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Article

Adsorption of Glyphosate in Water Using Iron-Based Water Treatment Residuals Derived from Drinking Water Treatment Plants

by
Fuguo Qiu
*,
Chaoran Li
,
Shunxi Wang
and
Shuang Li
Key Laboratory of Urban Stormwater System and Water Environment, Ministry of Education, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
*
Author to whom correspondence should be addressed.
Processes 2024, 12(7), 1352; https://doi.org/10.3390/pr12071352
Submission received: 5 June 2024 / Revised: 20 June 2024 / Accepted: 24 June 2024 / Published: 28 June 2024
(This article belongs to the Section Environmental and Green Processes)
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Abstract

:
Glyphosate, a broad-spectrum herbicide, poses a potential threat to human health and the ecosystem due to its toxicity. In this study, iron-based water treatment residuals (Fe-WTRs) were employed for glyphosate removal. The adsorption kinetics, isotherms, and thermodynamics, as well as the effects of pH, Fe-WTR particle size, and temperature, were explored. The results show that Fe-WTRs are an effective adsorbent for glyphosate adsorption, and the maximum uptake capacity was recorded as 30.25 mg/g. The Fe-WTR surface was positively charged, and low-valent iron dominated under acidic conditions, favoring glyphosate adsorption. Furthermore, smaller Fe-WTR particles (<0.125 mm) showed a faster absorption rate and 20% higher adsorption capacity than larger particles (2–5 mm). The kinetic analysis indicated that the adsorption process exhibits a two-step profile, conforming to the pseudo-second-order model, and the thermodynamic analysis indicated that it is a spontaneous, endothermic, and entropy-driven reaction. Finally, the Fourier transform infrared spectral analysis revealed that this process is mainly associated with the formation of metal phosphate through the ligand exchange of the phosphate groups of glyphosates with the hydroxyl groups of iron present in Fe-WTRs. In this study, we demonstrated the potential of Fe-WTRs as a cost-effective and efficient adsorbent for glyphosate removal.

