Recent advance in enhanced adsorption of ionic dyes from aqueous solution: A review

Abstract Adsorption technology is a hot topic in the field of dye removal, and higher adsorption capacity is one of the eternal themes. Ionic dyes account for more than 90% of the literature on dyes adsorption in recent decade. In this article, various physical and chemical mechanisms of dye adsorption are combed in detail, and electrostatic interaction is the major mechanism of ionic dyes adsorption. The key to improving the adsorption capacity is to increase the electrostatic interaction force/surface charge of adsorbents. Moreover, different modification methods to improve the adsorption of ionic dyes by changing the charge properties of adsorbents were reviewed, including grafting functional groups, composite, and adsorbent structural modulation. The practical prospects of each modification method/substance were highlighted and discussed from the perspectives of performance, preparation processes, cost, and toxicity. This review mainly includes (1) the classification and adsorption mechanisms of dyes; (2) types, performance and application prospects of grafted ligands and substances applied for compositing; (3) the performance and limitations of the structural regulation methods; (4) research progresses of amphoteric adsorbents; (5) the prospect of adsorbents development and industrial-scale application. This review attempts to provide a detailed reference for the modification of adsorbents via electrostatic interaction regulation. Graphical Abstract


Introduction
Clothing, food, housing, and transportation are inseparable from dyes. More than 700,000 tons of dyes are produced and used annually, and about ∼15% of them are discharged into waterbody (Jiang et al., 2019;. For instance, the annual consumption of reactive dyes accounts for ∼30% worldwide, while 20-60% of the reactive dyes are inevitably discharged (Li, Mu, et al., 2019). Especially, most dyes are difficult to be biodegraded and have teratogenic, carcinogenic, and mutagenic effects on humans and aquatic organisms.
Coagulation, membrane separation, adsorption, advanced oxidation, biodegradation and other technologies can be applied to remove dyes (Bulgariu et al., 2019;Praveen et al., 2022). The advantages and disadvantages of dye removal technologies were discussed in our previous review . Biodegradation is the main technology of dyeing wastewater with the advantage of high efficiency and inexpensive. Chemical oxidation is appropriate and efficient for the highly toxic azo dyes, but the cost is high and iron-containing sludge will be generated. Common physical methods include coagulation/flocculation, membrane separation and adsorption (Katheresan et al., 2018;Zhou et al., 2019). Coagulation/flocculation is a typical pretreatment process to remove large particles, but it will produce precipitated substances. Membrane separation is an advanced treatment process at the end of dyeing wastewater, which has high efficiency but high cost. As an advanced treatment process, adsorption is not only economically feasible but also high-efficiency . Therefore, dye adsorption has been a hot topic in recent years. Figure 1 shows different dyes classifications. As shown in Figure 2a, more than 30,000 articles have been published since 2012, and more than 4,000 articles have been published annually since 2019. The order of the number of publications is reactive > basic > direct > acidic > disperse > vat dyes. About 95% of the literature deals with the adsorption of ionic dyes (anionic and cationic dyes), and only about 5% is for nonionic dyes ( Figure  2b). In an aqueous solution, anionic dyes are negatively charged due to sulfonic acid groups (-HSO 3 ), while cationic dyes have positive charges due to the presence of protonated amine or sulfur containing groups . There may be unpaired electrons, incompletely saturated bonds, and/or certain functional groups on the surface of adsorbents, which also affect the surface charge and ultimately the adsorption capacity.
Electrostatic interaction based on charge attraction is the main driving force for the efficient separation of cationic or anionic dyes. Different adsorption forces such as π-π stacking, hydrogen bonding, and Van der Waals force also contribute to the adsorption of dyes (Armanious et al., 2014). Compared with other mechanisms, electrostatic interaction is indeed the most common and plays the most important role in the adsorption of ionic dyes. For instance, Xiao et al. summarized the mechanism of dye adsorption by metal-doped carbon. Electrostatic interaction is the main fore and the most common mechanism, even though other forces also participate in the dye adsorption process (Xiao et al., 2021). Moreover, electrostatic interaction also extensively exists in MOFs-based, carbon-based, metal-based and organic polymer-based adsorbents for dye removal . The universality of electrostatic interaction mainly stems from the fact that the strength of the mutual attraction between charges is far greater than other interactions. The strength is generally as follows: Van der Waals force < hydrogen bonding ≈ π-π stacking < Electrostatic interaction .
Modification of adsorbents based on charge regulation to improve the adsorption capacity has always been the target in the field of adsorption Duan et al., 2020). Zhou et al. reviewed the adsorption of dyes by different adsorbents such as minerals, biomass, agricultural and forestry wastes, industrial by-products, and emerging nanomaterials, and found that grafted and composited adsorbents have higher adsorption capacity . Ligand functionalization is the main strategy for MOFs modification, which can change not only the specific surface area but also the surface charge. And electrostatic interaction is the main driving force to improve the dye adsorption of MOFs-based adsorbents (Jiang et al., 2019). In the recent decade, there are countless literature on improving the adsorption capacity by modulating the surface charges of adsorbents, including grafting functional groups, compositing, and structural regulation. However, there is currently no review of the details of combing electrostatic interaction regulation strategies to enhance dye adsorption.
Although there are many reviews on improving the adsorption capacity of dyes in recent years, most of them deal with the modification strategies of specific materials (Bushra et al., 2021;Shi et al., 2022;Xiao et al., 2021). From the macroscopic point of view, there are few reviews that focus on combing and summarizing the universal modification methods based on the electrostatic interaction of adsorption of anionic/cationic dyes. In this review, the classification, properties, and adsorption mechanisms of different dyes are summarized. Recent progress in different adsorbent modification methods for enhancing ionic dye adsorption is summarized, including grafting ligands, compositing and structural regulation. The adsorption capacity, preparation processes, toxicity, and cost analysis are discussed and assessed. Moreover, we introduced the common amphoteric adsorbent modification methods and provided the progress of the industrial-scale adsorption of real dyeing wastewater. In the end, we proposed the most promising ligands and compositing substances and discussed the limitations and challenges for the adsorbent development and its industrial-scale application. Considering that the word limit and extensive reviews have basic knowledge of adsorption, the classification and properties of dyes are listed in Text S1 and the isotherm, kinetic and thermodynamic models are listed in Table S2.

