Recent progress in polydopamine-based composites for the adsorption and degradation of industrial wastewater treatment

Abstract

Polydopamine-based composites have attracted much attention due to polydopamine (PDA) similarity to 3,4-dihydroxy-L-phenylalanine (DOPA), a key structure that produces superior adhesion stem from mussel foot silk protein. Considering that PDA is rich in highly active functional groups such as catechol groups and amine groups etc., it is desirable to provide a large number of active sites through various strong interactions just like hydrogen bonding, electrostatic interactions, π–π stacking interactions, coordination or chelation, thereby removing pollutants such as heavy metals or organic synthetic dyes from industrial waste water. In fact, the introduction of PDA into wastewater treatment materials will greatly improve the pH usage range, accuracy, and biocompatibility of adsorbent, which has generated the interest of water treatment materials researchers. In this review article, based on the two methods of adsorption and degradation commonly used in water treatment, which mainly refer to separation of heavy metal ions and degradation of organic pollutants, we describe the special role of PDA in the process of removing pollutants from industrial wastewater materials in recent years. A brief summary and some key issues in water treatment field and must be considered in future research is given in the finally. We hope to provide researchers with ideas based on the outstanding advantages of PDA composite materials in the treatment of pollutants in wastewater, so that the role of PDA can be better explored.

Polydopamine; Water treatment; Adsorbent; Degradation; Membrane adsorption materials.

1. Introduction

The water pollution stems from the development of industry. Generally, the major industrial water pollutants include organic matter (dyes, pesticide, oil, etc.) and inorganic matter (metal ions, chloride, sulfide, etc.). Aiming at different pollutants, wastewater treatment methods are also different, such as degradation [1, 2], chemical precipitation [3, 4], adsorption [5, 6], biological treatment and membrane filtration [7], and so on. Due to easy operation, low cost, and fewer toxic by-products, adsorption and degradation have been proved to be the most effective and widely used as wastewater treatment [8]. A large number of literature reports on the use of the above two methods to remove industrial wastewater pollutants, and they have made positive contributions to the improvement of specific pollutants. For example, MoS2/Fe3O4 nanomaterials reported in many literatures [9, 10, 11] are often used as adsorbents because MoS2 has excellent adsorption capacity for metal ions. By introducing Fe₃O₄, the MoS2/Fe3O4 obtained was used by three researchers to adsorb Cu (II), Pb (II) and Hg (II) respectively, and their adsorption amounts reached 11.7 mg/g [9], 46.5 mg/g [10] and 425.5 mg/g [11]. Typical examples of degradation, inorganic materials such as Ag@ZnO nanomaterials，often act as catalyst to degrade the organic substance rhodamine B (RhB，a carcinogenic bright red dye) in wastewater by electron transfer under ultraviolet light [12]. Under the synergistic effect of Ag and ZnO, the degradation of RhB increases from 75% to 99% at pH 10. Same as in the previous example, the degradation rate of Ag@ZnO was also influenced by pH, with only 27% at pH = 2. For the degradation of metal ions, Chen [13] team combined porous boron nitride (p-BN) with zero valent iron (ZVI) to obtain p-BN@ZVI. Due to the high electronic supply capacity of ZVI, the degradation rate of Cr (VI) is increased from 20% to 98%, the aggregation of ZVI is also be solved. However, the degradation process was affected by anions such as CO₂⁻³ and SO₂⁻⁴, which led to the rapid reduction of the degradation rate to 50%. Therefore, the practical application of materials is challenged. These cases make us realize that the existing adsorption and degradation water treatment materials do not meet the actual production needs such as adsorption or degradation capacity, pH usage range, and biocompatibility, etc. It is easy to realized that the existing water materials either cannot meet the actual demand for pollutant removal capacity, or the removal capacity drops sharply under special conditions, or even fails. These problems force people to explore more effective water purification materials [14].

In fact, learning from nature, bionics, to manufacture advanced functional materials has become a fashionable method of material design in the past two decades. Muhammad and his team [15, 16, 17] used the idea of bionics to find a way to break through wastewater treatment capacity from plants. In 2017, Nepenthes rafflesiana pitcher and Nepenthes rafflesiana leaves were studied as novel adsorbents for methyl violet dyes, they showed better results (maximum adsorption amounts were 288.7 mg/g and 194.0 mg/g) and the main force of the adsorbent on the dyes was found to come from hydrophobic-hydrophobic interactions. They reported the adsorption performance of acid blue 25 dye adsorption by water lettuce, tarap peel and cempedak peel, and investigated the ability to regenerate the spent adsorbent cycle in 2018. Help for other researchers to remove water pollution by using bionic materials.

