Chapter 1: Biomaterials for Water Purification: Dyes, Heavy Metals, and Pharmaceuticals

Access to pure water is vital for the plant and animal kingdoms and is an emerging problem, as global demand increases. Many researchers and scientists are working to resolve the issue and to develop new materials with enhanced features for water purification.¹ Chitosan is a naturally occurring polysaccharide and possesses remarkable properties, which have attracted particular attention, such as wide availability, renewability, interesting structural features, low cost, and impressive performance in water treatment processes.² Chitosan is well known for its interesting properties, such as being a non-toxic biopolymer, biodegradable, and biocompatible, and it can be obtained commercially from shrimps.³ The polysaccharide polymer exhibits unique functionalities and structural features and is widely used in the fields of agriculture, biomedicine, and in environmental protection.⁴ Chitosan can be obtained from chitin by enzymatic and chemical methods and has also been used in biomedical applications. Chitosan compounds are highly soluble in water due to their polymeric structural features.⁵ The adsorption mechanism offered by chitosan and chitosan-based polymeric membranes has been widely explored for the removal of pollutants.⁶ Chitosan also possesses unique and attractive features that can be applied in the fields of agriculture,⁷ water purification,⁸ cosmetics,⁹ and biomedicine.¹⁰ It also possesses unique antibacterial properties and is a promising candidate for the removal of salts (desalination).¹¹ For example, Motshekga et al. synthesized a chitosan-supported Ag-ZnO nanocomposite with bentonite for the effective removal of disinfectants from water.¹² In another report, Kamal et al. synthesized a chitosan-ZnO nanocomposite coated onto microfibrillar cellulose and studied its antibacterial properties against Escherichia coli.¹³ Certain biosorbents with smaller particles show high capacity in the removal of pollutants from water due to their large surface area, which increases the availability of binding sites present on the adsorbents.¹⁴ Metals and heavy metals constitute a large portion of the pollutants found in waste water from industry. In particular, heavy metals cause environmental problems due to their high toxicity and non-biodegradable characteristics.¹⁵ At certain concentration thresholds heavy metals can cause serious health problems for humans, with adverse effects on cardiovascular, renal, gastrointestinal, peripheral, and central nervous systems.¹⁶ Cellular dysfunction occurs as a result of heavy metals combining with cellular components that contain essential elements, such as O, N, S etc., which in turn leads to modifications of enzymes or proteins.¹⁷ In aquatic bodies, elements such as Hg, Co, Cr, Al, Zn, Ni, Cu, Pb etc. are considered as priority elements to for removal from the water.¹⁸ Readily available biowaste products are used to remove or reduce the concentration of these heavy metals in waste water.¹⁹ 

This chapter discusses the use of biodegradable polymers, i.e., chitosan, for the effective removal of heavy metals, dyes, and pharmaceutical small organic compounds from waste water, as well as the use of biosorbents derived from biowaste and their mechanism of action in the removal process, the development of polymeric membranes with various sized nanomaterials, chitosan-based nanomaterials, chitosan-based nanocomposite membranes, and chitosan-based metal oxide membrane development. The polymeric membranes developed have been used to compare their performance with other polymeric membranes, for the treatment of water contaminated with heavy metals, dyes, and various pharmaceutical molecules.

Characterization of the polymeric membranes was carried out using different techniques. A Malvern Panalytical instrument was used for C, H, and N elemental analysis. Functional groups were identified using Fourier-transform infrared (FT-IR) spectroscopy (Agilent) in the wavenumber range 400 to 4000 cm⁻¹. The morphology of the as-developed polymeric membranes was observed using a Jeol JSMIT800 instrument. The crystallite size, phase, and microstructure of the as-developed polymeric membranes were observed with scanning electron microscopy (SEM) using a Jeol JSMIT800 instrument.

Elemental compositional analysis of the different chitosans (CTS) synthesized was performed with a CHN analyser. From the elemental compositional analysis, a significant decrease in C, H, N composition was observed. Table 1.1 summarizes the C, H, N composition of the various chitosan structures.

