Comparison of Two Types of Modified Zeolites and the Key Factors for Cd(II) Adsorption Processes in Micropolluted Irrigation Water

Abstract

Zeolites were modified by high-temperature roasting and chitosan loading, respectively. As a result, activated zeolite and chitosan-loading natural zeolite were obtained. They were used for the treatment of the micro-polluted irrigation water quality simulated by a low concentration (100 μg/L) of Cd(II) contamination. The static adsorption experiments showed that two types of modified zeolites were cost-effective and had high removal efficiency for low concentrations of Cd(II). The removal rates were 95.8% and 92.06%, respectively. The static adsorption experiments investigated the effects of modified zeolite dosage, pH, initial concentration of the solution, and adsorption time on the removal efficiency of cadmium ions. The dynamic adsorption experiments discussed the influence of factors such as dynamic adsorption medium type, influent filtration rate, and adsorbent amount on adsorption efficiency in the adsorption column. The dynamic adsorption experiments adopted intermittent operation to simulate the characteristics of micro-polluted irrigation water. The optimal operating conditions were determined as follows: single activated zeolite filter column or mixed medium (activated zeolite: chitosan loading natural zeolite = 4:1), filtration rate of 6 m/h, filter column height of 500 mm, adsorbent dosage of 30 g. The removal rate was more than 99.5%. The adsorption experiments were represented by Langmuir and Freundlich isotherm models. The adsorption results indicated that the adsorption of modified zeolite showed a better correlation with the Langmuir isotherm model than the Freundlich isotherm model. The adsorption process was described by pseudo-first-order and pseudo-second-order kinetic models, with the pseudo-second-order kinetic model being the predominant mechanism. The average concentration of Cd(II) in the effluent after filtration was 0.41 μg/L, which was far lower than the 0.01 mg/L stipulated in the standard for irrigation water quality (GB5084-2021), and met the requirements of the irrigation water quality standard. Activated zeolite and chitosan-loaded natural zeolite are good adsorbent materials that have broad application prospects in the treatment of micro-polluted irrigation water containing low concentrations of Cd(II).

1. Introduction

Water is an indispensable resource. However, the problem of water scarcity around the world is becoming increasingly serious. In recent years, environmental and climate change, population growth, and the wastage of water resources have further exacerbated the global scarcity of freshwater resources, leading to severe water shortages in many regions. China is one of the birthplaces of agriculture in the world, a predominantly agricultural country with a lengthy history, and one of the countries with the most severe population, resources, and environment in the world [1]. Sewage irrigation as an important alternative water resource has been widely used in farmland irrigation since 1972 [2]. Despite the short-term benefits, the risks associated with sewage irrigation far outweigh their advantages. However, due to objective factors such as water shortages, wastewater, the most stable and large resource, once became the main agricultural irrigation water source in water shortage areas [1]. Nevertheless, soil heavy metal pollution caused by sewage irrigation has become one of the problems that cannot be ignored. According to the 2014 National Soil Pollution Survey Bulletin, 39 out of 55 sewage irrigation areas were polluted, and the exceeding rate of soil points in cultivated land was 19.4%. Among them, farmland contaminated with the heavy metal cadmium reached 12,000 hm [2,3,4]. Cadmium is easily accumulated in animals and human organs through the food chain, such as the brain, kidney, and liver, which is harmful to the human body [5].

In order to improve heavy metal cadmium pollution, a variety of methods have been used to remove cadmium ions from wastewater. These methods include precipitation [6], chemical flocculation [7,8], flotation [9,10], ion exchange [11,12], electrochemical [13,14], membrane filtration [15,16], and so on. With the development of research methods, the adsorption method has been widely used to treat cadmium ions in wastewater. Adsorption is currently the most effective and economical method for treating wastewater containing Cd(II). It is inexpensive and can effectively remove Cd(II) from aqueous solutions using natural materials [17]. Adsorption processes offer flexibility in design and operation and, in many cases, will produce high-quality treated wastewater. Since adsorption is sometimes reversible, adsorbents can be regenerated by a suitable desorption process [18].

The selection of adsorbent plays a key role in the adsorption results. In addition to traditional adsorbents, some new adsorbents are also worthy of attention. Hashim K. S. et al. [19] used bottom ash (BA), a byproduct of the coal combustion process (in the furnace), to adsorb phosphate in sewage with good results. In their study [20], the authors synthesized poly (aniline-co-pyrrole) nanospheres (PACPNS) using the micro-emulsion polymerization technique. They found that PACPNS can be used as an efficient absorbent for removing lead (II) from aqueous media. In addition, poly (m-phenylenediamine)@ ZnO (PmPDA@ ZnO) nanocomposite prepared by in situ chemical oxidation polymerization can also effectively remove lead (II) in an aqueous solution [21]. Biochar Fe-Mn-Ce oxide-modified biochar composite adsorbents have the ability to remove arsenic from water [22]. Zeolite is an effective adsorption material with great application potential for removing heavy metals from wastewater [8]. Compared with Ca-montmorillonite, zeolite has a higher removal rate of the heavy metal ion Cu²⁺, which is proportional to the zeolite dosage [23]. In the study of removing Co(II) ions from an aqueous solution, zeolite has a higher adsorption capacity and better analytical ability than kaolinite.

Zeolite is a general term for a class of aluminum silicate minerals. Zeolites have many internal pores and a large specific surface area. Due to its special microstructure and macroscopic properties, zeolite has strong adsorption of heavy metal ions and cation exchange capacity [24]. However, there is an open framework in natural zeolite. A large number of impurities and water molecules are distributed in the pores of zeolite. The adsorption capacity of natural zeolite will be affected to some extent. Therefore, the adsorption capacity of heavy metals and other pollutants is limited [25]. Different modification methods have been used to enhance the adsorption capacity of zeolites, including high-temperature modification, chitosan loading modification, and acid-base modification. Chitosan is the only alkaline polysaccharide found in nature so far that is non-toxic and has excellent, strong adsorption [26]. However, chitosan is soluble in acidic solutions and has poor mechanical stability, which limits its application range [18]. Chitosan-zeolite has been used as a passivator to passivate Cd in paddy soil [27]. The loading of chitosan on zeolite can improve the mechanical properties of chitosan and make use of the porous and large specific surface area of zeolite to prepare a multifunctional composite material [28]. High temperatures can reduce the surface resistance of zeolite and improve its exchange adsorption capacity [29]. The high-temperature heat treatment method does not need to add other chemical reagents, which avoids the risk of secondary pollution of heavy metals caused by modifying heavy metal components. It is a green, safe, and feasible modification method [30]. In this article, natural zeolite was modified by two kinds of modification methods, and modified zeolite with simple preparation, a short reaction time, and high removal efficiency was prepared.

Compared with previous studies, the zeolite obtained through two modification methods has a simple structure, a short reaction time, and a high removal efficiency. The effect of the adsorbent amount on adsorption efficiency was also studied in the adsorption column test.

2. Materials and Methods

2.1. Experimental Materials

(1) Natural zeolite

The natural zeolite used in the experiment was clinoptilolite, which has light gray, milky white, and flesh-red color. The particle size ranged from 0.5 mm to 1.5 mm. The main components of the zeolite were as follows: SiO₂ (68–71%), Al₂O₃ (13–14%), and Fe₂O₃ (1–1.8%).

(2) Activated zeolite

The particle size was 0.3–0.45 mm.

High-quality natural zeolite ore was used, and it was crushed and granulated. Then, it was immersed in a 5% dilute hydrochloric acid solution for over 2 h. After that, it was subjected to high-temperature roasting in a high-temperature furnace at 350 °C for approximately 1 h.

(3) Chitosan loading natural zeolite

The chitosan solution of 1% concentration was taken in a 100 mL beaker, and the mass ratio of chitosan to natural zeolite was 0.05. The mixture was continuously stirred by magnetic force at room temperature for about 5 h. After full infiltration, the mixture was kept for 24 h, washed with deionized water until neutral, and placed in an electrothermal constant-temperature drying oven. After vacuum drying at 55 °C to a constant weight, the chitosan-loaded natural zeolite composite adsorption material was obtained.

The process of zeolite modification is shown in Figure 1.

2.2. Test and Analysis Methods

2.2.1. Preparation Method of Cd(II) Stock Solution of 100 μg/L

It is formulated to simulate cadmium-containing micropollution solutions in the adsorption experiment. The preparation method is to suck 0.1 mL of cadmium stock solution into a 1000 mL volumetric flask, dilute it with ultrapure water to the standard graduation line, and shake it well for standby.

2.2.2. Standard Method for Determination of Low-Concentration Cadmium in Water

The maximum cadmium concentration in the adsorption test is 100 μg/L, which is a trace amount. Therefore, the determination of cadmium ion concentration in water was analyzed by graphite furnace atomic absorption spectrometry specified in the standard examination methods for drinking water (GB/T5750.6-2006).

2.2.3. Parameters for Instrumental Analysis

Using the scanning electron microscope (EM-30) of South Korea’s COXEM Instrument Co., Ltd., the instrument’s working voltage was set to 10 kV, and the amplification was 3000 times. The X-ray diffractometer (D8Focus) of Bruker Instrument Co., LTD., Bremen, Germany, was used to test the radiation of copper target kα at a voltage of 40 kV and a current of 35 mA. The scanning range of 2θ was 8–80°, and the scanning step was 0.020°/step. A Fourier transform infrared spectrometer (IRAffinity 10) of Shimadzu Instrument Co., Ltd., Kyoto, Japan, was used for determination by the KBr tablet method, and the scanning wavelength range was 500–4000 cm⁻¹.

