Recovery of Cr(III) by using chars from the co-gasification of agriculture and forestry wastes.

The aim of the present work was to assess the efficiency of biochars obtained from the co-gasification of blends of rice husk + corn cob (biochar 50CC) and rice husk + eucalyptus stumps (biochar 50ES), as potential renewable low-cost adsorbents for Cr(III) recovery from wastewaters. The two gasification biochars presented a weak porous structure (A = 63-144 m g), but a strong alkaline character, promoted by a high content of mineral matter (59.8% w/w of ashes for 50CC biochar and 81.9% w/w for 50ES biochar). The biochars were used for Cr(III) recovery from synthetic solutions by varying the initial pH value (3, 4, and 5), liquid/solid (L/S) ratio (100-500 mL g), contact time (1-120 h), and initial Cr(III) concentration (10-150 mg L). High Cr(III) removal percentages (around 100%) were obtained for both biochars, due to Cr precipitation, at low L/S ratios (100 and 200 mL g), for the initial pH 5 and initial Cr concentration of 50 mg L. Under the experimental conditions in which other removal mechanisms rather than precipitation occurred, a higher removal percentage (49.9%) and the highest uptake capacity (6.87 mg g) were registered for 50CC biochar. In the equilibrium, 50ES biochar presented a Cr(III) removal percentage of 27% with a maximum uptake capacity of 2.58 mg g. The better performance on Cr(III) recovery for the biochar 50CC was attributed to its better textural properties, as well as its higher cation exchange capacity.