Abdelazeem S. Eltaweil*a,
Eman M. Abd El-Monaema,
Hala M. Elshishinib,
Hisham G. El-Aqapaa,
Mohamed Hosnyc,
Ahmed M. Abdelfatahc,
Maha S. Ahmedd,
Eman Nasr Hammade,
Gehan M. El-Subruitia,
Manal Fawzyc and
Ahmed M. Omer*f
aChemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt. E-mail: abdelazeemeltaweil@alexu.edu.eg
bDepartment of Environmental Studies, Institute of Graduate Studies and Research, Alexandria University, 163, Horrya Avenue, Alexandria, Egypt
cGreen Technology Group, Environmental Sciences Department, Faculty of Science, Alexandria University, 21511, Alexandria, Egypt
dHigher Institute of Science and Technology-King Mariout, Egypt
eChemistry Department, Faculty of Science, Menoufia University, Egypt
fPolymer Materials Research Department, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research and Technological Applications (SRTA-City), P. O. Box: 21934, New Borg El-Arab City, Alexandria, Egypt
First published on 15th March 2022
The huge development of the industrial sector has resulted in the release of large quantities of phosphate anions which adversely affect the environment, human health, and aquatic ecosystems. Naturally occurring biopolymers have attracted considerable attention as efficient adsorbents for phosphate anions due to their biocompatibility, biodegradability, environmentally-friendly nature, low-cost production, availability in nature, and ease of modification. Amongst them, alginate-based adsorbents are considered one of the most effective adsorbents for removing various types of pollutants from industrial wastewater. The presence of active COOH and OH− groups along the alginate backbone facilitate its physical and chemical modifications and participate in various possible adsorption mechanisms of phosphate anions. Herein, we focus our attention on presenting a comprehensive overview of recent advances in phosphate removal by alginate-based adsorbents. Modification of alginate by various materials, including clays, magnetic materials, layered double hydroxides, carbon materials, and multivalent metals, is addressed. The adsorption potentials of these modified forms for removing phosphate anions, in addition to their adsorption mechanisms are clearly discussed. It is concluded that ion exchange, complexation, precipitation, Lewis acid–base interaction and electrostatic interaction are the most common adsorption mechanisms of phosphate removal by alginate-based adsorbents. Pseudo-2nd order and Freundlich isotherms were figured out to be the major kinetic and isotherm models for the removal process of phosphate. The research findings revealed that some issues, including the high cost of production, leaching, and low efficiency of recyclability of alginate-based adsorbents still need to be resolved. Future trends that could inspire further studies to find the best solutions for removing phosphate anions from aquatic systems are also elaborated, such as the synthesis of magnetic-based alginate and various-shaped alginate nanocomposites that are capable of preventing the leaching of the active materials.
Fig. 1 (a) Sources of phosphate pollution and (b) Scopus database regarding the numbers of published articles about the removal of phosphate from 2001 to 2021. |
Numerous materials have been applied as adsorbents for the removal of phosphate, including clays, biochar, red mud, metal oxides, layered double hydroxides, cross-linked hydrophilic biopolymers, activated carbon residue, mesoporous silica spheres, lanthanum-based materials, metal–organic frameworks and nanomaterials.39–50 Among these adsorbents, natural biopolymers are polymeric organic molecules derived from renewable sources such as algae, the exoskeletons of crustaceans and shells,51 plants, microbial biomass, and animals, as depicted in Fig. 2. Biopolymers are made up of monomeric sections that are covalently bound together to form larger molecules.52,53
Owing to the exceptional structural and physical properties, availability, biocompatibility, and biodegradability of biopolymers, they represent a highly promising choice for the generation of sustainable materials with enhanced adsorption behavior.54–57 Consequently, biopolymer-based adsorbents have exhibited fabulous adsorption behavior not only toward phosphates but also toward a wide variety of noxious contaminants, including pharmaceuticals,51 hydrocarbons,58 pesticides,59 fluoride,60 nitrate,61 heavy metals,62,63 dyes64 and nitro-aromatic compounds.65 Amongst these brilliant biopolymers, anionic alginate (Alg) is a collective family produced from algae and bacteria at a comparatively low cost.66 Alg is a linear anionic polysaccharide that typically consists of 1,4-linked β-D-mannuronic acid and α-L-guluronic acid monomers. Alg was extracted for the first time in 1881 from kelp, then in the 1930s, it was extracted at a large scale from brown algae such as Laminaria digitata, Laminaria japonica, Ascophyllum nodosum, and Macrocystis pyrifera.67 The alginate extraction process involves several steps. First, the initial treatment is to convert salts of alginic acid in the algae into free alginic acid. Secondly, this is neutralized by sodium carbonate/hydroxide to form water-soluble sodium alginate. Finally, the soluble alginate is recovered via precipitation by calcium chloride to form calcium alginate that is later treated with sodium carbonate to produce sodium alginate.68 Notably, there are many alginate-based salts, including calcium alginate, potassium alginate, and ammonium alginate; however, sodium alginate is the most popular salt.69,70
Alginate demonstrates several unique and desirable properties, such as high chemical stability,71 biocompatibility, biodegradability,72 nontoxicity, low-cost manufacture,73 mild gelation, and chelating ability.74 Accordingly, alginate has been effectively applied in diverse applications, in medical, pharmaceutical, industrial, food, and water treatment sectors (Fig. 3).30,65,75–77
Alginate has been extensively used for the removal of noxious anions, as described in the following sections.
