## 1 Introduction  
During the past decade, pressure-driven membrane processes have been gaining more attention as a sustainable and efficient technology for water treatment to address the global challenges related to water scarcity and the pollution of aquatic environments [1]. In this scenario, ultrafiltration (UF) is a well-established technology for the purification of contaminated water bodies enabling an efficient and cost-saving low-pressure filtration. In fact, compactness, modularity, easy automation and high removal rate of turbidity, organic matters (such as humic substances), and viruses are the main features of UF process highly desired in a wide range of industrial applications [2, 3, 4]. The mechanism of separation in UF is basically governed by the pore size exclusion impaired by the trade-off between selectivity and permeability; thus, UF membranes (pore size from 20 nm to 0.1 \(\upmu\)m) are ineffective in rejecting small molecules and ions, such as heavy metals [5, 6]. Nevertheless, functional water-soluble polymers with reasonably high molecular weights are exploited as heavy-metal-binding macro-ligands, resulting in complexes easily rejected by UF membranes [7]. The drawbacks of this approach are: (i) the disposal of the by-products of water treatment consisting in heavy metals enriched slurries [8]; and (ii) membrane fouling, negatively impacting on the cost of the process (energy, operation, and maintenance), the lifespan of the membrane, and the performance of the filtration unit [9]. In fact, fouling is basically caused by adsorption, precipitation, or sedimentation of large metal-polymer complexes on the membrane surface generating a cake layer and/or blocking the pores, thus compromising the permeability of the membrane [10]. Definitively, future perspectives should be aligned towards the development of the next generation of antifouling UF membranes [11; 12]. Different strategies have been adopted for fouling mitigation, such as the chemical or physical surface modification of UF membranes (i.e., layer by layer, dip-coating, and plasma treatment) at the expense of a decline in permeability due to pore and porosity constriction [13; 14; 15; 16; 17; 18]. Moreover, concerns have arisen regarding the feasibility of membrane modification strategies on a large scale due to a lack of uniformity, reproducibility, stability, process control, and reasonable cost [19]. Polymer blending emerged as a simple, scalable, and economically viable method to produce a single-step high-performance membrane with antifouling behavior [20]. For instance, the blending of poly (vinylidene fluoride) (PVDF) with a quaternary ammonium compound assembled on carbon material was considered an effective strategy to fabricate anti-biofouling membranes for water and wastewater treatment [21]. Polystyrene (PS) with poly(methyl methacrylate) (PMMA) showed excellent results in controlling and hindering membrane biofouling [22], and poly (methyl methacrylate-co-2-hydroxyethyl methacrylate) blended with PVDF was studied for selective adsorption of Ru(III) [23]. Cellulose acetate (CA) has been extensively studied in UF membranes preparation because of its processability, biocompatibility, hydrophilicity, and low price, but its practical exploitation has been hampered by the absence of reactive functional groups on the polymer backbones responsible for selective interactions. On the other hand, poly (acrylic acid) (PAA) has been widely recognized as an ideal complexing agent for heavy metals thanks to a large number of carboxyl groups allowing one to efficiently adsorb or chelate heavy metal ions [24; 25]. Additionally, few studies have been focused on the evaluation of the PAA in membrane processes employed in heavy metal removal [26]. Herein, PAA was blended with CA for the preparation of UF membranes via non-solvent-induced phase separation (NIPS).