Statistical analyses on effective removal of cadmium and hexavalent chromium ions by multiwall carbon nanotubes (MWCNTs)

K.S Obayomi,J.O Bello,M.D Yahya,E Chukwunedum,J.B Adeoye

doi:10.1016/j.heliyon.2020.e04174

K.S Obayomi, J.O Bello + Show 3 more

Open Access

https://doi.org/10.1016/j.heliyon.2020.e04174

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Abstract

In this work, multiwall carbon nanotubes (MWCNTs) developed from cobalt-ferrite catalyst on activated carbon (from castor seed), was used as an adsorbent for the removal of cadmium and hexavalent chromium ions. The effectiveness of the adsorbent for the uptake of Cd(II) and Cr(VI)ions from aqueous solution was investigated in a process batch adsorption study. The developed activated carbon and MWCNTs were characterized by Brunauer-Emmett-Teller (BET) surface area analysis, Fourier Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) for the determination of surface area, functional group, and surface morphology, respectively. The BET surface area of activated carbon and developed adsorbent from Co–Fe/AC was 230.24 and 372.42 m2/g, respectively. The operational parameters evaluated on the adsorption efficiency were solution pH, temperature, adsorbent dosage initial metal ions concentration, and contact time. The adsorption of Cd(II) and Cr(VI) were found to have attained equilibrium positions in 60 min for the concentration range tested, respectively. The four linearized adsorption isotherm models; Langmuir, Freundlich, Temkin and Dubinin Radushkevich (D-R) tested, when compared, revealed that Langmuir isotherm fitted well to the experimental data judging from the higher correlation coefficient values (R2) and lower values of the error functions (chi-square (χ2), the sum of square error (ERRSQ/SSE) and the sum of absolute error (EABS))with monolayer adsorption capacities of 404.858 and 243.902 mg/g for Cd(II) and Cr(VI) ions, respectively. Adsorption kinetic models investigated by pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion showed the conformity of pseudo-second-order model to the process adsorption as informed by the higher values R2 and Adj, R2, maximum log-likelihood and smaller ERRSQ/SSE, χ2, Akaike information criterion (AIC), Bayesian information criterion (BIC), and Hannan-Quinn information criterion (HQIC). The intraparticle diffusion model plots indicated that intraparticle diffusion was not the only rate-limiting step. Thermodynamic adsorption parameters (ΔHo and ΔGo, ΔSo) showed that the adsorption of Cd (II) and Cr (VI) ions was spontaneous, endothermic, and increased in randomness between the adsorbate-adsorbent. The mean adsorption energy (E), the heat of adsorption (ΔHo), and activation energy (Ea) values, revealed the adsorption mechanism of Cd(II) and Cr(VI) onto MWCNTs as a combination of chemical and physical adsorption but dominated more by chemical adsorption.

Highlights

Water is important to the existence of living organisms in the world
The carbon conversion enhancement in the nucleation process of the Cr(VI) onto the adsorbent (CNTs) formation in a catalytic vapour deposition (CVD) reactor may be due to the incorporation of Co–Fe catalyst supported on activated carbon and produced from the castor seeds
It can be observed from the CNTs Scanning Electron Microscopy (SEM) image that activated carbon offers excellent support on the transition metals

Summary

Introduction

Water is important to the existence of living organisms in the world. human activities which include population growth, industrialization increase, and urbanization have caused rapid contamination, compromising the availability of potable water [1]. Heavy metals like chromium and cadmium when exceeding their tolerance level in water are extremely toxic. Chromium and cadmium showed toxic biological effects such as damage to bones and kidney, death in humans, acute effects in children, necrosis nephritis, bronchitis, abdominal pain, liver damage, skin irritation, respiratory cancer, and irritation of gastrointestinal mucosa [4]. Chromium exists as two stable species; Cr (III) which is a macronutrient, and the toxic hexavalent Cr (VI) [5]. Rapid development globally has led to the discharge of these heavy metals into water bodies, which is dangerous to humans and their environment. It is imperative to reduce these heavy metals or remove them completely using efficient and suitable technologies to prevent hazardous effects on the environment and human health. The reason being that they exhibit non-biodegradable and bioaccumulation tendencies in plants and animals [6, 7]

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