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Chemical treatment of teff straw by sodium hydroxide, phosphoric acid and zinc chloride: adsorptive removal of chromium Wassie, A. B. & Srivastava, V. C. Abstract 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, H3PO4 and ZnCl2 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, H3PO4 as well as ZnCl2-treated teff straw, and Freundlich isotherm best represented the isotherm data for NaOH-treated teff straw. Maximum Cr(VI) adsorption capacity of untreated, NaOH-, H3PO4- and ZnCl2-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 (De) were found to be 2.8 × 10-13, 2.59 × 10-14, 1.32 × 10-13 and 1.14 × 10-13 m2/s, respectively. The negative value of ∆Go for all the adsorbents indicates Cr(VI) spontaneous adsorption. Isosteric heat of adsorption (∆Hst,a) was found to vary with surface coverage (θ). ∆Hst,a increased for untreated, H3PO4- ZnCl2-treated teff straw, and decreased steadily with θ for NaOH-treated teff straw. Keywords Adsorption kinetics; Chemically modified absorbent; Chromium removal; Isosteric heat of adsorption; Isotherm study

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