Highly efficient and sustainable alginate/carboxylated lignin hybrid beads as adsorbent for cationic dye removal

Highlights

• Carboxylated lignin was prepared via peracetic acid oxidation of Kraft lignin.

• Optimum blend ratio of alginate/carboxylated lignin hybrid beads was 50:50.

• Hybrid beads showed the superior methylene blue adsorption capacity (613.0 mg/g).

• Reusability of hybrid beads was maintained 80% in the fifth times of regeneration.

Abstract

In this study, chemically modified lignin and alginate (Alg) hybrid spherical adsorbents exhibiting highly effective adsorption performance for cationic dyes were simply prepared. Carboxylated lignin (C-Lig) was prepared by simple peracetic acid oxidation. Thereafter, through encapsulation of C-Lig into Alg beads was conducted. As the encapsulated amount of C-Lig increased, a higher methylene blue (MB) removal capacity was observed due to the introduction of carboxylated anions on the surface. The adsorption isotherm of Alg/C-Lig beads followed the Langmuir model and exhibited a high monolayer maximum adsorption capacity (q_m) of 613 mg/g due to the driving force which is electrostatic interaction between the cationic dye and the anionic group of C-Lig. According to intraparticle diffusion theory, the adsorption behavior proceeded in surface adsorption and internal diffusion during 6 h of the adsorption process. Desorption of MB molecules was found to occur easily via a change in only the pH condition of the solution, and it was possible to maintain over 80% of stable adsorption efficiency even after 5 repeated adsorption/desorption cycles. The facile fabrication process of the Alg/C-Lig beads and superior MB removal performance imply that the Alg/C-Lig beads show a high possibility to purify dye wastewater especially cationic dye molecules.

Introduction

Dye molecules are an indispensable and important material for expressing the color of objects together with pigments. It has been responsible for the color of our society, such as dyed fabrics, colored cosmetics, and a paint for automotive [1,2]. However, if a dye remains in water, the dye that expresses color is considered to be a major pollutant, which adversely affects the ecosystem [3]. Every year, 700,000 tons of dye stuffs are produced worldwide and approximately 10% of this dye flows into wastewater during industrial process [4]. These unexpected dye pollutants can damage nature and humanity in a variety of ways [5,6]. The first problem is the damage caused by dye accumulation. Dye molecules enter lakes, rivers, and ponds to contaminate water flows, and accumulated dyes can completely destroy hydroponic plants and aquatic animals and can ultimately harm human health through the direction of the food chain [7]. The second issue is the change of water color due to dye pollutants. Dyestuffs can change the color of water in very small quantities. Clouded dye-contaminated water blocks sunlight and interferes with the photosynthesis of plants, turning the entire aquatic ecosystem upside down. Therefore, an efficient technology capable of treating dye contaminants that are present in water is essential.

As the damage from water pollution caused by dyes becomes serious, numerous studies have been conducted to find an ideal dye removal method to recover and reuse wastewater. Conventional dye removal methods can be divided into three categories: biological [8,9], chemical [10], and physical treatments [11]. Of the many tried and tested dye removal methods, the adsorption removal (physical method) has emerged as one of the preferred techniques for dye removal. Compared with other conventional dye removal methods, the adsorption process has an advantage of high removal efficiency and reusability of adsorbent. However, the high cost of adsorbents has limited its wide use and applications [12]. The development of high-performance adsorbents using various natural polymers from agriculture, forestry, and fisheries can not only contribute to the circulation of natural resources but also increase the economic benefit of the adsorption process.

Lignin is a naturally occurring heterogeneous poly-aromatic organic material presents in woody biomass along with cellulose and hemicellulose [13]. Currently, lignin is produced at 50–70 million tons per/year in the pulp and paper industry facilities, and it is expected that 225 million tons/year of lignin will be generated in 2030 with the increasing demand for ecofriendly biofuel production. Lignin has been used as the main fuel for electricity and heat generation required for the operation of paper and pulp mills. Furthermore, with the depletion of petroleum resources and increasing interest in green carbon resources, lignin's entry into the material world is steadily occurring. Lignin is an eco-friendly renewable raw material that has also attracted considerable attention in the field of adsorbent materials. It has been known that the abundant phenolic and aliphatic hydroxyl groups, and small amounts of carboxyl groups in lignin have a binding affinity for various heavy metal species and dye stuffs [14]. However, the adsorption performance of these lignin raw materials is greatly influenced by the wood type and the lignin extraction method, and the adsorption performance is insufficient compared with that of a commercially available adsorbent [6,15,16]. Lignin is also in the spotlight as a precursor of activated carbon having hierarchical porous structures. A lignin-based porous activated carbon can be prepared and used for removing contaminants through a chemical/physical activation method followed by a carbonization process [[17], [18], [19], [20]]. However, in this case, a large amount of energy consumption in the carbonization process is required, which can incur a large cost to produce activated carbon.

Various strategies should be established to increase the applicability of lignin to practical water treatment adsorbents. Typically, adsorption performance should be improved through chemical modification [6] and providing convenient processability (easy separation of adsorbent from the contaminant solution and efficient regeneration capability) through various lignin formulation fabrication techniques. In the case of chemical modification of lignin, a method of introducing a new functional group or an existing hydroxyl group into a functional group can be considered. Additionally, a method for grafting and polymerizing a functional polymer with lignin as a main chain has been attempted. To fabricate various types of lignin-based adsorbent materials, development of new solvent systems, blending with existing natural/synthetic polymers, and encapsulation or immobilization methods have been steadily studied.

Herein, to develop valorized lignin into water treatment materials, simple carboxylation of Kraft lignin and alginate (Alg) encapsulation strategy were carried out resulting in the lignin-based bioadsorbent for cationic dye removal. Furthermore, the optimal blend ratio of carboxylated lignin (C-Lig) and Alg with improved cationic dye removal performance and sustainable reusability was investigated. The prepared Alg/C-Lig hybrid adsorbent was analyzed by a field emission scanning electron microscope (FE-SEM), an energy dispersive X-ray spectroscopy (EDS), and a Fourier-transform infrared (FT-IR). A typical cationic dye; methylene blue (MB) adsorption behavior of carboxylated lignin (C-Lig) adsorbent was elucidated in terms of adsorption isotherms, kinetics, and thermodynamics. Finally, the reusability of the Alg/C-Lig hybrid beads was also studied systematically.