It is essential to solve the problem of phosphorus pollution in urban landscape water and reduce the degree of eutrophication. In this paper, lanthanum-modified bentonite (La-B) was prepared by high-temperature calcination and liquid-phase precipitation. Then La-B was modified with chitosan to prepare a low-cost environment-friendly functional material: lanthanum/chitosan co-modified bentonite (La-BC). It can reach the adsorption equilibrium within 30 min, and the maximum adsorption capacity is 15.5 mg/g (initial phosphate concentration 50 mg/L); when the target concentration is 2 mg/L, the removal rate can reach 98.5%. La-BC has a stronger anti-interference ability to common coexisting anions SO42−, HCO3, NO3 and Cl in the urban landscape water body. La-BC has excellent performance in weakly acidic to neutral water, and its pH applicable range has been improved, making it possible to apply in practical water. The fitting results show that the adsorption behavior conforms to the pseudo-second-order kinetic model and the Freundlich model. After 5 regenerations, the removal efficiency remained around 80%. In the actual water test results, the phosphate concentration can be controlled below 0.1 mg/L and the removal rate is above 75%. Due to its low cost and reusability, it has great potential in the practical application of phosphate removal from landscape water.

  • La-BC high-efficiency phosphorus removal agent was prepared by high temperature calcination and liquid phase precipitation.

  • La-BC has shown excellent competitiveness in actual landscape water application and regeneration experiments.

  • A possible mechanism of La-BC adsorption of phosphate has been studied and analyzed.

Graphical Abstract

Graphical Abstract
Graphical Abstract

As an important part of the ecological structure and spatial composition of lakes, urban landscape water is an important basis for urban landscape construction. As well as being an essential part of landscape diversity, it plays an irreplaceable role in the construction of the urban ecological environment and social and economic development (Smith et al. 2002; Zhao et al. 2003). In recent years, with continuous research on lake pollution control, it has been found that aquatic organisms such as algae in water are more sensitive to phosphorus than nitrogen. So the growth of aquatic organisms such as algae in water may mainly depend on total phosphorus but not total nitrogen (Carpenter et al. 1998; Fang et al. 2018). There is a clear correlation between algal stock and phosphorus content (Huang et al. 2015; Dotto et al. 2016). Therefore, the efficient treatment of low-concentration phosphate has become the key to solving the problem of water eutrophication.

With the development of technology, many efforts have been made to remove phosphate from water including employing chemical precipitation (Kim & Chung 2014; Lv & Yuan 2014), biological treatment (Zhang et al. 2015; Hongwei et al. 2018; Hu et al. 2018; Lo et al. 2018), ion exchange (Choi et al. 2011; Acelas et al. 2015; Kalaruban et al. 2016; Tang et al. 2016; Cheng et al. 2018) and adsorption (Delaney et al. 2011; Vianna et al. 2016; Cusack et al. 2018) and so on. Among these methods, adsorption is still a widely used method. In the treatment of water with low phosphate content, low-cost adsorbents are widely studied, such as clay adsorbents (Jang & Lee 2019), mesoporous alumina (Lee et al. 2010), fly ash (Xu et al. 2011) and metal composites (Liu et al. 2019) et al. The maximum adsorption capacity and adsorption equilibrium time of the above materials are unsatisfactory. Bentonite is a clay mineral that exists widely in nature and is abundant and inexpensive. Montmorillonite is the main mineral composition of bentonite, and its proportion is about 85–90% (Baglieri et al. 2013). Named because it is a water-bearing layered aluminosilicate mineral, its crystal structure belongs to the monoclinic system (Yıldız et al. 2005), and it is a dioctahedral layered silicate mineral. The crystal structure of montmorillonite is a 2:1 layered silicate composed of two layers of silicon-oxygen tetrahedral lamellae sandwiched by an aluminum (magnesium)-oxygen octahedral lamellae (Kasprzyk & Gajewska 2019). Because the charge carried by the silicon-oxygen structure itself is negative, it is excellent in the removal of cations and heavy metal ions. However, natural bentonite (Chen et al. 2018) has low adsorption capacity for the removal of phosphate anions due to its negative charge, which limits its application. Douglas et al. report the efficacy of rare earth elements, especially lanthanum, in removing various forms of phosphate from water bodies. The lanthanum phosphate compound, known as ‘Rhabdophane’, is a highly water-insoluble salt that forms even at low concentrations and pH (Kuroki et al. 2014). Therefore, lanthanum-modified bentonite has become a research hotspot in recent years in the removal of phosphate from water. However, its application in landscape water bodies still needs to be improved because its pH range is more acidic.

