Phosphate and ammonia nitrogen recovery from sewage sludge supernatants by coupled MgO-biomass ash and its potential as heavy metal adsorbent

3.1.1 Microstructural characterization of CMBA

As shown in Table 1, BA had a specific surface area of 65.12 m²/g, a total pore volume of 0.14 cm³/g, and an average pore diameter of 8.79 nm. With the increase of ${\text{Mg}}^{\text{2+}}$ addition, the specific surface area of CMBA increased from 50.18 to 59.79 m²/g, the total pore volume increased from 0.14 to 0.20 cm³/g, and the average pore size increased from 8.79 to 13.35 nm. These results agree with those of He et al. (2022). It may be because the added ${\text{Mg}}^{\text{2+}}$ hinders the formation of substances such as tar during calcination, thus promoting the formation of pore structures (Zhu et al., 2020). However, the specific surface area and total pore volume of CMBA gradually increased (≤1.25 M) and then decreased (>1.25 M) when the ${\text{Mg}}^{\text{2+}}$ addition varying from 0.25 M to 1.75 M. Meanwhile, the average pore size showed a decreasing trend. These results suggest that larger additions of ${\text{Mg}}^{\text{2+}}$ may form excess MgO covering the surface of BA, leading to pore blockage (Xu et al., 2018). In addition, the removal of phosphate and ammonia nitrogen by CMBA remained almost constant when the ${\text{Mg}}^{\text{2+}}$ addition varying from 1.25 M to 1.75 M, indicating that the specific surface area and total pore volume had a negligible effect on the removal effect. It was reported that the improvement of adsorption performance and removal of pollutants by heat treatment was not significantly related to the changes in specific surface area and pore volume, and the adsorption performance was mainly related to the surface properties (Lv et al., 2022).

The FTIR spectra of BA and yCMBA were shown in Fig. 2(a). The two peaks at 3423 cm⁻¹ and 1446 cm⁻¹ were due to the stretching vibrations of hydroxyl groups (Lv et al., 2022; Shirazinezhad et al., 2021). The signals at varying from 2921 to 2985 cm⁻¹ was due to the asymmetric stretching vibrations of aliphatic C—H (Li et al., 2017). The peaks at 1003 cm⁻¹ and 473 cm⁻¹ corresponding to the asymmetric stretching vibrations Si—O—Si and Si—O stretching vibrations, respectively (Li et al., 2019). The signals at varying from 692 to 879 cm⁻¹ was related to stretching vibrations of —CH₂— and C⚌C—H bonds (Li et al., 2017). From FTIR, the types of surface functional groups were reduced after making yCMBA compared to BA. However, a new peak appeared near 3693 cm⁻¹ for 200CMBA, which can be attributed to the stretching vibration of Mg—OH after modification. With the increase of calcination temperature, the absorption peaks of functional groups such as O—H became weaker or disappeared, indicating that the high temperature was not favorable for forming functional groups on the surface of BA. In addition, a new peak at 436 cm⁻¹ appeared when the temperature exceeded 300 °C, which was attributed to the stretching vibration of Mg—O, which overlaps with the SiO₂ diffraction peak at 473 cm⁻¹ (Zhu et al., 2020). It confirmed that the magnesium hydroxide loaded on the surface of BA was dehydrated to MgO at the calcination temperature of 400 °C.

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Fig. 2

(a) FTIR spectra of different CMBA. (b) XRD patterns of original biomass ash, Mg(OH)₂-BA and 400CMBA. (c) N₂ adsorption/desorption isotherms and the corresponding pore-size distribution (inset) of BA. (d) N₂ adsorption/desorption isotherms and the corresponding pore-size distribution (inset) of 400CMBA.

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3.1.2 Characteristics of BA before and after modification

The XRD spectra of BA, Mg(OH)₂-BA and 400CMBA samples were depicted in Fig. 2(b). Multiple characteristic diffraction peaks were observed in BA, of which 20.9°(1 0 0) and 26.6°(1 0 1) are relatively more intense and belong to SiO₂ (JCPDS 01–0649). In addition, four new characteristic diffraction peaks appear at 18.6°(0 0 1), 38.1°(1 0 1), 50.8°(1 0 2) and 58.6°(1 1 0), belonging to Mg(OH)₂ (JCPDS 07-0239). After calcination, the characteristic diffraction peaks at 18.6°(0 0 1), 50.8°(1 0 2) and 58.6°(1 1 0) were disappeared, and the characteristic diffraction peaks at 38.1°(1 0 1) was weaken. Meanwhile, three new characteristic diffraction peaks were observed at 36.9°(1 1 1), 42.9°(2 0 0) and 62.3°(2 2 2), which were confirmed to be MgO (JCPDS 04-0829) diffraction peaks, indicating the conversion of Mg(OH)₂ to MgO microcrystals during calcination. Moreover, the relative intensities of the lattice surfaces (1 0 0) and (1 0 1) in Mg(OH)₂-BA and 400CMBA were weaker than those of the unmodified BA due to the coverage of Mg(OH)₂ and MgO on the surface of BA.

