Adsorption Behavior of Hydroquinone by Diatomite-based Porous Ceramsite

Diatomite-based porous ceramsite is a new kind of environmental material. In this study, ceramsite was prepared by wet grinding, a rolling-ball method, and high temperature-calcination using diatomite as the main raw material with the addition of a pore-forming agent and sintering assistant. X-ray diffraction, scanning electron microscopy, and mercury injection, were used to analyze the structure and characteristics of the prepared materials. Using hydroquinone as the target pollutant, the adsorption behavior of diatomite-based porous ceramsite was investigated. Results indicated that the diatomitebased porous ceramsite had a pore size ranging from 500 to 3000 nm, a specific surface area of 6.14 m2.g-1, and a porosity of 47.8%. When pH was 7, the removal rate and adsorption capacity of the hydroquinone by the diatomite-based porous ceramsite was 91.2% and 4.56 m2.g-1, respectively. In the adsorption process of hydroquinone by diatomite-based porous ceramsite, the diffusion of a liquid membrane was dominant, which could be better described by the quasi-first-order kinetic equation. The Langmuir and Koble-Corrigan equations had a higher fitting degree of data for the adsorption isotherms. The adsorption characteristics of the diatomite-based porous ceramsite are in accordance with the fixed-point adsorption of a single molecular layer and belong to a heterogeneous composite adsorption system. The correlation coefficient R2 and k value of hydroquinone adsorption by the diatomite-based porous ceramsite determined by the liquid film diffusion model were 0.848 and 0.0417, respectively. INTRODUCTION Diatomite is unicellular algal debris that is deposited in the ocean or lakes and can form diatomite. Diatomite is a non-metallic porous mineral that has a small density, a large specific surface area, good adsorption properties, is chemically stable, and has pores with the size of 50 800 nm (Iuchaurroudo et al. 2016, Zahra et al. 2018, Tan et al. 2018, Font et al. 2018). Hydroquinone is an important chemical raw material and chemically synthesized intermediate that is widely used in photographic developer, antioxidants, and synthetic ammonia co-solvents in rubber. Hydroquinone is an important organic pollutant that is highly toxic and difficult to degrade. It can be easily oxidized into benzoquinone which is more toxic to the central nervous system and liver in humans. It is also one of the main pollutants in industrial wastewater which can cause great harm to organisms and the environment (Ergürhan et al. 2018, Yıldız et al. 2005, Xu et al. 2019, Li et al. 2013). Ceramsite is a new type of porous material that has a small bulk density, high porosity, good chemical and thermal stability, can be easily regenerated at low cost with good mechanical strength. Ceramsite can be made of clay, sludge, or fly ash that is mixed with sintering aids and pore-forming agents. These are ground, balled, and calcined at high temperature, and are widely used in construction materials, sewage treatments, the petrochemical industry, and gardening (Chen et al. 2010, Qin et al. 2015, Che et al. 2018, Li et al. 2019, Wang et al. 2019a, Wu et al. 2016, Podder & Majumder 2016). In this study, diatomite-based porous ceramsite was prepared and its adsorption effect on hydroquinone was elucidated. The results provide a theoretical basis for exploring the efficient utilization of diatomite, the optimal allocation of resources, and the sustainable development of the environment. MATERIALS AND METHODS Material and Reagent Diatomite was produced by Linjiang Beifeng Diatomite Company, Jilin Province, (The mass fractions of main compositions included: 70.38% SiO2, 2.37% Al2O3, 1.42% K2O, and 10.58% CaO). The pore-forming agent was carbon powder, provided by Nanjing Greifa Carbon Material Company; Bauxite powders (325 mesh), Al2O3 ≥ 65%, was produced by Yangquan Haotianwei Refractory Company; 2021 pp. 1527-1534 Vol. 20 p-ISSN: 0972-6268 (Print copies up to 2016) No.4 Nature Environment and Pollution Technology An International Quarterly Scientific Journal Original Research Paper e-ISSN: 2395-3454 Open Access Journal Nat. Env. & Poll. Tech. Website: www.neptjournal.com Received: 16-07-2020 Revised: 13-12-2020 Accepted: 26-05-2021


