Key technology of HX-NGS adsorption for treating organic matter in orchid carbon wastewater

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).

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).

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.

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.

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).

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.

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.

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.

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.

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).

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).

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 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).

• (1)

  SEM

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.