1. Introduction

Water pollution has become a significant environmental challenge in the 21st century due to urbanization and industrialization [1]. In the past few years, a variety of recently developed harmful contaminants have been discharged into the aquatic ecosystem [2]. Among them, herbicides are a class of emerging pollutants resulting from agricultural mechanization [3] that are mainly used to kill or destroy microorganisms and invasive weeds that affect crop production [4]. Globally, glyphosate has become one of the herbicides most frequently used due to its effectiveness in crops; however, although this substance is considered safer than other herbicides, its widespread use poses chronic and extended risks to humans and the biological environment [5,6].
A very large amount of glyphosate-contaminated wastewater is generated during the manufacture of this substance. Additionally, glyphosate can enter water bodies through various routes, such as spray drift, surface runoff, and wind erosion after its application on farmland [7]. Consequently, glyphosate can now be readily identified in both surface water and groundwater [8]. For instance, the concentration of glyphosate in surface water and groundwater in the United States was found to range from 2 to 430 μg/L [9], far exceeding the maximum permissible limit of glyphosate in drinking water, which is 0.7 μg/L [10]. Glyphosate minimally degrades in the environment, so it can pose a significant threat to human health and the ecosystem due to its toxicity [11,12]. Extensive evidence has suggested the presence of potential toxic effects of glyphosate on animal and human DNA, lysosomes [13], and kidneys [14], and it can cause endocrine disruption [15,16]. The International Agency for Research on Cancer (IARC) has classified glyphosate herbicide as a Group 2A agent, meaning that it is possibly carcinogenic to humans [17]. Therefore, it is critically important to properly treat glyphosate-polluted wastewater.
The currently available methods for removing glyphosate from wastewater include membrane separation [18], electrolysis [19], photocatalytic degradation [20], advanced oxidation processes [21], microwave radiation [22], ozonation [23], ultraviolet radiation [24], and adsorption [25]. Among them, the latter stands out by virtue of its adaptability, straightforward implementation, simplicity of use, and great efficiency, as well as the lack of an association with secondary pollution [26]. Adsorption has been widely demonstrated to be a low-cost and effective way of removing glyphosate from wastewater [25,27,28,29]. Adsorbents, such as synthetic clay substances [30], activated carbon [31], biochar [32], and metal organic frameworks [33], have been proposed to remove glyphosate from water. Nevertheless, the manufacture of these adsorbents requires high energy input or chemical materials, which might increase the operational costs. Thus, the ideal sorbent would be both economical and effective in removing glyphosate.
Currently, the common plant-based drinking water treatment process comprises coagulation, sedimentation, filtration, and disinfection. In recent years, there have been developments in coagulants, and commonly used ones include polymeric aluminum chloride, ferric chloride, ferrous sulfate, etc. Water treatment residuals (WTRs), an unavoidable byproduct generated by drinking water treatment facilities during the utilization of aluminum (Al) or ferric iron (Fe) salts as coagulants for water purification, has attracted increasing interest due to the very large amounts generated (approximately 3.6 million tons produced per year globally) [34]. Al- and/or Fe-rich WTRs have been widely recognized as a potent adsorbent for removing anionic organophosphate compounds, such as hexa-phosphate [35] and phosphate [34,36,37], from wastewater. As an anionic organophosphorus compound, glyphosate has strong affinity for trivalent cations such as aluminum and ferric ions [12]. A previous study showed that Al-rich WTRs are efficient adsorbents for removing glyphosate [28]. Compared with Al-rich WRTs, Fe-rich WTRs (Fe-WTRs) were shown to achieve a better capacity for absorption of phosphate from water [37]. Thus, it was hypothesized that Fe-WTRs might be potential effective adsorbents for glyphosate removal. However, currently, the application of Fe-WTRs in glyphosate removal and the corresponding mechanism are unclear.
Thus, this research study’s objective was to investigate the viability of glyphosate adsorption on Fe-WTRs. To this end, a series of laboratory experiments were conducted. The effects of factors including pH, Fe-WTR particle size, and temperature on glyphosate adsorption were evaluated. Furthermore, we also examined the adsorption kinetics, isotherms, and thermodynamic parameters of Fe-WTRs for glyphosate. Finally, the adsorption reaction was elucidated based on Fourier transform infrared spectroscopy. The outcomes provide the basis for a practical and useful approach to glyphosate treatment and the reuse of Fe-WTRs, addressing the knowledge gaps in the use of this waste type for glyphosate adsorption.

2. Materials and Methods

2.1. Fe-WTRs and Glyphosate Aqueous Solutions

The Fe-WTRs analyzed in this research study were sourced from a standard drinking water treatment plant located in Beijing, China. The water treatment process, consisting of coagulation, sedimentation, filtration, granular activated carbon filtering, and disinfection, aims to remove organic pollutants such as color, odor, and taste from water, ensuring that the turbidity of the treated water is less than 0.5, the odor threshold is less than 4, the color value is less than 5, and the organic matter removal rate is greater than 90%. After collection, a certain amount of naturally air-dried Fe-WTRs was placed in an oven, dried at 150 °C for 6 h, and then placed in a desiccator for future use.
Glyphosate was used as an adsorbent to investigate the adsorption characteristics of organic phosphorus and the adsorption mechanism of drinking water treatment residuals. The glyphosate used for preparing the designed glyphosate aqueous solutions was purchased from a company (Shanghai Pesticide Research Institute Co., Ltd., Shanghai, China), with a standard value of 99.3% and an expanded uncertainty (K = 2) of 0.6.

