Determining the heavy metal content in pyrolysis products of recycled plastics is essential for verifying the safety, regulatory compliance, and market acceptance of chemical recycling outputs such as pyrolysis oil, char, and wax. Because pigments, stabilizers, fillers, and inorganic contaminants present in waste plastics migrate and concentrate during thermal decomposition, accurate elemental analysis is critical for product quality control, hazardous waste classification, and environmental protection. This article explains the key sample types, target elements, digestion and instrumental methods, and quality assurance practices applied by accredited third-party laboratories.
Technical Overview
Pyrolysis is a core technology in chemical recycling, in which mixed and contaminated waste plastics are thermally decomposed at approximately 400 to 600 °C under oxygen-free or oxygen-limited conditions to generate pyrolysis oil, non-condensable gas, wax, and solid char. Recycled plastic feedstocks frequently contain heavy metals originating from inorganic pigments and colorants, lead and cadmium-based heat stabilizers in legacy PVC, zinc and antimony-based flame retardant synergists, titanium dioxide and metal-oxide fillers, printing inks, metallized layers, and cross-contamination from electronic and automotive plastic scrap.
During pyrolysis, these metals undergo complex partitioning. Non-volatile elements such as lead, chromium, nickel, copper, and zinc are predominantly retained and enriched in the solid char and ash, while more volatile species, including mercury, cadmium, arsenic, and selenium, can partially evaporate and re-condense in the oil, wax, aqueous condensate, or downstream deposits. As a result, every output stream of a pyrolysis plant can carry a characteristic heavy metal signature that must be quantified before the products can be sold as fuels, chemical feedstocks, or recovered carbon black substitutes, or before the residues can be disposed of or landfilled.
Heavy metal determination in pyrolysis products supports compliance with frameworks such as the RoHS substance restrictions evaluated through the IEC 62321 series and GB/T 26572, fuel and solid recovered fuel specifications, hazardous waste identification criteria such as GB 5085.3, and the elemental measurement procedures established in ISO, ASTM, EN, and EPA-based reference methods routinely implemented by accredited third-party laboratories.
Testing Samples
Pyrolysis Oil
Liquid condensate is the principal commercial product of plastics pyrolysis. Samples are typically collected from the condenser train, homogenized, and analyzed for dissolved and particulate-bound metals that affect downstream catalytic upgrading, refinery acceptance, and fuel stability.
Pyrolysis Char and Solid Residues
The carbon-rich solid fraction strongly accumulates ash and heavy metals. Testing of char determines whether it can be valorized as recovered carbon black, used as a solid fuel, or must be treated as a residue requiring controlled disposal. Cyclone fines, filter dust, and reactor coke are included in this category.
Wax and Heavy Condensate Fractions
Semi-solid waxes produced from polyolefin-rich feedstocks can entrain metal-containing particulates and condensed volatile species. Representative subsampling after controlled melting and homogenization is required to obtain reliable results.
Aqueous Condensate and Process Water
Moisture in the feedstock generates an aqueous phase that can dissolve soluble metal salts, ammonia, and chlorides. Its metal content governs wastewater treatment requirements and discharge compliance.
Recycled Plastic Feedstock
Pre-screening of incoming baled flakes, pellets, or agglomerate allows plants to predict metal partitioning, reject non-conforming batches, and fulfill mass-balance and due-diligence obligations before processing.
Off-Gas, Scrubber Liquor, and Deposits
Stack condensates, activated carbon from vapor polishing units, and heat-exchanger fouling deposits are analyzed to close the metal mass balance and verify emission control performance.
Testing Items
Standard elemental packages for pyrolysis products address the following groups of analytes:
- RoHS-restricted heavy metals: lead (Pb), cadmium (Cd), mercury (Hg), and hexavalent chromium (Cr(VI))
- Substances of very high concern and ECHA watch-list metals: antimony (Sb), arsenic (As), cobalt (Co), nickel (Ni), and their compounds
- Combustion and ash-related metals: chromium (total), copper (Cu), zinc (Zn), vanadium (V), manganese (Mn), tin (Sn), and thallium (Tl)
- Filler and pigment tracers: aluminum (Al), iron (Fe), titanium (Ti), barium (Ba), calcium (Ca), and magnesium (Mg)
- Leachable metal content evaluated through standardized extraction tests for waste classification and landfill acceptance
- Supporting parameters such as ash content, chlorine, and sulfur, which influence metal volatility and corrosion behavior
Testing Methods
Sample Preparation and Digestion
Accurate results depend on complete matrix destruction. Solid char, feedstock flakes, and filter dust are digested by microwave-assisted acid digestion with nitric acid, hydrochloric acid, hydrofluoric acid, and hydrogen peroxide mixtures following EPA 3052 or EN 13657 procedures, depending on the required total-content scope. Oils and waxes are either digested by closed-vessel microwave methods such as EPA 3051A or prepared by organic solvent dilution or dry ashing followed by acid uptake, consistent with ASTM D4951 and ASTM D5185 practice for petroleum streams. Where only a conservative, environmentally available concentration is needed, mild acid extraction (EPA 3050B) may be applied.
