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Ultraviolet Radiation-Assisted Preparation of a Novel Biomass Fiber to Remove Cadmium from Wastewater

Chinese Research Academy of Environmental Sciences, Beijing 100012, China
Qiantang River Basin Management Center of Zhejiang Province, Hangzhou 310016, China
Hangzhou Shenyuan Environmental Sci-Tech Co., Ltd., Hangzhou 310016, China
Authors to whom correspondence should be addressed.
Water 2023, 15(4), 811;
Submission received: 2 January 2023 / Revised: 15 February 2023 / Accepted: 17 February 2023 / Published: 19 February 2023
(This article belongs to the Special Issue Water Pollution and Bioremediation)


The heavy metal adsorbents developed based on biomass resources have valuable application prospects due to the characteristics of rich sources, renewability and low cost. In the present work, a carboxyl functioned loofah fiber (LF@AA) was synthesized via UV-induced polymerization, and its adsorption capacity for cadmium (Cd2+) was investigated systematically. This modification resulted in the effective combination of a loofah fiber template and polyacrylic acid (PAA), which promoted its adsorption of Cd2+ to significantly increase to 339.3 mg·g−1, and the applicable pH range was 4.0~7.0. Furthermore, the adsorbability of LF@AA remained stable at a high level after eight consecutive cycles. The adsorption kinetics and isotherm parameters revealed that the adsorption characteristics of cadmium conformed to the Weber–Morris and pseudo-second-order kinetics equations, and the adsorption process of cadmium conformed to Redlich–Peterson and Langmuir models. In addition, consequences of EDS, FTIR, and Zeta potential analysis reflected that the main adsorption mechanism should be ion exchange. Cd2+ was drawn to the adsorbent surface by electrostatic binding, and ion exchange occurred to form a bidentate chelate. This study suggests that it is reasonable and feasible to use natural biomass materials to develop efficient adsorbents to treat heavy metal pollution in wastewater.

