Introduction

Issues related to the quality of water are one of the major problems faced by humanity in the twenty-first century. The quality of our water resources is deteriorating day by day due to various anthropogenic activities, increasing industrialization and unplanned urbanization. Among the various organic and inorganic pollutants encountered in wastewater, toxic heavy metals and dyes are among the major pollutants that pollute the aquatic environment. Wastewater effluents containing dyes and heavy metals cause potential hazard to the environment and human health. Recently, numerous approaches such as ion exchange, membrane separation, electrochemical treatment and adsorption (Barakat 2011) have been studied for the development of effective technologies to decrease the amount of wastewater produced and to improve the quality of the treated effluent. However, most of them require substantial financial input and thus their use is restricted. Among all the treatment processes mentioned, adsorption is found to be effective, simple and relatively lower operation cost of pollutant removal (Meshko et al. 2001). Different types of materials such as activated carbon, zeolites, carbon nanotubes and molecularly imprinted polymers have been developed as adsorbents. In recent years, development of green adsorbents received widespread attention as they are valued for their renewability, low cost and non-toxicity (Chang and Juang 2004). Green adsorbents are manufactured in a more energy conservative way, pose few health problems and are recyclable. Cellulosic adsorbents have the proficiency to meet almost all the requirement for being green. Cellulose is the most abundant, natural biopolymer which is renewable, biodegradable and non-toxic. The primary occurrence of cellulose is the existing lignocellulosic material. One of the promising applications of lignocellulosic material is as an adsorbent for water purification or wastewater treatment due to its wide availability, renewability, sustainability and the possibility of surface modification. Several researches have been devoted to review the removal of organic and inorganic pollutants using lignocellulosic adsorbents (Abdolali et al. 2014). However, untreated lignocellulosic biomass is generally not functional and the adsorption capacity varies depending on the biomass source. When the size is minimized to the nanoscale, the high specific surface area of the polysaccharide adsorbents contributes to enhancing the adsorption capacity. This led to the emergence of nanocellulose as a new generation of bio-based adsorbents with potential applications in wastewater treatment. These nanomaterials have been extensively explored by researchers as an adsorbent for removal of various kinds of hazardous pollutants and the studies indicate that these materials possess high adsorption capacity, are environmental friendly and inexpensive (Lam et al. 2012). Cellulose-based materials are more attractive for water purification when it makes structural modifications to improve their existing properties or adding new potentialities to this material (Silva Filho et al. 2013). Benefiting the presence of abundant hydroxyl groups on the surface of micro- or nanocellulose offers a unique platform for significant surface modification to graft a myriad of functional groups or molecules onto the cellulosic structure thereby immobilizing pollutants. Here, we review the recent progresses related to the application of cellulose-based materials and their modified forms as an adsorbent for the removal of toxic heavy metals and dyes from wastewater. Herein the adsorption efficacies of various green adsorbents, cellulose-based green adsorbents, modified cellulose-based adsorbents and modified nano/micro-based adsorbents have been discussed. This article is an abridged version of the chapter published by Varghese et al. (2018) in the series Environmental Chemistry for a Sustainable World (https://www.springer.com/series/11480).

Heavy metals

The term heavy metal refers to any metallic element that has a density more than 5 g per cubic centimeter and is toxic or poisonous even at low concentration. These include lead (Pb), cadmium (Cd), zinc (Zn), mercury (Hg), arsenic (As), silver (Ag), chromium (Cr), copper (Cu), iron (Fe) and the platinum group elements. Most of the metals are non-biodegradable, highly toxic and carcinogenic in nature (Barakat 2011). Heavy metals cause serious health effects, including reduced growth and development, cancer, organ damage, nervous system damage, and in extreme cases, death. At higher doses, heavy metals can cause irreversible brain damage. Therefore, it is necessary to treat metal-contaminated wastewater before its discharge into the environment. Table 1 (Abdel-Raouf and Abdul-Raheim 2017) lists those heavy metals that are relevant in the environmental context, their source and toxicity effect.