Applying bionics to various functional polymer materials is also embodied in introducing special biological substances into polymer materials. Millions of years of evolution have allowed marine mussels to attach firmly to almost all solid materials, even on wet surfaces (Figure 1A). Tremendous efforts have revealed that the adhesion ability of mussels is closely related to the abundant dopa (3,4-dihydroxyphenyl-L-alanine) and lysine in silk foot protein [18]. The catechol group (Figure 1B) in dopa cooperates with the amino group (lysine) to form various interactions with the substrate, such as hydrogen bond, covalent bond, coordination, chelation, etc. [19], even generating a strong self-assembly structure inside the polymer, making it possible for the mussels to shape super adhesion on any solid surface in any environment.

Figure 1.

(A) Photo of Mussel; (B) Dopa adhesion diagram; (C) The structural formula for DA [20]. Reprinted (adapted) with permission from ref. 20, copyright (2014), the American Chemical Society.

As a result, dopamine (DA) (Figure 1C)has attracted the attention of chemical researchers. Under alkaline conditions (pH > 7.5), dopamine is oxidized by oxygen and spontaneously polymerizes to become polydopamine (PDA) (Figure 2). This structure is very similar to dopa in silk food protein, and the self-polymerization reaction has a relatively mild reaction conditions without any complicated equipment. Since PDA contains a variety of functional groups, such as catechol, amine and imine, these functional groups provide active sites for covalent modification of the desired molecules and also firmly anchor transition metal ions. In addition, with the strong reduction ability of metal ions, the PDA/metal ion functionalized polymer further realizes the formation of a variety of hybrid materials.

Figure 2.

Schematic diagram of dopamine self polymerization principle.

It is worth noting that the PDA functionalized polymer is a material with excellent biocompatibility. Therefore, there are a large number of reports about the introduction of dopamine and its derivatives into the polymer network in various ways for nearly two decades, and excellent results have been achieved. This paper reviews the research work of PDA-based composites in the field of wastewater treatment in the past four years by focusing on adsorption and degradation. The principles, design or preparation methods, advantages and characteristics of PDA functional materials used in this field are comprehensively summarized. And finally discusses the existing drawbacks and unsolved problems of PDA. We hope this article can give some new inspirations to scholars.

2. Application in adsorption materials

Many innovative thinking based on chemical adsorption has been applied to the design of water purification adsorption materials in recent years. Various elaborate structured polymers (membrane), such as core-shell microspheres [21], functional nanoparticles [22], bacteria, fungi, etc., serve as carriers for adsorbed substances [23]. Many works have achieved outstanding results. Nonetheless, it is still difficult to obtain water treatment materials with high adsorption rate, large adsorption capacity (the adsorption capacity is the amount of adsorbed mass per unit of adsorbent in mg/g, it is used to determine the adsorption ability of the adsorbent) and a wide usage range of pH. The key to these problems lies in the lack of a large number of active binding sites between most adsorbents and contaminants.

For metal ions, these active binding sites often include electrostatic adsorption [24], functional group grafting, complexation, and ion exchange, etc.; For organic matter, the purpose is mainly achieved through hydrogen bonding and hydrophobic interaction [25]. However, it is difficult for most adsorbent materials to meet the above structural requirements at the same time in practical applications.

To a certain extent, compared with other polymers, polydopamine shows more flexibility for the construction of target structural molecules. The most valuable feature of polydopamine lies in its molecular structure, which contains many functional groups, such as catechol groups, aromatic groups, amines and imines, etc. These groups will combine heavy metal ions and organic pollutants provide the necessary active reaction center through electrostatic interaction, coordination or chelation, hydrogen bond or covalent bond, π-π stacking interaction etc. [26], which stimulate the design of PDA in water treatment adsorption materials.

2.1. Nano-adsorption materials

One of the key factors for high adsorption capacity lie in the high specific surface area. Typical examples are carbon-based nanomaterials such as graphene (GO) and carbon nanotubes (CNTs), which are sought after by a wide range of researchers due to their extremely high specific surface area and excellent performance in the direction of energy storage applications [27, 28, 29, 30] and water treatment [31, 32]. And it has been shown that using nanomaterials to treat organic matter in wastewater is feasible. Elkodous and his team [32] developed a layered Co_0.5Ni_0.5Fe₂O₄/SiO₂/TiO₂ composite matrix combined with carbon dots, single-walled carbon nanotubes and reduced graphene oxide nanomaterials (Figure 3), respectively, for the removal of chloramine-T (a toxic water contaminant that can cause a variety of health problems) from water, loaded with 10% C-dots nanocomposites exhibited a high removal efficiency of 65%.

Figure 3.

Schematic diagram of the synthesis process of different nanocomposites [32]. Reprinted (adapted) with permission from ref. 32, copyright (2021), Elsevier B.V. All rights reserved.

However, the adsorption capacity of these carbon nanomaterials is limited by the number of functional groups. Considering the characteristics of PDA, the combination of the high specific surface area of carbon nanomaterials and the high level of active centers of polydopamine will definitely be beneficial to the adsorption capacity and efficiency of the adsorbent.

