The modification of biomass waste by cerium-based MOFs for efficient phosphate removal: excellent performance and reaction mechanism

Phosphate is a necessary nutrient for the growth of organisms, whereas excess phosphate would induce the eutrophication in aquatic ecosystems (Braun et al. 2018). So far, many methods have been applied to remove phosphate from water (Yue et al. 2017; Yang et al. 2021; Wei et al. 2022; Yunyun et al. 2022). Among them, adsorption method attracts extensive attention of researchers because of its simple operation, safety and efficiency. As a low-cost adsorbent, the use of waste materials is a great choice to achieve the circular economy through waste-to-resource supply chains. Among them, agricultural waste owning abundant surface functional groups (e.g., -OH and -CHO) can be easily modified to be a functional polymer with useful active sites (Zhou et al. 2022).

Pine sawdust (PS) is powder waste left during wood processing. With the rapid development of urban construction in recent years, the demand for processed wood is increasing sharply. In this case, a large amount of waste sawdust would be produced. The surface of PS, porous and full of functional groups, lights up a new idea for the reuse of the waste as adsorbent, which could be easily modified to greatly improve the stability and adsorption performance. Meanwhile, the agricultural straw is an important kind of agricultural waste. When crops mature, straw is usually burned or used for composting, which causes air, soil and water pollution. Let's take corn straw (CS) for example, of which the main components are cellulose, hemicellulose and lignin with a large number of hydroxyl groups. The straw is actually considered as a proper adsorbent for treating heavy metal pollution in water, which could also be implied in phosphate removal (Clauser et al. 2020; Xudong et al. 2022).

Usually, there are various modification methods for agricultural waste to become a waste-based adsorbent. Especially, metal loading and amine grafting are widely used to enhance the phosphate capacity of the pristine adsorbents (Liu et al. 2020; Zhicong et al. 2022). Metal (hydrogen) oxides (including composite compounds containing these substances) are considered as promising candidates for effective phosphate removal from water because of their excellent selectivity, specificity and non-toxic properties (Baile et al. 2020).

Cerium is the most abundant rare earth element in the Earth's crust. In terms of adsorption, Ce-based materials are usually used to remove toxic metal arsenic and are rarely used to adsorb phosphate (Vences-Alvarez et al. 2022; Yang et al. 2022). In fact, phosphorus and arsenic both belong to the same element group and form similar components in water (Jiaojie et al. 2021; Yang et al. 2022). Hence, there may be great prospects for the application of Ce-based materials in phosphate adsorption testing. Recently, Hong Ding et al. prepared Fe₃O₄@SiO₂ magnetic nanoparticles functionalized with mesoporous cerium oxide (Fe₃O₄@SiO₂@mCeO₂). The newly developed adsorbent had an excellent performance in adsorbing phosphate, and its maximum adsorption capacity calculated from the Langmuir model was 64.07 mg/g (Ding et al. 2017). Yanfang Feng et al. prepared nano-cerium oxide functionalized maize straw biochar (Ce-MSB) and utilized it to remove P from agricultural wastewater. The maximum adsorption capacity of Ce-MSB for PO₄³⁻ was 78 mg/g, implying that Ce-MSB was an effective adsorbent for P removal (Feng et al. 2017). Thus, Ce-based material is capable of phosphate removal, but the adsorption capacities in the literatures above are still not satisfactory. In our recent study, different valence states (Ce (III) or Ce (IV)) show great influence on the phosphate removal, and the trivalent state is preferable to capture phosphate (He et al. 2020). However, to our best knowledge, few studies have focused on the application of Ce (III)-based adsorbents. Meanwhile, in order to increase adsorption capacity, metal (hydrogen) oxides are often designed as nanoparticles with large specific surface areas. Considering the excellent characteristics such as high surface area and large pore volume, the metal organic frameworks (MOFs) and their derivatives with designed nanostructures draw the attention of researchers. But those powder particles easily cause secondary pollution during use in aquatic ecosystem. Using biomass as a carrier can not only prevent the leakage of nanoparticles, but also optimize the adsorption capacity of original biomass.

In this study, Ce-based MOFs and their derivatives were loaded on alkali treated CS or PS by hydrothermal synthesis to prepare CS-CD, PS-CD, CS-CT and PS-CT, which avoided the agglomeration phenomenon of Ce-based MOFs nanoparticles in the process of phosphate removal and obtained better adsorption. Their differences in nanostructures, surface physicochemical characteristics and phosphate adsorption properties were studied. The results showed that all the metal loaded agricultural wastes had a significant enhancement on the adsorption capacity of phosphate, for which CS-CT had best phosphate removal under the same conditions. But CS-CD and PS-CD showed great phosphate uptake under wide applicable scope of pH from 2 to 11. The possible adsorption mechanism was clarified by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and zeta potential.

