Efficient Removal of Toxic Textile Dyes using Silver Nanocomposites

In the present study biomimetic synthesis of silver nanoparticles using Solanum nigrum and Cannabis sativa were explored. The biosynthesized silver nanocomposites were characterized using UV-VIS spectroscopy, DLS, TEM, FTIR and XRD. The characteristic surface plasmon resonance of Solanum nigrum and Cannabis sativa were recorded at 430 and 445 nm respectively. The average diameter of silver nanocomposites were 34.13 ± 3.10 and 70.93 ± 3.57 nm for Solanum nigrum and Cannabis sativa leaf extract respectively. The release of textile dyes into our surrounding water bodies has toxic effect on human health and marine life therefore removal of these dyes is necessary to protect environment. The nanotechnology has provided a new platform for waste water treatment. Hence the biosynthesized nanocomposites were employed for dye removal from wastewater. The effects of various parameters, such as time of incubation, concentration, pH, and temperature, were studied. The biosynthesized silver nanoparticles are most effective in dye removal at alkaline pH 9 and at 60ºC. The textile industry effluents have high pH and temperature which makes these nanoparticles more appropriate in treatment of these industry effluents. To elucidate the reusability, the decolorizing efficiency of biosynthesized silver nanocomposites were investigated upto 3 cycles. dyes from its aqueous The


Introduction
Nanotechnology is the science for tailoring and manufacturing macro atoms into nano materials. Inorganic nano materials are gaining attention due to their novel and size-related physico-chemical properties as compared to bulk matter [1]. Due to their these novel properties of nano materials, they have been used in a wide range of potential applications in medicine, cosmetics, catalyst, renewable energy and biomedical devices [2][3][4][5][6].
Among nano materials, silver nano materials have been focused due to their distinctive physical, chemical and biological properties as compared to macroscopic materials [7]. Silver nanoparticles have distinctive physico-chemical properties, such as high electrical and thermal conductivity, chemical stability, catalytic activity and non linear optical behaviour [4,[8][9][10]. Besides this silver nanoparticles have also been used as antimicrobial agents, biosensors and waste water purification etc. [11][12][13]. Currently, different methods are known for the synthesis of silver nanoparticles by using chemical, physical and biological routes [14].
The chemical methods are the most popular but the use of toxic chemicals during synthesis produces toxic by-products [15]. The physical methods required ample amount of energy to maintain high pressure and temperature. Although chemical and physical methods may successfully produce pure well-defined nanomaterials, these are also cost oriented and not good for sustainable ecosystem [15]. To overcome the disadvantage of chemical and physical methods, the researchers have migrated to biological systems which utilize ecofriendly technique for the synthesis of nanoparticles [16]. Green nanotechnology is an alternative to the conventional physicochemical approaches for the synthesis of nanomaterials in a clean, benign and eco friendly environment [16]. Various industries, for example, plastics, paper, textile and dyestuffs, devour considerable volume of water, and utilize chemicals and dyes in manufacturing which generates substantial amount of waste water [17,18]. One of the essential contaminant to be perceived in waste water is color which are highly visible and undesirable even in very low concentrations of dyes [19].
It is very well known that dyes enriched wastewaters are extremely hard to treat, since the dyes are recalcitrant molecules (particularly azo dyes), stable to oxidizing agents and resistant to aerobic digestion. The release of dyes into our surrounding water bodies has toxic effect on human health and marine life [20].
Therefore removal of these dyes is necessary to protect environment. The nanotechnology has been provided a new platform for waste water treatment [21]. The researchers have explored the catalytic and dye degradation of silver nanoparticles [22]. So in the present study green synthesized silver nanoparticles are explored for its efficiency in removing the textile dyes.

Materials
Fresh Solanum nigrum and Cannabis sativa leaves were collected from Jamia Millia Islamia campus in the month of January. Silver nitrate, ethanol were procured from Hi-Media. air drying, chopped leaves were mixed to 100 ml 50% ethanol and were kept at 60°C for 10-15 minutes till the color of the solvent changes to light green. The mixture was cooled and filtered through Whatmann filter paper. This filtrate was used as precursor and was utilized in subsequent procedures for biosynthesis. For each plant synthesis of silver nanocomposites was carried out by mixing of leaf extract with freshly prepared aqueous solution of AgNO 3 (2 mM) in 1:9 ratio. Purification was done as described earlier in our lab [23].

