Water chemistry an introduction to the chemistry of natural and engineered aquatic systems

2 Inorganic Chemical Composition of Natural Waters:3 The Thermodynamic Basis for Equilibrium Chemistry 79... viii CONTENTS6 Fundamentals of Organic Chemistry for 9 Complexation Reactions

Water Chemistry

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Library of Congress Cataloging-in-Publication Data

ISBN 978-0-19-973072-8 (hardcover : alk paper) 1 Water chemistry.

I Arnold, William A II Title.

To our extended families:

Leo and Jeannette Brezonik (deceased)

Carol Brezonik Craig and Laura Nicholas and Lisa and Sarah, Joe, Billy, Niko, and Peter

Thomas and Carol Arnold

Maurice and Judith Colman; Lola Arnold (deceased)

Eric and Carly Arnold Lora Arnold and Alex and Ben

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2 Inorganic Chemical Composition of Natural Waters:

3 The Thermodynamic Basis for Equilibrium Chemistry 79

viii CONTENTS

6 Fundamentals of Organic Chemistry for

9 Complexation Reactions and Metal Ion Speciation 311

10 Solubility: Reactions of Solid Phases with Water 364

Part IV Chemistry of Natural Waters and Engineered Systems 449

13 Chemistry of Chlorine and Other

14 An Introduction to Surface Chemistry and Sorption 518

15 Aqueous Geochemistry II: The Minor Elements:

16 Nutrient Cycles and the Chemistry of Nitrogen and

17 Fundamentals of Photochemistry and Some

18 Natural Organic Matter and Aquatic Humic Matter 672

Appendix Free Energies and Enthalpies of Formation of Common

as it has developed over the past few decades These include nonequilibrium aspects(chemical kinetics) and organic chemistry—the behavior of organic contaminants andthe characteristics and behavior of natural organic matter In addition, most waterchemistry textbooks for environmental engineering students focus their examples onengineered systems and either ignore natural waters, including nutrient chemistry andgeochemical controls on chemical composition, or treat natural waters only briefly This

is in spite of the fact that environmental engineering practice and research focuses at least

as much on natural systems (e.g., lakes, rivers, estuaries, and oceans) as on engineeredsystems (e.g., water and wastewater treatment systems and hazardous waste processing).Most existing textbooks also focus on solving inorganic ionic equilibria using graphicaland manual algebraic approaches, and with a few exceptions, they do not focus on theuse of computer programs to solve problems

This book was written in an effort to address these shortcomings Our overall goals

in this textbook are to provide readers with (1) a fundamental understanding of thechemical and related processes that affect the chemistry of our water resources and (2)the ability to solve quantitative problems regarding the behavior of chemical substances

in water In our opinion, this requires knowledge of both inorganic and organic chemistryand the perspectives and tools of both chemical equilibria and kinetics The book thus

x PREFACE

takes a broader approach to the subject than previous introductory water chemistrytexts It emphasizes the use of computer approaches to solve both equilibrium andkinetics problems Algebraic and graphical techniques are developed sufficiently toenable students to understand the basis for equilibrium solutions, but the text emphasizesthe use of computer programs to solve the typically complicated problems that waterchemists must address

An introductory chapter covers such fundamental topics as the structure of wateritself, concentration units and conversion of units, and basic aspects of chemicalreactions Chapter 2 describes the chemical composition of natural waters It includesdiscussions on the basic chemistry and water quality significance of major and minorinorganic solutes in water, as well as natural and human sources and geochemicalcontrols on inorganic ions Chapters 3–7 cover important fundamentals and tools needed

to solve chemical problems The principles of thermodynamics as the foundation forchemical equilibria are covered first (Chapter 3), followed by a separate chapter onactivity-concentration relationships, and a chapter on the principles of chemical kinetics.Chapter 6 provides basic information on the structure, nomenclature, and chemicalbehavior of organic compounds Engineers taking their first class in water chemistrymay not have had a college-level course in organic chemistry For those that havehad such a course, the chapter serves as a review focused on the parts of organicchemistry relevant to environmental water chemistry Chapter 7 develops the basictools—graphical techniques, algebraic methods, and computer approaches—needed tosolve and display equilibria for the four main types of inorganic reactions (acid-base,solubility, complexation, and redox) The equilibrium chemistry and kinetics of thesemajor types of inorganic reactions are presented as integrated subjects in Chapters 8–11

Of the remaining eight chapters, six apply the principles and tools covered in the first

11 chapters to specific chemicals or groups of chemicals important in water chemistry:oxygen (12), disinfectants and oxidants (13), minor metals, silica, and silicates (15),nutrients (16), natural organic matter (18), and organic contaminants (19) The other twochapters describe two important physical-chemical processes that affect and sometimescontrol the behavior or inorganic and organic substances in aquatic systems: Chapter 14describes how solutes interact with surfaces of solid particles (sorption and desorption),and Chapter 17 describes the principles of photochemistry and the role of photochemicalprocesses in the behavior of substances in water

The book includes more material and perhaps more topics than instructors usuallycover in a single-semester course Consequently, instructors have the opportunity

to select and focus on topics of greatest interest or relevance to their course; werecognize that the “flavor” and emphasis of water chemistry courses varies depending

on the program and instructor Those wishing to emphasize natural water chemistry,for example, may wish to focus on Chapters 12, 15, 16, and 18 after covering theessential material in Chapters 1–11; others who want to focus on engineered systemsand contaminant chemistry may want to focus more on Chapters 13, 14, and 19 Within

several chapters, there also are Advanced Topic sections that an instructor may or may

not use With supplementary material from the recent literature, the book also may besuitable for a two-quarter or two-semester sequence

A strong effort was made to write the text in a clear, didactic style withoutcompromising technical rigor and to format the material to make the book inviting andaccessible to students We assume a fairly minimal prior knowledge of chemistry (one

PREFACE xiyear of general chemistry at the college level) and provide clear definitions of technicalterms Numerous in-chapter examples are included to show the application of theoryand equations and demonstrate how problems are solved, and we have made an effort toprovide examples that are relevant to both natural waters and engineered systems Theproblems included at the end of most of the chapters generally are ordered in terms ofdifficulty, with the easiest problems coming first Finally, we encourage readers to visitthe book’s companion Web site at www.oup.com/us/WaterChemistry, which containsdownloadable copies of several tables of data, an interface for the kinetics software,Acuchem, additional problems and figures, and other useful information.

Patrick L Brezonik and William A Arnold

University of Minnesota

February 2010

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We owe a great debt of gratitude to the individuals whose reviews of individual chaptersprovided many comments and suggestions that improved the content of the book, and weappreciate their efforts in finding errors We list them here alphabetically with chaptersreviewed in parentheses: Larry Baker (1, 2, 16), Paul Bloom (18), Steve Cabaniss (18),Paul Capel (4, 6, 19), Yu-Ping Chin (18, 19), Joe Delfino (1, 2), Baolin Deng (10, 11),Mike Dodd (13), Dan Giammar (7, 8), Ray Hozalski (13), Tim Kratz (12), Doug Latch(17), Alison McKay (4, 6), Kris McNeill (17), Paige Novak (6, 15), Jerry Schnoor (15),Timm Strathmann (3, 5), Brandy Toner (14), Rich Valentine (9, 11), and Tom Voice (8,9) Any remaining errors are the authors’ responsibility, but we sincerely hope that thereaders will find few or no errors We especially thank Mike Dodd for his extensivereview and suggestions for Chapter 13 and Paul Bloom and Yo Chin for their detailedreviews and input to Chapter 18

The senior author is happy to acknowledge the role that his previous book, Chemical

Kinetics and Process Dynamics in Aquatic Systems (CRC Press, 1994), played in

informing the writing of Chapters 5 and 17 and parts of Chapters 13 and 15 He alsoexpresses his thanks to the students in his 2008 and 2009 water chemistry classes, inwhich drafts of various chapters were used as the textbook, for their helpful commentsand for finding many errors Special thanks go to Mike Gracz, Ph.D student in Geologyand Geophysics, for his detailed reviews of the chapters The junior author hopes thatstudents in his future water chemistry classes do not find any mistakes

Several individuals were helpful in supplying data used in this book We are pleased toacknowledge Larry Baker, Joe Delfino, Paul Chadik, Charles Goldman, PatriciaArneson,and Ed Lowe for chemistry data used in Chapter 2; Tim Kratz and Jerry Schnoor fordissolved oxygen data used in Chapter 12; Rose Cory for fluorescence spectra and AbdulKhwaja for NMR spectra in Chapter 18; and Dan Giammar and Mike Dodd for some

of the problems at the ends of several chapters Several of the in-chapter examples

We happily acknowledge the excellent library system of the University of Minnesotaand the inventors of Internet search engines, which greatly facilitated our library researchand hunts for references, enabling us to continue this work wherever we could find anInternet connection.

