Hydrogel is considered as an important material in our world nowadays as it is used in many important and significant applications such as in tissue engineering and agriculture. There are hundreds of types of such materials, where most of them can be easily prepared. The main objective of this work is to prepare one of the hydrogel types which could be very useful in the agriculture of deserts where plants in dry places require water in order to grow up. There are many places around the world where raining occurs only once or twice a year. There are also places where it does not rain at all. Therefore, hydrogels are required in order to absorb water in large quantities either during raining or irrigation instead of escaping to underground and then eject them to the roots of plants over time as the plants require watering. In this research a hydrogel based on acryl amide, Poly 2-Acrylamide-2-Methyl-1-Propane Sulphonic Acid, (PAMPS) was prepared by using different percentages of a suitable cross-linking agent, Methylene-bis-Acrylamide. The cross-linker content is very important factor affects the rate and amount of absorbed water. The highest amount of absorbed water at 25°C was observed by using 0.6% cross-linking agent based on monomer mass. The temperature of absorbed water and its pH value are also essential factors that affect the rate and the amount of absorbed water and were investigated in this work. The highest amount of absorbed water was recorded at pH = 12 and at 60°C. The amount and the rate of water absorbed by Sodium Polyacrylate Hydrogel were also investigated at 25°C. The agriculture applications of hydrogel based on Sodium Polyacrylate were examined using Fenugreek seeds implanting.
Hydrogels are very important class of thermo-setting polymers due to their ability of swallowing great amount of water up to hundred times the authentic weight of the arid polymer and hence they are very useful and could benefit the agriculture of deserts by absorbing large quantities of water during rain and then eject them to the roots of plants over relatively long time. Hydrogel, which is also known as super absorbent polymer, is a type of polymer whose structure is three dimensional and it is capable of swelling and absorbing large amounts of water. Such absorption of water depends on many factors such as type of the polymer, temperature, pH of water, and other factors such as the cross linking density [
The monomer 2-Acrylamide-2-Methyl-1-Propane Sulphonic Acid, (AMPS) and the cross-linking agent, Methylene-bis-Acrylamide were obtained from Sigma Aldrich company as commercial grade while, the initiator, ammonium persulphate was purchased from Alfa Chemicals as commercial grade.
The monomer (AMPS), (5 g) was added to about 12.5 ml of distilled water in a conical flask. A specific amount of methylene-bis-acrylamide which acts as the cross linking agent was then added. The mixture was shaken well and inert nitrogen gas was then passed through the mixture for 2 minutes in order to remove oxygen and make an inert blanket. Then, quickly after passage of nitrogen gas, about 0.02 g of ammonium persulphate was added to the solution which acts as the initiator. The solution was then poured in a test tube and then covered with an appropriate cork. The test tube is then put in an oven at 60˚C for about 2 - 3 hours. Then after that time hydrogel is obtained and cut into disks.
In the typical practical work, six different samples of PAMPS of different methylene-bis-acrylamide (cross linking agent) amounts were prepared. The used amounts by weight of the cross linking agent were 0.03, 0.09, 0.15, 0.25, 0.35, and 0.55 g. after preparing the different samples, a certain weight of the hydrogel is placed in a beaker and 200 ml of water was added to each sample in order to determine the amount of water absorbed as time elapsed.
Methylene-bis-acrylamide (0.15 g) cross linked PAMPS hydrogel was prepared and then three certain weights of this sample were taken, where each sample was placed in a beaker in which 200 ml of water at 25˚C was added. One of the hydrogels was left with the water neutral (pH = 7). For the second hydrogel sample, the water to be absorbed was made acidic by adding some drops of hydrochloric acid (HCL) till the pH was adjusted to be 4. Finally, for the third hydrogel sample, the water to be absorbed was made alkaline by adding small pieces of sodium hydroxide chips (NaOH) till the pH was adjusted to be 12.
Methylene-bis-acrylamide (0.15 g) cross linked PAMPS hydrogel was prepared and then three certain weights of this sample were taken, where each sample was placed in a beaker in which 200 ml of water was added. One of the hydrogels was left with the water at room temperature 25˚C. For the second hydrogel sample, the water to be absorbed was heated to 40˚C in a water bath maintained at 40˚C. Finally, for the third hydrogel sample, the water to be absorbed was heated to 60˚C in a water bath maintained at 60˚C.
Sodium polyacrylate hydrogel can be obtained by extracting the super absorbent polymer found in a diaper. The diaper from which the sodium polyacrylate was extracted is Molfix diapers (Junior). Such diaper is made in Egypt by Hayat Egypt Hygienic Products (S.A.E) where the factory is located in sixth of October city. The sodium polyacrylate hydrogel obtained from this type of diaper is in the form of very fine white crystals.
An experiment was carried out to apply this methodology of hydrogels in implanting and observe the results comparing it to the other regular farming method. Hydrogel crystals are put beneath the soil surface, a single layer of hydrogel beneath the soil is enough, and traditional farming methods are applied starting from watering the soil and applying enough sunlight to the plant.
