Radiation induced silver nanoparticles in ionically crosslinked Alginate beads for efficient catalytic degradation of basic dye

Silver nanoparticles (Ag NPs) are widely used as engineered nanomaterials in many advanced nanotechnologies, due to their versatile, easy and cheap preparations combined with peculiar chemical-physical properties. Their increased production and integration in environmental applications including water treatment raise concerns for their impact on humans and the environment. In this study, gamma radiation dose of 20 kGy was utilized to induce the synthesis silver nanoparticles (Ag NPs) in the alginate micro beads to prepare calcium Alginate/Ag (Ca-ALG/Ag) nanocomposite beads. These beads were then used to degrade toxic basic dyes in waste water. Initially, Ca-ALG /Ag nanocomposite beads were synthesized and characterized using Ultra Violet-visible spectrum (UV-Vis), Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD) and Transmission Electron Microscope (TEM) for confirming the formation of catalyst 5 ml NaBH 4 and the optimum time for complete reduction was 30 min. The pseudo-first order, pseudo second order and intra particle diffusion model used to evaluate the kinetic models and the mechanism of the degradation. Results showed that the degradation kinetics best fit the pseudo-second-order model and Langmuir isotherm model. The Biopolymer-based nanocomposite beads of calcium alginate, and Ag NPs can be applied to reduce dyestuff, where it is economically viable and environmentally friendly.


Introduction
In recent years, nanotechnology and its uses are among the most excited research fields with application in many industrial parts. This is due to their unique physicochemical properties and another advantages such that they can be more specific and highly sensitive material for their selective applications [1]. Indeed, their applications have been applied in many technology areas, for instance, antimicrobial [2], and waste water treatment [3].
Engineered nanomaterials (ENMs) are typically defined as materials smaller than 100 nm in at least one dimension, and they have specific surface functionalizations and size-dependent properties, such as high reactivity and large surface-to-volume ratio, that satisfy their wide range of applications, including in sensing , optics energy, catalysis, biotechnology and so on [4].
Synthesized metal nanoparticles are carried out by various chemical, physical, and biological methods, such as pyrolysis [5], chemical vapor deposition and sol-gel [6]; that is, most of them use toxic reagent and are expensive. Therefore, high efforts in the synthesis of noble metal nanoparticles using environmentally friendly methods are required. These solutions generally are produced by the use of gamma radiation especially in preparation of Ag NPs [7].
Radiation synthesized silver nanoparticles incorporated with biodegradable sodium alginate lead to the formation of alginate micro beads [8]. The synthesis of alginate beads occurs through ionotropic gelation methods which are found to be simple and time-consuming. The incorporation of metal nanoparticles into the porous structures improves the physical and chemical nature of the nanocomposite hydrogel toward a variety of applications [9].
Since the end of the last century, a variety of wastes from chemical and pharmaceutical industries have been released into the environment through wastewater streams. These effluents mainly consist of surfactants, disinfectants, detergents, and personal care products. One of the major forms of pollutants is dye wastes and has become a major environmental threat drawing concerns globally [10]. Currently, most of the dye wastes are being treated using physical and chemical treatment methods but they fail to remove dyes completely considering that they are not degradable in nature [11]. Therefore, it is necessary to take sustainable initiatives that use principles from safe methods and tools to remove the dyes before releasing into the aquatic system.
For ages, catalysis has been one of the most common techniques used for the degradation of organic dyes into nontoxic substances in wastewater [12][6] [13].
Based on previous concepts and facts, this work focused on the clean preparation of silver nanoparticles (Ag NPs) within a biocompatible and environmentally friendly polymer alginate taking the advance of the ionotropic gelation of alginate to form micro beads. The prepared calcium alginate/Ag nanocomposite beads were fully characterized and examined as a highly efficient catalyst for fast degradation of basic dye.

Materials
Sodium alginate (SA), was purchased from Sigma-Aldrich. Calcium chloride dihydrate (CaCl2.2H2O) was purchased from Algomhoria Co. Egypt. Silver nitrate AgNO3 and sodium borohydride were purchased from Acros. Doubly distilled water was used throughout the study.
All reagents were of analytical grade or better and were used as received without further purification.

Preparation of calcium Alginate (Ca-ALG) beads
The preparation of sodium alginate beads by ionic crosslinking was followed as reported in literature with some modifications [14]. Sodium alginate (SA) stock solution was prepared by dissolving five grams of alginate powder in beaker and stirring with magnetic stirrer over night to give homogeneous viscous solution (5% wt/ v). By using a 10 ml syringe the SA was transferred drop wisely into a solution of CaCl2.2H20 (3% wt/v) with continuous stirring. Hydrogel beads were formed instantly; then they were left in contact with the solution for 30 min in order to complete the gelation process. Finally, the resulting beads were rinsed gently with distilled water and dried at 37 0 C for further work.

