Assessing activated charcoal toxicity to key soil organisms: Microorganisms, Collembola and earthworms

Although activated charcoal (AC) is a commonly used adjuvant in agrotechnical products, it is currently not comprehensively regulated. Under REACH, most terrestrial ecotoxicological tests are waived, as ACs are deemed chemically inert due to their insolubility. Secondary physical effects are not considered. The aim of this study was therefore to evaluate holistically the ecotoxicological hazard of two ACs (hard coal- and wood-based). The effects on different microbial driven soil functions (OECD TG 216 – Nitrogen transformation, ISO 15685 – Potential nitrification, amoA abundance, ISO 20130 – Exoenzymatic activities), Collembola survival and reproduction (OECD TG 232) as well as earthworm avoidance (ISO 17512-1), survival, weight and reproduction (OECD TG 222) were observed in a concentration range from 10 to 1000 mg kg −1 . In a first stage of testing, a sandy soil was used. The organism group with the strongest ecotoxicological response was then exposed to the ACs in a silty soil to investigate the soil impact on the ecotoxicological effect. Overall, Collembola and earthworm reproduction were mostly unaffected, while earthworm avoidance behaviour was significantly triggered (LOEC 100 to 1000 mg kg −1 ). As the microbial driven soil functions were strongest affected, they were investigated further. Spore germination (ISO 10832) of arbuscular mycorrhizal fungi was added to the workflow for additional information but indicated no impairment. Nitrogen related processes were mostly not significantly affected. Exoenzymatic activities showed comparable ecotoxicological responses to ACs in silty and sandy soils (LOEC ≤10 mg kg −1 ), suggesting relevant secondary physical effects. At the highest concentration tested, these were discussed to partially interfere with the photometric measurements.