Salisbury biochar did not affect the mobility or speciation of lead in kaolin in a short-term laboratory study

Salisbury biochar (produced from British broadleaf hardwood) with two different particle sizes (≤2 mm and ≤0.15 mm) was applied on a kaolin with three different lead (Pb2+) contamination levels (50 mg/kg, 300 mg/kg and 1000 mg/kg) at the dosage of 1% in w/w. The short-term impact of biochar on the mobility and speciation of Pb2+ in the kaolin was investigated using attenuation periods of 1, 7 and 28 days. The leachability and extractability of Pb2+ in carbonic acid leaching and EDTA extraction tests as well as the speciation of Pb2+ in soils were not significantly affected by biochar treatment during all periods. The insignificant effects of biochar on Pb2+ immobilisation were most likely attributed to the high adsorption capacity of Pb2+ on the kaolin and biochar failed to competitively adsorb Pb2+ against kaolin. The kaolin immobilised Pb2+ primarily through cation exchange, which represents the readily bioavailable fractions of Pb2+ in soils and may still pose environmental risks. This paper suggests the inefficiency of biochar treament on heavy-metal contaminated clay-rich soils. Therefore a laboratory treatablity study with respect to the soil type may be crucial when large-scale biochar applications in heavy-metal associated soil remediation are evaluated.