Mina Alikhania,
Ehsan Khoshkalam*b,
Jalal Sadeghib,
Laura Bulgariuc and
Hossein Eshghi
d
aDepartment of Chemistry, Payame Noor University, Tehran, Iran
bDepartment of Soil Science, Faculty of Agriculture, Ferdowsi University of Mashhad (FUM), Mashhad, Iran. E-mail: e.khoshkalam@alumni.um.ac.ir; Fax: +985138807147; Tel: +989104050217
cDepartment of Environmental Engineering and Management, Cristofor Simionescu Faculty of Chemical Engineering and Environmental Protection, Gheorghe Asachi Technical University of Iasi, 700050 Iasi, Romania
dDepartment of Chemistry, Faculty of Science, Ferdowsi University of Mashhad (FUM), Mashhad, Iran
First published on 5th August 2024
The primary goal of this study is to examine PO43− adsorption from aqueous solutions using zinc-doped carbon dots (Zn-N-CDs) as a new adsorbent and cost-effective technique. Zn-N-CDs were produced through a hydrothermal process and subsequently identified using various techniques. The effect of reaction time, temperature, pH, ionic strength, adsorbent dosage, initial PO43− concentration, and anion competition (NO3−, Cl−, HCO3−, and SO42−) on PO43− adsorption using Zn-N-CDs were investigated. The characterization results depicted that Zn-N-CDs have a spherical structure without obvious aggregation and revealed the amorphous nature of carbon dots with many pores. Zn-N-CDs had a high affinity for adsorbing PO43−, reaching equilibrium within 5 minutes. While PO43− adsorption reduced with an increase in temperature, it increased with pH and ionic strength. The optimal conditions for PO43− adsorption were determined to be pH 8, 100 mM KCl as an ionic strength, and 5 g L−1 of Zn-N-CDs. The presence of SO42− and HCO3− as competing anions slightly decreased PO43− adsorption. Thermodynamic studies revealed that PO43− adsorption onto Zn-N-CDs was endothermic, spontaneous, and disordered, as evidenced by ΔG° < 0, ΔH° > 0, and ΔS° > 0. The experimental data fit well with a pseudo-second-order kinetic model (R2 = 0.999) and the Freundlich isotherm model (R2 = 0.9754), signifying that PO43− adsorption onto Zn-N-CDs occurs through multi-layer and chemi-sorption mechanisms. Overall, Zn-N-CDs indicated a great capability to adsorb high concentrations of PO43− across a wide range of pH values, indicating their potential for environmental remediation.
Over the last few decades, various techniques, including biological, chemical, and physical treatment processes have been utilized to remove PO43−. However, these methods have become expensive and ineffective due to the production of sludge, the introduction of chemical reagents to the environment, and the generation of secondary harmful materials during the treatment process.4 The biological technique, for instance, takes a long time and results in biological sludge. Additionally, some decontamination techniques, including chemical reduction, may produce hazardous byproducts.5 To address these issues, scientists have turned their attention to adsorption treatment for water and wastewater purification. Adsorption techniques have proven to be highly efficient, sensitive, and selective in removing various organic and inorganic pollutants from water and wastewater.1,2,4 Different types of adsorbents, including industrial and agricultural wastes,6 polymeric exchangers,7 EL-MNP@Zeolite,8 La-CTS-ATP,9 hydrotalcite,10 and LDH11 have been used for PO43− removal. However, the high cost of producing and consuming these compounds, the multiple steps involved in the synthesis process, introducing harmful chemicals to the environment and the low efficiency in PO43− adsorption are the disadvantages of such adsorbents.12,13 Therefore, in order to be applicable at full scale, it is desirable to have environmentally friendly adsorbents.
