Effects of process parameters on the degradation of high salinity industrial wastewater

High salinity wastewater is characterized by high salt content, a large number of organic pollutants and dif ﬁ culty in biochemical degradation, which has become a major problem in industrial wastewater treatment. In this article, the electrochemical oxidation technology was used to treat high salinity wastewater. The effects of temperature, current density, pH and additives on the removal effect of high salinity wastewater were investigated to optimize the process parameters. The results show that the best degradation effect is when the current density is 21.43 mA cm (cid:1) 2 , pH ¼ 2, the temperature is 60 (cid:3) C, and electric ﬁ eld activates additional persulfate. After puri ﬁ cation of high salt wastewater, the evaporated salt can be utilized as a resource. The industrial cost of degradation was estimated, and its economic bene ﬁ ts were calculated. This work will provide a theoretical and experimental basis for treating high salt wastewater by boron-doped diamond (BDD) electrochemical degradation technology.


INTRODUCTION
High salinity wastewater refers to the wastewater with a total salt content of more than 1%, containing a large amount of organic matter, organic heavy metals and radioactive substances (Lefebvre & Moletta ). In the chemical industry, many pharmaceuticals and pesticide intermediates (such as chloramphenicol, ciprofloxacin, etc.) are produced, which result in a large amount of wastewater. High salinity organic wastewater will not only corrode buildings and industrial equipment but also inhibit or poison microorganisms. Untreated wastewater will pollute our living environment (Lefebvre & Moletta ; Ismail et al. ). The removal of organic pollutants from high salinity industrial wastewater is of vital importance to the ecological environment, and the purification of high salinity industrial wastewater will also facilitate the recycling of resources.
It is difficult to achieve the degradation of high concentrated organic wastewater with high salinity by traditional treatment technology. For example, a biological method is adopted for treatment, which mainly decomposes organic matter by cultivating salt-tolerant bacteria ( (1) and (2) (Enache et al. ; Zhou et al. ). Inorganic salts such as NaCl and Na 2 SO 4 in salt-containing wastewater can be used as supporting electrolytes so that no additional additives are needed in the electrochemical oxidation process, which is conducive to resource-saving.

Electrochemical degradation experiments
In the experiment of wastewater degradation, the prepared BDD electrode was used as the anode, and the cathode was the 304 stainless steel sheet with a size of 60 × 80 mm (it requires ultrasonic vibration in an ethanol solution to remove oil stains). The power supply used in this experiment was the DC stabilized power supply (RD-3020, China). A 500 mL glass beaker was used as the electrolyzer. The beaker was placed on the magnetic agitator (MS7-H550-Pro Dalong Instrument Co., Ltd, Beijing) and the speed was adjusted to 200 rpm.

Analysis of degradation effect
The total organic carbon analyzer (TOC-L, Shimadzu, where TOC 0 (mg L À1 ) is the initial TOC value of the wastewater, and TOC t (mg L À1 ) is the TOC value when the degradation time of wastewater is t.
The energy consumption of the experimental process is analyzed by the calculation formula (4) (Martínez-Huitle & Brillas ):  to the cathode peak current was also close to 1, indicating its good reversibility.

Influence of current density (J )
The increase of current density leads to increasing mineralization efficiency of pollutants on or near the electrode surface, but it will also increases energy consumption. Figure 2 shows the degradation effect of the BDD electrode on NaCl and Na 2 SO 4 industrial wastewater at the current density of 7.14, 14.25 and 21.43 mA cm À2 . The generation rate of ·OH and other strongly oxidizing substances (ROs) increases with the increase of current density, showing higher degradation efficiency. As shown in Figure 2(a) and 2(d), the color removal gradually gets better with the increase of current density. For NaCl industrial wastewater, the degradation time required to reach transparency under the current densities of 7.14, 14.25 and 21.43 mA cm À2 was 6, 3 and 3 h, respectively. For Na 2 SO 4 industrial wastewater, the degradation time required to reach transparency at the current densities of 7.14, 14.25 and 21.43 mA cm À2 was 24, 9 and 6 h. As can be seen from Figure 2 2(e) and 2(f), with the increase of current density, the chroma removal rate and the TOC removal of NaCl and Na 2 SO 4 industrial wastewater were gradually improved.
Under the condition that the current density was increased from 7.14 to 21.43 mA cm À2 , the mineralization efficiency of NaCl and Na 2 SO 4 industrial wastewater after the degra-

