MAGNETITE AND MANGANESE DIOXIDE NANOPARTICLE AMENDMENT 
IMPACTS ON ARSENIC, CADMIUM, AND GREENHOUSE GAS 
DYNAMICS IN PADDY SOIL 
 
 
 
 
 
 
By 
 
MARKUS W. KOENEKE 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
A THESIS 
 
Presented to the Department of Earth Science 
and the Graduate School of the University of Oregon 
in partial fulfillment of the requirements 
for the degree of 
Master of Science 
December 2020 
THESIS APPROVAL PAGE 
 
Student: Markus W. Koeneke 
 
Title: Magnetite and Manganese Dioxide Nanoparticle Amendment Impacts on Arsenic, 
Cadmium, and Greenhouse Gas Dynamics in Paddy Soil 
 
This thesis has been accepted and approved in partial fulfillment of the requirements for 
the Master of Science degree in the Department of Earth Science by: 
 
Matthew Polizzotto Chair 
Qusheng Jin Member 
Scott Bridgham Member 
and 
Kate Mondloch Interim Vice Provost and Dean of the Graduate School 
 
 
Original approval signatures are on file with the University of Oregon Graduate School. 
Degree awarded December 2020. 
 ii 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
© 2020 Markus W. Koeneke 
 iii 
THESIS ABSTRACT 
Markus W. Koeneke 
Master of Science 
Department of Earth Science 
December 2020 
Title: Magnetite and Manganese Dioxide Nanoparticle Amendment Impacts on Arsenic, 
Cadmium, and Greenhouse Gas Dynamics in Paddy Soil 
 
Rice contamination by arsenic and cadmium is a well-documented challenge that 
impact billions of people globally. Our goal is to quantify the extent to which magnetite 
and manganese dioxide could be used as rice paddy amendments that limit As and Cd 
mobility and methane emissions. To do this, a suite of anaerobic and aerobic batch 
incubations were conducted utilizing rice-paddy soil and varying quantities and 
combinations of nano-magnetite and -MnO2. In the anaerobic incubation, As release 
decreased for all treatments, and Cd was not mobilized. In the aerobic incubations, all As 
concentrations in solutions were low and MnO2 decreased the amount of available Cd. 
This research demonstrates that nanoparticle magnetite and MnO2 can be used to reduce 
As and Cd mobility in flooded paddy soils, while having varying impacts on greenhouse 
gas emissions. 
There is Supplemental Information that goes in-depth on my digestion and 
extraction methods, data, and other figures. 
 iv 
CURRICULUM VITAE 
NAME OF AUTHOR: Markus W. Koeneke 
GRADUATE AND UNDERGRADUATE SCHOOLS ATTENDED: 
University of Oregon, Eugene, OR 
North Carolina State University, Raleigh, NC 
DEGREES AWARDED: 
Master of Science, Earth Science, 2020, University of Oregon 
Bachelor of Science, 2017, North Carolina State University 
AREAS OF SPECIAL INTEREST: 
Soil and Water Chemistry 
PRESENTED PRESENTATIONS 
1. Koeneke, M., Zhang, S., Seyfferth, A., and Polizzotto, M. Iron and Manganese 
Amendments to Flooded Soils Decrease As Availability But Increase Cd Availability and 
Methane Production. Soil Science Society of America Annual Meeting, San Antonio, 
Texas, November, 2019. Poster presentation. 
2. Koeneke, M., Jones, MC., Gillispie, E., and Polizzotto, M. Elucidating the role 
of carbon sources on abiotic and biotic release of arsenic into Cambodian aquifers. 
American Geophysical Union Annual Conference, New Orleans, Louisiana, December, 
2017. Poster presentation. 
3. Koeneke, M., Jones, MC., Gillispie, E., and Polizzotto, M. Elucidating the role 
of carbon sources on abiotic and biotic release of arsenic into Cambodian aquifers. Soil 
Science Society of America Annual Meeting, Tampa, FL, October, 2017. Poster 
presentation. 
4. Koeneke, M., Jones, MC., Gillispie, E., and Polizzotto, M. Elucidating the role 
of carbon sources on abiotic and biotic release of arsenic into Cambodian aquifers. North 
Carolina State University Spring Undergraduate Research Symposium, Raleigh, NC, 
August, 2017. Poster presentation. 
5. Koeneke, M., Jones, MC., Gillispie, E., and Polizzotto, M. Elucidating the role 
of carbon sources on abiotic and biotic release of arsenic into Cambodian aquifers. North 
Carolina State University Spring Undergraduate Research Symposium, Raleigh, NC, 
April, 2017. Poster presentation. 
 
