Effect of pH on Nanofiltration Fouling of Combined Carbonate Species and Natural Organic Matter
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Abstract
Nanofiltration is an efficient system for removing inorganic and organic compounds depending on solution types and solution chemistry. Therefore, this research studied the effect of pH during nanofiltration (NF) fouling of combined carbonate species and natural organic matter using crossflow nanofiltration (HL4040FN, GE water and process technology). Solution conditions were controlled with ionic strength of 0.01 M, NOM concentration of 10 mg/L and various solution pHs of pH 4.3 (H2CO3 equiv. pt.), 6.3 (pKa1), 8.3 (HCO3− equiv. pt.), 10.3 (pKa2) and 11.3 (CO32− equiv.pt). Different solution pHs could change carbonate species from carbonic acid (H2CO3) at pH 4.3, to monovalent bicarbonate (HCO3-) at pH 6.3, then form divalent carbonate (CO32- ) at high solution pHs of 11.3. The increase in solution pHs from 4.3, 6.3, 8.3, 10.3 and 11.3 resulted in decreases of permeate fluxes of 29.31, 27.38, 22.23, 21.90 and 19.71 L/m2/h (LMH), respectively. The removal efficiencies of NOM for the range of solution pHs were approximately 96-98%. The sodium carbonate rejections measured by conductivity increased with increasing solution pHs, possibly caused by repulsion charge effects between negatively charged NF membrane and negatively charged carbonate species. Carbonate salt can form bicarbonate ions with increased solution pHs from 4.3 to 10.3, while carbonate species can dominate with high solution pHs of 11.3.
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