Corn straw, pine sawdust, cerium ammonium nitrate (NH₄)₂Ce(NO₃)₆), terephthalic acid (H₂BDC), Benzene-1,3,5-tricarboxylic acid (H₃BTC), N,N-dimethyl-formamide (DMF), sodium hydroxide, anhydrous ethanol, potassium dihydrogen phosphate, ammonium molybdate, potassium antimony tartrate, ascorbic acid, all chemical agents are analytical pure.

Four kinds of biomass cerium based materials, CS-CD, PS-CD, CS-CT and PS-CT, were prepared. All samples were synthesized in a polytetrafluoroethylene lined autoclave (50 mL).

Pretreatment of biomass materials: after washing the biomass materials with deionized water three times, the samples were soaked and stirred in 4% NaOH solution for 6 h, then after washing with deionized water to neutral, they were dried in a blast drying oven at 70 °C.

The preparation steps of CS-CD were as follows. First, 1 g ammonium cerium nitrate ((NH₄)₂Ce(NO₃)₆) was dissolved in 10 mL DMF and the mixed solution was placed in the reactor. Then 0.2857 g pretreated corn straw was put into the above solution and stirred for 2 h. Subsequently, 0.6 g terephthalic acid was dissolved in 5 mL DMF and transferred to the above reactor. After hydrothermal reaction at 150 °C for 12 h, the obtained precipitate was centrifuged and washed three times with DMF. Finally, the product was dried in a vacuum dryer at 50 °C for 24 h.

Preparation of PS-CD: Firstly, 1 g of ammonium cerium nitrate ((NH₄)₂Ce(NO₃)₆) was dissolved in 10 mL DMF and the mixed solution was placed in the reactor. Subsequently, 0.2857 g pretreated sawdust was put into the above solution and stirred for 2 h. The remaining steps were the same as the preparation of CS-CD.

Preparation of CS-CT: Firstly, 6 mL of 0.533 mol/L ceric ammonium nitrate solution was placed into the reactor, then 0.5009 g pretreated corn straw was put into the reactor and stirred for 2 h. Subsequently, 0.22 g benzene-1,3,5-tricarboxylic acid was dissolved in a mixture of 12 mL DMF and 2.57 mL formic acid, and then transferred to the above-mentioned reactor. After 12 h hydrothermal reaction at 100 °C, the obtained precipitate was centrifuged, washed with DMF and acetone. Finally, the product was dried in an air blast dryer at 50 °C for 24 h.

Preparation of PS-CT: First, 6 mL of 0.533 mol/L ceric ammonium nitrate solution was placed into the reactor, then 0.5009 g pretreated sawdust was put into the reactor and stirred for 2 h. The remaining steps were the same as the preparation of CS-CT.

The specific surface area, pore size distribution and pore volume were determined by N₂ adsorption-desorption tests on a pore structure analyzer (Micromeritics ASAP, 2020 Plus HD88, USA). The crystal structure of the adsorbent was studied by XRD (Bruker D8 Advance Diffactometer, Germany) and Cu Kα radiation source. The surface microstructures were characterized by field emission scanning electron microscopy (FE-SEM, ZEISS sigma500, Germany). XPS was used to record surface chemical composition and valence analysis (Thermo Fisher Science, ESCALAB 250 Xi, USA). Zeta potentials were measured using Zetasizer Nano-Z (Malvern, UK) at different pH ranges from 2 to 12. Using potassium bromide as reference material, FT-IR spectra of solid residues adsorbed by cerium-based biomass materials and phosphate were collected by FT-IR spectrometer (Thermo Fisher Science, Nicolet IS5, USA).

In the adsorption experiments, the phosphate solution was prepared with KH₂PO₄ and deionized water. Sample dosage was 1 g/L in the experiment. The samples were put into the thermostatic air bath at 135 r/min for 24 h at 25 °C. The pH value of the solution was about 6. Before analysis, 0.45 μm microporous membrane was used for filtration. Residual phosphate concentration in filtrate was determined by molybdenum blue spectrophotometry. The unit of phosphate concentration in solution was mg PO₄/L. Batch test results were obtained from the average of three repetitive trials.

