Phosphate adsorption improvement using a novel adsorbent by CaFe/LDH supported onto CO₂ activated biochar

Phosphate is known as a necessary nutrient for the growth of plants, wildlife, and humans. However, in excess, this nutrient can cause eutrophication, which is a persistent condition of surface waters and a worldwide environmental problem, due to accelerated growth of algae blooms or water plants, anoxic events, altered biomass, and species composition (Kim et al. 2020). Therefore, it is imperative to remove phosphate from the aquatic system to prevent related issues.

Several methods are applied to remove phosphate including electrocoagulation (Hashim et al. 2019), chemical precipitation, (Costa et al. 2019) membrane separation (Li et al. 2021), adsorption and biological methods (Shang et al. 2018). Among them, the adsorption process is a promising alternative, due to its low cost, ease of operation, and higher removal efficiency (Hashim et al. 2021). However, in the adsorption method, the key challenge is to develop a proper adsorbent, i.e., a sustainable, economical, and less toxic material (Cui et al. 2019).

Among various adsorbents, biochar and activated biochar stands out for being versatile materials rich in carbon produced from thermochemical processes through different types of biomasses, such as orange peel (Chen et al. 2022), banana peel (Ahmadi & Ganjidoust 2021), and rice husk (Leal da Silva et al. 2021). In addition, biochar generated from biomass pyrolysis is a low-cost, ecologically correct, and renewable material to adsorb heavy metals (Wu et al. 2019), nutrients (ammonium, nitrate, and phosphate) (Dai et al. 2020), dyes (Praveen et al. 2022), and organic contaminants from aqueous solutions (Hasanzadeh et al. 2022). However, the physicochemical properties of pristine biochar (pH, specific surface area, volatile matter, ash, and carbon content), which can significantly affect its pollutant adsorption capacity, are variable, mainly due to biomass species and pyrolysis operational conditions (Tomczyk et al. 2020). Generally, due to the negatively charged surface and contained little anion exchange capacity, biochar has a low capability to adsorb anions, such as phosphate (Bolbol et al. 2019). Therefore, the modification of biochar for improving the adsorption of anionic pollutants become an important practice for expanding the application of biochar technology.

To improve the adsorption capacity and functionality of biochars, lanthanum (La) (Liu et al. 2022), iron (Fe) (Shen et al. 2019), aluminum (Al) (Yu et al. 2019), magnesium (Mg) (Jiang et al. 2018), and calcium (Ca) (Xiong et al. 2017) have been used to modify the waste-derived biochar via simple impregnation or synthesis methods. These elements are selected because they have a strong affinity for phosphorus binding (Jasper & Sumithra 2020). Furthermore, the incorporation of layer double hydroxides (LDHs) in the biochar structure to remove phosphate, proved to promote a higher loading rate and more adsorption sites (Peng et al. 2019).

Layered double hydroxides (LDHs) are anionic clay with high anion sorption capacity that can be easily prepared in the laboratory by co-precipitation methodology (Shi et al. 2020). They are host-guest materials consisting of positively charged metal hydroxide sheets with intercalated anions and water molecules, and are generally represented as [M⁺²_1−xM⁺³_x(OH)₂]^+xA⁻ⁿ_x/n·mH₂O, where M⁺² is a divalent metal, M⁺³ a trivalent metal and A⁻ⁿ, an anion n valent; usually where the ratio between M⁺²/M⁺³ is 0.1 ≤ x ≤ 0.5 molecules (Zhang et al. 2021). Common divalent and trivalent cations used to construct LDH are Ca²⁺, Mn²⁺, Mg²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Al³⁺, Cr³⁺, Fe³⁺, respectively (Almojil & Othman 2019). However, LDHs directly used as adsorbents can be exfoliated during adsorption. Therefore, it is more effective to use it in the form of composites with recalcitrant materials, such as biochar (Meili et al. 2019).

