Properties of CuO nanoparticles–humic acid (CuONP–HA) flocs and subsequent effect on membrane fouling: Influence of aluminum species and solution pH

•The removal of CuONP–HA largely depended on Al(III) species and solution pH.•The larger and stronger CuONP–HA flocs were formed by Alb and Alc.•The more compact CuONP–HA flocs were formed as solution pH increased.•UF membrane would be fouled severely by CuONP–HA at high solution pH.

The CuO nanoparticle (CuONP) is one of the most commonly-utilized groups of nanoparticles. The adsorption between CuONP and humic acid (HA) make it more complicated to remove both CuONP and HA from water. Results investigated from this study indicated that the properties of CuONP–HA flocs mainly depended on Al(III) species and solution pH. Al polymer (Alb) and sol or gel species (Alc) were appropriate for aggregating large and strong CuONP–HA flocs. Poly aluminium chloride (PAC) could aggregate more compact CuONP–HA flocs as solution pH increased and the largest CuONP–HA flocs (with size over 680 μm) was formed at solution pH of 8.0. These flocs influenced the performance of subsequent ultrafiltration membrane. In addition, aluminum species would influence the properties of CuONP–HA flocs, but show little effect on subsequent ultrafiltration membrane due to the special properties of nanoparticles.