In this article, COD, volatile phenol and ammonia nitrogen concentrations of the wastewater from semicarbon are reported as 38,000; 6,400 and 5,700 mg/L, respectively. According to the field test, when the pH of the wastewater is 9, the field test temperature is 20 °C, the adsorption time is 30 min and the optimal dosing ratio of nitrogen-doped gasification slag (HX-NGS) to the wastewater is 1:4, HX-NGS has the best removal effect on COD, volatile phenol and ammonia nitrogen in the wastewater from the semicarbon. The removal rates of COD, volatile phenol and ammonia nitrogen are 94, 91 and 85%, respectively, and the concentrations of the remaining COD, volatile phenol and ammonia nitrogen are 2,280, 576 and 855 mg/L, respectively, after regeneration, the material HX-NGS has a good effect on the treatment of the wastewater from the semicarbon. The reuse rate of the adsorption material is at least eight times. The adsorption effect of the material HX-NGS conforms to the mechanism law of dynamics and thermodynamics.

  • Physical extraction is adopted without adding chemical agents, which will not lead to secondary pollution of water in the treatment link with no increase in solid and liquid wastes.

  • Moreover, the extraction materials can be recycled for use, with a strong adsorption capacities for oil and phenol.

  • The first adsorption can handle about 80% of polyphenol, SS, etc.

Graphical Abstract

Graphical Abstract
Graphical Abstract

The treatment of coke wastewater from the coal chemical industry is a common process. The concentration of pollutants in coke wastewater is about 10 times higher than that of coking wastewater and its composition is also more complex. It is more difficult to treat than coking wastewater. Its treatment method should be different from coking wastewater. However, due to the late rise of the blue coke industry, there is no mature blue coke wastewater treatment process at home and abroad at present (Li et al. 2018). The existing treatment methods still mainly refer to the coking wastewater treatment process with similar water quality, that is, physical, chemical and biochemical treatments and then advanced and reclaimed water recycling treatments. Typical process flows for the treatment of orchid charcoal wastewater, including oil removal, phenol and ammonia recovery, biochemical treatment, advanced treatment, desalination treatment and evaporative crystallization processes, have some serious problems in the promotion and application of various treatment processes at present, such as complex process flow, high primary investment cost, high operating cost, harsh reaction conditions, fouling or scaling, unstable operation, etc. Oil, phenol, and organic matters are also difficult to be treated to the national emission standard, which is also an urgent problem to be solved in the current field of wastewater treatment of blue charcoal (Li et al. 2018). Chinese coal chemical enterprises will discharge a large amount of wastewater and gasification fine slag, which will cause serious damage to the environment and there are a large number of pollutants. The situation of treatment of wastewater is urgent. The gasification fine slag of the coal chemical industry is used to ‘treat waste with waste’. After preparation and modification, the gasification fine slag has strong adsorption capacity, which can effectively treat the wastewater, minimize the cost and treat the wastewater up to standard. Then, carry out reasonable discharge and recycling of water resources (Zhengjin et al. 2020). It cannot only improve the ecological environment but also make high-end use of pollutants. YL-CGS (Yulin carbonaceous gasification fine slag) is a kind of solid waste material of coal chemical industry, which is a carbon-containing adsorption material screened in the high-temperature gasification process of coal chemical enterprises (Li et al. 2018). The processed coal undergoes a certain pressure and temperature in the gasifier, transforming the coal into H2, CH4, CO and other coal chemical products. The temperature in the gasifier during the gasification process is about 800–1,400 °C. At the bottom of the gasifier, condensed granular solids will slowly form and then be separated, purified and collected, which is the formation process of YL-CGS. In this article, a new type of adsorption material, HX-NGS (preparation of nitrogen-doped gasification slag) was prepared from the solid waste of the coal chemical industry, YL-CGS, as the raw material the experiment and then the high concentration of orchid charcoal wastewater was treated (Anipsitakis & Dionysiou 2003). The preparation and modification were carried out on the basis of the original material to maximize the adsorption capacity of the adsorption material (Anipsitakis & Dionysiou 2003). Through continuous optimization of the adsorption conditions, the best adsorption conditions were determined (Wang & Xu 2012). The indicators of COD, volatile phenol and ammonia nitrogen of high concentration orchid charcoal wastewater in this test are 38,000; 6,400 and 5,700 mg/L, respectively (Wang & Xu 2012).

YL-CGS adsorption test

The adsorption material is used to adsorb and treat the orchid charcoal wastewater. Under the conditions of different times, proportions, temperatures, stirring speeds, pH values, etc., the adsorption material and the orchid charcoal wastewater are fully mixed and then the solid–liquid separation is carried out with a suction filter (Wang & Xu 2012). The water quality indicators related to the filtered orchid charcoal wastewater are tested and the adsorption material is systematically characterized. In addition, it may be necessary to carry out a second round of adsorption for some of the higher concentrations of orchid charcoal wastewater to make the water quality meet the standard and explore the best conditions for adsorption treatment of orchid charcoal wastewater (Anipsitakis & Dionysiou 2003). Compare and analyze the adsorption differences between YL-CGS and HX-NGS, analyze the water quality after adsorption characterize adsorption materials and further study, analyze the adsorption kinetics and thermodynamic mechanisms (Wang & Xu 2012).

