Nanocelllulose Based Adsorbents for Heavy Metal Ions Removal

Heavy metal ion pollutions are of serious threat for our human health, and advanced technologies on removal of heavy metal ions in water or soil are in the focus of intensive research worldwide. Nanocellulose based adsorbents are emerging as an environmentally friendly appealing materials platform for heavy metal ions removal as nanocellulose has higher specic surface area, excellent mechanical properties and good biocompatibility. In this review, we briey compare the differences of three kinds of nanocellulose and their preparation method. Then we cover the most recent work on nanocellulose based adsorbents for heavy metal ions removal, and present an in-depth discussion of the modication technologies for nanocellulose in assembling high performance heavy ions adsorbent process. By introducing functional groups, such as amino, carboxyl, phenolic hydroxyl, and thiol, the nanocellulose based adsorbents not only remove single heavy metal ions through ion exchange, chelation/complexation/coordination, electrostatic attraction, hydrophobic actions, binding anity and redox reactions, but also can selectively adsorb multiple heavy ions in water. Finally, some challenges of nanocellulose based adsorbents for heavy metal ions are also prospected. We anticipate that the review supplies some guides for nanocellulose based adsorbents applied in heavy metal ions removal eld.


Introduction
Nowadays, heavy metal ions has become the most serious problem in water environment due to their toxicity and incompatibility, which not only cause badly problems of environmental but also threaten health of human (Miretzky and Cirelli 2009). Excessive intake of heavy metal ions can cause body damage and even death, such as, Minamata (Petrova et al. 2020) disease in Japan caused by the excessive intake of organic mercury (Hg),some lungs or gastrointestinal tract disease caused by the accumulation of Cd 2+ (Anetor 2012), and Alzheimer's and Parkinson's diseases (Du et al. 2018) caused by the excessive intake of Fe 3+ and Al 3+ .In addition, the rise of nuclear power plants will also cause many radioactive heavy metal pollution such as (Cs 137 , Pu 239 and U 238 ) (Buesseler et al. 2012).It is challenging to attenuate or eliminate heavy metal ion pollutions.
There are many methods to solve heavy metal ions pollution problems in wastewater, mainly include chemical precipitation, ions exchange, ultra ltration, occulation, electrodialysis, adsorption and reverse osmosis, etc.(Fu and Wang 2011) Among these methods, adsorption is the most widely used one due to the high removal e ciencies, exibility in the design and low cost (Shen et al. 2019). The adsorbents mainly include activated carbon, clay, biochar and polymers (Qin et al. 2019b). Recently, nanocellulosebased adsorbents become more and more popular as nanocellulose has higher speci c surface area, excellent mechanical properties and good biocompatibility (Jordan et al. 2019).
In this review, we rst summarize categories, characteristics and preparation methods of nanocellulose. Then, we present the most recent work on nanocellulose based adsorbents, and deeply highlight the roles of nanocellulose in nanocellulose based adsorbents. Finally, we conclude with perspectives on the remaining challenges and potential opportunities of nanocellulose based adsorbents.

Classi cations Of Nanocellulose
Cellulose is photosynthesized from CO 2 and water, and accumulated as a major component in plants (Isogai 2020). It is a crystalline biopolymer possessing long chains of β-D-glucopyranose units joined by β-1,4 glycosidic linkage in which inter-and intramolecular hydrogen bonding restrict its main chain motion (Garba et al. 2020). It is evident that the adsorption properties of native cellulose can be increased by converting it to the nanostructure (Kardam et al. 2014). According to the work of Vadakkekara (Vadakkekara et al. 2019) et al. the maximum adsorption capacity (115 mg/g) of maleic acid-modi ed nanocellulose for Pb 2+ is higher than that of maleic acid-modi ed macro(20mg/g) or micro cellulose(40mg/g). Nanocellulose (NC) is a cellulose material with at least one dimension in the nanometer size range separated from ber raw materials through physical, chemical or biological treatment (Li et al. 2010).