It has been reported that alginate-based adsorbents demonstrated acceptable adsorption aptitudes towards various pollutants comprising heavy metals, toxic organic dyes, pharmaceutical residues, aromatic compounds, and other anions such as nitrates and phosphates.78,79 As a type of water-soluble biopolymer, alginate possesses plenty of functional hydrophilic OH and COOH groups in its structure, which simplifies its physical and chemical reactions. Enormous numbers of modifications have been conducted, such as grafting, crosslinking, composite formation, and surface functionalization by other active groups to boost the adsorption characteristics of native alginate. Hence, this review sums up recent progress in the removal of phosphate from wastewater by alginate-based adsorbents. Various modification approaches are discussed, including the incorporation of clays, carbon materials, layered double hydroxides, metal ions, and magnetic materials into the alginate matrix. The morphology, and chemical and physical properties of the developed alginate adsorbent are clarified based on various characterization tools. Furthermore, the key adsorption parameters that control the phosphate adsorption efficacy onto alginate-based adsorbent are explained. Importantly, the proposed adsorption mechanisms of phosphate ions are thoroughly discussed.
Fig. 4 (a) The crosslinking process of Fe3+ to Alg network and interaction with Mt and (b) the possible MtIAlg network configuration. This figure reproduced from ref. 82 with permission from Elsevier. Copyright (2021). |
Kaolin (KN), a silicate clay, consists of different ratios of Al2O3, SiO2, MgO, and CaO. Owing to its ion-exchange ability, thermal stability, binding ability, and high surface area, KN is widely used as a good adsorbent.83,84 In this regard, Ilango et al.85 prepared zirconium crosslinked alginate/kaolin composite beads (Zr@AlgKN) (Fig. 5a) via a hydrothermal method to enhance its adsorbent properties by increasing its surface area. BET results showed that the specific surface area (SBET) of hydro-supported Zr@AlgKN composite beads (78.93 m2 g−1) was higher than that of in situ assisted Zr@AlgKN composite beads (67.15 m2 g−1). Furthermore, from the scanning electron microscopy (SEM) images (Fig. 5b and c) the particle sizes of in situ and hydro-assisted Zr@AlgKN were measured to be 1.597 and 1.362 mm, respectively. It was observed that the incorporation of Zr(IV) on AlgKN composite beads enhanced the adsorption capacity of the composite due to the high affinity of Zr(IV) to phosphate ions. The FTIR spectrum of Zr@AlgKN composite beads after the adsorption of phosphate ions (Fig. 5e) indicated the successful adsorption of phosphate since the characteristic bands were slightly shifted. Moreover, the presence of distinct phosphate ion peaks at 1032 and 560 cm−1 provides additional evidence for phosphate ion adsorption. The X-ray diffraction (XRD) of Zr@AlgKN (Fig. 5f) depicted the same crystalline peaks of both KN and Alg, indicating that the structures of KN and Alg were not changed.
Fig. 5 (a) Digital image for Zr@AlgKN, (b) SEM of Zr@AlgKN (in situ), (c) SEM of Zr@AlgKN (hydro), (d) FTIR spectra of Zr@AlgKN before and after the adsorption of phosphate, and (e) XRD patterns of Alg, KN and Zr@AlgKN. This figure reproduced from ref. 85 with permission from Elsevier. Copyright (2020). |
Moreover, it was found that the adsorption of phosphate reached its peak when the pH increased from 3 to 7 which may be ascribed to the electrostatic interaction between positive charges on the Zr@AlgKN (pHpzc = 5.64) and phosphate ions. In contrast, at pH above 7, the beads were negatively charged and surrounded by hydroxyl groups which could compete with phosphate ions for the active sites of the beads. This result agrees well with the studies by Youness et al.,86,87 and Qixuan et al.88 The adsorption of phosphate on Zr@AlgKN composite beads was evaluated in the presence of different co-anions, such as Cl−, HCO3−, SO42−, F− and HCrO4−. It was found that HCO3− showed the lowest competition with phosphate ions for the adsorption sites of the beads. While SO42− exhibited the highest competing effect on phosphate adsorption because it has high reactivity and electronic charge.
