Synthesis and characterization of crystalline carboxymethylated lignin–TEOS nanocomposites for metal adsorption and antibacterial activity
© The Author(s) 2016
Received: 16 February 2016
Accepted: 26 May 2016
Published: 13 June 2016
KeywordsLignocellulose wastes Sol–gel mechanism Composites Crystallinity Adsorption Antimicrobial agent
Agricultural wastes owing to low natural degradation rate hinder the carbon cycle and are commonly eliminated by incineration, leading to pollution. Agricultural waste materials, however, are good sources of low-cost adsorbents and value-added product obtained through physico-chemical modifications (Paul and Robeson 2008). A number of biodegradable biocomposites and biofilms have been prepared from natural mixtures (carbohydrates, proteins, lipids, and fibres) obtained in the flour form of raw materials of plant origin, such as cereals, tubers and rhizomes (Maniglia et al. 2014). The natural, renewable, biodegradable and biocompatible polymers have been used for applications such as remediation of industrial effluents (Kumar et al. 2015) and in the medical (Reis and Gomes 2004) field (used in tissue engineering for preparing scaffolds). They are preferred due to their good biocompatibility and mechanical properties similar to those of hard and soft tissues and easy fabrication into a variety of shapes with adjustable interconnecting porosity (Slavutskya and Bertuzzi 2014). When compared to pure polymers, polymer nanocomposites possess many attractive properties, such as enhanced barrier characteristics, increased moduli and strengths, high heat distortion temperatures, reduced gas permeability and decreased absorption in organic liquids (Gulyas et al. 2013). Rice straw (RS), one of the most widely available agricultural wastes, has an annual production of over 600 million tons (Zhang et al. 2010). The hard surface, small bulk density and high amorphous silica content make RS suitable for the synthesis of biopolymer-based biomaterials having wider applications in environmental remediation and therapy (Gupta et al. 2016).
Industrial effluents are the major sources of heavy metal contamination of water resources. Nickel, arsenic and cadmium are some of the toxic pollutants present in wastewater for electroplating, battery, mining and metallurgy, aircraft, pigments and ceramic industries (Salem and Awwad 2011). Bioaccumulation of these heavy metals in the food chain causes serious health hazards and disorders to all the biotic components of the ecosystem, including human beings. Cadmium toxicity results in a wide range of syndromes including renal dysfunction, hypertension, hepatic injury, lung damage and teratogenic effects. Excessive exposure to nickel can significantly increase the risk of lung, cardiovascular and kidney diseases in humans (Heidari et al. 2013). Chromium (Cr), cadmium (Cd) and nickel (Ni) compounds are also widely used in industries, such as leather tanning, electroplating, metal finishing, paint and pigments, and therefore discharge of these metals in large quantities into industrial effluents cannot be ruled out. Water containing a high concentration of these metals can cause serious environmental problems as well as induce toxic and carcinogenic health effects on humans. Therefore, the removal or minimizing the toxicity of the metal ions from wastewaters has been a matter of great concern.
Major methods that have been used to remove metal ions from wastewater include adsorption, chemical precipitation, ion exchange, electrolytic extraction and membrane filtration, yet most of them have disadvantages like high cost, large input of chemicals and incomplete removal. Effective treatment of wastewater containing these environmental pollutants has become a challenging task due to the cost of the treatment and, thus, demanding the development of new alternative cost-effective methods (Li et al. 2012).
A number of raw lignocellulose-based adsorbents have potent metal adsorption capacity (Sciban et al. 2014). Recent studies by several research groups have indicated that RS and agave bagasse have higher adsorption capacity for nickel, cadmium and chromium compared to sorghum or oat straw (Li et al. 2012). The concentration of binding sites in RS polymers could be increased, based on the facts that the natural adsorption capacity of some lignocellulosic materials may be improved with chemical treatments and surface modifications with basic solutions, minerals and organic acids, organic compounds and some oxidizing agents (Mendez et al. 2013).
The present work was undertaken with the aim of preparing inexpensive, effective, crystalline and porous lignin–TEOS based composites using lignin extracted from the RS, which could be an alternate source for treatment of environmental pollutants. The nanocomposite materials thus synthesized were characterized and their potential in industrial waste treatment was assessed by UV spectrum, FT-IR, SEM, XRD, AAS and PSD analyses. The approach was to replace the existing commercial lignin with extracted lignin, followed by chemical surface modification to synthesize biocompatible and biodegradable nanocomposites. Additionally, the antimicrobial activities against various microbial contaminants make them applicable as packaging materials for enhancing the shelf life of food stuffs.
Extraction of lignin from RS and surface modification through carboxymethylation
RS was collected from the rice mill of Bilaspur (C.G.) in India. The extraction of lignin from RS was performed using hot water by the method described earlier (Shweta and Jha 2015).
Surface treatment of lignin was performed as described by the method of Yadav et al. (2014) with minor modifications. In brief, 100 mg of hot water-extracted lignin (HL) was mixed with 72 % ethanol and continuously stirred for 30 min at 25 °C, followed by dropwise addition of NaOH (30 %, w/v) to this mixture. After shaking for 90 min, 15 mL of acetic acid was added and the resulting mixture was stirred for 3.5 h at 55 °C. The solution thus obtained was filtered and suspended in 95 % ethanol and neutralized with acetic acid. The final product was washed thrice with 95 % ethanol and the anionic sodium carboxymethylated lignin (CML) was dried overnight at 60 °C for further use in the synthesis of composite material.
Synthesis of CML–T nanocomposites using TEOS
CML–T composites were synthesized as described by the method described previously (Yang et al. 2014) with minor modifications. Initially, lignin was dissolved in 5 % (w/v) NaOH (pH = 13.0) in 1:1 ratio and subsequently heated at 70 and 95 °C for 1 h each. Different concentrations of TEOS (1, 2 and 3 %) were mixed in the solution with vigorous shaking at 300 rpm. The pH of the solution was adjusted to 6.0 with slow stirring for 3 h to form a gel in the presence of TEOS with aggregation of the mixture. The gel was filtered with Whatman filter paper (Grade: 42) with retention capacity of 1.5 µm and filtration rate at 4 mL/min. The gel was completely dried in cold at 4 °C for 1 week. Dried CML–T composites were crushed and stored in airtight glass vials used for further physico-chemical analyses. An alkaline solution of CML without TEOS was used as the control.
Spectral study of CML–T nanocomposites
The synthesized composites and the control samples were initially crushed in a mortar and pestle, dissolved in preheated deionized water (1:2, w/v) and sonicated using ultrasonic homogenizer (Biologics Inc. 3000) at 40 W with 60 % pulse rate for 30 min. The processed samples were then subjected to spectral analyses (Nassar and Youssef 2012).
Unmodified (extracted lignin), surface-modified lignin (CML) and dried CML–T composite samples were separately mixed with KBr at a concentration of 1/100 mg KBr (w/w) for FT-IR analysis. The spectra were taken in absorption mode in the range of 400–4000 cm−1 with a resolution of 8 cm−1 and 32 scans in an FT-IR spectrophotometer (Thermo Nicolet model Avtar 370 G) as described recently (Kim et al. 2014).
Particle size analysis (PSD)
The particle size of the CML–T composites was determined by a laser diffraction apparatus (Malvern Instruments Ltd, Zetasizer version 6.2, UK). Samples were dissolved in preheated deionized water (1:2, w/v) and sonicated at 40 W with 60 % pulse rate for 30 min (Abdel-Halim et al. 2009). To improve the sample dispersion, 80 % ethanol was used as the solvent. The surface charge and size of the composites were analysed using light scattering at room temperature (25 ± 2 °C) in triplicate and the results were presented as the mean value ± S.D.
X-ray diffraction (XRD)
Crystallinity of the samples (CML–T composites) was determined by X-ray diffraction (XRD) using X-ray diffractometer (PAN analytical 3KW X’pert powder, multifunctional). The X-ray was energized by 45 kV and 10 mA. The step-scan mode was 0.1° 2θ and the time of scan 1 s; 2θ ranged from 20 to 80° (Zhang et al. 2011).
Crystallinity index (I cr %)
Study of electrical conductivity, turbidity, effect of CML–T composites on soil and estimation of organic matters in nanocomposites
0.5 g of lignosulphonate, TEOS and composites were thoroughly mixed mechanically with a magnetic stirrer in a ultrasonic bath in 50 mL of deionized water for 1 h for breaking agglomerates at RT in the electric field with intensity ~100 V/m (Ichkitidze et al. 2013). Further electrical conductivity was measured by conductivity meter 304 (Systronics).
The turbidity could be determined by the clarity of the solution after mixing 0.5 g each of the control and test samples in 20 mL of deionized water (Snowden et al. 2012).