Depending on its cellulose source, processing conditions, size, function and preparation methods, it can be classi ed into three categories such as nano brillated celluloses (NFCs), bacterial nanocellulose (BNCs) and cellulose nanocrystals (CNCs). CNCs, also known as nanowhiskers, exhibit elongated crystalline rod-like shapes, and have high rigidity compared to NFCs since their amorphous regions are highly removed. Nano brillar cellulose, cellulose nano bers, and cellulose nano brils are some of synonyms for NFCs (Li et  To date, the three kinds of nanocelluloses are not only were used in conductive materials, oil-water separation, lter materials, food processing, sensor, capacitor, bio scaffold, and drug delivery elds, but also applied in the elds of heavy metal ions removal and become more and more popular. 3. Nanocellulose Based Adsorbents For Single Heavy Ion Removal 3.1 Nanocellulose based adsorbents for Cu 2+ removal Generally, natural nanocellulose is challenged by intrinsic hydrophilicity and inferior adsorption sites, which thus affect its performance in extraction of heavy metal ions (He et al. 2018 showed that its adsorption capacity for Cu 2+ decreased from 68mg/g to 45mg/g after 4 cycles. Wang et al. (2018a) obtained 9-CNF, 7-CNF and 5-CNF with different carboxyl contents according to different ratios of hydrochloric acid/ citric acid (v/v = 9/1, 7/3, 5/5) waste ginger ber. As a comparison, S-CNF was obtained by traditional sulfuric acid hydrolysis. Hereafter, CNF suspension freeze-dried to obtain aerogels. 3D network structures of all aerogels were physically cross-linked by hydrogen bonding with mesopores to macropores (Fig. 2). Among them, 7-CNF had the largest aspect ratio (144) and the largest carboxyl content (1.18 ± 0.1mmol/g) and largest negative zeta potential (-36 ± 3mV). Compressive properties (99.5kPa at 80% strain, 241.77 kPa at 90% strain) at a low density of 20.3 mg/cm 3 . Its maximum adsorption capacity for Cu 2+ reached 45.053mg/g due to the network capture effect, charge neutralization and chain bridging of high aspect ratio carboxylated CNF. Lei et al. (2019) obtained NFCs suspension after grinding through Endoglucanase enzymolysis, adjusted the concentration of sodium periodate to 10 (DNFC-1), 40g/L (DNFC-2) to controlled the content of aldehyde group. DNFC-2 has the largest aldehyde content (1.95 mmol/g), biggest speci c surface area (2.73 ± 0.08m 2 /g), and the surface charge density of (1.14 ± 0.07) ×10 − 5 eq/g. The maximum adsorption capacity of DNFC-2 for Cu 2+ was 26mg/g. Amino groups have a strong chelating ability to heavy metal ions, so increasing the amino group content can increase the adsorption capacity. Since polyethyleneimine (PEI) has plenty of primary, secondary and tertiary amines on the macromolecular chains, it was usually fabricated hydrogels or aerogels by crosslinking with an aldehyde or epoxy group for improving the adsorption capacity (Shao et al. 2017). Tang et al. (2020b) obtained a high amine group content(5.74mmol/g) of cellulose nano bril/PEI aerogel bead (CGP1.3) with the help of a cross-linking agent of 3-glycidyloxypropyl) trimethoxy silane (GPTMS) by quickly frozen with liquid nitrogen and freeze-dried methods. Its maximum Cu 2+ adsorption capacity reached163.40mg/g. Mo et al. (2019) used TO-CNF and Trimethylolpropane-tris-(2-methyl-1-aziridine) propionate (TMPTAP) to react through a ring-opening reaction at room temperature, and post-crosslinked PEI to obtain a 3D multi-wall structure TO-CNF/TMPTAP/PEI aerogel (TO-CTP)with pores. Since the aerogel had a large number of amino groups and oxygen-containing groups on the surface, its maximum adsorption capacity for Cu 2+ reached 485.44mg/g. Moreover, after being treated with EDTA-2Na, the regenerated aerogels also retained high removal e ciency for Cu 2+ over four desorption-regeneration cycles. Zhang et al.(2016) obtained TOCN by HCl hydrolysis and TEMPO oxidation, and then cross-linked with PEI under the action of glutaraldehyde (GA) cross-linking agent, then freeze-dried and grinded to get TOCN-PEI adsorbent. Because of the linking between PEI and -COOH, the carboxyl content of TOCN-PEI decreased from 1.88 to 0.85 mmol/g, and its total amount of amino groups was 4.06 mmol/g. Its maximum capacities for Cu 2+ reached 52.32mg/g due to its abundant carboxyl and amino groups. After HCl cleaning, its adsorption capacity could still reach 33mg/g. Tang et al.(2020c) controlled different H 2 O 2 oxidation time (3h, 6h, 9h), and oxidized pomelo peel to obtain anionic/carboxyl nanocellulose (POCNF-3, 6, 9), the obtained POCNF-6 had the highest carboxyl group content (1.711 ± 0.173mmol/g), highest aspect ratio (169) and largest negative zeta potential(30.5 ± 2.7). Hereafter, POCNF-6 crosslinked with PEI under the action of a glutaraldehyde crosslinking agent to obtain POCNF-PEI. Its maximum adsorption capacity for Cu 2+ reached 74.2mg/g. The Cu 2+ adsorption capacity of POCNF-PEI remained to 66.2 mg/g after four adsorption-desorption cycles.
The composite of two or more polymers has become a new development trend of biomaterials in order to obtain certain excellent properties that a single polymer cannot achieve (Cui et al. 2015). Polydopamine (PDA), formed by self-polymerization of dopamine, is rich in catechol and amine groups, which facilitate covalent conjugation or other noncovalent interactions with organic and inorganic materials (Liu et al. 2011). Juntao et al.(2019) used a bio-inspires coating strategy to introduce PDA particles into the surface of CNFs, and then cross-linked PEI to form porous aerogel (PDA-CNF-PEI). Its maximum adsorption capacity for Cu 2+ reached 103.5mg/g, and its porosity and density were 98.5% and 25mg/cm 3 , respectively. When it was regenerated for four cycles by using 0.1M HCl treatment, its adsorption e ciency for Cu 2+ still retained more than 91%. Mautner et al.(2016) modi ed the cellulose nano brils from the ber sludge with phosphoric acid to obtain phosphorylated cellulose nano brils, and then prepared nanopapers(CNF-P) through papermaking methods. Its maximum adsorption capacity for Cu 2+ could reach 19.6mg/g by the ion exchange capture action of phosphate groups in CNF-P (18.6 ± 2.3mmol/kg). After a simple washing of phosphoric acid, the adsorption capacity of Cu 2+ can still reach 19.4mg/g.

Nanocellulose based adsorbents for Pb 2+ removal
Francisco et al.(2020) obtained cellulose from raw agave leaves by benzyl alcohol extraction, sodium hydroxide alkali treatment and peracetic acid (PAA, H 2 O 2 + acetic acid + H 2 SO 4 ), and then high-pressure treatment with a micro uidization process to obtain cellulose nano brils and nanosheet (CNF/CNS). The results showed that the CNF/CNS surface contained a lot of carboxyl groups. At low initial concentrations of Pb 2+ (C0 < 100 ppm), the adsorption-mechanism is governed by electrostatic interactions between carboxylate groups and Pb 2+ ; meanwhile, at (C0 = 1000 ppm) mono and bi-dentate complexes dominate the adsorption-mechanism. Finally, when 110 < C0 < 1000 ppm, both mechanisms co-exist. According to Langmuir model, the maximum adsorption capacity for Pb 2+ was 43.55mg/g. Sharma et al. (2018b) used nitric acid/sodium nitrite to oxidize untreated jute bers to obtain NOCNF slurry. Although its crystallinity was very low (35%), its carboxyl content and surface charge were very high (1.15mmol/g and − 70mV). At a low NOCNF suspension concentration (0.23wt%), room temperature, pH = 7, the NOCNF could remove sharply Pb 2+ ions of from 50 to 5000ppm in the initial steps, and nally its maximum adsorption capacity could reach 2270mg/g. Graphene oxide (GO) was used to remove heavy metal ions due to its high speci c surface area and large number of functional oxygen groups that could provide active sites for heavy metal ions (İlayda et al. 2016). Yu et al. (2020) used Fe 3+ as crosslinking agent, carboxymethyl cellulose nano bril as ller, and wet-spinning method to obtain GO/CMCNF composite ber(CF). The ber exhibited enhanced mechanical property up to 648MPa. Its maximum adsorption capacity for Pb 2+ reached 99.0mg/g by electrostatic attraction, ion exchange and complexation of carboxyl groups and Pb 2+ .