PROFESSIONAL EXPERIENCE: 
Greenhouse Manager/Water Analyzer, LeapFrog Design, Current 
Lab Manager, UO Soil and Water Lab, 2018 
VOLUNTEER WORK 
Environmental Justice Caucus Co-Chair, GTFF 
Field Checker, Cascadia Wildlands 
 v 
ACKNOWLEDGEMENTS 
 
Thanks Matt Polizzotto, Qusheng Jin, and Scott Bridgham for being on my 
committee, assisting me on my path through this project in a countless number of ways. 
Special thanks to Sili Zhang, who without his help, I would have not been able to tackle a 
project this large. To Chelsea, Fatai, Charlotte, Jason, Katie, Rachel, Fusheng, and 
Gabbie, thanks for being such supportive lab mates and an overall joy to be around. I 
could have never done this without your support, help, love, and silliness. Thanks to 
Chris Russo and Jesse Muratli for all the assistance with analyzing my samples and the 
great conversations had on those long days. Thanks to Laura McCullough for the 
assistance running the GC. And lastly, thank you so much to all my friends and family 
who have supported and been there with me through this process. I love you all. 
 vi 
TABLE OF CONTENTS 
 
Chapter Page 
I. INTRODUCTION ......................................................................................................... 1 
II. METHODS .................................................................................................................... 4 
2.1 Sample Collection ................................................................................................. 4 
2.2 Anaerobic Batch Reduction Incubation ................................................................ 4 
2.3 Aerobic Batch Incubation ..................................................................................... 5 
2.4 Strong Acid Digestion (EPA 3050b) .................................................................... 5 
2.5 Sequential Extraction ............................................................................................ 6 
III. RESULTS ...................................................................................................................... 8 
3.1 Characterization of Soils....................................................................................... 8 
3.2 Experiment 1: Anaerobic Incubation .................................................................... 8 
3.2.1 Arsenic Dynamics in Soil ......................................................................... 8 
3.2.2 Cadmium Dynamics in Soil ...................................................................... 9 
3.2.3 Soil Carbon Emissions ............................................................................ 10 
3.2.4 Comparing Treatments ........................................................................... 12 
3.3 Experiment 2: Aerobic Study ............................................................................. 13 
IV. DISCUSSION .............................................................................................................. 15 
4.1 Anaerobic Study ................................................................................................. 15 
4.2 Oxic Study .......................................................................................................... 16 
4.3 Future Challenges ............................................................................................... 16 
V. CONCLUSION............................................................................................................ 18 
REFERENCES CITED .................................................................................................... 19 
Supplemental Information 
 vii 
LIST OF FIGURES 
 
Figure Page 
 
Figure 1 – Anaerobic experiment arsenic release ................................................. 9 
 
 
Figure 2 – Anaerobic experiment cadmium release ........................................... 10 
 
 
Figure 3 – Anaerobic experiment methane emissions ........................................ 11 
 
 
Figure 4 – Anaerobic experiment carbon dioxide emissions .............................. 11 
 
 
Figure 5 – Week 8 anaerobic experiment data compared ................................... 12 
 
 
Figure 6 – Aerobic experiment As and Cd released ........................................... 14 
 viii 
I. INTRODUCTION 
 
 
Rice is the most important food in the world for human consumption, with 
approximately 4 billion people relying on it as a staple food (1). Several challenges 
currently jeopardize global rice production, one being the accumulation of arsenic (As) in 
rice grain, which poses threats to human health (2). This challenge has been exacerbated 
by farmers using As-contaminated groundwater for irrigation, causing an increase in As 
concentrations in rice paddy soils and rice (3). Many solutions have been proposed to 
address this challenge, but there are other environmental quality and human health issues 
that can arise or be intensified. Some of these issues are increasing methane (CH4) 
emissions (4), cadmium (Cd) accumulation in rice grain (5), or a decrease in iron (Fe)  
and manganese (Mn) micronutrients in rice. Therefore, solutions are critically needed that 
comprehensively address all of these problems. 
 
