Lead Release and Contamination Risks Associated with Soil Application of Lead-Loaded Biochars

Biochar is increasingly used in environmental applications, such as remediating soils and water contaminated with toxic elements, because of its high adsorption capacity. However, the environmental fate of toxic elements in spent biochar if used as a soil amendment is poorly understood. In this study, wheat straw and cattle manure biochars, with their lead (Pb)-loaded counterparts (spent biochars) prepared in the laboratory, were applied to a Black Chernozemic soil in an 8-week incubation experiment to evaluate the extractability of Pb in the soil–biochar mixtures and the associated risk of Pb contamination. Pb loading lowered biochar pH by 3.70 units for the wheat straw biochar (from 9.91 to 6.21) and by 3.14 units for the cattle manure biochar (from 10.98 to 7.83), corresponding to approximately 5000-fold and 1400-fold increases in H+ activity. Both spent biochars increased organic acid-extractable Pb (phytoavailable) in the soil–biochar mixtures relative to the control, but to different degrees. Pooled across sampling times (geometric means), organic acid-extractable Pb was 20.3 mg kg−1 under the spent wheat straw biochar and 1.7 mg kg−1 under the spent cattle manure biochar, compared with 0.19 mg kg−1 in the control; potentially mobile Pb followed the same order. Between weeks 4 and 8, the potentially mobile Pb increased under both spent biochars, markedly under the spent cattle manure biochar, whereas phytoavailable Pb declined significantly in the control and the two pristine biochar treatments, indicating that the two operationally defined Pb pools changed differently over time. No significant differences in soil pH or electrical conductivity were detected among treatments after the incubation, and treatment mean pH differed by at most 0.12 units. Bulk soil pH is therefore unlikely to explain differences between the two spent biochars. These results do not show which biochar binds Pb more strongly because the amount of Pb retained by each biochar was not quantified; they do show that, under the conditions tested, land application of Pb-loaded biochar can increase extractable Pb in soil, that the magnitude of this increase differed between the two feedstocks, and that this risk should be assessed before spent biochars are used as a soil amendment.