Chitosan is the second-largest renewable polymer polysaccharide in nature. It is a linear aminopolysaccharide, usually formed by the deacetylation of chitin. It has many excellent properties, is non-toxic, biodegradable, and contains a large amount of free reactive amino groups (-NH2) and hydroxyl groups (-OH), making chitosan an excellent chelating agent for metals, cationic dyes and phosphates. In addition, chitosan can neutralize or reverse the negative charge of bentonite, the positive surface charge of chitosan under acidic conditions can be attributed to the protonation of on the chitosan chain (Xu et al. 2020). The -NH2 group on the chitosan chain is fully protonated at pH less than 6.2, and at pH 9.1 the total positive charge on the chain is equal to the total negative charge. This feature of chitosan enhances its adsorption capacity for negatively charged phosphates (Cabuk et al. 2017).

Our research goal is to develop a material suitable for the removal of low concentrations of phosphate adsorbent material in urban landscape water bodies, and explore its phosphate adsorption mechanism. In this work, a chitosan-modified lanthanum-modified bentonite (La-BC) material was successfully prepared by high-temperature calcination and liquid-phase precipitation. We have systematically studied the physical and chemical composition and adsorption mechanism through characterization and sequential batch experimental analysis. Its advantages of simple preparation, low cost, environmental protection, and wide pH application compared with conventional phosphate adsorbents make it have a strong competitive advantage in practical water applications. Under the premise of economic and practical use, La-BC can rapidly and efficiently remove excessive phosphate in urban landscape water bodies through the adsorption method, thus essentially solving the problem of eutrophication treatment of urban landscape water bodies.

Materials

Bentonite (Al2O3·4(SiO2)·H2O) was bought from Bentonite Factory (Shandong, China), chitosan (deacetylation≥ 90%) RuJi Biotechnology Development Co, Ltd (Shanghai, China). KH2PO4, CH3COOH, sulfuric acid (H2SO4), ascorbic acid (C6H8O6), ammonium molybdate tetrahydrate (NH4)6Mo7O24·4H2O), antimony potassium tartrate (C8H18K2O15Sb2), hydrochloric acid (HCl, 36.0%), sodium hydroxide (NaOH, 96.0%), ethyl alcohol absolute (C2H6O, 99.7%) were all purchased from Sinopharm Chemical Reagent Co.,Ltd, China. Lanthanum chloride hexahydrate (LaCl3·6H2O, 99.99%) was acquired from Aladdin Reagent Co, Ltd, Shanghai. Chemical reagents all were of AR grade or of the highest purity available and were used without further purification. A list can be seen in Table S1.

Preparation of La-B and La-BC

The lanthanum-loaded bentonite adsorbent was prepared by referring to the method of Zhang et al. (2014) and enhanced. First, a certain amount of bentonite was taken in a muffle furnace and calcined at 773.15 K for 2 h, so that the internal water and bound water in the spatial structure were precipitated. The specific surface area and porosity of bentonite can be enhanced by calcination at high temperature. The bentonite was then purified and activated with 5 wt% 343 K hydrochloric acid solution. By deepening the pores, the adsorbate molecules are rapidly dispersed into the pores.

Then, a LaCl3 solution with a concentration of 0.05 mol/L was prepared, and 1.0 g of activated bentonite was accurately weighed and added to the above solutions, and the pH was adjusted to 10 with 2 mol/L NaOH solution. Stir at room temperature for 2 h under the action of 200 rpm magnetic stirring, let stand for 12 h, and rinse the sediment with deionized water until the pH is neutral. After excluding the supernatant, the residue was calcined at 500 °C for 2 h, and then passed through a 100–200 mesh sieve after grinding to obtain lanthanum oxide-modified bentonite, designated as La-B.

