**1. Introduction**

Dyes are colored organic chemicals typically classified as anionic (acid, reactive, and direct dyes), cationic (all basic dyes), and non-ionic (dispersed dyes) based on their charge upon dissolution in aqueous solutions [1]. These complex molecules are widely used in many industrial fields, such as textile, paper, leather tanning, food processing, plastics, cosmetics, rubber, and printing. The contamination of the hydrosphere with dyes raises serious environmental and sanitary concerns due to their ubiquity, toxicity and deleterious effects on photosynthetic activity in aquatic life due to decreased sunlight penetration [2]. Particularly, methylene blue (MB), one of the most widely used basic dyes in the printing and textile industries, can cause a variety of harmful effects, such as eye burns, gastrointestinal tract and skin irritation [3]. As a typical direct azo dye, Congo red (CR) is mainly applied in a relatively large dosage for dyeing biological samples, and can increase the risk of cancer if absorbed into the human body [4]. Currently, there are various industrial methods for treating waste dye solutions, including adsorption, microbial treatment, chemical oxidation or reduction, flocculation precipitation, ozone oxidization, chemical precipitation, nanofiltration, catalytic degradation etcetera [5–7]. Due to

its simple operation and low cost, adsorption is widely used as a commercial way to remove organic dyes from aqueous solutions [8].

Recently, carbon nanomaterials have attracted a broad interest in pollutant adsorption. Among them, carbon nanotubes (CNTs) are comprised of one or several sheets of hexagonally packed carbon atoms rolled into concentric seamless cylinders; these exhibit strong mechanical properties, a high aspect ratio, excellent chemical stability and a large specific surface area, which make them desirable for dye adsorption [9–11]. The adsorption mechanism of organic compounds on carbon nanomaterials can be described as an interplay between different intermolecular forces (i.e., hydrophobic, van der Waals forces, π–π bonding, hydrogen bonding, and electrostatic interactions), whose contributions depend on the adsorbate nature and the surface chemistry of the nano-adsorbent [12]. Another type of carbon nanomaterial, graphene, is a planar honeycomb-shaped nanomaterial consisting of six-member rings with a large specific surface area (theoretical value of 2630 m<sup>2</sup> g<sup>−</sup>1) [13]. It is more easily prepared than CNT and shows an extensive application prospect in the removal of organic and inorganic pollutants [14] and dyes [15,16]. Graphene could be modified to adsorb pollutants via π–π, electrostatic interaction and hydrogen bonding. However, the cost of a carbon-based nano-adsorbent is high and the regeneration treatment of spent materials is often challenging [17]. Due to strong π–π bonds and van der Waals forces, graphene sheets tend to condense and restack. CNTs often group into bundles and tangle due to the same reason. Therefore, the available specific surface area of these carbon nanomaterial is much lower than the theoretical value, greatly restricting their practical applications [18]. Thus, providing a recycling function and reducing the aggregation potential of the carbon nanomaterials are vital ways to design high-efficiency adsorbents.

Nanocelluloses are a class of renewable nanomaterials derived from the most abundant natural polymer on earth. Their high specific surface area, bioavailability and surface reactivity make nanocelluloses interesting materials for pollutant adsorbents [19]. Since unmodified nanocelluloses expose abundant surface hydroxyl groups and perhaps other negatively charged groups such as sulfate or carboxylate groups depending on the preparation procedure [20,21], they are well-suited for the adsorption of cationic molecules. Recently, high surface-area cellulose nanofibril (CNF) aerogels prepared via 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) oxidation were shown to absorb cationic malachite green (MG) dye (212.7 mg g<sup>−</sup>1) due to electrostatic interactions between MG and negatively charged oxygen moieties on the CNF surface [22]. To make nanocellulose adsorbents for anionic dye, chemical modifications, such as introduction of positively charged amino groups, are usually required [23]. However, these positively charged nanocelluloses are no longer suitable for cationic dye adsorption.

While the above studies demonstrate the individual efficacy of carbon nanomaterials and nanocellulose adsorbents, improved adsorption performance and cost optimization may be realized by combining these nanomaterials together. Since carbon nanomaterials and nanocellulose exhibit different affinities for given molecules; hence, their combination can increase the variety of pollutants that may be adsorbed. Recently, CNFs were shown to improve the dispersion of carbon nanotubes [24,25] and graphene [26] in aqueous solutions. Hajian et al. [26] suggested the charges on the TEMPO oxidized CNFs induced an electrostatic stabilization of the CNF–carbon nanomaterial complexes to prevent aggregation. When oxygen-containing carbon nanomaterials, such as graphene oxide (GO) or oxidized CNTs, were used, there was a strong interaction between the oxygen-containing groups of carbon nanomaterials and hydroxyl groups of nanocellulose [27]. Recently, Wei et al. [27] synthesized GO/microcrystalline cellulose (MCC) aerogels in an LiBr aqueous solution. Hybrid GO/MCC aerogel had higher adsorption capacity of MB per unit mass of GO (2630 mg g−1) than pure GO when the content of GO was low (0.3 wt%). Hussain et al. [28] fabricated GO/CNF monoliths based on a urea-assisted self-assembly method. The maximum adsorption capacity of these hybrid monoliths to MB achieved 227.27 mg g−1. Wu et al. [29] exploited cellulose nanofiber as a cross-linker to interweave between reduced graphene oxide (rGO) layers and obtained hybridized monolith aerogels by a hydrothermal method. The hybridized monolith was able to adsorb not only hydrophilic dyes, but also hydrophobic organic oil. To our knowledge, the influence of carbon nanomaterials features, such as their morphology (nanotubes or nanoplates), on the adsorption behaviors of the CNF/carbon nanomaterial hybrids have not been explored.

Concerning industrial wastewater, different types of dyes could be found. Most of the previous works focus on single solute adsorption in pure water and are not representative of real-world wastewater effluents. During this study, two types of carbon nanomaterials, CNTs and graphene nanoplates (GnPs), are dispersed in water with CNFs using different mass ratios, respectively, to prepare hybrid aerogels through a simple freeze-drying procedure without the assistance of other agents. The aim is to understand the interactions of CNT and GnP with CNF and to explore the adsorption capacity of these hybrid materials to both anionic and cationic dyes in single and binary systems. The preparation, characterization, and adsorption assessments of CNF–CNT and CNF–GnP aerogels for MB and CR dyes are reported. Adsorption behaviors of dyes are inspected by kinetic models and adsorption isothermal models. The adsorption mechanism, contact time, and concentration of MB and CR dye to hybrid aerogels are investigated. Furthermore, desorption of both dyes from the adsorbents are studied.
