Enhanced Removal of Dichlorvos from Aqueous Solution using zinc-silver Bimetallic Nanoparticles Embedded in Montmorillonite-Biopolymer Nanobiocomposites: Equilibrium, Kinetics and Thermodynamic Studies

 

Sahithya K, Nilanjana Das*

School of Biosciences and Technology, VIT University, Vellore-632014, Tamil Nadu, India

 

ABSTRACT:

The present investigation is focused on the application of bimetallic zinc-silver (Zn-Ag) nanoparticles embedded in montmorillonite (MMT)-polysaccharide (chitosan-Ch and gum ghatti-Gg) nanobiocomposites for the removal of dichlorvos (DCV) from aqueous solution. Operating parameters viz., pH (4.0-11.0), contact time (30-360 min), temperature (20-50 °C), initial DCV concentration (20-140 mg/L), composite dosage (0.2-1.2 g/L) were optimized. Zn-Ag/MMT/Ch nanobiocomposite showed the maximum removal of DCV (96.6 %) followed by Zn-Ag/MMT/Gg (85.3 %) nanobiocomposite and Zn-Ag/MMT (64.3 %) nanocomposite. The process followed a heterogenous mode of DCV adsorption by all the composites. The kinetic studies indicated that the DCV adsorption followed pseudo-first order model. Intraparticle diffusion and Boyd plot suggested that the film diffusion was not the sole rate limiting step. The thermodynamic parameters indicated the feasibility and exothermic nature of DCV adsorption. The mechanism of adsorption process was further elucidated by FT-IR, AFM and EDX analysis. Packed bed column studies showed the removal of DCV up to 79.3 % using Zn-Ag/MMT/Ch under 12 cm bed height, 1 ml/min of flow rate and at 0 % dilution. Regeneration studies suggested that the Zn-Ag/MMT/Ch could be reused up to three cycles.

 

 

 

INTRODUCTION:

Dichlorvos (DCV, 2, 2-dichlorovinyl dimethyl phosphate) is an organophosphate insecticide widely used for maintenance and growth of agricultural products to control the internal and external parasites of farm animals and to eliminate insects threatening the household, public health and the stored products^1,2. DCV has shown adverse effects on the nervous system by inhibiting the action on acetyl-cholinesterase.

 

Due to the smaller size and high lipid solubility, DCV is readily absorbed by passive diffusion through lungs, gastrointestinal tract and skin. Absorption occurs readily through all routs of exposure including oral, nasal, dermal, intraperitoneal and intravenous³. The high doses of DCV may lead to headache, vomiting, nausea, blurred vision, excessive sweating, and tightness in chest and muscle tremors at high levels^4-6. The long term exposure may cause breathing problems, coma and even death⁷. The half-life of DCV in aqueous solution ranges from 10 to 40 h and is usually substantially higher in acidic conditions. Hence, significant amount of DCV can be accumulated in water bodies leading to severe water pollution and health hazards⁸. Traditional techniques used for the removal of pesticides include microbial degradation⁹, Fenton degradation¹⁰ and photocatalytic degradation¹¹. However, these techniques are associated with major limitations such as high cost, time consuming and generation of residual products¹². Adsorption has emerged as suitable, effective and economical method for the elimination of wide range of contaminates from aqueous solution¹³. Several research reports highlighted the application of various adsorbents for the removal of pesticides from aqueous environment^14-17. In recent years, clay-biopolymer based nanobiocomposites have become a subject of intensive research due to their ability of nanoscale dispersion in biopolymer matrix, which brings significant improvement in physical and functional properties of both the biopolymers and nanoparticles¹⁸. Clays have been used as nanofillers in biopolymer systems due to improved mechanical properties and low cost of the nanobiocomposites. Furthermore, clays are well known for their higher surface area, high binding capacities and unique layered structure^19,20. Biopolymers are the promising class of materials for removal of various organic or inorganic compounds from wastewater due to their improved properties such as non-toxicity and biodegradability²¹. Recent reports suggest the use of various biopolymer-clay nanobiocomposites and nanocomposites for the removal of pesticides^22-24.  So far, no report is available on the application of nanobiocomposites composed of bimetallic Zn-Ag nanoparticles, montmorillonite, chitosan and gum ghatti. Therefore, in the present study, the nanobiocomposites prepared from bimetallic Zn-Ag nanoparticles, montmorillonite, chitosan and gum ghatti have been used for the removal of DCV from aqueous environment.

