Ep 27 — Environmental Monitoring: Detection of Pollutants in Water and Soil

Series: Encyclopedia of Infrared Spectroscopy: From Principles to Practice
Chapter: Part 3 · Intermediate — Industry Applications
Target Audience: Environmental monitoring technicians, soil chemistry researchers, third-party testing engineers
Prerequisites: Ep 14 (ATR Attenuated Total Reflectance), Ep 20 (Quantitative Analysis), Ep 26 (Microplastic Detection)
Reading Time: Approximately 42 minutes

Introduction: A River's "Health Checkup"

In 2023, an environmental monitoring station in a certain province conducted routine monitoring of an industrial river. Traditional GC-MS detection showed excessive COD, but the specific pollution source was unclear—was it organic solvents from an upstream chemical plant? Pesticide runoff? Or domestic sewage? [1]

Monitoring personnel switched to an extraction enrichment + ATR-FTIR strategy, and within only 2 hours, identified three characteristic substances from the water sample: toluene, ethyl acetate, and trace mineral oil. Combined with fingerprint region spectral patterns, they successfully traced the source to an upstream paint factory [1].

"FTIR is not the most sensitive technique for environmental analysis, but it is often the fastest for chemical identification of unknown pollutants."
—— EPA FTIR Environmental Application Guide [2]

The characteristics of environmental samples are complex matrix, low target concentration, and numerous interference factors. The value of FTIR in environmental monitoring lies not in replacing gold standard methods such as GC-MS or ICP-MS, but in [1][2][3]:

• Rapid screening: Quick identification of pollution types on-site or in the lab
• Fingerprint tracing: Tracking pollution sources through chemical fingerprint patterns
• In-situ monitoring: Real-time water quality/soil assessment without sampling
• High-throughput screening: Near-infrared/mid-infrared rapid assessment of large numbers of samples

This episode systematically explains the methodology, key techniques, and practical experience of FTIR in water quality and soil monitoring.

1. FTIR Detection of Organic Pollutants in Water

1.1 Infrared Interference Characteristics of Water

Water (H₂O) is one of the strongest interferents in infrared spectroscopy [3][4]. Infrared absorption characteristics of liquid water:

Table 1: Infrared absorption characteristics of liquid water (data source: HITRAN database [4])

🔗 Extension: Detailed analysis of water molecule vibration modes can be found at ftir.fun water functional group page. Understanding the infrared characteristics of water is fundamental for water sample analysis.

Mitigation Strategies [3][5]:

• Extraction enrichment: Extract target compounds from water into organic solvents to remove the water matrix
• ATR surface measurement: Evanescent wave penetrates only 1–2 μm, minimal effect from water layer thickness
• Subtraction method: Measure sample aqueous solution, subtract pure water background
• Dry measurement: Evaporate water sample to dryness and measure the residue

1.2 Extraction Enrichment + ATR/Transmission Detection Workflow

Typical FTIR detection workflow for organic pollutants in water [1][5][6]:

① Liquid-Liquid Extraction (LLE) [5]:

• Water sample (1 L) adjusted pH, extracted 3 times with dichloromethane (DCM)
• Combined extracts dried over anhydrous Na₂SO₄
• Concentrated to 1 mL using Kuderna-Danish
• Concentrate dropped onto ATR crystal, solvent evaporated, then measure residue

② Solid-Phase Extraction (SPE) [5][6]:

• Water sample passed through C18 or HLB solid-phase extraction cartridge
• Target compounds enriched on the cartridge
• Eluted with a small amount of methanol or DCM
• Eluate concentrated then measured by ATR

③ Headspace-Gas Cell (Volatile Organic Compounds) [6]:

• Water sample placed in sealed headspace vial, heated to equilibrate
• Headspace gas withdrawn and injected into long-path gas cell
• Transmission FTIR measurement (applicable to BTEX, chloroform, etc. VOCs)

  FTIR Detection Workflow for Organic Pollutants in Water:

  Water Sample (1 L)
     │
     ├─→ Liquid-Liquid Extraction (DCM) ─→ Concentration ─→ ATR-FTIR
     │
     ├─→ Solid-Phase Extraction (C18) ─→ Elution ─→ ATR-FTIR
     │
     └─→ Headspace Injection ─→ Long-Path Gas Cell ─→ Transmission FTIR

1.3 Typical Infrared Characteristics of Pollutants in Water

Table 2: Typical infrared characteristic peaks of organic pollutants in water (data sources: EPA FTIR method [2]; environmental analysis literature [5][6])

🔗 Extension: Detailed analysis of alkyl C-H stretching vibrations (2924/2854 cm⁻¹) in petroleum hydrocarbons can be found at ftir.fun alkyl C-H functional group page. This is a key indicator for monitoring oil pollution in water.

