mechanistic understanding of fate and transport of selenium, arsenic, and sulfur in a pilot-scale

285 Pages·2015·3 MB·English

MECHANISTIC UNDERSTANDING OF FATE AND TRANSPORT OF SELENIUM,
ARSENIC, AND SULFUR IN A PILOT-SCALE CONSTRUCTED WETLAND TREATMENT
SYSTEM DESIGNED FOR FLUE-GAS DESULFURIZATION WASTEWATER

by

MADHUBHASHINI BUDDHIKA GALKADUWA

B.S., University of Peradeniya, Sri Lanka, 2006
M.S., Post Graduate Institute of Science, University of Peradeniya, Sri Lanka, 2008

AN ABSTRACT OF A DISSERTATION

submitted in partial fulfillment of the requirements for the degree

DOCTOR OF PHILOSOPHY

Department of Agronomy
College of Agriculture

KANSAS STATE UNIVERSITY
Manhattan, Kansas

2015
Abstract
Constructed wetland treatment systems (CWTSs) are an alternative adaptation for flue-
gas desulfurization (FGD) wastewater purification. A series of laboratory-based soil column
studies mimicking a pilot-scale CWTS was carried out to evaluate the performance of the
treatment system in detail. The main objectives of studies were to (1) understand the transport
characteristics, retention capacity and transformation of selenium and other FGD constituents in
the CWTS, (2) evaluate the effectiveness of soil treatments and influent flow rate on the
performance of the CWTS, and (3) develop a mechanistic understanding of the CWTS
performance through monitoring interrelationships of selenium (Se), arsenic (As), iron (Fe), and
sulfur (S). Ferrihydrite (1% w/w), and labile organic carbon (OC) were used as soil treatments.
Different influent flow rates, X (1.42 mL/hour), 2X, or 1/2X were used depending on the
objectives of each study. Deoxygenated 1:1 mixture of FGD: raw water was the influent. It was
delivered to the saturated columns with an upward flow. Effluent samples were collected
continuously, and analyzed for constituents of concern. End of these experiments, soil from
sectioned columns were used for total elemental analysis, sequential extraction procedure (SEP)
for Se, and synchrotron-based X-ray spectroscopy analyses. Results indicated a complete Se
retention by the columns. Boron, and fluorine partially retained whereas sodium, sulfur, and
chlorine retention was weak, agreeing with field observations. Some of the initially-retained Se
(~ 4 to 5%) was mobilized by changing redox conditions in the soil. Selenium fed with the
wastewater accumulated in the bottom 1/3 (inlet) of the soil columns and was mainly
sequestrated as stable forms revealed by SEP. Bulk-, and micro-XANES analyses suggested the
retention mechanism of Se from the FGD wastewater was via the transformation of Se into
reduced/stable forms [Se(IV), organic Se, and Se(0)]. Under wetland conditions, native soil As
was mobilized by reductive dissolution of As associated minerals. However, the ferrihydrite
amendment suppressed the native soil As mobility. Micro-XRF mapping integrated with As, and
Fe-XANES suggested that the mechanism of native soil As retention was the sequestration of
released As with newly precipitated secondary Fe minerals. A long-term study carried out with
X, 1/2X flow rates, and OC source indicated enhanced S retention by the slow flow rate (1/2X),
most likely due to the time dependency of biogenic S reduction. Further, bulk S-, As-, and Fe-
XANES revealed that long submergence period and the slow flow rate increased the formation of
reduced and/or biogenic S, realgar-like, and greigite-like species. These observations indicated
that modified flow rates could have a significant impact on the long-term trace element (such as
As) sequestration in the CWTS. Our studies provide useful information to improve the
performance, and longevity of a full-scale CWTS for FGD wastewaters.
MECHANISTIC UNDERSTANDING OF FATE AND TRANSPORT OF SELENIUM,
ARSENIC, AND SULFUR IN A PILOT-SCALE CONSTRUCTED WETLAND TREATMENT
SYSTEM DESIGNED FOR FLUE-GAS DESULFURIZATION WASTEWATER

by

MADHUBHASHINI BUDDHIKA GALKADUWA

B.S., University of Peradeniya, Sri Lanka, 2006
M.S., Post Graduate Institute of Science, University of Peradeniya, Sri Lanka, 2008

