The research results of Li Daping of Chengdu Institute of Biology, Chinese Academy of Sciences show that under natural conditions, polyacrylamide will undergo slow physical degradation (heat, shear), chemical degradation (hydrolysis, oxidation and catalytic oxidation) and biodegradation, resulting in Various oligomers and highly toxic acrylamide monomers cause great indirect or direct harm to the human body.
1. Decomposition and potential toxicity of polyacrylamide under natural conditions
In the past, polyacrylamide was generally considered to be a very stable polymer. In fact, under natural conditions, polyacrylamide would undergo slow physical degradation (heat, shear), chemical degradation (hydrolysis, oxidation, and catalytic oxidation) and Biodegradation (microbial enzymolysis). These degradations are mainly caused by the generation of free radicals to induce chain oxidation reactions, which leads to polymer backbone chain breakage and relative molecular mass reduction, and loss of viscosity in aqueous solution. In the stability study of polyacrylamide, it was found that polyacrylamide occurs simultaneously in aqueous solution Two kinds of chemical degradation reactions: ①hydrolysis reaction, which causes changes in the structure of the side groups, from amide groups to hydroxyl groups; ②oxidation reactions, which cause the breaking of the main chain, which reduces the relative molecular mass of the polymer. The oxidative degradation reaction has the characteristics of free radical chain reaction. It acts as an activator for peroxides, reducing organic impurities and transition metal ions, etc., generates active free radical fragments, and enters the polymer for oxidative degradation. The peroxides and carbonyl compounds produced in polymers are the main causes of oxidative degradation and photodegradation of polymers.
According to different uses of polyacrylamide, the relative molecular mass is generally between (200 ~ 2000) x104. Due to degradation, the relative molecular mass of the main chain breakage is greatly reduced, and a large amount of oligomers are generated. Further degradation of the oligomers will generate a large amount of acrylamide monomers. Acrylamide is a toxic chemical substance, and its toxicity has been extensively studied at home and abroad. For the concentration of acrylamide in the environment, each country has corresponding laws and regulations: The Occupational Safety and Health Act (OSHA) stipulates that the occupational exposure standard is the threshold-time weighted average (TLA-TWA) of acrylamide in the air is 0.3mg/m3 Our country Fei Weiquan and others proposed that the residual concentration of acrylamide in water should be less than 10 x10-9; the United Kingdom stipulates that the content of acrylamide in beverages is less than 0.25 x10-9; Japan stipulates that the content of acrylamide discharged into river water is less than 10 x10- 9.
Due to its good water solubility, acrylamide in the human discharge environment basically enters human surface water and groundwater, can be absorbed through the skin, mucous membranes, respiratory tract and oral cavity, widely distributed in human body fluids, and can also enter human embryos, causing Poisoning. The metabolism of acrylamide is mainly due to the reaction with glutathione to generate N-acetate-s-cysteine, which is catalyzed by enzymes and non-enzymes in the liver, brain and skin. It has been proven to be a chromosomal breaking agent, inducing chromosomal aberrations. It can cause neurotoxic reactions. The toxic reactions are sensory and motor disorders. The pathological manifestations are numbness of the limbs, paresthesia, dyskinesia, tremors, dull sensation and midbrain damage. Ingestion of water contaminated with enolamide can cause drowsiness, disturbance of balance, loss of mixed memory and hallucinations.
There is no doubt that polyacrylamide itself is safe and non-toxic, so its application range permeates all aspects of people's lives, and it is also used in food, medicine and cosmetics, and other cities directly related to human health. In fact, the far-reaching effects of migration and degradation of polyacrylamide in the environment have not yet been recognized, so it is necessary to carry out in-depth research on the biodegradation of polyacrylamide and find suitable treatment methods to eliminate its potential toxicity. .
2. Pollution of polyacrylamide and biodegradation at home and abroad
(1) Pollution of polyacrylamide.
