Soil Heavy Metal Determination Experiment: Analytical Methods, Standards, and Testing Procedures

Soil heavy metal determination is a core environmental testing experiment that quantifies toxic elements such as lead, cadmium, chromium, mercury, and arsenic in soil to evaluate contamination levels and regulatory compliance. This technical article presents the complete experimental workflow—from field sampling and pretreatment to acid digestion and instrumental analysis by AAS, ICP-OES, ICP-MS, and AFS—in accordance with GB, ISO, and EPA standards. It serves as a practical reference for accredited third-party laboratories, environmental researchers, and remediation professionals involved in soil quality assessment.

Technical Overview

The soil heavy metal determination experiment is a quantitative analytical procedure designed to measure the concentrations of metallic elements with densities greater than 4.5 g/cm³—commonly including cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn)—in soil matrices. Unlike organic pollutants, heavy metals are non-biodegradable and persist in soil for decades, entering the food chain through crop uptake, dust inhalation, and groundwater leaching, where they pose chronic risks to human health and ecosystems.

The experiment typically follows a five-stage workflow: representative field sampling, laboratory pretreatment (air-drying, grinding, and sieving), acid digestion to bring the target elements into solution, instrumental quantification, and data validation against certified reference materials (CRMs). In China, the interpretive framework is anchored by two risk-control standards: GB 15618-2018 for agricultural land and GB 36600-2018 for development land, which define screening values for the eight regulated inorganic elements. Internationally, ISO 11047, ISO 17294-2, and EPA Methods 3050B, 6010D, 6020B, and 7471B provide equivalent methodological bases for extraction and measurement.

Testing Samples

Agricultural Soil

Collected from cultivated farmland, vegetable plots, orchards, and greenhouse bases, agricultural samples are used to verify compliance with food-safety screening values for Cd, Hg, As, Pb, and Cr. Sampling typically follows a checkerboard or plum-blossom (diagonal) layout at depths of 0–20 cm for plow layers and 20–40 cm for deep verification, with 5–20 subsamples composited into a single sample of at least 1 kg.

Development and Construction Land Soil

For residential, commercial, and industrial redevelopment parcels, grid-based sampling (e.g., 40 m × 40 m units) is performed at multiple depths depending on suspected contamination pathways. These samples are assessed against GB 36600-2018 Class I or Class II screening and intervention values, and results directly determine whether risk management or remediation is required before land transfer.

Industrial and Mining Site Soil

Soil from smelters, electroplating facilities, pesticide plants, mining tailings, and waste-storage areas often exhibits multi-element enrichment and heterogeneous distribution. Dense systematic sampling combined with field portable X-ray fluorescence (XRF) pre-screening is used to delineate hotspots before confirmatory laboratory analysis.

Sediment and Special Matrices

Riverbed, lakebed, and estuarine sediments, as well as sludge-amended or greenhouse soils, may be analyzed under the same experiment but require adjusted moisture correction, additional homogenization, and, in some cases, speciation analysis (e.g., hexavalent chromium) to reflect bioavailability.

Testing Items

The experiment covers the eight priority regulated metals plus supplementary elements requested for risk screening or scientific research:

• Cadmium (Cd) – the most restrictive agricultural contaminant, often controlled at levels below 0.3–0.8 mg/kg depending on soil pH
• Mercury (Hg) – volatile element requiring dedicated digestion and cold-vapor or atomic fluorescence detection
• Arsenic (As) – metalloid measured by hydride-generation AFS or ICP-MS to meet sub-mg/kg limits
• Lead (Pb) and Chromium (Cr) – classic industrial contaminants, with Cr(VI) speciation available where needed
• Copper (Cu), Nickel (Ni), and Zinc (Zn) – essential trace metals that become phytotoxic at elevated concentrations
• Optional elements: selenium (Se), antimony (Sb), beryllium (Be), cobalt (Co), molybdenum (Mo), thallium (Tl), vanadium (V), and manganese (Mn)

Testing Methods

Sample Pretreatment

Field samples are air-dried at room temperature (below 40 °C to prevent mercury loss), debris and foreign matter are removed, and the soil is ground in an agate mill. The fraction passing a 0.149 mm (100-mesh) nylon sieve is retained for metal analysis, while a separate 2 mm fraction is used for pH and moisture determination. All contact surfaces must be non-metallic to avoid cross-contamination.