In order to study the effect of phosphate concentration on the adsorption capacity, 50 mg samples were placed in 50 mL phosphate solution of different concentration (50–500 mg/L). The adsorption experiments lasted for 24 h. Langmuir and Freundlich isotherm models were used to fit the data.

Among them, (mg/L) is the equilibrium phosphate concentration in the solution; (mg/g) is the equilibrium phosphate adsorption capacity; (mg/g) is the Langmuir constant related to the adsorption capacity; and n are the Freundlich model constants; is the constant of Langmuir model.

In the kinetic experiment, the initial phosphate concentration was 500 mg/L, and the time was controlled from 0 to 1,440 min. The kinetic mechanism was analyzed by using pseudo-first-order and pseudo-second-order adsorption models, as follows:

Among them, (mg/g) is the amount of phosphate adsorbed in a given time; is the equilibrium capacity (mg/g) and t (min) is the reaction time; (min⁻¹) and (g/(mg·min)) are the rate constants of the pseudo-first-order and pseudo-second-order adsorption models, respectively.

In the experiment of the effect of pH on phosphate adsorption, the initial pH value was adjusted by NaOH/HCl solution, ranging from 2.0 to 12.0, and the initial concentration of phosphate was 500 mg/L.

The influence of co-existing anions on the competitive adsorption of phosphate was studied in the experiment. The initial concentration of phosphate was 500 mg/L. The effects of Ca²⁺, Mg²⁺, NO₃⁻, Cl⁻, F⁻, SO₄²⁻, CO₃²⁻ on the competitive adsorption of phosphate were studied, with ion concentration of 0.1 mol/L.

The specific surface area is a critical characteristic, closely related to the size, shape and pore structure of the adsorbent (Zhang et al. 2018). As exhibited in Table 1, the surface area of pure CS and PS are extremely small, only 0.798 m²/g and 0.549 m²/g. In comparison, all the modified samples showed enlarged surface areas, which are proposed to be attributed to the porous structures of MOFs (Jiaojie et al. 2021). Notably, the specific surface areas of CS-CT and PS-CT are 8.518 m²/g and 6.250 m²/g, much higher than those of CS-CD and PS-CD. That reason may be related to the difference between CD and CT, in that BDC (acid radical ion of H₃BTC) has lower specific surface area than BTC (acid radical ion of H₂BDC) (Table S1). The BET reports show that the pore size and surface area increase after the combination of biomass and MOF, which can be beneficial to the adsorption of phosphate on composites (Kong et al. 2018). When the pH was in the range of 4–10, the main forms of phosphate were H₂PO₄⁻ and HPO₄²⁻ in aqueous solution (Figure S2). Due to the simulation of the molecular structure of H₂PO₄⁻ or HPO₄²⁻ (Figure S3), the size of the phosphate molecule was much smaller than the average pore diameters of all the samples, demonstrating that the phosphate was basically not trapped by the pores of adsorbents, but only captured through adsorption.

The pristine CS and PS are reacted with MOFs precursors by solvothermal method to obtain CS-CD, PS-CD, CS-CT and PS-CT. Powder XRD is conducted to verify the crystalline structures (Figure 2(b)). The XRD patterns of CS-CD and PS-CD are the same as those of CD, which are in accordance with the information of Ce₅(BDC)_7.5(DMF)₄ (CCDC: 912350) (D'Arras et al. 2014; Jiaojie et al. 2021). The inorganic node of the MOFs structure contains five cerium atoms in a line, where the Ce atoms lying at the extremities of the node coordinate eight oxygen atoms, and the three Ce atoms in the middle coordinate nine (Figure S4). The results demonstrate that the existence of CS and PS shows a negligible effect on the structures of the added Ce-based MOFs. Similarly, the diffraction peaks of CS-CT and PS-CT are well matched with the simulated spectra of crystal structure of [Ce (HCOO)]_n (JCPDS Card. 49-1245), which is in agreement with the peaks of CT. It is worth noting that the CT is not a MOF but a MOF derivative. A solvothermal react for 20 min leads the ammonium cerium nitrate and trimeric acid to convert into Ce-MOF-808, in which the Ce atom is six coordinated (Figure S5) (Hennig et al. 2013). However, after 12 h reaction, the MOF structure is totally changed, leaving formic acid cerium as the residue. The Ce (III) atoms of [Ce(HCOO)]_n are nine fold coordinated (Hennig et al. 2013). Thus, compared with CT, the unsaturated coordination numbers in CD might indicate the contribution of MOFs to the adsorption efficiency.

