Fabrication of smart magnetic nanocomposite asymmetric membrane capsules for the controlled release of nitrate

Environmental Nanotechnology, Monitoring & Management 8 (2017) 233–243

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Fabrication of smart magnetic nanocomposite asymmetric membrane capsules for the controlled release of nitrate

MARK

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Nahid Emamia, Amir Razmjoua, , Fatemeh Noorisafaa, Asghar Habibnejad Korayemb, Ali Zarrabia, Chao Jic a b c

Department of Biotechnology, Faculty of Advanced Sciences Technologies, University of Isfahan, Isfahan, Iran Department of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales, Sydney 2052, Australia

A R T I C L E I N F O

A B S T R A C T

Keywords: Controlled release Magnetic nanoparticles Membrane capsule Nanocomposite

Nitrate contamination of water resources due to excessive use of nitrogen-based fertilizers has become a global health issue. In this work, a recyclable and environmentally sensitive magnetic Fe2O3/PES (Polyethersulfone) nanocomposite asymmetric membrane like capsules (NAMCs) were prepared such that release nitrate in a controlled manner. Different characterization techniques such as SEM, EDAX, DSC, and surface energy measurement were used to systematically characterize the NAMCs. It was observed that the incorporation of only 0.5 wt.% Fe2O3 nanoparticles into NAMC provided a slow and steady release rate of nitrate. A further addition of nanoparticles (from 0.5 wt.% to 1.5 wt.%) into the capsules matrix resulted in an increase in the initial-release rate (IRR, from 3.17 to 4.17 μS/cm min) and maximum release level (MRL, from 36 to 52 μS/cm). The designed capsules also showed recyclability and the ability to respond to the environmental conditions. For instance, IRR increased from 3.17 to 5.31 μS/cm min for NAMCs when temperature increased only by 10 °C. These findings could be considered as a new and green approach to prevent nitrate contamination of water resources in the first place without exacerbating energy issues. Although this work is based on the controlled release of nitrate as the water resources contaminant, the NAMCs can also be used for other nutrients and fertilizers.

1. Introduction Plants need nitrogen compounds for producing proteins and nucleic acids. Insufficient nitrogen content turns them stunted and yellow. Gaseous nitrogen is stable and nearly useless for plants. They usually receive the nitrogen components from Nitrogen-fixing bacteria. Nitrogen-fixing bacteria such as Rhizobium are free living bacteria which can catalyze the combination of nitrogen with hydrogen and convert it into ammonium by molybdenum nitrogenase enzyme. Then, nitrifying bacteria such as Nitrosomonas and Nitrobacter in the denitrification process, convert ammonia into Nitrate. In the first step, Nitrosomonas hydrolyses the ammonia to hydroxylamine with ammonia monooxygenase (a transmembrane copper protein). Consequently, hydroxylamine oxidoreductase catalyzes the transformation of hydroxylamine into nitrite in the periplasm space. Then, Nitrobacter by oxidoreductase (a membrane-associated iron-sulfur molybdoprotein) performs the conversion of nitrite into Nitrate (Woznica et al., 2013). Some Nitrosomonase species are responsible for the conversion of urea to ammonia molecules (Moir, 2011).

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Although nitrogen fixing microorganisms resupply soil nitrogen, this process is not adequate for plants and modern agriculture. Furthermore, nitrogen loss occurs in the soil through leaching, denitrification, volatilization and run off (Raun and Johnson, 1999). According to Raun et al. (2002), the world efficiency of nitrate fertilizer in cereal grain production is only 33%. In addition, the increase in the population and industrialization of developing countries has led to a more demand for industrial food production, which requires an excessive consumption of nitrate. Farmers also have been applying too much of artificial nitrate fertilizers to sustain high yields. According to Food and Agriculture Organization of the United Nations (FAO), the world nitrogen fertilizer consumption increased from 111400000 in 2013–113100000 tons in 2014, at a growth rate of 1.5%. It is estimated to be around 119400 000 tons in 2018 at the annual growth of 1.4%. Since the nitrate is both soluble and mobile, it can enter the groundwater and pollutes the drinking water resources (Galloway et al., 2004). Nitrate contamination threats not only the environment but also human health. Studies have shown a significant correlation between nitrate level in drinking water and gastric cancer (Hansson et al., 1996;

Corresponding author at: Faculty of Advanced Sciences and Technology, University of Isfahan, Isfahan, Iran. E-mail address: [email protected] (A. Razmjou).

http://dx.doi.org/10.1016/j.enmm.2017.09.001 Received 22 April 2017; Received in revised form 3 August 2017; Accepted 9 September 2017 2215-1532/ © 2017 Published by Elsevier B.V.