Fig. 2(c) and (d) showed the N₂ adsorption/desorption isotherms and pore size distributions of BA and 400CMBA. It was observed that the N₂ adsorption isotherms of BA and 400CMBA were consistent with the type IV of the H3 hysteresis loop, which corresponds to the slot holes formed by the accumulation of lamellar particles. As seen from the insets of Fig. 2(c) and (d), the pore size distribution curves were broad, with a predominant number of mesopores and a low number of micropores and macropores for BA and 400CMBA. In addition, the zeta potential of BA was –24.90 mV and the zeta potential of the modified 400CMBA was –8.60 mV. The negative charge on the surface of the modified adsorbent material was reduced, which was beneficial to the phosphate removal from the wastewater.

The surface morphologies of BA and 400CMBA are shown in Fig. 3(a) and (b). By SEM analysis, it observed that the surface morphologies of BA residue and 400CMBA were significantly different. The microstructure of BA residue is rod-like, flaky, and block-like, with a relatively smooth surface. With the massive deposition of MgO particles, the surface pore size of 400CMBA decreased and partially showed a fluffy state. As seen from Fig. 3(b), nano-MgO existed in the form of sheets, which were uniformly loaded on the surface of BA, thus significantly increasing the total pore volume of BA (Xie et al., 2014). The EDS spectra of 400CMBA is shown in Fig. 3(c), and it can be observed that the distribution of element C is uniform. In contrast, the distribution regions of Mg and O are closely related, indicating the generation of nano-MgO. In addition, small peaks of Si, Cl, Ca, and K elements also appeared in the spectrum of Fig. 3(d). Thus, the elemental composition of 400CMBA consists of C (15.47 wt%), Mg (49.43 wt%), O (25.89 wt%), Si (5.40 wt%), Cl (2.28 wt%), Ca (1.05 wt%) and K (0.49 wt%).

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Fig. 3

(a) SEM images of BA. (b) SEM images of 400CMBA. (c) The selected area of 400CMBA for EDS analysis. (d) Elements content of the selected area in EDS analysis.

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3.2.1 Effect of pH value

The removal effects of 400CMBA on phosphate and ammonia nitrogen at different pH value are shown in Fig. 4(a). The trends of phosphate and ammonia nitrogen were similar at pH varying from 2 to 10, and the adsorption ability of 400CMBA on them was strongly dependent on the pH value. When the pH value changed from 2 to 3, the adsorption of phosphate and ammonia nitrogen by 400CMBA increased sharply. The equilibrium adsorption amounts of phosphate and ammonia nitrogen increased from 7.00 mg/g and 1.79 mg/g to 56.68 mg/g and 22.81 mg/g, respectively, and continued to increase at a relatively low rate until the pH value reached 7. The best adsorption effect was achieved at this time, with the adsorption amounts of phosphate and ammonia nitrogen peaking at 83.37 mg/g and 33.86 mg/g, respectively. When the pH value was increased, the adsorption capacity decreased at a relatively fast rate, especially for phosphate. And the adsorption amounts of phosphate and ammonia nitrogen decreased to 63.28 mg/g and 13.07 mg/g at pH 10. Considering the removal efficiency and operational feasibility, the initial pH value of 7 was chosen in the following experiments.

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Fig. 4

Effect of pH_initial value (a), adsorbent dosages (b), initial concentration (c,d), and coexisting ions (e,f) on the adsorption of phosphate and ammonia nitrogen removal.