The basic properties that can influence the behaviour of nanocomposites, i.e. texture, organic carbon, pH and electrical conductivity, were determined on the basis of soil fraction according to the standard procedure reported in the Methods of Soil Analysis (SSSA Book Series, Methods in Soil Analysis 1996). Soil sample was collected from the top layer (0–15 cm) of a local agricultural field in Bilaspur (C.G.). Initially, the soil sample was air dried and sieved through a 2 mm sieve to remove grasses, pebbles and stones. Further, it was homogenized using mortar–pestle. 5 g of pre-weighed soil sample was taken in six different flasks. 0.5 g of lignosulphonate, TEOS, CML–1T, CML–2T and CML–3T were mixed in each flask, dissolved in 50 mL of deionized water and kept for continuous shaking at RT for 24 h for soil analysis. The flask containing only the soil sample was taken as the control.
The content of organic matters in CML–T composites was estimated by Lowry’s method (for protein estimation), anthrone test (for carbohydrate estimation) and lipid estimation test (Sadasivam and Manickam 2008).
Scanning electron microscopy (SEM)
The surface morphology of the CML–T composites was visualized using an SEM (Zeiss) with an operating voltage at 10 kV. Images were taken at different magnifications as described earlier (Malarvizhi et al. 2014).
Well diffusion method (Nassar and Youssef 2012) was performed to determine the antimicrobial properties of the composites. The test microorganisms (Gram negative: Pseudomonas aeruginosa MTCC 741, Escherichia coli MTCC 739 and Gram positive: Bacillus subtilis MTCC 441 and Staphylococcus aureus MTCC 96 were spread over nutrient agar (NAM) plates. The well in the centre of the culture plates (pre-incubated at 28 ± 2 °C) was loaded with aqueous suspension containing different concentrations (10, 20, 30 and 40 μL) of CML–T composites prepared in sterile deionized water. The plates were further incubated at 28 ± 2 °C for 24 h and the diameter of the inhibition zone surrounding the well was subsequently measured with a ruler up to 1 mm resolution. The experiments were performed in triplicate and the results were expressed as mean ± SD.
Aqueous solutions of Cd (II) and Ni (II) ions for the batchwise experiments were prepared in a buffer solution (0.1 M HNO3 and 0.1 M HEPES [(N-[2-hydroxy ethyl] piperazine-N′-N-[ethane sulphonic acid])], pH 3.0. The pH of the buffer solution was adjusted with 0.1 M NaOH in the range of 1.0–5.0 for further experimentation. CML–T composites (20, 40 and 60 mg/L) were mixed with 25 mL each of metal solution (0.1 M) and kept for vigorous shaking at ambient temperature (25 + 2 °C) for different time intervals (0, 24, 48 and 72 h). The effects of the initial concentrations of nickel and cadmium ions on adsorption were studied by varying their concentrations at a constant pH value (Parajuli et al. 2005).
Determination of nickel and cadmium biosorption by AAS analysis
Study of the biosorption efficiency and kinetics of biosorbents was conducted under static conditions employing a glass vessel equipped with a rotary shaker. The adsorbent CML–T composites (20, 40 and 60 mg/L) were incubated under shaking conditions (120 rpm) with 100 mL of 0.1 M solutions of nickel and cadmium. The concentration of Ni (II) and Cd (II) ions in the solution was determined at fixed intervals by AAS analysis (atomic absorption spectrophotometer, SL-173, Shimadzu). The amount of Ni (II) and Cd (II) ions adsorbed, qt (mg/g) at different time intervals, was calculated as described in Eq. (4) (Salem and Awwad 2011).
Metal solutions were acid digested and subjected to AAS analysis for determination of total dissolved metal. The test samples were digested in a 1:1 ratio of aqua-regia. Subsequently, samples were kept under vigorous shaking for 24 h at ambient temperature followed by filtration (Whatman filter paper Grade 42 with retention capacity of 1.5 µm and filtration speed rapidity at 4 mL/min). The filtrates were collected in glass vials for AAS analysis (Nardis et al. 2004).
All the experiments were performed in triplicate. The yield obtained for CML–T composites was statistically analysed and their mean ± S.D. was determined. Analysis of average, standard deviation and standard error was performed using Graph Pad Prism 5.
Results and observations
Synthesis of carboxymethylated lignin–TEOS composites and their spectral study
Hot water-extracted lignin from RS was subjected to surface modification by carboxymethylation to obtain free functional groups to increase the binding efficiency (Kumagaia and Matsuo 2013). A higher percentage of CML could be achieved only if the reactivity of the extracted lignin was chemically enhanced, and one of the reactivity-enhancing processes used in our study was carboxymethylation for hybrid synthesis as described earlier (Atwood and Lehn 1996). Previous reports suggested that the lignin yield was dependent on the solvent used (Zhang et al. 2010). Increase in the concentration of alkali might reduce the complexity and molecular size of modified lignin and disrupt the chemical bonds present in lignin, cellulose and hemicelluloses. The carbonium ions in the resulting intermediates react with an electron-rich carbon to form stable carbon–carbon linkages, leading to condensation of lignin (Wang et al. 2013).
