Adsorption Processes in the Removal of Organic Dyes from Wastewaters: Very Recent Developments

The problem of the treatment of contaminated wastewaters is of the upmost worldwide interest. This contamination occurs via the presence of inorganic or organic contaminants of different nature in relation with the industry they come from. In the case of organic dyes, their environmental impact, and thus, their toxicity come from the air (releasing of dust and particulate matter), solid (scrap of textile fabrics, sludges), though the great pollution, caused from dyes, comes from the discharge of untreated effluents into waters, contributing to increase the level of BOD and COD in these liquid streams; this discharge is normally accompanied by water coloration, which low the water quality, and caused a secondary issue in the wastewater treatment. Among separation technologies, adsorption processing is one of the most popular, due to its versatility, easiness of work, and possibility of scaling-up in the eve of the treatment of large wastewater volumes. Within a miriade of potential adsorbents for the removal of organic dyes, this work presented the most recent advances in the topic.


Introduction
Nowadays, one of the most important environmental issues is related to water scarcity, water contamination and water quality. In many cases, water contamination is due to pollution by means of the presence of inorganic and/or organic compounds; particularly, the presence of organic dyes in waters, or in general, in effluents must be avoided since they are toxic (mutagenic, i.e. Azure B or Disperse Red 1, and carcinogenic, i.e. Basic Red 9 and Crystal Violet) against life and humans. Under the word "dye," is included substances which. produced a color on materials of diverse nature, being the textile industry (54%) which produced the highest quantity of dyes-bearing effluents, followed by the dyeing industry (21%), paper and pulp industry (10%), tannery and paint industry (8%) and the own dye manufacturing industry (7%).
Nanomaterial-based adsorbents are another type of materials that, due to their properties and adsorption capacities, have applications in the removal or organic dyes from waters. Including in these nanomaterials, carbon nanotubes (CNTs), grapheme sheets (GS), and metal oxides (MO) are found [10].
Carbon nanotubes presented a sp 2 allotropic carbon of graphite structure in cylindrical or tube shaped sheets. Based on the number of these sheets presented in the adsorbent, CNTs can be found as single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs). Typically, SWCNTs presented a diameter in the 0.4-10 nm range, whereas MWCNTs have a diameter in the 10-100 nm range and spacing between sheets in the 0.34-0.38 nm range.
The adsorptive effectiveness of these carbon nanotubes can be improved by functionalizing them or modifying some of their characteristics: specific area, charge density, porosity, and hydrophilicity. These modifications can be done by acid/oxidant treatment, combination with metals/MO, and grafting special functional groups, such as polymer and surfactants.
These carbon materials presented four characteristics adsorption sites in their surfaces. Thus, the adsorption process occurred at i) the external, and/or ii) internal surface of the nanotubes, iii) the interstitial pathways between individual nanotubes sheet, and iv) the external groove sites. In the case of MWCNTs, the space between the sheets can also be used to adsorb organic dyes.
Organic dyes uptake onto these nanotubes responded very often to the Langmuir and Freundlich isotherm models, and the adsorption kinetics is best fitted to the pseudo-second-order kinetic model.
Graphene is formed by a single layer of sp 2 allotropic carbon atoms arranged in a two-dimensional hexagonal honeycomb lattice structure.
Similarly to carbon nanotubes, single-layer (SLG) or multiple-layer graphene (MLG) materials can be yielded in a 2D structure from a graphite-based material. Other derived materials, such as graphene oxide (GO) and reduced graphene oxide (RGO), with enhanced adsorptive characteristics, can be produced by chemical oxidation of graphite and reduction of grapheme oxide, respectively. These two last materials, presented better adsorption characteristics than the above grapheme materials.
A graphene/wastepaper composite [11], had been used in the removal of methylene blue and Congo red from waters, with maximum capacities of 58 and 90 mg/g, respectively. Nanoribbons of graphene were used in the adsorption of methylene blue and orange II dyes [12], in this case the maximum capacities, presented for the adsorbent, were of 280 and 265 mg/g, respectively. Others graphene-based materials had been recently used in the adsorption of crystal violet (69 mg/g) [13], rhodamine B (963 mg/g) [14], etc.
It was found that for selected organic dyes, graphene-based adsorbent had an average 2-5 times higher dye adsorption capacity than carbon nanotubes and metal oxides.
