H2SO4 has an effect on the sorption of organic contaminants by coking coal (CC) in wastewater. This paper focused on the effect of pH on the removal of chemical oxygen demand (COD), phenols and ammonia. UV-vis spectra, Fourier transform infrared spectra, zeta potential and Brunauer, Emmett and Teller (BET) analysis were investigated to characterize the changes of CC properties and coking wastewater (CW) at different pH values. The results showed that the COD and phenol removal efficiencies increased with decreasing pH value, while the ammonia removal efficiency was decreased gradually. A new transmittance band in the region of 340–600 cm−1 was observed in UV-vis spectra of CW in acidic condition. The absolute value of the zeta potential as the solution was gradually increasing with the increasing of pH value. Surface area and total pore volume of CC which was immersed in acidic solutions measured by BET were much higher than that of raw CC. CC has a greater adsorption capacity to organic pollution in the acidic solution mainly by van der Waals forces and hydrogen bonding.

INTRODUCTION

Coking wastewater (CW) from coking, coal gas purification and by-product recovery processes in the coke plant has become one of the most severe environmental problems in China. CW is known to be a harmful pollutant because of its toxic, complex, malodorous, mutagenic and carcinogenic nature. According to the 2013 environmental statistics, the amount of wastewater discharged from the coke industry accounted for 240 million m3 (SEPA, 2013). CW contains complex inorganic and organic pollutants, such as ammonium sulfate, cyanide, thiocyanate, phenolic compounds, benzo(a)pyrene, polynuclear aromatic hydrocarbons (PAHs) and polycyclic nitrogen-containing acyclic compounds (Kim et al. 2008; Zhang et al. 2010; Chu et al. 2012). Most of the compounds in the CW may have long-term environmental and ecological impacts.

Biological treatment is the mature method of CW treatment, it mainly utilizes oxidation, decomposition, adsorption of the microorganism to degrade the compounds of CW. In China, anoxic-oxic (A-O), anaerobic-anoxic-oxic (A-A-O) and sequencing batch reactor processes have been extensively investigated for CW treatment (Yu et al. 1996, 1997; Lee & Park 1998). Although biological treatment is the primary action of the CW treatment system, and 94% of pollutants as assessed by chemical oxygen demand (COD) means are primarily removed during the biological treatment process (Burmistrz et al. 2014), the effluent still is not able to meet the reuse standard requirement (COD < 100 mg L−1). Hence, it is very important to choose appropriate methods for CW pretreatment to improve the treatment efficiency of biological processes (Wu & Zhu 2012).

Due to its efficiency in removing a wide range of organic compounds and easy to operate procedures, the adsorption technique is a frequently used method for treating wastewater (Zhang et al. 2010). Adsorption of organic compounds from wastewater by activated carbon and other porous carbon materials has been studied for several decades. Rudy et al. used various low-cost adsorbents of natural origin to remove PAHs from petrochemical wastewater (Rudy et al. 2008). Kaustubha et al. designed a loop airlift reactor to remove phenol from wastewater by means of its adsorption onto the surface of activated carbons (Kaustubha et al. 2008).

A suitable adsorbent is the prerequisite for technical application of an adsorption process. However, the high price and cost of regeneration is the biggest barrier to the implementation of commercial sorbents. An attempt to overcome this barrier was made by using materials with fairly good sorptive properties at the lowest possible price and without the need for regeneration.

Wang et al. described a sustainable wastewater treatment technology (Wang et al. 2014). It can be used both in pretreatment and depth processing of CW. This technique has been successfully applied to industrial processes. The operating process of this technology is shown in Figure 1. The sorbent of this technology is the raw material of coke which has a good sorptive properties as well as a low price. After adsorption, the coking coal (CC) goes back into the coking system instead of entering a regeneration process. Gao et al. had confirmed the feasibility of this technology, and the adsorption after biological treatment of CW with CC at 40 g·L−1, the COD removal was 75%, and the concentration was reduced from 178 to 44 mg·L−1; given an adsorption by CC of 250 g·L−1 prior to the biological treatment of CW, the elimination of COD and phenol was 58% and 67%, respectively (Gao et al. 2015).
Figure 1

The operation process of new technology.

Figure 1

The operation process of new technology.

After a series of preliminary experiments, the results showed the adsorption capacity of CC was different at different pH values of wastewater. So the aim of this study was (i) to explore the sorptive properties of CC at different aqueous environments of pH value and (ii) to discuss of the influence of pH value of CW and CC.

MATERIALS AND METHODS

Materials

CC and CW were taken from a coke plant in Mongolia, China. CC was the raw material of coke with a particle size of −3 mm, and it was ground up to obtain the mass of −0.074 mm fine coal. The mineral components, functional groups of the coal surface, surface area and pore volume were analyzed with an X-ray diffractometer (XRD), a Fourier transform infrared spectrometer (FT-IR) and Brunauer, Emmett and Teller (BET).

