Waste and Reaction Rates

Chemistry Background

Chemical reactions happen at many different speeds. Some happen in split seconds while others take years to occur. Sometimes it is useful to change the rate of a reaction. There are multiple different ways to accomplish a change in reaction rate that are explained by chemical kinetics. Chemical kinetics is the area of chemistry that is concerned with reaction rates and reaction mechanisms. There are five factors that influence the reaction rate. They are: the nature of reactants, the surface area, the temperature, the concentration, and the presence of catalysts.¹

Environmental background

In many cities and civilizations there is a problem of waste management. Waste management sites are often large and expanding. These sites are necessary because the average American citizen generates 4.6 pounds of trash in one day.² That means that there is a total of 251 million tons of new trash each year in America! No one wants to live near a waste management site so acquiring new locations can be difficult. There are currently 3091 active landfill sites in the United States and over 10,000 old sites.³ Other problems that are associated with waste management sites include increased amounts of rodents and pests and smell.
As well as social problems, waste management sites can cause environmental problems. If not monitored and contained properly sites can cause severe water and air pollution. There are two main categories of waste management sites. They are landfills and dumps. Dumps are locations where little or no environmental precautions are taken. Landfills, on the other hand, have been designed so that environmental impact is minimal. Dumps often pollute local watersheds because rainwater that seeps through trash picks up many chemicals. This toxic water is called leachate and if not controlled can contaminate local well water.⁴
Dealing with these problems can be difficult given the slow reaction rate for the decomposition of many trash substances. While most food substances decompose in 2-8 weeks, plastic bags take 10-20 years, diapers take 450 years, and glass containers can take up to 1 million years.⁵ Figure 1 shows what percentage of total waste each type of material made up before recycling was accounted for. These time periods are often cited as a reason to increase recycling procedures. However, in the near future people will continue to throw many materials away. Landfills are already overflowing so some ways to increase the speed of the decomposition process are necessary.

Nature of reactants

Some materials take much longer to decompose than others. The most common way for trash to be decomposed is by microorganisms that feed on trash. These microorganisms typically chose organic material such as food waste, wood, and paper as their first choice to eat. For this reason these materials decompose much faster than other nonorganic materials such as plastic and glass. Microorganisms will be much less likely to aide in the decomposition of these. Scientists are learning that depending on the nature of the reactant, different microorganisms are useful for decomposition. For example, recently scientists found a type of fungi that decomposes polyurethane.⁶ More research is being done to find other microorganisms that could decompose other plastic polymers.

The surface area

The surface area of a substance means the amount of it that is touching the outsides and is accessible to the surrounding environment. The larger the surface area, the quicker the material can decompose. This means that for trash to decompose more rapidly it should be broken into lots of smaller pieces rather than one big chunk. The more accessible the material is the easy it is for wind, rain, and other outside forces to break the material into smaller pieces. Also, more accessibility means that more enzymes needed for chemical decomposition will be able to work at once. Many small landfills use a process called shredding for this exact purpose.⁷ All the waste material is dumped through a shredder like the one shown in figure 2 before it is finally placed in the landfill. Because this takes a lot of time and energy many larger landfills do not use this process yet.

The temperature

Chemical reactions are affected by temperature. Most reactions happen faster at higher temperatures. This is true of decomposition of trash. A colder temperature where materials may freeze generally lowers the reaction rates. One reason for this is because microorganisms that help to decompose trash thrive at warmer temperatures. Also, temperature increasing means that the particles within the substance are moving faster. Faster movement increases that probability that the molecules will collide with enough force to break bonds internally and thus decompose.¹

The concentration

Increasing the concentration of something also increases its reaction rate. Usually this is considered for solutions. Remember that in order to increase the concentration, there must be more of the substance in the same amount of area. Increasing concentration increases the reaction rate because the particles within the material are closer together. This means they will collide more often and each collision has the possibility of a reaction occurring.¹ For solid waste products it can be hard to increase the concentration. However, sometimes materials are compressed into a smaller area. This means that if the area was the same there would be more of the material. Compression of trash is typically used as a means to save space but it could actually be making the decomposition happen faster also!

Presence of a catalyst

A catalyst is a substance that increases the rate of a chemical reaction without itself being permanently consumed.¹ This means that the same catalyst can be used over and over again to speed up the decomposition of trash. Microorganisms that live on garbage are useful because they contain enzymes. An enzyme is a substance produced by living organisms that acts as a biochemical catalyst^.1 Microorganisms that contain these enzymes are found naturally in waste management environments however; sometimes more are specifically added to an area.⁶ These microorganisms are introduced because of their catalytic function breaking apart materials. Without any microorganisms it would take a very long time for any garbage to decompose.

From ChemPRIME: 18.8:Increasing the Rate of a Reaction

References

1. Holt, Modern Chemistry, Houghton Mifflin Harcourt 2009.
2. Freudenrich C. “How Landfills Work” How Stuff Works. 2012 http://science.howstuffworks.com/environmental/green-science/landfill.htm
3. EPA, “Landfills” 2012 http://science.howstuffworks.com/environmental/green-science/landfill.htm
4. Heimlich J. “Landfill versus dump: some facts” Solid Waste Management http://www.guyenterprisegy.com/solidwaste/dumpvslan.htm
5. Green Eco Services, “Lifespan of litter” 2008. http://www.greenecoservices.com/how-long-does-it-take-for-trash-to-biodegrade/
6. Boyle R. “Amazonian Rainforest Fungus Eats Polyurethane” Popular Science” 2012. http://www.popsci.com/science/article/2012-02/rainforest-fungus-eats-plastic-potentially-solving-landfill-problems
7. SSI Applications “Solid Waste Shredding” http://www.ssiworld.com/applications/applications1-en.htm

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.