Initially, 1.0 g of chitosan was weighed, dissolved in a 3% acetic acid solution, and stirred at room temperature for 2 hours to make it uniform. Precisely weigh 1.0 g of La-B, add it to the above solution and maintain stirring for 3 h, adjust the pH to 9.0 with an appropriate concentration of NaOH solution, and let it stand overnight. It was washed with deionized water until neutral, then dried at 70 °Cfor 12 h, and then passed through a 100–200 mesh sieve after grinding, and the attained product was labeled La-BC.

Batch experiments

The batch equilibrium method was used for batch processing experiments. Unless otherwise specified, 25 mL of 2 mg/L KH2PO4 experimental simulated solution and 0.020 g of La-BC composite were taken in a 150 mL glass conical flask for adsorption studies. The blend was reacted for 120 min at a temperature of 298 K at a speed of 150 r/min in an air-bath thermostatic shaker. Then after centrifugation, the supernatant was collected and filtered through a 0.22 μm microporous membrane, and the supernatant was determined by the vanadium molybdenum phosphoric acid method. The concentration of the liquid was measured with a UV-Vis spectrophotometer at a wavelength of 400 nm (Rotzetter et al. 2013). To explore the adsorption capacity of the adsorbent material, the effect of adsorption time was observed in the range of 5–180 min. The concentration of the adsorbate solution was varied from 2 to 50 mg/L to find the maximum adsorption capacity of the adsorbent. The optimum dosage of the adsorbent is to vary the dosage of La-BC composite from 0.005 to 0.070 g under the initial conditions. The pH is an important parameter affecting adsorption performance and adsorbent performance. Therefore, the pH study was carried out by changing it from 2 to 10 using 0.01 M HCl/NaOH solution. The study on the effect of temperature was carried out between 278 K and 318 K. The effects of inorganic anions such as HCO3, SO42−, NO3 and Cl, which are common in water, which are easy to compete with phosphate for adsorption and adversely affect the removal of phosphate, were studied. Using water samples collected from Nanhu Lake and Jianhu Lake in Wuhan, China, the effectiveness of La-BC in phosphate removal from actual water bodies was verified. The value of Adsorption capacity (qe) is calculated by formula (1), and the elimination percentage is calculated by formula (2).
formula
(1)
formula
(2)

In the formula, the volume of the adsorbate solution is expressed as V (L), the initial concentration of phosphate ions is expressed as Ci (mg/L), the equilibrium concentration is expressed as Ce (mg/L), and the La-BC mass is expressed as m (g).

Regeneration and actual water experiments

To explore the regeneration ability of La-BC, 0.1 g of La-BC adsorbed phosphate was added to NaOH solution for 24 h (1 M, 298 K, 150 r/min), and then centrifuged to measure the phosphorus concentration in the supernatant, and the evaluated La-BC was repeatedly scrubbed to neutrality, dehydrated at 333 K, and the above operation was repeated 5 times.

Water samples were gathered from 3 different sections of the natural landscape water body Nanhu Lake and Jianhu Lake (Wuhan, China). Add 0.1 g La-BC to 50 mL of the filtered water sample, then put it into an air-bath incubator shaker at 150 r/min and react at 25 °C for 60 min. After centrifugation, take the supernatant. The phosphorus concentration in the filtrate was measured after the liquid was filtered through a microporous membrane, and the phosphate removal rate was computed.

Characterization

To determine the composition, morphology, surface chemical composition and changes before and after adsorption of the La-BC, a Fourier transform infrared spectrometer (Nicolet 6700) equipment in the United States was used to test the bentonite before and after loading with lanthanum, and the wave number ranged from 400 to 4,000 cm−1. The X-ray diffractometer (D8 Advance) manufactured in the United States was used to explore the crystal structure changes of bentonite before and after loading with lanthanum. An X-ray energy dispersive spectrometer (X-Max 50) machine in Germany was used to analyze the change of the chemical composition elements of the composite materials before and after the adsorption of phosphate. N2 adsorption-desorption tests on dry solid powder samples before and after lanthanum loading on bentonite using a specific surface area pore size analyzer (BET). Phosphate ions were analyzed using a UV-3100 UV-Vis spectrophotometer in Shanghai, China.

Material characterization

The N2 adsorption/desorption tests were carried out of La-BC are shown in Figure 1(a). The specific surface area of La-BC determined by nitrogen adsorption assay was 34.057 m2/g, and the average pore volume was 0.067 m3/g; the increase in the specific surface area of La-BC after modification was probably due to the introduction of chitosan which made the surface of the material rougher. The test results indicate that the N2 adsorption/desorption isotherm of La-BC is type IV with a giant H3 type hysteresis loop, which represents a mesoporous structure according to the classification of IUPAC (Wang et al. 2004; Moussout et al. 2018), and the mesopores are primarily derived from montmorillonite. The gaps in the interlayer structure of the stone indicate that La-BC is a porous material. To sum up, we can conclude that La-BC has excellent performance in physical adsorption.
Figure 1

(a) N2 adsorption/desorption isotherm of La-BC; (b) XRD of chitosan and La-BC; (c) FT-IR spectra before and after adsorption on phosphate of La-BC.

Figure 1

(a) N2 adsorption/desorption isotherm of La-BC; (b) XRD of chitosan and La-BC; (c) FT-IR spectra before and after adsorption on phosphate of La-BC.

Close modal

According to the XRD patterns of chitosan and La-BC in Figure 1(b), some sharp peaks appeared in the diffraction pattern of La-BC. Compared with the main peak of chitosan at 2θ = 19.9°, there is also a corresponding peak at the same position of La-BC. It can be observed that the peak is caused by chitosan. This indicates that chitosan may be introduced into the lanthanum-loaded bentonite material. There is a very sharp peak at 2θ = 26.6°, which is caused by SiO2 in bentonite and belongs to the characteristic diffraction peak of montmorillonite crystal plane. There is also a comparatively obvious peak at the position of 2θ = 27.0°, which may be caused by La2O3 loaded in the interlayer structure of bentonite, which belongs to the characteristic diffraction peak of lanthanum (Aghazadeh et al. 2014).

The infrared spectrum test results of La-BC before and after phosphorus removal are illustrated in Figure 1(c). The peak at 1,378 cm−1, which is not identified in the original bentonite spectrum, may be caused by the La-O vibration formed after lanthanum is loaded on the bentonite. The peaks at wavelengths between 1,200 and 400 cm−1 belong to the silicate band, and the peak at 1,036 cm−1 belongs to the stretching parallel of Si-O-Si. Several main typical peaks of chitosan appear in the infrared spectra before and after La-BC phosphorus removal. The characteristic peak at 3,443 cm−1 belongs to the overlap caused by the stretching vibration of -OH and -NH2, while the peak at 2,877 cm−1 is caused by the asymmetric stretching vibration of -CH3. The peaks at 1,649 cm−1 and 1,591 cm−1 respectively belong to the stretching vibration of -NH2-C = O and the bending vibration of -NH, during the deacetylation of chitosan (Zhang et al. 2005). These were all introduced by chitosan, indicating that chitosan was successfully introduced into lanthanum-loaded bentonite. In addition, the peak shape and peak intensity of La-BC did not change substantially before and after phosphorus absorption.

The SEM images of La-BC before and after phosphate adsorption are displayed in Figure 2(a) and 2(b). From the figure, it can be seen that the surface smoothness of the material La-BC is significantly diminished, which are numerous uneven wrinkles (Benucci et al. 2016; Zhang et al. 2016), and La-BC exhibits a sandwich structure before and after adsorption, which is caused by the interaction of lanthanum metal ions with the chitosan and bentonite polymeric matrix, and a zigzag structure after metal chelation. After adsorption of phosphate, the surface of this zigzag structure is further widened, which may be due to the chelation of lanthanum metal with the amino group of chitosan and the silicate group of bentonite, respectively. The adsorption of phosphate was then completed on the surface of La-BC material (Celis et al. 2012). The EDS spectra of La-BC before and after phosphate adsorption are demonstrated in Figure 2(c) and 2(d). After the adsorption process, the content signals of main elements such as O and Si practically did not modify while the content signals of La element reduced, and the signals of P element appeared in the EDS characterization chart, which shows that La element participates in the adsorption process of phosphate.
Figure 2

(a,b) SEM image of before and after adsorption on phosphate; (c,d) EDS spectra of La-BC before and after adsorption on phosphate.

