Experimental2.1. Reagents and apparatusCommercial multiwall CNTs were obtained from Sigma–Aldrich,USA, diameter 110–170 nm, longitude 5–9 m, 90%. Membrane fil-ter papers were Millipore of 0.45 mm pore size.Stock standard solutions of As(V) with a concentration of1000 mg L-1were obtained by dissolving appropriate amounts ofNa2HAsO4(Merck, Darmstadt, Germany). Working standard solu-tions were obtained by appropriate dilution of the stock standardsolutions.