Recent Advances on Chemically Functionalized Cellulose-Based Materials for Arsenic Removal in Wastewater: A Review

Abstract

Clean water is very important for the good health of society. In South Africa, it is estimated that people need 20 to 50 litres of safe water daily for basic hygiene, drinking, and cooking. In recent times, water bodies have harboured harmful pollutants, including oil, heavy metal ions, and dyes. As a result, this has become a major global concern. Societies with limited clean water are often forced to utilise contaminated water or buy filtered water, which might be a problem for poor residents. The health consequences that are related to contaminated water include Guinea worm disease, dysentery, cholera, etc. The side effects associated with the utilisation of unclean water are gastrointestinal diseases such as cramps, vomiting, and diarrhoea. The wastewater disposed of by chemical industries contains toxic elements such as arsenic. Wastewater that is released directly without treatment causes serious damage to the environment. Chronic arsenic poisoning can lead to keratinisation of the skin and even cancer. Cellulose biomass materials have the potential to become the greatest bio-based materials used in wastewater treatment applications. There are two major reasons that validate this statement: firstly, cellulose is a low-cost material that is abundant in nature, and, secondly, cellulose is an environmentally friendly material. However, these are not the only reasons that validate cellulose as a good candidate for wastewater treatment applications. Cellulose has a unique structure a large surface area, good mechanical properties and is degradable, renewable, and biocompatible. Cellulose also has an abundance of hydroxyl groups on its surface. These hydroxyl functional groups allow cellulose to be chemically modified in various ways, which results in the fabrication of nanocomposites with tunable characteristics. Since arsenic pollution has become a serious global concern, this review uniquely provides a broad discussion of the work that has been accomplished recently on the fabrication of functionalised cellulose-based materials designed specifically for the removal of arsenic heavy metal species from wastewater treatment facilities. Furthermore, the functionalised cellulose materials’ arsenic adsorption capacities are also discussed. These adsorption capacities can reach up to a maximum of 350 mg/g, depending on the system used. Factors such as pH and temperature are discussed in relation to the adsorption of arsenic in wastewater. The removal of As(V) was found to be effective in the pH range of 3.0–8.8, with a removal efficiency of 95%. Moreover, the removal efficiency of As(III) was reported to be effective in the pH range of 6–9. However, the effective pH range also depends on the system used. The selective extraction of cellulose from various sources is also discussed in order to verify the percentage of cellulose in each source. Future work should be focused on how the chemical modification of cellulose affects the toxicity, efficiency, selectivity, and mechanical stability of cellulose materials. The use of cheaper and environmentally friendly chemicals during cellulose functionalisation should be considered.

1. Introduction

In the current century, the key issue is the development and/or utilisation of renewable and biodegradable materials in order to decrease the burden of the environmental crisis [1]. The purpose is to use these renewable materials in various applications to counteract the demolition of the planet by industry. Due to its eco-friendliness, the cellulose extracted from various natural fibres provides solutions for the current environmental problems caused by the non-biodegradable materials that are used in different applications [2]. Cellulose is normally found in trees and plants, and it is inserted in lignins and hemicellulose. It is one of the most common natural organic materials, making up one-third of vegetables globally [3]. The cellulose structure is held up by a hydrogen bond, due to the multiple hydroxyl groups on its structure. The advantages of cellulose include its (i) biodegradability, (ii) low cost (iii), abundance (iv), sustainability, and (v) high aspect ratio [4]. Cellulose-based materials heavily contribute to the field of nanotechnology, including applications in medicine [5,6,7,8], food packaging [9,10,11,12,13,14,15,16], water treatment, which includes the removal of dyes and heavy metal ions from wastewater [17,18,19] and supercapacitor applications [20,21,22,23,24]. Figure 1 illustrates the applications of cellulose and cellulose-based materials.

Amongst all the applications of cellulose-based materials, the one that seems to currently require a lot of attention is wastewater treatment. Various contaminants such as detergents, heavy metal ions, food additives, dyes, pesticides, biomolecules, and polycyclic aromatic hydrocarbons are responsible for contaminating water [25]. Amongst all the treatments utilised for the removal of contaminants from water, cellulose based membranes are preferred due to their chemical inertness, hydrophilic surface, high strength [26,27,28,29], and ability to undergo surface modification; as a result, they ease the selective adsorption of the pollutant. Besides the primary original cellulose, cellulose may be divided into microfibrillated cellulose (MFC) and nanofibrillated cellulose (NFC). The diameter of MFC is normally in the region of 10–100 nm, and its length is roughly between 0.5 and 10 mm, while nanofibrillated cellulose has a diameter in the range of 4–20 nm with a length of 500–2000 nm [30]. It is documented in pieces of the literature that the functionalisation of cellulose takes place through the hydroxyl groups, and, once cellulose is functionalised, its applications are extended [30,31,32,33]. There are two categories for the functionalisation of cellulose viz, chemical and physical functionalisation [30]. The removal of unwanted material in an aqueous medium is normally accomplished with chemically modified cellulose. Various chemicals have been utilised for the chemical modification of cellulose, i.e., aminoethanethiol, diethylenetriamine, carboxymethylation, and 2,2,6,6-tetramethylpiperidine-1-oxylradical (TEMPO) [34,35,36,37,38,39,40,41,42,43]. This current review discusses the effect of various chemical functionalisations on cellulose for the removal of arsenic materials in wastewater. Since cellulose is being functionalised, the morphology of unfunctionalised and functionalised cellulose is also discussed in relation to water-purification efficiency. The sources of cellulose extraction are also discussed, since it is important to understand the percentage of cellulose in their original sources.

The development of the metallurgical and chemical industries as well as the exploitation of ores has led to an increased discharge of arsenic and its compounds into our mainstream water outlets [44]. The presence of copious amounts of arsenic in water bodies, especially wastewater, is due to numerous factors that include ore mining, fossil fuel combustion, the smelting of non-ferrous metals, the preparation of drugs containing arsenic, volcanic eruptions, and the usage of pesticides [44]. Arsenic is a toxic and carcinogenic element that exists in various forms in the environment [45]. The two most common forms of arsenic found in contaminated water are arsenate (As(V)) and arsenite (As(III)) [45]. As(III) is more resistant and has higher health risks upon exposure [45]. Arsenic pollution has become a huge problem for many countries [44]. The discharge of arsenic-containing water into the main water sources can inflict severely harmful effects on human health and may also cause an environmental imbalance [44]. Arsenic can alter major metabolic processes in cells and, thus, cause cell death [44]. Chronic arsenic poisoning can lead to keratinisation of the skin and even cancer [44]. Therefore, it is important to control arsenic concentrations in water environments to ensure safe drinking water [44]. The treatment of arsenic-containing drinking water is one of the best ways of ensuring that arsenic concentrations are kept in check in our drinking water [44]. In 2011, the World Health Organization (WHO) stipulated that the arsenic concentration in drinking water should not exceed 10 ppb [45].

Cellulose is an ideal low-cost source for the extraction of water purification materials. This is because cellulose is the most abundant and renewable natural polymer in the world [45,46,47]. Cellulose also contains numerous hydroxyl functional groups on its surface. These functional groups enable the modification of cellulose into nanocomposites, with tunable properties that may involve the removal of arsenic from wastewater [48]. Modified cellulose-based materials have a history of being used to remove toxic metal ions from water. Succinic anhydride-modified nanocellulose was used to effectively remove heavy metal ions, e.g., Cr(VI) and Pd(II), and dyes from water [49,50]. A carbon aerogel prepared from microcrystalline cellulose was also used to effectively remove Cr(V) and Pb(II) ions [51]. Carboxylated cellulose nanofibre (CNF), an oxidised form of nanocellulose, was discovered to possess an excellent adsorption capacity towards Uranium ions (UO₂²⁺) [52,53], Pb(II) ions [54], and Cd(II) ions [55]. Ferric hydroxide-coated CNFs also proved to be excellent candidates for the removal of phosphate from wastewater [56]. Furthermore, three-dimensional porous cellulose was discovered to be an effective adsorbent with a high capacity for the removal of cationic dyes [57]. It was also demonstrated that nanocomposites based on regenerated microfibrillated cellulose decorated with zinc oxide nanocrystals had a very high maximum capacity for the removal of As(V) [58].

2. Selective Studies on the Extraction of Cellulose from Various Sources

Cellulose fibres may be extracted into nano or micro cellulose by chemical and mechanical techniques [59,60,61,62,63,64,65,66,67,68]. There are different sources of cellulose fibres, which include wood, non-woody fibres, tunicates, agro-wastes or biomass, algae, etc. [69,70,71,72,73,74,75,76]. Figure 2 illustrates selective sources of cellulose fibres. Generally, the extraction route of cellulose consists of pre-hydrolysis, pulping, and bleaching (Figure 3). Generally, it is known that pre-hydrolysis is utilized to open the matrix with an acid or alkali material. Chemical pulping is the process whereby the lignin and hemicellulose are degraded into small as well as water-soluble molecules, which can be easily removed from the cellulose without depolymerising the fibres. Morán et al. [77] extracted cellulose from sisal fibres, with sisal fibre consisting of cellulose (50–74%), hemicellulose (10–14%), lignin (8–11%), pectin (1%), and wax (2%). As a standard in most extractions, when the fibre is received, it is washed with distilled water several times and dried in an oven. The pre-treatment of the sisal fibre was accomplished with sodium hydroxide in a solution of ethanol. The bleaching of the fibre was accomplished with sodium chlorite (NaClO₂). The nanofibres were produced by acid hydrolysis using sulphuric acid (H₂SO₄). After acid hydrolysis, the obtained cellulose nanofibres were reported to be in the range of 30.9 ± 12.5 nm.

The cellulose was extracted from Calotropis procera fibre (CPF) [78]. The CPF was found to have 64.1% cellulose, 9.7% lignin, and 19.5% hemicellulose. As reported by previous studies, the extraction of cellulose in this case also took place through pre-hydrolysis, pulping, and bleaching. The cellulose nanocrystals were also fabricated by acid hydrolysis with 63 wt% sulphuric acid (H₂SO₄). TEM revealed uniform needle-like CNCs with a length of 250 nm and a diameter of 12 nm. The extraction of microcrystalline cellulose from Washington fibre was reported by Azum and co-workers [79]. The production yield of the WMCC was found to consist of 82.5% α-Cellulose, 5.2% hemicellulose, and 2.1% lignin. The length and diameter reported in this study were in the ranges of 300–500 μm and 60–90 μm, respectively. The acid hydrolysis in this study was found to be responsible for rupturing the cellulose structure into various individual fibrils of smaller size. Song et al. [80] proved the effectiveness of microwave technology with regard to the extraction of lignin. The aim of that study was to make certain that the lignin was completely removed by microwave technology, and then nanocellulose extraction was further completed by acid hydrolysis using an ultrasonic device. Microwave technology was used due to its various advantages such as energy conservation, cleanness, speediness, and high efficiency [80,81]. There was a decrease in the content of lignin and hemicellulose after microwave treatment, which obviously led to an increase in the content of the cellulose. For example, there was 69.28% lignin extracted from the original kenaf fibre by microwave treatment, while there was 24.49% lignin content extracted by bleaching [80]. The extracted cellulose by microwave treatment and acid hydrolysis revealed a needle-like type of morphology with a length of around 300 to 600 nm, while the diameter was reported to be in the range of 10–60 nm. Luzi et al. [82] reported on the extraction of cellulose nanocrystals from pristine and carded hemp fibres. Fibre carding is the process whereby pristine fibre undergoes a mechanical process to disentangle and clean the fibre for further processing. During the process of carding, there is a separation of tufts into fibres, an alignment of the fibres, and the removal of any impurities. The aim of the study was to investigate the effect of the carding process in relation to the pre-treatment competency and the percentage yield of cellulose nanocrystals. The extraction method followed the well-known route that involves chemical treatment with the gradual removal of lignin and subsequent acid hydrolysis for the formation of cellulose nanocrystals. In this study, both chemical and enzymatic treatments of non-carded and carded hemp fibres were reported. Based on the visual images, the neat hemp fibres were found to be unorganised (Figure 4a), while, on the contrary, the carded hemp fibres were revealed to be well-aligned, with no dust or impurities (Figure 4b). A similar observation was seen in the FESEM images, whereby the non-carded hemp fibres showed unaligned fibres (Figure 4c), whereas the carded hemp fibres were well-separated and aligned without any impurities (Figure 4d). The fibre diameters for the uncarded and carded hemp were reported as 21 and 19 μm, respectively.