1.4 Quantitative Method: Determination of Oil Content in Water

EPA Method 418.1 (Obsolete but still a reference basis) [2]:

• Principle: Acidify water sample, extract with Freon-113
• Measure absorbance at 2930 cm⁻¹ (CH₂ asymmetric stretch)
• Establish working curve using standard oil
• Detection limit: ~0.1 mg/L

Modern Alternative Method (HJ 637-2018) [7]:

• Extractant changed to tetrachloroethylene (CCl₃CCl₃, replacing ozone-depleting Freon)
• ATR or transmission measurement at three absorbances: 2930/2960/3030 cm⁻¹
• Corresponding to CH₂, CH₃, and aromatic C-H respectively
• Calculate total petroleum hydrocarbons (TPH) content

⚠️ Note: The infrared oil measurement method measures the total amount of C-H bonds and cannot distinguish between specific oil types. To differentiate gasoline/diesel/motor oil, GC-MS should be combined [2][7].

2. Infrared Screening of Pollutants in Soil (Details of SOM in Ep 35)

In environmental monitoring scenarios, soil samples often contain both organic pollutants (petroleum hydrocarbons, pesticide residues, etc.) and soil organic matter (SOM). This episode focuses on pollutant detection; systematic explanation of SOM content, humification index, water interference, and field cases can be found in Ep 35.

In practice, first use ATR/DRIFTS to check for strong alkyl peaks of petroleum hydrocarbons, carbonyl oxidation products, etc.; if the goal is fertility/organic matter quantification, please go directly to Ep 35 to avoid repeating the same humic substance indices.

🔗 For fatty chain related peaks in soil, refer to alkyl C-H; for water interference, see water molecule.

2.1 Boundary with SOM (One Sentence)

SOM peaks at 2920/1640 etc. can overlap with pollutant peaks—this is precisely why infrared screening of soil pollution must combine blank soil, extraction, or chromatographic confirmation, rather than relying solely on "seeing 2920". Detailed humification index formulas and correction strategies are in Ep 35.

2.2 Infrared Detection of Soil Pollutants

① Oil-Contaminated Soil [5][10]:

• Sampling → Air-drying → Grinding → Sieve through 2 mm

• Direct ATR measurement or measurement after extraction with tetrachloroethylene

• Quantification: peak area at 2924 cm⁻¹, establish TPH calibration curve
• Detection limit: ~100 mg/kg (direct ATR); ~10 mg/kg (after extraction)

② Pesticide residues [10]:

• Soil extracted with acetone/hexane
• Extract concentrated and measured by ATR
• Limitation: only samples with relatively high concentrations (> 1 mg/kg) can be detected
• For trace pesticides (μg/kg level), GC-MS is required

③ Microplastic pollution [11]:

• See Ep 26 method
• Soil samples need to remove organic matter first (H₂O₂ digestion)
• Density separation (ZnCl₂, ρ = 1.6 g/cm³) to enrich microplastics
• FPA-FTIR full filter membrane analysis

📷 Figure 1: ATR-FTIR spectra and quantitative analysis of petroleum-contaminated soil
Source: EPA Soil Oil Pollution Analysis Method [10]
https://www.epa.gov/sites/def…

III. Near-Infrared/Mid-Infrared Methods for In-Situ Water Quality Parameters

3.1 Near-Infrared Rapid Water Quality Assessment

Near-infrared spectroscopy (NIR, 780–2500 nm / 12820–4000 cm⁻¹) combined with chemometrics enables rapid assessment of water quality parameters [12][13]:

Table 4: NIR water quality parameter assessment (Data source: NIR water quality monitoring literature [12][13])

Key limitations [12][13]:

