K2cr2o7

Cotton textile waste (CTW), used as the precursor of activated carbon, was mainly composed of warp and weft yarn mixtures, which was purchased from WUXI No. 1 Cotton Mill (Jiangsu, P. R. China). All chemicals (FeCl₃·6H₂O, HCl, HNO₃, H₂SO₄, H₃PO₄ and K₂Cr₂O₇) were of analytical reagent grade and obtained from Sinopharm Chemical Reagent Co. (Shanghai, P. R. China). The deionized water used was produced by an Ultra-pure water system (Milli-Q Advantage A10, USA). The purity of N₂ was higher than 99.9%.

The stock solution of Cr(vi) and NaCl were prepared by the chemicals of K₂Cr₂O₇ and NaCl respectively, and both of these chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd with analytical purity (AR). The working solution was prepared by diluting certain volumes of Cr(vi) and NaCl stock solutions with deionized water just before it was to be used. The final concentration of Cr(vi) in the working solutions ranged from 40 to 400 mg L⁻¹, and the concentration of NaCl was 0.01 M, which acted as the background electrolyte.
The used black soil sample was obtained from an undisturbed area located at the northeast of China (43°18′36′′N, 128°23′26′′E), where is located at one of the three major black soil regions worldwide (the other two are the great plains of Ukraine and the Mississippi River basin of America, respectively). This black soil sample can be classified as Mollisol soil according to USDA soil classification system, and it is the typical black soil in the region of Northeast China. The location, landform, climate and vegetation can be found in our previous published paper.^{29 (link)} The surface soil within the depth of 30 cm was sampled after the topsoil containing substantial plant roots had been removed. The soil sample was air-dried and ground, and then was passed through a 100-mesh sieve.