UV-VIS spectroscopy
The bioreduction of Ag+ ions by leaf extract will be monitored by measuring the UV-Vis spectra of the reaction medium. UV-VIS spectral studies were done by using a Mecasys Optizen 3220UV spectrophotometer. The experiments were performed thrice.

Dynamic light scattering
DLS measurements for determining the average size and size distribution of the silver nanoparticles were carried out using the spectroscatterer RiNA, GmbH class3B. The dried powder was dispersed in distilled water and all the analysis were done at 20ºC for 10 cycles. Experiments for DLS were repeated thrice.

Fourier transform infrared (FTIR)
Spectra of washed and purified silver nanocomposites were recorded on a Perkin Elmer FTIR spectroscopy using KBr pellets. To obtain good signal to noise ratio, 32 scans of nanosilver were taken in the range 400-4000 cm −1 and the resolution was kept as 4.0 cm −1 .

Removal of dyes by silver nanocomposites
Two local dyes used in textile industry (Yellow and blue) were collected from local vendors of Punjabi Bagh, New Delhi and 10 mg/ml of dyes were dissolved in distilled water. Parameters optimized included concentration of silver nanocomposites (1-4 µg/mL), pH (4.0 to 11.0) and temperature (40, 50, 60, 70 and 80ºC) at constant speed of agitation (180 rpm). The percent decolourization was calculated by taking untreated dye solution as control (100%).

Statistical analysis
ANOVA was performed followed by Dunnets test and values were represented as means of three replicate (n=3) ± SD. The significance level was maintained as p-value <0.05.

