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2. Materials and methods 2.1. Chemicals and reagents Table 1. Physicochemical properties of three different CNTs.CNTs typeOD (nm)Length (μm)PurityAshSSA (m2/g)Metal impurities constants (μg/g)CdCoCuNiPbZnMWCNTs36&plusmn;212&plusmn;2&gt;973&plusmn;0.5275&plusmn;120.017105.443N.D.11.4321.8750.56OH-MWCNTs36&plusmn;24&plusmn;1.5&gt;972&plusmn;0.3103&plusmn;4N.D.0.1011.754.7832.9890.101COOH-MWCNTs36&plusmn;24&plusmn;1.5&gt;972&plusmn;0.3115&plusmn;5N.D.0.186N.D.0.156N.D.N.D.N.D.: not detected;OD: outer diameter;SSA: specific surface area.Full-size tableTable optionsView in workspaceDownload as CSV Fig. 1.&#xA0;FE-SEM (A&ndash;C) image of CNTs (A: MWCNTs; B: OH-MWCNTs; C: COOH-MWCNTs) deposited on copper grid with carbon film and TEM (D&ndash;F) image of [http://www.apexbt.com/mg-132.html MG-132] same CNTs (D MWCNTs; E OH-MWCNTs; F: COOH-WCNTs) with bar scale 100&#xA0;nm.Figure optionsDownload full-size imageDownload as PowerPoint slide Aqueous suspensions of different MWCNTs were prepared based on the methods of Alpatova et al. (2010), 5&#xA0;mg CNTs were added to 50&#xA0;mL of ultrapure water in a [http://www.thefreedictionary.com/semilunar+valve semilunar valve] 100&#xA0;mL glass beaker with sonication for 12&#xA0;h (80&#xA0;W, 32&#xA0;kHz) in an ultrasonic cell disruptor (Nanjing Emmanuel Instrument Equipment Co., Ltd. Nanjing, China) until the absorbance at 400&#xA0;nm was unchanged. The absorbance was measured by UV&ndash;vis spectrometry (TU-1800, Persee, China). Before each use, the suspensions of CNTs were diluted to 0.5&#xA0;mg/L and a 30&#xA0;min sonication was carried out. The metal stock solution was prepared by dissolving a defined amount of ZnSO4&middot;7H2O into 0.1&#xA0;L of ultrapure water.