Copper adsorption on rice husk derived materials studied by EPR and FTIR

Copper (II) ions adsorbed on rice husk (RH), cellulose extracted from rice husk (RH-cellulose), rice husk heated to 300 °C (RHA300) and rice husk heated to 500 °C (RHA500) were investigated in order to understand their sites of adsorption. α-Cellulose and lignin extracted from coirpith were used as references for cellulose and silica components, respectively. Surface functional groups and the chemical environments of copper(II) ions were identified by Fourier transform infrared (FTIR) spectrophotometry and electron paramagnetic resonance (EPR) spectroscopy, respectively. EPR spectra of copper adsorbed on rice husk correspond to a mixture of chemical environments for the copper, whereas the spectra of RH-cellulose and α-cellulose were consistent with one major anisotropic Cu(II) component. Their spectral parameters (g⊥ = 2.085, g‖ = 2.369 and A‖ = 130 G) were different from those of RHA300 and lignin extracted from coirpith (g⊥ = 2.080, g‖ = 2.339 and A‖ = 135 G). Therefore, the spectra probably correspond to copper bound to CO, COC and/or COH groups of cellulose and to carbonyl groups (>CO) of RHA300 and lignin. EPR spectral parameters were refined by using Cu/Zn mixtures for Cu adsorbed on Mn-depleted RH-cellulose and Mn-depleted RHA300, which removed the interference from the sextet hyperfine splitting of 55Mn and broad features that result from dipolar interactions between neighbouring paramagnetic ions. The results demonstrate that oxygen-containing functional groups account for the binding of copper to rice husk and rice husk derived materials.