Handbook of Composites from Renewable Materials, Nanocomposites: Advanced Applications

Preface

The concept of green chemistry and sustainable development policy impose on industry and technology to switch raw material base from the petroleum to renewable resources. Remarkable attention has been paid to the environmental-friendly, green, and sustainable materials for a number of applications during the past few years. Indeed, the rapidly diminishing global petroleum resources, along with awareness of global environmental problems, have promoted the way to switch toward renewable resources-based materials. In this regard, biobased renewable materials can form the basis for a variety of eco-efficient, sustainable products that can capture and compete markets presently dominated by products based solely on petroleum-based raw materials. The nature provides a wide range of raw materials that can be converted into a polymeric matrix/adhesive/reinforcement applicable in composites formulation. Different kinds of polymers (renewable/nonrenewable) and polymer composite materials have been emerging rapidly as the prospective substitute for the ceramic or metal materials, due to their advantages over conventional materials. In brief, polymers are macromolecular groups collectively recognized as polymers due to the presence of repeating blocks of covalently linked atomic arrangement in the formation of these molecules. The repetitive atomic arrangements forming the macromolecules by forming covalent links are the building block or constituent monomers. As the covalent bond formation between monomer units is the essence of polymer formation, polymers are organic or carbon compounds of either biological or synthetic origin. The phenomenon or process of polymerization enables to create diverse forms of macromolecules with varied structural and functional properties and applications. On the other hand, composite materials, or composites, are one of the main improvements in material technology in the recent years. In the materials science field, a composite is a multiphase material consisting of two or more physically distinct components, a matrix (or a continuous phase) and at least one dispersed (filler or reinforcement) phase. The dispersed phase, responsible for enhancing one or more properties of matrix, can be categorized according to particle dimensions that comprise platelet, ellipsoids, spheres, and fibers. These particles can be of inorganic or organic origin and possess rigid or flexible properties. The most important resources for renewable raw materials originate from nature such as wood, starch, proteins, and oils from plants. Therefore, renewable raw materials lead to the benefit of processing in industries owing to the short period of replenishment cycle resulting in the continuous-flow production. Moreover, the production cost can be reduced by using natural raw materials instead of chemical raw materials. The waste and residues from agriculture and industry have been also used as an alternative renewable resources for producing energy and raw materials such as chemicals, cellulose, carbon, and silica. For polymer composites applications, an intensifying focus has been directed toward the use of renewable materials. Biobased polymers are one of the most attractive candidates in renewable raw materials for use as organic reinforcing fillers such as flex, hemp, pine needles, coir, jute, kenaf, sisal, rice husk, ramie, palm, and banana fibers which exhibited excellence enhancement in mechanical and thermal properties. For green polymer composites composed of inorganic reinforcing fillers, renewable resources-based polymers have been used as matrix materials. Significant research efforts all around the globe are continuing to explore and improve the properties of renewable polymers-based materials. Researchers are collectively focusing their efforts to use the inherent advantages of renewable polymers for miscellaneous applications. To ensure a sustainable future, the use of biobased materials containing a high content of derivatives from renewable biomass is the best solution. This volume of the book series Handbook of Composites from Renewable Materials is solely focused on the "Nanocomposites: Advanced Applications". Some of the important topics include but not limited to virgin and recycled polymers applied to advanced nanocomposites; biodegradable polymer–carbon nanotube composites for water and wastewater treatment; eco-friendly nanocomposites of chitosan with natural extracts, antimicrobial agents, and nanometals; controllable generation of renewable nanofibrils from green materials and their application in nanocomposites; nanocellulose and nanocellulose composites; poly(lactic acid) biopolymer composites and nanocomposites for biomedical and biopackaging applications; impact of nanotechnology in water treatment: carbon nanotube and graphene; nanomaterials in energy generation; sustainable green nanocomposites from bacterial bioplastics for food-packaging applications; PLA nanocomposites: a promising material for future from renewable resources; biocomposites from renewable resources: preparation and applications of chitosan–clay nanocomposites; nanomaterials: an advanced and versatile nanoadditive for kraft and paper industries; composites and nanocomposites based on polylactic acid obtaining; cellulose-containing scaffolds fabricated by electrospinning: applications in tissue engineering and drug delivery; biopolymer-based nanocomposites for environmental applications; calcium phosphate nanocomposites for biomedical and dental applications: recent developments; chitosan–metal nanocomposites: synthesis, characterization, and applications; multi-carboxyl functionalized nanocellulose/nanobentonite composite for the effective removal and recovery of metal ions; biomimetic gelatin nanocomposite as a scaffold for bone tissue repair; natural starches-blended ionotropically gelled microparticles/beads for sustained drug release and ferrogels: smart materials for biomedical and remediation applications. Several critical issues and suggestions for future work are comprehensively discussed in this volume with the hope that the book will provide a deep insight into the state-of-the-art of "Nanocomposites" of the renewable materials. We would like to thank the Publisher and Martin Scrivener for their invaluable help in the organization of the editing process. Finally, we would like to thank our parents for their continuous encouragement and support.

Vijay Kumar Thakur, Ph.D.

Washington State University—USA

Manju Kumari Thakur, M.Sc., M.Phil., Ph.D.

Himachal Pradesh University, Shimla, India

Michael R. Kessler, Ph.D., P.E.

Washington State University—USA

Chapter 1

Virgin and Recycled Polymers Applied to Advanced Nanocomposites

Luis Claudio Mendes* and Sibele Piedade Cestari

Instituto de Macromoléculas Professora Eloisa Mano – IMA, Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro, Brazil

*Corresponding author: [email protected]

Abstract

The study and development of nanostructured polymers is an expanding field. New strategies on advanced polymeric nanocomposites and hybrid materials have been created, to be used in different areas. Nanocomposites can improve characteristics of virgin and recycled polymers; they can also resemble biomaterials for medical or drug delivery applications. We studied some neat and modified materials that are seldom used as filler in nanocomposites – zinc oxide and zirconium phosphate – and added it to recycled polymers matrices – polycarbonate and poly(ethylene terephthalate). The use of nanoscaled fillers in polymer composites can improve properties like morphology, resistance to ultraviolet radiation, mechanical performance, crystallinity, and molecular mobility. Advanced nanocomposites can actually improve the effectiveness, sustainability, and performance of materials.

Keywords: Polymers, nanocomposites, advanced materials, recycling, sustainability

1.1 Introduction

Definitely, nanoscience and nanotechnology entered our lives in order to bring benefits for society. In particular, researches on polymeric nanocomposites intend to create solutions for daily problems. Polymeric nanocomposites can be considered advanced and sustainable composites. These materials are expanding. Several new strategies for developing advanced polymeric nanocomposites and hybrid (nanocomposites/microcomposites) have been created, in order to be used in many different areas (Thakur et al., 2012a,b; 2014a,b). Due to the high aspect ratio of the disperse phase, the properties are improved with low filler content. The disperse phase may resemble leaves – nanolayers – where only one dimension is in nanoscale; have the shape of nanotubes – two dimensions are in the nanometer range; and finally be a nanoparticle – three dimensions are on the nanometric scale. In polymeric nanocomposites, polymer – virgin, recycled, and renewable – is the phase that allows the incorporation and takes advantage of the properties which the nanosize substances can offer. Both academia and industry understand the importance of polymeric nanocomposites in the current state of society development.

