Cd(II) removal from amino acids-bearing wastewater: Critical evaluation and comparison of using magnetic carbon nanotubes vs. magnetite

Abstract

In the present work, two magnetic adsorbents (magnetic carbon nanotunes “MCNT” and magnetite “Mag”) were tested for cadmium “Cd(II)” removal from water in the presence of various amino acids “AA” (aspartic acid, glutamic acid “Glu”, glycine and cysteine). MCNT was prepared by initially oxidizing carbon nanotubes (CNT) by three different methods: nitric acid method, ammonium persulphate method, Hummer's method. Magnetite was then deposited on the oxidized CNT to produce the corresponding magnetic carbon nanotube adsorbents: NA-MCNT, APS-MCNT, HUM-MCNT, respectively. Adsorption of Cd(II) on various MCNT was investigated in the presence/absence of AA where various factors were studied: pH, oxidation method of CNT, type of AA, magnetite-to-CNT ratio. It was found that the presence of AA significantly reduced the adsorption capacity of MCNT towards Cd(II) at all pHs (reduction occurred from 49.8 mg g⁻¹ to 12.5 mg g⁻¹). Contrary adsorption capacity of Cd(II) on bare magnetite “Mag” was noticeably enhanced in the presence of AA (for Glu, enhancement occurred from 6.9 to 22.3 mg g⁻¹). Due to cheapness of Mag relative to CNT, it was recommended to use Mag rather than MCNT for Cd(II) removal from AA-bearing water. So that the optimum uptake method was proposed using Mag at pH 8 in the presence of Glu at concentration ratio (1:3 Cd(II): Glu). Mag was selective towards Cd(II) even in the presence of co-existing ions. The use of Mag for five cycles decreased adsorption efficiency to 80% which indicated that adsorption system was cost effective. Real water samples gave higher adsorption of Cd(II) in the presence of Glu than in distilled water. As a conclusion, it was recommended to use Mag rather than MCNT for Cd(II) removal in the presence of AA.