The use of plant-based biochars for the remediation of heavy metal-contaminated soil

Heavy metals (HMs) are common contaminants in soil. At elevated concentrations they can have a negative impact on ecosystem functioning and pose a risk to human health. Cost-effective management solutions are required to reduce the environmental risk from HM-contaminated soil. Among other organic amendments, biochar has been proposed as a suitable low-cost material with the potential to immobilise HMs in soil and improve soil conditions for restoration of degraded land and re-use of contaminated soil.
Biochar is a black carbon produced by thermal decomposition of feedstock -biomass destined for conversion to energy. It typically has a high specific surface area and is rich in oxygenated functional groups (e.g. carboxyl, hydroxyl). These properties make it useful for the treatment of HM-contaminated soils, where it can immobilize pollutants by different physical and chemical mechanisms, reducing metal toxicity towards plants and soil microorganisms, and thereby enabling the re-use of restored land. However, there are uncertainties that can limit the generalised use of biochar in the remediation of HM-contaminated soil. For example, biochars have different potentials to immobilize HMs, wherein the metal sorption characteristics are influenced by the feedstock properties and production conditions. Other factors, such as biochar dosage, soil properties and the presence of different HMs in soil, may also result in varied metal attenuation characteristics. The varied and contrasting results of biochar studies make it difficult to identify biochars with suitable characteristics for the immobilisation of HMs at contaminated sites that can also improve soil health.
This thesis investigates the mechanistic properties of plant-based biochars that influence the immobilisation of HMs in contaminated soil and the broader impact of biochar addition on soil health in the short- and long-term. Biochar sorption mechanisms were studied in batch sorption tests using synthetic soil pore water as the sorption media. Five plant-based biochars were used. Single and competitive sorption tests were conducted to determine the sorption capacities of these biochars for cadmium (Cd), lead (Pb) and zinc (Zn). A strong correlation was identified between selected biochar properties (pH, cation exchange capacity -CEC- and electric conductivity -EC-) and HM sorption.
Thereafter, four biochars were selected to assess their potential in alleviating HM toxicity towards plant growth in short-term incubations under controlled conditions. The effect of biochar on plant performance was evaluated using Arabidopsis thaliana, Lolium perenne and Sinapsis alba plants. The results demonstrated that biochars applied at 1% (w/w) reduced bioavailable Pb to 18-69 %, Zn to 60-80% and Cd to 33-43 % of the control. Nonetheless, plants still accumulated HMs in above-ground tissue. Higher biochar application rates of 3% (w/w) and 5 % (w/w) caused inhibition of plant growth, depending on the plant species. This negative effect was mainly associated with increased salinity due to soluble Na and K released from biochars with high mineral content, which may have induced osmotic stress - water deficit in plants caused by a high salt content in the soil.
Finally, the ability of biochar or compost as single amendments or the combination of both, to reduce HM toxicity was studied in a long-term outdoor pot experiment using soil highly contaminated with Pb and Zn from former mining activity. Plant performance was assessed using Lolium perenne and microbial community composition assessed using high-throughput DNA sequencing. The results demonstrated that biochar and compost consistently reduced bioavailable concentrations of HMs and limited plant uptake of HMs over an 18 month period. Biochar was shown to have higher stability during aging and to be a more efficient adsorbent (based on mg of HM immobilised per kg of amendment) than compost. However, compost had the greatest impact on microbial community composition and abundance.
Overall, this research supports the use of organic amendments, particularly biochar, in the restoration of HM-contaminated soil. Biochar application can provide favourable results, especially when applied to highly contaminated soil or soil contaminated with multiple HMs. In this situation the benefits of biochar, outweigh possible unfavourable outcomes to plant growth, such as osmotic stress. However, although these organic amendments can reduce the bioavailability of HMs in restored soil, plants can still accumulate HMs which may limit land use. Besides, land-use change constantly. Therefore, a “solution” to contamination that is prescribed in one current context may not provide the expected conditions for all future demands. Hence, the results of biochar application as an additive to restore soils contaminated with HMs are encouraging, but must be taken with caution because some aspects concerning the interaction with plants and long-term fate of HMs immobilised by biochar remain unclear.