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biodegradability of polyurethane coating and also change the release rate of fertilizers effectively. In all of the above-mentioned CRFs, nutrients release through diffusion, the disintegration of coating or by osmotic pressure (Jarosiewicz and Tomaszewska 2003). In spite of advantages attributed to the CRFs, their commercial implementations have encountered a few limitations and implications. It is difficult to adjust the release rate of CRFs according to the needs of different plants. They may release their content so fast or too slow that may be suitable for one species but not sufficient or harmful to another. In addition, the most polymers that are used in CRFs preparation are non-biodegradable and can be considered as residual wastes, which may compromise the environment. CRFs which are made from inorganic coating could have also side effects, for instance, used sulfur coated CRFs can increase the acidity of soil and affect the growth of plants. From an economic point of view, CRFs preparation cost is two to eight times higher than conventional fertilizers (Tomaszewska et al., 2002; Martin, 1997; Roberts, 2008). To address non-biodegradability of the PES coating, biodegradable polymers can be used but either their preparation process are expensive or they have limited availability. Therefore, there is still a continuing quest for finding affordable environmental friendly CRFs. It was shown that the addition of nanostructured materials into polymeric membrane matrix can not only alter the surface chemistry and structure of membranes but also can improve their performance (Razmjou et al., 2011; Low et al., 2015). Incorporation of magnetic nanostructured materials into asymmetric membrane capsules (AMCs) can not only change their structure and improve their release rate performance but also can provide a chance of recovering them from the soil around the plants by using a magnetic power at a very low cost. In this way, all of the cheap polymer wastes can be used to produce CRFs in a sustainable way and would not make their overall costs insurmountable. The idea of using synthetic membrane systems for controlling the release of a chemical content has been practiced in pharmacotherapy where a controlled release of a drug helps the diagnosis, treatment, cure or prevention of a disease. In pharmaceutical and drug delivery designing system, some of the poorly water soluble drugs such as doxazosin (Thombre et al., 1999a, 1999b), flurbiprofen (Choudhury et al., 2007), glipizide (Thombre et al., 1999a, 1999b) and indomethacin (Kaur et al., 2013) were successfully loaded in AMC and their releases were investigated in-vitro and in-vivo conditions. However, to the best of our knowledge, asymmetric membrane capsules have not been used for environmental and agronomical applications. Here, for the first time, the principles of preparing asymmetric ultrafiltration (UF) nanocomposite membrane were used to introduce a new class of CRFs. UF membranes use hydrostatic pressure in water separation and purification processes to retain large macromolecules (103–106 kDa), especially proteins (Ghosh and Cui, 2000). As presented schematically in Fig. 1(a), the asymmetric UF membrane compromises three regions of skin, finger and sponge layers. Based on the size exclusion mechanism, the surface pore size in the dense layer

Bruning-Fann et al., 1993). Carcinogenicity studies were conducted by Speijers et al. (Speijers et al., 1987). They showed that a high dose of both nitrite and nitrosatable precursors which was administered up to 1000 mg/l of drinking-water simultaneously can cause an increase in tumor incidence. The first strategy to address this issue is to reduce the amount of nitrate in water when it exceeds standard levels. The most common approaches are chemical denitrification (Murphy, 1991), ion exchange (Clifford and Liu, 1993), reverse osmosis (Bohdziewicz et al., 1999), electrodialysis (Elmidaoui et al., 2001), catalytic denitrification (Lemaignen et al., 2002) and biological denitrification (Kapoor and Viraraghavan, 1997). Although, these techniques are able to reduce nitrate level from water, they are energy intensive and have adverse environmental impact. Another strategy is to protect water resources from nitrate contamination in first place. One way to achieve this goal is to provide a sufficient amount of nitrate that plants need. Recently, controlled release fertilizers (CRFs) have attracted researchers as a novel approach to provide the required amount of nutrients for plants in a controlled manner (Deng et al., 2017). CRFs extent or delay the nutrient release by reducing the dissolution rate of fertilizers, minimizing the risk of water resources contamination by excess of nutrients. Coating of granular fertilizers with insoluble material is one of the most widely used approaches. A variety of inorganic such as graphene oxide film (Zhang et al., 2014) and sulfur (Rindt et al., 1968) and organic polymer materials such as chitosan (Melaj and Daraio, 2013), polyethylene (Salman et al., 1989), polysulfone (Tomaszewska and Jarosiewicz, 2002) and polyacrylamide (Abraham and Rajasekharan Pillai, 1996) have been used for coating of fertilizers. Polymer coatings consist of either semipermeable membranes or impermeable membranes with tiny pores. Generally, they are not degradable by soil microorganisms (Azeem et al., 2014) and it enables to improve constancy and longevity of the release by adjusting thickness of coating and polymer composition (Morgan et al., 2009; Lu et al., 2013). Compared to inorganic coated fertilizers, polymer coated fertilizers (PCF) have been more developed in controlled release technology. García et al. (1996) used the waste product of a paper pulp fabrication process as the coating material of their fertilizers. They found that kraft pine lignin had a high potential as coating material because of its low cost and biodegradability. The results showed that additives including linseed oil, dimerized, esterified, and natural resins improved the efficiency of the products. González et al. (2015) developed biochar as the supporting material for the preparation of CRFs. Three different polymers including ethyl cellulose, sodium alginate, and cellulose acetate were used to encapsulate impregnated urea with biochar. Authors concluded that ethyl cellulose had the lowest leaching rate of nitrate. Tomaszewska and Jarosiewicz (2002) prepared CRF based on polysulfone polymer, investigating the effect of coating porosity, coating thickness and temperature on the release of NPK (Nitrogen, phosphorus, and potassium) fertilizers. Biodegradable polyurethane was used by the incorporation of biomass as a coating material (Ge et al., 2002). Nutrient was completely released by the degradation of the coating. It was found that corn starch and bark can enhance the

Fig. 1. Schematic diagram of (a) asymmetric membrane flat sheet and (b) asymmetric membrane capsule (AMC).

