This article describes the commonly used experimental methods for assessing the biodegradability of polyacrylic acid polymers. It is recommended to use the carbon dioxide (PCD), oxygen consumption (COD) and infrared spectroscopy images of biodegradation to comprehensively analyze the biodegradability of polyacrylic acid polymers.
The volume of the bioreactor is 2L, the concentration of the tested substance is 1/1000, the concentration of the inoculated sludge is 500mg/L, the reaction temperature is room temperature (about 25°C), and the reaction time is 14 days. The experimental results show that the polyacrylic acid polymer studied can be completely biodegraded after 14 days.
Keywords: polyacrylic acid polymer, biodegradation, carbon dioxide production.
So far, domestic and international research on the biodegradability of organic substances has focused on dyes, coking wastewater, pesticides, heterocyclic compounds, and polycyclic aromatic hydrocarbons. There are few studies on the biodegradability of polyacrylic acid polymers.
At present, the evaluation index for the biodegradability study of organic matter can be measured according to factors such as the reduction of organic matter, the consumption of oxygen or the amount of carbon dioxide generated, and the content of high-energy phosphoric acid.
In addition, it can also be evaluated by observing the physiological and biochemical changes of the inoculum. The most used are BOD5, COD, dissolved organic carbon (DOC), carbon dioxide production (PCD), ATP content and so on.
Among them, nitrification has an effect on BOD, and the adsorption of microorganisms has an effect on the experimental results of DOC. The amount of carbon dioxide (PCD), as a new method for evaluating the biodegradation of organic matter, has the least influencing factors and can reflect the degree of complete inorganization of organic pollutants.
Therefore, this paper comprehensively analyzes the biodegradation of polyacrylic acid polymers by three indicators: oxygen consumption (COD), PCD value and infrared spectroscopy.
The research object of this experiment is a polyacrylic acid polymer, which is a milky white suspension. The main components include monomer (two-component monomer including hard monomer and soft monomer), initiator, emulsifier, carrier medium water. The monomers are mainly acrylates, and mainly include ester compounds such as methyl methacrylate, ethyl methacrylate, isobutyl methacrylate, hydroxypropyl acrylate, methyl acrylate, and butyl acrylate.
Further, the emulsion also contains a small amount of an emulsifier (sodium dodecyl sulfate), an initiator (azobisisobutyronitrile), and the like.
2 .1 CO2 production test method
2.1.1 Experimental device
The test device is shown in Figure 1. In order to remove CO2 from the intake air. A three-stage absorption device is used.
The first stage and the second stage were each absorbed with 150 mL of 1.00 mol/L NaOH solution. The third stage was absorbed by 0.10 mol/L Ba(OH) solution. This will not only test the absorption of the first two stages but also completely absorb CO2.
It is then washed in a water wash bottle to prevent lye from entering the bioreactor. To ensure good absorption, replace the absorption device regularly.
The CO2 produced during the biodegradation process uses a three-stage absorption device, and the third stage is absorbed by a 0.05 mol/L Ba(OH)2 solution in a volume of 100.0 mL. Replace the device every two days and titrate with 0.1000mol/L HCI solution
1. Gas flow meter, 2. CO2 first-stage front absorption bottle, 3. CO2 second-stage front absorption bottle, 4. CO2 three-stage pre-absorption bottle, 5. Washing bottle, 6. Biological reaction bottle, 7. Constant temperature water bath, 8 Sampling and sample tube, 9.CO2 primary absorption bottle, 10.CO2 secondary absorption bottle, 11.CO2 tertiary absorption bottle.
2.1.2 Biological media
In the bioreactor, add CaCl2 solution (27.50g L), magnesium sulfate heptahydrate solution (22.50g L), hexahydrate aluminum aluminide solution (0.25g L) each 5.0mL.10.0mL PH=7 phosphoric acid A salt buffer solution (8.50 g of dipotassium hydrogen phosphate and 44.70 g of disodium hydrogen phosphate dodecahydrate per liter of solution) and an appropriate amount of trace elements.
2.1.3 Biological cultivation
The activated sludge from the sewage treatment plant was used as an inoculum. Continuous aeration was carried out with an air pump of 135.0 L/h and domesticated for 10 to 15 days.
First, the sludge mixture was aerated for 24h. Two drops of 0.001 mol/L of the polyacrylic polymer solution and 10 ml of the culture solution were added every day for the next 2 days.
On the fourth day, add 5 drops of 0.001 mol/L polyacrylic acid polymer solution.
On the 5th and 6th days, 1.0 ml of a 0.001 mol/L polyacrylic acid polymer solution was added to acclimate culture. At the same time, a small amount of the originally activated sludge supernatant and 10.0mL of the culture solution were added. When the sludge mixture is uniformly dispersed brown sludge flocs, it indicates that the microorganisms have been initially domesticated and matured.
Thereafter, a 1.00 mL of a 0.001 mol/L polyacrylic polymer solution may be added for cultivation until it is used for the experiment.
2.1.4 Experimental conditions
In this experiment, the bioreactor bottle is divided into an endogenous reaction bottle (only 1.0L concentration of 500.0mg/L activated sludge mixture and biological medium) and biochemical reaction bottle (add 1.0L 0.0010mol/L polyacrylic acid) Polymer solution, 1.0L concentration is 500.0mg L activated sludge mixture and biological medium).
The reaction temperature was about 25°C (room temperature), the gas flow rate was 25.0L/h, and the reaction time was 14 days. The amount of carbon dioxide produced and the COD value was measured every two days.
2 .2 Determination of COD: Potassium dichromate standard method
2 .3 Infrared spectroscopy experimental method
Take the supernatant from the endogenous reaction flask and bioreactor for 14 days after biodegradation. Place in a watch glass and dry in an oven at 30°C-40°C (about 72 hours).
The dried samples were placed by tableting method, and infrared and spectroscopy experiments were performed on the blank and polyacrylic polymers, using a WQF-310 Fourier infrared spectrometer.
3. Results and discussion
3 .1 Carbon dioxide release curve and discussion analysis
The polyacrylic polymer solution is a mixture of a polymer and a monomer. During the first 2 days, microbes quickly experienced a stagnation period.
At the beginning of the stagnation period, a part of the microorganisms suitable for degrading monomers quickly adapt to the environment and quickly enter the end of the stagnation period. The cell material increases, the cell volume increases, its long axis grows at a particularly fast rate, the cell metabolism is strong, the respiratory rate, the nucleic acid, and protein synthesis rate is close to the log phase cells, and the cell division begins, with the monomer in the solution The microbes of nutrients are rapidly increasing in geometric progression.
At the same time, since the polymer is more difficult to degrade than the monomer, the enzymes that degrade the polymer are not required for the time being and are not subjected to induction synthesis, they exist at a very low concentration level or exist only as information in the DNA. Other microbes that are not adapted to the environment die.
In general, the total amount of microorganisms mainly degrading monomeric microorganisms increased in the first 2 days, and the metabolic activity was strong, the breathing rate and the synthesis speed were fast, and the CO2 amount was the highest in the first 2 days of the reaction.
After 2-4 days, after two days of degradation, the amount of monomer in the solution is greatly reduced. A large number of monomeric microorganisms, part of which begins to adjust their enzyme system to adapt to the lack of monomer and high polymer concentration. new environment.
Because the microorganism does not synthesize all the enzymes it can synthesize under any circumstances. They actually only synthesize enzymes that are necessary under certain environmental conditions, which is also the “economic” principle that microbes follow.
In this case, the synthesis of the previous polymerase is actually inhibited by a repressor acting on the DNA. The appearance of the inducer relieves the repressor’s repression of DNA, causing the gene fragment on the DNA to initiate its “command” process and begin the synthesis of polymer-induced enzymes.
On days 2-4, overall, the activity of the microorganisms decreased, and the amount of CO2 decreased on the image.
On days 4-6, the microorganisms that degrade the polymer are rapidly multiplied, and the polymer is rapidly degraded and fully utilized. This process is called secondary growth.
During this period, the microorganisms were rich in nutrients, the cell metabolism activity was enhanced, the speed of synthesizing new cell substances was accelerated, and the microbial growth was strong. On the image, the amount of CO2 has risen again.
On days 6-8, due to the rapid growth of microorganisms in the log phase, a large amount of polymer is consumed.
At the same time, the accumulation of metabolites is toxic to the bacteria itself, which is unfavorable to the growth of microorganisms, and the growth rate of microorganisms gradually decreases to zero, and the death rate gradually increases and enters the stationary phase.
After the quiescent period, due to the depletion of nutrients, the microorganisms use the stored material for endogenous respiration due to lack of nutrition. As a result, the amount of CO2 has decreased significantly.
