The role of kaolin and kaolin/ZnO nanoadsorbents in adsorption studies for tannery wastewater treatment

S Mustapha,S A Abdulkareem,J O Tijani,A K Mohammed,D T Shuaib,M M Ndamitso,A Sumaila

doi:10.1038/s41598-020-69808-z

Abstract

In the present study, comparative studies of kaolin and kaolin/ZnO nanocomposites for the adsorption of Cr(VI), Fe(III), COD, BOD, and chloride from tannery wastewater were investigated. ZnO nanoparticles and kaolin/ZnO nanocomposites were prepared by sol–gel followed by wet-impregnation methods. The prepared adsorbents were characterized using different analytical tools such as X-ray diffraction, Fourier transforms infrared, high-resolution transmission electron microscopy, energy dispersive spectroscopy, selective area electron diffraction and Brunauer Emmett–Teller (BET) and X-ray Photoelectron Spectroscopy (XPS). The HRSEM/EDS/XPS analysis confirmed successful immobilization of clay structural network on the lattice layers of zincite hexagonal structure of ZnO nanoparticles. BET measurement showed an increase in the surface area of kaolin/ZnO nanocomposites (31.8 m2/g) when compared to kaolin (17 m2/g). Batch adsorption studies were carried out by varying the parameters such as contact time, adsorbent dosage and temperature. The maximum removal of Cr(VI) (100%), Fe(III) (98%), COD (95%), BOD (94%) and Chloride (78%) was obtained at 15 min by kaolin/ZnO composites. While 78% Cr(VI), 91% Fe(III), 91% COD, 89% BOD and 73% Chloride were removed by kaolin under the same conditions. The kaolin/ZnO nanocomposites exhibited better adsorption performance than kaolin due to higher surface area of the former than the latter. It was found that the Jovanovic isotherm model fitted the adsorption experimental data most with the highest correlation (R2 > 0.99) for both nanoadsorbents and indicate the occurrence of adsorption on monolayer and heterogeneous surfaces. The mechanism for the adsorption of metal ions in tannery wastewater onto the nano-adsorbents was examined using Weber Morris intra-particle diffusion model and Boyd plot which showed that the adsorption process was both intra-particle and film diffusion controlled. The thermodynamic parameters such as enthalpy change showed that that adsorption of metal ions and other parameters was feasible, spontaneous and endothermic. The ZnO/clay nanocomposites exhibited excellent recyclable and re-useable properties even after six repeated applications and can, therefore, be applied in wastewater treatment for removal of heavy metals and other physicochemical parameters.

Highlights

Environmental contamination via natural and anthropogenic activities have been recognized as one of the major global problems confronting the human race and aquatic species
The ZnO nanoparticles loaded onto the matrix in the X-ray diffraction (XRD) pattern showed the presence of diffraction related to the zincite hexagonal structure irrespective of the pH
The present study proves that kaolin and kaolin/ZnO nanocomposites could serve as effective adsorbents for the sequestration of Cr, Fe, chemical oxygen demand (COD), biochemical oxygen demand (BOD) and chloride from tannery wastewater

Summary

Introduction

Environmental contamination via natural and anthropogenic activities have been recognized as one of the major global problems confronting the human race and aquatic species. Different conventional techniques for the removal of pollutants from wastewater include solvent extraction, ion exchange, chemical reduction, precipitation, electrolysis, electro-dialysis, micro and ultra-filtration, reverse osmosis, advanced oxidation process and membrane filtration[5,6] These conventional methods have some disadvantages such as lower efficiency and high costs which discourage many industries from adopting any treatment methods. Different adsorbents such as zeolites, goethite, graphene oxide, activated carbon, agricultural wastes/byproducts, industrial wastes, metals/metal oxides nanomaterials and many more were successfully used to remove the pollutants from wastewater Among these adsorbents is zinc oxide nanoparticles (ZnONPs), an n-type semiconductor metal oxide which exhibits excellent adsorption and photocatalytic activity in the absence or presence of s unlight[9]. The removal of pollutants in simulated wastewater by nanocomposites was found to be excellent but there is a need for further clarification on the adsorption mechanism

Methods

Results

Conclusion