Synthesis, Optical Characterization, and Adsorption of Novel Hexavalent Chromium and Total Chromium Sorbent: A Fabrication of Mulberry Stem Biochar/Mn-Fe Binary Oxide Composite via Response Surface Methodology

Shuiping Xu,Yanmei Ding,Yinian Zhu,Meina Liang,Linbo Han,Dunqiu Wang

doi:10.3389/fenvc.2021.692810

Abstract

In this study, a new generation chromium sorbent, mulberry stem biochar/Mn-Fe binary oxide composite (MBC-MFC), was fabricated by chemical precipitation on carbonized mulberry stem according to response surface methodology (RSM) results. RSM was more convenient to figure out the optimized preparation condition of MBC-MFC theoretically for achieving a maximum removal efficiency of Cr(VI) and total chromium (TCr), compared to labor-intensive orthogonal experiments. The RSM results showed that Fe/Mn concentration (CFe; CMn), MBC activation temperature after soaking in KOH solution (T), and pH during precipitation of Fe-Mn oxide were three main factors to significantly affect the efficiency of MBC-MFC (p &lt; 0.05) in Cr(VI) and TCr removal. With the selected condition (CFe = 0.28 mol/L; CMn = 0.14 mol/L; T = 790°C; pH = 9.0), MBC-MFC was synthesized with a large surface area (318.53 m2/g), and the point of zero charge values of MBC-MFC was 5.64. The fabricated MBC-MFC showed excellent adsorption performance of Cr(VI) and TCr in an aqueous solution. The maximum Cr(VI) and TCr removal capacity of MBC-MFC was 56.18 and 54.97 mg/g (T = 25°C, pH = 3.0, t = 48 h, and dosage = 0.10 g/50 ml), respectively, and the maximum Cr(VI) adsorption of MBC-MFC was 4.16 times that of bare MBC, suggesting the synergistic effects of Fe/Mn oxides and MB on the performance of MBC-MFC in Cr(VI) and TCr removal. The adsorption mechanism of MBC-MFC on chromium was mainly contributed by surface complexation and electrostatic attraction. Our study offers valuable outlooks to develop high-performance biochar-based sorbents for heavy metal removal and sustainable environmental remediation.

Highlights

Toxic heavy metal ions are very harmful to human health and environmental ecosystem (Wang et al, 2020; Liu et al, 2021), among which chromium-containing compounds are among the most ubiquitous contaminants (Kretschmer et al, 2019; Pei et al, 2020; Xu et al, 2021)
Optimization by response surface methodology (RSM)-PB The experimental scheme in Supplementary Table 3 was obtained through the RSM-PB, and twelve kinds of mulberry stem biochar (MBC)-MFC were prepared under different conditions
The MBC-MFC was successfully fabricated by chemical precipitation on the carbonized mulberry stem, acting as a novel adsorbent to remove total chromium (TCr) and Cr(VI)

Summary

Introduction

Toxic heavy metal ions are very harmful to human health and environmental ecosystem (Wang et al, 2020; Liu et al, 2021), among which chromium-containing compounds are among the most ubiquitous contaminants (Kretschmer et al, 2019; Pei et al, 2020; Xu et al, 2021). Chromium commonly exists in the industrial waste and natural environment in the form of Cr(III) and Cr(VI) (Andrade et al, 2019; Qiu et al, 2020; Zhang et al, 2020). Cr(III) is comparatively stable in an environment with low mobility and solubility, while Cr(VI) primarily exists in the form of oxyanions such as CrO42−, Cr2O72−, and HCrO4−, which are highly soluble and mobile (Li et al, 2020). The Ministry of Ecology and Environment of the People’s Republic of China has set the maximum concentration of Cr(VI) for the legal discharge of surface water as 50 ug/L (Lv et al, 2020)

Methods

Results

Conclusion