J. Cent. South Univ. (2021) 28: 1652-1664

DOI: https://doi.org/10.1007/s11771-021-4724-8

Preparation of chitosan modified fly ash under acid condition and its adsorption mechanism for Cr(VI) in water

JIANG Chun-lu(姜春露)^{1, 2}, WANG Rui(王瑞)¹, CHEN Xing(陈星)¹,ZHENG Liu-gen(郑刘根)¹, CHENG Hua(程桦)¹

1. School of Resource and Environmental Engineering, Anhui Province Engineering Laboratory for Mine Ecological Remediation, Anhui University, Hefei 230601, China;

2. College of Geoscience and Surveying Engineering, China University of Mining & Technology (Beijing), Beijing 100083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract:

Chitosan-coated fly ash (CWF) was prepared by the acid leaching-coating method. Chitosan and fly ash were crosslinked in the solution of acetic acid and sulfuric acid. The microstructure of CWF was conducted by scanning electron microscope (SEM) and X-ray diffraction (XRD). The removal of Cr(VI) from water by CWF was studied by adsorption experiments. The composite prepared by the experiment developed a pore structure and a crystal structure similar to SiO₂ and chitosan chain-like coating was formed on the surface of fly ash. The new modified material has larger surface roughness, specific surface area and more adsorption channels. The Cr(VI) was enriched in modified materials by electrostatic adsorption between CrO₄^2-, CrO₇^2- and —NH₃⁺ group and surface acid functional groups. The movement of Cr(VI) in solution is a diffusion process from the main body of the liquid phase to the surface of the liquid film.

Key words:

chitosan; fly ash (CWF); Cr(VI); adsorption; modification；

Cite this article as:

JIANG Chun-lu, WANG Rui, CHEN Xing, ZHENG Liu-gen, CHENG Hua. Preparation of chitosan modified fly ash under acid condition and its adsorption mechanism for Cr(VI) in water [J]. Journal of Central South University, 2021, 28(6): 1652-1664.

1 Introduction

Chromium (Cr) is a common heavy metal that is prevalent in wastewater from the electroplating, printing and dyeing, mining, metallurgy and energy industries [1-3]. Cr mainly exists as the form of trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) [4]. CrO₄^2- and Cr₂O₇^2- are the common occurrence forms of Cr(VI). CrO₄^2- and Cr₂O₇^2- are highly toxic, soluble and mobile chromates that can be enriched in organisms, and both of them can cause cancer, distortion and other health problems [5-7]. Cr(VI) compounds have been included in the List of Toxic and Harmful Water Pollution (the first batch) by the Ministry of Ecology and Environment of China. The maximum permissible concentrations of Cr(VI) is 0.05 mg/L according to the environment quality standard of surface water (GB 3838-2002) III class water standard limit [8]. The US National Environmental Protection Agency (EPA) also regards Cr(VI) as a priority pollutant [9]. Therefore, the pollution of Cr(VI) in water and its treatment have been a common concern of government departments, researchers and all sectors of society.

Common removal methods of Cr(VI) in water mainly include adsorption [10], biological treatment [11], ion exchange/complexation [12, 13] and electrochemical oxidation [14], among which adsorption treatment has attracted wide attention due to its low cost and high efficiency. Typical adsorbents are clay minerals [15], metal oxidants [16], biomass [2] and fly ash [17, 18], etc.

Fly ash is one of combustion products discharged from coal-fired power plant. The particle size is generally between 1 and 100 μm, and mainly composed of SiO₂, Al₂O₃, CaO, Fe₂O₃ and other oxides. The hydroxyl groups attached to these oxides adsorb metal ions by complexation, ion exchange and chemical bonding. RIBEIRO et al [19] showed that fly ash has the potential to be an effective and low-cost adsorbent by gasifying fly ash, but the adsorption efficiency of monomer fly ash is low [20]. LIANG et al [21] prepared fly ash into three-dimensional honeycomb-like modified material by modification, which improved the adsorption efficiency of fly ash on Cr(VI).