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Fig. 2. (a) AMC and (b) NAMC were loaded with nitrogen fertilizers and precipitated in pure water (the inset image on the right image shows how the NAMC were collected by a magnet).

2.2. Preparation of AMCs

controls the membrane separation performance. The lower the surface pore size is, the higher the rejection is. As a result, water permeation can be manipulated through controlling the skin layer and surface pores sizes (Razmjou et al., 2011). This water permeation control ability is exactly what a CRF needs. In contrast to UF processes where the driving force is hydrostatic pressure, in asymmetric membrane capsules, the osmotic pressure is the dominant driving force. Osmosis is the movement of solvent from a solution of lower concentration to the higher concentration solution through a semipermeable membrane. The release kinetic of AMCs followed zero order especially if the solubility of the drug is low (Kumar et al., 2012). As presented in Fig. 1(b), the AMC is a spherical system comprises a dens skin layer which is supported by an intermediate and a porous layer which not only can act as a drug reservoir but also provides the mechanical strength. Polyethersulfone (PES) was used to prepare the AMCs because of its unique physical and chemical properties and particularly its availability. PES has a broad range of typical applications i.e. headlight reflectors, dairy membranes, sensors, food trays, water purification membranes, manifolds, reverse osmosis membranes and hemodialysis membranes. Most of them are single use and end up as waste products. For example, hemodialysis membranes are used once for each patient and they end up in hospital recycling bins. All of these PES waste products which are cheap can be used to prepare AMCs. A simple and inexpensive method for the fabrication of a new class of CRFs is introduced using PES polymer as the matrix and γ-Fe2O3 nanoparticles as the nanofiller. Incorporation of γ-Fe2O3 nanoparticles into asymmetric membrane capsules (AMCs) can not only improve their release rate performance but also can provide a chance of recovering them from the soil around the plants by using a magnetic power at a very low cost. KNO3 was used as the model of nitrate fertilizers. Physical and chemical characteristic of the AMCs were also investigated systemically alongside their controlled release performance.

PES capsules were prepared by the well-known technique of phase inversion (Young and Chen, 1995). Because of the insolubility of KNO3 in the solvent, the KNO3 was dissolved in 5 wt.% distilled water before its addition to the casting solution. According to the binodal curve, the maximum concentration of water in casting solution should be below 8 wt.% for 16 wt.% PES to avoid early phase separation (Lau et al., 1991). In the first step, 5.0 wt.% water contained 1 wt.% KNO3 was added to 10 wt.% NMP. Separately, 16.0 wt.% of PES powder was mixed with 65 wt.% NMP at 60 °C followed by the addition of 4.0 wt.% PVP. The two solutions were mixed and stirred for 24 h at 150 rpm to obtain a homogenous and uniform solution, the final solution was then left to degas for 24 h before use. The AMCs were prepared by dropping a certain volume of casting solution using a designed in-house syringe with the tip diameter of 0.5 mm from the desired distance into a 50 ml saline coagulation bath. The prepared capsules were kept in glycerol for 2 h to preserve pores after drying and then dried for next step. It should be noted that the solvent and PVP come out of the capsules during the phase inversion process. Therefore, the final amount of nitrate in the capsule was 6.25 wt.% of the entire capsule. It should point out here that the nitrate content can be increased by simply adding more KNO3 into the casting solution. Also, the AMCs do not contain an organic solvent, due to complete phase separation, and thus it could not pose any damage to plants.

2.3. Preparation of magnetic nanocomposite asymmetric membrane capsules (NAMC) To reduce the aggregation of nanoparticles, the mechanical modification was carried out using the technique introduced by Razmjou et al. (2011). In brief, different concentrations of Fe2O3 nanoparticles (0.5, 1 and 1.5 wt.%) were ground and dispersed in NMP by using a water bath and probe sonicators for 20 and 15 min, respectively. Nanocomposites asymmetric membrane capsules (NAMCs) were formed by the addition of modified nanoparticles into the casting solution and stirred by a mechanical stirrer for 2hr. As can be seen in Fig. 2(a), the capsules are relatively spherical with the mean diameter of 1–1.2 mm. Inset image in Fig. 2(b) shows how the NAMCs can be collected using a magnetic power.

