The evolution of metal size and partitioning throughout the wastewater treatment train

Understanding the behavior of heavy metals in wastewater is critical for the development of metal removal and detection techniques. In this study, we characterize the dynamic and evolving size and partitioning behavior of lead (Pb), cadmium (Cd), and arsenite (As(III)) throughout the wastewater treatment train (WWTT). Metal concentrations were determined in three size fractions (>0.45 μm, 0.45 μm - 5 kDa, and <5 kDa), and the partitioning/complexation of the metals was quantified for the <0.45 μm fraction. Cd was found to be highly mobile, with the fraction of dissolved Cd gradually increasing throughout the WWTT. As(III) was also highly mobile, with its size distribution and partitioning remaining largely steady, except when FeCl₃ was used as a flocculation agent, which led to the formation of arsenic/iron complexes. However, Pb was found primarily in complex forms or adsorbed onto inorganic particulates. The WWTT had little impact on the size and partitioning of Pb, except that the formation of the Pb/iron complex occurred after flocculation with FeCl₃. An increase of water hardness slightly increased the metals in the dissolved fraction. Overall, this study provides insight into the evolution of metals throughout the WWTT, offering guidance to users and researchers regarding their treatment and detection.