Enhanced adsorption of cationic and anionic dyes via electrostatic interaction
The regulation of electrostatic interaction is a complex process, and its macro-control direction is to provide more charge. Generally, the strength of electrostatic interaction can be reflected by Zeta potential or the quantification of functional groups. Different adsorbents modification strategies, like grafting, composition or acid/base treatments, have different experimental conditions and reactions (Liu, Zhou, et al., 2020;Sadiq et al., 2021;. Taking grafting as an example, organic reaction between different functional groups is a common method, including esterification, amidation, and silylation. Moreover, grafting can be achieved with cross-linking agents, like glutaraldehyde and epichlorohydrin (Hokkanen et al., 2016).

Grafting functional groups
Grafting functional groups is the most common way to modify adsorbents. And the electronegative functional groups mainly include carboxyl groups (-COOH), sulfonic acid groups (-HSO 3 ) and phosphate groups (-PO 4 3− ).

Carboxyl group (-COOH).
Grafting-modified adsorbents with negatively charged carboxyl functional groups is a common and effective method for cationic dye adsorption. As shown in Table S3, carboxyl ligands mainly include polycarboxylic acid (CA, EDTA, and DTPA), acrylic acid, anhydride and carboxysilane. Our group has made a series of progress in the preparation of high-performance cationic adsorbents by regulating electronegativity via polycarboxylic acid and acrylic acid. It was found that carboxyl-rich CA could efficiently modify the adsorbent by simple esterification cross-linking. After cross-linking of WS with CA, the adsorption capacity of MB was increased by 3.7 times . Meanwhile, soluble CA and β-CD can be cross-linked into an insoluble polymer CD/CA. In the pH range of 2-11, the Zeta potentials of the CD/CA surface are all negative, and the electronegativity can be controlled by adjusting the ratio of CA and β-CD. The adsorption capacity of MB by CD/CA reached 345 mg/g . The surface of CD/CA is rich in hydroxyl and carboxyl groups, which provides the feasibility for further modification. Thereby, CD/CA-EDTS can be easily prepared by grafting EDTS on CD/CA. Its synthesis process only needs stir EDTS with CD/CA at 75 °C. The carboxyl group content of CD/CA was only 8.6 mmol/g, while that of CD/CA-EDTS reached 13.6 mmol/g. Consequently, the adsorption capacity for MB was increased from 380 to 641 mg/g . Notably, the electrostatic interaction between the above as-prepared adsorbents and dye molecules could be easily destroyed and rebuilt by eluting with HCl solution. Almost no significant decrease in adsorption capacity was observed, which also demonstrated the stability and reusability of the grafted adsorbents Huang et al., 2018). Succinic anhydride and maleic anhydride are the most widely used anhydrides in adsorbents modification. The chemical formula of anhydrides is R-CO-O-CO-R' , which can be hydrolyzed in an aqueous solution to form carboxyl group (-R-CO-OH). Carboxyl groups can be grafted with -OH or -NH 2 groups through esterification or amidation reactions Jiang et al., 2018). In addition to providing carboxyl groups, they have the advantage of simple preparation. For example, Chen et al. easily grafted succinic anhydride on passion fruit peel by stirring, the as prepared adsorbent could adsorb 1776 and 1756 mg/g of MB and MV, respectively .

Sulfonic acid group (-HSO3)/sulfur containing groups.
Sulfonates and sulfates are the two main forms of ligands containing sulfur-containing groups and sulfonic acid groups, most of which are surfactants (shown in Table S3). Li et al. obtained SLG by pretreatment of Na 2 SO 3 . The introduction of sulfonic acid groups significantly increased the Zeta potential of SLG, thereby the adsorption capacity of MB increased by more than 5 times, reaching 647 mg/g . SDBS is a representative substance with sulfonic acid groups. Its grafted aragonite microspheres could provide large numbers of negative charges for enhancing the adsorption of MB due to the introduction of -SO 3 − group in SDBS (Nagpal & Kakkar, 2020). Grafting SDS on montmorillonite dramatically decreased its Zeta potential (from −49.6 to −105.3 mV). Consequently, the adsorption capacity of BR13 increased by 1.5 times, reached 1111 mg/g (Bayram et al., 2020). It was reported that dithiocarbamate contains negatively charged xanthate groups, which have higher adsorption capacity compared with carboxyl groups. For instance, its grafted adsorbent prepared by Liu et al. achieved attractive adsorption performance for MB, CV, BF, and Methylene violet 3RAX, reaching 2624, 2553, 1729, and 1239 mg/g, respectively (Liu, Zhao, et al., 2020).

Phosphate group (-PO 4 3−
). The phosphoric acid group (-PO 4 3− ) is also an effective electronegative group (shown in Table S3), mainly including phosphoric acid and PA. PA is a type of nontoxic and environmentally friendly organophosphorus composite with six -PO 4 3− groups, which widely exists in plant seeds, grains, and beans. For instance, polycondensation products of PA and β-CD (PA-β-CD) realized rapid and high adsorption capacity for MB, basic green 4, Astrazon pink FG, and MV, reaching 1095, 2006, 1736, and 1930mg/g, respectively (Li, Yu, et al., 2022. The immobilizing phosphate (Na 3 PO 4 ) on Uio-66 (UiO-66-P) increased its charge density and increased its adsorption capacity for MB by 2.7 times (Yang, 2017). Table S3 shows some of the uncommon but excellent ligands. For instance, POMs are a class of discrete transition metal oxide clusters, the highly negatively charged oxygen-rich surface of which is expected to possess efficient and selective adsorption of cationic dyes (Zeng et al., 2018). IA is a natural monomer containing two carboxylic acid groups. Most research on IA involves the adsorption of heavy metals, but it also has excellent adsorption capacity. For example, ultra-high adsorption capacity for cationic dye MB (2439 mg/g) was obtained by crosslinked carrageenan/IA (Sharma et al., 2022). Moreover, catechol and L-glutathione are also outstanding ligands that contain catechol groups and mercaptan (-SH) groups, respectively (Hua et al., 2018;Kumari et al., 2020).

Dopamine.
DA is a kind of natural substance produced by marine mussels. Its abundant catechol moieties and amino groups provide adsorption sites for dyes and metal ions. Moreover, its adhesion capacity makes it easy to coat the surface of adsorbents . For example, our group constructed adsorbent CD-CA/PDA by self-polymerization of DA on the surface of CD-CA. Its adsorption capacities for MB, MG, CV, and Cu 2+ increased by 2.2 (583 mg/g), 2.8 (1175 mg/g), 11.2 (473 mg/g), and 1.4 (74 mg/g) times, respectively . Dong et al. coated DA on the surface of GO (PDA/GO). Its adsorption capacities for MB, MV, Malachite green oxalate, Coomassie brilliant blue, BF, and NR reached 1800, 2100, 2000, 2100, 1700, and 1400 mg/g, respectively, which is ten times higher than that of the GO or PDA monomers and five times that of commercial AC (Dong et al., 2014). It is worth mentioning that DA contains a large amount of -NH 2 groups which provide excellent adsorption capacity for anionic dyes. More detailed information on DA could be found in Section 2.2.2.3.
In 2007, it was first discovered that the adhesion of DA was derived from self-polymerization under weak alkaline conditions, which broadened its application field and gradually entered the area of environment (Lee et al., 2007). DA is a kind of amino acid secreted by mussels, green, natural and harmless. Due to its self-aggregation and adhesion properties, it is easy to compound modification and morphology control. Moreover, it can be used as a carrier for different metal nanoparticles. It has been proved that DA can effectively adsorb organic pollutants and metal ions, and can be applied to the modification of membranes.