Characteristics of the retained CML–T composites
Appearance of shape under scanning electron microscopy
Size (in nm)
% Removal efficiency
65.89 ± 2.37
58.19 ± 0.94
56.00 ± 0.96
81.79 ± 1.89
16.00 ± 0.36
70.72 ± 1.67
Peak positions of various modes of vibration in the FT-IR spectra of lignin, carboxymethylated lignin and CML–T composites during sol–gel reaction
699 and 767
–C≡C–H or C–H bend
Different functional groups of lignin
Phenolic group and carboxyl
=C–H bend and O–H bend
800 and 996
Phenolic group and carboxyl
Crystallinity indices, porosity and grain size of the CML–T nanocomposites
Grain size (nm)
Electrical conductivity, turbidity and effect of nanocomposites on organic matter
Study of electrical conductivity of lignosulphonate, TEOS and nanocomposites
Mili Q water
Specific surface area and pore diameter measurement indicated that the surface area of the composite was high. Therefore, the higher fraction of mesopores contributed to the high degree of adsorption capacity. Previous reports reveal that the turbidity is caused by the presence of particles and coloured material in water. The change in colour of composites with the increasing concentration of TEOS (Fig. 1) may be attributed to the high concentration of particles.
Effects (pH, temperature, electronic conductivity and total dissolved solids) of nanocomposites on organic component (soil)
Sample name (Conc. mg/ml)
Temperature (in °C)
Electronic conductivity and its temperature (°C)
Total dissolved solids (TDS) and its temperature (°C)
Soil sample (control)
249.4 µS (36.3)
539.8 ppm (37.1)
25 mg CML
0.806 µS (41.4)
164.5 ppm (37.2)
50 mg CML
0.801 µS (41.1)
171.9 ppm (37.8)
75 mg CML
0.721 µS (41.2)
193.2 ppm (37.7)
100 mg CML
0.711 µS (40.9)
216.6 ppm (38.1)
25 mg T
6.125 µS (36.5)
3.688 ppt (37.3)
50 mg T
6.098 µS (36.4)
4.198 ppt (37.9)
75 mg T
6.006 µS (35.2)
4.200 ppt (38.1)
100 mg T
5.890 µS (35.7)
4.319 ppt (38.9)
25 mg CML–1T
434.8 µS (36.5)
281.6 ppm (37.3)
50 mg CML–1T
433.9 µS (36.4)
293.7 ppm (37.8)
75 mg CML–1T
465 .7 µS (36.1)
280.6 ppm (38.1)
100 mg CML–1T
446.1 µS (36.6)
288.9 ppm (38.8)
25 mg CML–2T
248.5 µS (36.2)
153.0 ppm (37.2)
50 mg CML–2T
248.1 µS (36.2)
159.4 ppm (36.1)
75 mg CML–2T
244.2 µS (35.1)
167.9 ppm (37.9)
100 mg CML–2T
241.8 µS (34.8)
181.9 ppm (37.8)
25 mg CML–3T
524.4 µS (31.0)
317.4 ppm (37.3)
50 mg CML–3T
517.1 µS (30.9)
321.3 ppm (38.2)
75 mg CML–3T
514.9 µS (30.4)
333.9 ppm (38.8)
100 mg CML–3T
512.0 µS (30.1)
345.8 ppm (39.1)
The estimation of protein (by Lowry’s method), carbohydrate (by anthrone test) and lipid content, respectively, for CML–1T, CML–2T and CML–3T and control samples were found to be as follows: protein = 26.1 ± 0.21, 29.12 ± 0.23, 22.06 ± 0.26 and 10.36 ± 0.22 mg/mL, carbohydrate = 22.38 ± 0.12, 26.18 ± 0.21, 13.69 ± 0.01 and 12.89 ± 0.11 mg/mL, and lipid = 32.91 ± 0.03, 29.63 ± 0.22, 43.49 ± 0.24 and 23.69 ± 0.19 mg/mL (Sadasivam and Manickam 2008).
