Gypsum scaling on anion exchange membranes during Donnan exchange Meital Asraf-Snir n , Jack Gilron, Yoram Oren Department for Desalination and Water Treatment, Zuckerberg Institute for Water Research, Unit of Environmental Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel article info Article history: Received 10 July 2013 Received in revised form 12 December 2013 Accepted 24 December 2013 Available online 3 January 2014 Keywords: Ion exchange membranes Scaling CaSO 4 Donnan exchange Electrodialysis abstract The formation of gypsum (CaSO 4 Á 2H 2 O) scale in ion-exchange membranes was investigated under a Donnan exchange regime. Two types of anion exchange membranes (AEM), homogeneous AMV and heterogeneous MA-40, were used for the study. Counter-ion flux, scaling-cation concentration in the fouled membrane and membrane potential served as indicators for membrane scaling. Locations of scaling domains were dependent on the membrane structure, and lead to different effects on the membrane performance. Scaling in the homogeneous membrane was mainly characterized by surface deposits, resulting in a moderate decrease of the counter-ion flux. On the other hand, the heterogeneous membrane showed a high degree of internal precipitation resulting in a complete clogging of the membrane, thus leading to earlier and more severe decline in counter-ion flux. Significant amounts of calcium were found within the fouled MA-40, supporting the other indications of internal scaling. The potential difference across the homogeneous AMV did not change significantly after operating under scaling conditions. However, for the MA-40, an increase of the membrane potential was observed, suggesting that CaSO 4 scale blocks defects and imperfections in the heterogeneous matrix. Scaling experiments were conducted in a specially designed Donnan cell with optical apparatus for in-situ observation of scales. This allowed real-time observation of surface scaling on the ion exchange membranes. Gypsum growth centers were observed on the homogeneous AMV significantly earlier than flux decline appeared. This finding shows that it is feasible to detect onset of scaling on homogeneous anion-exchange membranes before the appearance of significant effects on membrane performance. & 2014 Published by Elsevier B.V. 1. Introduction Water reuse and desalination of saline water are well-known and important strategies to reduce the pressure on water resources, particularly in areas that have either no or limited surface or groundwater. In the past three decades, reverse osmosis (RO) has become the most widely used technology for desalination and wastewater treatment for water reuse, yet it is limited by the disposal of RO concentrates [1]. Conventionally, the RO concentrate is discharged into large water bodies such as seas, or treated by evaporation. However, both methods do not comply with the stringent environmental protection rules in the developed countries. In addition to the environmental issues, the cost of brine disposal and the use of conventional energy for volume reduction [2] are factors that must be taken into account. Estimations show that the cost of brine disposal to the sea ranges from 5% to 33% of the total desalination costs [3], while the disposal costs of inland RO desalination plants are higher than that for plants located near the sea [4]. Furthermore, in inland brackish water (BW) desalination plants located more than 80 km from the sea coast, the disposal of concentrates to the sea is not economically feasible, and thus the zero liquid discharge (ZLD) approach becomes mandatory. The idea behind the ZLD approach is to further desalinate the concentrate and bring it into dryness or semi-dryness in order to achieve the maximum possible wastes volume reduction. More- over, the removed water can be recycled to improve the recovery ratio of a BW-RO unit to more than 95% [5]. Perez-Gonzalez et al. reviewed the most recent treatment technologies of RO concen- trates, developed to overcome the environmental problems associated with direct discharge [6]. Recent studies have focused Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.memsci.2013.12.065 Abbreviations: AEM, anion exchange membrane; BW, brackish water; CEM, cation exchange membrane; DSC, differential scanning calorimetry; ED, electrodialysis; EDR, electrodialysis reversal; EDX, energy dispersive X-ray spectroscopy; IEM, ion exchange membrane; RED, reverse electrodialysis; RO, reverse osmosis; SEM, scanning electron microscope; WWPT, wastewater treatment plant; ZLD, zero liquid discharge n Corresponding author. Tel.: þ972 8 6563519; fax: þ972 8 6563503. E-mail addresses: asserafm@post.bgu.ac.il (M. Asraf-Snir), jgilron@bgu.ac.il (J. Gilron), yoramo@bgu.ac.il (Y. Oren). Journal of Membrane Science 455 (2014) 384–391