Novel graphene oxide/aminated lignin aerogels for enhanced adsorption of malachite green in wastewater
Graphical abstract
Introduction
Industrial pollution is a noteworthy matter in various fields, and they are discharged randomly into water or gas will cause great harm to nature and the human body. Each country has customized its own emissions targets, but the emission of some stubborn industrial pollution occasionally exceeds their limits [1]. Malachite green (MG), a kind of triphenylmethane dye, is mostly used in the fishery. It can cause people and animals to be cancerous and deformed if long-term overuse, so effectively removal of MG is an issue worth exploring [2]. Many methods have been exploited to decrease emission of pollutant from water, including membrane separation [3], chemical precipitation [4], electrostatic adsorption [5] and biological degradation [6], etc. The biomass adsorption method is widely used as its low cost, simplicity and biocompatibility; biomass adsorbents such as cellulose, sugarcane bagasse [7], Zea mays L [8] and lignin derivate [9] gradually replaced chemical adsorbents, however these absorbents were inefficient and difficult to reuse.
Graphene, a versatile material, has been developed for its outstanding properties and applied to various industries, such as biological sensors [10], electrochemistry [11], testing technology [12] and pollutant disposal [13], it is very difficult to make the single graphene or to maintain its form. Therefore, graphene oxide (GO), a stable precursor for the preparation of graphene, has been used in biomedicine [14], catalysis [15], and composites [16], etc. GO has the obvious adsorption effect on azo dyes and aromatic pollutants owing to the large amount of oxygen-containing groups on the edges and planes of GO [17], that makes GO easy to assemble and functionalize, and provides active sites for binding and coordination for adsorption [18]. Because of the high price, it should be used with other biomass materials.
Lignin is the second most abundant component next to cellulose in biomass [19]. The worldwide pulping and papermaking industry produce more than 50 × 104 tons annually, but only approximately 2 % of the lignin is used as dispersants or binding agents and the rest is used as low value fuel [20]. Although lignin is low price, renewable and abundant functional groups, and has an adsorption effect on metal ions and dyes, it is easy to form intermolecular hydrogen bonds, making it difficult disperse in water, thus lignin has low adsorption capacity as the adsorbent [21]. However, lignin can be applied in various fields to achieve high value utilization through the modification and functionalization. Amination is one of the modifications with great promise because the introduced amine groups are ionizable and positively charged under acidic conditions [22]. Among the methods, the Mannich reaction is the simplest and most direct. Making full use of aminated lignin (AL) can not only increase the utilization value of biomass resources, but also protect the environment.
Here, the low cost, recyclable and eco-friendly graphene oxide/aminated lignin aerogels (GALA) were prepared by photo-initiation, which was faster, energy-saving and green than thermal initiation, and the polymers of photo initiation had a more uniform structure and narrower molecular weight distribution [23]. In these aerogels, the abundant oxygen-containing groups of GO (hydroxyl, carboxyl and epoxy) and the functional groups of AL (amino, aliphatic hydroxyl and phenolic hydroxyl) generated the synergy through various molecular interactions such as π-π conjugation, hydrogen bonding and Van der Waal’s forces, and any toxic cross-linking agents were not added in the whole process. Moreover, while the addition of GO and AL were a small amount in the experiment, the improved adsorption effect was obtained, which not only reduced the cost, but also utilized the waste biomass lignin. To evaluate the adsorption capacity of the GALA, MG was chosen as the adsorbate, and the adsorption effect under the different adsorption conditions was studied. Meanwhile, the adsorption mechanism was discussed by the adsorption isotherm and kinetics models, and the recycle ability of the GALA was also investigated.
Section snippets
Materials
Graphite powder was provided from Damo Chemical Reagent Co., Ltd. (Tianjin, China). Alkali lignin was supplied by Tranlin Forest & Paper Co., Ltd. (Shandong, China). Acrylic acid (AA), ammonium persulfate (APS), N, N'-methylene bisacrylamide (MBA) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, china). Deionized water was used to prepare aqueous solution.
Preparations of the GO and AL
GO was prepared by the modified Hummers methods [24]. Briefly, Concentrated H2SO4 (23 mL) and graphite powder (1.0 g) were
Characterizations
The morphology of the GO and the structure of the GALA were displayed in Fig. 1. As illustrated in Fig. 1 (a) and (b), the structure of the GO was a flat layered surface with the smooth surface and crumpled edge due to the interaction of the oxygen-containing groups of the GO [28]. Fig. 1 (c) was the top surface of the GALA, which shown the round holes with smooth edge and large gaps. Compared with the top surface, the bottom surface of the GALA was rough in surface and loose in structure (Fig.
Conclusion
In this work, graphene oxide/aminated lignin aerogels (GALA) with a three-dimensional structure was successfully prepared by photo-initiation, providing an efficient adsorbent for removal of dyes from waste water. The maximum adsorption capacity and efficiency to MG were reached to 113.5 mg/g and 91.72 % at the optimal pH values. Then, the experimental data indicated that the adsorption isotherm was suitable for the Langmuir model, and the adsorption kinetic was agreed well with the
CRediT authorship contribution statement
Hang Chen: Conceptualization, Investigation, Writing - original draft, Writing - review & editing. Tanglong Liu: Investigation, Data curation, Methodology. Yi Meng: Conceptualization, Writing - review & editing. Yi Cheng: Software, Supervision. Jie Lu: Software, Supervision. Haisong Wang: Supervision, Project administration, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by Natural Science Foundation of China (Nos. 21978029 and 31770624), National Key R&D Program of China (No. 2018YFD0400703), the Program for Liaoning Excellent Talents in University (LR2016058) and Liaoning BaiQianWan Talents Program (201945).
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