Assefa Sergawie's research while affiliated with Addis Ababa Science and Technology University and other places

Heavy metals that are present in surface water and wastewater are becoming a severe environmental problem. Because of its toxicity, heavy metal removal has become the main priority for environmental concerns. Banana peels are low-cost agricultural waste that could be used for heavy metal adsorption in wastewater. Te main objective of this study is to evaluate the efective powdered banana peel for the removal of copper (II) from aqueous solutions and real wastewater. Te banana peels were collected from domestic waste and ground to get a particle size of 150 µm. Powdered banana peel waste adsorbent (PBPWA) contained moisture content, ash content, volatile matter, and bulk density of 3.8%, 3.5%, 37.5%, and 0.02 g/cm 3 , respectively. Te Fourier-transform infrared spectroscopy (FTIR) results showed that the alkyne, aldehyde, and amide functional groups were dominant in the powdered banana peel surface, and the scanning electron microscope showed the morphology of the adsorbent. Physico-chemical characteristics of the raw wastewater revealed that the concentration of Cu (II), Pb (II), COD, BOD5, and Cd (II) were 2.75 mg/L, 2.02 mg/L, 612.16 mg/L, 185.35 mg/L, and 0.01 mg/L, respectively. At pH 5, adsorbent dose of 2g/100 mL, initial copper (II) concentration of 80 mg/L, and contact time of 90 min, the maximum removal efficiency of synthetic wastewater was 96.8% and textile wastewater was 69.0%. The adsorption isotherm ftted well with the Langmuir isotherm model at R 2 = 0.99. The kinetics of copper (II) adsorption followed the second-order kinetic model better. Finally, these studies showed that banana peel bio-adsorbent is a potential adsorbent for heavy metal removal from synthetic and textile wastewater.

Recently, increasing the discharge of untreated dyes containing wastewater is causing a serious environmental problem in water systems. Textile industries generate a huge volume of wastewater that contains many pollutants such as suspended solids, acidity, alkalinity, dissolved solids, sulfate, chromium, COD, and different types of dyes including Congo red (CR). Normally, CR is mainly associated with many industries like paint production, paper manufacturing, and garment printing. Therefore, this study aimed to investigate the removal of CR from aqueous and textile wastewater using activated carbon developed from corn cobs under four adsorption factors named pH, adsorbent dose, contact time, and initial dye concentration. This activated carbon was developed from corn cobs using pyrolysis and the chemical activating agent. Carbonization was done using an electrical fume furnace at 500 °C and then treated with phosphoric acid at a ratio of 1:1 (w/v). The textile wastewater properties were described by the color of 1200 (pt-co), pH 9, BOD5 200 mg/L, COD 370 mg/L, and TDS 4970 mg/L. The corn cob activated carbon (CCAC) was characterized by ash 2.4%, moisture 4.9%, volatile 40.9%, fixed carbon 51.8%, carbon yield 59.3%, bulk density 290 mg/m3, and surface area 650 m2/g. FTIR results showed the presence of multi-functional groups such as hydroxyl, alcohol, ketone, pyrone, aliphatic, ether, and aromatic whereas the X-ray analysis indicated the amorphous nature of carbonaceous matter. The maximum CR removal was obtained at the experimental condition of pH 6.2, adsorbent dose 1.6, contact time 84 min, and initial dye concentration of 90 mg/L in conventional experiments studies whereas the highest dye removals were found to be 96% from the aqueous solution and 88% from real textile wastewater under the factorial approach. Langmuir isothermal and pseudo-second-order at R2 0.997 are in good agreement with experimental adsorption data. Finally, CCAC is a good adsorbent for the discoloration of dyes from textile wastewater with the potential to be scaled up.

Investigations on the spin states of octahedral Fe(II) complexes have received special attention due to their clear discrimination in the spin states of the d-orbitals. As a means to further understand the factors that influence the spin-crossover (SCO) phenomenon in Fe(II) systems, we herein report two mononuclear Fe(II) complexes, [FeL2](ClO4)2·2CH3OH (1) and [FeL2](BF4)2·CH3CN·CH3OH (2), derived from a novel N3-donor Schiff base ligand, 2,6-bis[(3-methylbenzylimino)methyl]pyridine (L) with varying counteranion and the diamagnetic [ZnL2](BF4)2 congener for a comparative investigation. The complexes have been synthesized and characterized by electrospray-ionization mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction (XRD) and magnetic susceptibility studies. Structural and magnetic investigations reveal that both 1 and 2 show Fe__N6 distorted octahedral geometry and are locked in the diamagnetic LS state throughout the entire explored temperature range from 1.8 to 400 K. The LS state of [FeL2]2+ is also confirmed by comparing the experimentally found structural parameters, NMR chemical shifts and excitation energies in the visible region with density functional theory (DFT) calculations.

