纳米化和酸洗对赤泥吸附Cd^(2+)动力学的影响

Kinetics of Cd^(2+) adsorption on red mud affected by nanorization and acidification

为揭示赤泥类钝化剂对Cd2+的吸附规律和动力学特性,研究了纳米化和酸洗处理,以及不同Cd2+添加质量浓度对赤泥类钝化剂吸附Cd2+动力学的影响。结果表明:准二级动力学方程和指数(Ⅱ)型方程可用于描述赤泥对Cd2+的吸附动力学特征。溶液中Cd2+的质量浓度从50mg/L增加到100mg/L,其平衡吸附量相应增加1倍,平衡时间也相应延长。纳米化赤泥、酸洗赤泥、酸洗纳米化赤泥的准二级动力学方程拟合参数K值(反应速率常数)与原状赤泥的比值分别为2.34、0.15、0.63,而指数(Ⅱ)型方程拟合参数-B(吸附量达到平衡吸附量36.8%时所需的时间)与原状赤泥的比值分别为0.53、2.09、0.85。赤泥类钝化剂既作用于Cd2+的吸附离子交换反应,又作用于表面络合反应,吸附速率受膜扩散和颗粒间扩散的联合控制。纳米化增大了赤泥的比表面积,提高了赤泥的吸附性能;酸洗处理也增大了赤泥的比表面积,但同时降低了赤泥的pH值,从而降低了赤泥的吸附性能。

This paper aims to report our investigation findings of the kinetic characteristics of Cd2 ＋adsorption for the red mud by way of nanorizing and acidifying process at different initial concentrations of Cd2 ＋ . For this purpose, we have applied three kinetics equations, that is, pseudo second-order kinetics equation, exponential equation II and parabolic diffusion equation. The results of our investigation indicate that the pseudo second-order adsorption equation and expo- nential equation II are fit best for describing the adsorption process. The reason is that, when the concentration increased to 100 mg Cd2＋/L from 50 mg Cd2＋/L, the adsorptive capacity in balance would also increase twice as much as it was before, correspondingly, if the equilibrium time can be made to prolong as it is needed. The pH of the solution would increase gradually before getting stable in the adsorption process. As to the pseudo second-order adsorption kineticequation, it was found that the values of their parameter K, which is referred to a rate constant, for nano-red mud, acidified red mud and acidified nano-red mud, were equal to 2.34, 0.15, 0.63 times of the original, respectively. As for the exponential equation II, the values of parameter - B, which is referred to the adsorptive time, can be found rising to 36.8 %, that is, the maximum of the adsorp- tive capacity, for nano-red mud, acidified red mud and acidified nano-red mud were 0.53, 2.09, 0.85 times of the original red mud, respectively. The above results imply that the adsorption processes of Cd2 ＋ to red mud may include ion exchange and surface complexation reactions. When the parabolic diffusion equation is used to describe the ion diffusion process in the medium, we can find that there exists a twofold linear relation between Qt and /t, which suggests that the intraparticle diffusion is not only a controlling step for the adsorption process, which might be in all probability controlled by both the film diffusion and intraparticle diffusion. In addition, nanorization may help to increase the specific surface area of the red mud, so that it could heighten the adsorption performance whereas the acidification tends to decrease the pH value of the red mud, hence resulting in a reduced adsorption performance.