Due to the similarities with the mineral constituents of bone tissue, enamel, and dentin of teeth, hydroxyapatite (HA) is an important class of biomaterial (Sato, Hotta et al., 2006; Fomin, Barinov et al., 2009; Brundavanam, Jiang et al., 2011). Besides immune response, other qualities – osteoinduction, osteoconductive, and osteointegration – indicate HA for using in medical or drug delivery devices. Biomimetic, hydrothermal, sol–gel, and precipitation processes have been studied as routes for producing collagen/HA composites for bone and dental repairs (Hilson, 1986; Orlovskii, Komlev et al., 2002; Ficai, Andronescu et al., 2010; Zhang, Tang et al., 2010). In order to prepare a collagen/HA nanocomposite as osteoinductive of the pulp–dentin complex, we investigated the influence of the presence of collagen (COLL) on structural and morphological characteristics (Mendes, Ribeiro et al., 2012).

Thermogravimetric (TG) analysis (Figures 1.1 and 1.2) of the materials showed that COLL has two stages of degradation. The first one (25–200 °C, 8%) was ascribed to the loss of water and the second one (270–500 °C, 65%) to the polymer chain degradation. The HA without collagen showed only one stage of degradation (150–235 °C) ascribed to the loss of water. In contrast, the HA synthesized with COLL showed two stages of degradation. The initial stage was similar to that of HA without collagen, and a second stage arose at higher temperatures (425–450 °C). We concluded that some chemical and/or physical interactions between components have happened, increasing the thermal stability of COLL (Sionkowska & Kozłowska, 2010).

Figure 1.1 TG curves of COLL, HA, and HA/COLL. (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Figure 1.2 DTG curves of COLL, HA, and HA/COLL. (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Scanning electron microscope (SEM) images (Figure 1.3) of the materials showed morphological differences between the two HAs. The HA synthesized in the presence of COLL showed morphology similar to regular microrods, while in the absence of COLL an irregular cluster was obtained. Ca/P ratios of 1.89 and 2.38 were calculated for HA without and with COLL, respectively. The addition of COLL influenced the HA chain growth, and different chain catenation was proposed as shown in Figure 1.4.

Figure 1.3 SEM photomicrographs of HA (a), COLL (b), and HA/COLL (c). (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Figure 1.4 Schematic representation of the feasible repeat unit of the HA: Ca/P = 1.89 and 2.38, without and with COLL, respectively. (Reproduced with permission from L.C. Mendes, G.L. Ribeiro, and R.C. Marques, Materials Sciences and Applications, Materials Sciences & Applications, 2012, 3, 8. ©2012, Scientific Research.)

Polycarbonate is a very important engineering polymer largely used in outdoor exposure. In order to improve its resistance against ultraviolet radiation and increase its mechanical performance, nanocomposites of recycled polycarbonate (rPC) and nano-zinc oxide (nZnO) were studied in a master’s dissertation (Carvalho, 2015). The effects of nZnO, gamma radiation, and UV light exposure were assessed. The action of UV light was monitored by variation of carbonyl group along the rPC chains, using carbonyl index (CI). Samples without UV exposition (Table 1.1) showed decrease of CI. Both nZnO nanoparticles and gamma radiation fostered a certain degree of degradation on the rPC chains.

Table 1.1 CI of the materials before UV light exposure (Carvalho, 2015).

(a)Percentage loss (in parenthesis)

The effect of UV light on the CI was surprising (Table 1.2). After 50 h, the CI decreased for all selected materials. The percentage loss (in parenthesis) followed this order: filled rPC (2% nZnO), neat rPC, irradiated rPC (30 kGy), and filled and irradiated rPC (2% nZnO and 30 kGy). After 100 h, there was a recovery of the CI. The percentage loss was lesser according to this order: filled rPC (2% nZnO), neat rPC, filled and irradiated rPC (2% nZnO and 30 kGy), and irradiated rPC (30 kGy). Thérias et al. (Collin, Bussière et al., 2012) suggested that the occurrence of cross-linking at longer exposure periods, based on the decrease of the absorption at 1186 cm–¹ (isopropylidene group) in their investigation of UV photoaging on PC. We assumed that the peroxide radicals produced from the reaction of rPC free radicals with oxygen have been activated by UV radiation. The activated species could have reacted with end groups of the rPC oligomeric chains (recombination) and/or with hydroxyl groups onto the nanoparticles surface through esterification reaction (Figure 1.5). These reactions could have fostered the increase of CI during the UV light exposure.

Table 1.2 rPC CI: (a) neat rPC, (b) irradiated (30 kGy), (c) filled (2% nZnO), and (d) filled and irradiated (2% nZnO and 30 kGy), after UV light exposure (Carvalho, 2015).

(a)percentage loss (in parenthesis)

Figure 1.5 Schematic representation of the reaction between rPC-free radical species with hydroxyl group upon ZnO surface during UV exposure. (Carvalho, 2015)

In order to be used in outdoor applications, weathering in Rio de Janeiro city and accelerated photoaging of a PET/PC blend was investigated (Pires, Mendes et al., 2015). The effect of UV radiation on the thermal and mechanical properties and morphology were assessed. The OM analysis (Figures 1.6 and 1.7) showed the coexistence of, at least, three phases – one rich in PET (matrix), the other rich in PC (dispersed droplet) and an interfacial region between them, made of PET/PC copolymer (compatibilizing agent) produced in situ.

Figure 1.6 OM images of PET/PC (80/20 wt/wt%) (unexposed): (a) 25 °C and (b) 280 °C. (Reproduced with permission from H. M. Pires, L.C. Mendes, S. P. Cestari, and V.J.R.R. Pita, Materials Research, 2015, 18, 4. ©2015, Materials Research Ibero-American Journal of Materials.)

Figure 1.7 OM images of PET/PC (80/20 wt/wt%) (exposed 2000 h): (a) 25 °C and (b) 280 °C. (Reproduced with permission from H. M. Pires, L.C. Mendes, S. P. Cestari, and V.J.R.R. Pita, Materials Research, 2015, 18, 4. ©2015, Materials Research Ibero-American Journal of Materials.)

Considering the weathering (Figure 1.8), the CI of PET showed a slight trend to decrease. The PC CI remained practically constant. For accelerated aging, the decline of the PET CI was more pronounced, while the PC CI was constant. They showed that PET phase acted as a shield against the PC degradation.