Alginate could be also mixed with nanocellulose to remove heavy ions in water due to its non-toxic, biodegradable, low cost and rich carboxyl groups . Then use the obtained AC to activate NCS/NCP. The SEM image showed that AC was dispersed in the NC matrix, forming a looser embedded network. Its adsorption mechanism was mainly the electrostatic attraction between adjacent hydroxyl groups and positively charged metal ions on the surface of the super-adsorbent, and its maximum adsorption capacity for Pb 2+ reached 24.94mg/g.
In order to improve the hydrophobicity and exibility of nanocellulose based adsorbents, Rani et al.(2019) used steam explosion method to extract CNCs from banana ber, and grafted butyl acrylate(BA) onto CNCs to obtain CNCs-g-nBA membrane with the help of ceric ammonium nitrate initiator. Its adsorption capacity changed with the different pH values of the solution. At low pH, the concentration of protons in the solution was high, and the metal binding site became positively charged, repelling Pb 2+ cations. As the pH raised, the negative charge density on the adsorbent increased due to the deprotonation of the metal binding site, and adsorption capacity enhanced, however, at pH > 6, the formation of aqueous metal hydroxide precipitation was the main mechanism for removal. At pH = 5, its maximum adsorption capacity with Pb 2+ reached 159.53mg/g, and after 3 cycles of desorption with 0.1N HCl and 0.1M EDTA, its adsorption capacity could still maintain a high potential.
Polyvinyl alcohol (PVA) is an inexpensive polymer which possesses desirable properties such as water solubility, biocompatibility, and biodegradability. The application of magnetic adsorbents technology has become one of the promising ways to solve environmental problems (Feng et al. 2010). Zhou et al.(2013) used TEMPO to oxidize MCC to rstly obtain carboxylated cellulose nano brils (CCNFs) and then obtain CCNFs-lled magnetic chitosan hydrogel beads (m-CS/PVA/CCNFs) through an instantaneous gelation method. It had a 3D porous structure ( Fig. 3(a)), m-CS/PVA/CCNFs hydrogel beads can be easily separated from water ( Fig. 3(b)). Its removals for Pb 2+ were mainly through amino chelation and carboxyl ion exchange. Its maximum adsorption capacity for Pb 2+ could reach 171.0mg/g. After four cycles with weak acid regeneration, its adsorption e ciency could still retain 90%.
Vadakkekara et al. (2020) obtained nanocellulose from jute ber, and then modi ed it with sodium itaconate to obtain sodium itaconate grafted nanocellulose(SINC) adsorbent. The SEM image showed that the SINC surface was irregularity and smoothness. Its maximum adsorption capacity for Pb 2+ was 85mg/g at the pH value of 5.5.

Nanocellulose based adsorbents for Cr 6+ removal
Cr is one of priority pollutants in water. Cr has two common oxidation states, Cr 6+ is highly toxic, mutagenic and carcinogenic to the ecosystem, however, Cr 3+ is a nontoxic substance .
For the removal of hexavalent chromium (Cr 6+ ) ions (Attia et al. 2010), most adsorbents have a higher e ciency when pH value was less than 3, but under neutral or alkaline conditions, the removal e ciency was relatively lower. Huang et al. (2020) oxidized sugarcane bagasse with sodium periodate and followed by cationization using Girard's reagent T to obtain cationic dialdehyde cellulose (c-DAC). There were high density of quaternary ammonium groups and aldehyde groups on the surface of c-DAC. The electrostatic attraction between the positively charged quaternary ammonium salt group and the negatively charged dichromate was the main mechanism of adsorption, and there was a strong binding a nity between the adsorbent and Cr 6+ . When the adsorption reached saturation, the surface charge on c-DAC was neutralized to form c-DAC-chromium ocs, which could easily be removed by decantation or low-cost gravity-driven micro ltration. The maximum adsorption capacity for Cr 6+ could reach 80.5mg/g, and it had stable adsorption performance under a wide pH range (2-10).