However, pristine biochar may have a limited surface area, leading to a smaller amount of LDH impregnated in its structure (Missau et al. 2021b). Therefore, a physical activation procedure with CO₂ can be applied as a biochar pre-treatment, before the LDH impregnation step, as it offers a large specific surface area (Ma et al. 2020). Various studies have already reported the use of different LDHs supported onto biochar to remove phosphate from aqueous solutions through the adsorption process such as MgFe (Rahman et al. 2021), MgAl (Alagha et al. 2020), and FeAl (Peng et al. 2019). Nevertheless, to the best of our knowledge, no available research has been conducted to investigate the adsorption of the phosphate by an adsorbent developed from CaFe/LDH supported onto previously treated biochar with CO₂ physical activation.

In this way, the main objective of this study was to enhance phosphate adsorption using a CaFe/LDH supported on physically activated biochar with CO₂. The newly synthesized adsorbent was characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET), and scanning electron microscopy (SEM) as measurement techniques to examine the surface and structure properties after the inclusion of CaFe/LDH layers into activated biochar. In the adsorption assays, the effects of pH (2.0–10.0) and adsorbent dosage (0.5–1.5 g L⁻¹) were evaluated. Kinetic curves were studied using pseudo first order (PFO), pseudo second order (PSO), pseudo n order (PNO), and Elovich models. Isotherms were constructed at four different temperatures (25, 35, 45, and 55 °C) and the curves were fitted with Langmuir, Freundlich, and Temkin models. The standard values of Gibbs free energy (ΔG⁰), enthalpy (ΔH⁰), and entropy (ΔS⁰) changes were estimated. In addition, the effect of competitive anions and reusability of the CaFe/LDH supported on physically activated biochar with CO₂ were also studied.

Eucalyptus saligna sawdust was kindly donated by the company Madeireira Haas (Venâncio Aires, Rio Grande do Sul, Brazil). In the present work the following analytical reagents grade were used: calcium chloride dihydrate (CaCl₂.2H₂O, Mw = 147.01, Dinâmica/Brazil), iron chloride hexahydrate (FeCl₃.6H₂O, Mw = 270.29, Dinâmica/Brazil), sodium hydroxide (NaOH, Mw = 39.99, Dinâmica/Brazil), hydrochloric acid (HCl 37%, Mw = 36.46, Dinâmica/Brazil), sodium sulfate (Na₂SO₄, Mw = 142.04, Alphatec/Brazil), sodium bicarbonate (NaHCO₃, Mw = 84.00, Alphatec/Brazil), and sodium carbonate (Na₂CO_3, Mw = 105.98, Dinâmica/Brazil). Synthetic phosphate solutions were prepared in various concentrations by dissolving potassium dihydrogen phosphate (KH₂PO₄, Mw = 136.08, Química Moderna/Brazil).

The chemical groups present in the adsorbent samples were identified by spectroscopy Fourier transform infrared spectroscopy (FT–IR) (Shimadzu, Prestige 21, Japan). X-ray powder diffractometry (XRD) (Rigaku, Miniflex 300, Japan), with Ni–filtered CuKα radiation (λ = 1.54051 Å, 30 kV, 10 mA), using 2θ = 5–100°, was performed to analyze adsorbents crystallinity. BET method was applied to determine the surface area of the samples, using N₂ adsorption isotherms at 77 K (New Win, Quantachrome, USA). Finally, the prepared samples' morphology was studied using scanning electron microscope (SEM) through VEGA-3 SBU equipment (TESCAN, Czech Republic) in an acceleration of 10 kV with gold coating treatment.

The production and activation of the biochar were performed in a 310 stainless-steel cylindrical reactor, with an intern volume of 0.030 m³, placed in a tube furnace (Sanchis/Brazil) as previously reported (Missau et al. 2021b). The pyrolysis parameters were based on preliminary tests considering the volume of the reactor and the thermogravimetric curve of the eucalypt sawdust in a previous study (Missau et al. 2021a). Therefore, pyrolysis operational conditions were fixed at 550 °C, for 120 min under N₂ atmosphere with a heating rate of 15 °C min⁻¹. Also based on preliminary tests and previous studies, the CO₂ biochar activation operational conditions were maintained at 900 °C, for 180 min under N₂/CO₂ atmosphere with a heating rate of 25 °C min⁻¹ (Lan et al. 2019).