Yulin coal chemical solid waste (YL-CGS) is used as the adsorption material to treat the orchid charcoal wastewater with waste treatment and other conditions and factors are fixed to explore the treatment of orchid charcoal wastewater under different pH, temperature, adsorption time and dosage ratio conditions (Zhengjin et al. 2020). The following is the removal effect diagram of COD, volatile phenol and ammonia nitrogen in the orchid charcoal wastewater treated by YL-CGS under different conditions and factors, as shown in Figure 1 (Anipsitakis & Dionysiou 2003; Wang & Xu 2012).
Figure 1

Effect of YL-CGS adsorption on water quality under different conditions.

Figure 1

Effect of YL-CGS adsorption on water quality under different conditions.

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The removal rate of COD, volatile phenol and ammonia nitrogen in the treatment of orchid charcoal wastewater by YL-CGS is the best when the pH of the orchid charcoal wastewater is 9, the field test temperature is 20 °C, the adsorption time is 30 min and the optimal dosage ratio of YL-CGS to the orchid charcoal wastewater is 1:4. According to the optimal conditions, the removal rates of COD, volatile phenol and ammonia nitrogen in the wastewater from the coke plant by YL-CGS were 69, 57 and 47%, respectively.

Preparation material HX-NGS adsorption test

HX-NGS prepared material

YL-CGS solid waste has certain adsorption capacity and effect as an adsorption material, but the value of COD, volatile phenol and ammonia nitrogen after treatment is still high and it is difficult to treat the water quality to meet the standard after multiple rounds of adsorption treatment (Wang & Wang 2020). It is particularly important to study and prepare new adsorption materials. In this article, YL-CGS is used as the carrier to prepare the new nano-targeted adsorption material HX-NGS.

HX-NGS material is prepared from solid waste materials produced by coal chemical enterprises, containing a large amount of carbon and metal oxides (Wang & Wang 2020). The average particle size of solid waste from the coal chemical industry is 180–210 meshes, the specific surface area is 112.9 m2/g, the cumulative pore volume of BJH adsorption is 0.23 cm3/g, the average pore diameter of BJH mesopore adsorption is 5.1 nm and the three-dimensional cross-linked micropores are 1–5 μm. The nanopore size on the skeleton is 60–200 nm. The following steps are the preparation process of HX-NGS material.

  • (1)

    Dispersion of YL-CGS

Carefully select 10 g of coal chemical solid waste (YL-CGS), wash 10 g of material with the proper amount of distilled water, dry it in a drying oven of 100 °C for 20 min and use a ball mill to disperse the coal chemical solid waste. In order to prevent the damage of micropore, mesopore and macropore structures of the material during ball milling, the ball mill rotation speed is 100–120 r/min and the ball milling time is 0.5–1 h (Zhengjin et al. 2020). Place the milled material in anhydrous ethanol solution for ultrasonic cleaning and separate the impurities precipitated at the bottom. The purpose of ultrasonic cleaning is to speed up the cleaning speed to remove impurities and dry the cleaned material at 90 °C for 30 min.

  • (2)

    YL-CGS hydrogen peroxide washing

In order to improve the pore size, porosity and specific surface area of the material, it is pre-oxidized from the inside to the outside to better open and close the pores, to a certain extent enhance the adsorption performance of the orchid charcoal wastewater and generate polar functional groups. The solid waste material is washed with 20% hydrogen peroxide and then dried at 90 °C for 20 min.

  • (3)

    YL-CGS acid leaching

In order to form a pore-forming mechanism, add about 16% hydrochloric acid solution for acid leaching, material: acid solution = about 1:3, acid leaching, stirring for 30–40 min, wash with distilled water to neutralize and then dry at 90 °C for 20 min (Feng et al. 2021).

  • (4)

    YL-CGS alkali leaching

Use 20% potassium hydroxide solution for alkali leaching, material:alkali solution = 1:3, alkali leaching and stirring for 30–40 min, wash with distilled water to neutralize and then dry at 90 °C for 20 min.

  • (5)

    YL-CGS ammonia water immersion

Use 30 mL of 10% ammonia water to soak the material, mix it manually and slowly for 5 min, then let it stand for 15 min, wash it with the proper amount of distilled water to neutralize and dry it in a drying oven at 100 °C for 30 min (Goodman et al. 2020).

  • (6)

    YL-CGS activation

The new nano-targeted adsorption material HX-NGS was prepared by mixing the material:urea = 5:1, adding nitrogen into a quartz tube and activating it for 30 min at 300 °C in a microwave tube furnace.

Optimization of HX-NGS adsorption test conditions

Effect of pH value on HX-NGS adsorption

According to the conditions of the test site, study the effect of pH in the orchid charcoal wastewater on the adsorption effect of HX-NGS, fix other conditions and study the removal effect of HX-NGS on COD, volatile phenol and ammonia nitrogen at pH 6, 7, 8, 9, 10 and 11, as shown in Figure 2 (Feng et al. 2021). After preparation and modification, the adsorption performance of the material HX-NGS is greatly improved and the adsorption effect of HX-NGS is better than that of YL-CGS under different pH conditions. With the gradual increase of pH, the removal rate of HX-NGS gradually increases. When the pH is about 11, the removal rate of HX-NGS for COD, volatile phenol and ammonia nitrogen decrease significantly (Yang et al. 2018). When the pH is neutral, weak alkaline or strong alkaline, the removal effect of HX-NGS becomes worse. After the adjustment of pH, sodium hydroxide solution is used, which leads to a chemical reaction of organic substances in the orchid charcoal wastewater, making its composition more complex and diverse. When the pH is 9, the removal effect of HX-NGS is better. The removal rates of COD, volatile phenol and ammonia nitrogen are 94, 91 and 85%, respectively. The test use of the orchid charcoal wastewater can be adjusted without pH adjustment. When pH is selected as 9, HX-NGS has the best removal effect on the orchid charcoal wastewater (Goodman et al. 2020; Zhengjin et al. 2020).
Figure 2

Effect of HX-NGS adsorption on water quality under different conditions.