A typical example is that Zhan and his collaborators designed PDA in the multi-walled carbon nanotube (MWCNT)/GO composite material, and finally obtained the product MWCNT-PDA/GO, which is used to adsorb Pb(II) and Pb(II) in wastewater [33]. The stability of MWCNT/GO composite material depends on its own tightness. If it is not tight enough, the stability of the material will decrease; If the density of adsorbent materials is moderate, the material will exhibit excellent stability. But excessive closeness is likely to produce extremely bad results. That is, the active sites that adsorb metal ions are blocked. In order to solve the contradiction between the stability of the adsorbent material and the easy deactivation of the active site, the author implemented a simple strategy to obtain a highly dense material (MWCNT-PDA/GO), in details, MWCNT was simply immersed in an aqueous solution of DA, and DA spontaneously polymerized on the surface of MWCNT-PDA to form a complex, which was finally introduced into GO. This approach allows a large number of catechols, amino groups and other groups in PDA provide active sites for metal ions through interactions such as electrostatic, hydrogen bonding, or chelation (Figure 4A). In addition, PDA exhibits an excellent dispersion effect on GO, which significantly improves the agglomeration problem of GO and further increases the adsorption active sites.

Figure 4.

(A) The adsorption of Pb (II) and Cu (II) on MWCNT-PDA/GO composite was studied by complexation (red) and Chelation (blue); (B) The adsorption capacity of Pb (II) and Cu (II) on MWCNT-PDA/GO composite changed with time [33]. Reprinted (adapted) with permission from ref. 33, copyright (2019), Elsevier B.V. All rights reserved.

Among many adsorption materials, MWCNT-PDA/GO shows outstanding adsorption capacity. Its adsorption capacity for Pb(II) and Cu(II) reached 350.9 mg/g and 318.5 mg/g, respectively, with the highest adsorption capacity (Figure 4B, Table 1).

Table 1.

Maximum adsorption capacity (Qm) of different carbon nano adsorbents for Pb (II) and Cu (II).

Adsorbent	Adsorbate	Qm (mg/g)
MWNCT-PDA/GO	Pb(II), Cu(II)	350.9, 318.5
EDTA-MCS/GO	Pb(II), Cu(II)	206, 207.2
MCGO	Pb(II)	79.8
GO	Cu(II)	53
TEPA-GO/MnFe2O4	Pb(II)	263.2
rGO-DTC/Fe3O4	Pb(II), Cu(II)	147.1, 113.6
GO/SiO2	Cu(II)	158.9

In catalysis, many hybrid materials, including transition metal-based materials, N-doped carbon materials, etc., are often used as redox reaction catalysts or catalyst supports [34], even implement additional chemical pretreatment methods in order to introduce nitrogen. The researchers believe that the vital reason for this design is that after adding an appropriate amount of heteroatoms N or transition metal-N to the carbon matrix, the number of active centers increases significantly [35, 36, 37]. Catechol groups, aromatic groups, amines and imines, etc., these groups in PDA with powerful chelating ability, load a variety of transition metal ions, making it an ideal choice for adsorbing metal ions in wastewater. For the example we mentioned in the introduction, the significant advantage of MoS₂ lies in its large specific surface area and nanometer structure, which has abundant binding sites when combined with metal ion. It usually adsorbs various metal ions jointly with Fe₃O₄. However, under acidic conditions, Fe₃O₄ will be exposed to the hydrogen ion atmosphere, resulting in the failure of adsorption, which limits its application to a large extent. Taking into account the outstanding adhesion ability of PDA, wang et al. attached PDA to the surface of Fe₃O₄ as a protective coating [38], grafted MoS₂ via a large number of functional groups, and then grown in situ to produce Fe₃O₄@PDA- MoS₂ nano-microstructure with a special core-shell structure (Figure 5A). The experimental results showed that the mass loss of nanospheres was reduced from 35.00% to 4.34% after being modified by PDA in nitric acid solution (Figure 5B). Polydopamine, undoubtedly, plays an important role in improving the stability of nanospheres in acid environments. In fact, this designed material can exist stably under any pH conditions.

Figure 5.

(A) Fe₃O₄@PDA- MoS₂ preparation diagram; (B) Stability of Fe₃O₄@ MoS₂ and Fe₃O₄@PDA- MoS₂ nanospheres in strong acid solution [38]. Reprinted (adapted) with permission from ref. 38, copyright (2019), Elsevier B.V. All rights reserved.

Further adsorption test of Pb(II), Fe₃O₄@PDA-MoS₂ nanospheres have the highest adsorption capacity (Table 2) among the numerous conventional adsorption materials listed. Due to the presence of PDA, the number of functional groups on the surface of the microspheres increased, so more and more MoS₂ could be grafted on the microspheres, which in turn resulted in more bonding sites between MoS₂ and heavy metal ions, the stability of the nanospheres was also improved.

Table 2.

Comparison of the maximum adsorption capacity of different adsorbents for Pb(II).