Acid Digestion Techniques

The choice of digestion governs whether results represent "aqua regia extractable" or "total" content:

• Aqua regia digestion (HCl:HNO₃ = 3:1) – the standard approach for GB 36600-2018 and GB 15618-2018 compliance screening
• Total digestion with HNO₃–HF–HClO₄ – dissolves silicate lattices for true total element content
• Microwave-assisted digestion (e.g., HNO₃–HCl–HF closed-vessel) per HJ 832-2017 – minimizes loss of volatile Hg and As and improves blank control
• Water-bath or hot-block extraction for hexavalent chromium under alkaline conditions to prevent redox interconversion

Instrumental Analysis

Instrument selection depends on target elements, detection limits, and sample throughput:

• ICP-MS (inductively coupled plasma mass spectrometry) – multi-element determination at ng/L-level solution detection limits, the preferred platform for HJ 803-2016 and EPA 6020B
• ICP-OES (inductively coupled plasma optical emission spectrometry) – simultaneous multi-element measurement suited to mg/kg-level contaminants such as Cu, Zn, and Cr
• GFAAS (graphite furnace atomic absorption) – single-element trace analysis of Pb and Cd per GB/T 17141
• FAAS (flame atomic absorption) – cost-effective determination of higher-concentration Cu, Zn, Pb, and Cd
• AFS (atomic fluorescence spectrometry) – the method of choice for Hg and As by cold-vapor and hydride generation per GB/T 22105
• Portable and benchtop XRF – rapid on-site screening used to guide laboratory sampling, not a substitute for confirmatory analysis

Quality Assurance and Quality Control

Each analytical batch must include a method blank, laboratory duplicates (relative deviation typically ≤ 20%), a certified reference material such as a national soil standard (GBW series) with recovery inside the certified uncertainty range, matrix spike recoveries of 70–130%, and multi-point calibration with correlation coefficients ≥ 0.999. Reporting limits must remain below one-quarter of the applicable screening values to be legally defensible.

Applications

• Agricultural land quality surveys and delineation of farmland requiring safe utilization measures
• Due-diligence site assessments prior to land transfer, redevelopment, or construction
• Verification sampling and acceptance testing for soil remediation projects
• Environmental impact assessments and pollutant-discharge compliance monitoring
• Scientific research on metal migration, bioavailability, and soil–crop transfer modeling
• Forensic and legal support for contamination liability disputes and insurance claims

FAQ

What is the difference between aqua regia extraction and total digestion?

Aqua regia dissolves most environmentally reactive metal species but does not fully attack silicate minerals, so results are slightly lower than total content; it is nevertheless the basis for GB and ISO screening-value comparisons. Total digestion with hydrofluoric acid releases all elements from the crystal lattice and is used when true total concentration is required, such as in background-level studies.

Which technique is best for mercury and arsenic in soil?

Microwave digestion followed by atomic fluorescence spectrometry or ICP-MS is recommended, because mercury is volatile and easily lost during open-vessel hot-plate digestion. Closed-vessel processing, gold-amalgam trapping, and strict blank control keep method detection limits at the 0.002 mg/kg level required by agricultural screening values.

How should soil samples be preserved before analysis?

Samples should be transported in polyethylene or glass containers, protected from light and heat, air-dried promptly upon arrival, and stored in a dry, contaminant-free cabinet. Samples intended for speciation analysis (e.g., Cr(VI)) must be refrigerated at 4 °C and tested within the holding period specified by the relevant method standard.

Which standards govern result evaluation?

For agricultural land, GB 15618-2018 sets risk screening and control values; for development land, GB 36600-2018 defines screening and intervention values. International projects may be evaluated against ISO 11047 or jurisdiction-specific EPA and EN criteria, and accredited third-party laboratories select the digestion–instrument pairing that matches the governing standard.