Preparation of low-cost functionalized diatomite and its effective removal of ammonia nitrogen from wastewater

A low-cost functionalization method was used to treat diatomite, and an efficient adsorbent for ammonia nitrogen was prepared by optimizing the functionalization conditions. The functionalized diatomite (DTCA-Na) was characterized by SEM, EDS, BET, XRD, FT-IR, and TG. The results demonstrate that DTCA-Na has excellent adsorption performance after being modified with H2SO4 (60.00 wt.%), NaCl (5.00 wt.%), and calcination at 400 °C for 2 h. While studying the effect of adsorption factors on the removal of ammonia nitrogen, the kinetic and thermodynamic behaviors in the adsorption process were discussed. The removal efficiency of the simulated wastewater with the initial ammonia nitrogen concentration of 10.00 mg L−1 by the DTCA-Na was more than 80% when the contact time was 60 min, pH was 6–10, the dosage of adsorbent was 1.00 g, and the temperature was 25 °C. The adsorption process of ammonia nitrogen was conformed to the pseudo-first-order and Langmuir isothermal model. The removal efficiency of ammonia nitrogen was still above 80% after 5 times adsorption–desorption experiments. The DTCA-Na has a brighter prospect of application in the field of ammonia nitrogen wastewater treatment due to its excellent adsorption performance and low-cost advantage.


Introduction
The rapid development of urbanization and industrialization in most countries has led to a large amount of waste water being discharged into rivers and lakes (Fan et al. 2021).These wastewaters contain many types of pollutants, such as ammonia nitrogen, phosphorus, dyes, bacteria, organic matter, etc. (Fiorentino et al. 2019;Rajasekhar et al. 2020;LaMartina et al. 2021).Among them, excessive ammonia nitrogen (NH 4 + -N) in water bodies will cause severe eutrophication and pose a serious threat to the water environment (Cheng et al. 2019;Li et al. 2020a).And it should be noted that water quality safety has an essential effect on human health and survival.Therefore, the technology development to remove NH 4 + -N from wastewater is an effective way to ensure the safety of water quality.
Over the decades, researchers have developed various techniques to remove NH 4 + -N from water, including physical, chemical, and biological techniques (Xiang et al. 2020).The main methods include air stripping method (Yin et al. 2018b), ion exchange and adsorption technology (Zhang et al. 2019), struvite precipitation method (Uludag-Demirer et al. 2008), nitrification and denitrification (typical activated sludge) (Zhou et al. 2011), microalgae treatment (Wang et al. 2019), etc.However, these methods are often difficult to have a wide range of applications due to the disadvantages of complex operation, high development and Communicated by Tito Roberto Cadaval Jr. Shuju Fang and Gufeng Li contributed equally to this work.maintenance costs, long processing time, and limitations of sewage categories (Ducey et al. 2010;Du et al. 2017;Fang et al. 2018;Zeng et al. 2018).In recent years, the adsorption method with low price, stable properties, and high efficiency has attracted extensive attention from researchers (Yin et al. 2018a).The nonmetallic mineral raw material are commonly used adsorbent materials (Xu et al. 2018), mainly including bentonite (Seredych et al. 2008), zeolite (Wu et al. 2021), and diatomite (Wu et al. 2005;Li et al. 2021).
Diatomite is a natural nonmetallic mineral with abundant reserves, and the main component is SiO 2 .Diatomite has excellent porosity, void volume, and specific surface area, and the surface is rich in silyl hydroxyl groups and hydrogen bonds (Danil de Namor et al. 2012;Zhang 2013;Wang et al. 2016).These characteristics make diatomite widely used in the field of adsorbents or filter materials (Gómez et al. 2014;Song et al. 2021).However, the various impurities contained in raw diatomite make it difficult to effectively adsorb pollutants.Therefore, researchers have effectively improved the adsorption properties of diatomite through multiple methods, such as surface modification and purification.For example, Ye et al. modified diatomite by ultrasound and sodium chloride solution to effectively remove phosphate from water (Ye et al. 2021).Xia et al. prepared porous cationic diatomite by covalently immobilizing poly-epichlorohydrin-dimethylamine onto diatomite and showed efficient adsorption performance for anionic dye wastewater (Xia et al. 2020).Li et al. have reported a new-type magnetic diatomite which was used to remove nitrobenzene from aqueous solutions (Li et al. 2020c).In general, most adsorbents for the NH 4 + -N are challenging to be applied in actual water treatment due to their disadvantage, such as complex preparation, highcost and limitation of water features (pH, temperature, etc.).Therefore, it is critical to develop low-cost and green ammonia nitrogen adsorbent.
To find a cost-effective adsorbent for NH 4 + -N, a new functionalized diatomite was prepared by acid leaching, sodium-activation and calcining method in this paper.First, the DTC A-Na with excellent adsorption performance was prepared by optimizing the functionalization conditions from the aspect of NH 4 + -N removal efficiency and various characterization tools.Secondly, the effects of time, pH, adsorbent dosage, temperature, initial concentration of NH 4 + -N, and regeneration times were investigated, and the kinetic and thermodynamic behaviors of the adsorption process were discussed.In addition, the possible mechanism of NH 4 + -N adsorption in water by DTC A-Na was initially explored.This study not only offers a novel approach for the effective utilization of cheap diatomite resources but also provides promising adsorbent for wastewater with NH 4 + -N.
The surface morphology and the elemental contents of diatomite were investigated by Nova Nano SEM450 (Thermo Scientific, USA).The specific surface area of diatomite was analyzed using TriStar II 3020 (Micromeritics, USA).2SX100e the X-ray diffractometer (Rigaku, Japan) was used to record X-ray diffraction pattern of samples.A Nicolet iS10 (Thermo Scientific, USA) was used to examine Fourier transform infrared spectroscopy.The thermogravimetry of samples was analyzed by STA449F3 (Netzsch, Germany).

