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 (PAA(linear)). PAA(linear) has 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 PAA(linear) in agricultural soil. Two C-13-labeled PAA(linear) of the average molecular weights of 530, 400, and 219,500 g mol(-1) were incubated in soil. Mineralization of PAAlinear was measured directly as the (CO2)-C-13 efflux from incubation vessels using an automatic system, which is based on C-13-sensitive wavelength-scanned cavity ring-down spectroscopy. After 149 days, the PAA(linear) with 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 delta C-13 signature of the microbial biomass (delta C-13(mic)), which was significantly enriched in the samples with short PAA(linear) (-13% against reference Vienna Pee Dee Belemnite, VPDB) as compared to those with long PAA(linear) (-16 parts per thousand VPDB). In agreement with other polymer studies, the results suggest that the biodegradation of PAA(linear) in 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.