SEM analysis was carried out in order to understand the change in morphology of the as-developed membranes. Figure 1.1(a and b) displays the change in morphology of chitosan and chitosan nanocomposites.

FT-IR was performed to understand the change in the functional groups of the modified and unmodified chitosan, and to provide reliable information for prediction of the structural changes taking place in the chitosan. Figure 1.2(a and b) shows unmodified and modified chitosan. The unmodified chitosan shows asymmetric and symmetric stretching of N–H groups appearing at 3400 cm⁻¹ and 3250 cm⁻¹, respectively, while the primary amine appears at 1650–1580 cm⁻¹ as a broader peak. The peaks in the region 1564 cm⁻¹ to 1628 cm⁻¹ relate to the bending vibration of the N–H peak. The modified chitosans, especially chitosan–glutaradehyde (CTS–GL), show a broad –OH peak in the range 3200 to 3400 cm⁻¹. For the modified chitosans the sharp appearance of the N–H bands disappeared due to the formation of cross-linking, while additional bands appeared in the region 2859 cm⁻¹ to 2955 cm⁻¹ due to the presence of CH₂ groups.

To understand the phase and crystallinity of the unmodified and modified chitosan, X-ray diffraction (XRD) spectra were recorded, as displayed in Figure 1.3. The dominant peaks appearing at 2θ values of 10.22° and 19.77° correspond to the unmodified chitosan membrane, while the modified chitosan–CAA shows strong peaks at 2θ = 8.259° and 22.145°, respectively. Modification of the structure of the chitosan can be inferred from the shift in the position of the 1st peak to a lower 2θ, value while the second peak is shifted to a higher 2θ value. The peak broadening in the XRD spectrum shows a high degree of functionalization inside the polysaccharide membrane.

Over the past decades different polymeric membranes have been developed and used efficiently in the purification of water or in water treatment processes. Certain conditions and features are generally needed to optimize the process of separation of contaminants and to improve the efficiency of the separation process.²¹ Nanoparticles (NPs) embedded into the polymeric matrix improve certain parameters, such as stability, flexibility, and separation grade, and can be used for effective removal of contaminants from waste water.²² There are different nanomaterial-based biosensors with various applications (Figure 1.4). Abedini et al. synthesized a new polymeric membrane using TiO₂-cellulose using a sonochemical technique and used it in the water treatment process.²³ The water flux greatly increased with increase in TiO₂ concentration up to 20 wt%. Chaloupka et al. reported antibacterial properties for Ag NPs dependent on their size, shape, and chemistry. Because of their chemoselective nature, Ag NPs are found to decrease the bacterial concentration and spread on the cell membrane.²⁴ Cao et al. synthesized a new membrane by inserting Ag NPs and vitamin C as a reducing agent.²⁵ Haider et al. reported the antibacterial activity of the polyethersulfone membrane against E. coli upon incorporation of Ag NPs.

Cu NPs are robustly used in the synthesis of polymeric membranes. Cu NPs and their derivatives present antibacterial and antifungal properties against different microorganisms.²⁷ Xu et al. synthesized membranes with antibacterial properties for Cu NPs incorporated in chelated polyacrylonitrile membranes. On the other hand, ZnO NPs have attracted much attention due to their diverse antibacterial and antifungal properties and their ability to attract hydroxyl groups, with a larger exterior system compared to other inorganic NPs.²⁸ Similar results were obtained for Al₂O₃ NPs and they showed significant improvement in hydrophilicity and fouling properties.²⁹ The most common methods used in the synthesis of polymers are interfacial polymerization, surface deposition, and structure entrapment.³⁰ Zinadini et al. reported the synthesis of Fe₂O₃ NPs on the formation of polymeric membranes.³¹ Niksefat et al. studied the morphology and performance of a sheet of Si NPs, and as the composition of Si increased the water permeability of the polymer and flux increment also increased.³² 

