Researches On Adsorption Of Cadmium By Willow Root Cell Wall And Kinetics Of Cadmium Uptake By Willow Root

Willow (Salix spp.) has large biomass production and high resistance to environmental stress as an important multipurpose tree species in afforestation, hence has been proved to be effective in the uptake and accumulation of metals from contaminated waters and soils. Suliu172(Salix jiangsuensis ’J172’), Hanliu (Salix matsudana), Weishanhu (Salix integra ’Weishanhu’) and Yizhibi (Salix integra ’Yizhibi’) were chosen as model plants to evaluate their potential for uptake of cadmium from hydroponic culture and relative uptake mechanism.Cadmium uptake showed a linear increase in the short time course, and a nonlinear and slow increase in the long time course. After one weeks cultivation, cadmium accumulation in different parts of willows generally followed the order of root> stem> leaf. Cadmium influx in willow roots increased with the increase of cadmium concentration in hydroponic solutions. A modified Michaelis-Menten equation was employed to describe the concentration-dependent kinetics of cadmium uptake through the roots. Cadmium influx could be resolved into linear and saturable components under concentration-dependent kinetics. The saturable component followed Michaelis-Menten kinetics, which indicated that cadmium uptake across the plasma membrane was transporter-mediated. The uptake capacity (Vmax/Km) jointly decided by the Vmax and Km followed in descending order of Hanliu>Weishanhu≈Yizhibi> Suliu172, indicating that their inherent potential of cadmium uptake reduced in turn.The effect of Mg, Mn, Fe(Ⅱ) on willow cadmium uptake is not significant, while Zn and Cu have an inhibition function on willow cadmium uptake, which may be the result of cadmium ion competing transport channels with other ions. Cadmium uptake of Yizhibi and Suliul72is likely to be through the Zn and Cu transport channels, also related to the Ca ion channels and carrier of root cell membrane. The effect of organic acid on the cadmium uptake of willow is complex, the kinds of organic acid, secreting mechanism and metabolism in plant all will influence the cadmium uptake, and different willows varieties, different kinds of organic acid, will cause different result. Low temperature and metabolic inhibitor inhibited the apparent uptake of cadmium in willow. Both active absorption and passive absorption occurred in the cadmium uptake by willow roots.Willow (Salix jiangsuensis ’J172’) roots (R), and root cell walls (RCW) and their three further isolated fractions (RCW1, RCW2, RCW3) were characterized by FTIR and used to adsorb cadmium from aqueous solution. Effects of contact time, initial cadmium concentration, and temperature on adsorption of cadmium were investigated using batch adsorption technique. Different models are used to fit experimental data. Results show that equilibrium time is obtained within40-60min for the different root materials. The cadmium adsorption capacity follows the order:RCW3> RCW2> R> RCW1>RCW. Adsorption dynamics analysis indicates that pseudo-second-order model described very well to the adsorption of cadmium on willow root materials(R>0.99). Intra-particle diffusion process is not the unique rate-limiting step. The adsorption process of RCW2and RCW3followed to the Langmuir model (R>0.91). The maximum cadmium adsorption capacities of RCW3were5.77,6.86and8.74mg/gat10,20and30℃, respectively, and RCW2were1.82,3.61and5.16mg/g. Equilibrium data of all the root materials were well described by the Freundlich (R>0.97), parameters shows a favorable adsorption except for RCW at10and20℃. Further thermodynamic analysis reveals that the adsorption process was endothermic and the cadmium adsorption from aqueous solution on RCW2and RCW3are a spontaneous and feasible process, while R, RCW, RCW1were not spontaneous.