砖红壤对Cr(Ⅵ)吸附-解吸反应动力学研究

研究了表面覆盖度、H2PO-4等因素对砖红壤吸附Cr(Ⅵ)反应动力学的影响,以及土壤与Cr(Ⅵ)反应时间对表面吸附态C r(Ⅵ)解吸量的影响,还对土壤吸附Cr(Ⅵ)机理进行了初步探讨.在所研究的pH值范围内,砖红壤对Cr(Ⅵ)的吸附量随离子强度增加而减少,这表明砖红壤吸附Cr(Ⅵ)机理中存在静电吸附.动力学实验表明,当Cr(Ⅵ)初始浓度较高时(0.1 mmo.ll-1),吸附反应速率开始比较快,持续约5m in,随后逐渐减缓.而当Cr(Ⅵ)初始浓度较低时(0.05 mmo.ll-1),土壤对其吸附速率十分迅速,5m in内的吸附量占总吸附量的96%以上.该结果表明,表面覆盖度越低,吸附反应速率越快.H2PO-4的存在将降低吸附反应速率和吸附量.H2PO-4对土壤表面吸附态Cr(Ⅵ)的解吸反应动力学也表现出双速率特征,即开始比较迅速而随后逐渐变缓.当延长土壤对Cr(Ⅵ)的吸附时间后,Cr(Ⅵ)吸附量虽然没有增加,但其解吸量却减少,这说明吸附态Cr(Ⅵ)滞留在土壤表面期间,其结合形态向难解吸态方向发生了转变.

KINETICS OF CHROMIUM ( Ⅵ) ADSORPTION AND DESORPTION ONTO LATOSOL

In this study, kinetic batch experiments were carried out to investigate the influence of surface coverage and phosphate on the adsorption of Cr (Ⅵ) on Latosol and the effect of residence time on its desorption by phosphate.The effect of background electrolyte concentration on Cr (Ⅵ) adsorption was also examined with the aim to provide microscopic evidence for the adsorption mechanism.The results showed that one of the probable mechanisms of Cr (Ⅵ) adsorption on the soil involved electrostatic adsorption because Cr (Ⅵ) adsorption exhibited a marked dependence on ionic strength.With the initial Cr (Ⅵ) concentration of 0.1 mmol · l~(-1), Cr(Ⅵ) adsorption was characterized by an initial fast slow phase followed by a slow phase.As the initial Cr(Ⅵ) concentration decreased to 0.05 mmol · l~(-1), the rate of Cr(Ⅵ) adsorption was significantly increased with the percentage of Cr( Ⅵ) adsorbed within 5 min accounting for 96.7% of the total Cr( Ⅵ) adsorbed by the soil.The rate and amount of Cr ( Ⅵ) adsorption were decreased in the presence of 0.1 mmol · l~(-1) phosphate.Accordingly, the results indicate that a few readily available sites with high-affinity for Cr( Ⅵ) on the soil surfaces are responsible for the rapid reaction while the slow reaction is controlled by those low-affinity sites.Desorption kinetics were also characterized by a rapid, followed by a slow reaction.The amount of Cr( Ⅵ) desorbed by phosphate decreased with increasing residence time, which suggested that some of Cr( Ⅵ) adsorbed on the soil surfaces were transformed to resistant or undesorbable forms.