Sorption of Water Pollutants Prepared by Kimberly Hetrick and Ljiljana Rajic
Objective The overall goal of this session is to learn about sustainable ways to deal with water contamination while specific goal is to understand sorption mechanisms and applicability of different sorption materials for water treatment. Standards Met: NGSS HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction. HS-PS1-6. Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium. HS-PS2-1. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution. (Science and Engineering Practices) HS-PS2-6. Communicate scientific and technical information (e.g. about the process of development and the design and performance of a proposed process or system) in multiple formats (including orally, graphically, textually, and mathematically). (Science and Engineering Practices) Background What do we want to keep out of our water? Often, drinking water sources (groundwater or surface water like rivers, lakes or reservoirs) can contain pollutants that can cause affect ecosystems and can cause different illnesses in humans. While we can see some of these contaminants (e.g., dye), some of them can not be noticed by organoleptic observation! Here are some examples of pollutants that can be found in drinking water (either at the source or at the faucet) and how they can affect us: Nitrates (NO - 3 ) causes blue baby syndrome Mercury (Hg) damages nervous system Lead (Pb) damages nervous system Copper (Cu) in high doses, can cause throat irritation or illness 2
Pesticides (large group of chemicals) High levels can result in headaches, cancer, reproductive harm, or endocrine disruption (behave like hormones) Some Disinfection Byproducts (made from chlorine and other disinfectants that are added in our drinking water to remove bacteria) Could affect liver and nervous system if ingested in large quantities What is Sorption? Many contaminants can be removed through a process known as sorption. You may be familiar with this process, because this is what occurs in many water filters, such as those you might keep in your refrigerator or on your faucet at home! Materials used in adsorption processes can run the gamut from activated carbon to sand, to even ceramic! Activated carbon is one of the materials we will use in this lab. Sorption Processes These filters work because some of the materials in the filter are able to sorb contaminants onto themselves, removing them from the water. Sorption can happen in two different manners. Absorption happens when a molecule is incorporated into the 3-d structure of another molecule. Adsorption happens when a molecule sticks onto the surface of another molecule, for example when a gas is taken in a solution. In this lab, most processes occur through adsorption, where the pollutants in the water are stick onto the surface of the material. 3
Physical vs. Chemical Sorption The phenomenon of adsorption is essentially an attraction of adsorbate molecules (green ellipses in the figure above, in our case food coloring) to an adsorbent surface (blue rectangle in the figure above, in our case activated carbon). There are 2 types of adsorption process: 1. Physical adsorption (van der Waals adsorption) where the individuality of the adsorbate and the adsorbent are preserved. This means they interact but each remains the same molecule as before the interaction. Adsorption by materials you will use in this lab will mostly rely on physical adsorption. 2. Chemisorption (activated adsorption) involves the formation of chemical bonds between the adsorbate and adsorbent. This means that each molecule changes as a result of this interaction. What affects the sorption process? There are many factors that can affect how well a material can adsorb a contaminant. Some of these factors are related to the characteristics of adsorbent surface, some to the characteristics of adsorbate molecules and some to the water that contains the contaminant. One of the biggest factors affecting sorption onto a filter material is how much surface area the material contains. Surface area is how much 2-dimensional area you would get if you spread a 3-dimensional object out flat. For filter materials, the more surface area contained in the molecules in the filter, the better the adsorption, because there are more sites for the contaminants to land and stick onto! 4
Fun fact! Activated carbon contains as much surface area as a whole soccer field in one spoonful of material! Another factor that affects sorption is surface charge. Some contaminants have positive or negative charges, as do the materials they stick to. Like a magnet, opposite charges attract, so sometimes it s useful to use a filter material with a positive or negative surface charge to attract contaminants with the opposite surface charge. For example: Here are some factors that influence the performance of active carbon in water: Compounds with high molecular weight and low water solubility (hydrophobic) are better absorbed. The higher the concentration of the compound to be removed, the higher the carbon consumption. Presence of other organic compounds which will compete for the available adsorption sites. The ph of the waste stream. For example, acidic compounds are better removed at lower ph. 5
Experimental part Materials 5x250 ml transparent reagent bottles (you can also use plastic water bottles but remind students to use funnel to put the sorbent in) Sand, kaolin (or other clay), activated carbon (can be found in pet stores), used coffee (all materials should be dry) Balance 100 ml graduated cylinder Food coloring (any color) Plastic tube or transparent vials (for 16 per group) 2x10 ml syringe Cotton balls Spectrophotometer/cuvettes (if not see below) Teacher s Prep 2-3 students per group. Prepare dye solution using food coloring. Assuming that drop of the dye contains 50 µg of dye prepare solution that students will use for the tests (each group 5X100 ml). Make standards from the solution, mark exact concentration of each and save solutions for students to compare color as they do the tests. Here are few suggestions how they can do that if spectrophotometer is not available: If small glass vials are available, you can fill them up with the defined volume of the solutions and students can compare the color of their sample (need to be the same volume to reflect the same length for the light path) by placing both vials against the white background. 6
If vials are not available, you can prepare standards in the transparent tube (e.g., close them with plastic screw) and have students filter their samples in the same type of tube and compare the colors. In both cases if color is between two concentration values, students can take the average. Students will have to filter their samples before comparing the color. If syringe filters are not available students can take sample with one 10 ml syringe (no need to use needle!), fill the other 10 ml filter with cotton ball up to 2 ml, add their sample and filter through cotton ball into the vial or tube (based on how they will compare the color). Notes! 1. Equilibrium will not be reached after 30 minutes but students will observe the changes in process kinetics. 2. Ideally, students would set sorption test for each sorbent and time interval. 7
Procedure In this lab you will be using different adsorbents (materials that can take chemicals from water) to purify water. You will work in pairs and test the performance of 4 materials with different sorption capacities: sand, kaolin (or other clay), used grinded coffee and activated carbon to remove food dye from water. Measure 2 g of each sorbent and put in the bottle (use funnel if it is a plastic water bottle). Mark each bottle. Measure 4 g of sorbent of your choice and put in the mark bottle (use funnel if it is a plastic water bottle). Take initial sample of the water containing dye and record in a Table below. Add 100 ml of water containing dye into each bottle and start shaking the bottles. Take sample after 10, 20 and 30 minutes and estimate the concentration by comparing the color of your sample to the color of the dye with known concentrations. Prepare a Table where you will record the concentration of dye after each time for each sorbent. Sorbent and mass (g) Initial (mg/l) 10 min (mg/l) 20 min (mg/l) 30 min (mg/l) Data and discussion Plot your data for all sorbents together as: concentration [mg/l] on y- axis and time on x-axis. This can be done in Excel or by hand. 8
Calculate removal efficiency of each material (RE, %) for each sample. Where c in is initial dye concentration and c t is concentration after each time interval. Now plot your data for all sorbents together as: RE [%] on y-axis and time on x-axis. This can be done in Excel or by hand. What do you notice? How do concentration and RE change with time? Did adding more sorbent make a difference in sorption efficiency? Why? Which material has the highest removal efficiency? If used coffee performs the same as activated carbon which one you will choose to use in your water filter prototype? Why? Hint for Teacher! Used coffee has been found to have high sorption capacity. Adsorption properties are found to be physical and affinity binding. Choosing used coffee would be more cost-effective and environmentally friendly solution. 9