Abstract:
Over the past few years, heavy metal ion (HMI) pollution has become a crucial matter due to their threat to human health and ecological systems.,. Furthermore, HMIs have been reported to be hazardous, persistent, and in some of the global health organizations reports, they have been declared as carcinogens. These HMIs include Pb (II) and Cr (VI) and they are very reactive and highly oxidizing in nature. Thus, the need to remediate these HMIs from wastewater using magnetic nano adsorbents.
In this study, three nano-adsorbents such as cellulose nano crystals (CNC), magnetite (M), and magnetic cellulose nanocrystal (MCNC) were synthesized for the removal of Pb (II) in wastewater. The magnetic cellulose nanocrystals (MCNCs) were synthesized using a co-precipitation method from the magnetite (Fe3O4) and cellulose nano crystals (CNCs) were used as a base for stability and easy dispersion of iron for the adsorptive removal of Pb (II) ions. Furthermore, to enhance the adsorption capacity and to improve selectivity of the CNC towards -targeting anionic Cr (VI) ions, the surface modification was conducted by crosslinking CNCs with 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), thereby oxidising the material to form a bridge with the grafting of the polyethyleneimine (PEI). The surface of the CNC-TEMPO-PEI was further magnetised by introducing iron on to the surface material via a co-precipitation method.
Fourier-transform infrared spectroscopic (FTIR) analysis revealed the presence of C=O, COOH, CH, OH and FeO stretching frequencies in MCNC, while powder X-ray diffraction (P-XRD) confirmed the formation of MCNC and the monoclinic type 1 cellulose with 1β lattice and magnetite cubic spinel phases of the CNC. Ultraviolet-visible spectroscopy (UV-Vis) showed the presence of both CNC and magnetite at 400 nm. The scanning electron microscopy (SEM) indicated a smooth fibroid surface of CNCs while magnetite (M) displayed 2 morphologies, the rod like and spherical morphology, indicating the presence of iron and oxygen. The MCNC were stable after 600 ⁰C as shown on the thermograms generated from the thermogravimetric analyser (TGA). Last, the Brunauer-Emmett-Teller (BET) displayed surface area, pore size and pore volume improvement of 56 m2/g, 98 Å and 0,1465 cm3/g. Å, respectively, for the MCNC.
Following the characterization of the MCNCs nanocomposites, the material was used for adsorptive removal of Pb (II). It was discovered that for the Pb (II) removal efficiency was 97 % with an acceptable precision of ≤ 3 %. The highest efficiency was obtained at optimal conditions of 60 mg dosage, 0,1 ppm concentration within a rapid contact time of 5 min at a temperature of 60 ⁰C and at a pH of 6. These parameters were optimised by using multivariate optimization tools (Minitab) and were also validated against the magnetite and the CNC. A maximum adsorption capacity of MCNC was also obtained at 47,70 mg/g for Pb (II) and the material was re-used for up to 4 cycles. The results revealed that the reaction followed Freundlich isotherms and Pseudo First Order kinetic model with a regression coefficient of 0,98 and 0,96 respectively. The adsorption thermodynamics studies indicated a spontaneous process and an exothermic reaction.
On the other hand, the MCNC-TEMPO-PEI was characterised with FTIR, P-XRD, TEM and SEM-EDS techniques. The FTIR confirmed a successful formation and the presence of COOH, OH, Fe-O band and NH2 groups on the nanocomposite. The P-XRD confirmed the crystal structure of CNC-TEMPO and the amorphous structure of both the CNC-TEMPO-PEI and the MCNC-TEMPO-PEI. The SEM-EDS results demonstrated the rod-like, oval and irregular cubic morphology for successful preparation of MCNC-TEMPO-PEI nanocomposite.
The adsorption performance of MCNC-TEMPO-PEI on Cr (VI) ions was investigated by using univariate optimization tools. The MCNC-TEMPO-PEI was efficient at 5 ppm, using a 30 mg dosage at 25 ⁰C within the acidic conditions at pH 2 within a rapid contact time of 15 min. The optimised parameters were further validated using 5 various adsorbent materials and the results indicated that the MCNC-TEMPO-PEI was the most efficient by exhibiting the highest adsorption capacity of 4,4 mg/g with a 98% removal. The interaction between the MCNC-TEMPO-PEI and the Cr (VI) ions indicated a chemisorption of the electrostatic forces governing the magnetic and ionic exchange interaction between of the adsorbate and the analyte. The Langmuir adsorption isotherm displayed a correlation coefficient of 0,94 following the PSO kinetic model against the adsorptive removal of Cr (VI) ions. The thermodynamic interaction indicated a non-spontaneous endothermic reaction with a favourable reaction. The adsorbent could be reused at least 8 times with a removal efficiency above 75 %. The results revealed that the real wastewater samples analysed from this study did not contain Cr (VI) ion.