The chemically extracted cellulose from both the neat and carded fibres was reported to be acicular, which symbolised the nanocrystals or nanowhiskers. Furthermore, it was reported that there was no difference in the shape or dimension between the extracted cellulose from the two fibres (i.e., the neat and carded hemp fibres). A similar observation was reported for the enzymatic treatments of the fibres, whereby both fibres revealed nanocellulose with the same shape and dimension. XRD results revealed that the carding process produced a high conversion of cellulose I. The effect of the reaction parameters, such as temperature, catalyst, time, and alkali treatment, were studied to find the optimum reaction conditions for cellulose extraction from agricultural waste [83]. Cellulose was extracted from rice husk (RH) with a montmorillonite K-10/LiOH solution followed by peroxide bleaching. The concentration range of the catalyst/alkali system varied from 0.1 g/4% to 0.5/12%. The maleic acid concentration was in the range of 5–20%, while the investigated temperatures were kept in the range of 50–90 °C. The investigated time range used in this study varied between 3 to 7 h. The optimum conditions were found to be 6 h, 80 °C, and 20% maleic acid as well as 10% of LiOH (in water). The percentage yield of cellulose was reported to be 68%. The effect of alkaline treatment on the extraction of cellulose from various agricultural by products such as rice straw, Phulae pineapple leaves, corncob, and Phulae pineapple peels was reported in a study [84]. The extracted cellulose from agricultural waste was compared with commercial cellulose. The treatment of the fibres, which included both the alkaline and bleaching processes, was performed by employing 1.4% of acidified sodium chlorite and 5% potassium hydroxide. The percentage yield of cellulose from various agricultural wastes, i.e., RS, CC, PL, and PP, was reported to be 32.26%, 38.18%, 16.60%, and 9.05%, respectively. The extraction of cellulose from bamboo pulp by mechanochemical activation and phosphotungstic acid hydrolysis was reported by Lu et al. [85]. The mechanochemical method is preferred because it is environmentally friendly, without the utilisation of solvents. The concentration of phosphotungstic acid and its reaction were found to play a key role in the yield of cellulose nanocrystals. The yield percentage of the cellulose nanocrystals increased with the reaction time. The yield percentage was found to be high when the reaction time was in the range of 4.5–5 h with the phosphotungstic acid in the concentration range of 10–15%. The lower yield percentage of the cellulose nanocrystals at lower acid concentrations is associated with fewer acid sites to break up the β-1,4-glycosidic bonds of the cellulose fibres. The mechanochemical process was also found to play a critical role in the production of cellulose, with the yield of the cellulose nanocrystals increasing between 1.5 and 2.5 h at various instances in the reactions. The mechanochemical process, by utilising the ball milling process, is able to promote a lot of stress, which was transferred into the cellulose, enhancing the cleavage of the chemical bonds in the cellulose and reducing the size of the cellulose. In summary, the optimum conditions to obtain the highest cellulose nanocrystals (CNC) are: a 13.5% concentration of phosphotungstic acid, 4.7 h reaction time, and ball time of 2.2 h, which yielded 88.4% CNC. In addition to the well-known sources of cellulose such as kenaf, bamboo, sisal, jute, sugarcane bagasse, hemp, etc., there are emerging sources of cellulose such as the halophytes. Singh et al. [86] utilised halophytes (viz, Tamarix aphylla, Juncus rigidus, and Thespesia populnea) as an emerging source of cellulose. Halophytes are sources of cellulose that are salt-loving plants. Cellulose extraction was completed from the source using various concentrations of acid, alkaline, and bleaching completed at different temperatures. The extraction of cellulose from the plant source was completed by the following treatment: (i) sodium chlorite, (ii) sodium hydroxide, (iii), and (iv) hydrogen peroxide. The extracted samples were further fractionated into α-cellulose and β-cellulose. Generally, the halophytes produced crude cellulose in the range of 39–48%. The key findings reported by the study were, for example, that T. aphylla needs a low concentration of sodium chlorite (NaClO2), hydrochloric (HCl) acid, and more heating time in order to remove the lignin and hemicellulose when compared with other halophytes. Furthermore, T.aphylla recorded the highest extraction of α-cellulose with the highest percentage crystallinity index, i.e., CI = 61.8%, whereas the lowest crystallinity was recorded to be 48%, for the extraction from T. populnea. In summary, the extraction of cellulose and the fabrication of cellulose nanocrystals (CNCs) depend on the source(s) of the cellulose, extraction method, reaction time, and type of acid utilised.

Table 1. Selective studies on the extraction of cellulose from various sources.