1. Introduction

With the rapid development of industry, the discharge of industrial wastewater containing high-density heavy metals has caused serious water pollution. Cadmium is extensively used in the field of alloy manufacturing, plastic stabilizer manufacturing, pigments and batteries; however, cadmium is very highly toxic, and causes grave harm to living organisms, because it is easily enriched in the vital organs of organisms and has potential carcinogenicity. The European Union treats cadmium as a high-risk carcinogen and toxic agent. The Environmental Protection Agency (EPA) had set its maximum concentrations in drinking water at 10 µg/L and planned to prune it down to 5 µg/L [1]. Therefore, it is meaningful to consider the removal of cadmium from wastewater. Various purification methods have been proposed for the treatment of heavy metals, e.g., chemistry precipitation, reverse osmosis, electrochemical treatment, and so forth [2]. Aside from these, adsorption is highly recommended because it is clean, requires low investment, highly efficient, and easy to operate [3].
Recently, there is a growing interest in the utilization of agricultural by-products or agricultural and forest product processing wastes, such as sugarcane bagasse, pineapple peel, fruit peel, straw, seed shells, and rice husks, to exploit cost-effective adsorbents for the separation and enrichment of heavy metal [4,5,6,7]. It is known that a biomass resource has splendid material sources, it is renewable, is low cost, and has particular properties, such as biodegradability, biocompatibility and homogeneity. However, most of these adsorbents do not have superlative performance in adsorption and/or reuse. Many studies on adsorbent modification methods have recently been carried out, such as heat treatment [8,9], acid treatments, grafting [10], metal oxide loading, and organic compound coating [11]. Among them, UV-induced polymerization is considered to have excellent efficacy because the grafted polymer contains abundant functional groups, resulting in the muscular coordination ability for metal ions [12]. This modification can form a uniform micro-villous layer on the surface without damaging the original structure of the substrate. Specifically, benefitting from properties of the elastic design, convenient control, and tight binding, the UV-induced polymerization not only can realize efficient adsorbent capacity but also keep the excellent properties of the selected substrate [13]. It has been reported that excellent adsorbents have been prepared based on natural minerals and natural polymers (e.g., attapulgite, diatomite, biomass fiber, and silica fume) by UV-induced polymerization [12,13,14,15].
Loofah sponge fiber (LF), which is the vascular bundle of the dried ripe fruit of Luffa cylindrica (L.) Roem is known for its complex interconnecting porous structure, which has conspicuous mechanical properties and practical surface chemistry characteristics at remarkably low densities [16,17,18]. Furthermore, LF is often used as a natural adsorbent due to its numerous advantages, including effectiveness, being innoxious, cost-effectiveness, multi-hole properties, ruggedness, and biodegradability [19,20]. At present, due to its microsponge structure and large number of micro-cell fibers, natural LF can be nominated as a great candidate for the separation of pollutants from aqueous solutions in the field of environmental protection. Researchers have used natural LF as an adsorbent or carrier to remove heavy metals, COD, and nitrogen from wastewater and have achieved good treatment results [21,22,23,24].
Natural LF is mainly composed of cellulose/hemicellulose, hemicellulose, lignin, extractives, and ashes, which do not include functional groups of adsorption, as well as ion exchange. However, the LF is rich in hydroxyl groups, which are easily modified by amino groups or carboxylate groups. Several strategies have been suggested to improve the adsorption capacity of natural LF, such as ultraviolet radiation grafts [15,25], microbionation [26,27], composite materials [28,29,30], activated carbon [31], and modified biochar [32]. However, the literature on the removal of cadmium by these agricultural by-products-based adsorbents is still scarce, and to the best of our knowledge, the adsorbent functionalized loofah sponge structure with carboxyl groups (LF@AA) for the recovery of cadmium has not been reported.
This paper aims to prepare LF@AA supported on LF’s three-dimensional natural polymer networks via UV-induced polymerization and verify its removal ability of Cd2+ from wastewater. The physical and chemical characterization of LF@AA was conducted. Several batches of adsorption experiments were carried out to study the effects of pH, ionic strength, cycle number, kinetics, and isotherms on adsorption. The regeneration of the materials was also studied for potential practical applications. Through this study, the mechanisms for Cd2+ removal were better understood.

2. Materials and Methods

2.1. Materials

The LF was obtained from Hubei Province, southern China. Sodium hydroxide (NaOH), acrylic acid (AA), ethanol, acetone, benzophenone (BP), hydrochloric acid (HCl), ferrous ammonium sulfate ((NH4)2Fe(SO4)2·6H2O), and cadmium nitrate (Cd(NO3)2) were of analytical grade and used as received.

2.2. Synthesis of LF@AA

Typically, the LF washed with ethanol and distilled water was dried at 105 °C for 12 h, and then, particles of 0.05–0.2 cm were screened. The LF&BP was prepared by immersing a certain quality of LF in a certain concentration of BP ethanol solution and drying it at 323.15 K in a vacuum environment. The grafting solution was produced by dissolving a certain amount of AA and ferrous ammonium sulfate into a 25% ethanol solution. The LF&BP soaked (24 h) in the grafting solution was irradiated with ultraviolet light (25 min) and then washed clean with boiling water to obtain the clean product (LF@AA). LF@AA was then soxhlet extracted (24 h) with acetone, soaked (30 min) in 2.0% NaOH solution, washed, and dried (8 h) in a vacuum at 80 °C. The graft ratio (%) was calculated as follows:
Grafting   yield   = ( W 2 W 1 ) × 100 / W 1
where W1 (g) and W2 (g) represent the original weight and the grafted weight of the sample, respectively.

