The effect of glycerol and boric acid in the composition of polyvinyl alcohol and starch cryogels on their biodegradation in soil, the number of microflora and plant growth

The problem of excessive accumulation of used synthetic polymer materials can be partially solved by replacing them with biodegradable materials based on renewable raw materials. One option is compositions based on synthetic and natural polymers-polyvinyl alcohol (PVA) and starch-with the addition of a plasticizer and/or cross-linking agent. An important feature of PVA and starch is the ability of their aqueous solutions to form cryogels after a freeze-thaw cycle-viscoelastic bodies that remain stable at positive temperatures. This article presents the results of experiments modeling the biodegradation of PVA and potato starch cryogels modified with boric acid and glycerol in soil. It is shown that cross-linking with boric acid makes PVA and PVA-starch cryogels more resistant to the effects of native soil microflora: polymer mass loss reached 24 % and 60 %, respectively, within a month. The addition of 5, 10, and 20 % of the plasticizer glycerol increased the biodegradability of PVA and starch cryogels to 79, 81, and 86 %, respectively. The activity of the degradation processes was evidenced by changes in the soil microflora population: a decrease followed by partial recovery in the presence of cryo-gels containing boric acid and active growth during the decomposition of the glycerol-containing formulations. Increasing the glycerol concentration led to an increase in soil microflora population by 5-50 times relative to the control. The reaction of the test plant (white mustard), however, indicated the phytotoxicity of the degradation products of the glycerol-containing cryogels: in the soils after their degradation, the dry aboveground matter was 60-70 % lower than the control. Soil phytoproductivity after degradation of cryogels containing boric acid changed at different stages: initially, it increased by 30-90%, then decreased by 15-40 % compared to the control. This may have been due to the gradual release of borate from the polymer samples: boron is a micronutrient for plants at low concentrations, but is toxic at high concentrations. Contribution of the authors: the authors contributed equally to this article. The authors declare no conflicts of interests.

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