Geochemical drivers of Mn removal in drinking water reservoirs under hypolimnetic oxygenation

This study addressed the geochemical drivers of Mn removal, including pH, alkalinity and the presence of mineral particles. We conducted laboratory experiments and field monitoring at two drinking water reservoirs in southwestern Virginia – Falling Creek Reservoir (FCR) and Carvins Cove Reservoir (CCR). In laboratory experiments in pH and alkalinity-adjusted nanopure water solutions, we observed substantial Mn removal within 14 days only under high pH conditions (pH≥10). In experiments with high pH and moderate to high alkalinity (> 80 mg/L CaCO3), near-total Mn removal occurred within 2 hours, at a rate of 0.25 mg/L-1 hr-1. Mn removal occurred alongside precipitation of microscopic (<5 μm diameter) and macroscopic (>100 μm diameter) particles. Elemental analysis of particles with energy-dispersive X-ray spectroscopy (EDS) supports their identification as Mn(IV) oxides (MnOx), which suggests Mn removal driven by oxidation. Elevated alkalinity in high pH solutions promotes Mn oxidation by maintaining high pH through buffering, which sustains conditions favorable for Mn oxidation. Our results also suggest sorption of Mn and mineral-catalyzed Mn oxidation by Mn oxides formed through oxidation by dissolved oxygen. In experiments using filtered and unfiltered water from the two reservoirs, we observed significant Mn removal in experiments with unfiltered water, suggesting that particles may remove Mn by catalyzing oxidation or nucleating Mn oxide precipitation. Mn removal occurred at 0.05 d-1 in unfiltered FCR water and 0.002 d-1 in unfiltered CCR water. We observed no Mn removal in filtered water from either reservoir. Scanning electron microscope (SEM) and EDS of visible particles from reservoir water experiments suggests that quartz and clay minerals present in the water column may nucleate or catalyze Mn oxide formation. Overall, this research shows that Mn removal under HOx operation is influenced by a variety of factors, including pH, alkalinity and suspended particles.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Geosciences

Department dc:contributor.department

Geosciences

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2023

Author dc:creator

• Ming, Cissy L.

Chair dc:contributor.committeechair

• Schreiber, Madeline E.

Committee members dc:contributor.committeemember

• Michel, Frederick Marc
• Gill, Benjamin C.
• Carey, Cayelan C.

• manganese
• reservoir
• pH
• alkalinity
• metals
• Carvins Cove Reservoir
• Falling Creek Reservoir
• hypolimnetic oxygenation
• autocatalysis

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:37340

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/115388