Abigail et al, 2016

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<p>Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 Contents lists available at ScienceDire Journal of the Taiwan Institute of Chemical Engineers Journal of the Taiwan Institute of Chemical Engineers ELSEVIER journal homepage: Application of rice husk nanosorbents containing CrossMark 2,4-dichlorophenoxyacetic acid herbicide to control weeds and reduce leaching from soil Evy Alice Abigail Melvin Samuel Ramalingam Division of Industrial Biotechnology, School of Biosciences and Technology, VIT University, Vellore, Tamil Nadu, India ARTICLE INFO ABSTRACT Article history: In the present study, the adsorptive behaviour of 2,4-D on to nanosized rice husk was evaluated for its Received 16 November 2015 effective usage as herbicide carrier. The rice husk waste which is known to cause serious environmen- Revised 17 February 2016 tal problem, was reduced to nanosize (n-RH) via a mechanical method and was loaded with 2,4-D to Accepted 14 March 2016 serve the purpose. The resulting nanosized rice husk was characterized before and after 2,4-D sorption Available online 8 April 2016 by scanning electron microscopy (SEM), Fourier transform infrared analysis (FT-IR), dynamic light scat- Keywords: tering (DLS) and zeta potential analyser. A series of sorption experiments were conducted for optimizing 2,4-D the n-RH to 2,4-D loading concentration where the weight ratio of 1:0.10 and contact time of 90 min was Rice husk optimised. The mechanism of 2,4-D sorption onto the n-RH carrier was evaluated using isotherm and Nanosorbent kinetic models and was found to be a monolayer mode of sorption following chemisorption process. The Nanocarrier sustained release and reduced leaching property of the DnRH nanoformulation was also checked and was Leaching found to exhibit better herbicidal activity against the tested target plant (Brassica sp.) compared with that Sustained release of the commercial 2,4-D, along with increased sustained release of 2,4-D in both water and soil. Based on the environmental friendliness of the carrier, sustained release and enhanced herbicidal activity, the current nanoformulation could be a boon for effective herbicide usage with reduced consumption rate. 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 1. Introduction countries, due to its presence as residues in food grains, vegetables and fruits [4]. Herbicide contamination has become the most significant envi- Once in the environment, the residual 2,4-D from the fields ronmental pollution in the recent years, due to substantial agro- contaminates the surface and ground water sources and subse- nomic and economic benefits [1]. Among the herbicides, quently leads to awful health hazards. Reports do exist on the de- dichlorophenoxyacetic acid (2,4-D), a phenoxyacetic acid tection of residual 2,4-D in the ground water in many countries viz. is the third most widely used herbicide across the globe in agricul- United states, Canada, India, Russia, Poland, Hungary and Germany ture and forestry, for combating a wide range of broad leaf weeds. [5]. The World Health Organization (WHO) has set the maximum Due to its ability to accumulate in organic matter, poor biodegrad- allowable concentration for 2,4-D as 100 ppb in drinking water [6]. ability and high water solubility, it is often detected in the natu- Despite its environmental and health hazards, the usage of 2,4-D in ral water bodies [2]. Although available at a low cost and highly agriculture is inevitable and their contamination of the ecosys- selective, it is a well-known and proven endocrine disruptor, po- tem and effects on humans should be fastened. tential mutagen, and carcinogen, and has genotoxicity [3]. 2,4-D is The conventional herbicide application methods are the major known to cause cytogenetic damage in human lymphocytes, irre- cause for herbicide dissipation from the applied soil surface to the versible eye damage, hepatotoxicity and nephrotoxicity and as well water bodies which eventually reaches the non-target organisms classified as a class 2B carcinogen by the International Agency for and the humans [7]. As the traditional 2,4-D removal methods such Research on Cancer (IARC). Although used for decades, the contin- as coagulation, sedimentation, filtration and disinfection are ineffi- ued usage of 2,4-D has drawn concerns recently, in a number of cient in removing 2,4-D from contaminated water sources [8], it is mandatory to prevent 2,4-D contamination by a suitable applica- tion method that will aid in decreasing the herbicide consumption * Corresponding author. Tel.: +91 9487044822. E-mail address: nanobiolab115@gmail.com, cramalingam@vit.ac.in (R. Chi- rate as well increase its bioactivity. dambaram). The exploitation of the controlled release technology offers 1 Evy Alice Abigail M. and Melvin Samuel S. have contributed equally. attractive ways to mitigate the problems associated with the http://dx.doi.org/10.1016/j.jtice.2016.03.024 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.