Varying solutions have been proposed to address these challenges, however each solution 
comes with complicating factors that can decrease the overall utility of that solution. 
Alternate wet-dry irrigation (AWD), where rice fields are drained to oxygenate the 
system, decreases As and Fe uptake, yield, water use, and CH4 emissions, but increases 
Cd uptake and N2O emissions (6-9). Silicon amendments, typically in the form of rice 
straw, rice husk, rice straw ash, and rice husk ash, have shown very promising results in 
decreasing As uptake by rice. This is due to many species of As and Si sharing a pathway 
for uptake into rice, which Si is prioritized for (10), with the exception of dimethylarsenic 
acid, which increases in grain concentration with this amendment.  In flooded paddy 
soils, rice with husk amendments took up 45% less As and reduced CH4 emissions, and 
in non-flooded paddy soils, rice took up 40-50% less Cd attributed to increasing pH and 
biomass content (11, 12). Biochar formed from the rice husk is another possible solution 
to this challenge that is promising. Studies have shown increases and decreases in both 
As, Cd and CH4 depending on how the biochar is made and manipulated (13)(14). 
Biochar can also decrease zinc, a micronutrient, uptake by rice and stimulate direct 
interspecies electron transfer, which can increase methane emissions, though it is poorly 
 1 
studied. (14)(15). The strategies stated above are useful for achieving certain objectives 
with growing rice, however they can also threaten rice or environmental quality. 
 
Iron and manganese oxides are two oxide minerals that impact redox chemistry 
and contaminant sorption, which suggest they can be used to solve As and Cd issues (16). 
Iron- and Mn-oxide amendments have been used to varying success on limiting As and 
Cd uptake by rice. Both Fe- and Mn- oxides have the ability to scavenge contaminants 
like As and Cd in soil and can keep them bound in the soil (17). Iron-oxides readily sorbs 
As, but, it can be reduced in a flooded system, which mobilizes the sorbed As (17). 
Manganese-oxide is a powerful oxidant, that can oxidize As(III) to As(V), Fe(II) to 
Fe(III) and sorb cations (18). Cadmium plant uptake occurs through the Fe(II) and Mn(II) 
transporters, so amendments could prevent mobilized Cd from being taken up by rice. 
Their reactivities and ability to sequester contaminants vary according to the specific 
oxide mineralogy, surface chemistry, and surface area. 
 
In the present study, we utilize nanoparticle Fe- and Mn-oxides to examine their 
utilities as soil amendments. Nano Fe- and Mn-oxides size can be orders of magnitude 
smaller than their bulk counterparts, resulting in reactivities, structures, and mobility in 
soils exceeding those of their bulk counterparts (19). Their small size results in very high 
surface areas and unique surface structures that can make them more reactive or act 
differently in the environment to the bulk-oxides. They have the ability to get into pore 
water and have a higher impact on contaminant sorption and reduction potential (19). In 
particular, nano-magnetite is an Fe-oxide that is very effective at sequestering As, and in 
reduced environments, when most Fe-oxides would undergo reduction and release the As 
back into solution, magnetite does not (20). Nano-MnO2 is a mineral that has been 
successfully shown to mitigate As accumulation in rice from a contaminated field 
through sorption and oxidation of Fe(II) to form Fe(III) oxides (21). No studies have 
looked into the impacts that both nano-magnetite and nano-MnO2 have on arsenic, 
cadmium, and greenhouse gas emissions (GHG). 
 2 
The overall goal of this work is to utilize magnetite and manganese dioxide as soil 
amendments to limit As and Cd mobility and monitor methane and CO2 emissions. To 
accomplish this goal, we examined the impacts of magnetite and MnO2 nanoparticle 
amendments on As and Cd mobility and availability and GHG emissions in oxic and 
anaerobic rice paddy soil incubations. Overall, we found that magnetite and MnO2 can be 
effective at limiting As and Cd mobilization and greenhouse gas emissions in oxic and 
anaerobic environments, although specific results are dependent on amendment 
concentrations. 
 3 
II. METHODS 
 
 
2.1 Sample Collection 
 
 
Bulk soil samples were collected from the University of Delaware RICE farm in Newark, 
DE, USA and shipped to Eugene, Oregon aerobically. The farm was naturally a grassland 
and the soil is an Utisol/Acrisol. 
 
