Chem 106 – Water Quality, Nutrients, Spectroscopy & Emerging Contaminants

Water Quality: Purpose & Definition

Goal: Characterize the water in a given watershed / sub-basin to decide whether it is fit for a specific use (drinking, irrigation, industry, recreation, aquatic life support, etc.)
Working definition of water quality = “What is in the water, and how well is it suited for the intended task?”
Suitability standards differ by use:
- Drinking: must not kill / sicken humans
- Irrigation: salinity, hardness, specific ions
- Industry: scaling, corrosion, conductivity, contaminants
- Recreation: clarity, pathogens, toxins
- Aquatic life: temperature, dissolved O₂, pH, nutrients, toxins

Three Families of Water-Quality Parameters

Physical (sense-based & bulk properties)
- Color/turbidity, temperature, odor/taste, suspended solids, light transmission, conductivity, etc.
Chemical (ionic & molecular)
- Major ions: hardness (Ca²⁺, Mg²⁺), alkalinity (HCO₃⁻/CO₃²⁻), Cl⁻, nutrients (N, P), pH, trace metals, organics, etc.
Biological (living organisms & their indicators)
- Microbes (bacteria, amoebae), algae & cyanobacteria, macroinvertebrates, fish presence/absence, toxins produced by organisms, biologically controlled parameters such as temperature or pH niches

Focus Parameters for Chem 106

Nutrients (N & P) → Thursday field lab measures phosphorus
Pilot/experimental topic: organic pollutants, esp. microplastics / PFAS; measurement attempted later with Raman + SERS

Nitrogen Cycle in Water Systems

Elemental N importance: amino acids → proteins, nucleic acids (DNA/RNA), chlorophyll, etc.
Atmospheric stock: ~80 % N₂ (diatomic, triple bond → chemically inert, high bond enthalpy ⇒ difficult to use directly)
Fixation pathways
- Biological: N-fixing bacteria/fungi in root nodules (legumes, clover, etc.) convert N₂ → NH₃/NH₄⁺ and/or NO₃⁻
- Physical: lightning breaks N≡N, forms NO₃⁻ via reaction with O₂
Transport: Once ionised (NH₄⁺, NO₃⁻) → soluble, mobile in runoff/groundwater
Denitrification: Different microbes convert NO₃⁻ → N₂ or N₂O (nitrous oxide)
- $N_2O$ is a potent greenhouse gas; excess fertiliser accelerates N₂O release
Anthropogenic inputs: synthetic fertilisers, manure, wastewater effluent, atmospheric deposition
Management problem: Balance is essential—too much N promotes eutrophication; too little limits productivity

Phosphorus Cycle & Forms in Water

Source: Weathering/dissolution of phosphate minerals, decomposition of plants/animals/manure; less atmospheric involvement than N
Biological role:
- Energy currency (ATP: adenosine triphosphate)
- Structural (phospholipid cell membranes)
- Also component of DNA/RNA backbones
Analytical species (commonly reported as “phosphate”):
- Orthophosphates (inorganic, dissolved): $PO<em>4^{3-}, HPO</em>4^{2-}, H<em>2PO</em>4^-$ (pH-dependent speciation)
- Organic phosphates (bound to C‐containing compounds; e.g.
  in manure, decaying biomass)
- Condensed / polyphosphates (industrial detergents, descaling agents; multiple P atoms)
Total phosphate = ortho + organic + condensed; labs usually target orthophosphate

Nutrient Over-Enrichment → Eutrophication

Question: “Can there be too much of a good thing?” → YES
Observable symptom: Massive algal (cyanobacterial) blooms (green, turbid water)
Consequences
- Some cyanobacteria release neurotoxins
- Light blocked → submerged vegetation dies
- Bloom collapse → decomposition consumes dissolved O₂ → hypoxia/anoxia (“dead zones”)
Examples
- Lake Erie bloom (2009, 2015) visible from satellite
- Gulf of Mexico hypoxic zone: 6 000–7 000 mi²; linked to Mississippi River nutrient runoff (agriculture from entire basin)
Recovery depends on climate & hydrology (cold winters help kill blooms in temperate lakes; tropical seas persist longer)

Spectroscopy Primer (Tools for Nutrient Measurement)

Spectroscopy: Study of interaction between electromagnetic radiation (EMR) & matter
- Core equation: $E = h \nu = \frac{hc}{\lambda}$
Absorption vs. emission vs. scattering; for lab we use absorption (spectrophotometry)
Instrument basics
1. Light source (lamp, laser, etc.)
2. Wavelength selector (prism, grating + slit)
3. Sample cell (path length $b$ , often 1 cm cuvette)
4. Detector (photodiode/CCD)
Beer–Lambert Law
- $A = \varepsilon \, b \, c$
- $A$ (absorbance) ∝ concentration $c$ provided $\varepsilon$ (molar absorptivity) & path length $b$ are constant
- Enables quantitative analysis via calibration curves
Visible spectra: Peak position indicates colour absorbed; peak height gives concentration

Upcoming Labs

Tuesday (Spectrophotometry Skills)
- Pocket-size “SpectroPlus” units
- Record spectra of coloured solutions; apply Beer’s Law to unknown concentration
- Pre-lab on Kappa; no procedure required (group exercise)
Thursday (Phosphate Determination)
- Strategy: Convert colourless orthophosphate → blue molybdenum complex (“molybdenum blue”)
- Measure absorbance in red region; use calibration to compute $[PO_4^{3-}]$ in campus water samples
- Deliverables:
  - Individual: Pre- & post-lab questions
  - Team: Formal technical lab report comparing 2 samples; discuss risk of eutrophication

Microplastics & PFAS (Emerging Chemical Concern)

Microplastics = small plastic particles; many are PFAS (poly/perfluoroalkyl substances)
Structural hallmark: Long C–C chains fully or partly substituted by F (e.g., PFOA)
Properties
- C–F bond extremely strong → “forever chemicals” (resist biodegradation)
- Toxicity: Bioaccumulate, some are carcinogenic
Regulatory limit (EPA): 4–10 ng L⁻¹ (≈ 4–10 ppt)
- Illustration: 1 eye-drop in 5 Olympic pools; 4 in in 33 Earth–Moon round trips
Traditional analysis: LC/GC + tandem mass spectrometry (>$1 M instruments, 85–495/sample, 15 min–3 h run time)

Alternative Detection – Raman & SERS

Raman spectroscopy
- Incident photon excites molecular vibration → scattered photon emerges with shifted frequency (Raman shift)
- Each molecular structure has a unique “barcode” of vibrational peaks → qualitative ID + quantitative via intensity
Surface-Enhanced Raman Spectroscopy (SERS)
- Nanoparticles of Au/Ag (10–20 nm) create localized surface plasmon resonances; electromagnetic field at surface amplifies Raman signal ~10⁶×
- Makes ppt-level PFAS detectable with inexpensive laser + camera setup

Quantum & Spectroscopic Quick-Reference

Photon energy–wavelength: E \propto \frac{1}{\lambda}$$ (shorter λ = higher energy)
Visible spectrum ≈ 400 nm (violet) → 700 nm (red)
Absorption peaks appear where photons have exactly the right energy to promote electronic transition
Emission (fluorescence) = excited electron relaxes, emits photon
Raman shift axis often given in cm⁻¹ (frequency difference from excitation)

Teamwork Expectations for Field/Analytical Projects

Teams listed in Brightspace → shared Google Drive folder contains “CHEM 106 Team Contract” (Lecture Assignment 2)
Contract components
- Team name (PG-13!)
- Member list & contact info
- Individual strengths & skills to contribute (leadership, calculations, writing, figure prep, instrument skills, safety)
- Personal improvement goals
- Communication norms & meeting times (in-person or virtual)
- Signatures (typed acceptable)
Effective teams leverage complementary skills, delegate tasks, maintain unified voice, and communicate continuously

Numerical & Statistical Data (For Study/Reports)

Atmosphere ≈ 80 % N₂
Lake Erie algal bloom (2015): visible over >100 km; Gulf of Mexico dead zone: 6 000–7 000 mi²
EPA PFAS limit: 4–10 ng L⁻¹ (ppt)
SERS enhancement: up to 10⁶× signal

Ethical & Practical Implications

Nutrient management: Balance food production vs. ecosystem health; agricultural best practices critical (Mississippi basin example)
PFAS: Industrial convenience vs. persistent global contamination; need cost-effective monitoring & remediation technologies
Spectroscopic methods: Provide early-warning data → inform policy, protect public health
Team science: Real-world analytical/environmental projects demand collaboration, safety culture, and clear communication