2.2.4. Data Processing Method

Calculation of Cd(II) Removal Rate

The removal rate of Cd(II) by zeolite under adsorption conditions can be calculated according to Formula (1).

$$\eta =\frac{C_0-C_e}{C_0}\times 100\%$$

(1)

where η (%) is the removal rate of cadmium ions, Ce (μ/L) is the residual cadmium concentration in the water sample, and C₀ (μ/L) is the initial concentration of cadmium.

Calculation of Cd(II) Adsorption Capacity

The adsorption capacity of the adsorbent can be calculated according to Formula (2).

$$q_e=\frac{\left(C_0-C_e\right)V}{m}$$

(2)

where q_e (μg/g) is the equilibrium adsorption capacity of a unit mass adsorbent, and C₀ (μ/L) is the initial concentration of cadmium. Ce (μ/L) is the equilibrium concentration of Cd(II) corresponding to q_e, V (L) is the solution volume, and m (g) is the adsorbent dosage.

Adsorption Isotherm Model

(1) The Langmuir adsorption isotherm model

The Langmuir adsorption isotherm model is an ideal chemical adsorption model that can approximate many practical processes. The Langmuir isotherm equation is based on three assumptions. A primary assumption of the Langmuir isotherm equation is that the adsorption reaction is limited to adsorption between a single layer of molecules.

Based on this assumption, the Langmuir isotherm equation can be derived in linear form.

$$\frac{C_e}{Q_e}=\frac{1}{k{Q}_m}+\frac{C_e}{Q_m}$$

(3)

where $Q_e$ (μg/g) is the adsorption capacity of the monolayer and $C_e$ (μg/L) is the equilibrium concentration at a given time. $Q_m$ (μg/g) is the maximum adsorption capacity of the monolayer, and $k$ is the adsorption equilibrium constant.

(2) The Freundlich adsorption isotherm model

The Freundlich adsorption isotherm model is another commonly used adsorption model that takes into account the non-uniformity of the solid surface. It is an empirical equation derived from extensive experimental data and is one of the most widely used equations for describing adsorption equilibrium, offering greater versatility. The linear form of the Freundlich isotherm equation, as derived, is given by Formula (4).

$$lg{Q}_e=lgk+\frac{1}{n}lg{C}_e$$

(4)

where $Q_e$ (μg/g) is equilibrium adsorption capacity, $C_e$ (μg/L) is equilibrium concentration at a given time, and $k$ is the adsorption constant. $n$ is the characteristic constant related to adsorption intensity or degree of adsorption, usually referred to as the Freundlich exponent.

Adsorption Kinetic Models

(1) Pseudo-first-order Reaction Kinetic Equation

The following formula represents the pseudo-first-order reaction kinetic equation:

$$lg(q_1-q_t)=lg{q}_1-\frac{k_1}{2.303}t$$

(5)

where $q_t$ (μg/g) is the adsorption amount at time t, and $q_1$ (μg/g) is the equilibrium adsorption amount. $t \left(\min \right)$ is reaction time, and $k_1$ (min⁻¹) is the pseudo-first-order kinetic adsorption rate constant.

(2) Pseudo-second-order Reaction Kinetic Equation

The following formula represents the pseudo-second-order reaction kinetic equation:

$$\frac{t}{\begin{array}{c}{q}_t\\ \end{array}}=\frac{1}{k_2{q}_2{}^2}+\frac{t}{q_2}$$

(6)

where $q_t$ (μg/g) is the adsorption amount at time t, and $q_2$ (μg/g) is the equilibrium adsorption amount. $t$ (min) is the reaction time, and $k_2$ ((g/(μg·min)) is the pseudo-second-order kinetic adsorption rate constant.

(3) Intraparticle Diffusion Equation

The following formula represents the intraparticle diffusion equation:

$$q=k_3{t}^{1/2}+C$$

(7)

where $k_3$ (μg/(g·s^1/2) is the intraparticle diffusion adsorption rate constant, and $C$ is constant.

2.3. Experimental Method

2.3.1. Static Adsorption Test

The reagent dosage and method of the static adsorption experiment are referred to in the literature [31]. A total of 100 mL simulated micro-polluted water (pH = 6) with a Cd(II) concentration of 100 μg/L was taken and put into a 250 mL conical flask. The modified zeolites with different masses were weighed, added to the water sample, and put into an air thermostatic oscillator. The conditions were set at 25 °C, 180 r/min, constant temperature oscillation for 180 min in order to fully contact and react the adsorbent and solution, and standing for 1 min. The supernatant at 1 cm from the liquid level was taken to measure the residual Cd(II) content.

2.3.2. Dynamic Adsorption Test

The test process is shown in Figure 2. Plastic filter column: diameter Φ = 30 mm, high H = 190 mm. The adsorption medium filled in the adsorption column was a mixture of single natural zeolite, single activated zeolite, activated zeolite, and chitosan-supported natural zeolite. The top of the adsorption column was paved with a 2 cm thick quartz sand (particle size 0.5–1 mm) layer, and the bottom of the adsorption column was paved with a 2 cm thick quartz sand (particle size 0.5 mm~1 mm) layer to play the role of a supporting cushion. The experimental reaction device for dynamic adsorption is shown in Figure 2.

