Application of Superabsorbent Polymers to minimize soil water stress

Transcription

1 Application of Superabsorbent Polymers to minimize soil water stress Maria Tomé Cosme Belard da Fonseca, Abel Rodrigues, Fátima C. Rosa, Miguel Casquilho Department of Chemical Engineering (DEQ), Instituto Superior Técnico, Universidade de Lisboa Abstract The benefits of a superabsorbent polymer s application (sodium polyacrylate) in agricultural or forest areas with dry weather, to improve water resources management efficiency, were studied, using weather stations data from Herdade das Barradas da Serra, located in Serra de Grândola (in the municipality of Grândola). The superabsorbent polymer (acronym used: SAP) was characterized in terms of its swelling capacity, both in water and in aqueous solutions of different and typical soil salts, as these can affect water absorbency. The results were applied in a model for prevision of the SAP s behaviour for water release into the soil. The model indicated that the polymer can increase, on average, soil moisture about fifteen times. For prevision, we selected some areas of interest for SAP s potential application in Grândola, Aljustrel, Beja, Ferreira do Alentejo and Serpa municipalities using geographical information based software. Keywords: Superabsorbent polymer; water resources; sodium polyacrylate; geographical information. 1 Introduction 1.1 Superabsorbent polymers Superabsorbent polymers (SAPs) can absorb and retain large quantities of water (about 100 to 1000 g of water per gram of initial dry polymer [Rosa et al., 2007]) and, in smaller scale, some quantities of aqueous solutions (electrolytes and brines) and biological fluids, even under load [Suda, 2007]. The absorbed fluid is not released easily or quickly as it is immobilized by sequestration rather than being retained in the polymer structure. SAPs are generally produced by polymerizing monomers such as acrylic acid, acrylic esters, acrylamide and a range of various unsaturated monomers. Early superabsorbents were made from chemically modified starch and cellulose and other polymers like poly(vinyl aclcohol) PVA and poly(ethylene oxide) PEO. Today s superabsorbent polymers are made from partially neutralized, very lightly cross-linked poly(acrylic acid), which has been proven to give the best performance versus cost ratio. The polymers are manufactured at low solids levels for both quality and economic reasons, and are dried and milled in to granular white solids, which swell to a rubbery gel when in contact with water [Elliott, 2004]. The cross-linked polyacrylates are the most important commercially superabsorbent polymers. Among them, only sodium polyacrylate can provide the most economical high charge-tomass ratio [Buchholz; Graham, 1998]. The main structure of this type of polymer is hydrophilic due to its carboxyl groups containing chains. The negative charges from neutralized carboxyl groups repel each other, causing the chains to expand. The cross-links between chains prevent endless swelling and thus the polymer doesn t June 2014

