Strong adsorption of phosphate by amorphous lanthanum carbonate nano-adsorbents

Phosphorus (P) is a key nutrient that supports the growth of organisms in natural ecosystems. However, excessive release of P into water bodies can lead to eutrophication, which may cause excessive growth of algae, depletion of dissolved oxygen, deterioration of water quality, death of fish and plants, and other serious environmental problems (Correll 1998; King et al. 2015; Ding et al. 2018). Since the discharge of industrial wastewater and agricultural runoff are the primary causes of increased P concentrations in aquatic ecosystems, many countries and regions have enforced stricter P discharge limits to reduce the concentration of P in receiving water bodies and prevent eutrophication (Dodds et al. 2009; Karydis & Kitsiou 2012). For example, the United States Environmental Protection Agency (USEPA) recommends that any streams entering a lake or reservoir should have a total P concentration not exceeding 0.05 mg P/L, and the European Union (EU) defines the cut-off for total P concentration in lakes as non-risk and risk conditions of eutrophication to be <0.01 mg P/L and >0.1 mg P/L, respectively (Loganathan et al. 2014). For discharge from wastewater treatment plants, the permissible concentration of P will be lowered from 1–2 mg P/L to 0.1 mg P/L in the EU under the Water Framework Directive (Shepherd et al. 2016). Therefore, the selective and effective removal of P has become a challenge in the treatment of P-containing water and wastewater and has attracted considerable research interest in recent years.

Several technologies and processes, including biological, chemical, and physical treatments, have been developed for the removal of P from water and wastewater. Biological methods such as conventional activated sludge processes can achieve nearly 100% removal of P. However, these processes are less effective for P at trace levels, because such levels of P are below the metabolic thresholds of microorganisms (Sidat et al. 1999; Dulekgurgen et al. 2003). Moreover, higher capital cost and maintenance are often needed during implementation of biological methods, and sludge production then becomes a concern. Therefore, chemical treatments are primarily utilized in P removal, but these methods suffer from difficulties in sludge disposal and effluent neutralization. Physical methods such as reverse osmosis and electrodialysis, have been demonstrated to be either too expensive or inefficient in P removal (Wang et al. 2013; Seminskaya et al. 2017). The adsorption process is promising for P removal due to its attractive advantages, such as simple operation, high removal efficiency, and fast adsorption rate, especially at low P concentrations (Chouyyok et al. 2010; Yang et al. 2011). Many inorganic and organic adsorbents, as well as industrial by-products and biological wastes have been tested for their efficacy in removing P from wastewater. The main disadvantage of these adsorbents is their limited ability to adsorb P, even if the ratio of adsorbent to P varies between 100:1 and 1,000:1 (Yan et al. 2010; Yue et al. 2010; Hussain et al. 2011). Another disadvantage of using an adsorption method is that the adsorbed P may leach back into the solution after a period of time, causing concerns about secondary pollution (Bhardwaj et al. 2014).

Lanthanum-based adsorbents are rare-earth metal-based adsorbents with high chemical stability that are gaining increasing attention regarding the adsorption of P from water and wastewater due to their high affinity for P, even at low P concentrations. Moreover, the lanthanum–phosphate complex has been found to be stable in the pH range 3–6, with no detectable release of P or its solubility products (Aneesh et al. 2009). Therefore, various lanthanum materials have been prepared and investigated for P adsorption; for example, lanthanum hydroxide (La(OH)₃), La(III) oxide (La₂O₃) and a variety of lanthanum-incorporated materials (Li et al. 2009; Tian et al. 2009; Zhang et al. 2011; Liu et al. 2013; Spears et al. 2013; Xie et al. 2014a, 2014b; He et al. 2016; Wang et al. 2016). Although these adsorbents are highly efficient in reducing P concentrations to trace levels, there are still some limitations that inhibit their wide application. For example, these adsorbents exhibit high P adsorption capacity only under acidic conditions, so their performance will be greatly reduced when treating sewage under neutral and alkaline conditions. Therefore, in recent years, it has been desirable to obtain new lanthanum-based adsorbents with enhanced adsorption capabilities over a wider pH range to overcome these disadvantages.

