PH-ALKALINITY EXPERIMENTS: Two sets of laboratory experiments were conducted with synthetic solutions to measure the effects of pH and alkalinity on Mn removal rate. The first experiment tested Mn removal under eight conditions of varying pH and alkalinity over a 14-day period (see Table 1 for full description). Samples were collected at 0, 1, 4, 7, 10 and 14 days. The second experiment included select conditions from the 14-day experiment, run over 24 hours (see Table 2 for full description). During the experiment, we collected samples at 0, 1, 2, 6, 12 and 24 hours.
The experiments were conducted in 500 mL acid-washed Erlenmeyer flasks containing 250 mL of 1 mg/L MnCl2 solution adjusted to varying alkalinity and pH (see Tables 1 and 2). Alkalinity of 1 mg/L MnCl2 solutions was adjusted by adding NaHCO3 crystalline solid and raised pH by iteratively adding 0.1 N NaOH. Alkalinity and pH were measured in initial solutions for each condition. Aluminum foil-covered flasks were incubated at room temperature on orbital shakers (150 rotations per minute, rpm).
At each sampling interval, ~1.5 mL of solution was withdrawn from each flask for pH measurement using an OHaus benchtop pH meter. Samples of 10 mL from each flask were removed and syringe filtered (0.22 μm nylon syringe filter) for analysis of Mn. Samples collected for Mn analyses were acidified with ~0.5 mL of 1:1 trace metals-grade HNO3. Analyses for Mn and other metals were conducted as described in “Chemical Analyses.” At the end of both experiments, 100 mL of solution from each pH and alkalinity condition described in Table 1 and 2 was titrated for alkalinity.
RESERVOIR WATER EXPERIMENTS: An experiment was conducted using hypolimnetic water from FCR and CCR to compare Mn removal rates between reservoirs. The 10-day experiment tested Mn removal rates in unfiltered and filtered water (0.45 μm) from both reservoirs. In mid-March 2023 (prior to stratification), water for experiments was collected at 9 m in FCR and 18 m in CCR using a 4 L Van Dorn sampler (Wildlife Supply Company; Yulee, FL). We immediately transferred water from the Van Dorn into opaque brown plastic Nalgene bottles then stored under chilled conditions (1-3 °C) until experimental setup ~36 hours later. All reservoir water experiments were spiked with 100 mg/L MnCl2 solution to achieve an Mn concentration of 1 mg/L and included a control with unaltered 1 mg/L MnCl2 in DI water (see Tipping et al. 1984, Godwin et al. 2020).
Experiments of 250 mL each were conducted in triplicate, in acid-washed 500 mL Erlenmeyer flasks. Flasks covered in aluminum foil were incubated at room temperature on orbital shakers (150 rpm). Sampling occurred at 0, 1, 4, 7, and 10 days. The procedure for sampling, sample preservation and collecting pH measurements remained consistent from the methods described for 14-day and 24-hour experiments.
ALKALINITY TITRATION: Alkalinity titration methods were adapted from the Massachusetts Water Watch Partnership Standard Operating Procedure for pH and alkalinity of low to medium alkalinity lakes (UMass Amherst 2016). After alkalinity samples warmed to room temperature, 100 mL of sample was transferred to a 150 mL Erlenmeyer flask. The sample was continually mixed at slow speed with a magnetic stir bar while drops of H2SO4 were added from a Hach digital titrator. Cartridges of 0.16 N H2SO4 were used for natural waters and experimental solutions without alkalinity adjustment, and 1.6 N cartridges for experimental solutions with alkalinity adjustment. Changes in pH with each addition of acid were monitored with an OHaus benchtop pH meter. Alkalinity in mg/L CaCO3 was calculated according to Equation 1. These endpoints represent the depletion of CO3-2 and HCO3-, respectively.
Equation 1:
Alkalinity = (2A-B)*0.1
Where A=total number of H2SO4 drops added to reach pH 4.5,
B=total number of H2SO4 drops added to reach pH 4.2.
CHEMICAL ANALYSIS: In 2022, samples were analyzed for Mn using an Inductively Coupled Plasma- Atomic Emission Spectrometer (ICP-AES; Spectro Analytical Instruments, Inc. ARCOS-II MultiView or ARCOS-III MultiView). Analytical procedures followed APHA Standard Method 3125-B (American Public Health Association et al., 1998). The minimum detection limit for Mn varied with each analysis, but fell between 0.001-0.004 mg/L Mn. Starting in 2023, samples were analyzed for Mn using an Inductively Coupled Plasma-Mass Spectrometer (ICP-MS; Thermo Electron iCAP-RQ) following APHA Standard Method 3125-B (American Public Health Association et al., 1998). The minimum reporting limits were: 50 µg/L for Na, 1 µg/L for Mg, 1 µg/L for Al, 100 µg/L for Si, 10 µg/L for K, 50 µg/L for Ca, 10 µg/L for Fe, 0.1 µg/L for Mn, 0.05 µg/L for Sr, and 0.05 µg/L for Ba. We used the minimum reporting limits as the lower threshold for data in 2023. Values that were below the minimum reporting limit, including negative values, were set to the minimum reporting limit for each element.
REFERENCES:
American Public Health Association (APHA), American Water Works Association, Water Environment Federation, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC.
Godwin, C.M., Zehnpfennig, J.R., and Learman, D.R., 2020, Biotic and Abiotic Mechanisms of Manganese (II) Oxidation in Lake Erie: Frontiers in Environmental Science, v. 8, p. 57, doi:10.3389/fenvs.2020.00057.
Tipping, E., Thompson, D.W., and Davison, W., 1984, Oxidation products of Mn(II) in lake waters: Chemical Geology, v. 44, p. 359–383, doi:10.1016/0009-2541(84)90149-9.
UMass Amherst. 2016. Massachusetts Water Watch Partnership: pH and Alkalinity for Lakes. Available at: https://www.umass.edu/mwwp/protocols/lakes/ph_alkalinity_lake.html. (Accessed: 25th April 2023)
