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DOI: 10.1039/D2EW00478J (Critical Review) Environ. Sci.: Water Res. Technol., 2022, 8, 2170-2189

Research progress of modified natural zeolites for removal of typical anions in water

Wenjing Lu , Chunhui Zhang *, Peidong Su , Xinling Wang , Wenlong Shen , Bingxu Quan , Zhelin Shen and Lei Song
School of Chemical & Environmental Engineering, China University of Mining & Technology (Beijing), Room 607, Yifu Science & Research Building, Ding 11 Xueyuan Road, Beijing 100083, China. E-mail: ZCHcumtb@hotmail.com; Fax: +86 10 6233 9331; Tel: +86 10 6233 9331

Received 21st June 2022 , Accepted 13th August 2022

First published on 25th August 2022

Abstract

Natural zeolite has been regarded as an adsorbent for cation removal in water. However, natural zeolite cannot effectively adsorb anions due to the negative charge. In recent years, great effort has been put into surface modification of natural zeolite to enhance the anion removal performance. Modification methods and research progress of anionic contaminant removal in water by zeolites were reviewed. The adsorption mechanisms were also generalized to provide useful implications in designing of zeolite adsorbents with excellent performance against anions. In addition, the prospect of natural zeolite application in wastewater treatment was also investigated. The main conclusion drawn was that the composite-modified zeolite exhibited the optimal performance. Tuning the pore size, modifying the pores, and introducing appropriate functional groups into the internal and external structure of zeolites were core to the design and enhancement of zeolite properties. However, attention needs to be paid to the treatment of the adsorbed saturated zeolite to prevent the generation of new pollution sources into the environment.


Water impact

Zeolite is an inexpensive and easily available natural sustainable material. The application of natural zeolites can be expanded by purposely modifying zeolites, which has significant implications for wastewater treatment. This review provides some strategies to improve the adsorption performance of zeolites for anion removal.

1. Introduction

An increasing number of anionic pollutants are being released into the ecosystem, and excess anionic pollutants pose a serious threat to the environment and human health.1 These anionic pollutants are considered to be important in wastewater quality standards. Some common anionic substances, including fluoride (F−),2 nitrate (NO3−),3 and phosphate (PO3−),4 are released into the aqueous environment via various industrial activities such as mining, metal plating, pigment synthesis, and battery manufacturing.5 They can be considered pollutants that affect humans and ecosystems when their concentrations reach certain levels. For example, excessive fluoride (>1.5) can cause dental fluorosis and fluorosis bone disease, cancer, Alzheimer's disease and other life-threatening diseases in severe cases.6 Human poisoning caused by fluoride has involved about 50 countries worldwide.7 High concentrations of nitrate in drinking water will pose a potential threat to human health. The World Health Organization recommends a nitrate limit of 50 mg L−1. Phosphate and nitrate concentrations higher than 0.5–1.0 mg L−1 will cause eutrophication.8 More than 30% of global lakes are eutrophic, and the trend is rapidly increasing.9 There are also some heavy metal oxygen anion pollutants, such as chromate,10 arsenate,11 and antimonite,12 that are present in industrial wastewater. These anionic contaminants are receiving special attention due to their toxicity and persistence in the environment. They can cause serious damage to the body's liver, lungs, and gastrointestinal tract.13 Therefore, sustainable and advanced water treatment technologies are necessary.

It has been found that adsorption,14 precipitation,15 ion exchange,16 membrane filtration,17 membrane separation,18 and electrolytic reduction19 can remove these anionic contaminants from water. Among them, adsorption is considered as an ideal technique for anion removal due to its simple operation, low energy consumption and low secondary pollution.20 The adsorption effect depends directly on the adsorbent, and a suitable adsorbent is very important. Sorbents can be divided into nano and non-nano materials.21 Nanomaterials showed ultrahigh adsorption performance of pollutants, but nanoparticles are easily agglomerated, which increases the difficulty of subsequent separation.22–26 Zeolite is an adsorbent with good ion exchange and adsorption properties.27–31 Zeolites show excellent adsorption performance in removing cationic pollutants due to the negatively charged surface.32 The methyl methacrylate (MMA)–Na-Y zeolite composite synthesized by Elwakeel et al.33 recovered up to 80–85% of Cu(II), Cd(II) and Pb(II) in drinking water. Khan et al.34 found that zeolite adsorbed U(VI) and Eu(III) in aqueous solution with a capacity of 24.39 mg g−1. Natural zeolite is also a friendly adsorbent, however, it contains impurities and presents problems of clogging, poor pore size connectivity and a negatively charged surface which makes adsorption ineffective and only on cations.35–42 Recently, modified zeolites have been reported to exhibit positively charged outer surfaces.35 Therefore, many surface modification methods of zeolites have been proposed in order to effectively remove anionic contaminants. Extensive studies have shown that suitable modification methods can change the charge properties of the zeolite outer surface.22,43–45 For example, Franus et al.46 showed that aluminum cations can effectively transform the zeolite surface charge from negative to positive, shifting the isoelectric point to the highest value. Li et al.47 prepared NaCl-modified zeolite to remove phosphate from piggery wastewater and demonstrated that the removal efficiency of modified zeolite for phosphate was up to 90.3%. Cheng and colleagues48 first investigated zeolite modification for nitrite removal, and acid modification of zeolite significantly increased the specific surface area and protonated the zeolite. Nitrite could be effectively adsorbed onto zeolite with an adsorption capacity of up to 54.5 mg g−1, which was more than 7 times higher than that of natural zeolite at 25 °C. These studies break the limitation that natural zeolites exhibit electronegativity and bring an entirely new stage in zeolite adsorption of anionic pollutants, and the method is expected to be further applied industrially.

Previous reports on zeolites have mostly focused on mesoporous or macroporous synthetic zeolites, but there is a need to improve the availability of natural zeolites as a low-cost and readily available natural adsorbent, which is in line with the concept of sustainable development. With an aim to obtain a general view about the removal of anionic contaminants from water by natural zeolites and their modified forms, we surveyed studies reported in the Web of Science and China National Knowledge Infrastructure (CNKI) from 2000 to 2022 and finally screened 159 research papers. This paper summarizes the research progress of surface-modified natural zeolites for anion removal in water. The modification method of natural zeolites in removing anions from water is highlighted. In addition, we summarized the adsorption mechanisms between the target anions and zeolites, and strategies to improve the adsorption performance of zeolites for anion removal. Finally, the prospects of zeolite utilization and its progress in combination with other processes for wastewater treatment are briefly presented. The discussion in this paper contributes to the selection and design of natural zeolite materials rationally and efficiently for anionic contaminant removal.

2. Surface modification methods for the removal of anions

The physicochemical characteristics of zeolites may be affected differently by different modification methods. The modification of zeolites not only changes the structure and size distribution of the internal pore structure but also changes their surface hydrophilicity/hydrophobicity and surface functional groups. Usually, surface modification methods can be divided into physical modification, chemical modification, and composite modification (Fig. 1).
image file: d2ew00478j-f1.tif
Fig. 1 The classifications of surface modification methods.

2.1 Physical modification

Thermal modification is generally carried out by muffle or microwave heating. High temperature can remove most of the water molecules, carbonates and some organic matter from the pores and channels of zeolites, reduce the surface resistance of zeolites and improve their exchange and adsorption capacity.49,50 Zeolites after thermal activation were found by Aziz et al.51 to have increased pore size and increased displacement between crystal structures due to the reduction of water molecules attached in the crystal structure by thermal activation. Too high temperature and long heating time would damage the original structure of zeolite. Typically, heating zeolites in the range of 200–600 °C for 2–5 h would not disrupt their structure. Natural zeolites were obtained from Sukabumi, Bandung, Indonesia, by Abdullah et al.52 The sorption ability of zeolite was enhanced through thermal activation by using a furnace. The adsorption capacity of zeolite was enhanced when the temperature was increased from 100 °C to 225 °C. Then, when the activation temperature was further increased from 225 °C to 600 °C, the adsorption capacity decreased. It was probably due to solidification of zeolite as a result of high temperature, while the degree of solidification increases with the increase in temperature. This solidification leads to the decrease in effective surface area of zeolite and then a decrease in its sorption ability. Ultrasonic modification is usually used as a supplementary means, which uses ultrasonic waves to eliminate impurities in the zeolite pores. It has the advantages of high heating speed and uniform heating. The increase of microwave power and heating time is beneficial to improve the adsorption capacity of modified zeolites, but excessive power and long heating time can destroy the crystal structure of zeolites.53 Normally, the specific surface area of zeolite can be significantly increased by heating in the ultrasonic power range of 400–560 W for 30–60 min without destroying its lattice structure.

2.2 Chemical modification

The adsorption capacity of zeolites can be enhanced via chemical modification like increasing the surface area and pore volume of zeolite as well as abundant surface functional groups. The specific modification mechanism is shown in Fig. 2. It can be seen that the modification strategies used to improve the anion adsorption performance of zeolites are (1) removing impurities and unclogging pore channels to facilitate the entry and transfer process of the target, (2) increasing the specific surface area to expose more sites to capture the target, and (3) introducing new functional groups to change the surface properties of zeolites, such as hydrophobicity, to provide new binding sites for the removal of the target. Currently, commonly used modification methods to improve the adsorption capacity of zeolites for anions include acid modification, salt modification, rare earth modification, cationic surfactant modification, chitosan modification and composite modification.
image file: d2ew00478j-f2.tif
Fig. 2 Modification mechanism of zeolite.
Acid modification. The most common methods to modify the surface properties of adsorbents are acid modification and alkali modification. Acid-modified zeolite is prepared by impregnation with acid solutions. The most commonly used acids are hydrochloric acid, sulfuric acid, nitric acid, oxalic acid, citric acid and acetic acid.54 The modification mechanism of zeolite by acid treatment mainly includes the following points. (1) The acid solution can dissolve impurities in the pores and dredge the channels. (2) The small radius of H+ can replace the larger-radius cations such as K+, Ca2+ and Mg2+ from layers to increase the effective space in the pore channel. (3) Zeolite impregnation in acid solutions leads to surface protonation and increases the positive charge density on the zeolite surface. The increase in the number of positive charges is favorable for zeolites to adsorb more anions via electrostatic adsorption.

The increase of acid concentration in the modification process is beneficial for improving the removal efficiency of pollutants. The removal rate of contaminants is highly dependent on the concentration of acid. The surface and internal pore structure of zeolite might be damaged and associated with excessed acid resulting in the reduction of removal rate. Moreover, acid treatment can easily change the pH value of the modified product, and improper operation can also bring secondary pollution. Cheng55 used different concentrations (0.1–2 mol L−1) of sulfuric acid solution to modify natural zeolites for removing nitrate from water. When the sulfuric acid concentration increased from 0.1 mol L−1 to 0.3 mol L−1, the removal of nitrate increased from 57.46% to 63.17%, but when the sulfuric acid concentration reached 2 mol L−1, the removal rate decreased to 47.60%. Acid with a concentration of less than 1 mol L−1 for multiple treatments not only can act as an acidifier but also can prevent the reduction of crystallinity and pore space.56 Patrylak and co-workers found that the volume of mesopores with a diameter of 2 to 10 nm increased (by a factor of two) after acid treatment by 1 and 3 mol dm−3 HCl, and that the volume of the zeolite increased even more (by a factor of three) when treated with a more concentrated acid (3 mol dm−3 HCl).57

Salt modification. Salt modification is carried out by immersing the zeolite in a salt solution so that the cations in the salt solution replace the alkaline earth metal ions or other metal cations in the zeolite. The ion exchange order is Cs+ > Rb+ > K+ > NH4+ > Ba2+ > Sr2+ > Na+ > Ca2+ > Fe3+ > Al3+ > Mg2+ > Li+.58 The most commonly used solutions are sodium chloride, magnesium chloride, aluminum chloride, and ammonium chloride.59,60 This modification method can remove water and inorganic impurities from zeolite channels, increase the active site and ion exchange capacity of zeolite, and reduce the silica–alumina ratio of zeolite. Removal of heavy metal ions often uses ions with smaller radii, such as NH4+, and Na+, and removal of inorganic anions usually uses cations with larger radii, such as Ca2+, Fe3+ and Al3+. Zhang et al.31 found that chemical modification of natural zeolite with CaCl2 significantly improved fluoride removal performance. The maximal fluoride adsorption capacity of CaCl2-modified zeolite (CZ) was 1.766 mg g−1 at the initial fluoride concentration of 100 mg L−1.
Rare earth modification. Rare earth modification involves mixing natural zeolites into a rare earth solution by stirring, drying and crushing. The commonly used rare earth materials include metals and oxides such as zirconium, lanthanum and cerium. Metal or metal oxide loading on the adsorbent surface can play a similar role to that of surface protonation. Positively charged metal ions are strongly adsorbed on the external surface of zeolite with a negative charge by attraction and/or ligand exchange,61 so that the external surface of zeolite exhibits a positive charge, which effectively improves its adsorption capacity for anions in water. Adding metal ions during the modification process can also increase the surface area and pore volume of zeolite, thus enhancing its adsorption capacity for the adsorbed material.62 Li et al.63 evaluated the surface properties of lanthanum-modified zeolite (LMZ), which exhibited a positively charged surface due to the introduction of lanthanum oxide. Chen et al.64 compared the fluoride removal effect of zirconium-modified zeolite, hydrochloric acid-modified zeolite, and natural zeolite, and the results showed that the fluoride removal capacity of zirconium-modified zeolite reached 19.84 mg g−1 and the saturation adsorption rate reached 93.4%, and its fluoride removal performance was superior to that of unmodified zeolite (56.4%) and acid-modified zeolite (84.4%).
Cationic surfactant modification. Surfactant-modified zeolites (SMZs) are prepared by mixing natural zeolites into cationic surfactants. The commonly used cationic surfactants are cetyltrimethylammonium (HDTMA), cetyltrimethylammonium bromide (CTMAB), and cetyl pyridinium bromide (CPB). The key to the modification is the loading of surfactant and the loading density on the zeolite surface. The form of the load can be divided into two types: a monolayer or hemimicelle (Fig. 3a) and a bilayer or admicelle (Fig. 3b). At low concentrations (surfactant concentration < critical micelle concentration (CMC)), the surfactant undergoes ion exchange on the zeolite surface and a monolayer of surfactant molecules is adsorbed and formed on the zeolite.65,66 At high concentrations (surfactant concentration > critical micelle concentration), the adsorbed surfactant molecules form a bilayer on the external surface of the zeolite.67 The formation of a bilayer causes a charge reversal on the external surface of the zeolite, with the modified zeolite exhibiting a positive charge on the external surface and still exhibiting a negative charge on the internal surface, which provides locations that will retain anions and repel cations, and additional cation exchange capacity is retained on the internal surface of the zeolite.68 Chao et al.69 investigated the possible mechanism of HDTMA on Na-zeolite. When HDTMA modifies the zeolite surface, the zeolite shows significant changes in texture, morphology and the nature of the surface charge. The aggregation of HDTMA on the external zeolite surface resulted in a significant decrease in the micropore surface area, total porosity and micropore volume of the HDTMA-modified zeolite samples (Na-H zeolite). The specific adsorption mechanisms can be divided into three types: (1) several intraparticle macropores are blocked by HDTMA. (2) HDTMA molecules can be adsorbed directly on the external surface of zeolite by electrostatic attraction, which leads to a continuous decrease in the number of adsorption sites associated with Al. (3) HDTMA can be adsorbed on the outer surface of Na-zeolite by electrostatic attraction after exchange with counter cations on the surface of Na-zeolite. SMZ is an effective adsorbent for a wide range of pollutants and to some extent removes oxygenated anions (chromate, antimonate, arsenate, selenate, phosphate, nitrate, etc.) and metal cations (lead, cadmium, zinc, copper, nickel, etc.). The specific adsorption mechanism of the oxygenate-containing anion on the SMZ surface is shown in Fig. 3c.
image file: d2ew00478j-f3.tif
Fig. 3 Conceptual model of cationic surfactant adsorption and removal of oxygenate-containing anions onto natural zeolite.