Figure 1.8 CI as a function of exposure time for (a) natural and (b) accelerated photoaging. (Reproduced with permission from H. M. Pires, L.C. Mendes, S. P. Cestari, and V.J.R.R. Pita, Materials Research, 2015, 18, 4. ©2015, Materials Research Ibero-American Journal of Materials.)

Aiming to create an anti-UV film to be used as protective coating for window frames of houses and buildings, we added different contents (1–3%) nanoparticles of zinc oxide (ZnO) to a recycled poly(ethylene terephthalate) – PET – and polycarbonate – PC – blend (Pires, Mendes et al., 2015). The variation of the rPET and PC CIs of the samples exposed to UV light is shown in Figure 1.9. The rPET and PC CIs of A0 and D0 samples (unfilled and UV exposed, 45/166 h) slightly decreased. The A1 and D1 samples (filled with 1% of nZnO and UV exposed, 45/166 h) showed the lowest values of rPET CIs, but no change was noticed in PC ones. The A3 and D3 samples (filled with 3% of nZnO and UV exposed 45/166 h) showed rPET CIs higher than P0 sample, while the PC ones were slightly lower. Fechine, Rabello et al. (2002) pointed out that PET underwent some oxidation and photo-oxidation reactions during UV aging. Diepens and co-workers showed the degradation of PC chains through photo-oxidation reactions (Diepens & Gijsman, 2011). The rPET ester and PC carbonate bonds were able to be attacked by the UV light.

The variations of CI were slight. This could be related to the small exposure time (approximately 2 and 7 days). Photolysis (degradation reaction) of rPET and PC chains predominated in samples A0, D0, A1, and D1. For A3 and D3 samples, the esterification reaction prevailed over photolysis and this could explain the increasing of PET CIs. In the case of A1 and D1, the PC CIs remained constant probably due to free radicals recombination since the esterification reaction is not operative for PC oligomers. The results indicated that the sample with 3% of nZnO attenuated the UV damage on the rPET/PC matrix.

The ½MSE is LFNMR technique – which improves the accuracy of the acquisition of the original signal – made easier the detection of high stiffness regions in samples. The ½MSE-FID was used to calculate the percentage of rigid and flexible phases of the polymeric matrix. As illustration, the ½MSE-FID curves of the samples D0, D1 and D3 are shown in Figure 1.10. For P0, the percentages of rigid and flexile phases were 94 and 6%, respectively. The percentage of the flexible phase of A0 sample increased 27% due to UV degradation. The expected protection in the nanocomposite with nZnO (1 wt%) was not observed – the ½MSE-FID pattern was similar to A0. A slight effect was noticed in the nanocomposite with nZnO (3 wt%) – the percentage of the flexible phase was 14%. For longer UV exposure times, the D0 sample showed 75 and 25% of rigid and flexible phase, respectively, evidencing progressive degradation. Even with the presence of the nZnO (1 wt%), the A1 sample showed an increase of the flexible phase (44%), indicating higher UV damage. Both A3 and D3 nanocomposites showed the same percentage of flexible phase (23%). This percentage was lower than those of A0 and D0. Then, the sample with highest content of nZnO was less susceptible to the action of UV light than the other samples, indicating potential application of this material as barrier to UV radiation.

Figure 1.9 CI: P0 sample (unfilled and unexposed), AO/D0 samples (unfilled and UV exposed, 45/166 h), A1/D1 samples (filled with 1% of nZnO and UV exposed, 45/166 h), and A3/D3 samples (filled with 3% of nZnO and UV exposed, 45/166 h). (Reproduced with permission from H. M. Pires, L.C. Mendes, S. P. Cestari, D. C. Rodrigues, G. C. Mattos, and R. P. Cucinelli Neto, Journal of Nanoscience and Nanotechnology, 2015, in press. ©2015, American Scientific Publishers.)

Figure 1.10 ½MSE-FID: D0 sample (unfilled and UV exposed at 166 h); nanocomposite with 1% of nZnO – D1 sample (UV exposed at166 h); nanocomposite with 3% of ZnO – D3 sample (UV exposed at 166 h). (Reproduced with permission from H. M. Pires, L.C. Mendes, S. P. Cestari, D. C. Rodrigues, G. C. Mattos, and R. P. Cucinelli Neto, Journal of Nanoscience and Nanotechnology, 2015, in press. ©2015, American Scientific Publishers.)

Zirconium phosphate nanofiller (ZrP) modified with long chain amine (octadecylamine, Oct) was incorporated to linear low-density polyethylene (LLDPE) and polyamide-6 (PA-6) in order to create new materials (Mendes, Silva et al., 2012; Mendes, Silva et al., 2014; Silva, 2015). We systematically investigated a lamellar ZrP synthesized at nanometric scale, and its intercalation with Oct. The platelet/surfactant interactions and the organization of the surfactant in the interlayer space were studied as a function of three amine:ZrP molar ratios (0.5:1, 1:1, and 2:1). We studied the influence of Oct on the structural, thermal, crystallographic, and morphologic characteristics of the zirconium phosphate. The FT-IR spectra of the samples are shown in Figure 1.11. The Oct absorptions (2957, 2918, 2850, 1470, and 720 cm–1) were registered. The absorptions around 3593 and 3511 cm–1 ascribed to the hydrogen bonds of P–OH and H–O–H groups (Díaz, Mosby et al., 2013) were absent in samples with higher amine:phosphate ratio. The PO–+H3N(CH2)17CH3 salt was obtained by replacing water molecules. The absorptions at 1251 and 1073 cm–1 disappeared. New bands at 1198 and 1140 cm–1 were detected due to the increase of amine content. We noticed for all modified samples the +H3N group absorptions (1562 and 1542 cm–1, asymmetric and symmetric angular deformation, respectively). These structural changes, and the enlargement and shift of peaks in the region of P–O and P–OH absorptions evidenced that the octadecylamine was intercalated inside the ZrP platelets. Then, chemical reactions and interactions between ZrP layers and long-chain amine were successful in similar to observed by Gérard and Espuche (2012).