The triangulo-{Er3} complex [Er3Cl(o-van)3(OH)2(H2O)5]Cl3·nH2O (n = 9.4; H(o-van) = o-vanillin) (1) was generated by an in situ method. The isolated Er(iii) complex 1 was characterized by elemental analysis and molecular spectroscopy. The results of single crystal X-ray diffraction studies have shown that 1 is built up of trinuclear [Er3Cl(o-van)3(OH)2(H2O)5]3+ complex cations, chloride anions and water solvate molecules. Within the complex cation the three Er(iii) central atoms are placed at the apexes of a triangle which are bridged by three (o-van)- ligands with additional chelating functions and two μ3-OH- ligands. Additionally five aqua and one chlorido ligands complete the octa-coordination of the three Er(iii) atoms. AC susceptibility measurements reveal that the compound exhibits slow magnetic relaxation with two relaxation modes.

Iron(II)-Schiff base complexes are a well-studied class of spin-crossover (SCO) active species due to their ability to interconvert between a paramagnetic high spin-state (HS, S = 2, 5T2) and a diamagnetic low spin-state (LS, S = 0, 1A1) by external stimuli under an appropriate ligand field. We have synthesized two mononuclear FeII complexes, viz., [Fe(L1)2](ClO4)2.CH3OH (1) and [Fe(L2)2](ClO4)2.2CH3CN (2), from two N6–coordinating tridentate Schiff bases derived from 2,6-bis[(benzylimino)methyl]pyridine. The complexes have been characterized by elemental analysis, electrospray ionization–mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR), solution state nuclear magnetic resonance spectroscopy, 1H and 13C NMR (both theoretically and experimentally), single-crystal diffraction and magnetic susceptibility studies. The structural, spectroscopic and magnetic investigations revealed that 1 and 2 are with Fe–N6 distorted octahedral coordination geometry and remain locked in LS state throughout the measured temperature range from 5–350 K.

A tetranuclear dysprosium Schiff base complex was isolated by reacting dysprosium chloride with 2-hydroxy-3-methoxybenzaldehyde and 2-(aminomethyl)pyridine in-situ under basic conditions. The isolated Dy(III) complex was characterized by elemental analyses, single crystal X-ray diffraction and molecular spectroscopy. The complex crystallizes in the triclinic space group P-1 with unit cell parameters of a = 10.2003 (4), b = 13.8602 (5), c = 14.9542 (6), α = 94.523 (3), β = 109.362 (4), and γ = 99.861 (3). The magnetic properties of 1 have been investigated by DC and AC susceptibility measurements. The DC measurements reveal weak exchange coupling of antiferromagnetic nature. In the AC measurement, the complex shows a slow relaxation of magnetization in the absence of an external magnetic field.

A dinuclear dysprosium(III) complex [Dy2(NO3)3(L)3]•nCH3OH (n = 1.20; HL = (2-[(2-hydroxy-propylimino)methyl]phenol)) (1) was isolated when the dysprosium nitrate reacted with a solution of salicylaldehyde and 1-amino-2-propanol in basic medium under...

Catalytic activities of graphitic carbon nitride (g-C3N4) are restricted thanks to inadequate visible light absorption and high rate electron–hole recombination. In this work, we synthesized porous g-C3N4 using polycondensation process. Structural and physico-chemical characteristics of the prepared g-C3N4 materials were studied via XRD, DRS, PL, FTIR, Raman spectroscopy, SEM, BET and CHN elemental analyzer. The prepared samples exhibited surprising catalytic activity for the photo-oxidation of rhodamine-B (RhB) in visible light irradiation. From the fabricated g-C3N4 materials, the g-C3N4-550 showed photodegradation efficiency of 100% towards the RhB pollutant in water within 30 min. No appreciable decrease of the photocatalytic efficiency of g-C3N4 was observed up to five consecutive cycles, confirming the synthesized g-C3N4 was highly stable. Thus, this work gave a simple process for large scale production of highly visible light responsive and stable g-C3N4 materials used for environmental remediation.

Magnetically separable all-solid-state Z-scheme g-C3N4/graphene/NiFe2O4 (CGN) nanocomposites have been synthesized using one pot hydrothermal method. The as-prepared pristine and nanocomposite samples were characterized using X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), UV–VIS diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface analysis, and Vibrational sample magnetometer (VSM) characterization techniques. The result reveals that the weight percent of NiFe2O4 have considerable effect on the optical property and photocatalytic activity of the nanocomposites. The CGN-25% nanocomposite shows the best photocatalytic activity and its activity is about 15, 7 and 6 folds higher than those of NiFe2O4, g-C3N4, and CN-25% respectively on the degradation of methyl orange (MO). This enhancement could be attributed to the synergistic effect between g-C3N4, graphene and NiFe2O4, which could accelerate photogenerated electron-hole separation and prolong the life of the photo-induced carriers to enhance their activity. The CGN-25% was recycled using magnet without significant loss and shows almost the same photocatalytic performance for six runs. Moreover, a possible photocatalytic mechanism was proposed. This could provide new insights to develop highly efficient and stable g-C3N4 and graphene based magnetic photocatalysts.