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
ICP-OES in accordance with ISO 11885, EPA 6010D, or HJ 781 is the workhorse technique for major and minor elements such as Al, Ca, Fe, Ti, Zn, Cu, and Pb, offering a wide linear range, robust plasma tolerance of residual carbon, and detection limits in the low mg/kg range, suitable for routine quality control of char and oil.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS following ISO 17294-2, EPA 6020B, or HJ 766 delivers sub-µg/kg sensitivity for trace contaminants including Cd, Hg, As, Se, Sb, and Tl. Internal standardization, collision-reaction-cell mode, and dilution strategies are used to control matrix effects from aromatic-rich pyrolysis oils and carbon-laden char digests.
Atomic Absorption Spectrometry (AAS)
Flame AAS under EPA 7000B covers higher-concentration metals, while graphite furnace AAS under EPA 7010 extends to trace Pb and Cd. Hydride generation techniques quantify As, Se, and Sb, and cold vapor AAS is a validated option for mercury, complementing the IEC 62321-4 approach used for polymeric matrices.
Direct Mercury Analysis
Thermal decomposition gold-amalgamation analyzers operating on the principle of EPA 7473 quantify total mercury in solids and viscous samples within minutes without acid digestion, minimizing volatilization losses and contamination risks inherent to wet chemistry.
Chromium Speciation
Hexavalent chromium is determined independently of total chromium by alkaline digestion under EPA 3060A or the alkaline extraction described in IEC 62321-7, followed by colorimetric detection with diphenylcarbazide by UV-Vis spectrophotometry per EPA 7196A, since Cr(VI) and Cr(III) differ fundamentally in toxicity and regulation.
X-Ray Fluorescence Screening
Energy-dispersive XRF screening per IEC 62321-3-1 provides rapid, non-destructive sorting of feedstock and char against concentration thresholds, guiding which samples require confirmatory wet-chemical analysis. Confirmatory ICP or AAS measurement remains necessary for compliance decisions near regulatory limits.
Leaching Assessment
Where residues are destined for landfill or by-product status, leachable metals are measured using batch extractions such as EPA 1311 (TCLP), EPA 1312 (SPLP), EN 12457, or the GB 5085.3 horizontal vibration procedure, and the eluates are quantified by ICP-OES or ICP-MS.
Quality Assurance
Accredited third-party laboratories support data validity through method blanks, duplicate digestions, matrix spikes with 75 to 125 percent recovery targets, calibration verification, interference checks, certified reference materials of comparable carbonaceous or oily matrix, and reported measurement uncertainty, with limits of quantification matched to the applicable regulatory thresholds.
Applications
- Qualification and routine release testing of pyrolysis oil for petrochemical feedstock and alternative fuel markets
- Classification of pyrolysis char as recovered carbon black filler, solid fuel, or controlled waste requiring stabilization
- Hazardous waste identification and landfill acceptance evaluation of solid residues and filter dusts
- RoHS and substance-restriction verification for recycled plastics destined for electronics, automotive, and packaging applications
- Mass balance and metal partitioning studies supporting pyrolysis process optimization, condenser temperature design, and gas-cleaning upgrades
- Environmental permitting support, emission-control validation, and wastewater discharge compliance for pyrolysis facilities
- Due diligence documentation for investors, certifiers, and buyers within chemical recycling value chains
FAQ
Why do heavy metals concentrate mainly in the pyrolysis char?
Most metals in plastics, such as Pb, Cr, Ni, Cu, and Zn, have very low vapor pressures at typical pyrolysis temperatures, so they remain in the condensed solid phase where they are progressively enriched as the polymer volatilizes. Only volatile species such as Hg, Cd, As, and some metal chlorides partition appreciably into the oil and gas phases, which is why both char and liquid outputs require testing.
What is the difference between total content and leachable content testing?
Total content analysis digests the entire sample and reports the overall metal inventory, which is relevant for product quality and substance restrictions. Leachable testing extracts the sample under simulated disposal conditions and reports only the environmentally mobile fraction, which drives hazardous waste classification and landfill criteria.
Can XRF screening replace laboratory digestion analysis?
XRF is an excellent rapid screening tool for sorting and trend monitoring, but it is semi-quantitative for light elements, cannot distinguish Cr(VI) from total Cr, and loses accuracy in heterogeneous or carbon-rich matrices. Regulatory submissions and marginal results should always be confirmed by ICP-OES, ICP-MS, or AAS following validated digestion methods.
Which method should be chosen for mercury in pyrolysis products?
Direct thermal decomposition analysis per EPA 7473 is preferred for solids and viscous samples because it avoids digestion losses of this volatile element, while ICP-MS or cold vapor AAS after closed-vessel digestion is appropriate for oils, provided gold stabilization is used to prevent mercury adsorption and memory effects.
What detection limits are achievable?
Typical limits of quantification are in the range of 0.1 to 1 mg/kg for ICP-OES, 0.001 to 0.01 mg/kg for ICP-MS, and around 0.005 mg/kg for direct mercury analysis, which is sufficient to demonstrate compliance with the low ppm thresholds commonly applied to RoHS substances and SVHC metals.
How should samples be taken and preserved before testing?
Liquid products should be collected in acid-washed glass or fluoropolymer containers, homogenized at controlled temperature, and protected from light and headspace loss of volatile metals. Solids should be ground to uniform particle size, moisture-corrected, and stored in sealed containers, with all samples accompanied by batch, location, and date documentation for traceability.