2.3. Characterization Analysis

The microscopic characteristics of the samples were respected by scanning electron microscopy (SEM, SU8010, Hitachi, Ltd., Tokyo, Japan) with an energy-dispersive X-ray spectroscopy (XPS, MULT1LAB2000, Thermo Fisher Scientific Inc., Waltham, MA, USA). The spectral changes of samples at different stages were observed via Fourier transform infrared spectrometry (FTIR, Nicolet iS50, Thermo Fisher Scientific Inc., Waltham, MA, USA), the spectra were implemented based on ATR (Attenuated Total Reflection), and the recording range was 4000 to 400 cm−1.

2.4. Factors Affecting Adsorption Properties

The effects of the Cd2+ concentration, pH value, interfering ions, and temperature on the adsorption performance of LF@AA were evaluated through adsorption experiments. The Cd2+ solution was prepared by dissolving Cd(NO3)2 salt into deionized water.
Typically, the LF@AA (1.0 g∙L−1) was added to a certain concentration of the Cd2+ solution and stirred for 4 h (150 rpm, 25 °C). After filtration, the concentration of Cd2+ in the supernatant was monitored with an inductively coupled plasma emission spectrometer (ICP-8100, Shimadzu Co., Ltd., Kyoto, Japan). The sorption capacity was calculated (Equation (2)).
q e   =   ( C 0 C e ) × V / m
where qe is the saturated adsorption capacity (mg∙g−1), C0 is the initial concentration of Cd2+ (mg∙L−1), Ce is the equilibrium concentration of Cd2+ (mg∙L−1), V is the volume of Cd2+ solutions, and m is the mass of the sorbent (g).
The pH of the solution was adjusted to a range of 2.0–7.0 with 1 M NaOH or HCl. The effect of interfering ions on Cd2+ adsorption was determined by adding the adsorbent to Erlenmeyer flasks filled with Cd2+ solution (400 mg∙L−1) and interfering ions (K+, Na+) of different concentrations from 0 to 100 mmol∙L−1.

2.5. Adsorption: Kinetic Study

Adsorption kinetics have always been considered an important tool to characterize the adsorption efficiency and application prospects of an adsorbent. In this work, the adsorption kinetics were assessed at regular intervals with Cd2+ concentrations of 200, 300, and 400 mg∙L−1; the pseudo-first-order and pseudo-second-order kinetic models are summarized as follows [33,34]:
q t = q e × ( 1 e k 1 t )
q t = k 2 q e 2 t / 1 + k 2 q e t
where qt (mg∙g−1) is the Cd2+ adsorption capacity at time t, t (min) is the contact time, and k1 (min−1) and k2 (g·mg−1·min−1) are the rate constants of pseudo-first-order and pseudo-second-order adsorption, respectively.
Furthermore, the Weber–Morris diffusion model was also used to fit the experimental data, to evaluate the rate-limiting step during the overall adsorption process [35]. The Langmuir, Freundlich, and Redlich–Peterson isotherm models were tested, and the specific methods are referred to related studies [36,37].

2.6. Adsorption: Equilibrium Study

The adsorption isotherms on LF@AA were obtained with different Cd2+ concentrations from 0 to 500 mg∙L−1 at 25, 35, and 45 °C. In this work, the Langmuir, Freundlich, and Redlich–Peterson isotherm models were tested, and the nonlinear forms are as follows:
q e = q m K L C e 1 + K L C e
q e = K F C e 1 / n
q e = K R P C e 1 + a R P C e β
where Ce is the equilibrium concentration of Cd2+ (mg·L−1), qe is the amount of Cd2+ adsorbed at equilibrium (mg·g−1), qm is the maximum adsorption capacity (mg·g−1), and KL (L·mg−1) is the Langmuir binding constant, which is related to the energy of adsorption; KF (mg1−1/n·L1/n·g−1) and 1/n are Freundlich constants representing the adsorption capacity and adsorption intensity, respectively; KRP (L·g−1), aRP ((L·mg−1)β), and β (0 < β < 1) are Redlich−Peterson isotherm constants.