Source of Cellulose	Type of Cellulose	Cellulose Extraction Method	Extraction Conditions	Results Obtained	Refs.
Date pits (DP)	Cellulose nanocrystals (CNCs)	Two methods were used: mechanical stirrer method (CNCs1) and soxhlet apparatus method (CNCs2)	- CNCs1: Delignification (6% NaOH, 4 h, 70 °C), bleaching (6% NaClO, 2 h, 70 °C), acid hydrolysis (64% H2SO4, 1 h, 25 °C) - CNCs2: Dewaxing (96% n-hexane, 8 h), delignification (2% NaOH, 3 h), bleaching (1.7% NaClO, 3 h), acid hydrolysis (64% H2SO4, 1 h, 25 °C)	- Acid hydrolysis formed CNCs with a cellulose microfibril packed structure. - CNCs1 and CNCs2 had a crystallinity index of 69.99% and 67.79%. - A high yield of CNCs (≥10%) was obtained using both methods.	[87]
Robinia pseudoacacia seed fibres	Cellulose fibres immobilised onto chitosan beads	Cellulose was extracted using methods by Gao et al. [25] and Almutairi et al. [32], with slight modifications; extracted cellulose was filtered and blended with chitosan at variable composition ratios	- Delignification: NaOH solution (5 wt%, LR = 1:50 (w/v)), 2 h, 80 °C - Further delignification and bleaching: Acetic acid and hydrogen peroxide solution (v/v = 1:1), 2 h, 90 °C	- Combining the cellulose with chitosan improved the removal of methylene blue from aqueous media. - Maximum uptake of methylene blue was 55 mg/g for cellulose–chitosan beads versus 35 mg/g for chitosan beads at 20 °C.	[88]
Bean forage	Cellulose nanocrystals	Two response surface superimposition methodologies were used with a rotatable composite central design (CCD) for both	- Alkaline and bleaching treatments: 1.75% w/w NaOH for 8.5 h with a bleaching time of 1 h at 70 °C - Acid treatments: 55% H2SO4, 30 min, 40 °C	- Percentages crystallinities obtained with the optimal conditions were 40.39% after the alkaline treatment, 42.02% after the bleaching treatment, and 49.36% after the acid treatment.	[89]
Banana stem	Cellulose pulp	Alkaline pulping using sodium hydroxide (NaOH)	17.7% NaOH and 10% EDTA, pulping of banana stem at 100 ± 5 °C for 30 min	- Pulping with EDTA under optimum conditions increased pulp yield, lignin removal, and cellulose content by approximately 18.5%, 1.1%, and 0.6%, respectively, as compared to pulping without EDTA. - Ester and C–N stretching bands from cellulose extracted with NaOH/EDTA were observed due to successful esterification of EDTA on the cellulose pulp. - The cellulose pulp extracted with NaOH/EDTA had a higher degree of polymerisation compared to the pulp extracted without EDTA. - Esterification with EDTA successfully protected the cellulose against alkaline hydrolysis by NaOH. - The addition of EDTA is a promising approach to improve the pulp yield with a high degree of polymerisation.	[90]
Calotropis gigantea fibre (CGF)	Cellulose nanocrystals (CNCs)	Combined ball milling defibrillation and SO42−/TiO2 nanosolid superacid catalyst-assisted method	Bleached CGF was milled with superfine zirconium beads in the presence of diluted H2SO4 and the synthesised SO42−/TiO2 catalyst	- The yield of CNC obtained from Calotropis gigantea fibre was 55.37%. - Rod-like CNCs with an average length of 242.06 nm and width of 8.80 nm were obtained. - The aspect ratio of the CNCs (length:width) was 27.5:1.	[91]
Rice straw	Microcrystalline cellulose (MCC) and cellulose nanocrystals (CNCs)	Extracted through alkaline treatment, bleaching, and acid hydrolysis, with slight modification to the process described by Chin et al. [26]	- Alkaline treatment: 4% aqueous solution of NaOH (w/w), 2 h at 80 °C under mechanical stirring, washed with deionised water until a pH of 7 is reached - Bleaching: Equal parts (v:v) acetate buffer (40 g NaOH, 75 mL glacial acetic acid, diluted to 1 L with distilled water) and sodium hypochlorite (1.7 wt% NaClO2), 80 °C for 2 h under mechanical stirring and washed with deionised water until a pH of 7 is reached - Acid hydrolysis: 64 wt% H2SO4 at the optimal temperature of 45 °C for 45 min and washed with deionised water until a pH of 7 is reached	Extracted CNC appear as long, well-defined rodlike crystals with a high aspect ratio (41).	[92]
Momordica Charantia plant stem	Cellulose fibres	Manual retting process	Fibres were soaked in water for 4 h, retted, washed with distilled water, sun-dried for 48 h, and oven-dried at 50 °C for 2 h	- Momordica charantia fibres had hemicellulose (7.3 wt.%.), lignin contents (14.32 wt.%), wax content (1.1 wt.%), moisture content (6.3 wt.%), and ash content (2.24 wt.%), which are mostly similar to other natural cellulose fibres. - The crystallinity index of Momordica charantia fibres was found to be 21.42%, which shows the presence of relatively moderate-crystalline cellulose. - The ultimate tensile strength of Momordica charantia fibres was 36.5 ± 1.04 MPa, which can be used as promising reinforcements in biocomposites.	[93]
Strelitzia reginae (SR) plant stem	Cellulose fibres	Retting process	- Step 1: SR stems were completely immersed in hot water for a period of one month at room temperature - Step 2: Fibres were separated using a wire brush in order to remove the surrounding lignin - Step 3: Fibres, which are up to 60 cm long were separated using a scrapper - Step 4: SR fibres were then washed and air-dried for 3 days	- The mean values of Young’s modulus exhibited by untreated fibres,1 h alkali treated fibres, and 4 h alkali treated fibres were 9.89 GPa, 12.08, and 18.39 GPa, respectively. - Tensile strength mean values exhibited by untreated fibres,1 h alkali treated fibres, and 4 h alkali treated fibres were 271.79, 306.23, and 421.39 MPa, respectively. - The XRD reveals the presence of cellulose with a crystallinity index of 70% for raw fibres and 72% for treated fibres.	[94]
Eriobotrya japonica leaves	Cellulose	Chemical extraction	- Alkali treatment: 4% wt. NaOH, at 80 °C for 2 h - Bleaching: Bleaching solution made of equal volumes of distilled water, acetic acid/sodium acetate trihydrate buffer (pH = 5), and sodium hypochlorite 1.7% wt, at 80 °C for 2 h	The final product of the extraction was a crispy and fragile white film.	[95]
Corn stalk	Corn stalk cellulose (CSC)	Pretreatment technology with immobilised enzyme	- Step 1: Pretreated corn stalk with immobilised enzyme was stirred in NaOH at 60 °C for 2 h - Step 2: Cellulose was by using a mixed solution of sodium chlorite and acetic acid at 70 °C for 1 h	The cellulose content obtained by immobilised enzyme pretreatment was 96.72%.	[96]
Sugarcane straw	Cellulose nanocrystals	Purification processes, followed by subsequent preparation of CNCs via sulfuric acid hydrolysis	- Step 1: Alkali-treated fibres (ATFs) were reacted with 64 wt.% H2SO4 solution in a solid–liquid ratio of 1:9 (w/v) at 45 °C for the desired time - Step 2: Samples were washed with distilled water and centrifuged (10,000 rpm, 10 min) until CNCs were obtained - Step 3: Obtained CNC suspension was treated with ultrasonic radiation for 5 min with different power strengths ranging from 0~80% (total power: 950 W)	- Alkali-treated fibres (ATFs) contained 886.33 ± 1.25 g kg−1 cellulose, and its morphological analysis revealed a smooth and slender fibrous structure. - CNCs obtained by treatment with 64 wt% sulfuric acid at 45 °C for 60 min were isolated in a yield of 21.8%, with a diameter and length of 6~10 nm and 160~200 nm, respectively. - Crystallinity index of these CNCs reached 62.66%, and thermal stability underwent a two-step degradation.	[97]
Populus tremula seed fibres	Cellulose polymeric material	Chemical extraction	- Dewaxing and delignification: wt.% NaOH with a liquor ratio of 1:50 (w/v) at 80 °C for 2 h - Bleaching: Mixed solution of CH3COOH and H2O2 (v/v = 1:1) at 90 °C for 2 h with a liquor ratio of 1:50 (w/v). The resulting product was filtered and oven-dried 60 °C for 6 h	- The crystallinity index values for untreated Populus tremula fibres and extracted cellulose were calculated to be 32.8% and 58.9%, respectively. - Extracted cellulose exhibited excellent adsorption capacities of methylene blue (140.4 mg g−1) and crystal violet (154 mg g−1).	[98]
Carpet wastes	Cellulose nanofibres (CNFs)	Supercritical carbon dioxide (Sc.CO2) treatment approach	- Pulping: 26% sodium hydroxide (NaOH), cooking ratio was 1:7 at 170 °C for 90 min - Bleaching: Sc.CO2 treatment 50 MPa at 60 °C for 2 h followed by mild hydrolysis and mild hydrolysis without Sc.CO2 treatment - Mild hydrolysis: 5% oxalic acid, and the washed fibre suspension was eventually homogenised for 6 h in an OV5 homogeniser to obtain CNFs	- The average fibre length and diameter of Sc.CO2-treated CNFs were 53.72 and 7.14 nm, respectively. - Untreated samples had a longer fibre length and diameter (302.87 and 97.93 nm). - Sc.CO2-treated CNFs also had a significantly higher thermal stability by more than 27% and zeta potential value of −38.9 ± 5.1 mV, compared to untreated CNFs (−33.1 ± 3.0 mV). - The Sc.CO2 treatment method is a green approach for enhancing the isolation yield of CNFs from carpet wastes and producing better quality nanocellulose for advanced applications.	[99]
Oat bran fibres	Nanofibrillated cellulose (NFC)	Chemical (i.e., acid, base, and bleach) or hydrothermal (i.e., microwave pretreatments and autoclave), followed by disintegration using high-pressure homogenisation from oat bran fibres	- Chemical pretreatment: 5% w/w KOH at 90 °C for 2 h and 1% w/w HCl at 80 °C for 2 h - Autoclave pretreatment: 120 °C for 2 h - Microwave pretreatment: 80 W for 5 min and stir at 600 rpm for 30 min - High pressure homogenisation: 500 bar for 20 passes	- Chemical pretreatment was an effective method for nanofibre extraction with good emulsion capacity. - Microwave with bleaching pretreatment was an alternative method for extracting nanofibres, which needs further study to improve the efficiency.	[100]
Areca nut husk fibres	Cellulose nanocrystals (CNCs)	Sulfuric acid hydrolysis	- Alkali treatment: 4% sodium hydroxide at 80 °C, under constant stirring for 3 h - Bleaching: 1.7% w/v NaClO2 solution, and CH3COOH buffer (2.7 g NaOH, and 7.5 mL of glacial acetic acid in 100 mL of distilled water) at 80 °C for 4 h - Acid hydrolysis: 65 wt% H2SO4 for 45 min at 45 °C, following which the solution obtained was chilled with H2O to terminate the process - Centrifugation: 10,000 rpm for 10 min to obtain CNCs	- The obtained CNCs were rod-like structures. - The diameter of the isolated CNC was 19 ± 3.3 nm; the length was about 195 ± 24 nm with an aspect ratio of 10.2 ± 6.8. - The zeta potential of CNC was −15.3 ± 1.2 mV. - CNCs had a higher crystallinity than the raw, alkali, and bleached fibres. - Thermogravimetric analysis revealed good thermal stability for the CNCs.	[101]

