INTRODUCTION

Water is very crucial for humans and other living beings. In the human body, more than 70% is fluid, such as blood, and all these fluids are mainly water. Bone marrow needs water to produce blood, and the blood transports oxygen from the lungs to the whole organism [1, 2]. In addition, water can maintain the temperature of the human body and provide natural materials with minerals. Water is not only important to the body; it also plays an important role in our daily activities, such as washing clothes and food, bathing for personal hygiene and solvent work for all reagents. Therefore, the water industry in this situation refers to a problem solver [3-5]. In the underdeveloped areas of the world, where there is no treatment facility, water-related diseases such as cholera, typhoid fever, and dysentery make a large number of people ill every year [6, 7]. Vitally, clean water is one of the most significant natural resources on the planet. Wastewater, which is essentially used water, is also an important resource, especially with recurring droughts and water shortages in many parts of the world. However, effluents contain many pollutants and can only be released after treatment. Wastewater treatment, therefore, has two important tasks: the restoration of water supply and the protection of the planet from toxins [8].

The heavy metal describes any metallic element with a relatively high density, which is harmful or toxic in small quantities. Heavy metals threaten as they appear to bioaccumulate. Bioaccumulation is an improvement for the compound in a biological organism during the period compared to the concentration of the chemical in nature. Substances that accumulate in living organisms when absorbed and collected faster than when degraded [2, 9, 10]. In human, exposure to lead may result in different biological effects, depending on the extent and duration of exposure. Different effects occur in a different dosage, with the growing fetus and the child becoming more sensitive than the adult. High amounts of contact can cause harmful biochemical impacts in mankind, leading to problems with hemoglobin synthesis, influence on the kidneys, joints and reproductive system, as well as a chronic problem on the nervous system [2, 11-13]. A number of reports indicate that a losing 2 IQ points could result in an increase in blood lead amounts of 10 to 20 μg/dL in young children [14-16].

In the United Kingdom, the typical daily exposure of lead in adults is expected to be 1.6 μg in the air, 20μg in drinking water and 28μg in food. Although the majority of people receive most of their lead intake, additional options can be essential in some populations, including water in areas with lead pipes and lead water, near air the source of emissions, land [17, 18].

Mercury is a poisonous material that has no identified performance in biochemistry or human physiology and does not naturally appear in living beings. Inorganic mercury poisoning is related to tremor, and /or minor psychological changes as well as miscarriages and birth defects [19-21]. Monomethylmercury and dimethylmercury are highly toxic and cause neurotoxic disorders [22, 23].

There are several methods of water purification, including adsorption technology, which is a wastewater treatment technique to remove many compounds from industrial wastewater. The adsorption is usually used for the removal or low concentration of non-degradable organic compounds in groundwater, in drinking water treatment, in process water or in tertiary treatment after, for example, biological water purification [24, 25].

Zeolites are among the most popular adsorbents in various industries. Zeolites are porous crystalline aluminosilicates comprising assemblages of SiO₄ and AlO₄ tetrahedra linked together by sharing oxygen atoms. In addition, the prospects for using nanomaterials with a diameter of less than 100 nm are currently extensively studied in many applications in various fields: physics, biology, cosmetics, chemistry, optical components, polymers pharmaceuticals, mechanics, toxicology and oil industry [26-28]. Nickel oxide (NiO), as a result of its high electrocatalytic properties, oxygen ion conductivity, biocompatibility, nontoxicity as well as high chemical stability is a potential candidate for the development of adsorption [29, 30]. Besides, the application of NiO for some adsorption process together with its beneficial is within the literature [29-33].

In this study, for the first time, green biobased template formed by decarboxylation of amino acids was utilized to prepare the crystalline microporous NiO/ZSM-5. The adsorption capacity was determined by isotherm models and thermodynamic parameters for the pollutant removal.

EXPERIMENTS

Material

Lead nitrate (Pb(NO₃)₂) and mercury standard solution were supplied by Merck, Germany. All other chemicals were sourced from Merck and Daejung (Busan, KOREA).

