1. Introduction
Dyes are widely used in the textile, tanning, paper, and plastic industries. The use of dyes is characterized by large losses of the latter, caused by their high solubility that also creates an economic and environmental problem of great concern. Consequently, the removal of the dye from waste water is an important environmental problem.
Different techniques and materials can be used for the treatment of water contaminated by pollutants such as photocatalysis through the use of photocatalytic agents [
1,
2,
3,
4,
5,
6].
Studies on the adsorption phenomenon have shown that the latter is superior to other techniques for the treatment of waste water, being low-cost, highly efficient, simple, easy to perform, and not influenced by toxic substances [
7]. There are several materials used in adsorption. Natural and synthetic zeolites are often used in adsorption to remove pollutants from water [
8,
9,
10,
11]. They are often used modified to optimize the adsorption process [
12,
13,
14,
15,
16,
17]. Recent studies report the removal of phenol from aqueous solutions in the presence of Cu (II) ions on synthetic NaP1 zeolite and NaP1 zeolite modified with chitosan [
18].
Synthetic materials called zeotypes, such as Engelhard silicate titanium, represent another important family of microporous materials with adsorbing properties. The ETS-4 and ETS-10 phases belong to this family, which differ in the size of the pores [
19,
20,
21,
22,
23,
24,
25,
26].
Carbonaceous materials represent another class of adsorbent materials. Due to their nonpolar nature, they are able to selectively adsorb nonpolar rather than polar substances [
27,
28,
29,
30]. Organic substances, as well as natural gluconate, have been successfully used for the removal of heavy metals from aqueous systems [
31].
Polymers with multilevel structures containing sub-micron pores and interconnected mesopores can be used for the adsorption of hazardous pollutants [
32,
33,
34,
35,
36,
37]. Functionalized organ-based materials, inorganic or mineral clays, have been shown to be potential absorbers in removing various pollutants in wastewater [
38,
39,
40,
41,
42]. Biochar can be used as a potential adsorbent material. It is a product of the thermal decomposition of organic material under the limited supply of oxygen at temperatures between 350 and 700 °C. Thanks to its porosity, high specific surface area, and cation exchange capacity, it can be used as an adsorbent material in the treatment of polluted water [
43,
44,
45]. Chitosan is a linear copolymer composed of (1–4)-linked d-glucosamine, and
N-acetyl-d-glucosamine is a polysaccharide prepared by the
N-deacetylation of chitin. It presents several characteristics, as nontoxicity, biodegradability, biocompatibility, bioadhesivity, and bioactivity. Different studies report chitosan and chitosan-based materials as important adsorbent materials thanks to the cationic character and the presence of reactive functional groups in polymer chains. In particular, one of the major applications is based on its ability to adsorb strongly heavy and toxic metals [
46].
Carbon nanotubes (CNTs) are attracting increasing interest in research as a new adsorbent material. Carbon nanotubes can be single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) [
47]. They are very versatile materials and can also be used in other sectors such as fiber reinforcements [
48,
49]. They represent very effective materials in the treatment of water contaminated by pollutants thanks to their high adsorbing capacity [
50,
51,
52,
53,
54,
55,
56,
57,
58], which is given to the CNTs by a large specific area and by hollow and stratified structures within them [
59,
60,
61], which makes an interesting alternative for removing organic and inorganic contaminants from water.
Textile dyes significantly compromise the environment by increasing the biochemical and chemical oxygen demand (BOD and COD), compromising photosynthesis, inhibiting plant growth, entering the food chain, and promoting bioaccumulation. They can promote toxicity, mutagenicity, and carcinogenicity [
62].
For this reason, the treatments currently used for the purification of textile wastewater require high efficiency but also economy.
In particular, among the different methods used, there is the bioremediation of textile dyes, through the transformation or mineralization of these contaminants by the enzymatic action of plant biomass, bacteria, extremophiles, and fungi [
63,
64,
65,
66,
67,
68].
