Removal of heavy metal ions from dilute aqueous solutions by polymer–surfactant aggregates: A novel effluent treatment process Li-Cheng Shen a , Xuan-Tung Nguyen b , Nicholas P. Hankins a,⇑ a Laboratory of Sustainable Water Engineering, Department of Engineering Science, Parks Road, The University of Oxford, OX1 3PJ, UK b Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore 117585 article info Article history: Received 23 April 2015 Received in revised form 24 July 2015 Accepted 28 July 2015 Available online 29 July 2015 Keywords: Polymer–surfactant system Complexation and flocculation Aqueous effluent treatment Dilute heavy metal ion removal Settling Coarse filtration abstract A novel treatment process for effluent streams that employs polymer–surfactant complexation and floc- culation has been developed and applied to remove dilute heavy metal ions, such as Cr(III), Zn(II) and Cd(II), from aqueous solutions. This process uses cationic polymers, such as poly diallyldimethylammo- nium chloride (PolyDADMAC) or poly ethylenimine (PEI), as a back-bone structure onto which anionic surfactants, such as sodium dodecyl sulphate (SDS), can form micelle-like aggregates. The resulting struc- tures, called polymer–surfactant aggregates (PSAs), have the ability to remove heavy metal ions from solution, and to form larger flocculated aggregates through a process of intermolecular association. The flocculated aggregates can then be separated from the effluent stream through a settling or coarse filtra- tion step. In the work presented here, it is shown that 99% of 11.2 ppm Cd(II) was removed under opti- mum dosage using 40 ppm PEI and 0.5 mM SDS. Surface tension measurement results indicate that the removal of heavy metal ions occurs by the PSAs when the surfactant concentration is well below the crit- ical micellar concentration. The results show that this process is effective for heavy metal ion removal at a pH range from 6 to 9, and is also effective in the presence of organic contaminants (acetone, ethanol and phenol) and in the presence of NaCl. Such a PSA complexation and flocculation process thus has potential application for the efficient removal of dilute heavy metal ions during process effluent water treatment. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Heavy metals, such as zinc, cadmium, and chromium, are included on the EPA list of priority pollutants [1]. These ions tend to accumulate in organisms, causing numerous diseases and disor- ders [2]. In wastewater treatment, treatment processes for toxic metal-contaminated water have received much interest in recent years in order to comply with stringent regulations, which are caused by direct contamination in water and indirect the subsoil mobility of ions [3,4]. Due to the requirement of discharge concen- trations for heavy metal ions to be at ppb levels, dilute heavy metal ion removal processes, such as chemical precipitation [5,6], ion exchange [7], adsorption [8], biosorption [9] and membrane filtra- tion [10–12], have been widely researched. Specifically, the uses of surfactant and polymer systems have each been individually inves- tigated for their abilities to remove heavy metal ions from aqueous solutions with the aid of ultrafiltration [13,14]. Meanwhile, the association between polymers and surfactants has been extensively studied, because this association plays a major role in many applications, such as drug delivery, the forma- tion of detergents and hair care products, and mineral recovery [15,16]. In weak systems, the non-ionic polymer and ionic surfac- tant mixtures lead to two break points at surfactant concentrations T1 and T3 in the surface tension plot [17]. T1 is the critical aggre- gation concentration (CAC), which is the starting point of the for- mation of micelle-like aggregates onto the polymer chain. T3 is the critical micellar concentration (CMC), which is when the sur- factant monomers start to form spherical or cylindrical micelles in the bulk solution. In strong systems, oppositely charged polymer and surfactant mixtures result in more complex surface tension behaviours. Many researchers have made significant contributions to the understanding of the interactions of polyelectrolyte and ionic surfactant at the air/water interface by neutron reflectivity and surface tension [18–20]. A classic example is the surface ten- sion changes in SDS/polyDADMAC mixtures [21,22]. With increas- ing SDS concentrations at low levels, a sharp decrease of surface tension was measured at various polymer concentrations. At low SDS concentrations until T1, the formation of SDS/polyDADMAC complexes at the interface enhances the air/water interface adsorption of SDS (Fig. 1). After T1 the interface is almost saturated by the complexes, SDS forms micelle-like aggregates on the bulk http://dx.doi.org/10.1016/j.seppur.2015.07.065 1383-5866/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: nick.hankins@eng.ox.ac.uk (N.P. Hankins). Separation and Purification Technology 152 (2015) 101–107 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur