A new method was developed for the coagulation of arsenic with serpentine and Fe(II). Excellent removal efficiency (>99%) and satisfactory stability of the sediments were accomplished for As(V) and As(III). A mechanism study showed that hydroxyls generated by the surface hydrolysis of serpentine mediated active iron hydroxides for arsenic adsorption, while the Fe-As and Mg-As chemical interaction contributed to the arsenic stabilization.