</p><p>E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 319 conventional application methods such as toxicity, harmful envi- the 2,4-D biosorption efficiency, the RH was reduced to nanosize ronmental impacts and less efficiency The usage of her- from microsize to serve the purpose. bicide nanoformulation for effectual herbicide delivery could in- Therefore, the objectives were framed to check the 2,4-D ad- crease the overall efficiency of the applied 2,4-D and avert all the sorption capacity of nanosized rice husk sorbents (n-RH) followed associated hazards. Among the nanoformulation methods, adsorp- by its characterization using scanning electron microscopy (SEM), tion of herbicide onto a carrier could avoid the undesirable loss of Fourier transform infrared analysis (FT-IR), dynamic light scatter- highly water soluble herbicides like 2,4-D [9], while their encapsu- ing (DLS) and zeta potential analysis. The adsorption isotherms lation in an appropriate carrier could improve the effectiveness of and kinetic models were also evaluated for describing the exper- the active ingredient and as well improve its photostability. Nano imental data and to understand the adsorption mechanism. The carriers using polymers such as chitosan/tripolyphosphate 10] and 2,4-D adsorbed n-RH adsorbent was utilized as a 2,4-D nanofor- [11] were considered as herbicide car mulation(DnRH) for controlled release in water and as well in soil rier for 2,4-D controlled release, but these polymers induces acidic for minimizing the herbicide pollution. Thus, the potential of the formation within the polymer matrix and hence, are not appro- agro-industrial waste rice husk nanoparticles for sustained release priate for many pesticides [12]. To round these issues, Yan et al. of herbicide as well its potential as soil amendment was revealed [13] prepared an herbicide carrier, with increased bioactivity and in this study. photostability, using an environmental waste cyanobacteria which offered more efficiency. And thus, in the verge of searching for a 2. Materials and methods suitable, eco-friendly and low cost carrier for effective 2,4-D deliv- ery, in the present study, the abundantly available agro-industrial 2.1. Chemicals and adsorbate waste, rice husk was chosen as herbicide carrier. In India, rice husk is an easily available agro-industrial waste 2,4-dichlorophenoxyacetic acid was purchased from Sigma generated in large quantities, which is estimated to be about 18- Aldrich, Bangalore, India. Rice husk was procured from a rice mill 22 million tonnes as a rice-milling by-product creating enormous in Vellore, India, for nanoparticle synthesis. All the other reagents environmental problems [14] It consists of cellulose (32%), hemi- were of analytical grade bought from Himedia, Pvt. Ltd. The stock cellulose (21%), lignin (21%), silica (20%) and crude protein (3%). solution was prepared by dissolving 2,4-D in methanol where the is granular in structure, insoluble in water with high chemical sta- required concentration was prepared by diluting the stock solution. bility and mechanical strength as well possess functional groups Before the addition of n-RH adsorbent to the aqueous medium, the like carboxyl, hydroxyl and amidogen which accounts for its pH of the solution was adjusted to desired values using 0.1 N HCI cellent adsorption capacity [15]. The enormously generated RH is or NaCl. usually disposed via landfill, leading to serious water and air pol- lution. Recently, researchers have become interested in using this 2.2. Preparation of rice husk nanosorbents industrial waste as a resource. Due to its excellent adsorption char- acteristics, mechanical strength and chemical stability, RH was re- The procured rice husk was sieved through 68-75 um sieve ported by a number of researchers for the removal of major water and the collected microparticles were subjected to a mechanical pollutants such as heavy metals and dyes [15] from aqueous en- method reported by Anton et al. [19], with minor modifications vironment. In this current study, we an attempt was made to use for obtaining nanosized adsorbents. For the rice husk nanosor- the rice husk for adsorbing 2,4-D herbicide, with an aim to check bent preparation, about 1 g of rice husk microparticles were its potential as a herbicide carrier and as well to evaluate its role suspended in distilled water at a pH of 3.0 which was set using in soil amendment. 