Soil Heavy Metal Polarographic Analysis: Principles, Methods, and Applications

Soil Extracts, Leachates, and Pore Water

DTPA, calcium chloride, and EDTA extracts used to estimate bioavailable metal fractions, together with soil leachates and pore waters, can often be analyzed directly by voltammetry with minimal further preparation.

• Samples are collected on a grid basis following HJ/T 166-2004, typically from the 0-20 cm plow layer or from depth intervals in profile studies.
• Non-metallic sampling tools (bamboo, wooden, or titanium) and polyethylene containers are used to prevent cross-contamination.
• A minimum sample mass of about 500 g to 1 kg is collected, transported to the laboratory, air-dried below 40 degrees Celsius, crushed, quartered, and sieved through 2 mm and 0.149 mm (100 mesh) nylon sieves.

Testing Items

Elements routinely determined by polarographic and stripping voltammetric techniques in soil digests and extracts include:

• Cadmium, lead, copper, and zinc — determined simultaneously by anodic stripping voltammetry in acetate buffer (pH about 4.5), with stripping peaks at approximately -0.6, -0.4, -0.02, and -1.0 V versus Ag/AgCl, respectively.
• Nickel and cobalt — determined by adsorptive cathodic stripping voltammetry using dimethylglyoxime (DMG) in ammoniacal buffer, with solution detection limits near 0.05 micrograms per liter.
• Chromium — hexavalent chromium is measured selectively via its catalytic adsorptive wave with DTPA; total chromium is determined after oxidative digestion and the trivalent fraction is obtained by difference.
• Arsenic — As(III) is determined on a gold film electrode, while total arsenic is measured after reducing As(V) with potassium iodide and ascorbic acid, an approach consistent with US EPA Method 7063 methodology.
• Selenium — determined as Se(IV) by cathodic stripping of the selenium-mercury compound on the HMDE.
• Mercury — determined by anodic stripping voltammetry on a gold electrode, consistent with US EPA Method 7472 methodology.
• Other elements — antimony, bismuth, thallium, tin, manganese, and iron can be measured with suitable supporting electrolytes and electrode systems.
• Speciation and bioavailability — coupling voltammetric detection with DTPA, CaCl2, or EDTA single extractions or BCR sequential extraction provides insight into mobile and plant-available metal pools.

Testing Methods

Sample Preparation

Air-dried soil is ground with an agate mill, homogenized by quartering, and sieved to below 0.149 mm. Moisture content is determined separately at 105 degrees Celsius so that all reported results can be converted to a dry-weight basis. Blanks and reference soils are carried through the complete preparation sequence.

Digestion and Extraction

For pseudo-total (environmentally relevant) metal content, 0.2 to 0.5 g of soil is digested in aqua regia (hydrochloric acid to nitric acid, 3:1) on a hot block according to ISO 11466 or HJ 803-2016, or in a closed microwave system according to ISO 12914. For true total content, a hydrofluoric-nitric-perchloric acid digestion dissolves the silicate lattice. Bioavailability assessments employ milder extractants such as DTPA. Digests are cooled, filtered, and diluted to a known volume with deionized water.

Voltammetric Determination

An aliquot of digest or extract is transferred to the cell, combined with the supporting electrolyte (commonly 0.1 mol/L acetate buffer at pH 4.5 for Cd, Pb, Cu, and Zn, dilute hydrochloric acid for copper and bismuth, or DMG-ammonia buffer for Ni and Co), and deoxygenated with high-purity nitrogen for 5 to 10 minutes. In stripping mode, metals are deposited onto the electrode at -1.0 to -1.2 V versus Ag/AgCl for 60 to 300 seconds with stirring, followed by a 10 to 15 second quiet period and a differential pulse or square-wave anodic scan. Each sample is measured in at least duplicate, with acceptable replicate peak precision typically within 10 percent relative standard deviation.

Calibration and Quantification

The standard addition method—two to four additions of a mixed metal standard—is preferred for matrix-rich soil digests because it compensates for residual matrix effects. External calibration is acceptable for cleaner extracts. Linear working ranges typically span two to three orders of magnitude. Peak identity is confirmed by the characteristic peak potential, and known interferences such as copper-zinc intermetallic compound formation are managed through supporting-electrolyte selection, deposition-potential adjustment, or the addition of masking agents such as ferrocyanide.

