Determination of Mercury Content in Chemical Fertilizers: Standards, Methods, and Laboratory Testing

Mercury is one of the most toxic heavy metals that can enter agricultural soils through phosphate-based, compound, and organic fertilizers, creating long-term risks to food safety, soil health, and human health. Accurate determination of mercury content in fertilizers is therefore essential for regulatory compliance with standards such as GB 38400-2019 and EU Regulation 2019/1009. This article provides a comprehensive technical guide covering contamination sources, sample types, digestion procedures, instrumental analytical methods, quality control, and practical applications of fertilizer mercury testing.

Technical Overview

Mercury contamination in fertilizers originates primarily from the geological composition of phosphate rock, which can carry naturally elevated mercury levels into single superphosphate, triple superphosphate, monoammonium phosphate (MAP), and diammonium phosphate (DAP). Additional sources include trace contamination in sulfuric acid used in acidulation, industrial by-product ammonium sulfate, certain trace element micronutrient salts, and waste-derived organic fertilizer feedstocks such as compost, manure, and biosolids. Because mercury is persistent in soil, highly toxic, and prone to methylation and bioaccumulation in the food chain, most major fertilizer markets impose strict limits on its concentration.

Key regulatory benchmarks include China's GB 38400-2019 standard, which restricts mercury in fertilizers to a maximum of 5 mg/kg, and the European Union's Fertilising Products Regulation (EU) 2019/1009, which typically limits mercury to 1 mg/kg for inorganic macronutrient fertilizer categories. The Minamata Convention on Mercury further drives global efforts to control mercury releases across product lifecycles. Analytically, mercury determination at trace levels is challenging due to the element's volatility, its tendency to be lost during sample preparation, memory effects in instrument sample introduction systems, and complex fertilizer matrices containing high concentrations of salts, phosphates, and organic matter. Reliable results therefore depend on controlled closed-vessel digestion and validated cold vapor or direct combustion techniques.

Testing Samples

Nitrogen Fertilizers

Urea, ammonium sulfate, ammonium chloride, ammonium nitrate, ammonium bicarbonate, and nitrogen solutions are tested to verify that upstream synthesis gas streams, by-product recovery, or filler materials have not introduced mercury contamination.

Phosphate Fertilizers

Single superphosphate (SSP), triple superphosphate (TSP), MAP, DAP, ground phosphate rock, and nitrophosphate products represent the highest-risk category because phosphate ore deposits can contain geologically elevated mercury. Both raw materials and finished products are routinely monitored.

Potash Fertilizers

Potassium chloride (MOP), potassium sulfate (SOP), and potassium nitrate generally exhibit low mercury background, but testing confirms compliance when products are marketed under strict ecological labels or for export.

Compound and Blended Fertilizers

NPK compound fertilizers, bulk blends, slow-release and controlled-release formulations, and water-soluble fertilizers are tested as finished goods, since blending multiple raw materials can cumulatively raise heavy metal loads.

Organic Fertilizers and Soil Conditioners

Compost, animal manure, biochar, humic products, and amendments derived from municipal or industrial waste streams require careful screening, as feedstocks can concentrate mercury from diverse anthropogenic sources.

Testing Items

The core testing item is total mercury content, reported in mg/kg on either an as-received or dry-mass basis. Laboratories typically offer the following scope:

• Total mercury (Hg) content as a single-parameter determination
• Combined heavy metal panels including arsenic, cadmium, lead, chromium, and nickel in accordance with GB 38400-2019
• Water-soluble mercury fractions for environmental mobility and risk assessment studies
• Mercury speciation screening in research contexts where methylation potential is under investigation
• Moisture content determination to support dry-basis reporting

Testing Methods

Sample Preparation

Representative laboratory samples are reduced by coning and quartering or rotary splitting, then ground to pass a 0.5 mm or 0.149 mm (100 mesh) sieve. Thorough homogenization is critical because mercury distribution in fertilizer matrices can be heterogeneous. Prepared samples are stored in sealed containers to prevent cross-contamination and moisture change.

Sample Digestion

Aqua regia-based digestion (hydrochloric acid and nitric acid, typically 3:1) is the standard approach, performed either in closed-vessel microwave systems or in water-bath digestion with an oxidation holding step. Potassium permanganate or potassium persulfate may be added to maintain mercury in its stable divalent ionic state and suppress volatilization losses. Closed-vessel microwave digestion at controlled temperature is strongly preferred for mercury because open acid digestion at high temperature can cause significant analyte loss. Relevant digestion references include ISO 16772 for mercury in soils by cold vapor atomic absorption spectrometry and EPA Method 7471B for solid samples.