illustrates selective studies on the extraction of cellulose from various sources.

3. Brief History of Arsenic

Arsenic is a substance that has been around for thousands of years [102]. The notion regarding arsenic as an infamous substance used for suicide and homicide began to gain popularity in the Middle Ages and the Renaissance period [103,104]. It became one of the most notorious poisons leading up to the mid-1850s, due to its toxic and discreet nature. Moreover, analytical testing methods for arsenic were only discovered in the mid-1700s, but it was not until the trial of Marie Lafarge in 1840 that arsenic-detection techniques, pioneered by James Marsh, were proven to be admissible in court. Despite the toxic nature of arsenic, it gained significant attention due to its widespread use across various industries such as the (i) pharmaceutical industries, (ii) agricultural sector (as a pesticide and insecticide), (iii) commercial industries (in the manufacturing of pigment products, glass, and semiconductors), and (iv) copper smelting industry (where it is released as a by-product) [103,105]. It was documented that Hippocrates, the “Father of Medicine”, was believed to have utilised arsenic paste to treat abscesses and ulcers [103,106]. Furthermore, in 1786, Fowler’s solution, used to treat diseases such as asthma, eczema, psoriasis, malaria, and syphilis, was reported to contain 1% potassium arsenite. In 1910, Paul Ehrlich presented the “magic bullet”, which was an arsenic-based drug used for the treatment of syphilis, until penicillin gained popularity in the 1940s. Following the success of Fowler’s solution in the treatment of leukemia in 1878, by lowering the white blood cell count, the investigation into arsenic trioxide as a viable chemotherapeutic drug for other cancers has been ongoing [103]. The agricultural use of arsenic began with the introduction of Paris Green, between 1867 and 1900, as an effective insecticide against the beetles and mosquitoes that destroyed the Colorado potato. The use of Paris Green declined as lead arsenates (less-toxic arsenic-containing pesticides) were introduced in the 1800s. However, by 1960, the use of lead arsenate-based pesticides also declined due to the health complications expressed by orchard workers that were associated with the arsenic remnants found in fruits. This eventually led to the ban of lead arsenate by the United States in 1988. Nonetheless, some arsenic-based pesticides are still used today, such as chromated copper arsenate (CCA), which is used as a wood preservative. Much like for the agricultural sector, the adverse health effects of arsenic in commercial products such as wallpaper pigments as well as in the copper smelting industry have been identified [105]. Studies have shown exposure to arsenic in an occupational setting as well as during commercial use can increase the burden of lung cancer, neurological abnormalities, and dermatitis. Reports by Bartolomco Gosio in 1893 illustrated how the volatised arsenic used in wallpaper pigments can emit arsenic-containing compounds such as Paris Green [103].

The dangers of ingesting Arsenic in high dosages has been well-known for many years. Ancient Chinese farmers used arsenic trioxide to kill fungi and rats in their rice fields in order to protect their crops and increase crop yields [107]. Napoleon Bonaparte’s death was suspected to be caused by arsenic poisoning [104,108]. However, its effects at low dosages only became clear in the 1980s [102]. The full impact of this element on society is only becoming clear now. Nowadays, it is known to be the most dangerous substance ever to be regulated by the US Environmental Protection Agency (EPA). High concentrations of Arsenic have been found in many drinking water sources around the world [109]. For example, in 2008, the number of Bangladeshi people affected by overexposure to Arsenic vastly exceeded the number of people affected by the Chernobyl disaster that occurred on 26 April 1986 [102].

Arsenic in the environment has become a global concern because of the widespread chronic arsenic poisoning found in many countries affecting large numbers of people [104]. In 1963, the World Health Organization (WHO) set the arsenic standard in drinking water as 50 ppb. However, in 1993, the standard was reduced to 10 ppb [104,105,110]. Based on analysis, the 50 ppb level was considered to cause arsenic poisoning and was not protective of overall public health [104]. Therefore, in 2006, U.S. water treatment techniques were required to meet the new 10 ppb maximum contaminant level (MCL) for arsenic, which is still being used today [111]. Since arsenic toxicity depends on its oxidation states and chemical forms, the measurement of all arsenic species in natural water is essential for future environmental monitoring and regulatory considerations [111].

Water containing high concentrations of arsenic has been treated using several techniques. The lime softening technique is one of the oldest effective methods used to reduce lime concentration. In 2015, Klerk et al. [112] used lime neutralisation to conduct a continuous circuit co-precipitation of As(V) ions with ferric ions. The two stages of the continuous experiments were operated at a pH of 4 and 8, respectively. The lowest residual concentration of arsenic was achieved when the Fe/As molar ratio was kept at 4.

Ion exchange technology has been considered as another effective way of removing arsenic from water, by using anionic exchange resins. However, this technique was only effective in the removal of As(V) from water, and it was not effective in the removal of uncharged As(III) [113]. One major drawback with the development of these systems is that they turned out to be expensive. During ion exchange water purification, the adsorption capacity was limited by the interference and competitive adsorption of other co-existing anions. The adsorbent regeneration process in this technology created a sludge disposal problem [113].

