Determination of Nickel Content in Soil: Analytical Methods, Standards, and Applications

Determination of nickel content in soil is a fundamental requirement for environmental site assessment, agricultural soil quality evaluation, and contaminated land remediation. Accredited third-party laboratories quantify total and extractable nickel using validated digestion procedures and instrumental techniques such as flame atomic absorption spectrometry (FAAS), ICP-OES, and ICP-MS, in accordance with standards including ISO 11047, EPA 3050B/6010D/6020B, GB/T 17139, and HJ 803. This article explains the sampling, pretreatment, digestion, instrumental determination, quality control, and regulatory context of soil nickel analysis.

Technical Overview

Nickel (Ni) is a naturally occurring transition metal whose concentration in soil is controlled primarily by geological parent material. Uncontaminated mineral soils typically contain approximately 5 to 100 mg/kg total nickel, while soils derived from ultramafic and serpentine rocks can naturally exceed 1,000 mg/kg. Elevated nickel levels, however, are frequently associated with anthropogenic activities, including nickel mining and smelting, stainless steel and alloy production, electroplating and metal finishing, battery manufacturing, fossil fuel combustion, waste incineration, and the land application of sewage sludge and phosphate fertilizers.

Although trace nickel is an essential micronutrient for plants and microorganisms, excessive concentrations are phytotoxic and can impair crop yield and soil ecology. From a human health perspective, nickel is a known cause of allergic contact dermatitis, and inhalation exposure to nickel-bearing dust is a recognized respiratory hazard. Mobile divalent nickel species may also leach from soil into groundwater.

For these reasons, nickel is a regulated substance in soil quality frameworks worldwide. In China, GB 15618-2018 establishes risk control values for agricultural land, while GB 36600-2018 sets land-use-specific risk screening values for development land. Comparable guidance exists under ISO and EPA frameworks. Because screening thresholds are numerically low and land-use dependent, accurate, defensible analytical data from accredited third-party laboratories are essential for regulatory compliance decisions.

Testing Samples

Agricultural and Greenhouse Soils

Farmland, orchards, vegetable bases, tea plantations, and organic farms are tested to evaluate soil quality, monitor the accumulation of nickel from fertilizers and amendments, and support producing-area safety programs for food safety compliance.

Industrial and Redevelopment Site Soils

Soils from electroplating and metal finishing plants, stainless steel and alloy processing facilities, mining and smelting areas, battery and chemical manufacturing sites, and former industrial land undergoing redevelopment are tested during Phase II site investigations and land-use change projects.

Sediments, Sludges, and Soil Amendments

River and lake sediments, dredged materials, sewage sludges, composts, and fertilizers are analyzed to determine nickel loads before land application or disposal, in line with sludge and waste utilization regulations.

Background and Geochemical Survey Samples

Regional baseline samples, environmental impact assessment background stations, and research samples for geochemical mapping are tested to establish reference concentrations against which contamination can be judged.

Testing Items

Depending on the assessment objective, laboratories typically report the following nickel parameters, expressed in mg/kg on a dry-weight basis unless otherwise specified:

• Total nickel content, determined after complete decomposition of the silicate matrix
• Aqua regia extractable nickel, the regulatory basis for most soil quality standards
• Water-soluble and bioavailable nickel, for example DTPA-extractable nickel for plant availability assessment
• Nickel speciation and fractionation by BCR sequential extraction, covering acid-extractable, reducible, oxidizable, and residual fractions
• Leachable nickel by TCLP (EPA 1311) or SPLP (EPA 1312) for mobility and waste classification purposes

Testing Methods

Sample Collection and Pretreatment

Sampling is performed in accordance with HJ/T 166 or the ISO 18400 series using multi-point composite designs. Non-contaminating tools such as plastic, wooden, or titanium implements are preferred to prevent cross-contamination. Samples are air-dried at room temperature or below 40 °C, stones and debris are removed, and the soil is homogenized and ground with agate equipment before sieving to below 0.149 mm (100 mesh) for the digestion subsample. Processed material is stored in clean polyethylene or glass containers.

Digestion and Extraction

• Aqua regia hot-block digestion (HCl:HNO3 = 3:1), following HJ 803 and ISO 12914, is the most widely used regulatory digestion; because silicates are not fully dissolved, results represent a conservative pseudo-total concentration.
• Microwave-assisted digestion in closed vessels using HNO3, HCl, and HF mixtures, following EPA 3051A and HJ 832, provides high recovery, low blanks, and excellent reproducibility.
• Total digestion with HF-HNO3-HClO4, following ISO 14869-1 and the digestion scheme of GB/T 17139, completely dissolves the silicate lattice and yields true total nickel.
• Single extractions with DTPA or deionized water and BCR sequential extraction are applied for bioavailability and speciation studies.

