Are biodegradable materials completely degraded in different environments? Is it harmful to the environment?Knowledge Science(Part II)

Do you really understand what is degradable plastic?Knowledge Science(Part I)
2020-07-06
Will biodegradable plastic degrade in the air or in use? Can biodegradable plastics only be composted? Knowledge Science(Part III)
2020-07-28

Are biodegradable materials completely degraded in different environments? Is it harmful to the environment?Knowledge Science(Part II)

1. Whether biodegradable plastics can only be composted?

Not necessarily. Biodegradable plastics can be biodegraded under composting conditions, can also be biodegraded under other conditions such as anaerobic digestion devices, and can also be biodegraded under natural conditions such as soil, seawater, etc., but in general, biodegradable plastics Only composting is wrong.
Like PHA, PBAT materials are more easily biodegradable under seawater, soil, and composting conditions, while PLA is more easily biodegradable under composting conditions. The degradation cycle or speed under seawater and soil conditions will be relatively slow.

2. Can biodegradable plastics be completely degraded in anaerobic digestion devices?

The main reason for biodegradable plastics to be degraded by microorganisms is that its chemical structure is different from traditional plastics, so it can be degraded by microorganisms. Most of the currently known biodegradable materials are degradable by microorganisms under anaerobic digestion devices.

3. Whether biodegradable plastics can be completely degraded in the soil

In the soil of nature, the conditions such as temperature, moisture, and microorganisms are uncontrollable. Compared with the situation where the conditions are controlled during the composting process, the conditions are relatively uncontrollable. At present, from the application of biodegradable mulch film and the results of laboratory and field degradation experiments, most biodegradable plastics are biodegradable under field soil conditions, such as PBAT, PHA, PCL, PBS, etc. under normal climatic conditions for 5 months Under the condition of buried soil, it can be completely degraded, and it will not affect the plants. The degradation of PLA materials is relatively slow and takes a long time, but after being blended with materials such as PBAT, from the experimental results, it can also be degraded in about half a year. Completely degraded. The following figure is the observation result of the experiment of using PMT, PBAT/PLA, PLA film buried in natural soil in Mentougou. From the photo, it can be seen that the temperature of the biodegradable plastic is adequate in the spring, summer and autumn in the north under the condition of sufficient soil moisture The following can be naturally degraded.

(Y.-X. Weng et al./Polymer Testing 32 (2013) 918–926)

4. Can biodegradable plastics be completely degraded in seawater?

There are also microorganisms that adapt to the high-salt environment in seawater. Materials like PHA and PBAT can be completely degraded in seawater at 25℃±3℃ in about 30-60 days. The specific experimental process observation photos can be seen in the figure below.

5. Will degradation products of biodegradable plastics cause secondary environmental hazards after degradation?

Everyone is particularly concerned about whether the products of biodegradable plastics will cause secondary harm to the environment. Therefore, when the degradation performance of biodegradable plastics is required, the index of heavy metal content is increased and strict requirements are imposed.

Heavy metals Limited/(mg/kgs)
Arsenic(As) ≤5
Cadmium(Cd)  ≤0.5
Cobalt(Co)  ≤38
chromium(Cr)  ≤50
Copper(Cu)  ≤50
Nickel(Ni)  ≤25
Molybdenum(Mo) ≤1
Lead(Pb) ≤50
Selenium(Se) ≤0.75
Zinc(Zi)  ≤150
Mercury(Hg) ≤0.5
Fluorine(F)  ≤100

The compostable plastic compost after composting was tested for ecotoxicity, including phytotoxicity test and earthworm toxicity test.

In addition to the degradation performance and heavy metal content regulations, the biodegradable biodegradable mulch in the soil is also regulated in the new international standard for ecological toxicity.
Therefore, from the current perspective, biodegradable plastics that meet the requirements of the standard will not cause secondary damage to the environment after degradation.

(To Be Continued…)