3. Results and Analysis

3.1. Analysis of Performance Characteristics of Modified Zeolite

3.1.1. Scanning Electron Microscopy Analysis

Scanning electron microscopy of natural zeolite (NZ), activated zeolite (AZ), and chitosan-loaded natural zeolite (CNZ) are shown in Figure 3a–c. The surface of natural zeolite was smooth, irregular, and flake-like, with certain holes, but the internal structure was relatively flat and dense. After acidification and high-temperature heating, the surface of activated zeolite was uneven, and the internal structure was dispersed and loose, with obvious holes. High-temperature heating caused the water in natural zeolite to evaporate and discharge from the zeolite pores. This process increased the internal channels and caused a loosening of the internal structure. Chitosan was loaded on the surface of natural zeolite and some thick flake pores with finer powdered small particles, namely chitosan particles, indicating that chitosan had been successfully loaded on the surface and some pores of natural zeolite.

3.1.2. XRD Analysis

The X-ray diffraction spectrum characterization is shown in Figure 3d. It can be seen from Figure 4 that the three zeolites had relatively strong diffraction absorption peaks at the same X-ray diffraction angle. It indicated that the crystal structure of natural zeolite (NZ) was not changed by activation zeolite (AZ) modified by high-temperature heating after acidification. The natural zeolite loaded with chitosan did not undergo chemical changes; only physical mixing occurred. In the X-ray diffraction spectrum of the chitosan-loaded natural zeolite (CNZ), a unique diffraction absorption peak appeared at the X-ray diffraction angle 2θ of 26.87°, which was consistent with the X-ray diffraction peak of chitosan in the literature [32]. This indicates that chitosan was loaded on natural zeolite.

3.1.3. FTIR Analysis

The infrared spectrum is shown in Figure 3e. As can be seen from Figure 3e, compared with natural zeolite (NZ), the infrared absorption peak positions of activated zeolite (AZ) and chitosan-loaded natural zeolite (CNZ) did not change significantly. The peak area of activated zeolite did not change significantly, and the peak area of chitosan-loaded natural zeolite increased. This indicates that the basic structure of activated zeolite was consistent with that of natural zeolite. According to the infrared spectrum data of chitosan in the literature [33], the vibration band containing -NH₂ and -OH appeared at 1586 cm⁻¹ and 3550 cm⁻¹ of natural zeolite loaded with chitosan, which was a typical active group in the molecular structure of chitosan. This phenomenon proves that chitosan has been loaded into the skeleton of natural zeolite.

3.1.4. Effect Analysis of Cadmium Ion Adsorption by Modified Zeolite

The specific surface area and pore size distribution were analyzed using a surface area and pore size analyzer. The samples were degassed at 120 °C for 60 min and then analyzed at a temperature of 77.35 K under vacuum conditions up to 5.0 cm³/g. Based on the experimental results, Figure 3d was obtained. Figure 3d shows the nitrogen adsorption/desorption isotherms of AZ, NZ, and CNZ. According to the latest classification by the International Union of Pure and Applied Chemistry (IUPAC), the adsorption/desorption isotherms in the graph are of type IV, which is characteristic of mesoporous materials. The presence of an $H_3$ hysteresis loop in the middle and late stages of the isotherms indicates highly irregular pore structures, which is considered a characteristic of slanted zeolite materials [34].

The BET data for the three materials calculated from the nitrogen adsorption-desorption isotherms are shown in

Table 1. Specific Surface Area of NZ, AZ, and CNZ.

Adsorption Material	BET Surface Area (m2/g)
Natural zeolite	35.2220
Activated zeolite	36.3772
Chitosan loading natural zeolite	36.6398

. The specific surface area of the activated zeolite and the chitosan-loaded natural zeolite is increased compared to the natural zeolite. The increase in the specific surface area of the chitosan-loaded natural zeolite may be due to the intercalation of chitosan in the zeolite, which increases the BET surface area [35]. Slanted zeolite exhibits an increased specific surface area and improved surface morphology after acid treatment and high-temperature heating, enhancing its adsorption performance.

3.2. Analysis of Static Adsorption Effect of Modified Zeolite on Cadmium Ions

3.2.1. Effect Analysis of Cadmium Ion Adsorption by Modified Zeolite

The experimental design temperature was 25 °C, the dosage of natural zeolite and activated zeolite was 6 g/L, and the dosage of chitosan-supported natural zeolite was 8 g/L. The adsorption reached saturation [31]. The removal effect of Cd(II) adsorbed by natural zeolite, activated zeolite, and chitosan-loaded natural zeolite is shown in Figure 4. Compared with natural zeolite, it can be seen from Figure 4 that the adsorption effects of Cd(II) on the two modified zeolites were greatly improved, reaching more than 90%. The maximum removal rate of activated zeolite was 95.8%, and that of chitosan-loaded natural zeolite was 92.06%. The adsorption equilibrium time of natural zeolite and activated zeolite was 180 min, while the adsorption equilibrium time of chitosan-loaded natural zeolite was 60 min. Chitosan-supported natural zeolite improved the adsorption reaction rate. The reason is that chitosan is a natural organic polymer product. The amino groups and hydroxyl groups adjacent to amino groups in the molecule have strong coordination abilities with metal ions and can form stable chelates with heavy metals.