Results and Discussion
In the present study two different plant assisted silver nanocomposites were analysed for removal of textile dyes. Although several reports have been published for the synthesis of silver nanocomposites using plants/plant extract, till date Solanum nigrum and Cannabis sativa plants have not been explored for biosynthesis of silver nanocomposites. Solanum nigrum is also known as Black Nightshade or Makoi is a perennial shrub and grows as a weed with agricultural crops, also considered an important shrub in Ayurveda.
Cannabis sativa is also annually herbaceous plant commonly known as marijuana, due to its psychotropic properties its is used in treatment of various ailments. The easy cultivation and availability throughout the year makes these medicinal plants a suitable candidates for synthesis of silver nanoparticles. The colorless solution of silver nitrate turns to dark brown due to the synthesis of silver nanocomposites. The surface plasmon vibrations are responsible for color change of solution.
The peak for surface plasmon resonance were recorded at UV-Vis spectrophotometer. The λmax for Solanum nigrum and Cannabis sativa leaf extract were at 430 nm and 445 nm respectively. The critical parameters such as time, temperature, concentration of leaf extract and silver nitrate which dictates the synthesis were optimized. The Figure 1 shows the effect of concentration of leaf extract and the best results were obtained at concentration of 7.5% for Solanum nigrum ( Figure 1A) and for Cannabis sativa was 5% ( Figure 1B).
Further the concentration of silver nitrate was varied. The results shows that as the concentration of silver nitrate increases the absorption intensity increases upto 2 mM for both the nanocomposites, beyond that intensity started decreasing (Figures 2A  and 2B). To study the effect of time on synthesis, the UV-Vis spectrum of the solution were recorded at different time period (Figures 3A and  3B). The results show that absorption increases up to 2 h for Solanum nigrum and 5 hr for Cannabis sativa, after that it is stable. Effect of incubation time has a significant role in synthesis this has been reported earlier also [24]. The controls of only Solanum nigrum and Cannabis sativa leaf extract and silver nitrate did not show any color and consequently did not have any peak in the visible region.
To determine the hydrodynamic radius of biosynthesized silver nanopartic the DLS were carried out. The DLS pattern reveals that silver nanocomposites synthesized by this method have average diameter of 34.13 ± 3.10 and 70.93 ± 3.57 nm for Solanum nigrum ( Figure 4A) and Cannabis sativa ( Figure 4B) leaf extract respectively. Polydispersity index (PDI) were determined to know the particle size distribution of these biosynthesized silver nanoparticles and PDI was found to be 0.28 for Solanum nigrum assisted silver nanoparticles and 0.33 Cannabis sativa assisted silver nanoparticles. The stability of colloidal solutions were determined by zeta potentials and were found to be +39.75 and +34.25 for Solanum nigrum and Cannabis sativa assisted silver nanoparticles.
These values of zeta potential indicate a long-term stability of the corresponding colloids, which may be due to biomolecules responsible for stabilization of nanoparticles. The size and shape of silver nanocomposites were also determined using TEM.
The TEM data reveals that biosynthesized silver nanocomposites are spherical in shape having 5-17 nm for Solanum nigrum ( Figure 5A) and 12-22 nm for Cannabis sativa ( Figure 5B).  Figure 6B). In all FTIR spectra the -OH peak is most abundant due to the wide presence of the-OH functional group in tannins, flavonoids and eugenol [25]. The other peaks are indicating the presence of organic moieties on the metal surface of particles [25]. The silver nanocomposites were known for its catalytic properties. Therefore the biosynthesized silver nanocomposites were exploited for the hazardous and toxic textile dye removal. The percentage of dye removal known to have been influenced by various parameters such as concentration of silver nanocomposites, time and temperature [13]. The concentration of silver nanocomposites are considered to be a vital factor, therefore effect of concentration of silver nanocomposites were studied. As the concentration of silver nanocomposites increases from 1 to 4 µg/ml, the efficiency of dye removal increases upto 3 µg/ml and after this the dye removal efficiency becomes constant (Figure 7). This may be due to achievement of equilibirium as a given amount of adsorbed has the capacity to adsorb fixed amount of adsorbent and after equilibirum it becomes constant. The Solanum nigrum mediated silver nanocomposites at 3 µg/ml for yellow, blue and mixture of both the dyes are 68.2%, 77.5% and 54.23% respectively while Cannabis sativa mediated silver nanocomposites at same concentration are 27.02%, 45.7% and 25.5%.
The effect of pH on dye removal was also studied, for this, the pH of the solution was adjusted to 4, 7, 9, 10 and 11 and 2 µg/ml silver nanocomposites were added to each solution and all were incubated at 60ºC for 2 hrs with constant agitation and the results obtained are shown in Figure 8. The maximum dye removal (yellow, blue and mixture) were obtained at basic pH that is at pH 9 for both the nanocomposites. This may be due to electrostatic interaction of dyes with nanocomposites which varies with the change in pH [13].
Hence, the alkaline pH was maintained for all other experiments. Another important parameter that taken into account is temperature which greatly influenced potential of dye removal and therefore batch experiments were carried out at different temperatures. The temperature of the solution was varied from 40-80ºC with constant agitation.  assisted silver nanocomposites respectively, after that there is decrease in efficiency. This might be explained by a probable increase in affinity of binding sites of the adsorbent for dye molecules with a corresponding increase in temperature [13]. Rise in temperature may also have resulted in a higher mobility of the dye molecules accompanied by a reduction in the retarding forces acting on the same thereby incrementing the adsorbent efficiency. The decrease of dye removal beyond 60ºC is may be due to denaturation of biomolecules present on silver nanocomposites at higher temperature. Figure 9: Effect of temperature of Solanum nigrum and Cannabis sativa assisted silver nanocomposites on dye removal. All the values are means of triplicate (n=3) ± SD. ANOVA significant at P ≤ 0.05. Figure 10 shows the visual observations of dye removal when incubated with Solanum nigrum and Cannabis sativa assisted silver nanocomposites at a concentration of 3 µg/ml, pH 9 at 60ºC for 2 hrs. The biosynthesized nanocomposites were studied for their reusability and the results shows that they can be reused upto 3 cycles efficiently. The study indicates that Solanum nigrum assisted silver nanocomposites are better in dye removal as compared to Cannabis sativa as the efficiency of dye removal for Solanum nigrum is two times more as compared to Cannabis sativa this may be due to small size of Solanum nigrum assisted silver nanocomposites. Solanum nigrum can be grown easily throughout the year thus it can be used for large scale synthesis of silver nanocomposites. These nanocomposites can be incorporated into membrane filters to enhance their efficiency.

Conclusion
Silver nanocomposites has been successfully prepared by green route using two medicinal plants leaf extract i.e., Solanum nigrum and Cannabis sativa. These biosynthesized silver nanocomposites were implemented for removal of textile dyes from its aqueous solution. The Reactive textile dyes are highly soluble in nature and the total dye is in not taken up by the textile fibres which result in 10-15% of dyes being discharged in wastewater. The study concludes that Solanum nigrum is more effective than Cannabis sativa in the reduction of water impurities from colored dyes. In totality, the study suggests achievement of an ecofriendly and highly efficient adsorbent which may be considered useful for the removal of dyes from textile industries.