INTRODUCTION
Diatomite is unicellular algal debris that is deposited in the ocean or lakes and can form diatomite. Diatomite is a non-metallic porous mineral that has a small density, a large specific surface area, good adsorption properties, is chemically stable, and has pores with the size of 50 -800 nm (Iuchaurroudo et al. 2016, Zahra et al. 2018, Tan et al. 2018, Font et al. 2018. Hydroquinone is an important chemical raw material and chemically synthesized intermediate that is widely used in photographic developer, antioxidants, and synthetic ammonia co-solvents in rubber. Hydroquinone is an important organic pollutant that is highly toxic and difficult to degrade. It can be easily oxidized into benzoquinone which is more toxic to the central nervous system and liver in humans. It is also one of the main pollutants in industrial wastewater which can cause great harm to organisms and the environment (Ergürhan et al. 2018, Yıldız et al. 2005, Li et al. 2013).
Ceramsite is a new type of porous material that has a small bulk density, high porosity, good chemical and thermal stability, can be easily regenerated at low cost with good mechanical strength. Ceramsite can be made of clay, sludge, or fly ash that is mixed with sintering aids and pore-forming agents. These are ground, balled, and calcined at high temperature, and are widely used in construction materials, sewage treatments, the petrochemical industry, and gardening (Chen et al. 2010, Qin et al. 2015, Che et al. 2018, Wang et al. 2019a, Wu et al. 2016, Podder & Majumder 2016. In this study, diatomite-based porous ceramsite was prepared and its adsorption effect on hydroquinone was elucidated. The results provide a theoretical basis for exploring the efficient utilization of diatomite, the optimal allocation of resources, and the sustainable development of the environment.

Material and Reagent
Diatomite was produced by Linjiang Beifeng Diatomite Company, Jilin Province, (The mass fractions of main compositions included: 70.38% SiO 2 , 2.37% Al 2 O 3 , 1.42% K 2 O, and 10.58% CaO). The pore-forming agent was carbon powder, provided by Nanjing Greifa Carbon Material Company; Bauxite powders (325 mesh), Al 2 O 3 ≥ 65%, was produced by Yangquan Haotianwei Refractory Company; Quartz was purchased from the Xinhui Mining Processing Plant in Lingshou County, China, and feldspar was obtained from Yantai Yitao Mining Company. The aforementioned raw materials were industrially pure. The analytically pure sodium silicate was used as a dispersant obtained from Beijing Tongguang Fine Chemical Company, and the analytically pure hydroquinone was purchased from Qingdao Yousuo Chemical Technology Company.

Sample Preparation and Characterization
Diatomite, the pore-forming agent, sintering aid, water, and dispersant (a mass ratio of 82: 6: 18: 120: 2), were added to the ball mill with a grinding medium (ZrO 2 ceramic ball with φ ≈ 8 mm) and ground for 20 min. The materials were then put into an oven and dried at 105°C. The diatomite-based porous ceramsite was obtained by dispersing, rolling, and calcining in a box resistance furnace at 1020°C for 2 h.
A Japan JSM-6490LV SEM was employed to observe the micromorphology of the ceramsite. The pore size distribution of samples was detected using an American Auto Pore IV 9500 mercuryporosimeter. An ultraviolet-visible spectrophotometer produced by LAMBDA of PerkinElmer Company in the United States was used to detect the absorbance of hydroquinone wastewater at 288 nm. The pH value of hydroquinone wastewater was measured with JY-pH 2.0 pH agent (Shimazu, Japan). The porosity of diatomite-based porous ceramsite was determined using the Archimedes method.

Adsorption Experiment
In this experiment, 1g of the diatomite-based porous ceramsite was placed into a 150 mL conical flask, and 50 mL of hydroquinone solution at a concentration of 50 mg.L -1 was added. The conical flask was sealed and placed into a constant temperature shaking table for shaking. The pH was adjusted to 2-9 with 0.1 mol.L -1 HCl or NaOH solution (25°C, 120 r.min -1 , 24 h). After the supernatant was filtered by a 0.45 μm membrane filter, its absorbance was measured to be 288 nm using an ultraviolet spectrophotometer. According to the Lambert-Beer law, if the absorbance of the maximum wavelength of a solution has a good linear relationship with the concentration of the solution, so the adsorption q e and the removal rate r of hydroquinone can be calculated from the absorbance.
Where: C 0 is the concentration of hydroquinone before adsorption (mg.L -1 ); C e represents the concentration of hydroquinone at adsorption equilibrium (mg.L -1 ); m is the mass of adsorbent (g); V is the volume of solution (L).