Quality Assurance and Validation

• Method blanks, reagent blanks, and full procedural blanks accompany every digestion batch.
• Certified soil reference materials are analyzed to verify accuracy; measured values should fall within the certified uncertainty range.
• Matrix spike recoveries are maintained between 80 and 120 percent, and laboratory duplicate relative deviations are generally required to remain below 10 to 20 percent in accordance with HJ/T 166-2004.
• Method detection limits are established from seven or more replicate measurements and documented together with measurement uncertainty in the final report.
• Quality control charts track calibration verification standards, blank behavior, and reference material recovery over time.

Applications

• Farmland soil safety evaluation and land classification against GB 15618-2018 screening and control values, particularly for cadmium and lead in rice and vegetable production areas.
• Contaminated-site investigation, risk assessment, and remediation target verification against GB 36600-2018 for industrial and redevelopment land.
• Environmental impact assessment and legacy pollution mapping around mining, smelting, and metal-processing facilities.
• Soil environmental quality surveys, geochemical baseline studies, and background monitoring networks.
• Monitoring of wastewater-irrigated zones and areas receiving reclaimed water or sewage sludge amendments.
• Remediation performance verification following soil washing, stabilization, thermal treatment, or phytoextraction.
• Scientific research into heavy metal mobility, bioavailability, and redox-driven speciation changes in flooded or amended soils.
• Emergency response screening of spill-affected soils using portable voltammetric analyzers for rapid on-site triage.

FAQ

What detection limits can polarographic analysis achieve in soil?

Stripping voltammetry typically reaches 0.1 to 1 microgram per liter in solution, which translates to approximately 0.01 to 0.1 mg/kg in soil depending on sample mass, dilution volume, and deposition time. This is one to two orders of magnitude below the strictest GB 15618 screening values for cadmium and lead in farmland.

How does polarography compare with AAS and ICP techniques?

Atomic absorption spectrometry and ICP-based methods offer higher throughput and broader element coverage, but polarography provides comparable or better sensitivity for electroactive metals at a fraction of the instrument and operating cost, determines several metals in a single scan, and can distinguish oxidation states—capabilities that optical and mass spectrometric methods generally require additional chemistry or chromatography to achieve.

Is the mercury electrode a safety concern?

Modern systems use very small, closed mercury drop or film cells with integrated waste collection, and mercury-free electrodes such as bismuth-film, gold, and glassy carbon electrodes are now validated alternatives for most soil applications. Mercury waste is handled and disposed of according to hazardous-waste regulations, so occupational and environmental risks are effectively controlled.

Why is aqua regia digestion used instead of total digestion?

Aqua regia extracts the environmentally relevant, acid-soluble metal pool, which correlates best with ecological risk and aligns with the derivation basis of GB 15618, GB 36600, and ISO 11466. Hydrofluoric acid digestion is reserved for true total content when geochemical mass-balance studies require it, because metals locked in silicate lattices are largely unavailable in the environment.

Can polarography distinguish chromium and arsenic species?

Yes. Cr(VI) is determined selectively through its catalytic adsorptive voltammetric wave, with total chromium obtained after oxidation and Cr(III) by difference. Similarly, As(III) is measured directly at a gold electrode and total arsenic after chemical reduction of As(V), enabling speciation profiles that total-concentration methods cannot provide.

How much sample is needed and how long does the analysis take?

A 0.2 to 0.5 g test portion is sufficient for most elements. Digestion requires one to three hours, and the voltammetric measurement itself, including deoxygenation, deposition, and replicate scans, typically takes 10 to 20 minutes per element group, allowing a modest batch of samples to be completed within a single working day.

What quality control documentation should a competent laboratory provide?

An accredited third-party laboratory should report results on a dry-weight basis together with method detection limits, blank values, duplicate precision, certified reference material recoveries, matrix spike recoveries, and an estimate of measurement uncertainty, all traceable to the quality management framework of HJ/T 166-2004 and the laboratory's accreditation scope.