Filtration assisted by complexation has proven to be a promising process for increasing the rejection of metal ions in wastewater treatment. However, a drop in permeate flux resulting from membrane pollution is commonly observed when complexing agents are used. The apparent efficiency of the filtration is low because of the integration of flux drop and rejection increase. Therefore, new agents that increase rejection and maintain flux are rather worth exploring. In this study, potassium sodium tartrate (PST) was chosen as the complexing agent to study both effects in nanofiltration for the removal of cadmium and zinc from wastewater. The effect of PST concentration, applied pressure, and feed concentration on metal rejection and permeate flux were examined. The results showed that the rejection increased and then decreased with increasing PST concentration. Unexpectedly, the permeate flux was found to increase over the entire range, which was quite different from previously reported complexation studies. Increasing the operating pressure did not improve metal rejection, but the flux increased linearly over the pressure range that was investigated. The rejection coefficient increased and then remained steady with increasing feed concentrations, with the flux remaining almost constant in the range. The contact angle (beta) between solid-liquid and gas-liquid interfaces was introduced to clarify the relationship between PST concentration and permeate flux. beta was inversely related to permeate flux and the decrement of beta caused by the addition of PST had a sigmoidal relationship with PST concentration. The results indicated that beta was a better indicator of permeate flux than other concentration polarization parameters. (c) 2013 Elsevier B.V. All rights reserved.