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Received Feb 23, 2018; Revised Apr 20, 2018; Accepted May 3, 2018

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

With the rapid development of global printing and dyeing industry (PDI) for decades, a variety of dyes are used in knitting silk, cotton, and so on [1–3]. Considerable industrial wastewater including dyes would have been generated during the processing of PDI [4–8]. Congo red 1-naphthalenesulfonic acid, 3, 3′-(4, 4′-biphenylenebis (azo) bis (4-amino) disodium salt, as one of the most widely used direct dyes in PDI for its high chromaticity, is the critical source contamination of wastewater with high chemical oxygen demand and high biological toxicity [9–14]. Therefore, it is of great significance to remove Congo red from aqueous solution.

It has been reported that physical adsorption is the dominant method for Congo red removal. The materials with a high-specific surface area and an abundant pore structure are explored to absorb Congo red to decontaminate industrial wastewater [15, 16]. For instance, activated carbon (AC), possessing a large specific surface area [17], good environmental friendliness [18], and mechanical stability [19], is a promising material for wastewater treatment [20]. However, its application is limited by high energy consumption and large greenhouse gas emissions during an AC preparation process [21]. Recently, biochar (BC), a precursor of AC, has attracted many researchers’ attention [22]. It has a pore structure with a large surface area and good environmental friendliness as well as AC. In addition, BC could usually be prepared by pyrolysis of biological organic materials under a moderate temperature with low cost. And pyrolytic BC could keep the original structure of the rich organic functional group and could be modified easily [23, 24]. When BC is used as an adsorbent to remove Congo red, its organic functional group would interact weakly with the organic molecule of Congo red. Hence, BC might show better research prospects than AC in terms of waste water purification, heavy metal ion adsorption, and soil restoration [24–28]. However, BC mostly exhibits a micropore and mesoporous structure, which seldom matches Congo red macromolecule. Therefore, it is necessary to enlarge...

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