Internal Micro-electrolysis Using Fe/C Material for Pre-Treatment of Concentrated Coking Wastewater

Untreated coking ef ﬂ uent presents a great challenge for sustainable development of the steel industry and envi-ronment preservation. In this study, an internal micro-electrolysis method using Fe/C materials was employed for pretreatment of real coking wastewater with high mass concentration. The Fe/C materials were prepared by Fe powder and graphite powder; and the characteristics of surface morphology, structure, composition of the synthesized materials were examined by Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Energy Dispersive X-ray Spectroscopy (EDS). The effects of factors namely dosage of Fe/C material, treatment time, initial pH and temperature were investigated via chemical oxygen demand (COD) and phenol removal ef ﬁ ciencies. Optimal treatment ef ﬁ ciency was attained at pH of 4, Fe/C dosage of 40 g/L, treatment time of 360 minutes and temperature of 25 o C. After the internal electrolysis process, the values of COD, BOD5, and phenol of the wastewater were 6500, 4850 and 0.1 mg/L, respectively.


INTRODUCTION
The effi cient and effective treatment of coal gasifi cation wastewater (CGW) plays a crucial role in developing the coal industry. Coking wastewater is one of the most diffi cult wastes to treat due to its complexity and widely variable composition, which contains a large amount of organic, aromatic and cyanide contents 1-5 . As such, combinations of physical, chemical and biological processes are usually employed to resolve the issue 6-7 . In physical methods, activated carbon, in the form of powder or granular, is often used to remove toxic substances such as aromatic compounds or cyanide from coking wastewater with high effi ciencies 8-10 . Meanwhile, chemical processes commonly adopt extraction, fl occulation, Fenton or oxidation by O 3 to treat the wastewater 11-14 . However, the two processes share a common shortcoming of having expensive operational costs and causing secondary pollution. Moreover, chemical processes also present diffi culties in the treatment of highly concentrated wastewater and require complex instruments that are diffi cult for upscaling.
Internal electrolysis has been an emerging technique suitable for the pre-treatment of wastewater containing pollutants with low biodegradability and at high concentrations. The technology applies to various industries such as textile, pharmaceuticals, paper, fertilizer and or pesticide 9, 11, 14 .
Internal electrolysis allows the following reactions to occur [7,8,13,15]: (1) The effect of electric fi elds: Micro-batteries in wastewater will generate electric fi elds and have the effect of causing charged pollutants to move to opposite electrodes. Then at the surface of the electrodes, there will be a redox reaction against charged pollutants. As a result, the chemical structure of the pollutants would be transformed or degraded.
(2) The reducing effect of hydrogen: Iron is a metal with strong reducing properties, in an acid environment, the following reaction: The reaction takes place at the electrodes and will produce hydrogen atoms [H], which have strong reducing activity. Then in solution, they will reduce pollutants. For example, grouped pollutants will be reduced and converted to am ino group compounds.
(3) The effect of metallic iron: The metals behind iron in the action series can exchange electrons on the metallic iron surface. Then metal ions with strong toxicity or organic substances will be reduced by iron to metal ions in a less toxic state. For example, Cr (VI) with has strong oxidizing properties, in an acidic environment, metal iron will react: . Then Cr 6+ with strong oxidizing properties will be converted to Cr 3+ with weak reducing properties.
(4) The reducing effect of Fe 2+ ions: Iron is oxidized to iron Fe 2+ , Fe 2+ has high reducing properties. For Cr (VI), the reduction reaction occurs as follows: . For pollutants such as azo dyes, the dye-generating radicals of the dye will be reduced by Fe 2+ and converted to amine compounds, at which point the color of the wastewater will be reduced. The reaction goes as follows: The fl occulation effect of iron ions. In the condition of acidic wastewater, metallic iron will corrode and produce more Fe 2+ and Fe 3+ ions. In the presence of O 2 , reactions will occur in alkaline environments: The newly born Fe(OH) 2 and Fe(OH) 3 have a high ability to adsorb organic substances. Through the internal electrolysis reaction, the pollutants are chemically changed and the newly formed substances will be fl occulated by Fe(OH) 3 .
(6) The effect of chemical precipitation: Fe 2 + and Fe 3 + ions in water when meeting with inorganic elements will precipitate into compounds such as FeS, Fe 3 [Fe(CN) 6 ] 2 and Fe 4 [Fe(CN) 6 ] 3 . This compound will quickly settle down and be easily removed.
After pre-treatment of coke wastewater with Fe/C materials, the content of COD, BOD 5 , and phenolic compounds, cyanide will be reduced, effi ciency for biological treatment.
The reaction effi ciency of the zero-valence iron (Fe o ) could be further improved by employing an internal electrolysis reaction combining iron with carbon element, which has more high voltage values 16- 19 .
In this study, we aim to improve the treatment effi ciency of coking wastewater by investigating several experimental conditions of the internal hydrolysis process. Considered parameters included pH, temperature, treatment time, dosage of Fe/C materials and their impacts were monitored to the removal of coking waste in real wastewater of a steel plant. The results are expected to aid in the justifi cation of internal electrolysis using Fe/C material into real applications and contribute to mechanism elaboration of the internal electrolysis reaction.