Figure 2

(a,b) SEM image of before and after adsorption on phosphate; (c,d) EDS spectra of La-BC before and after adsorption on phosphate.

Close modal

Response parameter influence study

The effect of adsorbent dosage

The experimental results of adsorption under different adsorbent dosage levels are shown in Figure 3. It can be seen that before the dosage of 0.8 g/L, the phosphorus removal efficiency is substantially improved, and then tends to be horizontal. The adsorption capacity decreased gradually with the increase of dosage. The main reason for the continuous increase of phosphate removal rate is that when the solution volume is constant, the increase of the adsorbent concentration increases the active adsorption sites per unit volume in the reaction system, which also increases the probability of effective collision between phosphate and adsorption sites. The continuous decrease of the unit adsorption capacity of La-BC is because the concentration and volume of phosphate in the solution are constant. When the dosage of adsorbent increases continuously, the phosphate in the solution is adsorbed to equilibrium, and a large number of active sites remain, so the unit adsorption capacity keeps decreasing. Some studies have also shown that as the adsorbent concentration rises, the collision and focusing increases, and the active sites per unit mass of adsorbent become fewer, so the unit adsorption capacity will also decline. In order to explore the effect of La-BC dosage on the adsorption effect of phosphate and the practical application cost, the optimal dosage level of the adsorbent La-BC in this research was determined to be 0.8 g/L considering the removal rate and adsorption capacity.
Figure 3

Effect of La-BC dosage on phosphate removal (Volume: 25 mL, pH = 3, time: 120 min, T = 298 K, same below).

Figure 3

Effect of La-BC dosage on phosphate removal (Volume: 25 mL, pH = 3, time: 120 min, T = 298 K, same below).

Close modal

The effect of pH

To explore the effect of pH of the reaction system on the phosphorus removal performance of La-BC, this research set up a pH effect adsorption experiment in the range of 2–10, and the results are shown in Figure 4. It can be demonstrated that the adsorbent La-BC has an excellent phosphorus removal effect in a weak acid neutral environment. The above phenomenon is caused by the different existing forms of phosphate in the water body with the change of pH, and the competition between OH and phosphate ions under alkaline conditions. The adsorbent La-BC after the introduction of chitosan has a large number of nitrogen-containing and oxygen-containing functional groups on the surface, and the change of pH is related to the degree of protonation on the surface of the material, which may strengthen the affinity for H2PO4 and HPO42−.
Figure 4

Effect of pH on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, time: 120 min).

Figure 4

Effect of pH on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, time: 120 min).

Close modal

Effect of initial concentration

The effect of initial phosphate concentration C0 on the phosphorus removal performance of adsorbent La-BC is shown in Figure 5. This is because when C0 is high, the amount of adsorbent in the solution is constant, and the number of active adsorption sites contained is restricted. If C0 continues to increase, the active adsorption sites on the surface of the adsorbent are basically inhabited, so the adsorption amount tends to balance ultimately, and the uncaptured phosphate in the solution continues to accumulate, resulting in a continuous decline in the removal rate. When the phosphate C0 in the solution was 2 mg/L, the removal rate of La-BC was 95.4%, and the adsorption capacity was 2.38 mg/g; when the phosphate C0 in the reaction system gradually expanded to 50 mg/L, the removal rate of phosphate by La-BC decreased to 24.8%, while the adsorption increased to 15.5 mg/g.
Figure 5

Effect of initial concentration on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Figure 5

Effect of initial concentration on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Close modal

Effect of contact time

In this research, the effect of contact time on the phosphorus removal effect of La-BC was explored. To study the removal rate of La-BC, adsorption experiments with different contact times were set in the range of 0–120 min; the experimental results are shown in Figure 6. The entire adsorption process can be roughly divided into three stages. The initial stage is 0–10 min, and the removal rate of phosphate by La-BC is sharply increased; In the middle stage (10–30 min), the growth rate of phosphate removal efficiency gradually flattened, which may be because the mass transfer kinetics decreased with the increase of phosphate captured on the surface of La-BC, and the nitrogen and oxygen-containing adsorption sites are gradually occupied, and the reduction of active adsorption sites reduces the probability of effective phosphate collisions. When the contact time was 30 min, the adsorption rate no longer increased, the removal rate was stable at about 97%, and the adsorption was in an equilibrium state, and the adsorption amount at this time was 2.44 mg/g.
Figure 6