The ¹H LFNMR domain curves of the PA-6 and nanocomposites are shown in Figure 1.11. Table 1.3 presents the T1H and percentage domain for all materials. Nuclear magnetic resonance is a powerful technique. It is feasible to evaluate chemical structure, chain conformation of organic substances, molecular structural organization, molecular dynamic, and relaxation of polymeric blends and nanocomposites. Particularly, hydrogen low field nuclear magnetic resonance allows to investigate and conclude on the organization, heterogeneity and particle dispersion, interaction between polymer, and nanofiller, releasing or restricted chain mobility and so on (Tavares, Nogueira et al., 2007). The domain curves showed two relaxation peaks. At relaxation time lesser than 10,000 ms the domain was related to the chain mobility of the amorphous phase, while those between 50,000 and 250,000 ms were associated to the chain mobility of the amorphous chains constricted among PA-6 lamellae and crystalline phase. This phase is currently responsible for controlling the relaxation process. The domain curves of the nanocomposites sharply showed that both domains were disturbed – in general shifted to higher relaxation time. With respect to the T1H, its behavior could be discussed in terms of the interaction between polyamide and ZrP/ZrPOct. Considering PA-6/ZrP, its amorphous phase showed increasing of T1H and T1H percentage domain. On the contrary, its crystalline phase presented decrease of both parameters. The result is ascribed to the interference of the ZrP on the PA-6 hydrogen bonds. The increase of free amine and carboxyl indices supports these findings. For ZrPOct nanocomposites, in general, its amorphous phase showed the increase of T1H and the decrease of T1H percentage domain. The crystalline phase displayed marked increase of T1H, but the T1H percentage domain was practically constant. The behavior could also be ascribed to the polymer/nanofiller interaction. Great interaction implies restriction on mobility and thus larger T1H values. The arising of new hydrogen bonds between the PA-6 and ZrPOct could not be enough to alter significantly the T1H percentage domain. The formation of intercalated and/or exfoliated nanocomposites could be expected.

Figure 1.11 FT-IR spectra of phosphate composites. (Reproduced with permission from L.C. Mendes. D. F. Silva, L. J. F. Araujo, and A. S. Lino, Journal of Thermal Analysis and Calorimetry, 2014, 118, 3, 1461. ©2014, Springer Science+Business Media.)

Table 1.3 TG/DTG data for phosphate compounds. (Reproduced with permission from L.C. Mendes. D. F. Silva, L. J. F. Araújo, and A. S. Lino, Journal of Thermal Analysis and Calorimetry, 2014, 118, 3, 1461. ©2014, Springer Science+Business Media.)

Nanocomposite based on linear low-density polyethylene (LLDPE) and lamellar α-zirconium phosphate (α-ZrP) modified with octadecylamine (Oct) was prepared. The long chain hydrocarbon amine was compatible with LLDPE chain and also increased the interlamellar spacing, allowing polymer intercalation. New diffraction peaks at low reflection angles in the LLDPE/ZrPOct sample strongly revealed that a nanomaterial was reached. The WAXD and ¹H LFNMR analysis showed the admittance of the LLDPE chains into the filler galleries, indicating the successful formation of a nanoscale material. The relaxation curves of the materials are shown in Figure 1.12. The absence of relaxation times related to the filler in the nanocomposite domain curve is an important indication that good dispersion and filler/polymer interaction have occurred, as reported by Rodrigues et al. (Rodrigues, Tavares et al., 2009). The relaxation time of the LLDPE/ZrPOct was also moved but the shift was smaller and the two peaks were slightly enlarged. This confirms that the LLDPE chains entered into the lamellae of the organically modified ZrPOct. The Oct plasticized the LLDPE chains – they are freer to move and the relaxation time decreased. Then, there is a strong indication that a partially intercalated/exfoliated material was obtained in LLDPE/ZrPOct.

Figure 1.12 ¹H LFNMR domain curves of LLDPE, LLDPE/ZrP and LLDPE/ZrPOct. (Reproduced with permission from L.C. Mendes, D. F. Silva, and A. S. Lino, Journal of Nanoscience and Nanotechnology, 2012, 12, 12, 8867. ©2012, American Scientific Publishers.)

From our point of view, the presence of nanotechnology in everyday life is irreversible. In addition, polymeric nanocomposites improve the effectiveness and performance of materials, also including sustainability.

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Chapter 2

Biodegradable Polymer–Carbon Nanotube Composites for Water and Wastewater Treatments

Geoffrey S. Simate

School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa

Corresponding author: [email protected]

Abstract

Synthetic polymers derived from petrochemical products have been used extensively in numerous applications of nanocomposites. However, environmental pollution resulting from the use of non-biodegradable polymers and limited availability of petroleum resources have become serious concerns now than ever before. Hence, in the recent past, a significant amount of research has been directed towards finding alternatives to non-renewable and non-degradable petroleum-based polymers. However, most of the biodegradable polymers have inferior mechanical properties and low thermal distortion temperatures, which limit their use in many applications. Thus, attempts to improve the properties of biopolymers have been carried out by many researchers. Several studies in this regard have paid particular attention to improving the physical behavior of biopolymers using the fundamental properties of carbon nanotubes (CNTs). This chapter discusses the synthesis and applications of biodegradable polymer–CNT composites for water and wastewater treatments.

Keywords: Biodegradable polymers, starch, cellulose, chitosan, carbon nanotubes, nanocomposites, wastewater treatment, heavy metals, dyes

2.1 Introduction

Synthetic polymers derived from petrochemical products have been used extensively in several applications as they are considered to be suitable alternatives to traditional metallic/inorganic materials (Thakur et al., 2014a; Thakur & Thakur, 2015). This is because polymers have several user-friendly and industrial advantages including low density, low abrasiveness, facile synthesis, low cost, and are easily modified (Thakur et al., 2014a; Thakur & Thakur, 2015). However, environmental pollution caused by the use of non-biodegradable polymers and limited availability of petroleum resources have become serious concerns now than ever before (Lu et al., 2009; Thakur et al., 2014b). Furthermore, the non-biodegradable polymers are unsuitable for short-term applications such as surgical sutures. Hence, in recent past, significant amount of research have been directed toward finding alternatives to non-renewable and non-degradable petroleum-based polymers. At the moment, many industries throughout the world that deal with polymers have begun designing and engineering new polymers that possess eco-friendly properties (Thakur & Singha, 2010a–e). Indeed, many biodegradable polymeric materials have been developed and a lot of them have already been industrialized (Yang et al., 2007; Thakur & Voicu, 2016).

