1. Introduction
Activated carbons are highly effective and universal adsorbents extensively used in various applications such as removing water pollutants, gas purification, separation or storage, catalysis, and many others [
1,
2]. Their porous structure, characterized by a sizeable specific surface area and pore volume and the presence of surface functional groups, defines the main properties of carbon adsorbents necessary for this particular application [
1]. Although activated carbons are often chosen adsorbents, their widespread use is limited due to their relatively large cost. One of the ways to reduce the cost of producing carbon adsorbents is to use agricultural by-products and waste as low-cost precursors [
3,
4,
5]. On a global scale, there is a growing need to find green and sustainable solutions to environmental issues.
Along with wheat and rice, corn is one of the three most important crops in the world. The United States remains the largest producer of corn. It is followed by China and Brazil. The European Union crop accounts for about six percent of the world production.
Corn is a versatile crop. It can be grown not only for use as a grain but as animal feed. A large number of farmers also grow it for silage. It is also used for the production of furfural [
6], bioethanol [
7], biogas [
8], lactic acid [
9], paper pulp [
10], and building materials [
11]. It also serves as an energy feedstock [
12].
Ahmed et al. [
13] report that corn residues such as cobs, straws, stalks, stovers, husks, silk fibers, and leaves have been used to obtain carbon adsorbents for purifying water from various chemicals. Cobs (31%), stalks (31%), and straw (28%) have been used most frequently to date, followed by husks (6%), silk (3%), and leaves (1%). These residues are used in crude [
14], chemically modified [
15], biocarbon [
16] and composites [
17]. However, the most applied adsorbents from corn residues are activated carbons (ACs) [
18,
19,
20,
21,
22,
23,
24]. Producing activated carbons (ACs) from renewable resources helps maintain environmental balance and utilize agricultural waste. Using local residual biomass as a raw material can enhance sustainability. Opting for lignocellulosic materials from nearby sources reduces the need for long-distance transportation, thus lowering the carbon footprint of the entire treatment process. Activated carbons are mainly obtained from corn cobs by chemical activation using KOH, NaOH, H
3PO
4, ZnCl
2, or FeCl
3. These carbons have a strongly developed specific surface area (under 1000 m
2/g) and can be successfully used for the removal of pollution from liquid and gas phases [
25,
26,
27,
28,
29,
30].
In contrast, in the present study, activated carbons produced from corn biomass (different parts of a corn plant) have been used as adsorbents to remove herbicides such as phenoxyacetic acid (PAA), 4-chlorophenoxyacetic acid (4CPA), and 2,4-dichlorophenoxyacetic acid (2,4-D) from aqueous solutions. Phenoxyacetic herbicides, especially 2,4-D, are the most popular and widely used herbicides today, next to glyphosate [
31,
32]. Unfortunately, their widespread use is associated with their frequent occurrence in the environment, which is highly undesirable due to their high toxicity. Among the many techniques for removing these pollutants from water, adsorption, especially on activated carbon, is considered the most popular and effective. This effectiveness largely depends on the physical and chemical properties of the adsorbent used. Therefore, the search for and production of new sustainable carbon materials as potentially effective adsorbents is currently a scientific trends.
The application of carbon adsorbents in the removal of water pollutants is much more effective than the use of other methods due to adsorption’s more convenient operating units in terms of convenience and simple design. On the other hand, as precursors, activated carbons were taking into account agricultural waste because of the much lower cost of carbon adsorbent production in comparison with the commonly used coal or wood methods. Agricultural wastes are considered a good choice, taking into account their availability, low cost, and the economic advantage of their useful exploitation, which also serves as a means of disposal. The method of activated carbons produced from different parts of the corn plant and their physical and chemical characteristics were described and discussed in earlier work [
20].
The aim of this work was to attempt to provide a sustainable solution to several environmental protection problems at the same time. The use of waste biomass was chosen to provide raw material for the production of activated carbon, which was then to be used as a low-cost adsorbent to purify water from organochlorine herbicides and their derivatives.
4. Discussion
Studies of the porous structure (
Table 2) showed that the value of every parameter, including
SBET,
Vt,
Vmi, and
Vme, varies significantly for each of the three tested activated carbons. The order of values is consistent for
SBET and
Vmi (maximum value for AC-K and minimum value for AC-S). In the case of other porous structure parameters
Vt and
Vme, the order of their values is different (maximum value for AC-L and minimum value for AC-K). All ACs were characterized by a high specific surface area S
BET (above 1600 m
2/g) and a high total pore volume
Vt (above 1.11 cm
3/g). The AC-K had the strongest developed S
BET surface area. The corn biomass yielded carbon with varying proportions of micro- and mesopores. AC-K had the highest proportion of micropores, while AC-L had the lowest.
The presence of an extensive pore system were confirmed by SEM analysis (
Figure 2). In the case of AC-K, most of the grains are up to 20 μm in size, while some of them are 100–150 μm in size. The shape of the grains is angular, their edges are sharp, and their surfaces are smooth. In the case of AC-L, most of the grains are up to 20 μm in size, while some up to 100 μm in size. Their shape is irregular, their edges are jagged, and their surfaces are irregular. In the case of AC-S, the grains are large, 100–300 μm in size, and finer grains are 50–100 μm in size. Their structure is porous, with large pores with a diameter of approx. 10 μm, and their surfaces are smooth.
Based on the surface chemistry parameters of the prepared activated carbons presented in
Table 3 and
Table 4, it can be noted that the use of various anatomical parts of corn as a precursor produces activated carbons with very diverse properties. Comparing the properties of the obtained activated carbons with those obtained in other works is quite difficult because they generally used one of the anatomical parts of the plant as a precursor, e.g., silk, leaves, or cobs. One activating factor was most often used there, e.g., CO
2 or KOH. Also, the conditions (temperature or time) were different.
