Aggregation and disaggregation of ZnO nanoparticles: Influence of pH and adsorption of Suwannee River humic acid Fatehah Mohd Omar a , Hamidi Abdul Aziz b , Serge Stoll a, ⁎ a F.-A. Forel Institute, University of Geneva, 10 route de Suisse, 1209 Versoix, Switzerland b School of Civil Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia HIGHLIGHTS • ZnO nanoparticle aggregation and dis- aggregation behavior is pH dependent. • SRHA environmental concentrations in- duce disaggregation of ZnO NPs. • Electrostatic repulsions and steric stabi- lization control ZnO disaggregation. GRAPHICAL ABSTRACT ZnO Nanoparticles and Aggregates SRHA ? Aggregation Disaggregation pH, [SRHA] abstract article info Article history: Received 12 July 2013 Received in revised form 14 August 2013 Accepted 14 August 2013 Available online 9 September 2013 Editor: Damia Barcelo Keywords: ZnO nanoparticles Suwannee River humic acid Zeta potential Aggregation Disaggregation The surface charge and average size of manufactured ZnO nanoparticles (NPs) were studied as a function of pH to understand the aggregation behavior and importance of the electrostatic interactions in solution. The interac- tions between ZnO and Suwannee River humic acid (SRHA) were then investigated under a range of environ- mentally relevant conditions with the ZnO nanoparticles pH PZC as the point of reference. The anionic charges carried by aquatic humic substances were found to play a major role in the aggregation and disaggregation of ZnO nanoparticles. At low concentrations of SRHA (b 0.05 mg/L) and below the pH PZC , anionic SRHA was rapidly adsorbed onto the positively charged ZnO NPs hence promoting aggregation. With similar SHRA concentrations, at pH PZC , SRHA was able to control the suspension behavior of the ZnO and promote partial disaggregation in small volumes. This was more distinguishable when the pH was greater than pH PZC as SRHA formed a surface coating on the ZnO nanoparticles and enhanced stability via electrostatic and steric interactions. In most cases, the NP coating by SRHA induced disaggregation behavior in the ZnO nanoparticles and decreased the aggregate size in parallel to increasing SRHA concentrations. Results also suggest that environmental aquatic concentration ranges of humic acids largely modify the stability of aggregated or dispersed ZnO nanoparticles. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide nanoparticles (NPs) have emerged as one of the most wide- ly used nanomaterials in commercial products with applications such as in catalysis, dye-sensitized solar cells, sensors, sunscreens, cosmetics, Science of the Total Environment 468–469 (2014) 195–201 ⁎ Corresponding author. Tel.: +41 22 379 0333; fax: +41 22 379 03 29. E-mail addresses: mohdoma9@etu.unige.ch (F. Mohd Omar), cehamidi@usm.eng.my (H. Abdul Aziz), serge.stoll@unige.ch (S. Stoll). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.08.044 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv