Removal of polycyclic aromatic hydrocarbons by nanofiltration membranes: Rejection and fouling mechanisms

Abstract

Polycyclic aromatic hydrocarbons (PAHs), as a group of micropollutants with high toxicity, are commonly detected in the environment and difficult to remove. In this work, five commercial polyamide (PA) nanofiltration (NF) membranes were used to treat three PAHs in the synthetic solution or coking wastewater and their removal mechanisms were discussed. By comparing water permeability and PAHs rejections of the NF membranes before and after PAHs adsorption, we found that the PAHs adsorption into the membrane was the dominant rejection mechanism at the initial filtration stage. Due to the limited adsorption capacity and increasing diffusion of PAHs in the membrane, the PAHs rejections would drop rapidly and then reach steady. Size exclusion mechanism was also important to PAHs removal by NF, especially for PAHs with weaker polarity and higher molecular weight. Although the polysulfone intermediate layer could adsorb substantial PAHs resulting in an increase of filtration resistance, the pore blocking and narrowing effects on the PA separation layer were still the main reasons for the permeability loss of NF membranes. However, the glucose rejection did not change obviously after the PAHs adsorption in the membrane. Through analyzing porosity and pore size distribution of the NF membranes before and after PAHs adsorption, we found that for the NF membrane with small pore size and narrow pore size distribution (e.g. NF270), PAHs adsorption produced indiscriminate pore blocking effect, and thus the average pore size and its distribution as well as glucose rejection did not change obviously after fouling formation; while for the NF membrane with large pore size and wide pore size distribution (e.g. NF10), PAHs adsorption occurred more seriously in the larger pores leading to a pore narrowing effect, but the reduced pore size was still too large to affect glucose rejections. The outcomes of this work suggested that the PAHs adsorption in both separation and intermediate layers of NF membranes should be avoided, and the NF membrane with narrow pore size distribution was preferred for PAHs removal.