Preparation and optimization of DTC A-Na
A certain amount of untreated diatomite was grinded to fine granularity in a grinder, and then made samples through a 100-mesh sieve to obtain the raw diatomite.Twenty grams of raw diatomite was put into six 100 mL conical flasks, respectively, and mixed with H 2 SO 4 solutions of different concentrations (30.00-80.00wt.%) to form homogenates.The homogenates were stirred for 60 min in a magnetic stirrer, and then been statically separated for 3 h; the top layer of suspension and bottom sediment were removed and washed to neutral, and then dried at 105 °C.Of the above acid-leached diatomite, 5.00 g was placed in 500 mL beakers separately, and then added La 2 O 3 , NaCl, and LaCl 3 with 5.00 wt.%, respectively.The mixes were kept stirring and heating at 95 °C for 2 h and then washed several times until neutral with distilled water.The samples dried at 110 °C were placed in a crucible and calcinated in a muffle furnace at different temperatures (100-700 °C) for 2 h.Subsequently, the prepared DTC A-Na was stored in a desiccator and for later use.

+ -N by DTC A-Na
The NH 4 + -N simulated wastewater was prepared using NH 4 Cl.The 50 mL NH 4 + -N simulated wastewater and a certain amount of DTC A-Na were added into a 100-mL conical flask, and then the conical flask was shaken in a constant temperature shaker.The effects of time (5-60 min), adsorbent dosage (0.50-3.00 g), pH (2-12), temperature (25-45 °C), and initial concentration of simulated wastewater (1.00-25.00mg L −1 ) on the removal of NH 4 + -N during the adsorption process were investigated, respectively.After the adsorption was completed, 10 mL of the solution adsorbed was centrifuged in a centrifuge tube at 1500 r/ min for 20 min.The NH 4 + -N content in the supernatant was determined by Nessler's reagent colorimetric method.The amount adsorbed (Q e ) and the removal efficiency (R) of DTC A-Na for NH 4 + -N at equilibrium can be calculated using the following equation: where C 0 and C e (mg L −1 ) are the initial and equilibrium concentration of NH 4 + -N in solution, respectively; V (L) is the volume of the NH 4 + -N wastewater; and m (g) is the dosage of adsorbent.

Regeneration experiments
The adsorbent deposits adsorbed on the simulated wastewater were desorbed using different regenerant (NaCl, Na 2 CO 3 , NaOH, HCl, and distilled water), separately.After desorption, the adsorbent was separated by centrifugation and washed 3 times with distilled water.The adsorbent was dried at 50 °C and set aside.Five adsorption-desorption cycles were completed in this way.