2.2 Anaerobic Batch Reduction Incubation 
 
 
A 2-month batch incubation experiment was conducted to assess how Fe and Mn 
nanoparticle treatments impact reduction potential and As and Cd dynamics in farm soils. 
The method was adopted from Gillispie et al. 2016 (22), and modified for this 
experiment. Two and a half grams of soil were added to 30 mL serum vials for 3 different 
treatments. The three treatments were a 50 nm manganese dioxide nanoparticle (US 
Research Nanomaterials Inc.), a 15-20 nm magnetite nanoparticle (US Research 
Nanomaterials Inc.), and a combination of the MnO2 (Mn) and magnetite (Fe) 
nanoparticles. Each treatment had 3 different concentrations, 0.025 g, 0.075 g, and 0.125 
g, for 5 sample dates where each date has a specific sample for it, in triplicate. Each 
sample was destructively sampled, so every vial was unique for each sample date, 
treatment, and replica. The solution was purged of oxygen by boiling 18 mΩ water and 
bubbling N2 through it, and contained HEPES buffer, a minimally reactive buffer to a pH 
around 6, and 10 mM potassium chloride (KCl) as an electrolyte. Samples and solutions 
were brought into an anaerobic chamber, purged with nitrogen (92%) and hydrogen gas 
(8%) to remove oxygen, and 25 mL solution was added to each sample. Soil-free controls 
and blanks were included in duplicate, where treatment in the soil free controls was 0.075 
g for Fe and Mn. Vials were crimp sealed in the anaerobic chamber, taken out of the 
chamber, and set up on shakers covered in tinfoil until sample day. 
 
On sample day vials were taken off the shaker and headspace concentrations of CO2 and 
CH4 were analyzed by gas chromatography (GC) using a flame ionization detector 
 4 
equipped with a methanizer (SRI Instruments, Torrance, CA, USA). For GC analysis, the 
instrument was calibrated using a mixed CO2 and CH4 gas with a calibration range of 
1,010-10,100 ppm CO2 and 99.9-999 ppm CH4. After samples were analyzed for gas they 
were brought back into the anaerobic chamber. In the anaerobic chamber samples were 
measured for Eh and pH, decanted into 50 mL centrifuge tube, centrifuged at 4,000 rpm 
for 10 minutes, decanted into a 50 mL syringe, pushed through a Thermo Scientific PTFE 
0.2 um filter, and then acidified with 5 drops of 12M HCl. Sample vials with the soil 
inside were left in glovebox to air-dry in an anaerobic environment. Solutions were taken 
out of the glovebox and refrigerated at 4°C. For Fe, Mn, As, and Cd solution analysis, 
samples were diluted at a 2:5 ratio in 2% HNO3, then taken to Oregon State University’s 
W.M. Keck Lab. Iron and Mn were analyzed via ICP-OES (Spectros Arcos II), and As 
and Cd solutions analyzed on an ICP-MS (Thermoscientific iCAP-RQ). 
 
Water content of the soil was calculated by weighing out 2.5 grams of soil, done with 6 
replicates, putting them in an over at 105°F for 24 hours, taking them out and reweighing 
the dry soil. 
 
2.3 Aerobic Batch Incubation 
 
 
A follow up 2-month batch incubation experiment was conducted to assess how Fe and 
Mn nanoparticle treatments impact As and Cd dynamics in an aerobic farm soil. The 
same method for the anaerobic study was used for this one with a few changes. Only one 
amount, 0.025 g, of amendment was used, the water for the solution was not purged with 
nitrogen, samples were set up outside of glovebox, and covered in parafilm with slits for 
oxygen exchange. Because of this there was no gas analyses done at the end of the study. 
Samples were processed, stored, and analyzed the same way as the aerobic study. 
 