Fabrication of Fe/C material
Chemicals including Fe powders with a particle size of less than 50 μm and 99.9% purity were obtained from Meiqi Industry and Trade Co.Ltd (Xihe Village, Beishankou Town, Gongyi, Henan, China), natural graphite powder with the particle size of smaller than 50 μm and 99.9% purity was obtained from Dingyida international trade (Dalian) Co., Ltd; and (NH 4 ) 2 CO 3 99.9% purity was purchased from Sigma Adrich.
Fe/C materials were prepared as follows. The fi rst following ingredients were mixed at a defi ned weight ratio: 95% Fe, 3% graphite and 1.5% bentonite binder and 0.5% (NH 4 ) 2 CO 3 99.9%. The mixture was pressed into blocks, at the pressure of 5 ton/cm 2 and dried at 80-105 o C for 2 hours. Dried blocks were then sintered at 500-600 o C, for 4 hours and subsequently stored in a desiccator for use in subsequent studies.

Collection of coking wastewater
Real coking wastewater was collected from a steel factory located in Thai Nguyen Province, Vietnam (Luu Xa Steel Co. Ltd). Some indicators of the wastewater sample were given as in Table 1.

Method and instruments
As-synthesized Fe/C sample material was characterized using several techniques and instruments including scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) methods (SEM-EDS, S4800, Hitachi) and XRD (Siemens/Bruker, D5000).
Values of COD, BOD 5

SEM-EDS analysis of Fe/C
SEM-EDS image analysis results are shown in Figures  1, 2 and Table 2. Visually, Fe and C powder particles were distributed relatively evenly on the material surface with the size of smaller than 50 μm. EDS analysis results Table 1. The characteristics of coking wastewater allowed to vary from 3 to 7. Other parameters included 4.0 g of Fe/C material for 100 ml coking wastewater (the sample N1), the temperature of 25 o C and shaking speed of 160 rpm, processing treatment time of 6 hours. Figure 4 illustrates phenol and COD variations to different pH values. From the pH range from 3 to 5, no clear changes in removal of COD and phenol were observed and the treatment effi ciency was about 24.5%, corresponding to the COD and phenol content of around 6500 and 0.3 mg/L respectively. Although higher pH could facilitate the subsequent biological treatment, current results support low pH for optimal COD and phenol removal. Because very low pH may raise the cost for pH adjustment, we selected a pH value of 4 for the next study. (

XRD analysis of Fe/C
The results of XRD analysis (Fig. 3) showed that Fe in the Fe/C sample was oxidized, as indicated by structures of

Impact of experimental parameters on treatment efficiency of coking wastewater.
Infl uence of pH pH value is an important parameter that may affect the reaction effi ciency. In this investigation, the pH was