In recent years one-dimensional (1D) NPs have gained much interest due to their attractive and unique features, such as flexibility, porosity, etc., and they are widely used in sensors for environmental monitoring, bioengineering, etc. One of these interesting materials is carbon nanotubes (CNTs), which display enhanced permeability, physiochemical, mechanical, and thermal properties without any decrease in their selectivity.³³ Celik et al. synthesized a CNT-functionalized polymeric membrane to increase permeability and decrease the fouling rate. The synthesized CNT-functionalized polysulfone polymeric membrane is widely used in the removal of heavy metals.³⁴ Shen et al. synthesized methyl methacrylate-reinforced CNT polymers and showed enhanced water permeability and increased flux, by 62%, compared with pristine composite membranes.³⁵ To exhibit high potential for water treatment processes, polymer membranes have been decorated with ZrO₂ and TiO₂ NPs.³⁶ Pan et al. reported the synthesis of thin-film composite membranes for the removal of various pollutants,³⁷ which also showed remarkable antibacterial properties against E. coli and Staphylococcus aureus.

In recent years, scientists and researchers have developed two-dimensional (2D) nanocomposite materials with superior properties, such as chemically active surface area, and superior transport properties, with respect to their shape, size, and structure³⁸ (Figure 1.5). When incorporated with polymeric membranes, the 2D nanocomposite materials showed superior contaminant removal from water.³⁹ 2D nanomaterials have also been used in the desalination process. Graphene is a 2D material that has been used in the desalination process due to its honeycomb-like structure.⁴⁰ The honeycomb structure facilitates 100% removal of salts from water, as well showing removal of 99% of organic contaminants when coated with PVDF.⁴¹ Graphene oxide (GO) is produced from the oxidation of graphite and possesses more hydrophobic functional groups, such as –NH₂, –COOH, –OH, etc. Water purification can be enhanced by incorporation of GO with the polymeric structure.⁴² Yang et al. reported the synthesis of GO in nanofiltration membranes, which were found to be effective in water and dye purification.⁴³ Xiaowei et al. reported the synthesis of chitosan dispersed with GO and this has been widely used in water purification systems.⁴⁴ Gao et al. synthesized di-vanadium pentoxide (V₂O₅) in the presence of hierarchal TiO₂ NPs for the effective removal of organic contaminants from water.⁴⁵ 

Currently there are larger number of inorganic materials that are three-dimensional (3D) in structure, such as silicates, zeolites, etc. Zeolites, in particular, are used in gas and liquid separation, desalination, and in the formation of polymeric membrane reactors.⁴⁷ Zeolites show enhanced permeability, greater selectivity, high thermal and mechanical properties, etc. The presence of zeolites in the structure increases the hydrophilicity, diversifies the molecular structure, and provides greater stability, as described by Huang et al.⁴⁸ Certain nanocomposite membranes have shown greater chemical reactivity (catalysis) when fixed with CO₂. Ahmad et al. synthesized polymeric membranes with amounts of silica at different concentrations.⁴⁹ The newly formed silica showed greater permeability than the simple membrane and the results showed that antifouling properties were enhanced, which in turn increased the oil and water separation.⁵⁰ Nanoparticles showing fluorescence and sensing properties can be used to detect heavy metals in water treatment applications.^51–55 There are many different 0D, 1D, and 2D nanoparticles used in water treatment, as shown in Table 1.2.

The process of water purification involves removal of contaminants from waste water.¹²⁰ In recent years diversified and novel improved polymeric membranes have been developed for water purification.¹²¹ Various membranes have been developed with biomaterials and other particles deposited on them for efficient removal of contaminants from water. However, the process of removal of large- and small-sized particles depends on the pore size of the particles.¹²² Many processes are used for the removal of contaminants from water, such as microfiltration, nanofiltration, ultrafiltration, etc. In the use of chitosan-like biomaterials, their physical character, adsorption ability, and selectivity determine the contaminant removal from waste water¹²³ (Figure 1.6).

