Investigating the Flint Water Crisis: Optimizing Solubility & Precipitation

Teacher Notes

Estimated Time: 60-90 minutes Materials Needed: Internet-connected device for simulation access, calculator (optional).

NGSS Alignment:

• Performance Expectation: 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.
• SEP (Constructing Explanations and Designing Solutions): Refine a solution to a complex real-world problem, based on scientific knowledge, student-generated sources of evidence, prioritized criteria, and tradeoff considerations.
• DCI (PS1.B Chemical Reactions): In many situations, a dynamic and condition-dependent balance between a reaction and the reverse reaction determines the numbers of all types of molecules present.
• CCC (Stability and Change): Much of science deals with constructing explanations of how things change and how they remain stable.

Evidence Statements Addressed:

• 1a. Students identify and describe potential changes in a component of the given chemical reaction system that will increase the amounts of particular species at equilibrium. (Students modify orthophosphate dose to shift equilibrium towards the solid $Pb_3(PO_4)_2$ product).
• 2a. Students describe the prioritized criteria and constraints, and quantify each when appropriate. (Students must achieve a lead concentration ≤ 15 ppb while minimizing the dose to save budget).
• 3a. Students systematically evaluate the proposed refinements to the design of the given chemical system. (Students test various doses and pH levels to evaluate the efficacy of precipitation).
• 4a. Students refine the given designed system by making tradeoffs that would optimize the designed system to increase the amount of product, and describe the reasoning behind design decisions. (Students select the optimal, minimum dose required and justify their choice based on Le Chatelier’s Principle and the constraints).

Part 1: Engage (Anchoring Phenomenon)

The Flint Water Crisis In April 2014, the city of Flint, Michigan, changed its municipal water source from Lake Huron to the Flint River to save money. The Flint River water was highly corrosive due to high chloride levels. Historically, treatment plants add a chemical called orthophosphate ($PO_4^{3-}$) to the water. This acts as a corrosion inhibitor by reacting with aging lead pipes to form a solid, insoluble mineral barrier—a passivation layer of lead(II) phosphate, $Pb_3(PO_4)_2$—on the inside of the pipes.

Because the city failed to add this inhibitor, the protective scale dissolved. Neurotoxic lead ($Pb^{2+}$) leached directly into the aqueous drinking water supply.

Think-Pair-Share:

1. Based on this text, what was the function of the “solid mineral barrier”?
2. What questions do you have about how a chemical added to the water can form a solid inside a pipe?

Part 2: Explore (Simulation Investigation)

Mission: Act as a municipal water chemist. The EPA safety limit for lead in drinking water is 15 ppb (parts-per-billion). Your goal is to determine the optimal orthophosphate dose to lower the aqueous lead concentration below this limit without using more chemicals than necessary (which wastes the city budget).

Instructions:

1. Open the Flint Water Crisis Simulation.
2. Familiarize yourself with the controls:
   • Phosphate Doser: Controls total injected orthophosphate $[PO_4]_{total}$.
   • Water Acidity (pH): Affects the active $[PO_4^{3-}]$ ions.
   • Macro & Micro Views: Observe the pipe scale and the particle interactions.
3. Start the simulation by pressing Play.
4. Set the pH to 7.0 (neutral).
5. Gradually increase the Phosphate Doser and observe the “Lead Level (ppb)”.
6. Record data in the table below to find the minimum effective dose.

Data Collection Table: Effect of Orthophosphate Dose at pH 7.0

Tip: You want the Lead Level to drop to 15 ppb or below, using the smallest dose possible on the slider.

Part 3: Explain (Sensemaking)

Using your data and observations from the simulation, answer the following questions:

1. Chemical Equation: The reaction forming the protective scale is: \(3Pb^{2+}_{(aq)} + 2PO_4^{3-}_{(aq)} \rightleftharpoons Pb_3(PO_4)_{2(s)}\) Based on your data, how did increasing the reactant ($PO_4^{3-}$) affect the equilibrium of this system? Describe this in terms of Le Chatelier’s Principle.
2. Macroscopic & Microscopic Connections: When you increased the dose to a SAFE level, what changes did you observe in the Macro View (the pipe) and the Micro View (the particles)?
3. The Role of pH: Reset the simulation. Set your optimal dose from your data table. Now, lower the pH (make the water more acidic, e.g., pH 6.0).
   • What happens to the Lead Level?
   • Does the water remain SAFE?
   • Explain why water acidity is a critical constraint for municipal water treatment systems relying on precipitation.

Part 4: Elaborate / Evaluate (Optimizing the Solution)

Final Report to the City Council

The City Council has asked for your formal recommendation. They need to know exactly how much orthophosphate to buy, but their budget is extremely tight.

Write a brief recommendation letter that includes:

1. The Proposed Solution: State your recommended optimal orthophosphate dose at a neutral pH (7.0).
2. Scientific Reasoning: Explain why this dose works to make the water safe. Explicitly use the concepts of solubility, precipitation, equilibrium, and Le Chatelier’s Principle in your explanation. Mention how the change in conditions (adding the chemical) increases the amount of solid product.
3. Tradeoffs and Constraints: Justify why you chose this specific dose instead of the maximum possible dose (0.050 M). Discuss the constraints of cost and safety.
4. Warning: Include a warning about what will happen if the pH of the river water suddenly drops (becomes acidic) and why they must monitor it.

Scoring Guidelines & Rubric

Final Report Rubric

Sample Student Responses

High Proficiency Example:

Dear City Council, I recommend an orthophosphate dosage of 0.005 M. At a neutral pH of 7.0, this is the lowest amount needed to bring the lead levels down to 14 ppb, which is under the EPA safety limit of 15 ppb. According to Le Chatelier’s Principle, adding more reactant ($PO_4^{3-}$) to the system causes a stress that shifts the equilibrium to the right. This produces more solid product ($Pb_3(PO_4)_2$), which forms the protective scale on the pipes and pulls the dissolved lead out of the water. I chose this specific dose because it perfectly balances our safety constraints with our budget constraints; adding more chemical would waste money since the water is already safe. However, we must continuously monitor the pH. If the river water becomes acidic (like pH 6.0), the active phosphate decreases, the equilibrium shifts back to the left, and the toxic lead will dissolve back into the water!

Low Proficiency Example:

We need to add orthophosphate to the water so the lead goes away. Put the doser up to 0.050 M to be safe. It makes a barrier on the pipe. If the water gets too acidic it will break.

Extension Options

• Economic Modeling: Have students calculate the total annual cost of orthophosphate treatment for the city of Flint (given a hypothetical cost per gallon and average daily municipal water usage) to understand the magnitude of the budget constraint.
• Alternative Passivation Strategies: Research how other cities with lead pipes (e.g., Chicago or Washington D.C.) manage their water chemistry. Compare orthophosphate treatment to other corrosion control methods like adjusting pH and alkalinity alone.