2. Materials and methods 2.1. Materials Polyethersulfone (PES, 58000 g/mol) as polymer was prepared from BASF Co. Ltd, polyvinylpyrrolidone (PVP, 40,000 g/mol) as pore former and iron (III) oxide nanoparticles (γ-F2O3, particle size < 50 nm) were purchased from Sigma Aldrich. N-Methyl-2-pyrrolidone (NMP) as a solvent, KNO3 as nitrate fertilizer, glycerol as pore preserver and formamide for surface free energy measurement were supplied from Merck. Distilled water and ethanol were provided from a local store.

2.4. Recyclability of NAMCs To show recoverability of NAMCs, the used NAMCs were redissolved in the solvent (NMP) and nitrate was reloaded again. The recyclability cycles (NAMCs preparation- nitrate release-magnetic recollecting-dissolving) were repeated three times. 235

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2.5. NAMCs characterization 2.5.1. Particle size distributions Average hydrodynamic particle size and polydispersity of Fe2O3 nanoparticles were measured by using a Malvern Zeta Sizer Nano (ZSN) which is based on dynamic light scattering (DLS). The particles were sonicated in ethanol for 15 min before measurements. 2.5.2. Field emission scanning electron microscopy analysis (FESEM) The structure and morphology of the NAMCs were studied by using a field emission scanning electron microscope (JEOL 7001F FEG) at 15 kV acceleration voltages. The capsules were prepared for cross section imagining by glaciating in liquid nitrogen and then fracturing. In order to make samples conductive, a thin layer of Gold–Palladium was deposited onto the samples during a sputter deposition process. To demonstrate the quality of the distribution of nanoparticles and also proof of the existence of Fe2O3, EDS (EDX Energy dispersive X-ray) spectra were collected using an Oxford instruments Aztec system EDAX on the cross section of the samples. SEM studies were performed for each sample on five randomly selected capsules.

Fig. 3. SEM images of AMC: Finger-like and honeycomb structure were shown.

3. Results and discussion

2.5.3. Contact angle geometry The wettability of the samples was studied by using sessile drop technique with the help of a contact angle measurement instrument. The mean of at least five measurements was reported for each sample.

3.1. Morphological study of NAMCs The SEM image of an AMC is shown in Fig. 3. As can be seen in the figure, the capsules comprise 4 regions: (ii) a skin layer (a thin layer at the outermost part of membrane), (ii) a finger like structure (under the skin layer), (iii) circular microvoids (beneath the finger like structure), and (iv) cavity and sponge like structure (at the center of capsules).

2.5.4. Surface free energy measurement Acid-base (van-Oss) approach was applied to calculate the surface free energy (SFE) of the AMCs and NAMCs. In this approach, three liquids of water, glycerol, and formamide having known parameters were used (van Oss et al., 1990). Surface free energies are calculated based on the solution of a set of three first order linear equations. The parameters of each equation was listed in Table 1 (Hołysz 2000) where “γ” refers to SFE and the superscripts d,+ and – refers to dispersive, acid and base components, respectively.

3.2. Particle size Fig. 4(a) shows the SEM image of Fe2O3 nanoparticles and their particle size distribution. From the SEM image, the majority of the particles have a hexagonal shape with the average particle size of less than 100 nm. According to the DLS result (Fig. 4(b), the hydrodynamic particle size of the nanoparticles is about 71 nm. The Polydispersity index is 0.184 which shows a narrow size distribution.

2.5.5. Thermal analysis Differential scanning calorimetry (DSC2010, TA instrument) at a heating rate of 10 °C/min up to 300 °C was used to observe the effect of incorporation of nanoparticles on the glass transition temperature (Tg). In order to improve the DSC signal by removing the thermal history in NAMCs, samples were heated and then were cooled to room temperature and reheated again to 300 °C (the Tg was obtained from the second run).

3.3. The dispersion of nanoparticles in the NAMC 3.3.1. Presence of Fe2O3 To verify the presence of Fe2O3 nanoparticles into the NAMCs, EDAX point analysis was performed. As shown in Fig. 5a, the Fe peak was not detected for NAMC with 0.0 wt.% (AMC). For NAMC sample with Fe2O3 nanoparticles, a peak around 6.4 keV belongs to the Kα energy of Fe and the one at 7.0 keV corresponds to Kβ energy of Fe were detected, which is an indication of nanoparticles existence (Fig. 5(b–c)). As can be seen in Fig. 5(a–d), by increasing the nanoparticle concentration, the peak intensity increases. There are also peaks around 0.2, 0.4 and 2.5 keV which belong to C, O and S of the PES matrix, respectively.

2.5.6. NAMCs performance analysis The technique that was used by Melaj and Daraio (2013), Calabria et al. (2012), Engelsjord et al. (1996) and Stanley et al. (2003) was employed for the detection of release rate of NAMCs. An electrical conductivity meter (AZ8351) was used to measure the conductivity of released KNO3 and to calculate the release rate of NAMCs. Released rate measurement was conducted over 4 h at 25 °C. The initial released rate (IRR) was obtained at the first 5 min of each experiment by calculating the slope of the release curves while the maximum release level (MRL) was measured when IRR approaches zero. IRR values were obtained by fitting a first order equation to the beginning of release curves.

