Detecting Free Radicals in Biochars and Determining Their Ability to Inhibit the Germination and Growth of Corn, Wheat and Rice Seedlings

Abstract

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.

Characterization of free radicals by electron spin resonance spectroscopy in biochars from pyrolysis at high heating rates and at high temperatures

“…Chars produced by pyrolysis of hemicelluloses had lower concentration of radicals than chars from pyrolysis of lignin due to the aromatic structure of lignin. This results suggest a significantly influence of lignin on the radical concentration of chars at the decay stage [8]. Radicals generated from the primary pyrolysis reactions can be trapped in the polyaromatic matrix of lignin [56], and stabilized by the delocalized unpaired electrons [57,58].…”

“…The mechanical strength of biochars affects significantly the application in metallurgical processes [3]. Higher free radical concentrations favor chemical activity and cross-linking reactions, which enhance the physical and mechanical strength of composites [8,9]. Little is known about the influence of feedstock type on the biochar production and pyrolysis byproducts.…”

Response difference of transgenic and conventional rice (Oryza sativa) to nanoparticles (γFe2O3)

“…For example, iron oxide nanoparticles (Fe 2 O 3 NPs) have been used extensively in many fields including microelectronics, biomedical imaging, and the detection and visualization of various phases and interfaces in oil reservoirs (Basnet et al 2013;Laurent et al 2008;Mahmoudi et al 2011). The increased prevalence of engineered nanoparticles (ENPs) has led to their release into the environment, which is potentially having an impact in living organisms (Shah et al 2014;Zhao et al 2014), especially plants Liao et al 2014;Martinez-Ballesta and Carvajal 2014). How these NPs may affect food safety and quality is of particular concern.…”

Asynchronous reductive release of iron and organic carbon from hematite–humic acid complexes

“…5), with g values of 2.00442, 2.00458 and 2.00295 for HA1, HA2 and HA3, respectively. Previous studies indicate g values are a good index to distinguish different types of radicals, with g b 2.003 for carbon-centered radicals and g N 2.004 for oxygencentered (Barclay and Vinqvist, 1994;Dellinger et al, 2007;Liao et al, 2014;Senesi and Steelink, 1989). The g is 2.0028 and 2.0026 for graphitic carbon and polycyclic aromatic carbon radicals, while it is normally above 2.004 for semi-quinone radicals (Delhaes and Marchand, 1968;Szent-Györgyi and Isenberg, 1960).…”