Carbon materials.
Carbon materials, such as GO and CNTs, are widely applied in the field of adsorption due to their special structural properties (laminated or tubular structure) and oxygen-containing functional groups (Table S4). In particular, GO contains a large number of negative charged oxygen-containing functional groups that strengthen its electrostatic interaction, such as -OH, -C = O-and -COOH groups (Sharma & Das, 2013). For example, the adsorption capacity of ZIF-8 for MG reached 1667 mg/g due to the larger specific surface area and π-π stacking. After compositing with GO and CNT, the adsorption capacity for MG was further improved by 1.2 and 1.98 times, respectively (Abdi et al., 2017).
Different carbon materials, metal materials and organic polymers can be composited with carbon materials. At present, the large-scale applications of GO and CNT are hindered by their cost. Although the adsorption capacity of biochar is not as good as that of GO or CNT, biochar is still an excellent substitute because of its low price, easy availability and easy preparation (Fang et al., 2020). Grafting functional groups, doping heteroatoms and compositing are good means to further improve the adsorption performance. Up to now, the controllable large-scale preparation of biochar still needs further exploration. In addition to the electrostatic interaction, π-π stacking also plays an important role in the adsorption process by carbon materials.

Alginate.
Alginates (including SA and Calcium alginate) are widely applied in the field of adsorption since their abundant carboxyl groups (Table S4). Meanwhile, alginates are green and cheap. For example, Ma et al. composite of lignin-containing cellulose and SA (LCNCs/SA) contained both sulfonate groups of lignin and carboxyl groups of SA, which could adsorb 1181 mg/g of MB (Ma et al. 2019). Readers can refer to Li and Yang (2021) and Quesada et al. (2020) for more information about SA.
As shown in Table S4, the research on alginate composites has shown an increasing trend since their excellent adsorption capacity. Alginate is a natural, safe, and green polysaccharide with strong viscosity. It can be composited with metal oxides, minerals, and carbon materials. In most cases, alginate is applied to gel AC powder into granular beads. Notably, although alginate is safe, easy to modify, and has excellent performance, it is expensive. SA is more than $4800 per ton, while calcium alginate is more expensive (Table 1). There is currently little literature concerned with the price of alginate.

Acid treatment
Acid treatment is a common method for the structural regulation of adsorbents, which can improve the acidity, oxygen-containing functional groups, and hydrophilicity of the adsorbent. Generally, acid treatment reagents include HNO 3 , HClO, H 2 SO 4 , HCl, and H 3 PO 4 . (Abegunde et al., 2020).
Adsorbents that can be treated by acid include inorganic minerals and carbon-based adsorbents, such as clays, montmorillonite, zeolite, attapulgite, fly ash, biochar (Hokkanen et al., 2016;Zhang et al., 2021). And organic adsorbents would be oxidized during acid treatment. For example, Deng et al. prepared SCS and PCS by impregnating cotton stalk (CS) with H 2 SO 4 and H 3 PO 4 , respectively. After the acid treatment of CS, the carboxyl content of SCS and PCS surfaces increased from 0.30 to 1.89 and 2.13 mmol/g, respectively. Consequently, the adsorption capacity of modified PCS and SCS on MB increased to 222 and 556 mg/g, respectively, while that of CS alone was only 147 mg/g (Deng et al., 2011). Sh. Gohr et al. modify AC with NaOH and chloroacetic acid, which provides AC a large number of hydroxyl and carboxyl groups. The presence of carboxyl groups strengthens its electronegativity, which facilitates the adsorption of MB and CV (Sh. Gohr et al., 2022). Due to the hazard of acids, acid treatment is not a recommended approach in practical applications.

Grafting functional groups
Most common ligands for anionic dyes adsorption include amides, imidazoles, polyamines and surfactants (Table S5). As for the adsorption mechanism of nitrogenous functional groups on anionic dyes, take -NH 3 as an example, -NH 3 can deprotonate into positively charged -NH 2 + to electrostatic attraction anionic dyes in a certain pH range (mainly acidic).

Common nitrogenous groups.
Nitrogenous groups include amino, quaternary ammonium, and tertiary amine groups. For instance, AM is an environmentally friendly and high-efficiency ligand. Its grafted WS (WS-AM) has an excellent adsorption capacity of 3276 mg/g for MO due to the introduction of large numbers of amino groups . Electrostatic interaction depends on pH of the solution. Generally, acidic conditions are conducive to the adsorption of anionic dyes. Taking WS-AM and WS-CA as examples, the isoelectric point of WS-AM is about 6.5, so its adsorption capacity for anionic MO increases from pH 5 to 2. The zeta potential of WS-CA gradually becomes negative when pH is greater than 3, thus its adsorption capacity for cationic MB increases sequentially from pH 4. PEI is another important excellent ligand since its macromolecular chain contains abundant primary, secondary, and tertiary amine groups (Zhang et al., 2016). Its long chain structure makes it easy to graft, crosslink, and modify (Kani et al., 2022). For instance, cellulose and PEI can be easily crosslinked by stirring in the presence of glutaraldehyde at room temperature. Surprisingly, the adsorption capacity after modification can be up to 83 times higher (Guo et al., 2018). Moreover, as shown in Table S5, pyrrole and imidazole are also important nitrogen-containing ligands.