Unlike the widely used lanthanum salts, such as La₂(SO₄)₃, La(NO₃)₃, La(OH)₃ and La₂O₃, LC has attracted intensive research interest in medical applications (Shao et al. 2011; Lee et al. 2013; Oka et al. 2016). LC is not only an effective P binder in the treatment of hyperphosphatemia of chronic kidney disease (CKD) (Behets et al. 2004; Tonelli et al. 2010) but is also tolerated by patients in long-term treatment (Guo et al. 2013; Hutchison et al. 2016). Being different from La₂(SO₄)₃ and La(NO₃)₃, the insolubility of LC results in low leaching of lanthanum ions during the adsorption process and thus is beneficial to the P removal in practical applications. Moreover, the pH buffering capacity of carbonate can limit the pH changes in the solution and keep the water body stable.

Based on previous studies of LC adsorbents for P removal from water and wastewater, the objectives of our work were to: (1) prepare an amorphous lanthanum carbonate adsorbent (simplified as LC) in co-existing Mg²⁺ through a facile coprecipitation method under different temperatures; (2) investigate the P adsorption performance (e.g., adsorption isotherms, adsorption kinetics, adsorption thermodynamics, physical models, effects of pH and co-existing ions); and (3) elucidate adsorption mechanisms of LC using a combination of XRD, FTIR and SAED analyses.

All the chemicals used in this study were of analytical grade and were used without further purification. Lanthanum chloride hexahydrate (LaCl₃·6H₂O) was provided by Aladdin Chemical Reagent Co., Ltd (Shanghai, China). Anhydrous sodium bicarbonate (NaHCO₃) and magnesium sulphate heptahydrate (MgSO₄·7H₂O), sodium hydroxide (NaOH) and anhydrous potassium dihydrogen phosphate (KH₂PO₄) were obtained from Shanghai Hushi Laboratory Equipment Co., Ltd (Shanghai, China). Deionized water was used throughout the study.

LC nano-adsorbent was synthesized via a conventional coprecipitation method. The Mg²⁺ solution (MgSO₄) was added to a 0.04 M La³⁺ solution (LaCl₃) with a molar ratio (Mg²⁺/La³⁺) of 0.05:1, 1:1, 2.5:1 and 10:1 at different temperatures (298 K, 313 K, 328, 343 and 358 K). A slow drip of 0.12 M NaHCO₃ was added continuously for 30 min to the solution while stirring and to maintain the pH value of the solution at approximately 11. When the precipitation reaction was complete, the mixed products were centrifuged at a speed of 5,000 r/min for 10 min and subsequently washed thoroughly with deionized water until the pH of the effluent solution was neutral. Finally, the washed samples were oven-dried at 318 K for 12 h to obtain the corresponding LC adsorbents. Using the same process as above, a reference sample of lanthanum carbonate, simplified as LC₀, was obtained without the coexistence of Mg²⁺.

Scanning electron microscopy (SEM, HITACHI SU8010, Japan) was used to investigate the surface morphology of the different adsorbents synthesized. The crystal structures of the nano-adsorbents were analyzed using a Bruker D8 Advance diffractometer (X'Pert Powder, PANalytical, Netherlands) with Cu Ka radiation (40 kV, 40 mA) over the 2θ range of 10–60°. The zeta (ζ) potential of the prepared nano-adsorbents was measured using a Zetasizer 3000HSA (Malvern Instrument Ltd, UK). SAED patterns were obtained by using a transmission electron microscope (TEM, HITACHI HT-7700, Japan) operated at an acceleration voltage of 200 kV. The functional groups of adsorbents were recorded on an FTIR spectrometer (Nicolet 5700, USA) in the range of 4,000 cm⁻¹ to 400 cm⁻¹ using the KBr tablet method.

A series of batch experiments were carried out to investigate the adsorption behavior of P onto LC. About 0.02 g of LC was put into a flask containing 100 mL P solution ranging from 20 to 50 mg P/L. The suspensions in the covered flasks were shaken in a thermostatic chamber at a specified temperature for 24 h at 250 rpm. After 24 h, the equilibrium pH was measured, and the suspensions were centrifuged. The supernatant was collected through filtration using a 0.45 μm syringe filter and analyzed to determine the residual P concentration by the Mo-Sb anti-spectrophotometer method using a UV-vis spectrophotometer (HACH DR900, American).