Table 1 Source, route of entry and toxicity effect of some heavy metals
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Dyes

Dyes are complex organic compounds which are purged from various industrial sources such as textile, cosmetic, paper, leather, rubber and printing industries to color their products. To meet industrial demand, it is estimated that 1.6 million tons of dyes are produced annually and 10–15% of this volume is discarded as wastewater. As a result, dyes are major water pollutants. Excessive exposure to dye causes skin irritation, respiratory problems and some dyes even increase the risk of cancer in humans (Rai et al. 2005). Thus, it is of utmost importance to remove dyes from wastewater effectively to ensure safe discharge of treated liquid effluent into watercourses.

Methods for pollutant removal

In response to the rising demands of clean and safe water, many different technologies are available for treating the pollutant-laden wastewater. Some of the widely used treatment technologies include biological treatments (McMullan et al. 2001), membrane process (Barakat 2011), chemical and electrochemical technology (Ku and Jung 2001), reverse osmosis (Sonune and Ghate 2004), ion exchange (Maranon et al. 1999), electrodialysis, electrolysis and adsorption procedures (Barakat 2011). Table 2 collects some of the common technologies that have been adopted by researchers for the removal of heavy metals and dyes from wastewater along with their advantages and disadvantages. Among the various decontamination techniques, adsorption process is regarded more prospective for water treatment due to its ease of operation, convenience and simplicity of design (Faust and Aly 1981).

Table 2 Treatment technologies for the removal of pollutants from wastewater and associated advantages and disadvantages
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Adsorption

The general mechanism of adsorption involves the transfer of the pollutant from bulk solution to the outer surface of the adsorbent, internal mass transfer from the outer surface to the inner pores of the adsorbent and the adsorption of adsorbate particles onto the active pores of the adsorbent. The overall rate of the reaction is determined by either film formation or intraparticle diffusion or both (Abdel-Raouf and Abdul-Raheim 2017). An ideal adsorbent for the adsorption of pollutants should include inexpensiveness, good mechanical and structural integrity to overcome water flow for a long time, high adsorption capacities with high rates, have a large surface area and possess a regeneration aptitude using cost-effective approaches (Mahfoudhi and Boufi 2017). Different materials have been tested as possible wastewater adsorbents such as polymeric, carbon-based, bio-based and inorganic materials. As per the above requirements, adsorption using low-cost materials with satisfactory adsorption properties and environmentally friendly nature has gained much attention and currently researchers have switched onto green adsorbents due to their abundance, biodegradability and non-toxic nature. Green adsorbents include low-cost materials originated from (1) natural sources (Sharma et al. 2011) (2) agricultural residues and wastes in particularly lignocellulosic biomass (Sud et al. 2008; Abdolali et al. 2014) and (3) low-cost sources (Bhatnagar and Sillanpää 2010) from which activated carbon adsorbents can be produced. Indeed green adsorbents were found to be inferior in terms of their adsorption capacity compared to commercial adsorbents such as activated carbons and structurally complex inorganic composite materials. But their cost-potential makes them competitive (Kyzas and Kostoglou 2014). Cellulosic adsorbents have the proficiency to meet almost all the requirement for being green and are thus potential materials for high end applications such as water purification.

Adsorption by green adsorbents

Interesting works have been reported regarding the adsorption of various heavy metals and dyes onto green adsorbents. Dried prickly pear cactus cladodes were explored for the biosorption of dyes from aqueous solutions (Barka et al. 2013). In another study, Ferrero explored the adsorption of methylene blue onto ground hazelnut shells and observed that adsorption capacities of methylene blue for hazelnut shells were five times higher than the respective amount reported for activated carbon obtained from the same material (Ferrero 2007). Animal bone meal was explored as a novel adsorbent for the removal of rhodamine B from wastewaters and the adsorption capacities obtained at different temperatures were close to 65 mg/g (El Haddad et al. 2016). El-Mekkawi and Galal investigated that the adsorption capacity of rutile TiO2 and Degussa P25 TiO2 for the removal of Direct Fast Blue B2RL and an adsorption capacity of 56 and 144 mg/g respectively was observed (El-Mekkawi and Galal 2013). In the light of the literature reviewed, the green adsorbents show higher adsorption ability for dyes than the metal ions. Table 3 depicts the adsorption capacities of various green adsorbents for the removal of dyes and heavy metals.