Ferreira-Neto et al.(2020) prepared a self-supported hybrid aerogel lm(BC/MoS 2 ) combining Bacterial nanocellulose-based organic macro/mesoporous scaffolds and MoS 2 nanostructures (Fig. 4). Since MoS 2 is a transition metal hydrogen disul de, a layered structure composed of stacked two-dimensional nanosheets (Wang et al. 2018c), this lm displays a large number of sul de(S 2− ) sites, which was easy to bind heavy metal ions by electrostatic or hydrophobic adsorptions. Its speci c surface area and pore volume were 97-137m 2 /g and 0.28-0.36cm 3 /g, respectively. Cr 6+ is removed through adsorptivephotocatalytic mechanism since MoS 2 showed effective visible light photoactivity ( Fig. 4(b)). Remove Cr 6+ ion (88% removal within 120min, Kobs = 0.0012min − 1 ) in photos assisted in-ow. Zhao et al.(2015) used bamboo pulp to perform ultrasonic and high shear homogenization to obtain CNFs (( Fig. 5 (a, b)). Then, the CNFs were modi ed with amino groups by Michael reaction, and nally the terminal ester groups were amidated with ethylenediamine (EDA). Repeated several times to obtain PAMAM-g-CNFs, and freeze-dried to obtain PAMAM-g-CNFs aerogel (Fig. 5 (c)). Poly(amidoamine) (PAMAM) (Han et al. 2012) contains a large number of amine groups and amide groups and is common dendrimers. The aerogel has a porosity of 99.39%, speci c surface aera is 82m 2 /g and a density of 0.01g/cm 3 . The aerogel removes Cr 6+ through ion exchange and redox reactions. First, the amino group is protonated under acidic conditions, and Cr 6+ is electrostatically attracted to the aerogel on the surface, part of Cr 6+ is reduced to Cr 3+ by the adjacent electron donor groups. The positively charged Cr 3+ is released in the water. Some Cr 3+ ions can still form stable complexes with the negatively charged groups of PAMAM-g-CNFs (Fig. 5(d)). These led to its maximum adsorption capacity of 377.36mg/g for Cr 6+ . Ram et al. (2018) obtained spherical nanocellulose(SNC) through acid hydrolysis, then oxidized it with sodium periodate, and further processed it with 2-aminoethane-1-sulfonic acid to obtain Schiff base. Finally, a SNC-Chemosensor was obtained after adding mercaptopropionic acid (MPA) and a speci c amount of lipase. Its adsorption mechanism mainly was the complexation of imine, sulfonate and thioester functional groups with Cr 6+ , furthermore, the phenomenon of charge transfer from the ligand to the metal also led to the naked eye color sensing in the range of 30ppb to 100ppm of Cr 6+ . Its maximum adsorption capacity for Cr 6+ was 130mg/5mg at pH = 7 and could be easily regenerated with 0.1M NaOH since it contained imine, thioester and sulphonate groups. polymer justi es its stability, low cost and eco-friendly nature. Compared with 197m 2 /g for NC, the speci c surface area of NCPY was signi cantly increased (488m 2 /g). The maximum adsorption capacity of NCPPY for Cr 6+ was 147.3mg/g possibly duo to the -OH and -NH 2 adsorption sites on its surface.   (Fig. 6). The maximum adsorption capacity of CNF-2MPTMA for Hg 2+ could reach 700mg/g. After using 0.1M aqueous disodium edetate dihydrate solution to remove Hg 2+ , the adsorption capacity of CNF-MPTMS sponges did not decrease signi cantly.

Nanocellulose based adsorbents for Hg 2+ removal
Chauhan (2018) obtained spherical nanocellulose (SNC) by the treating sequences with NaOH and mixed acid of sulfuric acid/hydrochloric acid and ultrasonication, and then enzymatically esteri ed SNC with 3mercaptopropionic acid (3-MPA) to obtain its ester derivative (SNC-3-MPA). 13 C-NMR (nuclear magnetic resonance) results showed that the thiol group was grafted onto C-6-0 of cellulose monomer rings. Since SNC has a higher speci c surface area than cellulose and the presence of thiol groups had a high a nity for Hg 2+ , so the removal rate of Hg 2+ at a concentration of 100 ppm could be as high as 98.6% within 20 minutes. It can be recycled with 0.1N HCl. The adsorbent can be regenerated and its reusability was studied up to nine cycles with cumulative adsorption capacity of 404.95 mg/g.