Adsorption kinetic and isotherm models
Adsorption kinetics models are conducive to understand the adsorption mechanism and kinetics process of DTC A-Na for NH 4 + -N.Hence, the pseudo-first-order kinetic, the pseudosecond-order kinetic, and the intraparticle models were used to fit the experimental data.The equations are as follows (dos Santos et al. 2014;Shaban et al. 2017;Ruíz-Baltazar 2018): (3) where Q t (mg g −1 ) is the adsorption amount of NH 4 + -N at time t; k 1 (min −1 ), k 2 (g (mg min) −1 ), and k p (mg (g min 1/2 ) −1 ) are the pseudo-first-order kinetic rate constant, the pseudo-secondorder kinetic rate constant, and the intraparticle diffusion rate constant, respectively.
The adsorption isotherm reflects the concentration relationship between adsorbent and adsorbate when adsorption reaches equilibrium at a certain temperature.Through the fitting analysis of Langmuir, Freundlich, and Temkin isothermal adsorption models, the adsorption characteristics of DTC A-Na can be expounded, which is helpful to the discussion of the adsorption mechanism.The relevant equations are as follows (Chen et al. 2016;Wibowo et al. 2017): where Q m (mg g −1 ) is the maximum adsorption capacity at a certain temperature; k L (L mg −1 ) is the Langmuir constant related to temperature; k F ((mg g −1 )(L mg −1 ) 1/n ) is the Freundlich constant which indicates adsorption capacity; n is calculated from the Freundlich equation related to adsorption intensity; k T (L mg −1 ) and b is the Temkin constant; A is equivalent to (R T lnk T ) b −1 and B is (R T b −1 ); R (8.314 J (mol K −1 )) is thermodynamic constant and T (K) is a temperature in Kelvin.
The thermodynamic study of adsorption can analyze whether the adsorption process is spontaneous and exothermic.The thermodynamic parameters are calculated using the following equations (Anastopoulos 2016;Lima et al. 2020): where k c (L g −1 ) is the thermodynamic equilibrium constant; ΔG (kJ mol −1 ) is Gibbs free energy change; ΔS (J mol −1 k −1 ) is entropy change; ΔH (kJ mol −1 ) is enthalpy change.
The apparent activation energy of the adsorption process was calculated using the Arrhenius equation as follows: where k (min −1 ) is the adsorption rate constant, E a (kJ mol −1 ) is the apparent activation energy of adsorption, and A t is the prefactor.( 6)