2.4 Strong Acid Digestion (EPA 3050b) 
 
 
The soil used for the experiment, and soils from week 8 samples, were analyzed for Fe, 
Mn, As, and Cd after they were acid digested following the EPA 3050b protocol. This 
 5 
shows the “environmentally available” but does not dissolve silicate structures. For the 
digestion, 0.5 g of wet soil was weighed and put in a 50 mL digestion tube. Two and a 
half milliliters of 18.2 MΩ water and 2.5 mL of 15.8 M Nitric Acid (HNO3) were added 
to tubes and the samples were vortexed. The samples sat over night for 16 hours in the 
acid covered with a watch glass. After the 16 hours, the samples were heated to 95°C in a 
DigiPrep Digestion Block for 15 minutes then removed from heat to cool for 10 minutes. 
After cooling, 2.5 mL of 15.8 M HNO3 was added to samples, then they were vortexed, 
and tubes heated to 95°C for 30 minutes. This step was repeated once. After the 
repetition, watch glasses were removed and tubes were heated to 95°C for 2 hours. 
Following the 2 hours the samples are removed from the heat to cool. Once cooled, 1.5 
mL of 18.2 MΩ water and 1 mL of 30% hydrogen peroxide were added to samples. 
Samples were then put back in the digestion block with watch glasses on and heated to 
95°C, adding 1 mL of 30% hydrogen peroxide when the effervescence stopped from the 
previous addition. This was done until 5 mL total of 30% hydrogen peroxide had been 
added to the samples. Once the effervescence of the 5th mL of 30% hydrogen peroxide 
stopped, samples were vortexed and heated uncovered to 95°C for 2 hours. After the 2 
hours, samples were taken off the digestion block and left to cool. Once cool, 2.5 mL of 
12 M hydrochloric acid (HCl) was added to samples, then samples put back on digestion 
block and heated to 95°C for 45 minutes. Samples were taken off of the digestion block, 
cooled, filtered with Whatman paper #41 into 50 mL centrifuge tubes and stored in the 
fridge. Samples were analyzed for As, Cd, Fe, and Mn via ICP-MS and ICP-OES, as 
described above. 
 
2.5 Sequential Extraction 
 
 
The chemical fractionation of solid-phase Fe, Mn, As, and Cd was achieved via 
sequential extraction adopted by Keon et at 2001, modified with the addition of a step for 
chemical fractions associated with crystalline Fe oxides (Mehra and Jackson, 1960) and 
without the monosodium phosphate step (Detailed in Supplemental Information). The 
extractants and steps detailed below target Fe, Mn, As, and Cd from (1) ionically bound, 
 6 
(2) Mn-Oxides (3) amorphous iron oxide, and (4) crystalline iron oxide. 
1. 1 M MgCl2, pH 8, 2 h, 25 °C two repetitions + one water wash 
2. 1 N HCl, 1 h, 25 °C one repetition + one water wash 
3. 0.2 M ammonium oxalate/oxalic acid, pH 3, 2 h, 25 °C in dark 
one repetition + one water wash 
4. Citrate-bicarbonate-dithionite (from Mehra and Jackson, 1960) 
 7 
III. RESULTS 
 
 
3.1 Characterization of Soils 
 
 
Data in Table S1, obtained through digestions and extractions, show associations between 
elements in the soil. These strong acid digestion data show the original soil had 881 mg/g 
Fe, 13.1 mg g-1 Mn, 227 ng g-1 As, and 3440 ng g-1 Cd. The soil has a pH of 6.67 and a 
wet-dry-ratio of 1.34 g : 1 g. 
 
3.2 Experiment 1: Anaerobic Incubation 
 
 
3.2.1 Arsenic Dynamics in Soil 
Across all incubations, As release from soil to solution was greatest for samples not 
treated with Fe or Mn nanoparticles (Figure 1), with nearly 500 ng As released per g soil, 
or roughly 9% of the total As in the soil (6 ug g-1, Table S1). In contrast, only 165 ng As 
per g soil was released in incubations with added nanoparticles, excepting the 1% and 3% 
MnO2 amendments, where 400 ng g-1 and 300 ng g-1 As released, respectively, by day 
28. For all treatments, As release primarily occurred in the first 28 days, after which 
dissolved As concentrations stabilized or slightly decreased. Magnetite amendments 
appeared to have the greatest inhibition of As release, as shown in Figures 1a and 1c, 
whereas MnO2-only treatments resulted in As release that approached levels observed 
without nanoparticle addition through 28 days (Figure 1b). However, by 8 weeks of 
incubation, As release was below 165 ng g-1 in all treatments (Figure 5c). Finally, As 
release generally was lessened with increasing nanoparticle concentration for all 
treatments. 
 8 
 
Figure 1: Anaerobic experiment arsenic release per gram of dry soil from magnetite treatment (A), from 
MnO2 (B), and from a combination of both magnetite and MnO2 (C). Error bars represent standard error of 
experimental triplicates. 
 