2 dissolve in water. The greater the degree of crosslinking, the lower the swelling capacity becomes and the greater is the mechanical strength of the polymer. When the superabsorbent polymers come in contact with water, there is hydration of C = OO - and Na + ions and the formation of hydrogen bonds [Elliott, 2004]. The sodium ions dissociated due to hydration of carboxylate groups, can move freely in the polymer network, thus increasing the osmotic pressure in the gel. The difference between this and the osmotic pressure outside is the driving force of swelling. The electrical neutrality is maintained because the ions are encapsulated into the gel (due to the weakly attraction to the carboxylate ions). When the amount of sodium ions outside the gel increases, the osmotic pressure decreases and hence swelling capacity is also reduced (for this reason, this is maximum in deionized water). The higher the degree of neutralization, the greater is the swelling capacity in general. The absorption capacity of the superabsorbent hydrogels can be significantly affected by several factors in external solutions such as salt concentration and its valence. Increasing the difference in concentration of mobile ions between the polymer and the external solution reduces the volume of the gel, because the swelling capacity decreases (charge screening effect). Additionally, in the case of multivalent cations, ionic cross-linking at surface of particles can occur. Increasing the charge, increases the degree of cross-linking. This ionic cross-linking is a more effective factor against swelling than the charge screening effect of the cation. The effect of ionic strength on the absorption capacity is also relevant. At low ionic strengths, the concentration of bund charges within the hydrogel network exceeds the concentration of salt in external solution, a large ion-swelling pressure causes the hydrogel to expand, thereby lowering the concentration of ions within the hydrogel. As the external salt concentration rises, the difference between the internal and external ion concentration decreases and the hydrogel deswells. The hydrogel continues to deswell with increasing external salt concentration until the mobile-ion concentrations inside and outside are approximately equal. These phenomenons can also be explained on the basis of repulsion between fixed charges groups on the hydrogel. As ionic strength increases, repulsion is shielded and the hydrogel deswells (charge screening effect) [Sadeghi; Koutchakzadeh, 2007]. The swelling ratio increases, generally, with increase of ph, although often the maximum is reached near the neutral ph range, the amount of water absorbed decreasing again in strongly basic medium. This latter phenomenon is due to the shielding effect exerted on the carboxylic anions by the cation (from the base used in obtaining high ph values), preventing effective anion-anion repulsions. At acidic ph values, polyacrylic acid s chains are collapsed. When the ph increases, the polymer network is electrically charged due to the deprotonation of the carboxylic protons, thus increasing the repulsive forces between adjacent groups which, in turn, causes the expansion of the polymer chains, leading to an increase in the swelling ratio [Roy et al., 2011]. The two main processes for producing polyacrylate superabsorbents are polymerization in bulk solution and suspension polymerization. These two processes have in common many factors, the most significant being the following: cross-linker and monomer concentration, initiator type and concentration, polymerization modifiers, relative reactivities of the monomers, basic polymerization kinetics and reaction temperature. In both cases, the polymerization of acrylic acid and its salts is initiated by free radicals, with a crosslinking agent, 2

3 and occurs in aqueous solution or suspension of aqueous droplets in a hydrocarbon solution [Buchholz; Graham, 1998]. SAPs have been used in such diverse applications as in diapers, feminine hygiene products, agriculture, horticulture, absorbents for domestic animals and in medicine (soft contact lenses, coatings for surgical instruments, wound dressings and bandages, among other examples) [Rosa et al., 2007]. Another application is the release, in the form of water-soluble substances, from the structure of the polymer and thus fertilizers and drugs can be incorporated into SAPs for controlled released. The SAPs can also be used to control the consistency of some products as diverse as cosmetics or cement. They may also serve as insulation of wires and underground cables [Suda, 2007]. An application of considerable interest is the use in the separation process of dilute organic solutions or organic materials, such as the removal of water from the cheese whey [Rosa et al., 2007]. The application of superabsorbent polymers in agriculture and forestry can improve ecological conditions in desertified areas or in the process of desertification (through life s support in ecosystems from these areas), aid the recovery of soils through reforestation, reduce the effects of drought, as water retention on soil surface increases, thus becoming more available to the plant and allowing a better crop development. The use of SAPs is responsible for two mainly benefits: it promotes a faster growth of plants, even with optimal conditions in terms of available water and prolongs the survival of plants subjected to water stress [Hütterman et al., 2009]. 2. Experiments 2.1 Experimental procedure The characterization of the sodium polyacrylate superabsorbent polymer consisted in determining its absorption capacity. Therefore, we compared the absorption rate in distilled water, tap water, in nitric acid solution (to evaluate the effect of lower ph values), solutions of salts commonly found in soil and also solutions of PK fertilizer (which has been used in the field work in Herdade das Barradas da Serra). We used a fixed amount of 0.05 g of SAP in all the experiments. Some of the samples (from the experiments with distilled water, tap water and aqueous solution of HNO 3 at 20 ml/l) were also subjected to desorption tests, through drying at room temperature, until all the absorbed water had evaporated and they could be used in further absorption tests. Thus, several absorption and desorption cycles were performed, which allowed the assessment of SAP s reusability capacity. The swelling experiments in solution were performed in the following conditions: in aqueous solutions of NaCl (at concentrations of 0.5; 1; 1.5; 2.5; 5; 10 and 300 g/l, the latter being close to the solubility product at room temperature); aqueous solutions of HNO 3 (at concentrations of 1; 2.5; 5, 10 and 20 ml/l); aqueous solutions of FeSO 4.7H 2 O (with concentrations corresponding to 0.5; 1; 1.5; 2.5; 5 and 10 g/l of FeSO 4 ); aqueous solutions of Na 2 SO 4 (at concentrations of 0.5; 1; 1.5; 2.5; 5 and 10 g/l); aqueous solutions of (NH 4 ) 2 SO 4 (at concentrations of 0.5; 1; 1.5; 2.5; 5 and 10 g/l); aqueous solutions of CaCl 2.2H 2 O (with concentrations corresponding to 0.5; 0.8; 1; 1,5; 2.5; 5 and 10 g/l of CaCl 2 ) and aqueous solutions of a NPK [ ] fertilizer (at concentrations of 5 and 10 g/l). The procedure was as follows: the sample was placed within a tea bag (then closed with a drawstring). Then the bag was immersed in water or aqueous solution and was weighed at one minute intervals, which have become two and five minutes in the latter half of the test. 3