Establishment of Adsorption Kinetic, Adsorption Thermodynamic and Dynamic Boundary Models
Adsorption kinetics, adsorption thermodynamic, and dynamic boundary models (Li et al. 2015, Jing et al. 2018, Zhu et al. 2016, Duan et al. 2011, Qu et al. 2018, Cheng et al. 2017, Chiara et al. 2019) of diatomite-based porous ceramsite to hydroquinone are listed in Tables 1-3 respectively. Fig. 1 shows the SEM image of the diatomite-based porous ceramsite. The diatomite particles basically maintain the original pore structure (The small holes are distributed in the plate-shaped particles). Due to high-temperature calcination, the smaller holes on the diatomite particles disappear. The plate-shaped particles and tiny particles (sintering aids and pore-forming agents) are stacked to form a large gap. A large Tables 1: Adsorption kinetics of diatomite-based porous ceramsite to hydroquinone.

Models Equations
Quasi-first-order kinetic model (2) Where: C0 is the concentration of hydroquinone before adsorption (mg.L -1 ); Ce represents the concentration of hydroquinone at adsorption equilibrium (mg.L -1 ); m is the mass of adsorbent (g); V is the volume of solution (L).

Models Equations
Quasi-first-order kinetic model Quasi-second-order kinetic model Elovich model Where qt (mg.g -1 ) is the adsorption capacity at the time of t, qe (mg.g -1 ) is the adsorption capacity at equilibrium; Ce and Ct (µmol.L -1 ) represent the adsorption concentrations at equilibrium and t, respectively; K and k0 denote the adsorption rate constant and a is the remaining constant.
In this experiment, 1g of the diatomite-based porous ceramsite was placed into a 150 mL conical flask, and 50 mL of hydroquinone solution at a concentration of 50 mg.L -1 was added. The conical flask was sealed and placed into a constant temperature shaking table for shaking. The pH was adjusted to 2-9 with 0.1 mol.L -1 HCl or NaOH solution (25 ℃, 120 r.min -1 , 24 h). After the supernatant was filtered by a 0.45 μm membrane filter, its absorbance was measured to be 288 nm using an ultraviolet spectrophotometer. According to the Lambert-Beer law, if the absorbance of the maximum wavelength of a solution has a good linear relationship with the concentration of the solution, so the adsorption qe and the removal rate r of hydroquinone can be calculated from the absorbance.
(2) Where: C0 is the concentration of hydroquinone before adsorption (mg.L -1 ); Ce represents the concentration of hydroquinone at adsorption equilibrium (mg.L -1 ); m is the mass of adsorbent (g); V is the volume of solution (L).

Models Equations
Quasi-first-order kinetic model Quasi-second-order kinetic model Elovich model Where qt (mg.g -1 ) is the adsorption capacity at the time of t, qe (mg.g -1 ) is the adsorption capacity at equilibrium; Ce and Ct (µmol.L -1 ) represent the adsorption concentrations at equilibrium and t, respectively; K and k0 denote the adsorption rate constant and a is the remaining constant.
In this experiment, 1g of the diatomite-based porous ceramsite was placed into a 150 mL conical flask, and 50 mL of hydroquinone solution at a concentration of 50 mg.L -1 was added. The conical flask was sealed and placed into a constant temperature shaking table for shaking. The pH was adjusted to 2-9 with 0.1 mol.L -1 HCl or NaOH solution (25 ℃, 120 r.min -1 , 24 h). After the supernatant was filtered by a 0.45 μm membrane filter, its absorbance was measured to be 288 nm using an ultraviolet spectrophotometer. According to the Lambert-Beer law, if the absorbance of the maximum wavelength of a solution has a good linear relationship with the concentration of the solution, so the adsorption qe and the removal rate r of hydroquinone can be calculated from the absorbance.
(2) Where: C0 is the concentration of hydroquinone before adsorption (mg.L -1 ); Ce represents the concentration of hydroquinone at adsorption equilibrium (mg.L -1 ); m is the mass of adsorbent (g); V is the volume of solution (L).