Flame Atomic Absorption Spectrometry (FAAS)

FAAS, specified in GB/T 17139, ISO 11047, and EPA 7000B, measures nickel at the 232.0 nm resonance line in an air-acetylene flame with deuterium or Zeeman background correction. The method detection limit is typically around 5 mg/kg, which is well suited to contaminated and moderately enriched soils and offers a robust, economical solution for routine monitoring.

Graphite Furnace Atomic Absorption Spectrometry (GFAAS)

Electrothermal atomization, covered under ISO 11047 and EPA 7010, extends detection to the sub-mg/kg level, making it suitable for background soils and low-level studies. Matrix modifiers and background correction are used to control interferences.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

ICP-OES, following EPA 6010D, determines nickel using analytical lines such as 231.604 nm and 221.648 nm with simultaneous multi-element capability. Detection limits for soil digests are typically below 1 to 2 mg/kg, and the technique is ideal for multi-metal site characterization and regional surveys.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS, following EPA 6020B and HJ 803, provides the lowest detection limits, generally well below 0.1 mg/kg in soil. Multiple isotopes (58Ni, 60Ni, 62Ni) allow interference confirmation, and collision or reaction cells resolve polyatomic spectral interferences. ICP-MS is the preferred technique for background studies, speciation work, and low-level regulatory assessment.

Field Screening with Portable XRF

Portable X-ray fluorescence analyzers operated under EPA Method 6200 enable rapid on-site triage at resolution in the tens of mg/kg range. Results approaching regulatory thresholds should always be confirmed by laboratory digestion followed by FAAS, ICP-OES, or ICP-MS determination.

Quality Control and Data Reporting

A comprehensive quality system includes method blanks, laboratory duplicates with relative deviation typically within 10 to 20 percent, matrix spike recoveries of 80 to 120 percent, certified reference materials such as GBW series soil standards, continuing calibration verification, and spectral interference checks. Results are reported as mg/kg on a dry-weight basis, with the digestion basis clearly identified so that data can be compared correctly against the applicable regulatory standard.

Applications

• Contaminated land investigation and human health and ecological risk assessment under GB 36600-2018 and related frameworks
• Compliance checking of agricultural soils against the pH-dependent risk control values of GB 15618-2018
• Post-remediation verification for soil replacement, solidification and stabilization, soil washing, and phytoremediation projects
• Farmland quality monitoring and producing-area safety programs supporting food supply chain assurance
• Land application decisions for sewage sludge, compost, and phosphate fertilizers
• Environmental impact assessment baseline surveys for mining, smelting, and industrial construction projects
• Geochemical mapping, regional background studies, and academic research on nickel mobility, fractionation, and bioavailability

FAQ

What is a normal nickel concentration in soil?

Most uncontaminated mineral soils contain between about 5 and 100 mg/kg total nickel, with typical background values falling between 10 and 60 mg/kg. Soils formed on ultramafic or serpentine parent material can naturally contain several thousand mg/kg. Interpretation should always reference local background statistics and the applicable screening standard.

What is the difference between total nickel and aqua regia extractable nickel?

Total nickel is measured after digestion with hydrofluoric acid that completely dissolves soil silicates, capturing the lithogenic fraction. Aqua regia extraction dissolves most adsorbed, precipitated, sulfide-bound, and organic-bound nickel but not the silicate lattice, typically yielding results five to fifteen percent lower. Most soil regulatory standards, including the Chinese soil quality standards, are based on aqua regia extraction, so the digestion basis must match the evaluation criterion.

Which regulatory limits apply to nickel in soil?

In China, GB 36600-2018 specifies risk screening values for development land, for example 150 mg/kg for sensitive first-class land use, with higher values applied to less sensitive land uses. GB 15618-2018 sets risk screening values for agricultural land that vary with soil pH. Exceedance of a screening value triggers detailed risk assessment or remediation rather than constituting automatic non-compliance, which is why precise analytical data are critical.

How should soil samples be collected and preserved for nickel analysis?

A sample of approximately 500 g is collected as a multi-point composite using clean plastic, wooden, or titanium tools, placed in polyethylene bags or glass jars, and transported protected from light and heat. Samples should be air-dried promptly after receipt. Because nickel is chemically stable in soil, no chemical preservation is required, but contact with galvanized or plated containers must be avoided.

What detection limits can be achieved?

Typical method detection limits are approximately 5 mg/kg for FAAS, below 1 mg/kg for GFAAS, around 0.5 to 2 mg/kg for ICP-OES in soil digests, and below 0.1 mg/kg for ICP-MS. These levels are sufficient for all regulatory thresholds and background monitoring applications.

How long does soil nickel testing take?

Standard turnaround is usually five to ten working days after sample receipt, including digestion, instrumental determination, quality control review, and reporting. Expedited services can generally be arranged with accredited third-party laboratories when project schedules demand faster results.