This study involved novel-designed sludge biochar (SB) adsorbed for arsenic removal with lower operating costs and higher adsorption efficiency properties. Generally, biochar only relies on micropores for pollutant adsorption, but physical adsorption is not highly efficient for arsenic removal. Therefore, in order to improve the removal efficiency of arsenic by SB, diethylenetriamine (DETA) and FeCl3 were used in this study to modify the surface of SB by an immersion method. The objectives of this research are to obtain optimum operation conditions by assessing the effect of different Fe content, pH and initial concentration on adsorbing arsenic. This study is the first to use Density Functional Theory (DFT) to simulate and verify the adsorption mechanism of arsenic by SB. Results showed the presence of amine/iron oxyhydroxides functional groups greatly promoted SB surface activity and its arsenic adsorption potential. The surface area, pore volume and pore size of the SB were estimated to be 525 m2 g-1, 0.35 cm3 g-1 and 8.71 nm, respectively. The DFT model result is the same as the result of arsenic adsorption performance with high adsorption energy (-246.3 kJmol-1) and shorter bond distances (1.42 Å), indicating strong chemical adsorption between arsenic and material. The reaction mechanism is divided into four pathways, including oxidation-reduction, complexation, electrostatic adsorption and pore adsorption.