Adsorbent	Conditions	Qm (mg/g)
Powdered active carbon	PH5.0 T303K	26.9
Celtek clay	PH6.0 T293K	10.08
EDTA-graphene oxide	PH6.8 T298K	479
Graphene oxide	PH5.5 T298K	125
Biochar-rGO	PH6.0 T298K	26.10
MoS2– Fe3O4 nanosheets	PH6.0	9.6
Fe3O4/MoS2 nanohybrid	PH6.5	46.51
Fe3O4@PDA- MoS2 nanospheres	PH∼5.5 T293K	508.9

Compared with Pb(II), the treatment of uranium in wastewater can be described as hard bones. Researchers have made progress in removing U (VI) [39, 40], however, adsorption capacity has not yet reached the ideal. Xu et al. also realized that PDA can be introduced into specific nanospheres [41], similar to the production procedure of the above example (Figure 6A), that is, first, polydopamine chelates Au nanoparticles to form a (PDA/Au) polymer film, and then wrap it on the surface of the microsphere h- Fe₃O₄ to obtain the final product h-Fe₃O₄@Au/PDA hollow nanosphere adsorbent. In this kind of principle, PDA plays the role of "bridge", connecting h- Fe₃O₄ and Au nanoparticles via electrostatic interaction, coordination chelation and hydrogen bond interaction. Xu and coworkers concluded that the adsorption capacity of the material for hexavalent U (VI), increased with time, and the maximum will beyond 90 mg/g (Figure 6B). Obviously, all of this should be attributed to the PDA.

Figure 6.

(A) Preparation of h- Fe₃O₄@Au/PDA nanospheres; (B) Under different time conditions, h- Fe₃O₄@Au/PDA adsorption capacity of hexavalent U [41]. Reprinted (adapted) with permission from ref. 33, copyright (2020), Elsevier B.V. All rights reserved.

In fact, there is a major disadvantage in this method, that is, PDA is uniformly attached to the surface of the hollow particles, which prevents the movement of U (VI) into the interior of the adsorbent. In other words, U (VI) only binds to the functional groups of PDA on the surface of the adsorbent, while the active groups inside the hollow spheres cannot be touched [42].

Based on the above problems, studies proformed by Yang et al. replaced the hollow microspheres with Aspergillus niger mycelium (AM) [43], which is a common biological material with a three-dimensional network structure and a stable structure. According to previous reports, AM/PDA microspheres were obtained by immersing AM in the PDA solution to cover the entire mycelium. This method also exhibits the result of higher adsorption capacity. Unsurprisingly, the existence of PDA gives a good explanation for the source of a large number of additional active sites (phenolic hydroxyl groups and amino groups, etc.). Moreover, the mesh cage structure of AM has a larger specific surface area that is conducive to dispersing and fully exposing the functional groups of PDA. In this way, U (VI) can penetrate into AM and combine with PDA, making PDA more fully utilized (Figure 7A). Compared with fungi/attapulgite, Fe₃O₄/PDA and many other adsorption materials, such high adsorption performance and high stability were also observed in AM/PDA when the pH is appropriate (Table 3). It is worth noting that the adsorption capacity of the microspheres for uranium ions is affected by the pH of the solution because PDA has an isoelectric point (pH = 4). At pH values below 4, most of the functional groups undergo protonation and cannot generate active sites that will bind to U ions. When pH values are higher than 4, phenolic hydroxyl groups on the surface of PDA undergo deprotonation to form a negative charge, which enhances the electrostatic interaction with the positively charged U (VI). Therefore, when the pH is in a wide range of 4–7 (Figure 7B), the minimum adsorption capacity of AM/PDA is also higher than the maximum adsorption capacity of AM. The improved pH usage range brought by PDA can help AM/PDA microspheres adapt to more complex environments.

Figure 7.

(A) Adsorption mechanism of U(VI) on AM/PDA; (B) Adsorption capacity of hexavalent u on A. niger, AMMs, AM/PDA microspheres at different pH [43]. Reprinted (adapted) with permission from ref. 43, copyright (2019), Elsevier B.V. All rights reserved.

Table 3.

Comparison of maximum adsorption capacity of different adsorbents for U(VI).

Adsorbent	pH	Qm (mg/g)
Aspergillus niger	5	6.79
Fungus/attapulgite	4	125.00
Fe3O4/PDA microspheres	6	193.27
Magnetic MnFe2O4	5	119.9
AM/PDA microspheres	5	250.7

2.2. Membrane adsorption materials

Membrane separation technology is also wildly used due to its extremely high selectivity, low energy consumption, easy operation and reusable characteristics [44, 45]. In this technology, the performance of the polymer membrane materials were largely determined the structure, morphology, separation and permeability of the separation membrane [46, 47, 48, 49]. It is the key to distinguish the quality of the separation effect (Figure 8). It is difficult for single-component membranes to meet the performance requirements of separation membranes such as permeability, selectivity, and anti-fouling performance etc. This principle forces people to take some modification measures. Surface modification not only equips the membrane surface with new properties, but also does not destroy its internal structure. However, the new surface coating constructed by covalent bonds adheres firmly, but the operation process becomes more complicated, the porosity decreases, and the roughness increases accordingly [50, 51, 52]. Correspondingly, the new coating obtained by surface coating forms a weak interaction with the substrate, and the formed composite exhibits instability.

