Relevance of nonfunctional linear polyacrylic acid for the biodegradation of superabsorbent polymer in soils

Biodegradability is a desired characteristic for synthetic soil amendments. Cross-linked polyacrylic acid (PAA) is a synthetic superabsorbent used to increase the water availability for plant growth in soils. About 4 % within products of cross-linked PAA remains as linear polyacrylic acid (PAAₗᵢₙₑₐᵣ). PAAₗᵢₙₑₐᵣhas no superabsorbent function but may contribute to the apparent biodegradation of the overall product. This is the first study that shows specifically the biodegradation of PAAₗᵢₙₑₐᵣin agricultural soil. Two¹³C-labeled PAAₗᵢₙₑₐᵣof the average molecular weights of 530, 400, and 219,500 g mol⁻¹were incubated in soil. Mineralization of PAAₗᵢₙₑₐᵣwas measured directly as the¹³CO₂efflux from incubation vessels using an automatic system, which is based on¹³C-sensitive wavelength-scanned cavity ring-down spectroscopy. After 149 days, the PAAₗᵢₙₑₐᵣwith the larger average molecular weight and chain length showed about half of the degradation (0.91 % of the initial weight) of the smaller PAAₗᵢₙₑₐᵣ(1.85 %). The difference in biodegradation was confirmed by the δ¹³C signature of the microbial biomass (δ¹³Cₘᵢc), which was significantly enriched in the samples with short PAAₗᵢₙₑₐᵣ(−13 ‰ against reference Vienna Pee Dee Belemnite,VPDB) as compared to those with long PAAₗᵢₙₑₐᵣ(−16 ‰ VPDB). In agreement with other polymer studies, the results suggest that the biodegradation of PAAₗᵢₙₑₐᵣin soil is determined by the average molecular weight and occurs mainly at terminal sites. Most importantly, the study outlines that the size of PAA that escapes cross-linking can have a significant impact on the overall biodegradability of a PAA-based superabsorbent.