Ball milling enhanced Cr(VI) removal of zero-valent iron biochar composites: Functional groups response and dominant reduction species

Jinlan Zhang; Lihong Xie; Qiyan Ma; Yiyang Liu; Jie Li; Zhifeng Li; Shangyi Li; Tingting Zhang

doi:10.1016/j.chemosphere.2022.137174

Ball milling enhanced Cr(VI) removal of zero-valent iron biochar composites: Functional groups response and dominant reduction species

Chemosphere. 2023 Jan;311(Pt 2):137174. doi: 10.1016/j.chemosphere.2022.137174. Epub 2022 Nov 8.

Authors

Jinlan Zhang¹, Lihong Xie¹, Qiyan Ma¹, Yiyang Liu¹, Jie Li¹, Zhifeng Li¹, Shangyi Li¹, Tingting Zhang²

Affiliations

¹ Department of Environmental Science and Engineering, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China.
² Department of Environmental Science and Engineering, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China. Electronic address: zhangtt@mail.buct.edu.cn.

PMID: 36368528
DOI: 10.1016/j.chemosphere.2022.137174

Abstract

Zero-valent iron biochar composites (ZVI/BC) have been widely used to remove Cr(VI) from water. However, the application of ZVI/BC prepared by the carbothermal reduction was limited by the non-uniform dispersion of ZVI on the biochar surface. In this work, ball milling technique was introduced to modify ZVI/BC. Results showed that after ball milling, the maximum Langmuir adsorption capacity for Cr(VI) was 117.7 mg g^-1 (298 K) which was 2.08 times higher than ZVI/BC. The initial adsorption rate of the Elovich model increased from 4.57 × 10² mg g^-1 min^-1 to 3.74 × 10⁹ mg g^-1 min^-1 after ball milling. Dispersibility of ZVI on biochar surface and contact between ZVI and biochar were improved by the ball milling, thus accelerating the electron transfer. Besides, ball milling increased the content of oxygen-containing functional groups in biochar, contributing to the chemisorption of Cr(VI). The response sequence of oxygen-containing functional groups was analyzed by two-dimensional correlation spectroscopy, indicating that Cr(VI) preferentially complexed with phenolic -OH. Shielding experiments showed that Fe (0) was the dominant reducing species with a contribution of 73.4%, followed by surface-bound Fe(II) (21.3%) and dissolved Fe²⁺ (5.24%). Density functional theory calculations demonstrated that ball milled ZVI/BC improved the adsorption affinity and electron transfer flux towards Cr(VI) by introducing phenolic -OH and Fe (0). Combining all the textural characterization, the Cr(VI) removal mechanism of the ball milled ZVI/BC could be proposed as adsorption, reduction, and precipitation. Eventually, stable Cr-Fe oxides (FeOCr₂O₃ and Cr_1·3Fe_0·7O₃) were formed. This work not only provides a simple method to modify ZVI/BC to remove Cr(VI) in water efficiently and rapidly, but also improves the mechanistic insight into the Cr(VI) removal by iron-carbon composites via the response sequence of functional group analysis and the quantitative analysis of reducing species.

Keywords: Ball milling; Biochar; Density functional theory; Hexavalent chromium; Two-dimensional correlation spectroscopy; Zero valent iron.