Fabrication of lanthanum/chitosan co-modified bentonite and phosphorus removal mechanism from low-concentration landscape water

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.

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.

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.

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

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.

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.

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 -CH_3. 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.

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.

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 R_L 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.

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.

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.