2.7. Regeneration

Eight cycles of regeneration and reutilization were performed to investigate the reusability of LF@AA. Typically, 20 mg of adsorbent was added to 20 mL of Cd2+ solution (400 mg/L), shaken (150 rpm) for 4 h, and separated by filtration. After filtration, the adsorption capacity of LF@AA was obtained by measuring the concentration of Cd2+ in the supernatant. Afterward, the adsorbent was washed alternately with 0.1 M HCl (desorption agent) and 0.1 M NaOH (active agent). After washing, the adsorbent continued to be reused. The regeneration study was conducted by suspending adsorbent-saturated Cd2+ into eluted solution with different concentrations (0.1–1.5 mol∙L−1) and vibrating at 25 °C for 6 h. Then, the adsorbent was separated via filtration, and the Cd2+ concentration was determined using the aforementioned determinate methods.
The desorption rate was calculated as follows:
Desorption   rate   ( % ) = C n V / m q n 1 × 100 %
where m (g) is the adsorbent weight, qn (mg/g) is the adsorption capacity, Cn (mg/L) is the equilibrium concentration of Cd2+ at cycle number n, and V (L) is the solution volume.

3. Results and Discussion

3.1. Adsorbent Characterization

After modification, the fiber of LF@AA was uniformly thickened, and after drying, it could be found that LF@AA still maintained the natural three-dimensional spongy structure of Luffa, which indicated that the ultraviolet photocatalytic grafting technology could be used to uniformly graft polyacrylic acid on the Luffa fiber skeleton. SEM/EDS analysis results of the loofah fiber samples at different stages are shown in Figure 1. Noteworthy differences were observed between the surface topography of the LF (Figure 1a) and that of LF@AA (Figure 1b). As represented in Figure 1a, the surface of LF was moderately smooth and has no discernible porous structures on it, whereas the LF@AA (Figure 1b) surface was rougher, irregular, striped, and porous, the porosity of the adsorbent was increased significantly, and the specific surface area and pore volume were increased correspondingly. The results of energy spectrum analysis showed that the composition of elements on the surface of LF had changed obviously. The proportion of C and O elements on the surface of LF was 45.96% and 54.04%, while the proportions of C and O elements became 32.62% and 34.89% after modification, and a large number of Na elements were discovered. This is because LF is mainly composed of cellulose, and its surface elements are mainly C and O. The grafting successfully introduced -COOH of polyacrylic acid. After NaOH treatment, carboxyl groups on the surface of LF existed in the form of -COO-Na, thus affecting the element composition of the surface of LF, which also indicates that successful grafting had occurred on the surface of LF [38].
It was revealed that the change in surface structure was due to the successful attachment of polymer chains to the surface [12,38,39]. Furthermore, after the adsorption was completed, EDS showed that the Cd element appeared on the surface of LF@AA (Figure 1c), with content as high as 31.63%, while Na content decreased significantly, indicating that ion exchange had occurred at the adsorption site [14]. Meanwhile, fiber bundles of LF@AA and Cd2+-adsorbed LF@AA were looser than natural loofah, suggesting that UV-induced polymerization has good effects on the disaggregation of fiber bundles.
Figure 2 shows the FTIR spectrum of natural loofah samples; for LF, some absorption peaks could be identified at ca. 3337 cm−1 (the stretching vibration of –OH and -NH2), ca. 2892 and 1638 cm−1 (the stretching vibrations of the C–H bond and carbonyl stretching of para-substituted ketones or aryl aldehydes) [40], and ca. 1162, 895, and 1018 cm−1 (belonged to asymmetric bridge C–O–C stretching, ß-glucoside linkage, and C–O, C–C stretching vibrations or C–OH bending in hemicelluloses, respectively) [39]. In the FTIR spectra of LF@AA, there appeared to be three new characteristic absorption peaks at 1696, 1450, and 1162 cm−1, which are related to the stretching vibration of C=O, stretching vibration, and symmetrical stretching of -COO groups [41], reflecting that carboxyl groups were grafted onto the LF. After Cd2+ adsorbed, the C=O stretching peak of LF@AA red-shifted about 94 cm−1 to 1602 cm−1. With a new peak at 1317 cm−1, it could be concluded that the carboxyl salt was established [42,43]. Meanwhile, the peak at 1160 cm−1 was no longer obvious, indicating that the adsorbed Cd2+ has formed a bidentate chelation with -COO- sites.

3.2. Effect of pH

The pH value of the working solution is one of the steering parameters that affect the adsorption capacity of adsorbate onto the adsorbent by influencing the degree of ionization, protonation of the functional groups of the adsorbents, adsorbent surface charge, and the types of metal [19]. Figure 3a demonstrates the effect of pH on the adsorption capacity of LF@AA and natural LF. To prevent the formation of precipitates by the combination of metal ions and hydroxyl ions, the initial pH was below 7.0. Under strongly acidic conditions (pH = 2.0), the adsorption capacity of LF and LF@AA for Cd2+ was at a low level, which was 24.1 mg/g and 143.7 mg/g, respectively. With the increase in pH, the adsorption capacity of LF@AA increased substantially, while that of LF did not increase significantly. In the pH range of 4–7, the adsorption capacity of LF@AA for Cd2+ could be kept at 315.0 mg/g, which is about 9 times that of LF. The adsorption capacity of LF@AA for Cd2+ was significantly increased compared with that of natural LF, indicating that the carboxyl group was effectively grafted with LF. The adsorption capacity of LF@AA was minimum at pH 2.0, which may be because there are countless positive charges (H+) on the surface of the adsorbent under strongly acidic conditions, and H+ has a higher concentration and mobility, which is preferentially adsorbed and competes with Cd2+ cations, hinders its adsorption, and thus provides a huge static repulsive force [19]. In addition, at low pH, the Coulomb repulsion also hinders the adsorption of metal ions.
The results of the Zeta potential measurement (Figure 3b) showed that the surface of LF@AA was negatively charged, the charge intensity built up over the increase in pH, and correspondingly, the adsorption capacity of positively charged Cd2+ was also enhanced. As expected, the adsorption capacity of LF@AA increased sharply with the increase in pH and reached a higher level at pH 4.0–7.0, indicating that the LF@AA could effectively remove Cd2+ from wastewater in a wide range of pH values. When the pH value is higher, the weak acidic carboxyl group is deprotonated, more negative binding sites are produced, the attraction for positively charged metal ions is correspondingly enhanced, and the adsorption capacity is enhanced [44]. On the other hand, the adsorption of the exposed surface functional groups, porous structure, and -COO- groups from grafting may enhance the adsorption capacity of LF@AA.

3.3. Effect of Ionic Strength

In a check to see the practical application properties of LF@AA, and offers further reveal the adsorption principle, the effects of ionic strength were studied. The result (Figure 4) showed that the adsorption efficiency of LF@AA is negatively correlated with the concentration of alkali metal ions. During the adsorption, the main adsorption mechanism should be ion exchange. Cd2+ was drawn to the adsorbent surface by electrostatic binding, and then, ion exchange occurred to form a bidentate chelate. When a large number of impurities in cations coexisted in an aqueous solution, there would be fierce competition between the target adsorbate and impurity cations. Moreover, as the main driving force between Cd2+ and LF@AA, electrostatic binding plays an important role. In the presence of Na+ and K+, the electrostatic shielding effect should be one of the principal factors leading to the obvious decrease in the adsorption of LF@AA [12,45]. Nevertheless, it was relevant that the adsorption capacity is maintained a high level, even when the alkaline metal ion concentration is approximately 25 times the initial concentration of Cd2+. This also suggests that the affinity of Cd2+ in the adsorption site is stronger than that of Na+ and K+ due to the higher valence, and it is preferred to form bidentate chelation with -COO- sites.

