Millions of people worldwide are exposed to chronic levels of arsenic poisoning due to drinking water with elevated concentrations of arsenic. To decrease these concentrations, various metal based compounds have been explored as arsenic adsorbents. We synthesized CuO nanoparticles and evaluated them as an adsorbent to remove As(III) and As(V) from groundwater. The CuO nanoparticles had a surface area of 85 m(2)/g and were 12-18 nm in diameter. Adsorption occurred within minutes and CuO nanoparticles effectively removed As(III) and As(V) between pH 6 and 10. The maximum adsorption capacity was 26.9 mg/g for As(III) and 22.6 mg/g for As(V). The presence of sulfate and silicate in water did not inhibit adsorption of As(V) but only slightly inhibited adsorption of As(III). High concentrations of phosphate (>0.2 mM) reduced the adsorption of arsenic onto CuO nanoparticles. X-ray photoelectron spectroscopy (XPS) indicated that As(III) was oxidized and adsorbed in the form of As(V) on the surface of CuO. The CuO nanoparticles were also able to remove arsenic (to less than 3 mu g/L) from groundwater samples. These results suggest that CuO nanoparticles are an effective material for arsenic adsorption and may be used to develop a simple and efficient arsenic removal method. (C) 2009 Elsevier Inc. All rights reserved.