In order to use the two functional groups(thiol and amino groups) in its molecular structure, Li et al. Ma et al. (2013) used TEMPO/NaBr/NaClO oxidation approach to prepared an aqueous suspension of 0.05wt% ultra ne cellulose nano ber. The diameter of ultra ne cellulose nano bers was found to be in the range of 5 to 10 nm, the average aspect ratio of the nano ber estimated from the Transmission electron microscope (TEM) image was about 160, and the carboxyl group content (1.4mmol/g). The maximum adsorption capacity for UO 2 2+ could reach167 mg/g due to the chelation between the carboxyl group and UO 2 2+ . After the UO 2 2+ adsorption, the surface of cellulose nano bers became covered with metal ionic crystals (Fig. 7). UO 2 2+ could still be used as a "cross-linker" to convert aqueous CNF into gel.  can reach 2550mg/g.
Arsenic is a metalloid that forms highly toxic compounds in many oxidation states including arsenate (As 3+ ) and arsenate (As 5+ ). (As 3+ ) is reported to be more toxic than (As 5+ ) and more di cult to remove CNF and positively charged partly deacetylated chitin nano ber to self-assemble into 3D biohybrid hydrogel (BHH) through electrostatic force at room temperature. Then freeze-drying to obtain biohybrid aerogel (BHA) (Fig. 10). The speci c surface area of BHA is 54m 2 /g, and the maximum adsorption capacity for As 3+ is 217mg/g.

Chai et al.(2020) used glutaraldehyde as a crosslinking agent to cross link the NC and PEI oxidized by
Tempo to obtain NC-PEI/GA nanoparticles. At pH 3, the adsorption equilibrium can be reached within 10min, and the maximum adsorption capacity for As 5+ is 255.19mg/g. The adsorbent could be easily regenerated and reused via NaOH treatment, and the regeneration e ciency remained relatively steady even after eight cycles. Lignin is conjugated polymer with a high consistency of aromatic groups that can interact with cations, moreover, the oxygen-containing groups (the hydroxyl, methoxy, and phenolic groups) are potential interaction sites of lignin for water puri cation (Naseer et al. 2019). Sirvio et al.(2020) used DES (sulfamic acid and urea) to treat lignin-rich groundwood pulp and sawdust to obtain sulfation of sulfated wood nano bers (SWNFs) and sulfated sawdust nano bers (SSDNFs). As A comparison, lignin-free bleached cellulose bers were treated with the same method to obtain sulfated cellulose nano bers (SCNFs). The adsorption capacity of three kinds of nano bers on Cu 2+ and Pb 2+ was investigated. The maximum adsorption capacities of SCNFs for Cu 2+ and Pb 2+ were 2.2 and 1.1mmol/g, respectively. However, due to the presence of lignin, the adsorption capacity of SWNFs and SSDNFs was increased to a certain extent.

Adsorbents for two heavy ions
The maximum adsorption capacity of SWNFs for Cu 2+ and Pb 2+ is 2.5 and 1.6mmol/g, and the maximum adsorption capacity of SSDNFs for the two are 2.2 and 1.4mmol/g, respectively.
Succinic anhydride is an active agent containing one anhydride group, which can react with the hydroxyl groups of cellulose. Yu et al.(2013) obtained CNCs by hydrolyzing cotton with sulfuric acid, subsequently, CNCs were modi ed with succinic anhydride, and the product SCNCs were then converted into the sodic form (NaSCNCs). The maximum adsorption capacities of NasCNCs for Pb 2+ and Cd 2+ were 465.1 mg/g and 344.8 mg/g, which were higher than those of SCNCs 367.6 mg/g and 259.7 mg/g. NaSCNCs could be e ciently regenerated with a mild saturated NaCl solution with no loss of capacity after two recycles.
The adsorption mechanism for SCNCs was a complexation process, while ion-exchange was the principal mechanism for the removal of heavy metal ions from NaSCNCs. Therefore, it was essential to convert the carboxyl groups into carboxylates for this adsorbent containing carboxyl groups. NPA31 aerogel were obtained. The maximum speci c surface area of the aerogel is 42.5m 2 /g, and it has good shape recover capacity (Fig. 10). The electrostatic attraction and cation exchange between carboxyl and amino groups on Cu 2+ and Pb 2+ are the reasons for the high adsorption capacity of aerogels. The maximum adsorption capacities of NPA22 for Cu 2+ and Pb 2+ are 175.44mg/g and 357.44mg/g, respectively. After three adsorption/desorption cycles, the adsorption capacity of NPAs still maintained more than 90%.