The digestion and extraction data of the post-experiment week 8 anaerobic incubation 
soils are shown in Table S2. The strong acid digestion released 6.47-7.51 ug g-1 As from 
the soils. The ammonium oxalate/oxalic acid (AMO), hydrochloric acid (HCl), and 
citrate-bicarbonate-dithionite (CBD) extractions released between 0.6-3.7 ug g-1 As. All 
magnesium chloride (MC) extractions released a maximum of 0.23 ng g-1 As. 
 
3.2.2 Cadmium Dynamics in Soil 
 
 
Across all incubations Cd release from soil to solution was low, with highest values in 
samples treated with Fe or Mn nanoparticles (Figure 2). The range of Cd released over 8 
weeks was 2-6 ng Cd released per g soil, or roughly 0.05-0.15% of the total Cd in the soil 
(4260 ng g-1, Table S1). In contrast, samples that did not receive a treatment released less 
than 1.6 ng of Cd per gram of dry soil across the entire experiment. The highest 
concentrations of Cd released occurred at 56 days for the 5% magnetite amendment, 14 
days for the 5% MnO2 amendment, and 56 days for the 5% magnetite/MnO2 treatment. 
These were the only samples that released over 3.15 ng of Cd across the incubation. For 
the magnetite amendment, the majority of the Cd was released after 14 days, with the 
exception of the 5% amendment, which released 1.63 ng Cd per g soil over 14 days. For 
the MnO2 amendment, there is a spike in cadmium that occurred between 7 and 14 days, 
with Cd release being 2.33- 5.65 ng per gram of dry soil, and increasing with amendment 
concentration. However, by 28 days, Cd levels lowered to 1.4-2.6 ng Cd per g soil, and 
 9 
those levels plateaued through 56 days. For samples treated with magnetite/MnO2, Cd 
release primarily happened between 7 and 14 days to 1.3-2.6 ng per g soil, which 
plateaued through 28 days, and slightly increased through 56 days to 2.2-3.6 ng per g 
soil. 
 
Figure 2: Anaerobic experiment cadmium release per gram of dry soil from magnetite treatment (A), 
MnO2 treatment (B), and Fe/Mn treatment (C). Error bars represent standard error of experimental 
triplicates. 
 
The digestion and extraction data of the post-experiment week 8 anaerobic incubation 
soils are shown in Table S2. The strong acid digestion data of released Cd ranging from 
83.5-105.6 ng L-1. HCl extractions released between 58.4-77.4 ng g-1 Cd, and the MC, 
AO, and CBD extractions released a maximum of 17.71 ng g-1 Cd. 
 
3.2.3 Soil Carbon Emissions 
 
 
Greenhouse gas emission data in Figures 3 and 4 vary with Fe and Mn treatments and 
concentrations. The data in Figure 3a, the Fe treatments, show CH4 emissions increased 
over time, and with concentration of Fe added. In Figure 3b, where Mn was added, these 
data show CH4 production is lower than when Fe is added, with the lowest CH4 produced 
in the 1% additions. The 1% Mn amendment sample CH4 total emissions are constant 
throughout each sampling date of the experiment, while the 3% and 5% Mn amendment 
samples increased CH4 emissions throughout the experiment. In Figure 3c, the Fe and Mn 
treatments, CH4 levels are consistent throughout the 56 days, suggesting that either 
 10 
almost all the CH4 was produced in the first 4 days. The data also show lower CH4 
emissions with lower Fe/Mn amendment concentrations added to samples. 
 
Figure 3: Anaerobic experiment CH4 emissions from magnetite treatment (A), MnO2 treatment (B), and 
magnetite and MnO2 treatment (C). Error bars represent standard error of experimental triplicates. 
 