The use of nanotechnology with the ability to produce nanoparticles with superior properties has been considered in surface absorption studies.14,15 Among different nanoparticles, carbon based materials, including carbon dots (CDs) and graphene, have emerged as alternatives to conventional adsorbent materials. Carbon dots have been receiving increased interest due to their distinct optical properties, low toxicity, simple synthesis, extremely small size, high biocompatibility, and cost-effective precursors.13,16,17 Owing to their distinctive physicochemical characteristics and the presence of numerous hydrophilic functional groups, CDs provide a multitude of surface adsorption sites to remove pollutants from the environment.18,19
Carbon dots have been utilized in numerous studies for the removal of heavy metals.20–22 The capability of carbon dots to adsorb heavy metals primarily depends on their chemical properties, particular surface area, pore volumes, and the existence of functional groups.22 Researchers have argued that the adsorption process of metal ions can occur due to physiosorption, electrostatic attraction and chemical sorption.13 Among these reactions, electrostatic attraction is considered pivotal in the adsorption mechanism between carbon dot-based adsorbents and metal ions, primarily due to the abundance of oxygen-containing groups with negative charges on the surface of these materials.13 However, carbon dots have not been used as an adsorbent for removing PO43−, and the reaction mechanism of these compounds with PO43− ions remains unknown.
On the other hand, some research has reported that the combination of metal ions with carbon-based materials and LDH can generate additional surface functional groups and increase the efficiency of PO43− adsorption.1,10,11 For instance, Nakarmi et al.1 by synthesized a zinc oxide-biochar nanocomposite, which removed 265.5 mg g−1 of PO43− from water solutions. In another research, doping Mn2+/Zn2+/Fe3+ oxy(hydroxide) onto LDH, drastically increased the PO43− adsorption, resulting in the adsorption of 82.3 mg g−1 of PO43−. Furthermore, Koilraj and Kannan10 argued that the presence of zinc ions in the ZnAl hydrotalcite could enhance PO43− adsorption during chemi-sorption mechanisms.
Furthermore, the recent study showed that modifying carbon dots with zinc ions can alter their physical and chemical properties.23 Based on our findings, so far no studies have been conducted regarding the adsorption of PO43− by these types of nanoparticles. Therefore, to provide a precise answer to the question of whether the modification of carbon dots with zinc ions has the ability to retain PO43− or not, this study aimed to prepare zinc-doped carbon dots (Zn-N-CDs), as a novel adsorbent for PO43− adsorption from water solutions. The main objectives of this research were: (1) synthesizing and characterization of Zn-N-CDs, (2) evaluating PO43− adsorption using Zn-N-CDs at different pH, electrolyte concentrations, and temperatures to determine the optimum dosage of the adsorbent, (3) investigating the kinetics, isotherms and thermodynamics studies of PO43− adsorption, and (4) understanding the main mechanisms of PO43− adsorption using Zn-N-CDs.
In continue, the experiment was done in different pHs (2–12). The pH of solutions was adjusted using HCl/NaOH 0.1 M. Then, the optimum pH was used for further batch experiments. For investigating the effect of ionic strength, the adsorption PO43− was tested at different KCl salt concentrations. The best ionic strength was then selected for determining of adsorbent dosage for PO43− removal. Additionally, the effect of different background solutions, including KCl, NaCl, and CaCl2, on PO43− adsorption was evaluated. Finally, the effect of initial PO43− concentration (50–500 mg L−1) was evaluated under all optimal conditions. After sorption, all samples were centrifuged (10000 rpm, 5 min) and the equilibrium PO43− concentration was measured (using UV-spectrophotometer at a wavelength of 880 nm) at their supernatant.5 The majority of the trials were done in triplicate. eqn (1) and (2) were used to achieve PO43− adsorption in percent (R%) and qt (mg g−1), respectively.5,10
![]() | (1) |
![]() | (2) |
![]() | (3) |
![]() | (4) |
qt = kpt1/2 + C | (5) |
In these equations, the terms qt, qe, k1, k2, kp, C, and t stand for the amounts of ions adsorbed at time t (mg g−1), equilibrium time (mg g−1), pseudo-first-order rate constant (min−1), pseudo-second-order rate constant (mg min−1), intra-particle diffusion rate constant (mg g−1 min−1), intercept, and time (min), respectively.
