Studying Radiation-Induced Modification of Polymer to Make Water-Superabsorber As a Soil Moisture Conditioner

VAEC-AR 03-27 VN0500036

STUDYING RADIATION-INDUCED MODIFICATION OF POLYMER TO MAKE WATER-SUPERABSORBER AS A SOIL MOISTURE CONDITIONER

Doan Binh*, Phain Thi Tim Hong*, Doan Thi The*, Tran Tich Canh*, Nguyen Quoc Hien*,Vo Thi Kim Lang* and Nguyen Duy Hang** *Reseavch and Development Center for Radiation Technology **Nuclear Research Institute

ABSTRACT: Study and preparation of a water-superabsorber based upon polymer in conditioning soil moisture has been carrying out in VINAGAMMA. The capability of absorbing of the water super-absorber is 300 g to 500 g of water per dry gram of hydrogel. The material commonly found in horticultural markets are sold as hydrogel or water super-absorber. Investigation of grafting acrylic acid monomer onto starch (starch-g-AAc) and acrylamide monomer onto carboxymelhyl cellulose (CMC-g-AAm) is first put through. The effect of composition of monomer/substrate of polymer on grafting per cent and on swelling ratio was taken into account. In addition, influence of absorbed dose, rate of absorbed dose, pH, salinity and temperature on swelling ratio was studied. Evaluation of enzymatic degradability of starch-g- AAc and CMC-g-AAm was tested in a-amylaza and cellulaza, respectively. Biodegradation of starch-g-AAc in the soil was also implemented in the greenhouse at 25-28°C. Fresh keeping ability of the sprig of orchid flowers in starch-g-AAc hydrogel was proved better than in the distilled water. As a result, observation of dried/fresh biomass of 'tall shoot" cabbage with or without the hydrogel added in the soil matrix regarding to irrigation regime, distribution in depth of soil layer and ratio of hydrogel/soil was made. The first stage result of the project implementation has been stimulating for 2 years.

1. GENERAL INTRODUCTION

Water absorption material has been using in agricultural applications for control of soil erosion, limitation of loss of nutrients and silt for plants; decrease of irrigation frequency and increase of water retention in prolonged arid soil or droughts. The water absorbed polymers mixed with the soil enhances stabilization of growing plants, mycorrhirae and soil bacteria. Water-superabsorbers are often called hydrogel that consist of (1) natural origin polymers, (2) semisynthetic polymers originated from cellulose, which was reacted cracked mineral oil products, and (3) synthetic polymers used commonly: polyvinyl alcohol, PVA; polyacrylamide, PAAm; polyacrylic acid, AAc; and polyethylene, PEO. Salts, pH, nutrients, and fertilizers strongly effect on the characteristics of hydrogel in the soil matrix. Hydrogel reduced salts in the soil even though the water-held capacity of the hydrogel decreases, In addition, the hydrogel removes heavy metals. The organs such as plants, mycorrhirae, bacteria and fungi have a response of addition of the hydrogel to soil medium. Hydrogel is increasing microbial clone of nitrogen fixation in the soil (Azotobacter, Azospirilla va Clostridium) but developed at a limited level. Hydrogel reduced the fluctuation of temperature in the soil. The rate of evapotranspiration reduced when the mycorrhirae was added to the hydrogel (Glomus mosseae, G. Fasciculatum). Hydrogel polymers can be degraded by fungi and bacteria in laboratorial conditions. In the world, hydrogel named "watersorb", 'Smart hydrogel", 'STA-Moist", and "Stockosorb" are using to produce seed germination, conditioners, fertilizer release