Recently, bentonite (BT) has drawn a great deal of interest as an applicable adsorbent since it is inorganic.89 In this context, Xu et al.74 scrutinized the adsorption performance of modified bentonite entrapped in Alg beads (Al–NaBT–Alg) towards the adsorptive removal of phosphate from wastewater. SEM images (Fig. 6a–d) showed a change in the Al–NaBT structure from lamellar to a porous surface with an internal network structure after encapsulation within Alg beads, reflecting an increase in the available adsorption sites. The isotherm study clarified that the adsorption of phosphate ions onto Al–NaBT–Alg beads obeyed the Sips model and the maximum adsorption capacity (qmax) was 15.77 mg g−1. Furthermore, the kinetics study revealed that the adsorption process best fitted intra-particle diffusion and film diffusion with three different steps. In the first step, the ions diffused from the bulk solution to the external surface of the beads. While the second step involved transferring the ions from the surface into the beads and the third step represented the equilibrium. It was postulated that the adsorption mechanism (Fig. 6e) of phosphate onto the fabricated beads was controlled by ligand exchange between H2PO4− and OH− on the surface of the beads and anion exchange between H2PO4− ions and anions (viz., Cl− and OH−) as elucidated in the following equations:
R–OH + H2PO4− → R–H2PO4 + OH− | (1) |
R–OH + R–Cl + 2H2PO4− → 2R–H2PO4 + OH− + Cl− | (2) |
Fig. 6 (a) SEM image of Al–NaBT powder, (b and c) the surface of Al–NaBT–Alg beads, (d) the cross-section of Al–NaBT–Alg beads, and (e) a schematic representation for the possible adsorption mechanisms of phosphate onto Al–NaBT–Alg beads. This figure reproduced from ref. 74 with permission from Elsevier. Copyright (2020). |
In another attempt, Xi et al.90 studied the removal of phosphate from an aqueous medium using Alg–MgO@BT composite beads. Two different ways of drying were used to prepare two kinds of beads: freeze-drying (Alg–MgO@BT) and oven drying (OAlg–MgO@BT). It was found that the SBET of Alg–MgO@BT, OAlg–MgO@BT, and Alg–BT were 59.5, 42.9, and 26.8 m2 g−1 and the pore volumes were 0.135, 0.126, and 0.066 cm3 g−1, respectively. This finding evinced that MgO improved the SBET and the pore volume of Alg–BT beads. This result was consistent with the cross-sectional images of Alg–BT, Alg–MgO@BT, and OAlg–MgO@BT which showed that Alg–MgO@BT and OAlg–MgO@BT possess more abundant mesopores than Alg–BT, implying that MgO provided more adsorption sites. It was observed that Alg–MgO@BT has enhanced adsorption capacity over OAlg–MgO@BT which may be attributed to the higher porosity of the freeze-dried beads. Furthermore, phosphate adsorption onto Alg–MgO@BT increased from 58% to 99% with an increase in the concentration of MgO from 0.05 to 0.2 mol L−1, respectively. It was concluded that MgO provided more active sites which agrees with Lihua et al.91 One of the difficulties of phosphate removal is its presence in multiple forms where it exists as H3PO4 (pH < 2), H2PO4− (pH 2–7), HPO42− (pH 7–11), and PO43− (pH > 11). Alg–MgO@BT showed a high adsorption capacity towards phosphate ions over a wide pH range (3–10) where the phosphate removal efficiency exceeded 99% at a pH lower than 7. This may be anticipated from the strong electrostatic interaction between H2PO4− and HPO42− species and the positively charged Alg–MgO@BT. In addition there is a possibility of these species exchanging with the hydroxyl groups on the surface of the beads, as shown in Fig. 7. Furthermore, by comparing XPS (X-ray photoelectron spectroscopy) before and after the adsorption of phosphate ions, the Mg-spectrum showed that MgO formed a complex with phosphate since the peaks characteristic of Mg–P appeared and the peak area of M–O decreased, suggesting that MgO contributed greatly to the adsorption process. Moreover, the removal efficiency of phosphate ions was tested in the presence of interfering ions such as Cl−, SO42−, NO3−, HCO3− and F−. HCO3− and F− showed the highest negative effect on the removal efficiency owing to their high affinity to MgO and ability to bond to Alg–MgO@BT with electrostatic attraction. In contrast, Cl− and NO3− had a negligible effect on the removal efficiency of phosphate due to their weak binding to the outer surface of Alg–MgO@BT.90
Fig. 7 The multiple forms of phosphate ions and the possible mechanism to adsorb phosphate ions onto Alg–Mgo@BT. This figure reproduced from ref. 90 with permission from Elsevier. Copyright (2021). |
In another study, Huan et al.92 synthesized alginate-immobilized Zr–BT hydrogel beads (NH–Alg–ZrBT). These beads were modified by using the Na2CO3/HCl pore-forming technique to enhance the adsorption capacity of the fabricated beads toward phosphate ions, as shown in Fig. 8. SEM images depicted that both NH–Alg–ZrBT and NH–Alg–BT have a porous structure due to the reaction between Na2CO3 and HCl and the release of CO2 according to the following equation:
