Environmental Process Analysis
Environmental Process Analysis serves as a bridge between introductory environmental engineering textbooks and hands-on environmental engineering practice. By learning how to mathematically and numerically model environmental processes and systems, readers will also come to better understand the underlying connections among the various models, concepts, and systems. HENRY V. MOTT, PhD, Professor Emeritus of Civil and Environmental Engineering of the South Dakota School of Mines and Technology, is now a consultant in private practice and part-time instructor for the Civil Engineering Department of the University of Minnesota. Dr. Mott has developed and delivered a broad range of undergraduate and graduate courses, drawing upon his engineering practice experience to develop opportunities for students to apply fundamental principles in problem solving and design. His research has focused on contaminant fate and transport, physical/chemical/microbial processes, and environmental chemistry.
Environmental Process Analysis
As the Earth's human population continues its exponential increase, the importance of water to the preservation of the standard of living we humans enjoy is becoming of utmost importance. Water is the substance without which we know life, as currently understood, could not exist. The examination of water ranges from the accounting of the vast quantities lying in the oceans and under the surface of the Earth to the minutest details of the structure of water, allowing understanding of its behavior in both natural and contrived systems. As related to environmental process analysis, water is the substance without which there could be no water chemistry. In environmental systems, it is generally water and how water might be affected by a situation or perturbation of a system that drives our desire to understand. Thus, given the importance of water to virtually all that is water chemistry, we will examine important properties of water as related to its structure.
Engineers use many of the physical properties of water in analyses of engineered systems; tables yielding values, correlated with temperature, of density, specific weight, viscosity, surface tension, vapor pressure, and bulk modulus of elasticity are found in most textbooks addressing fluid mechanics. These are mechanical properties but are often important in environmental process analysis. Consideration of the molecular structure and molecular behaviors within liquid water can yield fascinating insights as to why these mechanical properties are as they are. For example, the physical chemists (e.g., Levine, 1988; Williams et al., 1978) tell us that the ordering of the oxygen - hydrogen bonds as water freezes leads to a density of solid water (ice) that is lower than that of liquid water. Consider the alternate existence we would know if the crystallization of water behaved in a manner similar to the crystallization of many other liquids wherein the solid is more dense than the liquid.
The properties of water leading to its rather anomalous behavior relative to other liquids are those that also govern the behavior of water in interactions with solutes - constituents present in and intimately mixed within the water. The term "dissolved" seems to have functional definitions. In the past, we referred to dissolved solids as those not separable from liquid water by a particular glass microfiber filter. In another application, we "filter" sodium and other ions from seawater or brackish water using reverse osmosis. We might use a term like "solvated," suggesting that the solid somehow has a bond with water in the aqueous solution. It is the particular structure of water that leads to its ability to bond with "solvated" solids. The important properties of water stem from the unique arrangement of electron orbitals around the water molecule. Herein we could launch into a detailed investigation of the quantum chemistry surrounding the water molecule - at which point a typical engineering student's mind wanders to seemingly more relevant topics. Thus, we will restrict our discussions and associated understandings to the semiquantitative nature.
2.2 IMPORTANT PROPERTIES OF WATER
Based on Pauling's electronegativity scale (H = 2.2, O = 3.4), we may quite simply understand that hydrogen is quite content to contribute its lone electron to a bond with another atom while oxygen is quite intent upon acquiring two electrons to render its outer electron orbital to be like that of neon, a noble gas. Consequently, each hydrogen atom of a water molecule shares a pair of electrons with the oxygen and two remaining pairs of electrons are largely associated with the oxygen atom. A Lewis dot diagram for water is shown in Figure 2.1 . When we consider the three-dimensional nature of the wat