Using chemically modified spent grains as a new biosorbent to treat arsenate and arsenite ions was studied. The influences of pH, contact time, initial concentration and temperature were studied in batch experiments. The equilibrium process was described well by Langmuir isotherm model with maximum biosorption capacities of 13.39 and 4.86 mg/g of arsenate and arsenite on spent grains, respectively. The initial removal was rapid, and equilibrium was established in less than 180 min. Good correlation coefficients were obtained for the pseudo-second-order kinetic model. In the binary metal solutions, the finite sites on the surface of spent grains showed a greater preference for As(V) ions. The enthalpy of biosorption was exothermic and the increase in As(III) removal was larger than that of As(V) over the same rise in temperature. In this study, spent grains proved to be suitable for removal of As(V) and As(III) from the effluent of metallurgical industry.