0.1 mol of disodium phosphate/citric acid solution. The suspen- The noteworthy effect of adsorbent particle size on pollutant sion was agitated at 10,000 rpm for about 30 min, after which the removal from water has already been recorded. And, it is a proven residue was left to dry in a hot air oven at 60°C for in order fact that the particle size of the adsorbent has direct influence on to obtain the nano-rice husk sorbent (n-RH). the biosorption efficiency [16]. Increase in particle size simultane- ously increases the surface area of the adsorbent. Nanoparticle ad- 2.3. Preparation of 2,4-D nanoformulation (DnRH) using rice husk sorbents of various types has advantages such as tiny particulate diameter, large external surface area, reactive surface areas of high In order, to estimate the amount of 2,4-D sorbed onto the n-RH density, small internal diffusion resistance and great intrinsic re- sorbent, the experiments were conducted, by adding about of activity of surface sites. The nanoparticle adsorbents also has few n-RH to 2,4-D solution at varying weight ratios (0.05, 0.10 and 0.15) shortcomings viz. high energy consumption, strict operational con- in methanol and the percentage of adsorption was determined at ditions and high cost [17]. The usage of Spirulina platensis biomass regular intervals. Based on the preliminary experiments, a pH of nanoparticles for dye removal has been proved for their excellent 5.0 and temperature of 30°C was chosen for the study. The sam- biosorption potential than that of the microparticles with a max- ples collected at regular intervals were centrifuged and the super- imum adsorption capacity of about g [18]. In another natant was filtered prior to 2,4-D analysis in high pressure liquid study, Pleurotus ostreatus, edible mushroom was made as a nanoad- chromatography (HPLC). The amount of 2,4-D adsorbed on to the sorbent for Mn(II) removal from aqueous solution with maximum n-RH carrier was calculated by subtracting the concentration dif- adsorption capacity of 130.625 mg The notion to use rice husk ference between the initial and equilibrium solutions. A control ex- as a nanosorbent combines the advantages of low cost adsorbent periment without the n-RH sorbent was also run in parallel. The with high adsorption efficiency. The n-RH adsorbent also retains obtained equilibrium data was fitted with the isotherm and kinetic high-area-to-volume ratio, less capital with low operational costs models for understanding the mechanism of 2,4-D sorption onto [17]. To the best of our knowledge, no similar report on the use the n-RH sorbent. After attainment of equilibrium, the methanolic of nano-size rice husk as an adsorbent or as an herbicide carrier solutions were centrifuged and the pellet was dried at 60°C, to ob- is available till date. Additionally, to overcome the disadvantage of tain the 2,4-D nanoformulation (DnRH), which was then vortexed high energy consumption and preparation cost of nanoadsorbent, for before storage in air-tight containers. All the experiments the rice husk microparticles were prepared in nano-size via a sim- were performed in triplicates and the statistical analysis of the re- ple mechanical method. This method could reduce the machining sults employed analysis of variance (ANOVA) and the Bonferroni time and reduce associated pollution. Hence, in order to increase post-test (GraphPad Prism version 5.0 software).</p><p>320 E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 2.4. Characterization of the 2,4-D nanoformulations (DnRH) were weighed after washing and drying for dry mass estimation and the mean data was presented. The synthesized n-RH sorbent was characterized using dy- namic light scattering (DLS) (Model: Horizon JUNO 10G-HO), scan- 2.8. Leaching experiments in soil column ning electron microscope (VEGA 3 TESCAN) with EDAX (Bruker) and Fourier transform Infra-red analysis (FT-IR, model: ALPHA-T, A soil column (20 cm high) was constructed with the help of Bruker) in the spectral range of 400-4000 with a res- five PVC rings, (height and diameter of cm) sealed with water- olution for information on its average size, surface morphology and proof tapes and filled with air dried and sieved, red loam soil. functional group identification. The 2,4-D nanoformulation (DnRH) Each of the column was blocked with filter paper at the end and was also checked for changes in the morphology and functional then filled with soil. The columns were left to drain water for group modification via FT-IR and SEM-EDAX. 24 h prior to the DnRH application. The 2,4-D nanoformulation (DnRH) were applied at a concentration equivalent to field appli- 2.5. Sustained release of 2,4-D in water and soil cation rate After application, the column were precip- itated by adding 25 ml of water, which is equivalent to 70 mm of To evaluate the sustained release potential of 2,4-D nanofor- water, for 24 and 48 h. The columns were dismantled for quantifi- mulations (DnRH) in water, about 3.0g of the DnRH was added cation of 2,4-D as well as for the evaluation of herbicidal activity in of distilled water at room temperature with agitation in each rings. A control column was also set where the commercial At definite intervals, 5 ml of the distilled water was 2,4-D was applied at the same rate as the DnRH nanoformulation. withdrawn, and an equivalent amount of distilled water was re- For 2,4-D quantification in soil, about of soil from each ring placed to the flask. The withdrawn samples were measured for was extracted with equal amount of methanol followed by agita- 2,4-D concentration with the help of HPLC. The experiments were tion for 20 min at After agitation, the