In recent times, membrane technologies such as nanofiltration and reverse osmosis are increasingly becoming popular for the removal of arsenic from water [114]. The advantages of such techniques are their high removal efficiency, their easy operation, and the reduced amount of sludge generated during the water purification process [115]. However, the disadvantage is that the initial investment and operating costs of these membrane technologies are relatively high. During membrane water purification, high pressure is applied to force the contaminated water through the membranes. Discharge of the concentrate, membrane fouling, and flux decline are usually unavoidable in the membrane process [116]. Electrodialysis was also capable of removing both arsenic and other contaminants. The problem is that large amounts of insoluble coagulants were deposited on the cathode [117].

The adsorption process is considered one of the most promising techniques, of the many that are used to remove arsenic from water. This is because the adsorption technique is cheaper, highly efficient, and easy to operate [118]. Iron-based adsorbents for the effective removal of arsenic species from water have been extensively developed [119]. Some commercial-grade adsorbents such as granular ferric hydroxide (GFH) and zero-valent iron have been developed and produced on an industrial scale for commercial purposes. Unfortunately, most of the reported adsorbents do not make it to the practical field, in spite of their effective arsenic removal capacity, due to the interfering ions present in water [120]. Due to the specific chemical reaction between arsenic and iron in iron-based adsorbents, anions such as $\mathrm{C}{\mathrm{l}}^{-}$, $\mathrm{N}{\mathrm{O}}_3^{-}$, $\mathrm{S}{\mathrm{O}}_4^{2-}$, and $\mathrm{C}{\mathrm{O}}_3^{2-}$ were not considered as an interference to arsenic adsorption [121]. However, phosphate was found to strongly compete with arsenic for the adsorption sites and, thus, reduce the arsenic adsorption capacity of iron-based adsorbents [122]. Organic matter such as humic acid and fulvic acid were also shown to have negative effects on the adsorption of arsenic by slowing down the adsorption equilibrium [123].

4. Functionalised Cellulose for Removal of Arsenic

4.1. Preparation of Functionalised Cellulose

Arsenic as a metal is a toxic as well as carcinogenic element, which occurs in various forms such as the atmosphere, soil, and water. Water, especially drinking water, is a direct track for human beings to intake arsenic, which has become a major issue globally [124]. The diseases associated with arsenic include bladder cancer, skin cancer, and kidney cancer. According to the World Health Organization (WHO), in the year 2011, the finite arsenic concentration of 10 ppb in drinking water was prescribed. The various types of arsenic found in water include arsenate (As(V)) and arsenite (As(III)) [124]. Based on the above chronic diseases that are associated with arsenic, urgent methods are needed for the removal of arsenic from water. The well-known methods include coagulation, membrane separation, adsorption, and ion exchange [124,125,126,127]. The adsorption method is the most favoured method since it is cheap and has high efficiency and low energy consumption. However, the adsorbents materials used in the adsorption method pose various challenges that include a high amount of preparation, a complex preparation method, and low biocompatibility, which might result in secondary contamination in the water. As a result, there is a need for the development of an eco-friendly and inexpensive adsorbent to avoid the problems associated with the adsorption methods. Therefore, the cellulose materials extracted from various plants are ideal sources to remove unwanted materials such as arsenic from water. A novel adsorbent system containing cellulose with thiol groups, with the cellulose extracted from wood pulp, was utilised to remove arsenic As(III) [124]. Two biomass-based cellulose structured materials, i.e., (i) microscale dialdehyde cellulose-cysteine (MDAC-cys) and nanoscale dialdehyde cellulose-cysteine (NDAC-cys), were reported in this study. The morphology of the wood pulp cellulose, per SEM images, showed fibres with an average diameter of 34 μm, with a long and thick microscale. However, after periodate oxidation, the cellulose fibres became thinner with an average diameter of 30 μm. The two nanostructured cellulose-based adsorbents were reported to show a similar but excellent behaviour towards As(III) removal efficiency. A slight decrease was observed in the adsorption capacity of NDAC-cys for the removal As(III), when compared with MDAC-cys, due to the agglomeration of NDAC-cys. Magnetic iron-based nanoparticles were utilised for functionalising cellulose nanoparticles for the removal of arsenic. The nanosized iron nanoparticles arise from the inherent properties of the nanoparticles such as interfacial reactivity, a high surface area, and magnetism [50]. Hokkanen and co-workers [128] reported on the fabrication of the magnetic iron nanoparticles that modified microfibrillated cellulose (FeNP/MFC) for the removal of arsenate (As(V)) in an aqueous medium. The effects of various parameters such as the pH, initial As(V) concentration, and contact time were reported. According to Figure 5, the adsorption of As(V) was found to be strong in the acidic medium (viz, pH = 2). In an acidic medium, the charge of the adsorbent was positive; as a result, it was able to attract As(V), which was in the form of H2AsO4- in the pH range of 2–5. However, beyond a pH of 5.2, there was more domination of the negative sites, as evidenced by the low adsorption efficiency, which is associated with the repulsion attraction.

The very same study [128] investigated the effect of the contact time for As(V) removal by the modified MFC. There was an initial increase in the adsorption of AS(V) after 75 min, after which the system reached equilibrium (Figure 6).

Furthermore, the pseudo-first-order and pseudo-second-order models were utilised to verify the experimental results. The pseudo-first-order (PS1) model is employed for simulating sorption in liquid or solid systems [128,129], while the pseudo-second-order (PS2) model is used for the assumption of the rate-determining step.

The PS2 model was found to be better-fitted to the experimental results, since the qe in the equation was close to the experimental value. PS2 was based on the rate-determining step, which was assumed to be the chemical sorption, with valence forces being involved in the sharing of electrons between both the adsorbate and the adsorbent. To further enhance the removal efficiency of arsenic by cellulose, functionalised iron nanoparticles and magnesium-doped iron oxide nanoparticles were produced and confined on the surface of the cellulose nanofibrils [130]. The immobilisation of the cellulose fibrils by the iron oxide nanoparticles is to avoid the aggregation of the nanoparticles. The synthesis of the Mg-doped iron oxide nanoparticles was synthesised and immobilised on the cellulose nanofibrils in a porous aerogel. The iron oxide nanoparticles (IONPs) were incorporated into the cellulose nanofibrils aerogels at up to 12.5 wt.% of the IONPs. IONP dosages of 63, 31, and 16 mg/L were reported in this study. The As(III) and As(V) concentrations were reduced in the first 2 h, depending on the concentration of the sorbent due to more IONP sites. However, as the time increases, there seems to be less removal of As(III) and As(V); for example, an IONP dosage of 63 mg/L removed less than 10 µg/L of As(III) and As(V) after 12 h of contact time. Another important parameter that was reported to affect the removal of arsenic is the pH. There was a high removal rate for As(III) and As(V) in the pH range of 3–7; however, the adsorption drops above a pH of 7.

