MATERIALS SYSTEM
Introduction
The conditions on Earth over the last four and half billion years, the energy flow from the sun, and the relative abundance of different elements form the basis of all phenomena on Earth, including life. Transformations of materials aided by energy from the sun and the Earth's core are at the basis of many environmental phenomena. In this unit, we examine some of the most basic systems of materials: elements and compounds.
Most materials in nature, especially those in the biosphere (the part of the Earth which holds life) undergo chemical and physical changes constantly. Some materials, such as carbon and water, contribute to the cycle through various physical and chemical stages. Many of the environmental problems surrounding material use are actually disruptions of the cycles, which arise from taking or putting too much or too little material too fast or too slow into or from one or more of the phases in the material cycles.
Solar energy in the form of electromagnetic radiation streams in through the atmosphere onto the surface of the Earth providing energy at the rate of about 1 kilowatt per square meter at places of peak intensity. In one hour, we receive more solar energy spread over the land area of the United States than we get from the fossil fuels we burn in one year! Except for this vast and continuous input of energy, most of the material on the Earth remains at a constant amount, changing forms in some cases, going through cycles that keep many of these in forms that are accessible to life. So, as far as most materials are concerned, the Earth is a "closed system". However, solar energy is the vital part – the input into this closed system – that maintains the material system suitable for life.
Most of the matter on Earth generally remains on Earth, due to a continuous recycling of materials. Figure 1 shows the natural, large-scale processes that recycle materials.
Figure 1: Natural Recycling of Materials on Earth.
Through processes of transport and transformation in the atmosphere, absorption and settling in the oceans, and subduction and volcanism in the lithosphere, materials are recycled in nature through both physical and chemical changes. Residual heat in the core of the Earth and heat from radioactive processes provide energy from within the Earth. Solar energy drives the water cycle and the atmospheric currents, aided by the gases in the atmosphere. Water, carbon dioxide, nitrogen, chlorine, and sulfur are the main materials that cycle through atmosphere, oceans, and sediments.
In this unit, we examine several material cycles including cycles of water, carbon, and nitrogen. As materials cycle through, we note that the total quantity of matter (mass) remains the same; energy that is put in changes to work, often to rearrange forms of matter; and is eventually lost to the surroundings. Human intervention has disrupted natural environmental processes. Later on in this unit, we look at some of these disruptions and the impacts, as well as the new paradigm of industrial ecology, which seeks to recycle rather than discard materials as part of the industrial processes.
History of Materials on Earth
The planets of the solar system are believed to have been formed from materials that broke off from the sun about 4.5 billion years ago. The Earth is presumed to have been in a completely gaseous state, cooling rapidly and gathering dust and smaller pieces of material, growing in size initially. Remains of pieces not coalescing initially with the planets remain as large rocky asteroids in orbits between Mars and Jupiter. One estimate is that a mass the size of the Earth originally at 6000° K (temperature of the sun's outer layer or photosphere) should have cooled to about 1500° in about 15,000 years. In about 25,000 years the temperature of the surface would have reached very nearly that of the Earth at present.
At least sixty-six of the ninety-eight elements on Earth have been detected on the sun by means of spectroscopy. As the Earth cooled, the lighter atoms would tend to move away from the center more rapidly than the heavier ones, leading to a certain degree of layering. In the early period, significant amounts of hydrogen and helium--the main constituents of the solar nebula--remained on the planets. Some of the lightest atoms (hydrogen and helium for example) would escape into space, unless they combined chemically with other elements. Helium’s lack of ability to combine could be why we find little of this element on Earth, while the lighter but more reactive hydrogen -- which due to its weight should be able to escape with greater ease -- has been captured in the form of water and other compounds. To escape from the Earth's present gravitational pull, a molecule must have the "escape velocity" 11.3 km/second moving perpendicular to the Earth's surface.
Table 1 shows the most abundant elements on Earth and a comparison of their estimated concentration in the sun.
