Novel Multinutrient Fertilizer and Its Effect on Slow Release, Water

Sep 21, 2012 - This study was carried out to develop a novel slow-release fertilizer, which is based on natural attapulgite (APT) clay as a matrix, gu...
1 downloads 11 Views 4MB Size
Download PDF
Article pubs.acs.org/IECR

Novel Multinutrient Fertilizer and Its Effect on Slow Release, Water Holding, and Soil Amending Boli Ni, Shaoyu Lü, and Mingzhu Liu* State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China ABSTRACT: This study was carried out to develop a novel slow-release fertilizer, which is based on natural attapulgite (APT) clay as a matrix, guar gum (GG) as an inner coating, and guar gum-g-poly(itaconic acid-co-acrylamide)/humic acid (GG-g-P(IAco-AM)/HA) superabsorbent polymer as an outer coating. The coated compound fertilizer granules with diameter in the range of 2−3 mm possess low moisture content and high mechanical hardness. The effects of APT matrix, GG, and superabsorbent polymer coatings on nutrients release were explored. The influence of the product on water-holding capacity of soil was determined. The degradation behavior of the GG-g-P(IA-co-AM)/HA outer coating was assessed by examining the weight loss with incubation time in soil. The experimental data and analysis in this study indicated that the product prepared by a simple route can effectively reduce nutrient loss in runoff or leaching, improve soil moisture content, and regulate soil acidity and alkalinity level.

1. INTRODUCTION Agricultural intensification through the use of high-yielding crop varieties, chemical fertilizers and pesticides, irrigation, and mechanization, known as the “Green Revolution”, has been responsible for the dramatic increase in grain production in developing countries over the past three decades.1 The annual global use of fertilizers has increased from about 46 million tonnes in the 1960s to about 130 million tonnes in the 1990s; and will need to double by the year 2030.2 However, studies show that more than half of applied fertilizers are not taken up by the crops. Increased awareness of ecological principles results in an improved understanding of the complex relationship between farming and ecosystems, and society increasingly recognizes the danger of improperly managed nutrients that lead to deterioration of land and water resources.3−5 New fertilizer productsslow- or controlled-release fertilizersthat release nutrients at controlled rates to maintain maximum growth and minimum losses have been developed in the last several decades.6−8 There are two most important kinds of slow release fertilizers on the market: condensation products of urea and urea-aldehydes, and coated or encapsulated fertilizers. Urea-formaldehyde based products, as a main product of urea-aldehydes, have the largest share worldwide. In general, urea-formaldehyde fertilizers show a significant slow release of nitrogen combined with a good compatibility with most crops. In addition, they will not burn vegetation or interfere with germination due to their low solubility. However, it appears that part of the nitrogen contained may be released to the soil solution extremely slowly or even not at all. Other slow-release forms include sulfur-coated urea and plastic resincoated fertilizers.9 In comparison to the urea-formaldehyde fertilizers, coated fertilizers may present more favorable economics. However, application of sulfur-coated fertilizers may increase the acidity of the soil. In addition, plastic resincoated slow-release fertilizers may leave undesired residues of synthetic materials on the fields.10 Therefore, environmentally © 2012 American Chemical Society

friendly slow-release fertilizers should be developed. Some potential effects of the slow fertilizers in soil are as follows: long-lasting effectiveness due to slow release, adequate nutrients for crops, prevention of seedling damage, remarkable decrease with respect to fertilizer application rate, and the reduced chance of the fertilizer leaching (dissolving or passing) through the soil.11,12 Attapulgite is a crystalline hydrated magnesium aluminum silicate mineral with a fibrous morphology, and with a structure consisting of parallel ribbons of 2:1 layers.13 Attapulgite’s unique structure makes it present an intermediate cation exchange of about 30−40 meq/100 g. The high surface area, the charge on the lattice, and the inverted structure which leaves parallel channels through the lattice, give APT a high absorption capacity.14 A number of representative applications of APT include oil well drilling muds, latex paint thickener, petroleum refining, decoloring, and carrier for agricultural chemicals, etc. In this study, APT was used as fertilizer carrier in order to allow slow release of the fertilizer nutrients. In addition, it can provide many nutrient elements for plants due to its abundant elements content, such as K, P, Ca, Mg, Fe, and Mn. Hydrophilic polymers can swell and absorb water without dissolving, provided that chemical or physical cross-links exist among the macromolecular chains. For this characteristic, they have been widely used in many fields such as diapers and napkins for personal care, drug delivery systems in pharmaceutical area, and soil conditioner in agriculture.15,16 In modern agriculture, many hydrophilic polymers are used to enhance both the nutrition and water status of plants. It has been reported that hydrophilic polymers are effective in Received: Revised: Accepted: Published: 12993

February 6, 2012 July 29, 2012 September 21, 2012 September 21, 2012 dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000

Industrial & Engineering Chemistry Research

Article

borax solution in the rotating pan. Finally, GG-g-P(IA-co-AM)/ HA powder (below 110 mesh) as the outer coating was coated on the surface of the granules under rotation. The granulated product was fed into a dryer to remove excess water and to harden the granules. The product from the dryer was passed over a screen where the particles smaller or larger than the desired size were removed. The schematic illustration of the preparation procedure of SMF is shown in Figure 1.

increasing the water holding capacity, decreasing deep percolation, and reducing evaporation losses in sandy soils.17 Since the first hydrogels based on poly(hydroxyethyl methacrylate) developed by Otto Wichterle in the 1950s and later patented for use as soft content lenses,18 many efforts have been made by researchers toward obtaining novel hydrogels with preferable properties based on synthetic, natural, or hybrid polymers for specific applications.19,20 Guar gum (GG) is a nonionic polysaccharide consisting of a (1−4)-linked β-D-mannopyranose with branch points from their 6-positions linked to α-D-galactose. It is derived from the seeds of Cyamopsis tetragonolobus.21 It is commonly used as a thickening agent in cosmetics and food industry. Taking into consideration that GG is cheap, easily available, and nontoxic, in this study, GG was introduced in superabsorbent polymer networks to fabricate a novel composite which was expected to reduce cost and improve degradation property. In addition, GG was used as a fertilizer coating to allow slow release of the fertilizer nutrients. The objectives of this study were (1) to develop a novel slow-release compound fertilizer based on APT clay and hydrophilic polymers; (2) to test the physical and chemical characteristics of the fertilizer; and (3) to evaluate its efficiency in slowing nutrient release, improving water-holding capacity of soil, and adjusting pH value of soil.

Figure 1. Schematic illustration of preparation procedure for slowrelease multinutrient fertilizer (SMF).

2.4. Component Analysis of SMF. Content of nitrogen in the SMF was determined with a 501 ammonia-selective electrode. The phosphorus and potassium contents in the SMF were determined by an inductively coupled plasma (ICP) instrument (American TJA Corp., model IRISER/S). 2.5. Measurement of Water Absorbency of GG-g-P(IAco-AM)/HA. The accurately weighed superabsorbents (0.2 g, 40−90 mesh) were immersed into a certain amount of tap water and allowed to soak at ambient temperature for 60 min. The swollen polymers were filtered and weighed. The water absorbency (WA) was calculated using eq 1:

2. MATERIALS AND METHODS 2.1. Materials. Guar gum (GG, number molecular weight 5 000 000) was obtained from Beijing Guaerrun Technology Co., Ltd.. Itaconic acid (IA, chemical grade, Aladdin Reagent Co., Ltd., Shanghai, China) was used as received. Acrylamide (AM) and humic acid (HA) were provided by Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China. N,N′-methylene bisacrylamide (NNMBA) was recrystallized from 95% ethanol prior to use. Natural attapulgite (APT, supplied by Gansu Haozhou APT Co., China) was milled and sieved through a 200-mesh screen before use. All other chemicals were analytical grade and used as received. The soil used in the study is a representative sample from Lanzhou, which lies in northwest China. The soil texture is silt loam. The soil pH and electroconductivity are determined to be 8.13 and 2150 uS/cm, respectively. 2.2. Synthesis of GG-g-P(IA-co-AM)/HA Superabsorbents. The grafting reactions were carried out under a nitrogen atmosphere in a four-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube. In a typical reaction, the required amount of 0.6 g of IA was dissolved in 15 mL of distilled water in the flask. The solution was neutralized by 0.89 mL of KOH solution (7.3 mol/L). Then 2.4 g of AM, 0.015 g of ammonium persulfate (APS), 0.0075 g of NNMBA, and 0.2 g of HA were added to the flask and stirred constantly. After that, 0.1 g of GG was added. The water bath was heated slowly to 75 °C and maintained at this temperature for 3 h. Finally, the resultant polymers were dried, milled, screened, and stored for further use. 2.3. Preparation of Slow-Release Multinutrient Fertilizer (SMF). First, an amount of urea, KH2PO4, and APT were ground to powder and mixed well. Then the mixture was fed into a rotating pan with urea granules (about 1.0−1.3 mm) in batches. During this step, the granules as fertilizer cores with desired range of sizes were obtained under water atomization. Subsequently, GG powder (below 200 mesh) as the inner coating was adhered to the fertilizer cores by atomized 5%

