A raw granular activated carbon (RGAC) is modified by polyaniline to improve arsenate adsorption. It is found that the modification does not change the specific surface area. The content of the aromatic ring structures and nitrogen-containing functional groups on the modified GAC (MGAC) is increased. The surface positive charge density is dramatically increased in acidic solutions. The adsorption of As(V) onto both sorbents is highly pH dependent. The modification broadens the optimal pH range for the arsenate adsorption: pH of 3.0-6.8 and 4.0-6.6 for the MGAC at initial arsenic concentrations of 0.15 and 8.0 mg L-1 vs pH 6.0 and 6.5 for the RGAC at the same concentrations. The maximum adsorption capacity at an optimal pH of 5.5 is enhanced by 84%. The MGAC is able to greatly remove the arsenic at a trace level; the concentration of treated solution is far below the U.S. EPA standard. The presence of humic acid does not have a great impact on the arsenic adsorption dynamics. The modification significantly enhances the adsorption of humic acid onto the carbon. X-ray photoelectron spectroscopy analysis demonstrates that the arsenate is reduced to arsenite during the process. A fixed bed is able to reduce arsenate concentrations ranging from 120 to 1910 ppb to less than 10 ppb. The column adsorption behavior by the MGAC can well be described by a fixed-bed model.