Cationic surfactants.
Cationic surfactants can provide positive charges by -NH 2 groups and can be easily grafted . There are many types of cationic surfactants, including CTAB, CPC, PEI, cetylpyridinium chloride monohydrate, and alkyl dimethyl benzyl ammonium chloride. (Ain et al., 2020;Mahmoodi et al., 2019). As shown in Table S5, the grafting of CTAB can effectively improve the adsorption capacity (generally the adsorption capacity is >1000 mg/g). For example, the adsorption capacities of CR and MO by nickel-cobalt sulfide (Ni-Co-S) were only 193 and 243 mg/g, respectively. Chowdhury et al. further grafted CTAB on the surface of Ni-Co-S (Ni-Co-S/CTAB), the adsorption capacities of CR and MO dramatically increased to 1995 and 2223 mg/g, respectively .
There are few studies on ILs in the field of dye adsorption, but they have gradually attracted attention in recent years (Table S5). More details about ILs could be found in the cited reference (Ayati et al., 2019). For instance, SiO 2 imidazolized with 3-n-Hexadecyl-1-vinylimidazolium bromide showed high adsorption capacity for CR, reaching 2956 mg/g (Lei et al., 2022). Qin et al. copolymerized 1-Vinylimidazole into a series of adsorbents, in which the adsorption capacity of CR could reach 3698 mg/g (Qin et al., 2020).

Carbon materials.
The porous structure and oxygen-containing groups are conductive to the adsorption capacity of carbon materials. Notably, the functional groups on the surface of carbon materials will be reduced or destroyed by high temperatures, and the adsorption of anionic dyes by carbon materials is actually dominated by π-π stacking (Pete et al., 2021). The surface polarity will also decrease when oxygen-containing groups of carbon materials are destroyed, and their affinity for dye molecules will also decrease. Therefore, the purpose of composite modification of such carbon materials is mainly to provide specific surface area for dye adsorption, such as composite with organic conducting polymers or support for metal nanomaterials. For instance, the adsorption capacity of CNT to MO is not satisfactory. Although the conducting polymer polyaniline contains a large amount of positive charge, its poor dispersion limited its adsorption sites. The composite of polyaniline and CNT realized synergistic adsorption and reduced the agglomeration of adsorbent (Pete et al., 2021). Readers can refer to the cited reviews for more details on the properties and modification methods of carbon-based materials Xiao et al., 2021). More detailed information could be found in Table S6.

Chitosan.
Chitosan-based adsorbents have attracted much attention in recent years because of their abundant amino groups (shown in Table S6). Chitosan possesses the advantages of low toxicity, environmentally friendly, and biocompatibility. However, the chitosan monomer will gradually hydrolyze in the acidic solution (pH < 5.5). It is an effective approach to composite it with other chemically stable materials. And some reviews give a detailed overview of their relevant information (Sadiq et al., 2021;Saheed et al., 2021). Chitosan can be easily modified and composited with magnetic nanoparticles, zeolite, clay, and cellulose (Sadiq et al., 2021) For example, Lipatova et al. (2018) prepared composite CPF by loading chitosan on the surface of polyethylene terephthalate fiber. It can stably exist in the pH environment of 2-8. Moreover, the adsorption capacities for anionic DB86 and С.I. RB21 reached 1097 and 302 mg/g, respectively. Chitosan can also be combined with waste plastic-polystyrene (PS-CS), which can not only realize the immobilization of chitosan, but also recycle resources from waste. And the adsorption capacity of CR was up to 1080 mg/g. After several times of "adsorption-desorption" cycles, the performance of PS-CS was almost not affected . As shown in Tables S5 and S6, the adsorption capacity of chitosan composites generally lower than 1000 mg/g because the number of amino groups provided by chitosan is limited. Meanwhile, the price of chitosan is not attractive. Therefore, chitosan-based materials should be used in special and more demanding occasions.

Dopamine and its substitute.
The excellent properties of DA have been introduced in Section 2.1.2.1. Its catechol groups can effectively enhance the adsorption of cationic dyes, while its abundant amino groups can enhance the adsorption of anionic dyes (as shown in Table S6). For example, Yan et al. (Yan et al., 2015) prepared PVA/PAA@PDA by coating polyvinyl alcohol/ polyacrylic acid films with DA. Its adsorption capacity for Methyl blue was increased by 10 times due to electrostatic interaction.
Recently, catecholamines have been extracted from plant polyphenols, which have similar physical and chemical properties to DA (Long et al., 2017). For example, Long et al. copolymerized catechol and branched PEI on Fe 3 O 4 . As a joint result of the amino groups of catechol and PEI, its adsorption capacity of Methyl blue reached 345 mg/g (Long et al., 2017).

Conducting polymers (CPs).
The main types of CPs include PANI, polypyrrole (ppy), and polythiophene (PTh), among which PANI and ppy have been wildly studied. Their structures contain a large number of free flowing electrons, so they have conductivity and high charge density surfaces (Taghizadeh et al., 2020). They are cheap, easy to prepare and chemically stable, and have received a lot of attention in the environmental field. More detailed information can be found in Table S6 and the relevant reviews (Khan et al., 2021;Taghizadeh et al., 2020).

Structural regulation of adsorbents 2.2.3.1. Alkali treatment.
Alkaline treatment is a common modification method for carbonaceous adsorbents, which can provide positive charges. A review by Yahya et al. elaborated on their chemical modifications in detail (Yahya et al., 2015). Generally, alkali source includes NaOH, KOH, Na 2 CO 3 (Abegunde et al., 2020). For example, Gao et al. produced PAC and KAC by treating Enteromorpha prolifera derived biochar with H 4 P 2 O 7 and KOH, respectively. The adsorption capacity of alkali-treated KAC for Acid scarlet reached 2500 mg/g, which is more than 5 times that of acid-treated PAC (Gao et al., 2013). Alkali treatment is a simple and lowcost modification method, which can not only increase the positive surface charges of the adsorbents, but also change the surface area, pore structure and functional groups (Abegunde et al., 2020;Devi & Saroha, 2016). However, the production of lye will cause environmental pollution and extra treatment costs. Additionally, the increased effect of adsorption capacity by alkali treatment is not as significant as that of grafting (Hokkanen et al., 2016).

Doping.
Generally, the surface charge distribution of carbon materials and nanomaterials can be regulated by doping. A review by Xiao et al. has made a detailed introduction of doped carbon materials for enhancing dyes adsorption (Xiao et al., 2021). Meng et al. successfully prepared Cr-doped ZnO by solvothermal method. The doping of Cr 3+ increased the lattice defects of ZnO and increased the hydroxyl groups. Consequently, the adsorption capacity of MO was dramatically improved due to the electrostatic interaction between the protonated hydroxyl group (-OH 2 + ) ZnO and the sulfonic acid group (Meng et al., 2015). Notably, doping is not a mature technology. The relationship between the doping atoms and the doping quantity and the performance still needs to be further studied (Xiao et al., 2021).