2.2. Experimental Set-Up

Duplicate-batch parallel adsorption tests were conducted in 250 mL glass flasks placed in a thermostat shaker. To investigate the impact of pH on glyphosate adsorption, the pH of the glyphosate solution (20 mg/L) was adjusted to 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0 by adding sulfuric acid (0.1 mM) or sodium hydroxide (0.1 mM). Then, we checked whether the pH had changed after the solution had been allowed to stand for more than 6 h. If the pH was obviously fluctuating, we adjusted the pH to the corresponding pH value and allowed the solution to stand until the pH was stable. Moreover, the Fe-WTR concentration and particle size were fixed at 2.0 g/L and 1.0–2.0 mm, respectively. Then, all the bottles were kept in a thermostat shaker (25 ± 1 °C) with a rotation speed of 120 rpm for 24 h.
To examine the impact of the particle size of Fe-WTRs, 0.5 g samples of Fe-WTRs with different particle sizes (i.e., <0.125 mm, 0.125–0.5 mm, 0.5–1.0 mm, 1.0–2.0 mm, and 2.0–5.0 mm) were added to the bottles containing the glyphosate solutions (15 mg/L). The original pH of the glyphosate solution was altered to 7.0 ± 0.15. Then, all the bottles were placed in a thermostat shaker (25 ± 1 °C) and shaken at a rotation speed of 120 rpm for 10 d, with samples being taken at regular intervals.
To comprehend the influence of the temperature, the adsorption performances of Fe-WTRs at three different temperatures (i.e., 15 °C, 25 °C, and 35 °C) were evaluated and compared at the different glyphosate concentrations of 1, 5, 10, 20, 40, 60, 80, and 100 mg/L, separately. The pH of the aqueous solutions was fixed at 7.0 ± 0.15. All the bottles were placed in a thermostat shaker with a rotation speed of 120 rpm for 10 d.

2.3. Sample Analysis

The elements in the analyzed Fe-WTRs were determined with an X-ray fluorescence (XRF) spectrometer (ZSX Primus II; Rigaku, Tokyo, Japan) at dispersive scanning wavelength. The Fe-WTR specific surface area and pore size distribution were measured with an Autosorb station (Autosorb station 4; Conta, GA, USA), where the specific surface area (SSA) was calculated with the Brunauer–Emett–Teller (BET) method, while the Barrett–Joyner–Hallenda (BJH) model was used to calculate the pore size distribution curve. The glyphosate concentration was determined using a UV–visible spectrophotometer (DR 6000; Hach, CO, USA) with the spectrophotometric method described in a previous study [28].
The bonding sites between glyphosate and Fe-WTRs were determined by analyzing Fourier transform infrared (FTIR) spectra to reveal the possible adsorption mechanism. The Fe-WTRs and glyphosate samples obtained before and after adsorption were firstly freeze-dried (−40 °C) under vacuum conditions (10 Pa) for 20 h, mixed with KBr at a 1:100 mass ratio, and ground into fine particles. The samples were then scanned with an FTIR spectrophotometer (Nicolet iS5; Thermo Fisher company, Shanghai, China), and the infrared spectra were recorded in the region from 4000 to 400 cm−1.

2.4. Evaluation of Adsorption Isotherms, Kinetics, and Thermodynamics

2.4.1. Absorption Isotherms

The adsorption isotherms of glyphosate adsorption on Fe-WTRs at 15, 25, and 35 °C were evaluated according to three empirical isotherm models, namely, Langmuir (Equation (1)), Freundlich (Equation (2)), and Temkin (Equation (3)) [35].
q e = b q m C e / 1 + b C e
q e = k F C e 1 / n
q e = B l n k T C e
Here, qe and Ce are the equilibrium absorption amount (mg/g) and the concentration in the solution phase (mg/L) of glyphosate, respectively; b is the affinity constant for Langmuir adsorption (L/mg); qm is the Langmuir theoretical maximal adsorption capacity (mg/g); kF represents the Freundlich constants associated with capacity for adsorption ((mg/g)/(mg/L)1/n); 1/n is the adsorbent’s level of adsorption; B is the adsorption heat-related constant; and kT is the constant of Temkin adsorption equilibrium (L/mg).

2.4.2. Kinetics

The adsorption kinetics of glyphosate adsorption on Fe-WTRs were evaluated with two kinetic models, i.e., the pseudo-first-order kinetic model (Equation (4)) and the pseudo-second-order kinetic model (Equations (4)–(6)) [35].
l n q e − q t = l n q m − k 1 t
t / q t = 1 / k 2 q e 2 + t / q e
h 0 = k 2 q e 2
Here, qe and qt represent the glyphosate adsorption capacity of Fe-WTRs at equilibrium and at time t (mg/g), respectively; k1 and k2 are the rate constants of the pseudo-first-order and pseudo-second-order kinetic equations, respectively (g/mg·h); and h0 is the initial adsorption rate of glyphosate.