3.2.2. Performance Analysis and Comparison of Cadmium Ion Adsorption by Different Modified Zeolites

In recent years, different types of zeolites and modified zeolites with different modification methods have been used to adsorb cadmium ions (Cd(II)).

Table 2. The comparison of the adsorption performance of different modified zeolites.

Zeolite Types	Initial Concentration of Treated Wastewater (Static Experiment)	Removal Rate	Adsorption Capacity	Reference
Calcite (synthetic)	20 mg/L	>99%		[36]
NaOH modified rhodamene	100 mg/L	97%	112.36 mg/g	[37]
NaCl modified clinoptilolite	50 mg/L	>98%		[38]
NaOH modified clinoptilolite	40 mg/L	>99%	6.456 mg/g	[39]
Chitosan loaded natural clinoptilolite	300 mg/L	>99%	28.1 mg/g	[40]
H-type modified zeolite, Na-type modified zeolite	50 mg/L	>99%		[41]
Magnetic Fe3O4 nanoparticles coated with natural clinoptilolite zeolite	20 mg/L	~68.9%	107.8 mg/g	[42]
Sonochemically and NaCl-modified clinoptilolite	79.41 mg/L	>99%	21.47 mg/g	[43]
Activated zeolite	100 µg/L	>90%	158.5 µg/g	This study
Chitosan loaded natural zeolite	100 µg/L	>90%	118.1 µg/g	This study

shows the comparison of their adsorption performance when the adsorbent reaches adsorption saturation.

It can be seen from the comparison in Table 2 that under approximately the same experimental conditions (room temperature, pH = 6), the activated zeolite prepared in this paper and the composite chitosan-supported natural zeolite can adsorb lower concentrations of Cd(II)-containing solutions with higher removal rates. This shows that activated zeolite and chitosan-loaded natural zeolite are good adsorbent materials and have broad application prospects in the treatment of micro-polluted irrigation water containing low concentrations of Cd(II).

3.2.3. Comparison of Material Prices of Zeolite before and after Modification

Natural zeolite was modified by heating at a high temperature to obtain activated zeolite. Natural zeolite was loaded with chitosan to obtain chitosan-loaded natural zeolite, which formed a new adsorption material. The modified zeolite can improve the adsorption efficiency, but the calculation of the economic cost of operation should be considered in practical production and application. The prices of natural and modified zeolite were compared, as shown in

Table 3. The price of zeolite was modified before and after.

Adsorption Material	Price (yuan/kg)	Note
Natural zeolite	3.00	Purchase price
Activated zeolite	8.00	Purchase price
Chitosan loading natural zeolite	16.85	Calculate the price

Note: The chitosan used in the experiment was purchased at 280.00 yuan/kg.

.

It can be seen from Table 3 that the composite adsorption material formed by chitosan-supported natural zeolite has a high price, followed by activated zeolite. The activated zeolite is relatively economical and practical. Additionally, the chitosan-loaded natural zeolite composite adsorption material has the complexation and coordination adsorption of chitosan. After loading zeolite, both are adsorbed together, which improves the adsorption efficiency and reduces the amount of chitosan, reducing the investment cost and having a certain application value.

3.3. Analysis of Factors Affecting Adsorption

3.3.1. Influence of Modified Zeolite Dosage on Cadmium Ion Adsorption

The dosage of activated zeolite adsorbent was 0.05, 0.1, 0.2, 0.4, 0.6, and 0.8 g, respectively. The dosage of chitosan-loaded zeolite was 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 g, respectively.

The effects of the two types of modified zeolite on the adsorption of cadmium ions in water at different dosages are shown in Figure 5a. From Figure 5a, it can be observed that the removal efficiency of cadmium ions decreases gradually with an increase in the dosage of the two modified zeolites. However, the adsorption capacity increases gradually and tends to stabilize with increasing dosage. When the dosage is 8 g/L, chitosan-loaded natural zeolite (CNZ) achieves a removal efficiency of 91.35% for Cd(II) and an adsorption capacity of 114.2 µg/g. When the dosage is 9 g/L, CNZ achieves a removal efficiency of 91.52% for Cd(II) with an adsorption capacity of 101.7 µg/g. This shows a similar removal effect. Initially, increasing the dosage of CNZ leads to an increase in its adsorption capacity and higher removal efficiency of Cd(II). However, when the dosage of CNZ exceeds 8 g/L, the removal efficiency of Cd(II) no longer significantly increases, indicating adsorption saturation. Considering factors such as improving adsorption efficiency and reducing the usage of adsorbents, the optimal dosage of chitosan-loaded natural zeolite (CNZ) is chosen as 8 g/L for subsequent experiments. Similarly, the optimal dosage of activated zeolite is determined at 6 g/L. At this dosage, the removal efficiency of activated zeolite is 90.48%, with an adsorption capacity of 150.8 µg/g.