suspension was allowed done in triplicates and the average mean was expressed. The re- to settle for 30 min for supernatant removal. The procedure was re- lease experiments were carried out till the attainment of 100% re- peated thrice and the collected supernatant was concentrated in a lease of the adsorbed 2,4-D from the n-RH sorbent. rotary evaporator before quantification using HPLC. In case of sustained release of 2,4-D in soil from the nanofor- The herbicidal activity in the columns were also analysed by mulation (DnRH), a thin layer of pesticide free soil (10g) was sowing 10 seeds in each ring at different depths. The germination placed in a Buchner funnel. The soil was amended with the DnRH index was used as indicators for calculating the herbicide presence accounting to 0.5 and 1.0% by weight, respectively onto a nylon at all depths of the column. The percentage of growth inhibition filter cloth of 0.05 mm mesh size paved in the funnel. For the re- was determined using the formulae lease tests, about 40 ml of distilled water was sprayed in for nine times with an interval of in the funnel. The samples Growth inhibition (%) = Lc collected at regular intervals were extracted with equal amount Where, Lc and Lt are the heights of the control and test samples of methanol prior to 2,4-D concentration analysis with the help at any soil depth. of high pressure liquid chromatography (HPLC). For comparison, a control experiment with addition of technical grade 2,4-D was also 3. Results and discussion performed without the addition of n-RH. All the experiments were performed in triplicates and the statistical analysis of the results In an era, where there is a need to regenerate, recycle and reuse employed analysis of variance (ANOVA) and the Bonferroni post- resources, we have come up with a solution of using the agro- test (GraphPad Prism version 5.0 software). industrial waste as a carrier for herbicide. Herbicide pollution is an insolvable issue, as their use cannot be evaded or their pres- 2.6. Quantification of residual 2,4-D ence from the environment cannot be completely removed either. So, the best possible solution for the issue is to control the un- For the residual 2,4-D analysis, the supernatant collected at reg- controllable release of the herbicide into the environment. In or- ular intervals was filtered with a 0.22 um microporous membrane der to achieve a sustained and slow release of 2,4-D, it was ad- prior to analysis on reverse phase HPLC (C18 column, particle size sorbed onto rice husk nanosorbent to make up a 2,4-D nanoformu- of 5 um) coupled with a UV detector at 230 nm, where methanol: lation, which is environmentally friendly. There may arise a ques- water (70:30, pH 2.0) was used as mobile phase. The total percent- tion, for the need for using rice husk in nanoform for increasing age of 2,4-D release after nine watering were calculated using the the adsorption potential of 2,4-D instead of the usual pretreatment formulae methods. Many researchers have stated that the reduction of ad- sorbent size could aid in increased removal due to increase in its surface area [16]. So in the current study, the use of harsh chem- icals for the pretreatment of the adsorbents were averted, but the Where, is the amount of herbicide released in mg and is goal of increasing the adsorption potential was achieved via prepa- the initial amount of herbicide added to the soil in mg. ration of the rice husk in nanosize. Usually, after adsorption stud- ies, the sorbent with the solute is regenerated using chemicals for 2.7. Bioactivity of the new 2,4-D nanoformulation reuse of the sorbent. But, here in this study, the 2,4-D adsorbed on to n-RH nanosorbent (DnRH formulation) was utilized as a car- The herbicidal activity of the DnRH was tested on pots (10 cm rier for sustained of 2,4-D into the environment and hence could high with diameter of 12.5 cm) filled with of plant substrate be termed as a nanoformulation. The prepared Dn-RH formulation (soilrite mix). For the tests, fifteen seeds of non-target plant (Bras- was checked for its slow release potential in water and soil fol- sica sp.) and 10 seeds of target plant (Zea mays) was sown and lowed by its activity check against target plant. To the best of our grown for four days. After four days of growth, the DnRH nanofor- knowledge, this is the first report on the use of a rice husk waste mulation was applied at a concentration equal to the field appli- as herbicide carrier via adsorption phenomenon. This environmen- cation rate In the control pot, an equal quantity of rice tally friendly nanoformulation of 2,4-D prepared in the present husk nanosorbent, without 2,4-D was applied to analyse the influ- study could be a boon for the preparation of new agrowaste based ence of rice husk on the plant growth. After 14 days, the plants nanoformulation in the future.</p><p>E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 321 100 Table 1 0.05 Sorption isotherm parameters of 2,4-D on n-RH nanosorbent. 