WA = (M − M 0)/M 0

(1)

where M and M0 refer to the weight of the swollen and dried superabsorbents, respectively. 2.6. Effect of GG-g-P(IA-co-AM)/HA Superabsorbents on pH Value of Soil. Simulated soil solution samples with various pH values were adjusted with 1 mol/L HCl or NaOH aqueous. A 0.1-g portion of GG-g-P(IA-co-AM)/HA sample was immersed in 80 mL of soil solution with different pH values for 90 min. After the swollen hydrogels were filtered, the pH value of the filtrate was measured with a pH meter (pHS3B, Shanghai Precision Scientific Instrument CO., Ltd., Shanghai, China). 2.7. Determination of Average Crushing Strength for SMF. In the production of compound fertilizers, fertilizer granules are required to have a mechanical hardness that will sufficiently withstand normal handling without fracture. The mechanical hardness is defined as the force required to fracture the granule between two flat surfaces. The crushing strength test is a valuable tool in helping the fertilizer manufacturer tune the granulation process in order to produce products with maximum attainable hardness and lowest possible caking propensity.22 In this study, average crushing strength was measured using compressing equipment for thirty granules (2.5 ± 0.5 mm in diameter), which was determined to be 21.4 N. 2.8. Measurement of Moisture Content of SMF. The physical form of a fertilizer is important in regard to satisfactory 12994

dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000

Industrial & Engineering Chemistry Research

Article

3. RESULTS AND DISCUSSION 3.1. Morphology and Characteristics of SMF. Figure 2 shows the morphology of the samples without coatings (Figure

handling, transport, storage, and application to the soil. During their storage, some fertilizers tend to form a coherent mass from the form of powder or granules. This phenomena, termed caking, reduces the free flowability of the fertilizer.23 Moisture content of fertilizer is crucial because it can be used as a product quality indicator and management tool for its storage and shipment. In this study, moisture content of SMF was determined by drying 10 g of SMF in a vacuum drying chamber at 50 ± 2 °C for 2 h and weighing, which was determined to be 3.4%. 2.9. Slow Release Behavior of SMF in Soil. Release rates of nitrogen (N), phosphorus (via P2O5), and potassium (via K2O) from SMF were determined by burying 1 g of SMF in sealed plastic mesh bags approximately 6 cm below the surface of the soil at ambient temperature. Soil samples (0−10 cm depth) were collected, air-dried, sieved through a 26-mesh sieve, and stored in plastic containers. Throughout the experiment, the soil moisture was kept at 20%. After 0.5, 1, 3, 5, 10, 15, 20, 25, and 30 days, the mesh bags were retrieved and air-dried. Then the fertilizer granules were removed from the mesh bags and estimated for the nutrient content. The contents of nitrogen (N), phosphorus (via P2O5), and potassium (via K2O) from SMF released in soil were determined by the difference between the initial nutrient content in the fertilizer granules and the amount remaining when the release experiment finished. The method for determination of the content of nutrients was shown in Section 2.4. 2.10. Measurement of Water-Holding Capacity of Soil with SMF. Different amounts of SMF were mixed with 200 g of dry soil (below 26 mesh) and placed in a 4.5-cm diameter PVC tube. The bottom of the tube was sealed with nylon fabric and weighed (marked W1). The soil sample was slowly drenched by tap water from the top of the tube until water seeped out from the bottom. When no water seeped at the bottom, the tube was weighed again (marked W2). A control experiment without SMF was also carried out. Three fertilizer application rates (0.5, 1, and 2%) were examined. The waterholding capacity (WH %) of the soil was calculated from eq 2. WH% = (W2 − W1) × 100/200

Figure 2. Morphology of the samples without coatings (a), with inner coating of guar gum (GG) (b), and the final product with double coatings of GG and GG-g-P(IA-co-AM)/HA superabsorbents (c).

2a), with inner coating of GG (Figure 2b), and the final product with double coatings (Figure 2c). The physical characteristics of SMF are presented in Table 1. Table 1. Physical Characteristics of the Slow-Release Multinutrient Fertilizer (SMF) value

nitrogen content phosphorus (via P2O5) content potassium (via K2O) content APT content GG content GG-g-P(IA-co-AM)/HA content moisture content average crushing strength diameter of dry sample

17.9 ± 1.4 wt % 11.3 ± 0.7 wt % 8.2 ± 0.4 wt % 37 ± 3 wt % 7 ± 1 wt % 9 ± 1 wt % 3.4 ± 0.2 wt % 21.4 N 2.0−3.0 mm

3.2. FTIR Analysis of GG, HA, and GG-g-P(IA-co-AM)/ HA. The FTIR spectra of GG, HA, and GG-g-P(IA-co-AM)/ HA are shown in Figure 3. Some characteristic peaks in the GG spectrum (Figure 3a) can be ascribed to the following: 1018

(2)