MOs adsorbents applied for the treatment of organic dyes-bearing waters included, iron oxide (Fe 3 O 4 ), zinc oxide (ZnO), titanium dioxide (TiO 2 ), magnesium oxide (MgO), alumina oxide (Al 2 O 3) , and zirconium oxide (ZrO 2 ). Among them, iron oxide nanoparticles presented good properties i.e. high specific surface area, to adsorb organic dyes, and they are magnetic. This characteristic facilitated the dispersion of the nanoparticles in the aqueous solution, and their removal from it, when an external magnetic field is applied [15].
Other investigations described the use of nanohybrids of Cu x O/Fe 2 O 3 /MoC as materials used in the adsorption of reactive red 195A and reactive yellow Promising Techniques for Wastewater Treatment and Water Quality Assessment 4 84 (maximum capacities 435 and 278 mg/g, respectively) [16], also the use of Cr-doped ZnO in the adsorption of methyl orange (19 mg/g) and methylene blue (41 mg/g), the adsorption of methyl orange (833 mg/g) by a magnetic composite [17], and Fe 3 O 4 /PPy composites (eosin Y, methyl orange and brilliant green: 212, 149 and 264 mg/g, respectively) [18]. It was described in the literature [19], the usefulness of Ag 2 O as adsorbent of Congo red (181 mg/g), acid orange 7 (125 mg/g) and amido black 10B (83 mg/g), however this investigation, as many others, did not give any information about the desorption step.
The list of nanoparticles or nanomaterials used to remove organic dyes from waters seemed not to end [20,21], considering that a series of materials such as biomass, clay minerals, different wastes, etc., when modified with magnetic nanoparticles enhanced their respective adsorption capacity towards organic dyes, because they increased their surface area and porosity and with the addition of the magnetic nanoparticles, they adopt a new property, as is the magnetic character, which improve their separation from the treated water. Moreover, by the addition of adequate functional groups to these nanoadsorbents, basically on their surface, they further improve their respective capacities on the treatment of waters contaminated with organic dyes. Not being exhaustive, Table 3 summarized some of the results encountered in this field.
Polyaniline, polypyrrole and other conducting polymers, had been also used in the removal of organic dyes from waters. In fact, these polymers reacted with organic dyes due to the similarity of the conjugated molecular structures, of both types of compounds, which enhanced the reactivity between them. The use of these organic dyes as templates in the conducting polymer synthesis may affect both the conductivity and morphology control of the end product, specially in the case of polypyrrole.
It was described [22], how conducting polymers and organic dyes reacted: i. π-π interaction between the aromatic rings, ii. electrostatic ionic interactions, iii. hydrogen bonding, and.

Adsorbent Organic dye (removal efficiency, mg/g)
Magnetic baker's yeast biomass methyl violet (61) Modified  Some dyes used in the preparation of polyaniline are: methyl orange and green GS, whereas in the case of polypyrrole, the list included both mentioned above and Congo red, Thymol blue, cresol red and rhodamine B among others.
In the conducting role, polyaniline and polypyrrole are polycations, thus, it is expected that they normally reacted with anionic dyes, however, experimentally it was found that both cationic and anionic dyes reacted with these conducting polymers; it seemed that electrostatic ionic interactions are not the most important factor to explain this reactivity.
As it is mentioned above, these conducting polymers had been used, alone or in the composite forms, in the removal of numerous organic dyes from waters, and basically their success is due to that they have a relative low production cost.
In the case of polyaniline, the list of investigations related to the removal of organic dyes from waters included: Congo red, eosin Y, rose bengal, indigo carmine, etc. Polyaniline composites are classified along the non-conducting component, and included composites containing (in various forms): aluminum, bismuth, carbon, iron, silicium, natural polymers (cellulose), synthetic polymers, etc.
Pristine polypyrrole had been investigated in the removal of methyl orange, methylene blue, etc. Polypyrrole composites again contained in various forms: aluminum, carbon, titanium, zinc, etc. Table 4 presented a comprehensive (but not exhaustive) list of organic dyes adsorbed by polyaniline and polypyrrole composites.
The use of polyaniline-related materials, such as aniline oligomers and copolymers, polyaniline chemically modified, etc., had a further interest in the removal of organic dyes from waters. The use of polypyrrole-related materials in this environmental role has a significant minor development.