The senior author thanks the Department of Civil Engineering and his environmentalengineering colleagues at the University of Minnesota for a light teaching load over thepast few years, which enabled him to focus his time and efforts on writing the book.The junior author wishes he had the same luxury, but he still managed to squeeze in

a fair bit of writing Both authors thank their colleagues and especially their familiesfor their understanding and patience when the writing absorbed their time We alsoappreciate the great work and helpful attitudes of the following staff at OUP and theirassociates in moving our manuscript through the publication stage: Jeremy Lewis, editor;Hallie Stebbins, editorial assistant; Patricia Watson, copy editor; Kavitha Ashok, ProjectManager; and Theresa Stockton and Lisa Stallings, Production Editors

Finally, we acknowledge with gratitude our predecessors in writing water chemistrybooks, starting with Werner Stumm and James J Morgan and extending to morerecent authors: Mark Benjamin, Philip Gschwend, Janet Hering, Dieter Imboden, DavidJenkins, James Jensen, Francois Morel, James Pankow, Rene Schwarzenbach, VernonSnoeyink, and others, on whose scholarly efforts our own writing has relied, and thecountless researchers, only some of whom are cited in the following pages, responsiblefor developing the knowledge base that now enriches the field of environmental waterchemistry

Symbols and Acronyms

i fraction of XTpresent as species i

  beam attenuation coefficient of light at wavelength

 cumulative stability (formation) constant

± mean ionic activity coefficient of a salt

 interfacial energy or surface tension (Chapters 3, 14)

O ligand field splitting parameter

A macroscopic binding parameter for sorbate A (Chapter 14)

−1 radius of ionic atmosphere (Debye parameter), and characteristic thickness

of the electrical double layer

xvi SYMBOLS AND ACRONYMS

f fundamental frequency factor

Scatchard equation variable (= [ML/LT])

 extent of reaction

susceptibility factor (Chapter 19)

w flushing coefficient for water in a reactor (= Q/V)

Hammett constant

 characteristic time

 quantum yield

0 electrostatic surface potential

ω electrostatic interaction factor

a light absorption coefficient at wavelength

ai activity of i

ai size parameter for ion i in Debye-Hückel equation

A preexponential or frequency factor in Arrhenius equation

at wt atomic weight

at no atomic number

b.p boiling point

c, C concentration

c correction factor (Chapter 19)

Cp heat capacity

Cp change in heat capacity for a reaction

D wavelength-dependent distribution function for scattered light (Chapter 17)

D diffusion coefficient

D distribution coefficient (Chapter 19)

D dielectric constant (relative static permittivity)

Do permittivity in a vacuum

D obs observed distribution coefficient (Chapter 4)

D theor thermodynamic distribution coefficient (Chapter 4)

Da daltons (molecular weight units)

E change in internal energy

eg type of molecular orbital

e−aq a hydrated electron

E◦ electrical (reduction) potential under standard conditions

Eact energy of activation

Ebg band gap energy

E0(,0) scalar irradiance just below the water surface

esu electrostatic units

f fraction of a substance in a specific phase (Chapter 19)

fi fugacity of substance i

fi fragment constants for fragment i (Chapter 19)

foc fraction of organic carbon

F Helmholtz free energy

F Faraday, unit of capacitance (Chapter 14)

F1 Gran function (used in alkalinity titrations)

F Faraday’s constant

SYMBOLS AND ACRONYMS xvii

G Gibbs free energy

G() total irradiance (sun+ sky) at the Earth’s surface

G◦f free energy of formation under standard conditions

G◦ change in free energy (or free energy of reaction under standard conditions)

G= free energy of activation

kPa kilopascals (unit of pressure)

k Boltzmann constant (gas constant per molecule)

k◦i molar compressibility of i

k rate constant

K diffuse attenuation coefficient of light at wavelength

K thermodynamic equilibrium constant (products and reactants expressed in

terms of activity)

cK equilibrium constant expressed in terms of concentrations of products and

reactants

Kd solid-water partition coefficient

KH Henry’s law coefficient (= H−1)

KL gas transfer coefficient (units of length time−1)

K L Langmuir sorption constant

Koc organic carbon-water coefficient

Kow octanol-water partition coefficient

Kw ion product of water

l (light path) length

L0 ultimate (first-stage) biochemical oxygen demand

m molal concentration

M molar concentration

MT total mass

m/z mass-to-charge ratio

n number (of molecules, atoms, or molecular fragments)

n nuclophilicity constant (Chapter 19)

N normality (equivalents/L)

N(K) probability function for equilibrium constant K

NA Avogadro’s number

pε negative logarithm of relative electron activity; a measure of the free energy

of electron transfer, pronounced “pea epsilon”

pH negative logarithm of hydrogen ion activity

pHPZC pH of point of zero charge on surfaces

pHZNPC pH of zero net proton charge

pX negative logarithm of X

xviii SYMBOLS AND ACRONYMS

P primary production (Chapter 12)

q charge density in diffuse layer (Chapter 14)