2.4.3. Thermodynamics

The fundamentals of the thermodynamics of glyphosate adsorption on Fe-WTRs were examined based on the experimental data obtained at 288, 298, and 308 K and the following equations (Equations (7)–(9)) [35].
∆ G 0 = − R T l n K
∆ G 0 = ∆ H 0 − T ∆ S 0
l n K = ∆ S 0 / R − ∆ H 0 / R T
where ΔG0 is the conventional Gibbs free energy change (kJ/mol), ΔS0 is the entropy shift (J/mol·K), ΔH0 is the enthalpy shift (kJ/mol), K is the constant of equilibrium, R is the generic gas constant (8.314 J/mol·K), and T is the Kelvin absolute temperature. The equilibrium constant (K) was determined by converting the concentration (Ce) unit into mol/L and applying the Langmuir affinity constants (b) in the Langmuir isotherm model.

3. Results and Discussion

3.1. Main Components and Microscopic Analysis of Fe-WTRs

The main characteristics of Fe-WTRs are shown in Table 1. The content of Fe was relatively high, with Al and Fe being mainly derived from coagulants added by water plants. The contents of C, Si, and Ca in the examined sludge were also high, with silicon and calcium being mainly derived from the particles carried in raw water.
To determine the pH of the analyzed Fe-WTRs, samples with a particle size of less than 0.125 mm were weighed and placed in a beaker, and distilled water was added in a water–soil ratio of 1:2.5 (1 g:2.5 mL). The Fe-WTR samples were measured with a pH meter and determined to be weakly alkaline with a pH of 7.42. According to the BET specific surface area equation, the specific surface area of Fe-WTRs was 91.74 m2/g.

3.2. Glyphosate Adsorption Capacity of Fe-WTRs

3.2.1. Effect of pH

As presented in Figure 1a, the pH of the Fe-WTR solution greatly affected the adsorption of glyphosate. The adsorption capacity of the Fe-WTRs decreased drastically when the pH increased from 3.0 to 9.0. The amount of glyphosate uptake after 24 h at pH 9.0 was 12.15 mg/g, which was 63.16% lower than that at pH 3.0. Thus, this result indicates that the acidic conditions favored glyphosate adsorption on Fe-WTRs, for which there are two possible explanations. Firstly, the speciation of Fe-WTRs was found to be pH-dependent, and, as a consequence, the zeta potential of Fe-WTRs was found to vary with the pH conditions. When an adsorbent has a positive zeta potential, it can adsorb anions in the solution through electrostatic interactions. In order to understand the change in the electrochemical properties of Fe-WTRs during the adsorption process, the zeta potential under different pH conditions was measured. As presented in Figure 1b, the zeta potential of Fe-WTRs was positive under acidic pH conditions (pH < 5.80), favoring glyphosate adsorption, while it was negative at pH higher than 5.80, which was not beneficial for glyphosate adsorption. Secondly, the speciation of glyphosate in the aqueous solution was also pH dependent [38]. pH is a crucial factor in determining the form of glyphosate because this substance is zwitterionic in the natural environment. Figure 1c shows the morphological distribution of glyphosate under different pH conditions. At pH less than 0.78, most of the solution existed in the form of H4GPS; as the pH increased, the content of H4GPS decreased, and that of H3GPS-1 increased. At pH 0.78–2.29, the solution mainly contained H3GPS-1. At pH 2.29–5.96, as H3GPS-1 removed hydrogen from carboxylic acid groups (–COOH), H2GPS-2 content was predominant in the solution. At pH 5.96–10.90, as H2GPS-2 continued to separate hydrogen from phosphate groups (P–O–H), the amount of H1GPS-3 increased in the solution. Finally, at pH > 10.90, as H1GPS-3 removed hydrogen from amino groups, the solution mainly included GPS-4. Consequently, glyphosate uptake was hindered under high pH conditions due to the increase in the multivalent anion proportion. In conclusion, the acidic environment was more conducive to the adsorption of glyphosate on Fe-WTRs, which is consistent with the phenomenon observed in previous studies on glyphosate adsorption on Al-WTRs or naturally occurring iron-based adsorbents [28,39].
It was also noticed that, after adsorption, the solution pH increased in cases where the initial pH was less than 7.0, while the opposite phenomenon occurred when the initial pH exceeded 7.0 (Figure 1a). Under acidic conditions, the hydroxide ions in Fe-WTRs were released into the solution through the coordination exchange reaction with glyphosate, resulting in an increase in pH, as presented in Equations (10)–(12), while the protons in glyphosate dissociated under alkaline conditions, reducing the pH of the solution, as presented in Equation (13) [40].
WTR-(OH)2 + R-PO32− ⇋ WTR-(O3P-R) + 2OH−
Fe(OH)3(S) → Fe(OH)2+ + 2OH−
Fe(OH)3(S) → Fe(OH)2+ + OH−
H3GPS ⇋ H2GPS− + H+ ⇋ HGPS2− + 2H+