Models Equations
Quasi-first-order kinetic model Quasi-second-order kinetic model Elovich model Where qt (mg.g -1 ) is the adsorption capacity at the time of t, qe (mg.g -1 ) is the adsorption capacity at equilibrium; Ce and Ct (µmol.L -1 ) represent the adsorption concentrations at equilibrium and t, respectively; K and k0 denote the adsorption rate constant and a is the remaining constant.
In this experiment, 1g of the diatomite-based porous ceramsite was placed into a 150 mL conical flask, and 50 mL of hydroquinone solution at a concentration of 50 mg.L -1 was added. The conical flask was sealed and placed into a constant temperature shaking table for shaking. The pH was adjusted to 2-9 with 0.1 mol.L -1 HCl or NaOH solution (25 ℃, 120 r.min -1 , 24 h). After the supernatant was filtered by a 0.45 μm membrane filter, its absorbance was measured to be 288 nm using an ultraviolet spectrophotometer. According to the Lambert-Beer law, if the absorbance of the maximum wavelength of a solution has a good linear relationship with the concentration of the solution, so the adsorption qe and the removal rate r of hydroquinone can be calculated from the absorbance.
(2) Where: C0 is the concentration of hydroquinone before adsorption (mg.L -1 ); Ce represents the concentration of hydroquinone at adsorption equilibrium (mg.L -1 ); m is the mass of adsorbent (g); V is the volume of solution (L).

Models Equations
Quasi-first-order kinetic model Where qt (mg.g -1 ) is the adsorption capacity at the time of t, qe (mg.g -1 ) is the adsorption capacity at equilibrium; Ce and Ct (µmol.L -1 ) represent the adsorption concentrations at equilibrium and t, respectively; K and k0 denote the adsorption rate constant and a is the remaining constant.
Where q t (mg.g -1 ) is the adsorption capacity at the time of t, q e (mg.g -1 ) is the adsorption capacity at equilibrium; C e and C t (μmol.L -1 ) represent the adsorption concentrations at equilibrium and t, respectively; K and k 0 denote the adsorption rate constant and a is the remaining constant.
In this experiment, 1g of the diatomite-based porous ceramsite was pl hydroquinone solution at a concentration of 50 mg.L -1 was added. T constant temperature shaking table for shaking. The pH was adjusted (25 ℃, 120 r.min -1 , 24 h). After the supernatant was filtered by a measured to be 288 nm using an ultraviolet spectrophotometer. Accord of the maximum wavelength of a solution has a good linear relationsh adsorption qe and the removal rate r of hydroquinone can be calculate (2) Where: C0 is the concentration of hydroquinone before adsorption hydroquinone at adsorption equilibrium (mg.L -1 ); m is the mass of ad
Where qt (mg.g -1 ) is the adsorption capacity at the time of t, qe (mg equilibrium; Ce and Ct (µmol.L -1 ) represent the adsorption concentr and k0 denote the adsorption rate constant and a is the remaining co Tables 2: Adsorption thermodynamics of diatomite-based porous ceramsite to hy In this experiment, 1g of the diatomite-based porous ceramsite was place hydroquinone solution at a concentration of 50 mg.L -1 was added. The constant temperature shaking table for shaking. The pH was adjusted to 2 (25 ℃, 120 r.min -1 , 24 h). After the supernatant was filtered by a 0.45 measured to be 288 nm using an ultraviolet spectrophotometer. According of the maximum wavelength of a solution has a good linear relationship w adsorption q e and the removal rate r of hydroquinone can be calculated f