The contaminant removal rate is related to the surfactant loading rate (depending on the concentration of the surfactant solution). The adsorption of nitrate by HDTMA-modified zeolite increases as the dose of HDTMA increases. When the loading of HDTMA is 25% of the effective cation exchange capacity (ECEC) of zeolite, the adsorption of nitrate by zeolite is <0.3 mg g−1, while when the loading of HDTMA is 150–200% of the ECEC of zeolite, the adsorption of nitrate by zeolite increased to 5 mg g−1.70 In addition, the counterbalance ions have an important influence on the micelles formed of ionic surfactants. Li et al.67 found that HDTMA-Br and HDTMA-Cl formed complete bilayers on zeolites, while HDTMA-HSO4− showed incomplete bilayer formation on zeolites. They also discovered that the counterbalance ions (Cl−, Br− and HSO4−) also affected the chromate adsorption, as follows: HDTMA-HSO4− > HDTMA-C1− > HDTMA-Br− (28, 16 and 11 mmol kg−1, respectively). Thus the effect of counteracting ions must be considered when surfactants are used for modification. The removal of oxygenate ions by SMZ has been extensively studied, and its partial adsorption capacity is shown in Table 1. The adsorption capacity of modified zeolite is closely related to the surfactant, the type of contaminant and the variety of zeolite.

Table 1 Adsorption capacity of SMZ for different anionic pollutants
Surfactant Zeolite Pollutant Adsorption capacity (mg g−1) Removal efficiency (%) Temperature (°C) pH Ref.
HDTMA-Br Clinoptilolite Nitrate 2.70 75.00–80.00 25 5.00–6.00 71
HDTMA Clinoptilolite Nitrate 5.00 — — — 70
HDTMA-Br — Nitrate 3.47 — 25 — 72
CPB — Nitrate 9.36 — 25 6.00 66
HDTMA-Br Clinoptilolite Chromate 5.44 — 25 7.00 67
HDTMA Clinoptilolite Antimonate 2.10 39.00 25 7.00–8.00 62
HDPB Clinoptilolite Chromate 2.82 — 20 5.00 73
HDPB Chabazite Chromate 14.31 — 20 5.00 73
HDTMA Clinoptilolite Arsenate 0.04 — 25 7.40 74
CTAB — Chromate 15.41 83.40 25 5.00 75
CPB — Phosphate 0.23 — 25 6.00 75

Chitosan modification. Chitosan-modified zeolites have attracted more and more research interest in recent decades. Chitosan is a copolymer composed of glucosamine and N-acetylglucosamine.76 Chitosan can adsorb the negatively charged colloidal particles in water due to the protonation of the amino group of chitosan into a positively charged polyelectrolyte with high charge density in acidic solution. However, chitosan has relatively low mechanical strength. Zeolite as a natural mineral has the advantages of high mechanical strength, high impact resistance, large specific surface area and many active sites. The silica oxygen structure of zeolites inherently carries a negative charge; hence natural zeolites are less effective in removing anionic contaminants from water.77 The zeolite modified by chitosan will form a very thin coating layer on the surface of the zeolite to improve the removal efficiency of anionic pollutants from wastewater. It can combine the advantages of both zeolite and chitosan. Studies by Peng et al.78 have demonstrated that surface coating with chitosan, that is, even if only part of the zeolite surface is covered with chitosan, can greatly improve the adsorption of fluoride. Han et al.79 explained the mechanism of chitosan-coated Na-X zeolite. The Si–OH (in Na-X zeolite) was mineralized by C–OH (in chitosan) and formed an interpenetrating network, which makes chitosan immobilized on the Na-X zeolite. The surface groups of Al–OH and –NH2 of the adsorbent were protonated in acid solution, and the positive charge was attained at the adsorbent surface; this allows the modified zeolite to adsorb anions, such as As(V). Arora et al.80 used sulfuric or hydrochloric acid to protonate chitosan-coated zeolite (Ch-Z) and tested its ability to adsorb nitrate from water; the study showed that Ch-Z has a maximum nitrate adsorption capacity of 37–46 mg g−1 at 20 °C. Ch-Z has a nitrate exchange capacity comparable to that of other weak base anion exchangers and can capture nitrate.

