Stability of oxidized single-walled carbon nanotubes in the presence of simple electrolytes and humic acid Minghua Li, C.P. Huang * Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA ARTICLE INFO Article history: Received 21 May 2010 Accepted 13 August 2010 Available online 18 August 2010 ABSTRACT The stability of nanoparticles is closely related to the fate, transport, and bio-toxicity of nanomaterials in the aquatic environment. Surface treatments and the presence of natural organic matter (NOM) have significant impacts on the aggregation behavior of nanoparti- cles. The aggregation kinetics of oxidized single-walled carbon nanotubes (O-SWCNT) was investigated in the presence of humic acid (HA). Results indicated that O-SWCNT was relatively stable in water with a critical flocculation concentration (CFC) of 0.16, 4.2 · 10 3 and 5.4 · 10 5 M for NaCl, CaCl 2 and AlCl 3 , respectively. The aggregation of O- SWCNT was not sensitive to pH over the range of 3–8. The presence of HA can enhanced the stability of O-SWCNT dependent on the type of electrolyte present. The CFC of NaCl increased from 0.16 in the absence of HA to 0.19, 0.23, and 0.25 M at HA concentration of 1, 5, and 20ppm, respectively. However, the effect of HA on the aggregation of O-SWCNT was negligible in the presence of CaCl 2 and AlCl 3 . The findings highlighted the importance of surface treatments, water chemistry, and the presence of NOM on the aggregation behavior of nanomaterials, which should be considered when dealing with the fate and environmental impacts of engineered nanomaterials. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The colloidal stability of nanoparticles in the aquatic environ- ment is critical to determining the industrial applications as well as the environmental impacts of these materials. Many applications of nanoparticles, such as nano-composite mate- rials and drug delivery, require stable suspensions to utilize the unique properties of these nanomaterials. In addition, the capability to remain in suspension controls the fate and transport of nanoparticles in natural aquatic systems. Studies indicated that most nanoparticles remain insoluble and read- ily aggregate in the aquatic environment, which may limit their mobility and transport. The ecological impacts of nano- particles are also closely related to colloidal stability, which in turn determines their bioavailability [1,2]. Among the various engineered nanomaterials, ENP, carbon nanomaterials, including fullerene and carbon nanotube (CNT) are being ac- tively pursued due to their unique and extraordinary mechanical, thermal, and electrical properties that render them attractive for a wide range of novel applications in var- ious industries. The surface physical and chemical properties of nanopar- ticles have significant bearing on the aggregation behavior of nanomaterials. Numerous studies have demonstrated that oxidation reactions between CNT and oxidation agents such as strong acids and ozone can change the particle size as well as surface properties of CNT, which in turn will affect the col- loidal stability in the aquatic environment [3–6]. Xing et al. [7] treated multi-walled carbon nanotube (MWCNT) with ultra- sound-assisted oxidation and reported the formation of polar oxygen groups on the tube surface. The treated MWCNT showed delay in aggregation and settlement. Li et al. [8] 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.08.032 * Corresponding author: Tel.: +1 302 831 8428; fax: +1 302 831 3640. E-mail address: huang@ce.udel.edu (C.P. Huang). CARBON 48 (2010) 4527 – 4534 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon