Adsorption Mechanism of Cr(VI) onto Coir Pith

Abstract

ABSTRACT Reductive adsorption of Cr(VI) on coir pith (hereafter CP) was examined as a function of pH, ionic strength, and temperature. The CP contains 1.33 meq g− 1 phenolic, 0.43 meq g− 1 of lactonic, and 0.35 meq g− 1 carboxylic sites. Thus the CP surface is enriched with electron-donating oxygen functionalities. As evidenced by infrared (IR) spectroscopy, the Cr(VI) → Cr(III) conversion is facilitated by CP sites that are enriched with O─ O functional groups. The adsorption of reduced Cr(VI) was found to occur via C─ O─ functional groups first forming innersphere complexes with the CP surface, yielding keto (> C═ O) groups on the CP surface. The reductive adsorption of Cr(VI) was almost completed within 3 to 4 h, and it was dependent on pH and background ionic strength, yielding the highest monolayer coverage (9.56E-7 mol m− 2) at pH 3.7 in 0.1 M NaNO3. The ΓCr(III) followed the order with respect to the ionic strength: Γ0.1 M > Γ0.01 M > Γ0.001 M. The initial rate constant, k i , increased with temperature as k i 313 K > k i 303 K > k i 293 K > k i 283 K.