2.3 Composite modification

In addition to modification using a single approach, there are also composite modifications that combine multiple approaches. The composite modification can prepare more efficient adsorbent materials. Research in recent years has also gradually moved from single modification to composite modification. Table 2 shows the anionic contaminants removed by composite modification. The composite-modified zeolite has better adsorption effect on the oxygen anion. Li et al.81 observed the role of aluminum-modified zeolites in fluoridated water treatment. They prepared three different types of zeolites, HCl-modified zeolite, Al2(SO4)3-modified zeolite, and HCl–Al2(SO4)3 composite-modified zeolite, by modifying zeolites with HCl and Al2(SO4)3. Compared with natural zeolites (fluoride removal rate of 14.48%), all three modifications could improve the fluoride removal capacity (the removal rates were 23.86%, 26.5%, and 40.6%), but the HCl–Al2(SO4)3 composite-modified zeolite showed the best performance. This was mainly because the hydrochloric acid removed impurities from the natural zeolite, which increased the specific surface area of the zeolite, and the zeolite was immersed in aluminum sulfate, making it a good carrier for hydrated aluminum chloride. Tang et al.82 also found that the fluoride removal efficiency using NaCl–Al2(SO4)3 composite-modified zeolites was significantly higher than that using NaCl and Al2(SO4)3 alone, and the prepared composite-modified zeolites had good water temperature and pH adaptability. The specific surface area and pore volume of 13X zeolite impregnated with binary salt (LiCl + CaCl2) were reduced, but the adsorption capacity of the composite adsorbent with binary salt was 0.8–1.1 times higher than that of pure LiCl composite adsorbent. After 12 cycles, the adsorption amount could still reach 91.8% of the initial amount. The binary salt-modified zeolite has excellent adsorption performance. Arcibar-Orozco and colleagues83 investigated the synthesis of a chitosan–zeolite (CZ) composite modified with La(III) ions (CZ-La) to enhance its adsorption of fluoride. They proposed a possible mechanism for the anchoring of La on CZ. Chitosan loading on zeolite resulted in the presence of hydroxyl groups on CZ through the exchange of the hydroxyl groups present in the composite, forming O–La bonds and displacing the H+ ions in the solution; these ions contributed by chitosan can form chelates with metal ions and cause an increase in pH in lanthanum solution. The mechanism of fluoride adsorption on CZ-La is related to the solution pH. When pH < pHpzc, the composite surface will acquire a positive charge favorable to fluoride ion adsorption, and the adsorption mechanism is mainly electrostatic attraction at this time. The surface of CZ-La is negatively charged, creating a strong repulsive force between the active site and the fluoride ion during the adsorption at high pH. Therefore, fluoride removal occurs mainly as a result of the ion exchange mechanism. Fluoride is bound to lanthanum through the displacement of OH−, thus forming lanthanum trifluoride.
Table 2 Adsorption capacity of composite-modified zeolites for different anionic pollutants
Method Pollutant Initial concentration of pollutant (mg L−1) Removal efficiency (%) Adsorption capacity (mg g−1) Temperature (°C) pH Ref.
6% NaCl + 15% Al2(SO4)3 Fluoride ion 20.00 59.60 0.45 25.00 6.00–9.00 84
Muffle (450 °C) + 2.0 mol L−1 AlCl3 + muffle (450 °C) Fluoride ion 5.00 48.68 2.40 25.00 — 85
Muffle (550 °C) + 1.0 mol L−1 HCl + 1.0 mol L−1 FeCl3 Fluoride ion 10.00 83.70 1.65 25.00 5.00–7.00 86
3.0 mol L−1 HCl + 0.2 mol L−1 Al2(SO4)3 Fluoride ion 1.63 40.06 — — — 81
0.01 mol L−1 HCl + Al2(SO4)3 + CaSO4 (ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1) Fluoride ion 10.00 — 8.62 25.00 4.00–8.00 87
Al2(SO4)3 + chitosan Fluoride ion 5.00 91.70 0.23 20.00 6.00–7.00 88
0.5 mol L−1 MgCl2 + 0.5 mol L−1 AlCl3 Chromate 25.00 80.00 4.52 25.00 4.00–6.00 89
Muffle (873 K) + 0.5 mol L−1 MgO + microwave oven (2450 MHz) Phosphate <60.00 86.30 — — 9.00 90
0.5% La2O3 + muffle (450–500 °C) Phosphate 5.00 99.00 25.00 — 4.00–8.00 91
MgCl2 + AlCl3 (mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4) Phosphate ≤10.00 ≥51.00 — 25.00 — 92
La(NO3)3 + ZrO2 Phosphate — — 21.20 25.00 7.50 93

The high removal capacity is an advantage of rare earth modification, but the high cost limits its industrial application. Zeolite modified with chitosan also has good adsorption effect, but poor mechanical strength is also a problem that needs to be solved urgently. However, acid modification and salt modification as well as physical modification methods with good economic prospect have poor removal effect. Therefore, we propose a composite modification to resolve these disadvantages.