In this way, the reactor was packed with biochar, placed inside the tube furnace, and heated from room temperature to the desired activation temperature under N₂ atmosphere (heating rate of 25 °C/min). Then, when the desired activation temperature (900 °C) was reached, CO₂ was allowed to flow into the reactor and the furnace was kept at the defined temperature for the required activation time. After the activation period was completed, the flow of CO₂ was stopped and the activated carbon product was cooled to room temperature under the flow of N₂.

The co-precipitation method was applied to synthesized CaFe/biochar (CO₂) adsorbent (Hoxha et al. 2020) at room temperature (25 °C). The production of the adsorbent material started with the dissolution of 8.820 g of calcium chloride and 8.108 g of iron chloride in 40 mL of ultrapure water (resistivity of 18.25 MΩ·cm⁻¹) for 30 min (Park et al. 2021). A proportion of 2 mol of Ca and 1 mol of Fe (2:1) was fixed. Then 2 g of physically activated biochar with CO₂ was added to the previous mixture and a sodium hydroxide solution (3 M) was dripped until the final mixture reached a pH = 11. After being stirred constantly for a further 24 h, the solid product was vacuum filtered, washed with ethanol and ultrapure water to remove unwanted residues, and posteriorly dried (24 h at 105 °C). To complete the synthesis, the dried solid material was macerated and sifted (average diameter < 0.50 mm).

The main parameters of the adsorption process, such as pH, adsorbent dosage, adsorption kinetics, and equilibrium profile were evaluated. This study is important to highlight the optimal conditions for phosphate removal with CaFe/biochar (CO₂). Batch adsorption experiments were carried out with a shaker incubator (Tecnal – TE 4200, Brazil), under constant stirring at 200 rpm.

The pH effect was analyzed ranging from 2 to 10. These values were adjusted with 0.1 mol L⁻¹ NaOH and HCl (Liu et al. 2022). For this study, 1 g L⁻¹ of adsorbent was added to 25 mL of a 25 mg L⁻¹ phosphate solution and kept under constant stirring for 240 min at 25 °C. Finally, the samples were filtered and the phosphate concentration of the solution was measured by a UV–visible spectrophotometer (Shimadzu, UVmini-1240, Japan) at a wavelength of 880 nm (Jiang et al. 2018). The point of zero charge (pH_pzc) of the CaFe/biochar (CO₂) was determined to establish the ideal pH for phosphate adsorption. The methodology involved was previously published (Loulidi et al. 2020).

Since real wastewater usually contains a mixture of different constituents, it is important to study the competing adsorption of common coexisting anions (Xu et al. 2019). Therefore, three commonly co-existing anions were selected including chloride (Cl⁻), carbonate (CO₃²⁻), and sulfate (SO₄²⁻) aiming to explore the effect of competitive anions on phosphate adsorption (Wang et al. 2020). The effect of ions was first explored individually and then simultaneously at a concentration of 0.1 M. Solution without co-existing anions was used as a blank test (Li et al. 2016). Experiments were performed with an adsorbent dosage of 0.93 g L⁻¹ mixed in 25 mL of phosphate solution (pH 2.15) at a concentration of 50 mg L⁻¹ and 25 °C with a constant speed of 200 rpm for 2 h.

The evaluation of adsorbent regeneration performance is vital for practical applications. Therefore, to determine the reusability of the tested adsorbent, regeneration cycles were carried out on phosphate-loaded samples. Firstly, adsorption was performed with a 25 mg L⁻¹ phosphate solution under constant stirring at 200 rpm for 2 h at 25 °C with an adsorbent dosage of 0.92 g L⁻¹ and a solution pH = 2.15. Solutions were then filtered and recovered. CaFe/biochar (CO₂) was over-dried at 80 °C for 2 h and was added into 1 M NaOH solution, with a dosage of 5 g L⁻¹, and stirred for 1 h at 25 °C (200 rpm) (Rahman et al. 2021). This CaFe/biochar (CO₂) was used in another adsorption test, which was repeated four times.

