## 1 Introduction  
Chromium (Cr) is a typical heavy metal pollutant in industrial wastewater, mainly from metallurgy, electroplating, leather tanning, mining, wood preservation and textile industries [1]. It predominantly exists in the trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) oxidation states in the environment. Cr(VI) poses a great threat to human health and the ecosystem due to its higher solubility, mobility and toxicity than those of Cr(III) [2]. At present, various techniques have been applied for the Cr(VI) removal, such as chemical precipitation, ion exchange, redox and adsorption. Among these techniques, adsorption is regarded as the most effective and simplest method to remove Cr(VI) from industrial wastewater [3, 4].  
It is critical for the adsorption method to obtain an adsorbent with low price, high adsorption capacity and easy accessibility. In the context of green chemical production, biochar has attracted the attention of researchers due to its economic and environmental sustainability. Biochar is a highly aromatic and carbonized solid matter formed from waste biomaterials by pyrolysis under the condition of oxygen deficiency [5]. Studies reveal that biochar has good adsorption capacity for heavy metal ions due to its well-developed pore structure, large specific surface area and negative charge. The efficiency of heavy metal removal by biochar is influenced by the properties and dosage of biochar, types and initial concentrations of heavy metals, and pH in aqueous solution [6, 7].  
Nascent biochar was relatively ineffective in removing oxyanions such as Cr, Se and As in aqueous solution. Thus, modifying the biochar to enhance its oxyanions adsorption capacity has been explored [8; 9]. Fe-modified biochar has been an inexpensive and highly effective method in enhancing Cr(VI) removal [10]. It has been shown that Fe-modified biochar has a good capability to remove Cr(VI) from acidic aqueous solutions [11; 12]. Therefore, the pH of aqueous solutions was usually adjusted below 7 (e.g., PH = 2) in the treatment of wastewater containing Cr(VI), which caused acid pollution and increased treatment costs. In addition, some industrial wastewater often contain ions, e.g., Ca\({}^{2+}\), Na\({}^{+}\) and Cl\({}^{-}\), which may interfere with the adsorption and removal of Cr(VI) [13]. However, the efficiency of Cr(VI) adsorption and removal under such industrial wastewater conditions has been rarely studied.  
Rice is widely grown in China; burning its straw in the open air will bring serious air pollution. Study on its recycling has been received wide attention. The aim of this study is to prepare Fe-modified biochar from rice straw biochar by impregnation method and to investigate the effect of different influencing parameters (i.e., modifier mass ratio, adsorbent dosage, pH and ionic strength) on the efficiency of Cr(VI) removal. Fe-modified biochar was characterized using scanning electron microscopy with energy dispersive X-ray (SEM-EDX), the Brunauer-Emmett-Teller method (BET), Fourier transform infrared (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Adsorption kinetics and isotherms were investigated in order to reveal the adsorption mechanism. This study will provide a theoretical basis and application for the Cr(VI) removal by Fe-modified biochar from industrial wastewater.