Chitosan is an organic polymer coagulant aid that is non-toxic and has the function of electric neutralization and adsorption bridging. Cellulose is hydroxyl at the second carbon position in the carbon chain (C2), and chitosan is acetylamino and amino at C2. Chitosan has many unique properties, such as biodegradability, cell affinity and biological effect [16, 22]. However, its slow dissolution rate and high price limit its wide application. Some researchers have attempted to treat polluted wastewater by using chitosan in combination with bentonite, montmorillonite, diatomite, fly ash and other materials [23]. For example, LIU et al [24] used chitosan/CoFe₂O₄ to prepare magnetic composite materials to adsorb dye wastewater. CHRISTOU et al [25] utilized PVP/chitosan to synthesize nanofibers to adsorb uranium.

Combining the advantages of fly ash and chitosan, this paper attempts to prepare a chitosan-coated fly ash (CWF) composite by attaching chitosan to an acid-modified fly ash surface by a coating method. The effect of pH and Cr(VI) concentration on the adsorption efficiency of CWF is discussed through batch adsorption experiments. The characterization of the modified materials was conducted by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the interaction between the adsorption material and Cr(VI) was described by an isothermal adsorption model (Langmuir, Freundlich) and diffusion model. The relationship was further described from the point of view of kinetics and thermodynamics to reveal the mechanism of removing Cr(VI) from water by CWF.

2 Materials and methods

2.1 Materials

Fly ash material: Fly ash was collected from a coal-fired power plant located in Linhuan Industrial Park, Huaibei, China. As shown in Table 1, the chemical constituents were determined by X-ray fluorescence spectrometry (FluoroMax-4P). From Table 1, it can be seen that the fly ash was used as Class C ash (high-calcium ash).

Table 1 Total content of some elements in fly ash (wt%)

Reagents: Sodium hydroxide, sulfuric acid, glacial acetic acid, and ethanol were of analytical purity (Aladdin Industrial Corporation). Chitosan (degree of deacetylation≥96.5%) was purified by analysis (Chemical Reagent Co., Ltd., China Pharmaceutical Group). Potassium dichromate and diphenylcarbazide were of high purity (China Pharmaceutical Group Chemical Reagent Co., Ltd.).

2.2 Method

2.2.1 Material preparation and optimization method

The preparation of fly ash coated with chitosan is mainly divided into two steps: acid modification of the fly ash and chitosan coating. There are two specific preparation methods. One is acid modification of fly ash. The fly ash was ground through a 0.074 mm (200 mesh) sieve, dried in a 373 K constant temperature oven for 24 h, cooled to room temperature, and activated in a muffle furnace at 573 K for 1 h. 2 mg/L H₂SO₄ solution was selected, and the activated fly ash was acid-immersed with a solid-liquid ratio of 3 g: 10 mL. After standing at room temperature for 24 h, the 0.45 μm microporous membrane was filtered and washed, and dried in a constant temperature drying oven at 373 K. After cooling to room temperature, the acid-modified fly ash was finely ground with a mortar. The other is chitosan coating. Different M_A: M_B was dissolved in 200 mL acetic acid solution of different concentrations, slowly dripped with 5% NaOH reagent until the solution was flocculated, and pH was about 9. Slow down and stirred for 20 min. 0.45 μm microporous filter membrane was washed to neutral. The modified fly ash was prepared by drying in constant temperature drying chamber at 373 K, grinding and passing 200 mesh sieve. The optimum material ratio of the modified fly ash was obtained by adsorption experiments at 298 K and pH=7. To accentuate the adsorption effect, the initial concentration of Cr(VI) was set at 0.1 mg/L. Considering the amount of chitosan (A), the amount of acid-modified fly ash (B) and the concentration of acetic acid (C), each factor was divided into three factors. The L₉ (3³) orthogonal experiment with three factors and three levels was designed (Tables 2 and 3).

According to the orthogonal experimental scheme, nine kinds of composite-modified materials with different proportions were prepared by adding 200 mL Cr(VI) solution into a conical bottle and adding 50 g of the modified material. The supernatant was absorbed by the batch method, and the concentration of Cr(VI) was determined by 150 min on a constant temperature shaker ((298±1) K) with an oscillation frequency of 60 r/min. The mass concentration of Cr(VI) before and after adsorption was determined by diphenylcarbazide spectrophotometry. The removal rate and adsorption amount at time t of Cr(VI) were calculated by Eqs. (1) and (2), respectively. The adsorption effect of CWF with different ratios on Cr(VI) was compared, and the optimum combination of chitosan-modified fly ash material was obtained.

