What Do You Think? Investigate GOALS

Transcription

1 Activity 8 Disinfection GOALS In this activity you will: Consider biological factors affecting the water supply. Adjust the ph of a water supply to the accepted level for drinking water. Reflect on the consequences of disinfecting water and the complications involved with ph. Create your theoretical pathway for treating water. What Do You Think? You have focused your attention on keeping the water within the chemically safe guidelines of the United States Environmental Protection Agency. However, many regard disinfection, the removal of harmful organisms and viruses in natural water, as the most crucial step in the water-treatment process. Why would many regard disinfection as the most critical stage in a water-treatment process, considering all you have studied thus far in the chapter? How do you think disinfection relates to basic chemistry? Record your ideas about these questions in your log. Be prepared to discuss your responses with your small group and the class. Investigate 1. Most commonly in the U.S., water-treatment plants use chlorine to kill bacteria. Three forms of chlorine may be added to water: chlorine gas, Cl 2 (g), sodium hypochlorite, NaOCl, and calcium hypochlorite, Ca(OCl) 2. a) In your log, write balanced equations showing how each of the three compounds can react with water to produce hypochlorous acid, HOCl(aq). 919

2 H 2 Woes Safety goggles and a lab apron must be worn at all times in a chemistry lab. In this activity you will be handling bacterial samples to test their growth in treated and untreated water. You must wear plastic or rubber gloves when handling the samples. Avoid breathing fumes. Wash your hands and arms thoroughly after the activity. 2. Obtain three ml samples of previously untreated water from a natural source. 3. Using one of the chlorinating substances, treat the first ml sample of natural water using a batch method. Vary amounts of the chlorinating substance used to get a variety of results throughout the class. Do not filter the water sample. Just let it sit in the batch. 4. Pour the second ml sample in a shallow dish to treat with ultraviolet irradiation. Use a source of ultraviolet irradiation such as a mercury lamp. Expose the water sample in the shallow dish to the source of ultraviolet light for a few minutes. Vary times throughout the class or within your group to get a variety of results. 5. Do nothing to the third sample. Leave it as natural as possible. 6. Obtain an agar plate but do not open it yet. The agar plate is a sterilized petri dish with a shallow jelly protein (agar) layer. The protein will encourage the growth of any bacteria present in the water samples. Flip over the agar plate and divide the plate into three portions by writing on the glass or plastic dish bottom with a dark wax pencil. Label the three portions to represent the three samples: natural, UV, and HOCl. 7. Obtain three sterile swabs. Dip the first swab into the natural water source. Swirl it within the source and remove. Using gloves, carefully open the plate. Quickly and gently roll the swab in the section marked natural. Be careful not to tear into the agar. Recover the agar plate as soon as you are finished swabbing. UV HOCl natural 920

3 Activity 8 Disinfection 8. Repeat Step 7 for the two treated water samples. 9. Save your treated and untreated water samples for Part B of this activity. 10. Tape the lid so it will not fall off. Place the plate upside-down (lid-side down) in an incubator at 37 C or warm area of the room for 24 h. Observe the plate. Do not open the plate. a) Record your observations in your log, noting any differences in the treated and untreated samples. Part B: ph and Disinfection For chlorination to be effective, HOCl must be the dominant chemical in solution. However, it too can dissociate in water: HOCl(aq) H (aq) OCl (aq) Adjusting the ph before and after treatment is an important consideration that must be included in your final process diagram for the treatment of water in your particular area of the world. 1. Copy the table below into your log. a) In your log, determine at which ph, the initial or the final, that hypochlorous acid, HOCl(aq), would be present in highest concentrations. Explain your reasoning. 2. Use ph test paper or a ph probe to measure the ph of the two treated samples and one untreated sample from Part A of this Investigate section. a) Record the ph. b) Is the water sample basic or acidic in each sample? c) Calculate the concentration (M) of H (aq) and OH (aq) in each sample. Show your work. 3. To adjust the ph of each ml sample to an acceptable ph (in this case you will target the neutral ph of 7.0), you will use 0.05 M solution of NaOH or HCl. a) Which source will you use to adjust the ph to 7.0? b) Calculate the concentration (M) of the appropriate solution you would add to adjust the ph to 7.0. Show the work behind your calculations. Have your calculations approved before moving forward to Step Using your approved calculations, conduct each neutralization reaction. Stir each sample to mix completely. Measure the new ph of your sample. a) Record this value in the table in your log. Safety goggles and a lab apron must be worn at all times in a chemistry lab. Dispose of the materials as directed by your teacher. Clean up your workstation. Wash your hands and arms thoroughly after the activity. Sample Initial ph Basic or acidic? [H ] [OH ] Source to adjust ph Concentration of the source Final ph untreated HOCl UV 921