3.3.2. Influence of Solution pH on Cadmium Ion Adsorption

The experimental conditions include an activated zeolite dosage of 6 g/L, a chitosan-loaded natural zeolite dosage of 8 g/L, and an initial Cd(II) concentration of 100 μg/L. The effect of solution pH on the adsorption of cadmium ions is shown in Figure 5b.

From Figure 5b, it can be observed that with a pH value less than 4, the removal efficiency of cadmium ions by both activated zeolite and chitosan-loaded zeolite increases as the pH value increases. This is because when the pH is low, the competition between hydrogen ions and cadmium ions for the adsorption active sites reduces the removal efficiency of cadmium ions. As the pH value increases and the concentration of hydrogen ions decreases, the competition weakens, resulting in a corresponding increase in the removal efficiency. When the pH value is six, both types of modified zeolite achieve the highest removal efficiency for Cd(II). The removal efficiency of activated zeolite is 95.8%, and the adsorption capacity reaches its maximum at 159.67 µg/g. The removal efficiency of chitosan-loaded natural zeolite (CNZ) for Cd(II) is 92.06%, with an adsorption capacity of 115.1 µg/g. When the pH value exceeds 9, the removal efficiency and adsorption capacity decrease.

3.3.3. The Influence of Initial Cadmium Ion Concentration on Adsorption Efficiency

For the tests, the activated zeolite (AZ) dosage was set at 6 g/L, and the chitosan-loaded zeolite (CNZ) dosage at 8 g/L, with a pH of 6. The influence of different concentrations of cadmium ions (20 μg/L, 40 μg/L, 60 μg/L, 80 μg/L, and 100 μg/L) on the adsorption efficiency was evaluated.

The effect of initial cadmium ion concentration on the removal of Cd(II) is shown in Figure 5c. From the graph, it can be observed that as the initial concentration of cadmium ions in the solution increases, both types of modified zeolites exhibit a gradual decrease in the adsorption removal rate and adsorption capacity for Cd(II). The main reason for this is that the dosage of modified zeolite is fixed and its adsorption capacity is limited. When the solution concentration increases, the adsorption amount also increases, leading to a decrease in the adsorption rate of Cd(II) by the modified zeolites.

3.3.4. Influence of Adsorption Time on Cadmium Ion Adsorption Efficiency

Adsorption time is an important factor in the removal process of pollutants from wastewater as it affects the ion adsorption kinetics and the economic efficiency of the adsorption process [44]. The adsorption times selected were 10 min, 20 min, 30 min, 45 min, 60 min, 90 min, 120 min, 150 min, 180 min, and 210 min. The influence of adsorption time on the adsorption efficiency of cadmium ions is shown in Figure 5d. The adsorption reaction reaches equilibrium after a certain time, where the ion concentration in the aqueous solution remains constant. Different adsorbents have different equilibrium times for adsorption. Initially, the removal efficiency of Cd(II) increases rapidly, but with an increase in adsorption time, the removal efficiency gradually approaches around 95%. Comparing the modified zeolite and chitosan-loaded zeolite, the removal efficiency and adsorption capacity of the modified zeolite increase gradually with adsorption time and reach equilibrium at around 180 min. On the other hand, the equilibrium time for chitosan-loaded zeolite is around 60 min, and further increasing the adsorption time does not increase the adsorption capacity or removal efficiency of cadmium ions.

3.4. Mechanism Analysis of Modified Zeolite for Cadmium Ion Removal

3.4.1. Adsorption Isotherm Models

The adsorption isotherms of activated zeolite (AZ) and chitosan-loaded zeolite (CNZ) were plotted at a temperature of 25 °C and an initial pH of 6. The data were fitted using the Langmuir and Freundlich adsorption models, and the fitting results are shown in Figure 6a,b.

From Figure 6a,b, it can be observed that the correlation coefficient of the Langmuir adsorption isotherm model is higher than that of the Freundlich adsorption model. This indicates that the adsorption processes of activated zeolite (AZ) and chitosan-loaded zeolite (CNZ) for low-concentration Cd(II) in water are better described by the Langmuir adsorption isotherm model.

Table 4. The adsorption isotherm constant of the Langmuir and Freundlich of AZ and CNZ.