3.3.2. Fe2O3 nanoparticles distribution across the NAMCs The distribution of FeO3 nanoparticles in the NAMC cross-sections was studied by EDAX mapping analyzing. Fe elements were shown by the purple color spots. As shown in Fig. 6, increasing the concentration of nanoparticles resulted in an increase in the purple color intensity of the spots, which is an indication of agglomeration of the particles. Although the nanoparticles were agglomerated, the agglomerations themselves are also dispersed to some extent. It should be pointed out here that the EDAX analyzing can provide qualitative information regarding the nanoparticles distribution across the NAMCs. To have rough quantitative information about the nanoparticles distribution across the NAMCs, the thermogravimetric analysis is conducted as

Table 1 Acid-base (van-Oss) parameters of three known liquids. Liquid

γ TOT

γd

γ+

γ−

Water Formamide Glycerol

72.80 58.00 64.00

21.80 39.00 34.00

25.50 2.28 03.92

25.50 39.6 57.40

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Fig. 4. (a) SEM image of hexagonal particles and (b) particle size distribution of Fe2O3 nanoparticles.

As can be observed in Fig. 7, the addition of nanoparticles can successfully increase the hydrophilicity of all NAMCs. The lowest contact angle was observed for NAMC with 0.5 wt.% Fe2O3 nanoparticles. It was reported that the magnetic nanoparticles without any surface modification, have an intrinsic hydrophobic nature (Ma and Liu, 2007), therefore, it is expected that the nanoparticles addition into the NAMC leads to a higher contact angle and lower surface free energy. The surface free energy (SFE) of NAMCs did not change significantly although there is a marginal reduction for NAMCs with 0.5 wt.% Fe2O3 nanoparticles. As a result, the reduction in contact angle and increase in hydrophilicity is attributed to the surface morphology rather than surface chemistry. Moreover, SFE and roughness can both have an effect on the wettability of the surface. It seems in our case the effect of surface structural changes is more prominent than the surface chemistry alteration. According to 3D capillary effect (Chaharmahali 2012; Bico et al., 2002), when surface micro and nano-roughness increases, water droplet imbibes into inner pores and causes its spread out on the surface more rapidly, and consequently, the water contact angle becomes lower. For NAMCs containing 0.5 wt.% nanoparticles there is an increase in roughness which leads to a better imbibition of water into surface channels and makes the surface more wettable.

below. 3.3.3. Differential scanning calorimetry (DSC) analysis To detect the impact of nanoparticles on glass transition temperature (Tg) on membrane capsules, DSC analysis was performed. Tg is the temperature that materials transform from hard state to rubbery state. The Tg of each sample was shown in Table 2. PES is known as a high thermal stable polymer and has a glass transition temperature around 225 °C (Li et al., 2005). As observed in Table 2 and Fig. S2, by blending of Fe2O3 in the capsules, the glass transition temperature significantly increased, which is an indication of thermal stability improvement after addition of nanoparticles into the polymer matrix. However, a further increase in the particles loading did not improve the Tg. According to Hamming et al. (2009), there is an association between the interaction of nanoparticles and their surrounding polymer and Tg. Since both PES and Fe2O3 are hydrophobics (Ma and Liu, 2007), an attractive interaction between nanoparticles and polymer can occur. In addition, there was also a reduction of the free-volume region and the mobility of the polymer chain, respectively, which could result in an increase in Tg (Hamming et al., 2009). 3.4. NAMCs surface hydrophilicity

3.5. Effect of nanoparticles concentration on the release performance To study the effect of incorporation of nanoparticles on the wettability and release rate of NAMCs, the contact angle for curvy surfaces was measured by the method introduced by Xu et al. (2002) (see Fig. 7).

To examine the effect of magnetic nanoparticles on the NAMCs structure as well as on the release rate, different concentrations of Fig. 5. EDAX spectra of (a) 0.0 wt.%, (b) 0.5 wt.% (c) 1.0 wt.% and 1.5 wt.%Fe2O3 nanoparticles in NAMC.

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Fig. 6. EDAX mapping for (a) 0.5 wt.% (b) 1.0 wt.% and (c) 1.5 wt.% Fe2O3 nanoparticles in NAMC.