In 8-12 days, some microorganisms use the cellular material after the lysis of dead microorganisms to synthesize the nutrients they need, and the amount of CO2 also increases.
Between 14 days, a large number of metabolites accumulate due to the degradation of polyacrylic acid polymers, resulting in the production of bacteria toxic. Therefore, it can be seen from the figure that after 6 days, the microbial activity in the bioreactor is lower than that in the endogenous reaction bottle, and the number is small, but the trend is consistent.
3.2 Analysis of COD Value Curve
The polyacrylic acid polymer solution has two parts of a monomer (easy to rapidly degrade) and a polymer (which is not easily degraded rapidly).
According to the COD image analysis of Figure 3: First, on the second day, due to a large amount of monomer degradation during the two days, the microorganisms present in the mixture are mainly microorganisms degrading the monomer type, and represent this part of the COD of the polymer. The value is not reflected, the polymer is not oxidized by the strong oxidant, so the COD value measured in the polyacrylic polymer only represents the COD value of the microorganism in the mixture and the COD value of the monomer which is not completely degraded, compared with the endogenous respiration process. The microbial COD value is less.
On the fourth day, after the increase of the inducing enzyme microorganisms, the polymer is decomposed into small molecular weight organic substances, which can be oxidized by potassium dichromate. At the same time, the number of microorganisms increases, and a part of the organic matter COD value is converted into the COD value of the cellular material. So on the fourth day, the COD value reached a peak.
On the sixth day, the polymer was degraded in a large amount, and the decomposed small molecular weight organic matter was also rapidly degraded, and the residual small amount of monomer COD value was further converted into a simple inorganic substance. At this time, the COD value was greatly decreased.
On the eighth day, the microorganisms in the solution were in a stationary phase, and the growth rate of the microorganisms in the stationary phase gradually decreased, but it was still growing, but the activity was poor. The total number of microorganisms reached a maximum during this period, and for a certain period of time, newborn microorganisms and death. The number of microorganisms is equivalent. The COD value also showed a slight increase due to the increase in the number of microorganisms.
After eight to fourteen days, the microorganisms in the biochemical reaction bottle enter the endogenous respiration period, that is, the decay period and the microorganisms in the decay period are less or less proliferated, or autolyzed, and the COD value is basically kept constant.
At the same time, a large number of metabolites accumulated by the degradation of polyacrylic acid polymers are toxic to microorganisms, and the number of microorganisms is small. Therefore, the biochemical COD value in the biochemical reaction bottle is lower than the COD value in the endogenous reaction bottle.
3 .3 Infrared spectroscopy analysis
The infrared spectral curves of the blank and polyacrylic polymers are shown in Figures 4 and 5.
It can be seen from Fig. 4 and Fig. 5 that at the wavenumbers of 2426 cm-1, 1650 cm-1, 1384 cm-1, 1139 cm-1, and 835 cm-1, there are such absorption peaks on both images, and there is no significant difference.
These wavenumbers reflect the functional groups in the bulk structure of the microorganism. In the polyacrylic acid polymer, there is a large peak at a wavenumber of 3492 cm-1, and there is no such peak in the blank.
At this wavenumber, a large amount of NH2– is present, and NH2– is a product of microbial metabolism. The presence of NH2– indicates:
After 14 days of reaction, the microbial metabolites in the biochemical reaction flask were more than the metabolites in the endogenous reaction flask. The organic functional group in the polyacrylic acid polymer is absent, indicating that it has been degraded.
In this study, the biodegradability of polyacrylic polymers in the environment was studied by the aerobic biodegradation method, and the following conclusions were drawn.
In the aerobic environment, it can be seen from the COD curve and the PCD curve that the COD value and PCD value of the 6-day acrylic polymer is greatly reduced, and it can be inferred that most of the organic matter has been degraded.
It can be seen from the infrared spectrum that the polyacrylic acid polymer has good biodegradability and can be completely degraded after 14 days.
At the same time, based on the PCD curve image, some new ideas are proposed: the carbon dioxide amount (PCD) histogram analysis method.
Through the comprehensive analysis of the biodegradation rules of polyacrylic polymers, it is shown that the combination of the COD curve, PCD curve, and infrared spectrum can be used to comprehensively analyze and evaluate the aerobic biodegradability of polyacrylic acid polymers.