Regulation of MOFs.
Many approaches can regulate the structure of MOFs, including surface modification, immobilization of functional materials, replacement or grafting of metal sites or organic ligands, etc. (Oveisi et al., 2019) For example, Qiu et al. added acetic acid during the preparation of Uio-66 to change the surface charge distribution. Acetic acid treatment increased the content of hydrogen ions of UiO-66, so that the Zeta potential of acid-treated U-200 reached 21.2 mV, while that of untreated U-0 was lower than 1 mV. Consequently, the adsorption capacity of MO was improved by 1.4 times (Qiu et al., 2017). At present, MOFs still has a long way to go for its industrial application, especially its ecological and environmental effects need to be further studied.

Comparison of adsorption capacity
1. Carboxyl groups, sulfonic acid groups and phosphoric acid groups are common electronegative functional groups for cationic dyes adsorption (Table S3). Different ligands have different effects on adsorption. ①The adsorption capacity of the carboxyl-containing ligands followed by acrylic acid > acid anhydride > polycarboxylic acid. Acrylic acid has higher adsorption capacity since its property of self-aggregability. Acid anhydrides are superior to polybasic acids since they can be easily grafted by stirring Kyzas et al., 2015), while the grafting of polybasic acids usually requires esterification and cross-linking reactions . For instance, the content of carboxyl groups increased by 2.2 times by stirring succinic anhydride with passion fruit peel for 6 hours . ②The capacity of sulfonic acid groups grafted adsorbents could reach 1000 mg/g, which has obvious advantages compared with polybasic acids (CA, EDTA, DTPA), because the electron withdrawing ability of sulfonic acid groups is stronger than that of carboxyl groups. Sodium dodecyl sulfonate and SDS are common sulfur-containing ligands and industrial products widely used as surfactants in toothpaste, shampoo and shower. Their grafted adsorbents usually have excellent adsorption capacity (>2000 mg/g). ③Although 2,2'-benzidinedisulfonic acid, phytic acid, and xanthic acid also have a good ability to increase the adsorption capacity, they are not conventional chemicals and do not have the potential for large-scale application in water treatment. Table S5, the variety of cationic ligands is less than that of anionic ligands.

As shown in
Most nitrogenous ligands have long-chain structures, such as PDMAEMA and DTAC. AM, PEI, and CTAB are common and excellent ligands, and their grafted adsorbents generally have a good adsorption capacity of more than 1000 mg/g. AM can be polymerized into polyacrylamide and PEI is in long chain. CTAB, as a surfactant, could be easily grafted on the solid surface and can provide large numbers of positive charges. ILs grafted adsorbents have more excellent adsorption capacity since higher charge density. As shown in Table S5, vinylimidazole ILs are typical types of cationic ILs, and their adsorption capacity could reach an excellent 2000 ∼ 4000 mg/g. However, the liquid state limited the practical application of ILs since they are inconvenient to store and use.

Comparison of grafting process
Generally, the grafting processes of most ligands are relatively simple. Carboxyl groups could form chemical bonds with hydroxyl or amino groups in presence of initiators (e.g. potassium persulfate and ammonium persulfate) . Acid anhydride could be easily grafted by stirring for several hours at room temperature . Sodium dodecylbenzenesulfonate and SDS can also be grafted by stirring with the carrier at room temperature (also known as surface impregnation) (Nagpal & Kakkar, 2020).
The grafting of AM could be easily achieved by stirring for several hours in the presence of initiator . PEI could be grafted in the presence of cross-linking agents, such as glutaraldehyde and epichlorohydrin (Long et al., 2017;Mahmoodi & Saffar, 2020). CTAB can be easily and quickly grafted by surface dipping method . However, the preparation of ILs is a complicated process, requiring organic synthesis from 1-vinylimidazole or further modification (Lei et al., 2022).
Notably, the modification of adsorbents is a complex process. Actually, the grafting reaction in experimental processes is not always successful, and grafting could not always positively promote the adsorption performance (many articles will not show this part of the content). Excluding the reason of graft failure, the change of physical and chemical properties of the adsorbents after grafting, such as specific surface area, particle size, pore size distribution, and dispersion, will also affect the adsorption performance. For example, poly(pyrene) porous organic polymers (PyPOP) can be sulfonated into a polymer with a larger specific surface area but a smaller pore size (PyPOP-SO 3 H). Although PyPOP-SO 3 H has a large amount of negative charge, their adsorption promotion effects on cationic dyes CV and MB are quite different. The adsorption capacity of sulfonated PyPOP-SO 3 H for CV was increased from 260 to 383 mg/g by about 1.5 times, whereas the adsorption capacity for MB was increased by 42 times (from 19 to 780 mg/g). The effect of -SO 3 H grafted on the adsorption of cationic dyes is not proportional to the number of positive charges. The driving force of the adsorption of CV by PyPOP may come from the pores, while MB is completely dependent on the negative charge provided by -SO 3 H .

Comparison of toxicity and cost
Information on the toxicity and cost of different ligands or materials is of great importance. As shown in Table 1, carboxyl-containing ligands are relatively low toxic and their acute toxicity levels are "moderately hazardous" and "slightly hazardous. " The LD50(rat, oral) of succinic anhydride and maleic anhydride were 1510 and 400 mg/kg, respectively. Although their acute toxicity level is still "moderately hazardous, " they are more toxic compared with other ligands from the value of LD50(rat, oral). Combined with the price and toxicity of DTPA and succinic anhydride, they are not suitable for large-scale application. Meanwhile, CA, EDTA and acrylic acid are safe, cheap and excellent graft ligands. As excellent ligands for sulfur-containing groups, sodium dodecyl benzene sulfonate and SDS have wide application space due to their lower toxicity and cheap price. Especially, sodium dodecyl benzene sulfonate has a very attractive price of 263.9 $/t. AM, PEI, and CTAB have best adsorption properties among different amino-containing ligands. Their prices are all in an acceptable range, but the ecotoxicity needs more attention from the value of LD50(rat, oral). For instance, the LD50(rat, oral) of AM is 150-180 mg/kg, which is "highly hazardous." However, AM is widely used as a flocculant in water treatment, and the content in drinking water is required to be less than 0.25 μg/L. There are few studies on AM residues in water of AM grafted adsorbents (Sriwiriyarat & Madmanang, 2020). The LD50(rat, oral) of CTAB reached 410 mg/kg, although it is ranked as "moderately hazardous," its toxicity is close to "highly hazardous". Therefore, PEI may have good application prospects because, apart from the long chain structure, its price is the lowest in Table 1. SA, DA, and chitosan, as excellent composite materials, stand out among different modifiers. As shown in Table 1, as natural materials, they have very low toxicity and better ecological friendliness. However, SA and chitosan are not attractive in price, reaching as high as 4861.1 and 15,000 $/t, respectively. In contrast, DA hydrochloride has a very attractive price and a good application prospect.
In conclusion, CA, EDTA, acrylic acid, sodium dodecyl benzene sulfonate, SDS, PEI, and DA hydrochloride are excellent ligands or materials. Their grafted or composited adsorbents have not only excellent adsorption capacity, but also simple and controllable preparation process, and most importantly they have attractive cost. Notably, although AM has strong toxicity, as long as the residual concentration of AM in solution is reasonably controlled, its low cost, good adsorption effect and simple modification process make it have a good application prospect.