Effect of contact time on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Figure 6

Effect of contact time on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Close modal

Influence of interfering ions

The effects of common coexisting anions SO42−, HCO3, NO3 and Cl in urban landscape water bodies on the phosphorus removal effect of La-BC are shown in Figure 7. It can be seen from the figure that the coexisting anions SO42−, HCO3, NO3 and Cl of low concentration (2 mg/L) and high concentration (100 mg/L) have little effect on the phosphorus removal effect of La-BC, and the removal efficiency is above 90%. When the coexisting anion concentration was 100 mg/L, the high concentration of coexisting anions reduced the phosphate removal rate by about 5% in comparison with interfering ions at low concentration. It can be seen that La-BC had strong resistance to the four co-existing anions mentioned above, indicating that La-BC has good selective adsorption on phosphate.
Figure 7

Effect of interfering ions on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Figure 7

Effect of interfering ions on phosphate removal of La-BC (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Close modal

Thermodynamic model studies

In order to explore the effect of reaction temperature on the adsorption of phosphate by La-BC, adsorption experiments were set up at 278, 288, 298, 308 and 318 K, respectively. The results are shown in Figure 8(a). It can be concluded that the temperature has slight effect on the phosphate adsorption, and the phosphorus removal efficiency slightly increases with the increase of temperature. The reason is that the rise of temperature makes the target ions in the solution move faster, and at the same time, the diffusion resistance decreases, and the ability of ions to diffuse into the pores is upgraded, which increases the collision probability between phosphate and nitrogen- and oxygen-containing functional groups on La-BC. The basic thermodynamic parameter calculations are listed in Table S2. The relationship between lnKd and 1/T is illustrated in Figure 8(b) and lnKd negatively with 1/T. With the rise in temperature, the value of △G is always less than 0, which signifies that the La-BC adsorption phosphate process is spontaneous; the value of △H, positive, indicates that La-BC adsorption phosphate is a heating process, the increase of temperature is favorable to the adsorption process; the △S value is positive, which signifies that La-BC adsorption phosphate process is entropy production, the disorder of the system is moderately reinforced.
Figure 8

(a) Effect of temperature on phosphate adsorption of La-BC (dose: 0.015 g, volume: 25 mL, pH = 3, time: 120 min); (b) plot of ln Kn vs. 1/T for the adsorption of phosphate.

Figure 8

(a) Effect of temperature on phosphate adsorption of La-BC (dose: 0.015 g, volume: 25 mL, pH = 3, time: 120 min); (b) plot of ln Kn vs. 1/T for the adsorption of phosphate.

Close modal

Dynamic model research

The data of the adsorption time and adsorption amount of phosphate on the adsorbent La-BC were brought into the pseudo-first-order model, pseudo-second-order model, Elvich model and intraparticle diffusion model for data fitting. The fitting results are shown in Figure 9 and Table S3 (Zhao et al. 2014).
Figure 9

(a) pseudo-first-order model; (b) pseudo-second-order model; (c) Elvich model; (d) intraparticle diffusion model (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Figure 9

(a) pseudo-first-order model; (b) pseudo-second-order model; (c) Elvich model; (d) intraparticle diffusion model (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Close modal

According to the results, it can be seen that the pseudo-second-order kinetic model can better fit the adsorption of phosphate by La-BC than the other three models, and its correlation coefficient R2 = 0.999, The results in the table show that the equilibrium adsorption capacity obtained by calculation is 2.525 mg/g, while the adsorption capacity in the experiment is 2.46 mg/g, and the theoretical value deviates from the experimental value by 2.6%. This indicates that the pseudo-second-order kinetic model is more suitable for simulating the adsorption kinetics of La-BC on phosphate, and it also shows that the adsorption rate of La-BC on phosphate is regulated by chemical reaction. Notably, there is the transfer and sharing of electron pairs between the active sites on the adsorbent surface and the adsorbed ions.

