pH dependence of glyphosate adsorption from aqueous solution using a cationic cellulose microfibers (cCMF) biosorbent

Introduction

The world demand for food has been growing as the population increases, according to the United Nations (UN) in 2050 the population will be greater than 9 billion people [1]. In recent decades, Brazil has been seen as a country of constant prominence in agricultural production, exporting a wide variety of grains to many countries around the world [2]. In order to maintain these levels of production, it is necessary to apply several alternatives for agricultural management, which includes the use of various agricultural inputs, such as different types of fertilizers and pesticides [2].

The indiscriminate use of these products can generate numerous problems, including contamination of water, soil, and atmosphere, generating toxic effects that harm living organisms and their ecosystems [3]. However, the use of pesticides is still important for world agricultural production. One of the most commonly used pesticides to increase agricultural production is the herbicide glyphosate [4].

Glyphosate, or N-(phosphonomethyl)glycine, is a broad-spectrum, non-selective systemic herbicide that suppresses or eliminates many types of weeds [4, 5]. It was created in 1950 and it is widely used nowadays, being the herbicide with the highest volume of production worldwide [6, 7]. Although, glyphosate has great potential to contaminate water bodies, acting not only at the planting level but also preventing enzymatic functions of animals that come in contact with the effluent, in addition to causing direct or indirect harm to humans, increasing the risks of allergic reactions, as well as heart and respiratory problems [8, 9].

Water pollution by the excess of this herbicide has become a constant concern in recent years. In this context, there is a demand to create new alternative methods to remove glyphosate from wastewater [10]. Several procedures can be used to remove pesticides from water, such as advanced oxidation processes [11, 12], photocatalytic degradation [13, 14], and ozonation [15], however, through the advancement of technology, biosorbents have gained notoriety as an effective and low-cost method [16, 17].

Biosorption is a physicochemical process for removing substances via adsorption from aqueous solutions or air by natural and renewable materials [18]. Among the most common sources for obtaining biosorbents, there is cellulose, present in all plant species. Thus, the use of agricultural residues, such as green coconut husk [19], orange peel [20], grape pomace [21], and pineapple peel [22] are a promising alternative for the removal of a huge variety of contaminants from water.

In order to reach higher efficiency in the adsorption capacity of contaminating agents, cellulose-based adsorbents have been modified by different methods and/or functionalization of fibers such as carboxylation [23], acetylation [24], mediated oxidation by TEMPO [25], cationization [26] and others. Notable works based on cationic cellulose applied to nitrates [27], fluoride, sulphate, and phosphate [28] adsorption are found with high performance.

Therefore, this work proposes to study the removal of the herbicide glyphosate from an aqueous medium using functionalized cellulose microfibers with cationic properties (cCMF), at different pH values. In addition, this work aims to contribute to the class of lignocellulosic materials applied in adsorption processes, showing the possibility of using an agroindustrial residue applied in water treatments.

Experimental

Results and discussion

Optical microscopy allowed the first observation of the fibers at lower magnification in order to identify the most notable changes throughout the obtaining processes (Fig. 1). The sugarcane bagasse samples are raw fibers that underwent a micronization and drying process at a mild temperature of 60.0 °C. At this stage, it was possible to reduce the size of the fibers between 0.11 and 0.15 mm to simplify the process of obtaining functionalized cellulose microfibers.

Fig. 1:

Optical microscopy images (10× objective lens) and SEM images (magnification of 100×) of the isolated cellulose after bleaching (A); dialdehyde cellulose (DAC) microfibers (B); and cCMF microfibers (C).

Besides the cellulose, hemicellulose, and lignin contribute to forming the structure of the biomass, making the fibers opaque due to a matter of density in the fiber structure, occupying the spaces between the cellulose fibrils. This fact was observed by the change in the fibers after the chemical treatment stages, which includes the delignification and bleaching of the fibers, ensuring that the other components were removed, and isolating the cellulose.