3.2.2. Effect of Fe-WTR Particle Size

The performance of Fe-WTRs in glyphosate adsorption based on different particle sizes is presented in Figure 2. As shown, the glyphosate adsorption capacity was greatly dependent on the particle size. Specifically, a two-stage kinetic profile was observed in all the tested groups, where the first 50 h was characterized by fast adsorption and the following stage by slow adsorption. During the first stage, faster glyphosate adsorption was observed for smaller particle sizes. Equilibrium was attained at about 240 h. Additionally, glyphosate adsorption on Fe-WTRs was improved with smaller particle sizes. The highest adsorption amount of 30.25 mg/g was obtained with a particle size of <0.125 mm, and this was reduced by 20% with a particle size of 2.0–5.0 mm. The better adsorption capacity of the smaller particle Fe-WTRs was attributed to the higher specific surface area. This parameter is generally considered to be a key factor in determining adsorption behavior and potential, and the active site and surface area suitable for removing glyphosate are inversely proportional to particle size [32]. Hence, the surface area and active sites beneficial for the adsorption of glyphosate increase with a decrease in particle size [41]. Similar phenomena have been also observed in previous studies in which WTRs were used to remove other pollutants, such as hexa-phosphate [35] and phosphate [36,37], from water.

3.2.3. Adsorption Isotherms

Three isotherm models were used to characterize the adsorption process of glyphosate on Fe-WTRs at different temperatures, and the outcomes are illustrated in Figure 3 and Table 2. The Langmuir isotherm best fitted the experimental data (R2 > 0.97) compared with the other two isotherm models, suggesting monolayer uptake and uniform energy of the adsorption sites according to the ligand exchange mechanism. Additionally, it was noticed that the Langmuir affinity constant, which represents the affinity and adsorption energy of the adsorption reaction [35], was found to be higher at higher temperature. This result indicates that elevated temperature conditions are more favorable for glyphosate adsorption on Fe-WTRs. The theoretical maximum adsorption capacity (mg/g) increased from 27.78 at 15 °C to 36.10 at 25 °C to 53.19 at 35 °C, implying that the glyphosate adsorption process of Fe-WTRs is endothermic.

3.2.4. Comparison of the Adsorption Capacity of Fe-WTRs with Other Adsorbents for Glyphosate

It was found that glyphosate adsorption on aluminum-based water treatment residual (Al-WTRs) has been reported in the literature with a theoretical maximum adsorption capacity of 85.9 mg/g [28]. Although the glyphosate adsorption capacity of Fe-WTRs in this study was lower, importantly, the use of Fe-WTRs as an adsorbent matrix in water bodies poses a smaller threat of secondary pollution than that of Al-WTRs, as aluminum elements could have toxic and harmful effects on the human body and the environment [42]. In addition, the adsorption capacity of Fe-WTRs for glyphosate is very close to that of naturally occurring iron-based adsorbents in nature [39], and Fe-WTRs have advantages in terms of waste reuse and sustainability development. Overall, Fe-WTRs represent a potentially cost-effective and efficient glyphosate adsorbent.