A DISSERTATION

submitted in partial fulfillment of the requirements for the degree

DOCTOR OF PHILOSOPHY

Department of Agronomy
College of Agriculture

KANSAS STATE UNIVERSITY
Manhattan, Kansas

2015

Approved by:

Major Professor
Ganga M. Hettiarachchi
Copyright
MADHUBHASHINI BUDDHIKA GALKADUWA
2015
Abstract
Constructed wetland treatment systems (CWTSs) are an alternative adaptation for flue-
gas desulfurization (FGD) wastewater purification. A series of laboratory-based soil column
studies mimicking a pilot-scale CWTS was carried out to evaluate the performance of the
treatment system in detail. The main objectives of studies were to (1) understand the transport
characteristics, retention capacity and transformation of selenium and other FGD constituents in
the CWTS, (2) evaluate the effectiveness of soil treatments and influent flow rate on the
performance of the CWTS, and (3) develop a mechanistic understanding of the CWTS
performance through monitoring interrelationships of selenium (Se), arsenic (As), iron (Fe), and
sulfur (S). Ferrihydrite (1% w/w), and labile organic carbon (OC) were used as soil treatments.
Different influent flow rates, X (1.42 mL/hour), 2X, or 1/2X were used depending on the
objectives of each study. Deoxygenated 1:1 mixture of FGD: raw water was the influent. It was
delivered to the saturated columns with an upward flow. Effluent samples were collected
continuously, and analyzed for constituents of concern. End of these experiments, soil from
sectioned columns were used for total elemental analysis, sequential extraction procedure (SEP)
for Se, and synchrotron-based X-ray spectroscopy analyses. Results indicated a complete Se
retention by the columns. Boron, and fluorine partially retained whereas sodium, sulfur, and
chlorine retention was weak, agreeing with field observations. Some of the initially-retained Se
(~ 4 to 5%) was mobilized by changing redox conditions in the soil. Selenium fed with the
wastewater accumulated in the bottom 1/3 (inlet) of the soil columns and was mainly
sequestrated as stable forms revealed by SEP. Bulk-, and micro-XANES analyses suggested the
retention mechanism of Se from the FGD wastewater was via the transformation of Se into
reduced/stable forms [Se(IV), organic Se, and Se(0)]. Under wetland conditions, native soil As
was mobilized by reductive dissolution of As associated minerals. However, the ferrihydrite
amendment suppressed the native soil As mobility. Micro-XRF mapping integrated with As, and
Fe-XANES suggested that the mechanism of native soil As retention was the sequestration of
released As with newly precipitated secondary Fe minerals. A long-term study carried out with
X, 1/2X flow rates, and OC source indicated enhanced S retention by the slow flow rate (1/2X),
most likely due to the time dependency of biogenic S reduction. Further, bulk S-, As-, and Fe-
XANES revealed that long submergence period and the slow flow rate increased the formation of
reduced and/or biogenic S, realgar-like, and greigite-like species. These observations indicated
that modified flow rates could have a significant impact on the long-term trace element (such as
As) sequestration in the CWTS. Our studies provide useful information to improve the
performance, and longevity of a full-scale CWTS for FGD wastewaters.
Table of Contents
List of Figures .......................................................................................................................... xii
List of Tables ............................................................................................................................ xx
Abbreviations ......................................................................................................................... xxii
Acknowledgements ............................................................................................................... xxiii
Dedication .............................................................................................................................. xxiv
Chapter 1 - Introduction ..............................................................................................................1
References ..................................................................................................................................7
Chapter 2 - Literature review ..................................................................................................... 14
Coal-fired power plants ......................................................................................................... 14
Flue-gas desulfurization (FGD) systems ................................................................................ 15
Flue-gas desulfurization wastewater ...................................................................................... 17
FGD wastewater treatment strategies ..................................................................................... 20
Constructed wetland treatment systems (CWTSs) ................................................................. 22
Selenium ............................................................................................................................... 32
Selenium in the environment ............................................................................................. 32
Selenium and health .......................................................................................................... 36
Selenium adsorption in soil ................................................................................................ 37
Selenium precipitation ....................................................................................................... 42
Selenium in wetlands ......................................................................................................... 42
Arsenic .................................................................................................................................. 44
Arsenic in the environment ................................................................................................ 44
Arsenic and health ............................................................................................................. 47
Arsenic adsorption in soils ................................................................................................. 48
Arsenic precipitation ......................................................................................................... 51
Reductive dissolution of arsenic ........................................................................................ 52
Biogeochemical cycling of sulfur, and iron............................................................................ 54
Sulfur cycling .................................................................................................................... 54
Iron cycling ....................................................................................................................... 55
References ................................................................................................................................ 58
viii
Chapter 3 - Transport and transformation of selenium and other constituents of flue-gas
desulfurization wastewater in a constructed wetland treatment system ................................ 71