In the following steps, the fibers suffer the functionalization processes. In the oxidation step (Fig. 1B), NaIO₄ is used as a strong oxidizing agent in the proportion of 2.0 mmol g⁻¹. This concentration is based on previous studies in the literature [32]. Nevertheless, it is natural that the fibers decrease in size or suffer partial degradation even under the action of an oxidizing agent since the oxidative attack has the principle of decreasing hydrogen bonds and causing a decrease in intermolecular interactions [32].

For the last functionalization step of fiber cationization (Fig. 1C), the dialdehyde groups are replaced by the quaternary ammonium salt, Girard T reagent. This replacement affords a positive charge along the fiber that was characteristically neutral. As the previous synthesis step, it causes the exit of two water molecules from each aldehyde group, leading to another fragmentation process, which explains why the fibers become even smaller in length and width when observed under an optical microscope.

Fibrils are formed from the packaging of long polymeric cellulose chains [33] and the SEM analysis (Fig. 1A–C) confirms that the structure maintains the fiber characteristics despite going through two chemical processes, the delignification of the in natura fiber and the bleaching of the pulp.

Yield calculations were based on the initial mass of sugarcane bagasse in natura (100 % w/w) after successive chemical steps. Table 1 shows the results obtained.

It is possible to observe the loss of the total mass obtained at the end of the synthesis caused by chemical processes of cellulose fibers isolation as well as for the groups inserted in the cellulosic structure in the oxidation and cationization process. The process of bleaching cellulose fibers obtained 60.0 % (w/w) of yield, that is the amount of mass of sugarcane bagasse, used at the beginning of the process, transformed into bleached cellulose fibers. This happens due to the removal of lignin and hemicellulose present in the sugarcane bagasse. After the oxidation of the bleached fibers, there is a tendency to form smaller fibers, which leads to a total yield of 31.27 % (w/w).

The degree of oxidation was also calculated following the methodology described above. This data shows how much the cellulose fibers were oxidized in the structure, making it possible to evaluate the efficiency of the reaction since an oxidation of 2.0 mmol g⁻¹ was initially calculated. This oxidation breaks the bond between carbons 2 and 3 of the anhydroglucose unit and simultaneously creates two aldehyde groups in this structure, allowing further functionalization, such as cationization (Fig. 2) [34].

Fig. 2:

Synthesis of cCMF by the oxidation of cellulose structure and cationization of DAC.

Finally, the cationization process of microfibers shows a high degree of yield, since the fibers are slightly degraded. In this step, the aldehyde groups are broken down for the insertion of the quaternary ammonium groups. This reaction obtained a yield of 89.94 % concerning the initial mass of DAC involved and 28.12 % of the general yield.

The FTIR spectra of different products from the cCMF obtaining process are shown in Fig. 3. Cellulosic materials tend to have several small peaks between 800 and 1800 cm⁻¹ with two peaks of greater intensity drawing attention in this region of the spectrum [32].

Fig. 3:

FTIR spectra of samples at different stages of microfiber production.

The peak present at 1029 cm⁻¹ refers to the symmetrical stretching of the C–O vibrations present mainly in hemicellulose and lignin; the peak at 1634 cm⁻¹ may be related to vibrations of the aromatic groups present in the structure; the peaks at 1723 cm⁻¹ and 895 cm⁻¹ are due to the nonconjugated C=O stretching assigned to hemiacetals and hydrated forms of aldehyde groups [35]. Another outstanding peak appears at 2901 cm⁻¹, which indicates a vibrational stretching of the C–H groups of cellulose and the symmetrical stretching of the CH₂ and C–H groups. A very characteristic band appears around 3335 cm⁻¹ which represents strong vibrations in the intermolecular and intramolecular OH bond present in cellulosic structures, as well as moisture present in the material from absorbed water molecules [32]. Around 931 cm⁻¹ there is a discrete signal that refers to the vibrational mode of N–N present in the Girard T group [32, 35, 36], proving the formation of cCMF.

To determine the point of zero charge (Fig. 4), the average value between the final pH points that tended to the same value (pH 4, 5, 6, and 7) was calculated [30].

Fig. 4:

Point of zero charge (pH_PZC) of the cationic cellulose microfibers produced to adsorb the glyphosate molecules.