Table 3 Adsorption capacities of various green adsorbents for the removal of dyes and heavy metals
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Adsorption on cellulose

Cellulose-based materials are abundant, cheap and have low or little economic value. Different forms of cellulosic materials are used as adsorbents such as fibers, leaves, roots, shells, barks, husks, stems and seed as well as other parts also. Jalali and Aboulghazi investigated the feasibility of sunflower stalks for lead (Pb) and cadmium (Cd) metal ion adsorption (Jalali and Aboulghazi 2013). Batch adsorption studies were conducted to study the effect of contact time, initial concentration, pH and adsorbent doses on the removal of Cd(II) and Pb(II) metal ions at room temperature. Elemental mercury adsorbents were successfully synthesized from the coconut husk wastes using different surface treatment methods (Johari et al. 2016). The surface morphology and surface functional groups of these adsorbents significantly changed after mercerization and bleaching treatments and resulted in different adsorption performances. The elemental mercury adsorption capacity for coconut pith and coconut fiber adsorbents observed the following trend: coconut pith-NaOH (956.282 mg/g) > pristine coconut pith (730.250 mg/g) > coconut fiber-NaOCl (639.948 mg/g) > coconut fiber-H2O2 (634.347 mg/g) > coconut fiber-NaOH (611.678 mg/g) > coconut fiber-H2O2 (531.277 mg/g) > coconut pith-NaOCl (501.126 mg/g > coconut fiber (431.773 mg/g). Table 4 shows the adsorption capacity of various lignocellulosic adsorbents, the major source of cellulose, for the removal of dyes and heavy metals. Most of the adsorption studies were conducted using untreated cellulosic materials and only a few of them show good adsorption potential. However, the performance of these adsorbents has been remarkably affected upon physical and chemical treatment. Table 4 shows the adsorption capacity of various lignocellulosic adsorbents, the major source of cellulose, for the removal of dyes and heavy metals.

Table 4 Results on the removal of dyes and heavy metals by different cellulose-based adsorbent
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Adsorption by modified cellulose

Cellulose is abundant in hydroxyl groups which can anchor other functionalities through a variety of chemical modifications. Modification of cellulose involves the direct modification and monomer grafting. Direct cellulose modification in the preparation of adsorbent materials is esterification, etherification, halogenation, oxidation, alkaline treatment and silynation (O’Connell et al. 2008; Hokkanen et al. 2016). Chemically modified cellulose bearing Schiff’s base and carboxylic acid groups was synthesized for the removal of Cu(II) and Pb(II) from aqueous solutions (Saravanan and Ravikumar 2016). This novel green adsorbent was synthesized by periodate oxidation of cellulose followed by condensation reaction with p-aminobenzoic acid for the Schiff’s base forming reaction. In another study, chemically modified cellulose bearing Schiff base extracted from Sesbania sesban plant was synthesized via a novel method using 2-hydroxy-5-methyl benzaldehyde for the removal of Cd(II) (Naeimi and Amini 2018). The cellulose biomass exhibited the highest metal ions uptake capacity of 9.39 mg/g at pH value of 4.0, biomass dosage of 0.01 g/L and cadmium concentration of 150 mg/L. Bediako et al. (2016) developed an adsorbent via carbomethylation and cross-linking reactions from waste lyocell fabric to produce carbomethyl cellulose adsorbent for the removal of Cd(II) (Bediako et al. 2016). This adsorbent displayed approximately 17 times greater metal uptake than the original material and at neutral and alkaline pH, maximum Cd(II) uptake was displayed (Table 5).

