STUDY OF THE CHROMIUM(VI) REMOVAL FROM AQUEOUS SYSTEMS BY COBALT NANOPARTICLES

The cobalt nanoparticles (NPs-Co) were synthesized and applied in the removal of Cr(VI). The obtained nanomaterials presented magnetic properties and were characterized by the techniques of Transmission Electron Microscopy and X-ray diffraction. The NPs-Co, with a diameter of less than 50 nm, had a composition based on metallic cobalt and cobalt oxide. The NPs-Co were applied in the removal of Cr(VI) being evaluated different parameters such as time of equilibrium, initial pH, dose of NPs-Co, concentration of Cr(VI), and system temperature. The removal of Cr(VI) was favored at lower pH values, being about 100% at pH 2.0. With the increase of Cr(VI) concentration from 40.0 to 300.0 mg L-1 the removal decreased from 80 to 15%. The NPs-Co dose promoted an increase in removal reaching 90% at a dose of 1.0 g L-1. The removal process was spontaneous and endothermic, and the Langmuir isotherm model fit better to experimental data. The maximum removal capacity was approximately 70.0 mg g-1. The removal kinetics followed the pseudo-second order model. The mechanism of Cr(VI) removal occurred through adsorption by electrostatic interactions between CrO42- anion and Co-NPs, followed by reduction of Cr(VI) to Cr(III) and, lastly, co-precipitation of Cr(III) as Cr(OH)3 on the NPs-Co.

本研究合成了钴纳米颗粒（NPs-Co），并将其应用于六价铬（Cr(VI)）的去除。所制备的该纳米材料具备磁响应特性，并通过透射电子显微镜（Transmission Electron Microscopy）与X射线衍射（X-ray diffraction）技术完成了表征。该钴纳米颗粒的粒径小于50 nm，物相组成为金属钴与氧化钴。在将NPs-Co用于Cr(VI)去除的实验中，研究人员考察了多项影响参数，包括平衡时间、初始pH值、纳米颗粒投加量、Cr(VI)初始浓度以及反应体系温度。Cr(VI)的去除效率在低pH条件下更优，当pH为2.0时，去除率可达约100%。当Cr(VI)初始浓度从40.0 mg·L⁻¹升高至300.0 mg·L⁻¹时，其去除率从80%降至15%。提升NPs-Co投加量可显著提高Cr(VI)去除率，当投加量为1.0 g·L⁻¹时，去除率可达90%。该Cr(VI)去除过程为自发吸热过程，且实验数据更符合朗缪尔等温模型（Langmuir isotherm model）。该材料对Cr(VI)的最大吸附容量约为70.0 mg·g⁻¹。Cr(VI)的去除动力学过程符合准二级动力学模型（pseudo-second order model）。Cr(VI)的去除机制主要分为三步：首先，CrO₄²⁻阴离子与钴纳米颗粒之间通过静电作用发生吸附；其次，Cr(VI)被还原为三价铬（Cr(III)）；最终，Cr(III)以氢氧化铬（Cr(OH)₃）的形式在NPs-Co表面发生共沉淀。