 

MATERIALS AND METHODS:

All the chemicals were analytical grade and used without further purification. Dichlorvos (DCV, purity 99.9%), chitosan of high molecular weight (Ch), gum ghatti (Gg), and montmorillonite (MMT) with surface area of 20-40 m² g^-1 were purchased from Sigma Aldrich Chemicals, India. Zinc nitrate (Zn (NO₃)₂) and silver nitrate (AgNO₃) were obtained from Sisco Research Laboratories. The standard solution of DCV (1000 mg L^-1) was prepared using deionised water and the experimental solutions were obtained by successive dilutions of standard DCV solution.

 

Synthesis of Zn-Ag/MMT nanocomposite:

Zinc-silver bimetallic nanoparticles/MMT nanocomposite were synthesised using a simple oxido-reduction method described by Srivatsan et al.²⁵ with minor modifications. Briefly, 10 % (w/v) montmorillonite was dispersed in 80 ml solution containing 240 mg of porcine gelatine. Zn (NO₃)₂ solution (10 mmol/5 ml 0.01 N HCl) solution was added to the above suspension followed by AgNO₃ (10 mmol/5 ml H₂O). After the reaction of 10 min, the pH was raised to 9.5 using 1 N NaOH and the solution was stirred for 2 h at 60 °C. The precipitate formed was centrifuged, washed three times with deionised water and freeze dried at 60 °C to obtain a fine powder of nanocomposite.

 

Preparation of Zn-Ag/MMT-biopolymer nanobiocomposites:

Chitosan based nanobiocomposite (Zn-Ag/MMT/Ch) was prepared by adding 10 % (w/v) Zn-Ag/MMT nanocomposite in 100 ml containing 2 % (v/v) glacial acetic acid and magnetically stirred for 30 min at room temperature. Chitosan (2 %) was added to the suspension and the mixture was stirred for 60 min to obtain a homogenous suspension after which the contents were dried at 60 °C for 4 h. Gum ghatti based nanobiocomposite (Zn-Ag/MMT/Gg) was prepared by addition of 1 % (w/v) gum in 100 ml distilled water containing 10 % (w/v) Zn-Ag/MMT nanocomposite. The pH of the solution was maintained at 4.0 using 1N HCl. The suspension was left overnight allowing the swelling of gum in the solution and dried at 60 °C for 4 h.

 

CHARACTERIZATION:

X-ray diffraction (XRD) patterns of the bimetallic Zn-Ag nanoparticles were recorded on Bruker D8 Advance diffractometer with Cu-Kα radiation in the 2θ range of 20°-80° with a scanning rate of 4 min and step size of 0.02. The thermal behaviour of composites was studied using the thermogravimetric analyser under helium atmosphere at a heating rate of 10 °C min^-1 and differential scanning calorimetry. The BET surface areas of composites were calculated following the standard procedure²⁶. The point zero charge (pH_PZC) of composites was evaluated following the standard method²⁷.

 

Spectroscopic Studies:

The involvement of various functional groups in the adsorption of DCV was studied using FT-IR spectra recorded on Avatar 330 model FT-IR spectrophotometer (Thermo Nicolet Co., USA). The surface elemental composition of composites was analyzed using Noran System Six model Energy Dispersive X-ray Microanalysis System (Thermo Electron Corporation, Japan). Accelerating voltage was kept constant at 15 kV, to facilitate the emission of secondary X-rays. The surface topologies of composites before and after adsorption were analyzed using Atomic force microscope (Nanosurf easyscan-2, Netherlands).

 

Adsorption Experiments:

The batch adsorption experiments were carried out in triplicates and the results were reported as an average. For adsorption experiments, a known weight of adsorbent was mixed with 50 ml of DCV solutions in Erlenmeyer flask at room temperature (30 °C±2) and shaken for a set period of time. The effect of operating parameters viz., pH (4.0-11.0), contact time (30-360 min), initial DCV concentration (10-140 mg L^-1), reaction temperature (10-50°C) and adsorbent dosage (0.2-1.2 mg L^-1) were investigated. At the end of pre-determined time interval, the samples were centrifuged at 10,000 rpm for 10 min and the supernatant was analyzed for the residual DCV concentrations using high performance liquid chromatography (HPLC). The uptake capacity (q) of adsorbent and the removal percentage (R) of DCV were calculated using the following equations.

         C₀ - C_f

q= ----------------- X  V

                M

 

         C₀ - C_f

R= ----------------- X  100

                C₀

 

_{Where, C0 (mg L-1) and Cf (mg L-1) are the DCV concentrations before and after adsorption respectively; V (L) is the volume of DCV solutions and M (g) is the weight of the adsorbents.}

 

Equilibrium, Kinetics and thermodynamic studies:

The equilibrium data were analysed using two-parameter isotherms; Langmuir28, Freundlich29 and Dubinin-Radushkevich (D-R) models30. The kinetic experiments were conducted at optimum conditions and samples were withdrawn at equal intervals for analysis of pesticide concentration. Pseudo-first order31, Pseudo-second order32, Intraparticle diffusion and Boyd plot33 have been used for the modeling of the kinetic data for adsorption of DCV on nanocomposite and nanobiocomposites. The fundamental thermodynamic parameters such as Gibbs free energy (∆G), enthalpy (∆H) and entropy (∆S) were calculated to evaluate the thermodynamic feasibility and the nature of the adsorption process using standard equations.

 

Packed Bed Column Studies:

For packed bed column studies, the industrial wastewater was collected from DCV production unit of pesticide manufacturing company, Chennai, India. The concentration of DCV in the effluent was analysed using high performance liquid chromatography (HPLC). Experiments were conducted in a glass column (15.0 cm in length) having an internal diameter 3.0 cm which was packed with Zn-Ag/MMT/Ch. The column efficiency was studied at various bed heights (4 cm, 8 cm, and 12 cm), flow rates (1 mL/min, 3 mL/min and 5 mL/min) and dilutions (0 %, 25 % and 50 %). The area under breakthrough curves was measured to calculate the total DCV adsorbed. The experimental data was further analysed using a Bed Depth Service Time (BDST) model and Thomas model.

 

_{Desorption and regeneration studies:}

In order to study regeneration and to evaluate the performance of the adsorbent, desorption and regeneration experiments were carried out using deionised water as desorbing agent in a column mode. After every cycle of adsorption and desorption, the column was washed and the generated bed was used in next cycle. The same procedure was repeated for five consecutive cycles.

 

RESULTS AND DISCUSSION:

Characterization:

X-ray diffraction patterns of nanoparticles were determined in the range of 20 to 80 θ and the results are presented in Fig. 1. The XRD pattern of both the ZnO and Ag nanoparticles exhibited well defined peaks thereby, indicating their purity. The XRD pattern of ZnO nanoparticles had peaks at 2θ values of 31.7, 34.4, 36.2, 47.5, 56.5, 62.8 and 67.9 which correspond to the 100, 002, 101, 102, 110, 103 and 112 planes respectively (Fig. 1a). For the Ag nanoparticles, the peaks obtained were at 2θ values of 38.0, 44.2, 64.3 and 77.3 which correspond to the 111, 200, 220 and 311 planes respectively (Fig. 1b). The intensity of the ZnO peaks at 2θ values of 31.7, 36.25 and 56.5 reflected high degree of crystallinity in nanoparticles. In case of Ag, peaks at 2θ values of 38.0 and 64.3 indicated high degree of crystallinity in Ag nanoparticles. The diffraction peaks obtained for bimetallic Zn-Ag nanoparticles had corresponding characteristic peaks which were obtained in both the individual nanoparticles which suggested the co-existence of both Zn and Ag nanoparticles (Fig. 1c). It was also noted that, bimetallic Zn-Ag nanoparticles retain their respective crystalline structures as seen in the individual nanoparticles, suggesting the interaction of one metal on the other forming a bimetallic nano-complex of ZnO and Ag.

 

The surface areas of the composites viz., Zn-Ag/MMT/Ch, Zn-Ag/MMT/Gg and Zn-Ag/MMT were measured as 66 m² g^-1, 43 m² g^-1 and 38 m² g^-1 respectively by BET. TGA analysis suggested a lower weight loss percentage in case of Zn-Ag/MMT/Ch (29.3 %) followed by Zn-Ag/MMT/Gg (41.5 %) and Zn-Ag/MMT (49.9 %). The pH_PZC values of Zn-Ag/MMT, Zn-Ag/MMT/Ch and Zn-Ag/MMT/Gg was found to be 5.6, 6.9 and 5.7 respectively which suggested the composites posses’ positive charge below the pH_PZC and negative charge above the pH_PZC.

 

 

Fig. 1:  X-ray diffraction pattern of (a) ZnO nanoparticles, (b) Ag nanoparticles and (c) bimetallic Zn-Ag nanoparticles

 

Effect of parameters:

Fig. 2a displays the effect of initial solution pH on percentage removal of DCV. Alkaline condition favoured the adsorption of DCV by Zn-Ag/MMT nanocomposite and Zn-Ag/MMT/Ch nanobiocomposite. Maximum removal was noted at pH 7.0, 8.0 and 9.0 by Zn-Ag/MMT/Gg nanobiocomposite, Zn-Ag/MMT nanocomposite and Zn-Ag/MMT/Ch nanobiocomposite respectively. The results suggested that pH values lower than pH_PZC led to maximum removal of DCV molecules from solution which could be due to the electrostatic attraction between the negatively charged composite surfaces and positively charged DCV molecules³⁴. The effect of contact time on removal of DCV by composites was studied in the range of 30-360 min. The removal efficiency of Zn-Ag/MMT nanocomposite and Zn-Ag/MMT/Ch nanobiocomposite reached equilibrium at 120 min where as Zn-Ag/MMT/Gg nanobiocomposite showed maximum removal at 150 min (Fig. 2b). It was observed that the adsorption of DCV was significantly rapid in the initial stages due to abundant availability of active sites on composites surface and with gradual occupancy of these sites the adsorption becomes less efficient in the later stages³⁵. The influence of temperature on the removal of DCV is presented in Fig.

 

2(c). Maximum removal was noted at 30 °C for all the composites. The decrease in DCV adsorption at high temperature was due to the decrease in attractive forces responsible for the DCV adsorption or decrease in the number of binding sites on the adsorbent. Fig. 2d depicts the effect of initial DCV concentration on the percentage removal. The removal was rapid at early stages due to the availability of more number of DCV molecules for adsorption process and gradually decreased with increase in concentration³⁶. Maximum uptake was noted at 60 mg L^-1 by Zn-Ag/MMT nanocomposite and Zn-Ag/MMT/Gg nanobiocomposite whereas Zn-Ag/MMT/Ch showed maximum removal at DCV concentration of 80 mg L^-1. The effect of composite dosage on DCV removal is presented in Fig. 2(e) and the effect of composite concentration was studied in the range of 0.2 to 1.2 g L^-1. Maximum DCV removal was noted at 0.6 g L^-1 for all the Zn-Ag/MMT composites. The enhanced removal with the increase in composite dosage was observed which was due to the availability of exchangeable ions and increased number of active sites on the composites surface³⁷. The removal efficiency was found to be decreased beyond the optimum concentration which cloud be due to aggregation of composite thereby leading to a decreased surface area for the DCV adsorption³³.

 

Fig. 2: Effect of (a) pH, (b) contact time, (c) temperature, (d) initial DCV concentration and (e) dosage on removal of DCV

 

Equilibrium, kinetic and thermodynamic studies:

Among the two parameter isotherms tested, Freundlich isotherm model was found to exhibit the best fit with high correlation coefficient (R²) and low error (APE) values thereby suggesting a heterogenous mode of DCV adsorption on all the composites (Fig. 3a).  The results showed that the adsorption capacity was higher in case of Zn-Ag/MMT/Ch nanobiocomposite (high K_F value, 8.94) compared to other composites (Table 1). Langmuir isotherm model exhibited a poor fit with the experimental data due to low R² values and high error values. D-R isotherm model though having high R² values exhibited a poor fit due to high error values for all the composites. In order to examine the diffusion mechanism involved in the adsorption process, various kinetic models were tested. The kinetic parameters including correlation coefficients (R²), k₁, k₂ and calculated q_e,cal values of pseudo-first order, pseudo-second order and intraparticle diffusion are determined by linear regression as shown in Table 2. It can be observed that the q_e,cal values of two kinetic models viz., pseudo-first order and pseudo second order are very close to the experimental q_e values. Pseudo-first order exhibited best fit compared to pseudo second order with high R² and low error values suggesting the involvement of physical forces in the adsorption of DCV by all the composites (Fig. 3b). The nature of diffusion was further evaluated using intra-particle diffusion model and Boyd plot.  Both the models exhibited a good linearity indicating their significant role in the adsorption. Boyd curves did not pass the through the origin, which suggested the involvement of both the diffusion mechanisms in adsorption of DCV by Zn-Ag/MMT composites (Fig. 3c-d).

 

Fig. 3: (a) Freundlich isotherm model, (b) Pseudo-first order, (c) Intraparticle diffusion, (d) Boyd plot of DCV adsorption onto composites

Table 1: Equilibrium isotherm model parameters for DCV adsorption on Zn-Ag/MMT nanocomposite and nanobiocomposites.

 

Table 2: Kinetic model parameters for DCV adsorption on Zn-Ag/MMT nanocomposite and nanobiocomposites.

 

The experimental data obtained at different temperature (10 to 50 °C) were used to estimate the thermodynamic parameters of adsorption and are presented in Table 3. The values of enthalpy and entropy were calculated from the slope and intercept of the plot of log(q_e/C_e) vs 1/T (Fig. 4). The negative ∆G values of all the studied temperatures suggested that the adsorption of DCV onto nanocomposite and nanobiocomposites were thermodynamically feasible and spontaneous. The positive values of ∆H indicated the adsorption of DCV was endothermic in nature whereas, the positive values of ∆S suggested that the increased randomness at the solid and liquid interface during the adsorption of DCV by all the composites. Similar trend was observed for pesticide adsorption on artificially prepared resin³⁸.

 

Fig. 4: Thermodynamic studies of adsorption of DCV onto Zn-Ag/MMT composites

Table 3: Thermodynamic parameters of DCV adsorption on Zn-Ag/MMT nanocomposite and nanobiocomposites.

 

Spectroscopic studies:

FT-IR spectra representing various functional groups of composites before and after DCV adsorption are shown in Fig. 5a-f. Major participation of primary alcohols (O-H stretch at 3616.53 cm^-1) was observed in case of Zn-Ag/MMT nanocomposite. In case of Zn-Ag/MMT/Ch and Zn-Ag/MMT/Gg, amines (N-H stretch) were found to play a vital role in the DCV adsorption (Fig. 5e-f). Considerable stretches at 776.88, 775.94 and 704.11 cm^-1 suggested the presence of chlorine groups on all composites thereby indicating the adsorption of DCV (Fig. 5d-f). A noticeable change in transmittance was found to be higher in case of Zn-Ag/MMT/Ch as compared to the other composites indicating the major participation of their functional groups in adsorption process.

 

 

Fig. 5: FT-IR analysis

 

AFM analysis was done to gain a deeper insight with respect to the surface properties in case of Zn-Ag/MMT/Ch nanobiocomposite before (Fig. 6a, c) and after (Fig. 6b,d) DCV adsorption. Heterogenous distribution of Zn-Ag nanoparticles on the MMT/Ch surface was noted which further suggested the heterogenous surface roughness. The loss of surface roughness was noted after DCV adsorption which was due to the patchy and high surface coverage of adsorbed DCV. The results of AFM were found to lie in accordance with the equilibrium studies.

 

 

Fig. 6: Surface topology of Zn-Ag/MMT/Ch (a) before and (b) after DCV adsorption. AFM images of 3d layer Zn-Ag/MMT/Ch (c) before and (d) after DCV adsorption

The changes in elemental composition of Zn-Ag/MMT/Ch nanobiocomposite surface before and after DCV adsorption were analyzed using EDX (Fig. 7). The spectrum of nanobiocomposite before adsorption showed the presence of C, O, Mg and Ag elements on the surface (Fig. 7a). The adsorption of DCV was confirmed by the presence of P and Cl peaks as shown in Fig. 7b. A significant decrease in C, Ag and Mg peak intensities was noted which confirmed the involvement of chitosan, bimetallic nanoparticles and MMT of the nanobiocomposite in the DCV adsorption process.

 

Fig. 7: EDX spectra of Zn-Ag/MMT/Ch (a) before and (b) after DCV adsorption

Packed bed column studies:

The removal of DCV was studied as a function of parameters: Bed height (4 cm, 8 cm and 12 cm), flow rate (1 mL/min, 3 mL/min and 5 mL/min) and effluent dilution (0 %, 25 % and 50 %) and the breakthrough curves are presented in Fig. 8. The efficiencies were determined in terms of breakthrough time (t_b), exhaustion time (t_e), total amount of pollutant sent to the column (M_total), amount of pollutant adsorbed (M_ad) and removal (%). As shown in Table 4 maximum removal of 79.3 % was noted at a bed height of 12 cm, flow rate of 1 mL/min and 0 % dilution. In the present study, the column data was evaluated using BDST model and Thomas model. Fig. 9a, shows the plot service time vs bed height at various dilutions. BDST model exhibited a good fit with correlation coefficient values <0.96. The column capacity was noted to be 311.2 mg/L and removal was calculated to be 7*10^-3 L/mg/min (Table 5). A low rate value indicated that a longer bed was needed to avoid a breakthrough. Based on the plot of ln((C_o/C_t)-1) vs volume of effluent treated (V_eff) shown in Fig. 9b. The rate constant (K_TH) which characterizes the rate of solute transfer from liquid to solid phase was found to increase with an increase in dilution which suggested a higher adsorption rate at higher pesticide concentration due to the availability of more functional groups on the surface of Zn-Ag/MMT/Ch at a low DCV concentration.

 

 

Fig. 8: Breakthrough curves for DCV adsorption onto Zn-Ag/MMT/Ch (a) at different bed heights (flow rate-1 mL/min; dilution- 0 %), (b) at different flow rate (bed height- 12 cm; dilution- 0 %) and (c) at different dilutions (bed height- 12 cm; flow rate-1 mL/min)

 

Fig. 9:  (a) BDST model, (b) Thomas model

Table 4: Column parameters obtained at different bed heights, flow rates and dilutions for DCV adsorption by Zn-Ag/MMT/Ch.