Removal rate:

                         (1)

Adsorption amount:

                           (2)

where η is the removal rate; ρ₀ and ρ represent the mass concentration of Cr(VI) ions in solution before and after adsorption, respectively, mg/L; m is the mass of the adsorption material, g; V is the volume of the solution, mL; and q_t is the adsorption amount at time t, mg/kg.

Table 2 Orthogonal experimental scheme

Table 3 Results of orthogonal table L₉(3³)

2.2.2 Component analysis

The concentration of Cr(VI) was determined by diphenylcarbazide spectrophotometry with an ultraviolet spectrophotometer (TU-1901, China). An X-ray diffractometer (Smart Lab 9kw) was used to analyze the mineral composition before and after the modification of the adsorbent material. The emitter material was a Cu target, and the tube pressure was 40 kV, the tube current was 100 mA. The scanning range was from 10°to 90°of 2θ, at a speed of 0.01°/s. The samples were subjected to continuous scanning to test the changes in the mineral composition before and after modification. The content of main elements in fly ash before and after modification was analyzed by an X-ray photoelectron spectrometer (ESCALAB 250Xi).

2.2.3 Particle size and structure analysis

The particle size distribution before and after the modification of fly ash was measured by a laser particle size analyzer (Beckman Coulter LS13320). The cold field emission scanning electron microscope (S-4800) was used to characterize the structure of the adsorbent before and after modification. The magnification was 1000×; the acceleration voltage was 2.0 kV; and the working distance was 16.3 mm.

2.2.4 Analysis of factors affecting adsorption

At 298 K and pH=7, the adsorption experiments of Cr(VI) with an initial concentration of 0.001 mg/L were carried out with fly ash and chitosan-coated fly ash as adsorbents. The solid-liquid ratio was 1 g:4 mL. Considering the influence of pH and the initial concentration of pollutants on the adsorption of Cr(VI) by CWF, the range of pH was set to 2-10. Based on the World Health Organization (WHO) drinking water quality standard with a total Cr limit of 0.05 mg/L [26], the batch adsorption experiments were carried out with 0.001, 0.01, 0.05 and 0.1 mg/L of Cr(VI) solution.

2.3 Data processing

The experimental data were processed and analyzed by Microsoft Excel 2010 and SPSS 18.0, and plotted by Corel Draw X6 and Origin 8.5. The retrieved minerals in fly ash and modified materials were analyzed by the Jade 6.0 program.

3 Results and analyses

3.1 Effect of material ratio on equilibrium adsorption capacity of Cr(VI)

The orthogonal adsorption experimental results of the optimum ratio of modified materials are shown in Table 2. S_i is the sum of experimental results corresponding to level i on any column (i=1, 2, 3), s_i=S_i/3, R=s_max-s_min. Through the value of R, it can be concluded that there is a significant order of influencing factors in this experiment, which is A>C>B. Therefore, the factors affecting the adsorption capacity (q) and efficiency of the modified materials are in order of amount of chitosan, concentration of acetic acid and amount of fly ash. According to the value of s, A₂, C₂, B₃ are the maximum influential values in each factor. The optimum combination is A₂C₂B₃, and the proportion of each component is A: C: B=2 g: 3 %: 15 g, named CWF-2c15.

Figure 1(a) shows the adsorption results of different adsorbents for Cr(VI). The test results show that the adsorption efficiency of the composite CWF-2c15 is superior to that of the fly ash monomer [27, 28]. The possible mechanism of Cr(VI) adsorption of composite materials is shown in Figure 1(b). Chitosan is a cationic polymer alkaline polysaccharide polymer. Amine matrix substrates make chitosan in acidic solution have more charged groups, and polyelectrolyte charge density increased, resulting in the change of its structure and performance. In case of collision, polyer molecules will be absorbed on the particle surface, and grow outward in a long chain, and formed particles-polymer particles structure (cellular chain structure) [29, 30]. The functional group NH₂ in the chitosan molecule can be protonated into —NH₃⁺ in acidic solution, which increases the electrostatic attraction among the CrO₄^2-, Cr₂O₇^2- and —NH₃⁺ groups and increases the adsorption capacity. At the same time, the surface acidic functional groups (silanol-based Si—OH, Al³⁺, Fe³⁺ protonated amino —NH₃⁺) of modified fly ash can exert multi-component adsorption properties [31].