3.3. Adsorption Kinetics

The kinetics of glyphosate adsorption on Fe-WTRs with different particle sizes were evaluated using the pseudo-first-order and pseudo-second-order kinetic models [28], and the outcomes are displayed in Figure 4 and Table 3. Both models were capable of sufficiently describing the kinetic data from the experiments. The pseudo-first-order formula could suitably separate the two-stage kinetic profiles under all the conditions (R2 > 0.95). In comparison, the pseudo-second-order formula could adequately reflect the entire experimental kinetic dataset (R2 > 0.99). Additionally, the pseudo-second-order formula for equilibrium uptake (qe,cal) as fitted by the model showed a strong correlation with the experimental results (qe,exp), while the pseudo-first-order formula could not correctly forecast the equilibrium uptake. These results suggest that the pseudo-second-order model is more appropriate for articulating the kinetic mechanism of glyphosate adsorption on Fe-WTRs and that adsorption could be involved in the chemical reaction [43,44]. In terms of the overall trend, the adsorption rate constant (K) decreased with the particle size, which suggests that this adsorption process may be related to the contact area of the adsorbent [32].

3.4. Thermodynamic Properties

The thermodynamics of glyphosate adsorption on Fe-WTRs was evaluated, and the results are illustrated in Figure 5 and Table 4. As shown, the ΔG0 values under all the conditions were negative, demonstrating that glyphosate adsorption by Fe-WTRs is a spontaneous process. Furthermore, when ΔG0 was negative, the reaction happened more readily. The ΔG0 values achieved at the temperatures of 288 K, 298 K, and 308 K were −17.01 kJ/mol, −17.9 kJ/mol, and −18.79 kJ/mol, respectively. The results suggest that glyphosate is more prone to be absorbed on Fe-WTRs at high temperatures, which is in line with the discussion in Section 3.2.3. In addition, we investigated the thermal effects at constant pressure based on ΔH0. The positive value of ΔH0 suggests that the process of adsorption is endothermic, which could explain the higher adsorption capacity found under the higher temperature conditions (Figure 3). ΔS0 reflects the orderliness of a material during adsorption [45]. Thus, the positive value for ΔS0 shows that the shift in entropy is the motivating factor for the adsorption process since the level of disarray at the contact interface between the solid and the solution increased upon glyphosate uptake. During absorption, the hydroxyl groups of glyphosate were dissociated in the solution and were then adsorbed on Fe-WTRs, resulting in the dissociation of the bonds with water, thus increasing the disorder of the system [35,45].