Moreover, wastewater contaminated with azo dyes, mainly from the textile industry, has been the subject of research using photodegradation and sonication techniques for their treatment [
69,
70]. Other studies have successfully highlighted the use of biopolymers [
71,
72] and microgel [
73]. Photo-electrochemical treatments and electrocoagulation have recently been reported [
74,
75]. Some studies have successfully reported the combination of multiple methods [
76].
Adsorption processes are always a valid option for the remediation of water contaminated by dyes. Many studies successfully report the use of microporous materials [
77,
78,
79,
80]. Other studies have used different adsorbent materials such as sawdust, activated carbon, and carbon nanotubes [
81,
82,
83].
In particular, previous studies report the treatment of water contaminated by the diazo dye called Reactive black-5 through different methods such as photocatalytic degradation [
84], microbial decolorization [
85], adsorption with activated carbon and bone carbon [
86], and by the Fenton oxidation process coupled with biological treatment [
87].
In this work, the removal of the azo dye called Reactive Black-5 was studied, using carbon nanotubes.
The use of carbon nanotubes as adsorbent materials has the advantage of generally being fast and cheap, as they can be easily regenerated and reused. Furthermore, they do not allow the introduction of, in the systems to be purified, new substances, which can subsequently be harmful to the environment, nor substances that can react with other molecules [
88,
89].
The aim was to study the influence of process parameters such as the initial concentration of the dye in the solutions to be purified, contact times, and stirring speed in order to optimize the adsorption processes. A further object was to confirm both the reversibility of the adsorption of Reactive Black-5 on the carbon nanotubes and whether, during the adsorption process, the dye molecule undergoes degradation phenomena.
2. Experimental
The experimental activity consisted of several consecutive phases: (a) Procurement, preparation, and preliminary characterization of raw materials; (b) execution of adsorption tests with carbon nanotubes in solutions with different concentrations of dye, for different contact times and different stirring speeds; (c) characterization of the phases after the adsorption process.
2.1. Materials
2.1.1. Multiwalled Carbon Nanotubes (MWCNTs)
The multiwalled carbon nanotubes used in this study were the same ones that have been synthesized and characterized in our previous publications; for further information, please refer to these articles reported in the references [
61,
83]. Specifically, they were synthesized by the catalytic chemical vapor deposition (CCVD) method of ethylene. The catalyst was prepared utilizing NaY zeolite previously dried at ca. 130 °C for 24 h. The precursor salts –Fe(NO
3)
3*9H
2O (Merck, Darmstadt, Germany, assay ≥98%) and (CH
3COO)
2Co*4H
2O (Merck, Darmstadt, Germany, assay ≥98%) were dissolved separately in ultrapure water. After 30 min of sonication, the solution of iron nitrate was poured into the solution of cobalt acetate followed by a 30 min sonication. Finally, the required quantity of NaY was added to the solution in a mortar and remained there for 5 min. At the end, the Fe-Co (5 wt%) catalyst deposited on NaY was dried in an oven at 130 °C.
The reaction temperature of MWCNTs was 700 °C, the time of the reaction was 20 min, the flow rate of ethylene was 800 mL/min, the quantity of catalyst Fe-Co/NaY was 0.25 g and carrier gas N
2 was 416 mL/min. Purification of synthesized nanotubes, to dissolve the NaY zeolite, was carried out as reported in previous studies [
61]. In particular, the carbon nanotubes were immersed in a hydrofluoric acid solution (40 wt%) for three days. Subsequently, after being washed with distilled water, they were dried in an oven at 130 °C for 18 h.
The yield of MWCNTs was computed as:
where m
in is the initial mass of the catalyst and m
out is the total mass including MWCNTs. The yield is equal to 1450%, showing that this catalyst is very efficient in MWCNT production.
The TGA and DTA curves of the purified MWCNTs are shown in
Figure 1.