2.11. Soil Burial Degradation of GG-g-P(IA-co-AM)/HA Superabsorbents. The soil burial degradation test of the superabsorbents was monitored by the dry weight loss. The GG-g-P(IA-co-AM)/HA superabsorbents with different amounts of GG and HA used in this experiment were prepared in a tube. They were cut into disks and then dried. The dried disks (9−10 mm diameter and 0.5−0.6 mm thick, about 0.1 g) were buried 6 cm beneath the surface of the soil at ambient temperature. The soil moisture was kept at 20% by spraying water. After 10, 20, 30, 40, 60, 90, and 120 days, the disks were taken out, washed with distilled water, and vacuum-dried to a constant weight. The percentage of degradation (PD %) of the superabsorbents was calculated from eq 3 PD% = (W0 − Wt) × 100/W0

characteristic

(3)

where W0 and Wt are the weight of superabsorbent disks before and after degradation, respectively. 2.12. Scanning Electron Microscopy. The surface morphologies of original and degraded superabsorbent polymers were examined by scanning electron microscopy (SEM). The samples were coated with gold and then observed in a JSM-5600 LV SEM (Japan).

Figure 3. FTIR spectra of guar gum (GG) (a), humic acid (HA) (b), and GG-g-P(IA-co-AM)/HA (c). 12995

dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000

Industrial & Engineering Chemistry Research

Article

cm−1 for the stretching vibration of C−O in the rings, 1080 cm−1 for C−O(H) stretching vibration, 1157 cm−1 for C−O− C stretching vibration, and 1657 cm−1 for −OH bending vibration.24 The characteristic peaks at 1705, 1594, 1246, and 1032 cm−1 in the spectrum of HA (Figure 3b) could be assigned to the CO stretching of −COOH group, −COO− asymmetric stretching vibration, phenolic C−O(H) stretching vibration, and C−O(H) stretching vibration, respectively.25 For GG-g-P(IA-co-AM)/HA (Figure 3c), it is noted that the characteristic peaks of PIA (1559 cm−1 for the −COO− asymmetric stretching vibration) and PAM (1665 cm−1 for the CO stretching of −CONH2 groups, 1612 cm−1 for the bending of the amide bands) are observed. Meanwhile, there exists the characteristic peaks of GG (1126 cm−1 for C−O−C stretching vibration), indicating the grafting reaction of IA and AM on GG. However, the characteristic bands of HA overlap with those of SA-g-P(AA-co-AM). Taken together, these results demonstrate that GG-g-P(IA-co-AM)/HA has been successfully prepared. 3.3. Water Absorbency Determination. The water absorbency as a function of GG content was investigated for GG-g-P(IA-co-AM)/HA superabsorbents, as shown in Figure 4.

Figure 5. Effect of humic acid (HA) content on water absorbency.

with higher content of HA. The reason for the change can be explained as follows: HA contains a large number of hydrophilic groups, such as carboxyl, hydroxyl, and amido, which could react with GG-g-P(IA-co-AM) during the polymerization process and then the hydrophilicity of the polymeric network is improved. When the content of HA in the superabsorbents is larger than 6.7 wt %, the decrease of water absorbency is attributed to the fact that more HA may result in the cross-linking degree increasing and the excessive HA only acts as a filler of the composite and reduces the water absorbency. As a kind of organic fertilizer, HA is an important biological input for almost any agricultural program. Both liquid and granular forms can be applied to the root zone at seeding. In addition, both forms can be used alone or incorporated into fertilizer blends. Many research studies demonstrated that HA can increase soil fertility and humus content, improve soil porosity, and promote root and top growth.26,27 Therefore, in this study, the introduction of HA in the superabsorbent polymers not only enhanced water absorbency capacity, but also provided a kind of organic fertilizer for plant growth. 3.4. Effect of GG-g-P(IA-co-AM)/HA Superabsorbents on Soil Acidity and Alkalinity. Soil pH is a value that refers to the acidity or alkalinity level in the soil. Soil pH is important to plants because of its effect on nutrient availability and the toxicity of related elements of ions.28 Previous studies showed that soil nutrients are most available to plant roots when soil pH is in the range of 5.5−7.0.29 For example, the nutrient phosphorus is most available for plant uptake at pH 6.0−6.8. The aluminum, which is not a nutrient but is a component of most soil minerals, becomes increasingly soluble and toxic at soil pH < 5.0. Certain nutrients such as iron, manganese, and copper also increase in availability for plant uptake as pH decreases. In this study, the effect of GG-g-P(IA-co-AM)/HA superabsorbents on pH value of soil was investigated and results are shown in Table 2. As can be seen from Table 2, the various pH values of the simulated soil solution are adjusted to about 7.0 after treating with the superabsorbents. This is because GG-gP(IA-co-AM)/HA superabsorbent polymers contain large amounts of −COOH and −COO− groups, which can react with OH− and H+ of the soil solution under basic and acidic conditions, respectively. This means that the superabsorbents

Figure 4. Effect of guar gum (GG) content on water absorbency.