3.5. FTIR Spectral Analysis

Figure 6 illustrates the FTIR spectra of Fe-WTRs, glyphosate (GPS), and glyphosate-loaded Fe-WTRs (Fe-WTRs + GPS), as well as the difference spectrum (Fe-WTR-GPS minus Fe-WTRs). For the Fe-WTR spectrum, the peaks at 3407 cm−1 and 1654 cm−1 could be attributed to the presence of free H2O molecules in Fe-WTRs [46], while the peaks at 1089 and 1050 cm−1 represent the bending mode of the bridging –OH groups for Fe–OH–Fe in Fe-WTRs [47]. As for the glyphosate spectrum, the bands at 800–1800 cm−1 belong to the characteristic peaks of glyphosate. The peak at 1734 cm−1 could be attributed to C=O stretching vibration [48]. The peaks at 1560 and 1484 cm−1 could be due to NH2+ deformation [49]. The peaks at 1422 and 1245 cm−1 correspond to –COOH and CH2 groups, respectively [49]. The peak at 1154 cm−1 could be due to P–O stretching vibration [50,51]; that at 1090 cm−1 to PO3 stretching vibration [48,49]; that at 917 cm−1 to P-O5 stretching vibration, CH2 deformation, and CCNC skeletal vibration [39,49]; and that at 805 cm−1 to POH deformation [49].
It has been widely reported in previous studies on various of adsorbents, such as Al-WTRs, amino-MIL-101(Fe) [52], goethite [50], and graphene [53], that the most active group in the glyphosate adsorption process is the phosphate group. In this study, a significant change was observed in the peaks at 800–1200 cm−1 of the Fe-WTRs + GPS spectrum after adsorption, implying that glyphosate interacts with the active sites of the Fe-WTR surface mainly through the phosphate groups. Additionally, the peaks at 1089 cm−1 and 1050 cm−1 were found to be weaker after adsorption, which was possibly due to the replacement of –OH with a phosphate group through ligand exchange. Noticeably, new peaks, including 1149 cm−1 and 1007 cm−1, emerged upon adsorption. The peak at 1149 cm−1 could be explained by the existence of phosphate bidentate [54] and that at 1007 cm−1 by the formation of γ-FeOOH [55]. These outcomes demonstrate that the phosphate group of glyphosate could combine with the ferric ion in Fe-WTRs to form ferric phosphate, which might be one of the main reactions by which Fe-WTRs to adsorb and remove glyphosate from water, as also presented in Figure 7.

4. Conclusions

In this study, we investigated the roles of Fe-WTRs in glyphosate adsorption. The results show that Fe-WTRs represent an effective absorbent for glyphosate removal. Acidic environments are more advantageous for glyphosate adsorption since the more positive groups at acidic pH facilitate the interactions between the negatively charged surface of Fe-WTRs and the positive glyphosate. The size of the Fe-WTR particles greatly affected adsorption removal efficiency, and better adsorption performance was achieved with smaller particles. The kinetic analysis suggested that the adsorption of glyphosate on Fe-WTRs complies with the pseudo-second-order model. It was revealed that adsorption has a two-stage kinetic profile, with fast adsorption in the first 50 h and then a stage of slow adsorption. The thermodynamic analysis revealed that glyphosate adsorption on Fe-WTRs is a spontaneous, endothermic, and entropy-driven process. The FTIR spectral analysis indicated that ligand exchange occurs between glyphosate and Fe-WTRs, where the ferric ion of Fe-WTRs combines with the phosphate group of glyphosate.

Author Contributions

F.Q.: formal analysis, funding acquisition, project administration, and supervision; C.L.: methodology, writing—original draft, software, visualization, and data curation; S.W.: writing—review, editing, and investigation; S.L.: writing—review, editing, and investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This research study was financially supported by the National Natural Science Foundation of China (51278024).

Data Availability Statement

All data are fully available and can be found within the manuscript.