3. Adsorption effect for typical anionic pollutants in wastewater

3.1 Fluoride

The main source of human fluoride intake is drinking water, and high concentrations of fluoride in drinking water can cause the phenomenon of fluorosis. Drinking water with high fluoride concentration for a long time will cause dental fluorosis, and in serious cases, it will also cause fluorosis of bone, cancer, Alzheimer's disease and other diseases that endanger human life.94,95 Many studies on the effectiveness of natural and modified zeolites for fluoride removal have been conducted in the past two decades. The conclusions of some studies on the adsorption of fluoride from wastewater by modified zeolites are shown in Table 3. With many modified zeolites, fluoride ions can be effectively removed, achieving adsorption amounts ranging from 1.44 to 22.2 mg g−1.
Table 3 Comparison of different modified zeolites for fluoride removal
Modification method Modifier Adsorption condition (optimal) Regeneration method Kinetics Thermodynamics Adsorption model Ref.
C 0 denotes the initial concentration of fluoride; q denotes the adsorption amount of adsorbent; R denotes the removal rate of fluoride; d denotes the amount of adsorbent; pH denotes the initial pH value of the solution; T denotes the temperature; and t denotes the contact time.
Salt modification CaCl2 C 0 = 100.00 mg L−1; q = 1.77 mg g−1; d = 50.00 g L−1; pH = 5.00–7.00; T = 25.00 °C; t = 6.00 h NaOH Pseudo-second-order kinetics, intra-particle diffusion models Spontaneous, endothermic Langmuir, Freundlich 31
Salt modification FeCl3 C 0 = 10.00 mg L−1; pH = 6.94; t = 2.00 h HCl Pseudo-first-order model — Langmuir 103
Salt modification KAl(SO4)2 C 0 = 20.00 mg L−1; q = 0.53 mg g−1; T = 25.00 °C; t = 5.00 h KAl(SO4)2 — — — 104
Salt modification Al2(SO4)3 C 0 = 50.00 mg L−1; d = 2.00 g L−1; pH = 6.00–9.00; q = 22.20 mg g−1; T = 25.00 °C; t = 12.00 h — Pseudo-second-order model — Freundlich 105
Salt modification MgSO4 C 0 = 80.00 mg L−1; pH = 6.00; q = 0.80 mg g−1; T = 20.00 °C; d = 0.20 g L−1; R = 71.65% NaOH Pseudo-second-order kinetics Spontaneous, exothermic Freundlich, Dubinin–Radushkevich 99
Salt modification Al2(SO4)3 C 0 = 80.00 mg L−1; pH = 6.00; q = 0.88 mg g−1; T = 20.00 °C; R = 80.34% NaOH Pseudo-second-order kinetics Spontaneous, exothermic Freundlich, Dubinin–Radushkevich 99
Salt modification FeCl3 C 0 = 50.00 mg L−1; q = 1.27 mg g−1; R = 83.00%; T = 28.00 °C; t = 4.00 h — Pseudo-second-order model — Langmuir 106
Composite modification ZrOCl2, chitosan C 0 = 5 mg L−1; q = 10.75 mg g−1; pH = 6.50; t = 2.00 h; T = 30.00 °C — Pseudo-first-order model, pseudo-second-order kinetic model Spontaneous, endothermic Freundlich, Langmuir 107
Rare earth modification ZrOCl2·8H2O C 0 = 10.00 mg L−1; d = 5.60 g L−1; q = 2.20 mg g−1; pH = 5.00; T = 30.00 °C — Pseudo-second-order model — Langmuir 108
Rare earth modification La3+ C 0 = 100.00 mg L−1; d = 7.50 g L−1; t = 25.00 min; T = 40.00 °C; pH = 4.00 — — Spontaneous, exothermic — 109
Rare earth modification Ti(SO4)2 C 0 = 53.76 mg L−1; d = 4.00 g L−1; pH = 3.00; q = 6.12 mg g−1; T = 25.00 °C; t = 12.00 h — Pseudo-second-order model — Freundlich 105
Rare earth modification Ti C 0 = 80.00 mg L−1; pH = 6.00; q = 1.64 mg g−1; T = 20.00 °C; R = 94.78% NaOH Pseudo-second-order kinetics Spontaneous, exothermic Freundlich, Dubinin–Radushkevich 99
Rare earth modification La3+ C 0 = 100.00 mg L−1; q = 27.00 mg g−1; D = 10.00 g L−1; t = 19.00 h — Pseudo-second-order kinetic model Spontaneous, endothermic Freundlich 109
Organic modification Chitosan C 0 = 40.00 mg L−1; q = 4.16 mg g−1; d = 0.40 g L−1; t = 3.00 h; T = 25.00 °C; pH = 4.50–5.50 — Pseudo-second-order model — Freundlich, Relidich–Peterson 110
Organic modification CTAB C 0 = 10.00 mg L−1; q = 2.99 mg g−1; T = 20.00 °C; pH = 5.00; d = 0.02 g L−1; t = 1.00 h — Pseudo-second-order model Spontaneous, exothermic Langmuir 111
Composite modification H2SO4, Fe2(SO4)3 C 0 = 10.00 mg L−1; q = 0.73 mg g−1; d = 80.00 g L−1; pH = 6.00–8.00; t = 2.00 h — — — Freundlich 96
Composite modification NaCl, Al2(SO4)3 C 0 = 20.00 mg L−1; q = 0.48 mg g−1; t = 40.00 min; pH = 4.00 — — — — 112
Composite modification AlCl3, muffle (450 °C) C 0 = 5.00 mg L−1; q = 2.40 mg g−1; d = 30.00 g L−1; R = 48.68%; T = 25.00 °C — — — — 85
Composite modification Al2(SO4)3, HCl C 0 = 1.63 mg L−1; d = 50.00 g L−1; T = 25.00 °C; t = 60.00 min — — — — 81
Composite modification Al2(SO4)3 C 0 = 10.00 mg L−1; q = 8.62 mg g−1; T = 25.00 °C Al2(SO4)3 Pseudo-second-order kinetics Spontaneous, endothermic Freundlich 87
CaSO4 pH = 4.00–8.00; d = 1.50 g L−1; t = 6.00 h NaOH, HCl
Composite modification NaOH, Al2(SO4)3 C 0 = 80.00 mg L−1; d = 20.00 g L−1; q = 1.44 mg g−1; T = 20.00 °C; pH = 6.00 Pseudo-second-order model Spontaneous, exothermic Freundlich 113
Composite modification ZrOCl2, chitosan C 0 = 5.00 mg L−1; q = 10.75 mg g−1; pH = 6.50; t = 2.00 h; T = 30.00 °C — Pseudo-first-order model, pseudo-second-order kinetic model Spontaneous, endothermic Freundlich, Langmuir 85
Composite modification Al2(SO4)3, chitosan C 0 = 5.00 mg L−1; q = 0.23 mg g−1; d = 20.00 g L−1; pH = 6.00–7.00; T = 20.00 °C; R = 91.70%; t = 8.00 h — Pseudo-second-order model Spontaneous, exothermic Freundlich 88

Salt solutions, rare earth elements, cationic surfactants and chitosan are mainly used to modify zeolites to enhance their fluoride removal performance. The earliest methods and the most commonly used modification methods are acid modification, alkali modification, and salt modification. Reagents such as acid and alkali salts mainly modify the endopore structure of zeolites, i.e., they change the acidic position of zeolites or limit the endopore diameter of zeolites. There are many reports about the use of salt solutions to modify zeolites.87,96–98 The adsorption capacity of salt-modified zeolite is about 0.5–1.7 mg g−1 for fluoride, and natural zeolite hardly has any adsorption capacity for F−; the adsorption capacity is only 0.021 mg g−1.96 In general, the adsorption performance of rare earth-modified and chitosan-modified zeolite is better than that modified with acid and alkali salts and other reagents alone, but higher costs make it unsuitable for promotion in practical engineering. Composite modification is a more popular method among researchers due to its higher adsorption capacity than a single modification method. The adsorption of fluoride ions by some modified zeolites can reach 8–11 mg g−1, mainly in the combination of inorganic modification (acid modification, alkali modification, salt modification, thermal modification) and organic modification + inorganic modification.85,87 However, there is also the problem of poor economics when it comes to composite modification methods such as rare earth elements and chitosan.

The adsorption effect of modified zeolite is influenced by the initial concentration of fluoride ions,87 adsorbent dosage,31 initial solution pH,99 temperature,99 and contact time.81 The adsorption of modified zeolites increased with increasing fluoride concentration, and after reaching a certain concentration, the adsorption capacity decreased. This is due to the limited availability of adsorption sites.87,97 The fluoride removal efficiency of modified zeolites increases with increasing adsorbent dosage, which was obviously due to an increase in the number of active sites available for fluoride adsorption,31,98 but it was not the case that more adsorbent dosage is better. Studies have shown that modified zeolite has better adsorption effect in weak acid and neutral solutions (pH 4–7).100 Optimal pH depends on the type of loaded metal ion.101 For example, the optimal pH values are 5–7 for CaCl2-modified zeolites and 6–7 for HDTMA-Br-modified zeolites,97 while the complex salt-modified zeolites have high adsorption capacity in the pH range of 4 to 8.87 Different conclusions have emerged from scholars regarding the effect of coexisting ions on the adsorption capacity. Onyango et al.102 showed that Cl− and NO3− enhance the adsorption of F− by Al3+- or La3+-modified zeolites; SO42− has no effect on Al-modified zeolites but weakly enhances La-modified zeolites; HCO3− has a significant effect on Al zeolites and no effect on La zeolites; PO3− slightly affects both zeolites. Arfin87 reported the effect of common anions in drinking water on the adsorption capacity of Al3+ and Ca2+ composite modified zeolites (CAZ). The presence of HCO3−, CO32− and PO43− led to a decrease in the adsorption capacity of zeolites, which is mainly due to the competing effects of anions and the change in solution pH. However, the presence of Cl−, SO42− and NO3− had no effect on the adsorption of CAZ. Overall, the adsorption capacity of CAZ decreases by approximately 40% in the presence of all anions. In summary, the adsorption effect of the modified zeolite on fluoride ions is influenced by many factors, and there is no uniform conclusion on the specific trend and magnitude of the effect. Future studies should further clearly show the effect mechanism of fluoride removal by modified zeolites.

