Sustainable tannin-based coagulants synthesized through Mannich reaction using melamine as an amine source for water treatment applications
Adedayo Bello, Tiina Leiviskä
Abstract
This study explored the potential of melamine, as the nitrogenating source for the cationization of pulverized quebracho and spruce tannins. The influence of modification conditions on the properties of the coagulants was studied by varying the formaldehyde and melamine ratios with different activation times and temperatures. Based on considerations of charge density and shelf life, the most viable modifications were established as coagulants synthesized with a 1:0.52 formaldehyde to melamine molar ratio at 70 °C. At optimal conditions, the charge density of the quebracho and spruce coagulants was 2.22 meq/g and 1.04 meq/g, respectively, and the residual formaldehyde content in the coagulants was low. The developed synthesis of this study demonstrated a clear advantage over previous methods due to the rapid modification step. X-ray photoelectron spectroscopy (XPS) confirmed the emergence of an amine which signifies a successful Mannich reaction in the coagulant. Although optimal modification conditions for the coagulants were established at 70 °C (5-min activation time), the average molecular weight could not be determined for these conditions. Nevertheless, electrospray ionization mass spectrometry (ESI-MS) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry measurements revealed that the tannin-based coagulants obtained at lower temperatures (23 °C and 45 °C) possessed low average molecular weight (approx. 800–900 Da). Furthermore, ESI-MS and MALDI-TOF spectra showed that the Mannich modification resulted in the depolymerisation of the quebracho tannin, leading to a reduction in units of higher mass fractions in the synthesized coagulant, which contrasted with the spruce tannin. Jar test experiments with surface and industrial process waters demonstrated that the tannin coagulants enhanced particle settling effectively.