4 H 2 Woes Chem Words pathogens: living organisms that cause illness, disease, or death to another host organism. disinfection: the destruction of disease-causing organisms. trihalomethanes: compounds like chloroform, CHCl 3, which contain three halogen atoms bonded to carbon. DISINFECTING WATER Sources of Pathogens Natural waterways are expected to have abundant amounts of both harmless and pathogenic bacteria. The closer the waterways are to livestock and cities, the more difficult it becomes to keep the pathogens out. Overflows of storm water from sanitary sewer systems offer an additional challenge in trying to keep sources of consumable water clean. It is crucial that you go through disinfection steps in the water-treatment process. In this activity, you explored chlorination and ultraviolet irradiation. These, along with a treatment with ozone (O 3 (g)), are the three most popular forms of disinfection, though each has its drawbacks and complications. Disinfection Disinfection refers to destroying disease-carrying microorganisms. There are a variety of ways water-treatment plants disinfect water, but the two of the more popular methods throughout the world are chlorination and ultraviolet disinfection. These disinfectants are popular because they are effective in killing the harmful organisms and they are inexpensive. Many water-treatment plants in the United States use chlorine to kill bacteria. Three forms of chlorine that may be added to water are chlorine gas, Cl 2 (g), sodium hypochlorite, NaOCl, and calcium hypochlorite, Ca(OCl) 2. All of these produce hypochlorous acid, HOCl(aq), which appears to be the chemical that kills the bacteria. For example, when Cl 2 (g) is bubbled into water, the following equilibrium is established: Cl 2 (aq) H 2 O(l) Cl (aq) H (aq) HOCl(aq) E. coli bacteria Treating water with chlorine can lead to byproducts that may be hazardous. One class of these is called trihalomethanes, such as chloroform, CHCl 3. In chloroform three of the hydrogens of methane, CH 4, are replaced with chlorine. Chloroform, once used as an anesthetic, is a known carcinogen 922

5 Activity 8 Disinfection or cancer-causing compound. Watertreatment facilities can minimize the amounts of these compounds in a municipal water supply by carrying out the chlorine treatment near the end of the overall water purification process and by filtration with activated carbon. An alternative way to avoid the introduction of chlorinated organic compounds into the water supply is to kill bacteria using ultraviolet irradiation. Ultraviolet irradiation involves exposing shallow amounts of water to a minimal amount of radiation at a wavelength outside the visible spectrum, usually around 260 nm. This exposure alters the RNA and DNA of bacteria and viruses, killing them or making them unable to infect. In Part A you explored the effects of chlorination and ultraviolet irradiation on a sample of water and drew conclusions on how well each method kills the bacteria in natural water supplies. Disinfection Complications Although chlorination is the most widely used method for killing microbes in water-treatment plants in the U.S., it can create new contaminants by chlorinating organic compounds in the water. Often, water-treatment plants add chlorine both early in the process to kill large numbers of bacteria and at the end when there are many fewer organic molecules that might become chlorinated. The chlorinated forms of organic compounds may be more hazardous to human health than the original compounds were. That is because some chlorinated compounds are resistant to further reactions. Hence, when they enter the body, they remain largely intact. They will gradually accumulate in tissue and interfere with the action of many different hormones in the body. Since many agricultural and industrial products also contain chlorinated hydrocarbons, contaminants of this type would have to be dealt with even if no additional ones were formed in the chlorination of water. One technique that is used to remove such contaminants is to adsorb them on an activated carbon filter. 923