3.2 Nitrate

Nitrate pollution in surface water and groundwater has become a global environmental problem. High concentrations of nitrate in drinking water will pose a serious threat to human health.114,115 The best method for nitrate removal is adsorption with inexpensive adsorbent materials such as zeolite. However, natural zeolites are not effective in removing nitrates116 because the inherently negatively charged surface of zeolites enhances the electrostatic repulsion between the adsorbed anionic substances such as nitrate and the active sites. Most of the current studies have focused on the preparation of modified zeolites to achieve efficient nitrate removal. Commonly used acid solution, alkali solution, salt solution and other modification methods, such as hydrochloric acid modification + thermal modification of the modification method, showed almost no adsorption for nitrates.117 This is because zeolite is a microporous material; the ionic radius of nitrate is larger than the microporous radius of zeolite, and nitrate cannot enter the interior of zeolite, resulting in lower removal efficiency. Inorganic modification cannot change the endopore diameter of zeolite effectively. Thus, some scholars have tried to add large metal organic compounds that cannot enter the zeolite pore channel to achieve the modification purpose. Zeolites modified with organic substances, especially those modified with surfactants, have attracted attention in recent years because they have good adsorption capacity for oxygen anions with large ionic radii as well. It has been shown that the maximum removal rate of nitrate by HDTMA-Br-modified clinoptilolite zeolites could reach 80%,71 and the removal capacity increases with the loading of surfactant.118 The surfactant loading on the zeolite surface significantly affects the adsorption capacity of zeolite for nitrate. Natural zeolites and zeolites covered with monolayer surfactant were not effective in removing nitrate from aqueous solutions. However, zeolites with mottled bilayers (irregular bilayer surface configuration) or bilayer surfactant coverings could effectively remove nitrate.119 It was found that counter ions play an important role in stabilizing the adsorbed surfactant bilayer, and surfactant counter ions affect the aggregation number, which in turn affects the size and shape of micelles and the critical micelle concentration.115 Different counter ions form bilayers with different densities on zeolites,120 as the trend of the effect of antagonistic ions (Br−, Cl−, HSO4−) is HDTMA-Br > HDTMA-Cl > HDTMA-HSO4, which indicates that Br− makes the HDTMA bilayer more stable than Cl−.68 In addition, the adsorption capacity of surfactant-modified zeolite was little affected by pH. A slight decrease in nitrate adsorption occurred with increasing pH.117,121 Competing ions also barely affect the amount of nitrate adsorbed by the modified zeolite, but slow down the nitrate kinetics.71,122 However, Mirzayi et al.123 found that various binary (nitrate/bicarbonate, nitrate/sulfate, nitrate/phosphate) and quaternary systems (nitrate/bicarbonate/sulfate/phosphate mixtures) reduce the sorption capacity of natural diabase zeolites for nitrate, and phosphate had the greatest effect on the reduction of nitrate uptake. This was due to the presence of competitive adsorption. It was also controlled by factors such as the geometry and charge density of the anion. However, the chitosan-modified zeolite, though effective in removing nitrate, was not selective for nitrate.80 Therefore, the modification of zeolite with cationic surfactants will be a key technology for the application of natural zeolite for nitrate removal, and it will also have a broad application prospect.

3.3 Phosphate

Phosphorus is an essential nutrient for the growth of aquatic organisms, but excessive phosphorus can cause eutrophication in surface water bodies, damaging the ecological environment and endangering human health. In recent years, studies have shown that rare earth modification and cationic surfactant modification can effectively adsorb phosphate from water. The results of experiments by Quan124et al. showed that HDTMA-modified zeolite had the highest adsorption capacity, followed by salt-modified (MgCl2 + AlCl3) zeolite, and natural zeolite had the lowest. The removal rate of phosphate by HDTMA-modified zeolite can be maintained at more than 80%.71 HDTMA as a large organic cation cannot enter the inside of the zeolite pores, and whose N-terminal end of the cation is adsorbed on the negatively charged zeolite surface, forming a micelle-like coverage on the zeolite surface. The phosphate is removed by forming a precipitate with the surfactant. After desorption, the cationic surfactant-modified zeolite can be used as a fertilizer carrier for phosphorus slow release to maximize the benefits of resource utilization.125 When the zeolite adsorption process is combined with the activated sludge process to remove phosphorus from wastewater, surfactant-modified zeolite with a single layer of coverage is more effective than surfactant-modified zeolite with a bilayer or incomplete bilayer coverage.126 This contradicts the findings that the majority of bilayer-covered surfactant-modified zeolites are effective for removing oxygen anions. Further studies on the interaction mechanism between surfactant-modified zeolite and polyphosphoric bacteria and between surfactant modified zeolite and phosphorus-accumulating organisms are needed. In addition, the study proved that lanthanum- and cerium-modified zeolite of rare earth elements also have better phosphorus removal effect.127 This was because lanthanum and cerium form functional groups containing hydroxyl groups on the zeolite surface, and PO43− was exchanged with hydroxyl ions and then ligated with rare earth ions. A study by He et al.128 showed that coexisting anionic species may compete with phosphate for adsorption of lanthanum-modified zeolites, and their interfering effect on phosphate adsorption eventually reduces the efficiency of phosphate adsorption. The presence of Cl− and NO3− had a slight effect on phosphate removal (the efficiency of phosphate removal decreased from 98.9% to 94.6% when the concentration of Cl− and NO3− was increased from 0.5 mM to 20 mM), which could be attributed to the fact that multivalent anions were more easily adsorbed than monovalent anions. However, when the HCO3− concentration was 20 mM, the phosphate removal efficiency was reduced by 18%. This was due to the electrostatic repulsion between the deprotonated surface hydroxyl groups and the highly charged phosphate ions caused by the high pH (due to the presence of HCO3−). For different types (concentrations) of phosphorus-containing wastewater, the phosphorus removal effect of modified zeolite is shown in Table 4. Studies on phosphate removal by modified zeolites are mostly focused on laboratory scale, and there are still relatively few studies on phosphate adsorption in actual wastewater.
Table 4 Phosphorus removal effect of modified zeolite
Sewage type (concentration) Modifier Removal rate (%) Adsorption capacity (mg g−1) Ref.
Experimental water, 5 mg L−1 La2O3 99.00 50.00 129
Experimental water, 30 mg L−1 Ce(SO4)2·4H2O 99.00 0.99 130
Experimental water, ≤10 mg L−1 HDTMA ≥85.00 — 124
Experimental water, ≤10 mg L−1 MgCl2 + AlCl3 ≥51.00 — 124
Pig farm wastewater, ≤60 mg L−1 MgO 86.30 — 131
Experimental water, ≤5 mg L−1 HDTMA-Br ≥80.00 — 125
Experimental water, 10 mg L−1 LaCl3 + AlCl3 95.86 2.43 132
Domestic sewage, 20 mg L−1 HDTMA 56.60 — 133
Experimental water, 10 mg L−1 FeCl3 — 3.60 60
Experimental water, 20 mg L−1 La — 0.31 127
Experimental water, 20 mg L−1 Ce — 0.33 127
Experimental water, 5 mg L−1 La2O3 99.04 15.06 134
Experimental water, 5 mg L−1 ZrOCl2·8H2O — 6.87 135

The pH of the solution has a significant effect on the amount of phosphorus adsorbed compared to other anions.132 This is because the pH will determine the form of phosphorus present as shown in the following equation:136 H3PO4image file: d2ew00478j-t1.tif H2PO4− + H+image file: d2ew00478j-t2.tif HPO42− + 2H+image file: d2ew00478j-t3.tif PO43− + 3H+ (pK1 = 2.15, pK2 = 7.20, pK3 = 12.33).