Optimization of preparation conditions
The removal efficiency of NH 4 + -N by diatomite purified under different modification approaches is shown in Table S1.Compared with raw diatomite (DT R ) and washed diatomite (DT W ), the NH 4 + -N removal efficiency of the diatomite leached (DT A ) with H 2 SO 4 (60.00 wt.%) was significantly increased.The result indicated that acid leaching is an effective way to improve the adsorption performance of diatomite.What cannot be ignored is that the removal effect of the DT A for NH 4 + -N is closely related to the concentration of H 2 SO 4 .As it is depicted in Fig. 1a, when the mass fraction of H 2 SO 4 was less than 60.00%, the removal efficiency of NH 4 + -N by the DT A increased with the increased of H 2 SO 4 concentration.The removal efficiency of NH 4 + -N by the DT A can reach to peak when using H 2 SO 4 with a concentration of 60.00 wt.%.When concentration of H 2 SO 4 exceeded 60.00 wt.%, the removal efficiency of NH 4 + -N from the DT A showed an obvious decreasing trend.The adsorption capacity was limited because the H 2 SO 4 concentration was too low to dissolve some impurities in the DT R , such as iron oxide, aluminum oxide, and alkaline earth metal oxides (Abdellaoui 2018).However, if the acidity is too strong, the structure and pores of diatomite are extremely susceptible to corrosion and dissolution.In addition, metal impurities such as iron and aluminum are easily passivated in concentrated H 2 SO 4 and difficult to remove, resulting in an insufficient acid leaching effect.Therefore, H 2 SO 4 with a concentration of 60.00 wt.% was chosen for the refinement and purification of the DT R .
Although the adsorption performance of diatomite enhanced after pickling, the removal efficiency of NH 4 + -N was still insufficient.From the data in Table S2, it can be seen that by introducing different types of modifiers to further improve the adsorption capacity of diatomite, the modification effect of NaCl and La 2 O 3 on diatomite was better than that of LaCl 3 ; however, NaCl was the preferred choice due to cost advantage.The modification effect often depends on the concentration of the modifier, and the impact of NaCl concentration on the adsorption property of diatomite was illustrated in Fig. 1b.The removal efficiency of NH 4 + -N increased with the increased of the concentration of NaCl; the removal efficiency basically no longer increased when the mass fraction of NaCl was 5.00%.As the concentration of NaCl increased, the amount of Na + exchanging with Mg 2+ , Ca 2+ , and other cations was increased, and the sites providing NH 4 + -N for adsorption also increased.When the mass fraction of NaCl reached 5.00%, there were insufficient ions for Na + exchange and no more adsorption sites, so the removal efficiency was saturated.The best modifier, NaCl with a concentration of 5.00 wt.%, was found to be the most efficient, energy-saving, and low-cost.
Calcination has been an essential means of enhancing the permeability of diatomite (Zheng et al. 2018), which facilitated the diffusion of NH 4 + -N from the pores to the interior of the diatomite, thus helping to enhance the adsorption capacity.Therefore, the diatomite after acid leaching and sodium-activation was calcined in a hightemperature muffle furnace.The relationship between the removal efficiency of NH 4 + -N and the calcination temperature could be observed in Fig. 1c.The highest removal efficiency was achieved when the calcination temperature was 400 °C.The calcination temperature was too low to effectively improve the NH 4 + -N adsorption performance; if the calcination temperature was too high, the structure of diatomite may be destroyed, which lead to difficulty in adsorption of NH 4 + -N.From the viewpoint of high efficiency and energy saving, the adsorbent material was prepared under calcination at 400 °C.In summary, the DT R was first leached with H 2 SO 4 (60.00 wt.%), then modified with 5.00 wt.% NaCl, and finally calcinated in a muffle furnace at 400 °C for 2 h to obtain DTC A-Na .Figure 2 describes the preparation method of DTC A-Na .