6 H 2 Woes Chem Words acid rain: rainfall containing sulfuric and nitric acid added through incorporation of SO 3 and NO 2 into water droplets in clouds. The carbon, which may simply be in the form of charcoal, is activated by heating it to drive off previously adsorbed species. This purification step is expensive to perform, and the use of activated carbon might be limited in some localities to small filters in homes on just the water used for drinking and food preparation. In order to minimize the extent of contamination by chlorinated organic compounds, ozone has been used as alternative means for killing bacteria. Ozone is frequently more lethal for bacteria than hypochlorous acid, which is the principal agent created by various chlorination techniques. Ozone can react with organic compounds to create new contaminants, but they are generally not as hazardous as the polychlorinated organic compounds, and are more readily degraded. However, alternatives to chlorination are more costly to use. Also, they cease to kill bacteria after being applied at the water-treatment plant. That contrasts with HOCl, which persists in the water supply lines where the potential always exists for more bacteria to enter the water. ph Complications Checking Up 1. Name three ways in which pathogens can enter the water supply. 2. What purpose does disinfection serve in the treatment of water? 3. What is a disadvantage of treating water with chlorine? 4. Describe two methods, other than chlorination, that can be used to treat water. 5. Why is activated charcoal of limited value in the watertreatment process? 6. What ph levels are considered safe for potable water? The Public Health Service Act has mandated an acceptable ph range of Water with a ph outside of this range may be corrosive to pipes or hazardous to human health. In light of all the substances that must be added to water during the purification process, it should not be surprising that the ph may need adjustment at the end of the process. For example, raising the ph was demonstrated in Activity 6 as a means for lowering the concentration of Mg 2, and in Activity 7 for removing toxic-metals ions like Cr 3, Pb 2, Ni 2, and Zn 2. The chlorination step can also change the ph. Natural waters differ in ph by large amounts in different parts of the country. Water supplies tend to be alkaline in areas such as the Midwest where limestone is common, particularly if groundwater is used as the water source. Rainwater, on the other hand, may be acidic in some areas because the NO, NO 2, and SO 2 emitted from coal and oil-fired power plants react with oxygen to form NO 2 and SO 3 which is absorbed by water droplets, forming sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ). When precipitation occurs in areas polluted by these emissions, the water has a lower ph. This is acid rain. One of the consequences of acid rain is that lakes in many parts of the world have become increasingly acidic, even killing fish as the ph goes below 5.5. Water can also be acidified as it runs over certain minerals like pyrite, FeS 2. Coal deposits 924

7 Activity 8 Disinfection are rich in this mineral, and acid discharges from mining areas are common and are known as acid mine drainage. The sequence of reactions producing acidic water with pyrite is: Chem Words acid mine drainage: the discharge of acidified water from mines. 2FeS 2 (s) 7O 2 (g) 2H 2 O(l) 2Fe 2 (aq) 4H (aq) 4SO 4 2 (aq) 4Fe 2 (aq) O 2 (g) 10H 2 O(l) 4Fe(OH) 3 (s) 8H (aq) When water has been acidified in this way, it tends to dissolve toxicmetal ions, which would be insoluble under basic conditions. No matter whether the complication is natural or from the purification process, the ph must be adjusted into the acceptable range before entering the drinkable water supply. What Do You Think Now? At the beginning of this activity you were asked: Why would many regard disinfection as the most critical stage in a water-treatment process, considering all you have studied thus far in the chapter? How do you think disinfection relates to basic chemistry? Look back at your answer to the questions. Would you change your answers to these two questions after performing the investigation and reading the information in the Chem Talk section? If so, how? If not, explain. What would be the cost if you do not consider disinfection in your part of the world? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC Describe whether growth was observed on the agar plate for the untreated water sample, the treated water samples, or both. Explain why the ingestion of chlorine gas (Cl 2 ) is extremely toxic in the human body. Bacteria are often eliminated from water systems by bubbling chlorine gas into the water. Explain with the equation from the Chem Talk section why the system should not be acidic. 925