In order to determining accurately adsorption capacity of Zn-N-CDs for PO43− removal, adsorption data was fitted to the Freundlich (eqn (6)), Langmuir (eqn (7)), and Temkin (eqn (8)) isotherm models.1
![]() | (6) |
Ce/qe = Ce/b + 1/KLb | (7) |
![]() | (8) |
The equilibrium adsorption capacity and concentration of PO43− at equilibrium are denoted by qe (mg g−1) and Ce (mg L−1), respectively. The experimental constants KF and 1/n are used in the Freundlich equation. The terms b (mg g−1) and KL in the Langmuir equation refer to the highest potential for adsorption and the constant of binding energy, respectively. bT, KT are the Temkin isotherm constants. R is the gas constant (J mol−1 K−1) and T is the temperature (K). RT/BT is the heat of adsorption (J mol−1).
The adsorption process of PO43− on the Zn-N-CDs was evaluated by calculating thermodynamics parameters including ΔG° (Gibbs free energy, kJ mol−1), ΔH° (enthalpy change, kJ mol−1), and ΔS° (entropy change, J mol−1 K−1). These parameters were calculated as follow (eqn (9) and (10)).11
![]() | (9) |
ln(Kd) = −ΔH°/RT + ΔS°/R | (10) |
After the PO43− adsorption, there was a shift towards lower wavelengths (Fig. 1a), which could have been caused by a chemical interaction resulting in the formation of complexes containing acidic oxygen and dehydration.29 Adsorption of PO43− significantly reduced the intensity on Zn–O stretching vibration, indicating the interaction between PO43− and the surface functional groups of Zn-N-CDs. Additionally, it was observed that a new band emerged at 1050 cm−1, which was attributed to the presence of new C–O–P bonds in the structure of Zn-N-CDs.11 Additionally, the peak at 630 cm−1 supported the Zn–O–P stretching vibration and demonstrated that P and the Zn-doped carbon dots can interact during of the adsorption process.30
Fig. 1c depicts the fluorescence emission spectra of Zn-N-CDs at various excitation wavelengths (310–420 nm). The difference in the excitation wavelength only resulted in a rapid decline in the intensity of the emission wavelength, with no alteration in peak position. Hence, the findings indicate that the excitation wavelength influences the fluorescence intensity of the Zn-N-CDs. Also, this observation demonstrates the remarkably uniform surface structure and narrow size distribution of Zn-N-CDs.32 In addition, the maximum fluorescence intensity was obtained at λex = 360 nm. Furthermore, an assessment of the quantum yield (QY) of the Zn-N-CDs indicated that it exhibited a significantly high QY. The QY of Zn-N-CDs using quinine sulfate (QY = 0.54) as standard was 47.54% at λex = 360 nm. Details about this investigation are listed in Table S1 on ESI.† The quantum yield (QY) achieved for the Zn-N-CDs is deemed satisfactory for analytical applications.
The TEM images of Zn-N-CDs showed spherical particles that were uniformly dispersed without obvious aggregation (Fig. 2S†).33 The diameter of the Zn-N-CDs varied between 4 and 5 nm, and an average diameter of 4.3 nm was determined using the particle size distribution plot and fitting the histogram to a Gaussian model (Fig. 2S†). Also, the particle size of Zn-N-CDs was determined to be about 17 nm by using DLS measurement (Fig. 3S†). The hydrodynamic diameter of particles in a solvent is typically larger than the particle size measured in a vacuum. The presence of a hydration layer on the Zn-N-CDs surface in an aqueous solution leads to an increase in particle size.34
The results of electron disperse X-ray (EDS) analysis before and after adsorption of PO43− (500 mg L−1) are shown in Fig. 2c and d. The results depict that there were decreases in carbon (38.26–13.94 wt%); chlorine (20.19–5.11 wt%) and increases in oxygen (21.42–33.68 wt%) and zinc (8.84–29.61 wt%) contents. After the PO43− adsorption, the reduction of the chloride content occurred due to the anions exchange between PO43− and chloride from the surface of Zn-N-CDs.11 The increase in the weight percentage of zinc (from 8.84 to 29.61 wt%) after PO43− adsorption suggests that some of the zinc ions in the aqueous solution may have been adsorbed onto the surface of the carbon dots along with the PO43−, which will be discuss in the following sections. Furthermore, the presence of P elements (8.49 wt%) and the increase in oxygen percentage (from 21.42 to 33.68 wt%) indicate the adsorption of PO43− ions onto the absorbent surface.