Figure 1 (a) Adsorption results of Cr(VI) on different materials; (b) Adsorption mechanism of Cr(VI) on different materials

3.2 Component analysis

Figure 2 shows the energy spectrum analysis of the SEM results before and after modification of the fly ash. The results of the composition analysis of the main elements are shown in Table 4. As shown in Figure 2 and Table 4, the main elements of energy-dispersive X-ray spectroscopy (EDS) Area 1 and EDS Area 2 before and after modification are composed of Si, Al and other metal elements, with high O content and no other anions. The metal elements and Si elements mainly exist in the form of oxides, and there is no significant change in the content of the elements and the phase composition before and after modification [31].

Figure 2 Energy spectrum analysis:

Table 4 Comparison of fly ash content before and after modification

Figure 3 shows the XRD pattern of fly ash before and after modification. From Figure 3, it can be seen that there are many diffraction peaks in the adsorbent matrix, and the main components are quartz (SiO₂), mullite and magnetohydrite, which are also the main components of fly ash [32]. Figure 3 also shows that there is no new diffraction peak except for the difference in the intensity of the diffraction peak. This result shows that the mineral types of the adsorbent modified by chitosan have not changed and the adsorption process of the adsorbent is mainly physical adsorption. Physical and chemical interactions between the silica matrix (fly ash) and polysaccharide (chitosan) can form an interpenetrating network to support the immobilization of chitosan on fly ash [33, 34]. It can be inferred that the binding of fly ash with positively charged chitosan is a bonding process [35]. A new functional group is formed on the surface of fly ash/chitosan. Silanol-based Si—OH and Al³⁺ exhibit multi-component adsorption performance, which is more effective for the adsorption of Cr(VI).

Figure 3 X-ray diffraction pattern of CWF-2c15

3.3 Particle size and structure description

Figure 4 shows the percentage and accumulative percentage of modified and unmodified fly ash in each particle group. As shown in Figure 4, the particle size distribution of modified and unmodified fly ash is concentrated below 200 μm. Compared with unmodified fly ash, the growth of the percentage of CWF-2c15 particles sizes lower than 15 μm ranged from 51.91% to 74.30% and the reduction greater than 50 μm ranged from 17.87% to 2.87%. The results showed that the smaller particle size of the CWF-2c15 was conducive to formation of aggregated structures. Under the action of acetic acid, chitosan easily adheres to the surface of fine fly ash, forming a chitosan coating and depositing on the pore wall of the adsorbent, which increases the number of micropores, specific surface area and adsorption capacity of fly ash [36].

Figure 4 Particle size distribution before and after modification

The SEM images of modified and unmodified fly ash are shown in Figure 5. For the unmodified fly ash as seen in Figure 5(a), the fly ash displays a uniform granular shape, the overall surface is smooth and the pore passages are few. In Figure 5(b), the surface of the fly ash changes after chitosan acid modification, showing agglomeration. The change indicates the increase of surface roughness, the specific surface area (due to the formation of a large number of pores) and the absorption in the pores. This is because chitosan exerts the capacity of coagulation and adsorption under acidic conditions. Protonated amino —NH³⁺ increases the electrostatic attraction among the CrO₄^2-, Cr₂O₇^2- and —NH³⁺ groups and improves the adsorption capacity. The absorption capacity of the chitosan/fly ash composite to Cr(VI) is enhanced by changing the microstructure of fly ash [37].

Figure 5 SEM of fly ash:

4 Discussion

4.1 Effect of pH and Cr(VI) concentration on adsorption

The pH value is particularly important during the adsorption process because it affects the form of Cr(VI) in the solution [38-40]. CrO₄^2- is stable in solution with pH>8.2, when 5 2O₇^- and HCrO₄^-, and when pH<5, Cr(VI) exists in the form of H₂CrO₄ [41]. The pH value affects the adsorption capacity and zeta potential of adsorbents. With increasing pH, the protonation degree of adsorbents decreases, which hinders anion adsorption [42].