Acknowledgments

The authors acknowledge the School of Environment and Energy Engineering, Beijing Univ. of Civil Engineering and Architecture, for providing the necessary research infrastructure.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Impacts of pH on glyphosate adsorption (a), zeta potential of Fe-WTRs (b), and glyphosate speciation (c).
Figure 1. Impacts of pH on glyphosate adsorption (a), zeta potential of Fe-WTRs (b), and glyphosate speciation (c).
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Figure 2. Glyphosate adsorption on Fe-WTRs with different particle sizes.
Figure 2. Glyphosate adsorption on Fe-WTRs with different particle sizes.
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Figure 3. Fe-WTR adsorption isotherms for glyphosate at different temperatures.
Figure 3. Fe-WTR adsorption isotherms for glyphosate at different temperatures.
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Figure 4. Pseudo-first-order (a) and pseudo-second-order (b) kinetic models used to fit the experimental kinetic data.
Figure 4. Pseudo-first-order (a) and pseudo-second-order (b) kinetic models used to fit the experimental kinetic data.
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Figure 5. The variations in standard enthalpy and entropy during glyphosate adsorption on Fe-WTRs were determined by plotting the Van ’t Hoff plot of LnK against 1/T.
Figure 5. The variations in standard enthalpy and entropy during glyphosate adsorption on Fe-WTRs were determined by plotting the Van ’t Hoff plot of LnK against 1/T.
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Figure 6. Fourier transform infrared (FTIR) spectra of Fe-WTRs, glyphosate (GPS), and GPS-loaded Fe-WTRs (Fe-WTRs + GPS) and the difference spectrum (Fe-WTRs + GPS minus Fe-WTRs).
Figure 6. Fourier transform infrared (FTIR) spectra of Fe-WTRs, glyphosate (GPS), and GPS-loaded Fe-WTRs (Fe-WTRs + GPS) and the difference spectrum (Fe-WTRs + GPS minus Fe-WTRs).
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Figure 7. The adsorption reaction proposed in this study.
Figure 7. The adsorption reaction proposed in this study.
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Table 1. Characteristics of Fe-WTRs.
Table 1. Characteristics of Fe-WTRs.
Chemical ElementSSA and Pore Volume
(Particle Size < 0.125 mm)
Fe
(%)		Si
(%)		Al
(%)		Ca
(%)		Mg
(%)		C
(%)		O
(%)		N
(%)		pHBET-N2
(m2/g)		MPSA
(m2/g)		TPV
(cm3/g)		MPV
(cm3/g)		TAAPW
(nm)
15.908.816.742.780.4310.5051.901.267.4291.7487.740.990.114.77
Table 2. Parameters of the three isotherm models with the best-fit isotherm.
Table 2. Parameters of the three isotherm models with the best-fit isotherm.
Temperature
(°C)		LangmuirFreundlichTemkin
qmbR2kFnR2BkTR2
1527.780.0070.990.481.480.955.690.080.96
2536.100.0080.980.801.560.987.740.090.93
3553.190.0090.971.011.460.9610.770.110.95
Table 3. Parameter of pseudo-first-order and pseudo-second-order kinetic models.
Table 3. Parameter of pseudo-first-order and pseudo-second-order kinetic models.
ModelPseudo-First-Order ModelPseudo-Second-Order Model
Parameterqe,exp
(mg/g)		Fast StageSlow Stagek2
(g/mg·h)		qe,cal
(mg/g)		h
(mg/g·h)		R2
k1f
(1/h)		qe,cal (mg/g)R2k1s (1/h)qe,cal (mg/g)R2
Particle size (mm)<0.12530.250.05422.870.990.0129.610.980.00430.583.700.99
0.125–0.528.330.05227.230.990.0108.080.950.00428.653.060.99
0.5–126.430.02520.280.970.0067.640.980.00227.781.280.99
1–225.860.02922.220.950.01114.040.960.00226.671.420.99
2–524.360.02627.640.980.0079.940.960.00225.511.170.99
The K1f and K1s values were calculated for the pseudo-first-order model to fit the fast stage and slow stage, respectively.
Table 4. Thermodynamic parameters of glyphosate adsorption on Fe-WTRs.
Table 4. Thermodynamic parameters of glyphosate adsorption on Fe-WTRs.
Temperature (K)K or b (L/mol)ΔG0 (kJ/mol)ΔH0 (kJ/mol)ΔS0 (J/mol·K)R2
2881209.93−17.018.6188.970.99
2981397.07−17.90
3081527.28−18.79
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Qiu, F.; Li, C.; Wang, S.; Li, S. Adsorption of Glyphosate in Water Using Iron-Based Water Treatment Residuals Derived from Drinking Water Treatment Plants. Processes 2024, 12, 1352. https://doi.org/10.3390/pr12071352

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Qiu F, Li C, Wang S, Li S. Adsorption of Glyphosate in Water Using Iron-Based Water Treatment Residuals Derived from Drinking Water Treatment Plants. Processes. 2024; 12(7):1352. https://doi.org/10.3390/pr12071352

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Qiu, Fuguo, Chaoran Li, Shunxi Wang, and Shuang Li. 2024. "Adsorption of Glyphosate in Water Using Iron-Based Water Treatment Residuals Derived from Drinking Water Treatment Plants" Processes 12, no. 7: 1352. https://doi.org/10.3390/pr12071352

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