4 After the experiment, the bag was immersed overnight and then weighed again and removed if there wasn t a significant change in the mass of the swollen SAP. The polymer was supplied by BASF. NaCl, Na 2 SO 4 and CaCl 2.2H 2 O salts were supplied by Merck (pro analysi). Nitric acid (HNO 3 at 65%), FeSO 4.7H 2 O and (NH 4 ) 2 SO 4 were supplied by Panreac. The amount of water absorbed per gram of dry polymer was calculated at each instant using Eq. (1). Fig. 2 Swelling ratio (g/g) in tap water. (1) in which Swelling is the amount of water absorbed per gram of dry SAP (g/g), M 2 is the mass of the swollen polymer (SAP + water) and M 1 is the mass of dry polymer (initial mass). The amount of absorbed and desorbed water was plotted as a function of time (minutes and days, respectively). 2.2 Cycles of absorption and desorption The results obtained in the absorption and desorption cycles are shown in Fig. 1, 2 and 3 (absorption) and 4, 5 and 6 (examples of desorption tests). Fig. 3 Swelling ratio (g/g) in HNO 3 solutions. Fig. 4 Fist desorption experiment after absorption in distilled water. Fig. 1 Swelling ratio (g/g) in distilled water. Fig. 5 Second desorption experiment after the first swelling test in tap water. 4

5 Fig. 6 Third desorption experiment after absorption in HNO 3. Fig. 10 Swelling ratio (g/g) in FeSO 4 solutions. 2.3 Absorption capacity in salt solutions Fig. 7, 8, 9, 10, 11 and 12 show the results of the absorption experiments with salt and fertilizer solutions. Fig. 11 Swelling ratio (g/g) in CaCl 2 solutions. Fig. 7 Swelling ratio (g/g) in NaCl solutions. Fig. 12 Swelling ratio (g/g) in PK fertilizer solutions. Fig. 8 Swelling ratio (g/g) in Na 2 SO 4 solutions. Fig. 9 Swelling ratio (g/g) in (NH 4 ) 2 SO 4 solutions. 2.4 Discussion It can be concluded, from analysis of the experimental results, that the polymer reaches a higher absorbency in distilled water (the average of the maximum value is 260 g/g), followed by absorption in tap water, which presents similar values (the mean maximum value is g/g) and is lower in salt solutions, such as expected. It is apparent that the absorption rate decreases with increasing ion concentration in solution and decreasing the ph (by increasing the concentration of nitric acid), which is in agreement 5

6 with theory and articles referred to in the introduction. Through analysis of the swelling curves for solutions of different ions with equal concentration (Fig. 13), it can be concluded that the SAP swelling ratio is lower in the presence of divalent ions (Ca, Fe (II)) as those obtained with monovalent ions (for example, the value of absorption rate achieved in ammonium sulphate solution after twenty minutes is g/g, which is 1.47 times higher than 88.3 g/g, the value for chloride calcium solution), as expected. Fig. 13 Swelling ratio (g/g) in solutions with concentration of 1 g/l. Observing the curves of the swelling ratio of SAP in tap water, it can be remarked that there is a decrease in swelling capacity, as the polymer is subjected to further tests of absorption, which is in agreement with the results reported in the literature [Bulut et al., 2009]. The decrease in absorption capacity was not observed, however, in the experiments with distilled water (but the swelling was slower as the experiments progressed) and was not significant in the case of SAP immersed in a solution of HNO 3. It was established that the average time for the polymer to lose all water is about ten days, placed at room temperature (during spring and summer and placing the SAP in watch glasses). 3 Field work The fieldwork comprised the data collected through sensors placed on BS1.2 installment of Herdade das Barradas da Serra, for monitoring the effect of a SAP (sodium polyacrylate) and also the development of a model for predicting the effect of SAP in soil moisture using data from a weather station near the site under consideration, since the equipment placed at Herdade das Barradas the Serra suffered damage that hampered obtaining conclusive results. The model was also applied to four other municipalities in the Alentejo: Aljustrel, Beja, Serpa and Ferreira do Alentejo. The effect of superabsorbent polymer in the field was evaluated through its application in a portion of the Herdade das Barradas da Serra (in the hills of Grândola), parcel called BS1.2, and whose geographical coordinates are N/ W. A breeding pasture was made in February 2012, accompanied by NPK ( ) fertilization at normal levels used for this purpose. The polymer was applied in two of four quadrants Q1 and Q2, each with an area of 2500 m 2. First, 4 kg of SAP were applied on half of Q1 and Q2 and then 2 kg were applied in Q1 and other 2 kg in Q2. (Fig. 14). The quadrant Q1 received 120 kg of fertilizer (100 kg on the whole area and Fig. 14 Scheme implementation of SAP in parcel BS1.2 [Lino, 2012]. 6

7 20 kg mixed with SAP on the lower half of the quadrant) and 6.25 kg of seed mixture. The Q3 quadrant received a similar amount of seed but the total amount of fertilizer was 100 kg. The Q4 quadrant received no treatment, thus being the control area. 3.1 Collection and processing of data relating to the application of SAP at Herdade das Barradas da Serra In June 2013, the data from weather stations (installed in Herdade das Barradas Serra in September 2012) were collected. These data were plotted in three graphics, as function of a time series, for each day of available data (September to November 2012): precipitation and soil moisture, with and without SAP (Fig. 15), the difference in the soil temperature and air temperature in quadrants Q1 and Q3 (Fig. 16), and the temperature soil in the parcel with SAP and solar radiation (Fig. 17). Fig Rainfall (mm) and soil moisture (% v) data in parcels with or without SAP (1 Sept Nov. 2012). Fig. 16 Soil and air temperature differences between parcels with or without SAP (1 Set Nov. 2012). Fig. 17 Parallel variation of solar radiation and soil temperature (1 Set Nov. 2012). Through analysis of Fig. 15, it appears that the humidity of the soil increased following the most significant rainfall episodes (which occurred on the following days: 23/9, 25/9, 26/9, 11/10, 17/10 18/10, 19/10, 21/10, 2/11, 3/11 and 4/11), and that soon after the fall of rain, the portion without SAP has higher moisture content due to accumulation of water by gravity and soil moisture on the portion with SAP is closer to the steady-state value, reached a few days later. In the days after the precipitation phenomena, it is observed that soil moisture stabilizes, with convergence of the curves of the two parcels and a gradual increase in the case of the SAP parcel, as expected. Air temperature and soil temperature were higher in the portion without SAP than in the parcel with SAP due to different sun exposures [Rodrigues, 2013]. 3.2 Model prediction of the effect of SAP in soil moisture As the model basis, we considered the application of 4 kg of SAP in 2500 m 2. Through chemical analysis of ground portion BS1.2, we determined the amount of sodium, calcium and iron in the soil (taking into account its bulk density and setting a control volume of 0.1 m 3, with a control area of 1m 2 ). We calculated the average of SAP maximum values of water absorption obtained in 7