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Fig. 4(a) also shows the change of solution equilibrium pH value after adsorption of phosphate and ammonia nitrogen at different initial pH value. At an initial pH value of 2, the solution equilibrium pH value was only 2.87, which limited the reaction. This may be because the surface of BA was still negatively charged in an aqueous solution with an initial pH of 2. At the same time, the phosphate in the solution exists in the form of H₃PO₄ and lacked electrostatic adsorption. Therefore, the adsorption capacity of 400CMBA for phosphate was small (Zhu et al., 2020). At an initial pH value varying from 3 to 8, the equilibrium pH of the solution was near 9, and the MgO on the surface of BA underwent a strong protonation reaction $\equiv \mathrm{M}\mathrm{g}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \equiv {\mathrm{M}\mathrm{g}\mathrm{O}\mathrm{H}}^{+}+{\mathrm{O}\mathrm{H}}^{-}$ (Li et al., 2016c). It shows that 400CMBA can maintain a higher equilibrium pH under lower initial pH conditions. This indicates that 400CMBA has alkalinity self-sufficiency and can adsorb phosphate and ammonia nitrogen in a wide initial pH range (Li et al., 2022a, 2022b). With the increase of the initial pH value from 3 to 10, H₃PO₄ was gradually deprotonated to ${{\text{HPO}}_{\text{4}}}^{\text{2-}}$ and ${{{\text{H}}_{\text{2}}\text{PO}}_{\text{4}}}^{\text{-}}$ . At this time, the positively charged 400CMBA on the surface would show a strong affinity for the negatively charged phosphate, leading to an increase in the adsorption capacity of 400CMBA on phosphate (Li et al., 2016b). However, due to the increasing pH value, the increasing concentration of ${\text{OH}}^{\text{-}}$ and phosphate produced a violent competition for adsorption sites on the positively charged adsorbent surface, and the electrostatic adsorption between phosphate and adsorbent weakened, eventually leading to a decrease in phosphate adsorption (Xiong et al., 2017). Although the positively charged surface of 400CMBA may not be favorable for the adsorption of positively charged ammonia nitrogen considering electrostatic adsorption, the best adsorption capacity of ammonia nitrogen was achieved at pH 7. It might indicate that the adsorption of ammonia nitrogen was controlled by different processes (Li et al., 2017). Furthermore, the adsorption capacity of 400CMBA for ammonia nitrogen decreases when pH > 8, which may be due to the loss of ammonia nitrogen as NH₃ gas under strongly alkaline conditions (Li et al., 2022a, 2022b). In summary, the adsorption of phosphate by 400CMBA was mainly through electrostatic adsorption, and the adsorption of ammonia nitrogen was affected by a combination of effects. 400CMBA showed strong adsorption capacity at a wide initial pH value varying from 5 to 9, which was favorable for applying 400CMBA in sewage sludge supernatants.

3.2.2 Effect of adsorbent dosages

Investigated the effect of 400CMBA dosages on the phosphate and ammonia nitrogen adsorption, and the results are shown in Fig. 4(b). 400CMBA dosages showed a proportional decrease in adsorption capacity with increasing dosages from 0.5 to 0.9 g/L. In contrast, the equilibrium removal efficiency of phosphate and ammonia nitrogen showed a significant increasing trend. At the dosage of 0.7 g/L, the removal efficiency of phosphate and ammonia nitrogen reached 97.26% and 39.50%, respectively. It may be attributed to the increase in the dosage resulting in 400CMBA providing more adsorption sites and a larger surface area, thus favoring the removal of more phosphate and ammonia nitrogen (Edet and Ifelebuegu, 2020). The decrease in adsorption capacity was attributed to the splitting effect of the concentration gradient between the adsorbent and the adsorbate (Albadarin et al., 2012). However, when the dosage exceeded 0.7 g/L, the reaction tended to equilibrium and the removal efficiency of both did not increase significantly. Therefore, the optimal adsorbent dosage was 0.7 g/L, considering the adsorption effect and economic cost.

3.2.3 Effect of initial concentration

The equilibrium adsorption capacity of 400CMBA for phosphate and ammonia nitrogen at different temperatures with the initial concentrations are shown in Fig. 4(c) and (d). The equilibrium adsorption capacity of 400CMBA increased with the initial concentration of phosphate and ammonia nitrogen, and then gradually reached the equilibrium. The increase may be due to the rise in the concentration of phosphate and ammonia nitrogen, which increases the collision with the adsorption sites. Also, the concentration gradient of phosphate or ammonia nitrogen between the solid and liquid phases increased mass transfer driving force, which facilitated the reduction of mass transfer resistance to increase the adsorption capacity (Younes et al., 2020). However, the removal of phosphate and ammonia nitrogen by 400CMBA increased and then decreased with increasing initial concentrations, reaching the maximum removal efficiency when the initial concentrations of phosphate and ammonia nitrogen were near 60 mg/L. It suggested that the adsorption of phosphate and ammonia nitrogen by 400CMBA may be more suitable for higher concentrations of sewage sludge supernatants. The adsorption capacity of 400CMBA decreased when the temperature increased from 298.15 to 318.15 K, indicating that the increase in temperature was not favorable for the removal of phosphate and ammonia nitrogen by 400CMBA.

3.2.4 Effect of coexisting ions

The results of the effect of single coexisting substances on the removal of phosphate or ammonia nitrogen by 400CMBA are shown in Fig. 4(e) and (f). From Fig. 4(e) and (f), the cations ( ${\text{K}}^{+}$ , ${\text{Ca}}^{\text{2+}}$ , ${\text{Mg}}^{\text{2+}}$ ) had a facilitating effect on the removal of phosphate. When the concentration of cations was increased from 0 to 10 mM, the removal of phosphate risen from 97.26% to 98.55%, 99.9% and 99.23%, respectively. It had been reported that the presence of ${\text{K}}^{+}$ in the solution competes with ammonia nitrogen to form K-struvite (Huang et al., 2019), resulting in a slight increase in phosphate removal efficiency. When ${\text{Ca}}^{\text{2+}}$ was alone in the solution, it immediately reacted with phosphate to form amorphous calcium phosphate and prevents the formation of struvite crystallization (Li et al., 2020). ${\text{Mg}}^{\text{2+}}$ played an essential role in the process of struvite crystallization. The large amount of ${\text{Mg}}^{\text{2+}}$ may lead to the conversion of struvite crystallization into magnesium phosphate and promote phosphate removal (Xie et al., 2021). In the presence of ${\text{Cl}}^{\text{-}}$ , the phosphate removal efficiency of 400CMBA had little change, indicating that the effect of ${\text{Cl}}^{\text{-}}$ on the phosphate adsorption process can be ignored. It is well known that the anions adsorbed on the adsorbent via outer sphere complexes are strongly influenced by the ionic strength and coexisting anions, and these coexisting anions can also form outer sphere complexes by electrostatic forces. In contrast, the anions adsorbed on the adsorbent via inner sphere complexes have low sensitivity to ionic strength and coexisting anions (Du et al., 2017). Apparently, inner sphere complexes were involved in the phosphate adsorption process on 400CMBA, while ${\text{Cl}}^{\text{-}}$ was weakly bound to the surface position of metal hydroxides to form outer sphere complexes, which leads to ineffective competition for ${\text{Cl}}^{\text{-}}$ (Li et al., 2016a; Lu et al., 2014). When ${{\text{NO}}_{\text{2}}}^{\text{-}}$ was present, the phosphate removal efficiency decreased slightly. It has been reported that elements in the same main group have similar chemical properties and molecular structures (Zhang et al., 2013). When they were present together, intense competition occurs on the surface of the adsorbent. Clearly, the results of this study were not consistent with this conclusion. In addition, when ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ was present, the phosphate removal efficiency of 400CMBA significantly decreased with the concentration of ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ . It may be due to the strong competition of ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ for adsorption sites on the adsorbent surface. Chemically, the ionic radius of ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ (0.230 nm) is larger than that of ${\text{Cl}}^{\text{-}}$ and ${{\text{NO}}_{\text{2}}}^{\text{-}}$ (both less than 0.200 nm) and like that of ${{{\text{H}}_{\text{2}}\text{PO}}_{\text{4}}}^{\text{-}}$ (0.238 nm). Therefore, the presence of ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ competed for positive charge sites on the adsorbent surface and reduced phosphate removal (Li et al., 2016a; Li et al., 2017). Overall, the results indicated that in the coexistence of ${\text{Cl}}^{\text{-}}$ , ${{\text{NO}}_{\text{2}}}^{\text{-}}$ and ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ presence, the phosphate removal efficiency decreased according to ${\text{Cl}}^{\text{-}}$ > ${{\text{NO}}_{\text{2}}}^{\text{-}}$  >  ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ .

As seen in Fig. 4(f), the presence of cations had a negative effect on the removal of ammonia nitrogen. When the concentration of cations increases from 0 to 10 mM, the removal efficiency of ammonia nitrogen decreased from 39.5% to 26.34%, 21.15% and 31.35%, respectively. The reduction of ammonia nitrogen removal efficiency was mainly due to the formation of K-struvite, amorphous calcium phosphate and magnesium phosphate. In addition, Fig. 4(f) showed that the anions ( ${{\text{SO}}_{\text{4}}}^{\text{2-}}$ , ${\text{Cl}}^{\text{-}}$ , ${{\text{NO}}_{\text{2}}}^{\text{-}}$ ) had little effect on the adsorption of ammonia nitrogen, and no significant changes were found.

3.2.5 RSM model

The experimental design of the Box-Behnken model and the predicted and actual values of phosphate and ammonia nitrogen removal are shown in Table S2. A quadratic regression model was used to explore the relationship between the independent variables and the responses, and the following equations were obtained. $$\mathit{Phosphate}removalrate=96.71-0.3237X_1+7.82X_2+0.9187X_3-0.015X_1{X}_2+0.2275X_1{X}_3-1.66X_2{X}_3-4.99{X_1}^2-8.47{X_2}^2-0.9977{X_3}^2$$ $$\mathit{Ammonium}removalrate=39.22-0.2962X_1+2.95X_2-0.2225X_3-0.0675X_1{X}_2+0.0950X_1{X}_3+0.3850X_2{X}_3-2.96{X_1}^2-4.17{X_2}^2-1.69{X_3}^2$$

Analysis of variance (ANOVA) was used to test the model's reliability, and the results are shown in Tables S3 and S4. According to the ANOVA and significance check of the regression models, the F-values of the regression models for phosphate removal and ammonia nitrogen removal were 68.92 and 222.79, respectively, with P-values less than 0.001, indicating that the models reached a significant level and were statistically significant, and could be used to substitute the actual points of the test for the result analysis. The multivariate correlation coefficients ${\text{R}}^{\text{2}}$ of the two models were 0.9888 and 0.9965, respectively. The corrected coefficients of determination ${{\text{R}}^{\text{2}}}_{\text{adj}}$ of the models were 0.9745 and 0.9920, indicating that the two models explained 97.45% and 99.20% of the variation in response values, respectively, further indicating that the regression models were suitable for analysis, prediction and optimization of phosphate and ammonia nitrogen removal efficiency.

The F-values for pH, adsorbent dosing and initial concentration in the phosphate removal model were 0.55, 321.55 and 4.44, respectively, with P-values of 0.48, <0.0001 and 0.07. It indicated that the effect of adsorbent dosages on phosphate removal reached an extremely significant level, and the significant degree of effect was: adsorbent dosages > initial concentration > pH. The F-values of the three single factors in the ammonia nitrogen removal model were 6.84, 678.85 and 3.86 with P-values of 0.03, <0.0001 and 0.09, respectively. It showed that the effects of pH value and adsorbent dosages on phosphate removal reached a significant level, and the significant degree of effect was: adsorbent dosages > pH value > initial concentration.

To comprehensively consider the effects of three factors, pH, adsorbent dosages, and initial concentration, on the removal of phosphate and ammonia nitrogen by 400CMBA, the graphical analysis was made by Design-Expert software. The three-dimensional response surface plots are shown in Fig. 5. As shown in Tables S3 and S4, the p-values of the interaction between pH value and adsorbent dosages in the phosphate and ammonia nitrogen removal models were 0.98 and 0.69, respectively, which were >0.05, indicating that there was an interaction between the two but not significant. The p-values of the interaction between pH value and initial concentration in the phosphate and ammonia nitrogen removal models were 0.72 and 0.57, respectively, which were >0.05, indicating an interaction between the two but not significant. The p-values of the interaction between pH value and adsorbent dosages in the phosphate and ammonia nitrogen removal models were 0.03 and 0.04, respectively, which were less than 0.05, indicating a significant interaction between them.

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Fig. 5

Interactive effects three variables on the phosphate (a) and ammonia nitrogen (b) removal rate.

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The optimization function of Design Expert was used to predict the optimal adsorption conditions for 400CMBA based on ensuring the maximum removal of phosphate and ammonia nitrogen by 400CMBA. The removal efficiency of phosphate and ammonia nitrogen was 98.48% and 39.74%, respectively, at pH 6.96, adsorbent dosages of 0.74 g/L and initial concentration of 60.16 mg/L. Considering the feasibility and economic cost of the experiment, made certain adjustments to the experimental results. The optimal adsorption conditions for 400CMBA were finally determined as follows: pH 7.00, adsorbent dosages 0.70 g/L, and initial concentration 60.00 mg/L.

3.2.6 Adsorption kinetics

Kinetics is often used to elucidate the mechanisms that occur during adsorption processes, such as mass transfer processes and diffusion control (González-Hourcade et al., 2022). Therefore, the adsorption kinetics of phosphate and ammonia nitrogen by 400CMBA was studied by the pseudo-first-order model, pseudo-second-order model, general-order model and IPD model. In the kinetic experiment, the reaction time reached 420 min when the initial concentration of phosphate and ammonia nitrogen varying from 20 to 100 mg/L. The curves and parameters are shown in Fig. 6 and Table 2, respectively.

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Fig. 6

(a) Adsorption kinetics of phosphate by 400CMBA. (b)Adsorption kinetics of ammonia nitrogen by 400CMBA. (c) Intraparticle diffusion model of phosphate and ammonia nitrogen by 400CMBA. Condition: Initial pH of 7.0, 0.7 g/L adsorbent dosage, 25 °C.

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In the first 60 min of the reaction, 400CMBA had a faster adsorption rate for phosphate and ammonia nitrogen, and the removal rate of phosphate exceeded 74% and the removal rate of ammonia nitrogen exceeded 23% within 60 min. Subsequently, the removal rate increased relatively slowly, and the removal rates of phosphate and ammonia nitrogen exceeded 96% and 29% at 420 min, respectively. This result was like that reported by Li et al. (2017). The initial rapid adsorption may be due to the electrostatic attraction between the positively charged ${\text{MgOH}}^{+}$ and the negatively charged phosphate ions (Li et al., 2016a). The subsequent slow growth of the removal rate indicated that the adsorption process may involve intraparticle diffusion (Zhang et al., 2009). In addition, the maximum removal efficiency was achieved when the initial concentration of phosphate and ammonia nitrogen was 60 mg/L, with 97.25% and 39.50%, respectively. Comparing the removal effects at different initial concentrations between 20 mg/L and 100 mg/L, it was observed that at higher concentrations, the removal of phosphate and ammonia nitrogen was better by 400CMBA.

Table 2 shows the kinetic model parameters used to fit the experimental data. The applicability of the kinetic model was evaluated according to ${{\text{R}}^{\text{2}}}_{\text{adj}}$ and SD. The higher ${{\text{R}}^{\text{2}}}_{\text{adj}}$ and the lower SD indicate that the difference between the experimental and theoretical values of q_e is small (given by the models), so there is an optimal suitable model. Based on these parameters, under the experimental conditions adopted, the ${{\text{R}}^{\text{2}}}_{\text{adj}}$ value of the general-order kinetic model was the highest and the SD value was the lowest. Therefore, the general-order model was more suitable for fitting the adsorption process of phosphate and ammonia nitrogen by 400CMBA.

The pseudo-first-order model is an adsorption analysis method based on the solid adsorption capacity proposed by Lagergren. It believes that the mass transfer resistance in the particles limits the adsorption (Sakthivel et al., 2021). The pseudo-second-order model assumes that the adsorption rate is controlled by chemical adsorption, indicating that the adsorption mechanism is the main reason affecting the adsorption (Feng et al., 2018). The general-order model describes the adsorption process in the same order as the chemical reaction (Guy et al., 2022). Therefore, the adsorption process may be chemisorption, and the adsorption rate was influenced by the ligand of the active site on the adsorbent surface and the adsorbate (Di et al., 2022). It likes the results reported by Koh et al. (2019) and Xu et al. (2020a, 2020b) regarding various phosphate adsorbents. In chemical reactions, the reaction order is determined by experiments. With the change in adsorbate concentration, the n (adsorption rate order) values of the general-order kinetic equation were different, and it was difficult to compare the kinetic parameters of the model. Therefore, the adsorption kinetics of phosphate and ammonia nitrogen by 400CMBA were compared with t_0.5 and t_0.95. t_0.5 is the time to reach 50% saturation, and t_0.95 is the time to reach 95% saturation. t_0.5 and t_0.95 were calculated and displayed according to the best fitting model (general-order model).

When the initial concentration was 20 mg/L, the t_0.5 and t_0.95 of phosphate were 29.314 min and 112.795 min, respectively, and the t_0.5 and t_0.95 of ammonia nitrogen were 25.226 min and 198.076 min, respectively. When the initial concentration was 60 mg/L, the t_0.5 and t_0.95 of phosphate were 23.794 min and 123.371 min, respectively, and the t_0.5 and t_0.95 of ammonia nitrogen were 37.081 min and 234.321 min, respectively. When the initial concentration was 100 mg/L, the t_0.5 and t_0.95 of phosphate were 30.846 min and 153.011 min, respectively, and the t_0.5 and t_0.95 of ammonia nitrogen were 28.917 min and 225.552 min, respectively. Observing these values, the adsorption was relatively rapid, and the time for phosphate to reach adsorption equilibrium was faster than that for ammonia nitrogen to reach adsorption equilibrium. In order to ensure that the adsorption process has enough time to reach the balance between 400CMBA and nutrients, the established contact time should be slightly higher than t_0.95. In order to achieve 95% saturation, the equilibrium should be established under the condition that the adsorbent is completely saturated. Therefore, the contact time of 240 min was set for further adsorption isotherm experiments.

For the ion adsorption process from the aqueous phase to the solid phase three mechanisms were generally involved: film diffusion, intraparticle diffusion and active site adsorption. The third step was speedy and was usually neglected. The results are shown in Fig. 6(c). At different initial concentrations, the rate constant K of the adsorption process of phosphate and ammonia nitrogen by 400CMBA decreases according to K_id1 > K_id2 > K_id3, indicating that the adsorption process consists of three parts: rapid adsorption, intraparticle diffusion and equilibrium stage. Comparison of the relevant parameters of the IPD model showed that the fitted results for phosphate and ammonia nitrogen adsorption by 400CMBA were linear but did not pass the origin, indicating that phosphate and ammonia nitrogen adsorption was not only controlled by intraparticle diffusion but also by a combination of mechanisms (Table S5). In addition, the higher ${{\text{R}}^{\text{2}}}_{\text{adj}}$ in the second stage showed that ammonia nitrogen could pass through the pores and voided on the surface of the 400CMBA material and further entered the interior of the material through a combined process of intraparticle diffusion and film diffusion. In summary, the adsorption of phosphate and ammonia nitrogen by 400CMBA was chemisorption controlled by several processes, including internal and external diffusion.

3.2.7 Adsorption isotherms

Adsorption isotherms are useful tools to understand how the interaction between adsorbents and adsorbates occurs (Cavalcante et al., 2022). It can provide reliable information for the possible mechanism and pathway of adsorption. The adsorption process of phosphate and ammonia nitrogen by 400CMBA was further studied by the equilibrium isothermal model. Langmuir, Freundlich and Sips models were used to evaluate the practicability of the equilibrium data of phosphate and ammonia nitrogen for 400CMBA adsorbent. The equilibrium isotherms of phosphate and ammonia nitrogen on 400CMBA at different temperatures (298.15, 308.15, 318.15 K) are shown in Fig. 7, while the three isotherm equilibrium parameters of phosphate and ammonia nitrogen are shown in Table 3.

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Fig. 7

(a) Isotherms of adsorption of phosphate. (b) Isotherms of adsorption of ammonia nitrogen. Condition: Initial pH of 7.0, 0.7 g/L adsorbent dosage, time of contact of 30 min.

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The Langmuir isotherm model assumes that monolayer adsorption occurs on a uniform adsorbent surface and there is no interaction between adsorbates (Salomón et al., 2020). The Freundlich isotherm model is an empirical equation based on multilayer adsorption on heterogeneous surfaces (Venkateswarlu et al., 2022). The Sips model is usually used to describe chemical adsorption on heterogeneous surfaces (Zhu et al., 2020). ${{\text{R}}^{\text{2}}}_{\text{adj}}$ and SD values were used to evaluate the applicability of the isotherm model. Based on ${{\text{R}}^{\text{2}}}_{\text{adj}}$ and SD values, the Sips model had the highest ${{\text{R}}^{\text{2}}}_{\text{adj}}$ value and the lowest SD value under three temperature conditions. This indicates the superiority of the Sips model in describing the adsorption of phosphate and ammonia nitrogen by 400CMBA. It means that the theoretical q_e value of the Sips model is closer to the experimentally found value. Thus, the adsorption of phosphate and ammonia nitrogen on the 400CMBA surface was a non-homogeneous chemisorption (Yin et al., 2019). The adsorption process was consistent with the Freundlich model at low phosphate and ammonia nitrogen concentrations and with the Langmuir model at high concentrations. It suggested that the adsorption of phosphate and ammonia nitrogen on 400CMBA may be influenced by various mechanisms, including physical and chemical adsorption, which is consistent with the kinetic fit (He et al., 2022).