Radiation preparation of calcium-Alginate/Ag (Ca-ALG/Ag) nanocomposite beads
Certain amount of the calcium alginate beads prepared in the previous step was taken and soaked in a beaker containing 30 ml of silver nitrate solution (1M). The beaker were covered carefully with aluminum foil and left for 24 h for complete loading of Ag ions in alginate beads and then exposed to 20 kGy gamma radiations to enhance the reduction of silver ions to silver nanoparticles. The formed Ca-Alginate/Ag nanocomposite beads were then removed and washed thoroughly using stainless steel grid with distilled water and left to dry at room temperature for further steps.

Catalytic degradation of basic dye
The degradation of basic dye carried out at room temperature. In this study, a volume of 40 ml of the aqueous solution of basic dye with the concentration of 1000 mg/L was introduced in a beaker containing 0.1g of Ca-ALG/Ag nanocomposite beads as a catalyst, and then the mixture was stirred for 30 min, after which 5 ml of NaBH4 (0.5 M) was added and the absorbance of the dye solution was monitored using UV-Vis spectroscope over the 300-700 nm range to measure basic dye solution absorbance samples at 5 min interval during catalytic reduction. The reaction mixture was stirred vigorously using magnetic stirrer for during the entire reaction time. Samples were collected at appreciate intervals and were centrifuged to remove catalyst particles with high-speed centrifugal. After short definite intervals of time, an aliquot of the solution was transferred to a quartz cuvette and its UV-Vis absorbance spectrum was recorded at the specific wavelength of the dye used. The extent of dye degradation was calculated using the relation: Where Ao is the initial absorbance of dye solution and At is the absorbance at different time intervals.
For the effect of dosage of catalyst various amounts of Ca-ALG/Ag nanocomposite beads (0.1, 0.15, 0.2 g) on the degradation % was used.
The effect of reducing agent content (NaBH4) on the degradation % was also, studied by varying the volume (3, 4, 5 ml) with respect to the total volume of dye sample (40 ml). Figure 1 illustrates the photographic preparation and dye degradation by Ca-ALG/Ag nanocomposite beads pseudo-first-order kinetics for adsorption/degradation process of dyes applied using equation 3 [15]: Where qe and qt (mg g -1 ) are the amount of dye degraded at equilibrium and time t (min), respectively, and k1 is the rate constant (min −1 ).
For pseudo-second-order kinetics, linear form applied as in equation 3: k2 (g/mg min) is the equilibrium rate constant. The values of qe and k2 are inferred from the plot of (t/qt) versus t.

Isothermal fitting of data
Equilibrium isotherm equations are used to describe experimental sorption data. Langmuir isotherm linearized model form is given by Equation (5): = + Where, Ce (mg/L) and qe (mg/g) are the liquid phase and solid phase concentrations of dye at equilibrium. qm is the constant related to the single layer adsorption/degradation capacity, and b is a constant for degradation energy.

characterizations
Fourier transform infrared (FT-IR) spectroscopy was measured between 4000-400 cm -1 , using a which was then dried at room temperature. The surface morphology of different beads was taken with a JSM5400 instrument by JEOL-Japan. A sputter coater was used to pre-coat conductive gold onto the surface before observing the microstructure at 10 kV. A UV/VIS spectrometer recording the spectra over the 300-700 nm range was used to measure acid dye solution samples at 30min interval during oxidation degradation.

IR spectroscopy
In the FTIR spectrum of calcium alginate/Ag beads is shown in Figure 2

Transmission Electron Microscope (TEM)
TEM technique is used for the morphological evaluation of nanoparticles and in addition estimation of particle size [18]. Figure 5 represents the TEM image of the Ag NPs incorporated in the Ca-alginate beads by gamma radiation. It is cleared from Figure 5 the formation of similar particles nearly equal in size which estimated to be 15.5 nm.