Models Equations
Quasi-first-order kinetic model Quasi-second-order kinetic model Where q t (mg.g -1 ) is the adsorption capacity at the time of t, q e (mg.g -1 equilibrium; C e and C t (µmol.L -1 ) represent the adsorption concentratio and k 0 denote the adsorption rate constant and a is the remaining const Tables 2: Adsorption thermodynamics of diatomite-based porous ceramsite to hydroq Where qe (mg.g -1 ) is the adsorption capacity at equilibrium, qm (mg.g -1 ) is the maximum adsorption capacity; Ce represents the   Where, F is the adsorption fraction at time t, F = qt/qe; k (min -1 ) is the rate constant. Where, F is the adsorption fraction at time t, F = qt/qe; k (min -1 ) is the rate constant. Where q e (mg.g -1 ) is the adsorption capacity at equilibrium, q m (mg.g -1 ) is the maximum adsorption capacity; Ce represents the concentration of adsorbate in the solution at equilibrium; k L is the Langmuir isotherm constant (related to the energy of adsorption); k F stands for the Freundlich isotherm constant (related to adsorption capacity and strength of adsorbents); k Th is the Toth isotherm constant.

Micromorphology of the Diatomite-Based Porous Ceramsite
Tables 3: Dynamic boundary model of diatomite-based porous ceramsite to hydroquinone.

Models Equations
Liquid film diffusion Intragranular diffusion Chemical reaction Where, F is the adsorption fraction at time t, F = qt/qe; k (min -1 ) is the rate constant.
number of three-dimensional pores form an organic hole between the aggregated particles and the particles.

Pore Structure Characteristics of the Diatomite-based Porous Ceramsite
The pore diameter distribution curves of the diatomite-based porous ceramsite (Fig. 2) are mainly concentrated at 500 to 3,000 nm. The results of the pore size distribution, porosity, and specific surface area of the diatomite-based porous ceramsite (before and after toner addition) by the mercury injection are displayed in Table 4. It can be seen, after adding the toner, the pore size is widely distributed, the specific surface area is larger, and the porosity is higher. Thus the adsorption capacity of the material is greatly improved. icles (sintering aids and pore-forming agents) are stacked to form a large gap. A large number ores form an organic hole between the aggregated particles and the particles.  particles and tiny particles (sintering aids and pore-forming agents) are stacked to form a large of three-dimensional pores form an organic hole between the aggregated particles and the parti

Pore Structure Characteristics of the Diatomite-Based Porous Ceramsite
The pore diameter distribution curves of the diatomite-based porous ceramsite (Fig. 2) are m 500 to 3,000 nm. The results of the pore size distribution, porosity, and specific surface area o porous ceramsite (before and after toner addition) by the mercury injection are displayed in Ta after adding the toner, the pore size is widely distributed, the specific surface area is larger, and Thus the adsorption capacity of the material is greatly improved.  ceramsite increase with increasing pH. When the pH is 7, the removal effect is most efficient. The adsorption capacity and removal rates are 4.56 mg.g -1 and 91.2%, respectively. When the pH is greater than 7, the adsorption capacity of the diatomite-based porous ceramsite to hydroquinone decreases significantly. This may be due to the degree of dissociation of hydroquinone being lower under acidic conditions. With an increase in pH, the degree of dissociation of hydroquinone increases, which is conducive to adsorption. In the alkaline environment, the hydroxyl group of hydroquinone is further dissociated. The hydrogen bond between the hydroquinone and the solvent water takes up too many adsorption sites, which leads to a decrease in the removal rate of hydroquinone by the diatomite-based porous ceramsite. Therefore, the pH value of the solution was adjusted to 7 in subsequent experiments.