8 H 2 Woes How do you know? Make specific reference to your data from both Part A and Part B of this activity. How do you know if your treatments were effective in making the natural water supplies more drinkable? Why do you believe? Disinfection is something you have certainly come upon before, especially chlorination. Name something other than your tap water where chlorination is used for disinfection. Why should you care? Explain the significance of adjusting the ph at the end of the water treatment. Reflecting on the Activity and the Challenge This final activity of the chapter has brought together a number of concepts and recalled several of the purification steps introduced in earlier activities. It should be clear by now that there is not one universal prescription for purifying water. The steps chosen must depend on the characteristics of the water source, which in turn depend on the geology of the area and the extent of pollution added by agriculture, industry, a dense population, mining, and so on. As much organic material as possible should be kept out of the water, and chlorination or other steps to kill bacteria should be put off until near the end of the water-treatment process. If disinfection were done before flocculation, a higher concentration of persistent organic pollutants would remain. After a number of precipitation and filtration steps, the hydrocarbons susceptible to chlorination should be much less abundant. Finally, all the purification steps can affect the ph, and it should be adjusted with the addition of a safe and inexpensive acid or base at the end. 1. What advantage does chlorination of a water supply have over treatment with ultraviolet light or ozone? 2. How can acidifying water make it more toxic? 3. If the UV lamp is effective in killing bacteria, does it work as well in preventing new bacteria from entering the water sample as the chlorine treatment did? Explain. 4. Why is the treatment of natural water with ozone or ultraviolet irradiation not widely used in the U.S.? 926

9 Activity 8 Disinfection 5. A less expensive way to raise the ph of treated water is to use Mg(OH) 2 (s) or Ca(OH) 2 (s) as a base, and a less expensive way to lower the ph is to add CO 2 (g). a) Give the reactions or ionization steps that these components undergo in dissolving. b) Discuss any advantages, other than lower cost, in using these rather than NaOH and HCl. 6. HOCl is a weak acid. All weak acids dissociate into an equilibrium and have a special equilibrium constant associated with them called K a. Hypochlorous acid dissociates into ClO (aq) and H (aq) and has a K a of a) Write the equilibrium equation for the dissociation of HOCl. b) Write the equilibrium constant expression (mass-action expression) for HOCl. c) In a M solution of HOCl(aq), the concentration of ClO (aq) is M. What are the equilibrium concentrations of ClO (aq) and H (aq)? d) What is the ph? 7. According to LeChatelier, what happens as the ph is raised in the dissociation of hypochlorous acid? Why is this important to consider in disinfection? 8. The most serious consequence of acid mine drainage is the dissolution of toxic metals in streams that have been acidified. Suppose that acidified water flows over Cd(OH) 2 (s). a) Calculate the equilibrium values of [H ], [OH ], and [Cd 2 ] at a ph of 6.5. For Cd(OH) 2, use K sp = [Cd 2 ][OH ] 2 = b) Calculate the equilibrium value of [H ], [OH ], and [Cd 2 ] at a ph of 4.5. c) Compare the two results. What do they mean? 9. Preparing for the Chapter Challenge Now that you have considered biotic challenges and the importance of adjusting the ph, you are ready to complete your Chapter Challenge. Look back at the diagram you ve been working on through the purification activities. Where would you work ph adjustments into your scheme? Why? How? Develop your disinfection treatment and justify. Inquiring Further Reverse osmosis and membrane processes Membrane processes are becoming more common in water-purification processes. Membrane processes involve the use of membranes to remove small particles, molecules, and ions from water supplies. There are a number of these processes, including reverse osmosis, which vary based on the particle size you are trying to remove. These membrane processes can be employed to create drinking water from salt water, remove excess iron, nitrate, manganese, and toxic metals, filter out pathogens and viruses, and purify water of organic contaminants. Research reverse osmosis and the membrane processes. Create a poster that illustrates and explains the basic concept and lists the advantages and disadvantages of these techniques. 927