Journal of Environmental Sciences 2011, 23(Supplement) S128–S131

Denitrification of coking wastewater with micro-electrolysis Yanli Lv ∗, Yanqiu Wang, Mingjun Shan, Xue Shen, Ying Su School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China. E-mail: [email protected]

Abstract The denitrification for the coking wastewater was conducted by means of original battery principle with Fe-C micro-electrolysis. Fe-C serves as positive and negative electrodes, by which NO2 − -N and TN were reduced to nitrogen, and then the purpose of denitrifieation for coking wastewater was realized. The influences of pH value, carbon particle size, Fe/C ratio (mass ratio), reaction time and coagulation pH value on removal rate of NO2 − -N and TN were investigated. Coking wastewater originated from Jiamusi Coal Chemistry Engineering Company. The optimum conditions of treatment were as follows: the initial pH was 3.0, the dosage of Fe 73.5 g/L, reaction time 70 min, mass ratio of Fe/C ratio 1.0:1.3, coagulation pH 9.0 and sedimentation time 40 min. Under those conditions, nitrogen removal efficiencies of NO2 − -N and TN were beyond 50% and 45%, respectively. Key words: micro-electrolysis; coking wastewater; denitrification; reaction condition

Introduction

1 Experimental

Coking wastewater comes from the high temperature carbonization of coal, purification of coal gas and the process of refining chemical products. It is shown from MRI-spectrogram that there are dozens of inorganic and hundreds of organic compounds in the wastewater. The inorganic ones are mostly nitrogen, thiocyanate, sulfide, cyanide and so on, while the organic ones include mainly single-ring or polycyclic aromatic compounds, heterocyclic compounds with N, S, or O, etc. (Toh and Ashbolt, 2002). Coking wastewater contains a huge mount of ammonia-N, and can cause eutrophication of the water body. Therefore, denitrification is one of the important disposal processes for coking wastewater. A/O and A2 /O are widely adopted for coking wastewater treatment at present. The biochemical-reaction in the processes is traditional nitrification and denitrification. Ammonia nitrogen is oxidized to nitrite (NO2 − ) and nitrate (NO3 − ) by the action of aerobiotic nitrobacteria. Then NO3 − and NO2 − are transformed into N2 , which can emit to the air, by the action of anaerobic denitrifying bacteria with the consumption of organic carbon resource (Van De Graaf et al., 1995). The processes have such problems as high reflux ratio of nitrate solution refluence, external carbon resource requirement because of low C/N ratio, long flow, and high capital investment and running cost. Micro-Electrolysis Method (Lu and Wei, 2001; Zeng et al., 2006; Zhang et al., 2007) is the electrochemical treatment process which using the principle of metal corrosion to form primary battery for removal organic compounds and nitrogenous substances of coking wastewater.

1.1 Experimental materials

* Corresponding author. E-mail: [email protected]

In the experiment, the average influent ammonia concentration is 330–370 mg/L in the wastewater which originated from a coke plant in Jiamusi, Heilongjiang Province. The reactor is self-made micro-electrolytic reactor (Fig. 1). 1.2 Experimental methods Micro electrolytic cell (Fe/C ratio (m/m) is 1:0.7–1:1.9) was added into coking wastewater, and stirred by mechanical stirrer at the agitation velocity of 180 r/min, the mixing time for 20–90 min. Then, pH value are adjusted to 8–9.5 with NaOH. Subsequently, it was precipitated for 20–50 min. Finally, the water quality index of supernatants was determined. During the experimental process, water quality indices, including NH4 + -N, NO2 − -N, NO3 − -N and TN, were studied in detail. 1.3 Debitrification mechanism of micro-electrolysis Denitrification mechanism of micro-electrolysis denitrfication system includes four mechanisms. (1) Electrochemical corrosion properties: During the process, iron particles are used as the anode which is corroded and carbon as the cathode (Wang et al., 2004; Gu et al., 1998; Xu et al., 2003). The reaction as follows: the anode: Fe–2e −→ Fe2+ , E0 (Fe2+ /Fe) = −0.44 V the cathode: 2H+ + 2e → H2 , E0 (H+ /H2 ) = −0.00 V the presence of oxygen and acidic condition: O2 + 4H+ + 4e → 2H2 O, E0 (O2 /H+ ) = 1.23 V the presence of oxygen and alkaline condition: O2 +2H2 O+ 4e −→ 4OH− , E0 (O2 /OH− ) = 0.4 V (2) Function of iron reduction: iron is actively metal

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Denitrification of coking wastewater with micro-electrolysis

Stirring motor Electrolyzer

Fig. 1

Governor

Self-made micro electrolytic cell.

