向日葵根分泌物明显抑制针铁矿吸附Cd2+。随着pH的升高,针铁矿对Cd2+的吸附量升高,针铁矿对Cd2+的平衡吸附量pH在3~6时增长的幅度较大,pH>7后,对Cd2+的吸附量的增长趋于稳定。在溶液初始pH为3~8的范围内,向日葵根分泌物抑制针铁矿对Cd2+的吸附。针铁矿等温吸附Cd2+符合Langmuir方程和Freundlich方程,吸附动力学符合二级速率方程。根分泌物的存在明显抑制针铁矿对Cd2+的吸附强度和吸附速率。表明针铁矿对Cd2+的吸附是一个熵增的吸热反应;根分泌物的存在则降低了吸附反应的自发性。
Goethite is one of the most important heavy metal host minerals. Root exudates are the active organic component in rhizosphere. The effect of root exudates on adsorption behavior of heavy metals on soil or minerals is still little known. The present study showed that adsorption of Cd2+was inhibited in the presence of Helianthus annuus. Adsorption capacity of Cd2+to goethite increased with increasing of the initial solution pH. Root exudates reduced Cd2+adsorption capacity at all experimental pH. Both the Langmuir and the Freundlich isotherms represented well the experimental data and the pseudo-second order equation described well the kinetic data. Isothermal and kinetic studies showed that root exudates inhibited adsorption strength and adsorption rate. Adsorption of Cd2+on goethite is a endothermic process whereas the spontaneity of the reaction was reduced in the presence of root exudates.
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