170 The Annual Report for 2003, VAEC VAEC-AR 03--27 material, promoters, reduction of soil compression, antirunning off pesticide and nutrients. 2. EXPERIMENTS 2.1 Kaw materials-chemicals Corn starch (France), acrylic acid (German), KOH (China), enzyme: a-amylaza (activity of 3000 IU, Vietnam), cellulasl 1.5L (capacity of J500 NCU/g, Denmark), carboxymethyl cellulose (China), acrylamide (German) 2.2. Equipment Irradiator (Gamma, SV-ST Co-60/B), Hungary at VINAGAMMA, Hochiminh city. 2.3. Method 2.3.1. The formation of hydrogel from AAc grafted on starch (starch-g-AAc) 2.3.2. The formation of hydrogel from AAm grafted on CMC (CMC-g-AAm) 2.3.3. Irradiation: Irradiation is carried out at a range of absorbed doses of 4.8, 11.1, and 14.6 kGy at the dose rate of 1.6 kGy and absorbed dose of 11.1 kGy at different dose rates of 0.27,0.55, 1.11, and 1.6kGy/hr. 2.3.4. Removing the ungrafted polyacrylic acid or polyacrylamide with Sohxlet extraction:

Grafting per cent (%G) = [(in, - ms)xl00]/ms Where n\: initial dry weight of substrate polymer(g), m,: dry weight of extracted sample (g) 2.3.5. Swelling ratio is calculated by weight of equilibrium absorbed water for I g of postexlracted sample expressed as gram of water/gram of dried sample. 2.3.6. Evaluation of enzymatic degradation: The residue of enzyme-degraded sample is washed by distilled water, dried at 65 "C and weighed to constant weight of the sample. 2.3.7. Biodegradable evaluation of sample buried underground: 5g of starch-g-AAc packed in a stainless steel net was buried underground at 3 cm deep in the greenhouse at 25-28"C. At a given period of time sample was dug up; soil washed out of it. Eventually sample was dried and weighed. 2.3.8. Relationship between moisturizing capacity and plant growth of "tall shoot" cabbage in respect of irrigation regime and in response to the ratio of composition of the hydrogellsoil: Sowing land with the cabbage seed (from China), trays of 50x30x15cm contain the mixture of hydrogel and soil in the ratio of 0.5/1000, 1/1000, 2/1000 (w/w). Hydrogel was laid on soil layer on the surface (0 cm), 3-5 cm, and 10-15 cm in depth. Experiment was arranged in lots and irrigation regimes of 2,3,4 times/week. A water amount used was 1.5 1 for one watering time in a tray.

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3. RESULTS AND DISCUSSION

3.1. Effect of the rate of monomer/substrate (M/S) on grafting percent at 11.1 kGy grafting percent increases with increasing the rate of M/S. At the same rate of M/S, grafting percent of Arc onto starch, cellulose and CMC is higher than that of Aim onto similar substrates. Because radiolysis of producing the free radicals of AAc is easier than that of AAm. 3.2. Effect of rate of M/S on swelling ratio i At 11.1 kGy the rate of M/S increases with increasing swelling ratio. When AAc or AAm grafted onto CMC, the rate of M/S increases with reducing the swelling ratio due to the presence of ion Na+ in CMC involving hydration of the hydrogel. On contrary, graft of AAc or AAm onto starch, the rate of M/S increases with increasing significantly swelling ratio. Swelling ratio is attainable of 300-500 times while rate of M/S is selected. 3.3. The effect of absorbed dose on grafting per cent and swelling ratio at the same rate of M/S (2/1) The results of the figure 1 show that water absorption reduced with increasing the dose at the higher doses of 4.8-5.0 kGy. The crosslinking of the part of grafted PAAc or PAAm is probably occurred. Grafting per cent of AAc or AAm on substrates tends to a maximum point as the absorbed dose is about 9-11 kGy. The absorbed dose is higher than 11 kGy the grafting per cent reduced because of degrading the substrate. 3.4. Influence of pH on swelling ratio of starch-g-AAc or CMC-g-AAm Increase of pH resulted in increasing the swelling ratio but not remarkably if compared to their swelling ratio in distilled water. 3.5. Effect of swelling ratio in distilled water on various temperatures It is indicated that it takes about 2 hrs for an equilibrium swelling ratio of starch-g-AAc or CMC-g-AAm at 5°C but it takes about 6 hrs for the equilibrium swelling ratio of these hydrogels at 30°C. Otherwise the swelling ratio of starch-g-AAc is the highest at 5"C. it is possible that a part of free water is converted to freezing water. At 5°C the structure of starch-g-AAc becomes porous. 3.6. Effect of absorbed dose rate on swelling ratio of the hydrogel The results show that the swelling ratio at 0.55 kGy/hr is more substantial than that at a lower or higher absorbed dose rate. The decrease of swelling ratio of starch-g- AAc at a high absorbed dose rate is due to crosslinked PAAc. Swelling ratio of CMC- g-AAm hydrogel increases with increasing absorbed dose rate.

