Application of Enteromorpha polysaccharides as a new coagulant aid to remove silver nanoparticles: role of dosage sequence and solution pH

Silver nanoparticles (AgNPs) in surface water cause a serious threat to the health of humans and aquatic organisms. However, it is difficult to remove AgNPs completely since they could adsorb onto the surface of humic acid (HA) and meanwhile release Ag(+) into water. In this paper, Enteromorpha polysaccharides (Ep) were applied as a coagulant aid with polyaluminum chloride (PAC) to solve this problem. The influences of Ep dosage, dosing sequence and solution pH on the coagulation efficiency, kinetics and removal mechanism of AgNPs were discussed systematically. Results showed that when Ep was applied, AgNPs could be removed effectively due to charge neutralization of PAC hydrolysate and the bridging-sweeping role of Ep gel network. When Ep was added 30 s after PAC dosing, the coagulation efficiency was about 10–20% higher than that of the reverse order. Under this condition, flocs sizes achieved 450 μm when the solution pH was 6.0, which is much larger than that using Ep–PAC. Additionally, Ep showed an ability to promote the re-aggregation of broken flocs, and AgNP–HA flocs exhibited larger sizes, better shear resistance, higher recovery ability and denser structure at pH 6.0. Factorial analysis results indicated that PAC dosage had the greatest impact on HA and AgNP removal, while Ag(+) removal is more sensitive to Ep dosage.