Na2CO3 + 2HCl → 2NaCl + H2O + CO2↑ | (3) |
Fig. 8 Illustration of the fabrication process of NH–Alg–ZrBT. This figure reproduced from ref. 93 with permission from Elsevier. Copyright (2021). |
It was found that the SBET of NH–Alg–ZrBT and Alg–ZrBT were 53.83 and 52.39 m2 g−1, respectively, while NH–Alg–BT showed a lower SBET of 30.43 m2 g−1, indicating that Zr(IV) plays a significant role in enhancing the SBET of the beads. The pore volumes of NH–Alg–ZrBT (9.97 × 10−2 cm3 g−1) and NH–Alg–BT (8.66 × 10−2 cm3 g−1) were larger than that of Alg–ZrBT (5.64 × 10−2 cm3 g−1), proving that the pore-forming process improved the porosity of the beads. This result explained the highest adsorption capacity (42.2 mg g−1) and removal efficiency (94.4%) of NH–Alg–ZrBT.93
One more study was intended to develop Alg beads for the efficient removal of phosphate from wastewater.94 This modification of the alginate depended on (i) the high chemical stability and mechanical strength of polyvinyl alcohol (PVA), (ii) the high affinity of La towards phosphate, and (iii) the high specific surface area and the cation exchange capacity of palygorskite. La/PVA/Alg/Pal beads exhibited a high adsorption capacity toward phosphate ions, reaching 33.20 mg L−1 at pH 4 and 25 °C. Furthermore, the selectivity of La/PVA/Alg/Pal beads toward phosphate ions was scrutinized in the presence of coexisting ions, showing that the removal efficiency of phosphate was 100% in the presence of Cl−, NO3−, and SO42−. In contrast, F− exhibited a negative effect on the removal of phosphate due to the high affinity of La towards F−. Moreover, a kinetics study revealed that the experimental data fitted the pseudo-second order (R2 = 0.968–0.984), indicating that the adsorption mechanism of phosphate onto La/PVA/Alg/Pal beads was chemisorption. The FTIR spectrum of La/PVA/Alg/Pal beads after the adsorption of phosphate implied the high affinity of La towards phosphate since the distinguishing peak of La–O was shifted from 526 to 514 cm−1. Furthermore, the shifting of the hydroxyl peak from 3398 to 3410 cm−1 suggested ligand exchange between hydroxyl groups and phosphate anions.95 In yet another attempt, Hanna Siwek et al.94 prepared an alginate/goethite hydrogel composite (Alg/α-FeOOH) taking into account the advantages of α-FeOOH, including high mechanical stability, selectivity towards phosphate, and large specific area. To evaluate the mechanical stability of Alg/α-FeOOH, the hydrogel was soaked in deionized water and two different types of natural water (hard and soft water) for 28 days. A slight decrease in the mechanical stability of the soaked hydrogel was noticed in deionized water and soft water. While the mechanical stability of the hydrogel in the hard water declined significantly, owing to the high concentration of Na, K, and Mg in hard water, which contributed to monovalent cation exchange with Alg/α-FeOOH causing the hydrogel to dissolve. Moreover, it was reported that the removal efficiency of phosphate by Alg/α-FeOOH hydrogel was 97.6%. Furthermore, there was no significant change in removal efficiency over a wide pH range (4–9) except for a slight decrease in phosphate removal aptitude (96.3%) at pH 10.94
M–OH2+ + H2PO4− → ≡M–O–P(OH)3+ + OH− | (4) |
Fig. 9 (a) VSM measurement, (b) effect of different concentrations of coexisting ions on phosphate adsorption, and (c) the proposed adsorption mechanisms of phosphate onto La–MgFe2O4/BC@Alg. This figure reproduced from ref. 100 with permission from Elsevier. Copyright (2021). |
In another attempt, Ilango et al.101 investigated the removal of phosphate by magnetic Fe3O4/alginate composite beads which were modified by amine groups to improve their adsorption capacity (Fe3O4/Alg@NH2). It was reported that the calculated qmax of phosphate under the Langmuir isotherm was 32.07 mg g−1 at 50 °C. The adsorption of phosphate was confirmed by the FTIR spectrum of Fe3O4/Alg@NH2 after the adsorption of phosphate, at which a shift occurred in the bands belonging to OH, NH2, and Fe2O3. Furthermore, the energy-dispersive X-ray (EDX) analysis of Fe3O4/Alg@NH2 after the adsorption process revealed the discriminative peak of phosphate at 2.0 keV with a high percentage (6.82%). It was observed that in a pH range of 2–7, Fe3O4/Alg@NH2 showed a high adsorption capacity, reaching 30.14 mg g−1, which may be attributed to Lewis acid–base interaction of Fe3O4 and NH2 (Lewis acid) and H2PO4− (Lewis base).101
This explanation is in line with Huayong et al.'s102 study that suggested the occurrence of the adsorption of phosphate onto Fe3O4/PAM/Alg–Zr beads via a chemical interaction represented in Lewis acid–base interaction between Zr(IV) and phosphate and ligand exchange, in addition to a physical interaction represented in electrostatic interaction between phosphate ions and Fe3O4/PAM/Alg–Zr beads.