Element / Mass Number / Atomic % in Sun's Photosphere / Atomic % on EarthH / 1 / 8.76 / 2.7
He / 4 / 18.7 / (10-7)
C / 12 / 0.003 / 0.1
N / 14 / 0.01 / 0.0001
O / 16 / 0.03 / 48.7
Na / 23 / 0.0003 / 0.7
Mg / 24 / 0.02 / 8.2
Al / 28 / 0.0002 / 2.4
Si / 28 / 0.006 / 14.3
S / 32 / 0.006 / 14.3
K / 39 / 0.00001 / 0.1
Ca / 40 / 0.0003 / 2.0
Fe / 56 / 0.0008 / 17.9
Cu / 64 / 0.000002 / 1.4
Zn / 65 / 0.00003 / (small)
Atomic percent = % of total # of atoms
Table 1: Proportion of Elements in Earth and Sun.
Much of the Earth's material is in combination, as molecules. Even when the gases oxygen and nitrogen occur as elements in the atmosphere, they occur as molecular compounds O2 and N2, rather than in the atomic form (O and N).
Seismology has given us much of our knowledge of the interior of the Earth. The core is approximately 3,500 km in radius with an average density 10.72 g/cc. The mean radius of the Earth is 6,371 km. The mantle, which is therefore about 2900 km thick has an average relative density of about 2.7 g/cc near the surface. The core, whose temperature is between 2000 and 4000° K, consists of molten heavy metals such as iron (Fe), nickel (Ni), and uranium (U), and minerals containing these metals as well as compounds of silicon (Si), aluminum (Al), and magnesium (Mg) with oxygen, carbon, and sulfur.
Table 1 shows that the three most abundant elements on Earth are oxygen, iron, and silicon. However carbon, which is only 1 in every 1000 atoms, is the basic molecule of all life. The chemistry of carbon and its capacity to form numerous components are described in the Ecological System. It is interesting to note that silicon, in the same chemical family as carbon, abounds on Earth in the form of sand (SiO2) and other rocks. While carbon chemistry has given us live intelligence, we have used silicon chemistry for artificial intelligence -- as silicon is the basic material for computers.
It is the coincidence of the strong hydrogen-oxygen bond and carbon chemistry, coupled with the abundance of these three elements and the Earth's gravity, distance from the sun (ensuring a particular temperature range), and speed of rotation (ensuring day and night) that gave us a water planet that could evolve our life forms!
One of the basic tenets of nature is a recycling of materials that play a role in ecosystems--water, carbon, nitrogen, oxygen, and to a smaller extent, materials such sulfur and phosphorus.
There are numerous other materials -- elements and compounds -- that we otherwise mine or extract and use in a variety of ways. They range from carbon-based materials, like oil, gas, and coal derived from carbon that has been sequestered by plants, or metals such as aluminum, iron, and uranium. Depending on the use, these materials may be dispersed into the atmosphere or Earth during processes like the burning of coal, or built into structures that slowly erode, such as buildings or monuments.
The early atmosphere of the Earth contained water and other compounds including nitrogen, carbon dioxide (CO2), methane (CH4), and ammonia (NH3). The gases in our atmosphere - H2O, CO2, and N2 - have different primary reservoirs. Water is mostly in the ocean reservoir; CO2 in sedimentary rocks as carbonates, and N2 as gas in the atmosphere.
In this unit we first describe the natural cycle of materials that pass through biological and geological cycles. Then we describe the use of materials in industrial processes, and how, over the last few decades, an examination of environmental impacts have led to some recycling of materials in the industrial setting.
Mass Balance Technique
The law of conservation of matter states that matter is conserved -- that is, neither created nor destroyed. Thus, if we know the amount of material that enters a chain of processes, and keep an account of all the amounts in different paths, we can calculate quantities of materials that are hard to measure. For example, we can calculate the amount of material discharged into the atmosphere if we know the amounts that went in, the transformations, and the waste streams to land and water. This method is called the Mass or Material Balance technique.