Influence of pH
The hydroxyl radicals (·OH) tend to be extremely sensitive Use HCl and NaOH to adjust the initial pH value of NaCl industrial wastewater, and use H 2 SO 4 and NaOH to adjust the initial pH value of Na 2 SO 4 industrial wastewater. When the pH value of NaCl industrial wastewater was 2, 7 and 10, the required degradation time to reach transparency was 3, 6 and 6 h, respectively. When the pH value of Na 2 SO 4 industrial wastewater was 2, 7 and 10, the required degradation time to reach transparency was 12, 24 and 24 h, respectively. It can be seen from Figure 3 2Cl À ! Cl 2(g) þ 2e À ⇌ Cl 2(aq) þ 2e À In the acid solution, the main substances generated are Cl 2 and HClO. ClO À mainly exists in alkaline conditions.
The standard potential of Cl 2(aq) (E 0 ¼ 1.36 V vs. SHE) and HClO (E 0 ¼ 1.49 V vs. SHE) is higher than that of ClO À (E 0 ¼ 0.89 V vs. SHE). Therefore, these substances oxidize organic matter indirectly faster in acidic media than in alkaline media (Boxall & Kelsall ). In addition, a small amount of toxic by-products such as chlorite, chlorate and perchlorate may be produced during the degradation process.
During the degradation process, some special reactions will also occur in the Na 2 SO 4 industrial wastewater. The oxidation reaction of SO 4 2À occurs at the anode, as shown in formula (8)

Influence of persulfate additive
Persulfate (PS) itself has a certain oxidation property, which it reacts slowly with organic pollutants. Persulfate can be activated easily by the BDD electrode to produce highly active free radical, sulfate radical SO 4 ÀÁ . Its oxidation approaches even beyond ·OH, the strong oxidizing group produced by the BDD electrode (Wang & Wang ).

Cost assessment
Degradation costs are critical for industrial applications.   USD per ton). So the content studied in this chapter is feasible in industrial production.
Persulfate has been regarded as a potential method because it can improve the degradation efficiency of wastewater. However, the actual cost assessment found that although the use of persulfate improves the degradation efficiency of high salt wastewater, the cost comparison actually does not have much advantage over other optimization methods.
The process parameters studied in this chapter are conducive to significantly reducing the treatment cost of high salt industrial wastewater by electrochemical degradation of the BDD electrode, laying a foundation for efficient industrial degradation of high salt industrial wastewater.
In Table 1, W 1 is the electricity consumed by 1 ton of wastewater reaching 80% mineralization efficiency and the unit is kWh; W 2 is the electrical energy consumed by  Degradation conditions: J ¼ 7.14 mA cm À2 , T ¼ 25 C, pH ¼ 7.
generating 1 ton of salt (about 140 g salt can be produced after evaporation of 1 L high salt industrial wastewater) and the unit is kWh; F 0 is the cost of PS consumed by 1 ton of salt and the unit is USD; F 1 is the electricity cost consumed by 1 ton of salt (calculated using electricity prices during valley period) and the unit is USD. Because the treatment prices of solid waste and hazardous waste are calculated in 1 ton, the electricity cost is calculated based on the cost of 1 ton of salt produced by the electrochemical degradation.
F 2 is the sum of the solid waste treatment cost for 1 ton of salt, electricity cost and persulfate cost, and the unit is USD.

CONCLUSION
In this work, BDD electrodes were prepared using HFCVD technology. The effects of current density, temperature, pH, external electric field activation of persulfate technology and other environmental conditions on the degradation effect of NaCl and Na 2 SO 4 industrial wastewater were studied. The specific conclusions are as follows: 1. As the current density increased, the degradation effect of BDD electrodes on NaCl and Na 2 SO 4 industrial wastewater gradually increase. With the increase of current density from 7.14 to 21.43 mA cm À2 , the mineralization efficiency of NaCl and Na 2 SO 4 industrial wastewater was increased by 14.40 and 24.70% after 24 h degradation, respectively.
2. The pH value affects the degradation of high salt industrial wastewater by the BDD electrode. Acidic conditions are favorable for degradation, followed by neutrality and alkaline conditions are the worst. Under the condition of pH ¼ 2, the required degradation time for the NaCl and Na 2 SO 4 industrial wastewater to reach transparency is 3 and 12 h, and the mineralization efficiency after degradation for 24 h is 83.21 and 85.72%, respectively.
3. As the temperature increased, the degradation effect of BDD electrodes on NaCl and Na 2 SO 4 industrial wastewater gradually increased. With the increase of temperature from 25 to 60 C, the mineralization efficiency of NaCl and Na 2 SO 4 industrial wastewater after degradation for 24 h was increased by 9.64 and 17.56%, respectively. 4. Compared with single electrochemical oxidation (BDD), the external electric field-activated persulfate technology (BDD þ PS) is more dominant in terms of degradation of wastewater. Compared with BDD technology, the mineralization efficiency of NaCl and Na 2 SO 4 industrial wastewater using BDD þ PS technology was increased by 9.39 and 15.62%, respectively. 5. In actual production, the results of the comparison of the total cost to electrochemically degrade 1 ton of high salt industrial wastewater by the BDD electrode under better conditions are as follows: F T ¼60 C < F pH¼2 < F J¼21:43 mA cm À2 < F BDDþ0:1M PS . After degradation, evaporation and crystallization, the purified salt can be used as industrial salt. The use of persulfate improves the degradation efficiency of high salt wastewater, but there is no advantage compared with other methods from the cost comparison. The total cost is significantly lower than the cost of hazardous waste treatment.
The process parameters studied in this work help significantly reduce the treatment cost of high salt industrial wastewater degraded by electrochemical oxidation of BDD electrodes and lay the foundation for the high-efficiency industrial degradation of high salt industrial wastewater.