Fig. 10 (a) Digital photo of Zn–Fe LDHs@Alg, (b) SEM of average particle size of Zn–Fe LDHs@Alg, (c) SEM of Zn–Fe LDHs@Alg surface, (d) SEM of Zn–Fe LDHs@Alg after phosphate adsorption, and (e) the proposed adsorption mechanism of phosphate. This figure reproduced from ref. 117 with permission from Elsevier. Copyright (2021). |
In another attempt, Han et al.113 highlighted the adsorptive removal of phosphate ions onto PVA-modified Mg–Al LDH/Alg beads (Mg–Al LDH–PVA/Alg). The stabilities of both Mg–Al LDH–PVA/Alg and Mg–Al LDH/Alg in different concentrations of phosphate solution (50, 100, 150 and 200 mg L−1) were examined to investigate the effect of PVA on the stability of the beads. The results showed that by increasing the concentration of phosphate from 50 mg L−1 to 200 mg L−1, the deformation percentage (Fig. 11) of Mg–Al LDH/Alg increased to reach 100%. In contrast, the deformation percentage of Mg–Al LDH–PVA/Alg was 7.8% which is most likely due to the high tensile strength and flexibility of PVA in water. Hence, the blending of Alg with PVA could improve the strength and durability of hydrogel beads.
Fig. 11 Deformation of the beads: (a) Mg–Al LDH/Alg beads, (b–e) Mg–Al LDH/Alg beads after phosphate adsorption, (f) before the reaction of LPAB, and (g–j) Mg–Al LDH–PVA/Alg beads after phosphate adsorption. This figure reproduced from ref. 113 with permission from the Korean Society of Environmental Engineers. Copyright (2012). |
In the light of this result, Mg–Al LDH–PVA/Alg beads could be regenerated for many cycles, which renders them a cost-effective adsorbent, agreeing with Idris et al.'s118 study that reported the high reusability of PVA/Alg beads. A recyclability test showed the excellent adsorption performance of PVA/Alg beads after the 7th cycle.
In yet another attempt, Shan and his coworkers128 inspected the adsorption performance of zirconium-crosslinked GO/Alg (Zr–GO/Alg) aerogel beads for removing phosphate. There was excellent synergy between the strong affinity and selectivity of multivalent Zr(IV) towards phosphate adsorption and the perfect mechanical strength of GO.129,130 Accordingly, Zr–GO/Alg beads exhibited an ultra-adsorption capacity of 189.06 mg g−1 at 25 °C using 0.2 g L−1 adsorbent dosage. In this case, the pH of the medium was a crucial factor that significantly affected the adsorption behavior of the beads, as depicted in Fig. 12a. It was found that Zr–GO/Alg beads reached a peak adsorptive performance at pH = 2.03 due to the strong electrostatic attraction between the protonated active groups on the Zr–GO/Alg surface (pHpzc = 7.2) and H2PO4− species as well as the ligand exchange between H2PO4− and Cl−. Meanwhile, a continuous decrease in the adsorption capacity occurred when the pH increased from 2 to 7, due to the weaker electrostatic attractions. When pH > 7 the adsorption capacity for phosphate dramatically declined due to the strong electrostatic repulsion between the deprotonated surface of Zr–GO/Alg and phosphate ions as well as OH− groups that strongly competed with phosphate ions for the binding sites of the beads. For a further understanding of the phosphate adsorption mechanism, XPS spectra of Zr–GO/Alg before and after the adsorption process were estimated. The XPS wide spectrum (Fig. 12b) revealed the peak belonging to P 2p, confirming the adsorption of phosphate onto Zr–GO/Alg beads. Whereas, the Zr 3d-spectrum (Fig. 12c) showed a slight negative shift of Zr 3d5/2 and Zr 3d3/2 peaks, implying the domination of the complexation mechanism in the adsorption process.131 Based on the aforementioned results, the adsorption mechanism of phosphate onto Zr–GO/Alg beads was mainly dominated by electrostatic interactions, complexation, and ligand exchange.
