Reduction of heavy metal uptake from polluted soils and associated health risks through biochar amendment: A critical synthesis

Biochar is a carbonaceous material produced from the slow/fast pyrolysis of biomass under limited or no supply of oxygen. It has diverse uses from climate change mitigation to contaminant remediation from the environment. However, biochar's role in bringing these changes depends on its physicochemical properties, suggesting the use of function-specific biochar for achieving a specific target. This review summarizes the basic properties of biochar and how they are affected by different modification strategies. It also looks at remediation of heavy metal pollution using biochar, mitigation of heavy metal toxicity to plants including their impacts on physiological attributes, and human health mitigating effects. Biochar production conditions and feedstock is known to control the physicochemical properties of biochar. For instance, biochar's ash content increases with increasing pyrolysis temperature while there is a significant linear relationship between biochar's pH and pyrolysis temperature (r2 = 0.53, n = 112). Moreover, the cation exchange capacity of biochar decreases with the increase in pyrolysis temperature. When applied to soil, biochar interacts with soil and contaminants and can fix contaminants depending on their functionality. Biochars with positive surfaces (e.g. mineral doped biochars) have a large capacity to fix anionic contaminants while biochars with negative surfaces (e.g. acid-modified biochar) mostly fix cationic contaminants. The meta-analysis suggests that the overall effect of biochar application to polluted soils is the reduction of pollutant uptake by plants, with some exceptions for Fe and Mn. The reductions were estimated at 26.2% (Cd), 25.8% (Cu), 56.0% (Cr), 41.5% (As), 3.03% (Pb), 18.3% (Zn), 33.0% (Ni), and 22.8% (Mn). The underlying mechanisms for this reduction in the bioavailability of heavy metals in soils are diverse with charged metals being fixed through ion exchange, physical entrapment on biochar's surfaces and changes in soil chemistry. Amending heavy metal polluted soils with biochar reduced the overall daily intake of heavy metals (12.5%), hazard quotient (30.0%), and cancer risk (30.6%). However, these effects can be quite diverse depending on biochar properties, soil properties and the chemistry of concerned heavy metals. Altogether, our study provides a fundamental understanding of biochar mediated changes in heavy metal bioavailability in the contaminated soils and their subsequent effects on plants and animals.