Biochar can benefit human society as a carbon-negative material and soil amendment. However, negative biochar impacts on plant germination and growth have been observed, and they have not been fully explained. Therefore, protocols to avoid these risks cannot be proposed. We hypothesized that the free radicals generated during charring may inhibit plant germination and growth. Significant electron paramagnetic resonance (EPR) signals were observed in the biochars derived from several types of common biomass (corn stalk, rice, and wheat straws) and the major biopolymer components of biomass (cellulose and lignin), but not in the original materials, suggesting the ubiquitous presence of free radicals in biochars. EPR signal intensity increased with increasing pyrolysis temperature, and it was dominantly contributed by oxygen centered in the mixture of oxygen- and carbon-centered free radicals as the temperature increased. The free radicals in biochars induced strong ·OH radicals in the aqueous phase. Significant germination inhibition, root and shoot growth retardation and plasma membrane damage were observed for biochars with abundant free radicals. Germination inhibition and plasma membrane damage were not obvious for biochars containing low free radicals, but they were apparent at comparable concentrations of conventional contaminants, such as heavy metals and polyaromatic hydrocarbons. The potential risk and harm of relatively persistent free radicals in biochars must be addressed to apply them safely.

Generation of environmentally persistent free radicals (EPFRs) on solid particles has recently attracted increasing research interest. EPFRs potentially have high reactivity and toxicity. However, the impact of EPFRs on organic contaminant behavior is unclear. We hypothesized that EPFRs in biochars can degrade organic contaminants and play an important role in organic contaminant behavior. We observed obvious degradation of p-nitrophenol (PNP) in the presence of biochars, through the detection of NO3(-) as well as organic byproducts. The extent of PNP degradation was correlated to the intensity of EPR signals of biochar particles. tert-Butanol (a •OH scavenger) did not completely inhibit PNP degradation, indicating that •OH could not fully explain PNP degradation. The decreased PNP degradation after tert-butanol addition was better correlated with reduced PNP sorption on biochars. PNP degradation through the direct contact with EPFRs in biochar particles could be an important contribution to the PNP concentration reduction in the aqueous phase. The coating of natural organic matter analogue (tannic acid) on biochars did not considerably inhibit PNP degradation, suggesting the ability of biochars to degrade PNP in soil and natural water. Similar EPFR-promoted degradation was observed for five different types of biochars and one activated carbon, as well as one additional chemical (p-aminophenol). Therefore, organic chemical degradation by EPFRs in biochars can be a common process in the environment and should be incorporated in organic chemical fate and risk studies.

Ischemic stroke is one of the leading causes of human death and disability worldwide. So far, ultra-early thrombolytic therapy is the most effective treatment. However, most patients still live with varying degrees of neurological dysfunction due to its narrow therapeutic time window. It has been confirmed in many studies that endothelial progenitor cells (EPCs), as a kind of adult stem cells, can protect the neurovascular unit by repairing the vascular endothelium and its secretory function, which contribute to the recovery of neurological function after an ischemic stroke. This paper reviews the basic researches and clinical trials of EPCs especially in the field of ischemic stroke and addresses the combination of EPC application with new technologies, including neurovascular intervention, synthetic particles, cytokines, and EPC modification, with the aim of shedding some light on the application of EPCs in treating ischemic stroke in the future.

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