Figure 2

Effect of HX-NGS adsorption on water quality under different conditions.

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Effect of temperature on HX-NGS adsorption

According to the field test, when other conditions are fixed and pH is selected as 9, HX-NGS has the best removal effect on the orchid charcoal wastewater. Now we study the effect of temperature on the adsorption of HX-NGS and determine the removal effect of HX-NGS on the organic matter in the orchid charcoal wastewater at the test temperatures of 20, 30, 40, 50 and 60 °C. According to the test results, the adsorption effect of HX-NGS is better than that of YL-CGS under different temperature conditions, which indicates that the preparation of modified HX-NGS has good performance. The test temperature is about 20 °C and the testing effect is the best at this temperature. The removal rates of HX-NGS for COD, volatile phenol and ammonia nitrogen are 94, 91 and 85%, respectively (Goodman et al. 2020). With the increase of temperature, the removal rate of HX-NGS begins to decline, especially when the temperature is above 40 °C, the removal rate of HX-NGS increases, making the temperature rise and the organic substances in the orchid charcoal wastewater undergo physical and chemical reactions. When the test temperature is 60 °C, the removal rate of HX-NGS for COD, volatile phenol and ammonia nitrogen decreases to 80, 74 and 68%, respectively. The increase of test temperature is very unfavorable to the removal of COD, volatile phenol and ammonia nitrogen. Therefore, the best adsorption test temperature for HX-NGS is 20 °C (Ahmed et al. 2012; Xu et al. 2020).

Effect of HX-NGS adsorption time

To study the removal effect of HX-NGS on the organic matter in the orchid charcoal wastewater under different time conditions, fix other conditions and study the removal rate of HX-NGS on the organic matter in the orchid charcoal wastewater at 20, 30, 40, 50 and 60 min. According to the test results, when the adsorption time of HX-NGS is 20 min, the removal rates of COD, volatile phenol and ammonia nitrogen in the orchid charcoal wastewater are 90, 88 and 80%, respectively, with the extension of time. When the adsorption time of HX-NGS is 30 min, the adsorption of HX-NGS on the orchid charcoal wastewater is in a saturated equilibrium state. When the adsorption time reaches 60 min, the removal rate of HX-NGS on COD, volatile phenol and ammonia nitrogen is basically the same as that of adsorption for 30 min (Ahmed et al. 2012). The removal rates of HX-NGS on COD, volatile phenol and ammonia nitrogen are 94, 91 and 85%, respectively, so the best effect is to select the adsorption time of HX-NGS for 30 min.

Effect of dosing ratio on HX-NGS adsorption

Now we study the influence of the dosage ratio of HX-NGS, strive to use the least HX-NGS material to treat more orchid charcoal wastewater, maximize the utilization of resources, fix other conditions and discuss the removal effect of HX-NGS on COD, volatile phenol and ammonia nitrogen in orchid charcoal wastewater when the ratio of HX-NGS and orchid charcoal wastewater is 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7, as shown in Figure 2. When the dosing ratio of HX-NGS to the wastewater of the blue coke is between 1:1 and 1:4, HX-NGS has a good effect on the removal of various organic substances in the wastewater of the blue coke and the overall removal rates of COD, volatile phenol and ammonia nitrogen are maintained at about 94, 91 and 85%, respectively, which is far better than the adsorption effect of YL-CGS. When the dosage ratio of HX-NGS to the wastewater of blue charcoal is between 1:5 and 1:7, the adsorption treatment effect of HX-NGS is poor. When the dosage ratio of HX-NGS to the wastewater of blue charcoal is 1:7, the removal rate of HX-NGS for COD, volatile phenol and ammonia nitrogen is 79, 74 and 70%, respectively and the removal effect is relatively poor at this time. It can be concluded that the turning point effect of the dosing ratio of HX-NGS and orchid charcoal wastewater is about 1:4 and the optimal dosing ratio of HX-NGS and orchid charcoal wastewater is 1:4.

Isothermal adsorption characteristics of materials

As we all know, COD is the core index in the process of water treatment and the level of COD determines the quality of water treatment. In order to study the law and performance of HX-NGS in the adsorption of COD in the wastewater of blue charcoal and its isothermal adsorption characteristics of COD in the wastewater of blue charcoal, the selected blue charcoal wastewater has lower COD and the selected blue charcoal wastewater COD in the test is 522 mg/L. The first and second-order kinetic models were used to analyze the kinetic process of HX-NGS adsorption of COD in the orchid charcoal wastewater. The isothermal adsorption equation describing the adsorption process of HX-NGS to COD in the wastewater from the blue coke mainly includes the Freundlich and Langmuir formulas.