The TGA and DTA curves of the purified MWCNTs show the oxidation of the nanotubes at 600 °C and a loss of 100%, demonstrating the efficiency of the purification.
The single carbon nanotubes have an external diameter of ca. 20 nm and the average length is equal to 20 μm. The BET surface of the nonpurified MWCNTs is equal to 108.78 m
2/g, while that of the purified sample is 118.25 m
2/g [
61]. The isotherms of N
2 adsorption are shown in
Figure 2. Adsorption isotherms show a typical profile of porous materials, but no particular differences are appreciated for purified and nonpurified nanotubes. These curves are typical of physisorption type II in the IUPAC classification. The first part tends to saturation, while the sudden increase is significant of the formation of multimolecular layers.
2.1.2. Reactive Black-5 Dye
The dye used is a commercial product, called Reactive black-5 (Sigma Aldrich, Darmstadt, Germany). It is an azo compound and it has a high molecular weight of 991.82 and the following formula: C
26H
21N
5Na
4O
19S
6 (
Figure 3a).
Figure 3b shows the UV-visible spectrum of Reactive Black 5. Three bands can be clearly identified at 307 (a), 481 (b), and 600 (c) nm.
TGA and DTA data of the Reactive Black-5 are reported in
Figure 3c. The two exothermal peaks at 220 and 547 °C are due to the oxidative degradation and final oxidation of Reactive Black 5.
2.2. Instruments
The UV spectrophotometer (UV-3100PC Shimadzu, Kyoto, Japan) was used to measure the residual concentrations of dye after adsorption tests. All experiments were performed in triplicate and the results were reported as mean values.
Differential thermal analysis (DTA) and thermogravimetric analysis (TG) (Shimadzu-60, Kyoto, Japan) were performed with an air flow of 50 mL/min, applying a heating rate of 10 °C/min.
The N2 adsorption isotherms of the samples were performed by Micromeritics ASAP 2010 (B.E.T-Unterschleißheim, Germany). All the samples, before being analyzed, were pretreated under vacuum at 200 °C for 12 h. The total organic carbon (TOC) and total inorganic carbon (TIC) content of the product solutions were measured by a TC analyzer (Shimadzu, Kyoto, Japan).
2.3. Preparation of Reactive Black 5 Solutions
Seven different solutions have been prepared with the following concentrations in distilled water (
Table 1).
2.4. Adsorption of Reactive Black-5 on MWCNTs
Here, 0.03 g of MWCNTs was added into 10 mL of Reactive Black-5 solutions with different concentrations in a stirrer. The speed was 350 and 500 rpm. Finally, the MWCNTs with the adsorbed Black-5 product were filtered, dried at 125 °C for 24 h, and analyzed by TG and DTA. The filtrate was analyzed using UV-visible absorption spectroscopy.
4. Conclusions
The results obtained allowed us to draw the following conclusions:
The use of MWCNTs for the removal of the Reactive-Black-5 dye has proven to be highly effective. Adsorption can even reach 100%.
Adsorption tests on both purified and nonpurified MWCNTs have been carried out to verify that the presence of catalytic and zeolitic particles in the nonpurified MWCNTs has no value in the dye adsorption process. It has been shown that the catalyst, used for the synthesis of nanotubes, has no particular influence on the dye removal process. The parameters, on the other hand, such as stirring speed and adsorption times, are important. The MWCNTs with the adsorbed colorant were examined using TGA and DTA techniques. In each case, the presence of adsorbed colorant was confirmed.
Another important aspect to underline is the possible recycling of post-treatment nanotubes: As seen from the thermal analyses, the combustion temperature of the dye is lower than that of the nanotubes, so exposing the post-treatment nanotubes to temperatures of around 500 °C, it is possible to destroy the adsorbed molecules without damaging the nanotubes, which can be subsequently reused.
The adsorbed Reactive Black 5 could be partially desorbed using DMSO. However, some of the colorant is destroyed, as it was shown by analyzing the TOC and using UV-visible.