Degradation And Sorption Of P-Nitrophenol On Biochars: The Overlooked Role Of Persistent Free Radicals

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.Biochar is defined as "charcoal for application to soil" which was obtained by pyrolyzing biomass materials in the early years.Owing to the carbonation process,biochar contains considerable concentrations of persistent free radicals.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 biochar sorption systems,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 PNP dissipation.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.Chars and other black carbons are known to be capable of transforming certain organic compounds.Such reactivity has been attributed to reaction of persistent free radicals(PFR)originating from pyrolysis with molecular oxygen to give hydrogen peroxide,which then back-reacts with PFR sites to generate reactive oxygen species(ROS),notably hydroxyl radical.PFR and the ROS they may generate have been linked to the toxicity of inhaled fine particles and may participate in transformations of pollutants.We undertook a detailed study of the formation and decomposition of H2O2 and the degradation of p-nitrophenol(PNP)by lignin and cellulose chars made anoxically at 500 ? and aged in moist air or under vacuum for up to a month at room temperature.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.Lignin char was generally more reactive than cellulose char,and the iron impurity had no effect on reactivity.Hydrogen peroxide was both produced and destroyed.In air,the peak net production of H2O2 declined steeply over the first 24 h of aging and then leveled off,whereas the rate of PNP degradation declined more gradually over the month-long aging period.Vacuum storage had little effect on reactivity.Degradation of PNP took place predominantly in the sorbed state and was not catalytic.Pretreatment of the chars with H2O2 removed some but not all reactivity toward PNP.We conclude that direct reaction of PNP with the solid plays a major role,while ROS derived from H2O2 play only a minor role in PNP degradation.The stoichiometry between PFR and PNP is quite small(<0.1%),indicating that other reactive(non-radical)sites are primarily responsible for PNP degradation.Non-radical sites are also predominantly responsible for H2O2 decomposition.