Zeolite	Langmuir Model			Freundlich Model
	$\mathit{K}$ (g/μg)	${\mathit{Q}}_{\mathit{m}}$ (μg/g)	${\mathit{R}}^2$	$\mathit{l}\mathit{g}\mathit{k}$	$1/\mathit{n}$	${\mathit{R}}^2$
AZ	−3.72	149.25	0.9965	0.6587	0.5339	0.8734
CNZ	−4.6	112.4	0.9997	1.1054	−0.0460	0.8253

presents the calculated parameters of the adsorption isotherm models for the modified zeolites. It can be seen that the maximum theoretical adsorption capacity of activated zeolite for Cd(II) is 149.25 μg/g, while that of chitosan-loaded zeolite is 112.4 μg/g.

3.4.2. Adsorption Kinetic Models

Figure 6c–e represents the pseudo-first-order kinetic, pseudo-second-order kinetic, and intraparticle diffusion equations for the two modified zeolites, respectively. By calculating the slopes and intercepts of these linear equations, the adsorption rate constants, equilibrium adsorption capacities, and correlation coefficients $\left(R^2\right)$ under different kinetic models were obtained. The parameter values are shown in

Table 5. A comparison of kinetic model rate constants.

Zeolite	${\mathit{q}}_{\mathit{e}}\mathit{e}$	Quasi-First-Order Kinetic Equation			Pseudo-Second-Order Kinetic Equation			Internal Diffusion Dynamics Equation
		${\mathit{K}}_1$	${\mathit{q}}_1$	${\mathit{R}}^2$	${\mathit{K}}_2$	${\mathit{q}}_2$	${\mathit{R}}^2$	${\mathit{K}}_3$	$\mathit{C}$	${\mathit{R}}^2$
AZ	158.50	0.0101	86.46	0.9339	0.0003	163.93	0.9826	7.7312	50.829	0.9432
CNZ	118.1	0.009	5.29	0.2958	0.014	117.65	0.9990	0.8714	107.17	0.4274

.

From the linear fits and correlation coefficients obtained in Figure 6c,d, it can be observed that the linear correlation coefficient of the pseudo-second-order kinetic equation for activated zeolite is 0.9826, while that for chitosan-loaded zeolite is 0.9998, indicating a good fit. Therefore, the pseudo-second-order adsorption kinetic mechanism plays a major role in the adsorption of Cd(II) by activated zeolite (AZ) and chitosan-loaded zeolite (CNZ).

The linear fitting of the intraparticle diffusion kinetic model shown in Figure 6e shows that neither of the linear fits for the two modified zeolites passes through the origin. Therefore, it can be inferred that intraparticle diffusion of particles is not the sole rate-controlling step in the adsorption process of Cd(II) solution by the two modified zeolites. Overall, the adsorption process of Cd(II) solution by the two modified zeolites is likely controlled by the combined effects of surface adsorption at active sites and intraparticle diffusion adsorption.

3.5. Analysis of Dynamic Adsorption Effect of Modified Zeolite on Cadmium Ions

3.5.1. Effect of Adsorption Medium on Cadmium Removal

Chitosan-loaded natural zeolite and activated zeolite have good adsorption properties. Chitosan-loaded natural zeolite can improve the adsorption reaction rate, but the price is higher. Activated zeolite has a good adsorption effect and a relatively low price. Therefore, four adsorption columns were installed, each utilizing different adsorption media. These media included single chitosan-loaded natural zeolite, single activated zeolite, mixed activated zeolite (AZ), and chitosan-loaded natural zeolite (CNZ). The arrangement involved placing activated zeolite above and chitosan-loaded natural zeolite below in the columns. Columns and operating parameters are shown in

Table 6. Four kinds of adsorption column filter cylinders and their operation parameters.

Adsorption Medium	Mass (g)	Cost (yuan)	Filter Material Height (mm)	Filter Speed (m/h)	Peristaltic Pump Speed (rpm)	Traffic (mL/min)
A single CNZ	50	0.84	800	4	80	46.67
A single AZ	50	0.40	800	4	80	46.67
Mixed media 1	50 (AZ:CNZ 1:1)	0.62	800	4	80	46.67
Mixed medium 2	50 (AZ:CNZ 4:1)	0.49	800	4	80	46.67

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The dynamic adsorption system was operated intermittently. The contaminated solution containing 100 μg/L (Cd(II)) was injected, respectively. Four adsorption columns were taken every 30 min to filter the effluent, and the Cd(II) concentration was detected—each continuous operation of 12 h after stopping 12 h from continuing to run. Four adsorption columns were operated for 26 h. The dynamic adsorption effect of Cd(II) is shown in Figure 7a. It showed that the four adsorption columns did not reach adsorption saturation, and the concentration of Cd(II) in the water after adsorption was far lower than 0.01 mg/L specified in the Standard for irrigation water quality (GB5084-2021). The dynamic adsorption effects of the four adsorption columns on Cd(II) from high to low were mixed medium 2, mixed medium 1, single chitosan-loaded natural zeolite, and single activated zeolite. Their average removal rates of Cd(II) in water were 95.10%, 93.78%, 93.22%, and 90.98%, respectively. Mixed medium two had a higher removal rate of Cd(II) in water. In addition, from Table 3, the economic costs of the adsorbents for the four adsorption columns from low to high are mixed medium 2, single activated zeolite, mixed medium 1, and single chitosan-loaded natural zeolite. Therefore, considering the economic cost and removal effect, mixed medium 2 was selected as the adsorption filter column for subsequent dynamic operation.

