Cotton cultivation is one of the most developed agricultural activities in Chaco, Argentina. The economy of this State is mostly based on cotton crop. Chaco produces 40 to 60% of all the harvested cotton in Argentina. Big companies have settled down in this region because of the cotton manufacture and its value chain development [1]. Even though the efficiency of the ginning process has reduced the amount of generated waste, the volume is still significant. Most of the ginners incinerate their waste in open hearth furnaces causing air pollution. In other cases, ginners make the disposal on ground where the waste is used to feed cattle or make bricks. Nowadays, there are many hosiery manufacturers producing and selling their own brands in Chaco. However, there is still a huge potential for development the industry. Because of that, the State’s role is important for the incentive for its development [2].

Textile industries use dyes, chemicals and water in their manufacturing processes. The degradation resistance of dyes has reached a high level. Currently the life time of the color is comparable with that of the colored clothes [3]. This is due to the development of synthetic dyes from petroleum derivatives. Their characteristics are superior to the natural ones [4].

The presence of dyes in water bodies is aesthetically undesirable and reduces the penetration of solar radiation [5] because of which the photosynthesis and photolysis are drastically affected [6]. The textile industry produces large quantities of colored effluents [7] of great persistence and high biological oxygen demand [8]. From a health point of view, dyes are toxic [9], carcinogenic and have mutagenic effects [10]. Also, they have detrimental effects on humans like afflictions in eyes, skin [11], respiratory system and irritation of the digestive tract [12]. To treat colored effluents there are several methods such as: coagulation, flocculation, reverse osmosis [13], precipitation, oxidation, reduction [14], membrane filtration, ultrasonic treatments, anaerobic and aerobic treatments, biochemical degradation, adsorption, microbiological decomposition and ozonation [15]. However, the adsorption method is considered the most efficient to remove color in industrial effluents [16]. The adsorbent usually used to treat them is activated carbon [17], but its cost is relatively high [18]. For this reason, the research on alternative materials [19], with natural origin and similar properties, easy to acquire and with minimal economic value has been greatly strengthened in recent years [20].

Adsorption with agroindustrial waste has become an adequate method for separation of contaminants in solution [21], allowing efficient retention, as well as the possible recovery and recycling of adsorbent materials, which also indicate environmental benefits [22]. Low cost adsorbents for elimination of dyes such as Methylene Blue (MB) in waste waters from textile, paper, printing and others industries [23], have been studied by several authors [24].

In this research, waste cotton fibrils, a mixture of short fibers, shells and dust, generated by the cotton industry, were tested as adsorbent material to treat the colored effluents of the textile industry. The most efficient method for cleaning the material was determined. Then this material was characterized and MB adsorption tests were carried out to evaluate the process of dye removal at different pH.

Valorizing these solid wastes with difficulty of disposal, and using them in the treatment of effluents of the same industry will contribute to the process of circular economy [25].

Bags of waste from cotton fibrils were collected. Due its nature as waste material, cotton fibrils are too dirty to be used directly in MB adsorption tests.

The design of a pre-treatment was required for the preparation, followed by the characterization. Later, the absorption and kinetics parameters were properly studied.

Preparation and characterization of the adsorbent

A study of a suitable and reproducible pre-treatment of the waste of cotton fibril was performed in order to discard factors that could alter its valuable properties as adsorbent. Then, the material obtained was characterized.

Particle size classification

Figure 1 shows the adsorbent material at different sizes.

Figure 1.

Original and two different particle size samples of waste cotton fibrils

Only the finest fraction (particle size smaller than 0.85 mm) was selected, since this would allow a greater contact surface and better removal efficiency. In this way, a more homogenous and material free of larger foreign particles was obtained.

Determination of the washing technique

Table 1 shows the results of the colorimetric analysis of the adsorbent in order to determine the best washing method. In addition, it exhibits the percentage of color removal of each type of washing.

Table 1.

Colorimetric analysis of adsorbent

Washing method	Initial color	Final color	% removal
HNO3 2M	2630	354	86
Ambient temperature water	2630	2000	24
Boiling temperature water	2630	477	82

Although that the highest percentage of removal was obtained with the nitric acid solution (86%), it was decided to work with boiling water (82%) because the difference in removal is not significant. In addition, the use of water instead of a strong acid minimizes the contamination of the effluents. The consumption of water is 0.1 L per g of CA.

Image analysis

Figure 2 shows Scanning Electron Microscopy (SEM) images of CA. The fibrous and rough surface in CA that could be observed may be appropriate for use as an adsorbent material.

Figure 2.

SEM images of CA: magnification 2.00 KX (a) and magnification 10.00 KX (b)

Zero charge point determination

The pH value is used to describe the zcp only for systems where H⁺/OH⁻ are the pH determining ions [22]. With the presence of basic functional groups, the adsorption of cationic dyes is favored at pH higher than the zcp. In terms of pH lower than the zcp, adsorption of anionic dyes is favored because the surface of the adsorbent is positively charged [29].

From the two experimental lines obtained (Figure 3) the zero charge point calculated results equal to 7. For the MB adsorption onto CA, it is expected that basic pH is favorable. These results are coincident with those obtained by Baghdadi et al. [30] on modified cotton.

TXRF studies

It was found that CA in contact with pure water released only traces of calcium and potassium chlorides. This is quite appropriate for its use in water treatment since it does not contaminate them with toxic species.

Figure 3.

