粘土颗粒吸附直接染料的分形特征

粘土颗粒对直接耐晒黑和直接大红染料的吸附均可分为快速的边缘覆盖和慢速的晶层吸附2个过程,整个吸附过程符合准-二级反应动力学方程式,除了伊利土-直接耐晒黑染料的吸附体系之外,其它体系的吸附过程中以晶层吸附为主的阶段均具有类分形特征.体系的非线性吸附等温线符合Langmuir型吸附等温模式,并且除了蒙脱土吸附直接大红染料之外,也可以用分形Langmuir吸附等温模式获得较好的模拟效果.在不同原始浓度下直接大红染料在粘土颗粒表面发生的边缘覆盖和晶层吸附这2个过程的程度是不同的,从而导致了吸附等温线在原始染料为150mg·L-1时出现最大的吸附量,而且颗粒边缘覆盖的直接大红染料的比例较高.染料进入粘土颗粒的晶层后,扩大了晶层间距,改变了其表面微孔几何结构.颗粒表面棱角变得光滑,结构疏松,片层结构逐渐减少,上述效果在蒙脱土颗粒上或直接耐晒黑染料吸附时表现地更为显著.而且吸附染料后蒙脱土颗粒对N2的吸附量比伊利土下降的幅度小,其比表面积、孔体积和平均孔径的下降比例也比伊利土小,其中平均孔径的差异比较明显.吸附染料之后粘土颗粒表面分形维数Ds均升高了,晶层膨胀和开孔作用抵消了"表面粗糙度屏蔽"、"孔阻塞效应"机理降低Ds的影响.增加了表面粗糙度.

Fractal characteristics of adsorption of direct dye compounds onto clay particles

The adsorption processes of direct fast light black G and direct scarlet 4BE onto illite and montmorillonite clay particles were investigated. The adsorption could be divided into two parts, one is high-speed boundary coverage, and the other is slow-speed interlayer adsorption. Not only is a pseudo-second-order reaction kinetic equation applicable for all the entire processes, but also fractal-like properties were found in the main interlayer adsorption stage, except for direct fast light black G adsorption onto illite. Classical and fractal forms of the Langmuir adsorption model were used to analyse the non-linear adsorption isotherms of direct dye compounds adsorption onto the clay particles, and good simulation effects were found for them, except for the fractal model of direct scarlet 4BE onto montmorillonite particles. For direct scarlet 4BE absorbing onto both clay particles at an initial concentration of 150 mg·L-1, a highest amount adsorbed were given in each adsorption isotherm, and the amount adsorbed by boundary covering is higher than that by interlayer adsorption. This may be ascribed to different amounts adsorbing onto clay particle surface through boundary coverage and interlayer adsorption mechanisms at different initial concentration of direct scarlet 4BE. Furthermore, the incorporation of dye molecules between the layers might lead to expansion of the interlayer distance and geometric structure alterations. The surface of the dye-adsorbed clay particles show smooth edges, loose structure and few layers, which is most marked on the surface of adsorbed montmorillonite particle or clay surfaces adsorbing direct fast light black G. After adsorption of dye compounds, the volume of N2 absorbed onto montmorillonite decreased less than onto illite, so the corresponding specific surface area and pore volume show the same trend. The average pore diameter of illite after dyes adsorption decreased more obvious than that montmorillonite absorbed dyes. The surface fractal dimensions Ds for clay particles with dyes absorbed are higher than for the clays without dyes absorbed because the interlayer expansion and pore opening counteract the decrease in Ds through surface roughness screening and pore blocking mechanisms. Therefore the surface roughness of the clay particles is increased after adsorption of the dyes.