Removal Efficiency and Mechanism of Cr(VI) from Aqueous Solution by Maize Straw Biochars Derived at Different Pyrolysis Temperatures

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.

Biochar from A Freshwater Macroalga as A Potential Biosorbent for Wastewater Treatment

“…This characteristic decides about the potential applications of biochar. Available literature confirms that biochars produced from bio-waste, due to their properties-such as porous structure, high specific surface area and abundant functional groups, have been widely used to remove toxic metals from wastewater [6,8,12,14,30].…”

“…For instance, Jung et al (2016) also observed this phenomenon for biochar produced at 200, 400, 600 and 800 • C from marine brown macroalga Undaria pinnatifida [13]. The ash content significantly increases with pyrolysis temperature mainly due to the minerals forming ash, which remain after biochar carbonization [30].…”

“…The increase in pyrolysis temperature did not affect significantly the textural properties of the material and all samples had a total pore area of about 20 m 2 g −1 . Usually, with the increase in pyrolysis temperature, the specific surface area also increases, mainly due to the destruction of organic functional groups and formation of micro-pores in the produced biochars [30]. The total volume intrusion increased along with the temperature rise and varied from 0.557 cm 3 g −1 to about 0.806 cm 3 g −1 .…”

“…Biochar is a type of porous carbon similar to activated carbon [14,30]. The surface area with the pore size and pore volume affecting its sorption characteristics [35].…”

Removal of Hexavalent Chromium from Aqueous Solutions Using Sulfonated Peat

“…However, the modified peat exhibited a greater adsorption efficiency to Cr(VI) than the natural peat. Therefore, based on the above results, it is suggested that chemisorption dominated the adsorption process [1,40,42].…”