(452g) Removal of Toxic Heavy Metal Cations from Simulated FGD Wastewater Using Functionalized Silica

Flue-gas desulfurization (FGD) process employed by electric power generation utilities produces wastewater containing significant levels of soluble heavy metal ions that are being regulated to parts-per-billion levels due to their toxicity. In order to address this issue, removal of several heavy metal ions was investigated by employing high-surface area amorphous silica grafted with sulfur-containing functional groups, which have been shown to have high affinity towards heavy metal ions. These adsorbents were tested for the removal of single component as well as multiple component heavy metal ions.

The silica support was grafted with 3-mercaptopropyltrimethoxysilane and 2-mercaptobenzothiazole. Thermogravimetric Analysis (TGA) and Potentiometric Mass Titrations (PMT) were performed on functionalized silica to determine surface loadings and the point of zero charge (PZC), respectively. The removal of Hg²⁺ at high concentration (10,000 µg/L) in different matrices was investigated. The effect of FGD matrix ions (Ca²⁺, Mg²⁺, Na⁺, SO₄^2-, Cl^-, NO₃^-) on removal efficiency of Hg²⁺ is also reported. Simultaneous removal of Hg²⁺ along with the several other heavy metal cations has been reported at concentrations relevant for the FGD wastewater (500 µg/L each of Cu²⁺, Cd²⁺, Hg²⁺, Tl⁺, Pb²⁺, Ni²⁺, Zn²⁺). 99% of Hg²⁺ was captured using these adsorbents for both concentration ranges. The adsorbents were able to selectively capture other heavy metal cations at low concentrations as well. The concentrations of heavy metal ions were determined using the Inductively Coupled Plasma â?? Mass Spectroscopy (ICP-MS). Preliminary regeneration studies of spent adsorbents were performed using dilute HCl indicating limited regeneration and suggesting their disposal without regeneration.