Table 2 Glass transition temperature of control and NAMC with different concentrations of Fe2O3 nanoparticles. Samples 0.0 wt.% 0.5 wt.% 1.0 wt.% 1.5 wt.%

Glass transition temperature, Tg (°C) Fe2O3 Fe2O3 Fe2O3 Fe2O3

228 235 233 234

± ± ± ±

0.5 1.0 0.5 1.0

Fig. 8. Effect of nanoparticle concentration on the release performance of NAMC.

concentration such that they increased from 3.17 to 4.74 μS/cm min and 36–52 μS/cm when the concentration of nanoparticles increased from 0.5 wt.% to 1.5 wt.%, respectively. However, increasing the concentration of nanoparticles in NAMCs resulted in an agglomeration of nanoparticles which reduced the impact of nanoparticles on the structural changes. These findings imply that adjusting the concentration of nanoparticles into NAMCs can provide us with a chance to manipulate the release rate of nitrate. This ability to control the release performance of the NAMCs could be related to the effect of nanoparticles addition on the NAMCs structure and morphology particularly on the skin layer thickens. As presented in Fig. 9, the addition of nanoparticles into the NAMCs resulted in a significant reduction in the length of finger-like structures. Skin layer thickness also changed substantially after the addition of nanoparticles. From Figs. 8 and S1 in supplementary data, it could be concluded that the slowest and steadiest release belongs to NAMCs with 0.5 wt.% nanoparticles content which is associated with the thickest skin layer (6.3 ± 0.5 μm). How could nanoparticles change NAMCs structure? As mentioned, NAMCs are prepared by using the phase inversion method. This method is based on the phase separation by immersion precipitation which is a process of controlled polymer transformation from a liquid phase to solid phase (van de Witte et al., 1996). When PES as polymer plus NMP as solvent (Casting solution) is dropped into a coagulation bath containing non-solvent water, a solvent and nonsolvent exchange occurs and precipitation takes place to form AMCs with a different structure of the skin layer, finger and sponge like structures. The combination of phase separation parameters and mass transfer affect the structure of capsules. One of the important parameters during the capsules formation via phase inversion process is the solubility parameter between solvent (NMP) and non-solvent (water). If

Fig. 7. Average contact angle and surface free energy of NAMC with different concentrations of Fe2O3 nanoparticles.

Table 3 Effect of different parameters on the IRR and MRL. Different effective parameters on release rate

IRR (μS/cm min)

MRL (μS/cm)

AMC

NAMC

AMC

NAMC

Skin layer thicknesses

0.0% Ethanol 30% Ethanol 50% Ethanol 90% Ethanol

4.74 1.42 0.82 0.25

3.17 1.42 0.82 0.25

52 25 19 11

36 21 13 10

Temperature

25 °C 30 °C 35 °C 4 7 10

4.74 4.80 5.65 2.52 4.74 0

3.17 4.31 5.31 1.57 3.17 0

52 59 68 23 52 16

36 47 54 20 36 14

– – – –

4.47 3.17 3.62 4.17

– – – –

52 36 40 45

pH

Nanoparticles concentration

0.0 wt.% 0.5 wt.% 1.0 wt.% 1.5 wt.%

NPs NPs NPs NPs

nanoparticles (0.5, 1 and 1.5 wt.%) were added into the capsules matrix. The effect of different parameters on the IRR and MRL are shown in Table 3. The control capsules have the highest IRR (4.74 μS/cm min) and MRL (52 μS/cm). According to Fig. 8, the addition of nanoparticles resulted in an increase in both IRR and MRL regardless of their 238

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Fig. 9. SEM images of NAMCs with different concentrations of Fe2O3 nanoparticles (SLT stands for skin layer thickness).

Fig. 10. Effect of skin layer thickness on the release rate of (a) AMC and (b) 0.5 wt.% Fe2O3 nanoparticles in NAMC.

investigated the effect of nanoparticles addition into the structure and property of membranes.They showed that with the increase of nanoparticles concentration, the skin layer thickness increased. According to the authors, nanoparticles with a concentration higher than 3 wt.% can increase the viscosity of casting solution, leading to a decrease in phase inversion rate, and consequently, resulting in an increase of the skin layer thickness. Also, Li et al. (2007) observed the same effect by adding nanoparticles into poly(phthalazine ether sulfone ketone) UF membrane. They found that nanoparticles with high surface area to volume ratio can make a highly viscous casting solution, which delays the solvent-nonsolvent counter diffusion velocity during the phase inversion process and thus a thicker skin layer.

the difference between solubility of solvent and nonsolvent is low, the counter diffusion of solvent in coagulation bath and nonsolvent in AMC occurs easily. This higher counter diffusion velocity results in AMCs with the higher porosity, thinner skin layer and the formation of fingerlike structures especially in the support layer (Chaharmahali, 2012). On the contrary, if the difference between solubility of solvent and nonsolvent is high, delayed demixing happens, which leads to the formation of dense and thick skin layer (Kools, 1998; Deshmukh and Li, 1998). The addition of nanoparticles could delay the counter diffusion velocity of solvent and non-solvent via two ways: (a) increasing the hydrophobicity of the polymeric matrix and (b) increasing the viscosity of the casting solution. Considering the fact that our nanoparticles have an intrinsic hydrophobic nature (Ma and Liu, 2007), their addition into polymeric matrix reduces the demixing rate of NMP as solvent and water as non-solvent and consequently resulted in a thicker skin layer. It is also reported that nanoparticles can increase the viscosity of casting solution and delays the demixing of solvent and non-solvent, which result in the formation of the thicker skin layer. Yang et al. (2007) prepared nanocomposite PSF (Polysulfone) UF membranes and

