A quick review of the applications of nano crystalline cellulose in wastewater treatment

Cellulose, as the most abundant natural biopolymers in the world, has attracted much attention in recent years, as concerns have been raised on environmental issues associated with the use of petroleum related products. Nano crystalline cellulose (NCC) is a special type of cellulose that is derived from renewable and sustainable resources such as cotton and wood. NCC has a large specific surface area with plentiful hydroxyl and anionic sulfate ester groups, which make NCC a perfect substrate for preparing composite absorbing and/catalytic materials for wastewater treatment. This paper reviews recent research on the applications of NCC in wastewater


INTRODUCTION
With the rapid development of the global economy, water pollution is becoming increasingly severe.Water pollution is not only hindering the further development of the economy, but is also affecting people's health and survival. 1The United Nations have declared that a water crisis would be the first global crisis in the 21 st century. 2 Effective control of water pollution is an urgent matter.Biotechnology, adsorption, ion-exchange, electrochemical, and chemical deposition are common methods to treat wastewater. 3Recently, there is much attention on utilizing renewable, biodegradable and sustainable materials for wastewater treatment.Natural polymers such as cellulose and starch are good examples. 4Among these, nano crystalline cellulose (NCC) has been found to be cost-effective in water treatment. 5ellulose, a colorless, odorless, thermally and chemically stable matter, is the most abundant natural biopolymer in the world. 6Cellulose molecules are formed by D-pyran glucose connecting through the β-1, 4 glycosidic bonds (Fig. 1). 7Anhydroglucose is the structural unit of cellulose.There are three hydroxyl groups in anhydroglucose, wherein esterification and oxidation reactions can occure. 8g. 1 The structure of cellulose molecule NCC refers to nano crystalline cellulose with one dimensional space existing on the nanoscale (1-100 nm), prepared from cellulose by acid hydrolysis or high strength ultrasonic processing. 9,10Wang et al 11 prepared NCC by an alkaline method, which can prevent excessive acid hydrolization of NCC.NCC has the key properties of cellulose, but it also has a large specific surface area with plenty of surface hydroxyl and anionic sulfate ester groups for physical and chemical reactions.This paper reviews recent research regarding NCC's applications in wastewater treatment.

Treatment of dyeing wastewater
Dyes are widely used in leather, paper making, textile, paint, etc. 12 Dyeing wastewater is one of the more noticeable industrial pollution existing in the environment.Dye wastewater causes serious pollution to the environment due to its complex chemical composition, high coloring effect, high organic concentration and being difficult to degrade.Low decoloring rate is a big challenge for dyeing wastewater treatment. 13Common methods for dyeing wastewater treatment are electrolysis, adsorption, flocculation, chemical oxidation and biological treatment. 14Among them, the adsorption method is widely used, as it does not produce secondary pollution, uses less energy, and has a high decoloring efficiency. 15i 16 treated the acid red GR dyeing wastewater with dialdehyde-NCC (DANC) and amino-NCC (ANCC), which was modified with NaIO 4 and ethylenediamine.The results showed that NCC and DANC had little effect in removing the dye from the wastewater.However, ANCC showed high adsorption potential for acid red GR (the adsorption capacity was 79.72%), and its adsorption capacity for Congo red was even higher.
Composite materials, which combine NCC with materials of strong adsorption property, are good candidates for wastewater treatment.As an organic polymer flocculant, polyvinylamine (PVAm) can achieve good flocculation at a low dosage (2%).Jin et al 17 prepared a novel nanogel with a diameter ranging between 200 to 300 nm with NCC and amphoteric PVAm (Fig. 2).The maximum adsorption capacity of nanogel for congo red 4BS, acid red GR and reactive light K-4G were 869.1 mg/g, 1469.7 mg/g and 1250.9 mg/g, respectively.
Polyhedral oligomeric silsesquioxane (POSS) is a nano structure with inorganic and organic hybrid molecules.Compositing with nanopolymer, POSS can improve the comprehensive performance of composites.Xie et al 18 used multi-N-methylol-POSS (R-POSS) prepared with NCC hybridized POSS as a novel bio-adsorbent to treat dyeing wastewater.The results showed that R-POSS can effectively reduce the concentration of reactive dyes from the wastewater.The maximum adsorption capabilities of Reactive Blue B-RN, Reactive Yellow and B-4RFN were 14.40 mg/g, 16.61 mg/g, 11.28 mg/g, respectively.Mohammed et al 19 fabricated NCC-ALG hydrogel beads by mixing the NCCs and alginate solution (ALG).Alginate is a biodegradable polymer material widely used in printing and dyeing wastewater treatment.The NCC-ALG hydrogel was used in an immobilization column to treat methylene blue.The experimental result demonstrated that the maximum adsorption capacity was 255.5 mg/g.

Treatment of wastewater containing heavy metal ions
The rapid development of the industries leads to increasing heavy metal ion water pollution.Heavy metals in polluted water body can transport through the food chain into the human body and accumulate there to cause serious health problems.Cu 2+ , Pb 2+ , Cd 2+ , Cr(Ⅵ), and Co(Ⅱ) are some of the common heavy metal ions existing in wastewater.Removal of these heavy metal ions from wastewater often adopts chemical precipitation, coagulation-flocculation, filtration, ion exchange, adsorption and biological membrane methods. 20Among them, adsorption is a frequently used method to remove heavy metal ions due to its cost-effectiveness, low energy consumption and absence of harmful by-products. 21iu et al 22 found that after loaded the phosphate groups on the surface of NCC by enzymatic treatment, the efficiency of the modified NCC for removing Cu 2+ , Fe 3+ from mirror manufacturing wastewater reached up to 99%.Hokkanen et al 23 noted that the maximum adsorption of the NCC modified by succinate anhydride for Zn 2+ , Ni 2+ , Co 2+ , Cu 2+ , Cd 2+ were 1.500 mmol/g, 0.716 mmol/g, 1.304 mmol/g, 1.879 mmol/g, 1.954 mmol/g, respectively.
Itaconic acid copolymerizing with various vinyl monomers can be used as a dyeing wastewater absorbent. 24Anirudhan et al 25 synthesized P(MB-IA)-g-MNCC, a cellulose based adsorbent, via itaconic acid polymerization onto magnetic NCC, with a maximum removal capability of 262.27 mg/g for Cd 2+ .Furthermore, Anirudhan et al 26 used 2-mercaptobenzamide to treat P(MB-IA)-g-MNCC (Fig. 3) to synthesize a novel agent for wastewater treatment.The novel agent can completely remove Hg(Ⅱ) from wastewater at a dosage of 2.0 g/L or lower.
Hybrid materials consisting of inorganic nanomaterials and NCC can inherit excellent characteristics from the sources, and are widely used for adsorption treatment of wastewater. 27Anirudhan 28 fixed carboxyl functional groups on the cellulose/nano bentonite mixture as an adsorbent to remove Co(Ⅱ) in the wastewater of nuclear power plants.The results showed that the adsorbent can effectively remove Co (Ⅱ) (greater than 90%) when the adsorbent dosage is very low (0.25 mg/L).Hokkanen 29 synthetized a NCC based calcium hydroxylapatite to remove Cd(Ⅱ)from wastewater, and the maximum removal was found to be 2.208 mg/g.Donia et al 30 prepared magnetic NCC by mixing Fe (Ⅱ)/Fe (Ⅲ) with cellulose as adsorbent which had maximum adsorption capacities of 2.0 mmol/g, 1.5 mmol/g and 1.2 mmol/g for Hg(Ⅱ), Cu(Ⅱ) and Ag(Ⅱ), respectively.

Treatment of domestic wastewater
Chemical oxygen demand (COD) of domestic wastewater can cause eutrophication of receiving water bodies, destroying the ecological balance of the water bodies.Some pollutants in sewage can lead to resistant genes in microorganisms.Through energy cycle, resistant genes can develop in human bodies, resulting in resistance to natural immune responses in the human body. 31uopajarvi et al 32 prepared double carboxylic NCC (DCC) via periodate and hypochlorite oxidation ( Fig. 4 ) .DCC effectively removed COD and reduced the turbidity of municipal wastewater.the removal rate of COD was about 63%, slightly higher than the removal rate of 60% using a electrochemical process. 33