Figure 8.

Schematic diagram of membrane adsorption principle.

As mentioned above, these significant advantages of PDA originate from mussels. Dopa spontaneously deposits on almost all substrate surfaces under natural conditions, regardless of inorganic and organic, or even the surface of polytetrafluoroethylene or super-hydrophobic substances. The thickness and stability of the film can be precisely controlled. The rich and diverse functional groups found in PDA ensure chemical reactions with a wide range of substances. More importantly, biocompatibility and hydrophilicity of the material is also a vital factor in determining the suitability for specialized applications such as water treatment and biomedical field.

Studies proformed by Zhu Liping's team [53] have demonstrated the mechanism model of the auto-polymerization of dopamine on the surface of polymer membrane materials since 2011. In line with these results, many scientists have carried out research on the preparation and structure regulation of polymer membrane materials according to the DA self-polymerization-composite method, and have achieved remarkable results. For instance, because of its high specific surface area and excellent permeability, titanium carbide (MXene) membrane has outstanding performance in solvent nanofiltration, but the property of removing pollutants needs to be improved [54]. Based on this problem, Feng et al. have implemented a simple strategy to bond polydopamine and rGO, then PDA and MXene are combined through electrostatic and hydrogen bond interactions to obtain 2D-2D laminated composite film composed of rGO/PDA/MXene (Figure 9A) [55]. Noteworthily, PDA functions as a "bridge" again. In fact, as illustrated in Figure 9B, considering that the performance of pure rGO membrane (RPM-0) and pure MXene membrane (RPM-10) have their limitations respectively, the former has a low rejection rate and the latter has insufficient water flow. After building the PDA bridge, MXene and rGO can be effectively combined, which not only takes advantage of their large specific surface area, but also takes into account the high permeability of MXene, thereby providing more reaction sites for dye adsorption (Figure 9C). In this way, a high rejection rate can be achieved under the condition of high water flow value. Studies by Feng and Co-works have confirmed the interception rates of rGO/PDA/MXene for four pollutants, including methyl red (Figure 10A), methyl orange (Figure 10B), Congo red (Figure 10C) and Evans blue (Figure 10D) (methyl red, methyl orange and Congo red are a common acid-base indicator; Evans blue is a biological stain), all exceeding 95%. The water flow rate also rises gradually.

Figure 9.

(A) The connection between MXene, PDA, GO; (B) Filtration of rGO/PDA/MXene on nylon membrane; (C)rGO/PDA/MXene Schematic diagram of dye removal by rGO/PDA/MXene [55]. Reprinted (adapted) with permission from ref. 55, copyright (2020), Elsevier B.V. All rights reserved.

Figure 10.

Rejection rate and water flux of (A) methyl red (B) methyl orange (C) Congo red (D) Evans blue with different RGO/PDA/MXene Composites [55]. Reprinted (adapted) with permission from ref. 55, copyright (2020), Elsevier B.V. All rights reserved.

Needless to say, such a procedure can be applied to the adsorption of other metal ions or dye molecules. For instance, Sha et al. have used PDA to combine MXene with Bi₆O₇ to make an adsorbent named MXene-PDA-Bi₆O₇ [56]; In the same year, Huang et al. prepared MXene-PDA-Ag₂O_X [57]. Both Bi₆O₇ and Ag₂O_X have the ability to adsorb iodine ions. PDA connects Bi₆O₇ and Ag₂O_X with MXene respectively, which has a high specific surface area. In particular, it is pointed out that Bi₆O₇ and Ag₂O_X were uniformly distributed on the surface of MXene due to the presence of PDA, the presence of PDA makes Bi₆O₇ and Ag₂O_X uniformly distributed on the surface of the MXene. In particular, Bi₆O₇ and Ag₂O_X were uniformly distributed on the surface of MXene respectively due to the abundant functional groups of PDA, and then were reduced to metal nanoparticles in situ, which further contributed to increase the adsorption area and increase the adsorption capacity. As illustrated in this report, for Mxene-PDA-Ag₂O_X, the adsorption rate is slightly lower, but the adsorption capacity is more than 70 mg/g (Figure 11B). Meanwhile, for Mxene-PDA-Bi₆O₇, the adsorption rate is relatively high, and it takes about 20 min to complete the adsorption process, while the adsorption capacity is slightly lower, 60 mg/g (Figure 11A).

Figure 11.

(A)Adsorption kinetics of iodide on MXene, MXene-PDA and MXene-PDA-Bi₆O₇ [56]; (B) Adsorption kinetics of iodide on MXene, MXene-PDA and MXene-PDA-Ag₂O_X [57]. Reprinted (adapted) with permission from ref. 56 and ref. 57, copyright (2020), Elsevier B.V. All rights reserved.

