Preparation of siliceous lignin microparticles from wheat husks with a facile method

Highlights

• Preparation of mesoporous siliceous lignin microparticles from wheat husks by a facile method.

• SiO₂ acted as the skeletal structure and lignin as filler in the siliceous lignin microparticles.

• The sorption ability of siliceous lignin microparticles to heavy metals improved.

Abstract

In order to make full use of the agricultural byproducts from wheat production process, mesoporous siliceous lignin microparticles from wheat husks were synthesized with a facile method in this paper. The optimum conditions for the samples' formation were pH 2 in the presence of 2 mol/L HCl and 90 °C for 6 h. The samples with various pore structures show an amorphous form with diameters of approximately 365 nm and a specific surface area of 117.44 m²/g. The sorption ability of the siliceous lignin microparticles towards heavy metal ions is much better than wheat husk soda lignin. Such siliceous lignin microparticles originating from wheat husks can be potentially used to decontaminate environmental objects.

Introduction

The recovery of useful materials from earth-abundant substances is of strategic importance for industrial processes. In recent years, a special concern has been manifested towards green hybrid materials from a renewable resource. Based on the use of all kinds of agricultural byproducts, predominant efforts are focused on synthesis of biologically active compounds or hybrid materials that can be applied in different fields (Bledzki et al., 2010, Wu et al., 2013). Wheat husk (WH), a kind of the main agricultural byproducts in wheat producing process, is scattered in all the wheat producing countries, which is accounted for about 15–20% of wheat (Bledzki et al., 2010). In China, only some of wheat husks are used as cattle food. Most of the WHs are usually burnt out in the open air or stacked on farmland, releasing large amounts of hazardous substances, occupying land resources, and polluting the environment. Considering that WHs are mainly composed of organic cellulose, hemicellulose, lignin, and inorganic silicon compounds, there is a great potentiality to recover the WHs to prepare task-specific products, such as silica-based and carbonaceous composite materials.

Lignin is ranked as the third of natural polymer present in plants following cellulose and hemicelluloses and thus has a great potential for exploitation (Buranov and Mazza, 2008). As lignin has a complicated three-dimensional structure and is chemically and physically heterogeneous, it is difficult to isolate the lignin with cellulose and hemicelluloses from plants. Generally, strong alkaline or strong acidic solution can be used to depolymerize the macromolecules and destroy its native chemical structure for isolating the lignin at high temperature. Lignin is usually obtained as a byproduct in the process of chemical pulp producing. The presence of significant amounts of different oxygen-containing groups on the lignin allows its physical adsorption, hydrogen bonding, coordination and covalent linking, and acidic-basic interaction with other compounds (Buranov and Mazza, 2008, Ferdous et al., 2002, Galina et al., 2009, Norgren and Edlund, 2014). The lignin as an adsorbent has been extensively studied to remove metallic contaminants in wastewater (e.g. from electroplating factories) by sorption and precipitation (Betancur et al., 2009, Crist et al., 2005, Li et al., 2015, Zghida et al., 2004). However, during the wastewater treatment, the wastewater’s pH value strongly influences the performance of the lignin adsorbent in contaminant sorption (Betancur et al., 2009, Guo et al., 2008, Wu et al., 2008) and the solubility of the lignin in treated wastewater (Garcia et al., 2009; Liu et al., 2013). If the pH value of the treating wastewater is not suitable, not only will the sorption ability of lignin adsorbent be decreased, but also the dissolved lignin as a pollutant from the adsorbent will pollute the wastewater again. In order to improve the sorption capacity, the pH adaptability and thermal stability of lignin adsorbent, a lot of lignin/inorganic and lignin/organic hybrid materials have been prepared (Galina et al., 2009, Klapiszewski et al., 2015, Zhang et al., 2011b, Silveira et al., 1995). Recently, much attention has been paid on silica/lignin hybrid materials (Galina et al., 2009, Jesionowski et al., 2014, Klapiszewski et al., 2013, Klapiszewski et al., 2015). Addition of silica into lignin adsorbent will make the functional oxygen groups on silica surface interact with lignin and enhance the stabilization of the lignin. The hybridization of silica (as a skeleton) with lignin is also accompanied with the formation of more active sites on the hybrid materials’ surface, which improves lignin’s ability to adsorb heavy metal ions, organic compounds and many other pollutants.

In the previous published works (Galina et al., 2009, Jesionowski et al., 2014, Silveira et al., 1995), different starting materials of silica and lignin had been used to prepare the silica/lignin hybrid materials. Monomeric or oligomeric organosilicon compounds and soda lignin are used as the starting materials of the silica and lignin, respectively. Furthermore, rice husks (Qu et al., 2010) or black liquor of rice straw pulping (Zhang et al., 2013) is also investigated as the only starting material of both the silica and lignin. Qu et al. (2010) had synthesized a lignin/silica hybrid material from rice husks as raw material by a sol-gel method. The mass ratio of lignin of the obtained sample was 41.67%. Zhang et al. (2013) had prepared lignin-modified silica nanoparticles from black liquor of rice straw pulping using CO₂ gas as a precipitating reagent. Lignin contents of the obtained products were about 1.1% –2.7%. Galina et al. (2009) had studied the sorption capacity of silica/lignin hybrid adsorbent and the addition of 10% silica into lignin adsorbent had improved the sorption capacity remarkably.

In this work, a facile method has been developed to synthesize the WH-based siliceous lignin microparticles (denoted as WHSiLMP hereafter) using wheat husks as the starting material of both lignin and silica. In this method, the pH value of the solution for sequential precipitation of lignin and silica from the alkaline treatment of wheat husks is easily modulated at ambient temperature, followed by a self-associating reaction between lignin and silica in situ at temperature of 50–100 °C. The mass ratio of lignin in the obtained products can be controlled from 20% to over 70%, and the lignin in the WH alkaline treating solution was used more effectively. In the obtained siliceous lignin materials, the Si compound guest molecule could adjust both the microsurface and the bulk structure of the lignin host matrix and influence the performance of the siliceous lignin materials as an adsorbent. Hence, the structure of the WH-based siliceous lignin microparticles (WHSiLMP) obtained at optimum synthesis conditions had been analyzed. The sorption capacity of the wheat husk soda lignin (WHSL) and the WHSiLMP towards heavy metals was tested, respectively. This work would supply a new method for the recovery of useful materials from the agricultural byproducts in wheat production process.