Depending on their origins, biodegradable polymers may be classified into three major categories: (1) synthetic polymers (Wee et al., 2006; Yang et al., 2007), (2) polyesters produced by microorganisms (Solaiman et al., 2003; Yang et al., 2007), and (3) polymers originating from natural resources such as starch, cellulose, chitin, chitosan, lignin, and proteins (Peniche et al., 2003; Yang et al., 2007; Thakur et al., 2014a). Indeed, most biodegradable polymers have very good properties that are comparable to many petroleum-based polymers (Moura et al., 2008). However, some of the biodegradable polymers have poor physical properties such as high brittleness, poor processability, poor moisture and gas barrier, and low thermal distortion temperatures, which limit their applications (Lindblad et al., 2002; Yang et al., 2007; Moura et al., 2008; Mekonnen et al., 2013). Thus, over the years, research efforts seeking to improve the properties of biodegradable polymers have been undertaken (Bhattacharyya et al., 2008; Moridi et al., 2011; Lin et al., 2011a,b). Essentially, considerable research efforts have been made to develop green composites from different renewable resources (Thakur et al., 2012a–c). Several studies in this regard have been focused on improving the physical behavior of biodegradable polymers using the fundamental properties of additives (Cao et al., 2007, 2008; Moridi et al., 2011). Additives or reinforcements of particular interest in this Chapter are carbon nanotubes (CNTs). Unique physical properties combined with high aspect ratios and low density of CNTs have rendered them attractive for a new generation of multifunctional and high-performance engineering composites (Kim et al., 2009). For example, the inclusion of CNTs gives good conductivity in the polymer matrix. The resulting conductive polymer nanocomposites possess properties similar to those of some metals and inorganic semiconductors, whilst retaining polymer properties such as flexibility and easiness of processing and synthesis (Parga et al., 2014). Essentially, in a biopolymer/CNT composite, the CNTs and a biopolymer are like a symbiosis; CNTs help to improve the properties (e.g., mechanical strength) of a biopolymer, while a biopolymer helps to reduce the cost of CNTs for adsorption, and the fabricated composite solves the problem of separating CNTs from aqueous medium (Gupta et al., 2013).

Besides reinforcement, research studies have also established that CNTs are important components in self-healing polymer nanocomposites (Thakur & Kessler, 2015). Ideally, self-healing materials are a class of new and emerging smart materials that have the ability to repair damage caused by mechanical usage over time similar to the self-healing functionality that is observed in many living organisms (Gosh, 2008; Pandey & Takagi, 2011; Thakur & Kessler, 2015). The CNTs are particularly suitable in the preparation of self-healing polymer nanocomposites because of their high surface to volume ratio, and unique thermal, optical, mechanical and electrical properties (Thakur & Kessler, 2015).

This Chapter discusses the synthesis and applications of some of the biodegradable polymer-CNT composites for water and wastewater treatment. Though synthetic polymers and natural polymers that contain hydrolytically or enzymatically labile bonds or groups are biodegradable (Lu et al., 2009), this chapter will specifically focus on biodegradable polymers derived from natural renewable resources. Moreover, natural polymer-based materials are found abundantly in nature (Singha & Thakur, 2008; Thakur et al., 2012a–c).

2.2 Synthesis of Biodegradable Polymer–Carbon Nanotube Composites

2.2.1 Introduction

As already stated, there are various ways that biodegradable polymers can be classified depending on their origin, i.e., natural, synthetic, or microbial polymers. This introductory section will only briefly define natural polymers.

Natural biodegradable polymers are polymers formed naturally during the growth cycle of living organisms (Rosa & Lenz, 2013; Vroman & Tighzert, 2009), and these polymers are also called biopolymers (Vroman & Tighzert, 2009). The formation of biopolymers generally involves enzyme-catalyzed reactions and reactions of chain growth from activated monomers which are formed inside the cells by complex metabolic processes (Rosa & Lenz, 2013). Classes of natural polymers based on their sources (plants or animals) are shown in Table 2.1. As already discussed, the most interesting attribute of natural polymers is the positive environmental impact arising from the fact that these natural resources are renewable and can be eliminated easily at the end of their life cycle (Singha & Thakur, 2008; Thakur et al., 2013a–e). Caloric value of some natural polymers may also be recovered (Pappu et al., 2015). This chapter will only focus on starch, cellulose and chitosan as they are the widely used biopolymers.

Table 2.1 Classification of natural polymers based on their source.

2.2.2 Starch–Carbon Nanotube Composites

Starch is a low-cost, renewable, and biodegradable polymer which is found abundantly in plants where it is stored in granule form and acts as an energy reserve (Ma et al., 2008a; Vroman & Tighzert, 2009; Rosa & Lenz, 2013). It is mainly composed of two homopolymers: amylose (a mostly linear molecule) and amylopectin (a major and branched component), both of which contain α-D-glucose units (Lu et al., 2009; Rosa & Lenz, 2013). Starch is totally biodegradable and is an environmentally friendly material (Vroman & Tighzert, 2009). Starch granules exhibit hydrophilic properties due to strong intermolecular association resulting from hydrogen bonding formed by the hydroxyl groups on the granule surface (Nabar et al., 2006; Lu et al., 2009). Owing to its hydrophilicity, the internal interactions and morphologies of starch are readily changed by water molecules, and thus its glass transition temperature, the dimension and mechanical properties depend on the water content (Lu et al., 2009). Since starch is highly sensitive to water and has relatively poor mechanical properties compared to other petrochemical polymers, its use tends to be limited (Vroman and Tighzert, 2009). To improve the properties of starch, various physical or chemical modifications of starch have been investigated (Lu et al., 2009). This chapter only deals with CNT additives and/or reinforcements.

A number of studies have shown that CNTs can significantly improve the tensile, shear, flexural, fracture toughness, and thermal properties of polymer composites (Kim et al., 2009). For example, Ma et al. (2008a) observed that the addition of functionalized multiwalled CNTs (MWCNTs) to a starch matrix led to improvements in tensile strength and Young’s modulus. In a study by Ma et al. (2008b), the introduction of MWCNTs in a glycerol plasticized-starch restrained starch re-crystallization, and improved the tensile strength, Young’s modulus and the electrical conductivity of the composite.

Famá et al. (2011) used very small quantities of MWCNTs (0.027 and 0.055 wt%) in starch-based nanocomposites, but the resulting nanocomposites exhibited highly improved tensile and impact properties as a consequence of wrapping the MWCNTs with a starch complex. This showed that there was good dispersion of the filler in the starch matrix and excellent adhesion between phases were achieved. In fact, it is well known that the size, shape, and interfacial adhesion between the polymer and the filler greatly influence mechanical properties of the final composite (Fu et al., 2008; Siqueira et al., 2007; Famá et al., 2011). In general, smaller-size fillers like CNTs lead to materials with better mechanical properties, such as higher Young’s modulus (Wisse et al., 2006; So et al., 2007; Famá et al., 2011).

Cao et al. (2007) studied the use of MWCNTs as filler-reinforcement to improve the performance of plasticized starch. The plasticized starch /MWCNTs nanocomposites were successfully prepared by solution casting and evaporation. The results indicated that the MWCNTs dispersed homogeneously in the plasticized starch matrix and formed strong hydrogen bonding with plasticized starch molecules. Compared with the pure plasticized starch, the tensile strength and Young’s modulus of the nanocomposites were enhanced significantly from 2.85 to 4.73 MPa and from 20.74 to 39.18 MPa with an increase in MWCNTs content from 0 to 3.0 wt%, respectively. The value of elongation at break of the nanocomposites was also higher than that of plasticized starch and reached a maximum value when the MWCNTs content was 1.0 wt%. Besides the improvement in mechanical properties, the incorporation of MWCNTs into the plasticized starch matrix also led to a decrease in water sensitivity of the plasticized starch-based materials.