3.4. Kinetic Study

The adsorption equilibrium time is one of the crucial factors affecting the mass production of an adsorbent. The effect of different reaction times (1–480 min) and different pollutant concentrations (200–400 mg·L−1) on the adsorption capacity was investigated. The results (Figure 5) indicated that the adsorption capacity of Cd2+ increased rapidly at the beginning of the reaction, and the adsorption capacity was close to saturation at 50 min. After 50 min, the adsorption capacity increased slowly with the increase in contact time, and the adsorption reaction gradually approached equilibrium. The saturated adsorption capacity increased with the increasing initial Cd2+ concentration. When the initial Cd2+ concentration increased from 200 mg·L−1 to 400 mg·L−1, the saturated adsorption capacity correspondingly increased from 180.8 mg·L−1 to 330.9 mg·L−1. This suggests that the porous structure of modified LF@AA and a large number of -COO- groups produced by grafting provide convenient conditions for the adsorption of Cd2+.
The change tendency of the adsorption rate may be explained by the following reasons: firstly, under the action of ion exchange and charge neutralization, Cd2+ quickly migrated and adsorbed to the adsorbent surface. While the adsorption capacity on the adsorbent surface was close to saturation, the metal ions diffused into the pores of LF@AA and were gradually adsorbed on the inner surface. Internal diffusion is more difficult than surface adsorption, and this adsorption process takes a longer time, and a prolonged adsorption process leads to slow growth of the adsorption capacity in the later period [46].
To further explore the adsorption mechanism of LF@AA for Cd2+, the kinetic model and intraparticle diffusion model were carried out to fit the adsorption process. Table 1 and Figure 6a shows the results of the pseudo-first-order and pseudo-second-order kinetic models fitting in three different initial Cd2+ concentrations. The constants of these kinetic models (Table 2) show that compared with qe,calc,1, qe,calc,2 comes closer to the experimental value (qe,exp). Moreover, compared with the pseudo-first-order equation, the pseudo-second-order equation had larger R2 values (0.9974~0.9862), which suggests that the pseudo-second-order equation had a better fitting effect on the adsorption process of Cd2+. To be more specific, chemisorption may be the rate-determining step in Cd2+ adsorption, which involves the exchange of electrons between heavy metal ions and LF@AA. In addition, the adsorption rate is in direct proportion to the square of the number of free sites, which accords with the term (qe − qt)2 in the pseudo-second-order model [46].
The relationship between qt and t0.5 of Cd2+ adsorption under different pollutant concentrations was studied. The results (Figure 6b) showed that despite the different initial concentrations, the qt and t0.5 could be fitted by a high slope straight line and a low slope straight line in turn. In the first stage, the adsorption rate was fast, and the adsorption capacity reached about 75% of the equilibrium adsorption capacity in the first 40 min, which was due to the rapid binding of adsorption sites on the adsorbent surface with Cd2+ in this stage. With the progress of adsorption, a large number of sites on the surface of the adsorbent were occupied. To achieve further adsorption, Cd2+ needs to diffuse into the pores of the adsorbent to complete the adsorption. Therefore, the rate control step in the second stage is mainly the internal diffusion process, and the adsorption rate becomes slower. The relevant parameters of the Weber–Morris model are contained in Table 2. It can be seen that R2 was greater than 0.9 under different initial pollutant concentrations, indicating that the Weber–Morris model is suitable for fitting the adsorption process of LF@AA. Furthermore, inner diffusion is more time-consuming than outer diffusion [47]. Therefore, the results of the kinetic analysis showed that both external and internal diffusion played a part in the process of cadmium adsorption by LF@AA, and the internal diffusion should be dominant.

3.5. Adsorption Isotherms

The effect of temperature on the adsorption properties of LF@AA was analyzed. The results (Figure 7) showed that the saturated adsorption capacity was positively correlated with temperature. When the temperature increased from 303.15 K to 313.15 K and 323.15 K, the saturated adsorption capacity increased from 345.0 mg·g−1 to 347.7 and 352.8 mg·g−1, respectively. The adsorption capacity was increased with the increase in temperature, but the effect was not significant. Generally speaking, the activity of metal ions and adsorbents increased with the increase in temperature, and the interaction between solvent and solid surface decreased with the increase in temperature. Therefore, the higher the temperature, the easier the transfer and diffusion of adsorbates to the adsorbent surface and the more adsorption sites are exposed, which enhances the possibility of adsorbent adsorption.
Redlich–Peterson, Langmuir, and Freundlich models were utilized to fit the isotherm adsorption curve of Cd2+. The fitted-curve and model parameters are shown in Figure 8 and Table 3, respectively. The results showed that the average R2 values of the Redlich–Peterson and Langmuir models were greater than 0.97, indicating a significant correlation. In addition, the error between the calculated values of qm of Langmuir fitted-curve and the experimental results was less than 3%, indicating that the adsorption characteristic of LF@AA is consistent with monolayer adsorption. The Redlich–Peterson isotherm has the has the characteristics of both Langmuir and Freundlich isotherms [37]. The β values of Redlich–Peterson and Langmuir isotherms converged with the increase in temperature, which may be due to the uniform distribution of binding sites on the surface of the adsorption medium.

3.6. Regeneration

Reusability is an important consideration in determining whether a new adsorbent can be used in practice. The reusability of LF@AA was verified based on eight consecutive adsorption cycles. The result (Figure 9) shows that the total adsorption capacity of LF@AA for Cd2+ decreased slightly from 339.3 to 318.8 mg/g after eight consecutive cycles, only decreasing by 6.0%, indicating that LF@AA had a stable adsorption efficiency in the reuse process. Compared with the reported results, the reusability of LF@AA is also quite satisfactory [21,48]. Therefore, LF@AA has a large adsorption capacity and can be reused many times, which has the potential to become a cost-effective heavy metal adsorbent.

4. Conclusions

In this work, a novel three-dimensional natural polymer network-based adsorbent was successfully prepared via UV-induced polymerization, allowing a uniform polymer brush on the matrix to be obtained, which resulted in an outstanding adsorption capacity of 339.3 mg·g−1 and widely applicative pH range (4.0–7.0) for the removal of Cd2+. Moreover, the coexistence of cations (Na+ and K+) in solutions is unfavorable to the adsorption of Cd2+, and the order is Na+ > K+. According to the characterization analysis, the main adsorption mechanism should be ion exchange. The Cd2+ electrostatically adsorbed on the surface of the adsorbent was synthesized into a bidentate chelate under the action of ion exchange. The results of the adsorption kinetics analysis showed that the adsorption process of Cd2+ was divided into external diffusion and internal diffusion and followed the pseudo-second-order kinetic and the Weber–Morris equations. The adsorption isotherm parameters showed that the adsorption process followed the Langmuir and Redlich–Peterson models. In addition, the excellent reusability of LF@AA was also confirmed after eight-time regeneration. The excellent adsorption performance of LF@AA makes it an attractive adsorbent for the treatment of water contaminated with Cd2+ and realizes the high added-value utilization of natural cellulose.

Author Contributions

Conceptualization, H.X.; Data curation, J.W.; Formal analysis, H.X. and M.X.; Funding acquisition, F.M.; Investigation, H.X.; Methodology, H.X.; Project administration, F.M.; Resources, C.L.; Software, M.X.; Supervision, F.M.; Validation, Y.Y.; Visualization, H.X. and C.L.; Writing—original draft, H.X.; Writing—review & editing, C.L. and F.M. All authors will be informed about each step of manuscript processing including submission, revision, revision reminder, etc., via emails from our system or assigned Assistant Editor. All authors have read and agreed to the published version of the manuscript.


This research was funded by Joint Research Project II on Ecological Environment Protection and Restoration of the Yangtze River, grant number (2022-LHYJ-02-0506-09).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. SEM/EDS analysis of LF (a), LF@AA (b), and Cd2+ adsorbed LF@AA (c).
Figure 1. SEM/EDS analysis of LF (a), LF@AA (b), and Cd2+ adsorbed LF@AA (c).
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Figure 2. FTIR of LF, LF@AA, and Cd2+-adsorbed LF@AA (LF@AA-Cd).
Figure 2. FTIR of LF, LF@AA, and Cd2+-adsorbed LF@AA (LF@AA-Cd).
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Figure 3. (a) Adsorption capacity at different pH values. (b) The Zeta potential of the adsorbent varies with pH values.
Figure 3. (a) Adsorption capacity at different pH values. (b) The Zeta potential of the adsorbent varies with pH values.
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Figure 4. Adsorption capacity of Cd2+ by LF@AA at different ionic strengths.
Figure 4. Adsorption capacity of Cd2+ by LF@AA at different ionic strengths.
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Figure 5. Effects of initial concentration and contact time on Cd2+ adsorption.
Figure 5. Effects of initial concentration and contact time on Cd2+ adsorption.
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Figure 6. Fitting of kinetics models (a) and intraparticle diffusion model (b) for Cd2+ adsorption on LF@AA.
Figure 6. Fitting of kinetics models (a) and intraparticle diffusion model (b) for Cd2+ adsorption on LF@AA.
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Figure 7. Adsorption capacity at different temperatures.
Figure 7. Adsorption capacity at different temperatures.
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Figure 8. Non-linear fitting curves of isotherms for 303.15K (a), 313.15K (b) and 323.15K (c).
Figure 8. Non-linear fitting curves of isotherms for 303.15K (a), 313.15K (b) and 323.15K (c).
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Figure 9. Effect of recycling times on adsorption capacity.
Figure 9. Effect of recycling times on adsorption capacity.
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Table 1. Coefficients of kinetic models.
Table 1. Coefficients of kinetic models.
qe,exp (mg·g−1)Pseudo-First-Order ModelPseudo-Second-Order Model
k2 (g·mg−1·min−1)qe,cal,2
Table 2. Parameters of Weber–Morris model.
Table 2. Parameters of Weber–Morris model.
C0 (mg·L−1)First Stage (External Diffusion)Second Stage (Intraparticle Diffusion)
ki,1(mg·g−1·min−0.5)R2ki,2 (mg·g−1·min−0.5)R2
Table 3. Parameters of the adsorption isotherm.
Table 3. Parameters of the adsorption isotherm.
T (K)Langmuir Isotherm ConstantsFreundlich Isotherm ConstantsRedlich−Peterson Isotherm Constants
qm (mg·g−1)KL (L·mg−1)R2KF (mg1−1/n·L1/n·g−1)1/nR2KRP(m·mg−1)αRP (Lβ·mg)βR2
25 °C335.91.02010.9706162.280.17560.8339421.571.47570.95840.9768
35 °C339.61.02780.9699163.800.17760.8309427.221.47810.95810.9760
45 °C345.21.13860.9723168.220.17610.8231447.531.45400.96980.9736
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Xue, H.; Xu, M.; Yu, Y.; Wan, J.; Liu, C.; Meng, F. Ultraviolet Radiation-Assisted Preparation of a Novel Biomass Fiber to Remove Cadmium from Wastewater. Water 2023, 15, 811.

AMA Style

Xue H, Xu M, Yu Y, Wan J, Liu C, Meng F. Ultraviolet Radiation-Assisted Preparation of a Novel Biomass Fiber to Remove Cadmium from Wastewater. Water. 2023; 15(4):811.

Chicago/Turabian Style

Xue, Hao, Min Xu, Yueyang Yu, Jiancheng Wan, Chen Liu, and Fansheng Meng. 2023. "Ultraviolet Radiation-Assisted Preparation of a Novel Biomass Fiber to Remove Cadmium from Wastewater" Water 15, no. 4: 811.

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