Effect of operating variables on functions of sodium alginate granules based on drinking water treatment residues

Drinking water treatment residues (DWTR) have been regarded as a promising adsorbent for phosphorus control. The granulation of DWTR is a crucial step for its application in the filtration system. Traditional granulation processes could not achieve the balance between stable structures and adsorption capacities. Encapsulation of DWTR powders into alginate beads overcame these drawbacks. However, previous studies ignored the effect of preparation parameters on the properties of DWTR particles. In this study, the effects of sodium alginate (SA) concentrations, alginate types, Fe3+ concentrations, and gelation times on structures and mechanical stabilities of DWTR granules were investigated in depth based on their morphologies and pore structures, and compressive strength. The results showed that SA concentrations influenced the viscosity of DWTR-SA solution, and that granulation of DWTR with low SA concentration resulted in incomplete crosslinking, but granulation of DWTR with high SA concentration could change the shape of final granules. 2% SA was chosen as an optimum concentration for DWTR granules preparation. The ratios of M/G in SA affected the microstructures and mechanical strength of resultant granules. The presence of high G-block content in SA resulted in the formation of a dense crosslinking structure, which increased mechanical strength but decreased specific surface area. The morphologies of DWTR granules did not change as Fe3+ concentrations increased, and the granules with excellent mechanical strength were obtained at FeCl3 concentrations of 2% (w/v). The DWTR spheres could maturate quickly and are stable after 4 h. The resulting granular DWTR exhibited a comparable P adsorption capacity (26.89 mg-P/g) to the original powder, which was better than traditional P adsorption materials and pellet DWTR produced by high-temperature calcination. The results should be helpful for engineers to choose the best granulation parameters for their applications.

Automated summary

This study investigated the effects of sodium alginate (SA) concentrations, alginate types, Fe3+ concentrations, and gelation times on the structures and mechanical stabilities of DWTR granules. The results showed that low SA concentrations resulted in incomplete crosslinking, while high SA concentrations changed the shape of the granules. The presence of high G-block content in SA increased the mechanical strength but decreased the specific surface area of the granules. Fe3+ concentrations did not affect the morphology of the DWTR granules. The resulting granular DWTR exhibited a comparable P adsorption capacity to the original powder, outperforming traditional P adsorption materials and high-temperature calcination produced pellet DWTR.