Evidence of humic acid-aluminium-silicon complexes under controlled conditions

The chemistry of silicon (Si), the second most abundant element in soil after oxygen, is not yet fully understood in the soil-water-plant continuum. Although Si is widely accepted as an element that has little or no interaction with natural organic matter, some data seems to show the opposite. To identify a potential interaction between natural organic matter and Si, batch experiments were achieved at various pH and Si concentrations, involving also Al3+ as a common ion in soil and using humic acid (HA) as a typical model for natural organic matter. Several complementary techniques were used to characterize the possible complexes formed in the dissolved or solid phases: molecular fluorescence spectroscopy, Si-29 solid-state NMR, Fourier transform infrared spectroscopy, quantification of Si, Al and organic carbon, and nanoparticle size distribution. These tools revealed that humic acid indeed interacts, but weakly, with Si alone. In the presence of Al, however, a ternary complex HA-Al-Si forms, likely with Al as the bridging atom. The presence of Si promotes the maintenance of both Al and dissolved organic matter (DOM) in solution, which is likely to modify the result or the kinetics of pedogenesis. Such complexes can also play a role in the control of Al toxicity towards plants and probably also exists with other metals, such as Fe or Mn, and other metalloids such as As.

Automated summary

The chemistry of silicon in the soil-water-plant continuum is not fully understood. Some data suggests that silicon has interactions with natural organic matter, contrary to the widely accepted belief. Batch experiments were conducted to investigate the potential interaction between natural organic matter and silicon, involving different pH and silicon concentrations. The results showed that natural organic matter, specifically humic acid, weakly interacts with silicon alone, but forms a ternary complex with aluminum in the presence of aluminum. These complexes have implications on pedogenesis and the toxicity control of aluminum towards plants.