Molecular binding mechanisms of manganese to the root cell wall of Phytolacca americana L. using multiple spectroscopic techniques

The root cell wall (RCW) of Mn hyperaccumulator Phytolacca americana L. (P. americana) plays an important role in immobilizing and detoxifying excessive Mn, but the molecular binding mechanism of Mn to RCW has been little studied. This study investigated the effect of varied pH on Mn adsorption by the isolated RCW from P. americana in batch experiments, and explored the binding mechanisms of Mn to RCW using Fourier transform infrared spectroscopy (FTIR), synchrotron-based X-ray absorption near-edge structure (XANES), and extended X-ray fine structure spectroscopy (EXAFS). Results showed that Mn binding capacity depends on solution pH, with an optimal pH of 5.0-6.0. Experimental isotherm data could be successfully modeled by the Langmuir and Freundlich equations; the estimated maximum Mn adsorption capacity was 5.446 mg g−1 according to the established Langmuir isotherm. FTIR spectroscopy demonstrated hydroxyl and carboxyl groups were probably involved in the Mn binding process. XANES results showed that Mn remained as Mn(II) after adsorption on RCW, without any change of oxidation state; EXAFS analysis further revealed that Mn was complexed to RCW via bidentate inner-sphere coordination with carboxyl, which provides new structure information of Mn adsorbed on biomaterials and accounted for high Mn accumulation on RCW of P. americana.