APPLICATIONS OF NCC-BASED CATALYTIC MATERIALS IN WASTEWATER TREATMENT
Cellulose-based composites have excellent mechanical properties, therefore, they are often used in thermoplastic composite materials as reinforcers. 37,38The hydroxyl groups and porosity on the surface of NCC provides an excellent mechanical support and templates for catalysts 39 , ensuring the dispersion of inorganic nanoparticles and stability of catalysts, simultaneously.Phenol, an ingredient used in paints, inks and detergents, is a volatile organic pollutant existing in water.Due to its volatility and low solubility, most of the phenol eventually finds its way into the air which has a negative impact on the human body and the environment. 41ynergy between NCC and titanium dioxide can effectively improve the decomposition of the phenol under light irradiation, Mohamed et al 42 prepared an environmentally sound nanometer photocatalytic film via phase transformation using regenerated cellulose from waste newspapers and TiO 2 nanorods.The film could degrade phenol in water solution under the ultraviolet and visible light irradiation.Siddiqa et al 43 prepared a TiO 2 /NCC composite with high porosity, low density, high hydrophobic and reproducibility.It can effectively remove organic pollutants and oil pollutants in water.The maximum degradation capacities of the TiO 2 /NCC composite towards these two pollutants were 128.3 wt% and 1885 wt%, respectively.Nanogold particles, which is fully dispersed in NCC has an oustanding catalytic capability.Wei et al 44 synthesized cellulose/nanogold compounds to catalyze sodium borohydride for the degradation of nitro phenol in water, and found that the maximum rate constant and crossover frequency was 0.0147 s -1 and 641 h -1 respectively.The composite was environmentally friendly, easy to prepare, low in cost, and had broad prospects in the industrial and medical fields.

APPLICATIONS OF NCC-BASED MEMBRANE IN WASTEWATER TREATMENT
Membrane separation technology is also good method for water treatment due to its high efficiency, low energy consumption, less space requirement, good operability and having no secondary pollution.Membrane technologies have developed rapidly in recent years.Membrane technologies have been widely used in the petroleum, metallurgy, lighting, food and pharmaceutical industries, as well as in desalination, environmental protection and other fields.Aside from its use in wastewater treatment as an adsorbent, NCC can also be used to prepare membrane materials, or to improve the properties of membrane materials.The electrostatic spinning method is a common technology for forming membranes.Zhao et al 45 prepared cellulose acetate membranes, NCC acetate membranes and porous NCC acetate membranes using the electrostatic spinning method with specific surface areas of 0.0024 m 2 /g, 3.76 m 2 /g, 68.74 m 2 /g, respectively.The removal rates of RhB from wastewater were 8.9%, 32.0%, 75.2%, respectively, for the three types of membranes.As a natural polymer flocculant with characteristics of being hydrophilic, nontoxic, biodegradable and achieving good flocculation, chitosan can help to quickly purify water. 46Karim et al 47 prepared a biomembrane by cladding NCC crystals with micropores (diameter from 10 to 13 nm) on chitosan as functional groups and used it for water treatment.The maximum removal rate of Victoria Blue 2B, Methyl Violet 2B, Rh6G reached 98%, 84% and 98%, respectively.
Xu et al 48 prepared polyvinyl alcohol/NCCs/silver (PVA/NCCs/AgNPs) nanocomposite films using a simple method.The tensile strength of the PVA film increased from 57.04 MPa to 81.21 MPa after NCCs/AgNPs was added, and the antimicrobial properties of the membrane improved dramatically.The composite membrane's resistance to gram-negative escherichia coli and gram-positive staphylococcus aureus reached 96.9% and 96.9%, respectively.Mautner et al 49 prepared NCC paper (commonly known as nanopapers) using BC, wood based NCC and TEMPO oxidation NCC as raw materials.The nanopaper had a cut-off molecular weight for organic pollutants in wastewater ranged from 6 to 25 kDa, similar to those for commercial UF membranes.

CONCLUSIONS
As a renewable and biodegradable natural material, nano crystalline cellulose (NCC) has a large specific surface area with plenty of hydroxyl groups and anionic sulfate ester groups on the surface, which make it a perfect substrate for preparing composite materials for wastewater treatment.Many studies in the literature have shown that NCC-based composite materials are highly effective and efficient reagents in treating wastewater, in particular for dyeing wastewater, domestic wastewater and wastewater from metallurgy industries.

Fig. 3
Fig. 3 Synthesis of P(MB-IA)-g-MNCC (copy right, Elsevier, reproduced with permission) 26 Moreover, DCC can remain stable in suspension for a long time in the wastewater.Application of DCC can significantly reduce the consumption of chemical reagents.Rathod et al 34 prepared NCC with seaweed as raw material to treat wastewater containing tetracycline hydrochloride.The maximum adsorption achieved was 7.73 mg/g under a 2hour treatment.Anirudhan et al 35 synthesized polyester with methacrylic acid-co-vinyl sulfonic acid grafted to magnetite/NCC material (P(MAA-co-VSA)-g-MNCC)).The polyester can effectively absorbed immunoglobulin (lgG) antibodies from wastewater.The process of 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO) oxidation can expose more negatively charged groups of cellulose without changing the structure of the fiber characteristics.Carlsson et al 36 used NCC oxidized by TEMPO to treat urban wastewater containing aspirin.The results showed that NCC with a surface in rich of carboxyl groups can significantly accelerate the degradation of aspirin.