Elsevier

Industrial Crops and Products

Volume 184, 15 September 2022, 114967
Industrial Crops and Products

pH graded lignin obtained from the by-product of extraction xylan as an adsorbent

https://doi.org/10.1016/j.indcrop.2022.114967Get rights and content

Highlights

  • Lignin, the by-product of xylan extraction from corn stalks, can be used as a useful adsorbent.

  • The pH of lignin precipitation has an impact on its adsorption capacities.

  • Lignin obtained at pH 5 has a better adsorption performance than that obtained at pH 3.

Abstract

As an important agricultural residue, corn stalks have been continuously used in various industries. In this work, lignin, the by-product of xylan extraction from corn stalks, was used as a useful adsorbent for the removal of methylene blue from aqueous solutions. Lignins were precipitated from the hydrolysates of pH 3 (LG3) and 5 (LG5), hoping to explore the difference in adsorption performance between the two. Detailed discussions were performed to provide a firm evaluation of adsorption models followed by batch experiments to analyze the methylene blue adsorption performance. Results showed that the lignin obtained at pH 5 had a better adsorption capacity compared with pH 3, which was mainly caused by the difference in specific surface area. When the optimal adsorption conditions (pH: 10; temperature: 35 °C; adsorption time: 90 min) were achieved, the maximum adsorption capacity of LG5 can reach 134.3 mg/g, which was much higher than those of most unmodified lignins extracted from other biomass energy sources. Regarding the adsorption behavior, the adsorption data were fitted by pseudo-second-order kinetics model and the isotherm adsorption equilibrium followed the Langmuir model. Overall, these results indicate that lignin, the by-product of xylan extraction, offers a great potential for removing methylene blue from wastewater.

Introduction

Every year, a large amount of wastewater containing dyes, especially methylene blue (MB), will be discharged (Katheresan et al., 2018, Singh et al., 2018). Compared to other dyes, MB is a well-known, extremely carcinogenic thiazide pollutant (Li et al., 2020a, Li et al., 2020b) that has been widely used in many industries for different purposes (for instance: dyeing of cotton, wool and fabrics; coloring of paper, as a hair coloring agent, etc.). This chemical dye (MB) is bring about potential threats to environment and human health. Hence, the removal of MB from water systems attracts wide attention (Yagub et al., 2014, Zhou et al., 2019). Based on the advantages of low cost and high efficiency, the adsorption offers a promising way to remove methylene blue in water bodies (Klapiszewski et al., 2015). Commonly used adsorbents for the removal of MB are mainly concentrated in green polymers and nanomaterials of natural origin (Ciesielczyk et al., 2017).

Lignin, as an extract derived from biomass resources (Gillet et al., 2017), is playing an indispensable role in the field of adsorption due to its safety and availability. Furthermore, active functional groups such as alcoholic hydroxyl and phenolic hydroxyl on the lignin surface make it an ideal adsorbent to remove heavy metal ions (Klapiszewskia et al., 2017) and purify wastewater (Ge and Li, 2018). A good deal of researches have shown that unmodified lignin extracted from agricultural waste can be directly used as an adsorbent. As mentioned in the literature, the maximum adsorption capacity for MB of Klason lignin extracted from Eucalyptus grandis sawdust was 30 mg/g (Zhang et al., 2016) and the value of organosolv lignin from rice straw can reach 40.02 mg/g (Cemin et al., 2021). Inspiringly, the adsorption performance of lignin can be further improved by physical or chemical modification which includes the introduction of other functional groups (Ogunsile and Bamgboye, 2017, Wang et al., 2018, Wang et al., 2020), production of activated carbon (Gao et al., 2013, Li et al., 2020a, Li et al., 2020b), magnetization (Meng et al., 2019, Wang et al., 2019) and lignin-based adsorbent materials made by compounding with polysaccharides (Albadarin et al., 2017, Bartczak et al., 2017). Recent studies have shown that chitosan modified lignin-biochar composite from corncob has an excellent adsorption capacity of 499.8 mg/g (Liu et al., 2022), and that of activated carbon from mangosteen peel wastes can even reach 871.49 mg/g (Zhang et al., 2021). Although modification can enhance the adsorption properties of lignin, the complex operations and expensive costs limit its application on a large scale. A high-performance lignin that can be used in a native form for removing dyes from wastewater are long-awaited.

As the main crop in China, corn produces a lot of agricultural residues while producing high yields (Jia et al., 2018). Nowadays, we have gradually realized the loss caused by directly discarding corn stalks, and there are also some regulations prohibiting the burning of them (Wu and Hu, 2021). Due to these changes, the comprehensive utilization of corn stalks has attracted more and more attention (Jiang et al., 2019). As a biomass material, corn stalks have been developed for applications in many fields. In addition to being used as feed (Nazli et al., 2018) and filler (Chen et al., 2021), they can even be applied for power generation (Chen et al., 2019, Bolat et al., 2021) and other composite materials (Sawalha et al., 2018). Which completes the transform of turning waste into treasure, enabling the corn stalks to maximize energy efficiency.

Compared with other non-wood raw materials, the percentage of hemicellulose in corn stalks is particularly prominent, about 26.59% (Ye et al., 2008). The main degradation product of hemicellulose is xylan (Yi and Zhang, 2012). In recent years, many studies have focused on the development and utilization of xylan from agricultural residues (Liu et al., 2016, Zhang et al., 2018, Alves et al., 2020). It is worth noting that the content of lignin (23.38%) in corn stalks is similar to hemicellulose, a large amount of lignin will be left during the extraction of xylan (Arnaud et al., 2020). If this part of lignin, produced during the process of xylan extraction, was converted to be a useful adsorbent, that will promote the full-value utilization of corn stalks and lead to a positive impact on sustainable development (Hu et al., 2019).

In our previous work (Liu et al., 2021), the xylan was extracted from corn stalks by alkaline extraction and the influence of mechanical pretreatment on the yield of xylan was evaluate. We discovered from previous studies, the lignin can be precipitated by adjusting the pH of the hydrolysate to 3 according the nature alkali soluble and acid-precipitation. However, it is more favorable for xylan at pH 5. Studies (Gomes et al., 2020, Khaire et al., 2021) have concluded that the pH of hydrolysate has an influence on the purity of xylan. Remarkably, little research has examined whether there is a difference in the adsorption performance of the lignin collected at different pH values during that process.

The main aim of the our work was to investigate the adsorption capacity of lignin from corn stalks and compare the difference in adsorption performance of lignin precipitated from different pH hydrolysates. To achieve this goal, the lignin was collected by pH grading method (the lignin was precipitated from the hydrolysate with a pH of 3 or 5) in the first step, after that, lignin was used as an adsorbent for the removal of methylene blue. The comparative research on adsorption capacity of these two lignins was comprehensively investigated. Meanwhile, we optimized the adsorption conditions (pH, time, temperature and the amount of adsorbent) and studied the adsorption kinetics and adsorption thermodynamics. Finally, the reasons for the differences in adsorption capacity between them were explained by the characterization of specific surface area.

Section snippets

Materials

The raw materials of corn stalks were collected from Sichuan Jinxiang Seri chemical Chemical Co., Ltd. The methylene blue was purchased from Tianjin Tianxin Fine Chemical Development Center. Meanwhile, analytical chemicals such as sodium hydroxide, hydrochloric acid, glacial acetic acid and 95% ethanol were bought from Guangzhou Chemical Reagent Co., Ltd, without further treatment.

Preparation of lignin adsorbent at different pH

First, corn stalk filaments (3–4 mm) were prepared with the help of a refiner. Second, the alkaline treatment of

Effect of pH

Fig. 1 shows the influence of pH on the methylene blue removal performance. According to this plot, the adsorption capacity of LG5 was significantly better than that of LG3 within the entire experimental pH range. The adsorption capacity (Qt) of them reached minimum under highly acidic (pH = 2) condition. When the pH of the methylene blue dye reached 10, the removal rate was close to 70%, and the maximum adsorption amount reached 134.3 mg/g. The pH of MB determines the charge and electric

Conclusion

This study reported the extraction of lignin from corn stalks hydrolysate with different pH values and its use in methylene blue removal from aqueous solutions by adsorption. The as-prepared lignin played an effective role in removing methylene blue with the presence of reactive functional groups such as phenolic hydroxyl groups. The pH of hydrolysate, as one of the conditions for the extraction of lignin, had a great influence on the adsorption performance of lignin. The adsorption amount

CRediT authorship contribution statement

Huiming Fan: Methodology, Funding acquisition, Writing – review & editing. Fengyu Li: Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Hanyi Huang: Investigation, Writing – original draft. Jingkun Yang: Investigation, Writing – original draft. Deli Zeng: Investigation, Writing – original draft. Jianan Liu: Supervision, Investigation, Funding acquisition. Hongyan Mou: Formal analysis, Resources, Writing – review & editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This work was financial supported by Guangzhou Science and Technology Plan Project (No. GZDD201808).

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