| Main constituents (lignin, cellulose and hemicelullose) were extracted from rice straw, and characterized by elemental analysis and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT). Sorption of pyrene on rice straw and its main constituents was studied. The results showed that there are great differences in the properties of the three constituents, with lignin of higher aromaticity and lower polarity, while cellulose and hemicellulose of greater polarity and aliphaticity. Sorption of pyrene also differed a lot due to the varied characteristics of the sorbents. All isotherms were fitted well with Freundlich equation. Lignin showed the greatest sorption capacity for pyrene, with kF being 5.04×104, approximately 100 times greater than that of cellulose. Sorption of pyrene on hemicellulose was even a little smaller than that on cellulose. At low solute aqueous concentration (ce= 0.01 Sw), the sorption of pyrene on rice straw was controlled by lignin, and sorption coefficient (kd) was a little smaller than that predicted by the kd value on lignin and its mass fraction. This suggested that some sorption sites such as alkyl and aromatic centers were covered when the lignin existed mixed in the rice straw. But at high concentration (ce= 0.5 Sw), the kd value of pyrene on rice straw was markedly greater than the sum of those on three constituents, suggesting that partition on other constituents could not be neglected. Moreover, pyrene sorption isotherm on lignin was nonlinear (Freundlich exponent, n = 0.89), while sorption on the other three sorbents was more linear (n > 0.96). A negative relationship between n and aromaticity was observed, which illustrates that specific effect related with aromaticity is the main reason for nonlinearity. Organic carbon normalized sorption coefficients (Koc) increased with increasing aromaticity and decreased with increasing polarity of the sorbents. |
