Synergistic fouling mitigation of co-contaminants of ultrafine microplastics and organics in seawater pretreatment using ferrous iron/peracetic acid

Abstract

The increasing occurrence of ultrafine microplastics (MPs, 3 μm) and algal organic matter (AOM), such as humic acid (HA), in seawater poses a growing challenge to conventional desalination pretreatment, as their interactions can lead to MP–HA co-contaminants that significantly reduce the effectiveness of traditional coagulants (e.g., FeCl₃). Furthermore, ultrafiltration (UF) of MP–HA co-contaminants showed a synergistic increase in the modified fouling index (MFI), exceeding the sum of individual effects. This intensified fouling was due to MPs serving as scaffolds for HA, which was immobilized through non-covalent interactions such as hydrogen bonding, π–π stacking, and electrostatic attraction, resulting in reduced porosity and a denser cake layer. To address this issue, this study proposed an advanced coagulation using ferrous iron/peracetic acid (Fe²⁺/PAA) for the treatment of MP-HA (20 mg/L HA and 10 mg/L MPs) in seawater (both synthetic seawater and real seawater). At an optimal dosage of 0.2 mM Fe²⁺/0.1 mM PAA, the Fe²⁺/PAA system demonstrated superior coagulation performance compared to 0.2 mM Fe³⁺, achieving more effective charge neutralization (Fe³⁺: -14.7 mV; Fe²⁺/PAA: -5.8 mV) and forming larger, denser flocs (Fe³⁺: 74.2 μm; Fe²⁺/PAA: 104 μm), resulting in significantly improved coagulation performance (e.g., turbidity removal of 83.9 %; 19.0 % for Fe³⁺). Regarding the unique mechanisms of Fe²⁺/PAA in seawater, we found that a high Cl⁻ concentration of 25 g/L (428 mM) markedly influenced the dominant reactive species by scavenging •OH radicals, thereby increasing the proportion of Fe^ⅣO²⁺ from 13.7 % to 40.3 %, highlighting the critical role of Fe^ⅣO²⁺ in enhancing coagulation performance. Furthermore, compared to Fe³⁺, Fe²⁺/PAA primarily mitigated membrane fouling caused by cake layer formed by MP-HA, resulting in a 1.8-fold improvement in membrane flux by the end of filtration. Crucially, trials in real natural seawater demonstrated that Fe²⁺/PAA preserved its enhanced coagulation and fouling‐control effectiveness under authentic marine conditions. Collectively, this study reveals the synergistic fouling effects of MP–HA in seawater desalination and offers theoretical and technical guidance for applying Fe²⁺/PAA to address this emerging challenge.