Figure 6 shows the adsorption capacity of CWF-2c15 in the pH range of 2-10, at t=298 K, c=0.05 mg/L. The results show that the fly ash has good adsorption performance when the pH ranges from 4 to 7. The adsorption capacity is the strongest at pH=5, which is consistent with the previous results of Ref. [43]. When the pH=2-5, the adsorption capacity of CWF-2c15 gradually increases with increasing pH. When pH is raised from 2 to 5, the content of HCrO₄^- and Cr₂O₇^2- in the solution keeps increasing, and the protonated CWF-2c15 has a good adsorption effect on Cr(VI) chromium ions under acidic conditions [44]. When pH=5-10, the adsorption capacity gradually decreases with increasing pH, which may be due to the addition of lye. CrO₄^2- and Cr₂O₇^2- preferentially combine with metal cations (M) in the solution (Eqs. (3) and (4)), and inhibiting the adsorption effect of CWF-2c15, resulting in a continuous decrease in its adsorption capacity:

           (3)

          (4)

Figure 7 represents the relationship among adsorption capacity, adsorption efficiency and time at different concentrations of Cr(VI). As shown in Figure 7(a), the adsorption capacity of the modified material increases with an increase in the initial concentration of the pollutant, which proves that the adsorbent is suitable for a wide range of concentrations of Cr(VI) and is not limited by the initial concentration of Cr(VI). As shown in Figure 7(b), when the concentration of Cr(VI) is 0.1 mg/L, the removal rate η_0.1<η_0.05 is due to the low dosage of adsorbent, which is not enough to absorb excessive pollutants [45]. Therefore, in practical applications, it is necessary to increase the amount of adsorbent to maintain a high removal rate.

Figure 6 Relationship between q and pH

Figure 7 Effect of concentration of Cr(VI) on:

4.2 Isothermal adsorption model

The isothermal adsorption equation is a mathematical equation that expresses the relationship between adsorption capacity and solution concentration at a fixed temperature. The Langmuir and Freundlich adsorption equations are typical representatives of isothermal adsorption equations [46, 47]. The relationship between the adsorption amount and the concentration of the adsorbed substance in the solution is the adsorption isotherm, which can be used to determine the action intensity of the adsorbent and the adsorbed substance. The Langmuir and Freundlich adsorption isotherm equations are obtained on the basis of the theoretical hypothesis and model of physical chemistry. By fitting the experimental data, some constants reflecting the adsorption mechanism, the structure of the adsorption layer and the macro surface structure of the adsorbent can be obtained.

The Langmuir adsorption equation assumes that the adsorption of substances on the solid surface is a monolayer and that there is no interaction between the adsorbed molecules. The adsorbent surface is uniform, and the adsorption equilibrium is dynamic equilibrium. The equation is expressed as Eq. (5):

                               (5)

where q is the adsorption capacity, mg/kg; c is the equilibrium adsorption concentration of the solution, mg/L; and a, b are constants.

The Freundlich adsorption equation corresponds to a wider range than Langmuir. The Freundlich model is an empirical equation based on the adsorption process on the homogeneous surface. The equation is expressed as below:

                                (6)

where K₀, n are constants.

Figure 8 shows the fitting diagram of the above model to the experimental data, the fitting parameters are shown in Table 5. From the results, Freundlich isotherm adsorption equation has better fitting effect on the experimental data, which shows that the adsorption is based on the physical process of the modified material surface. At 298 K, the maximum adsorption capacity of CWF on Cr(VI) is 0.48 mg/kg, and the theoretical adsorption capacity can reach 0.50 mg/kg. There are some deviations between them because there is a trace chemical reaction (Eq. (7)) during the adsorption process, which converts part of Cr(VI) into Cr (III) under acidic conditions [48]:

 (7)

Figure 8 Adsorption isotherm fitting diagram

4.3 Diffusion model

The internal diffusion of particles is mainly through micropores of particles. The molecules adsorbed on the outer surface of the particles migrate along the surface to the micropore. In the process of solid-liquid reaction, the first step is the diffusion of liquid membrane, and then through the liquid membrane to the solid-liquid two-phase interface through molecular diffusion, and participate in the reaction at the solid-liquid interface. The diffusion resistance of liquid membrane can only play a role in a short period of time at the initial stage of adsorption,which can be ignored when studying internal diffusion. There are three characteristics of internal diffusion: the direction of diffusion is random; the concentration of adsorbed substance does not change with the particle position; and the internal diffusion coefficient is constant [49].

Under the condition of T=298 K and pH=5, the internal diffusion model was used to fit the experimental data, and the diffusion mode of Cr(VI) in the adsorption process was studied. The internal diffusion model equation is shown as below:

                           (8)

where K_id is the diffusion rate constant; C is the reaction constant.