The hydrophobicity/hydrophilicity of the membrane determines the adsorption efficiency of the membrane. Excessive hydrophobicity will inevitably lead to inadequate water flow, which is not conducive to the adsorption efficiency [58, 59]. A typical example is that polypropylene (PP) membrane has the characteristics of high porosity, but its properties of super hydrophobicity seriously affects the passage of aqueous solution through the membrane pores [60]. PDA will be on stage again. That is, soak the PP film in the PDA aqueous solution, and the hydrophilic properties of the surface can be changed with a simple operation [61]. In light of the results of previous research, PDA reveals a higher adsorption efficiency for iodide ions due to electrostatic interactions and electrophilic substitution reactions (Figure 12).

Figure 12.

Schematic diagram of iodine removal mechanism of PP/PDA membrane [60]. Reprinted (adapted) with permission from ref. 60, copyright (2020), Elsevier B.V. All rights reserved.

Similar to the AM/PDA mentioned above, under different pH conditions, the removal effect of PP/PDA on iodide ions is always better than that of PP (Figure 13A). At pH < 4, the protonation of amino groups makes PDA coating with positive charge and adsorption with negative iodide ion through electrostatic action; At pH > 4, deprotonation of phenolic hydroxyl groups causes the coating to become negatively charged and iodide ions can be absorbed via replacing phenolic hydroxyl group by electrophilic substitution reaction (Figure 12). That is, PP/PDA exhibit excellent adsorption of iodide ions in any pH environment, through electrostatic interactions and electrophilic substitution reactions.

Figure 13.

(A) The removal efficiency of iodine ion by PP and PP/PDA at different pH [60]; (B) Adsorption of Cu(II) on different membranes [63]. Reprinted (adapted) with permission from ref. 60, copyright (2020), Elsevier B.V. All rights reserved. Reprinted (adapted) with permission from ref. 63, copyright (2015), CNKI All rights reserved.

Polyvinylidene fluoride (PVDF) fiber membrane prepared by electrospinning has the characteristics of high porosity and high surface area. However, the inherent hydrophobic characteristics of PVDF fiber membrane limit its development in wastewater treatment [62]. This problem was soon solved. For instance, Ma Fangfang et al. adhered PDA uniformly to the surface of a single fiber and then obtain PVDF/PDA fiber membrane by the same principle [63], in which a large number of hydrophilic functional groups such as phenolic hydroxyl and amino groups on PDA improved the hydrophilicity ability of PVDF. In contrast, due to the strong coordination between amino group and Cu(II) in PDA, the adsorption capacity of PDA/PVDF membrane for Cu(II) is significantly higher than that of PVDF membrane alone (Figure 13B).

The optimal adsorption conditions and the strongest adsorption capacity of each of the above adsorbent materials can be visualized in Table 4. The adsorption capacity of the adsorbent materials is positively related to the temperature and reaches adsorption equilibrium at a certain value, which is related to the fact that the adsorption reaction is a heat adsorption reaction. The best pH conditions are at neutral or weakly acidic, which is related to the isoelectric point (pH = 4) of PDA. In general, PDA can be used to achieve an increase in adsorption capacity in two ways. To sum up, PDA also plays the role of "bridge" in membrane adsorption materials. By combining the two materials with excellent adhesion, it achieves the purpose of efficient removal of pollutants. In addition, the hydrophilicity of PDA improves the water flow of the membrane, thus improving the removal efficiency.

Table 4.

Optimal adsorption conditions and maximum adsorption capacity of PDA adsorbent materials.

Adsorbent	Adsorbate	Optimal temperature	Optimal pH	Initial concentration of pollutants (mg/L)	Qm (mg/g)	Adsorption isotherm (R2)
MWNCT-PDA/GO	Pb(II) Cu(II)	298.15K	7	400 400	350.9 318.5	0.996 0.995
Fe3O4@PDA- MoS2	Pb(II)	293.15K	5.5	200	508.9	0.995
h-Fe3O4@Au/PDA	U(VI)	313.15K	7	85	90	0.999
AM/PDA microspheres	U(VI)	293.15K	5	160	250.7	0.980
MXene-PDA-Bi6O7	I-	348.15K	5	25	60	0.977
MXene-PDA-Ag2OX	I-	300.15K	5	30	70	0.954
PP/PDA	I-	333.15K	7	/	80	0.990
PDA/PVDF	Cu(II)	298.15K	/	/	26.7	/

3. Application in degradation materials

Degradation is also one of the common methods for industrial wastewater treatment. This method usually undergoes chemical reactions to decompose organic matter in the water into non-polluting small molecules, even CO₂ and H₂O, or reduce the valence of heavy metal ions to eliminate toxicity. For instance, one of dyes with high chroma and strong toxicity named RhB, which is easy to dissolve in water but extremely difficult to degrade naturally. Therefore, it seriously affects water quality and reduces photosynthesis of aquatic organisms. In 2019, Li's team [64] utilized the catechol and other groups in the PDA to self-assemble to form hydrogen bonds with the hydroxyl groups in the PVA, and then gold ions were reduced by catechol groups to atomic Au nanoparticles (NPs) to obtain PVA-PDA@Au nanospheres. Here, PDA is a non-toxic reducing agent. The catechol groups chelate Au nanoparticles to better disperse Au NPs and fix them on the surface of the microspheres. Au NPs can also catalyze NaBH₄, which is a source of hydrogen, releasing hydrogen atoms. Eventually, the generated active hydrogen reacts with dye molecules to decompose RhB into H₂O, CO₂ or other small molecules (Figure 14A).