Characterization of the DTC A-Na
To investigate the relationship between the structural properties and the adsorption performance of diatomite samples prepared in experiments, the SEM images are presented in Fig. 3.There were many porous structures on the surface of the DT R ; however, these pore structures were covered by diatomite fragments and impurities from Fig. 3a.According to the Fig. 3b, acid leaching not only removes metal oxide impurities from the DT R due to the reaction of metal oxide impurities with acid but also optimally improves the surface morphology (Zhang 2013).Figure 3c shows the SEM of diatomite after acid leaching and sodium-activation (DT A-Na ).It can be observed that the surface and pores of the DT A-Na were covered with some lumpy particles due to sodium-activation.In Fig. 3d, the calcined DT A-Na (DTC A-Na ) displayed the optimal surface morphology.The porous structure of the DT A-Na grew more clear and complete, which could enhance the permeability of diatomite.In addition, Table 1 summarizes the energy spectrum analysis of diatomite samples.The main elements of diatomite were Si and O, as well as containing small amounts of metallic elements, such as Al, Fe, and K.The content of metallic elements in the DT A reduced visibly compared with DT R , which was attributed to the dissolution of these metal oxides by the acid.The sodium was detected in the DT A-Na and the DTC A-Na , indicating that sodium was loaded onto the surface of diatomite after sodium-activation.
The N 2 adsorption/desorption isotherms are represented in Fig. S1, and the BET measuring parameters of all diatomite samples are listed in Table 2.The N 2 adsorption/ desorption isotherms of diatomite samples belong to type IV isotherms, and all appear to be the H3 hysteresis loop belonging to the IUPAC classification standard (Thommes et al. 2015).The occurrence of the H3 type hysteresis loop is linked to the presence of clay and other flaky particles in the material, which is consistent with the nature and SEM features of diatomite.Compared with the BET parameter of the DT R , the specific surface area of the DTC A-Na increased from 13.72 to 20.26 cm 2 g −1 , and the pore volume went from 0.1257 to 0.1606 cm 3 g −1 .To some extent, the functionalized Fig. 2 The preparation process of functionalized diatomite process changed the morphology of diatomite.The increased specific surface area and pore volume of DTC A-Na indicated that it had more adsorption sites and better adsorption effect.Furthermore, as Na + was introduced and entered into the surface and pores, the specific surface area and pore volume of the DT A-Na decreased considerably compared with that of the DT A .This corresponds to diatomite samples content in EDS.
To further analyze the structure and composition of the diatomite samples, XRD, TG, and FT-IR were used for characterization, and the results are shown in Fig. 4. It can be seen from Fig. 4a that the XRD patterns were broadened diffraction peaks, indicating that the diatomite is in noncrystalline form, and the characteristic diffraction peaks of quartz are evident at 26-27°.Some fragmented and blurred diffraction peaks appeared in the DT R , indicating that the DT R contained many different impurities; among the diffraction peak was the sharpest at 32°, which was probably an organic impurity peak and disappeared after acid leaching.The diffraction peaks of the remaining three diatomite samples have reduced impurity peaks and were smoother, which explained that functionalization could significantly reduce impurities due to the dissolution of metal oxides and soluble impurities and the  volatilization of some impurities by calcination.The diffraction peaks of all samples basically coincided, indicating that functionalization improved the microscopic morphology and composition without destroying the basic compositional skeleton.
The functional groups of samples were analyzed by FT-IR from Fig. 4b.The peak patterns of the four samples were not significantly different and were consistent with the conclusion of the XRD analysis above, indicating that the basic skeleton of diatomite remains unchanged after functionalization and has good stability.Since diatomite has water absorption, the absorption peaks at 3463.67 cm −1 and 1650.84 cm −1 were attributed to the stretching vibration absorption peak and bending vibration absorption peak of -OH of water adsorbed, respectively.The absorption bands at about 1114.70 cm −1 , 810.95 cm −1 , and 476.35 cm −1 represented the antisymmetric stretching, symmetric stretching, and bending vibrations of Si-O-Si bond (Saidi et al. 2012), respectively.The DT A-Na showed a characteristic absorption peak at 2374.04 cm −1 , which was assigned to an asymmetric stretching vibration absorption peak of CO 2 and disappeared after calcination.
The thermal behavior of diatomite samples in terms of TG analysis was observed in Fig. 4c.The first weight loss of the DT R , DT A , DT A-Na , and DTC A-Na were as follows: 3.57%, 3.61%, 3.21%, and 2.75% in order, indicating that the functionalization could remove part of the water.
The second weight loss peak all appeared in the range of 300-450 °C.The temperature of the second heat absorption peak of the four samples increased gradually, and the weight loss rates were 18.90%, 21.80%, 21.18%, and 10.21%, respectively.This stage was combined with water burning removal.These results indicated that calcination temperatures above 400 °C could destroy the structure of diatomite.

Effect of contact time and temperature
Adsorption time and temperature are often the limiting factors for the use of adsorbents in practical water treatment.As shown in Fig. 5a, the removal efficiency of NH 4 + -N in simulation wastewater by the DTC A-Na adsorption at various temperatures and contact time.The increasing trend of the removal efficiency was significant within 20 min.After the adsorption time exceeded 20 min, the removal efficiency tended to level off.The amount of NH 4 + -N adsorbed on the DTC A-Na was increased with contact time and reached equilibrium at 60 min.The main reason why the adsorption rate of the adsorbent was fast first and then slower may be that the adsorbent initially provides more adsorption positions and lower steric hindrance.When the adsorbent continued to adsorb + -N increased minimally with temperature, implying that the adsorption process was inherently heat-absorbing.However, the increase of temperature could not significantly improve the adsorption capacity of NH 4 + -N, indicating that temperature had little effect.For high-efficiency and energy-saving, the best contact time for the adsorption was 60 min at 25 °C.

Effect of adsorbent dosage
As can be seen from Fig. 5b that the NH 4 + -N removal efficiency showed an overall positive correlation with the amount of adsorbent dosing when the DTC A-Na dosage increased from 0.50 to 3.00 g.The removal efficiency increased with the increase of the adsorbent dosage.When the adsorbent dosage was greater than 1.00 g, the removal efficiency maintained a small increase.This is related to the effective adsorption of NH 4 + -N at the adsorption site on the surface of the DTC A-Na .More adsorbents cause more adsorption sites, and thus, the adsorption capacity is significantly increased.Regarding the cost of adsorption treatment, 1.00 g was chosen as the optimum dosage.