Study on adsorption isotherm model

The adsorption isotherm model of La-BC for phosphate was carried out at 298 K. The Langmuir, Freundlich and Temkin models were used to fit the experimental data in Section 3.2.3. The fitting results are shown in Figure 10, and the relevant parameters of the adsorption isotherm model are shown in Table S4.
Figure 10

(a) Langmuir model; (b) Freundlich model; (c) Temkin model (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Figure 10

(a) Langmuir model; (b) Freundlich model; (c) Temkin model (dose: 0.02 g, volume: 25 mL, pH = 3, time: 120 min).

Close modal

It can be seen from the figure that the correlation coefficients R2 of the above three models are 0.981, 0.999 and 0.918, respectively. Among them, the Freundlich model has the largest correlation coefficient, indicating that the Freundlich model can better simulate the isotherm adsorption behavior of La-BC on phosphate than the other two models. This decent degree of fit also suggests that La-BC has a non-uniform surface and thus multiple adsorption equilibrium during the following adsorption of phosphate (Debnath et al. 2019). Table S5 lists the separation factor RL adsorbed at different phosphate concentrations at 298 K, whose values are in the range of 0–1, demonstrating that the adsorption energy of La-BC on phosphate occurs spontaneously and belongs to supportive adsorption. Meanwhile, the Langmuir model fits the experimental data to a high degree and the maximal phosphate adsorption capacity calculated by the Langmuir model is 16.129 mg/g, which is also close to the experimental value (15.475 mg/g).

In addition, we summarize some typical phosphate adsorbents in Table 1, and it can be clearly seen that La-BC outperforms other adsorbents in terms of peak adsorption capacity and adsorption rate.

Table 1

Comparison of La-BC with other adsorbents

AdsorbentSorption capacity (mg/g)Equilibriumtime (min)Reference
Phoslock® 10.5 200 Haghseresht et al. (2009)  
La(III)-modified pillared clay 10 300 Tian et al. (2009)  
Fe(III)-modified bentonite 11.5 120 Zamparas et al. (2012)  
Al(III)-modified bentonite 12.7 360 Yan et al. (2010)  
ZnAl-LDH 84 360 Seftel et al. (2018)  
CaT-Z 7.57 7,200 Mitrogiannis et al. (2017)  
Magnetic diatomite and illite clay 11.89 30 Chen et al. (2016)  
Fe-La 89.41 60 Wang et al. (2018)  
La-BC 15.5 30 This work 
AdsorbentSorption capacity (mg/g)Equilibriumtime (min)Reference
Phoslock® 10.5 200 Haghseresht et al. (2009)  
La(III)-modified pillared clay 10 300 Tian et al. (2009)  
Fe(III)-modified bentonite 11.5 120 Zamparas et al. (2012)  
Al(III)-modified bentonite 12.7 360 Yan et al. (2010)  
ZnAl-LDH 84 360 Seftel et al. (2018)  
CaT-Z 7.57 7,200 Mitrogiannis et al. (2017)  
Magnetic diatomite and illite clay 11.89 30 Chen et al. (2016)  
Fe-La 89.41 60 Wang et al. (2018)  
La-BC 15.5 30 This work 

Recycle experiment and actual water test

The effect of NaOH desorption solution at different concentrations on the desorption effect of La-BC after adsorption of phosphate was studied experimentally, and the results are shown in Figure 11(a). To test the reusability of La-BC, five adsorption-desorption cycle experiments were performed on La-BC in this study, and the results are shown in Figure 11(b). From the relationship between the number of La-BC adsorption-desorption cycles and the phosphate removal rate, with the increase of the number of adsorption-desorption cycles, the removal rate of phosphate by La-BC was slightly reduced. The decrease in the phosphate removal rate may be due to the strong binding force between the nitrogen/oxygen functional group of the adsorbent and phosphate, and NaOH is insufficient to completely decorate it, resulting in the number of adsorption active sites and the phosphate removal rate decreasing. Nonetheless, after 5 adsorption-desorption cycles, the removal rate of La-BC for phosphate remained at about 80%, indicating that the La-BC prepared in this experiment had excellent reusability for phosphate adsorption.
Figure 11

(a) Effect of NaOH concentration on phosphate desorption (dose: 0.1 g, volume: 50 mL, time: 120 min); (b) effect of adsorption-desorption cycle times on La-BC (dose: 0.1 g, volume: 50 mL, time: 24 h); (c) phosphorus removal of La-BC in actual water samples (dose: 0.1 g, volume: 50 mL, time: 60 min).