Figure 14.

(A) Mechanism of RhB degradation by PVA-PDA@Au nanoparticle; (B) With the change of time, the removal effect of different samples on RhB; (C) Degradation of different concentrations of TBBPA by PVA-PDA@Au nanospheres [64]. Reprinted (adapted) with permission from ref. 64, copyright (2019), Elsevier B.V. All rights reserved.

Li and his colleagues concluded that the two types of microspheres, PVA and PVA-PDA, have very weak removal effects on RhB, with a very small degradation rate of 3.2% (Figure 14B), and only 11.3% of Au NPs. However, since the presence of PDA, the degradation rate of RhB by PVA-PDA@Au has increased significantly. The degradation rate reached 99.5% after 5 min of degradation, and 100% after 60 min. In addition, PVA-PDA@Au nanospheres could be completely degrade tetrabromobisphenol A (TBBPA, a common flame retardant, is carcinogenic) at a lower concentration (25 mg/L) (Figure 14C).

Similarly, Wang's team successfully synthesized a new type of catalyst named Al-MOF/Fe₃O₄/PDA@Ag [65], which has excellent degradability for ciprofloxacin (CIP), norfloxacin (NOR) and methyl orange (MO). CIP and NOR are two important antibiotics that are present in large quantities in the effluent of pharmaceutical plants. MO is a well-known acidic anionic dye widely used in the textile, printing, paper, food and pharmaceutical industries. Their removal is imperative in order to minimize the pollution of water resources by antibiotics and dyes. PDA reduces Ag ions to Ag NPs due to its excellent reducibility, and then the amino, carboxyl and other functional groups combine with Ag NPs, which not only improves the dispersion of Ag NPs, but also increases the degradation efficiency (Figure 15A). During the degradation process, the electrons on BH^4- was transferred to the adsorbed organic molecules by Ag NPs, and the organic molecules accept electrons continuously. Eventually, organic molecules were reduced to H₂O, CO₂ and other small molecular compounds to achieve the purpose of degradation.

Figure 15.

(A) Reduction and dispersion of Ag+; Effects of different catalysts on the removal of (B) CIP, (C) NOR and (D) Mo [65]. Reprinted (adapted) with permission from ref. 65, copyright (2020), Elsevier B.V. All rights reserved.

With the help of PDA, Al-MOF/Fe₃O₄/PDA@Ag can greatly improve the degradation rate of CIP, NOR and MO, even without the electron contribution of BH^4- (Fig.15B, C, D). With the presence of BH^4-, the degradation rates of the above three organic compounds were significantly increased to 80.7%, 80.1% and 98.2%.

In 2020, Zhao et al. designed a water treatment degradation agent that can not only degrade organic dye pollutants but also reduce Cr (VI) under photocatalysis [66]. The process is not complicated, that is, BioBr first binds firmly to a PDA-modified cotton fabric (CF) through a solvothermal reaction, and obtain a degradable material called CF/PDA/BioBr (CFPB). BiOBr is a promising semiconductor photocatalyst for visible light remediation of environmental pollutants due to its unique properties such as suitable band gap and response to visible light. After being irradiated by light, BioBr will produce vacancies (h+), O^2-, and OH radicals. These substances degrade organic dyes through oxidation reactions and reduce Cr(VI) to Cr(III). It is worth emphasizing that PDA strengthens the combination of BioBr and CF. Furthermore, the conjugated structure of PDA makes a great contribution to the capture of visible light, and its electron transport ability promotes the transfer of electrons from the conduction band (CB) of BiOBr to the HOMO of PDA (Figure 16A). Under light conditions, RhB was completely degraded after 20 min, and Cr (VI) was reduced by 96% after 110 min (Fig.16B, C).

Figure 16.

(A) Possible mechanism of photocatalytic degradation of RhB and reduction of Cr (VI) by CFPB under visible light irradiation; (B) Photocatalytic degradation of RhB by different materials; (C) The photocatalytic degradation of CFPB in the mixture of RhB and Cr (VI) was tested at pH 2.5 [66]. Reprinted (adapted) with permission from ref. 66, copyright (2020), Elsevier B.V. All rights reserved.

In the same way, Usman et al. coated Fe/rGO with DA polymerization induced by pH to form nano PDA@Fe/rGO degradation materials [67], which generates O^2- and OH free radicals to degrade trichloroethane (TCA) in sodium percarbonate system. Here, the key to increasing the degradation rate of TCA is that PDA promotes the generation of free radicals. The data reveals that with the increase in the amount of PDA@Fe/rGO (between 0.1 g/L and 0.4 g/L), the removal rate of TCA shows an upward trend and reaches the highest value of 98%, which is much higher than the 86% of Fe/rGO (Fig.17A, B).