Amphoteric adsorbent
Amphoteric adsorption has great practical significance. Cationic and anionic dyes are usually mixed in a certain ratio for the dyeing process . For example, in the process of textile printing and dyeing, acrylic fibers are mainly dyed with cationic dyes, while protein fibers (wool, cashmere, rabbit hair, and silk) mainly rely on anionic dyes coloring . The wool-acrylic blended fabric is a kind of common fabric in daily life, and their dyeing needs to go through anionic dye dyeing followed by cationic dye dyeing (Ozturk et al., 2015). In recent years, researchers have developed a large number of amphoteric functionalized adsorbents based on the principle of electrostatic interaction. As shown in Figure 3, the electronegative and electropositive groups can regulate the selective adsorption of ionic dyes.
Electropositive amino groups and electronegative carboxyl groups are commonly used active groups for the preparation of amphoteric adsorbents (shown in Table S7). Our group has successfully prepared a multifunctional β-cyclodextrin-based polymer (CD/CA-g-PDMAEMA) functionalized with anionic carboxyl groups and cationic tertiary amino groups. The charge distribution on the adsorbent surface can be tuned by changing the pH values, giving the material selective adsorption properties. The adsorption capacity of the anionic MO was 166 mg/g at pH 4.0, and the adsorption capacity of the cationic MB reached 336 mg/g at pH 11.0 . Chitosan and sodium alginate contain a large amount of -NH 2 and -COOH groups, respectively. For example, Zhao et al. prepared fibrous chitosan/sodium alginate composite foam by freeze-drying. The adsorption capacity of the amphoteric adsorbent for anionic Acid Black 172 and cationic MB reached 817 mg/g and 1488 mg/g, respectively . Chang et al. synthesized an amphoteric adsorbent by compositing chitosan and carboxyl-rich polyacrylate with GO, and the optimal adsorption capacity for cationic MB and anionic Food yellow 3 reached 297 and 280 mg/g, respectively (Chang, Chen, et al., 2020).
Amphoteric adsorption has made some progress in recent years, but most of them only stay in the laboratory stage. Real printing and dyeing wastewater have complex components and different types of dyes, thus more case studies on amphoteric adsorption or selective adsorption are needed. Meanwhile, more researches on process selection and regulation are needed since amphoteric adsorption is highly dependent on pH values.

Dyes adsorption mechanisms
The dyes adsorption behaviors are the result of the combined action of multiple mechanisms, including π-π stacking, hydrogen bonds, hydrophobic interactions and Van der Waals forces (as shown in Figure 4).
Electrostatic interaction is the main mechanism of adsorption of ionic dyes, which is also the focus of this article. It is characterized by the mutual attraction of positive and negative charges to realize the adsorption and separation of ionic dye molecules. Electrostatic adsorption is closely related to the pH value of the solution and the pKa of the dye molecules . Thereby, the determination of the Zeta potential is an important mean to study the surface charge properties. Generally, when the solution pH is smaller than the isoelectric point, the adsorbent surface will be positively charged, which is conducive to the adsorption of anionic dyes; when the solution pH is larger than the isoelectric point, the adsorbent will be negatively charged, which is conducive to the adsorption of cationic dyes. Studies have shown that most nonionic dyes are negatively charged in solution, which means they can be absorbed by electrostatic interaction (Kim et al., 2004). For instance, Qian et al. found that the adsorption of Disperse Blue by cationic polymer/bentonite composite adsorbent was pH dependent. According to Zeta potential, Dispersed Blue had more negative charges on the surface at low pH due to protonation (Li et al., 2010). It is worth mentioning that the adsorption capacity of nonionic dyes is often not very high. Actually, due to high toxicity, the main treatment technology of nonionic dyes in industry is coagulation or oxidation pretreatment coupled with biological treatment (Liu & Zhu, 2017). π-π stacking is a weak interaction force commonly presented in carbon-based adsorbents, mainly caused by the π-electrons interaction between the C = C bond and the aromatic ring of contaminants Qu et al., 2022). For example, Minitha et al. investigated the adsorption of anionic and cationic dyes by GO and rGO. It was found that the electrostatic interaction is the dominant mechanism between GO and cationic dyes (oxygen-containing functional groups of GO), while π-π stacking played an important role in dyes adsorption by rGO (C = C bonds) (Minitha et al., 2017).
The essence of hydrogen bond is an electrostatic force, which is a dipole-dipole interaction formed between hydrogen atoms and other atoms with high electronegativity. But the hydrogen bonding force is unlikely to be the main adsorption driving force. It is because the hydrogen bond between the anionic dye and the water molecule is greater than that between the adsorbent and dye. Non-aromatic hydroxyl groups or functional groups containing heteroatoms and lone pairs of electrons in the adsorbent may combine with the dye in the form of hydrogen bond (Li, Nu, et al., 2019).
Hydrophobic interaction is rare in dye adsorption. Most dyes have strong hydrophilicity for easy coloring. Disperse dyes are a kind of dyes without water-soluble groups. As shown in Figure  2b, there are few studies related to the adsorption of dispersed dyes. For example, the region of the benzene ring and ester group (-COOCH 3 ) in the cationic dye Rh 6 G is hydrophobic. Zha et al. prepared hydrophobic UiO-66-NHCOR via iso stearin chloride functionalization. The contact angle of the hydrophobically modified Uio-66 increased from 32° to 158°. The adsorption capacity of the hydrophilic Uio-66 for Rh 6 G was only 243 mg/g, while the hydrophobic UiO-66-NHCOR increased its adsorption capacity to 478 mg/g. FTIR showed that in addition to electrostatic forces, the hydrophobic groups of Rh 6 G had a strong tendency to aggregate with the hydrophobic domains of UiO-66-NHCOR (Zha et al., 2019).
Van der Waals force is a weak and widespread intermolecular force. However, it unlikely to be the dominate force for dye molecules adsorption. For example, Toumi et al. found that the adsorption of AB80 by olive cake waste is the result of the joint action of hydrogen bond, Van der Waals force and electrostatic force (Toumi et al., 2018). Table 2 summarized the adsorption mechanism between different adsorbents and ionic dyes. Furthermore, most adsorption process is spontaneous through adsorption thermodynamics (Table  S2) (Anastopoulos & Kyzas, 2014). From the perspective of adsorption force strength, the strength is generally as follows: Van der Waals force < hydrogen bonding ≈ π-π stacking (0-10 kJ/mol)< Electrostatic interaction (10-50 kJ/mol) . At the same time, π-π stacking is also a common mechanism, because most dyes have aromatic rings. Hydrogen bond and hydrophobic interactions rarely exist in isolation. However, compared with π-π stacking and hydrogen bond, electrostatic interaction is one of the most easily regulated adsorption mechanisms.