Furthermore, the zeta potentials of CNFs and IONPs were reported (Figure 7B). CNFs were found to be negatively charged in the pH range of 3–11 because of the carboxylate groups on the CNFs’ surface. In summary, there seems to be a better adsorption of arsenic at a lower pH due to the electrostatic charges between arsenic and IONP at a lower pH. At a higher pH, there is a possible electrostatic repulsion that may result in low adsorption. Another type of nanoparticle that has been utilised together with cellulose for arsenic removal is nanoscale zero-valent iron (nZVI). The nZVI was favoured for the removal of arsenic due to a high affinity towards arsenic, low cost, and ease of availability [131]. The reason that nZVI cannot be employed alone but is accompanied by cellulose is because of the ease of aggregates and rapid deactivation. The study conducted by Chai and co-workers [131] investigated the removal of arsenic (III)/V) from ground water by employing nZVI functionalised cellulose. In order to improve the affinity between the two components, i.e., nZVI and cellulose, nZVI was tied up into the polydopamine-coated cellulose nanocrystals. The composite was synthesised by the in situ reduction method (Figure 8a).

4.2. Adsorption Capacity of Functionalised Cellulose Materials

The SEM images of the neat CNCs showed smooth rods with an average size of 16–30 nm in diameter and 200–1000 nm in length (Figure 8c). The functionalised cellulose with PDA kept the rod-like shape of the nanocrystal (Figure 8d); however, the size of the CNCs was found to be large due to a thin layer of PDA that was created on the surface of the CNCs. The incorporation of nZVI into the PDA-modified CNCs showed bright particles of nZVI evenly spread on the surface of CNC–PDA. Several factors such as pH and temperature were found to be the other factors affecting the removal of As. The effect of pH was investigated in the range of 3–11. The neat cellulose nanocrystals (CNCs) recorded a zeta potential value of −37.2~−18.6 mV, while CNCs–PDA recorded a zeta value of +0.85~−27 mV. In the investigated pH range (viz, 3–11), neat CNCs revealed a low adsorption capacity for both As(III) and As(V), and this behaviour was attributed to a low zeta potential. With the functionalisation of cellulose with PDA, there was a steady improvement in the adsorption of arsenic, i.e., 1.77 mg g⁻¹ for As(III) and 2.20 mg g⁻¹ for As(V), when compared with neat CNCs. The effect of temperature was investigated with the following temperatures: 25, 35, and 45 °C. The temperature had an effect on the removal of As(III) and As(V). For example, the CNCs–PDA–nZVI had an enhancement for As(III), with an adsorption capacity increase from 285 mg g⁻¹ to 318 mg g⁻¹ with a temperature increase from 25 to 35 °C. Furthermore, it was revealed that the maximum adsorption capacity was recorded at the temperature of 35 °C; above this temperature, there was a reduction in the adsorption capacity. It was further revealed that the As(V) removal by CNCs–PDA–nZVI and nZVI was more affected by the temperature than the As(III) removal. The removal of As(III) and As(V) was investigated by the one-step synthesis of cellulose–iron oxide [132]. The adsorption capacity of As(V) was found to decrease with an increase in pH; however, the adsorption capacity of As(III) had the highest adsorption in the pH range of 6–9. At low pH, the adsorption is low due partially to the dissolving of Fe₂O₃, which might be an indication of the decrease in the active sites as a result of the low adsorption capacity of As(III). At a pH between 6 and 9, the active sites are not dissolved nor destroyed, and, as a result, there is a high adsorption capacity of As(III). Similarly, Fe(III)-loaded ligand exchange cotton cellulose adsorbent (Fe(III) LECCA) was fabricated for the adsorption of arsenate anions [133]. The SEM image of Fe(III) LECCA revealed large pores (viz, 5–30 μm) on their surface and in the inner part, which is advantageous for enhancing the hydrophilic surface in water solutions. Parameters such as time and pH were discussed in this study in relation to the adsorption of As(V). The removal efficiency of As(V) were reported to be above 95% in the pH range of 3.0–8.8, which is contrary to the previous study [133], whereby the adsorption was very high at a pH of 6–9. Furthermore, the effect of time on the adsorption of Fe(III) LECCA was also analysed and reported. There is rapid adsorption of the As(V), and the removal is almost complete at around 15 min (Figure 9). The increase in the adsorption was monotone as well as continuous. Based on the above observations, it was assumed the type of adsorption for As(V) was the monolayer adsorption.

Table 2. Various studies on the adsorption of arsenic by functionalised cellulose materials.