2.2.3 Cellulose–Carbon Nanotube Composites

Cellulose is the most abundant renewable and biodegradable polymer on earth, and has a widespread industrial uses because of its properties (Halász & Csóka, 2013). Essentially, it is a linear polymer with very long macromolecular chains of one repeating unit, cellobios (Vroman & Tighzert, 2009). Cellulose molecule consists of β-1, 4-D-linked glucose chains, with molecular formula of (C6H10O5)n, through an acetal oxygen covalently bonding C1 of one glucose ring and C4 of the adjoining ring (O’Sullivan, 1997; Samir et al., 2005). Ideally, cellulose differs from starch in that glucose units are linked by β-1,4-glycosidic bonds in cellulose, whereas the bonds in starch are predominantly α-1,4 linkages (Babu et al., 2013).

Cellulose is the predominant constituent in cell walls of all plants (Babu et al., 2013). In plant cell walls, approximately 36 individual cellulose molecule chains connect with each other through hydrogen bonding to form larger units known as elementary fibrils, which are packed into larger microfibrils with 5–50 nm in diameter and several micrometers in length (Zhou & Wu, 2012). Cellulose is insoluble and infusible thus, it is usually converted into derivatives to make it more processable (Ghanbarzadeh & Almasi, 2013). The main derivatives of cellulose such as ethers, esters, and acetals are produced by reaction of one or more of the three hydroxyl groups present in each glucopyranoside repeating unit (Vroman & Tighzert, 2009; Ghanbarzadeh & Almasi, 2013).

Just like other biopolymers, nanoparticles such as CNTs have been used as additives to enhance the performance of cellulose and/or cellulose derivatives (El-Din, 2015). Combining the nanofillers with biobased polymers enhances a large number of physical properties, including barrier properties, flame resistance, thermal stability, solvent uptake, and rate of biodegradability, relative to unmodified biopolymer (Babu et al., 2013). In the recent past, significant efforts have been made in the fabrication of CNT–cellulose nanocomposites by dispersing CNTs into various cellulose matrices. For example, Lu & Hsieh (2010) successfully incorporated MWCNTs in to ultrafine cellulose fibers by electrospinning MWCNT-loaded cellulose acetate (CA) solutions. The MWCNT/cellulose composite produced had increased specific surface area, and a much improved water wettability. The mechanical properties of the fibers were also greatly enhanced with increased MWCNT loading levels. Young’s modulus and tensile strength of the nanocomposite fibers were also enhanced significantly. Pushparaj et al. (2007) fabricated an integrated CNT–cellulose composite that could serve as a building block for a variety of thin mechanically flexible energy storage devices such as supercapacitor, battery, hybrid, and dual-storage battery-in-supercapacitor devices. The robust integrated thin-film structure allowed not only good electrochemical performance, but also the ability to function over large ranges of mechanical deformation, temperatures, and a wide variety of electrolytes.

Zhang et al. (2007) had used dry-jet wet-spinning method to fabricate cellulose/MWCNT composite fibers. The results indicated that the addition of MWCNTs increased the tensile strength of the fibers. However, as the MWCNT loading increased, the tensile strength decreased, which might be attributed to the aggregation of MWCNTs. The results also showed that the storage moduli of all the cellulose/MWCNT composite fibers were higher than that of the pure cellulose fiber, and this increase in modulus was particularly significant at higher temperatures. The electrical conductivity of the cellulose/MWCNT composite fibers also showed significant improvements, though higher MWCNT loadings did not increase the conductivity of the cellulose/MWCNT composite fibers further, possibly because of the misalignment and poor dispersion of the MWCNTs.

Imai et al. (2010) reported the production of CNT/cellulose composite materials using a paper making process. Higher electric conductivity and permittivity values compared to those of polymer-based composite materials were achieved in the study without decreasing the mechanical strength of the paper. The unique CNT network structure is thought to have been the reason for the high conductivity and permittivity values. Fugetsu et al. (2008) also used a common papermaking process to fabricate an electrically uniform CNT/cellulose composite paper. The composite paper was capable of shielding electromagnetic interference (EMI) over the tested range of 15–40 GHz, with absorption as the essential shielding mechanism. The incorporation of CNTs was also found to strengthen the physical properties of the cellulose paper such as its tensile strength and stiffness. The air resistance of the composite, however, was diminished compared to cellulose paper alone. Nevertheless, this problem can be avoided by using CNTs with larger outer diameters (Fugetsu et al., 2008).

Oya and Ogino (2008) obtained a cellulose/CNT composite using the ‘washi’ making process by adding SWCNTs into the pulp suspension. The resultant paper was electrically conductive; however, the electrical conductivities were not uniform. Several other researchers have also reported the production of CNT/cellulose composite materials using methods such as filtration, Langmuir–Blodgett deposition, weaving, and spin coating of dispersed CNT/cellulose solutions (Minami et al., 2006; Yun & Kim, 2007, 2010; Anderson et al., 2010; Virtanen, 2010). The results showed that the incorporation of CNTs in cellulose matrices helped to improve their properties such as thermal stability and mechanical strength.

2.2.4 Chitosan–Carbon Nanotubes Composites

Chitosan is the second most abundant natural polymer on earth (Xiao et al., 2012) that has been used in many forms and applications such as particles, films, gels, membranes, or scaffolds for different targeted applications (Thakur & Thakur, 2014). It is produced commercially by partial deacetylation of chitin, the naturally occurring polysaccharide found in the shells of crabs, lobsters, shrimps, and insects or in the cell wall of fungi and microorganisms (Adeosun et al., 2012; Ghanbarzadeh & Almasi, 2013; Rosa & Lenz, 2013). Structurally, chitosan is a linear polysaccharide composed essentially of β(1-4)-linked glucosamine units together with some proportion of N-acetylglucosamine units depending on the degree of deacetylation of the polymer (Peniche et al., 2003). Unlike most other natural polymers, chitosan is a polymer with a positive charge in aqueous solution (Li et al., 2011). Figure 2.1 shows the structure of chitin and chitosan (Nair & Laurencin, 2007).

Figure 2.1 Chemical structure of chitin and chitosan (Nair & Laurencin, 2007).

Though chitosan is becoming increasingly important because of its promising properties, including its low toxicity, biocompatibility, biodegradability, and nonantigenicity (Xiao et al., 2012), its applications are still limited because of its insolubility in most solvents (Vroman & Tighzert, 2009). Furthermore, its poor mechanical properties also limit its application in a wide range of applications (Xiao et al., 2012). However, chitosan can be chemically modified because of the presence of amino and hydroxyl reactive groups (Vroman & Tighzert, 2009). Currently, two methods are used to prepare chitosan composite materials with improved physicochemical, mechanical, electrical, and thermal properties. One method is the blending of chitosan with other polymers (Xiao et al., 2012). The other method, which is particularly important in this chapter, is the incorporation of nanofillers such as CNTs which are frequently used to reinforce chitosan matrix. For example, Xiao et al. (2012) used ionic liquids to dissolve chitosan and cellulose and to disperse MWCNTs which resulted in the formation of chitosan/cellulose/MWCNT composite membranes and fibers. The characterization results indicated that the incorporation of MWCNTs improved the thermal stability, mechanical properties, and electrical conductivity of the composite materials. Overall, the study provided a green method for preparing high-performance polymer nanocomposites with largely improved tensile properties.