Cold Vapor Atomic Fluorescence Spectrometry (CV-AFS)

Atomic fluorescence spectrometry with potassium borohydride reduction is one of the most widely applied techniques for fertilizer mercury determination, offering detection limits at the sub-microgram-per-kilogram level. The digested mercury is reduced to elemental mercury vapor, swept by argon carrier gas into an atomizer, and excited by a mercury hollow cathode lamp, with fluorescence measured at 253.7 nm. Methods of this type are described in Chinese agricultural standards such as NY/T 1978 for heavy metals in fertilizers.

Cold Vapor Atomic Absorption Spectrometry (CV-AAS)

In CV-AAS, mercuric ions in the digest are reduced with stannous chloride or sodium borohydride, and the generated mercury vapor is transported to a quartz absorption cell where absorbance is measured at 253.7 nm. The technique provides robust performance for routine fertilizer quality control with detection limits suitable for verifying compliance with mg/kg-level regulatory limits.

Direct Thermal Decomposition Analysis

Direct mercury analyzers following EPA Method 7473 and ASTM D6722 combust a weighed solid sample in an oxygen stream at approximately 850 degrees Celsius with catalytic decomposition. Mercury vapor is concentrated on a gold amalgamation trap, thermally desorbed, and quantified by atomic absorption. This digestion-free approach completes each analysis in five to ten minutes, minimizes reagent blanks and mercury loss risks, and is ideal for high-throughput screening of phosphate rock and finished fertilizers.

ICP-MS and ICP-OES

Inductively coupled plasma mass spectrometry enables simultaneous determination of mercury alongside arsenic, cadmium, lead, chromium, and nickel from a single digestion, which improves laboratory efficiency for full GB 38400-2019 panels. Gold or thiourea stabilization is added to digests to control mercury memory effects and ensure quantitative recovery. ICP-OES offers lower sensitivity but remains useful for higher-concentration screening.

Quality Control

Validated analyses incorporate method blanks, duplicate samples, matrix spikes, certified reference materials from recognized national metrology institutes, multi-point calibration with independent verification standards, and ongoing control charting. Measurement uncertainty budgets and method detection limits are documented for each matrix class.

Applications

Mercury content determination in fertilizers supports a wide range of commercial, regulatory, and scientific purposes:

• Incoming raw material inspection for phosphate rock, acids, micronutrient salts, and organic feedstocks
• Finished product release testing and internal quality control at production facilities
• Regulatory compliance demonstration against GB 38400-2019, EU 2019/1009, and destination-market requirements for exports
• Fertilizer product registration and ecological certification programs
• Market surveillance and port inspection by governmental authorities
• Soil environmental risk assessment and mercury mass-balance studies in agricultural systems
• Research into mercury behavior, methylation, and plant uptake in soil-crop systems
• Waste-derived fertilizer approval programs verifying that recycling streams meet safety thresholds

FAQ

What is the maximum permitted mercury content in fertilizers?

In China, GB 38400-2019 sets a limit of 5 mg/kg for mercury in fertilizers. Under the EU Fertilising Products Regulation 2019/1009, mercury limits are typically 1 mg/kg for inorganic macronutrient fertilizers, with stricter values for some organic categories. Manufacturers should always verify the specific limit applicable to the destination market and product category.

Which analytical method is most suitable for fertilizer mercury testing?

Cold vapor atomic fluorescence spectrometry offers excellent sensitivity and is widely used for regulatory testing, while direct thermal decomposition analyzers provide rapid digestion-free screening suited to high sample volumes. ICP-MS is preferred when a full multi-element heavy metal panel is required from a single digestion. Method selection should match the required detection limit, throughput, and budget.

How is mercury loss during sample preparation prevented?

Mercury is volatile and can be lost during open digestion. Best practice uses closed-vessel microwave digestion with aqua regia, controlled temperatures, and oxidizing additives such as potassium permanganate to keep mercury in a stable ionic form, along with prompt analysis of digests in sealed vessels.

What detection limits can be achieved?

Depending on the technique, method detection limits typically range from approximately 0.001 mg/kg for atomic fluorescence and direct analyzers to around 0.005 to 0.01 mg/kg for cold vapor atomic absorption and ICP-MS, all comfortably below regulatory thresholds.

How much sample is required and what is the typical turnaround time?

A minimum of 100 to 500 grams of representative sample is generally recommended to account for heterogeneity. Routine mercury determination by accredited third-party laboratories is usually completed within three to seven working days, with express services available for urgent compliance checks.

Why do phosphate fertilizers show higher mercury levels than other fertilizer types?

Phosphate rock deposits form in marine geological environments that naturally concentrate mercury and other trace metals. During acidulation, a portion of this mercury carries through into the finished phosphate fertilizer, which is why phosphate products and their raw materials receive the most intensive monitoring.