Langmuir isotherms

Langmuir isotherm equation:
formula
(1)
In the formula, Ae is the adsorption amount when HX-NGS adsorption reaches equilibrium, in mg/kg; Ye is the concentration of COD in HX-NGS adsorption equilibrium, in mg/L (Table 1); Am is the maximum adsorption capacity of a single molecular layer, in mg/kg; Sl is the Langmuir constant, in mg/L. It can be seen from Figure 3 that Ye/Ae and Ye have a linear relationship, with R2 = 0.967, which shows that the adsorption of HX-NGS on COD is consistent with the Langmuir isotherm equation and HX-NGS adsorption mainly occurs in the active region of the HX-NGS material surface (Table 2) (Ahmed et al. 2012).
Figure 3

Langmuir adsorption isotherm.

Figure 3

Langmuir adsorption isotherm.

Close modal

Freundlich isotherm

formula
(2)
where n is a constant related to temperature and other conditions, the value of n is generally less than 1; Sf is a constant related to the bet and temperature of HX-NGS. It can be seen from Figure 4 that n = 0.846 and Sf = 1.901. N is located at [0, 1], indicating that the adsorption conditions are conducive to the adsorption of COD, but its R2 = 0.856, which is lower than the Langmuir isotherm, indicates that the Langmuir isotherm is more consistent with the adsorption process of HX-NGS for COD in the orchid charcoal wastewater than the Freundlich isotherm (Wang & Xu 2012).
The isothermal adsorption equation of the adsorption process of COD in the orchid charcoal wastewater by adsorption materials mainly includes the Freundlich and Langmuir formulas (Figure 5). The adsorption isotherm curve has a certain linear relationship (Figure 6), which is consistent with the principle of isothermal adsorption (Table 3). (Ahmed et al. 2012). Through a detailed study of the curve relationship of the adsorption isotherm, it has important practical significance for the follow-up study of COD adsorption kinetics and thermodynamics.
Figure 4

Freundlich adsorption isotherm.

Figure 4

Freundlich adsorption isotherm.

Close modal
Figure 5

Relevant parameters of fitting curve of isothermal adsorption equation for COD.

Figure 5

Relevant parameters of fitting curve of isothermal adsorption equation for COD.

Close modal
Figure 6

COD adsorption capacity.

Figure 6

COD adsorption capacity.

Close modal
Figure 7

Comparison of removal effects of YL-CGS and HX-NGS.

Figure 7

Comparison of removal effects of YL-CGS and HX-NGS.

Close modal

When the pH of the wastewater is 9, the field test temperature is 20 °C, the adsorption time is 30 min and the optimal dosage ratio of material HX-NGS to the wastewater is 1:4, the removal rate of COD, volatile phenol and ammonia nitrogen in the wastewater treated by HX-NGS is the best.

Through the optimization of the test conditions, it can be found that the adsorption performance of HX-NGS is far better than that of YL-CGS. Based on the optimal conditions, the removal rates of COD, volatile phenol and ammonia nitrogen in the blue charcoal wastewater by YL-CGS are 69, 57 and 47%, respectively and the removal rates of COD, volatile phenol and ammonia nitrogen in the blue charcoal wastewater by HX-NGS are 94, 91 and 85%, respectively. The adsorption performance of HX-NGS is 25, 34 and 38% higher than that of YL-CGS (Figure 7). The treatment effect of HX-NGS is far better than that of YL-CGS (Rosenthal & Pitts 1971). The best conditions of HX-NGS in the adsorption treatment process are basically the same as those of YL-CGS. After the preparation and modification process of YL-CGS into HX-NGS, the adsorption capacity of the material is greater and the pore size, porosity, specific surface area, etc. are significantly increased.

HX-NGS regeneration test results

The core of the recycling of adsorption methods is material regeneration. After the adsorption of HX-NGS is saturated, the sustainable use of the material becomes the key. If the material cannot be adsorbed once or twice, it will cause a great waste of resources and generate a large amount of solid waste (Rosenthal & Pitts 1971; Goodman et al. 2020). To a certain extent, it is equivalent to transferring the harmful substances in the wastewater of the blue carbon to the solid adsorption materials, which does not fundamentally solve the core problem of the treatment of the blue carbon wastewater. Therefore, the regeneration of HX-NGS has become the core of the research in this article. If the materials can continue to be recycled and only a small amount of HX-NGS needs to be regularly replenished, the process cost will be greatly reduced and the resource utilization of the Yulin blue charcoal wastewater will be realized, which can not only achieve the vision of ‘treating waste with waste’, but also completely solve the practical problems in industry and respond to the policy of ‘carbon peak, carbon neutralization’. The regeneration method used in the test is to use ‘methanol-high-temperature activation’ for regeneration. First, under certain conditions, use 30 mL of methanol to soak 10 g of adsorbed saturated material for 15 min, then use a 160 r/min stirrer to stir the solid–liquid material for 10 min, use ultrasonic equipment to soak and radiate for 5 min and then conduct solid–liquid separation between the material and the solution. After methanol regeneration, use distilled water to clean the material HX-NGS until there is no residual methanol in the hole. Then dry it in a drying oven at 100 °C for 30 min and finally use a microwave tube furnace to activate it at 500 °C for 40 min for regeneration. The separated solution in the material is recycled for the relevant organic matter, the temperature is controlled by distillation to recycle the desorber and the relevant organic matter. The methanol desorption solution generated from regeneration is recycled separately and refined to the next batch for regeneration and utilization. The completed material HX-NGS can start a new adsorption experiment. As shown in Figure 8, with the increase of material reuse times, the removal rate of COD, volatile phenol and ammonia nitrogen in the orchid charcoal wastewater of HX-NGS gradually decreases. When the number of HX-NGS regeneration times reaches 8, the removal rates of COD, volatile phenol and ammonia nitrogen are 62, 56 and 50%, respectively (Rosenthal & Pitts 1971). At this time, the removal effect decreases compared with the raw material, but still has a certain adsorption capacity. Through comprehensive research and judgment, the number of HX-NGS regeneration times is determined to be 8; The ‘methanol–high-temperature activation’ regeneration mode can make HX-NGS regeneration complete and the adsorption performance recovers well.
Figure 8

Regeneration times of HX-NGS.