Synthesis of NiO/ZSM-5 material

Synthesis of ZSM-5. At first 1.4 g NaOH was dissolving in 8 mL bio-cadaverine (green template) in 65 mL of distilled water (making seeding gel). Then, 13.1 g silicic acid was added in portion with vigorous stirring. The resulting mixture was shaken for 1 hour at room temperature and aged at 100 ^oC for 10 hours. Then, the fixed amount of NaOH, NaAlO₂ and silicic acid were added to the mixture. The resulting mixture was stirred at room temperature for 1 hour. Then, seeding gel was included to the synthesis gel. The resulting mixture was treated in an autoclave at 180 ^oC for 40 hours. The prepared sample was filtered, dried and calcined.

Synthesis of NiO/ZSM-5. The NiO/ZSM-5 adsorbent was synthesized by incipient wetness impregnation. A solution of Ni(NO₃)₂.6H₂O was slowly added to the ZSM-5 powder. The sample was then placed in the oven at 100^oC for 12 hours. The resulting sample was then ground to powder and placed in the muffle furnace at 550 ^oC for 5 hours.

Adsorption experiment

To prepare a stock solution with various concentrations of Pb solution, we added 12.65 g of lead nitrate (Pb(NO₃)₂) (analytical reagent grade) to the nearest 0.1 mg. This material is then transferred to a 1 L volumetric flask, diluted with distilled water, and mixed thoroughly. The stock solution of mercury (II) with different concentrations was prepared by dissolving 0.25 g to 0.09 g of HgCl₂ in a 100 mL volumetric flask using double distilled water.

To demonstrate the adsorption properties of NiO/ZSM-5 in an aqueous solution of Pb(II) and Hg(II), a series of experiments were carried out to evaluate the effects of pH, contact time as well as initial concentration on pollutants. Batch tests were completed by closed types independently for each pollutant. The filter paper was applied for the filtration of the solution. The residual concentration of pollutants in the solution was regarded by atom fluorescence spectrophotometer (AFS, BRAIC) with an online reducing unit (AF-610A, China) with NaBH4 and HCl solution. The calibration, as well as performances of APS, is according to Ref. [34, 35].

The amount of adsorption capacity, q_e (mg.g^-1), was calculated using Eq. (1):

 (1)

All parameters are determined in our previous work [3, 4, 36]. Moreover, setting pH was performed employing 0.1 M HCl and 0.1 M NaOH.

Characterization techniques

X-Ray diffraction (XRD, Bruker D8) is used for the investigation of crystalline materials using CuKa radiation (40 kV and 50 mA). FT-IR was performed using a Nicolet 370 spectrophotometer. This technique measures the absorption of infrared radiation by the sample material versus wavelength. The TEM image of the prepared sample was tested using the JEOL JEM-2010 200 kV Transmission Electron Microscope. The XRF study was accomplished using X-ray fluorescence (XRF, unisantis, XMF-104) to identify the percent of each component of the composite.

RESULT AND DISCUSSION

Characterization of the adsorbent

Fig. 1 shows the XRD patterns of the samples. The representative peaks happen at 2θ= 22.95, 23.8 and 24.35. The intensity of ZSM-5 signals decreased slightly after the introduction of nickel during synthesis which is coincided with different Ref. [37, 38]. After NiO loadings, no new crystalline phase was detected.

The TEM image of the NiO/ZSM-5 adsorption surface was provided in Fig. 2. As shown, the size of the NiO particles is between 20 and 35 nm, which agrees properly with the estimated results of the Scherrer’s equation.

Table 1 represents the physicochemical properties for the synthesized sample. These results prove that the surface area and the actual content of NiO over the NiO/ZSM-5 sample are 348 m²/g and 2.8 wt.%. From the XRF analysis, the Si/Al ratio of the modified sample is approximately similar to that of the gel solution and the Ni content is almost equivalent.

The FT-IR spectra of ZSM-5 and NiO/ZSM-5 are represented in Fig. 3. The IR spectrum indicates that the ZSM-5 has different vibrational bands, e.g. at 1234, 1075, 792, 546, 450 cm^-1 which corresponded properly to Ref [39, 40]. All vibrational bands in the NiO/ZSM-5 framework were slightly shifted to a lower wavenumber than those of ZSM-5. These changes show that NiO particles can be incorporated into the ZSM-5 material [41].