As we can see from Figure 4 (reaction condition include the neutralization of IA, 70%; weight ratio of IA to AM, 1:4; HA, 6.7%; initiator, 0.5%; cross-linker, 0.25%; reaction time, 3 h; and reaction temperature, 75 °C), the water absorbency decreases almost linearly with an increasing GG content. The reason is that the water absorbency of GG is much lower than that of P(IA-co-AM). That is, the hydrophilicity of −OH in GG is much lower than that of −CONH2, −COOH, and −COO− in the P(IA-co-AM). Meanwhile, GG could function as a crosslinking agent itself in the grafting reactions due to its abundant hydroxyl groups. Therefore, by increasing the GG content, the degree of cross-linking of the superabsorbents increases and the water absorbency decreases. In this study, the main purpose of introducing GG into the superabsorbents is to make the product cheaper and improve its degradability. The effect of amount of HA in the superabsorbent composites on the water absorbency is shown in Figure 5. As we can see from Figure 5 (reaction conditions include the neutralization of IA, 70%; weight ratio of IA to AM, 1:4; GG, 3.3%; initiator, 0.5%; cross-linker, 0.25%; reaction time, 3 h; and reaction temperature, 75 °C), the water absorbency increases when the HA content is below 6.7%, and decreases 12996

dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000

Industrial & Engineering Chemistry Research

Article

Table 2. pH Value of Soil Solution (80 mL) Before and After Treating with GG-g-P(IA-co-AM)/HA Superabsorbents (0.1 g) pH before treating after treatingb

a

4.0 7.09

5.0 7.13

6.0 7.08

7.0 7.19

8.0 7.19

9.0 7.20

10.0 7.30

a

1 kg of dry soil (below 70 mesh) was added to 2.5 L of tap water with stirring for 30 min and the mixture was allowed to sit undisturbed. The supernatant soil solution with various pH values were adjusted with 1 mol/L HCl or NaOH aqueous. b0.1 g of GG-g-P(IA-co-AM)/HA sample was immersed in 80 mL of soil solution with different pH values for 90 min. After the swollen hydrogels were filtered, the pH value of the filtrate was measured with a pH meter.

can buffer soil acidity or alkalinity so as to develop a more optimal pH for plant growth.24 Consequently, GG-g-P(IA-coAM)/HA superabsorbents used in this study not only absorb water and increase water availability to crops but also adjust pH value of the soil as a kind of soil amendment. 3.5. Slow-Release Behavior of the Fertilizer in Soil. The release rate of uncoated and different coated fertilizer substrates was monitored in soil at room temperature. Figure 6 shows the release profiles of the fertilizer substrates entrapped in APT matrix (Figure 6a), coated with GG single coating (Figure 6b), and coated with GG and GG-g-P(IA-co-AM)/HA superabsorbents double coatings (Figure 6c). As shown in Figure 6a−c, the release rate of nitrogen is higher than that of phosphorus and potassium, which confirms reasonably well the solubility of urea and KH2PO4 (water solubility at 20 °C is 105 and 22.6 g/100 mL, respectively). This is because fertilizer solubility may affect the solute concentration gradient between the solution in the fertilizer granules and the external environment thus affecting the flux of nutrient release. As shown in Figure 6a, urea and KH2PO4 reached a steady state of releasing equilibrium within about 3 and 15 days, respectively. This is mainly due to the high adsorption capacity of APT. As mentioned before, APT has high special surface area and there are many parallel channels through the lattice. When water diffused into fertilizer cores, part of nutrients would be adsorbed by APT, which consequently slowed the release rate of nutrients. When the APT matrix granules were coated by GG (Figure 6b), the release rate of nutrients decreased slightly: about 93.3 ± 2.6 and 94.2 ± 2.8 wt % for P and K within 30 days, respectively. When the fertilizer cores were coated with GG and GG-g-P(IA-co-AM)/HA superabsorbent polymers double coatings (Figure 6c), the product showed preferable slow-release properties: nutrient N reached release equilibrium during 20 days; nutrients P and K released 88.2 ± 2.3 and 92.4 ± 2.6 wt % after being incubated in soil for 1 month, respectively. It is obvious that the superabsorbent materials used as coatings played an important role in retarding nutrients' release. Due to the hydrophilic nature of the polymers, GG-gP(IA-co-AM)/HA superabsorbents can absorb part of nutrients and then the nutrients release into soil through the dynamic exchange of free water. Furthermore, electrostatic interactions between the charged KH2PO4 and the superabsorbent networks controlled nutrients' release. 3.6. Effect of SMF on Water-Holding Capacity of Soil. The efficiency of the use of rain and irrigation water by plants is of great importance in semiarid and arid regions, where shortage of water is frequently experienced and water is often the limiting factor determining the size of the cultivated area.30

Figure 6. Slow-release behaviors of the fertilizer substrates entrapped in attapulgite (APT) matrix (a), coated with GG single coating (b), and coated with GG and GG-g-P(IA-co-AM)/HA superabsorbents double coatings (c).

In recent years, superabsorbent polymer materials have been intensively studied and used to enhance both the nutrition and water status of plants. Previous studies have shown that superabsorbents are effective in increasing the water-holding capacity, decreasing deep percolation, and reducing evaporation losses in sandy soils.17,31 Taking into account the interesting particular characteristics of the water absorption capacity of the 12997

dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000

Industrial & Engineering Chemistry Research

Article

SMF, we studied its effect on water-holding capacity of soil. The water-holding capacity of soil was determined from the swelling properties of the soil−SMF mixture after saturation. As shown in Figure 7, the water-holding capacity of soil is 45.2 ±

Figure 8. Weight loss of superabsorbents with different amounts of GG and HA with incubation time during soil burial degradation: (a) P(IA-co-AM); (b) GG-g-P(IA-co-AM), 3.3% GG; (c) GG-g-P(IA-coAM)/HA, 3.3% GG, 6.7% HA. Figure 7. Water-holding capacity of soil with different slow-release multinutrient fertilizer (SMF) application rates.

is obvious that introduction of GG and HA in the superabsorbent polymers enhanced degradation properties of the composite. This is because when the SMF was applied in soil, GG and HA of the superabsorbent interacted with surrounding fluids by first absorbing water, which initiated their degradation process. The absorption of water made the material more flexible and caused dimensional change, which facilitated the interaction of superabsorbents with soil microorganisms. SEM pictures of all the three superabsorbent materials surface changes before and after burying in soil for a period of 4 months are shown in Figure 9. As we can see, there are many wrinkles on the surface of the superabsorbents d, e, and f, which were formed during the process of drying and swelling of the samples. Cracks and pitting are clearly visible on the surface of the polymers with GG and HA (Figure 9e and f) after burying in soil for 4 months. It is obvious that introduction of GG and HA in the superabsorbent polymers improved degradation properties of the composite, which can be applied in agriculture as a new kind of coating material to alleviate environmental pollution.

1.8, 47.1 ± 2.4, 49.4 ± 2.8, and 52.5 ± 2.1% for SMF application rates of 0, 0.5, 1, and 2%, respectively. It is noted that the water-holding capacity increased with increasing mixing ratio of the soil−SMF. However, the superabsorbents in soil retained much less water than those in tap water. In the treatment, each SMF granule is surrounded by soil particles and subjected to a confining pressure by these particles.32 Meanwhile, the presence of ions (such as Na+, K+, Ca2+, and Mg2+) in soil solution has a negative effect on the absorbency behavior of the superabsorbents.33 Therefore, the swelling degree of the superabsorbents in soil is limited compared with that in tap water. However, compared with the control (soil without SMF), the SMF effectively improves the water-holding capacity of soil, even though at low application rate. In conclusion, the results of our studies showed that the use of SMF will be useful for reducing water losses and increasing plant establishment in drought-prone environments. 3.7. Soil Burial Degradation of GG-g-P(IA-co-AM)/HA Superabsorbents. Nowadays most superabsorbent materials are nonbiodegradable poly(acrylic acid) and polyacrylamide based products. The development of new products and materials, especially those which are nonpetrochemical reserves and based on renewable resources, is of increasing interest and deserves the attention of academic and industrial research.34 In this study, GG and HA were introduced in the network to improve the degradability of the superabsorbents. The degradation process was evaluated by a simple method of weight loss, which has been employed widely by other researchers.35−37 Figure 8 shows the weight loss of superabsorbents with different amounts of GG and HA with incubation time during soil burial degradation. As shown in the figure, a decrease in weight with time demonstrates the degradability of the superabsorbents. During the periods investigated, the sample c with 3.3% GG and 6.7% HA loses weight faster than the others, whereas the sample a without GG and HA shows little weight loss. After four months, the final percentages of degradation (PD %) are 4.5 ± 0.7, 10.5 ± 1.1, and 16.8 ± 1.5 wt % for the samples a, b, and c, respectively. It

4. CONCLUSION This work presents a slow-release multinutrient fertilizer (SMF) prepared by a simple route. The nutrients (H2NCONH2 and KH2PO4) were entrapped in an attapulgite (APT) clay matrix. Guar gum (GG) and guar gum-gpoly(itaconic acid-co-acrylamide)/humic acid (GG-g-P(IA-coAM)/HA) superabsorbent polymers were used as coatings. The product contained 17.9 ± 1.4 wt % nitrogen, 11.3 ± 0.7 wt % phosphorus (via P2O5), and 8.2 ± 0.4 wt % potassium (via K2O). The product had preferable slow-release properties: nutrient N reached a steady state of releasing equilibrium within 20 days; nutrients P and K released 88.2 ± 2.3 and 92.4 ± 2.6 wt % after being incubated in soil for 1 month, respectively. The results indicated that this new approach shows bright future in utilization of a natural resource, APT, in the production of fertilizer cores, which could significantly reduce the production cost. More importantly, the introduction of APT can slow fertilizer nutrients release due to its adsorption capacity. Moreover, the partially degradable superabsorbent 12998

dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000

Industrial & Engineering Chemistry Research

Article

(3) Chien, S.; Prochnow, L.; Tu, S.; Snyder, C. Agronomic and environmental aspects of phosphate fertilizers varying in source and solubility: an update review. Nutr. Cycl. Agroecosyst. 2010, 1. (4) Kundu, M. C.; Mandal, B. Nitrate Enrichment in Groundwater from Long-Term Intensive Agriculture: Its Mechanistic Pathways and Prediction through Modeling. Environ. Sci. Technol. 2009, 43, 5837. (5) Chatterjee, R. Projecting the future of nitrogen pollution. Environ. Sci. Technol. 2009, 43, 1659. (6) Du, C.-w.; Zhou, J.-m.; Shaviv, A. Release Characteristics of Nutrients from Polymer-coated Compound Controlled Release Fertilizers. J. Polym. Environ. 2006, 14, 223. (7) Ibrahim, A. A.; Jibril, B. Y. Controlled release of paraffin wax/ rosin-coated fertilizers. Ind. Eng. Chem. Res. 2005, 44, 2288. (8) Khan, M.; Kim, K.-W.; Mingzhi, W.; Lim, B.-K.; Lee, W.-H.; Lee, J.-Y. Nutrient-impregnated charcoal: An environmentally friendly slow-release fertilizer. The Environmentalist 2008, 28, 231. (9) Choi, M. M. S.; Meisen, A. Sulfur coating of urea in shallow spouted beds. Chem. Eng. Sci. 1997, 52, 1073. (10) Trenkel, M. E. Controlled-Release and Stabilized Fertilizers in Agriculture; International Fertilizer Industry Association: Paris, 1997. (11) Al-Zahrani, S. M. Utilization of polyethylene and paraffin waxes as controlled delivery systems for different fertilizers. Ind. Eng. Chem. Res. 2000, 39, 367. (12) Guo, M. Y.; Liu, M. Z.; Liang, R.; Niu, A. Z. Granular ureaformaldehyde slow-release fertilizer with superabsorbent and moisture preservation. J. Appl. Polym. Sci. 2006, 99, 3230. (13) Gan, F.; Zhou, J.; Wang, H.; Du, C.; Chen, X. Removal of phosphate from aqueous solution by thermally treated natural palygorskite. Water Res. 2009, 43, 2907. (14) Murray, H. H. Traditional and new applications for kaolin, smectite, and palygorskite: A general overview. Appl. Clay Sci. 2000, 17, 207. (15) Sannino, A.; Demitri, C.; Madaghiele, M. Biodegradable cellulose-based hydrogels: Design and applications. Materials 2009, 2, 353. (16) Lü, S.; Liu, M.; Ni, B. An injectable oxidized carboxymethylcellulose/N-succinyl-chitosan hydrogel system for protein delivery. Chem. Eng. J. 2010, 160, 779. (17) El-Shafei, Y. Z.; Al-Omran, A. M.; Al-Darby, A. M.; Shalaby, A. A. Influence of upper layer treatment of gel-forming conditioner on water movement in sandy soils under sprinkler infiltration. Arid Soil Res. Rehab. 1992, 6, 217. (18) Wichterle, O.; Lim, D. Hydrophilic Gels for Biological Use. Nature 1960, 185, 117. (19) Chen, Z.; Liu, M.; Ma, S. Synthesis and modification of saltresistant superabsorbent polymers. React. Funct. Polym. 2005, 62, 85. (20) Tan, H.; DeFail, A. J.; Rubin, J. P.; Chu, C. R.; Marra, K. G. Novel multiarm PEG-based hydrogels for tissue engineering. J. Biomed. Mater. Res. A 2010, 92A, 979. (21) Panariello, G.; Favaloro, R.; Forbicioni, M.; Caputo, E.; Barbucci, R. Synthesis of a New Hydrogel, Based on Guar Gum, for Controlled Drug Release. Macromol. Symp. 2008, 266, 68. (22) Walker, G. M.; Magee, T. R. A.; Holland, C. R.; Ahmad, M. N.; Fox, N.; Moffatt, N. A. Compression testing of granular NPK fertilizers. Nutr. Cycl. Agroecosyst. 1997, 48, 231. (23) Lafci, A.; Gürüz, K.; Yücel, H. Investigation of factors affecting caking tendency of calcium ammonium nitrate fertilizer and coating experiments. Nutr. Cycl. Agroecosyst. 1988, 18, 63. (24) Wang, W.; Wang, A. Synthesis, swelling behaviors, and slowrelease characteristics of a guar gum-g-poly(sodium acrylate)/sodium humate superabsorbent. J. Appl. Polym. Sci. 2009, 112, 2102. (25) He, M.; Shi, Y.; Lin, C. Characterization of humic acids extracted from the sediments of the various rivers and lakes in China. J. Environ. Sci. (China) 2008, 20, 1294. (26) Valdrighi, M. M.; Pera, A.; Agnolucci, M.; Frassinetti, S.; Lunardi, D.; Vallini, G. Effects of compost-derived humic acids on vegetable biomass production and microbial growth within a plant (Cichorium intybus)-soil system: a comparative study. Agric., Ecosyst. Environ. 1996, 58, 133.

