Preparation of hydrogels based on poplar cellulose and their removal efficiency of Cd(II) from aqueous solutions

Rapid industrialization and improvement of people's living standards have caused many environmental pollution problems. In reality, heavy metal ions usually coexist in environmental and industrial wastewater. Heavy metals in industrial wastewater are one of the major pollutants (Tchounwou et al. 2012; Shafiq et al. 2018; Hefny et al. 2020; Khan et al. 2021; Liu et al. 2022). Untreated sewage is discharged into rivers, lakes, and seas, causing serious deterioration of water quality. Heavy metals are difficult to biodegrade. Among them, cadmium (Cd), a toxic heavy metal, can be enriched in the human body through the biological diffusion effect of the food chain (Amin et al. 2022). Cadmium not only interacts vigorously with proteins, enzymes, and other substances in the human body, making them lose their vitality, but also accumulates in some organs of the human body, leading to slow poisoning and death (Anwar et al. 2010; Verma et al. 2018). Briefly, heavy metal water pollution not only harms human health but also reduces crop yield and quality and accelerates the degradation and destruction of the ecological environment. Heavy metal water pollution prevention and control is an urgent matter (Arumugam & Jongsung 2018; Musa et al. 2020).

To cope with this situation, various water treatment technologies have been explored and applied to the treatment of water pollution. The commonly used treatment methods include absorption, ion exchange, extraction, and membrane chromatography. The adsorption method is superior to other technologies in terms of initial cost, flexibility, design difficulty, operation method, and toxicity (Li et al. 2018; Liu et al. 2019; Musa & Zurina 2020; Shirell et al. 2020; Alharby et al. 2021; Tipplook et al. 2021; Lin et al. 2022). Common adsorbents are active white clay, bleaching powder, diatomaceous earth, and other natural mineral products, as well as active carbon, organic silicon, activated alumina, zeolite molecular sieve, adsorption resin, and humic acid. The use of these adsorbents has been greatly limited due to their limited adsorption capacity or difficult biodegradation.

Recently, hydrogels, as a new adsorbent, have attracted much attention (Alharby et al. 2021; Ma et al. 2021; Rahman & Rimu 2020; Wang et al. 2021; Weerasundara et al. 2021). Hydrogel is a kind of highly hydrophilic three-dimensional (3D) network structure gel (Tie et al. 2022). It expands rapidly in water and can hold a large volume of water without dissolving in this swelling state. Several new types of gels with excellent mechanical properties have been developed, such as topological hydrogels, double-network hydrogels, composite hydrogels, macromolecular microsphere composite hydrogels, hydrophobic association gels, and homogeneous chain hydrogels. Among them, composite hydrogels have attracted extensive attention due to their high strength and diversified composite methods. It is worth noting that the various active functional groups on the gel polymer skeleton and the 3D network structure ensure that more sites are involved in the adsorption process. These advantages of gels make them widely used as adsorbents in the field of environmental protection. The removal of heavy metal ions by hydrogels mainly depends on the presence of a large number of functional groups (such as –OH, –COOH, –NH_2, and –SO₃H) in hydrogels for adsorption, ion exchange, and chelation of heavy metal ions. The adsorption of heavy metal ions on hydrogels shows the advantages of high adsorption capacity, fast adsorption, and recycling, so the adsorption of heavy metal ions on hydrogels has become a hot spot in recent years (Wang et al. 2020; Kim et al. 2022; Muhammad et al. 2022; Peng et al. 2022; Qiu et al. 2022).

In order to prepare biodegradable and environmentally friendly adsorbents, it is very important to select biodegradable and environmentally friendly raw materials or environmentally friendly preparation methods (Musa et al. 2019; Sabet & Kamran 2019). Poplar is a kind of renewable resource in nature, which contains abundant cellulose and lignin. Cellulose and lignin contain a large number of hydroxyl, carboxyl, and other active functional groups, which play an important role in the process of adsorption (Yu et al. 2000; Abia et al. 2003; Abdić et al. 2018). Polyaspartic acid (PASP) belongs to a group of polyamino acids (Yang et al. 2019). PASP is easily broken by microorganisms and fungi because of the peptide bonds in its structural backbone, and the final degradation products are ammonia, carbon dioxide, and water, which are harmless to the environment (Jv et al. 2019). Therefore, it is widely used (Ye & Wang 2016; Hao & Li 2019). It can be used in water treatment, medicine, agriculture, daily chemical, and other fields. As a new green water treatment agent, it can chelate calcium, magnesium, copper, and other multivalent metal ions. Vitamin C (VC) is a polyhydroxyl compound. The molecular structure of VC has an enediol structure, a lactone ring, and two chiral carbon atoms. Therefore, it is not only active in nature but also has an optical rotation.