An example of a process from everyday life is sewage treatment (see Figure 2). Wastewater is generated in your homes and is collected with the sewer system and transported to a treatment plant. When asked what happens to the sewage at the plant, most people say that the pollutants are removed from the water and the relatively "clean water" is then discharged to a water body. But what happens to the pollutants that are removed? In the treatment process, these pollutants are transferred from the water to the air, and to solid material known as sludge, or biosolids. And, a small amount remains in the "clean water." These waste products must be taken care of so that they do not affect the environment. A mass balance can be used to determine how much pollutant is emitted in each of its various forms.
Figure 2: Schemes of a waste water sewage treatment plant.Another example (though historic) is the steel industry in Pittsburgh. The processing of steel requires vast amounts of water that then need disposal. As a result, many of the early "steel towns" were along rivers, since they provided both the water and the means for disposal. Prior to the environmental regulations in the USA, the “process water” was disposed directly to the rivers. However, one of the earliest regulations was the Clean Water Act, which prohibited such disposal without treatment to remove the process waste contained in that water. Since such treatment was expensive, the next option was to use the waste process water as cooling water since vast quantities of water were also needed for that purpose. However, this process led to air pollution as the water evaporated and transported the impurities into the air. After air pollution legislation was passed, the industry operators needed to remove the waste impurities.
These are just two examples of the human search to find a sink for pollution that we are coming to realize does not exist. Many environmental problems have been caused by neglecting to think of the pollutants in terms of the conservation of matter and a mass balance.
A mass balance is an accounting of material for a specific system boundary. In other words, you are keeping track of all sources of the material that enter the system, all sinks of the material that leave the system, and all storage of the material within the system. A mass balance can be done for four scenarios, or combinations of those scenarios as follows:
· Dynamic (flows change over time)
· Steady State (flows do not change over time; the system is in equilibrium)
· Conservative pollutants (the pollutant does not change form over time; no reactions)
· Non-conservative pollutant (the pollutant changes form over time due to chemical, physical, or biological reactions)
The dynamic scenario is the most difficult to model mathematically. For our purpose, only the steady state conservative and steady state non-conservative scenarios are discussed to illustrate how the technique can be applied to environmental systems.
Steady State Scenario
The accounting system to track pollutants is as follows:
input rate = output rate + reaction rate
The reaction rate is equal to 0 if the pollutant is conservative. The reaction rate can be + or – if the pollutant is non-conservative.
EXAMPLE: Two streams enter a lake in the system represented below. The main stream has a flow of 10 m3/s, and a chloride concentration of 20 mg/L. The tributary stream has a flow of 5 m3/s and a chloride concentration of 40 mg/L. What is the chloride concentration leaving the lake system? Note that chloride is a conservative pollutant. The answer is obtained by balancing the sinks and sources of pollutants to the lake system as follows:[10 m3/s]*[20 mg/L] / + / [5 m3/s]*[40 mg/L] / = / [C mg/L]*[10 m3/s + 5 m3/s]
200 / + / 200 / = / C*[15]
C / = / 400/15 = 26.7 mg/L
Often the reaction rate is due to biological degradation also known as a decay rate. The decay rate is often modeled as a first order reaction, which means that the amount that decays is proportional to the amount present at any time. In other words:
Ct = [C0]*e-[k*t]
Therefore for a steady state non-conservative pollutant, the equation needs and additional term to account for the decay as follows:
Decay rate = -[k*C*V]
k = reaction rate
C = concentration at time
V = volume of the system modeled
Input = [5 m3/s]*[10 mg/L] + [0.5 m3/s]*[100 mg/L] = 100 [m3.mg/L.sec]
Output = [5 m3/s + 0.5 m3/s]* [C] = 5.5 * [C]
Decay = -[0.2 1/day] * [C] * [10 * 106 m3] = -23.1 * [C]
Input = Output + Decay
C = 3.5 mg/L
Material Cycles in Nature