Abstract................................................................................................................................. 71
Introduction........................................................................................................................... 72
Materials and Methods .......................................................................................................... 75
Collection of soil and FGD wastewater .............................................................................. 75
Set-up of column study ...................................................................................................... 76
Analysis of influent and effluent ........................................................................................ 77
Soil sampling and analysis ................................................................................................. 78
Sequential extractions procedure ....................................................................................... 78
Selenium bulk-XANES analysis ........................................................................................ 79
Selenium micro-XRF and micro-XANES analysis ............................................................. 80
Results and Discussion .......................................................................................................... 81
Characterization of FGD influent ....................................................................................... 81
pH of effluents and soils .................................................................................................... 82
Breakthrough curves (BTCs) ............................................................................................. 82
Distribution of Se in the soil columns ................................................................................ 86
Selenium fractionation ....................................................................................................... 87
Selenium solid state speciation using synchrotron-based XANES ...................................... 89
Conclusions........................................................................................................................... 92
Acknowledgements ............................................................................................................... 93
References ................................................................................................................................ 95
Chapter 4 - Minimizing arsenic mobility using ferrihydrite in a pilot-scale constructed wetland
treatment system designed for flue-gas desulfurization wastewater ................................... 122
Abstract............................................................................................................................... 122
Introduction......................................................................................................................... 123
Materials and Methods ........................................................................................................ 126
Collection of soil and FGD wastewater ............................................................................ 126
Synthesis of 2-line ferrihydrite......................................................................................... 127
Setting-up of soil column study ....................................................................................... 127
Drying and re-wetting experiment ................................................................................... 128
ix
Analysis of influent and effluent ...................................................................................... 128
Soil sampling and analysis ............................................................................................... 129
Arsenic micro-XRF mapping and micro-XANES analysis ............................................... 129
Arsenic bulk-XANES analysis ......................................................................................... 130
Iron bulk-XANES analysis .............................................................................................. 130
Statistical Analysis .......................................................................................................... 131
Results and Discussion ........................................................................................................ 131
pH of effluent and soil ..................................................................................................... 131
Behavior of selenium ....................................................................................................... 132
Behavior of arsenic .......................................................................................................... 133
Behavior of iron .............................................................................................................. 135
Behavior of sulfate .......................................................................................................... 136
Behavior of Se and As in drying and re-wetting soils ....................................................... 136
Results from soil analysis ................................................................................................ 137
Selenium distribution within the soil columns .............................................................. 137
Arsenic distribution within soil columns ...................................................................... 138
Spatial distribution of As, Fe, and Mn in the soil ............................................................. 139
Soil arsenic speciation ..................................................................................................... 140
Soil iron speciation .......................................................................................................... 143
Conclusions......................................................................................................................... 144
Acknowledgements ............................................................................................................. 144
References .............................................................................................................................. 146
Chapter 5 - Understanding of the retention of trace elements by sulfate reduction in a pilot-scale
constructed wetland treatment system designed for flue-gas desulfurization wastewater ... 180
Abstract............................................................................................................................... 180
Introduction......................................................................................................................... 181
Materials and Methods ........................................................................................................ 184
Soil and FGD wastewater collection ................................................................................ 184
Packing of soil columns and FGD wastewater feeding ..................................................... 185
Analysis of influent and effluent ...................................................................................... 186
Long-term soil column study ........................................................................................... 187
x