Figure 8

Regeneration times of HX-NGS.

Close modal
Figure 9

FTIR spectra of YL-CGS and HX-NGS.

Figure 9

FTIR spectra of YL-CGS and HX-NGS.

Close modal
Figure 10

XRD spectra of YL-CGS and HX-NGS.

Figure 10

XRD spectra of YL-CGS and HX-NGS.

Close modal

In order to realize the resource recycling of the orchid charcoal wastewater process system, the main components of the methanol regeneration desorption solution are the desorption agent methanol and the recovered phenols, ammonia nitrogen, coal tar and other organic substances, as well as the water brought in by the HX-NGS channel and methanol. For this experiment, take a certain amount of desorption liquid to carry out the desorption agent recovery and tar crude phenol and other material recovery experiments: use the segmented temperature distillation method for separation and separate the fractions with different boiling ranges for recovery (Bonhoeffer & Reichardt 1928). The experimental results are as follows: the received fractions are mainly ammonia within 80 °C, middle fractions (methanol mixture and low boiling point organic matter) at 80–98 °C, middle and rear fractions (water and a small amount of crude phenol) at 98–145 °C and 150–255 °C rear distillate (a small amount of crude phenol oil).

Adsorption material characterization

Infrared spectrum characterization and analysis of YL-CGS and HX-NGS

It can be seen from the infrared spectrum that the functional groups of various materials are not affected as a whole before and after the treatment of YL-CGS and HX-NGS. Take the infrared spectrum analysis of materials and raw materials prepared by changing the temperature, time, reagent solubility and other conditions of material preparation as an example. The infrared absorption peak of YL-CGS is basically the same as that of the prepared modified HX-NGS, which indicates that changing the oscillation temperature and time does not significantly damage the functional groups of the material itself (Figure 9) (Bonhoeffer & Reichardt 1928). The main functional groups of the raw material YL-CGS are at 617; 1,102; 1,650; 2,038 and 3,463 cm − 1, respectively. After the front-end treatment, the HX-NGS modified with urea at high-temperature has little change in the position of its characteristic peak, at about 621; 1,120; 1,639; 2,029, 3,455 and 3,862 cm − 1.

The absorption peak at 1,102 cm − 1 is the asymmetric stretching vibration of SiO-Si bond, which is consistent with the aluminosilicates contained in YL-CGS. The absorption peaks at 621 and 617 cm − 1 in the low wavenumber region are the symmetric stretching and folding vibrations of Si–O–Si and Si–O–Al bonds; the high wavenumber region 3,463 and 3,455 cm − 1 is the stretching vibration of –OH. The HX-NGS infrared spectrum chart shows that the absorption peak area of the hydroxyl group of the material prepared by doping urea is slightly smaller than that of the raw material YL-CGS and when the microwave oven activation time is about 30 min, the absorption peak area of the hydroxyl group of the HX-NGS prepared by doping urea becomes smaller. The reduction of total oxygen functional groups, especially carboxyl groups, on the carbon surface of HX-NGS is conducive to its adsorption of volatile phenols and other phenolic substances, which is consistent with the removal effect of volatile phenols from the orchid charcoal wastewater adsorbed by modified HX-NGS prepared by doping urea. There is an absorption peak of the nitrogen conjugated system at 2,029 cm − 1 before HX-NGS adsorbs the orchid charcoal wastewater, which is consistent with the later EDS determination results, indicating that HX-NGS is loaded with nitrogen functional groups. There is no absorption peak at 2,029 cm − 1 in the infrared spectrum of HX-NGS after the test adsorption of wastewater, which indicates that the nitrogen-containing functional group may have a chemical reaction with the chemical substances in the orchid charcoal wastewater. The characteristic peaks of different HX-NGS are similar before and after the adsorption of orchid charcoal wastewater. The absorption peak of Si–O–Si asymmetric stretching vibration occurs at 1,009–1,138 cm − 1, so 1,120 and 1,102 cm − 1 are the positions of the aluminosilicate absorption peak of HX-NGS aluminosilicate polymer (Bonhoeffer & Reichardt 1928).

X-ray diffraction characterization analysis of YL-CGS and HX-NGS

It can be seen from the graph analysis that the XRD spectra of YL-CGS and HX-NGS before and after processing, HX-NGS at 2θ, the characteristic peaks at the angles of 21°, 26° and 43° are (1 3 0), (0 1 2) and (4 2 0) and YL-CGS at 2θ, the characteristic peaks with angles of 20.9°, 26°, 29°, 39° and 43° are (0 0 6), (0 1 2), (−3 − 2 2), (2 4 0) and (3 3 2) (Figure 10). The XRD spectrum is characterized by wide dispersion, indicating that the inorganic mineral components in the material are almost completely melted during the coal gasification process and most of the silicon dioxide and metal oxides are amorphous.