Kinetic tests were carried out in a 500 mL conical flask into which 200 mL of P solution and 0.04 g or 0.10 g of LC were added. The initial P concentrations were 20 and 50 mg P/L. The covered flask was shaken in a thermostatic chamber at 298 K for 24 h at 250 rpm. After each specified reaction time, a 1 mL sample was removed for analysis. The sample was then filtered through a 0.45 μm syringe nylon-membrane filter and the filtrate was analyzed for P concentration.

The effect of solution pH on P removal was investigated in the same way as for the adsorption measurements. The initial P concentration was 20 mg P/L, with pH ranging from 3.0 to 11.0. The suspensions were adjusted to the desired pH values with 0.1 M HCl or NaOH. Then, 0.02 g LC was added to the flask containing 100 mL P solution. The covered flask was shaken in an incubator shaker at 298 K for 24 h. The equilibrium pH was measured, and the suspension was centrifuged and analyzed for the residual P concentration in the supernatant.

To study the effect of common anions on P removal efficiency, 0.02 g LC was added into 100 mL P solution containing 0.01 M coexisting ions that were prepared by dissolving sodium salt forms of F⁻, Cl^−, NO₃⁻, SO₄²⁻, and HCO₃⁻ into 20 mg P/L solution. After the covered flasks were shaken for 24 h at 298 K, the suspensions were filtered, and the filtrate was analyzed for P concentration.

To mimic the typical P concentration of real wastewater, an initial P concentration of 3.2 mg P/L effluent was selected to demonstrate the applicability of LC nano-adsorbents. All batch adsorption experiments were investigated in the same way as for the adsorption measurements. After adding various amounts of LC to the flask containing 100 mL P solution, the covered flask was shaken in an incubator shaker at 298 K for 24 h. The suspension was then centrifuged and analyzed for the residual P concentration.

The morphology and structural characteristics of LC obtained under various Mg²⁺ concentrations are summarized in Figure 1. For comparison, the XRD patterns of LC₀, which was prepared without the coexistence of Mg²⁺, is also illustrated in Figure 1. Compared with the hexagonal layered structure of LC₀ (Figure 1(a)), SEM results for LC showed a dependence on the molar ratio of C(Mg²⁺)/C(La³⁺). When the molar ratio of C(Mg²⁺)/C(La³⁺) increased from 0.05:1 to 10:1(Figure 1(b)–1(d)), the particle size of LC was reduced from hundreds of nanometers to tens of nanometers. Although all LC samples maintained a layered nanostructure similar to LC₀, the shapes of each sample became more irregular and smaller with increasing Mg to La molar ratio, indicating that coexistence of Mg was a significant parameter for controlling the LC nanostructure.

The structure of LC prepared at different reaction temperatures was also confirmed by XRD analysis (Figure 2(a)). A sharp peak at 10.27°, which is the most characteristic reflection of La₂(CO₃)₃·8H₂O (PDF #25-1400) and corresponding to the (002) plane, was observed in all samples, indicating the successful synthesis of LC nano-adsorbents. Other broad peaks were located at 18.51°, 20.79°, 21.24°, 27.08°, and 29.06°, corresponding to the (020), (004), (022), (220) and (222) reflections of La₂(CO₃)₃·8H₂O, respectively. Since there were no peaks corresponding to MgCO₃ or La(OH)₃ in Figure 1, it was reasonable to consider that the resulting product consisted only of LC. After the preparation temperature changed from 298 to 358 K, it was noted that there was a broadened and weakened peak at 10.27°, indicating that the prepared LC gradually lost its crystallinity with increased temperature. When the temperature reached 358 K, LC was completely transformed into an amorphous nanostructure. Since the amorphous phase is considered to be more favorable than the crystalline phase in P adsorption, the preparation of LC with this microscopic morphology should be more effective in removing P (Yang et al. 2017).

The SEM results of LC samples confirmed the same conclusions as from the preparation temperature (Figure 2(b)–2(d)). When the preparation temperature was between 298 and 313 K, LC had a well-defined layered structure, with the diameter ranging from 200 to 500 nm. Continuing to raise the preparation temperature to 328 K (Figure 2(c)), some small particles appeared and aggregated on the residual layered structure of LC. As the reaction temperature reached 358 K (Figure 2(d)), LC completely transformed into spherical nanometer microspheres with a uniform particle size of about 60 nm.