Fig. 12 (a) Effect of pH on the adsorption of phosphate on Zr–GO/Alg aerogel beads and XPS of Zr–GO/Alg before and after the adsorption process. (b) Wide spectra and (c) Zr 3d spectra. This figure reproduced from ref. 128 with permission from Elsevier. Copyright (2019). |
Biochar (BC) is one more carbonaceous material that is derived via the thermal treatment of biomass like wood, marine algae, agricultural wastes, rice husk, and wheat straw under a limited oxygen atmosphere.120,132,133 The structure of BC is distinguished by the presence of abundant oxygen-containing functional groups, high porosity, large surface area, and high mechanical strength.123 Hence, BC has been established as a green adsorbent and propitious candidate for the cost-effective removal of phosphate. Notably, the adsorption capability of a BC-based adsorbent mainly depends on its natural source and pyrolysis temperature.134,135 In this regard, Jung et al.136 fabricated BC from three different types of marine macroalgae (Laminaria japonica, Undaria pinnatifida and Porphyra tenera) and studied the impact of pyrolysis temperature on their structural properties and adsorption behavior. Laminaria japonica-derived biochar (LBC) exhibited the highest removal efficiency for phosphate (97.02%) owing to its higher Ca/P and Mg/P ratios which increased the ionic strength and boosted the binding strength to phosphate.137–139 In addition, LBC was fabricated at different temperatures of 200, 400, 600, 800 °C to infer the effect of pyrolysis temperature on its physical properties and adsorption efficiency.139 It was found that LBC-600 attained the highest removal efficiency, reaching 97.95%. Such a superb removal efficiency may be assigned to the higher SBET of LBC-600 of 79.95 m2 g−1. Furthermore, the incorporation of LBC-600 into Alg beads (LBC-600@Alg) improved its SBET (242.34 m2 g−1), suggesting the availability of more adsorption sites to adsorb phosphate. As a result, the qmax of LBC-600@Alg towards phosphate ions reached 116.15 mg g−1 at 30 °C. It was observed that LBC-600@Alg beads exhibited excellent adsorptive behavior at pH < pHpzc (8.42) owing to the strong electrostatic attraction forces between the protonated binding sites (i.e., hydroxyl and carboxyl) on the beads and the H2PO4− species. Whereas at pH > pHpzc, there were robust electrostatic repulsion forces between the deprotonated binding sites of LBC-600@Alg and the PO43− anion. Thus, electrostatic interaction dominated the adsorption mechanism of phosphate onto LBC-600@Alg, disagreeing with Cui et al.'s study.123 Phosphate adsorption onto Mg–BC@Alg beads exhibited excellent removal efficiency of 97.1% over a wide pH range from 3 to 10. This behavior indicated that electrostatic interaction had a negligible effect on the adsorption process. It was hypothesized that phosphate ions adsorbed onto Mg–BC@Alg via the complexation mechanism between phosphate ions and Ca2+, Mg2+ and Al3+ played a significant role in the adsorption process. In addition, ligand exchange occurred between Mg(OH)2 onto the Mg–BC@Alg surface and H2PO4− as in the following equation:140
Mg(OH)2 + 2H2PO4− → Mg(H2PO4)2 + 2OH− | (5) |
Fig. 13 SEM of Alg–La@PEI beads: (a and b) before adsorption and (d and e) after adsorption. EDX mapping of Alg–La@PEI beads: (c) before adsorption and (f) after adsorption. (g) The plausible adsorption mechanism. This figure reproduced from ref. 141 with permission from Elsevier. Copyright (2020). |
The adsorption selectivity of SA–La@PEI towards phosphate was tested in the presence of interfering ions such as NO3−, HCO3−, SO42−, and Cl−, showing good selectivity towards phosphate with a slight decrease in the adsorption capacity in the presence of NO3−, SO42−, and Cl− ions. However, there was an observable decline in the adsorption capacity in the presence of HCO3− that may be related to the basic nature of bicarbonate that raised the pH of the medium and altered the surface potential of the adsorbent.147 Moreover, SA–La@PEI beads showed a promising removal efficiency of up to 78% after 5 cycles, implying excellent mechanical stability of the fabricated beads.
In another study, Luo and his coworkers146 fabricated interpenetrated Zr–Alg beads decorated with polyethylene glycol beads (PEG-modified PNIPAM/Alg–Zr), merging the excellent adsorption capability with good mechanical and chemical stability. In detail, PEG-modified PNIPAM/Alg–Zr beads exhibited a superb adsorption capability provided by Zr4+ ions that have a strong affinity towards phosphate.129 Good mechanical stability is accomplished by the formation of interpenetration networks. In addition, the macro-porosity formed by PEG during the polymerization facilitated the transfer of ions from the bulk solutions to the active adsorptive sites, boosting the adsorption efficiency. More importantly, PEG-modified PNIPAM/Alg–Zr beads showed