3.5.2. Effect of Filtration Rate on Cadmium Removal

A fast or slow filtration rate would affect the adsorption effect. The mixed medium 2 (AZ: CNZ 4:1) was selected as the filter material of the adsorption column, and three identical adsorption columns were installed. The filtration rates were adjusted to 4 m/h, 6 m/h, and 8 m/h, respectively. The intermittent operation was adopted. After each continuous operation of 8 h, the operation was stopped for 12 h and then continued. The effluent was filtered by three adsorption columns every 1 h, and the concentration of Cd (II) was measured. The dynamic adsorption effects of Cd (II) on three adsorption columns are shown in Figure 7b. It showed that the three adsorption columns did not reach saturation. The Cd (II) concentration in the water after adsorption was far lower than the 0.01 mg/L specified in the Standard for irrigation water quality (GB5084-2021). The dynamic adsorption effects of Cd(II) on three adsorption columns with different filtration rates were 6 m/h, 4 m/h, and 8 m/h, respectively. The average removal rates of Cd(II) in water were 97.48%, 96.44%, and 96.07%, respectively. The filtration rate was too fast, and the contact time between the adsorption medium and the cadmium solution was shortened, which was not conducive to effective adsorption. However, the filtration rate was too slow, and the filtration efficiency was not high.

3.5.3. Effect of Adsorption Medium Dose on Cadmium Removal

The dosage of the adsorption medium determines the height of the filter column. The removal effect of Cd (II) solutions with low concentrations is also different when the height of the filter column is different. The mixed medium 2 (AZ:CNZ 4:1) was selected as the adsorption column filter material, and three different doses of adsorption columns were installed. Columns and operating parameters are shown in

Table 7. The three different adsorption column filter cylinders and their operation parameters.

Filter Column Number	Dose (g)	Filter Material Height (mm)	Filter Speed (m/h)	Peristaltic Pump Speed (rpm)	Traffic (mL/min)
Filter column1	50	800	6	105	70
Filter column2	30	500	6	105	70
Filter column3	15	200	6	105	70

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The dynamic adsorption system was operated intermittently and stopped for 12 h after 8 h of continuous operation. Three adsorption filters were operated for 12 h. The effluent was filtered by three adsorption filters every 1 h, and the concentration of Cd (II) was measured. The dynamic adsorption effect of the adsorption column on Cd (II) is shown in Figure 7c. It can be seen from Figure 6c that the three adsorption columns did not reach adsorption saturation. The dynamic adsorption effects of the three adsorption columns on Cd (II) from high to low were filter column 1, filter column 2, and filter column 3. The average removal rates were 97.48%, 95.14%, and 79.57%, respectively. The large amount of adsorbent is conducive to removing Cd (II) from the solution. However, with the increase in the dosage of the adsorption medium, the flow resistance of the adsorption column is also increased, and the investment cost is also increased. Therefore, under the condition that the effluent quality meets the standard and the removal rate is relatively high, the large dosage of the adsorption medium cannot be excessively pursued in the practical application, and 30 g of adsorbent can be selected.

4. Conclusions and Outlook

(1) Both modified zeolites have good adsorption properties under similar experimental conditions. The adsorption rate of chitosan-loaded natural zeolite was higher than that of activated zeolite, which was relatively economical. Activated zeolite and chitosan-loaded natural zeolite are good adsorbent materials with broad application prospects in treating micro-polluted irrigation water containing low concentrations of Cd(II).

(2) Dynamic adsorption experiments show that the high flow rate is not conducive to adsorption, and the increase in adsorbent dosage is beneficial to removing Cd(II) from the solution. The mixed modified zeolite filter column has greater adsorption capacity and a better adsorption effect than the single modified zeolite filter column. In dynamic column adsorption, the diameter of the adsorption column is 30 mm, and the height is 190 mm. The adsorption medium utilized has a particle size ranging from 0.3 to 0.45 mm. The operation is conducted intermittently. The optimum operating conditions are as follows: single activated zeolite filter column or mixed medium (AZ:CNZ = 4:1), the filtration rate is 6 m/h, the height of the filter column is 500 mm, and the adsorbent amount is 30 g.

(3) By simulating the intermittent operation mode of farmland irrigation, the two modified zeolites had an obvious indigenous removal effect on the solution containing 100 μg/L Cd(II), and the removal rate reached more than 99.5%. The average concentration of Cd(II) in the effluent after filtration was 0.41 μg/L, much lower than the 0.01 mg/L stipulated in the Standard for irrigation water quality (GB5084-2021).

(4) Zeolites modified to remove heavy metals are environmentally friendly and promising materials. It is important to analyze the performance of modified zeolites in real water from different sources to study their behavior under different environmental conditions in the future.