For water treatment, changes in the structure of chitosan with hydrogels or beads,¹²⁴ nanoparticles,¹²⁵ membranes,¹²⁶ fibres,¹²⁷ powders,¹²⁸  etc. are needed. The increased adsorption capacity of the chitosan-supported membranes is due to their elemental composition, surface area, pore volume, size, etc.¹²⁹ Saifuddin et al. reported structural variation in chitosan when doped with Ag NPs and this was used for the removal of pesticides in agriculture. Magnetically activated C-doped chitosan can be used as an adsorbent for the removal of antibiotics and smaller pharmaceutical compounds, such as ciprofloxacin, amoxicillin, erythromycin, etc. Nadavala et al. synthesized a novel chitosan-based membrane with sodium alginate and calcium chloride, which is widely used in the removal of phenols.

Pollutants, such as inorganic oxides, and organic pollutants, such as dyes, drugs, pesticides, detergents, and phenols, play a vital role in the water purification process. For the removal of organic contaminants, the concentration, pH, and contact time influence the absorptivity for removal of the pollutant from waste water. Studies carried out by Bibi et al. summarized the adsorption capacity of modified chitosan membrane-based systems with polyvinyl alcohol (PVA), nanotubes, and silane, which were responsible for the removal of naphthalene.¹⁵³ Fan et al. reported the synthesis of magnetic graphene oxide-modified chitosan for the effective removal of methylene blue.¹⁵⁴ In their reports, the modified chitosan with magnetic graphene oxide was characterized by SEM, XRD, and FT-IR techniques. The change in the parameters for the modified chitosan showed higher adsorption of methylene blue when compared with unmodified chitosan. Chitosan with GO nanocomposites doped with Cu NPs were reported by Khan et al. for the effective removal of 4-nitrophenol compounds from water.¹⁵⁵ For the effective removal of polyethylene glycol (PEG) and PVA, chitosan supported by a silica membrane was used by Herman et al.¹⁵⁶ The study showed enhanced permeability, selectivity, and increased mechanical features for the membrane. The study also showed that higher amounts of PEG resulted in increased pore diameter and increased the flux rate. However, increase in PVA concentration decreased the pore diameter and flux rate. Rosdi et al. synthesized chitosan polymeric membrane with silica added for the effective removal of Pb(ii) ions from waste water.¹⁵⁷ The composite membrane showed greater selectivity and effective removal of Pb(ii) ions compared with the unmodified chitosan. An evacuation permeation process was carried out for the novel development of chitosan membranes containing zeolite, which was further loaded with glutaraldehyde, for the removal of trace metal ions, such as Cr, As, Pb, etc.¹⁵⁸ Carbon nanotubes were loaded with chitosan for the development of polymeric membranes and are effective for the removal of heavy metal ions.¹⁵⁹ A special membrane was also developed with chitosan-biochar (CBS). The as-prepared CBS composite has shown higher absorptivity, which can be attributed to the large pore size of the CBS, which is extensively used in the removal of heavy metals, antibiotics, and pharmaceuticals from waste water.¹⁶⁰ The diverse applications of chitosan, the added compounds, and the material type are summarized in Table 1.3.

Various metal oxides when combined with chitosan show significantly improved adsorption ability and are widely used in the removal of contaminants from waste water. Among these, ZnO, MgO, Fe₃O₄, etc. show a significant improvement because of their high semiconducting characteristics. Among the various metal oxides, ZnO–chitosan has shown remarkable properties due to its increased surface area and semiconducting properties¹⁶¹ (Figure 1.7). The as-formed ZnO–chitosan hybrid materials are effective in the removal of organic contaminants from waste water, compared with unmodified chitosan. On doping with ZnO, the hybrid material showed an increase in the number of active centres, formation of a surface-strengthening effect, and improved surface corrosion resistance properties.

