Soil Based Remediation of Wastewater

Ganga HettiarachchiDepartment of Agronomy

2017 Kansas Environmental Conference 08/10/2017

• Soils can be a source, sink, or interacting medium for many potential contaminants and pollutants• Trace elements of concern• Nutrients• Pesticides, organic compounds• Greenhouse gases: carbon dioxide, methane

• Focus of this presentation- redox-based treatments

Introduction

Flue gas desulfurization process

FGD system

Coal-fired power plants

FGD wastewater

Air pollution Water pollution

A set of technologies used to remove sulfur dioxide from exhaust flue gases of coal-fired power plants or any other sulfur dioxide emission processes

FGD wastewater

• High salinity

• Presence of trace elements of concernselenium, boron etc.

• Other major and minor constituents sulfur, calcium, sodium, chloride, bromide, etc.

• Chemical composition vary from site to site

4

Selenium• Micronutrient for humans &

animals

Narrow range between deficiency and toxicity

Become toxic very easily

• Sources of Se to the environment

Combustion of coal and petroleum fuels

• A constituent in FGD wastewater

• Kansas surface water quality standard (acute) 20 µg/L

Se toxicity-deformation of nails

Se toxicity to birds

Deficiency Toxicity

Se deficiency: Keshan disease

5

Wastewater Treatments

• ↑Regulations of effluent water quality →

Need for more economical wastewater

treatment systems

Picture courtesy: KSU Civil Eng.

Contaminant removal:Redox-based solutions

Redox – The oxidation/reduction status of a system Influences biological activity

Microorganisms: Influence on redox - All use an electron acceptor as part of their metabolism – O2, NO3

-, Fe3+, Mn4+, SO42-

Elements that change oxidation state in soilsRedox sensitive major elements:Nitrogen Sulfur Oxygen Iron ManganeseCarbonRedox sensitive trace elements:Selenium, Arsenic, Mercury, Chromium, Uranium, Cobalt, etc.

Reducing conditions can alter the chemistry of many soil constituents

Indirectly influence: phosphorus, lead, zinc, cadmium, etc.-Elements that chemisorb or precipitate

Change in oxidation state can

• Change element to a gaseous form, or vice versa• Insoluble ↔ soluble• Chemisorbed (strongly bound) ↔ Not chemisorbed• Cation ↔ Anion• Toxic ↔ Not toxic• Bioavailable ↔ Not bioavailable

Indirect effects:Reductive dissolution of Fe and Mn oxidesFormation of sulfide precipitatesFerrous phosphates/ferric phosphate ppt.

• Oxidized conditions Selenate [Se(VI)] Oxidized species

• Mild to highly reduced conditions Selenite [Se(IV)] , organic Se,

elemental selenium[Se(0)], and Reduced speciesselenide [Se(-II)]

Selenium behavior/speciation

Oxidation state mobility, bioavailability

10

Constructed wetland treatment systems (CWTS)

• Feasible approach to treating wastewater economically and environmentally

• Remove contaminants by physical, chemical, and biological treatment mechanisms

• Eggert et al., 2008 at Clemson University found CWTS efficiently remove selenium and mercury in FGD wastewater

11

Study Approach- Saturated soil columns

Influent solution: 1:1 mixture of FGD:rawwater

Influent rate 1.42 mL/hr, bottom-up feeding

For further info.: Galkaduwa et al., 2017.J. Environ. Qual. 46: 384-393

12

Findings

Breakthrough curve for Selenium

Pore volumes0 1 2 3 4 5 6 7 8 9 10

C/C

0

0.0

0.2

0.4

0.6

0.8

1.0Top soilEngineered soil C = Effluent concentration

C0 = Influent concentration

1 pore volume:Top soil = 10 daysEngineered soil = 9 days

Pore volume = volume of pores occupied by the solution

~ 100 % selenium retention

BTC for Boron

Pore volumes0 1 2 3 4 5 6 7 8 9 10

C/C

0

0.0

0.2

0.4

0.6

0.8

1.0Top Soil Engineered soil

After 100 days Top soil removed 81% Engineered soil removed 73% of B in the influent

Boron concentration in the influent 3.3 mg/L

Flushing experiment & soil sampling

• Two columns were flushed with river water for additional 100 days

• End of experiments Soil columns were partitioned into 6 sections

Columns partitioning Column section

15

Selenium- X-ray analysis

Beamline (“lab”)

Soil sample sandwiched between two Kapton tapes

Data collectionSample set up

Conducted at Advanced Photon Source, Argonne National Laboratory

Se-XANES analysis

Species K-edge energy (eV)

Sodium selenateSe(VI) 12664Sodium seleniteSe(IV) 12662Elemental SeSelenium mineralSe(0)

12658Seleno-DL-cysteineSe(Organic)/ Se(II)

12659.5Iron selenideSelenium sulphideSe(-II)

1265712658

Energy(eV)

12655 12660 12665 12670 12675 12680

Nor

mal

ized

Abs

orba

nce

Se (Organic)

Se (-II)

Se (-II)

Se (0)Se (0)

Se (IV)

Se XANES spectra for reference standards

Se(VI)

Se bulk- XANES

Speciation of sequestrated Se: Se(IV) and Se(0)

Continued submergence would reduce Se further

Se-XANES for Top soil columns (Bottom section)

Energy (eV)

12650 12660 12670 12680

Nor

mal

ized

Abs

orba

nce

Top soil (Original)Se(IV) = 40%Se(Organic) = 60%

100 days of FGDSe(IV) = 68%Se(0) = 32%

FlushedSe(IV) = 40%Se(0) = 60%

Se(

IV)

Se(

Org

anic

)S

e(0)

Soil spectraFit

Comparison: A pilot-scale CWTS

• Jeffrey Energy Center, St. Mary’s, Kansas

• Pilot-scale CWTS to treat FGD wastewater

19

Comparison of % removal of constituents by pilot-scale CWTS vs soil columns

Soil type Selenium % Boron % Fluoride %

Chloride%

Sulfate%

Top soil * 100 19% 78% -11 ~3

Engineered soil *

100 15% 67% -14 -11

* % removal of B, and F- - after 100 days of flushing with river water

• By CWTS

• By soil columns

Selenium%

Boron%

Fluoride%

Chloride %

Sulfate%

80 17 72 -3 -17

20

Challenges

Days

0 10 20 30 40 50 60

As c

once

ntra

tion

(µg/

L)

0

10

20

30

Non-treatedFh-treated

Soil arsenic mobilization due to long-term saturation

Fh= ferrihydrite(iron oxide)

Challenges (cont.)Variable performance due to high salinity?

Enhanced boron retention

Relatively poor and variable selenium retention

Relatively high arsenic release

Paredez et al., 2017Journal of Water Science and Technology (Accepted in July 2017)

Possible solutions

• Pretreatment options to remove salts

• Oxidizing zone for arsenic retention

• Maximize performance by adding

promising treatments such as iron

compounds

Modifications to treatment train

Acknowledgement

• Contributors and collaborators• Advanced Photon Source• Westar Energy, Burns & McDonnell