Chemical and operational aspects in running the polymer assisted ultrafiltration for separation of copper(II)–citrate complexes from aqueous media

The polymer assisted ultrafiltration (PAUF) process for treatment of wastewaters containing metal ions chelated with citric acid has been studied giving particular attention to the chemical and operational aspects. The polyethylenimine (PEI) polymer bound quantitatively the copper(II)–citrate chelate at pH 5.5 forming the ternary PEI–Cu–citrate complex as discussed considering the stability constants. The ternary complex was separated by UF membranes producing a permeate with very low metal concentration. Best performance was obtained with the Iris 30 membrane (permeate flux 167.4 L h−1 m−2; rejection 98.8%) by operating at 4 × 105 Pa (4 bar). The limiting concentration (maximum retentate concentration for obtaining a copper(II) concentration in the permeate lower than 1.3 mg L−1 corresponding to 2.04 × 10−5 mol L−1), was 250 mg L−1 (3.93 × 10−3 mol L−1) for Iris 30 membrane and 400 mg L−1 (6.29 × 10−3 mol L−1) for Iris 10 membrane by operating with both constant trans-membrane pressure and constant permeate flow rate modes. The polymer regeneration was carried out with good results by operating with the diafiltration method and using both the membranes, but the Iris 30 membrane, operated at 4 × 105 Pa (4 bar), performed a little better owing to the higher flux. The copper ion present in the diafiltration permeate was recovered by oxidising the citrate in a membrane photoreactor. The obtained results permitted to draw an integrated membrane process combining complexation–UF–diafiltration–photocatalysis to treat metal containing wastewaters (e.g. washing water from soil remediation) for obtaining water recyclable in the process itself or safely discharged, recycling the polymer and recovering the metals.