which occurs the reaction as follows in the presence of acidic condition (Johnson, 1996; Quan and Yang, 1989; Tang et al., 1998): Fe + 2H+ −→ Fe2+ + H2 In the oxidation conditions, reaction occurs as follows: Fe2+ − e → Fe3+ (3) Reductive effect of hydrogen: atomic hydrogen ([H]) generated from the electrode-reactions has strong activity, and reductive-action occurs between [H] and NO2 − , NO3 − in coking wastewater (Jin, 1989). (4) Coagulation effect of iron ions: Iron ions is an efficient flocculation agent, and has excellent effect of adsorption and decoloration (Chin et al., 1998). Fe2+ + 2OH− −→ Fe(OH)2 4Fe2+ + 8OH− + O2 + 2H2 O −→ 4Fe(OH)3 Fe(OH)2 and Fe(OH)3 are high efficient flocculants, which has good decoloring and flocculation effect.

2 Results and analysis In this study, micro-electrolysis method had an unstable effect on the removal of NH4 + -N and NO3 − -N, but had a remarkable effect on the removal of NO2 − -N and TN. NO2 − -N and TN were reduced to nitrogen, and therefore denitrification for coking wastewater was realized. The influence of pH value, carbon particle size, Fe/C ratio (m/m), reaction time and coagulation pH value on removal rate of NO2 − -N and TN were studied.

Table 1 that NO2 − -N and TN of effluent were significantly reduced under different pH conditions. The result meant micro-electrolysis process was effective for denitrification of coking wastewater. To determined the best pH conditions, the denitrification effect was calculated, and the result was shown in Fig. 2. The experiment showed that initial pH value greatly influenced the denitrification effect. As shown in Fig. 2, the removal rates of TN and NO2 − -N were 50.8% and 68.5%, respectively, with the highest removal rate and the highest chemical cost (15 yuan RMB/m3 ) at pH 1.0. It was too costly to popularize. Meanwhile, the removal rates of TN and NO2 − -N were higher at pH 3.0 than that at pH 4.0, 5.0 and 6.0. And the removal rates of TN and NO2 − -N at pH 3.0 were 32.1% and 45.6%, respectively, with the lower chemical cost (0.64 yuan RMB/m3 ). Thereby, the optimum initial pH value was 3.0. 2.2 Influence of carbon particle size On the base of the principle of micro-electrolysis reactions, a bigger cathode area can increase the number of iron carbon micro cell under anode surface area fixed in certain extent. The influence of carbon particle size for the effect of nitrogen was investigated (Table 2). It was known that NO2 − -N and TN of effluent gradually decreased when carbon particle diameter from 40 to 80 mesh, however, NO2 − -N and TN of effluent have an increasing trend when carbon particle diameter from 80 to 200 mesh. The experimental results showed that a bigger particle size of carbon particles could increase the cathode surface, and the optimum particle size of carbon particles was 80 mesh. The impact on removal rate of NO2 − -N and TN with Table 1 Water quality of influent and effluent with different initial pH values pH

Influent NO2 − -N (mg/L)

Effluent NO2 − -N (mg/L)

Influent TN (mg/L)

Effluent TN (mg/L)

1.0 2.0 3.0 4.0 5.0 6.0

151.5 151.5 151.5 151.5 151.5 151.5

47.7 49.1 82.4 109.7 116.0 122.4

252.7 252.7 252.7 252.7 252.7 252.7

124.4 125.1 171.6 211.3 214.2 221.4

100

Removal rate (%)

2.1 Influence of initial pH value On the base of the principle of micro-electrolysis reactions, there are differences among different primary battery reactions of nitrogen heterocyclic compounds in coke plant wastewater under different pH values. Under lower acid condition, the hydrogen ions in original battery reaction are deficient, and the function of iron reduction is poor. Under strong acid condition, original battery reaction is promoted, but flocculation is destroyed, which cause so many problems such as larger amount, higher operation cost, and secondary pollution. Therefore, lower nitrogen removal efficiency was obtained. The influence of pH value on NO2 − -N and TN of influent and effluent were listed in Table 1. It was seen from

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90

TN

80

NO2--N

70 60 50 40 30 20 10 0

1.0

2.0

3.0 4.0 5.0 Initial pH value Fig. 2 Removal rate under different pH values.

6.0

Journal of Environmental Sciences 2011, 23(Supplement) S128–S131 / Yanli Lv et al.

