Photocatalytic Removal of Heavy Metals from Private Well Water

Heavy metal contamination of surface water and groundwater is a growing widespread threat to ecological and human health. 43 million Americans currently source their drinking water from a non-EPA-regulated private well. Because of rapid industrialization and their multiple agricultural, medical, and technological applications, many harmful metals are now frequently found in these wells at concentrations above the EPA regulatory limit. Common methods of removing heavy metals include ultrafiltration, reverse osmosis, and adsorption; however, these methods suffer from high cost, incomplete removal of both organic and inorganic pollutants, and high energy use. Traditional advanced oxidative processes (AOPs) and chemical precipitation are also used to treat heavy metals, but their main drawback is the need for sourcing and dosing of expensive, hazardous liquid chemicals. Photocatalytic AOPs with low-cost, nontoxic catalysts have shown promise in treating organic pollutants but also in the adsorption and reduction of metal ions without the production of polluting intermediates.

The purpose of this study was to assess the effectiveness of immobilized titania on a silica fiber substrate (P25/QF) in the reduction and removal of Cr(VI) and Pb(II) in aqueous solutions. Chromium and lead were chosen because they ranked among the most dangerous heavy metals for their severe toxicity and carcinogenic nature in very small amounts. P25/QF photocatalysts were placed in batch reactors with aqueous solutions of potassium chromate and lead nitrate under varying light conditions. Contact times included 30, 60, 90, and 120 minutes while metal concentrations (5 mg/L) and pH (5) were kept constant. We report that when irradiated with a 15 W/cm², 254nm UVC light, at pH 5, the P25/QF is able to remove up to 86.9% and 64.2% of the initial concentration of Pb(II) and Cr(VI), respectively. In the dark, only 9.4% of the initial Cr(VI) concentration was removed from the aqueous solution on average; in the case of Pb(II), up to 48.5% of the initial concentration was removed. At time intervals of 90 & 120 minutes, photocatalysts exposed to Cr(VI) and UVC light produced a grey-green deposit, characteristic of nontoxic, trivalent Cr species. At the same time intervals, photocatalysts in Pb solutions under UVC light displayed a tan color resembling Pb monoxide species. Further studies will be conducted to investigate the fouling effects of these metal ions on the photocatalytic surface. In addition, potential metal ion removal mechanisms will be discussed.