Desorption and regeneration
The purpose of desorption is to regenerate the adsorbent, increase the utilization cycles, and finally realize the minimum cost of wastewater treatment. The cost includes both the wastewater treatment and the disposal of saturated adsorbents. Meanwhile, the behavior of desorption and regeneration is conductive to elucidate the adsorption mechanism indirectly. At present, there are two main desorption techniques: thermal desorption and solvent elution. Thermal desorption technology is aimed at those adsorbents with strong thermal stability, such as AC and metal particles. Thermal desorption process relies on heat (or steam) to separate contaminants from the surface of the adsorbent or to crack them . Solvent elution is one of the most common regeneration methods, including acid, lye and organic solvents, such as HCl, H 2 SO 4 , HNO 3 , NaOH, methanol, ethanol, acetic acid, and acetone (Bushra et al., 2021). The reason why the adsorbent can be regenerated by solvent elution is that the dye adsorption behavior dominated by electrostatic interaction is reversible. The elution process of methanol, ethanol and acetone is similar phase dissolution principle, which uses organic solvent to dissolve organic dyes. Generally, acids have better desorption properties for cationic dyes, and basic solvents have better desorption properties for anionic dyes . Under acidic conditions, the positive charge of adsorbents is conducive to the adsorption of anionic dyes, and lye can weaken electrostatic interactions, and vice versa. For example, Huang et al. investigated the desorption behavior of adsorbed MB on CD/CA by 0.5 M HCl, ethanol and 0.5 M HCl + ethanol mixture (0.5 HCl in ethanol) respectively. HCl has the best elution performance, and the adsorption capacity of CD/CA has almost no loss after five adsorption-desorption cycles. Whereas, the adsorbent capacity regenerated by ethanol or 0.5 M HCl + ethanol mixture decreased significantly (decreased by 20-30%/5 cycles desorption) . As solvent elution does not destroy the structure of adsorbent, the adsorption capacity of adsorbent can generally be completely regenerated when it is thoroughly eluted. Therefore, the adsorption capacities of most ionic dye adsorbents are generally above 90% after five or six cycles of "adsorption-desorption" regeneration. For instance, surfactant has amphipathicity (hydrophilia and hydrophobicity), which can be stable and efficient adsorbed on the surface of solid materials. The Ni-Co-S/SDS composite can exist stably even after washing with methanol. The adsorption loss of the composite was less than 5% after five repeated sorption-washing process, which proved the stability of the surfactant grafted by impregnation method (Chowdhury et al., 2019). The similar stability of surfactant impregnated adsorbents has also been observed in many studies (Tang et al., 2017;. In addition, advanced oxidation, electrochemical oxidation, biodegradation, and other methods can also be applied for adsorbents regeneration. However, such regeneration technologies are not universal. For example, although biodegradation is economical and environmentally friendly, microorganisms and incompletely degraded dye fragments can still occupy the adsorption sites (Abegunde et al., 2020). The premise of adsorbent regeneration by advanced oxidation method is that the adsorbent can undergo advanced oxidation reactions. For instance, Gu et al. prepared a Fe/porous carbon hybrid with adsorption capacity of 1301 mg/g for tetracycline, and H 2 O 2 was added for advanced oxidation regeneration after adsorption saturation. After three times of regeneration, the adsorption capacity remained 92.5% (Gu et al., 2021). Notably, advanced oxidation or electrochemical oxidation regeneration technology still has limitations for large-scale application.
Actually, thermal desorption is the main regeneration technology of industrial AC . And the main regeneration technology in the literature is solvent elution. Both methods have advantages and disadvantages. For example, carbonaceous materials are suitable for thermal desorption regeneration, but the adsorbent structure may be damaged during this process, producing toxic and harmful gases and consuming energy. The greatest advantage of solvent elution is low energy consumption and high efficiency, but it consumes plenty of solvents, produces waste liquid, and increases additional costs. At present, the new and efficient adsorbent regeneration technology or optimized process combination is worthy of further research.

Industrial-scale adsorption of dyeing wastewater
The ultimate goal of a technology is practical application. As a promising and widespread technology, there are few studies on industrial-scale treatment of dyeing wastewater by adsorption. On the one hand, the water quality of real dyeing wastewater is complex which contains many unknown and uncontrollable variables; on the other hand, adsorption is one kind of tertiary treatment and has not obtained much attention.
Typical dyeing wastewater treatment includes three-stage treatment processes: physical separation, biochemical reaction, and tertiary treatment (decolorization). Adsorption is an important single application unit for decolorization. The printing and dyeing process goes through processes including desizing, scouring, bleaching, dyeing, and finishing. Each unit discharges dyeing wastewater, including dyes, dye auxiliaries, and salts (60-100 g/L NaCl and Na 2 CO 3 ). For example, half of the dyes used in cotton textile dyeing are reactive dyes, and the pH of the effluent is as high as 10 ∼ 11 (Allègre et al., 2006). The leather dyeing process consumes a lot of water (7 L H 2 O/1 kg leather), and the concentration of the dyes generally exceeds 5000 mg/L (Piccin et al., 2012). Piccin et al. collected real printing and dyeing effluent from a Brazilian tannery. The pH of the dyeing wastewater is about 4, the salinity is about 2%, the total organic carbon exceeds 1800 mg/L, the total Cr concentration is about 40 mg/L, and the dyes concentration is about 440 mg/L (Piccin et al., 2016). Kyzas et al. collected the effluent from Dyeing-Finishing Mills of Thessaloniki (Greece), which includes anionic reactive dyes with a pH up to 10. Acrylamide and PEI grafted chitosan (named CS-AM and CS-PEI, respectively) were prepared as adsorbents. The adsorption capacity for the wastewater followed by CS-PEI > CS-AM > AC. The dyes concentration of the wastewater after CS-PEI adsorption could be lower than 1 mg/L (Kyzas et al., 2011). Kyzas et al. also collected coffee grounds to adsorbents above dyeing wastewater. The different between the removal efficiency of the real dyeing wastewater and the simulated dyeing wastewater was only 2%. Almost 99.9% decolorization can be achieved when the adsorbent dosage was 10 g/L. Almost no loss of the adsorption capacity in 10 cycles of regeneration experiments (Kyzas et al., 2012). Aparna Roy collected wastewater from a local textile mill in Samudragarh (India), which contains different dyes (azo, direct, reactive, disperse, and vat dyes), salt, acid and different chemical additives. The designed adsorbent, tannin grafted jute fiber, could achieve 99.5% decolorization efficiency of the above wastewater. Rapid regeneration can be achieved by elution with alkaline solution. The surface oxygen-containing functional groups promoted the electrostatic interactions with dye molecules (Roy, 2021). Zaharia et al. applied fly ash to treat the textile wastewater collected from a Romanian textile factory. The wastewater is composed of Remazol arancio 3R (33 g/L), Remazol rose RB (22 g/L), Na 2 CO 3 (20 g/L), NaOH (5 g/L), and other additives. The fly ash achieved 83% decolorization efficiency, and 61.1% chemical oxygen demand removal efficiency. The effluent can be directly charged to the public biological treatment plant for further treatment (Zaharia & Suteu, 2013).
Meanwhile, the most important condition for the industrial application of adsorbents is the cost. There is currently little literature on adsorbent costing. The cost includes many aspects, such as the cost of the adsorbent itself (preparation, modification, and raw material transportation cost), and the application cost (energy consumption, adsorption capacity, durability, and regeneration cost). At present, the most widely used adsorbent AC has disadvantages such as high price and difficult regeneration. It is of practical significance to develop low-cost and high-performance adsorbents. Detailed information related to the industrial application of adsorbents and cost could be found in the reviews Zhou et al., 2019).
Therefore, the composition of dyeing wastewater is complex. Adsorption as an advanced treatment process has important practical significance. In the future, more attention should be paid to the development of cheap and efficient adsorbents to replace the traditional AC, and more attention should be paid to the adsorption efficiency of adsorbents in real dye wastewater and industrial scale.

