Mechanisms of Alleviating Gypsum Scaling by Antiscalants during Membrane Desalination: Implications on Agricultural Drainage Water Reuse

Agricultural drainage water has great potential as a freshwater supply via reverse osmosis (RO) membrane desalination. However, high calcium and sulfate concentrations in the drainage water lead to gypsum (CaSO_4(s)) scaling during the RO process. This study investigated the effects of three antiscalants, i.e., two phosphonate-based (DTPMP and NTMP) and one polymer-based (PAA), as well as pH and natural organic matter (NOM) on alleviating gypsum scaling during RO desalination of drainage water, and illustrated the gypsum inhibition mechanism of three antiscalants. Results showed that 1 μM of DTPMP was sufficient to prevent gypsum scaling within 24 h of RO desalination, while both NTMP and PAA required 5 μM of dosage. At acidic pH 3, the permeate flux with 5 μM of DTPMP remained relatively stable, whereas the flux with NTMP and PAA decreased by 35 and 80%, respectively. Furthermore, the presence of NOM did not significantly affect the antiscalant inhibitive capacity. The gypsum inhibition mechanism of DTPMP and NTMP was primarily contributed by negative charge repulsion, with higher pH increasing the total charge of antiscalant aqueous species, thereby strengthening the repulsive forces among calcium, sulfate, and gypsum nuclei. In contrast, PAA’s gypsum inhibition mechanism involved both negative charge repulsion and crystal lattice distortion, which distorted gypsum crystals into irregular shapes and smaller sizes, preventing the formation of large-size gypsum precipitates under neutral and alkaline conditions, but deteriorating membrane scaling under acidic conditions. Ultimately, an ideal antiscalant for preventing gypsum scaling during RO desalination of agricultural drainage water would preserve higher negative charges without changing the precipitate morphology.