Comparative study on the characteristics and environmental risk of potentially toxic elements in biochar obtained via pyrolysis of swine manure at lab and pilot scales

Formation mechanisms and degradation methods of polycyclic aromatic hydrocarbons in biochar: A review

Biochar has been widely used in soil amendment and environmental remediation. Polycyclic aromatic hydrocarbons (PAHs) could be produced in preparation of biochar, which may pose potential risks to the environment and human health. At present, most studies focus on the ecotoxicity potential of biochar, while there are few systematic reviews on the formation mechanisms and mitigation strategies of PAHs in biochar. Therefore, a systematical understanding of the distribution, formation mechanisms, risk assessment, and degradation approaches of PAHs in biochar is highly needed. In this paper, the distribution and content of the total and bioavailable PAHs in biochar are reviewed. Then the formation mechanisms, influencing factors, and potential risk assessment of PAHs in biochar are systematically explored. After that, the effective strategies to alleviate PAHs in biochar are summarized. Finally, suggestions and perspectives for future studies are proposed. This review provides a guide for reducing the formation of biochar-associated PAHs and their toxicity, which is beneficial for the development and large-scale safe use of environmentally friendly biochar.

Combined magnetic biochar and ryegrass enhanced the remediation effect of soils contaminated with multiple heavy metals

Biochar is a very promising material for soil remediation. However, most studies mainly focus on the adsorption ability of biochar on one heavy metal, which is difficult to evaluate the actual remediation effect since soils were contaminated with multiple heavy metals. In order to improve the soil remediation efficiency, we used the joint remediation method of magnetically modified biochar and ryegrass to remediate the soil polluted by compound heavy metals (chromium, nickel, copper, zinc, arsenic and cadmium), and evaluate the effect on the process of organic carbon mineralization in polluted soils. It was found that magnetic biochar and ryegrass together decreased the concentrations of Cr, Ni, Cu, Zn, As, and Cd in soils by 24.12 %, 23.30 %, 22.01 %, 9.98 %, 14.83 %, and 15.08 %, respectively, and reduced the available fractions. Ryegrass roots were the main accumulation part of heavy metals, and the order of enrichment effect was ranked as Zn > As > Cr > Cu > Ni > Cd. In addition, magnetic biochar can maintained the stability of the organic carbon pool, and inhibited the emission of volatile organic compounds from ryegrass. Overall, this study indicates that magnetic biochar spheres combined with ryegrass is an effective method for heavy metals co-contaminated soils, and has the excellent remediation ability for actual co-contaminated soils.

Phytoremediation plants (ramie) and steel smelting wastes for calcium silicate coated-nZVI/biochar production: Environmental risk assessment and efficient As(V) removal mechanisms

Arsenic is one of the most common and harmful pollutants in environment throughout the world, especially in aqueous solutions. In this study, two kinds of industrial solid wastes (Oxide scale (OS) and Blast furnace slag (BFS)) and one kind of phytoremediation plant waste (Ramie stalk) were used to prepare an environmentally friendly, low-cost, and efficient calcium silicate coated nano zero-valent iron (nZVI)/biochar composite (BOS) for As(V) adsorption. The potential environmental risks of BOS and their effects on removal of arsenic ions from aqueous media were investigated. The adsorption mechanism was explored and discussed based on XRD, SEM-EDS, XPS, etc. The results suggested that the environmental risk and heavy metals toxicity in BOS by co-pyrolysis were significantly reduced compared to the original materials, and no additional contaminant was observed in the subsequent experiments. Simultaneously, the BOS showed excellent As(V) removal capacity (>99%) and regenerative properties. The As(V) removal mechanisms are mainly ascribed to the complexation and co-precipitation between Fe and As, and the hydrogen bond between CO functional group of BOS and As. The mechanism of enhanced nZVI activity for As(V) removal was revealed. A protective layer of Ca₂SiO₄ was formed on the surface of nZVI during the co-pyrolysis process to prevent the passivation of nZVI. During the reaction process, the Ca₂SiO₄ covering the nZVI surface would be continuously detached to expose the fresh surface of nZVI, thus providing more redox activity and adsorption sites. This study provides a new way to treat and recycle industrial steel solid wastes and phytoremediation plant wastes, and the produced calcium silicate coated-nZVI/biochar composite is proposed to be a very promising material for practical remediation of As(V)-contaminated water bodies.