Q hydraulic flow rate

r ratio of peak areas determined via gas chromatography

R gas constant

R respiration (Chapter 12)

s substrate constant (Chapter 19)

s wavelength-dependent light-scattering coefficient

S saturation ratio

Sc dimensionless Schmidt number (kinematic viscosity/diffusion coefficient)

t2g type of molecular orbital

t½ half-life

tc critical time (time to achieve maximum DO deficit in Streeter-Phelps

model)

TOT X total concentration of X in all phases of a system

U10 wind velocity 10 m above the surface

V◦i standard molar volume for i

XT total concentration of all species of X in solution

Xmax maximum sorption capacity

y amount of O2consumed at any time in biochemical oxygen demand test

z depth (Chapter 17)

z, Z charge (on an ion)

ZAB collision frequency between A and B

Acronyms

SYMBOLS AND ACRONYMS xixBNC base neutralizing capacity

BOD biochemical oxygen demand

BTEX benzene, toluene, ethylbenzene, and xylene

CAS Chemical Abstract Service

CCM constant capacitance model

CD-MUSIC charge distribution multisite complexation (model)

CFSTR continuous-flow stirred tank reactor

cgs centimeter-gram-second, system of measure

CDOM colored (or chromophoric) dissolved organic matter

CP-MAS NMR cross-polarization-magic angle spinning nuclear magnetic

resonance (spectroscopy)CUAHSI Consortium of Universities for the Advancement of Hydrologic

Science, Inc

DBP disinfection by-product

DDT dichlorodiphenyltrichloroethane

DEAE diethylaminoethyl (functional group)

DFAA dissolved free amino acid

DHLL Debye-Hückel limiting law

DIC dissolved inorganic carbon

DOC dissolved organic carbon

DOM dissolved organic matter

DON dissolved organic nitrogen

DOP dissolved organic phosphorus

EAWAG German acronym for Swiss Institute for Water Supply, Pollution

Control, and Water ProtectionEDHE extended Debye-Hückel equation

EDTA ethylenediaminetetraacetic acid

EfOM effluent organic matter

EPC equilibrium phosphorus concentration

EPICS equilibrium partitioning in closed systems

EPI Suite Estimation Programs Interface Suite

EXAFS extended x-ray absorption fine structure spectroscopy

FITEQL nonlinear data fitting program

FMO frontier molecular orbital (theory)

FT-ICR MS Fourier transform ion cyclotron resonance mass spectrometryGC-MS gas chromatography-mass spectrometry

xx SYMBOLS AND ACRONYMS

GCSOLAR public domain computer program to calculate light intensity and

rates of direct photolysis

HMB heteropoly-molybdenum blue

HMWDON high-molecular-weight dissolved organic nitrogen

HOMO highest occupied molecular orbital

HPLC high-performance liquid chromatography

HRT hydraulic residence time

HSAB (Pearson) hard-soft acid-base (system)

IAP ion activity product

ICP inductively coupled plasma

IHSS International Humic Substances Society

IMDA imidodiacetic acid

IUPAC International Union of Pure and Applied Chemistry

LC-MS liquid chromatography-mass spectrometry

LED light-emitting diode

LFER linear free energy relationship

LFSE ligand-field stabilization energy

LMCT ligand-to-metal charge transfer (process)

LUMO lowest unoccupied molecular orbital

MCL maximum contaminant level

MEMS microelectromechanical system

MINEQL computer program to calculate mineral equilibria

MINEQL+ commercially available equilibrium computer program based on

MINEQLMINTEQA2 public domain computer program based on MINEQL

NADP National Atmospheric Deposition Program

NCH noncarbonate hardness

NDMA N-nitrosodimethylamine

NMR nuclear magnetic resonance (spectroscopy)

NOM natural organic matter

NTA nitrilotriacetic acid

NTU nephelometric turbidity unit

PAH polycyclic aromatic hydrocarbon

PBDE polybrominated diphenylether

PCB polychlorinated biphenyl

PCDD/Fs polychlorinated dibenzodioxins/furans

PCE perchloroethylene or tetrachloroethylene

PCU platinum-cobalt color unit (Hazen unit)

SYMBOLS AND ACRONYMS xxiPES potential energy surface

PFOA perfluorooctanoic acid

PFOS perfluorooctane sulfonic acid

PON particulate organic nitrogen

POP persistent organic pollutant

PP particulate phosphorus

PPC products of proton consumption

PPCPs pharmaceutical and personal care products

PPR products of proton release

RPHPLC reverse-phase high-performance liquid chromatography

SC specific conductance (same as EC)

SEC size-exclusion chromatography

SII specific ion interaction

SMILES Simplified Molecular Input Line Entry System

SMP soluble microbial products

SRFA Suwannee River fulvic acid

SRP soluble reactive phosphate (expressed as P)