The Fe treatments produced CO2 concentrations greater than the 0-treatment sample over 
the first 28 days, then significantly less than the 0-treatment sample on day 56 due to a 
huge spike (Figure 4). Concentration of the Fe amendment did not have an observed 
impact on CO2 production. The data in Figure 4b, the MnO2 nanoparticle amendment, the 
data show CO2 production was 0 across all days and concentrations except the 0% 
sample. The data in Figure 4c, the Fe/Mn amendment samples, we see very small 
amounts of CO2 produced, all below 2,000 ppm, except the 0% sample, that slightly 
increase with time, and are highest in the 1% treatments. 
 
Figure 4: Anaerobic experiment CO2 emissions from magnetite treatment (A), MnO2 treatment (B), and 
magnetite and MnO2 treatment (C). Error bars represent standard error of experimental triplicates. 
 11 
 
 
 
3.2.4 Comparing Treatments 
The nanoparticle magnetite and MnO2 amendments decreased As concentrations, 
increased Cd concentrations, decreased CO2 emissions and CH4 emissions were increased 
and decreased depending on treatment and concentration. These data in Figure 5a show 
As concentrations of samples treated with Fe or Mn significantly lower than the no 
treatment sample. While As concentration decreased, data in Figure 5b show that Cd 
concentration increased in about every sample with the exceptions of the 1% and 5% Mn 
treatments where they were about the same as the treatment blank. Magnetite treatments 
increased CH4 emissions, with the 5% amendment increasing CH4 production the most 
and the 1% and no treatment samples producing the least. For the samples with Mn added 
(including Fe/Mn), the 5% treatment was similar to the no treatment, but in the 3 and 1% 
treatment samples were lower than the no treatment. The 1% Fe and Fe/Mn treatments 
had the least CH4 produced by a decent amount. In Figure 5d, CO2 week 8 data, all 
samples were much lower than the no treatment, given a huge spike in CO2 produced in 
the 8 week no treatment samples. There was CO2 produced in all the magnetite 
treatments, there was no CO2 produced in the MnO2 samples, and very small 
concentrations of CH4 produced for the Fe/Mn data for the week 8 samples. 
 
 
 
 
 
 12 
 
Figure 5: Week 8 anaerobic experiment data compared across treatments for As (A), Cd (B) released into 
solution and CO2 (C), and CH4 (D) emissions from the soil. Error bars represent standard error of 
experimental triplicates. 
 
3.3 Experiment 2: Aerobic Study 
 
 
In our follow up aerobic incubation experiment, As concentrations were low through all 
treatments, magnetite increased Cd concentrations, and MnO2 decreased Cd released 
(Figure 6). 
As concentrations in solution throughout the study ranged from 0.2-0.7 ng g-1. Cd 
concentrations in solution were low through the first 14 days of the study, and then spike 
at 28 days. All of our Cd results are significantly higher than the anaerobic study, which 
had a spike at 28 days, but MnO2 treated samples reduced Cd concentrations relative to 
the magnetite and no treatment samples. 
 13 
 
Figure 6: Aerobic experiment As released (A), Cd released (B) from soils with each treatment. Error bars 
represent standard error of experimental triplicates. 
 14 
IV. DISCUSSION 
 
 
4.1 Anaerobic Study 
 
 
Soils treated with nanoparticle magnetite amendment had little As released into solutions, 
with minimal impacts on Cd dynamics. However, depending on amendment 
concentration, it can increase CH4 emissions. In the 1% Fe sample, we see methane 
emissions similar to that of the no treatment, so lower concentration amendments of 
magnetite have the potential of sequestering As in reduced environments while having 
little impact on cadmium mobility and possible CH4 production. 
 
Nanoparticle MnO2 amendments reduced As concentration in solution without 
mobilizing Cd, reduced CH4 production, and inhibited CO2 production. Arsenic readily 
binds to Fe-oxides, which Mn-oxides can oxidize if reduced (23). This can explain how 
we measured no Fe in solution in samples treated with Mn, and low As concentrations in 
solution. Mn also reduced the amount of CH4 produced for the 1% amendment and was 
similar emissions in all the other concentrations (Figure 3). For this treatment there was 
no CO2 produced throughout the entire experiment (Figure 4), suggesting that Mn could 
be toxic at treatment concentrations to certain microbes (24). Lower concentrations of 
Mn should be studied to see impacts of smaller amendments on GHG emissions and 
contaminant mobility in paddy soils. 
 
