Temporal and spatial inhibitory effects of zinc and copper on wastewater biofilms from oxygen concentration profiles determined by microelectrodes

To understand the temporal and spatial toxic effect of heavy metals on the microbial activities of biofilms, microelectrodes were used to measure the inhibitory oxygen (O₂) concentration profiles resulted from the effects of zinc (Zn²⁺) and copper (Cu²⁺). Using the O₂ microprofiles as bases, the spatial distributions of net specific O₂ respiration were determined in biofilms with and without treatment of 5 mg/L Zn²⁺ or 1 mg/L Cu²⁺. Results show that microbial activities were inhibited only in the outer layer (∼400 μm) of the biofilms and bacteria present in the deeper sections of the biofilms became even more active. The inhibition caused by the heavy metals was evaluated by two methods. One was derived from the oxygen influx at the interface and the other was based on the integral of the oxygen consumption calculated from the entire O₂ profile. The two methods yielded significantly different results. We argue that the integral method results in more accurate assessment of toxicity than the surface flux determination.