Environmental chemistry Study Guide

📖 Core Concepts Environmental Chemistry – Study of chemical/biochemical processes in natural environments (air, water, soil) and how human or biological activity alters them. Contaminant vs. Pollutant – Contaminant: any chemical present above natural background. Pollutant: a contaminant that causes harmful effects. Receptor & Sink – Receptor: medium (soil, organism, fish) that experiences the contaminant’s impact. Sink: material that retains or transforms the contaminant (e.g., carbon sink, sediments). Baseline Environment – The natural, un‑impacted chemical composition of a system; essential for detecting anthropogenic change. Key Water‑Quality Indicators – Dissolved Oxygen (DO), Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), Total Dissolved Solids (TDS), pH, nutrients (N, P), heavy metals. Analytical Foundations – Quantitative chemical analysis (wet‑chemical, spectroscopic, chromatographic) provides the data for all environmental assessments. 📌 Must Remember Contaminant ≠ Pollutant – All pollutants are contaminants, but not all contaminants are pollutants. DO vs. BOD vs. COD DO = O₂ actually present (mg L⁻¹). BOD = O₂ required by microbes to degrade organic matter (5‑day test, mg L⁻¹). COD = O₂ needed to chemically oxidize organics + inorganics (mg L⁻¹). Heavy Metal Toxicity – Cu, Zn, Cd, Pb, Hg are routinely monitored; even trace ppb levels can be hazardous. Eutrophication Trigger – Excess N or P → algal bloom → ↓ DO → fish kills. Analytical “Gold Standards” Metals: AAS, ICP‑OES, ICP‑MS. Organics: GC‑MS, LC‑MS (including tandem & high‑resolution). Regulatory Limits – PAHs often limited to low ppb (µg L⁻¹) because many are carcinogenic. 🔄 Key Processes Sampling → Preservation → Analysis Collect water/soil/air sample. Preserve (acidify, cool, filter) to prevent transformation. Choose appropriate analytical method (metal vs. organic). BOD Test (5‑day) Incubate sealed sample at 20 °C. Measure DO at start and after 5 days. BOD = DOinitial – DOfinal. ICP‑MS Metal Determination Digest sample (acid digestion). Nebulize into plasma → ionize → mass‑filter → detect ion current → quantify via calibration curve. GC‑MS PAH Identification Extract organics (solid‑phase extraction). Separate on GC column → ionize in MS → match mass spectra to library. 🔍 Key Comparisons AAS vs. ICP‑OES vs. ICP‑MS AAS: single‑element, moderate sensitivity, low cost. ICP‑OES: multi‑element, higher throughput, detection ppb. ICP‑MS: ultra‑trace detection (ppt), isotopic info, most expensive. BOD vs. COD BOD: biological, slower, only biodegradable organics. COD: chemical, fast, includes non‑biodegradable organics. GC‑MS vs. LC‑MS GC‑MS: volatile, thermally stable compounds (PAHs). LC‑MS: non‑volatile, polar or thermally labile compounds. ⚠️ Common Misunderstandings “All contaminants are hazardous.” – Some are benign at current concentrations; toxicity depends on dose and exposure pathway. Confusing BOD with COD. – BOD measures biological oxidation; COD measures total chemical oxidation. Assuming AAS can detect trace metals at ppt levels. – It is limited to ppb; use ICP‑MS for ultra‑trace work. Thinking pH alone tells water quality. – pH is important but must be evaluated with DO, nutrients, and metals. 🧠 Mental Models / Intuition “Chemical River” Analogy – Imagine a river as a conveyor belt: sources (up‑stream) add chemicals, reactions (mid‑stream) transform them, sinks (down‑stream sediments, biota) remove them. Track each segment to diagnose problems. Dose‑Response Ladder – Low dose → no effect → sub‑lethal → lethal; helps remember why “contaminant” may not be a “pollutant” until concentration crosses a threshold. 🚩 Exceptions & Edge Cases Metal Speciation – Toxicity depends on chemical form (e.g., Hg⁰ vs. Hg²⁺). Total metal concentration alone can be misleading. Delayed Toxicity – Some contaminants (e.g., PCBs) bioaccumulate; effects may appear years later. Urban Runoff “First‑Flush” – The initial stormwater carries the highest pollutant load; sampling after the first 10 % of runoff may underestimate concentrations. 📍 When to Use Which Metal analysis → If only a few elements needed → AAS; many elements or low‑level detection → ICP‑OES or ICP‑MS. Organic pollutant screening → Volatile/thermally stable → GC‑MS; polar, non‑volatile → LC‑MS. Water‑quality field screening → Portable DO meter, pH probe, colorimetric kits for BOD/COD. Biological impact → Use bioassays/immunoassays for rapid toxicity; PCR for microbial contamination source tracking. 👀 Patterns to Recognize High BOD + Low DO → Likely recent organic waste input (e.g., sewage, agricultural runoff). Elevated nutrients + algal bloom → Nutrient leaching/eutrophication. Sharp spikes of heavy metals after rain → Urban runoff or storm‑driven sediment transport. PAH presence without petroleum odor → Subsurface oil seepage or historic spill. 🗂️ Exam Traps Choosing “BOD” when question mentions “total oxidizable matter.” – Correct answer is COD, not BOD. Assuming any detectable metal is a pollutant. – Remember pollutant requires adverse effect; trace background levels may be benign. Mix‑up of analytical acronyms – ICP‑OES ≠ ICP‑MS; the former is emission, the latter is mass spectrometry. Overlooking “first‑flush” effect in runoff questions. – Answers that ignore the disproportionate pollutant load in early stormwater are usually distractors. Confusing “sink” with “receptor.” – Sink retains/transforms contaminant; receptor suffers the impact. --- Use this guide for a rapid, confidence‑building review right before your exam. Good luck!