Carboxymethyl Inulin Biopolymers: A Green Alternative for Phosphonate Calcium Carbonate Growth Inhibitors

Abstract

There is a growing demand for environmentally friendly alternatives for phosphonate-based crystal growth inhibitors (or antiscalants) used in reverse osmosis desalination, which are nontoxic, biodegradable, and free of phosphorus. The effectiveness of such an alternative, carboxymethyl inulin (CMI) biopolymers, in inhibiting calcium carbonate crystallization (CaCO3) was compared to that of nitrilotris(methylenephosphonic acid) (NTMP) and 1-hydroxyethane-1,1-diphosphonate (HEDP). Their inhibiting effect on the seeded growth of calcite was investigated using the constant composition method at different solution compositions. The values of inhibited growth rates could be related well to a Temkin adsorption isotherm. Compared to HEDP and NTMP, CMI-20 and CMI-25 biopolymers exhibited an equally stronger inhibitory effect on the seeded growth of calcite, despite their different degrees of carboxylation. The ability of the inhibitor molecules to mitigate the spontaneous precipitation of CaCO3 in a synthetic membrane concentrate was investigated as well. The results show that the ability of the biopolymers to mitigate precipitation is controlled by their degree of carboxylation. About 2.7 times more NTMP than CMI-25 is needed to achieve a similar precipitation inhibition. At high concentrations, both NTMP and CMI-25 induced twinning in the precipitating calcite crystals. This demonstrates that CMI biopolymers are effective CaCO3 growth inhibitors, indicating their potential as a green alternative for phosphonates.