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Manganese Oxide-Coated Sand Geomedia for Treatment of Contaminated Stormwater


Managed aquifer recharge is an increasingly popular means of replenishing groundwater supplies by intentionally infiltrating stormwater or tertiary wastewater effluent into the subsurface. Although this practice can recharge depleted aquifers, it poses potential risks to water quality, especially when practiced with stormwater from city streets which may contain contaminants derived from automobiles, asphalt, biocides and consumer products. To provide a means of protecting water quality during managed aquifer recharge, engineered geomediamaterials designed to passively remove contaminantscan be introduced to recharge basins. Manganese oxide-coated sand is a promising engineered geomedia because manganese oxides can oxidize certain organic contaminants and adsorb metal cations. This dissertation describes the use of a manganese oxide-coated sand produced using a novel, room-temperature synthesis for the passive treatment of urban stormwater.

The useful lifetime of manganese oxide-containing geomedia may be limited. After oxidizing organic contaminants and exposure to other solutes typically present in stormwater (e.g., Ca2+, CO32-, natural organic matter), the initially high reactivity of the manganese oxide-containing geomedia decreased. The passivation appeared to be related to the reduction of Mn(IV) sites and subsequent accumulation of Mn(II/III). To address the loss of reactivity, methods for regenerating the reactivity of manganese oxide-coated sand were evaluated (Chapter 2). Hypohalites produced the most reactive regenerated phase when dosed in sufficient abundance to fully oxidize all Mn in the failed geomedia to the +4.0 oxidation state. Permanganate also oxidized Mn(III)-rich manganese oxides, but the resultant mineral phase was less reactive with bisphenol A than that produced by HOCl or HOBr. These results imply that mineralogy, in addition to oxidation state, is important to manganese oxide reactivity.

Chlorine was evaluated as a means of in situ regeneration by oxidizing reduced Mn without removing the geomedia from columns (Chapter 3). Chemical analyses indicated that the average manganese oxidation state of the geomedia coating increased after exposure to hypochlorous acid (HOCl). The regenerated geomedia demonstrated similar reactivity and longevity to virgin geomedia. X-ray absorption spectroscopy and X-ray diffraction indicated that the virgin and regenerated geomedia coatings had similar manganese oxide structures. These more reactive minerals were nanocrystalline and lacked long-range mineral order, whereas the failed geomedia coating exhibited greater crystallinity and resembled the manganese oxide cryptomelane. These results suggest that manganese oxide-coated sand may be capable of oxidizing organic contaminants urban stormwater for many years and that it is possible to regenerate the oxidative capacity of manganese oxide-coated sands without excavating stormwater recharge systems.

Manganese oxide-coated sand was also assessed for its ability to remove metal contaminants from stormwater. The presence of Cu, Zn, Cd and Pb in urban runoff pose potential risks to humans and aquatic organisms. Batch and column experiments were conducted to evaluate the capacity of manganese oxide-coated sand to adsorb metals in a stormwater matrix containing background ions and natural organic matter (Chapter 4). In batch tests the geomedia adsorbed over 95% of Cu, Cd and Pb in conditions typical of natural organic matter-free stormwater. In the presence of natural organic matter, however, the geomedia had a lower capacity for metal removal. Results from column tests indicate that, even in the presence of natural organic matter, a typical stormwater infiltration system containing manganese oxide-coated sand would have a lifetime of over 20 years before the complete breakthrough of Cu and Zn and of over 800 years before the complete breakthrough of Cd or Pb. In field applications, the geomedia could become saturated or release adsorbed metals because of changes in chemical conditions. Therefore, chemical treatments were evaluated for their ability to recover adsorbed metals from the geomedia and restore the adsorption capacity of the manganese oxide-coated sand. A mild acid wash (i.e., pH 3 HCl) restored the adsorptive capacity of the geomedia with minimal degradation of the Mn coating. The regenerated geomedia demonstrated similar performance to the virgin geomedia.

The research described in this dissertation indicates that manganese oxide-coated sand may be effective for treating urban stormwater during managed aquifer recharge. High levels of removal of organic contaminants and toxic metals could be sustained for decades with regenerative treatments. In managed aquifer recharge systems, other geomedia will likely be used in conjunction with manganese oxides, and their contributions to pollutant remediation may be complementary or inhibitory. For instance, biochar could protect manganese oxides from fouling by natural organic matter, but anoxic conditions induced by upstream carbonaceous geomedia could reduce manganese oxides. Further research is needed to determine the optimal configurations of geomedia in unit process treatment trains in order to target contaminants across a range of stormwater qualities.

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