Antimicrobial activity of CML–T nanocomposite against Gram-positive and Gram-negative bacteria: a comparative study for size of clearance zone as observed by the well diffusion method
Sample concentration (mg/µl)
P. aeruginosa (MTCC 741) (mm)
S. aureus (MTCC 96) (mm)
E. coli (MTCC 739)
B. subtilis (MTCC 441) (mm)
One of the plausible reasons of antibacterial activities of CML–Ts could be their nanoscale size and larger surface area of pathogen, due to which they could easily reach the nuclear content of bacteria (Shameli et al. 2012). In the polymeric matrix, some researcher reported that silicate ions released from the surface of the composite are responsible for their antibacterial activity (Uppuluri et al. 2015). Their mode of antimicrobial action may be related to their ability to inactivate microbial adhesions, enzymes, cell envelope transport proteins, etc. due to their complexation with polysaccharides (Cowan 1999). Lignin is also a known antibacterial agent and tends to inhibit the growth of a wide spectrum of microorganisms and, therefore, may lead to synergistic increase in the antimicrobial activity as one of the components of CML–T composites (Jesionowski et al. 2015).
Adsorption kinetics of CML–T nanocomposites
Our findings clearly suggest that the most efficient biosorbent hybrid for Ni (II) was CML–3T, as it showed the highest level of rough surface with approximately 75 % adsorption at pH 5.0 within 96 h, and for Cd (II) CML–2T was the best biosorbent as it showed 81.79 ± 4.6 % metal adsorption at pH 5.0 in 72 h. This could be possibly because of the crystalline and porous structures of CML–T nanocomposites suitable for biosorption for cationic metal ions at different concentrations.
Regeneration and disposal of adsorption materials
In the present study, RS-based lignin and TEOS composite (CML–T composites) proved to be a good adsorbent of heavy metal ions (cadmium and nickel) from wastewater. The presence of hydroxyl (–OH) and methyl groups (–CH3) in the CML–T nanocomposites provided binding sites for the metal ions. The maximum metal removal (81.79 %) is obtained at 72 h of contact time and at pH value of 5.0, which is at par with any synthetic adsorbent. It can be thus considered as a viable alternative to activated carbon, ion exchange resin and other synthetic adsorbents used for this purpose. Modification of the –OH groups of lignin through carboxymethylation and synthesis of composite from the modified lignin are important for the reaction parameters. The hybrid is thermally stable and its metal sorption capacity is strongly influenced by the crystalline and porous structure of the adsorbent, rather than the composition and quantity of dispersed superficial functional groups. The composites also possess antibacterial property and are proposed to be used as packaging materials to protect valuable stuffs from bacterial contamination. The results therefore suggest a potential application of nanotechnology in the development of natural biopolymer-based biodegradable materials like biofilters for wastewater treatment by removal or reduction in the toxicity of metals present in effluents. This will increase the availability of co-products suitable for producing various chemicals, thus reducing our dependence on non-renewable energy sources.
The results emphasize that it is beneficial to extract lignin using agro-wastes (RS) to produce value-added products which minimize the environmental pollutions created by otherwise unmanageable agricultural wastes. Application of the modified lignin in combination with TEOS could be furthered for the production of advanced biodegradable, eco-friendly biomaterials with tailored properties for making biofilters or biomembranes, which could be utilized for removal of several other environmental pollutants or eliminating the chance of bacterial contaminants.
particle size determination
scanning electron microscopy
HJ provided the concept of study, designed experimental plan and contributed to analysis and inference of results whereas KS contributed through execution and optimization of experiments along with analysis of data and manuscript preparation. All authors read and approved the final manuscript.
The authors wish to thank the Department of Biotechnology, Government of India for financial support under the DBT-BUILDER Project (No. DBT/BT/PR7020/INF/22/177/12). DST-INSPIRE Senior Research Fellowship (DST Registration No. IF110479) to one of us (KS) by the Department of Science & Technology (DST), Ministry of Science & Technology, Government of India is thankfully acknowledged. Technical support from national facilities at STIC, Cochin (India), for FT-IR analysis, National Institute of Immunology (NII), Delhi, for particle size analysis and the National Institute of Technology (NIT), Raipur, for XRD analysis is acknowledged. The Head of Department, Department of Biotechnology is gratefully acknowledged for his moral support during the work.
The authors declare that they have no competing interests.
“This work was supported by the Department of Science & Technology (DST) and Department of Biotechnology, Ministry of Science & Technology, Government of India, in the form of DST-INSPIRE Senior Research Fellowship [DST Registration No. IF110479] to one of us (KS)” and DBT-BUILDER (Sanction Order No. DBT/BT/PR7020/INF/22/177/12) Grant, respectively.
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