Cellulose Type(s)	Cellulose Functionalisation Procedure	Cellulose-Based Adsorbent	Arsenic Adsorption Capacity	Refs
Microfibrillated cellulose (MC), nanocellulose (NC)	Precipitation of magnetite (MG) on an amino terminal branched organic structure (L), either linked by maleic acid residue on NC surface (NC-MA/L) or linked by oxalyl bridge on MC surface (MC-O/L) to produce NC-MA/L-MG and MC-O/L-MG adsorbents, respectively	Magnetite-loaded amino modified nano/microcellulose adsorbents	- Adsorption capacity of arsenate, As(V), was in favour of NC-MA/L-MG (85.3 versus 18.5 mg g−1), while MC-O/L-MG exhibited faster kinetics (0.541 versus 0.189 g mg−1min−1). - As(III) removal of 68.3 mg g−1 was obtained for NC-MA/L-MG and 17.8 mg g−1 was obtained for MC-O/L-MG.	[134]
Carboxymethyl cellulose (CMC), microcrystalline cellulose (MCC), hydroxyethyl cellulose (HEC), industrial grade cellulose powder (CP)	2-line ferrihydrite (FH) nanoparticles were incorporated in the biopolymeric confinement of microcellulose to act as active sites for As(III) and As(V) adsorption	Functionalised microcellulose-reinforced 2-line ferrihydrite composites	- The maximum adsorption capacity of carboxymethyl-ferrihydrite composite (CMCFH) by batch studies was 143 mg/g and 83 mg/g for As(III) and As(V), respectively.	[135]
Cellulose fibre	Microwave irradiation (MW) was used to facilitate the grafting of the hyperbranched polyethylenimine (HPEI) coating agent with a large molecular size	Hyperbranched polyethylenimine (HPEI)-modified cellulose fibre (CellMW-HPEI) adsorbent	- The adsorption capacities of the obtained CellMW-HPEI fibres for As(III) and As(V) were found to be 54.13 and 99.35 mg·g−1, respectively, which are much higher than those of other biosorbents reported previously.	[136]
Cellulose nanocrystals (CNCs)	By using the “bridge joint” effect of iron ions, the cellulose-nanocrystal-containing high-performance adsorbents were synthesised via the co-precipitation method, which enhanced the cross-linking action of cellulose nanocrystal and polyethyleneimine (PEI)	Three cellulose nanocrystal-containing adsorbents were prepared	- CNC-PEI-Fe(III) adsorbent has the best removal effect on As(III)/As(V), and the adsorption capacity is 142.42/78.71 mg/g.
Nanocellulose	pH-sensitive nanoparticles were synthesized via a simple single-graft method, in which nanocellulose treated with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO-NC) was cross-linked with glutaraldehyde (GA) and polyethyleneimine (PEI)	pH-sensitive nanoparticle based on nanocellulose, involving cross-linking polyethyleneimine and glutaraldehyde	The As(V) adsorption capacity of the nanoparticles reached approximately 255.19 mg g−1 at pH 3, which was five times greater than that achieved with the As(V) solution at its initial pH (44.33 mg g−1).	[137]
Cellulose	Ethylation of glycidylmethacrylate grafted aminated titanium dioxide densified cellulose (Et-AMPGDC)	Amino-functionalised glycidylmethacrylate-grafted-titanium dioxide densified cellulose for the adsorptive removal of arsenic(V) from aqueous solutions	The maximum adsorption capacity was evaluated to be 108.70 mg/g.	[138]
Cellulose filter paper	3-mercapto-propanoic acid (MPA) was covalently grafted to the cellulose filter paper (Cell) by esterification process through the formation of O-acylisourea intermediate	3-mercapto-propanoic acid (MPA)-modified cellulose filter paper (MPA–Cell paper) for arsenate removal from drinking water	The modified cellulose filter paper performed well at nearly a neutral pH, for arsenate removal through adsorption, and demonstrated a significant arsenate uptake capacity of 92.59 mg/g.	[139]
Cellulose sponge	Cellulose sponge was modified by coating with magnetite microparticles and then used as an effective adsorbent for the elimination of As(V) from aqueous solution	Magnetite microparticles decorated cellulose sponge as an efficacious filter for improved arsenic(V) removal	The maximum adsorption capacity of the adsorbent was 349.9 mg/g, which is substantially higher than the results from previous reports.	[140]
Nanocrystalline cellulose (NCC)	Nanocrystalline cellulose (NCC) was selectively oxidised using sodium periodate followed by grafting of diethylene triamine (DETA) to obtain their amine derivatives in 80–85% yield	Diethylene triamine grafted dialdehyde nanocrystalline cellulose (DETA-g-DA-NCC) for As(III and V) removal from aqueous solution	The adsorption capacities were 10.56 and 12.06 mg/g for As(III) and As(V), respectively.	[141]
Cellulose	The Fe(III)-AM-PGMACell was prepared through graft copolymerisation of glycidyl methacrylate (GMA) onto cellulose (Cell) in the presence of N,N′-methylenebisacrylamide (MBA) as a cross linker using benzoyl peroxide initiator, followed by treatment with ethylenediamine and ferric chloride in the presence of HCl	Iron(III)-coordinated amino-functionalised poly(glycidyl methacrylate)-grafted cellulose (Fe(III)-AM-PGMACell) for the adsorption of arsenic(V) from aqueous solutions	- More than 99.0% adsorption was achieved from an initial concentration of 25.0 mg/L. - The maximum adsorption capacity of 78.8 mg/g was achieved at 30 °C. - The desorption of As(V) was achieved at over 98.0% with 0.1 M NaCl solution.	[142]
Cellulose	Cross-linked dithiocarbamate-modified cellulose sorbents cross-linked with epoxy or complexed with iron, and the quantities of the modifiers were varied between 3.0 and 10 mol%	Dithiocarbamate (DTC) modified cellulose sorbents that can selectively arsenite AsIII ions separate from water	The maximum sorption capacity of the epoxy-cross-linked sorbent, calculated from the Langmuir isotherm equation, was 600 μmol g−1 (45 mg g−1) at 25 °C.	[143]
Wheat straw cellulose	Fluorophore, 3-bromofluoranthene was grafted on cellulose (BF@CFs) for removal of arsenite As(III) ions in water	Fluoranthene decorated fluorescent nanofibrous cellulose probe (BF@CFs)	- BF@CFs exhibited good adsorption of As(III) ions, with maximum adsorption of 171.2 μg g−1 at 35 min under optimised conditions. - BF@CFs displayed 95.2% removal efficiency, with 2 μg L−1 concentration of As(III) ions at room temperature and a neutral pH. - BF@CFs retained adsorption 97.3% efficiency after five adsorption/desorption cycles, displaying excellent reusability and stability and strengthening its potential as a dual-functional sensor and adsorbent.	[144]

summarises various studies on the adsorption of arsenic by functionalised cellulose membranes.

5. Conclusions and Future Recommendations

Hazardous arsenic concentrations, most notably above 10 µg/L, threaten millions of people’s health globally. The use of environmentally friendly adsorption technologies is considered a solution for arsenic adsorption. Cellulose-based materials are preferred for the adsorption of arsenic due to their renewability, abundance, biodegradability, and low cost. Cellulose also has numerous hydroxyl functional groups on its surface. These hydroxyl functional groups enable the modification of cellulose in various ways, which leads to the fabrication of cellulose-based materials with tunable characteristics. One of the tunable characteristics is the adsorption of arsenic from wastewater. Various functionalised materials such as magnetite-loaded amino, 3-mercapto-propanoic acid, diethylene triamine grafted dialdehyde, and magnetic iron nanoparticles are some of the materials utilised for the functionalisation of cellulose for arsenic removal. Factors such as pH, reaction time, and temperature were found to affect the removal of arsenic, depending on the cellulose system used. The pH’s efficiency in arsenic removal depends on the type of material used for functionalising the cellulose (i.e., the cellulose system used for the removal of arsenic) and the type of arsenic to be removed. For example, in one study, it was proven that when Fe₂O₃ was used as a functionalising material for cellulose-based materials, the removal of As(V) was found to be effective at a pH of 3.0–8.8, with a removal efficiency of 95%. However, the removal efficiency of As(III) was reported to be effective in the pH range of 6–9. In some other cases, the effective efficiency for removal of As(V) was reported in the pH = 2–5, which emphasises the variation in pH in relation to the various systems used. For future recommendations, there is a need for the incorporation of two or more adsorbents materials for cellulose functionalisation, in order to enhance the removal efficiency of arsenic and selectivity. Furthermore, there is a need for more investigation into the recycling and reuse of functionalised cellulose systems for arsenic removal, as this is important for the environment and economy. In spite of the work that has been done, wastewater treatment technologies involving functionalised cellulose-based biomaterials still face a number of challenges that need to be addressed. Unfortunately, most of the current research is focused on the fabrication, functionalisation, and mechanism of the wastewater treatment of cellulose-based materials. Some of the chemicals used to modify the cellulose could be harmful to the environment and human health. Therefore, future work should be focused on how cellulose’s chemical modification affects its toxicity to the environment and human health. Another focus area could be the efficiency of the removal of these functionalised cellulose-based materials. The selectivity of the cellulose materials is important for the complete removal of all toxic pollutants from wastewater. Furthermore, research could be channeled towards studying the longevity of these cellulose materials by investigating their mechanical stability under various media conditions. The use of cheaper and environmentally friendly chemicals during cellulose functionalisation should be considered.