According to the BJH analysis, the carbon dots exhibited a surface area of 107.17 m2 g−1 and a total pore volume of 0.18 cm3 g−1. The average pore size was observed to be 6.76 nm. As shown in Fig. 3a, the adsorption–desorption isotherms can be classified as type IV according to the Brunauer–Deming–Deming–Teller (BDDT) classification, with a prominent hysteresis loop at medium pressure (Fig. 3a). This suggests the presence of a mesoporous structure within the carbon dots.36
The high surface area and mesopores in the Zn-N-CDs provide ample adsorption sites and efficient mass transfer, respectively, which are crucial for effectively removing PO43− from aqueous environments. The mesoporous structure facilitates the diffusion of phosphate-containing species into the porous network, while the large surface area enhances the adsorption capacity of the Zn-N-CDs.
The experimental results were described by three noticeable kinetic models (Table 1). A key element in establishing which kinetic model is suitable for explaining experimental results is the regression coefficient (R2).5 As a result, the high regression coefficient (0.999) pseudo-second-order model was better suited to describe data. At 21, 35, and 45 °C, the regression coefficients of the pseudo-second-order model were significantly higher than those of the pseudo-first-order and intra-particle diffusion kinetic models. This strong match indicates that chemisorption interactions which entail the creation of chemical bonds between the adsorbate and the adsorbent are primarily responsible for the PO43− adsorption onto Zn-N-CDs.8,9 Furthermore, comparing the adsorption capacity predicted by the kinetic models (qe.cal) with the adsorption capacity determined from experimental data (qe.exp) is also important.5 Table 1 shows that the qe.cal values for the pseudo-second-order at temperatures 21, 35, and 45 °C are 112.36, 100, and 79.37 mg g−1, respectively, which are much closer to qe.exp values obtained from experimental data. In addition, at 21 °C, 35 °C, and 45 °C, the pseudo-second-order kinetic model's rate constants (K values) were found to be 0.038, 0.033, and 0.023, respectively. The pseudo-second-order model's lower K values emphasize the relative speed of the adsorption process in addition to demonstrating the model's adequacy in explaining the experimental results.8 Therefore, the high regression coefficient, adsorption capacity (qe.cal), and lower K values of the pseudo-second-order indicate that the PO43− adsorption by Zn-N-CDs follows the pseudo-second-order kinetic equation. Comparable results for green-synthesized iron nanoparticles,8 Zinc Oxide Betaine-Modified Biochar Nanocomposites (ZnOBBNC),1 halloysite nanotubes (HNTs),3 and Mg-Al-LDH11 have been documented in the literature.