80 0.10 Langmuir isotherm Freundlich isotherm 0.15 qm (mg/g) (L/mg) R2 Kf n R2 60 24.75 0.0012 0.998 2.871 0.944 0.996 40 Temkin isotherm D-R isotherm 20 (L/mg) R2 qm (mg/g) B R2 0.0084 24.298 0.931 98.49 0.002 0.990 0 Contact time (min) bent. Based on the equilibrium data, the Langmuir isotherm model Fig. 1. Adsorption of 2,4-D onto n-RH nanosorbent at various initial concentrations fitted best with the adsorption data with a high regression co- at different time intervals. efficient (R2) of 0.998 higher than that of Freundlich isotherm (0.996) and Temkin isotherm (0.931) regression coefficient. Based on the R2 value, the isotherm models conformity could be given 3.1. Adsorption of 2,4-D onto n-RH nanocarrier as Langmuir> Freundlich> Temkin isotherm model. Thus, the ad- sorption data was found to be in good conformity with Langmuir To understand the optimal initial 2,4-D concentration and equi- isotherm. Similar conformity of 2,4-D adsorption data to Langmuir librium time for 2,4-D adsorption on to the n-RH nanosorbent isotherm was also observed by other researchers [21,22]. The D-R the adsorption process was estimated at various ranges of n- isotherm was also applied to find out the type of sorption of pro- RH to 2,4-D concentration ratios such as 1:0.05, 1:0.10 and 0.15 (%, cess taken place, where the mean energy of sorption, E was calcu- weight) at regular time intervals. From the results (Fig. 1), it was lated for analysing the sorption process. noticed that the adsorption (%) rapidly increases at the initial hours and reached equilibrium within 90 min. This pattern could be due Meansorptionenergy : E = (-2K)- to the presence of more valent adsorption sites on the n-RH sor- bent, which got occupied as the contact time increased. At an n-RH From the D-R isotherm model, an E value of 15.81 to 2,4-D ratio of 1:0.10 (%), the sorption was found to be quick and which falls under the ion-exchange reaction range of rapid until the equilibrium was obtained. The quick equilibrium So, the type of sorption involved on 2,4-D adsorption onto the within 90 min might be due to the increased surface area of the nanosorbent is a chemical sorption process. nanosize of n-RH sorbent. The surface area increases with the size The kinetic models were also examined as the physical and/or of the nanosorbent [16]. This phenomenon of adsorption based on chemical characteristics of the nanosorbent as well as the mass the movement rate of the pollutant has also been reported by De- transport process could aid in understanding the mechanism of ad- hghani et al. [20]. In case of the present study, the further prepa- sorption. So, to determine the mechanism of 2,4-D adsorption onto ration of nanoformulation was performed at an optimal n-RH to n-RH nanosorbent, the kinetic models such as pseudo first order, 2,4-d concentration of 1:0.10 (%) and contact time of 90 min, based pseudo second order and intraparticle diffusion models were ex- on the results obtained. ploited using the equations In order, to understand the interaction between the 2,4-D molecules and n-RH sorbent, the widely used isotherms models viz. Langmuir, Freundlich, Temkin and D-R isotherm were utilized. Pseudo first order model, log = Among these, the first three models draws the logarithmic equilib- rium concentration in the liquid and solid phase [20]. The Lang- Pseudo second order muir isotherm is used for describing the monolayer sorption into the surface of the n-RH sorbent with an assumption of finite num- Intraparticle diffusion model, ber of identical sites. On the other hand, the Freundlich isotherm is based on heterogeneous surface adsorption whereas the Temkin Where qt and qe are the amount of 2,4-D adsorbed at time isotherm takes into account the adsorbent-adsorbate interactions. 't' and at equilibrium (mg/g), K1 K2(g/mg/min) and Kid The Langmuir, Freundlich, Temkin and D-R isotherm models are (mg/g/min) are the pseudo first order, pseudo second order equa- given by the following equations tion and intraparticle diffusion rate constant. The regression coef- ficients of the kinetic parameters of the used models are tabulated Langmuir: in Table 2. From the results, the pseudo second order model was found to fit best the adsorption data with higher regression coeffi- Freundlich: cient than the pseudo first order model. The pseudo second order Temkin: model did appear linear but failed to achieve the criteria of pro- D ducing the sync between the experimental and calculated qe value, whereas the pseudo second model achieved both the criterion and Where, (mg/g) and qm(mg/g) are the amount of 2,4-D ad- so was considered as best fit. On the other hand, the intraparticle sorbed per g of n-RH sorbent and amount required for monolayer diffusion kinetic model was utilized for understanding the limiting formation; Ce (mg/L) is the concentration of the solution at equi- factors for 2,4-D adsorption onto the n-RH adsorbent. The results librium; K, and B are the adsorption constants; Kf (L/kg) and (Table 2) revealed that the intraparticle diffusion is not the only 1/n are the adsorption coefficient and adsorption constant; E is the rate limiting step. As the plot was not found to pass through the Polanyi potential with the formula (E=RT In(1+1/Ce)); KD is the origin, although regression coefficient was found > 0.99. This sug- sorption energy. gests that the adsorption of 2,4-D onto the n-RH sorbent is a com- Table 1 depicts the conformity of the adsorption data fitted bination of film diffusion and intraparticle diffusion process, with with the adsorption data of 2,4-D adsorption onto n-RH nanosor- the domination of the latter.