Heavy metal adsorption in a multi-metal solution by bentonite-kaolin-zeolite pellets

The use of adsorbent pellets of natural bentonite, kaolin and zeolite is analyzed in this work to study the removal capacity of heavy metals from wastewaters. Adsorbent pelets made with different aluminosilicates and zeolite proportions, and treatments were compared in an adsorption column system. The adsorbent with the highest removal efficiency had a proportion of 67% zeolite, 29% bentonite and 4% kaolin and no chemical treatment. Contrary to the expected acid activation reduced the adsorption capacity of the adsorbents, probably due to the aggressiveness of the process that collapsed the internal structure. Using the adsorbent with the highest adsorption efficiency the surface response analysis with a central composite rotatable design was applied to define optimal operating conditions. The four factor evaluated were contact time, adsorbent dose, pH values and heavy metals initial concentration; for lead, copper and cadmium these values are; 150,150 and 240 min; 25, 25, 25 g/mL; 4.3, 4.3, 4.3; and 4, 7, and 2 mg/L respectively. Equilibrium adsorption was analyzed with the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models. Type I linearization of the Langmuir isotherm had the better fit for lead, copper and cadmium predicting maximum adsorption capacities of 7.27 mg/L, 1.45 mg/L and 0.28 mg/L respectively. Freundlich model also had a high coefficient of determination for lead, copper and cadmium (0.97-0.99); this suggests that adsorption sites can be characterized both as monolayer and multilayer, and adsorption surface as heterogeneous. The kinetics showed that after 300 minutes removal efficiency begins to stabilize for all three metals. Lead and copper data fitted better with the pseudo second order model and cadmium with the pseudo first order model. Main adsorption mechanisms could be cation exchange and physical processes such as Van der Waals forces. The comparison between the obtained results and other researches highlights the need to improve the pelets adsorption capacity. The latter could be done optimizing acid activation conditions and a thorough analysis of both thermal and chemical treatment effects.

本研究分析了天然蒙脱石、高岭土和沸石吸附剂的使用，以探究其对废水中重金属去除能力的影响。对不同比例的铝硅酸盐和沸石混合吸附剂以及处理方法在吸附柱系统中进行了比较。具有最高去除效率的吸附剂含有67%沸石、29%蒙脱石和4%高岭土，且未进行化学处理。出乎意料的是，酸活化过程反而降低了吸附剂的吸附能力，这可能是由于该过程对内部结构的破坏性所致。利用最高吸附效率的吸附剂，采用中心复合旋转设计进行了表面响应分析，以确定最佳操作条件。评估的四个因素为接触时间、吸附剂剂量、pH值和重金属初始浓度；对于铅、铜和镉，这些值分别为150、150和240分钟；25、25、25克/毫升；4.3、4.3、4.3；以及4、7和2毫克/升。平衡吸附分析采用Langmuir、Freundlich、Temkin和Dubinin-Radushkevich模型。对于铅、铜和镉，Langmuir等温线的I型线性化与数据拟合最佳，预测的最大吸附容量分别为7.27毫克/升、1.45毫克/升和0.28毫克/升。Freundlich模型对铅、铜和镉也具有高确定系数（0.97-0.99），这表明吸附位点既可以表征为单层，也可以表征为多层，吸附表面为非均质。动力学研究表明，在300分钟后，所有三种金属的去除效率开始稳定。铅和铜的数据与伪二级模型拟合更好，而镉与伪一级模型拟合更好。主要的吸附机理可能是阳离子交换和如范德华力等物理过程。将获得的结果与其他研究进行比较，突显了提高吸附剂吸附能力的必要性。这可以通过优化酸活化条件以及彻底分析热处理和化学处理的影响来实现。