In this study, PASP/CMPP hydrogels and VC/CMPP hydrogels were prepared by using glutaraldehyde as a cross-linking agent, and used for the treatment of wastewater containing Cd(II), in order to understand the removal behavior of Cd(II) by hydrogels. Therefore, the main objective is to compare the removal efficiencies and adsorption capacities between PASP/CMPP and VC/CMPP on the adsorption properties of Cd(II).

NaOH and KMnO₄ were purchased from Tianjin Damao Chemical Pharmacy Factory. PASP, glutaraldehyde, Cd(NO₃)₂, and VC were purchased from Aladdin Reagent (Shanghai) Co., Ltd, China. Anhydrous ethanol was purchased from Tianjin Kermel Chemical Reagent Co., Ltd, China. Potassium bromide, HNO₃, and CH₃COOH were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All chemicals were used without further purification. The poplar was collected from the north side of the comprehensive experimental building on the campus of Heze University. Water was purified with a Hitech-Kflow water purification system (Hitech, China).

First, the collected poplar was washed with water to remove the dust and dried in an electric thermostatic air-blowing drying oven. Second, it was pulverized with a pulverizer, sifted through a 100-mesh sieve, and then set aside. Third, 2 g of poplar sawdust was weighed and immersed in 100 mL of 15% NaOH solution for 12 h. Fourth, the mixture was centrifuged, washed, and then transferred to a round-bottom flask. Finally, 20 mL of ethanol and 2 mL of acetic acid were added and the mixture was stirred using a magnetic stirrer for 30 min at room temperature. The final sample obtained is denoted as CMPP.

0.1 g CMPP was weighed and added to 50 mL of 0.06 mol/L KMnO₄ solution, and the mixture was stirred using a magnetic stirrer at 50 °C for 15 min. Under the conditions of continuous magnetic stirring at 70 °C for 3.5 h, 1.5 g of PASP and 1 g of glutaraldehyde were measured and added to the above solution. The sample was frozen and lyophilized in a freeze dryer for 72 h. The material obtained after freeze-drying is denoted as the PASP/CMPP hydrogel. For comparison, VC/CMPP was prepared in the same way by substituting PASP for VC.

The sample morphology was analyzed using a JSM-6700F scanning electron microscope (SEM, JEOL, Japan) under the conditions of 1 kV acceleration voltage and gold spraying. Fourier transform infrared spectra (FTIR) (Nicolet 5700 Spectrometer, USA) of samples were recorded in the range of 500–3,900 cm⁻¹. The N₂ adsorption–desorption isotherms were determined using a Autosorb IQ-MP system (Quantachrome Instruments, USA), and the test samples were degassed at 120 °C for 5 h under vacuum before measurement. The specific surface area (A_s) and pore volume (V_p) of the samples were calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

The heavy metal ion, Cd(II), was used as the target to test the adsorption capacity. The cadmium removal test was carried out at room temperature. The initial concentrations of Cd(II) from 30 to 300 mg/L were prepared by dissolving Cd(NO₃)₂ in deionized water. A certain amount of adsorbent was weighed and put into a polyethylene tube containing a certain volume of the above solution and oscillated at 150 r/min in a thermostatic oscillator until the specified time. The initial pH values of the solutions were adjusted to 5.5 with 1 mol/L of HCl and 1 mol/L of NaOH solution. Then, an appropriate amount of the solution was filtered through a 0.45-μm filter membrane, and the concentration of cadmium ions present in the filtrate was tested by an atomic absorption spectrophotometer (AAS) (AAS-3600, Shanghai Metash Instruments Co., Ltd, China) equipped with an air-acetylene flame.

The tests shall be conducted three times and the average of overall measurements was taken as the final value, with a relative error less than 5.2%.

The PASP/CMPP hydrogel was successfully synthesized by using alkalized poplar sawdust (CMPP) as the substrate and PASP as the modifier. The effective results, provided by the characterization analysis of SEM and BET, indicate that the surface of the PASP/CMPP hydrogel has a large number of loose pores and a larger pore volume, which are conducive for adsorption. Under the same adsorption conditions, the adsorption capacity of the PASP/CMPP hydrogel was higher than that of the VC/CMPP hydrogel. The sorption kinetics for Cd(II) follow a pseudo-second-order, and sorption isotherms follow the Langmuir and Freundlich models. There is an obvious effect of solid concentrations on the sorption kinetic curves and sorption isotherms. The PASP/CMPP hydrogel has a promising application in the removal of Cd(II) in water. This research provides insights into the synthesis of an efficient and biodegradable functional adsorbent for the simultaneous removal of heavy metal pollutants in wastewater.

This work is supported financially by the Science and Technology Program for Colleges and Universities of Shandong Province (No. J18KA104).

F.Z. conceived and designed the experiment. She performed the experiments and carried out the data analysis. J.T. and D.H. carried out characterization experiments on the samples. L.W., C.Z., and W.H. have polished the grammar of the manuscript. All authors have read and agreed to the published version of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.