3.6. Effect of skin layer thickness on the release performance As previously mentioned, in asymmetric UF membranes the skin layer plays a critical role in the control of the mass transfer rate; and appears to have a similar role in AMCs. To study the effect of skin layer thickness on the release rate of AMCs, the coagulation bath was 239

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Fig. 11. SEM images of AMCs in different concentrations of ethanol. (a) 0%, (b) 30%, (c) 50% and (b) 90%v/v of ethanol. The inset images show finger like and skin layer thickness (SLT).

and MRL. Comparing the SEM images of NAMCs and AMCs, the effect of the addition of ethanol into the coagulation bath on the general structure of NAMCs is similar to that of AMCs (See Fig. S1 in supplementary data). However, the skin layer thickness of AMC changes from 3.3 ± 0.5 μm to 10.0 ± 0.5 μm while it changes from 4.5 ± 0.5 μm to 11.0 ± 0.5 μm for NAMC. This could be attributed to the intrinsic hydrophobic nature of nanoparticles, which makes a delay in the counter diffusion velocity of solvent and non-solvent when NAMCs are formed (see Fig. S1 supplementary data).

prepared with different concentrations of ethanol (0, 30, 50 and 90%v/ v). As observed in Table 3 and Fig. 10(a), the addition of ethanol into the coagulation bath resulted in a significant reduction in the IRR and MRL of AMCs. The significant reduction in IRR can provide a chance for slower and steadier release of nitrate ions. Therefore, the controlled release of nitrate ions is possible through the manipulation of skin layer during the phase inversion process. To study how the skin layer changes through phase inversion process, SEM images were taken from the cross-section of AMC precipitated in the different concentrations of the ethanol solution. According to Fig. 11, the finger-like structures become more suppress and shorter as the ethanol concentration in water bath increases. Also, the skin layer thickness increases from 3.3 ± 0.5 to 10.0 ± 0.5 μm as the ethanol content increases. These results are in agreement with the previous studies on the effect of solvent-nonsolvent interaction on the membrane formation (Deshmukh and Li, 1998; Razmjou et al., 2011). Deshmukh and Li (1998) showed that the increase of ethanol content from (0%v/v to 50%v/v) in water bath changes the morphology of PVDF hollow fiber membrane from short finger-like to the sponge-like structure. Also, Razmjou et al. (2011) investigated the effect of isopropanol on the diffusion rate. They showed that a reduction in the pore size and porosity, as well as the suppression of microvoids and a dense skin layer, occur with the addition of isopropanol in the coagulation bath. As explained in Section 3.5, reducing the solvent-nonsolvent counter diffusion velocity could significantly alter the capsules structure particularly the skin layer thickness. It is known that the addition of alcohol into the coagulation bath can significantly reduce the demixing rate of solvent and nonsolvent during the phase inversion process (Chaharmahali 2012). The effect of changing the solvent-nonsolvent counter diffusion velocity on the nitrate release performance of 0.5 wt.% Fe2O3 nanoparticles was also investigated [Fig. 10(b) and Table 3]. Similar to the AMCs, reducing the solvent-nonsolvent counter diffusion velocity by the addition of ethanol into the coagulation bath can reduce the IRR

3.7. Effect of environmental conditions Any new CRF should be designed in a way that it can adapt itself to its surrounding environmental conditions. For example, when the temperature of weather changes the capsule should be able to adjust its release rate, or when the concentration of nutrients surrounding the plants is high or low, the capsule should be able to manipulate its release rate. 3.7.1. Effect of temperature on the release performance To investigate the effect of temperature on the nitrate release performance of AMC and 0.5 wt.% NAMC, samples were placed in a water bath at different temperatures ranging from 25 °C to 35 °C. Their release behavior was presented in Fig. 12(a and b). As shown in Table 3, the effect of temperature on the release performance of AMCs and NAMCs is significant such that IRR increased from 4.74 to 5.65 μS/ cm min for AMCs and from 3.17 to 5.31 μS/cm.min for NAMCs when temperature increased from 25 °C to 35 °C. Also, MRL increased from 52 to 68 (μS/cm min) for AMCs and from 36 to 54 (μS/cm.min) for NAMCs as temperature rose from 25 °C to 35 °C. This might be related to the solubility and osmotic pressure of KNO3. It is known that the solubility of salts increases when temperature increases (Treptow, 1984; Chatelier, 1884), and thus it leads to a higher IRR and MRL. The 240

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Fig. 12. Effect of temperature on the release performance of (a) AMC and (b) NAMC and the effect of pH on the release performance of (c) AMC and (d) NAMC.

the basic solution was significantly higher than that of the acidic solution, the nitrate release of capsules in the basic solution was substantially slower than when they are in the acidic solution. This suggests that the capsules can adapt themselves intelligently when they are facing different environmental conditions. In a better word, when the concentration of ions such as nitrate surrounding the plants increases the capsule reduces the release rate of its content, whereas when its concentration reduces the capsule rises its release rate. This result is contradictory with results obtaining from Oertli and Lunt (1962) which showed that nutrient release is independent of pH. It seems that their fertilizers coating were not affected by the osmotic pressure. But, our AMCs were significantly affected by the osmotic pressure which results in a change of pH.