Compared with CS and PS, of which the removal capacities are only 5.4 mg/g and 5.6 mg/g with the initial phosphate concentration of 500 mg/L, the adsorption capacities of CS-CD, PS-CD, CS-CT and PS-CT reach 157.49, 161.24, 191.24 and 151.85 mg/g, respectively, indicating the loaded nanoparticles greatly improve the phosphate removal capacity of biomass due to the abundant active sites on its surface. When the initial phosphate concentration increases from 0 to 150 mg/L, the phosphate adsorption of the four samples grows sharply. Those phenomena might be related to the strong attraction between Ce (III) and phosphate through the chemisorption process to make adsorbents more effective for removing phosphate under lower concentration, showing a fast increase of adsorption capacities at the beginning stage of isothermal curve. After the initial concentration exceeded 150 mg/L, the rate of the growth slowed down. In addition, the isotherm data are fitted using Langmuir and Freundlich models, and the fitting results are shown in Table 2. The values of R² for composite materials of CS-CD and CS-CT are higher than 0.95, which proves that the CD or CT nanoparticles are doped uniformly in CS (Zhu et al. 2019; Koh et al. 2020). In contrast, the R² of the composite material of PS-CD and PS-CT is slightly lower, indicating the chemical heterogeneity of the adsorbents. What's more, the values of Freundlich constant n are more than 3.5, implying a favorable adsorption condition for all the samples. Moreover, the maximum adsorption capacities of CS-CD, PS-CD, CS-CT and PS-CT calculated by Langmuir model are 181.38, 183.27, 225.55 and 186.24 mg/g, respectively. The reason why CS-CT can exhibit the best adsorption performance may be affected by both the specific surface area and the content of trivalent cerium. Besides, the study of phosphate adsorption property showed that all the biomass wastes doped with metallic compound had significant enhancements on the adsorption capacities of phosphate, much higher than many reported biomass waste adsorbents (Table S2).

In order to better analyze the kinetic adsorption behavior of phosphate removal by Ce-MOF/biomass, the pseudo-first-order kinetic equation and pseudo-second-order kinetic equation are used to fit the adsorption experimental data. As shown in Figure 3(b), the phosphate capture of CS-CD and PS-CD is more rapid. In the first 10 minutes, about 91, 90, 13 and 13% phosphate are removed on the surfaces of CS-CD, PS-CD, CS-CT and PS-CT. The adsorption equilibrium is completed within 30 minutes for CS-CD and PS-CD. Relatively speaking, the phosphate uptake by CS-CT and PS-CT still increases until the samples are saturated at about 24 h. The reason why the biomass modified by two kind cerium-based nanoparticles shows such different speed in adsorption is due to the different structures between them. CT has no unsaturated coordination bond, so its combination with OH⁻ in water can only be achieved by its slow hydrolysis. While as a typical MOF structure, the surface of CD contains unsaturated coordination bonds (Jiaojie et al. 2021). Once immersed in water, these sites will capture OH⁻ immediately, so the reaction is very rapid. As shown in Table 3, the kinetic fitting results also verify the characteristics of the two kinds of adsorbents. For the adsorption reaction of CS-CD and PS-CD, the R² values of the pseudo-first-order kinetic model are higher than those of the pseudo-second-order kinetic model, which is because their too fast reaction rate makes their adsorption process more similar to the physical adsorption process. On the contrary, the fitting parameters R² of the pseudo-second-order kinetic model of CS-CT and PS-CT are higher than those of the pseudo-first-order kinetic model, indicating that the adsorption process of the two materials conforms to the conventional chemical adsorption process (Wen et al. 2016; Wang et al. 2020).