Polyacrylamide has a very bad influence on the ground engineering while producing and improving the recovery factor for the oil field. The polyacrylamide injected into the formation enters the ground oil-water separation and water treatment terminal with the mixed liquid of the original and water, which greatly improves the viscosity and emulsification of the mixed liquid, makes the separation of oil and water more difficult, and causes the oil content of the produced water to seriously exceed the standard. The direct impact of polyacrylamide on the environment is that it has to be drained out of the local water body during the production of the oil field. Because the preparation of polyacrylamide in the oil field requires fresh water and some low-permeability formations, some parts contain a higher degree of dependence. The polyacrylamide produced water was discharged. Most polyacrylamide enters the underground oil layer. Because of the stratum structure, it is difficult to avoid its penetration into the groundwater layer. The long-term retention of polyacrylamide in surface water and groundwater will surely The local water environment has caused serious pollution. In addition to the large amount of polyacrylamide used in oil fields, water treatment, papermaking, textiles, mining and many industries that directly affect human health have no relevant data on the discharge and possible impact of polyacrylamide. The public's understanding still stays in the benefits of polyacrylamide for production and life. For a long period of time, water-retaining agents like solid water will be widely used in the process of tree planting and afforestation in water-scarce and dry areas. Application. Polyacrylamide that can remain in the environment through various ways will slowly degrade and release toxic acrylamide monomers, which will have a huge and long-term impact on the local environment. However, this has not yet caused Enough attention.
(2) Research on polyacrylamide biodegradation at home and abroad.
In the past, polyacrylamide was generally considered to be toxic to microorganisms, and there are few public literature reports on the biodegradation of polyacrylamide at home and abroad. Literature searches at home and abroad have found that the early MagdaliniukS (1995) and others have proposed the non-biodegradability of polyacrylamide, but Japan’s Kunichika N (1995) and others, at 30°C, polyacrylamide,
K2HPO4, MgSO4. 7H20 , NaCl, FeSO4. 7H20 mixture is used as a medium to separate Enterobacter agglomerans and Azomonas mac-rocytogenes strains that can use water-soluble polyacrylamide as the sole carbon source and nitrogen source from activated sludge and soil; After 27 hours of cultivation, the entire biological system consumes 20% of the total organic carbon, and the average molecular weight of polyacrylamide is reduced from 40x104 to 200x104; experiments show that microorganisms can only use part of polyacrylamide, but not the amide part of it Even the low concentration of polyacrylamide cannot be fully utilized. Jeanine LKay-Shoemake et al. used polyacrylamide as the soil microbial growth substrate in the experiment, polyacrylamide can only be used by microorganisms as the only nitrogen source, but it cannot be degraded as a carbon source. The possible reason is polypropylene The amide is first converted into long-chain polyacrylate, which can be used by microorganisms as a nitrogen source. In China, the experiments of Huang Feng (2002) and others showed that the saprophytic bacteria (TCB) were activated continuously for 5 times and incubated in a 1000 mg/L polyacrylamide solution for 7 days at constant temperature, which can make the solution viscosity loss rate reach 11. 2% However, the biodegradation of polyacrylamide by TGB is slow, and the viscosity loss rate of polyacrylamide solution caused by TGB within 30 days is still not more than 12%; sulfate reduction bacteria (SRB) bacterial amount reaches 3.6x104mL-1 Incubating at a constant temperature of 30% for 7 days can make the loss rate of 1000 mg/L polyacrylamide viscosity reach 19.6%, but the loss rate of polyacrylamide viscosity does not increase with the increase of cultivation time.
So far, the research on polyacrylamide at home and abroad has basically remained at the initial stage. As a stable polymer material, polyacrylamide has extremely strong biological resistance. Even polyacrylamide that has been degraded into small molecules still has this characteristic. At present, the application scope and scale of polyacrylamide are showing a rapid growth trend, while its accumulation and migration in the environment
The toxicity caused by migration and transformation will gradually be revealed, and it will bring immeasurable long-term harm to the ecological environment. The research results show that biocatalysis and oxidation play an important role in the conversion of polyacrylamide. It plays a central role in the field of environmentally sound disposal of environmental pollutants. Due to the special environmental adaptability, high reproduction rate and variability of microorganisms, the degradation and harmlessness of microorganisms will become an effective means to solve the potential toxicity problems of environmental pollution and conversion caused by polyacrylamide.