When pH ≤ 2.15, the main form of phosphorus was PO43−; when 2.15 < pH < 12.33, the main forms were H2PO4− and HPO42−. Different modifiers have different optimal adsorption pH ranges owing to the different affinities of the adsorbents for different valence states of phosphorus. For instance, the lanthanum/aluminum-modified zeolite adsorbent (La/Al-ZA) achieved good adsorption at pH 4–9, which indicates that La3+ has a strong affinity for divalent phosphorus.125,132 The magnitude of the effect of various anions in solution on phosphorus adsorption may be related to the affinity of the adsorbent for the anions.132

The magnitude of the effect of various anions on phosphorus adsorption may be related to the affinity of the adsorbent for the anions. The size of the affinity depends on the valence of the ion and the radius of the hydrated ion.125 In general, the higher the valence state, the smaller the radius of the hydrated ion and the stronger the adsorption effect. For example, although Cl− was a monovalent ion, its radius (0.181 nm) was much smaller than that of SO42− (0.240 nm), so Cl− had more influence than SO42−. Table 5 shows the adsorption of competing ions on modified zeolites. As can be seen from the table, there is still debate about the effect of competing ions on phosphorus adsorption, and conflicting results have been reported in the literature. The effect of competing ions on phosphorus removal from modified zeolites needs to be further investigated.

Table 5 Adsorption of competing ions on zeolite
Modification method Competing anion Effectiveness Ref.
FeCl3 Cl−, SO42−, HCO3− Cl− and SO42− had little effect; HCO3− had an inhibitory effect and the removal rate was reduced by 8% 60
ZrOCl2·8H2O K+, Na+, Ca2+, SO42−, HCO3− K+, Na+, Ca2+, and SO42− had no effect; HCO3− significantly inhibited 131
ZrOCl2·8H2O Cl−, SO42−, HCO3−, SiO3− Cl−, SO42− had no effect; HCO3−, SiO3− had an inhibitory effect 104
LaCl3 + AlCl3 Cl−, SO42− Slightly inhibited by 0.1 mmol L−1 Cl−, SO42−; significantly inhibited by 0.50–2.0 mmol L−1 Cl−, SO42− 132
HDTMA NO3−, Cl−, SO42−, HCO3− NO3−, Cl−, SO42−, HCO3− do not affect the removal rate but affect the kinetics 71
CPB SO42−, HCO3− SO42− was obviously inhibited, but increasing the pH could eliminate the effect; HCO3− was slightly inhibited, and increasing the pH could not eliminate the effect 137
HDTMA-Br NO3− Inhibition occurred with C(NO3−) >50 mg L−1 125

3.4 Other anionic pollutants

In addition to the anions mentioned above that can cause water pollution, there are many other anions, such as arsenate and chromate, that can also be harmful to humans and the environment, causing non-negligible harm and pollution. Arsenate and chromate are mainly from natural evolution and substandard industrial wastewater discharge. Arsenate in solution exists mainly in trivalent and pentavalent forms; pentavalent arsenic is easily aggregated in a high oxygen environment, and trivalent arsenic is easily concentrated under reducing conditions.138 The toxicity of trivalent arsenic is much higher than that of pentavalent. A moderate amount of arsenic helps the synthesis of hemoglobin and can promote human growth and development, but too much arsenic can cause arsenic poisoning. Chromate exists in aqueous solution in two oxidation states, Cr(III) and Cr(VI). Cr(VI) has carcinogenic and genetic mutagenic effects on organisms.139

Several attempts have been made in the past to modify zeolite surfaces with organic substances to allow the adsorption of oxygenate-containing anions.140–143 Modification of zeolites with cationic surfactants is the most common method for removing oxyacid-containing anions.67,144,145 As an example, zeolites are treated with HDTMA, ODTMA and CPB and used to remove chromate, sulfate and arsenate ions with promising results.146–148 The adsorption capacity of chromate and arsenate after modification can reach between 2.82 and 14.31 mg g−1, while the adsorption capacity of unmodified zeolite is less than 1 mg g−1.73,74,149 Zeolite adsorption of arsenate and chromate is highly dependent on pH,144 surfactant loading,73,144 and counteracting ions.67,118 The effects of different factors on the adsorption of chromate and arsenate are shown in Table 6. The pH of the system has a dramatic effect on the morphology of Cr and As, and zeolites have the strongest adsorption capacity for them when the monovalent form is dominant. Also, it has been found that acidic pH and neutral pH are the best conditions to achieve high adsorption of Cr and As. The loading level of HDTMA affects the shape of the surfactant layer on the zeolite surface, and the counteracting ions affect the stability of the HDTMA bilayer. The attraction between the counteracting ion and the head group of HDTMA can counteract the repulsive force of the head group, reducing the diffusion of surfactant out of the bilayer and decreasing the competition between chromate and HDTMA in solution. Therefore, when surfactants are used to modify zeolites, the intense influence of counterions must be considered.

Table 6 Effect of different factors on the adsorption of anionic pollutants by modified zeolites
Anion type Modifier Influencing factor State of existence Absorbing situation Conclusion Ref.
Chromate HDTMA pH pH = 3.00: HCrO4− (dominant species) The adsorption efficiency was highest when pH = 3.00; when the pH increased, the adsorption power decreased At lower pH, most of the Cr(VI) species were present in the monovalent form (HCrO4−) and only one exchange site of thezeolite was required for adsorption; at high pH, most of the Cr(VI) species were present in the divalent form (Cr2O72−, CrO42−) and requires two exchange sites of zeolite to be adsorbed; moreover, it was affected by the competition of OH− 144
pH = 5.00: HCrO4−
pH = 2.00–6.00: HCrO4− and Cr2O72− (in equilibrium)
pH >6.00: CrO42−
pH = 7.00: CrO42−
Arsenate HDTMA pH pH = 3.00–6.00: H2AsO4− (dominant species) The highest removal rate at pH = 8.00 At pH = 8, the monovalent (H2AsO4−) form of As(V) dominated and only one adsorption site from the modified zeolite was required; at pH = 12, As(V) species were present in the divalent (HAsO42−) and trivalent (AsO43−) forms, requiring two and three adsorption sites from zeolite for adsorption to occur, in addition to competition from OH− 144
pH = 4.00: H3AsO4 (dominant species) At pH = 12.00, the lowest removal rate
pH = 6.00–8.00: H2AsO4− and HAsO42−
pH = 8.00: H2AsO4− (dominant species)
pH = 8.00–11.00: HAsO42− (dominant species)
pH = 12.00: HAsO42− and AsO43−
Chromate, arsenate HDTMA HDTMA load level — The adsorption capacity was strongest when the loading = 100.00% of zeolite ECEC At lower HDTMA levels (50%), the lower amount of HDTMA attached to the zeolite led to reduced adsorption; at higher HDTMA levels, excess, loosely bound HDTMA was easily released from the zeolite into the aqueous solution, which in turn led to competition between chromate and HDTMA in solution 65, 144
The adsorption capacity decreased when the loading = 50% and 200% of zeolite ECEC
Chromate HDTMA Counter ions (Cl−, Br−, HSO4−) — Adsorption capacity: HDTMA-HSO4 > HDTMA-Cl > HDTMA-Br (28.00, 16.00 and 11.00 mmol kg−1 respectively) The role of counter ions in stabilizing HDTMA bilayer adsorption could not be ignored 67