Effect of pH
The pH of the water environment is an indicator that has to be taken into account when adsorption was used to treat water pollution.The experimental results of the NH 4 + -N removal efficiency on the DTC A-Na in the pH range 2-14 are shown in Fig. 5c.It has been demonstrated that the highest removal efficiency of NH 4 + -N was achieved by the DTC A-Na in the pH range of 6-10, indicating that neither highly acidic nor highly alkaline environments were conducive to NH 4 + -N adsorption.When the pH value was small, the solution contained a large amount of H + , and both H + and NH 4 + were cations; they will compete for the adsorption position (Du et al. 2005).Furthermore, H + is more likely to enter the micro-pores of the adsorbent due to the ionic radius r (H + ) < r (NH 4 + ), resulting in a lower removal effect.Excessive OH − concentration in high pH solutions, NH 4 + and OH − combine to form ammonia (Huang et al. 2010), so that the concentration of NH 4 + decreases and the amount of NH 4 + adsorbed by the adsorbent per unit time decreases.The study results revealed that most of the aqueous environments were favorable for capturing NH 4 + -N using the DTC A-Na without adjusting the pH value of the effluent.As shown in Fig. 5d, NH 4 + -N removal efficiency of the DTC A-Na at the different initial concentration of wastewater was investigated.The low concentration of NH 4 + -N in the simulated wastewater was not conducive to the full performance of the adsorbent.However, as the concentration of NH 4 + -N increased, more NH 4 + -N could be adsorbed, so the removal efficiency increased when the initial concentration of NH 4 + -N was increased from 1.00 to 10.00 mg L −1 .When the concentration of NH 4 + -N in wastewater was too high, surplus NH 4 + -N was not adsorbed due to the limited adsorption sites of a certain amount of adsorbent.Hence, the removal efficiency tended to decrease when the initial concentration of NH 4 + -N was > 10.00 mg L −1 .The initial concentration of NH 4 + -N was fixed at 10.00 mg L −1 during the adsorption process, considering the need to exploit the adsorption performance of the DTC A-Na effectively.

Desorption and circulation experiments of adsorbent
Regeneration performance is one of the critical indicators to evaluate the practicality of adsorbents.The effect of regenerant and its concentration on the regeneration of the DTC A-Na was studied by an adsorption-desorption experiment to evaluate the regeneration performance.The effect of several inexpensive regenerants (NaCl, Na 2 CO 3 , NaOH, HCl, and distilled water; the mass fraction of all regenerants were 5.00%) on the resolution and resorption of NH 4 + -N are listed in Table S3 for comparison.All five regenerants have a certain desorption regeneration effect on the DTC A-Na after adsorption.What calls for special attention is that distilled water showed the worst desorption effect, indicating that the NH 4 + -N adsorbed could not be easily removed from the aqueous solution.The best desorption effect for NH 4 + -N was NaCl, and the desorption rate and the removal efficiency were 96.19% and 81.42%, respectively.
The mass fraction of regenerant has a significant impact on the desorption effect of the adsorbent.As depicted in Fig. S2, when the mass fraction of NaCl solution was 5.00%, the desorption rate and removal efficiency of the DTC A-Na were more than 85.00%, and the regeneration effect was the best.The findings revealed that NaCl solution with a concentration of 5.00 wt.% solution could be chosen to desorb the NH 4 + -N adsorbed.The adsorption-desorption regeneration recycling experiments of the DTC A-Na are shown in Fig. 6.After repeated five cycling experiments, the removal efficiency of NH 4 + -N was maintained at more than 82.00%.The excellent regeneration performance served as a valuable reference point for the actual sewage treatment of the DTC A-Na .

