Using biochar to manage and remediate an urban soil contaminated by lead

The contamination of soils by lead (Pb) is a widespread issue which poses a risk to urban communities globally. Recently, the remediation of this contaminant in-situ using chemical immobilisation has emerged as a promising option. This approach to remediation works by adding an amendment, usually phosphorus-based, in aim of inducing a shift in Pb speciation to a form which poses a lower risk to human health. A wide range of conventional phosphorus-based amendments, mostly mined or manufactured phosphates have been assessed for this purpose in the past. This approach is not without critique, with an increasing number of studies bringing in to question the sustainability of using these types of amendments. Specifically, there is concern that excess phosphorus could enter the environment and result in poor environmental outcomes. Biochar has garnered attention as a means to sequester carbon, manage waste and remediate contamination. Biogenic waste products are often rich in phosphorus and the conversion of these wastes streams to biochar has emerged as a method to capture phosphorus so that it can be applied to the land and recycled back in to the soil system for one or more of the aforementioned benefits. This study is motivated by recent efforts to immobilise Pb contaminated soils in-situ by adding phosphorus based amendments. Phosphorus plays an important role in the immobilisation of this element and there is a growing need to appraise alternative P sources for this use. Specifically, this study aims at assessing biochars produced from poultry litter and biosolids as sources of P to reduce the risk which Pb in soil poses to human health by comparing their efficacy in reducing in vitro bioaccessible Pb against conventional treatments. In order to understand any improved environmental outcomes by using these biochars for this purpose, impacts on soil quality were assessed, together with the potential for the release of excess P in to the environment. Overall, this research has identified two problematic waste streams (poultry litter and biosolids) that could be valorised towards the remediation of Pb contaminated soils and signals acidic biochars, a product which to date has found less applications than their alkaline counterparts, as more suitable for Pb remediation. The results of this research indicate that the application of biochars produced at 500°C increased the activity of soil enzymes, signaling an increase in soil quality. Consistent with previous authors, the results indicate that phosphorus-rich biochars act as a slow release source of P, whereby the availability of P in the receiving soil was not affected following treatment with biochar, indicating poor environmental outcomes such as the release of excess P in to the environment could be avoided using biochars for this purpose. The application of an amendment for the purposes of immobilizing heavy metals in soil should consider wider environmental outcomes to ensure that a net environmental benefit is achieved. Under a holistic approach to assessing remediation options, these wider environmental outcomes will help to equip researchers and industry alike in their endeavour to reduce the burden posed by contaminated soils in to the future