Semi-quantitative analysis of microbial production of oxalic acid by montmorillonite sorption and ATR-IR

Interactions between organic acids and clay minerals significantly influence elemental cycle on Earth. Oxalic acid has been recognized as one of the most important secretions of soil microorganisms, for both typical bacterium Enterobacter sp. and fungus Aspergillus niger. This study examined the ATR-IR spectra of solid and aqueous oxalic acid. Then, sorption of dissolved oxalic acid, microbial secretion, and simulative acid solution onto montmorillonite were also studied by ATR-IR. The sorption significantly elevated intensity of the characteristic peak of oxalate at 1318 cm−1. Then, the intensity ratio (R) of 1318/1635 cm−1 was proposed as an accurate indicator for semi-quantitatively analyzing oxalic acid concentration. R values of ~0.13 and ~0.20 (for non-microorganism system) represented the oxalic acid concentrations of 600–800 ppm and ~2000 ppm respectively. Additionally, only oxalic acid with >800 ppm concentration can be identified appropriately if mixed with montmorillonite for 6 or 12 h, whereas 24 h shaking can decrease the detection line to as low as 100 ppm. Finally, we proposed an equation of Rcorrection = C0 ∗ Rmicrobe oxalic (with a coefficient C0 of 3.171) to estimate oxalic acid secreted by microorganisms. The coefficient was necessary due to the interference from multiple organic acids in microbial secretion. This equation worked successfully for both Enterobacter sp. and Aspergillus niger. It therefore is a reliable method for semi-quantitatively estimating microbial production of oxalic acid via montmorillonite sorption and ATR-IR technique.