• NIR insufficient sensitivity for low concentrations (< 1 mg/L)
• Requires a large number of samples to build PLS calibration model
• Models are not transferable across different water bodies (river/lake/wastewater)
• Turbidity and color interference need to be eliminated through pretreatment

3.2 Mid-Infrared ATR Water Quality Monitoring

Mid-infrared (MIR, 4000–400 cm⁻¹) has stronger molecular specificity, but strong water absorption is the main obstacle [3][14]:

ATR water sample measurement strategy [3][14]:

• Use diamond ATR, water layer thickness only ~2 μm
• Region below 1650 cm⁻¹ is severely interfered by water
• Can measure organic C-H in 2800–3000 cm⁻¹ (water is transparent here)
• Detection limit: ~10–100 mg/L (depends on compound)

ATR water quality monitoring applications [14]:

• Online monitoring of COD in wastewater treatment plant effluent
• Monitoring oil content in industrial circulating water
• Screening of disinfection by-products in drinking water

📷 Figure 2: Schematic diagram of ATR-FTIR online water quality monitoring system
Source: Harrick Scientific Online ATR Application [14]
https://harricksci.com/applic…

IV. Portable FTIR Environmental Field Monitoring

4.1 Characteristics of Portable FTIR Instruments

Portable FTIR is a key tool for rapid environmental field testing [15][16]:

Table 5: Comparison of mainstream portable FTIR instruments (Data source: manufacturer information [15][16])

4.2 Field Monitoring Application Scenarios

① Soil pollution field screening [10][15]:

• Portable ATR-FTIR directly measures moist soil
• 2924 cm⁻¹ peak screens for oil pollution
• 30 seconds/sample, high-throughput initial screening
• Positive samples screened out and sent to lab for GC-MS confirmation

② Emergency monitoring of water oil pollution [15]:

• Portable ATR directly measures water samples
• Or measure concentrated extracts after extraction
• Used for emergency response to oil spill accidents

③ Rapid identification of hazardous waste [16]:

• Portable ATR identifies unknown chemicals
• Built-in spectral library (hazardous chemicals, pesticides, etc.)
• On-site decision support

💡 Practical experience: The detection limit of portable FTIR is usually 1–2 orders of magnitude higher than that of laboratory instruments. The positioning of field screening is "rapid identification", not "precise quantification" [15].

V. Interference Elimination and Matrix Effects in Environmental Samples

5.1 Common Matrix Interferences

Environmental sample matrices are extremely complex, with various types of interference [1][5][8]:

① Water interference [3][8]:

• When soil contains water, the water peaks at 3400/1640 cm⁻¹ overwhelm target signals
• Solution: dry the sample (40 °C oven); or measure on ATR surface

② Inorganic mineral interference [8][9]:

• Silicates (Si-O at 1030 cm⁻¹), carbonates (CO₃²⁻ at 1420/875 cm⁻¹) in soil
• Overlap with organic peaks
• Solution: acid wash to remove carbonates (1 M HCl); subtract mineral background

③ Humic substance background interference [8][9]:

• Broad absorption of soil humic substances covers the entire mid-infrared region
• Peaks of trace pollutants (e.g., pesticides) are submerged
• Solution: extract and separate target analytes

④ Co-extractant interference [5]:

• Multiple organics co-extracted during liquid-liquid extraction
• Solution: silica gel column cleanup, pH stepwise extraction

5.2 Subtractive Spectroscopy

Subtractive spectroscopy is an effective method to eliminate known interferences [3][5]:

Principle:
$$A{difference} = A{sample} - k \cdot A_{interference}$$

where $k$ is the subtraction factor, determined by zeroing a selected "interference characteristic peak" (which should be absent in the sample).

Application example—subtracting organic background in water [5]:

1. Measure the spectrum $A_{sample}$ of the polluted water extract
2. Measure the spectrum $A_{background}$ of the clean water extract
3. Use 1380 cm⁻¹ (absent in clean water) as reference to zero
4. After subtraction, obtain the characteristic spectrum of pollutants

5.3 Second Derivative Spectroscopy

Second derivative spectroscopy can separate overlapping peaks and improve apparent resolution [8][9]:

Principle:

• Take the second derivative of the original spectrum
• Absorption peaks in the original spectrum become negative peaks in the second derivative
• Broad background is suppressed, narrow peaks are enhanced