Infl uence of temperature
In this investigation, 4.0 g of Fe/C material was used, and 100 mL of wastewater was added into 250 mL fl asks under shaking (160 rpm). The reaction was performed under the temperature from 25 to 40 o C. The results are shown in Figure 5. COD and phenol content in wastewater seemed to deplete more quickly under the temperature of 40 o C than under 25 o C. However, after 6 hours, the attained processing effi ciencies from the two examined temperatures were almost identical, reaching the COD of around 6500-6550 mg/L and phenol of around 0.1-0.3 mg/L.
As the dosage increased from 30 to 100 g per 1 liter of wastewater, removed COD removed in coking wastewater increased from 1350 mg/L to 2250 mg/L and phenol removal effi ciency was also improved from 25 to 99.95%.
Identifi cation of a suitable material dosage carries important implications for optimization of treatment effi ciency and process scalability. An increased amount of used internal electrolysis materials might result in a larger surface area of the reaction electrode and necessitate a larger volume for wastewater storage. On the other hand, stronger electrochemical oxidation may produce excessive [OH*] and [OH -], Fe 2+ . For optimal processing effi ciency, 40 g of Fe/C materials per l liter of wastewater was selected as the appropriate dosage for the next study.

Infl uence of time
The infl uence of time on treatment effi ciency was investigated in the reaction time ranging from 1 to 8 hours. Other conditions included dosage of 4.0 g of Fe/C material, pH value of 4, reaction temperature of 25 o C and shaking speed of 160 rpm. The results are shown in Figure 7.
Within the fi rst hour, a decline in COD and phenol was moderate but then accelerated as the reaction time elapsed past 1 hour. After 2 hours value of COD dropped from 8500 mg/L to about 6800 mg/L and phenol content dropped from 75 mg/L to about 18 mg/L. In the period from 2 to 4 hours, COD and phenol decomposition performance was stagnant and eventually reached the Temperature plays a crucial role in practical wastewater treatment since temperature infl uences both treatment effi ciency and the economic aspects of the process. As a result, technological processes are often designed to refrain from veering the process temperature from natural conditions. The reaction rate is proportional to temperature, but the change is small. Given that the fi nal treatment indicators achieved after 6 hours were indistinguishable between 25 and 30 o C, a temperature of 25 o C was selected as the condition for subsequent studies. This temperature is also consistent with that of ponds and streams in Vietnam.

Infl uence of Fe/C dosage
In this investigation different dosages including 0.5, 1.0, 3.0, 4.0, 5.0, 7.5 and 10 g were selected. The experiment commenced by adding 100 mL of wastewater into 250 mL fl asks, followed by pH adjustment to 4. The reaction time was 6 hours, the agitation speed was 160 rpm and the temperature was set to 25 o C. Experiment results are presented in Figure 6. minimum after 6 hours, at which point phenol was no longer detected and COD was approximately 6500 mg/L. Table 3 summarizes the main indicators of the wastewater treated with Fe/C electrolytic material. Overall, COD treatment effi ciency was approximately 24.5%. However, NH 4 + treatment effi ciency was still modest. This highlights the importance of pretreatment processes using internal electrolysis for enhancing biological degradability of coking wastewater 17, 18 .

CONCLUSIONS
The Fe/C material was successfully prepared from Fe powder and graphite powder. The samples were characterized for surface characteristics, structure and composition by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS), respectively. The synthesized material was applied for the treatment of real coking wastewater and the process was optimized by varying several factors including pH, time, temperature and dosage of Fe/C. Optimal effi ciency was achieved after 6 hours of treatment, at pH 4.0, with the dosage of 40 g Fe/C material per 1 liter of wastewater and at the temperature of 25 o C. COD and phenol were reduced from 8800 mg/L to 6500 mg/L and from 110 mg/L to 0.1 mg/L, respectively. Interestingly, the treatment effi ciency of toxic ingredients such as phenol, cyanide is very effective at 99.95%, after 6 hours of treatment. The initial concentration of phenol is 110 mg/L high, and then 0.1 mg/L after the internal electric treatment, which is favorable for the biological process of further treatment. This is the preeminence of the internal electrolysis process to treat coke wastewater. Current results implied the stability of the internal electrolysis in the treatment of coking wastewater and demonstrated that the process can be used to improve the biodegradability of coking wastewater.

ACKNOWLEDGMENTS
This work was completed with fi nancial support from the Ministry of Education and Training of Vietnam, under project B2019-TN A-10. Figure 7. Infl uence of time on the effi ciency of coking wastewater treatment Table 3. The characteristics of coking wastewater after pretreatment by the Fe/C material, pH 4, 40 g/L Fe/C, 100 mg/L of PAM (Polyacrylamide, Xunyu Group Co., Limited, China) fl occulant, 6 hours treatment