Due to their environmentally friendly and availability, polymer derivatives based on polysaccharides are also of interest in the removal of organic dyes from contaminated waters. Thus, a variety of modified polysaccharides, i.e. chitosan, starch, dextran, cellulose, have been investigated as adsorbents in this role; however, and despite some of pullulan characteristics such as: high solubility and flexibility of the backbone when compared with other polysaccharides, they are not amply used in waters purification [27].
Polyaniline  Pullulan, having a chemical formula (C 6 H 10 O 5 ) n , is a linear, non-ionicpolysaccharide consisting of maltotrioseunits:α-(1 → 6)-linked(1 → 4)-α-d-triglucosides. The known pullulan derivatives are: i) soluble ionic pullulan derivatives: this type of compounds can be synthesized by chemical modification of the polysaccharide. It can include i.i) various content and length of grafted chains, and i.ii) various content of tertiary amine groups, ii) pullulan microspheres: they can be formed by suspension cross-linking of the previously grafted pullulan with cationic moieties (P-g-pAPTAC); iii) nonionic thermosensitive pullulan copolymer: it was prepared by graft-polymerization of p(N-isopropylacrylamide) onto the pullulan. Here, cerium(IV) was used as initiator. The resulted thermosensitive material has the (P-g-pNIPAAm) acronysm; iv) nonionic pullulan-graft-polyacrylamide hydrogel: this pullulan was synthesized by free radical polymerization in presence of a crosslinking agent and calcium carbonate.
Pullulan derivatives showed high removal efficiency of organic dyes contaminants, i.e. P-g-APTAC microspheres were used in the adsorption of azocarmine B (maximum capacity: 114 mg/g), acid orange 7B (65 mg/g) and methyl orange (55 mg/g); pullulan-graft-polycrylamide hydrogel was used in the adsorption of methylene blue and reactive blue with maximum capacities (70° C) of 399 and 356 mg/g, respectively.
The properties of metal-organic frameworks (MOFs), made of them interesting materials for the removal of organic dyes from waters. Some of these properties are: thermal stability, high surface area and porosity, nanosized cavities, etc. The metal centers of these materials provided additional coordination locations aimed to fixing organic dyes, whereas one step ahead in the practical use of these MOFs is provided them with magnetic properties via the incorporation of magnetic materials within the framework. Some examples of the use of these materials are given in Table 5.

Conclusions
During the first quarter of 2020 year, an important number of dye adsorption procedures have been proposed in a series of published papers, all of them, claiming successful dye removal. However, in the opinion of the authors of the present review some criticism to the papers must be accounted for: i. comparison of the maximum dye capacity presented different numbers, see Table 6. Thus, the successfulness of some of these adsorbents can be questioned if compared with the figure presented by other materials, ii. near 52% of the published papers included some data about the desorption step, that is, the other 48% of the published papers are uncompleted in terms of the overall adsorption-desorption process. Authors must aware that as important is the adsorption step than the desorption one, and in all the cases in which the latter is included, the authors do not give any comment about what to do with the now dye-bearing desorption solution. In the manuscripts, organic dyes simply go to one solution (the feed one) to another solution (the desorption one) (except in the case of degradation occurs), iii. all the reviewed manuscripts lacked the investigation of a sometimes key variables on adsorption processes and in batch mode, as the stirring speed and how the phases mixed can be. This is because, with the correct stirring speed, the.
iv. thickness of the aqueous film layer reached a minimum and the adsorption reached a maximum. Together with the above, the form in which the phases are mixed also ensured the best contact, and thus the best solute transfer, between the phases involved in the process.
It is needed to mention here, that in Refs. [63,64] a heavy scientific fault was detected: the authors of the manuscripts investigated, besides the adsorption of organic dyes, the adsorption of Cr 6+ , written as such, when this element in the VI oxidation state, never exists as a cation in aqueous solutions. This fault also is responsibility of the corresponding reviewers and of the Editors.
In practical terms, the real bottlenecks in the usefulness of these adsorbents are: i. lack of information about the desorption step, ii. evident loss of adsorption capacity under continuous adsorption-desorption cycles, iii. adsorbent cost and possibility of production at large scale to scale-up the adsorption-desorption process, iv. environmental friendship of the own adsorbent, v. lack of information on the use of the purification of real waters bearing organic dyes.