Pseudo-first-order | Pseudo-second-order | Intraparticle diffusion | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
T (°C) | qe.exp | qe.cal (mg g−1) | K1 (min−1) | R2 | qe.cal (mg g−1) | K2 (g min−1 min−1) | R2 | C | Kp (mg g−1 min−1/2) | R2 |
21 | 110.62 | 5.37 | −0.0003 | 0.1615 | 112.36 | 0.038 | 0.999 | 65.553 | 6.6668 | 0.3305 |
35 | 98.02 | 7.09 | −0.0003 | 0.2271 | 100 | 0.033 | 0.999 | 57.902 | 6.0064 | 0.3393 |
45 | 79.46 | 10.84 | −0.0004 | 0.4255 | 79.37 | 0.023 | 0.999 | 43.45 | 4.9799 | 0.3841 |
T (K) | ΔG° (KJ mol−1) | ΔH° (KJ mol−1) | ΔS° (J mol−1 K−1) | |||||||
294 | −16.16 | 0.163 | 54.14 | |||||||
308 | −15.87 | |||||||||
318 | −14.8 |
Table 1 shows the thermodynamic parameters of PO43− adsorption onto Zn-N-CDs. The negative values of ΔG°, at 21 °C, 35 °C, and 45 °C specify that the PO43− adsorption onto Zn-N-CDs is a spontaneous process. This means that the process may occur without the need for external energy input, emphasizing an essential aspect of efficient adsorption processes. As the temperature increased, the ΔG° values decreased, indicating that higher temperatures were less favorable for adsorption. The endothermic feature of adsorption was confirmed by the positive result of ΔH° (0.163). The increase in randomness at the solid/solution interface during the adsorption of PO43− onto Zn-N-CDs is indicated by the positive value of ΔS° (54.14).8,11,38,39 As is common for adsorption events where new configurations between adsorbate and adsorbent are produced, these results suggest that the adsorption process is not only energy-dependent (endothermic) but also leads to a more disordered state at the interface.1,8,11 These finding also align with the adsorption of PO43− onto other adsorbents, such as Mg-Al-LDH,11 IONP,38 FeMg2Mn-LDH,39 and green-synthesized iron nanoparticles.8
As FT-IR spectrum (see Fig. 1a), Zn-N-CDs contain numerous active adsorption sites such as –OH, –NH2, and zinc hydroxide. These functional groups have a decisive role in PO43− adsorption.1,4,10,11 In acidic pH, the functional groups can be protonated, which helps to retain PO43− electrostatically.1 However, the surface zeta potential of the adsorbent at different pH (Fig. 2e) depicts that the Zn-N-CDs contain negligible surface positive charge at pH 2 and at pH 4 (also known as pHpzc), and the net surface charge is almost neutral. Therefore, the capability of the adsorbent to adsorb PO43− electrostatically, at highly acidic pH, is relatively low and the Zn-N-CDs, in a wide variety of pH, including negative surface charge. It means that increasing pH accelerates the deprotonation of the functional groups of the adsorbent, and the net surface charge is negative. For this reason, electrostatic attraction might not be the main mechanism to remove PO43− from the solution. On the other hand, by increasing pH and deprotonation of the surface functional groups, it is expected that the electrostatic repulsion force between PO43− and Zn-N-CDs increases, which can reduce the adsorption of PO43− on the negative surface charge of particles. However, by increasing pH, the amount of adsorption dramatically increased. Therefore, to explain this occurrence, other possible interactions between adsorbent and adsorbate should be mentioned.
Accordingly, the distribution of PO43− species in a solution as a function of pH is crucial (eqn (11)–(13)).3,7 At pH = 2, the dominant species is H3PO4. Meanwhile, the H3PO4 is uncharged, the low adsorption capacity at highly acidic pH could attributed to this phenomenon.1,9 By increasing pH, the concentration of H2PO4− and HPO42− gradually increase. Since, the net surface charge of Zn-N-CDs is negative, PO43− adsorption mainly occurs through chemisorption interactions,1,10,40 which will be discussed in the following lines.
H3PO4 ↔ H2PO4− + H+pKa1 = 2.15 | (11) |
H2PO4− ↔ HPO42− + H+pKa2 = 7.2 | (12) |
HPO42− ↔ PO43− + H+pKa3 = 12.35 | (13) |
Generally, metal oxide (hydroxides) have high affinity to adsorb PO43− ions.10 PO43− could react with zinc hydroxides through ligand exchange. In addition, active sites of zinc hydroxides and PO43− ions act as Lewis base and Lewis acid, respectively. As a result of these reactions, Zn–O form bonds with oxygen ions of PO43−. Consequently, monodentate-mononuclear (MM), bidentate-mononuclear (BM), and bidentate-binuclear (BB) inner-sphere complexes could be formed (eqn (14)–(16)).10,11 The fact that PO43− adsorption increases with increasing pH might therefore be due in part to these processes. Additionally, inner–sphere complex interactions could raise the equilibrium pH by releasing OH ions into the solution.2,11 However, based on Fig. 4b, the equilibrium pH remained steady during the adsorption process. This discrepancy might be explained through the role of zinc and PO43− ions in the solution. Exploring these roles could unravel the main mechanisms of PO43− adsorption.
