Article pubs.acs.org/crystal
New Findings about Nucleation and Crystal Growth of Reverse Osmosis Desalination Scales with and without Inhibitor E. A. Abdel-Aal,*,† H. M. Abdel-Ghafar,† and B. E. El Anadouli‡ †
Central Metallurgical Research and Development Institute (CMRDI), P.O. Box: 87 Cairo, Egypt Cairo University, Faculty of Science, Cairo, Egypt
Downloaded by STOCKHOLM UNIV on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 14, 2015 | doi: 10.1021/acs.cgd.5b01091
‡
ABSTRACT: Nucleation and crystal growth of CaSO4·2H2O (gypsum) was studied under simulated conditions of scale formation using a reverse osmosis desalination technique. Calcium chloride and sodium sulfate were mixed with concentrated sodium chloride solution (40 g/L), and the reaction mixture turbidity was measured at different time intervals to determine the induction time of precipitated gypsum. Induction time was measured under different high supersaturation ratios ranging from 3.92 to 6.71. The induction time decreases exponentially with increasing the supersaturation. Using crystallization equations that relate induction times with supersaturation ratios, the free energy barrier and critical nuclei radius were calculated with and without addition of a scale inhibitor (trisodium phosphate, TSP). Rates of nucleation at a 4.47 supersaturation ratio are 0.4 × 1028 and 0.5 × 1028 nuclei/cm3·s with and without TSP addition, respectively.
1. INTRODUCTION Formation of scales on reverse osmosis (RO) membranes is a major problem that affects performance efficiency. The major compound that forms scale on RO membranes is calcium sulfate dihydrate (gypsum), followed by calcium carbonate. In the reverse osmosis process, it has a permeate and reject streams. In the reject stream, feedwater total salts contents are concentrated. If supersaturation of calcium sulfate dihydrate occurs and the limit of its solubility is exceeded, then the scales will be formed on the reverse osmosis membrane. Because of scale formation, fouling of the reverse osmosis membrane occurs.1−4 A limited number papers about the basic science of scale formation are available. However, many literatures that describe the methods for predicting scale formation in water treatment systems are available.1−4 In addition, many literatures discuss primary and secondary nucleation and growth of crystals of CaSO4·2H2O using pure chemicals at room temperature and in aqueous solutions.4−8 On the other hand, some additives may act as inhibitors for scale formation by one of several possible mechanisms.9 These mechanisms are reviewed elsewhere.3 Moreover, nucleation and crystal growth of CaSO4·2H2O using pure chemicals and under simulated conditions of production of phosphoric acid industrially with and without additives such as crystal modifiers, promoters, and inhibitors are also extensively studied.10−20 In addition, nucleation and crystal growth of CaC2O4·H2O using pure chemicals with and without additives are also studied.21−23 Amjad and Hooley24 studied the effect of different scale inhibitors on crystallization of CaSO4·2H2O at 25 °C. It was reported that inhibition of CaSO4·2H2O growth was observed © XXXX American Chemical Society
in supersaturated solutions with low contents of polyelectrolytes.4,24 Also, it was reported that the effect of scale inhibitor dose on the duration of the induction time is very significant.1−4,24 This work aims to study the nucleation fundamentals of calcium sulfate dihydrate in a synthetic saline solution of NaCl with high concentration simulating a reverse osmosis desalination system with and without addition of commercial scale inhibitor (trisodium phosphate, TSP).
2. EXPERIMENTAL SECTION 2.1. Materials. Pure chemicals with analytical grade of NaCl (ADWIC, Egypt), CaCl2 (Merck, Germany), and Na2SO4 (SigmaAldrich) were used in this study. Commercial trisodium phosphate (TSP) salt, Na3PO4·12H2O (SUNDIA, China), was used as a scale inhibitor to prepare different concentrations (5, 10, and 20 mg/L). Chemical analysis of TSP is shown in Table 1.
Table 1. Chemical Analysis for Na3PO4·12H2O Scale Inhibitor (TSP) constituent
content
P2O5 Na2O Cl− SO42− Fe Pb As
18.1% 16.5% 0.3% 0.2% 4.01 mg/L 1.52 mg/L