8 laboratory experiments with solutions of NaCl, Na 2 SO 4, CaCl 2 and FeSO 4, and the average for sodium ions was calculated with the results obtained for NaCl and Na 2 SO 4. We used the laboratory data obtained for solutions of 0.5 g/l, as it s the closest to the ions soil concentration (0.03 g/l of sodium, g/l of iron and 0.01 g/l of calcium). From these values and using a simple rule of proportion we calculated the maximum absorption in solution of sodium, iron and calcium for the amount of SAP considered in the model. The model was developed for spring (which is the season when water stress is most significant for annual crops) using precipitation and soil moisture data collected in spring These daily data were obtained from a meteorological station, near the study area, in Serra de Grândola. The moisture data was collected at a depth of 10 cm. We related precipitation and soil moisture through the following rule: in the days when it does not rain, soil moisture takes a constant value (a baseline, which value is 0.04 mm), which is the average of soil moisture data; when there is little rain, soil moisture takes the value of the next data field day; when it rains heavily, the soil moisture has two days late (compared to field data). There is little rain when the precipitation value belongs to the first two ranges of data that is, ranging from 0 to 0.7mm, and 0.7 to 1.9 mm. A day with heavy rain is one whose values are defined within the last interval, ie from 1.9 to 8 mm (Fig. 18). We estimated the SAP behaviour based on the absorption values obtained for sodium, calcium and iron. The ultimate aim was to calculate the total soil moisture, which is defined as calculated soil moisture (obtained from field data by the previously enunciated rule) added to water released by SAP. It was considered that, in the presence of sodium ion there is saturation when it rains, and thus the SAP does not lose water; however, in the presence of calcium and iron, it was established that there is always water release since it was found through the absorption tests in solutions with these ions that the SAP swelled to a certain extent followed by desorption. Based on laboratory tests of desorption, it was established that if there is no rainfall in the spring, the SAP loses all its water in 9 or 5 days in the presence of sodium or calcium and iron ions, respectively. The 5 days time for complete water loss is due to the deswelling verified in presence of calcium and iron, as stated earlier, and also because the amount of water absorbed by the SAP in solutions of divalent ions is smaller and therefore the desorption time is shorter. When there is no rainfall the SAP releases a ninth or a fifth of total moisture per day, in sodium presence or in calcium and iron presence, respectively. When the SAP has lost all water and there is no rainfall on the following day, then there s only the soil moisture contribution for the total moisture. When there is rainfall again, the Fig. 18 Rainfall, soil moisture and calculated soil moisture in the spring of Fig Soil moisture data, calculated soil moisture and average total soil moisture with 4 kg of SAP/2500 m 2, during the spring of

9 SAP achieves maximum absorption value. The mean value of the total soil moisture (Fig. 19) is calculated from the three different cases described above (presence of sodium, calcium and iron ions). 3.3 Representation in ArcMap We used the ArcGIS software developed by ESRI corporation [Belard-Fonseca et al., 2014], to determine locations (and related areas) in five municipalities in the Alentejo (Grândola, Aljustrel, Beja, Serpa and Ferreira do Alentejo) in which it is profitable to apply SAP superabsorbent to minimize the drought stress of the soil. We have created maps picturing rainfall, soil moisture and soil moisture with SAP contribution (Fig. 20, 22 and 23), with respect to the spring of 2009 using ArcMap, an application provided by said software. We selected the five municipalities mentioned above and its respective annual rainfall data from the Environment Atlas, available on the Portuguese Environment Agency website. These annual rainfall data have five classes and thus we created five rainfall classes for the field data. We calculated the accumulated rainfall values for each class. The soil moisture values were also organized in five classes, dependent from the rainfall ones, and the accumulated values were calculated. The rivers located in the five municipalities were added to the ArcMap file as well as photo-plots (provided by the National Institute for Agricultural and Veterinary Research, which is in charge of this project). These photoplots provided information for the use and occupation of the soil. Thus, using the photo-points and ArcGIS was possible to select areas of unirrigated agriculture, grassland and fallow for potential application of SAP (Fig.. 21). To determine which, between the selected areas, have a greater need for application of SAP we considered its soil moisture values and a code from 1 to 4 was created (where 1 corresponds to the greatest need in absolute terms and 4 to lower requirements in absolute terms) (Fig. 24). Fig. 20 Rainfall (mm) during spring in the five counties considered in the study. Fig. 21 Selected zones for potential application of SAP. Fig. 22 Soil moisture (mm) in the selected zones during spring. Fig. 23 Soil moisture with SAP contribution (mm) in the selected areas during spring. Fig. 24 Areas most in need of SAP implementation. 4 Conclusions We confirmed that SAPs may contribute to minimize soil water stress, because their ability to absorb water (and its subsequent release) is significant even in the presence of salts. The model 9

10 that has been developed for forecasting has shown that soil moisture with SAP is on average about fifteen times higher than in the soil without application of SAP. The municipalities wherein the said water stress is greater, and therefore the implementation of SAP is most needed, are Aljustrel, Ferreira do Alentejo, Beja and Serpa (in unirrigated agriculture and fallow areas). In the future, the rules for developing the model may be improved and the selection of areas most in need of placement of SAP, depending on soil moisture, can be refined. Despite the limitations (for example, the absence of absorption capacity data for all ions present in the analysed soil) and the simplifications that were made, this model can be a useful tool for monitoring and management the need for artificial irrigation and, if applied to a longer period of time, also anticipate the need for replacing the SAP in soil (due to loss of absorptive capacity over time). Acknowledgments The authors thank BASF for kindly supplying the sodium polyacrylate superabsorbent polymer used in all the experiments and field work. References BELARD-FONSECA, Maria, RODRIGUES, Abel, CASQUILHO, Miguel, ROSA, Fátima, GODINHO-FERREIRA, Paulo (2014) Aplicação de polímeros superabsorventes para minimização de stress hídrico no solo, EUE2014 (Encontro de Utilizadores ESRI), Culturgest, Lisboa, Maio. BUCHHOLZ, Frederic L.; GRAHAM, Andrew T. (ed.). (1998) Modern Superabsorbent Polymer Technology, Wiley-VCH. BULUT, Y. et al. (2009) Synthesis of clay-based superabsorbent composite and its sorption capability, Journal of Hazardous Materials, 171, , Elsevier. ELLIOTT, M. (2004) Superabsorbent Polymers, BASF, accessed online on March HÜTTERMANN, Aloys et al. (2009) Application of Superabsorbent Polymers for Improving the Ecological Chemistry of Degraded or Polluted Lands, Clean Journal, 37 (7), pp , Wiley-VCH Verlag GmbH & Co. LINO, Inês Sena (2012), Superabsorbent Polymers: Forestry and Agricultural Application, Department of Chemical Engineering, Instituto Superior Técnico. RODRIGUES, Abel (2013), Relatório Preliminar das Intervenções Realizadas na Herdade das Barradas da Serra. ROSA, Fátima, BORDADO, João M., CASQUILHO, Miguel (2007) Polímeros Superabsorventes-Potencialidades e aplicações, Dossier Comunicações-Química, accessed online on June ROY, P. K. et al. (2011) Removal of Toxic Metals Using Superabsorbent Polyelectrolytic Hydrogels, Journal of Applied Polymer Science, Vol. 122, , Wiley Periodicals, Inc. SADEGHI, M.; KOUTCHAKZADEH, G. (2007) Swelling kinetics study of hydrolyzed carboxymethylcellulose-poly(sodium acrylate-coacrylamide) superabsorbent hydrogel with salt sensitivity properties, J. Sci. I. A. U (JSIAU), Vol 17, No. 64, pp , SUDA, K., (2007) Superabsorbent Polymers and Superabsorbent Polymer Composites, Science Asia 33, Supplement 1, pp