As shown in
Similarly, In case of 0.09 g cross linking agent, the figure shows a high and constant slope for the first 7 hours, while after 7 hours; the slope started to decrease
showing another constant slope from the hour 21 till 150 hours of the experiment. The volume of water absorbed per unit weight of the polymer could increase the polymer mass to one hundred and forty one times its original weight after 240 hours. While, In case of 0.15 g cross linking agent, the figure shows a high and constant slope for the first 6 hours, while after 6 hours; the slope started to decrease showing another constant but lower slope from the hour 52 till 118 hours of the experiment. The volume of water absorbed per unit weight of the polymer could increase the polymer mass to one hundred and twenty three times its original weight after 240 hours. But, In case of 0.25 g cross linking agent, the figure also shows a high and constant slope for the first 7 hours, while after 7 hours; the slope started to decrease showing a constant decrease through- out the experiment.
The volume of water absorbed per unit weight of the polymer could increase the polymer mass to one hundred and ten times its original weight after 240 hours. In case of 0.35 g cross linking agent, the figure also shows a high and constant slope for the first 5 hours, while after 5 hours; the slope started to decrease and became unsteady. The slope then became constant but lower from the time 52 hours till 118 hours, then decrease and became constant again till the end of the experiment. The volume of water absorbed per unit weight of the polymer could increase the polymer mass to one hundred and three times its original weight after 240 hours. Finally, in case of 0.55 g cross linking agent, the figure shows a high and constant slope for the first 4 hours, while after 4 hours; the slope then was still decreasing till reaching 71 hours after which the slope is fixed till the end of the experiment. The volume of water absorbed per unit weight of the polymer could increase the polymer mass to fifty three times its original weight after 240 hours.
Generally, the previous figure, showed the different curves of the different cross linking densities PAMPS hydrogels. For the same time intervals, the 0.03 g cross linked PAMPS hydrogel showed the highest volume of water absorbed per unit weight followed by the 0.09 g, 0.15 g, 0.25 g, 0.35 g and finally 0.55 g cross linked PAMPS hydrogel which was the least volume of water absorbed per unit weight.
Generally the amount of absorbed water decreases as the cross-linking density increase due to decrease of the rooms available to absorb water as illustrated in
Cross Linking Density | Volume of Water Absorbed per Net Weight of Polymer (ml/g) |
---|---|
0.03 g | 223.112 |
0.09 g | 141.296 |
0.15 g | 123.706 |
0.25 g | 109.60 |
0.35 g | 102.98 |
0.55 g | 53.40 |
highest volume of water absorbed per unit weight of polymer (about six times that absorbed in neutral water, followed by the hydrogel in case of acidic water, and then in case of neutral water which shows the least volume of water absorbed per unit weight. This may be attributed to the acidic character of the polymer which contains a sulphonic acid group.
When using 0.2 g of this hydrogel in 200 ml tap water at 25˚C and observing the amount of water absorbed every 3 minutes, a relationship between the volume absorbed and time is drawn as shown in
The volume of water absorbed per unit weight of the sodium polyacrylate hydrogel increases with time until reaching 24 minutes. After this time the volume of water absorbed does not change with time showing that the hydrogel has reached saturation. The volume of water absorbed per unit weight of the polymer increases the polymer mass 205 times its original weight after 24 minutes only.
In the experiment that was carried out to apply this methodology of hydrogels in
implanting and observe the results comparing it to the other regular farming method. Hydrogel crystals are put beneath the soil surface, a single layer of hydrogel beneath the soil is enough, and traditional farming methods are applied starting from watering the soil and applying enough sunlight to the plant.
As shown in
The two pots were left for a longer period to observe an effective change keeping both pots to enough sunlight. Seven days later, it was observed that the pot with hydrogel grew more and was healthy, while the other plant died and stopped growing.
The relation between water absorbed by PAMPS hydrogel and time in case of different cross linking densities was investigated. For the same time intervals, the 0.03 g cross linked PAMPS hydrogel showed the highest volume of water absorbed per unit weight followed by the 0.09 g, 0.15 g, 0.25 g, 0.35 g and finally 0.55 g cross linked PAMPS hydrogel which was the least volume of water absorbed per unit weight. The study of the different PAMPS hydrogel in case of different water pH was also investigated. It was found that at the same time intervals the hydrogel in case of alkaline water shows the highest volume of water absorbed per unit weight followed by the hydrogel in case of acidic water, and then in case of neutral water which shows the least volume of water absorbed per unit weight. The study of the different PAMPS hydrogel in case of different temperatures was also investigated. For the same time intervals the hydrogel in case of water at temperature 60˚C shows the highest volume of water absorbed per unit weight followed by the hydrogel in case of water at temperature 40˚C, and then in case of water at room temperature which shows the least volume of water absorbed per unit weight.
Radwan, M.A., Al-Sweasy, O.H. and Elazab, H.A. (2017) Preparation of Hydrogel Based on Acryl Amide and Investigation of Different Factors Affecting Rate and Amount of Absorbed Water. Agricultural Sciences, 8, 161- 170. https://doi.org/10.4236/as.2017.82011