The diffraction peaks of YL-CGS and HX-NGS are at 2θ. The positions of the characteristic peaks are the same. HX-NGS is obtained after treatment and the relevant impurities are less. YL-CGS contains more SiO2 crystals than HX-NGS. YL-CGS and HX-NGS in 2θ: the characteristic peaks at 20.9° and 21° are similar to those of Na2Si4O29•10H2O, so the substance Na2Si4O29•10H2O is also contained in YL-CGS and HX-NGS. YL-CGS and HX-NGS in 2θ: the characteristic peak at 26° is similar to that of CaAl2Si2O8•H2O. According to the analysis of the XRD spectrum, there are aluminosilicates in YL-CGS and HX-NGS and YL-CGS is richer than HX-NGS in aluminosilicates. After HX-NGS adsorbs the orchid charcoal wastewater, the original Al(OH)3, Al2O3 and other substances in HX-NGS decrease, indicating that HX-NGS contains very small amounts of CaAl2S22O8•4H2O, SiO2, K2Ca5•(SO4)6•H2O, Ca2Fe9O13 and other substances. Compared with YL-CGS material, HX-NGS has less impurity in the coal chemical industry, a higher carbon content and purity and improved adsorption capacity.

Table 1

Isothermal adsorption experiment effect of HX-NGS on COD

Ye (mg/L) 15 20 45 55 70 80 
Ae (mg/kg) 12.5 17.4 25 32 45 50 58 61 
Ye (mg/L) 15 20 45 55 70 80 
Ae (mg/kg) 12.5 17.4 25 32 45 50 58 61 
Table 2

Parameters related to the isothermal adsorption of HX-NGS on COD

Langmuir
Freundlich
SlAmR2SfnR2
0.0703 58.877 0.967 1.901 0.846 0.856 
Langmuir
Freundlich
SlAmR2SfnR2
0.0703 58.877 0.967 1.901 0.846 0.856 
Table 3

Experimental data of HX-NGS filtration of orchid charcoal wastewater

Time (h) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 
COD (mg/L) 522 479 356 293 176 95 54 47 36 19 10 
At (mg/g) 9.06 20.3 25.78 36.25 41.32 42.01 42.92 44.98 45.82 47.03 
Time (h) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 
COD (mg/L) 522 479 356 293 176 95 54 47 36 19 10 
At (mg/g) 9.06 20.3 25.78 36.25 41.32 42.01 42.92 44.98 45.82 47.03 

Characterization and analysis of YL-CGS and HX-NGS by field emission scanning electron microscope

The Figure 11 shows the scanning electron micrographs of YL-CGS and HX-NGS, respectively. The surface of YL-CGS is relatively rough, with spherical structures of different sizes, an uneven distribution and the channels are scattered. The surface of HX-NGS is relatively smooth and some spherical surfaces become smooth and there will also be some damage. The increase of pore channels becomes obvious and it is easy to observe. The number of small balls in the hole increases and it is easy to observe. There is flocculent residual carbon on the surface and there are many mineral beads attached. The main components are mostly aluminosilicates. The surface of spherical materials is smooth and the inorganic mineral components are amorphous (Bonhoeffer & Reichardt 1928). It is observed from the figure that the hole shape of YL-CGS is not obvious and the shape is irregular. The figure shows that HX-NGS significantly increases the diameter of the hole.
Figure 11

(a) and (b) The SEM morphology of YL-CGS and (c) and (d) the SEM morphology of HX-NGS.

Figure 11

(a) and (b) The SEM morphology of YL-CGS and (c) and (d) the SEM morphology of HX-NGS.

Close modal

Table 4 shows that YL-CGS contains C, O, Si, S and some metal elements. The combination of SEM and EDS proves that the spherical structure in YL-CGS conforms to the morphology of SiO2. There are substances such as elemental carbon, silicon dioxide and aluminum oxide on the surface of the material and the content of carbon is the highest. The morphology of carbon elements on the surface of YL-CGS and HX-NGS materials is observed and the carbon content of YL-CGS materials is less than that of HX-NGS materials. According to the SEM morphology, it can also be seen that the morphology of HX-NGS at different multiples is very close to that of fruit shell-activated carbon and coconut shell-activated carbon and its appearance is layered with the homogeneous and massive distribution. In the process of material preparation, the corresponding acid–base solution was added and the chemical reaction greatly reduced the impurities such as silicon dioxide, aluminum oxide, calcium oxide, etc. In YL-CGS, which made the original dispersion state of raw materials becomes concentrated and compact. HX-NGS was prepared by high-temperature activation, its carbon content reached 71.09% and its adsorption capacity was greatly enhanced. The particle size on the generated material skeleton reaches the nanometer level and is mainly mesoporous adsorption (Korhonen et al. 2011).