Catalytic degradation of basic dye
Heterogeneous catalytic degradation processes comprise several steps, such as physical or chemical adsorption, chemical reactions occurring between the species formed at the catalyst surface followed by degradation and reaction products formation (such as halides, metals, inorganic acids, organic compounds), and adsorption/desorption of these products. Therefore, the description of heterogeneous catalysis with specific adsorption modelling tools also involves the consideration of secondary chemical processes (pseudo-adsorption), to which is added the influence of substrate morphology, especially in the inter-particle diffusion stage [19].
Basic dye (sandocryl dye) was selected as a model pollutant to investigate the catalytic ability of the Ca-ALG/Ag nano-catalyst beads with NaBH4 as the reducing agent. NaBH4 was used intensively in the literatures as the reducing agent because of high electron injection capability of the BH −4 ions. It was seen that addition of Ca-ALG /Ag nano-catalyst beads and NaBH4 rapidly weakened the blue color of dye solution to a colorless one after only 30 min. The sandocryl dye absorption peak intensity at 644 nm decreased with time. The time-dependent UV-Vis spectra screening the reduction outline of sandocryl dye by NaBH4 over Ca-ALG/Ag nano-catalyst beads is presented in Figure 6. As can be seen from the figure, the absorption intensity became zero after 35 min, indicating the completion of the degradation reaction. Hence, it could be concluded that the nanocomposite beads efficiently catalyzed the reduction of basic dye by relaying of electrons from BH -4 species to Sandocryl via the Ag NPs, similar work was done by PRANJAL SAIKIA et al, based on using Au/CeO2-TiO2 nano-hybrid for fast degradation of methylene blue [20]. Figure   7 shows the effect of time on the absorbance of sandocryl dye during degradation and the degradation % achieved with time using Ca-ALG/Ag nanocomposite beads as a catalyst.

Effect of different parameters on degradation process
In the case of fast catalytic degradation of dyes it is very important to study the effect of dosage of catalyst and also the content of the reducing agent. Figures 8 and 9 represent the catalytic degradation of sandocryl dye using different amounts of Ca-ALG/Ag nanocomposite beads as a catalyst and various contents of NaBH4 as a reducing agent. It is concluded from these results that

Kinetic study
Study of adsorption degradation kinetics is important because the rate of adsorption or degradation (which is one of the criteria for efficiency of adsorbent) and also the mechanism of adsorption can be concluded from kinetic studies [21]. Figure  In degradation systems, there is the possibility of intra-particle diffusion being the rate limiting step [22]. Intra-particle diffusion model for the degradation process of sandocryl dye using Ca-ALG/Ag nanocomposite beads as a catalyst is shown in Figure 11 and its kinetics parameter presented in table 1 and it is noticed that the plot nearly had one strait portion which means that the catalytic degradation of sandocryl dye by using Ca-ALG/Ag nanocomposite beads as a catalyst is controlled mainly by the intra particle diffusion step. Also, it is noticed from Figure 11 that the intercept is very near to zero which confirms that the fast degradation in our case is generally controlled by intra-particle diffusion mechanism [23].

Isothermal study
Two-parameter isotherm models (Langmuir eq. 4, and Freundlich eq. 5) [24] were tested in the fitting of the degradation data of Sandocryl dye by Ca-ALG/Ag nanocomposite beads titanium [25] [26]. , , are the Langmuir, Freundlich, and Timken, adsorption equilibrium constants, respectively; and are the equilibrium concentration, and the adsorption capacity, respectively. The parameter represents the maximum monolayer degradation capacity (mg g −1 ) and 1/ the adsorption intensity, which provides an indication of favorability and capacity of the adsorbent/adsorbate system [27]. The parameter is related to the adsorption heat; gives the mean adsorption free energy (kJ mol −1 ). And 2 is the corresponding sum of squares error obtained in the fitting experimental data of each model. Figure 12 shows the plotting of the Adsorption isotherm models fitted to experimental degradation of sandocryl dye using Ca-ALG/Ag nanocomposite beads. From the data obtained in Table 1 and monitoring the values of R 2 which was 0.98 and 0.91 for Langmuir, and Freundlich isothermal models respectively. It is concluded from these results that the degradation process of sandocryl dye is best fitted by the Langmuir isotherm model.

Conclusions
Based on our experimental results, the Ca-ALG/Ag nanocomposite beads were successfully synthesized using gamma radiation without using toxic chemicals. The catalytic performance of Ca-ALG/Ag nanocomposite beads for the catalytic degradation of sandocryl dye was tested as a model reaction with NaBH4 as a reducing gent. Catayltic degradation is strongly dependent on different parameter such as time, dose of adsorbent and volume of NaBH4. The catalytic degradation efficiency of sandocryl blue dye was fast in the present condition, 30min and using 0.1 g of catalyst. Results showed that a pseudo-second-order adsorption kinetic was predominant in the adsorption of sandocryl dye onto the nanocomposite beads. The experimental equilibrated adsorption capacity of the nanocomposite beads agrees with Langmuir isotherm model. The efficiency of Ca-ALG/Ag nanocomposite beads as a promising candidate for the catalysis of organic dyes by NaBH4 through the electron transfer process is established in the present study.