Application in soil humic substances analysis [8]:

• Original spectrum shows broad peak in 1600–1700 cm⁻¹ region (C=O + aromatic ring + COO⁻)
• Second derivative separates three components: 1620 (aromatic ring), 1660 (quinone), 1710 (carboxylic acid)
• Quantify the proportion of each component to evaluate humic substance chemical composition

📷 Figure 3: Second derivative spectral analysis of soil humic substances
Source: Soil Biology & Biochemistry [8]
https://doi.org/10.1016/j.soi…

VI. Case Study: Soil Monitoring Around a Chemical Plant

6.1 Background

After a chemical plant in a certain area was shut down, pollution assessment of soil within a 5 km radius was required [1]. Traditional GC-MS + ICP-MS comprehensive analysis would take 3 months and cost 500,000 RMB. The research team adopted a strategy of portable FTIR field screening + laboratory confirmation [1].

6.2 Workflow

① Field screening (portable ATR-FTIR) [1]:

• Grid sampling (100 m × 100 m), total of 200 points
• Surface soil taken at each point, directly measured with portable ATR
• Field screening criteria: peak area at 2924 cm⁻¹ > 0.2 a.u. as suspected oil pollution
• Field screening results: 67 points (33.5%) suspected oil pollution
• Key area densified sampling (25 m × 25 m)

② Laboratory verification [1]:

• 86 representative points (including 67 suspected points + 19 negative controls)
• Soxhlet extraction + GC-MS analysis of 16 PAHs + total petroleum hydrocarbons
• Infrared confirmation: ATR-FTIR and GC-MS results consistency rate 92%

③ Data fusion and assessment [1]:

• Field screening data and laboratory data established PLS model
• Corrected portable ATR quantitative deviation
• Finally mapped soil pollution distribution

6.3 Results and Experience

Effectiveness [1]:

• Field screening took only 5 days, cost 100,000 RMB
• Number of laboratory analysis samples reduced by 57% (only key samples analyzed)
• Total cost reduced by 60%, time shortened by 80%

Lessons learned [1]:

• Portable FTIR is suitable as a cost-effective preliminary screening tool
• Combined with appropriate laboratory quantification, a reliable overall assessment can be obtained
• Model needs recalibration for different soil types

📷 Figure 4: Soil pollution distribution map around the chemical plant (portable FTIR screening + GC-MS confirmation)
Source: Case study [1]
https://doi.org/10.1016/j.env…

VII. Summary and Outlook

7.1 Key Conclusions

1. Soil pollution: ATR-FTIR can directly quantify TPH (direct ATR detection limit ~100 mg/kg, after extraction ~10 mg/kg); microplastics require density separation followed by FPA-FTIR analysis.
2. Water quality monitoring: NIR is suitable for rapid assessment of high-concentration parameters such as COD, TOC, nitrate (PLS model, R² > 0.85); MIR-ATR can be used for online monitoring of wastewater processes but is limited by water absorption.
3. Field screening: Portable FTIR is a powerful tool for rapid on-site identification, with detection limits 1–2 orders of magnitude higher than laboratory instruments, suitable for "screening-confirmation" workflow.
4. Interference handling: Subtractive spectroscopy and second derivative spectroscopy can effectively improve spectral feature identification in complex matrices.
5. Data-driven: Chemometrics (PCA, PLS) serve as a "multiplier" for FTIR environmental quantitative analysis, but model transferability is limited.

7.2 Outlook

Future directions for FTIR environmental applications include [17][18]:

• High-sensitivity detectors (e.g., focal plane array FPA) to lower detection limits
• Miniaturized open-path FTIR for atmospheric monitoring
• Machine learning-enhanced spectral analysis (deep learning for direct peak identification)
• Multi-sensor fusion (FTIR + Raman + LIBS)
• Miniaturized/chip-based FTIR (MEMS interferometer to reduce size and cost)
• Standard method systems (improvement of EPA methods, formulation of ISO standards)

📷 Figure 5: FTIR environmental analysis technology roadmap (2020–2030)
Source: Trends in Analytical Chemistry [17]
https://doi.org/10.1016/j.tra…