ΔpH vs. pH initial

Adsorption studies

Figure 4 shows the dosage curves obtained. The % removal decreases as the concentration of the MB solution does. It is observed in the figures that the initial MB concentration that reaches 80% removal is 400 mg/L at pH = 4 and 7, and at pH = 10 is 700 mg/L. As predicted by the zcp study, a higher removal of MB at basic pH was observed.

Figure 4.

Experimental dosage curves obtained (the lines drawn are just to guide the view)

Adsorption isotherms allow to evaluate the adsorbent capacity and to understand how the process is carried out. Figure 5 shows the experimental equilibrium curves obtained by plotting the retained amount (q_e) [eq. (2)] vs. the equilibrium concentration (C_e) [31]:

$q_{\text{e}}=\frac{C_{\text{i}}-C_{\text{f}}[\frac{\text{mg}}{\text{L}}]}{\text{adsorbent}\text{mass}[mg]}\text{Vol}[L]$ (2)

Langmuir [32], Freundlich [33] and BET [34] models were used to evaluate the MB adsorption process. The mathematical expression of the linearized Langmuir isotherm is presented in eq. (3):

$\frac{C_f}{q_e}=\frac{1}{b{Q}_m}+\frac{C_f}{Q_m}$ (3)

The mathematical expression of the linearized Freundlich isotherm is presented in eq. (4):

$\ln q_e=ln{k}_F+\frac{1}{n}ln{C}_f$ (4)

The mathematical expression of the linearized Brunauer-Emmett-Teller (BET) isotherm is presented in eq. (5):

$\frac{C_f}{q_e(C_i-C_f)}=\frac{1}{K{X}_m}+\frac{K-1}{K{X}_m}\times \frac{C_f}{C_i}$ (5)

Figure 5.

Experimental isotherms obtained from MB adsorption onto A (the lines drawn are just to guide the view)

Table 2 shows the obtained parameters from the different models. The Langmuir model has the better correlation and describes the adsorption of MB on CA, which is indicative of the process that could be associated with chemisorption. This equation is derived by assuming that there is no interaction between the adsorbed molecules and it predicts an adsorptive capacity of a monolayer [35].

For the kinetic studies the pseudo-first order [36] and pseudo-second order [37] models were used. Eq. (6) is the linearized equation for the pseudo-first model and eq. (7) for the pseudo-second one:

$ln\left(q_e-q_t\right)=ln{q}_e-k_{1}t$ (6)

$\frac{t}{q_t}=\frac{1}{K_2{q}_e^2}+\frac{1}{q_e}t$ (7)

Table 2.

Adsorption parameters from the different models tested

Model	pH	R2	Qm Xm [mg MB/g ads.]	b, KF K	N
Langmuir	4 7 10	0.9689 0.9966 0.9177	227 189 303	0.0445 0.0803 0.0710	-
Freundlich	4 7 10	0.9716 0.9357 0.9438	-	19.468 0.0008 32.660	1.959 0.453 1.994
BET	4 7 10	0.0463 0.8157 0.9689	435 156 227	1.9167 12.800 0.0449	-

Table 3 shows the results obtained from the fit of kinetic models to the experimental data. The pseudo-second order model best explains the kinetics of MB adsorption on CA, since the correlation coefficient R² is close to 1 and the q_e parameter is similar to the experiment. The pseudo-second order model assumes that adsorption capacity is proportional to the number of active centers of the adsorbent surface and the adsorption rate is controlled by chemical adsorption [38].

Table 3.

Kinetic parameters at 25 °C

Model	pH	R2	k1 or k2	qe (experimental) [mg MB/g ads.]	qe [mg MB/g ads.]	te [min]
Pseudo first order	4 7 10	0.8992 0.9735 0.9534	0.0076 0.0034 0.0037	175 164 282	155 57.4 269	1,380 1,140 1,320
Pseudo second order	4 7 10	1 1 0.9837	0.0002 0.0059 0.0019	175 164 282	185 164 313	1,380 1,140 1,320

In this work, a process of adsorption of a dye, MB, on residues of cotton fibrils was studied. In this way, it is intended to contribute to the use of a difficult-to-dispose residue and, at the same time, offering to the textile industries the possibility of a low-cost treatment of their effluents.

Washing with boiling water is considered the most appropriate conditioning method for these adsorbents. This technique gives the opportunity of elimination of the natural color from the material and removes the undesirable contaminants. Also, it is one of the procedures that causes less environmental impact.

Considering the zero charge point equal to 7, it can be estimated that the cationic dyes, such as MB, would have a higher adsorption in alkaline medium. This was verified on the results obtained in the adsorption studies at different pH.

The Langmuir model was the best for describing the adsorption of MB onto the waste cotton fibrils. It is an indicator of the phenomenon which could be associated with chemical adsorption.

The pseudo-second order model best explains the kinetics of this process, this model assumes the adsorption capacity is proportional to the number of active centers of the adsorbent surface and the adsorption rate is controlled by chemical adsorption. This is consistent with the results obtained from the adsorption isotherms.

Further studies on filled reactors and the corresponding scale-up will be made. It is also very important to study the possibilities of reusing the adsorbent in several cycles and its final disposal in some useful ways for the planet, taking into account the circular economy principles.

Desorption adsorption cycles should be studied for adsorbent recovery. It is also necessary to study the different existing technologies to dispose the saturated adsorbents, as well as studying their alternative use in the manufacture of new materials for construction, such as cement floors.

Financial support from Universidad de Buenos Aires (UBACyT 2017-2019 N° 20020160100143BA; 2018-2020 N° 20020170200204BA) and from Universidad Tecnologica Nacional (MSUTIRE0005259TC), as well as the collaboration of Las Marias Textil SRL, with the residual material.