Table 4

Weight percentage of adsorption material

Element Type
CONaAlSiNSKCaFe
HX-NGS 71.09 16.97 0.47 1.58 1.73 4.31 3.85 
YL-CGS 43.1 15.5 0.96 8.2 10.9 4.72 0.22 9.3 7.1 
Element Type
CONaAlSiNSKCaFe
HX-NGS 71.09 16.97 0.47 1.58 1.73 4.31 3.85 
YL-CGS 43.1 15.5 0.96 8.2 10.9 4.72 0.22 9.3 7.1 

  • (1)

    SEM

According to Table 4, it is found that the nitrogen content of HX-NGS is higher than that of YL-CGS. According to the literature, HX-NGS loaded with nitrogen-containing groups, such as pyridine, quaternary nitrogen, pyrrole and other groups, will improve the alkalinity of HX-NGS, thus enhancing the adsorption capacity of phenolic compounds. HX-NGS contains a large amount of residual carbon. The naked eye observation shows that HX-NGS is in black powder form. The observation chart shows that the structure of HX-NGS is similar to that of coconut shell-activated carbon and the surface has a pore structure. The morphology of HX-NGS is a regular and smooth, spherical structure and the surface is attached with small particles. The pore distribution of HX-NGS is relatively dense and the number of pores is more than that of YL-CGS, so HX-NGS can be used as a porous adsorption material. After preparation, the carbon content of HX-NGS has been greatly improved, the pore density of the material is relatively concentrated and evenly distributed and the treatment capacity of the orchid charcoal wastewater has been improved, which shows that the preparation method of the material is correct and feasible.

Specific surface area and micropore characterization analysis of YL-CGS and HX-NGS

Figure 12 shows that YL-CGS and HX-NGS adsorption isotherms belong to Class IV adsorption isotherms (Wang & Xu 2012). There are narrow slit mesoporous structures and relatively wide pore structures in YL-CGS, forming the discoid phase (discoid molecules are oriented parallel to the direction perpendicular to the molecular plane, forming a liquid crystal state of columnar aggregation) and producing capillary condensation (refers to the liquefaction and condensation phenomenon when the gas encounters structures such as pores and porous media). The hysteresis loop curve shows that there are slit-like pores on the surface of HX-NGS, with an irregular shape and wide pore size distribution. The figure shows that the adsorption capacities of YL-CGS and HX-NGS increase with the increase of P/Po and the adsorption capacity of HX-NGS is higher than that of YL-CGS. It can be seen from the table that the BET-specific surface area of YL-CGS is 112.90 m2/g (Table 5). Under optimal conditions, the BET-specific surface area of HX-NGS is 630.24 m2/g and the specific surface area of HX-NGS is 517.34 m2/g higher than that of YL-CGS. According to the principle rules of pore size distribution, 0–2 nm is microporous, 2–50 nm is mesoporous and above 50 nm is macroporous. It can be seen from the analysis table of specific surface area and pore size that the adsorption cumulative pore surface area (Figure 13), adsorption cumulative pore volume and average pore diameter of mesoporous adsorption of HX-NGS are higher than those of YL-CGS. The average adsorption pore diameter of BJH of HX-NGS and YL-CGS is between 2 and 50 nm, which indicates that the adsorption material is mainly mesoporous. There are narrow slit mesoporous structures and part of wider pore sizes in HX-NGS. The two-pore structures are discoid, resulting in capillary condensation. The adsorption curve further confirms that YL-CGS and HX-NGS are mainly mesoporous (Korhonen et al. 2011; Ahmed et al. 2012). After preparation, HX-NGS adsorbed mainly micropores and mesopores from orchid charcoal wastewater. The specific surface area of the material has been greatly improved, the purity of the carbon content of the material is high and the adsorption capacity for orchid charcoal wastewater is good.
  • (2)

    EDS

Figure 12

YL-CGS and HX-NGS adsorption and desorption curves.

Figure 12

YL-CGS and HX-NGS adsorption and desorption curves.

Close modal

Study on adsorption kinetics and thermodynamics

Study on the first-order kinetic model of HX-NGS adsorption capacity

Lagergren's first-order rate equation based on solid adsorption capacity is the most common one, which is applied to the adsorption kinetic equation of the liquid phase. The model formula is as in the following equation:
formula
(3)
Integrate Equation (3) and use the boundary conditions. When t = 0, At = 0, e = t, Ae = At.
formula
(4)
where Ae represents the equilibrium adsorption capacity, in mg/kg; At represents the adsorption amount, in mg/kg at time t; and S1 represents the first-order adsorption rate constant (Korhonen et al. 2011).

Study on the second-order kinetic model of HX-NGS adsorption rate

Dynamic model formula is shown in the following equation:
formula
(5)
By integrating Equation (3) and using boundary conditions, when t = 0, At = 0, e=t, and Ae=At, we can get the following equation:
formula
(6)
where Ae represents the equilibrium adsorption capacity, in mg/kg; At is the adsorption amount at t, in mg/kg; and S2 is the secondary adsorption rate constant.

Adsorption thermodynamics

In HX-NGS adsorption thermodynamics, the commonly used thermodynamic temperature unit is Kelvin, which is the adsorption thermodynamic temperature scale or the absolute temperature scale. In the international system of units, the temperature unit is also called the thermodynamic temperature scale, with the symbol T, the unit Kelvin and the abbreviation K (Tang & Wang 2018).
Table 5

Specific surface area of YL-CGS and HX-NGS

TypeSample
BET (m2/g)BJH adsorption cumulative pore volume (cm3/g)BJH average pore diameter of mesoporous adsorption (nm)
YL-CGS 112.90 0.23 5.1 
HX-NGS 630.24 0.29 5.9 
TypeSample
BET (m2/g)BJH adsorption cumulative pore volume (cm3/g)BJH average pore diameter of mesoporous adsorption (nm)
YL-CGS 112.90 0.23 5.1 
HX-NGS 630.24 0.29 5.9 
Table 6

Fitting parameters of two adsorption kinetics

Parameters of first-order dynamic equationAt (Experimental value)Parameters of second-order dynamic equation
S1 Ae R2 47.03 S2 Ae R2 
0.0578 138.5 0.878 0.022 35.06 0.9991 
Parameters of first-order dynamic equationAt (Experimental value)Parameters of second-order dynamic equation
S1 Ae R2 47.03 S2 Ae R2 
0.0578 138.5 0.878 0.022 35.06 0.9991 
Figure 13

Aperture distribution curve of YL-CGS and HX-NGS.