Figure 11

(a) Effect of NaOH concentration on phosphate desorption (dose: 0.1 g, volume: 50 mL, time: 120 min); (b) effect of adsorption-desorption cycle times on La-BC (dose: 0.1 g, volume: 50 mL, time: 24 h); (c) phosphorus removal of La-BC in actual water samples (dose: 0.1 g, volume: 50 mL, time: 60 min).

Close modal

To analyze the actual water removal effect of La-BC, we tested water samples obtained from three different sections of Nanhu Lake and Jianhu Lake (Wuhan, China). The test results are shown in Figure 11(c). Compared with laboratory test results, La-BC has a lower removal rate of phosphate in actual landscape water samples. The reasons include the following aspects: First, the pH of the actual landscape water is generally neutral, and the competition between coexisting ions and phosphates for adsorption sites in the water may be enhanced under neutral conditions; second, the actual landscape water frequently contains some complex organic components, such as humic substances, phenolic compounds and other soluble organic substances, etc., which will affect the mass transfer effect of phosphate in the water body on the La-BC surface and its impact on the target. adsorption of pollutants. In general, La-BC has a good phosphorus removal effect on the water samples of these two landscape water bodies, which further proves that La-BC has a good application prospect in low-concentration phosphorus-containing water bodies.

Exploration of the reaction mechanism

The surface of La-BC contains a large number of nitrogen-containing and oxygen-containing functional groups, and the metal lanthanum has an excellent affinity for phosphate, so La-BC has a good adsorption effect on phosphate. The possible mechanism of La-BC adsorption of phosphate is shown in Figure 12, which mainly includes electrostatic attraction, ion exchange and chemical bonds generated by the formation of inner layer complexes. On the one hand, La2O3 will combine with water in the solution to form hydrated lanthanum oxide. When the solution is acidic, the surface of hydrated lanthanum oxide will be positively charged, which can electrostatically adsorb with H2PO4 under the action of electrostatic force. On the other hand, on the surface of La2O3, coordinatively unsaturated ions are easily coordinated with water in the solution, thereby generating a hydroxylated surface. For stable compounds, this effect belongs to the chemical specific adsorption of ion exchange and can be represented by the following formula (3):
formula
(3)
Figure 12

Potential adsorption mechanism on phosphate of La-BC.

Figure 12

Potential adsorption mechanism on phosphate of La-BC.

Close modal

Eventually, phosphate and La are united in the inner layer, forming intricate complexes with strong chemical bonds. La-BC has a good affinity for phosphate under the action of electrostatic adsorption, ion exchange and bonding of the inner layer complexes.

In summary, we provide a low-cost, recyclable, ecological and effective adsorbent option for the phosphorus removal process in the eutrophication treatment of urban landscape water bodies. In the test of low-concentration phosphorus content of 2 mg/L, the phosphorus removal rate of La-BC achieved 98.5% at 30 min. The reaction mechanism of the adsorbent primarily involves electrostatic attraction, ion exchange and the formation of inner layer complexes, and the adsorption process is spontaneous and endothermic. In addition, compared with classic phosphorus remover and La-B, La-BC exhibited broader pH adaptability, particularly under weak acid and neutral conditions, with a rapid adsorption rate and stronger anti-interference performance. The excellent performance of La-BC at lower phosphorus concentrations makes it highly competitive in the eutrophication treatment of urban landscape water bodies.

This work was supported in part of the project titled ‘Special research on key technology research and application of urban lake landscape water quality improvement’ (ZJHM-2019-056-001 = KJFZ-2019-027-001-001). The authors thank the funding source of this research and the research platform provided by Wuhan University of technology.

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

The authors declare there is no conflict.

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Supplementary data