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

Abstract

The aim of the present work was to assess the efficiency of biochars obtained from the co-gasification of blends of rice husk\u2009+\u2009corn cob (biochar 50CC) and rice husk\u2009+\u2009eucalyptus stumps (biochar 50ES), as potential renewable low-cost adsorbents for Cr(III) recovery from wastewaters. The two gasification biochars presented a weak porous structure (ABET\u2009=\u200963–144\xa0m2\xa0g−1), 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\xa0mL\xa0g−1), contact time (1–120\xa0h), and initial Cr(III) concentration (10–150\xa0mg\xa0L−1). High Cr(III) removal percentages (around 100%) were obtained for both biochars, due to Cr precipitation, at low L/S ratios (100 and 200\xa0mL\xa0g−1), for the initial pH 5 and initial Cr concentration of 50\xa0mg\xa0L−1. 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\xa0mg\xa0g−1) 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\xa0mg\xa0g−1. 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.