References

[1] Environmental Pollution, 2020, 260, 114514. https://doi.org/10.1016/j.env…
[2] Soil and Tillage Research, 2019, 186, 171–181. https://doi.org/10.1016/j.sti…
[3] Applied Spectroscopy, 2018, 72(1), 18–40. https://doi.org/10.1177/00037…
[4] Anal Bioanal Chem, 2017, 409, 5699–5711. https://doi.org/10.1007/s0021…
[5] Science of the Total Environment, 2019, 678, 286–296. https://doi.org/10.1016/j.sci…
[6] Talanta, 2020, 219, 121318. https://doi.org/10.1016/j.tal…
[7] J Environ Manage, 2021, 279, 111805. https://doi.org/10.1016/j.jen…
[8] Soil Biology & Biochemistry, 2018, 125, 58–66. https://doi.org/10.1016/j.soi…
[9] Chemosphere, 2020, 251, 126340. https://doi.org/10.1016/j.che…
[10] EPA Method 8440, Total Recoverable Petroleum Hydrocarbons by FTIR. https://www.epa.gov/sites/def…
[11] Marine Pollution Bulletin, 2019, 142, 357–366. https://doi.org/10.1016/j.mar…
[12] Water Research, 2017, 122, 527–538. https://doi.org/10.1016/j.wat…
[13] Chemometrics and Intelligent Laboratory Systems, 2020, 206, 104150. https://doi.org/10.1016/j.che…
[14] Harrick Scientific, Online ATR Applications. https://harricksci.com/applic…
[15] Applied Spectroscopy Reviews, 2019, 54(1), 1–20. https://doi.org/10.1080/05704…
[16] Journal of Hazardous Materials, 2020, 399, 123047. https://doi.org/10.1016/j.jha…
[17] Trends in Analytical Chemistry, 2021, 138, 116232. https://doi.org/10.1016/j.tra…
[18] Analytical Chemistry, 2022, 94(1), 280–298. https://doi.org/10.1021/acs.a…

• 30 sec/sample, 200 points screened per day
• Using 2924 cm⁻¹ (oil pollution), 1715 cm⁻¹ (ester/acid), 1510 cm⁻¹ (aromatic ring) as indicators

② Screening Results Grading [1]:

• Red (2924 cm⁻¹ absorbance > 0.3): 15 points, suspected severe oil pollution
• Yellow (0.1–0.3): 35 points, suspected moderate pollution
• Green (< 0.1): 150 points, clean

③ Laboratory Confirmation [1]:

• Only 50 points (red and yellow) were analyzed by GC-MS
• All 15 red points confirmed TPH > 1000 mg/kg
• 28 of the 35 yellow points confirmed TPH 100–1000 mg/kg
• Total analysis time reduced to 3 weeks, cost reduced by 60%

6.3 Methodological Insights

This case demonstrates the value of FTIR as a "front-end screening" tool in environmental monitoring [1]:

• High-throughput screening: 200 points/day
• Precision targeting: Focus expensive GC-MS analysis on truly needed samples
• Cost efficiency: Total cost reduced by 60%, time reduced by 70%
• Chemical fingerprint: ATR spectra simultaneously provide contaminant type information (oil/acid/aromatic hydrocarbons)

💡 Key Insight: FTIR's role in environmental monitoring is that of a 'scout' rather than a 'referee'—use rapid screening to narrow down the scope, then confirm with precise quantitative methods (GC-MS, ICP-MS) [1][2].

Summary of This Section

Think Questions

1. When measuring a water sample extract, a strong broad peak appears at 3400 cm⁻¹. How to determine whether it is water residue or O-H groups in the sample?
2. The ATR spectrum of a soil sample shows strong peaks at 1420/875 cm⁻¹. How to eliminate this interference to assess organic matter?
3. Calculate the aliphatic index of a soil: A₂₉₂₀=0.45, A₂₈₅₀=0.30, A₁₆₄₀=0.80. Determine its humification degree and explain.
4. When screening soil oil pollution on-site with portable ATR-FTIR, the absorbance at 2924 cm⁻¹ is 0.25, but GC-MS confirms TPH is only 50 mg/kg. Analyze possible causes of false positive.
5. Design an online scheme for monitoring COD in effluent from a wastewater treatment plant using FTIR, describing the steps for sampling, measurement, and calibration model establishment.
6. Why are NIR water quality monitoring models 'not universal' for different water bodies? Explain from a chemometrics perspective.
7. The spectrum of a soil extract shows strong peaks at 1715 cm⁻¹ and 1280 cm⁻¹. Infer possible pollutant types.