Figure 9 shows the kinetic model of adsorption and the fitting parameters are shown in Table 6. The diffusion process of pollutants can be divided into three stages. In the first stage (0-25 min), the slope of fitting curve is the largest. There is a maximum adsorption rate in this stage because of the sufficient adsorption sites on the surface of CWF-2c15. The concentration of hexavalent chromium in the solution is the highest and the mass transfer power is the largest [50]. When the initial concentration of pollutants increases, the mass transfer power and the adsorption rate increases. In the second stage (25- 90 min), the slope of fitting curve decreases obviously, which shows that the adsorption sites on the surface of CWF-2c15 and the concentration of hexavalent chromium in the solution gradually decrease, and the internal diffusion resistance inside the solution also increases. The adsorption process is controlled by membrane diffusion and intragranular diffusion [51]. The third stage (90 min later) is the final equilibrium stage. The adsorption sites on the surface of CWF-2c15 material reach saturation in this stage, the adsorption and desorption rates are basically the same, and the slope of fitting curve approach to zero.

Adsorption mainly occurs in the first two stages, and the time of this process is about 90 min. It can be concluded that the diffusion of hexavalent chromium in solution is mainly membrane diffusion and internal diffusion, and the adsorption process conforms to the pseudo-second-order dynamic model, because the fitting lines of the equation did not pass through the origin [52].

Table 5 Isothermal adsorption parameters of CWF-2c15

Figure 9 Adsorption kinetic model diagram

For pollutant diffusion, there are two diffusion modes: intraparticle diffusion and liquid membrane diffusion. The intraparticle diffusion is mainly carried out through the micropore of the particle, and the molecules adsorbed on the outer surface of the particle migrate along the surface to the inside of the micropore. In the solid-liquid reaction, the solute molecules move from the main body of the liquid phase to the surface of the liquid membrane, then reach the solid-liquid interface through the liquid membrane by molecular diffusion, and then participate in the reaction at the solid-liquid interface.

The Boyd diffusion model was usually used to explain this diffusion mechanism [53, 54]. The diffusion model equation is shown in Eq. (9):

              (9)

where B(t) is the time constant; F(t) is the ratio of the amount of pollutant adsorbed on the adsorbent at time t to the amount adsorbed at equilibrium, F(t)=q_t/q. F(t) is substituted into the Boyd model to obtain B_t.

The adsorption and removal of pollutants are controlled by intraparticle diffusion when the Boyd model fitting straight line pass through the origin. On the contray the adsorption and removal of pollutants are controlled by liquid membrane diffusion when the straight line does not pass through the origin.Figure 10 shows the fitting results of the test data with Boyd. Figure 10 shows that there is a perfect straight line fitting relationship between B_t and t, but the straight line does not pass through the origin. This result shows that the diffusion process is controlled by the diffusion of the liquid membrane. The diffusion process of the solute molecule from the main body of the liquid phase to the surface of the liquid membrane completes, which is conducive to a smooth adsorption process [55].

4.4 Adsorption thermodynamic model

The adsorption thermodynamic model mainly judges the reaction mode by calculating the free energy △G, the adsorption heat △H and the adsorption entropy change △S in the adsorption process [56]. The adsorption thermodynamic equations are shown as follows:

                          (10)

                         (11)

                        (12)

where △G is the free energy, kJ/mol; △H is the adsorption heat, kJ/mol; △S is the adsorption entropy change, kJ/mol; R is the general gas constant, 8.314 J/(mol·K); T is the absolute temperature, K; and K_f is the Freundlich constant.

As shown in Table 7, the reaction free energy △G<0 and the absolute value have no correlation with temperature. The adsorption process is spontaneous, which is indicated by △G_{298 K}>△G_{288 K}>△G_{308 K}. At 298 K, a turning point appeared. After exceeding this temperature, an increase in temperature was beneficial to the reaction and promoted adsorption. When △H<0, it is proven that the adsorption process is exothermic, which is consistent with the results of the Freundlich model fitting. When △S>0, the adsorption and chemical transformation of ions in liquid become more disordered and random when the reaction reaches adsorption equilibrium, which is the result of the increase in the temperature after adsorption. Thermodynamic parameters △H<0 and △G<0 indicate that the removal of Cr(VI) by CWF-2c15 is a spontaneous exothermic reaction. And the higher the temperature is, the greater the spontaneous degree is. △S>0 indicates that an irreversible entropy-increasing reaction occurred [57].