Figure 13

Aperture distribution curve of YL-CGS and HX-NGS.

Close modal
At 293 K (20 °C), 303 K (30 °C) and 313 K (40 °C), the adsorption law of temperature on HX-NGS was studied. The adsorption equilibrium constant Sy was calculated by Equation (7):
formula
(7)
The fitting parameters of the Langmuir equation are used to calculate, where Yay is the concentration of COD absorbed by HX-NGS in the water when the adsorption equilibrium is reached and the standard Gibbs free energy ΔG can be calculated by the following formula:
formula
(8)
where T is the absolute temperature, in K; R is the ideal gas constant and the value is 8.314 J/(mol·K). The relationship between the equilibrium constant Sy and temperature is expressed by Van's Hoff equation in the following equation:
formula
(9)
where, ΔS is entropy change, in J/(mol·K); ΔH is enthalpy change, in J/mol. ΔS and ΔH are calculated according to the slope and intercept of 1/T regression line of lgSy. The Van's Hoff curve of HX-NGS adsorbing COD is shown in Figure 14. The ΔS and ΔH values are listed in Table 7 (Korhonen et al. 2011). From Table 7, the negative value of ΔH indicates that the adsorption of COD in the orchid charcoal wastewater by HX-NGS is an exothermic process.
Table 7

Thermodynamic parameters of HX-NGS to COD

T/KΔG (kJ/mol)ΔH (kJ/mol)ΔS [J/(mol·K)]
293 (20 °C) −11.562 − 1.1022 12.9 
303 (30 °C) −11.635 
313 (40 °C) −11.988 
T/KΔG (kJ/mol)ΔH (kJ/mol)ΔS [J/(mol·K)]
293 (20 °C) −11.562 − 1.1022 12.9 
303 (30 °C) −11.635 
313 (40 °C) −11.988 
Figure 14

Van's Hoff curve of COD adsorption by materials.

Figure 14

Van's Hoff curve of COD adsorption by materials.

Close modal

Summary

It can be seen from Equation (1) that the Langmuir isotherm equation S1 = 0.0703 of HX-NGS for adsorption of COD in the orchid charcoal wastewater is positive, indicating that the reaction can occur spontaneously at room temperature, and HX-NGS can automatically adsorb COD in the orchid charcoal wastewater (Wang & Zhuan 2020). According to the second-order kinetics, the maximum adsorption capacity of HX-NGS to the orchid charcoal wastewater is 45.79 mg/kg, which is very close to the experimental value at 47.03 mg/kg (Table 6); the maximum adsorption calculated by the Langmuir isotherm equation is close to the maximum adsorption calculated by the second-order kinetic model and the relative error is small; Table 7 shows that, ΔH is a negative value, which means that the adsorption of COD in the orchid charcoal wastewater by HX-NGS is an exothermic process; ΔG is also negative, indicating that HX-NGS adsorption can occur spontaneously (Lim & Huang 2007). In addition, the determination of the adsorption kinetic model should not only be judged by R2 of the fitting equation, but also be comprehensively analyzed in combination with other parameters and phenomena.

To sum up, the framework of HX-NGS is nano-porous adsorption, mainly mesoporous adsorption. The pore-forming mechanism of nano-adsorption materials, a solid waste-based new nano-adsorption material technology, can increase the adsorption capacity by more than 35%. With the large specific surface area and complex pore structure of the new nano-adsorption material, HX-NGS can selectively adsorb 90% volatile phenol and 80% ammonia nitrogen, etc. (Lim & Huang 2007) in the wastewater from the treatment of modified blue carbon by preparing the modified orchid charcoal wastewater to target the adsorption of volatile phenol and ammonia nitrogen, etc. Under the optimal conditions, the removal rates of COD, volatile phenol and ammonia nitrogen in the wastewater treated by HX-NGS were 94, 91 and 85%, respectively and the concentrations of the remaining COD, volatile phenol and ammonia nitrogen were 2,280; 576 and 855 mg/L, respectively. After the regeneration of material HX-NGS, the recycling rate of adsorption material is at least eight times and the adsorption effect of material HX-NGS conforms to the mechanism law of kinetic thermodynamics.

How time flies! The writing of the thesis is inseparable from the earnest instruction and enthusiastic help of the tutors. Here, I would like to express my heartfelt thanks to them. First of all, I would like to thank my thesis advisor. This thesis has devoted a lot of effort to both the topic selection and every experimental link in the middle. I should not only follow-up my experimental progress, but also pay attention to the validity of experimental data, and provide guidance for my next experiment. It can be said that without the enthusiastic guidance and strict requirements of the advisor, I cannot successfully complete this thesis. In addition, I would also like to thank my experimental partners. Sometimes, although the results may not be satisfactory, it is important that we have worked hard and become better ourselves. All the efforts are meaningful, thank you!

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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