For the Langmuir model, the maximum adsorption capacities were 259.73, 237.43 and 205.86 mg/g for phosphate and 99.32, 92.25 and 87.49 mg/g for ammonia nitrogen on 400CMBA at temperatures of 298.15, 308.15 and 318.15 K, respectively. Usually 1/n represents the adsorption strength, between 0 and 1. The smaller the 1/n value, the better the adsorption performance. In this study, the 1/n of the Freundlich model is less than 1, which confirms that the adsorption conditions are good, and the smaller 1/n value of phosphate indicates that the affinity of 400CMBA to phosphate is stronger than that to ammonia nitrogen (Ahmed et al., 2021).

3.2.8 Adsorption thermodynamics

The adsorption thermodynamics allows further analysis of the influence of adsorption systems and conditions such as adsorbate, adsorbent, and solvent on the adsorption process. Among them, determined the change in Gibbs free energy changes (ΔG), entropy (ΔS) and enthalpy change (ΔH) used to the spontaneity of the adsorption process, the nature of the adsorption process and the applicability of the adsorption process (Edet and Ifelebuegu, 2020). Investigated the effect of thermodynamic temperature on the adsorption properties of 400CMBA on phosphate and ammonia nitrogen by varying the temperature between 293.15 K and 318.15 K. The results are shown in Fig. 8(a), (b) and Table 4. Fig. 8(a) showed that the removal efficiency of both adsorbates showed a trend of increasing and then decreasing with increasing temperature, and reached the best removal effect at the reaction temperature of 298.15 K. When the temperature was low, the number of activated molecules in the reaction system increased with the increase of the reaction temperature, which was favorable to the formation of struvite crystallization. Continuing to increase the reaction temperature, a significant decrease in both phosphate and ammonia nitrogen removal and adsorption capacity occurred, probably because the solubility product of struvite crystallization was inversely proportional to the ambient temperature, which was not favorable for struvite crystallization when the temperature was higher (Hanhoun et al., 2011). Fig. 8(b) demonstrated the Van't Hoff plots for phosphate and ammonia nitrogen at different temperatures. As shown in Table 4, the negative value of ΔG indicated that the adsorption process of phosphate was spontaneous. While the negative ΔG values of ammonia nitrogen gradually tended to positive values, meaning that its adsorption was spontaneous but less efficient at higher temperatures. Negative values of ΔH and ΔS indicated the exothermic nature of phosphate and ammonia nitrogen adsorption and the reduced stoichiometry of the interface between 400CMBA and the solution. The results agreed with the findings of Isik et al. (2021). It had been reported that ΔH values in the range of 2.1 to 20.9 kJ/mol indicate physical adsorption, while ΔH values in the range of 20.9 kJ/mol to 418.4 kJ/mol indicate chemisorption (Yu et al., 2022). In the present study, the ΔH of phosphate was –56.68 kJ/mol, indicating the involvement of chemisorption in the phosphate adsorption process. While ΔH of ammonia nitrogen was –11.27 kJ/mol, indicating that physisorption was involved in the adsorption process of ammonia nitrogen, confirming that the adsorption of ammonia nitrogen by 400CMBA was not only controlled by the formation of struvite crystallization.

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Fig. 8

(a) Effect of temperature on removal capacity. (b) Relationship curve between lnK and 1/T.

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3.2.9 Regeneration research

Renewability is an important index to evaluate the performance of adsorbents. The commonly used solvents are NaOH, HCl and alcohols (such as methanol, ethanol). In this study, 0.1 M HCl was used as the desorption solution, and the samples were subjected to 5 adsorption–desorption cycles. The effect of reuse times of 400CMBA on its adsorption capacity is shown in Fig. 9. It can be seen from the figure that the removal ability of 400CMBA to phosphate and ammonia nitrogen showed a large downward trend after 5 cycles, which may be because phosphate and ammonia nitrogen occupied more adsorption sites in the pores of 400CMBA (Li et al., 2021). After the fifth cycle, the removal rates of phosphate and ammonia nitrogen were less than 50% and 25%, respectively. These results indicate that the reusability of 400CMBA is not high. However, to our knowledge, 400CMBA after adsorption of phosphate and ammonia is a potential slow-release fertilizer. The effect of the practical application of recycled 400CMBA in soil on seed growth needs further study.

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Fig. 9

Effect of reuse times of 400CMBA on removal capacity.

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