Future prospects
Due to limited space, some details have not been discussed in depth. Some excellent ligands or materials are not described, such as biomass and polymers (gum polysaccharide, starch, and polyamidoamine dendrimer) (Duan et al., 2019;Gao et al., 2021;Song et al., 2018;Zeng et al., 2022).The development and application of high-efficiency adsorbents still face many practical problems that need to be solved.
1. It was found that the adsorption capacity of polymer grafted adsorbents tends to be more excellent, which is related to the higher number of available functional groups provided by their long-chain. Although there are many functional groups available, there may be steric hindrance leading to waste of active sites. There are currently few studies on the relationship between steric hindrance and the number of reactive functional groups. 2. More attention should be devoted to the toxicity of the graftable ligands. For example, although the adsorption capacity of succinic anhydride grafted adsorbents can reach 2000 mg/g, it has high toxicity. And some natural substances, such as CA, chitosan, DA, PA and alginate, are environmentally friendly and have excellent adsorption performance, which are worthy of further study. 3. Many studies are lack of investigation of the potential industrial-scale application. Conventional dyeing wastewater contains different inorganic salt ions. Furthermore, the current research lacks an assessment on the industrial-scale preparation and application of adsorbents. 4. At present, the regeneration of the adsorbent in the experimental research usually involves solvent elution, and this kind of regeneration method is difficult to be practically applied. It's urgent to try new effective and economically feasible adsorbents regeneration methods. At the same time, the safe disposal of waste adsorbents also needs further research. 5. The stability of the adsorbent is an essential parameter. Most studies only investigate the stability of adsorbents from the perspective of the regeneration and cyclic experiments. In some cases, the adsorbent may crack and disintegrate. In our opinion, the stability of adsorbent should pay attention to its mechanical stability, including acid resistance, alkali resistance, high temperature resistance and mechanical wear resistance. However, there is little research focuses on this aspect. 6. Cost is the most important factor for adsorbents. At present, we focus more on the original price of the adsorbent and do not evaluate the economic benefits from its whole life cycle. In fact, in addition to the adsorbent cost itself, the cost components of an adsorbent include synthesis/modification, transportation, storage, labor, enterprise profit, desorption, treatment and disposal of the saturated adsorbent, etc. In the process of continuous regeneration cycles, the actual cost of an adsorbent is constantly compressed. It is a reasonable way to calculate the cost of treating unit volume dyeing wastewater from the perspective of the life cycle of an adsorbent. 7. The existing adsorption models cannot well explain the complex adsorption behaviors. New characterization methods, numerical simulations or theoretical calculations are needed to inject new insights.

Conclusion
This review provides a general overview of adsorbents modification strategies for enhancing adsorption of ionic dyes via electrostatic interaction, including grafting functional groups, compositing with special substance and structural regulation. The review summarizes the progress in the field of dyes adsorption over the last decade, and the practical application prospects of different modification methods are discussed from the perspectives of adsorption capacity, modification process, toxicity and cost. It was found that ligands containing carboxyl and sulfonic acid groups are conductive to enhancing cationic dyes adsorption. It can be concluded that CA, EDTA, acrylic acid, sodium dodecyl benzene sulfonate, and SDS are excellent grafting ligands. Their grafted adsorbents have not only excellent adsorption capacity, but also simple and controllable preparation process, and most importantly they have low cost. Amino-containing ligands are often applied for the adsorption of anionic dyes. Among different ligands, AM, PEI, and CTAB have excellent improvement effects, and the adsorption capacity of their grafted adsorbents can generally exceed 1000 mg/g. AM and PEI has good application prospects since its low cost, good adsorption performance and simple modification processes. It's worth noting that AM has high acute toxicity, but as an industrial flocculant for water treatment, it is still an excellent ligand as long as the free concentration of AM in water is reasonably controlled. Substances with special properties are often introduced to composite with other adsorbents. For instance, carbon-based materials are rich in oxygen-containing functional groups, alginate contains carboxyl groups, chitosan contains amino groups, and DA contains catechol moieties and amino groups. However, the price of alginate and chitosan hindered their practical application. DA hydrochloride is a promising, cheap and excellent material. The controllable large-scale preparation of biochar still needs further exploration. It's also worth noting that in addition to the electrical interaction, the π-π stacking also plays an important role in adsorption by carbon-based materials. Up to now, the structural regulation of adsorbents is not yet a mature technology, such as doping and morphology regulation, since their regulation and large-scale preparation still have a long way to go.

Funding
This work was supported by Program of Shanghai Outstanding Technology Leaders (Grant No. 20XD1433900), the National Natural Science Foundation of China (Grant No. 21906056, 51778230), and the Science and Technology Commission of Shanghai Municipality (22ZR1418600).