A critical review of the interactions between rhizosphere and biochar during the remediation of metal(loid)contaminated soils

Biochar has a large specific surface area,well-developed pore structure,abundant surface functional groups,and superior nutrient supply capacity,which is widely available and environmentally friendly with its advantages in waste resource utilization,heavy metal(loid)remediation,and carbon storage.This review focuses on the interactions between biochar(including raw biochar,functional biochar(modified/engineered/designer biochar),and composite biochar)and rhizosphere during the remediation of soil contaminated with heavy metal(loid)s(Pb,As,Cd,Hg,Co,Cu,Ni,Zn,Cr,etc.)and the effects of these interactions on the microbial communities and root exudates(enzymes and low-molecular-weight organic acids(LMWOAs)).In terms of microorganisms,biochar affects the composition,diversity,and structure of microbial communities through the supply of nutrients,provision of microbial colonization sites,immobilization of heavy metal(loid)s,and introduction of exogenous microorganisms.With regard to root exudates,biochar provides electron transfer support between the microorganisms and exudates,regulates the secretion of enzymes to resist the oxidative stress stimulated by heavy metal(loid)s,ameliorates rhizosphere acidification caused by LMWOAs,and promotes the activity of soil enzymes.The roles and mechanisms of biochar on rhizosphere soils are discussed,as well as the challenges of biochar in the remediation of heavy metal(loid)-contaminated soils,and the issues that need to be addressed in future research are foreseen.

Arsenic removal from water and soils using pristine and modified biochars

Arsenic(As)is recognized as a persistent and toxic contaminant in the environment that is harmful to humans.Biochar,a porous carbonaceous material with tunable functionality,has been used widely as an adsorbent for remediating As-contaminated water and soils.Several types of pristine and modified biochar are available,and significant efforts have been made toward modifying the surface of biochars to increase their adsorption capacity for As.Adsorption capacity is influenced by multiple factors,including biomass pyrolysis temperature,pH,the presence of dissolved organic carbon,surface charge,and the presence of phosphate,silicate,sulfate,and microbial activity.Improved As adsorption in modified biochars is attributed to several mechanisms including surface complexation/precipitation,ion exchange,oxidation,reduction,electrostatic interactions,and surface functional groups that have a relatively higher affinity for As.Modified biochars show promise for As adsorption;however,further research is required to improve the performance of these materials.For example,modified biochars must be eco-friendly,cost-effective,reliable,efficient,and sustainable to ensure their widespread application for immobilizing As in contaminated water and soils.Conducting relevant research to address these issues relies on a thorough understanding of biochar modifications to date.This study presents an in-depth review of pristine and modified biochars,including their production,physicochemical properties,and As adsorption mechanisms.Furthermore,a comprehensive evaluation of biochar applications is provided in As-contaminated environments as a guide for selecting suitable biochars for As removal in the field.