a thermo-sensitive behavior which affected their adsorption performance and the swelling ratio of the beads.148 At low temperatures, the beads became a swollen gel, facilitating the diffusion of phosphate into their matrices, and thus increasing their adsorption capacity. On the other hand, the beads shrank owing to the hydrophobicity of PNIPAM at temperatures higher than the vapor phase transition temperature, so the adsorption capability of the beads declined. As a result, the adsorption capacity of PEG-modified PNIPAM/Alg–Zr beads decreased from 36.90 to 34.00 mg g−1 when the temperature was raised from 25 to 65 °C. PEG-modified PNIPAM/Alg–Zr beads reached 99% desorption efficiency in 40 min at 55 °C using 0.2 M NaOH. This excellent result may be anticipated from the compactness of the adsorptive sites after the shrinkage of the beads, so phosphate ions become weakly bound to the active sites, and thus easily desorbed.149 It was found that the higher adsorption capacity of phosphate was 36.06 mg g−1 at pH 2 since the predominant species were H2PO4− and H3PO4.150 So, electrostatic attraction occurred between H2PO4− and the positively charged PEG-modified PNIPAM/Alg–Zr surface (pHpzc = 3.8). In contrast, the adsorption aptitude of phosphate onto talc/Alg/La beads was almost the same over a wide pH range. Talc/Alg/La demonstrated a superior phosphate removal efficacy of up to 85% at pH values of 4, 7, and 10. Moreover, talc/Alg/La beads retained their physical and chemical properties in both acidic and alkaline conditions, thus verifying their excellent chemical and mechanical stability and applicability for wastewater treatment.151
Fig. 14 The main modifications for alginate-based adsorbents and the most common adsorption mechanisms of phosphate ions by alginate-based adsorbents. |
Moreover, it was found that a variation in the optimum pH to adsorb phosphate ions onto alginate-based adsorbents may be attributed to the polyprotic nature of phosphate, resulting in electrostatic interaction/repulsion forces between phosphate and the adsorbents. Furthermore, in some cases, phosphate adsorption was not sensitive to pH change, which is most likely due to the domination of other adsorption mechanisms, such as ion exchange, complexation, precipitation and Lewis acid–base interaction. Hence, one or more of these mechanisms could surpass electrostatic interaction, hindering the effect of pH on the adsorption efficacy. Moreover, studying coexisting ions on the adsorption efficiency of phosphate implied that SO42− caused a significant hindrance to the adsorption process owing to its high charge density and large ionic size. Nonetheless, some studies confirmed that alginate-based adsorbents still had a high selectivity towards phosphate ions in the presence of diverse interfering ions. Moreover, based on concrete results, the incorporation of layered double hydroxides, clays and carbon materials into the alginate matrix significantly improved the adsorption performance of the developed alginate adsorbents toward phosphate ions.
The experimental conditions of the adsorption of phosphate by various alginate-based adsorbents in addition to the adsorption capacities, kinetics, and isotherms are summarized in Table 1.
Modification strategy | Adsorbent | SBET m2 g−1 | qmax mg g−1 | pH | Eq. time (min) | Kinetics model | Isotherm | Adsorption mechanism | Ref. |
---|---|---|---|---|---|---|---|---|---|
Clays-modified alginate | Alg–MgO@BT beads | 59.5 | 70.5 | 3–10 | 300–1200 | Pseudo-2nd order | Langmuir | Mg–P complexation formation | 90 |
Electrostatic attraction | |||||||||
Ligand exchange | |||||||||
Zr@AlgKN beads (hydro) | 78.9 | 37.2 | 2–7 | 30 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 85 | |
Surface complexation | |||||||||
Ion exchange | |||||||||
MtIAlg beads | — | 48.7 | 2–10 | 30 | Pseudo-2nd order | Langmuir | Ligand exchange | 82 | |
Surface complexation | |||||||||
Electrostatic attraction | |||||||||
La/PVA/Alg/Pal beads | 5.8 | 33.2 | 3–6 | 720 | Pseudo-2nd order | Freundlich | Electrostatic attraction and ligand exchange (acidic condition) | 95 | |
Lewis acid–base interaction (alkaline condition) | |||||||||
Al–NaBT–Alg beads | 17.0 | 15.8 | 3 | 360–2880 | Pseudo-2nd order | Sips | Ligand exchange | 74 | |
Anion exchange | |||||||||
Electrostatic attraction | |||||||||
NH–Alg–ZrBT beads | 53.8 | 63.6 | 3–9 | 600–3000 | Pseudo-2nd order | Langmuir | Electrostatic attraction | 93 | |
Inner-sphere complexation | |||||||||
Ligand exchange | |||||||||
MgCl2–BT@Alg microspheres | 46.2 | 46.6 | 3–10 | 1440 | Pseudo-2nd order | Freundlich | Chemical precipitation | 123 | |
Ligand exchange | |||||||||
Talc/Alg/La beads | — | 16.4 | 4 | 21600 | Pseudo-2nd order | Langmuir | Electrostatic attraction and ligand exchange (acidic condition) | 152 | |
Complexation (alkaline condition) | |||||||||