Specific surface areas were 6.92 m²/g for pristine biochar, 251.43 m²/g for biochar (CO₂), and 156.28 m²/g for CaFe/Biochar (CO₂). After activation, the surface area of biochar dramatically increased by more than 30 times via CO₂ activation. This result is related to a strong interaction in the CO₂-carbon reaction (Maniscalco et al. 2020). However, when LDH was supported onto biochar the surface area decreased, which indicated that the voids on the surface of the biochar were filled with the loaded nano-material CaFe/LDH. This is because after loading the layered double hydroxides, the pores on the biochar were filled, and the BET surface area of the material became smaller (Huang et al. 2019). This behavior was further confirmed by the SEM images.

Results showed that the adsorption capacity decreased with increasing pH. This trend is attributed to the pH-dependent distribution of phosphate species in the solution and the point zero charge of adsorbent (pH_pzc) (Jiang et al. 2018). The predominant phosphate species are H₂PO₄⁻ in the pH range of 2.15–7.20, and HPO₄²⁻ in the pH range of 7.20–12.33, respectively (Zhang et al. 2019). Compared with HPO₄²⁻, H₂PO₄⁻ possessed lower adsorption free energy and is more easily adsorbed (Jung et al. 2016). Furthermore, when the solution pH was lower than pHpzc (7.35), the surface of CaFe/Biochar (CO₂) samples became protonated and more positively charged with decreasing pH value, further bonding with phosphate ions through electrostatic attraction (Li et al. 2020). The results are in accordance with other works found in the literature (Yang et al. 2018, 2020). Consequently, it was selected the pH of 2.15 of the phosphate solution to perform the adsorbent dosage, kinetic, and isotherm experiments.

Many kinetic models can be used to describe the adsorption kinetic curves. In this article, four models were considered: PFO, pseudo second order (PSO), pseudo n order (PNO), and Elovich. The kinetic parameters for phosphate adsorption onto CaFe/Biochar (CO₂) are presented in Table 1. As can be seen, based on the higher values of determination coefficient (R² > 0.9940), adjusted determination coefficient (R²_adj > 0.9926), and in the lower values of average relative error (ARE < 1.8248%) the experiment data fit with PNO kinetic model better than the others. In general, the PNO model provides different values of n (order of adsorption rate) when the initial concentration of adsorbate changes, corroborating the experimental findings (Wamba et al. 2017).

The Langmuir and Freundlich isothermal models were selected because they are commonly used to examine the process of phosphate transfer from the liquid to biochar surfaces (Faheem et al. 2020). The Temkin model presumes that adsorption is a multi-layer process (Wang & Guo 2020). Therefore, for this reason, the Temkin isotherm model was also selected. Their obtained parameters are shown in Table 2. Considering the higher values of determination coefficient (R² > 0.99), adjusted determination coefficient (R²_adj > 0.99), and the lower values of average relative error (ARE < 11.44%), the Freundlich isotherm model revealed better fitting than the others for phosphate adsorption, elucidating multisite adsorption with a heterogeneous surface (Cordova Estrada et al. 2021). Furthermore, the depicted values of 1/nF indicated a highly favorable adsorption process of CaFe/biochar (CO₂) for phosphate (Wang et al. 2018). The best adsorption capacity was 99.95 mg g⁻¹ at 55 °C. Meanwhile, it should be noted that the adsorption capacity of CaFe/biochar (CO₂) for phosphate outcompeted most of the previously reported adsorbents, as presented in Table 3. In this way, it shows that LDH (CaFe) supported onto physically treated biochar with CO₂ can be used as a promising adsorbent for phosphate removal from aqueous solutions.