172 The Annual Report for 2003, VAEC VAEC-AR 03-27

M 2a.

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10 Absorbed dose, kGy Figure 1: Relalionsliip between water-uptake and absorbed dose at the same composition ratio (2/1)

3.7. Ability of enzymatic degradation of slarch-g-AAc or CMC-g-AAm Table I shows that loss of weight of starch -g-AAc is around 78% with the use of a-amylase for 96 hrs. Under the same condition, the weight loss of control sample is 17% without any enzyme. The sample is degraded by aerobes or dissolved from a part of degradable one. CMC-g-AAm is lost about 36% with cellulase enzyme if compared to 7.3% weight loss of control sample.

Table 1: Enzymatic degradation of starch-g-AAc or CMC-g-AAm in the laboratory

Weight loss, % Time, hr Starch-g-AAc CMC-g-AAm a-amylase Control Cellulase 1.5L Control 0 0 0 0 0 24 35.8±70.7 13.61 1.0 20.710.3 0 48 40.1 1 1.9 11.010.1 24.4 10.4 0 72 52.210.6 15.413.7 24.810.8 3.910.6 96 78.012.2 17.0 1 0.4 36.012.1 7.310.5

3.8. Biodegradation of starch-g-AAc in the moisture soil in the greenhouse It is indicated that starch-g-AAc is degradable because of presence of bacteria and fungi in the soil to degrade them into smaller molecules such as CO2, H2O. 71.6% weight loss of starch-g-AAc has been testing for 72 days. It is anticipated that fragment of starch degraded quickly with a results of breaking down PAAc through reduction- oxidation reaction in the soil. Residual part is a probable PAAc that has been crosslinked.

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3.9. Table 2 shows that dry and fresh biomass of "tall shoot" cabbage in the depth of 3-5cm, 10-15 cm yields a higher harvest than that of 0 cm deep.

Table 2: Observation of dry and fresh biomass of tall shoot cabbage when starch-g-AAc hydrogel used

Depth, cm 0 3-5 10-15 Control

Irrigation 2 3 4 2 3 ' 4 2 3 4 2 3 4 regime, t/w

Humidity,% 17,86 22,05 23,00 18,12 22,02 23,30 16,91 20,43 24,10 20,80 20,00 20,00

Fresh biomass.g 9,27 10,11 12,45 8,94 24,59 53,87 31,89 62,03 63,34 9,83 20,10 30,84

Dry biomass.g 0,85 0,97 1,49 0,95 2,15 4,27 2,75 5,69 6,11 0,70 1,80 2,52

Note: t/w is the abbreviation of times/week, biomass is expressed as gram/10 cabbage trees

4. CONCLUSION

Starch-g-AAc hydrogel is considered meeting equilibrium swelling ratio of 300-500 times and the following conditions: rate of AAc/starch (2/1), absorbed dose (9-11 kGy), absorbed dose rate (more 0.55 kGy/hr). The hydrogel can be degradable a-amylase enzyme with the weight loss of 78.0% after incubation in 92hrs and be biodegradable in the soil of 78% after 120 day bury. The hydrogel can absorb and desorb water under various conditions. The water absorption of the hydrogel decreases in the presence of salts and cations. The hydrogel acts as conditioner of soil through use it for seedling the "tall shoot" cabbage, accelerating the seed germination, and supplying water availability of stabilization and establishment of plants in droughts, reduced irrigation and saving water.

174 The Annual Report for 2003, VAEC