Zeta (ζ) potential analysis was employed to determine the pH at the point of zero charge (pH_pzc). As shown in Figure 3, the pH_pzc of LC was 8.9. Compared with the value of LC₀ (pH_pzc = 7.3) and other La-incorporated materials reported in previous studies (He et al. 2016), the pH_pzc of LC shifted towards more alkaline conditions. In the case where the pH of the solution was lower than the pH_pzc of LC, the overall surface of LC would be protonated and positively charged, leading to static electric forces between positive LC and negative phosphate ions. While the pH value of the solution was higher than the pH_pzc, the functional groups of LC would gradually deprotonate and form a negative charge, causing electrostatic repulsion between the negatively charged surface sites and electronegative P species, thereby greatly reducing the adsorption of P on LC.

Adsorption kinetics are used to evaluate the efficiency of adsorption between an adsorbent and adsorbate. The kinetic experiments with LC were conducted at two initial P concentrations of 20 mg P/L and 50 mg P/L. From Figure 4(a), it can be seen that P adsorption onto LC was divided into two stages: an initial rapid stage followed by a gradually slower stage until the adsorption equilibrium was achieved. In the rapid stage, P removal increased rapidly during the first 2 h, with a P adsorption capacity of 98.1 mg P/g in the 20 mg P/L solution and 111.8 mg P/g in the 50 mg P/L solution. Thereafter, the increase in contact time did not significantly increase the adsorption capacity of LC, and P was slowly adsorbed during the final reaction period. In contrast, the P adsorption onto LC₀ reached equilibrium after about 12 h and then increased slightly, resulting in a P adsorption capacity of 97.4 mg P/g in the 20 mg P/L solution and 105.4 mg P/g in the 50 mg P/L solution. The kinetic characteristics of LC; that is, the shortened equilibrium time and increased adsorption capacity, were attributed to the smaller nanoparticle size and higher pH_pzc value that helped LC have more active sites and much stronger forces that enhanced the interaction between LC and P.

To further study the P adsorption onto LC, the kinetic data recorded with different initial concentrations was fitted to the pseudo-first-order and pseudo-second-order models (Figure 4(b)–4(c)). The corresponding parameters and correlation coefficients are listed in Table 1. According to the correlation coefficients, the pseudo second-order model simulated the P adsorption process better than the pseudo first-order model, suggesting that chemisorption or chemical bonding between adsorbent active sites and P might dominate the adsorption process. Based on the rate constants of the pseudo second-order model (0.392 and 0.299 g/(mg·h) for initial P concentrations of 20 and 50 mg P/L, respectively), less time was needed for LC to achieve effective P removal. This performance result is beneficial for potential applications, as the rapid adsorption process can shorten the dephosphorization time in water and wastewater treatment.

Based on the optimized conditions, the adsorption isotherms were studied at 298, 308 and 318 K and the results are shown in Figure 4(d) and 4(e). The adsorption data were further fitted by Langmuir and Freundlich models, and the obtained parameters are reported in Table 2. The P adsorption isotherms were better fitted by the Langmuir model (R² > 0.9997) than the Freundlich model (R²> 0.7961), suggesting a monolayer P adsorption onto LC. The maximum adsorption capacities (q_m) estimated by the Langmuir model were 112.9 mg P/g, 110.5 mg P/g, and 109.4 mg P/g at 298 K, 308 K, and 318 K, respectively. It is thus reasonable to consider that the adsorption process on LC should be exothermic in nature.

Compared with the q_m values of other reported La-modified adsorbents (Table 3), the fact that the P adsorption capacity of LC was higher than those reported for lanthanum hydroxide (107.5 mg P/g) (Xie et al. 2014b) and lanthanum oxide (47.0 mg P/g) (Xie et al. 2015), demonstrated the superiority of LC for P removal.

All the thermodynamic parameters are listed in Table 4. The negative ΔG values indicated that the adsorption process of phosphate was spontaneous under the experimental conditions. The value of ΔH° was negative in the temperature range of 298–318 K, confirming that the adsorption process of P on LC is exothermic in nature. The result was supported by the decrease in the adsorption capacity of P with the ascending temperature (Table 2). The positive value of ΔS° indicates that the adsorption process increased the degrees of freedom of the system.

