Soil Bioavailable Heavy Metal Analysis: Extraction Methods, Standards, and Applications

Available (bioavailable) heavy metal analysis measures the fraction of metals in soil that can be released into the soil solution and taken up by plants, soil organisms, or humans, providing far greater environmental and food-safety relevance than total concentration testing alone. Using selective extraction chemistries such as DTPA, calcium chloride, ammonium nitrate, and BCR sequential extraction, accredited third-party laboratories quantify available Cd, Pb, As, Cu, Zn, Ni, Cr, Hg, and other elements in soil. This article explains the analytical principles, standard methods, sample requirements, instrumentation, quality control, and typical applications of soil available heavy metal analysis.

Technical Overview

Heavy metals in soil rarely exist as free ions. They are distributed among water-soluble, exchangeable, carbonate-bound, iron/manganese oxide-bound, organically complexed, and residual mineral forms. Only a portion of the total metal burden can actually desorb into the soil solution, be absorbed by plant roots, ingested by soil organisms, or dissolved into leachate. This mobile, plant-accessible, or organism-accessible portion is defined as the available fraction (also called the extractable, phytoavailable, or bioavailable fraction), and it is determined by soil available heavy metal analysis.

Total heavy metal concentration is frequently a poor predictor of biological effect. A soil containing high total cadmium held largely in residual silicate lattices may pose less crop risk than a soil with lower total cadmium dominated by exchangeable and water-soluble forms. Availability is controlled by soil pH (the single most influential factor), redox potential, organic matter content, cation exchange capacity, clay mineralogy, iron/manganese/aluminum oxide content, carbonate levels, phosphate status, contamination source, and the aging time since the metal entered the soil. Consequently, risk assessments under frameworks such as GB 15618-2018 and GB 36600-2018, which establish screening values on a total-content basis, are increasingly supplemented by available-fraction data to refine site-specific risk conclusions.

Because no single chemical reagent can isolate a truly "bioavailable" pool, availability is measured operationally: the result is defined by the extractant composition, extraction ratio, contact time, and extraction pH used. The principal analytical strategies include single extraction with dilute salt or chelating agents to estimate plant uptake, leaching tests to estimate contaminant mobility toward groundwater, sequential extraction to partition metals into operationally defined geochemical fractions, and in vitro gastrointestinal extraction to estimate oral bioaccessibility for human health risk assessment. Advanced passive sampling techniques such as diffusive gradients in thin films (DGT) provide an additional kinetically based measure of the labile metal pool that often correlates closely with plant accumulation.

Testing Samples

Agricultural and Greenhouse Soils

Paddy soils, upland vegetable fields, orchards, and protected greenhouse soils are the most frequently tested matrices. Greenhouse soils under long-term intensive fertilization and irrigation often accumulate elevated available Cd, As, Cu, and Zn, and available-fraction data directly support safe production decisions for rice, leafy vegetables, and other accumulator crops.

Contaminated Site and Industrial Soils

Soils from former electroplating, smelting, mining, pesticide manufacturing, tanning, and battery recycling sites require characterization of both total and leachable metal pools. Available-fraction and leaching results feed into human health and ecological risk assessment models and into the design of stabilization or immobilization remedies.

Sediments, Sludges, Composts, and Amended Soils

River and lake sediments, sewage sludges, livestock manures, composts, and soils receiving repeated organic or phosphatic fertilizer amendments are tested to evaluate metal transfer pathways when such materials are applied to land. Extraction schemes designed for soil improvers and growing media, such as the CaCl2/DTPA (CAT) extraction in EN 13651, are applicable to these matrices.

Sample Requirements and Preservation

• Recommended quantity: 500 g to 1 kg of sieved (<2 mm) soil per sampling point, or sufficient material for all requested extraction schemes plus archived reserve.
• Collection: use bamboo, wooden, stainless-steel, or plastic tools; avoid galvanized or brass implements that can contaminate Zn, Cu, and Pb.
• Preparation: air-dry at room temperature (below 40 °C) to constant mass, gently disaggregate, and pass through a 2 mm nylon sieve. Availability testing must not be performed on finely ground (<0.149 mm) samples intended for total digestion, because excessive grinding alters extraction behavior.
• Storage: dried samples in clean polyethylene or glass containers at room temperature; determine moisture (dry-matter) content separately according to HJ 613 or ISO 11465 so results can be reported on a dry-mass basis.

Testing Items

Testing items cover the available, extractable, leachable, and bioaccessible pools of regulated and environmentally significant metals and metalloids, together with auxiliary parameters needed for data interpretation.

• Available elements: Cd, Pb, Cr, Ni, Cu, Zn, As, Hg, Co, Mn, Fe, Mo, Se, Tl, Sb, V, Be, and Al.
• Extractable pools: DTPA-extractable, EDTA-extractable, CaCl2-extractable, ammonium nitrate-extractable, Mehlich-3-extractable, and water-soluble fractions.
• Leachable pools: TCLP and SPLP leachate concentrations of the eight RCRA metals plus other elements of concern.
• Speciation fractions: BCR F1 (acid-soluble/exchangeable), F2 (reducible, Fe/Mn oxide-bound), F3 (oxidizable, organic/sulfide-bound), and F4 (residual).
• Oral bioaccessibility: gastric-phase and gastrointestinal-phase extractable Pb, As, Cd, and Ni for human health risk assessment.
• Hexavalent chromium: alkaline-extractable Cr(VI) as the toxicologically significant chromium species.
• Auxiliary parameters: soil pH (HJ 962 or ISO 10390), organic matter, cation exchange capacity, and particle size distribution, all of which govern metal availability.

Testing Methods

Sample Preparation and Quality Basis

Samples are air-dried, sieved to <2 mm, and homogenized. Because extractable metal levels are low, rigorous contamination control is essential: procedural blanks, dedicated plasticware, acid-washed vessels, and ultrapure reagents are used throughout. All results are converted to a dry-soil mass basis using concurrently determined moisture content.

Single-Extraction Methods

• DTPA extraction (0.005 mol/L DTPA + 0.01 mol/L CaCl2 + 0.1 mol/L TEA, pH 7.3, typically 1:2 soil-to-solution, 2 h shaking): the standard approach for plant-available Cd, Pb, Cu, Zn, Fe, and Mn in near-neutral and calcareous soils, referenced in GB/T 23739 for available Pb and Cd and NY/T 890 for available Zn, Fe, Mn, and Cu. Extracts are analyzed by ICP-OES, ICP-MS, or atomic absorption spectrometry.
• Dilute calcium chloride extraction (0.01 mol/L CaCl2, 1:10, 2 h): an unbuffered method reflecting the actual soil solution ionic strength; the measured pool correlates well with plant uptake across soil types and is widely used for Cd and Zn mobility evaluation.
• Ammonium nitrate extraction (1.0 mol/L NH4NO3): standardized in ISO 19730 and DIN 19730 and used in several European regulatory frameworks as an unbuffered transfer-factor screening test for Cd, Pb, Zn, and other trace elements.
• EDTA extraction (0.05 mol/L, typically 1:5, 1 h): a strong chelating extraction that removes the labile pool bound to organic matter and carbonates, useful for long-term availability assessment of Cu, Pb, and Cd.
• Mehlich-3 and Mehlich-1 multi-element extraction: routine agronomic extractants for acid to neutral soils, permitting simultaneous determination of available metals and nutrients in a single extract.
• Water-soluble extraction (deionized water, e.g., liquid-to-solid ratio 2 to 10 L/kg per ISO/TS 21268 leaching procedures): quantifies the most mobile, immediately bioavailable pool.

Sequential Extraction (BCR Protocol)

The three-step BCR sequential extraction, developed under the European Standards, Measurements and Testing (SM&T) Programme, partitions metals into: Step 1 (F1), 0.11 mol/L acetic acid, exchangeable and acid-soluble; Step 2 (F2), 0.5 mol/L hydroxylamine hydrochloride, reducible Fe/Mn oxide-bound; Step 3 (F3), hydrogen peroxide oxidation followed by 1.0 mol/L ammonium acetate, oxidizable organic and sulfide-bound; followed by residual (F4) digestion with aqua regia. The sum of the four fractions is validated against the total or aqua-regia-soluble content, and certified reference material BCR-701 is used for method verification. The F1 fraction is a widely accepted proxy for the immediately available pool.

Leaching and Bioaccessibility Tests

• TCLP (EPA Method 1311): acetic-acetate buffer leach at liquid-to-solid ratio 20:1 for 18 h, simulating co-disposal leaching conditions for hazardous-waste classification.
• SPLP (EPA Method 1312): dilute acid sulfate/sulfate-nitrate leachant simulating acid rain infiltration, appropriate for in-situ mobility assessment.
• LEAF methods (EPA Methods 1313, 1314, 1315, and 1316): parameterized leaching protocols describing metal release as a function of pH, liquid-to-solid ratio, and mass transfer, used for detailed fate-and-transport modeling.
• In vitro bioaccessibility: EPA Method 1340 (0.4 mol/L glycine solution at pH 1.5, 37 °C, 1 h gastric phase) for Pb and As in soil, and unified bioaccessibility method (UBM)-type gastrointestinal assays for multi-element oral exposure assessment.
• Hexavalent chromium: alkaline digestion (EPA Method 3060A) followed by colorimetric determination with 1,5-diphenylcarbazide (EPA Method 7196A) or equivalent instrumental methods.

Instrumental Determination

Extracts contain metals at microgram-per-liter to milligram-per-liter levels, so sensitive multielement instrumentation is required. ICP-MS provides the lowest detection limits (sub-microgram-per-kilogram soil equivalents) and is preferred for Cd, Pb, Tl, and Ni in dilute salt extracts. ICP-OES offers high throughput for Cu, Zn, Mn, and Fe in chelating extracts. Flame and graphite-furnace atomic absorption spectrometry serve as validated alternatives, hydride-generation or cold-vapor atomic fluorescence spectrometry is used for available As, Se, Sb, and Hg, and UV-visible spectrophotometry is used for Cr(VI). Diffusive gradients in thin films (DGT) deployments, followed by elution and ICP-MS measurement, supply a dynamic, flux-based availability index.

Quality Assurance and Quality Control

• Method blanks, reagent blanks, and extraction blanks in each batch.
• Certified reference materials such as BCR-701 (sequential extraction) and NIST SRM 2711a for recovery verification where matrix-matched availability CRMs exist.
• Matrix spikes, duplicate extractions (typically relative deviation within 20%), and multi-point calibration with internal standards (for example In, Rh, Re) on ICP-MS.
• Reporting limits verified against extract dilution factors and moisture correction; results reported in mg/kg dry soil with extraction conditions stated.
• Testing performed by accredited third-party laboratories operating under ISO/IEC 17025 management systems.

Applications

• Farmland soil environmental quality and food safety: evaluating available Cd, Pb, and As before and during safe crop production, supporting selection of low-accumulation varieties and soil conditioners.
• Risk screening and detailed assessment: refining exceedance conclusions under GB 15618-2018, GB 36600-2018, and the HJ 25 series investigation and risk assessment guidelines, where available-fraction data reduce uncertainty in exposure modeling.
• Remediation design and verification: demonstrating the effectiveness of stabilization and passivation amendments, which reduce the available fraction without necessarily changing total content.
• Fertilizer, manure, compost, and biosolids land application: controlling metal loading and availability to receiving soils under relevant soil improver standards such as the EN 13651 extraction framework.
• Groundwater protection: leaching tests (TCLP, SPLP, LEAF) informing contaminant migration potential toward underlying aquifers.
• Ecological and human health risk assessment: oral bioaccessibility results adjusting conservative default absorption assumptions for ingestion pathways.
• Research and monitoring: soil chemistry studies, geochemical baseline surveys, aging and immobilization kinetics, and plant uptake prediction model development.

FAQ

What is the difference between total and available heavy metal analysis?

Total analysis digests the entire soil matrix (typically with hydrofluoric acid or aqua regia) and reports the complete elemental inventory, whereas available analysis uses mild extractants such as DTPA, CaCl2, or ammonium nitrate to dissolve only the mobile, plant- or organism-accessible pool. The available fraction is usually a small percentage of the total and is the more direct indicator of biological risk.

Which extraction method should be selected?

For plant-availability screening of neutral to calcareous agricultural soils, DTPA (GB/T 23739, NY/T 890) is the conventional choice. Dilute CaCl2 and ammonium nitrate (ISO 19730) better reflect the unbuffered soil solution and correlate strongly with crop uptake. Mehlich-3 suits acidic soils and multi-element surveys. For waste classification and mobility, TCLP or SPLP applies, and for human ingestion pathways, in vitro bioaccessibility assays such as EPA Method 1340 are appropriate.

How long does testing take?

Routine single-extraction analysis of eight to ten elements typically requires five to ten working days after sample receipt, including extraction, instrumental measurement, and quality-control review. BCR sequential extraction and LEAF multi-point leaching programs require additional extraction cycles and may take two to four weeks.

Can available-fraction results be used for regulatory compliance?

Chinese screening values in GB 15618-2018 and GB 36600-2018 are based on total concentrations, so compliance decisions rely on total-content data. Available-fraction results are used to refine risk characterization, prioritize follow-up investigation, verify remediation performance, and support land-use decisions, and unbuffered ammonium nitrate extraction underpins trigger values in several European jurisdictions.

Why do total and available results sometimes disagree across similar soils?

Availability is governed by pH, organic matter, oxides, and aging rather than by total content alone. Two soils with identical total cadmium can differ several-fold in DTPA- or CaCl2-extractable cadmium simply due to differences in pH or iron oxide content, which is precisely why available-fraction analysis adds diagnostic value beyond total screening.