Table 6 Parameters of particle diffusion model of CWF-2c15

Figure 10 Linear fitting diagram of B_t and t

Table 7 Thermodynamic parameters in adsorption process

5 Conclusions

The adsorbent material (CWF-2c15) was prepared by chitosan-coated fly ash synthesized by crosslinking modification. The material ratio is chitosan: acetic acid: acid modified fly ash =2 g: 3%: 15 g. When pH=5, the adsorption rate of Cr(VI) in water can reach 87%. Compared with the raw materials of fly ash, the component of CWF-2c15 basically unchanged. However, the particle size reduced, and a chain structure with chitosan as coating is formed on the surface. The modified adsorbent CWF-2c15 enlarges its specific surface area and porosity, stabilizes its crystal structure, improves its adsorptive capacity and is conducive to the adsorption of Cr(VI). The modified material adsorbs Cr(VI) chromium by the electrostatic attraction between CrO₄^2-, Cr₂O₇^2- and —NH₃⁺ groups and the multi-component adsorption function of surface acid functional groups, and this process is mainly a physical process. The movement of Cr(VI) in solution is a diffusion process from the main body of the liquid phase to the surface of the liquid film, which is a self-heating and irreversible entropy increase process.

Contributors

JIANG Chun-lu provided the funding acquisition, concept, and reviewed and edited the draft of manuscript. WANG Rui conducted the data curation and wrote the first draft of the manuscript. CHEN Xing conducted the literature review and wrote the first draft of the manuscript. ZHENG Liu-gen provided the methodology of the manuscript. CHENG Hua supervised the manuscript.

Conflict of interest

JIANG Chun-lu, WANG Rui, CHEN Xing, ZHENG Liu-gen, CHENG Hua declare that they have no conflict of interest.

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(Edited by ZHENG Yu-tong)

中文导读

酸性条件下壳聚糖改性粉煤灰的制备及其对水中Cr(VI)的吸附机理

摘要：以壳聚糖和粉煤灰为原料，经乙酸和硫酸交联改性，采用酸浸-包覆法制备壳聚糖包覆粉煤灰(CWF)。利用扫描电子显微镜(SEM)和X射线衍射(XRD)测试了CWF的微观结构，通过吸附实验研究了CWF对水中Cr(VI)的去除效果，结合吸附模型、扩散模型和动力学模型探讨了吸附机理。结果表明：实验制备的复合材料具有孔隙结构，并在粉煤灰表面形成类似SiO₂和壳聚糖链状涂层的晶体结构；新型改性材料具有较大的表面粗糙度、比表面积和更多的吸附通道；改性材料通过CrO₄^2-、Cr₂O₇^2-和—NH₃⁺基团之间的静电吸引以及表面酸官能团的多组分吸附功能吸附Cr(VI)；Cr(VI)在溶液中的运动是液相主体向液膜表面扩散的过程。

关键词：壳聚糖；粉煤灰；改性；六价铬；吸附

Foundation item: Project(41602310) supported by the National Natural Science Foundation of China; Project(2017M611044) supported by the China Postdoctoral Science Foundation

Received date: 2020-07-01; Accepted date: 2021-02-03

Corresponding author: JIANG Chun-lu, PhD, Associate Professor; Tel: +86-551-63861511; E-mail: ahuclj@ahu.edu.cn; ORCID: https://orcid.org/0000-0002-5375-5103

Abstract: Chitosan-coated fly ash (CWF) was prepared by the acid leaching-coating method. Chitosan and fly ash were crosslinked in the solution of acetic acid and sulfuric acid. The microstructure of CWF was conducted by scanning electron microscope (SEM) and X-ray diffraction (XRD). The removal of Cr(VI) from water by CWF was studied by adsorption experiments. The composite prepared by the experiment developed a pore structure and a crystal structure similar to SiO₂ and chitosan chain-like coating was formed on the surface of fly ash. The new modified material has larger surface roughness, specific surface area and more adsorption channels. The Cr(VI) was enriched in modified materials by electrostatic adsorption between CrO₄^2-, CrO₇^2- and —NH₃⁺ group and surface acid functional groups. The movement of Cr(VI) in solution is a diffusion process from the main body of the liquid phase to the surface of the liquid film.