Adsorption kinetic studies
In order to discuss the adsorption mechanism of the DTC A-Na , adsorption experimental data were fitted by the pseudo-first-order kinetics, the pseudo-second-order kinetics, and the intraparticle diffusion model.The kinetics models were calculated using Eqs.(3)-( 5).
The trend of NH 4 + -N adsorption by the DTC A-Na with contact time is shown in Fig. S3a.The adsorption process could be divided into a fast adsorption phase at the beginning and a subsequent slow equilibrium phase.The adsorption capacity of NH 4 + -N reached a plateau after about 60 min.
The nonlinear fitting curve of kinetics models for adsorption experimental data are illustrated in Fig. S3 (b-c), and the kinetic parameters are shown in Table 3.The pseudo-firstorder kinetics was accorded with the adsorption process of NH 4 + -N by the DTC A-Na , with the correlation coefficient (R 2 ) greater than 0.9400 at different temperature.In contrast, the R 2 of the pseudo-second-order kinetics was poor correlation and not suitable for describing this adsorption process.The kinetic fitting results showed the arrival of the adsorbate from the solution to the adsorbent surface was controlled by surface diffusion, and the only factor that determined the adsorption rate was the concentration of NH 4 + -N because the temperature had little effect.Several research papers have reported the same conclusion that the pseudo-firstorder kinetics was more consistent with the adsorption process (Cadaval et al. 2015;Guo et al. 2019;Hu et al. 2020).
The intraparticle diffusion kinetics curve fitting (Fig. S3d) reveals that the adsorption process of the DTC A-Na for + -N was composed of two stages.In the first stage, the NH 4 + -N in aqueous phase spread rapidly to the DTC A-Na surface, it is commonly referred to as the liquid film diffusion process.The intraparticle diffusion process was illustrated in the second stage; the NH 4 + -N dispersed into interior pores due to the occupied adsorption sites of the surface.According to the intraparticle diffusion model, if the fitted straight line passes through the origin, intraparticle diffusion is the only rate-limiting element.Figure S3d is shown that intraparticle diffusion was not the only rate-controlling step in the adsorption of NH 4 + -N by the DTC A-Na .In addition, the intraparticle diffusion parameters at different temperatures are reported in Table 4, which was represented that the second stage was the slowest step because k p,2 of the second stage was quite lower than k p,1 of the first stage.These results were demonstrated that the mechanism of the NH 4 + -N adsorption on the DTC A-Na was complex and not determined by a single factor.

Adsorption isotherm studies
First, the relationship of the equilibrium concentration of NH 4 + -N on the equilibrium adsorption amount at 25 ℃ is presented in Fig. S4a.Furthermore, in Fig. S4 (b-d), the Langmuir, Freundlich, and Temkin adsorption isotherm models were used to nonlinear fit the experimental data, and the equations for the adsorption isotherms were Eqs. ( 6)-( 8), sequentially.The adsorption isotherm correlates the equilibrium concentration of NH 4 + -N with the equilibrium adsorption amount by different models and studies the distribution of adsorbed molecules in the liquid-solid phase at the equilibrium adsorption state.The isotherm parameters of the adsorption of NH 4 + -N by the DTC A-Na are presented in Table 5.In terms of correlation coefficients, the adsorption process was more consistent with the Langmuir isothermal adsorption model, indicating that the adsorption sites were uniformly distributed on the adsorbent surface, and the adsorption process was unimolecular layer adsorption.The amount of adsorbed NH 4 + -N increased rapidly at lower solution concentrations, indicating a good affinity between NH 4 + -N and the adsorbent surface.At higher concentrations, NH 4 + -N adsorption reached a plateau, showing a maximum saturated adsorption capacity.The maximum adsorption capacity obtained from the Langmuir was 0.7162 mg g −1 .

Thermodynamic studies and adsorption activation energy
To further understand thermodynamic properties of the adsorption process by thermodynamic parameters, such as ΔH, ΔS, and ΔG, the calculation methods are as Eqs.( 9)-( 10), thermodynamic parameters were calculated in Table 6.A positive value of ΔH and ΔS reflected that the adsorption of the DTC A-Na for NH 4 + -N was essentially a

