Environmental Risk Assessment and Remediation

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Modeling of Nucleation Rate of Supersaturated Calcium Sulfate Solutions.

Reverse osmosis treatment of the Central Arizona Project (CAP) water provokes scaling because of its high content in salt. Experiments simulating various solutions of calcium sulfate (CaSO4) made by mixing of sodium sulfate (Na2SO4) and calcium chloride (CaCl2) reinforced this idea. An increase of the degree of supersaturation of the solution from 8.67 to 42.08 decreased the induction time for precipitation of calcium sulfate solutions. The use of Flocon® 135 as antiscalant increased the induction time. The increase was directly related to the concentration of antiscalant. Working with 11.06 degree of supersaturation, a dose 0 ppm of antiscalant has an induction time of 217 minutes. A dose of 0.5 ppm of Flocon 135 inhibits completely the crystal growth in solution. The use of 21.07 degree of supersaturation showed an induction time of 36 minutes for 0 ppm of antiscalant, 78 minutes for 0.25 ppm of antiscalant, 91 minutes for 0.5 ppm of antiscalant, 119 minutes for 0.75 ppm of antiscalant, 139 minutes for 1 ppm of antiscalant, 243 minutes for 1.25 ppm of antiscalant, and crystal growth and nucleation rate are completely inhibited by use from 2 ppm or more of antiscalant. The use of a solution of 42.08 degree of supersaturation gave the following results: 11 minutes to nucleation for 0 ppm of antiscalant, 19 minutes for 0.5 ppm of antiscalant, 24 minutes for 1 ppm of antiscalant, 36 minutes for 1.5 ppm of antiscalant, 84 minutes for 2 ppm of antiscalant, and 123 minutes for 2.5 ppm of antiscalant. The nucleation rate was 2.1 × 109 nuclei/cm3.min for 8.67 degree of supersaturation to 1.26 × 1027 nuclei/cm3.min for 42.06 degree of supersaturation. Logarithmic plots of the induction time (tind ) as function of 1/[ln(S)]2 for calcium sulfate without and with addition of antiscalant for fixed temperature provides a linear trend with slope B as a constant proportional to the surface tension and molar volume. Modeling of the nucleation rate allows predicting the induction time by knowing a unique constant value (C=22.86) associated to each nucleation rate (from 5.05 × 1011 to 1.22 × 1028). Each nucleation rate values are related to a specific degree of supersaturation (from 8.67 to 42.06) by this equation: 

Author(s): David Jonathas