STP standard temperature and pressure

SUVA specific ultraviolet absorption

TCP 2,4,6-trichlorophenol

TDP total dissolved phosphorus

TDS total dissolved solids

TOC total organic carbon

TON total organic nitrogen

TST transition state theory

U.S EPA U.S Environmental Protection Agency

USGS U.S Geological Survey

VOC volatile organic compound

WWTP wastewater treatment plant

XANES x-ray absorption near-edge spectroscopy

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Units and Constants

Units for physical quantities

Fundamental quantities Some derived quantities

Important constants

Avogadro’s constant (number) 6.022× 1023mol−1

xxiv UNITS AND CONSTANTS

Gas constant per mole, R 8.314 J mol−1K−1or 1.987 cal mol−1K−1

Gas constant per molecule, k,

called the Boltzmann constant

1.3805× 10−23J K−1

Gravitation constant (of the

Earth)

9.806 m s−2Melting point of water 0◦C or 273.15 K

Molar volume of an ideal gas at

0◦C and 1 atm

22.414 L mol−1or 22.414× 103cm−3mol−1Molecular vibration period 1.5× 10−13s

Permittivity of a vacuum,ε0 8.854× 10−12J−1C2m−1

Planck’s constant, h 6.626× 10−34J s

Relative static permittivity of

water, D, also called the

dielectric constant)

80 (dimensionless) at 20◦C

Speed of light (in a vacuum), c 2.998× 108m s−1

Conversion Factors

Energy-related quantities

1 newton (unit of force)= 1 N = kg m s−2

1 joule (unit of energy)= 107erg= 1 N m = kg m2s−2= 1 volt coulomb (V C) =0.239 calories (cal)= 9.9 × 10−3L atm−1= 6.242 × 1018eV

1 cal= 4.184 J

1 watt= 1 kg m2s−3= 1 J s−1= 2.39 × 10−4kcal s−1= 0.86 kcal h−1

1 entropy unit= 1 cal mol−1K−1= 4.184 J mol−1K−1

Pressure

1 atm= 760 mm Hg = 1.013 × 105Pa (pascals)= 1.013 bars

1 mm Hg= 1 torr

1 Pa= 10−5bars= 1 N m−2

Some useful relationships

RT ln x = 2.303RT log x = 5.709 log x (kJ mol−1)= 1.364 log x (kcal mol−1) at 25◦C

(298.15 K)

(RT/F) ln x = 2.303RT/F log x = 0.05916 log x (V at 25◦C, or 59.16 mV at 25◦C)

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Water Chemistry

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Prologue

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Introductory Matters

Objectives and scope of this chapter

This chapter sets the stage for the rest of the book by addressing four main topics First, abrief introduction to the history of water chemistry and its relationship to other branches

of environmental chemistry provides context for the topics treated in this book Second,

a description of the unique properties of water and their relationship to its molecularand macroscopic structure provides an appreciation for the complexity of the mediumthat supports the chemistry we wish to understand Third, many different units—somecommon, some standard chemical units, and some unique to environmental engineeringand chemistry—are used to report concentrations of chemicals in water We introducethese units and show how to use them and make interconversions among them as afirst step in describing the chemistry of water quantitatively Finally, we introduce themajor types of chemical reactions occurring in natural and engineered water systemsand briefly describe the kinds of equations used to quantify the equilibrium conditions

to which they tend and rates at which they occur

Key concepts and terms

• The “overlapping neighborhoods” of water chemistry, geochemistry,

biogeochemistry, soil chemistry, and many other branches of the environmentalsciences

• The unique and extreme physical-chemical properties of water

• Liquid water as a structured fluid caused by extensive hydrogen bonding to formwater “clusters”

5

• Concentration units unique to water chemistry: mg/L as CaCO3; mg/L as N, P, Cl,

or other elemental (atomic) components of ions and molecules

• Associative reactions: acid-base, solubility (precipitation and dissolution),complex formation and dissociation

• Redox processes: oxidation as a loss of electrons; reduction as a gain in electrons

• Sorption: a phase-transfer process

• Gas transfer and Henry’s law

As a recognized field of inquiry, water chemistry developed in the mid-twentieth century(late 1950s and early 1960s) at the dawn of the “environmental era.” Its origins assubareas of specialization in many other disciplines date back, however, to the earlytwentieth century and even before For example, the chemical composition of lakesand the oceans has been of interest to limnologists and oceanographers since the earlydevelopment of those sciences in the late nineteenth century Similarly, geochemistslong have been interested in the composition of natural waters, in addition to long-standing interests in the composition of geosolids Water chemistry also was a significantcomponent of the field of environmental engineering (or sanitary engineering, as it wasknown prior to about 1960), in part because of the important role of chemical processes indrinking water treatment In all these examples, however, chemistry played a supportingrather than a central role Analytical and descriptive aspects of chemistry were the prime

considerations, and chemistry was viewed primarily as a tool to be used by scientists and

engineers in the above disciplines rather than a subject worthy of intellectual inquiry inits own right