Adsorption Kinetic Analysis
According to the equations in Table 1, the non-linear kinetic adsorption models such as the quasi-first-order dynamic model, the quasi-second-order kinetic model, the Elovich model, and the double-constant model were respectively fitted. The non-linear kinetic adsorption of hydroquinone by the diatomite-based porous ceramsite is illustrated in Fig. 4, and the relevant parameters are shown in Table 5. Fig. 4 shows that the adsorption process of hydroquinone by the diatomite-based porous ceramsite occurs in two stages. The adsorption capacity increases rapidly in the range from 0 to 120 min. When t is 120 min, the adsorption capacity reaches 4.35 mg.g -1 . After 120 min, the adsorption capacity increases slowly and gradually become stable. Finally, at 1,440 min, the adsorption capacity reaches 4.4 mg.g -1 . This is because in the first stage, there is a large number of adsorption sites on the surface of ceramsite, and the adsorption amount increases rapidly. In the second stage, hydroquinone in the solution occupies a large number of adsorption sites on the surface of ceramsite, resulting in slow adsorption. According to Table 1, the correlation coefficient (R 2 = 0.979) of the quasi-first-order kinetic equation is greater than that of the quasi-second-order kinetic equation (R 2 = 0.953), the Elovich model, and the double constant.
The adsorption process of hydroquinone by the diatomite-based porous ceramsite can be accurately demonstrated by quasi-first-order and quasi-second-order kinetic equations. The quasi-first-order kinetic model is based on the assumption that the adsorption process is controlled by diffusion steps. The quasi-second-order kinetic model is based on the assumption that the adsorption rate is controlled by the chemical adsorption mechanism. The quasi-second-order kinetic model includes all adsorptions, such as liquid film diffusion, intraparticle diffusion, and chemical adsorption (Zhang et al. 2018, Pholosi et al. 2020). The adsorption of

dsorption of Hydroquinone by the Diatomite-Based Porous Ceramsite
ffect of pH on the adsorption of hydroquinone by the diatomite-based porous ceramsite. When of 2-7, the adsorption capacity and removal rate of the hydroquinone by the diatomite-based ease with increasing pH. When the pH is 7, the removal effect is most efficient. The adsorption rates are 4.56 mg.g -1 and 91.2%, respectively. When the pH is greater than 7, the adsorption ite-based porous ceramsite to hydroquinone decreases significantly. This may be due to the of hydroquinone being lower under acidic conditions. With an increase in pH, the degree of uinone increases, which is conducive to adsorption. In the alkaline environment, the hydroxyl e is further dissociated. The hydrogen bond between the hydroquinone and the solvent water sorption sites, which leads to a decrease in the removal rate of hydroquinone by the diatomitete. Therefore, the pH value of the solution was adjusted to 7 in subsequent experiments.  Table 1, the non-linear kinetic adsorption models such as the quasi-first-order dynamic  The adsorption process of hydroquinone by the diatomite-based porous ceramsite can be acc by quasi-first-order and quasi-second-order kinetic equations. The quasi-first-order kinetic m assumption that the adsorption process is controlled by diffusion steps. The quasi-second-order k on the assumption that the adsorption rate is controlled by the chemical adsorption mechanism hydroquinone by the diatomite-based porous ceramsite is dominated by liquid film diffusion. The quasi-first-order kinetic model can describe the whole adsorption process and is, therefore, more accurate in describing the adsorption process of hydroquinone by the diatomite-based porous ceramsite.

Adsorption Thermodynamic Analysis
The adsorption isotherm equations (Langmuir, Koble-Corrigan, Toth, and Redlich-Peterson) were used to fit the experimental data of the adsorption of hydroquinone by the diatomite-based porous ceramsite (Cui et al. 2019, Mudzielwana et al. 2019, Raganati et al. 2018, Lonappana et al. 2018, Wang et al. 2019b). The fitting results are shown in Fig. 5, and the fitting parameters are summarized in Table 6.
From Fig. 5 and Table 6, the Langmuir and Koble-Corrigan equations can well fit the data of the adsorption isotherm. The Langmuir equation is an adsorption isothermal model based on single molecular layers, that is, adsorption only occurs on the outer surface of the adsorbent. The Koble-Corrigan equation can be used to present the composite adsorption system of uniform adsorption and uneven adsorption, which shows that the adsorption process is more complex and related to the porous composite characteristics of ceramsite. The adsorption characteristics of diatomite-based porous ceramsite conform to the fixed-point adsorption of a single molecular layer. Each active adsorption site on the ceramsite only adsorbs one molecule, and there is no transfer or interaction between single molecules adsorbed on the site. The  adsorption site on the ceramsite only adsorbs one molecule, and there is no transfer or interaction between single molecules adsorbed on the site. The maximum adsorption capacity is a fixed value. Meanwhile, the adsorption on the surface of ceramsite is heterogeneous and belongs to a heterogeneous composite adsorption system.