Through the XRD analysis, the CC was mainly composed of amorphous coal with some minerals including quartz, kaolinite, illite and pyrite. The functional groups were mainly C—O, C=O, –OH, etc., and the surface area, pore volume and mean pore size were 11.015 m2·g−1, 0.01614 cm3·g−1 and 5.8603 nm, respectively.

CW was treated by ammonia distillation, and the wastewater was deep brown in color and emitted a foul odor. Table 1 shows the analytical results of the CW.

Table 1

Water quality of the CW

Parameter Value 
COD (mg·L−13,822 
Phenol (mg·L−1588 
Ammonia (mg·L−1169 
Cyanide (mg·L−12.45 
Color Deep brown 
pH 8.78 
Biochemical oxygen demand (BOD)/COD 0.2 
Parameter Value 
COD (mg·L−13,822 
Phenol (mg·L−1588 
Ammonia (mg·L−1169 
Cyanide (mg·L−12.45 
Color Deep brown 
pH 8.78 
Biochemical oxygen demand (BOD)/COD 0.2 

Figure 2 presents the gas chromatograms of CW. Numerous aromatic compounds were detected in the water sample, among which the content of phenol was the highest. And there were a lot of large molecular organic components in the wastewater, for example, aniline, methylphenol, benzofuran, quinoline, isoquinoline, dimethylphenol, pyridine, benzimidazole, benzopyrene, carbazole, etc. These large molecular organic components always pose a challenge for biodegradation, so CW was a kind of organic wastewater, which was difficult to be bio-degraded.
Figure 2

Gas chromatograms of CW.

Figure 2

Gas chromatograms of CW.

Methods

CW with different pH values, adjusted by H2SO4 or NaOH, was introduced to a mixing tank with a volume of 1 L, and the pH of wastewater was adjusted to different values, then 200 g CC, which was the raw material of coke with the particle size of −0.074 mm, was added (Gao et al. 2015). The average time the wastewater remained in the mixing tank was 5 min, and the mixing speed was 30 Hz. The effluent wastewater with the CC was pumped into a filter (XMY2m2/450-U). The schematic for this process is presented in Figure 3. After filtration, the filter liquor and CC were analysed further.
Figure 3

Schematic of CC adsorb CW. 1-screw feeder; 2-mixing tank; 3-pump; 4-filter press; 5-analysis of CW; 6-analysis of CC.

Figure 3

Schematic of CC adsorb CW. 1-screw feeder; 2-mixing tank; 3-pump; 4-filter press; 5-analysis of CW; 6-analysis of CC.

All CW samples were analysed as soon as possible after filtration. The wastewater quality indicators including COD, ammonia and phenol, and these parameters were determined by potassium dichromate oxidation, salicylic acid spectrophotometry, and the 4-AAP spectrophotometric method (National Environment Bureau Water and Wastewater Monitoring Analysis Committee, 2002).

To investigate the influence of different pH values on wastewater, H2SO4 and NaOH were used to adjust the pH of wastewater to 2 and 11, respectively. UV-vis spectra of CW samples were then measured in the range of 190–1,100 nm by using a UV-vis spectrophotometer (UV-4802 s, Unico) with a 1 cm quartz cell.

To explore the influence of pH value on coal, added 10 g CC with particle size −0.074 mm to 500 ml different pH deionized water at 25 °C, mixed for 30 min by magnetic stirrer and let stand, then took some supernatant and CC which was filtered and dried to measurements. Supernatant was taken zeta potential measurements by electrophoresis, and each experiment was repeated five times and on average. Meanwhile, CC was taken FT-IR, BET test. FT-IR study mainly explored the change of surface groups at different pH value. Specific surface area measurements were made by BET methods of adsorption of N2 at a temperature of 77 K.

RESULTS AND DISCUSSION

Effect of wastewater pH on the adsorption results

Figure 4 shows the changes in pH value, COD, ammonia and phenol concentrations at different initial pH after adsorption. The CW pH value played an important role in the removal of COD, ammonia and phenol compounds. With a decrease in pH value, the efficiency of COD and phenol removal increased significantly, while the removal of ammonia was decreased. The concentration of COD and phenol decreased obviously after adsorption. And the decreased tendency at pH = 2–7.5 was more sharply than pH = 7.5–10. At pH = 2, the removal of COD and phenol were 65% and 73%, respectively. When the pH was 9, the removal of COD and phenol were only 23 and 25%. Ammonia removal was always low. This finding may be explained by the fact that the CC was primarily based on organic matter, and the surface contained a plurality of functional groups, carbon and oxygen atoms included. However, ammonia was an ionic contaminant, and it was also strongly hydrophilic. Therefore, the removal efficiency of ammonia was far less than COD (Gao et al. 2015).
Figure 4

Adsorption results of wastewater with different pH value.

Figure 4

Adsorption results of wastewater with different pH value.

The value of pH is one of the crucial factors affecting the performance of adsorption. So it is important to explore the effect of pH value on CW and CC.