The region of zero charge of cCMF is between pH 4 and 7, with the isoelectric point pH = 5.4 ± 0.016. As the pH of the solution increases, the charges on the microfibers become more negative, however, due to the high cationicity of the fibers, there is still a positive charge present on the surface of the fibers which can interact by electrostatic interactions between the negatively charged adsorbate and positively charged quaternary ammonium functional groups on the cellulose surface (N(CH₃)₃ ⁺) [32, 36].

Adsorption test

The results obtained in the glyphosate adsorption test showed that at higher pH values the removal efficiency increases (Fig. 5).

Fig. 5:

Adsorption capacity (q _e) of glyphosate (50 mg L⁻¹) and removal efficiency (%) as a function of pH values (2, 6, 10, and 14).

According to the literature, glyphosate molecule changes depend on the pH values [37, 38], as shown in Fig. 6. At pH below 2, the molecule is fully protonated with three hydroxyl groups in equilibrium, conferring a neutral charge to the molecule. Considering that cCMF presents positive charges at acid pH [39], the possibilities of interaction between both species involved slightly decrease, resulting in an adsorption capacity substantially lower, with a value of removal efficiency of 6.79 % (q _e = 2.039 mg g⁻¹). As the pH increases to a range between 2-2.6, a process of deprotonation of the molecules begins, releasing at least one proton from the glyphosate, and becoming the pesticide molecules more negative [40]. This tendency of deprotonation continues and at pH higher than 10.6 the glyphosate molecule is totally deprotonated, showing a negative charge, which favors its adsorption in cationic adsorbents [37, 38]. In this sense, the removal efficiency increases almost linear (Fig. 5). Thus, at pH 14 the fibers reached a maximum removal value of 59.21 %.

Fig. 6:

Chemical structure of glyphosate under different pH conditions.

Parallelly with glyphosate behavior, the cCMF detains the cationic group (N(CH₃)₃ ⁺) which is responsible for the positive charge, justifying this increase in the removal process. Then, even though increasing the pH value of the middle consequently increases negative charges, there is this cationic group distributed along microfibers that promotes the possibilities of adsorption by charge disparity [36, 39]. As mentioned, sugarcane bagasse was subjected to several chemical processes, including cationization, a step very important to which is responsible for removal efficiency. In this way, the interaction between cationic cellulose microfibers and glyphosate is favored at higher pH, such as 14, where the positive charge of the microfibers interacts with the negative charges of glyphosate.

SEM images showed the microfibers before and after interaction with glyphosate (Fig. 7). The images prove that the glyphosate interacted with the fibers by adsorption process, increasing the average fiber diameter (AFD) from 9.545 ± 2.491 μm (Fig. 7A) to 12.715 ± 2.738 μm (Fig. 7B). The presence of glyphosate can be observed by the formation of a rough layer on the surface of the microfibers.

Fig. 7:

SEM images (magnification of 3000×) of cCMF before (A) and after (B) glyphosate (50 mg L⁻¹) adsorption, both at pH 14.

The presence of the glyphosate layer adsorbed on the surface of the cCMF allows, through simple separation processes such as filtration, precipitation, or centrifugation, to remove the contaminant and the biosorbent from the aqueous solution.

Conclusions

The cationic microfibers were obtained from sugarcane bagasse, an agroindustrial residue, with a total yield of 28.12 % cCMF. Morphological analyzes showed the transformation of micronized bagasse to cationic cellulose microfibers (cCMF), confirming the initial fibers’ defibrillation and increasing the surface area of the adsorbent after interaction with glyphosate. The adsorption test showed that the removal of the herbicide glyphosate from an aqueous medium using cCMF is more efficient at pH 14, obtaining a removal efficiency of 59.21 %. The interaction between the positive charges remaining in the microfibers and the deprotonated glyphosate molecules elucidates the higher removal on basic pH. In this way, the new biosorbent proposed in this work proved to be an alternative to the decontamination of polluted waters with the herbicide glyphosate.

Notes

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.