Dynamic Boundary Analysis
A Dynamic boundary model can be used to describe the adsorption behavior of porous adsorbents. The adsorption process on the surface of the diatomite-based porous ceramsite is divided into three steps: 1) liquid film diffusion, 2) intragranular diffusion, 3) adsorption chemical reaction on the active groups inside the particles. According to Table   1, the linear fitting of the dynamic boundary model is shown in Fig. 6, and the fitting parameters are summarized in Vol. 20, No. 4, 2021 • Nature Environment and Pollution Technology maximum adsorption capacity is a fixed value. Meanwhile, the adsorption on the surface of ceramsite is heterogeneous and belongs to a heterogeneous composite adsorption system.

Dynamic Boundary Analysis
A Dynamic boundary model can be used to describe the adsorption behavior of porous adsorbents. The adsorption process on the surface of the diatomite-based porous ceramsite is divided into three steps: 1) liquid film diffusion, 2) intragranular diffusion, 3) adsorption chemical reaction on the active groups inside the particles. According to Table 1, the linear fitting of the dynamic boundary model is shown in Fig. 6, and the fitting parameters are summarized in Table 7.
From the fitting parameters in Table 7, the liquid film diffusion model exhibits a higher fitting degree for the adsorption of hydroquinone by the diatomite-based porous ceramsite with a correlation coefficient R 2 =0.848, followed by the fitting degrees of the internal diffusion model of particles, and the chemical reaction model, which indicated that the adsorption rate of hydroquinone on the surface of ceramsite is mainly controlled by the liquid film diffusion (Hosokawa et al. 2018, Sun et al. 2017. This data demonstrated that the adsorption mechanism of hydroquinone by diatomite-based porous ceramsite is mainly through micropores on the ceramsite surface and the charge adsorption on the material surface (Jegasothya et al. 2004. The K value can be used to measure the speed of adsorption. The K value of the liquid film diffusion model is 0.04173, which is much larger than that fitted by the intragranular diffusion and chemical itted by the intragranular diffusion and chemical reaction models. These results indicated that the sion is relatively rapid in the adsorption process, while the intragranular diffusion and chemical tively slow in the adsorption process.  larger than that fitted by the intragranular diffusion and chemical reaction models. These results indicated that the liquid film diffusion is relatively rapid in the adsorption process, while the intragranular diffusion and chemical reactions are relatively slow in the adsorption process.  Time/min larger than that fitted by the intragranular diffusion and chemical reaction models. These results indicated that the liquid film diffusion is relatively rapid in the adsorption process, while the intragranular diffusion and chemical reactions are relatively slow in the adsorption process.   reaction models. These results indicated that the liquid film diffusion is relatively rapid in the adsorption process, while the intragranular diffusion and chemical reactions are relatively slow in the adsorption process.

CONCLUSION
Diatomite-based porous ceramsite has a high porosity, a specific surface area, and a better adsorption performance for hydroquinone. When the pH is 7, the removal rate and adsorption capacity of hydroquinone in the sample can reach 91.2% and 4.56 mg.g -1 , respectively. The quasi-first-order kinetic, and the quasi-second-order kinetic equations have a high fitting degree for the adsorption data of the diatomite-based porous ceramsite. The diffusion of the liquid films dominates the adsorption process of hydroquinone by ceramsite and the quasi-first-order kinetic equation is more accurate. The Langmuir and Koble-Corrigan equations can well fit the adsorption isotherm data. The adsorption characteristics of the diatomite-based porous ceramsite conform to the fixed-point adsorption of a single molecular layer. The adsorption on the surface of ceramsite is heterogeneous and seen as a heterogeneous composite adsorption system. The fitting results of the dynamic boundary model and the experimental data indicated that the liquid film diffusion model has a high fitting degree for the adsorption process of hydroquinone by the diatomite-based porous ceramsite, with a correlation coefficient of R 2 = 0.848 and adsorption rate of k = 0.04173. The adsorption mechanism of hydroquinone by the diatomite-based porous ceramsite mainly occurs in the micropores on the ceramsite surface, and the charge adsorption on the material surface.