Effect of pH value on CW

UV-vis spectra

Figure 5 presents the UV-vis spectra of samples with a pH of 2 and 10. Two adsorption bands were observed in the UV region of 340 and 600 nm. The effect of sulfuric acid in the absorption spectrum was just a slight red shift. When the wavelength range of 340–586 nm, the adsorption of CW with pH = 2 were greater than raw CW and CW with 11. This may be due to the wavelength range of 340–600 nm represented the abundance of aromatic rings in organic matter structure and these compounds were almost phenols and material containing acid group (James 1972; Sun et al. 2008; Kumru et al. 2015). In acidic conditions, the dissociation process of easily ionized compounds such as phenol was inhibited, and these compounds were in molecular state. Thus, that was beneficial to adsorb onto the surface of CC, and increasing the adsorption quantity.
Figure 5

UV-vis spectra of different pH samples.

Figure 5

UV-vis spectra of different pH samples.

Effect of pH value on CC

FT-IR spectroscopy

As shown in Figure 6, the FT-IR spectrum displayed a number of absorption peaks, indicating the complex nature of CC examined. The FT-IR spectroscopic analysis showed a strong band at 1,026–1,581 cm−1 (ash stretching at 1,026 cm−1, for example kaolin; C—O in phenol, alcoholic, ether and aliphatic stretching at 1,177 cm−1; C=C which was on the aromatic hydrocarbons stretching at 1,430 cm−1; C=O stretching at 1,581 cm−1). A medium strength absorption peak at 2,840–3,690 cm−1, indicative of −OH in the carboxyl group (Ashkenazy et al. 1997; Tirkistani 1998; Padmavathy et al. 2003; Won et al. 2006).
Figure 6

FT-IR absorption spectrum of the CC.

Figure 6

FT-IR absorption spectrum of the CC.

The analysis of the infrared spectrum shows that the coal contained a lot of oxygen-containing functional groups, for example —OH, C—O, C=O and —COOH. CC which was soaked in acid solution had not added new absorption peak, so acid may have had no effect on the quantity of functional groups of CC.

Zeta potential

The zeta potential of CC was first determined as a function of pH, and these results are presented in Figure 7. This figure shows that the absolute value of the zeta potential as the solution was gradually increasing with the increasing of pH value. This was mainly because the negative functional groups of coal surface zeta potential were —OH, —COOH, —C—O, etc. (Cai et al. 2010). With the increasing concentration of H+, the ionization of functional groups was restricted. Thus, the absolute value of the zeta potential was decreased.
Figure 7

Zeta potential of CC at different pH of deionized water.

Figure 7

Zeta potential of CC at different pH of deionized water.

In acidic conditions, the surface electronegativity of CC was low, and the functional groups might be in the form of molecules. So it might be more conducive to adsorb the organic pollutants through the role of hydrogen bonding and van der Waals forces.

BET analysis

Table 2 shows surface area (m2·g−1), total pore volume (cm3·g−1) and mean pore size (nm) of CC before and after it was soaked in deionized water of different pH values. Surface area and total pore volume of CC which was immersed in acidic solutions were much higher than that of raw CC. This may be because CC consisted of many minerals, and these minerals could have a reaction with H+ in solution. So the surface area and total pore volume were greater after immersed in acidic solution, which might be more conducive to adsorption through van der Waals force and hydrogen bond.

Table 2

BET results of CC before and after it was soaked in different pH value of deionized water

Coal types Surface area, m2·g−1 Total pore volume, cm3·g−1 Mean pore size, nm 
Raw CC 11.015 0.01614 5.8603 
CC after soaked in pH of 2.01 19.276 0.02271 5.5001 
CC after soaked in pH of 3.37 19.150 0.02177 5.5772 
CC after soaked in pH of 4.30 17.729 0.02099 5.5722 
CC after soaked in pH of 10.39 11.272 0.01655 5.8720 
Coal types Surface area, m2·g−1 Total pore volume, cm3·g−1 Mean pore size, nm 
Raw CC 11.015 0.01614 5.8603 
CC after soaked in pH of 2.01 19.276 0.02271 5.5001 
CC after soaked in pH of 3.37 19.150 0.02177 5.5772 
CC after soaked in pH of 4.30 17.729 0.02099 5.5722 
CC after soaked in pH of 10.39 11.272 0.01655 5.8720 

CONCLUSIONS

pH is one of the crucial factors affecting the performance of adsorption. With a decrease in initial pH, the efficiency of COD and phenol removal increased significantly. When the natural pH was 2, the removal of COD and phenol were 65% and 73%, respectively.

H2SO4 had an effect on CC, including the existence form of functional groups, the surface area, and total pore volume. Meanwhile, it also had an effect on the form of organic pollutants, for example, phenol. By increasing the surface area and total pore volume, as well as suppressing the ionization of functional groups, CC adsorbed the organic substances in CW by van der Waals forces and hydrogen bonding. Therefore, the adsorption process of CC may be a combination of physical and chemical adsorption.

ACKNOWLEDGEMENTS

This research was supported by the China National Fundamental Research Funds for the Central Universities Grant (No. 2014XT05), the National Key Technology R&D Program for the 12th Five-Year Plan of China with Project (No. 2014BAB01B00), and the 111 Project (No. B12030).

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