The parameter n is a stoichiometric coefficient that can provide additional information to interpret the adsorption mechanisms of P on LC. Referring to the values listed in Table 5, it was found in single adsorption systems that the temperature had a positive effect on this parameter, which increased with the adsorption temperature from 308 to 318 K. Moreover, all estimated values of n were less than 0.5, indicating that P can be shared on at least two functional groups of LC. In contrast, the increment of temperature caused a decrement of the parameter D_m. This trend has two possible explanations: (1) an increment of the parameter n led to a decrement of the density of functional groups, or (2) the thermal collisions affected and reduced the occupied adsorption functional groups.

pH is an essential parameter in optimizing chemical processes for removing ions from aqueous solutions. Due to the competition between hydroxyl ions and the phosphate ions on the surface of the adsorbent, P uptake usually increases with the decrease of pH. However, as shown in Figure 5, LC performed well for P removal over a wide pH range. For example, when the solution pH was increased from 4.0 to 11.0, the removal efficiency reached almost 99% at the initial P concentration of 20 mg P/L. The fact that LC showed high removal efficiency under both acidic and alkaline conditions indicates that LC could overcome the limitations of use under acidic conditions and show strong and stable P removal capacity over a wide pH range.

For comparison purposes, the results of pH on LC₀ are also given in Figure 5. Since it is generally believed that the adsorption reactions at lower pH are much more favorable than the adsorption reactions at higher pH values (Fang et al. 2017), it is not surprising that acidity was the most favorable condition for P removal by LC. The adsorption efficiency approached 93% at pH of 3.0–7.0. The P removal efficiency then decreased sharply to 36% with increasing pH from 7.0 to 11.0. Compared with other lanthanum-containing materials, similar results have been observed concerning the effect of pH for P removal (Liu et al. 2011; Qiu et al. 2015; Zhang et al. 2016).

As the pH_pzc value increased, the surface charge of LC tended to change from negative to positive. As such, more H₂PO₄⁻ and HPO₄²⁻ could be captured onto LC through electrostatic attraction and ligand exchange (Tokunaga et al. 1997; Chen et al. 2012). Benefiting from both the proper P species distribution and the large Zeta (ζ) potential of LC versus pH, LC could exhibit high P adsorption capacity of negatively charged phosphate over a wide pH tolerance range.

The influence of adsorbent dosage on P removal was investigated as the amount of LC varied from 0.025 to 0.30 g/L. Increased adsorbent dosage implied a greater surface area and a greater number of P binding sites available. As shown in Figure 6, P removal increased sharply with increasing LC dose from 0.025 to 0.20 g/L. Thereafter, the removal efficiency of P increased slowly and approached the maximum value of 99.9% for an initial P concentration of 10 mg P/L.

The effect of dosage on adsorption capacity is also presented in Figure 6. When the dosage varied from 0.025 to 0.15 g/L, the adsorption capacity of LC remained almost unchanged. As the dosage increased and became greater than 0.15 g/L, the adsorption capacity was reduced by 86%. Since there was an intersection between the P removal curve and the adsorption capacity curve, the optimal value of LC could be obtained so as to have both high P adsorption capacity and removal efficiency. As a result, the dosage selected was 0.15–0.20 g/L for further studies.

Coexisting anions, such as F⁻, Cl⁻, SO₄²⁻, NO₃⁻, and HCO₃⁻, are commonly present with P in natural water and wastewater. These anions could potentially compete with phosphate for the adsorption sites. The interferences of coexisting anions, including F⁻, Cl⁻, SO₄²⁻, NO₃⁻, and HCO₃⁻, were studied in the experiment. To simulate the real wastewater conditions, the concentration of co-existing ions was 0.01 M, which was 10 times the initial concentration of P. From the results shown in Figure 7, it can be seen that F⁻, Cl⁻, SO₄²⁻ and NO₃⁻ had no, or only just a slight influence on the P adsorption, suggesting that such anions, even at relatively high concentrations, did not present significant competition to the P adsorption on LC. However, the P removal efficiency of LC₀ was reduced by 25% in 0.01 M HCO₃⁻ solution. This conclusion agreed well with a previous study on P removal by La-based materials (Koilraj & Sasaki 2017), indicating that LC also possesses strong selectivity towards P, thereby offering great support in the practical utilization for effluent wastewater treatment.

