Characterization of chromium species and distribution during Cr(VI) removal by biochar using confocal micro-X-ray fluorescence redox mapping and X-ray absorption spectroscopy

Peng Liu; Carol J Ptacek; David W Blowes; Y Zou Finfrock; YingYing Liu

doi:10.1016/j.envint.2019.105216

Characterization of chromium species and distribution during Cr(VI) removal by biochar using confocal micro-X-ray fluorescence redox mapping and X-ray absorption spectroscopy

Environ Int. 2020 Jan:134:105216. doi: 10.1016/j.envint.2019.105216. Epub 2019 Oct 31.

Authors

Peng Liu¹, Carol J Ptacek², David W Blowes², Y Zou Finfrock³, YingYing Liu²

Affiliations

¹ School of Environmental Studies & State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Rd., Wuhan, Hubei 430074, PR China; Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave. W., Waterloo, ON N2L 3G1, Canada. Electronic address: pengliu@cug.edu.cn.
² Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave. W., Waterloo, ON N2L 3G1, Canada.
³ CLS@APS Sector 20, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA; Science Division, Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, SK S7N 2V3, Canada.

PMID: 31677596
DOI: 10.1016/j.envint.2019.105216

Abstract

Biochar is an effective, environmentally sustainable material for removing Cr(VI) from water. Potential removal mechanisms include surface reactions or reactions within the biochar structure with direct bonding of Cr(VI) or reduction of Cr(VI) and bonding of the reduced Cr forms. Diffusion process and Cr(VI) and Cr(III) distributions in biochar particles have not been elucidated. Aqueous Cr(VI) removal experiments followed by solid-phase analyses were conducted to evaluate the effectiveness of raw and modified oak wood biochar for removing aqueous Cr(VI) and to further determine removal mechanisms. Results showed that concentrations of Cr(VI) decreased from ~50 to <0.02 mg L^-1 at initial pH 2 after 1 d using raw oak wood biochar, with 8.8% of the initial aqueous Cr reduced to Cr(III) in the solution. Similarly, effective removal of Cr(VI) was observed using polysulfide-modified biochar; whereas ~54% of initial Cr(VI) was removed using HNO₃-treated biochar. Bulk X-ray absorption near-edge structure (XANES) analysis showed Cr is present as Cr(III) within the unmodified biochar, whereas confocal micro-XANES analysis showed the existence of Cr(VI) (0-36%) in selected spots and Cr(0) (43%) in one spot within a biochar sample collected after 30 min. Extended X-ray absorption fine structure (EXAFS) results showed the atomic structure of Cr within the unmodified biochar was similar to Cr(OH)₃, with O and Cr in the first and second shells. Confocal micro-X-ray fluorescence imaging (CMXRFI) results indicated total Cr (tCr) was heterogeneously distributed in the imaged area with a higher intensity close to the particle surface. Redox mapping results indicated no Cr(VI) in the unmodified biochar collected at 30 min; Cr(III) was the primary form and also remained close to the surface at later time. The removal mechanisms likely involve electrostatic attraction and diffusion inside the particle, followed by reduction and ion exchange reactions.

Keywords: Biochar; Confocal micro-X-ray fluorescence imaging; Hexavalent chromium; Redox mapping; X-ray absorption spectroscopy.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Adsorption
Charcoal
Chromium
Oxidation-Reduction
Water Pollutants, Chemical
X-Ray Absorption Spectroscopy*

Substances

Water Pollutants, Chemical
biochar
Chromium
Charcoal
chromium hexavalent ion