Chemistry • Year 12 • Module 8 • Lesson 8
Heavy Metal Contamination & Analysis
Apply AAS calibration data, compare remediation strategies, and reason through cause-and-effect chains linking heavy metal chemistry to ecological risk.
1. Interpret an AAS calibration curve for lead (Pb)
A chemist is monitoring lead contamination in water sampled near an older Sydney suburb where original lead-soldered plumbing is still in service. Five standard solutions and one unknown sample were run through the AAS instrument. The calibration data and the resulting curve are shown below. 9 marks
| Standard | Pb concentration (mg L−1) | Absorbance |
|---|---|---|
| S1 | 0.00 (blank) | 0.000 |
| S2 | 0.005 | 0.098 |
| S3 | 0.010 | 0.196 |
| S4 | 0.020 | 0.391 |
| S5 | 0.040 | 0.782 |
| Unknown U | ? | 0.275 |
Figure 1.1. AAS calibration curve for Pb using a hollow cathode lamp at the lead-specific wavelength (217 nm). Adapted from standard regulatory monitoring protocol (ADWG, 2011).
1.1 Using the graph, estimate the concentration of Pb in Unknown U. Show your reasoning. 2 marks
1.2 Compare the estimated concentration of Unknown U to the ADWG maximum contaminant level for lead (0.010 mg L−1). What action, if any, should the water authority take? 2 marks
1.3 The calibration curve passes through the origin and is linear. Identify the law this linearity reflects, and state one condition under which this linear relationship would break down. 2 marks
1.4 Explain why a hollow cathode lamp containing a lead cathode is used, rather than a general-purpose white light source, when monitoring lead in water. 3 marks
2. Cause-and-effect chain — mercury in an aquatic food web
Complete the cause-and-effect chain below. Each arrow represents a “so…” step. Fill in the missing effect boxes. 5 marks
So… phytoplankton and small invertebrates…
So… the concentration in fish tissue…
So… tissue Hg in the top predator…
Overall outcome (so…): Why does a low Hg concentration in water not mean low Hg risk for a top predator or for humans eating that predator?
3. Interpret monitoring data — Broken Hill lead-contaminated soil
The Broken Hill region in western NSW has a legacy of lead contamination from historic mining and smelter operations. Soil and blood-lead data from a 2019 remediation assessment are shown below. 7 marks
| Monitoring zone | Soil Pb (mg kg−1) | Blood Pb in children <5 yr (µg dL−1) | Distance from smelter (km) |
|---|---|---|---|
| Zone A (near smelter) | 4 800 | 12.4 | 0.5 |
| Zone B | 2 100 | 8.1 | 1.8 |
| Zone C | 890 | 5.3 | 3.5 |
| Zone D | 310 | 3.2 | 6.0 |
| Zone E (background) | 95 | 1.8 | 12.0 |
Data adapted from NSW EPA legacy-site monitoring reports; reference blood Pb level of concern (NHMRC) = 5 µg dL−1.
3.1 Describe the trend in soil Pb concentration and blood Pb level as distance from the smelter increases. 2 marks
3.2 Identify the zones where children’s blood Pb levels exceed the NHMRC reference level of concern (5 µg dL−1). Justify prioritising these zones for remediation. 2 marks
3.3 Suggest one likely pathway by which soil lead in Zone A enters the bloodstream of children living in that zone. 1 mark
3.4 A chemist proposes using AAS to verify the soil Pb levels. Describe, in order, the two sample preparation and measurement steps needed before the AAS instrument can give a reading. 2 marks
Q1.1 — Estimated Pb concentration of Unknown U
Read from the graph: the dashed horizontal line at absorbance = 0.275 intersects the calibration line directly above approximately 0.014 mg L−1. [1 mark for correct reading within ±0.001 mg L−1; 1 mark for showing the graphical or interpolation method.]
Alternatively by calculation using slope: slope = 0.782 / 0.040 = 19.55 (absorbance per mg L−1); concentration = 0.275 / 19.55 = 0.0141 mg L−1.
Q1.2 — Comparison with ADWG MCL
0.014 mg L−1 exceeds the ADWG MCL for lead of 0.010 mg L−1 [1 mark]. The water authority should issue a do-not-drink advisory, investigate the source (likely old lead-soldered plumbing), and either replace the infrastructure or implement a point-of-use treatment to reduce Pb below the MCL [1 mark for any appropriate action with reasoning].
Q1.3 — Law and breakdown condition
The Beer–Lambert law [1 mark]. It breaks down at high concentrations where absorbance exceeds approximately 0.8–1.0, because at high analyte densities the relationship between absorbance and concentration becomes non-linear (due to inter-analyte interactions or instrument detector saturation) [1 mark]. Accept also: stray light or very low signal-to-noise ratios at very low concentrations.
Q1.4 — Why a lead-specific hollow cathode lamp
Each element has its own unique set of electron energy levels and therefore absorbs light at unique, characteristic wavelengths [1 mark]. A hollow cathode lamp with a lead cathode emits light predominantly at those lead-specific wavelengths [1 mark]. Using this element-specific source ensures that only lead atoms in the atomised sample absorb the light significantly, minimising interference from other metals or matrix components in the water sample [1 mark].
Q2 — Cause-and-effect chain
Step 1: Phytoplankton and small invertebrates take up methylmercury from the water into their tissues — this is bioaccumulation, which can concentrate Hg to levels already higher than the surrounding water.
Step 2: Fish consuming many contaminated invertebrates accumulate higher Hg in their tissue than the invertebrates they ate — biomagnification is occurring; each trophic step multiplies the tissue concentration further.
Step 3: Top predators consuming many small fish accumulate the highest tissue Hg concentrations of any organism in the food web, far exceeding water Hg concentrations.
Overall outcome: Because bioaccumulation within an organism and biomagnification across trophic levels can increase tissue concentration by orders of magnitude relative to water, a water Hg level that appears low (or even below MCL) can translate to dangerously high tissue concentrations in top predators and in humans consuming them. [Award 1 mark per correctly completed step + 1 mark for overall outcome = 5 marks total.]
Q3.1 — Trend description
Both soil Pb concentration and blood Pb in children decrease as distance from the smelter increases [1 mark]. Zone A (closest) has the highest values (4800 mg kg−1 soil; 12.4 µg dL−1 blood), while Zone E (background, 12 km) has the lowest (95 mg kg−1; 1.8 µg dL−1) [1 mark for supporting data values].
Q3.2 — Zones exceeding reference level
Zones A (12.4), B (8.1) and C (5.3) all exceed the NHMRC reference level of 5 µg dL−1 [1 mark]. These zones should be prioritised because lead causes irreversible neurological damage in young children, and blood Pb levels at these concentrations are directly associated with cognitive and developmental harm — remediation is most urgent where measured harm is occurring now [1 mark].
Q3.3 — Pathway into bloodstream
Accept any one of: ingestion of contaminated dust or soil particles (hand-to-mouth behaviour common in young children); inhalation of fine lead-bearing dust; ingestion of lead from contaminated vegetable gardens or play areas. [1 mark]
Q3.4 — Sample preparation and measurement steps
Step 1: Acid digestion — treat the soil sample with concentrated nitric acid (HNO3) and heat to dissolve all lead species into a homogeneous aqueous solution [1 mark]. Step 2: Atomise the digested solution (inject it into the AAS flame or graphite furnace) so that free ground-state Pb atoms are produced; then measure the absorbance of the Pb-specific wavelength emitted by the hollow cathode lamp and compare to the calibration curve [1 mark].