influence of temperature on the release rate of encapsulated materials was studied by Husby (2000). It was found that different encapsulated fertilizers have different release patterns as a result of temperature changes. Oertli and Lunt (1962) hypothesized that the temperature changes the properties of the polymer coating and leads to a change in the release rate. According to Tomaszewska and Jarosiewicz (2002), as temperature increases the solubility of nutrient within polymer and diffusion rate increase. Christianson (1988) studied the effect of different factors such as pH, soil moisture, and temperature on the nitrogen release. It was shown that the release rate becomes twice when the temperature increases by 15 °C due to the temperature dependence of diffusion coefficient of coating and solubility of urea. Zou et al. (2015) showed that temperature has a considerable effect on the nitrogen solubility rate of coated urea based on modified PVA and the inorganic materials. They found that as temperature increased from 10 °C to 30 °C, the number and size of pores of membrane coating became larger due to the higher swelling rate of PVA/starch cross-linked fluid, which resulted in nitrogen release increasing. Using a first order kinetic reaction equation they showed that the solubility of nitrogen increases with temperature increasing. This result supports the previous researchers demonstrating that nitrogen solubility is higher with the increase of temperature (Sato et al., 1999).

3.8. Assessment of the recyclability of NAMCs The industrial scale production of most of the CRFs is still limited because of the high price of raw materials and preparation costs as well as environmental pollution of coating polymers. Therefore an inexpensive polymer, a low-cost method and also environmentally friendly polymers for fabrication of CRFs is essential. It was found that most of the coating polymers being used in CRFs have environmental hazardous for making a considerable amount of accumulation of their residual wastes up to 50 kg/ha/year (Majeed et al., 2015). As mentioned before, to address the issue, biodegradable polymers have been suggested. Many biodegradable polymers such as chitosan (Melaj and Daraio, 2013), polylactic acid (Hanafi et al., 2000), poly(3-hydroxybutyrate)/ethylcellulose (Costa et al., 2013), polydopamine (Jia et al., 2013) have been used for biodegradable polymer-coated fertilizers. However, they are either expensive or their associated preparation approach is not economical. In a better word, interesting chemistry does not bear very well to high volume-low value product, such as

3.7.2. Effect of pH on the release performance To examine the effect of pH on the release performance of nitrate for AMC and NAMCs, samples were placed in solution with three different pH of 4, 7 and 10. The pH of the solution was adjusted by using HCl and NaOH. The initial EC of acidic and basic solution before exposing to capsules were 9 and 75 μS/cm, respectively. As can be seen in Fig. 12(c and d) and Table 3, the release performances of the capsules were changed such that the IRR and MRL reduced for both acidic and basic solutions because of change in osmotic pressure. Since the initial EC of 241

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Fig. 13. Recyclability of NAMCs during the three cycles of NAMCs preparation-nitrate release-magnetic recollecting-dissolving.

fertilizer. Although NAMC matrix is a non-biodegradable polymer, they can be collected around plants using a magnetic power after releasing their content and they can be easily recovered and reused. As shown in Fig. 13, the release rate was increased for cycle 2 and 3, which is due to the accumulation of non-released nitrate from the previous cycles. These results indicate that NAMCs will promise a new class of environmental friendly CRFs. NAMCs enables to reduce cost, improve marketability and also solve the environmental issue of their non-biodegradability. It's worth mentioning that the residual wastes of other PES products, for example, hollow fiber membranes used in hemodialysis can also be employed for the construction of NAMCs. 4. Conclusion The new concept of using nanocomposite asymmetric membrane capsules (NAMCs) for the controlled release of nitrate as a solution to prevent water resources contamination was introduced. Nitrate was successfully loaded into NAMC and its release was successfully controlled. A systematic characterization was conducted to identify the key parameters which their adjustment can control the release rate of nitrate. It was found that the skin layer thickness and nanoparticles concentration have the contributory effect on the release performance of the NAMCs. The addition of only 0.5 wt.% Fe2O3 nanoparticles into NAMC provided a slow and steady release rate for nitrate. Increasing the nanoparticles concentration from 0.5 wt.% to 1.5 wt.% into the capsules matrix resulted in an increase in the IRR (from 3.17 to 4.17 μS/cm min) and MRL (from 36 to 52 μS/cm). The designed capsules also showed recyclability and the ability to respond to the environmental conditions. The capsules were also environmentally sensitive, such that they adapt themselves to a different temperature, pH, and nitrate concentrations. This study proves that application of NAMCs is a green approach which has a great potential to efficiently address the nitrate contamination issue and also shows promise for an extension to other fertilizers. Acknowledgements Authors acknowledge the use of facilities within the Centre for Electron Microscopy also the financial support from Isfahan university research console. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enmm.2017.09.001. 242

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