Influence of chemical speciation on the separation of metal ions from chelating agents by nanofiltration membranes T. Balanyà a , J. Labanda b , J. Llorens b , and J. Sabaté a a Departament d’Enginyeria Agroalimentària i Biotecnologia, Universitat Politècnica de Catalunya, Castelldefels, Spain; b Departament 5 d’Enginyeria Química, Universitat de Barcelona, Barcelona, Spain ABSTRACT The simultaneous separation of various metal ions (nickel, copper, calcium, and iron) from chelating agents (EDTA and citric acid in water streams using Nanofiltration membranes is analyzed. Assuming that multiply-charged species are highly rejected, chemical speciation com- 10 putations reproduce the observed patterns of metal and ligand rejection at different pH values and concentrations. The separation of metal ions from citric acid is achieved in acidic conditions, where multiply-charged free metal ions and neutral or singly charged free chelating species are abundant. Overall, speciation studies help to evaluate the applicability of Nanofiltration for recycling chelating agents used for metal extraction. ARTICLE HISTORY Received 15 November 2017 Accepted 17 July 2018 KEYWORDS Nanofiltration; metal; chelate; speciation 15 Introduction Several techniques that include precipitation, adsorp- tion, ion-exchange, electrochemical treatments, and membrane filtration have been proposed to separate or concentrate heavy metals from contaminated waters 20 produced by several industries. [1–5] Among them, Nanofiltration is a membrane process that show high retention of multiply-charged ions but their efficiency is influenced by factors such as other ions (e.g., sodium, calcium, iron, sulfate, nitrate, and chloride), pH, and 25 ionic strength. [6,7] Further, some water streams contain chelating agents because they are used to prevent the formation of insoluble salts incrustations or to extract metal ions from liquid streams or from solids such as contaminated soil, sludge, and sediment. 30 Nanofiltration of solutions containing metal ions such as Cu, Ni, Mn, Sr, Co, rare earth elements, and radio- nuclides in the presence of various complexing agents (e.g., EDTA, NTA, DTPA, citric acid, NH 3 , CN – , and phosphates) using membranes with different molecular 35 weight cut-off (MWCO) values (i.e., 150–1000 Da) has been studied by several authors. [7–16] Most of them reported that the addition of a chelating agent increases metal rejection because the chelate species is larger than the free metal ions. Consequently, metal rejection rises 40 as the pH increases due to complexation between the metals and the chelating agent. Clearly, the chelating agents must be separated from the metal ions for their recovery. This process involves fragmentation of the complex, which can be achieved at low pH. 45 Additionally, Nanofiltration membranes must facilitate very distinct rejection for metal ions and ligands. Most of the aforementioned studies investigated only metal ion rejection, and little attention was given to the rejec- tion of chelating agents. 50 In a previous work, [17] the present authors examined the separation of copper ions from citric acid in syn- thetic solutions using several Nanofiltration membranes of different pore size and nature. Membranes with nar- row pore sizes (e.g., SelROⓇ MPF-34, Koch) simulta- 55 neously rejected both the copper ions and citrate. However, both species leaked through a ceramic mem- brane with larger pores (i.e., CERAM INSIDEⓇ, Tami). Using a SelROⓇ; MPF-36 (Koch) at pH≈ 2, Cu 2+ and citric acid experienced high and low rejection, respec- 60 tively, even though they have similar radius: 0.365 [18] and 0.344 nm, [19] respectively. According to the manufacturer, [20] this membrane has a MWCO of 1000 Da, an estimated mean pore radius of 0.86 nm, [21] an isoelectric point (IEP) of ~ 5–6, [21] and is positively 65 charged at pH≈ 2. Our previous results were understood in terms of the more prominent role played by the electrostatic and dielectric interactions, rather than steric hindrance, between this membrane and the solutes. It would be interesting to extend this study to solutions 70 that contain several different metals and test other che- lating agents. CONTACT J. Sabaté jose.sabate@upc.edu Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsst. SEPARATION SCIENCE AND TECHNOLOGY 2018, VOL. 00, NO. 00, 1–10 https://doi.org/10.1080/01496395.2018.1502781 © 2018 Council of Scientific and Industrial Research, Govt. of India