Soil heavy metal total content determination is a core analytical experiment that quantifies the total concentrations of toxic metals and metalloids—such as cadmium, lead, chromium, mercury, arsenic, copper, zinc, and nickel—in soil samples through acid digestion and instrumental analysis. This article provides a comprehensive technical overview of the experimental workflow, including sample pretreatment, digestion protocols, instrumental determination methods, quality control requirements, and the relevant national and international standards, making it an essential reference for environmental laboratories, remediation practitioners, and researchers.
Technical Overview
Soil heavy metal total content determination refers to the quantitative measurement of the total mass fraction of heavy metals and metalloid elements bound within a soil matrix, encompassing all chemical forms including exchangeable, reducible, oxidizable, and residual fractions. Unlike bioavailable or extractable fraction analysis, total content determination requires complete or near-complete decomposition of the soil matrix, which is achieved through aggressive acid digestion. The experiment typically follows a standardized workflow:
- Field sampling and sample transport in accordance with technical specifications for soil environmental monitoring (HJ/T 166-2004, HJ 25.2-2014)
- Laboratory pretreatment: air-drying, removal of debris and stones, grinding with non-contaminating apparatus, and sieving
- Acid digestion using aqua regia, reverse aqua regia, or hydrofluoric-acid-based total decomposition systems
- Instrumental determination by atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma optical emission spectrometry (ICP-OES), or inductively coupled plasma mass spectrometry (ICP-MS)
- Data validation and reporting against risk screening values specified in GB 36600-2018 (development land) and GB 15618-2018 (agricultural land)
The results form the scientific basis for soil pollution investigation, land-use risk assessment, remediation target setting, and post-remediation acceptance evaluation. Internationally equivalent procedures are described in ISO 11466 (aqua regia extraction), ISO 14869-1 (total dissolution with hydrofluoric acid), and the widely adopted digestion methods of the US EPA series (3050B, 3051A, 3052).
Testing Samples
Agricultural and Forest Land Soil
Cultivated land, orchards, vegetable bases, tea plantations, and forest soils are tested to verify compliance with GB 15618-2018 risk screening values, particularly in areas influenced by mining, smelting, sewage irrigation, or long-term fertilizer and pesticide application. These samples are typically collected from the surface layer (0–20 cm) with subsoil profiles collected as required.
Residential and Industrial Development Land Soil
Soil from proposed residential, commercial, and industrial sites is tested in accordance with GB 36600-2018, which classifies land into the first land-use class (sensitive uses such as residential and schools) and the second class (industrial and other non-sensitive uses), each with distinct screening and intervention values.
Contaminated Site and Remediation Soil
Soil from suspected contaminated sites—former electroplating facilities, metal smelters, chemical plants, gas stations, and landfills—is tested during pollution investigation, and remediated soil is re-tested for acceptance verification against remediation targets.
Special Matrices
Stream and lake sediments, sewage sludge, compost, mine tailings, and solid waste-derived soil amendments share similar digestion and determination procedures, generally following the combined soil and sediment method standards such as HJ 803-2016 and HJ 680-2013.
Testing Items
The most commonly determined elements in soil heavy metal total content experiments include:
- Priority control elements: arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb)
- Basic inorganic items of GB 36600-2018: As, Cd, Cr, Cu, Hg, Ni, Pb, and hexavalent chromium (Cr(VI), determined separately as a speciation analysis)
- Extended "other items" elements: antimony (Sb), beryllium (Be), cobalt (Co), manganese (Mn), thallium (Tl), vanadium (V), zinc (Zn), and silver (Ag)
- Supplementary elements: selenium (Se), bismuth (Bi), molybdenum (Mo), tin (Sn), copper (Cu), and nickel (Ni)
Results are conventionally reported in mg/kg on a dry-weight basis, with moisture correction performed on a parallel subsample dried at 105 °C. Element selection depends on the land-use scenario, the pollution source characteristics, and the applicable regulatory standard.
Testing Methods
Sample Pretreatment
Air-dried samples are spread in a clean, dust-free environment at room temperature and protected from direct sunlight and cross-contamination. After removing plant roots, stones, and foreign debris, the sample is ground using an agate mortar or mechanical grinder (avoiding chromium-steel contact parts when Cr, Ni, or Co are target elements). The ground material is passed through a 2 mm (10 mesh) sieve for homogenization and moisture determination, and further through a 0.25 mm (60 mesh) or 0.149 mm (100 mesh) sieve for digestion. Samples for mercury determination are dried and sieved at low temperature to prevent volatilization losses.
Acid Digestion
- Aqua regia digestion: A 3:1 mixture of hydrochloric acid and nitric acid is used for hot-plate or block digestion according to HJ 803-2016 and ISO 11466, suitable for the majority of heavy metals in risk-assessment contexts; HJ 781-2016 covers 22 elements by ICP-OES using the same extraction
- Reverse aqua regia microwave digestion: A 1:3 mixture of hydrochloric acid and nitric acid under microwave heating per HJ 680-2013, offering closed-vessel recoveries for volatile elements such as Hg, As, Se, Sb, and Bi
- Total digestion: A hydrofluoric acid–nitric acid–perchloric acid (HF–HNO3–HClO4) system per ISO 14869-1 and GB/T 17141-1997 decomposes silicate lattices completely, representing true total content; EPA Method 3052 provides the microwave equivalent
- Water-bath digestion: Aqua regia extraction in a boiling water bath per GB/T 22105 for mercury and arsenic determination by atomic fluorescence spectrometry
- Direct thermal decomposition: Solid sampling direct mercury analysis per EPA Method 7473 requires no digestion for total mercury
The choice between aqua regia extraction and HF total digestion must be stated clearly in the report, because aqua regia dissolves most anthropogenic and exchangeable metal pools but does not fully attack silicate minerals, which may yield slightly lower results for lithogenic elements such as Cr and Ni.
Instrumental Determination
- Flame atomic absorption spectrometry (FAAS): suitable for Cu and Zn at mg/kg levels (GB/T 17138-1997) and for Pb, Cd at elevated concentrations
- Graphite furnace atomic absorption spectrometry (GFAAS): trace-level determination of Pb and Cd (GB/T 17141-1997, EPA 7010), typically with matrix modifiers such as ammonium dihydrogen phosphate or palladium nitrate
- Atomic fluorescence spectrometry (AFS): hydride generation AFS for As, Se, Sb, Bi and cold vapor AFS for Hg (GB/T 22105 series, HJ 680-2013), offering excellent sensitivity for hydride-forming elements
- ICP-OES: simultaneous multi-element determination for major and trace metals (HJ 781-2016, EPA 6010D), ideal for high-throughput screening
- ICP-MS: ultra-trace multi-element determination with the widest elemental coverage (HJ 803-2016, EPA 6020B), using internal standards such as In, Rh, and Bi, and collision/reaction cell technology to resolve polyatomic interferences such as ArCl+ on arsenic-75
Quality Assurance and Quality Control
A valid experiment incorporates procedural blanks (at least one per batch of 20 samples), parallel duplicates at a frequency of at least 10 percent with relative deviations typically within 20 to 30 percent, matrix spike recoveries within the method-specified range (commonly 70–130 percent), certified soil reference materials of the GBW series, continuing calibration verification standards, and interference-correction checks. Calibration curves must meet the correlation coefficient requirement of the method standard, normally not less than 0.999. All results are corrected for moisture content and reported with method detection limits and uncertainty statements where applicable.
Applications
Soil heavy metal total content data support a wide range of environmental and regulatory applications:
- Soil pollution status investigation and detailed investigation for land redevelopment and urban planning
- Classification of agricultural land into priority protection, safe utilization, and strict control categories
- Environmental impact assessment and soil pollution prevention plans for construction projects
- Baseline surveys before industrial operations and compliance monitoring during operation
- Verification of remediation effectiveness after soil replacement, solidification/stabilization, washing, or thermal treatment
- Fertilizer, compost, and soil conditioner quality evaluation, including limit-value compliance testing
- Scientific research including pollution source apportionment, geochemical baseline studies, and ecological risk assessment models such as the geoaccumulation index and potential ecological risk index
Accredited third-party laboratories routinely perform these experiments under ISO/IEC 17025 management systems, ensuring that reported data are legally defensible for regulatory filing and land transaction purposes.
FAQ
What is the difference between total content and available (extractable) content?
Total content measures all forms of an element after aggressive digestion of the soil matrix, whereas available content measures only the fraction extractable by mild reagents such as DTPA (e.g., GB/T 23739-2009 for available Pb and Cd) or dilute acids, which approximates the fraction potentially bioavailable to plants. Total content is the basis of regulatory risk screening; available content is mainly used in agronomic and plant-uptake studies.
Why can aqua regia results differ from HF total digestion results?
Aqua regia does not completely dissolve silicate and some refractory mineral lattices, so lithogenic portions of elements such as Cr, Ni, and V may be underestimated by several percent to tens of percent. HF-based digestion destroys the silicate matrix entirely and yields true total values. Most national screening standards accept aqua regia extraction results, but the digestion system must always be declared in the test report.
What are typical detection limits?
With ICP-MS, method detection limits in soil generally reach 0.01–0.1 mg/kg for Cd and Tl, 0.1–1 mg/kg for Pb, Cr, Cu, Ni, and Zn, and below 0.01 mg/kg for Hg by AFS or direct mercury analysis. GFAAS achieves approximately 0.01–0.05 mg/kg for Cd and 0.1 mg/kg for Pb. Actual limits depend on digestion volume, final dilution, and instrument performance.
How much sample is required and how long does the experiment take?
A minimum of 500 g of sieved soil (100 g if only a limited element list is required) is recommended. The standard workflow, including pretreatment, digestion, instrumental analysis, and quality control review, is typically completed within 5 to 10 working days per batch.
Which elements require special handling?
Mercury and arsenic are volatile or hydride-forming and are prone to loss during open-vessel digestion; closed-vessel microwave digestion or water-bath digestion with AFS detection is therefore preferred. Hexavalent chromium is a speciation parameter that requires alkaline extraction rather than total digestion, and it must be requested separately from total chromium.
How are results compared with regulatory limits?
Measured dry-weight concentrations are compared directly against the screening and intervention values in GB 36600-2018 for development land or GB 15618-2018 for agricultural land, taking into account soil pH for certain elements. Results below screening values indicate generally acceptable risk, whereas exceedances trigger detailed risk assessment or intervention requirements.