</p><p>322 E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 Table 2 Kinetic parameters of sorption for 2,4-D sorption onto n-RH nanosorbent. Initial 2,4-D concentration (mg/L) Pseudo First order kinetics Pseudo Second order kinetics Intraparticle Diffusion qe (mg/g) K1 R2 qe (mg/g) K2 R2 Ki C R2 50 50.5 0.0214 0.794 50.76 0.173 0.99 5.05 4.62 0.980 100 112.201 0.0201 0.8805 97.087 0.160 0.999 8.75 8.33 0.983 150 147.91 0.017 0.7291 149.25 0.108 0.997 12.61 15.37 0.975 120 a -100 b 1.0 -90 0.9 100 -80 0.8 -70 80 0.7 -60 0.6 60 -50 0.5 -40 0.4 40 -30 0.3 -20 20 0.2 -10 a 0.1 1 10 100 1000 10000 0.0 -150 -100 -50 50 100 150 200 Diameter (nm) Zeta Potential (mV) Fig. 2. Size distribution (a) and zeta potential (b) of n-RH nanosorbent in water. A 10 um 10 um Fig. 3. Scanning electron microscopic image of n-RH nanosorbent before (a) and after (b) 2,4-D sorption. 3.2. Characterization of rice husk (n-RH) nanosorbent and 2,4-D (Fig. 2b). With the help of SEM analysis, the cell surface of the n-RH nanoformulation n-RH sorbent was found as irregular squares with smooth surface individually (Fig. 3a). This could be due to the carbon coating of The morphology, size and composition of the prepared rice the samples for SEM analysis, which would have resulted irregular husk nanosorbent was characterized before and after 2,4-D adsorp- nanoparticles visualization. So, the polydispersity which was no- tion. For adsorbing 2,4-D onto the rice husk, it is mandatory to ticed in DLS analysis could not be recorded back in SEM analysis. understand the properties of the nano rice husk adsorbent. The The EDAX spectrum of n-RH sorbent depicted the presence of C, size and morphology of the n-RH sorbent was observed by dy- Si, and P (figure not shown). The 2,4-D adsorption was found namic light scattering (DLS). The n-RH prepared via the mechan- to induce changes on to the morphological and surface character- ical method was found to have an average hydrodynamic particle istics of the adsorbent and thereby resulted in aggregate forma- size of 42.1 nm (Fig. 2a). It was also noticed to be of uniform size tion (Fig. 3b). This aggregation might affect the surface properties and so, could result in a water dispersable formulation. A nega- of the n-RH adsorbent and reduce its access to 2,4-D molecules. tive value of 28.8 mV was recorded in a zeta potential analyser The EDAX spectrum of n-RH nanosorbent showed the presence</p><p>E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 323 Table 3 FT-IR spectrum of rice husk nanosorbent before and after 2,4-D adsorption. Wavenumber Assignment of functional groups Rice husk nanosorbent Rice husk nanosorbent after 2,4-D adsorption 3342.64, 3302.13, 3172.90 3267.41, 3211.48 -OH stretching 2918.30, 2848.86 C-H stretching of alkanes 2337.72 2335.80 NCO stretching 1627.92, 1710.86 1631.78 C=0 1512.19 1517.99 C=C stretching vibration of alkenes and aromatic functional groups 1462.04, 1427.32 1467.83 -CH2 and -CH3 stretching 1074.35 1149.57 Asymmetric stretching of Si-O-Si stretch of tetrahedran 891.11 795.74 Symmetric bending 412.77, 460.99 518.85, 557.43, 570.93 -Si-H, -CHOH stretching, Si-O-Si stretching A B 0 100 0.5 1 1.5 2 -0.2 80 -0.4 60 -0.6 40 -0.8 -1 20 -1.2 0 -1.4 Time (days) -1.6 1:0.05 1:0.10 1:0.15 In t Fig. 4. Cumulative sustained release (a) of 2,4-D at various concentrations and release kinetics (b) at optimal concentrations from DnRH formulation in water. of C, O, Si, Cl and P, revealing 2,4-D on the surface of n-RH the optimum 2,4-D weight ratio of 0.10 (%) in the preliminary ex- sorbent. periments, it was worth reporting a sustained release of 2,4-D from The IR studies carried out for the n-RH sorbent revealed the the n-RH sorbent for about 6 days without any polymer coating. In presence of various groups on its surface which are described the control experiment, nearly 93.4% of the crude 2,4-D was found in detail in Table 3. The band stretching in the range of 1410- to get dissolved in water. The sustained release of 2,4-D from the 1520 cm- indicated the presence of angular deformation of amide nanosorbent could be due to the strong interactions between the N-H groups. The vibrations in the range of 900-1200 cm-1 repre- 2,4-D molecule and n-RH sorbent. In contrast, in a study by Yan sented the involvement of C=0 bonds of hydroxyl, phosphate, sul- et al. [13], an attempt to use cyanobacterial waste as pesticide fonate groups and cyclic structure of polysaccharides. Different re- carrier was undertaken and a sustained release of 