Yao et al.(2016) used TEMPO-mediated oxidized kraft eucalyptus pulps obtain CNFs hydrogel with carboxylate content (0.65mmol/g). Then Dialdehyde CNFs hydrogel (DACs) was obtained by oxidizing CNFs with NaIO4. CNFs and DACs hydrogels were freeze-dried to obtain porous CNFs and DACs aerogels (Fig. 11). The speci c surface area for CNFs and DACs aerogels was 185.1m 2 /g and 134.4m 2 /g. In the studied concentration range (0.6-1.0 mmol/L), a removal capacity of 0.75 mmol/g for Pb 2+ and 0.58 mmol/g for Cu 2+ was obtained. According to the pseudo-second-order, the theoretical maximum sorption capacity was 38.36 mg/g and 157.73 mg/g for Cu 2+ and Pb 2+ , respectively.

Derami et al. (2019) incorporated of PDA particles into Gluconacetobacter hansenii broth under aerobic
and static conditions. PDA particles were grown in situ on the BNC membrane (Fig. 12). The catecholamine group on the surface of PDA particles has a strong a nity to lead ions. Adsorption of PDA/BNC was tested in a mixed solution of Pb 2+ , Cd 2+ . The PDA/BNC membrane removed 5.3 g of Pb 2+ from water per square meter of the membrane area. The lowest performance was observed for Cd 2+ with 2.1 g of ions removed per square meter of the membrane area. After regeneration with 0.1M sodium citrate solution, the regenerated membranes exhibited excellent contaminant removal e ciency even after 10 cycles of ltration (about 90% of the initial performance retained).  (Fig. 13). CA can be dissolved in a variety of solvents, so it can become nano bers through electrospun. The adsorption mechanism of the membrane is coordination/chelation between metal ions and amidoxime/hydroxyl groups. As an emerging materials platform for heavy metal ions removal, nanocellulose based adsorbents exhibit many advantages, but challenges are to be properly resolved in the future (Fig. 16). For instance, given strong negative ion groups, nanocellulose based adsorbents have a high electrostatic attraction ability, offering desired adsorbing sites for heavy metal ions. However, hydrophilic negative ion groups will decrease the hydrophobic ability and stability of the adsorbents in water. Possible solutions are, but not limited to, modifying the nanocellulose or integrating assemble processes to enhance the crosslinking behaviors among nanocellulose and the other furnishes.

Adsorbents for three heavy ions
Surface modi cations of nanocellulose, such as, oxidation, phosphorylation and amination, could promote the adsorbing sites of nanocellulose based adsorbents, but likely result in the rapid decrease of their desorption ability. To increase the recycling times of adsorbents, more acid washing is required, which will also cause harm to the environment. To obtain the high desorption capacities, we can rstly try to nd some groups which have different binding a nity for heavy metal ions, then assemble these groups into the multilayer 2D/3D nanocellulose based adsorbent aerogels or composites, nally prepare the high adsorption/desorption ability adsorbents.
To meet the requirements of recycling, selective adsorptions or desorption for different heavy metal ions, precisely controlled assembles of nanocellulose based adsorbents with tailorable hydrophilicities and mechanical properties are desired for the industrial applications. For example, to optimize the assembling process, crosslinking agent and functional additive kinds in adsorbent networks have to be properly selected to modulate the suitable porous structures and adsorption capacities. In addition, computational modeling and advanced in situ characterizations would be bene cial, which, in turn, will further guide researchers to the rational design of cellulose-based adsorbents for heavy metal ions removal. Availability of data and material: Not applicable.
Code availability: Not applicable.

Compliance with Ethical Standards
Con ict of Interest: The authors declare that they have no con ict of interest.
Ethical approval: This article does not contain any studies with human participants or animals performed by any of the authors.      Schematic of the self-assembly process for the production of BHH and BHA. (a-b) TEMPO-and mechanical-exfoliation to produce TOCNF; (c-d) NaOH-and mechanical-exfoliation to produce PDChNF; (e) electrostatic force induced self-assembly gelation of nano bers with ionic interactions between TOCNF and PDChNF; (f) digital images illustrate the ultra-light BHA was captured by a marker pen due to static electricity; (g) scanning electron microscopy (SEM) image of BHA exhibits a highly porous structure(Zhang et al. 2019).