(643e) Sorption and Desorption of Chromium (III) from Aqueous Solution by Agro-Waste Materials

Agro-waste materials such as sorghum straw (SS), oats straw (OS) and agave baggasse (AB) contain several functional groups (carboxyl, carbonyl, acetamide, amino, hydroxyl, etc.) which are able to bind heavy metal ions present in aqueous solution. The major advantages of these materials are: low-cost, high efficiency and regeneration. The agro-waste materials studied in this research (water washed and acid washed biosorbents) were chemically characterized and their chromium sorption capacity was obtained. The characterization of these residues included the determination of functional groups, equilibrium constants, surface charge distribution and ATR-FTIR analysis. Batch experiments were carried out to determine the biosorbents chromium (III) sorption capacity at 25°C and pH 4 and 3. Also, the regeneration of partially saturated biosorbents was carried out by using HNO₃, NaOH and EDTA at different concentrations and temperatures. Finally, the sorption mechanism of chromium (III) was discussed.

The results show that the main functional groups can be associated to carboxyl, nitrogen, and hydroxyl groups. Nitrogen could not be associated to a specific group due to the great amount of sites (e.g. amine, phosphoryl, amide, etc.) linked to a particular pk_a value. On the other hand, the acid treatment reduced the biosorbents amount of functional groups that have pk_a values around 7.0; this was observed in the ATR-FTIR spectra (1610 cm^-1) and in the surface charge distribution at pH 7. In addition, the pH_PZC was shifted to acid pH values.

The maximum chromium (III) sorption capacity, for acid washed OS, was 12.9 and 8.3 mg/g at pH 4 and 3, respectively: the sorption capacity decreased at pH 3 since H⁺ ions competed with chromium (III) ions for the biosorbents sorption sites. On the other hand, the biosorbents regeneration was affected by temperature, for instance, at 25 and 35°C the chromium (III) recovered, from acid washed OS, was 51 and 74%, respectively, but at 55°C the biosorbent was completely regenerated. Based on the ATR-FTIR spectra and previously results reported in literature, it was suggested that chromium (III) has special affinity for the biosorbents oxygen-containing sites.

Finally, the agro-waste materials studied herein presented excellent characteristics to remove cations and anions in solution, and have a chromium (III) sorption capacity similar to adsorbents like activated carbons and ion exchange resins. Plus the used biosorbents can be efficiently regenerated by an EDTA solution at 55°C.

Keywords: Chromium (III), agro-waste materials, sorption, regeneration.