Aiming to understand and classify the adsorption of phosphate onto the CaFe/biochar (CO₂) adsorbent, the thermodynamic values were estimated for all investigated temperatures, as displayed in Table 4. The equilibrium constant (k°) was estimated from the Freundlich isotherm (Tran et al. 2017). It was found that the adsorption process is spontaneous and more favorable at high temperatures, based on the negative ΔG⁰ values. Based on the ΔH° magnitude, it is possible to infer the adsorption mechanism. The positive value of ΔH° (lower than 20 kJ mol⁻¹) indicates endothermic adsorption, and also that the ion exchange during the adsorption was physical, more exactly, Van der Waals interactions (Tran et al. 2016). The positive value of ΔS⁰ indicates that some rearrangements occurred in the solid-liquid interface during the adsorption process (Silva et al. 2018). These results corroborate with the isothermal profiles.

According to other studies, phosphate has been considered to form inner-sphere complexes with metal hydroxides, whereas chloride often was weakly bound with surface sites of metal hydroxides forming outer-sphere complexes, which led to ineffective competition (Lu et al. 2014; Li et al. 2016). To explain the behavior that SO₄²⁻ and CO₃²⁻ ions had on phosphate adsorption, their ionic radius must be taken into account (H₂PO₄⁻ − 0.238 nm; SO₄²⁻ − 0.230 nm; and CO₃²⁻ – 0.189 nm) (Herrera et al. 2022). The decrease in phosphate adsorption by sulfate ions was likely due to the strong competition of this ion for the binding sites on the adsorbent surface, proper to their similar ionic radius. On the other hand, the carbonate ion did not show significant competition in phosphate adsorption, possibly because its ionic radius is smaller than that of the phosphate ion. In addition, the results of the ion mixture showed that the adsorbent has good potential for utilization, as it maintained a satisfactory phosphate removal, approximately 70%. The results were consistent with other study to some extent (Li et al. 2020).

In summary, the results demonstrated that the impregnation with CaFe/LDH onto biochar physically activated with CO₂ was satisfactory, as it increased the removal of phosphate from the aqueous solution. Therefore, this work showed that CaFe/biochar (CO₂) can be used to minimize environmental problems, such as eutrophication. Furthermore, this new adsorbent can be a great potential material for the removal of phosphate in aqueous solutions.

To enhance phosphate removal to minimize environmental problems, such as eutrophication, this work developed a new adsorbent material based on CaFe/LDH supported onto a physically activated biochar with CO₂.

Eucalyptus saligna sawdust was used to obtain the biochar by a pyrolysis process, which was the feedstock to produce the activated biochar with physical activation (CO₂).

Finally, the CaFe/LDH was supported onto physically activated biochar by co-precipitation methodology. This new adsorbent was employed as a potential adsorbent to remove phosphate from the aqueous solution.

The phosphate adsorption was favored at a solution pH of 2.15 and an adsorbent dosage of 0.93 g L⁻¹. The kinetic profiles for the different initial phosphate concentrations indicated that the process rapidly reached equilibrium, when compared to others adsorbent materials already published in the academic literature, and it was well represented by the PNO model. The equilibrium was in a better fit with the Freundlich model. According to the thermodynamic study, the adsorption was spontaneous, favorable, and endothermic, with a better adsorption capacity of 99.55 mg g⁻¹ obtained at 55 °C.

Therefore, the novel material prepared in this work could be employed as an alternative adsorbent to remove phosphate from aqueous and minimize environmental issues.

The authors are grateful for the financial support provided by CNPq (National Council of Scientific and Technological Development), CAPES (Brazilian Agency for Improvement of Graduate Personnel), FAPERGS (Foundation for Research Support of the State of Rio Grande do Sul), SDECT (Department of Economic Development, Science and Technology of the State of Rio Grande do Sul) and MADEIREIRA HAAS LTDA.

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

The authors declare there is no conflict.