To evaluate the practicability of the prepared LC for real sewage treatment, LC was applied to domestic wastewater containing low concentration P. The initial concentration of P in the collected sewage sample was about 3.2 mg P/L. The evolution of P concentration after being treated by different doses of LC is shown in Figure 8. A small dose of 0.5 g/L LC realized the removal of P pollutants to less than 0.5 mg P/L, fully meeting the discharge standard of China (0.5 mg total P/L) (Wu et al. 2017). The relatively low dose and satisfactory sewage treatment efficiency indicated that LC is a promising candidate for the practical water purification process.

Multiple analytical methods (XRD, FTIR and SEAD) were utilized to investigate the mechanism of LC for superior adsorption performance.

The P adsorption mechanism was investigated by the microscopic technique of FTIR spectroscopy. Figure 9(a) shows the FTIR spectra of LC before and after P adsorption. Without P adsorption, the FTIR spectrum of LC had strong hydroxyl stretching (3,425 cm⁻¹) and bending (1,631 cm⁻¹) vibrations of physically adsorbed H₂O, and the strong antisymmetric stretching vibration (1,474 cm⁻¹ and 1,369 cm⁻¹) and weak bending vibration (845 cm⁻¹) of CO₃²⁻. After P adsorption, the intensities of all CO₃²⁻ peaks saw an obvious decline. At the same time, new sharp bands located at about 1,050 cm⁻1, corresponding to the asymmetric vibration of P–O and P = O bonds of PO₄³⁻, appeared after adsorption, indicating that P was successfully captured by the adsorbent.

The XRD patterns of LC before and after P adsorption were further studied, and the results are depicted in Figure 9(b). There are typical changes of XRD diffraction peaks before and after P adsorption. The intensities of peaks representing LC were reduced, indicating their participation in the adsorption process. However, characteristic diffraction peaks assigned to LaPO₄·xH₂O at 2θ = 14.6°, 29.5° and 41.5° were also observed (Zhang et al. 2016). The appearance of La phosphate compounds after adsorption indicated that P anions in solution were combined with La cations on LC that contributed to the phosphate removal from the solutions.

The corresponding selected area electron diffraction pattern (SAED) is also shown in Figure 9(c) and 9(d). Before dephosphorization, a series of concentric rings in the SAED pattern indicated that the prepared LC may have an amorphous structure. After dephosphorization, the corresponding spot ring SAED pattern can be indexed as a mixed structure of single crystal and polycrystalline nanoparticles.

Many researchers have investigated the effect of pH on P adsorption capacity and found that lanthanum-based adsorbents have high P removal efficiencies only at lower pH values. In order to obtain a highly efficient adsorbent for P removal under neutral and alkaline conditions, LC nano-adsorbents were prepared in the presence of Mg²⁺ cations at different temperatures. Characterization results revealed that an amorphous structure and nanoparticle size were highly effective for LC dephosphorization. The optimal conditions for the synthesis of LC were a ratio of 1:1 (mol/mol) for Mg²⁺ to La³⁺, 0.04 M LaCl₃ concentration, and at a 358 K reaction temperature. Batch adsorption results showed that the maximum P adsorption capacity of LC was 112.9 mg P/g. In a 20 mg P/L solution, the P adsorption equilibrium was reached within 2 h with an adsorbent dosage of 0.2 g/L. LC exhibited high P removal performance over a wide pH range from 3.0 to 11.0 as well as excellent selectivity for P in the presence of common competing ions. Thermodynamic analyses indicated that the phosphate adsorption process was exothermic and spontaneous in nature. Results of statistical physics modeling suggested that P can fully interact with two or more functional groups from the adsorbent surface. The analysis also demonstrated the feasibility for real sewage treatment, residual P concentration in sewage could be decreased to below 0.5 mg P/L, meeting the stringent criterion for eutrophication prevention. In summary, LC can serve as a promising adsorbent for efficient and preferable P removal under a wide range of pH conditions.

This work was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment, China (2018ZX07208). We thank LetPub (www.letpub.com) for its linguistic assistance and scientific consultation during the preparation of this manuscript.

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