Through analyzing the adsorption kinetic data presented in
Table 5, it can be seen that significantly higher
R2 values (≥0.996), as well as lower
χ2 values (≤0.0175), were obtained for the pseudo-second-order model. A higher agreement between the theoretically calculated adsorption capacity (
qe2(cal)) and the experimental value (
qe(exp)) was also observed for the PSO model. These results indicate that PAA, 4CPA, and 2,4-D adsorption kinetics on all three corn-derived activated carbons follow the pseudo-second-order model. This is in general agreement with the literature data. As shown in reviews [
5,
31,
32], the adsorption of phenoxyacetic herbicides on activated carbons (but also other non-carbon adsorbents) mostly followed the PSO model.
When comparing the adsorption rates of the individual herbicides on ACs, it can be observed that the
k2 values increase in the order PAA < 4-CPA < 2,4-D. In previous studies on the adsorption of these three phenoxyacetic acids on lignite [
34], the opposite trend in the adsorption rate was observed (2,4-D < 4-CPA < PAA). This indicates that the adsorption rate of a specific adsorbate is an individual matter and depends on the type and physicochemical properties of the adsorbent used. The surface chemistry of the adsorbent and its porous structure determine the rate and adsorption efficiency. All three herbicides were adsorbed slowest on AC-K and fastest on AC-S (AC-K < AC-L < AC-S). This observed tendency seems to be closely related to the porous structure of these ACs (
Table 2). The adsorption rate increases with the increased mesopore volume (
Vme) of the activated carbons. The mesopores are mainly responsible for the transport of the adsorbate molecules into the micropores, and, in general, their greater number (greater volume and greater proportion of the total porosity) accelerates the adsorption process.
The intraparticle diffusion models, including the Weber–Morris and Bangham, suggest that the adsorption rates of PAA, 4CPA, and 2,4-D on ACs obtained from corn biomass are influenced by both pore diffusion and film diffusion, but the diffusion in the film is the rate-limiting step that controls the whole adsorption process.
The experimental adsorption isotherms were described using the Langmuir and Freundlich [
35] models, and it was found that the Langmuir isotherm described the equilibrium data better than the Freundlich isotherm model. The adsorption capacity determined from the Langmuir and Freundlich equations makes it possible to compare the adsorption efficiency of each adsorbate on the adsorbent surface and also to compare the adsorption capacities of the adsorbents used. Thus, by analyzing the data in
Table 6 (both
qm and
KF values), it can be seen that all three herbicides used were adsorbed best on the AC-K and least on the AC-S. The adsorption capacity of these activated carbons towards these adsorbates increases in the order AC-S < AC-L < AC-K and, as in the kinetic studies, seems to be closely correlated with the porous structure of the ACs used (
Table 2). The adsorption efficiency increases with increasing values of the BET surface area as well as the micropore volume of the studied activated carbons.
A comparison of the adsorption of phenoxyacetic acids on the ACs shows that PAA was adsorbed least and 2,4-D most preferentially. This observed order (PAA < 4CPA < 2,4-D) appears to be positively correlated with the molecular weight of the adsorbates and their hydrophobicity while inversely correlated with their solubility in water (
Table 1). The adsorption efficiency increased with the number of chlorine atoms attached to the aromatic ring of the molecule. The Cl substituents reduce the electron density of the benzene ring, which increases the molecule’s hydrophobicity and reduces its solubility in water. The more hydrophobic compound has more affinity for the hydrophobic surface of the activated carbon. Indeed, this is the case; the best-adsorbed herbicide was 2,4-D, which is the most hydrophobic compound—it is the most poorly soluble in water and has the highest octanol–water partition coefficient (2.37). In contrast, the least adsorbed compound was the most hydrophilic one (PAA), which is the most soluble in water and has the lowest log
p value (1.48). These findings are in general agreement with the results reported in other papers. The same adsorption sequence (PAA < 4CPA < 2,4-D) was observed on goethite [
37], lignite [
33], and various commercial activated carbons [
38]. However, it should be noted that among these phenoxyacetic herbicides, 2,4-D is by far the most commonly studied, while the other two (PAA and 4CPA) are rarely investigated. For this reason, the available comparative material is inadequate.
Based on numerous literature data [
5,
31,
32], it is known that the mechanism of 2,4-D adsorption on the surface of activated carbons includes van der Waals forces, electrostatic interactions, n-π and π-π interactions, and hydrogen bond formation. Since the physicochemical properties of PAA and 4CPA are very similar to those of 2,4-D, it can be speculated that their adsorption mechanism is analogous.
Table 7 shows the adsorption capacities of various activated carbons and other carbon materials reported in the literature as adsorbents for the removal of PAA, 4CPA, and 2,4-D from water. Most of the adsorbents produced from corn biomass available in the literature were cited. A comprehensive and more detailed comparison of different adsorbents can be found in recently published reviews [
5,
31,
32].
The pH dependence of adsorption observed in
Figure 7 indicates that the process is most efficient at pH 2, where the interaction of undissociated herbicide molecules with the positively charged AC surface is most favored. Above pK
a, more and more herbicide molecules become anions, resulting in a gradual decrease in adsorption efficiency. Finally, in the pH range from about 7 to 10, adsorption does not change and stabilizes at a low level. The relatively low adsorption efficiency observed in this range results from repulsive electrostatic interactions between the herbicide anions and the negatively charged surface of the activated carbons. A similar effect of pH value on the adsorption of 2,4-D was reported on lignite [
33], carbon black [
43], and other activated carbons [
42,
44].