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Electrochemical Detection and Removal of Heavy Metals in Water

  • Author(s): Ma, Shengcun
  • Advisor(s): Jassby, David
  • et al.

Measurements of heavy metal concentrations in municipal wastewater (MWW) are currently performed by obtaining a grab-sample of water, followed by analysis using inductively coupled plasma – mass spectrometry (ICP-MS) or atomic absorption spectroscopy. However, these methods require expensive instruments and dedicated technicians, which are not typically on-site. Therefore, several rapid heavy metal detection techniques have been developed. However, these methods require the metals to be in their free forms to achieve reasonable results. Unfortunately, metals in complex water streams are generally not in their free forms. Therefore, we firstly systematically investigated the evolution of Pb(II), As(III), and Cd(II) throughout the wastewater treatment train (WWTT) in terms of the size fraction these metals were found in, as well as the metal’s partitioning behaviors. Cd(II) was found to be highly mobile, with the fraction of dissolved Cd(II) 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 formations of arsenic/iron complexes. However, Pb(II) 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. These results provided insightful information on the evolution of size and partition distribution of metals during the wastewater treatment train.

Based on these results, we developed processing methods that enable the detection of Pb(II) and Cd(II) using anodic stripping voltammetry. We found that vacuum ultraviolet (VUV)/H2O2 could release Cd from its complex, and strong acid could release Pb(II). Pb(II) and Cd(II) are then accurately detected and quantified using anodic stripping voltammetry by utilizing a bismuth subcarbonate/reduced graphene oxide nanocomposite incorporated in a Nafion matrix electrode. Detection results were benchmarked against state-of-the-art metal detection methods, and were found to be highly accurate (>88%).

Lastly, we fabricated electrically conductive membranes that could effectively reject As(III) due to As(III)’s high mobility. Applying cathodic potentials significantly increased As(III) rejections in synthetic/real tap water, a result of locally elevated pH that is brought upon through water electrolysis at the membrane/water interface. The elevated pH conditions convert H3ASO3 to H2AsO3-/HAsO32- that are rejected by the negatively charged membranes. In addition, it was found that Mg(OH)2 that precipitates on the membrane can further trap arsenic. Importantly, almost all As(III) passing through the membranes is oxidized to As(V) by hydrogen peroxide produced on the cathode, which significantly decreased its overall toxicity and mobility. Although the high pH along the membrane surface led to mineral scaling, the fouling could be partially removed by backwashing the membrane.

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