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In this study, teff (Eragrostis tef) straw has been chemically treated and tested as an adsorbent for Cr(VI) removal. Chemically treatment of teff straw was done by NaOH, H 3 PO 4 and ZnCl 2 solutions. Scanning electron micrograph and X-ray diffraction were used for anatomical characterization, whereas Fourier transform infrared spectroscopy was used for surface change characterization of adsorbents. Effects of different experimental parameters like pH (2-12), initial Cr(VI) concentration (100-900 mg/L), adsorbent dose (2.5-20 g/L), contact time (15-360 min) and temperature (288-318 K) were studied. Temperature increment was found to stimulate the adsorption process. Langmuir isotherm was found to give better representation over wide range of temperature for untreated, H 3 PO 4 -as well as ZnCl 2 -treated teff straw, and Freundlich isotherm best represented the isotherm data for NaOH-treated teff straw. Maximum Cr(VI) adsorption capacity of untreated, NaOH-, H 3 PO 4 -and ZnCl 2 -treated teff straw was found to be 86.1, 73.8, 89.3 and 88.9 mg/g, respectively. Respective values of average effective diffusion coefficient (D e ) were found to be 2.8 9 10 -13 , 2.59 9 10 -14 , 1.32 9 10 -13 and 1.14 9 10 -13 m 2 /s, respectively. The negative value of DG o for all the adsorbents indicates Cr(VI) spontaneous adsorption. Isosteric heat of adsorption (DH st,a ) was found to vary with surface coverage (h). DH st,a increased for untreated, H 3 PO 4 -and ZnCl 2 -treated teff straw, and decreased steadily with h for NaOH-treated teff straw.

Efficient utilization of agricultural residue is the need of today’s environment. Teff straw is one such agricultural residue which is available in high amount in east African continent particularly Ethiopia. In the present study, combination of heat and acid treatment has been used to extract nano-silica from teff straw. X-ray fluorescence (XRF) analysis showed presence of high amount of silicon dioxide (≈52%) in raw teff straw. Thermal treatment at 600°C for 4 hour increased the SiO2 concentration to ≈ 92% in its ash. Further acid treatment increased the concentration to ≈97%. Fourier transform infrared (FTIR) spectroscopy also confirmed increase in SiO2 after thermal and acid treatment. X-ray diffraction (XRD) analysis showed the silica of amorphous nature in teff straw ash before acid treatment (S-BAT) whereas crystallinity increased after acid treatment (S-AAT). Transmission electron microscopy (TEM) showed presence of uniform nano-disks shaped particles of 50 nm average particle size in acid treated teff straw ash. Compared to other agricultural residues high amount of silica availability as raw and its small diameter anatomy structure will make teff straw better source of silica material at lower heat and chemical treatment.

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Teff straw, a by-product of Teff, mainly available in Ethiopia, has not been studied much for biosorbent production. The present study has investigated the effects of modification and optimization of process parameters (viz., concentration of modifying agent (H3PO4 and KOH), modifying temperature, and modifying time) on the Cr (VI) removal efficiency of using chemically activated Teff straw biosorbent by RSM followed by BBD. The maximum Cr (VI) removal was obtained using an H3PO4-modified Teff straw biosorbent of 92.5% with 2 M concentration of the modifying agent, 110°C, and 4 h. Similarly, maximum Cr (VI) removal using KOH-modified Teff straw biosorbent of 95.2% was obtained with 1.5 M activating agent concentration, 105°C activation temperature, and 3.5 h activation time. In addition, the effects of adsorption parameters (viz., biosorbent dosage, temperature, initial concentration of Cr (VI), and contact time) were investigated. The maximum removal efficiency was attained at 2 g of biosorbent dosage, 4 h contact, 75 mg/L of initial Cr (VI) concentration, and 25°C sorption temperature. In addition, isotherm, kinetic, and thermodynamic studies for Cr (VI) biosorption were studied. The experimental adsorption data were well fitted with the Langmuir isotherm and pseudo-second-order kinetic model with higher correlation coefficient in both untreated and chemically modified Teff straw biosorbent. The investigated thermodynamic parameters ( Δ H o , Δ S o , and Δ G o ) confirmed that Cr (VI) metal ions’ adsorption process onto Teff straw biosorbent was spontaneous and endothermic.