with a current density at 47.6 A/m2, but only 74% of removal was obtained with a current density at 15.9 A/m2, This shows that electrolysis can produce more Al3+ at a higher current density and then a greater amount of aluminium hydroxide can improve electrocoagulation.

Concentration (mg/L)

Removal efficiency (%)

The results indicate that the reaction performance is dependent on initial pH values, where the lower pH values lead to faster reactions and better efficiency. Theoretically, pH values of the solution affect the appearance of HA directly. An aromatic ring is the basic unit of HA; it is a reticular macromolecule polymer connected by hydrogen bonds between functional retentions. The most active functional retentions are carboxyl and phenolic hydroxyl groups. As a consequence dissociation of H+ form carboxyl or hydroxyl relates to the pH value of the solution. When the pH value is lower, carboxyl and hydroxyl radicals exist in the chemical form of -COOH and -OH respectively. When pH value are higher, they exist in ˉ ˉ the form of -COO and -O . Obviously, under conditions of a higher pH, HA takes on a more negative charge and more Al3+ is consumed to neutralize the

(a) Concentration

Fig. 6

4

515

(b) Removal efficiency

Effect of current density on humic acid removal

Conclusions

Our results demonstrate that HA in groundwater can be removed effectively by an electrocoagulation method. The electrode interval, current density and the initial pH value are important variables that affect HA removal efficiency. Given a small electrode interval and/or a high current density, the lower pH value leads to a better removal rate of HA. With the

current density at 47.6 A/m2 and an electrode interval of 1.0 cm, the removal rate reached 97.8%.

Acknowledgements The authors would like to thank Prof. Zhao Xuan for useful discussions and advice and Li Jingbo for her help with the experiment. (Continued on page 520)

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Journal of China University of Mining & Technology

TOUGH2 is very suitable to simulate water movements in an unsaturated state. Therefore this module can be widely used to study water movements in unsaturated zones. Chemical reactions were not considered in our study. These reactions are different from leachant

Vol.17 No.4

movement in copper ore. Further transport and interaction between leachant and copper ore will be carried out in the future, from which we should obtain useful data beneficial to the exploitation of copper and other depleted ores.

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