≡Zn–OH + H2PO4− ↔ ≡Zn–H2PO4− + OH− (MM) | (14) |
≡Zn–(OH)2 + HPO42− ↔ ≡Zn–HPO4 + 2OH− (BM) | (15) |
(≡Zn)2–OH + HPO42− → (≡Zn)2–HPO4 + OH− (BB) | (16) |
Most likely, a portion of the zinc cations might release into the solution from the carbon dots structure. The EDS spectrum (see Fig. 2c and d) also confirmed that the percentage of zinc ions in the solution, during the adsorption of PO43−, was increased. Free zinc cations are unstable in the solution, which are quickly surrounded by water molecules, resulting in the formation of water–zinc complexes. Proton ions generated by these processes could buffer the solution (eqn (17) and (18)).41 On the other hand, dissociation of PO43− ions, due to increasing pH, might help to buffer the system.7
[Zn(H2O)6]2+ + H2O ↔ Zn(OH)aq+ + H+ | (17) |
Zn(OH)aq+ + H2O ↔ Zn(OH)2(aq) + H+ | (18) |
By improving pH from 2 to 8, the concentration of zinc species in the solution such as Zn(OH)+ and Zn(OH)2 gradually increase.41 The adsorbed PO43− on the Zn-N-CDs surface, might act as a new adsorption site for zinc hydroxides in the solution forming a surface precipitate. Meanwhile, the zinc ion that has been adsorbed could play as a sorption site for the PO43− which is still in the solution.10,11 As a result of these reactions (anion exchange, complexation, and precipitation), hopeite (Zn3(PO4)2·4H2O) can develop on the surface of Zn-N-CDs structure.10 In addition, there is the possibility of ion exchange between PO43− in the solution and chloride ions that are adsorbed onto the Zn-N-CDs (eqn (19)) [5]. These interactions and possibilities indicate that the adsorption of PO43− using the synthesized particles might occurred through multi-layer adsorption, which will be discussed in the Sections 3–6. Furthermore, in another study, when Zn-Mg-Al-LDH was utilized to remove PO43, zinc ions played a crucial role in the buffering of a system and multi-layer adsorption of PO43−.
≡2CD–Zn–OH + ZnCl2 + 3H2PO4− → ≡3CD–Zn–H2PO4 + 2Cl− + 2OH− | (19) |
The results of FT-IR spectrum, FESEM, and EDS spectrum confirmed these potential mechanisms. Moreover, PO43− can react with monovalent and divalent cations in the solution and form precipitation on the surface of Zn-N-CDs, which will be discussed in the following section. After reaching pH 8, the amount of PO43− adsorption drastically decreased. Indeed, in basic pH, the functional groups on the surface of Zn-N-CDs are dissociated, leading to an increase in the negative charge on the surface. As a result, the adsorption of PO43− is reduced due to the enhanced repulsion between the negatively charged surface of Zn-N-CDs and the phosphate ions.1,9 Furthermore, OH− ions in the basic pH, compete with PO43− ions on active adsorption sites.9 Other research also confirmed this possibility. When HA520E-Fe (hybrid polymeric anion exchanger impregnated with hydrated Fe(III) oxide) was used to remove PO43−, by increasing pH, the adsorption process also increased; however, after pH 7 the efficiency of PO43− adsorption drastically decreased.7 By explaining the above phenomena and clarifying the interactions, the probable mechanisms of PO43− adsorption on Zn-N-CDs are illustrated in Fig. 5.