Freundlich
Langmuir Temkin heat absorption process and the degree of disorder at the solid-liquid interface was increased, which was consistent with the result on the effect of temperature on the NH 4 + -N removal efficiency.In Table 6, the ΔG at different temperatures were all negative, which indicated that the adsorption of NH 4 + -N by the DTC A-Na was favorable and spontaneous (Huang et al. 2014).In addition, the Arrhenius activation energy (E a ) of 4.25 (kJ mol −1 ) was obtained for the adsorption process of NH 4 + -N by plotting ln k against T −1 according to the Eq. ( 11), which was used to speculate the type of adsorption process.The activation energy of the NH 4 + -N adsorption process by the DTC A-Na was much less than 40 kJ mol −1 , indicating that the process requires very little energy and NH 4 + -N is mainly physically adsorbed to the DTC A-Na (Boparai et al. 2011;Kim and Kim 2019).The discussion of thermodynamics and activation energy illustrates that the adsorption of NH 4 + -N by the DTC A-Na is a complex process rather than a single simple one.

Analysis of the DTCA -Na adsorption mechanism
A schematic diagram of the adsorption mechanism for the removal of NH 4 + -N from water by the DTC A-Na is shown in Fig. 7.The NH 4 + -N removal was revealed to be a complex adsorption process based on the characterization results and the discussion of the kinetics and thermodynamics.First, it is demonstrated that the adsorption of NH 4 + -N by DTC A-Na mainly exhibited the physical adsorption process that included the liquid film and intrapore diffusion of the adsorbent surface.Second, Na + introduced to the surface of diatomite are able to exchange ions with NH 4 + -N, as the following chemical reaction (Ren et al. 2021): Meanwhile, the FT-IR result showed that the surface of diatomite is rich in a large number of -OH, Si-O-Si, and other functional groups, which not only lead to a negatively charged surface to attract NH 4 + -N with positively charged in solution, but also immobilize NH 4 + -N to the adsorbent surface (Rakhshaee et al. 2006;Zhao et al. 2019).
Therefore, the adsorption process of NH 4 + -N by the DTC A-Na is a multi-step process including physical adsorption and chemical adsorption.Among them, liquid film diffusion, intraparticle diffusion, and electrostatic adsorption belong to physical adsorption due to Van der Waals forces (Lyu et al. 2020;Li et al. 2021), while ion exchange and hydrogen bonding based on functional groups are chemisorption (Xia et al. 2017).The functional modification of diatomite by acid leached-Na-activation-calcination can not only effectively improve the adsorption performance but also construct a new adsorption pathway.Effective and diverse adsorption methods improve the adsorption efficiency of the DTC A-Na for NH 4 + -N.

Conclusion
The DTC A-Na prepared in this study not only has low modification cost and simple process but also can efficiently remove ammonia nitrogen from water.According to various characterization results, the morphology, pore size, specific surface area, and stability of diatomite after functionalization were significantly improved.At room temperature, the removal efficiency of ammonia nitrogen in wastewater with a pH of 6-10 and an initial ammonia nitrogen concentration of 10.00 mg L −1 by 1.00 g of adsorbent can reach over 80.00% in 60 min.The pseudofirst-order kinetics and Langmuir models may effectively match the adsorption process, and combined with thermodynamic characteristics to suggest that the adsorption process of DTC A-Na was a complicated process based on physical adsorption with chemisorption.Besides, the DTC A-Na can be regenerated and utilized at least five times.This research provides a new way for the development and application of diatomite and meets the needs of current green and economical environmental governance.

Fig. 1
Fig. 1 Effect of functionalization conditions on the functionalized diatomite preparation.a Effect of H 2 SO 4 concentration.b Effect of the mass fraction of NaCl.c Effect of the calcination temperature

Fig. 3
Fig. 3 Scanning electron micrographs of the DT R (a), DT A (b), DT A-Na (c), and DTC A-Na (d) under 60,000 × conditions

Fig. 4
Fig. 4 Characterization of functionalized diatomite.a The XRD patterns.b The FT-IR spectra characterization.c The thermogravimetric analysis

Fig. 5
Fig. 5 Influence of adsorption factors on the NH 4 + -N removal efficiency.a Effect of contact time and temperature.b Effect of adsorbent dosage.c Effect of pH.d Effect of initial concentration of NH 4 + -N

Fig. 6
Fig. 6 Effect of regeneration times on the removal efficiency

Fig. 7
Fig. 7 The schematic diagram of the adsorption mechanism of the DTC A-Na