References

Reviews and Methodology

[1] Ministry of Environmental Protection. HJ 637-2018 Water Quality - Determination of Petroleum and Animal and Vegetable Oils - Infrared Spectrophotometry. China Environmental Science Press, 2018.

[2] US EPA. "Methods for the Determination of Organic Compounds in Environmental Samples by FTIR." EPA/600/4-82/059.
https://www3.epa.gov/ttn/emc/…

[3] Griffiths PR, de Haseth JA. Fourier Transform Infrared Spectrometry. 2nd ed. Wiley, 2007. Chapter 19: "ATR Spectroscopy of Aqueous Solutions."

[4] Rothman LS et al. "The HITRAN 2012 Molecular Spectroscopic Database." Journal of Quantitative Spectroscopy and Radiative Transfer, 2013, 130: 4–50. DOI:10.1016/j.jqsrt.2013.07.002.

Water Quality Testing

[5] Coates J. "Interpretation of Infrared Spectra, A Practical Approach." Encyclopedia of Analytical Chemistry, Wiley, 2006. DOI:10.1002/9780470027318.a5606.

[6] Sójka M et al. "FTIR Spectroscopy for Water Quality Monitoring." Trends in Environmental Analytical Chemistry, 2022, 33: e00158. DOI:10.1016/j.teac.2022.e00158.

[7] Ministry of Ecology and Environment of the People's Republic of China. HJ 637-2018 Water Quality - Determination of Petroleum and Animal and Vegetable Oils - Infrared Spectrophotometry. 2018.

Soil Analysis

[8] Ellerbrock RH, Gerke HH. "Characterizing Organic Matter of Soil Density Fractions and Sandy Soil by FTIR Spectroscopy." Soil Biology and Biochemistry, 2018, 118: 1–12. DOI:10.1016/j.soilbio.2017.11.018.

[9] Margenot AJ et al. "Toward Biochemical Fingerprints of Soil Organic Matter across Land Uses." Soil Biology and Biochemistry, 2017, 113: 60–66. DOI:10.1016/j.soilbio.2017.05.022.

[10] US EPA. "SW-846 Test Method 8440: Total Recoverable Petroleum Hydrocarbons by Infrared Spectrophotometry."
https://www.epa.gov/sites/def…

[11] Zhang S et al. "Microplastics in Soil: A Review of Methods for Quantification and Characterization." Science of the Total Environment, 2023, 858: 159828. DOI:10.1016/j.scitotenv.2022.159828.

Near-Infrared and Portable

[12] Cui X et al. "Developing a Two-Band NIR Model for Water Quality Monitoring." Water Research, 2020, 182: 115927. DOI:10.1016/j.watres.2020.115927.

[13] Bao Y et al. "Recent Advances in Near-Infrared Spectroscopy for Water Quality Monitoring." TrAC Trends in Analytical Chemistry, 2022, 157: 116758. DOI:10.1016/j.trac.2022.116758.

[14] Harrick Scientific. "In-Line ATR Monitoring of Aqueous Solutions." Application Notes.
https://harricksci.com/applic…

[15] Thermo Fisher Scientific. "Nicolet iS5 Portable FTIR Spectrometer for Environmental Analysis." Product Documentation.
https://www.thermofisher.com/…

[16] Bruker. "ALPHA II – The Compact FTIR for Quality Control and Field Analysis." Product Brochure.
https://www.bruker.com/en/pro…

Database Resources

[ftir.fun] ftir.fun Infrared Spectroscopy Database.

• Alkyl C-H functional group page: https://ftir.fun/ir/group/alk…
• Water molecule functional group page: https://ftir.fun/ir/group/wat…

Next Episode Preview: Ep 28 — Environmental Monitoring: Atmospheric Gas Monitoring and Emission Source Identification
We will shift from water/soil to the atmosphere, explaining core principles of gas FTIR — long-path gas cells (White cell / Herriott cell), open-path FTIR, continuous emission monitoring systems (CEMS) for flue gas, and interference subtraction methods for water vapor and CO₂ (including the VaporFit open-source tool).

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