Carson et al. (2009) covalently grafted SWCNTs to chitosan. Firstly, oxidized CNTs were reacted with thionyl chloride to form acyl-chlorinated CNTs which were, subsequently, dispersed in chitosan and covalently grafted to form a SWCNTs/chitosan composite material. Figure 2.2 is a summary of the synthetic scheme for CNT–chitosan composite. The FTIR, SEM, TEM, and solid-state C-13 NMR positively confirmed the bonding of the CNTs and the chitosan. The composite was characterized by the TGA and it exhibited thermal stability.

Figure 2.2 Synthetic scheme for carbon nanotubes–chitosan composite (Carson et al., 2009).

Zarnegar and Safari (2015) synthesized chitosan-modified magnetic CNTs (chitosan–MCNTs) for use as a heterogeneous catalyst for the synthesis of 1,4-dihydropyridines (DHPs). The catalyst was easily separated by an external magnet and the recovered catalyst was reused several times without any significant loss in activity. A combination of the advantages of CNTs, chitosan, and magnetic nanoparticles provided an important methodology for carrying out catalytic transformations. Several other researchers have also produced magnetic chitosan and magnetic CNTs–chitosan nanocomposites (Zhu et al., 2013; Parga et al., 2014; Zarnegar and Safari, 2014).

Venkatesan et al. (2011) prepared chitosan grafted with functionalized MWCNTs (f-MWCNTs) and hydroxyapatite (Hap) (f-MWCNT-g-chitosan/HAp) scaffolds via the freeze-drying method. The cross-linkages in the f-MWCNT-g-chitosan/HAp scaffold were observed by FTIR spectroscopy. The water uptake, retention ability and degradation of composite scaffolds decreased, whereas thermal stability increased with an addition of HAp and f-MWCNT. Cell proliferation in composite scaffolds was twice that of pure chitosan. The results suggest that the f-MWCNT-g-chitosan/HAp composite scaffolds are promising biomaterials for bone tissue engineering. Armendariz et al. (2013) also studied the influence of MWCNTs on chitosan and the potential application of the chitosan/MWCNT composite as a biomaterial for bone tissue engineering. The presence of MWCNT in a chitosan matrix improved its mechanical properties and sustained osteoblast growth and differentiation.

Shawky et al. (2011) prepared composite beads of chitosan by incorporating different CNTs, i.e., SWCNTs, MWCNTs, and carboxylic MWCNTs (MWCNT–COOHs). The TGA results showed that the chitosan/CNT composites were slightly more thermally stable than chitosan alone. Wang et al. (2005) successfully prepared biopolymer chitosan/MWCNTs nanocomposites by a simple solution–evaporation method. The incorporation of CNTs into the chitosan matrix greatly enhanced the mechanical properties, including the tensile modulus and strength of the nanocomposites.

Baek et al. (2008) synthesized chitosan particle/CNT composite materials by electrostatic interactions between chitosan particles and f-MWCNTs. In this case, positively charged chitosan particles and negatively charged f-MWCNTs were reacted with each other by controlling the pH of solutions. The electrorheological (ER) behavior of f-MWCNTs-coated chitosan microspheres were observed under controlled applied electric field by optical microscope. The results showed that f-MWCNTs induced ER properties onto the chitosan microspheres. Liu et al. (2008) electrodeposited CNTs and chitosan onto a carbon paper electrode so as to fabricate a conductive and compatible CNT/chitosan nanocomposite biocathode material for microbial fuel cells (MFCs). The prepared CNT/chitosan nanocomposite biocathode significantly enhanced the activity towards oxygen reduction and demonstrated its potential as effective cathode material for MFC. Essentially, the MFC tests revealed that the electricity generation capacity of the nanocomposite cathode was superior to the MFC control.

The synthesis of MWCNTs/chitosan nanocomposite by direct compounding method was reported by Salam et al. (2011). In this study, suspensions of pure chitosan and pure MWCNTs made separately by sonication were mechanically mixed. The morphological results showed that the formation of MWCNTs/chitosan nanocomposites was successful. The TGA analysis was performed to estimate the homogeneity of the MWCNTs/chitosan nanocomposite and its thermal stability. The thermal stability of the MWCNTs/chitosan nanocomposite showed decomposition in two stages, but at higher temperature compared to pure chitosan and at a lower temperature compared with pristine MWCNTs. The MWCNTs/chitosan nanocomposite was also found to be homogeneous.

Magnetic hydroxypropyl chitosan/oxidized MWCNT composites were fabricated by Wang et al. (2015). The results of FTIR, XRD, and SEM showed that the magnetic hydroxypropyl chitosan/oxidized MWCNT composites were successfully synthesized.

Several other researchers have also found CNTs to be promising nanofillers for the preparation of chitosan nanocomposites because of their excellent mechanical, electrical, and thermal properties (Zhang et al., 2004; Liu et al., 2005a,b; Tan et al., 2005; Kandimalla & Ju, 2006; Ke et al., 2006; Qian & Yang, 2006; Hao et al., 2007; Tkac et al., 2007; Wu et al., 2007; Arias et al., 2009; Chatterjee et al., 2009; Ghica et al., 2009; Janegitz et al., 2009; Chatterjee et al., 2010; Vural et al., 2010; Chatterjee et al., 2010; Kuralay et al., 2011; Li et al., 2011; Aryaei et al., 2013; Chen et al., 2013; Mallakpour & Madani, 2015; Siregar et al., 2015). These studies evaluated CNT/chitosan nanocomposites for many applications including water treatment, biosensors, electronics, scaffolds in bone regeneration, drug delivery, etc.