Figure 17.

(A) Under the same conditions, the removal efficiency of TCA by different materials; (B) Effect of different doses of PDA@Fe/rGO on TCA removal efficiency [67]. Reprinted (adapted) with permission from ref. 67, copyright (2021), Elsevier B.V. All rights reserved.

The various degradation materials are summarized in Table 5. The effect of temperature on degradation is rarely discussed in the article, and the authors note in the case of Al-MOF/Fe3O4/PDA@Ag materials that temperature does not significantly affect the degradation rate, which is very beneficial for the degradation materials in practical applications. The optimum pH of the degradation materials also differed significantly, indicating that, unlike the adsorption materials, the pH change in the degradation materials had a greater effect on the pollutants than on the materials themselves. the degradation rates of all materials showed high levels ($>$ 80%) after PDA addition. It is not difficult to draw a conclusion, that is, conjugated π-structure and electron transport capability of PDA-based wastewater treatment materials not only reduce and fix metal ions by phenolic hydroxyl and amino groups but promote the transfer of electrons and visible light capture to accelerate the degradation rate of dye molecules. The synergistic action of the two ways makes the purification capacity of organic pollutants and metal ions in industrial wastewater greatly improved.

Table 5.

Optimal degradation conditions and maximum degradation rate of PDA degradable materials.

Degradation materials	Degraded	Optimal temperature	Optimal pH	Initial concentration of pollutants (mg/L)	Degradation rate
PVA-PDA@Au	RhB TBBPA	/	3–7	100 25	99.5% 100%
Al-MOF/Fe3O4/PDA@Ag	CIP NOR MO	313.15K	7 5 3–11	10 10 20	80.7% 80.1% 98.2%
CFPB	RhB Cr (VI)	/	2.5–6.6 2.5	10 10	100% 96%
PDA@Fe/rGO	TCA	/	10	400	98%

4. Concluding remarks

Human beings learn from the natural world to solve complex problems and design more advanced functional materials. Inspired by the strong adhesion of marine mussels, the excellent chemical reactivity of PDA has gradually attracted many researchers of water treatment materials. As an environmentally friendly bio-based material, PDA is equipped with a special function that is strongly anchored on the surface of various materials. When the PDA-based water treatment agent is anchored on the surface of nanomaterials, the phenolic hydroxyl and amino groups in the PDA strongly chelate metal ions, providing more active sites for the adsorption of metal ions. Therefore, the adsorption capacity of metal/metal ions is increased. When self-precipitating on the surface of the membrane material, PDA acts as a bridge to firmly bond the membrane with the substrate, the hydrophobicity of the membrane is changed, and the amount of water penetration increases, thereby increasing the adsorption capacity and adsorption efficiency. For organic pollutants, PDA exerts its reducing properties to reduce metal ions into metal nanoparticles, and chelate and fix them to avoid agglomeration; In some degradable materials, PDA improves the degradation efficiency by accelerating electron transfer.

In conclusion, the molecular structure of polydopamine and its derivatives is similar to the key components of marine mussels to achieve super adhesion performance, and it has been proven that research in this field has an extremely bright future. But just as there are no perfect people in the world, there are no perfect materials in experiments, and there are still several problems with PDA, as follows：

• 1.The molecular mechanism of DA self-polymerization. Although PDA has been studied for many years, the mechanism of DA self-polymerization has not been well explained and demonstrated. This is what has affected the progress of PDA in other research directions.
• 2.The PDA discoloration problem. The large number of phenolic hydroxyl groups on the PDA will undergo oxidation to quinone under natural conditions and the quinone will bring about a deepening of the color. It is debatable whether the problem of color change will affect the use of the material in the actual process.
• 3.Effect of pH. Although PDA has greatly improved the pH range of the material, the adsorption capacity and degradation rate vary at different pH, and the maximum adsorption capacity and degradation rate are not always available at any pH. Of course, this is not only a problem for PDA-based materials, but also for other water treatment materials.

Although there are still a lot of work to be solved urgently, we have considerable reasons to believe that these difficulties will be overcome soon. Once that day comes, polydopamine will provide new tools to address bottlenecks that have long plagued scientists in practical applications. Finally, we hope that this review will provide a framework for research on dopamine in industrial wastewater treatment, thereby encouraging more scientists in related fields to carry out dopamine-related research, and ultimately promoting the development of dopamine in many disciplines in the future.

Declarations

Author contribution statement

All authors listed have significantly contributed to the development and the writing of this article.

Funding statement

This research was funded by National Nature Science Foundation of China (21978139); Natural Science Foundation of Shandong Province in China (ZR2019MB030, ZR2020QB112); College Students' innovation and entrepreneurship training programs.

Data availability statement

No data was used for the research described in the article.

Declaration of interests statement

The authors declare no competing interests.

Additional information

No additional information is available for this paper.