Table 5 Adsorption capacities of various modified cellulose adsorbents for the removal of heavy metals and dyes
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Adsorption by modified nano/microcellulose

Carboxycellulose nanofibers prepared from untreated Australian spinifex grass using a nitro-oxidation method were found to be an effective medium to remove Cd2+ ions from water. A low concentration of nanofiber suspension could remove large concentrations of Cd2+ ions in less than 5 min. The maximum Cd2+ removal capacity of the cellulose nanofibers was around 2550 mg/g and the highest removal efficiency of 84% was exhibited when the Cd2+ concentration was 250 ppm (Sharma et al. 2018a). Sun et al. (2017) prepared a cellulosic adsorbent by halogenation of microcrystalline cellulose followed by the functionalization with pyridone diacid for the removal of Pb(II) and Co(II) from aqueous solutions. The content of carboxyl groups in this cellulosic adsorbent was determined to be 1.32 mmol/g, which was responsible for the high adsorption toward metal ions. In another study, TEMPO-oxidized fibrous cellulose modified with polyethyleneimine via cross-linking with glutaraldehyde also exhibited a higher adsorption of Cu(II) at pH 5 than the polyethyleneimine grafted cellulose (Zhang et al. 2016). The functionalized nanocellulose obtained by selectively oxidizing the C2 and C3 hydroxyl groups followed by oxidizing the aldehyde groups to form 2, 3-dicarboxyl groups demonstrated a maximum adsorption capacity for Cu2+ at pH 4 (Sheikhi et al. 2015). Mautner et al. (2016) synthesized phosphorylated nanocellulose papers for copper adsorption from aqueous solutions. The nanopaper ion-exchangers were able to adsorb copper ions in dynamic filtration experiments on passing water containing copper ions through the nanopapers.

Organic dye pollutants display cationic, anionic or non-ionic properties and pose a significant environmental problem in many parts of the world. Cationic dyes are removed using nanocellulose functionalized with anionic moieties. Carboxylated nanocellulose synthesized via TEMPO-mediated oxidation resulted in a significantly higher uptake of 769 mg/g at pH 9 of the cationic dye methylene blue, compared to nanocellulose with sulfate groups on their surfaces with an adsorption capacity of 118 mg/g at pH = 9 (Batmaz et al. 2014). Carboxylated nanocellulose produced by citric acid/hydrochloric hydrolysis of microcrystalline cellulose was used for the adsorption of methylene blue (Yu et al. 2016). Anionic dyes are usually removed using nanocellulose functionalized with cationic moieties. Cationic nanocellulose prepared via successive sodium periodate oxidation followed by reaction with ethylenediamine displayed a maximum uptake of 556 mg/g of acid red GR (Jin et al. 2015a). The same functionalization method was explored by Zhu et al. on dialdehyde functionalized cellulose powder, but using hyper-branched polyethyleneimine. The adsorbent displayed a high Congo red adsorption of 2100 mg/g and a high cationic basic yellow adsorption of 1860 mg/g (Zhu et al. 2016).

Nanocellulose, in the form of carboxycellulose nanofibers prepared using nitro-oxidation method exhibited the highest adsorption capacity of 2550 mg/g for removal of Cd2+ ions from water. Among the dyes, carboxylated nanocellulose synthesized via TEMPO-mediated oxidation resulted in the maximum adsorption capacity of 769 mg/g for the cationic dye methylene blue. Table 6 shows the adsorption capacities of nano/microcellulose and their modified counterparts.

Table 6 Modified and unmodified nanocellulose adsorbents for the removal of heavy metals and dyes
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Conclusion

In this review, applications of cellulose-based adsorbents for the removal of heavy metals and dyes from wastewater have been reviewed based on a substantial number of relevant research articles published up till now. Cellulose adsorbents are of specific significance owing to their abundant availability, ease of modification and application potential. As evident in the literature reviewed, most of the adsorption studies are limited to batch-scale only and are not fully developed at pilot and industrial scales for the treatment of real industrial effluents. Moreover, the actual industrial effluents were laden with several pollutants which require much work to investigate the selectivity of adsorbents in real effluents. Cellulose-based adsorbents contain many hydroxyl groups that can activate various reactions on chemical modification. Upon chemical modification, the adsorption capacity of these adsorbents has enhanced as a result of the increase in active binding sites on modification and addition of new functional groups that favor the higher uptake of pollutants. However, the pollution caused by the various modification methods was seldom reported. Currently, research is focused to synthesize modified cellulose and nanocellulose-based adsorbents using eco-friendly chemicals for wastewater treatment.