4. The adsorption mechanism of modified zeolite for anionic pollutants

Understanding the mechanism of adsorption is very significant because without proper knowledge of adsorbate–adsorbent interactions, it is absolutely impossible to design materials for future applications or developments. The adsorption mechanism of anionic pollutants on modified zeolites is mainly conducted by electrostatic interactions, anion ion exchange, and a combination of these interactions (Fig. 4). Studies on the mechanism of microporous zeolite adsorption have focused on four aspects: adsorption kinetics, adsorption thermodynamics, adsorption constitutive relationship and adsorption selectivity. The adsorption of anionic contaminants by modified zeolites is closely related to the surface properties of zeolites, including pore volume, specific surface area, surface charge, and functional groups. Physical modifications, such as heating and microwave heating, can affect the micropore structure, pore volume, and specific surface area of zeolites, which in turn can affect the adsorption mechanism, such as pore filling or intraparticle diffusion.150 Usually, chemical modification is used to enhance the adsorption capacity of zeolites by increasing the number of adsorption sites, thus facilitating electrostatic surface interactions as well as hydrophobic, covalent, and complex interactions between zeolites and contaminants.151 From most studies, most of the adsorption kinetic models for modified zeolites for anionic contaminants correspond to the pseudo-second-order kinetic model, and some adsorption processes are also consistent with the intraparticle diffusion model.152 This suggests that the adsorption of anionic pollutants is mostly controlled by chemical mechanisms, and both intraparticle diffusion and surface diffusion are involved in the adsorption of anions, whereas the adsorption thermodynamics suggests that the Langmuir and Freundlich models are more suitable as adsorption isotherm models for modified zeolites. It shows that the adsorption process is mostly monolayer chemisorption or multilayer physical adsorption. When different anionic contaminants are adsorbed, the main adsorption mechanisms may be different and there are sometimes simultaneous removal mechanisms. For example, the main mechanism of arsenate and arsenite removal by zeolites modified by active agents was indicated to be ion exchange.153 The mechanism of zirconium-modified zeolites for fluoride removal is the bonding of fluoride ions to hydroxyl groups protonated by ZrO2.154 The mechanisms of p-arsanilic acid degradation and synchronous adsorption removal of the formed inorganic arsenic species were proposed by Zhou et al.155 The main adsorption mechanisms may not be the same when adsorbing different anionic contaminants. Thermodynamic analysis shows that these adsorption processes are mostly spontaneous, heat-absorbing reactions.
image file: d2ew00478j-f4.tif
Fig. 4 Adsorption mechanisms of anionic pollutant on modified zeolite.

5. Application in combination with other materials/processes

The pollution components in actual wastewater are complex and variable, and combination of processes or materials has become a more popular way to improve the treatment effect on the actual wastewater. In recent years, materials/processes containing natural/modified zeolites have been widely used in wastewater treatment plants for wastewater, domestic wastewater and surface water like rivers and lakes, which mainly adsorb ammonia nitrogen and organic matter, as shown in Fig. 5. By using anion exchange resin before Na-type zeolite, the efficiency of ammonia nitrogen removal from actual municipal wastewater could be increased from 78% to 95%.156 The zeolite-anammox process provides higher nitrogen removal and lower sludge productivity than conventional nitrification and denitrification wastewater treatment processes, resulting in lower operating costs for wastewater treatment plants.157 The combination of dielectric barrier discharge (DBD) and Fe-based zeolite catalysts for processing coking wastewater containing high concentrations of ammonia nitrogen and phenol resulted in an optimum removal rate of 75.11% for ammonia nitrogen (10.93% for zeolite alone and 9.67% for DBD alone) and 56.67% for phenol (20.61 for zeolite alone and 3.54 for DBD alone).158 The use of DBD coupled with Fe-based zeolite catalysts for the simultaneous treatment of mixed wastewater with ammonia and phenol is an effective way.158 The HFO/calcite/zeolite mixture has a high adsorption capacity of at least 31.7 mg g−1 for phosphate in water, which in addition effectively reduces the risk of phosphorus release from sediments into the overlying water under anoxic conditions.159 It can be seen that zeolite has been widely used in various wastewater treatment fields in combination with other processes/materials.
image file: d2ew00478j-f5.tif
Fig. 5 Application of combination process/materials of zeolites in wastewater treatment.

6. Conclusions and perspectives

Contrary to most reports about synthetic zeolites, natural zeolites do not perform as well as synthetic zeolites. However, the low cost and local availability of natural zeolites make them more in line with today's sustainable development concept. Improving the performance of natural zeolites and expanding the application areas of natural zeolites are of environmental importance. We have comprehensively analyzed and summarized the adsorption performance and mechanism of natural and natural modified zeolites. The results show that natural zeolites have almost no affinity for anionic contaminants, while modified zeolites have better adsorption capacity for anions. Composite modification seems to be a more advantageous modification. Electrostatic interactions, anion ion exchange, and indicated complexation are the main adsorption mechanisms. Modified zeolites exhibit advantages such as high adsorption capacity and fast adsorption kinetics for anionic pollutants (F−, NO3−, PO43−, arsenate, chromate, etc.) mainly due to the porous structure of zeolites with abundant adsorption sites and functional groups. Therefore, we can further improve the adsorption capacity and kinetics of zeolite by adjusting the pore size, modifying the pores, and introducing appropriate functional groups into the internal and external structures of zeolites.

Natural zeolites are a cheap and readily available material, and their rich and variable surface properties make them promising in the field of water treatment. Regeneration of natural zeolites, which were originally treated as waste, does not seem to be a significant part of the research. This is because the cost of regeneration may be higher than the cost of preparation. However, a number of challenges remain before zeolites can be applied to actual wastewater treatment. Most reported zeolite pore sizes are limited to the micropore range, which is smaller than the pore size of the target oxygen anion, hindering their migration and diffusion within the zeolite and limiting their industrial applications. With many contamination components in the actual wastewater, zeolites are susceptible to coexisting ions and pH values, resulting in reduced adsorption capacity and even structural damage. Zeolite adsorption saturation can be transformed into a new pollution source if disposed of improperly.

Some strategies can be considered to address the above challenges: (1) inserting mesoporous materials into the zeolite structure to increase the pore size; (2) introducing functional groups to enrich the adsorption sites and improve the stability; (3) combining with other materials/processes to make up for the lack of structural properties of zeolite; (4) finding a safe, non-polluting regeneration solution for recycling and cost reduction. Or develop a stable encapsulation method as a practical alternative for the final and safe disposal of zeolites.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 52170096) and the Fundamental Research Funds for the Central Universities (No. 2022YJSHH07).

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