Figure 9. SEM images of all the three superabsorbent materials surfaces before (a, b, c) and after (d, e, f) burying in soil for a period of 4 months: P(IA-co-AM) (a, d); GG-g-P(IA-co-AM), 3.3% GG, (b, e); GG-g-P(IA-co-AM)/HA, 3.3% GG, 6.7% HA, (c, f).

materials used as coatings played an important role in retarding nutrients' release, as well as in holding water in soil. In addition, the superabsorbents could be used as soil amendment to regulate the pH value of soil to neutral level. Furthermore, the introduction of HA in the superabsorbent polymers not only enhanced water absorption capacities, but also provided a kind of organic fertilizer for plant growth. All of the results of the present work indicate that the SMF as a kind of environmental friendly compound fertilizer may be expected to have wide applications for sustainable development of modern agriculture.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-931-8912582. Tel: +86-931-8912387. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the financial support of the Special Doctorial Program Funds of the Ministry of Education of China (grant 20090211110004) and Gansu Province Project of Science and Technologies (grant 0804WCGA130).

■

REFERENCES

(1) Matson, P. A.; Naylor, R.; Ortiz-Monasterio, I. Integration of environmental, agronomic, and economic aspects of fertilizer management. Science 1998, 280, 112. (2) Ayoub, A. T. Fertilizers and the environment. Nutr. Cycl. Agroecosyst. 1999, 55, 117. 12999

dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000

Industrial & Engineering Chemistry Research

Article

(27) Vlcková, Z.; Grasset, L.; Antosová, B.; Pekar, M.; Kucerík, J. Lignite pre-treatment and its effect on bio-stimulative properties of respective lignite humic acids. Soil Biol. Biochem. 2009, 41, 1894. (28) Haynes, R. J.; Naidu, R. Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions: A review. Nutr. Cycl. Agroecosyst. 1998, 51, 123. (29) Aciego Pietri, J. C.; Brookes, P. C. Relationships between soil pH and microbial properties in a UK arable soil. Soil Biol. Biochem. 2008, 40, 1856. (30) Andry, H.; Yamamoto, T.; Irie, T.; Moritani, S.; Inoue, M.; Fujiyama, H. Water retention, hydraulic conductivity of hydrophilic polymers in sandy soil as affected by temperature and water quality. J. Hydrol. 2009, 373, 177. (31) Bakass, M.; Mokhlisse, A.; Lallemant, M. Absorption and desorption of liquid water by a superabsorbent polymer: Effect of polymer in the drying of the soil and the quality of certain plants. J. Appl. Polym. Sci. 2002, 83, 234. (32) Bhardwaj, A. K.; Shainberg, I.; Goldstein, D.; Warrington, D. N.; J.Levy, G. Water Retention and Hydraulic Conductivity of CrossLinked Polyacrylamides in Sandy Soils. Soil Sci. Soc. Am. J. 2007, 71, 406. (33) Ni, B.; Liu, M.; Lü, S.; Xie, L.; Zhang, X.; Wang, Y. Novel slowrelease multielement compound fertilizer with hydroscopicity and moisture preservation. Ind. Eng. Chem. Res. 2010, 49, 4546. (34) Marci, G.; Mele, G.; Palmisano, L.; Pulito, P.; Sannino, A. Environmentally sustainable production of cellulose-based superabsorbent hydrogels. Green Chem. 2006, 8, 439. (35) Umare, S. S.; Chandure, A. S. Synthesis, characterization and biodegradation studies of poly(ester urethane)s. Chem. Eng. J. 2008, 142, 65. (36) Duarte, A. R. C.; Mano, J. F.; Reis, R. L. Enzymatic degradation of 3D scaffolds of starch-poly-([var epsilon]-caprolactone) prepared by supercritical fluid technology. Polym. Degrad. Stab. 2010, 95, 2110. (37) Shogren, R. L.; Doane, W. M.; Garlotta, D.; Lawton, J. W.; Willett, J. L. Biodegradation of starch/polylactic acid/poly(hydroxyester-ether) composite bars in soil. Polym. Degrad. Stab. 2003, 79, 405.

13000

dx.doi.org/10.1021/ie3003304 | Ind. Eng. Chem. Res. 2012, 51, 12993−13000