In dually treated samples that received nanoparticle magnetite and MnO2 amendments, 
As and CH4 concentrations were reduced (Figures 1 and 3) relative to no treatment 
samples, while minimally impacting Cd relative to the oxic study (Figures 2 and 6b), and 
limiting CO2 production (Figure 4). The 1% amendment had the lowest CH4 emissions 
out of all samples over 56 days, and had similar results for As and Cd release as the 3% 
and 5% treatment concentrations. Limiting the treatment concentration while decreasing 
As and Cd released, and CH4 production is optimal for a solution. Further research into 
lower and varying concentrations of these amendments can optimize this strategy. 
 15 
4.2 Oxic Study 
 
 
In the oxic incubation, Cd concentrations increased by over an order of magnitude under 
all treatments relative to the anaerobic incubation, and the magnetite amendment 
increased [Cd] to levels higher than the samples that did not receive an amendment. 
There is a spike in cadmium that occurred between 2 and 4 weeks into the incubation, so 
oxidizing a field with this treatment for harvest might not result in cadmium uptake by 
rice, though this needs further investigating. Also, only one treatment concentration was 
used in the oxic study, so further investigation needs to be done on the concentration of 
Mn amendments to paddy soil and their impact on Cd. 
 
4.3 Future Challenges 
 
 
Growing rice in the future faces a multitude of challenges, many related to environmental 
quality, and we need solutions ready to address these challenges to ensure we have a 
well-nourished global population. Managing rice paddy fields to address contaminant 
fate, water scarcity, climate change, and rice yields are a hard task to achieve 
simultaneously (25, 26). Results from our study indicate that using MnO2 amendments to 
alleviate the cadmium contamination from AWD irrigation practices can reduce Cd 
mobility, however none of our amendments demonstrated that they can reduce Cd 
concentrations in that environment. 
 
In our study, all treatments in the anaerobic incubation demonstrated that they can 
address some of the challenges presented to rice growing, though more research is needed 
to investigate the possible externalities from these amendments. Future research needs to 
look into the use of Fe and Mn nanoparticles applied to paddy soil at smaller amendment 
concentrations to investigate if we see similar results. We recommend these amendments 
are tested on field studies with flooded irrigation to see if what we observe in the 
laboratory translates to the field. We advocate for the use of magnetite at low 
concentrations in flooded systems to alleviate As contamination, and MnO2 for lowering 
soil emissions, however, there needs to be more research into the implications of varying 
 16 
the amendment concentration. By addressing these questions, we will be able to improve 
the quality of rice without decreasing yield and limiting negative impacts from this 
practice. 
 
Overall a combination of Fe and Mn seems like it could be ideal at addressing this issue, 
as you can gain benefits from them both in different ways. The 1% Fe treatment seems 
like it is ideal for an anaerobic system, but it does not reduce CH4, and Cd can still be 
mobilized upon oxidation of the field. The 1% Mn seems like too large of a concentration 
to pair with the Fe, as it impacts microbes more and results in high Mn in solution, 
therefore, further investigations into Mn amendments at lower concentrations, paired with 
Fe seem like the ideal way to address the issue. Honing in on ideal concentration for the 
amendments can prevent rice from taking up As and Cd as well as lower CH4 emissions. 
 17 
V. CONCLUSION 
 
 
Global rice contamination by As and Cd is a well-documented challenge with no known 
solutions that address both contaminants. The driving factors that cause the 
contaminations are tied to irrigation practices that influence redox processes and 
contaminant sorption in paddy soils. Our results demonstrate that nanoparticle magnetite 
can be used to sorb As in reduced environments, while not mobilizing Cd, though you 
risk increasing CH4 emissions. They also demonstrate that in an oxic environment, 
nanoparticle MnO2 can reduce Cd availability. Both amendments had varying influence 
on CH4 and CO2 emissions so future research needs to elucidate the impacts of these 
amendments on emissions in a field study. Solutions to address these challenges are 
critical and many parts of the world are relying on them for growing rice in a safe matter. 
 18 
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