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Water quality of influent and effluent with different size of carbon

Particle diameter (mesh)

Influent NO2 − -N (mg/L)

Effluent NO2 − -N (mg/L)

Influent TN (mg/L)

Effluent TN (mg/L)

40 60 80 100 120 160 180 200

152.3 152.3 152.3 152.3 152.3 152.3 152.3 152.3

137.1 129.4 114.5 114.6 115.7 114.9 114.4 116.8

254.8 254.8 254.8 254.8 254.8 254.8 254.8 254.8

231.8 221.1 194.6 196.9 200.7 198.9 199.2 200.1

different carbon particle diameter conditions are shown in Fig. 3. From Fig. 3, we obtained favorable nitrogen removal effect and the removal rates of TN and NO2 − -N with 23.6% and 67.6% under the chosen conditions. Moreover, the settle ability of carbon particle was favorable after reaction. 2.3 Influence of Fe/C ratio The amount of carbon was increased, the number of the original battery in the system was improved, and the effect of primary battery reaction was raised. Different tests were made on the influences of Fe/C ratio (1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5 1:1.6, 1:1.7, 1:1.8, 1:1.9, m/m). The optimum Fe/C ratio was 1:1.3, and removal efficiencies of TN and NO2 − -N were 50% and 58% (Fig. 4). 2.4 Influence of reaction time It’s well known that significant effects of oxidationreduction reactions were obtained when the reaction time of micro-electrolysis was extended. The experimental purpose was that NO2 − -N or NO3 − -N is converted to N2 , but not converted to NH4 + -N. Therefore, the reaction time could not be too long. Different reaction time was studied (Table 3). Table 3 showed that in the beginning, NO2 − -N and TN of effluent decreased with the extension of reaction time. But when reaction exceeded 70 min, NO2 − -N and TN of effluent increased with the extension of reaction time. Based on micro-electrolysis reaction mechanism,

Removal rate (%)

Table 2

100 90 80 70 60 50 40 30 20 10 0

Vol. 23

TN NO2--N

1:0.7 1:0.8 1:0.9 1:1.0 1:1.1 1:1.2 1:1.3 1:1.4 1:1.5 1:1.6 1:1.7 1:1.8 1:1.9

Fig. 4

Fe/C ratio Removal rate under different Fe/C ratios.

Table 3 Water quality of influent and effluent with different reaction time Reaction time (min)

Influent NO2 − -N (mg/L)

Effluent NO2 − -N (mg/L)

Influent TN (mg/L)

Effluent TN (mg/L)

20 30 40 50 60 70 80 90

155.6 155.6 155.6 155.6 155.6 155.6 155.6 155.6

146.2 149.3 137.5 127.9 122.4 105.7 121.8 127.9

254.8 254.8 254.8 254.8 254.8 254.8 254.8 254.8

249.7 244.6 230.0 214.0 204.8 189.8 203.8 214.0

the longer reaction time is, and the more thoroughly the redox reaction is carried out. Thus, NO2 − -N is converted to NH4 + -N when reaction time was too long. As shown in Fig. 5, the optimum reaction time was 70 min, and removal efficiencies of TN and NO2 − -N were 25.5% and 32.1%. 2.5 Influence of coagulation pH value Under alkaline condition, Fe2+ , Fe3+ produced by electrolysis carried out coagulation reaction and became downy precipitation (Fe(OH)2 , Fe(OH)3 ). Thus, coagulation reaction had good decoloring and flocculation effect. Ammonia nitrogen from coking wastewater was further removed through the above effect. According to the iron ion coaguflocculation mechanism, Fe(OH)2 colloid was firstly generated, and then Fe(OH)3 colloid was generated. The experiment result was shown in Fig. 6. Figure 6 shows that coagulation effect of iron ions can well remove pollutants in coking wastewater, and

45

TN

40

NO2--N

50 TN

35 30

Removal rate (%)

Removal rate(%)

50

25 20 15 10 5 0

40

Fig. 3

60

80 100 120 160 180 200 Carbon particle size (mesh) Removal rate under different carbon particle sizes.

40

NO2--N

30 20 10 0

20

30

40

50 60 70 80 Reaction time (min) Fig. 5 Removal rate under different reaction time.

90

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Denitrification of coking wastewater with micro-electrolysis

100 TN

90

NO2--N

Removal rate (%)

80 70 60 50 40 30 20 10 0

8.0

8.5 9.0 9.5 Coagulation pH value Fig. 6 Removal rate under different coagulation pH values.

the optimum coagulation pH value condition was 9.0. Removal efficiencies of TN and NO2 − -N were 49.1% and 56.0 %, respectively.

3 Conclusions In conclusion, compared with traditional denitrification process, a new treatment method with micro-electrolysis for denitrification of coking wastewater was proposed, which has so many advantages as the wide scope of application, the good processing effect, the low cost and the convenience of operating and maintaining. The optimum conditions of treatment were as follows: the initial pH was 3.0, reaction time 70 min, mass ratio of Fe/C ratio 1:1.3, coagulation pH 9.0. Under those conditions, removal efficiencies of TN and NO2 − -N were over 45% and 50%, respectively. Acknowledgment This work was supported by Anshan Science and Technology Foundation (No. 08SF06.)

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