	4 cm	8 cm	12 cm
	Bed height (flow rate-1 mL/min; dilution-0 %)
tb (min)	60	180	240
te (min)	180	300	360
Mtotal	27.0	45.0	54.0
Mad	12.9	30.8	42.8
Efficiency (%)	48.0	68.5	79.4
	1 mL/min	3 mL/min	5 mL/min
	Flow rate (bed height-12 cm; dilution-0 %)
tb (min)	240	180	120
te (min)	360	300	240
Mtotal	54.0	135	180
Mad	42.8	30.9	23.3
Efficiency (%)	79.4	22.8	12.9
	0 %	25 %	50 %
	Dilution (bed height-12 cm; flow rate-1mL/min)
tb (min)	240	180	120
te (min)	360	300	240
Mtotal	54	71.5	18.0
Mad	42.8	24.2	11.5
Efficiency (%)	79.4	71.5	63.9

 

Table 5: Bed Depth Service Time model and Thomas model parameters for adsorption of DCV onto Zn-Ag/MMT/Ch at different effluent dilutions.

Effluent dilutions
	0 % dilution	25 % dilution	50 % dilution
BDST parameters
Slope	15.0	15.0	15.0
Intercept	4.0	126.6	179.6
No (mg/L)	311.2	238.9	158.6
Ka (L/mg/min)	7*10-3	1*10-4	8*10-5
R2	0.98	0.96	0.99
Thomas parameters
Veff	240	180	120
Co	150	113	75
KTH (min-1)	1*10-4	2*10-4	3*10-4
Qo (mg/g)	17.1	12.8	9.8

 

Desorption and regeneration studies:

Regeneration of used adsorbent was usually carried out in order to reuse it, avoiding the cost of a new acquisition and minimizing the amount of waste. The reusable nature of Zn-Ag/MMT/Ch was tested for five cycles by altering between adsorption and desorption using a column mode of operation (Fig. 10).  The breakthrough time (t_b), exhaustion time (t_e), total amount of DCV sent to the column (M_total), amount of DCV adsorbed (M_ad), total quantity of DCV desorbed (M_d), removal and desorption percentages for five cycles were calculated and presented in Table 6. Maximum removal of 79.3 % was noted in first cycle after which a decreasing trend was observed. The decrease in DCV removal was found to be insignificant till the third cycle, after which a drastic decrease was noted which suggested the Zn-Ag/MMT/Ch could be reused up to three cycles.

 

Fig. 10: Reuse of Zn-Ag/MMT/Ch during five regeneration cycles (bed height- 12 cm; flow rate- 1mL/min; dilution- 0 %)

 

Table 6: Adsorption and desorption process parameters for different sorption-desorption cycles.

Parameters	No. of cycles
	1	2	3	4	5
tb (min)	240	180	180	120	60
te (min)	360	300	240	240	180
Mtotal (mg)	54.0	44.3	34.5	25.2	23.4
Mad (mg)	42.8	33.7	23.8	12.7	5.4
Md (mg)	36.5	29.8	24.0	14.8	11.0
Removal (%)	79.3	76.0	69.1	50.6	23.1
Desorption (%)	57.9	55.3	53.4	41.2	40.9

 

CONCLUSION:

In the present study, bimetallic Zn-Ag nanoparticles embedded in MMT-biopolymer nanobiocomposites showed an efficient removal of DCV from aqueous environment. Maximum DCV removal was noted in case of Zn-Ag/MMT/Ch under optimized conditions. Equilibrium adsorption data exhibited a best fit to the Freundlich isotherm model, indicating a heterogeneous mode of adsorption of DCV onto the composites. The kinetic data indicated the involvement of physical forces. Intra-particle and film diffusion modes were found to be active in the DCV adsorption. Endothermic nature of the DCV adsorption was well defined by thermodynamic studies. The regeneration studies suggested that the Zn-Ag/MMT/Ch could serve as potential remediation agent for the efficient removal of DCV from aqueous environment.

 

ACKNOWLEDGEMENT:

The authors gratefully acknowledge VIT University for providing necessary laboratory facilities for smooth conduct of work.

 

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

 

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Accepted on 15.03.2017           © RJPT All right reserved