In addition, the PO43− adsorption capacity on Zn-N-CDs was evaluated in different background solution (concentration = 100 mM) including NaCl, KCl and CaCl2 salts (Fig. 6b). The results show that the PO43− adsorption in the presence of CaCl2 > KCl > NaCl. The adsorption capacity of PO43− in the CaCl2, KCl, and NaCl were acquired 148.89, 131.71, and 128.46 mg g−1, respectively. The surface charge density of the cations could explain this occurrence, which Na+ > K+ > Ca2+ has a surface charge density, respectively. Therefore, when these cations expose into the solution, they are surrounding with water molecules and from H2O-cations complexes. Any cation that has a lower surface charge density has a relatively smaller hydration radius. This means that the cation with a smaller hydration radius can better approach the surface of the adsorbent and neutralize the negative surface charge of the particles. The neutralization of the negative charge on the surface means the decline of the thickness of the electric double layer (EDL) of the absorbent, and also the reduction of the electrostatic repulsion force between the absorbent and the adsorbate.42,44 Ca2+ compared to K+ and Na+ has lower hydration radius, which at the solid–solution interface could decrease surface negative charge and electrostatic repulsion forces. Therefore, PO43− ions can easily approach the surface of the adsorbent to form chemical adsorption.44 This reaction was confirmed in a previous study by Zong et al.2 who investigated PO43− adsorption onto SCBC-La. They reported that increasing NaCl concentration as a background solution did not increase the adsorption capacity of PO43−. However, in another study, when Ca2+ ions were chosen as a background solution, the PO43− adsorption improved.42
As mentioned before, by decreasing the electrical double layer and approaching PO43− to the charged surface, the adsorption capacity increased. This issue leads to cascades of reactions including electrostatic adsorption, precipitation, and complexation.44 Based on literature, metal oxyhydroxides have high affinity for adsorbing Ca2+.42 The adsorption of Ca2+ on charged surface could stimulate the adsorption of PO43− and vice versa.42 The adsorbed Ca2+ by producing surface positive charge increase the electrostatic adsorption of PO43−. Furthermore, the Mendez & Hiemstra44 argued that ternary complex formation could occur on the surface of metal oxyhydroxides. Indeed, Ca2+ act as a bridge and cation-bridged ternary complex (M–Ca–PO4) could develop on the surface. On the other hand, by increasing pH, the PO43− surface speciation can change from monodentate-protonated to bidentate-deprotonated leading to increase the surface negative charge.42,44 Consequently, PO43− act as a bridge and PO4-bridged ternary complex could form (M-PO4-Ca).44 Furthermore, interaction of Ca-PO4 increase with rising pH and solid phase can precipitate as octa-Ca-PO4 (Ca4H(PO4)3·2.5H2O), β-tri-Ca-PO4 (Ca3(PO4)2), and hydroxyapatite (HPA) on the surface of Zn-N-CDs.44 Additionally, some researches claimed that both ions, Ca2+ and PO43−, are bonded monodentately to the charged surface, but there is also an additional chemical reaction (lateral interaction) between the adsorbed cations and anions.44,45
Based on the data obtained from the isotherm models (Table 2), the experimental data well fitted to the Freundlich model when R2 is 0.9754, and this value is higher than the R2 values of the Langmuir (R2 = 0.906) and the Temkin model (R2 = 0.8368). As mentioned above, the Freundlich model suggests that the surface of Zn-N-CDs is heterogeneous and that the adsorption of PO43− ions occurs in multiple layers.43 Additionally, the high value of KF (5.477) in the Ferundlich model indicates that PO43− could easily adsorb onto Zn-N-CDs. Furthermore, the 1/n value in the Ferundlich model indicates the adsorption intensity or surface heterogeneity. When 1/n > 1 and 1/n = 1, it presumes that the adsorption process is unfavorable and irreversible, respectively.43,47 However, when 0 < 1/n < 1 demonstrates that the adsorption process is favorable even at high concentrations of PO43−. Constantly, these obtained data specify that the Ferundlich model might clarify the adsorption mechanism. Since this model assumes multilayer adsorption, this assumption can confirm the previously discussed adsorption mechanisms (Section 3.3). As we mentioned before, PO43− ions adsorb onto the surface of Zn-N-CDs via electrostatic and/or ligand exchange. Furthermore, the adsorbed PO43− ions could serve as new adsorption sites for zinc ions in the solution. These processes can continue with PO43− adsorption by adsorbed zinc ions. These reactions involve multi-layer adsorption procedures, which are consistent with the information provided by the Freundlich model.
