Polysaccharides derived from Enteromorpha prolifera for the removal of silver nanoparticle-humic acid contaminants by a coagulation-ultrafiltration process

Silver nanoparticles (AgNPs) pose serious health risks to humans as the adsorption between AgNPs and humic acid (HA) makes it difficult to remove them from surface water. To solve this problem, polysaccharides extracted from a marine alga, Enteromorpha prolifera (denoted as Ep), were used to eliminate the AgNP–HA composite contaminant via a coagulation-ultrafiltration (C-UF) process. The structure of Ep, AgNP–HA removal mechanism and membrane fouling were analyzed. The results indicated that the backbone of Ep was composed of (1 → 4)-linked L-rhamnopyranose, (1 → 4)-linked D-xylose and (1 → 4)-linked glucuronic acid. With the charge neutralization of PAC hydrolysates and the bridging-sweep role of Ep, AgNPs could be removed completely by the C-UF process. The coagulation performance and membrane flux were the highest when the PAC and Ep dosages were 2.0 mg L−1 and 0.3 mg L−1, respectively. In addition, when Ep was applied in the C-UF process, the flocs exhibited larger sizes, faster growth rates, better recovery ability and looser structures, which resulted in lower cake resistance and less pore blocking of the UF membrane. Consequently, the membrane flux could be improved by about 25–30% due to the addition of Ep.