81% and 61.1% gions of the spectra showed changes in the intensity in case of n- of avermectin for 4 and 13 days was obtained, with and without RH after 2,4-D adsorption. A peak in the region of 3000-3500 carbopol coating. Hence, the results of the present study could of corresponds to the N-H from amino group and also a bonded OH great importance where a higher sustained herbicide release was group stretch, where clear shifts and stretching were found after obtained without any polymer coating for increased number of n-RH adsorbent contact with 2,4-D. A weak peak at 2918.30 and days. 2848.86 cm in n-RH adsorbent denoted the involvement of C- The release pattern of 2,4-D from the n-RH sorbent was fit- H stretch of the alkanes. The peak shifts in the region of 1400- ted with the Korsmeyer-Peppas (K-P) kinetic model (Fig. 4b) for attributed to the amide (-CO-) group and (-NH-) indi- obtaining further insight into the release pattern mechanism. The cating the binding of 2,4-D molecules on to the surface of n-RH ad- K-P model is a semi-empirical model used for analysing the be- sorbents. Also, small shifts were noticed in the region of 1076 haviour of 2,4-D release form the n-RH sorbent, which is given by corresponds to the -CO- group vibrations in the 2,4-D molecule. the equation, where is the fraction of 2,4-D The above indications demonstrate the role of functional groups released in time, t; k is the kinetic constant and n is the exponent such as amino, carboxylic, hydroxyl and carbonyl groups in the which explains the type of release mechanism. From the linearized D biosorption process. These results were in accordance with that model equation, a release coefficient (n) value of 0.5 was obtained of the kinetic results supporting the involvement of chemisorption indicating the 2,4-D release due to diffusion process. The value of process during the adsorption of 2,4-D on to the n-RH sorbent. release constant (k) was about 0.13 depicting a faster release rate, respectively. Thus, the present sustained release pattern of 3.3. Sustained release of 2,4-D in water and soil 2,4-D from an agro-industrial waste nanosorbent could be advanta- geous over the other costly polymeric systems and other nanopes- The sustained release pattern of different ratios of n-RH to ticide formulations due to its ecofriendliness and increased bioef- D from the n-RH nanosorbent was checked in water (Fig. 4a). The ficacy for a longer time. release of 2,4-D from the n-RH sorbent was slow and the sus- The n-RH nanosorbent was also checked for its sustained re- tained in the range of 4, 6 and 10 days for 2,4-D concentrations lease via a thin soil layer amended with 0.5% and 1.0% (wt) of at weight ratios of 0.05, 0.10 and 0.15 (%), respectively. Based on DnRH formulation and the release profiles are shown in Fig. 5.</p><p>324 E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 100 80 60 Control I 40 Control II 20 0.5 % n-RH % n-RH watering number Fig. 5. Sustained release of 2,4-D via soil amended with DnRH formulation. From the results, the control experiment, where the technical ent depths of column viz. 0-4,4-8, 8-12 and 12-16cm for DnRH grade 2,4-D was applied was found to get released very quick formulation and the control (2,4-D technical). Highest concentra- and completely by end of the nine waterings. On the contrary, as tion of 2,4-D was found with the top segment which was found hypothesized, the 2,4-D release from n-RH nanosorbent was de- reduced in the following bottom segments. The greater retention creased in the range 0.5% n-RH followed by 1.0% n-RH sorbent of 2,4-D on the top most segment could be due to the presence (Fig. 5). This could be due to the potential of n-RH adsorbent in of colloids in the soil (Fig. 7a). The obtained results indicate the sustaining the release of 2,4-D. Although a very small amount of n- bioavailability of the DnRH formulation in field conditions where RH was amended in soil, it had played a significant role in slowing increased persistence of the herbicide could have enhanced the down the rapid release of 2,4-D at both the weight ratios tested. bioavailability of the active herbicide to the target plants for pro- Hence, the n-RH nanosorbent could be considered as a good soil longed duration. But in the control column with technical grade amendment for enabling sustained release of herbicides like 2,4-D, 2,4-D, the residual herbicide concentration was detected in almost with high leaching capacity. Thus, a reduced release of 2,4-D from all the four segments, which depicts the reason for its ground wa- the n-RH nanosorbent from soil compared to that of the aqueous ter contamination potential. The lowest residual 2,4-D concentra- phase was noticed during the experiment. This could be due to tion was noticed in the lowest 12-16cm column segment. This the fact of competition between the molecules of 2,4-D for sorp- demonstrates the ability of n-RH nanosorbent in controlling the tion sites in the soil amended with n-RH nanosorbent. Therefore, herbicide run-off to the nearest