There are significant differences among the four adsorbents affected by pH, which is closely related to the species of phosphate in aqueous solutions, the surface charge and the deprotonation process of metal-based adsorbents. The distribution of phosphate species is closely related to pH value (Figure S2). H₂PO₄⁻ is the dominant species in the pH range of 2.1–7.2, while HPO₄²⁻ is the dominant species from 7.2 to 12.3 (He et al. 2020). The PO₄³⁻ only prevails when pH was higher than 12.3. Thus, all phosphate species are negatively charged except for H₃PO₄ formed at pH < 2.1. In addition, the zeta potential and protonation/deprotonation of the adsorbents are deeply dependent on the value of pH. It can be seen from Figure 4(b) that the isoelectric points(pH_pzc) of CS-CD, PS-CD, CS-CT and PS-CT are 2.73, 2.87, 2.21 and 2.08, respectively. As we all know, when pH < pH_pzc, the adsorbents with positively charged surface can easily capture the negatively charged phosphate through the driving force of electrostatic attraction. When pH is acidic, due to protonation, the trivalent cerium ions, on the one hand can directly form CePO₄ precipitation with phosphate; on the other hand, Ce (III) at the unsaturated site combines with OH⁻ in solution to form Ce(OH)₃ by hydrolysis, and then achieve adsorption through the ion exchange of OH⁻ and negatively charged phosphate (Jiaojie et al. 2021). Moreover, it can be seen from Figure 4(a) that CS-CD and PS-CD show good adsorption behaviors in a wide pH range. However, in contrast, CS-CT and PS-CT only have better adsorption effect under acidic conditions. Why don't CT added composites have stable adsorption effect at alkaline pH, for they are also materials loaded with trivalent cerium? The reason may lie in the difference between CD and CT. CD has a large number of unsaturated adsorption sites, which guarantee the phosphate removal by coprecipitation and ion exchange under alkaline conditions. Whereas main component of CT is proved to be Ce(HCOO)₃ by XRD. This nanoparticle doesn't contain unsaturated adsorption sites, so the effect of ion exchange or coprecipitation under alkaline conditions is not good. Under alkaline conditions, OH⁻ seriously affects the adsorption effect of CT. Therefore, it can be seen that the adsorption processes of CD loaded materials are jointly regulated by electrostatic attraction, coprecipitation and ion exchange, showing great adsorption capacity in a wide pH range.

The economic analysis of the adsorbents was calculated by Equation (S1), which was closely related to the cost of energy, chemicals and operating process (Kobya et al. 2009; Özyonar & Karagozoglu 2011). The material price was shown in Table S3, and the cost of all materials was selected as the maximum permissible limit. In addition, CS and PS are agricultural and forestry wastes, so their costs were not considered. At the same time, the labor costs were not considered because of less manpower in this process.

As shown in Table S3, the total estimated costs of materials required for CS-CD, PS-CD, CS-CT and PS-CT preparation were 0.3571, 0.3571, 0.3410 and 0.3410 $/kg, and the costs of removing 1 g P from the solution were calculated to be $2.81, $2.78, $1.37 and $1.66, respectively, which were lower than those documented for the efficient adsorption ($3.41) and electrochemical-adsorption ($3.16) (Kalaruban et al. 2017).

Through the analysis of FTIR, XRD, XPS and adsorption experimental data, it can be seen that trivalence is the dominant cerium valence state. Because of the strong attraction between Ce (III) and phosphate, all the composites can realize great adsorption by inner-sphere complexation through precipitation and ion exchange. Meanwhile, on account of the unsaturated coordination bonds in the MOFs structure, CD loaded CS and PS reveal excellent adsorption behaviors in a wide pH range. While as MOFs derivatives, the CT loaded adsorbents do not possess unsaturated active site and show unsatisfactory performance in phosphate capturing under alkaline condition.

In summary, cerium based MOFs and their derivatives are loaded on CS or PS to prepare biomass/Ce-MOF adsorbents by hydrothermal synthesis. Due to the biomass carriers, the agglomeration of metal based nanoparticles is avoided. The differences of nanostructures, surface properties and phosphate adsorption capacities are also studied. The results show that the loading of Ce (III)-based nanoparticles significantly increases the phosphate removal by pristine biomass, among which CS-CT exhibits the best adsorption performance, which may be due to the highest specific surface area and the higher content of trivalent cerium. However, the most sustainable removal capacities are preserved by CS-CD and PS-CD under wide pH scope from 2 to 11. Because of the designed characteristics of CD, it contains a large number of unsaturated adsorption sites, which could achieve the phosphate uptake by coprecipitation and ion exchange even under alkaline conditions. In conclusion, biomass/Ce (III)-based nanoparticles have the advantages of simple preparation and high phosphate adsorption performance. It is a promising composite for phosphate removal in water.

The authors gratefully acknowledge the Fundamental Research Funds for the Central Universities (Grant No. 300102281201), the China Postdoctoral Science Foundation (Grant No. 2021M692510) and the Young Talent fund of University Association for Science and Technology in Shaanxi, China (Grant No. 20200413).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.