Advances in ecological and health risks of biochar during environmental applications

Abstract

Biochar is a carbon-rich material synthesized through pyrolysis of biomass residues under the oxygen-limited conditions, which exhibited high specific surface area and abundant surface oxygen-containing functional groups. It widely existed in natural environment (such as soil, sediments, and rivers) and played important roles in various biogeochemical processes,nutrient availability, migration and transformation of chemicals. As a new kind of functional material, biochar has been well studied in numerous areas, including energy, materials, catalysis, and environmental applications. In recent years, the environmental applications of biochar have received considerable attention due to their low cost, high sorption capacity, and geological abundance. Numerous previous studies have reported that biochar possessed nutrient elements to enhance the soil fertility and exhibited promising adsorption capacity to remove organic and inorganic pollutants. However, recent studies indicated that the large-scale application of biochar also resulted in numerous potential environmental risks, for example, releasing inorganic and organic pollutants. Therefore, in this paper, we systemically review the application potential and significance of biochar in diverse sectors, including global climate change, environmental pollution remediation and soil amendment. Biochar has a long lifetime in soil varying from hundreds to thousands of years, and thus improves the carbon sequestration. For the abundant nutrient elements (such as N, P, K, Ca), large pore volume and surface area, biochar could effectively improve the fertility of soil. Meanwhile, biochar exhibited promising adsorption capacity to remove heavy metal ions and organic contaminants due to its high surface areas and abundant functional groups. The main adsorption mechanisms of heavy metal ions by biochar include ion exchange, complexation, coprecipitation, and electrostatic effects. While the π-electron donor-acceptor interactions, electrostatic interactions, hydrogen bonding, electrostatic interactions, and hydrophobic effects are the main adsorption mechanisms of organic contaminants on biochar. Then the risk factors of biochar are summarized to clarify its ecological and health risks. The endogenous pollutants such as organic pollutants, heavy metals, environmental persistent free radicals (EPFRs), nanoparticles and colloidal particles produced or transformed during the preparation of biochar are the main factors affecting the environmental application risks of biochar. Meanwhile, the potential environmental risks and adverse influence on soil and aquatic biota are also discussed in detail to evaluate the short-term and long-term effects of biochar. For example, contaminants in biochar could cause plasma membrane damage, and thus inhibiting the germination, growth and even resulting the death of plants. Furthermore, the addition of biochar may affect the pH, porosity of soil and then the soil ecosystem. The microbial community change in activity, community structure and abundance are significantly influenced by the soil ecosystem alterations. It has been demonstrated that the presence of biochar could inhibit the activity and abundance of soil microorganisms. In particular, the exposure risks of biochar are highlighted to fully understand its negative effects on human health. Some researchers indicated the potential cancer risk caused by contact with biochar during its production or transport process. The purpose of this review provides effective information for the comprehensive assessment of biochar in the application of environmental purification, and scientific guidance for its safe and effective utilization in the future.