water bodies when compared to the results imply that the n-RH nanosorbent amended soil sorption the technical 2,4-D applied test column. highly and could be exploited for sustained release of herbicides After analysing the residual 2,4-D concentration in the soil and as well act as a soil amendment for reducing the associated column segments, the herbicidal activity in the segments was environmental contamination. analysed with Brassica sp. seeds and the germination index was recorded. A germination index of about 95 % was achieved all the 3.4. Bioactivity of the new 2,4-D nanoformulations column segments in case of the column treated with distilled wa- ter (Fig. 7b). In case of the column segments treated with technical The prepared DnRH nanoformualtions were also checked for 2,4-D and n-RH nanoformulations, inhibition in the growth of the phytotoxic effect against one target plant (Brassica sp.) and one plant was noticed in the depth of 0-4 and 4-8 cm. On the contrary, non-target organism (Zea mays). In case of Zea mays, an ger- in the other column segments of 8-12 and 12-16cm increased mination index of 92 % was obtained in case of free 2,4-D growth was found in case of DnRH formulation than that of tech- and the DnRH nanoformulation. This confirms that the DnRH nical grade 2,4-D treated column. This could be due to reduced nanoformulation does not affect the plant development nor change leaching of the adsorbed 2,4-D from the n-RH nanosorbent com- the activity of the herbicide (Fig. 6). On the other hand, the ger- pared to that of the free 2,4-D herbicide. Thus, the use of carriers mination index of Brassica sp. exposed to technical grade 2,4-D for herbicide delivery could increase herbicidal activity and reduce and DnRH formulation caused 100% mortality but the activity was their frequent application as well. So, the use of agro-industrial found enhanced, in case of the 2,4-D nanoformulation (Fig. 6). The waste based nanosorbent could aid in reducing the environmental treatment where only the free n-RH nanosorbent was applied did pollution without any effect on its bioefficacy. not cause any effect on the plant's germination index (94%). The Additionally, the use of agro-industrial waste based pesticide results reveal the enhanced potential of 2,4-D nanoformulation in carriers also calms the question on the fate of metal and polymer target species while the non-target plant was not affected at all. based nanoparticles in the environment. As the metal and polymer This enhanced herbicidal effect in case of 2,4-D nanoformulation based nanoparticle pesticide carriers though effective in delivery could be due to the reduced soil sorption or increased bioavail- were feared in terms of their toxicity and fate upon introduction ability of 2,4-D in the soil. in the environment. The advantages in using the agro-industrial wastes as carrier is advantageous than the polymer based sys- 3.5. Leaching experiments in soil column items, as the sustained delivery of the delivery could be achieved at a very low cost with no consumption of harmful chemicals. In order to consider the performance of the leaching of 2,4- The induced acidic environment inside the polymer systems was D nanoformulation, soil column were set up with n-RH amend- not found appropriate for many herbicides. These disadvantages ment at 1.0% (wt). The leaching of 2,4-D was evaluated at differ- could be corrected with simple utilization of the easily available</p><p>E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 325 A B C D (i) Zea mays A B C D (ii) Brassica Sp. 0.35 Zea mays 0.1 Brassica Sp. 0.3 0.08 0.25 0.2 0.06 0.15 0.04 0.1 0.02 0.05 0 0 A-Control B-2,4-D C-n-RH D-n-RH: A-Control B-2,4-D C-n-RH D-n-RH: (iii) Dry weight 2,4-D 2,4-D Fig. 6. Herbicidal activity of 2,4-D evaluation in (i) non-target and (ii) target plant (Brassica sp.) (A) control, (B) free 2,4-D, (C) unloaded n-RH nanosorbent and (D) DnRH formulation and (iii) the dry mass of the (E) non-target (Zea mays) and (F) target plant (Brassica sp.). 80 1 2 3 4 2,4-D technical I 1.0 % n-RH 60 40 20 1 2 3 4 o Soil Column depth Fig. 7. Soil column experiments with free 2,4-D and DnRH formulation (a) 2,4-D quantification and (b) herbicidal activity on target plant (Brassica sp.) at different soil depths. agro industrial waste rice husk carrier. The environmental nuisance 4. Conclusions caused by rice husk as well the herbicide contamination issue could be solved hand in hand, by adopting the present strategy. In conclusion, the DnRH nanoformulation prepared using nano- Therefore, it is clearly demonstrates that these n-RH nanosorbents sized rice husk via adsorption phenomenon showed high and could aid in safer and effectual delivery of toxic but mandatory reversible sorption to 2,4-D. This unique property of the n-RH herbicides. nanosorbent could enable it to be used as a good carrier for</p><p>326 E.A. Abigail M et al./Journal of the Taiwan Institute of Chemical Engineers 63 (2016) 318-326 incorporating herbicides. 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