Magnetic materials-modified alginate | Fe3O4/PAM/SA–Zr beads | 24.7 | 42.2 | 2 | 1800 | Pseudo-2nd order | Freundlich | Electrostatic attraction (physisorption) | 102 |
Ligand exchange (chemisorption) | |||||||||
La–MgFe2O4/BC@Alg beads | 158.8 | 26.8 | 5–8 | 800–950 | Pseudo-2nd order | Langmuir | Electrostatic attraction | 100 | |
Inner-sphere complexation | |||||||||
Ligand exchange | |||||||||
Alg/α-FeOOH composite | — | 36.0 | 4–10 | 15840 | — | Freundlich | Electrostatic attraction | 94 | |
AFMAlg beads (hydro) | 79.4 | 30.1 | 5 | 30 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 101 | |
AFMAlg beads (in situ) | 65.0 | 25.1 | Surface complexation | ||||||
Layered double hydroxides-modified alginate | ZnFe-LDHs@Alg beads | 1.1 | 94.6 | 4–7 | 60 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 117 |
Surface complexation | |||||||||
Ion exchange | |||||||||
LDH–Alg beads | — | 1.0 | 4.9–8.9 | 720 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 114 | |
Anion exchange | |||||||||
Anion intercalation | |||||||||
Mg–Al LDH–Alg/PVA beads | — | 1.9 | 5.7 | 480 | Pseudo-2nd order | Freundlich/Langmuir | — | 116 | |
LDH–PVA/Alg beads | — | 0.4 | 4.9 | 720 | — | Freundlich | Electrostatic attraction | 113 | |
Anion exchange | |||||||||
Anion intercalation | |||||||||
Carbon materials-modified alginate | Zr–GO/Alg beads | — | 189.1 | 2.03 | 720 | Pseudo-2nd order | Freundlich | Ligand exchange | 128 |
Electrostatic attraction | |||||||||
Chemical precipitation | |||||||||
LBC–Alg beads | 242.3 | 157.7 | 6 | 1440 | Pseudo-2nd order | Langmuir and Freundlich | — | 136 | |
EMB–Alg beads | 177.5 | 169.9 | 4 | 1440 | Pseudo-2nd order | Sips | — | 153 | |
TATGO@Alg composite beads | 45.3 | 58.5 | 7 | 30 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 126 | |
AC@Alg beads | — | 133.3 | 10 | 20 | Pseudo-2nd order | Langmuir | — | 154 | |
AC@Alg beads | — | 0.34 | 4 | 45 | — | Freundlich | — | 155 | |
Multivalent metals-based alginate | Alg–La@PEI beads | — | 121.2 | 3 | 402 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 141 |
Chemical precipitation | |||||||||
Fe–CS–Alg beads | — | 84.7 | 3–6 | 40 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 156 | |
Surface complexation | |||||||||
Ion exchange | |||||||||
Alg/FeCl3 capsules | — | — | 4–10 | 2880 | — | Freundlich | Electrostatic attraction | 157 | |
Chemical precipitation | |||||||||
Zr(IV/red mud@Zn–alginate beads | — | 13.6 | 8 | 120 | Pseudo-2nd order | Langmuir | — | 158 | |
nZrO2–Al–Alg beads | — | 173.0 | 7 | 35 | Pseudo-2nd order | Freundlich | — | 159 | |
LaC@Alg beads | 22.7 | 42.5 | 5 | 120 | — | — | Chemical precipitation | 160 | |
CaCO3–Alg beads | — | 0.12 | 10 | 150 | Pseudo-2nd order | Freundlich | — | 161 | |
Alg–Ca–Ce | — | 41.4 | 2 | 240 | Pseudo-2nd order | Langmuir | Electrostatic attraction | 162 | |
Lewis acid–base interaction | |||||||||
PEG-modified PNIPAM/Alg–Zr bead | 34.1 | 47.4 | 2 | 1800 | Pseudo-2nd order | Langmuir | Electrostatic attraction | 163 | |
Inner-sphere complexation | |||||||||
Ligand exchange | |||||||||
(PVA–SA–LH) hydrogel beads | — | 7.9 | 4 | 1440 | Pseudo-2nd order | Freundlich | Electrostatic attraction | 164 | |
Lewis acid–base interaction | |||||||||
Alg–nZVI beads | — | — | 12 | 60 | — | — | — | 165 |
This review evinces the viability of recent ways of developing the adsorbability of alginate toward phosphate ions. However, we hold to the notion that the aforementioned modified forms of alginate still suffer from flaws; most of them need to be shaped as beads, hydrogels, or membranes to provide perfect separation, even though this decreases the surface area of the adsorbents. In addition, there is a probability that these fine materials can leach from the pores of the shaped alginate forms into water, causing another type of contamination. Furthermore, we could not neglect the economic aspect since most of the materials used to modify alginate are costly.
Therefore, based on this in-depth study, we recommend some serious points that should be taken into consideration in future investigations:
✓ Sustainable studies to fabricate efficient alginate-based adsorbents with promising adsorbability and selectivity toward phosphate ions.
✓ Improving the durability of the shaped alginate forms to avoid leaching of the incorporated materials.
✓ From the economic aspect, scalable fabrication of alginate-based adsorbents at an affordable cost is an important issue.
✓ Developing a magnetic-based alginate to provide practical, perfect, and easy separation which will increase the reusability of the adsorbents is another case from the economical point of view.
Finally, it is crucial to mention the lack of column experiments in the majority of the published research papers, though fixed-bed studies are essential for real applications from an industrial perspective.
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