One of the seminal papers in the transformation of water chemistry from itssupporting role in science to a science having its own intellectual merit is “ThePhysical Chemistry of Seawater,”1 written in 1960 by Lars Gunner Sillen (1916–1970), a prominent inorganic coordination chemist from Sweden In this paper Sillenexamined the geochemical origins of seawater and insightfully described the chemistry

of seawater as resulting from a “geotitration” of basic rocks by volatile acids, such

as carbon dioxide and acids from volcanic emissions This descriptive model led tomany other articles on the geochemical origins of natural waters and to the long-heldinterests of aquatic chemists and geochemists in chemical models2 and weatheringprocesses.3 Sillen’s paper also described the “speciation” of a wide variety of metalions in seawater—that is, the nature of the chemical complexes in which they occur

in seawater Those ideas led to many further studies to quantify and explain thechemical composition of natural waters, and these efforts ultimately resulted in thedevelopment of computer codes used to calculate chemical speciation in complicatedaquatic systems

At the same time, the field of environmental engineering was evolving fromits narrower predecessor field, sanitary engineering, which had been focused on

INTRODUCTORY MATTERS 7drinking water and wastewater treatment, to encompass the broader goals of under-

standing environmental systems (including atmospheric and terrestrial components)

and developing the technical tools to manage, protect, and, where necessary, restoreenvironmental quality Leaders of this emerging field realized that a more fundamentalscientific approach was needed to develop the understanding needed to provide soundscientific underpinnings for environmental policy and management To the considerableextent that the field of water chemistry was developed by scientists and engineersworking in or associated with environmental engineering programs, these considerationsalso played an important role in developing the new science of water chemistry.Academic programs initiated around 1960, like the ones led by Werner Stumm (seeBox 1.1) at Harvard University and G Fred Lee at the University of Wisconsin,espoused a fundamental approach to water chemistry that emphasized scientific rigorand quantitative approaches, involving the two cornerstones of physical chemistry—thermodynamics and kinetics These programs also emphasized the commonality ofchemical principles across all kinds of natural and engineered water systems and forgedlinks with marine chemists, limnologists, soil chemists, and scientists in other relatedfields Thus, water chemistry is linked to a wide range of earth and environmentalsciences (Figure 1.1) Moreover, a multidisciplinary perspective that includes theprinciples of chemistry but is not limited to them is now understood to be essential

for a holistic understanding of the biogeochemical processes that affect the composition

of aquatic environments

No field of science can exist for long without an organizing framework articulated in

a textbook Building upon an important foundation provided by an earlier geochemistrytext by Garrels and Christ,4 Stumm and Morgan provided this broad perspective in

1970 in the first text of the field, Aquatic Chemistry.5 Through three editions, the

most recent published in 1996, Aquatic Chemistry continued to set a high standard

for the field In the sense of being quantitative and focused on both understanding

systems and solving problems, the text has an engineering orientation, but it also

provides a multidisciplinary approach to understanding the chemistry of water innatural and engineered environments Several other textbooks, modeled to greater

or lesser extents after Stumm and Morgan’s text, have been published over the pastquarter century.6−11Some provide a more didactic approach suitable for students withlimited academic backgrounds in chemistry,6,11and all focus mostly (or exclusively) on

inorganic equilibrium chemistry

In contrast to the focus of water chemistry textbooks on inorganic and equilibriumchemistry, as the field itself has continued to develop and mature, increasing emphasishas been placed on two other major subjects: (1) the kinetics of chemical reactions—

natural waters as dynamic systems, and (2) the behavior of organic compounds that

contaminate natural waters as a result of their production and use by humans The list ofsuch compounds is too long to enumerate, but categories of current interest includepesticides, polyhalogenated aromatic compounds, chlorinated solvents, polycyclicaromatic hydrocarbons, and more recently, a variety of pharmaceuticals, antibiotics,personal care products, and perfluorinated compounds Despite the importance ofthese contaminants for water quality and ecosystem health and the massive amounts

of research undertaken for more than 40 years by environmental engineers andscientists to understand the behavior, fate, and effects of these compounds, the book

8 WATER CHEMISTRY

Box 1.1 Werner Stumm

Werner Stumm (1924–1999) is widely considered to be the founding father ofwater chemistry His contributions to the development of the field are bothbroad and deep, not only in terms of the span of his research contributions,but equally important as the mentor of many students who became leaders inthe field and his authorship (with James Morgan, his first Ph.D student) of the

important textbook Aquatic Chemistry (1970) Stumm was born in Switzerland

and received his Ph.D in inorganic chemistry from the University of Zurich

in 1952 under G Schwarzenbach, a coordination chemist who pioneered in theuse of complexing agents (e.g., EDTA) in analytical chemistry Stumm spent

15 years at Harvard University (1956–1970), where he developed a strongresearch and teaching program, mentoring many of the future leaders of waterchemistry and environmental engineering He returned to Switzerland in 1970 as

a professor at the Swiss Federal Institute of Technology and Director of EAWAG,the Swiss Institute for Water Supply, Pollution Control, and Water Protection,which he led until his retirement in 1992 Under his direction, EAWAG becamethe preeminent research institute for aquatic sciences in the world, recruitingoutstanding scientists and engineers to its staff and attracting numerous scientistsfor sabbatical visits Stumm’s approach to aquatic chemistry was fundamentaland multidisciplinary He emphasized molecular-level studies and application

of the principles of physical chemistry to develop both an understanding ofchemical processes in natural systems and science-based applications in watertechnology Stumm emphasized an ecosystem perspective that integrates theunderstanding of chemical, geochemical, biological, and physical processesoccurring within aquatic systems He was the author, coauthor, or editor of

300 research publications and 16 books during his illustrious career, and hisresearch spanned most of the field of water chemistry: ionic equilibria, kinetics ofiron and manganese oxidation, corrosion chemistry, coagulation and flocculationprocesses, evolution of the chemical composition of natural waters, phosphoruscycling and eutrophication, acid rain and its effects of chemical weatheringand lake chemistry, and chemical processes at water-solid interfaces (aquaticsurface chemistry) He received many awards during his life, including honorarydoctorates, the Tyler Prize, Stockholm Water Prize, and the Goldschmidt Medal

INTRODUCTORY MATTERS 9

WATER CHEMISTRY

Water and wastewater treatment chemistry

Atmospheric chemistry

Atmospheric Science

Limnological chemistry

Aquatic

Ecology

Fate and behavior

of organic chemicals

Environmental Engineering

Marine

chemistry

Oceanography

Aqueous geochemistry

Geology,

Geochemistry

Soil Science

Soil and sediment chemistry

Biogeochemistry

Fate and behavior

of inorganic chemicals

Fundamental fields of chemistry

Surface chemistry

Acid deposition

Nutrient chemistry

Figure 1.1 Overlapping neighborhoods of the subfields in water chemistry and relateddisciplines The size of the ovals is not intended to indicate the importance of a given subfield

or discipline, and the extent (or lack) of overlap of ovals reflects drawing limitations morethan the extent of concordance among subfields and disciplines In the interest of clarity,not all active and potential interactions are illustrated; the double-headed arrow shows oneobvious interaction not otherwise indicated in the diagram—between nutrient chemistry andenvironmental engineering and its subfield of water/wastewater treatment

by Schwarzenbach et al.12is the only comprehensive text on the environmental aquaticchemistry of organic contaminants (although a few earlier books and monographs dealtwith components of the subject)

By design, the scope of the present book is broader than other introductory waterchemistry texts As described above, environmental organic contaminant chemistry

is a major focus of research in water chemistry and environmental engineering, and

a substantial fraction of professional practice in these fields involves cleanup orremediation of sites degraded by organic chemicals Consequently, the authors believe it

is important for an introductory textbook on water chemistry to cover this subject matterand to describe the types of chemical reactions, including photochemical processes,whereby such compounds are transformed in aquatic environments Similarly, becausewater chemistry has expanded beyond its origins in equilibrium chemistry, we coverthe principles of chemical kinetics in some detail and devote significant portions of thetext to describing the rates at which processes occur in water, as well as the equilibriumconditions toward which they are headed

10 WATER CHEMISTRY

Water is by far the most common liquid on the Earth’s surface, and its unique propertiesenable life to exist Water is usually regarded as a public resource—a common good—because it is essential for human life and society However, water also is an economicresource and is sold as a commodity, and water rights in the American West arecontinuous source of conflict Beyond these perspectives, water holds a special place

in human society It is not an exaggeration to speak of its mystical and transcendentproperties Water has spiritual values in many cultures and is associated with birth,spiritual cleansing, and death The fact that about 70% of the Earth’s surface is covered

by water and only 30% by land makes one wonder whether “Planet Earth” more properlymight be named “Planet Water.” For chemists, water is a small, simple-looking, andcommon molecule, H2O, but they also know it has many unusual and even uniqueproperties, as discussed below For civil engineers, water is a fluid to be transported viapipes and channels, and when it occurs in rivers and streams, it is viewed partly as anobstacle to transportation, for which bridges need to be designed and constructed, andpartly as an energy-efficient means of transportation and shipping Scientists in manyother disciplines have their own viewpoints about water that reflect how it interacts withtheir science

Water is the medium for all the reactions and processes that comprise the focus ofthis book In this section we focus on the properties of water itself We then describe itsmolecular structure and show how that structure leads to the macroscopic structures ofliquid, solid and gaseous water and their unusual physicochemical properties

Compared with other small molecules, water has very high melting and boiling pointtemperatures:13

Compound Formula Mol Freezing Boiling

of vaporization and fusion, and dielectric constant (or “relative static permittivity”); seeTable 1.1 The latter characteristic measures the attenuation rate of coulombic forces in

a solvent compared to attenuation in a vacuum, and it is important for the dissolution ofsalts in water The high dielectric constant of water permits like-charged ions to approacheach other more closely before repulsive coulombic forces become important than isthe case in solvents with low dielectric constants Consequently, it is a key propertyenabling water to be such a good solvent for salts

In general, the unusual physical properties of liquid water reflect the fact that watermolecules do not behave independently Instead, they are attracted to each other and tomany solutes by moderately strong “hydrogen bonds.” The hydrogen bonds in ice and

Hydrogen bonding is a consequence of the basic molecular structure of water Theangle between the two O–H bonds (105◦) in water is greater than the 90◦expected for

perpendicular p-orbitals (Figure 1.3) This is caused by repulsion between the hydrogen atoms and indicates that there is some hybridization of the s and p orbitals in the

electron shell of the oxygen atom Oxygen’s four remaining valence electrons occupytwo orbitals opposite the hydrogen atoms in a distorted cube arrangement This explainsthe molecule’s large dipole moment These electron pairs attract hydrogen atoms ofadjacent water molecules and form hydrogen bonds with lengths of 1.74 angstroms(measured in ice by x-ray diffraction), which leads to the three-dimensional structurefound in ice and liquid water

The structure of ice is known with great accuracy.14Ice-Ih, the form that occurs underenvironmental conditions, has a structure in which each water molecule is surrounded

by the oxygen atoms of four adjacent water molecules in a tetrahedral arrangement(Figure 1.4) Extending this arrangement in three dimensions gives rise to a fairly

12 WATER CHEMISTRY

Table 1.1 Physical properties of water∗

Property Value Comparison with Environmental

other liquids importance

Very high Moderates climateLatent heat of

fusion 330 J/g

stabilizes airtemperaturesLatent heat of

maximum at

4◦C

Causes freezing to occurfrom air-water surface;controls temperaturedistribution and watercirculation in lakes andoceans

Surface tension 72.8 dyne/cm at 20◦C

72.8 mN/m

Very high Affects adsorption, wetting,

and transport acrossmembranesDielectric

constant§

80.1 at 20◦C(dimensionless)

Very high Makes water a good

solvent for ions; shieldselectric fields of ionsDipole moment 1.85 debyes High compared

with organicliquids

Cause of abovecharacteristics andsolvent properties ofwater

Viscosity 1.0 × 10−3Pa·s

1 centipoise (cP)

at 20◦C

2–8× higher thanorganic liquids

Slows movement of solutes

midvisible range

Allows thick zone forphotosynthesis andphotochemistryThermal

conductivity

0.6 W m−1K−1 High compared

with organicliquids

Critical for heat transfer innatural and engineeredsystems

∗Adapted from Horne.14

§ Also called relative static permittivity.

INTRODUCTORY MATTERS 13

0.96 Å

104.5°

Figure 1.3 H-O bond angle and lengths in the H2O molecule (left) lead to high polarity

of the molecule with the hydrogen atoms (right: positive, light gray mesh) on one sideand the unshared electron pairs (right: negative, dark gray mesh) on the opposite side.(See color insert at end of book for a color version of this figure.)

Figure 1.4 Tetrahedralarrangement ofhydrogen-bonded watermolecules in ice leads to anopen hexagonal ring structure

in crystalline ice-Ih Source:Wikimedia Commons, fileHex ice.GIF (public domain).(See color insert at end ofbook for a color version ofthis figure.)

open—i.e., low-density—crystalline structure of repeating hexagonal rings (hence thesubscript h in Ih), each of which contains six water molecules The density of icecalculated from measured bond lengths and the three-dimensional structure agrees withthe measured density of ice

In contrast to the rigid crystalline structure of ice, gaseous water has no structurebeyond that of the individual water molecules themselves, except for occasional,

of environmental chemistry. .. dioxide and acids from volcanic emissions This descriptive model led tomany other articles on the geochemical origins of natural waters and to the long-heldinterests of aquatic chemists and geochemists... coagulation and flocculationprocesses, evolution of the chemical composition of natural waters, phosphoruscycling and eutrophication, acid rain and its effects of chemical weatheringand lake chemistry, and