MgO NPs are used in water purification treatments, especially for removal of organic contaminants, due to their high chemical stability and non-toxicity.¹⁶³ MgO NPs were dispersed into a chitosan polysaccharide matrix for the effective removal of reactive blue dye in a very short contact time.¹⁶⁴ Additionally, Fe₃O₄ NPs also showed good adsorption characteristics, especially when used in the magnetic separation process. Abu El-Soad et al.²⁰ prepared chitosan-functionalized recyclable biomaterial for the adsorption of Cu(ii) and Zn(ii) ions in aqueous media (Table 1.4).

Adimule et al. synthesized nanocomposites of rare earth metals and they exhibited enhanced binding sites with considerable increase in their specific surface area.^165–169 These can be further applied by researchers to modify chitosan and study their applications.

Biosorption isotherm models have been utilized to study the initial concentration of dye influencing the biosorption process. The initial concentration of the dye was studied in the range 50–400 mg L⁻¹ at a constant time interval of 400 minutes. With increase in the concentration of the initial dye, the biosorption capacity of the polymer substantially increased. At the initial dye concentration on the polymer, more binding sites are available for biosynthesis of the dye molecule. Transformation from the liquid state to the solid state occurred with the initial biosorption process. Increased mass transfer of the dye molecule occurred with respect to initial dye concentration. Further biosorption capacity of the dye remained constant after some time, which can be attributed to the saturation state of the chitosan polymer, with no more binding places available.¹⁷⁰ Due to saturation of the binding sites, excess dye molecules are not adsorbed. Similar behaviour has been reported for chitosan–PVA, chitosan–MgO, and chitosan-activated C.¹⁷¹ 

Chitosan performance for both anionic and cationic dyes is summarized in Table 1.5. An optimal pH range of 8–11 is required for the removal of cation dye using chitosan polymer, whereas a pH of 2–6.8 (acidic range) is optimal for the removal of the anionic dye. The biosorption process takes place in the temperature range 25 °C–45 °C. The process of biosorption is affected by contact time, initial pollutant concentration, solution temperature, etc. The dye removal efficiency depends upon the solution pH. The pH of the solution affects the charge of the polymer composites and this suggests that the dye removal process is chemisorption.¹⁷² Chemisorption involves electronic interaction and exchange of the charges between the chitosan polymer and the dye molecules. Physical absorption takes place between the binding sites on the polymer.¹⁷³ 

A common definition for the presence of pharmaceuticals in water is a high concentration of organic components, microbial activity, difficult to biodegrade, microbial toxicity, high amount of salt, etc. Removal of pharmaceutical contaminants requires purpose-built high-temperature incineration and cleaning of the passing flue gas.

Reverse osmosis and nanofiltration are also effective processes for the removal of pharmaceuticals from waste water. The pharmaceuticals enter the water supply by excretion.

In this chapter, chitosan supported with various nanomaterials, composites, metal oxides, and other substances has been discussed, along with its importance in the development of polymeric membranes and applications in the purification of water. This approach aims to incorporate the effectiveness of the biological, renewable source chitosan and increase its efficiency in the removal of most metals, including heavy metals, and dyes from waste water by incorporation of various sized (0D, 1D, 2D and 3D) nanomaterials and nanocomposites. The role, mechanism, and stability of the polymeric membrane has been discussed. Chitosan-supported and composite membranes have been found to be efficient for the removal of contaminants from waste water, especially dyes, pharmaceuticals, and heavy metals.

Dr Vinayak Adimule and Dr Shanshanka Rajendrachari conceived the ideas, wrote the paper, and carried out grammatical corrections in the manuscript. Miss Nidhi Manhas was involved in the manuscript arrangement and further corrections, such as typographical and grammatical errors.

Data from the present study are presented in the chapter and further data can be obtained from the corresponding author on request by e-mail.

All authors declare that they have not received any funding from any source or organization.

All authors declare that they do not have any conflict of interest.

All the authors thank Bartin University, Bartin, Turkey and School of Sciences, IGNOU, New Delhi for constant encouragement and support.