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, acetamido, 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. Also, the regeneration of partially saturated biosorbents was carried out by using HNO3, NaOH and EDTA at different 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. In addition, acid treatment shifted the pHPZC 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 increased with rising the temperature. 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. In conclusion, the agro-waste materials studied herein could be used to remove chromium (III) and they can be efficiently regenerated by an EDTA solution at 55°C.