Isotherm models | ||||||||
---|---|---|---|---|---|---|---|---|
Freundlich | Langmuir | Temkin | ||||||
KF (mg g−1) | 1/n | R2 | Qmax (mg g−1) | KL (L mg−1) | R2 | BT (KJ mol−1) | KT | R2 |
5.477 | 0.550 | 0.9754 | 122.417 | 0.014 | 0.906 | 16.999 | 0.395 | 0.8368 |
![]() | ||
Fig. 7 The effect of competing anions on PO43− adsorption ([Zn-N-CDs] = 5 g L−1, [PO43−] = [HCO3−] = [SO42−] = [NO3−] = [Cl−] = 5.2 mM, I = 100 mM KCl, shaking time = 5 min, 21 °C). |
Moreover, the occupying of active adsorption sites by PO43−, SO42−, and HCO3− through chemisorption, especially PO43− ions, could increase the net negative surface charge of the nanoparticles, which leads to the increasing electrostatic force repulsion between adsorbent and adsorbate. While ions such as NO3− and Cl− have electrostatic adsorption, and this kind of adsorption is highly sensitive to the presence of cations, anions, and concentration of media solution, the increase of negative surface charge prevents the adsorption of NO3− and Cl− ions.5 In addition, in other studies, NO3− and Cl− ions have been reported not to be a good competitor for PO43− adsorption due to outer-sphere complexes.5,11 On the other hand, some literature has reported the presence of coexisting anions, resulting from increasing ionic strength and declining the thickness of electrical double layer, could increase PO43− adsorption.2,5 As mentioned before, PO43− can adsorb through monodentate-mononuclear (MM), bidentate-mononuclear (BM), and bidentate-binuclear (BB) inner-sphere complexes. These interactions not only occupy the active adsorption sites but also increase net negative surface charge, which improves the colloidal stability of nanoparticles in an environment. Since the Zn-N-CDs have a high capability to adsorb PO43−, even in the presence of competing anions, this phenomenon shows that the Zn-N-CDs are promising nano-adsorbents for application in water and wastewater treatment.
Materials | Maximum adsorption capacity (mg g−1) | pH | Equilibrium time (min) | References |
---|---|---|---|---|
ZnOBBNC | 265.50 | 7 | 15 | 1 |
Ferrihydrite | 50 | 7 | 120 | 43 |
La-CTS-ATP | 102.9 | 5 | 50 | 9 |
EL-MNP@Zeolite | 59.88 | 2–8 | 15 | 8 |
Silica and ligand embedded composite | 159.12 | 4 | 60 | 4 |
SCBC-La | 58.8 | 3 | 1080 | 2 |
Ca-BC | 13.61 | 8.5 | 4320 | 48 |
CTAB-modified IONP | 18.69 | 2 | 90 | 38 |
Mn2+/Zn2+/Fe3+/Mg-Al-LDH composite | 82.3 | 7.5 | 60 | 11 |
ZnAlZr4-HT | 91 | 2.3 | 120 | 10 |
Zn-N-CDs | 122.417 | 8 | 5 | Current study |
Overall, the data presented in this study indicate that Zn-N-CDs could be a promising material for the removal of PO43− from aquatic and terrestrial environments. However, the low yield and challenges in obtaining carbon dots as a solid powder due to their hydrophilic surface groups pose limitations on their large-scale use. Further research is needed to fully explore the potential of carbon dots in this field. Future studies should focus on assessing the impact of carbon dots on various biological species, such as microbes and aquatic organisms, in field-scale trials. Additionally, the effectiveness of phosphate removal by carbon dots in the presence of other impurities and potential compounds that may be present in real wastewater, such as organic matter and heavy metals, should be investigated.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra02428a |
This journal is © The Royal Society of Chemistry 2024 |