Abstract
The removal efficiency and mechanism of Cr(VI) removal from aqueous solution on semi-decomposed maize straw biochars pyrolyzed at 300 to 600 °C were investigated. The removal of Cr(VI) by the biochars decreased with pyrolysis temperature increasing from 300 to 600 °C, and the maximum removal capacity of Cr(VI) for maize straw biochar pyrolyzed at 300 °C was 91 mg/g at pH 2.0. The percentage removal of Cr(VI) rapidly decreased with pH increasing from 2.0 to 8.0, with the maximum (>99.9%) at pH 2.0. The variation of Cr(VI) and Cr(III) concentrations in the solution after reaction showed that Cr(VI) concentration decreased while Cr(III) increased and the equilibrium was reached after 48 h, while the redox potential after reaction decreased due to Cr(VI) reduction. X-ray photoelectron spectroscopy (XPS) semi-quantitative analysis showed that Cr(III) accounted for 75.7% of the total Cr bound to maize straw biochar, which indicated reductive adsorption was responsible for Cr(VI) removal by the biochars. Cr(VI) was firstly adsorbed onto the positively charged biochar surface and reduced to Cr(III) by electrons provided by oxygen-containing functional groups (e.g., C=O), and subsequently part of the converted Cr(III) remained on the biochar surface and the rest released into solution. Fourier transform infrared (FTIR) data indicated the participation of C=O, Si–O, –CH2 and –CH3 groups in Cr(VI) removal by the biochars. This study showed that maize straw biochar pyrolyzed at 300 °C for 2 h was one low-cost and efficient adsorbent for Cr(VI) removal from aqueous solution.
Highlights
Hexavalent chromium (Cr(VI)) is identified as a priority hazardous pollutant that mainly discharges from metallurgy, metal processing, leather tannery, dyeing, electroplating, etc. [1]
The produced biochars had high ash contents (50.5–67.9%) and low relative carbon contents (24.14–26.44%) probably because maize straw samples were semi-decomposed in one farm field
Increased from 24.14% to 26.44%, while the content of hydrogen, oxygen, and nitrogen decreased, indicating the increasing pyrolysis temperature resulted in accelerated carbonization, deoxygenation, and dehydrogenation reactions during the pyrolysis process [1]
Summary
Hexavalent chromium (Cr(VI)) is identified as a priority hazardous pollutant that mainly discharges from metallurgy, metal processing, leather tannery, dyeing, electroplating, etc. [1]. Various methods (such as reduction, adsorption, precipitation, membrane separation) have been applied for Cr(VI) removal from wastewater. Among these methods, adsorption is considered as a low-cost and high-efficiency method that uses absorbents derived from solid waste [3]. Biochar is a carbon-rich, porous, and stable solid residue, which is usually prepared by direct thermal decomposition of biomass feedstock under an N2 environment below 900 ◦ C [4]. Due to its properties (e.g., high specific surface area, porous, abundant functional groups), biochar has been widely used to remove heavy metals and organic contaminants in recent years [1,5,6,7].
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