Abstract
The effect of NaCl at extremely high concentrations from 3.5 to 14 wt. % on the crystallization of CaCO3 was investigated in depth. The static test experiment verified that the Ca2+ retention efficiency (η) of NaCl on CaCO3 scale increased from 31.06% (3.5 wt. %) to 41.56% (14 wt. %). Based on the calculation of supersaturation rations, the high concentration of NaCl could reduce the activity coefficients of [Ca2+] and [CO32−], thus reducing the actual concentration of CaCO3. The CaCO3 deposition rate constants (k) showed that NaCl slowed down the rate of CaCO3 crystallization. The X–ray diffraction (XRD) testing disclosed that the growth of (1 0 4) and (1 1 0) faces from calcite was impeded, while the formation of (1 1 1) face from aragonite was induced by the increasing concentration of NaCl. The inductively coupled plasma optical emission spectrometry (ICP–OES) results indicated that Na+ could be doped into CaCO3, leading to the one–dimensional crystal growth. It was further proved that NaCl heightens the efficiency of the typical phosphate inhibitors (2–phosphonobutane–1,2,4–tricarboxylic acid (PBTCA) and 1–hydroxyethane–1,1–diphosphonic acid (HEDP)) on prohibiting the scale of CaCO3.
Highlights
Seawater is widely used in the reverse osmosis desalination plants [1,2,3,4,5,6], multi-stage-flash desalination process [7], circulating cooling water systems, and other water treatment systems [8].In these processes, seawater is inevitably concentrated by two to four times
The degree of mineral supersaturation could be quantified by the supersaturation ratio (SR), which was the ratio of dissolved CaCO3 ionic activity product (IAP) to the solubility product (Ksp ), as shown in Equation (4) [32]: h i h i γCa Ca2+ γCO3 CO2−
The ionic strength of NaCl solution in this study was 0.72–3.75 mol/kg and, γCa and γCO3 could be calculated by the Pitzer model, which was applicable to ionic strength from 0.01 to 6 mol/kg [33]
Summary
Seawater is widely used in the reverse osmosis desalination plants [1,2,3,4,5,6], multi-stage-flash desalination process [7], circulating cooling water systems, and other water treatment systems [8] In these processes, seawater is inevitably concentrated by two to four times. Sheikholeslami et al discovered that NaCl at high concentrations (3.5–5.2 wt.%) delayed the crystallization of CaSO4 [20,21] It is of great importance if the CaCO3 scaling could be delayed when seawater is concentrated by 2–4 times. Until now, only Visscher et al critically evaluate the CaCO3 solubility data at extreme concentrations of NaCl [22] In addition, no one had studied the scale inhibition effect of characteristic inhibitor in NaCl solution with such high concentrations. Thereby, the η values of HEDP and PBTCA in NaCl solution improved enormously, and they were as high as 83.50% and 92.16%, respectively, under the optimal conditions
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