ENVIRONMENTAL CHEMISTRY OF SOILS ENVIRONMENTAL CHEMJSTRY OF SOILS Murray B McBride :'\l"ew York Oxford OXFORD UNIVERSITY PRESS 1994 :.9 Oxford University Press Oxford New York Toronto Delhi Bombay Calcutta Madras Karachi Kuala Lumpur Singapore Hong Kong Tokyo Nairobi Dar es Salaam Cape Town Melbourne Auckland Madrid and associated companies in Berlin Ibadan Copyright © 1994 by Oxford University Press, Inc Published by Oxford University Press, Inc 198 Madison Avenue, New York, New York 10016-4314 Oxford is a registered trademark of Oxford University Press All rights reserved No part of this publication may be reproducted, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Oxford University Press Library of Congress Cataloging-in-Publication Data McBride, Murray Environmental chemistry of soils / Murray McBride p em Includes index ISBN 0-19-507011-9 Soil chemistry I Title S592.5.M39 1994 631.4'I-dc20 93-1552 987654 Printed in the United States of America on acid-free paper TO JAN AND DEVIN 3.,.,d to the many generations of farmers in my family, '" ,·.hose inherited knowledge of the soil s not to be found in a book Preface -\n ordinary housefly, if placed within a hollow clear glass globe having a single hole for outlet, generally escapes rather promptly But a bee, under like circumstances, seldom :-[...]... equilibrium: (1.31) 1 A chemical component, i, of a system is any chemical species whose quantity can be varied independently of all other components ENVIRONMENTAL CHEMISTRY OF SOILS 14 The activity of a component i, given by ai, is defined in terms of chemical potentials: Ili = 117 + (1.32) RT In ai where 117 is the standard state chemical potential of i and R is the gas constant It can then be shown... (sultbydryl), R-S- ENVIRONMENTAL CHEMISTRY OF SOILS 18 soil Depending on the composition of the solution, a large fraction of the soluble metal ions in soil solutions may actually be complexed with inorganic or organic ligands For example, the toxic metal cadmium (CdH ) has a tendency to complex with as many as four Cl- ions The reactions can be written as four step-wise additions of one Cl- ligand,1... dwldt, the number of grams of molecules transferred across a boundary per second, the equation describing Fick's law of diffusion defines a diffusion coefficient, D: dw = _Da(dC) dt d.x (1.105) where dcl d.x is the concentration gradient and a is the cross-sectional area of the boundary The diffusion coefficient can be interpreted in terms of the equation ENVIRONMENTAL CHEMISTRY OF SOILS 28 D = RT 67rrNo'Y/... driving force The chemical potential of any chemical component, I i, of a system is the partial molar free energy: (1.30) where ni is the number of moles of component i in the system and nj is the number of moles of all other components In systems with several phases (a phase is defined as a chemically and physically uniform part of the system), the chemical potential of each component is equal in all... ~xample, the chemical reaction A + 2B- C ( 1.98) ENVIRONMENTAL CHEMISTRY OF SOILS 26 with a rate law of R = d[C] =k[A][BF dt If the concentration ofB is maintained at a high level such that [B] \ R ~ ( 1.99) » [A], then: k'[A] (1.100) and the reaction appears to be first-order This allows equation 1.97 to be used to determine the value of k' The simplified analysis of kinetics given here is only valid if the... calculated for a given temperature T and pressure P from the molecular properties of the system The molecular disorder of the system is measured by the entropy S, while the energy of the system is measured by E or H As the system undergoes reaction, these measures of disorder and energy change ENVIRONMENTAL CHEMISTRY OF SOILS 12 Weakly Hydrolysing r: 12 ~ La3 + Co • Mn2+ • Fe2 + Zn2+ Co2+ -~ Ni 2+... equal to half of the difference between the number of bonding and antibonding electrons For the hydrogen molecule, Hz, this is J1!(2 - 0) = 1 That is, the bond order is unity As bond order increases, the strength of the covalent bond increases Ionic Bonding This form of bondin{results if the outer valence electrons of two atoms, A and B, are of very different energies, so that transfer of one or more... ions with low charge (e.g., K+) The electric field of ions can induce dipole moments in uncharged, nonpolar molecules by distorting (polarizing) the electron distribution within the molecules ENVIRONMENTAL CHEMISTRY OF SOILS 6 WATER DIPOLE HYDRATED CATION HYDRATED ANION Figure 1.2 Separation of charge, q, for the H 2 0 dipole, and the orientation of these water dipoles around cations and anions in... those of the pure minerals For example, substitution of AI3+ into FeOOH appears to stabilize this mineral, resulting in a lower solubility product ENVIRONMENTAL CHEMISTRY OF SOILS 16 However, different structural forms, despite identical chemical compositions, can have different solubility products In the example given above, 10- 34 is the solubility product for gibbsite, one crystalline form of AI(OH)3'... necessary to the definition of thermodynamically exact quantities 1 The Debye-Huckel Model for Activity Coefficients In spite of the indifference of classical thermodynamics to the existence of ions, the quantification of soil chemical processes such as ion exchange requires that individual ion activities in solution at least be estimated This can be done using the Debye-Huckel theory of activities, which