1 Introduction

A major fraction of the natural organic matter (NOM) in surface or ground waters is composed of humic acid (HA), and which is complex macromolecular product of the chemical and biological degradation of plants and animal residues. HA is a substance which is amorphous, brown or black, hydrophilic, acidic and dispersive molecular weight. Humic acid adsorbed on the surface of particles can alter their surface properties. Increase of humic acid uptaking were assumed to be the neutralization of surface charges and the compression of diffuse double layers. According to its properties the familiar treatment on humic acid is adsorption, coagulation, micro-filtration and so on ^[1-3]. But the removal efficiency for humic acid of these methods are not obvious, so it is important to study new technique to remove humic acid for the safety of drinking water.

In the past decades electrocoagulation has been applied for the treatment of many kinds of wastewater [4-7]. When aluminum is used as electrode, the electrolytic dissolution of the aluminum anode produces Al³⁺, which is transformed initially into Al(OH)3 and finally polymerized

Al→Al³⁺+3e                              (1)

Al³⁺+3H₂O→Al(OH)₃+3H⁺                  (2)

nAl(OH)₃→Al_n(OH)_3n                      (3)

Al_n(OH)_3n+mF^-→Al_nF_m(OH)_3n-m+mOH^-        (4)

 Main reaction in cathodic compartment is as follows:

2H₂O+2e→H₂+2OH^-                        (5)

Fig. 1 Main stages involved in the electrocoagulation^[10]

Fig. 2 Schematic representation of direct and indirect electro-oxidation process

The main stages involved in the electrochemically assisted coagulation are shown in Figure 1. A generalized scheme for direct and indirect electro-oxidation processes was demonstrated in Figure 2.

According to researches before humic acid in water are generally removed by electrocoagulation-adsorption, net rooling-sweeping and electro-oxidation action. 

2 Materials and methods

Humic acid for experiment is produced by 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 0.1mg/L sodium hydroxide. The concentration of humic acid was analyzed by using Ultraviolet Spectrophotometer (UV-3100). The humic acid concentration has a good relativity with absorbency at 254nm (y＝42.0x-0.083，R2＝1) [11]. Different concentration and pH value of humic acid solution can be confected, and pH value of solution was tested by WTW526pH meter.

The experimental setup is schematically shown in Figure 3. The setup was made of plexiglass Aluminum plate was chosen as electrodes (it can be embedded several electrode planks and the inter-electrode distance is changeable) each with a dimension of 70 mm×120 mm×3 mm. Magnetic stirring at 300 rpm maintained a homogeneous solution in the reactor. The temperature in experiment was  20±2℃.

The water sample was injected from the top of the reactor and  flow out in a given reaction time. The effluent solution filtrated by 0.45μm microfiltration membrane was analyzed by ultraviolet spectrophotometer at a wave length of 254 nm. The removal rate of humic acid was calculated by the following formula:

               (6)

Where η_HA stand for removal rate of HA, A0 is absorbency of solution before treatment, A1 is absorbency of solution after treatment.

Figure 3 Schematic diagram of experimental set-up

1. direct current power supply  2. electrodes 

3. magnetic stirrer

3 Results and discussion

3.1 Effect of inter-electrode distance

When current density was 20.5A/m2, the humic acid initial concentration was 10mg/L the removal efficiency of humic acid under different inter-electrode distance was shown in Figure 4.

Fig. 4 Effect of inter-electrode distance on HA removal

The results indicated that the removal process was more efficient with the inter-electrode distance being decreased. When the inter-electrode distance was 1.0cm, the removal efficiency of humic acid reached 79% after 20min reaction, while the removal efficiency was only 56% when the distance was 3.0cm. These results can be explained that the floatation function of air bubble produced at the cathode area was big, and the flocs were easy to floated when the inter-electrode distance was small. At the beginning of reaction, the removal efficiency of humic acid changed with reaction time obviously. For short inter-electrode distance the current become too high and  it probably causes short circuit. Therefore, the inter-electrode distance in experiment thereafter was 1.0 cm. 

3.2 Effect of influent pH

The influent pH value is one of the important factors affecting the performance of electrochemical process. The influence of pH on the electrocoagulation was tested on the synthetic solution by simple addition of acid (HCl) or alkaline solution (NaOH). In experiment  pH value ranged  from 3.0 to 9.5. When current density was 20.5A/m², and the humic acid initial concentration was 20mg/L, the results were shown in Figure 5.

Reaction time/min

Fig. 5 Result of humic acid removal under different pH value

The results indicate that the reaction was quick and the removal efficiency was high when the initial pH was low, while the removal rate decreased and removal efficiency reduced gradually with pH increased. This is mainly because of  the pH influence on appearance of humic acid directly.

Aromatic ring is the basic unit of humic acid, and it is a reticular macromolecule polymer which connected by hydrogen bonds between functional retention. The most of  active functional retentions are carboxyl and phenolic hydroxyl group, and dissociation of H+ relates to pH value of solution. When pH value is lower, carboxyl and hydroxyl radical exist in -COOH and -OH form respectively, When pH value is higher, they can exist in -COO－ and -O－ form. It is obvious that when pH value is higher, humic acid takes more negative charge, and it needs more Al3+ to neutralize the negative charge. So treatment effect and efficiency are all descended under higher pH value.

3.3 Effect of current density

When the inter-electrode distance was 1.0cm, and humic acid initial concentration was 20mg/L, the relation between removal efficiency and current density presented in Figure 6. As expected,  for a given time the removal efficiency increased significantly with increasing of current density. When the current density was 47.6A/m2 , about 94.6 % of HA was removed after 25min reaction but it needed much more time under lower current density. This indicated that at high current density more aluminum ion was produced by electrolysis. And then aluminum hydroxide improved electrocoagulation.

Fig. 6 Effect of current density on humic acid removal

4 Conclusions

The experimental results showed that inter-electrode distance, current density and influent pH are important variables that affect humic acid removal efficiency. The inter-electrode was smaller and the current density was higher, then the removal rate was higher. In experimental condition the removal efficiency decreased with increasing of pH value. When current density was 47.6A/m2 the effluent humic acid concentration decreased from 20mg/L to 0.43mg/L, the removal rate reached 99.8%. The results indicate that humic acid in groundwater can be removed effectively by electrocoagulation.

Acknowledgement

The authors would like to thank Prof. Zhao Xuan for useful discussions and advices and Li Jingbo for her experimental help.

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