Effect of variables on the adsorption process

Fig. 4 indicates the effect of pH on the pollutant adsorption on NiO/ZSM-5. As shown, the adsorption capacity was low in the strongly acidic solution for both impurities. Improving the pH to 8 ensures better adsorption capacity. At low pH, the availability of H⁺ would compete with Pb²⁺ and Hg²⁺ for adsorption sites on the surface of the adsorbent.

Fig. 5 presents the effect of contact time on adsorption capacity. As shown, the adsorption equilibrium was completed within 180 min for Hg²⁺ and 220 min for Pb²⁺. These data prove that the equilibrium is attained in a short time.

The amount of adsorbed pollutants for various initial concentrations onto NiO/ZSM-5 was determined and shown in Fig. 6. To show the effect of adsorbent on water uptake, a specified amount of NiO/ZSM-5 sample was added to a 50 mL dilute aqueous solution (pH=8). The initial concentration of Hg (II) solution varies from 0.06 mg/L to 30 mg/L. Moreover, the initial concentration of Pb (II) solution ranged from 20 mg/L to 200 mg/L. The pollutant removal increases with an increase in the initial concentration of Hg²⁺ and Pb²⁺. Main driving force for mass transfer is responsible for the adsorbent capacity increase.

Adsorption isotherms

The equilibrium data were examined by both Langmuir and Freundlich models, following two linear Eqs:

 (2)

 (3)

All parameters are shown in our previous work [42-47]. The fitting results are shown in Figs. 7 and 8 and in Table 2. According to R², it is clear that the Langmuir model is more suitable for both pollutants than the Freundlich model. These findings show that the adsorption is homogeneous and localized in a monolayer. Besides, the larger the constant K_L, the higher is the adsorption energy, reflected by a quick increase in adsorption at low adsorbate concentrations [44]. Furthermore, 1/n values is in the range 0.1 < 1/n < 1, what signifies that adsorption was favorable [48]. The maximum monolayer adsorption capacity (q_m) was detected to be 64.52 mg/g for mercury and 277.78 mg/g for the lead.

Table 3 and 4 show comparison of mercury and lead adsorption capacity of this study and other adsorbents.

Adsorption Thermodynamics

The impact of temperature on the adsorption capacity of NiO/ZSM-5 nanocomposite at pH=8 is presented in Fig. 9. As observed in Fig. 9a, the adsorption capacity raises from 60 to 106 for Hg²⁺ and 210 to 256 for Pb²⁺ with rising temperature. This reveals the endothermic nature of the adsorption. Thermodynamic parameters, ∆H°, ∆G°, and ∆S° can be approximated using equilibrium constants that correlate with temperature:

 (4)

 (5)

 (6)

The ∆G° values at different temperatures were calculated according to equation (5). ΔH° and ΔS° are calculated from the slope and intercept of equation (6) of ln K_d vs T⁻¹ (Fig. 9b) and the calculated values are given in Table 5. The results show that the adsorption process of all examined metal ions was effective and spontaneous in nature as proved by the negative values of ∆G° and positive ∆H°. Additionally, the positive ΔS° showed that the degrees of freedom improved at the solid-liquid interface during the adsorption procedure.

CONCLUSION

To improve the removal of mercury and lead in water, a green and novel NiO/ZSM-5 adsorbent was synthesized. The XRD and FTIR analysis confirmed that no new crystalline phase was detected after the NiO charges, which means that the skeletal structure of the molecular sieve does not change. Due to the experimental results, the removal of pollutant was low in the strongly acidic solution. Increasing pH to 8 leads to an improvement of the adsorption capacity. In addition, pollutant removal increased with the initial concentration of Hg²⁺ and Pb²⁺. The isotherm models showed that the Langmuir model is better suited for both pollutants than the Freundlich. These results show that the adsorption is homogeneous and localized in a monolayer. In addition, the thermodynamic analysis revealed that the process is beneficial, spontaneous and endothermic. The entire outcomes recommend that the existing green and novel nanocomposite verified to be a potential adsorbent for the removal of toxic heavy metals from the aquatic environment.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.