2.3 Applications of Biodegradable Polymer–Carbon Nanotube Composites in Water and Wastewater Treatments

2.3.1 Removal of Heavy Metals

Heavy metals, termed as elements with atomic density greater than 6 g/cm³ (Gardea-Torresdey et al., 2005; Akpor & Muchie, 2010) or conventionally defined as elements with metallic properties and an atomic number greater than 20 (Tangahu et al., 2011), have serious implications on human health due to their acute and long-term toxicity (Ndlovu et al., 2013). Therefore, it is imperative that heavy metal ions are removed from wastewaters before they are released into the environment so as to avoid their entry into the food chain (Shawky et al., 2011). Though there are many conventional methods that are being used to remove metal ions, including oxidation, reduction, precipitation, membrane filtration, ion exchange, and adsorption, this chapter deals with the removal of heavy metals using biodegradable polymer/CNT composites. However, the application of both starch/CNT and cellulose/CNT nanocomposites in the field of water and wastewater treatment is hardly studied. Therefore, the chapter will only focus on the use of chitosan/CNT nanocomposites which have been widely studied for water and wastewater treatment. Both CNTs and chitosan can adsorb and remove heavy metals from aqueous environments (Salam et al., 2011). Actually, several studies show that CNTs are better adsorbents than activated carbon for heavy metals adsorption (Li et al., 2003; Lu et al., 2006) and the adsorption kinetics is fast on CNTs due to the highly accessible adsorption sites and the short intraparticle diffusion distance (Qu et al., 2013). On the other hand, chitosan is known to have good complexing ability through interactions of its high-content amino and hydroxyl groups with heavy metals from various waste waters (Rhazi et al., 2002a,b; Gamage & Shahidi, 2007; Kandile & Nasr, 2009; Elwakeel, 2010). Therefore, chitosan and CNTs are expected to have synergic effects on the adsorption properties of chitosan/CNT nanocomposites for heavy metals.

The following paragraphs give highlights of some of the research studies that utilized chitosan/CNT nanocomposites to remove heavy metals from effluents. In addition, Table 2.2 is a summary of the chitosan/CNT nanocomposites that have been used to remove heavy metals.

Table 2.2 Studies of the adsorption of heavy metal ions using chitosan/carbon nanotube nano composites.

Qm = maximum sorption capacity (mg/g), ND = not determined; MWCNT-COOH = carboxylic MWCNT

Salam et al. (2011) prepared and packed MWCNTs/chitosan nanocomposites into a glass column and successfully removed Cu²+, Zn²+, Cd²+, and Ni²+ ions from aqueous solution. The adsorption capacities in the study were found to be in the following order: Cu²+ > Cd²+ ≈ Zn²+ > Ni²+. The results also suggested that the fabricated MWCNTs/chitosan nanocomposite could be used for different environmental applications.

Shawky et al. (2011) studied the removal of mercury (Hg) using composite beads made from chitosan with different types of CNTs. In this study, a protected cross-linking method was used for the preparation of the chitosan/CNT beads through the reaction of the beads with Hg²+ ions as the protector. The results showed that beads prepared by the protected cross-linking technique removed 2.5 times more Hg²+ ions from solution than beads prepared by normal cross-linking. The equilibrium adsorption isotherm data of the beads exhibited a better fit to the Langmuir isotherm model than the Freundlich isotherm model.

Popuri et al. (2014) developed a chitosan/MWCNTs composite sorbent by mixing chitosan and f-MWCNTs in 1% acetic acid solution. The adsorption capacity of the composite sorbent was compared with the chitosan beads. The obtained composite adsorbent was used successfully for the removal of Cu²+ ions from aqueous solutions. The adsorption equilibrium data conformed well to the Langmuir model. The maximum capacities of chitosan beads and chitosan/MWCNT composite were found to be 178.57 and 454.55 mg/g for Cu²+ ions, respectively, which shows that chitosan/MWCNT composite performed better than chitosan beads. The kinetic studies indicated that the sorption of Cu²+ ions on chitosan and chitosan /MWCNTs followed a pseudo-second-order model.

The removal of U⁶+ ions from wastewaters using chitosan-modified MWCNTs was studied by Chen et al. (2013). The study found that sorption of U⁶+ ions onto MWCNT–chitosan was dominated by inner-sphere surface complexation rather than by ion exchange or outer-sphere surface complexation. The sorption of U⁶+ ions onto chitosan/MWCNT composite was found to be strongly dependent on pH and independent of ionic strength. The maximum ion removal was at pH of 7, and using the Langmuir model, the maximum adsorption capacity was found to be 41 mg/g.

Wang et al. (2015) fabricated magnetic hydroxypropyl chitosan/oxidized MWCNT composite and applied it as the adsorbent to study the adsorption characteristic of Pb²+ ions in aqueous solution. The results revealed that the adsorption process was strongly dependent on pH of the solution; and that the optimal pH and contact time were found to be 5.0 and 120 min, respectively. The adsorption of Pb²+ ions onto the magnetic hydroxypropyl chitosan/oxidized MWCNT composite was well described by the pseudo-second-order kinetic model, which suggested that the main rate determining step was chemisorptions. The experimental results also showed that the Sips model was more suitable than the Langmuir, Freundlich, and Dubinin–Radushkevich models. The thermodynamic parameters of the adsorption calculated such as free energy (ΔG, –2.304 to –5.078 kJ mol–1), enthalpy (ΔH, 39.03 kJ mol–1), and entropy (ΔS, 138.7 J mol–1 K–1) indicated that the adsorption process was endothermic and spontaneous. In a similar study, Alkhatib et al. (2010) produced a MWCNT/chitosan nanocomposite by immobilizing chitosan polymers onto the surface of the carboxyl oxidized MWCNTs. The nanocomposite was used as an adsorption material for Pb²+ ions removal from aqueous solutions. The residual concentration of Pb²+ ions (0.008 mg/L) in the treated solution was found to be much less than the maximum allowable concentration for the drinking water (0.01 mg/L).

The CNT/chitosan nanocomposites have also been used in electrochemical techniques for assessing the concentration of metal ions. In a study by Janegitz (2011), f-CNT paste electrodes modified with chitosan cross-linked with glutaraldehyde was used to determine the concentrations of Hg²+ ions in natural and industrial wastewater samples, and Cd²+ ions in sediments, human urine, natural, and industrial wastewater samples by anodic stripping voltammetry (ASV). Ideally, the ASV is a voltammetric method for quantitative determination of specific ionic species. The analyte of interest is electroplated on the working electrode during a deposition step, and oxidized from the electrode during the stripping step. The technique quantitatively determines the amount of analyte by measuring the current consumed during the stripping process (Ellis, 1973). It could be seen from the study by Janegitz (2011) that the proposed CNT paste electrode system had a good detection limit with a low deposition time as compared to most of the other methods for the electrochemical determination of Cd²+ and Hg²+ ions. In a similar study by Janegitz (2009), the paste electrode prepared with f-CNTs and chitosan cross-linked with epichlorohydrin showed a sensitive, precise, and accurate response for Cu²+ determinations in industrial wastewaters, natural water, and human urine samples.

2.3.2 Removal of Organic Pollutants

A wide range of organic compounds are currently being used, and many of these become potential water pollutants when they are released into water bodies (Luan et al., 2012). Water pollution due to organic contaminants is a serious issue because of acute toxicities and carcinogenic nature of the pollutants (Yang, 2011; Ali et al., 2012). Among the different types of organic pollutants, dyes (e.g.,