Removal of chromate and selenate in natural water using iron-bearing mineral-biochar composites

Feasibility tests of iron-biochar composites for removal of chromate (CrO 4 2− ) and selenate (SeO 4 2− ) were conducted through a series of batch experiments. Two types of iron-biochar composites were synthesized using zero-valent iron [Fe(0)] and Fe 3 O 4 via co-pyrolysis with rice straw. Fe(0)-biochar composites removed chromate and selenate via sorption and reduction. Even biochar alone could reduce selenate to selenite and its reduction products, probably due to localized electrons in biochar. The Fe 3 O 4 -biochar composite did not effectively remove chromate and selenate, nor did dissolved humic acid and salts affect the removal of chromate and selenate by the Fe 3 O 4 -biochar composite. In contrast, changes in pyrolysis temperature and pH enhanced the removal. The iron-biochar composites and biochar alone may be sustainable long-term options for remediation of toxic anionic metals in natural environments.