Sorption Behaviors And Molecular Mechanisms Of Organic Pollutants With Varying Dissociation Properties Onto Biochars And Their Quantification

Biochars is a carbon-rich material formed when a biomass(e.g.,woodchip,manure)is pyrolyzed under oxygen-limited atmosphere at low temperatures.The substance is increasingly concerned due to its great potentials for soil improvement,carbon sequestration and polluted soil remediation.In this dissertation,research advance on the roles of biochars in controlling soil organic pollution was reviewed,and it was noted that,aqueous sorption of organic pollutants on biochars was not well understood.The characteristics of biochar surface acid/base groups and their roles in sorption,the kinetics of organic pollutant uptake and the mechanisms of ionizable chemicals sorption on biochars are largly unknown,though they are critical for biochar applications to abate organic pollution in soil and water environments.To solve these problems,a series of biochars were produced by pyrolyzing pine wood,rice straw,dairy manure and orange peel under limited-oxygen condition at different temperatures.The derived-biochar samples were characterized by thermo gravimetric analysis,element content,ATR,FTIR,SEM,BET-N2 adsorption-desorption,acid-base titration and Zeta potential analysis.Sorption isotherms of naphthalene and 1-napthol as model hydrophobic organic chemicals onto biochars from different precursors were determined,and their sorption capabilities,sorptive mechanisms,and structure-property relationships were elucidated.Then,sorption kinetics of naphthalene on biochars was investigated and differed sorption rates were illustrated.To demonstrate ionizable organic pollutants sorption on biochars,pH sorption edges of anilines,phenols and sulfonamides on biochars were measured and the role of acidic/basic groups was specifically analyzed.The main original conclusions of this work were:(1)An integrated method composed of FTIR-ATR,Zeta potential,dissolved organic carbon and acid-base titration analysis was established to characterize the the category,content and pKa of acidic/basic groups on the surface of biochars.Analysis of dairy-manure derived biochars made at a series of temperatures(100-700?)showed that there were four acidic/basic groups on biochar surface with pKa of 1.44-2.51,5.15-6.47,7.36-9.51 and 10.1-12.5,attributing to the deprotonation of protonized carboxyl,carboxyl,esterified(or hydrogen bond connected)carboxyl(or hydroxyl)and hydroxyl,respectively.The content of each acidic/basic groups with close pKa for different biochars were comparable(with difference<3 folds),but the surface density of acidic/basic groups sharply decreased when producing temperature of biochar increased.(2)Sorption kinetics of hydrophobic organic contaminants on biochars was elucidated and it was found that,sorption was fast for biochars or biomass obtained at low temperature(100?or 150?),slow for biochars made at medium temperatures(200-350?)and exhibited a variety of rates,including fast and slow,for biochars made at high temperatures(500-700?).Naphthalene exhibited a fast rate of sorption to raw biomass owning a high oxygen content and a small surface area,due supposedly to the solute partition into a swollen well-hydrated uncarbonized organic matter.The partial removal of polar-group contents in biochars made at relative low temperatures(250-500?),which increased the compactness of the partition medium,decreased the diffusion of the solute into the partition phase to result in a slow sorption rate.The ash content of biochars was reversely related with sorption equilibrium time of low temperature biochars because the diffusion distance was shorter when biochars had higher ash content and smaller size of organic partition phase.With biochars made at high temperature(700?),which display low oxygen contents and high surface area,the solute sorption were dominated by pore diffusion.The rates ofpore diffusion exhibited a transition from fast to slow due probably to the enhanced retardation when the pore size decreased.Slow sorption was unable to be described by Elovich,pseudo-first-order model,pseudo-second-order model or particle diffusion model,but followed a proposed two-compartment model consisted of fast and slow steps.(3)The classical "speciation model" underestimated the contribution of hydrogen bonding interactions between ionisable groups of tested chemicals and carboxyl(or hydroxyl)on biochar surface to aqueous sorption of anlines,phenols and sulfonamides on biochars.The amino of anilines and hydroxyl of phenols-which protonated or deprotonated at pH around the pKa of chemicals and owned comparable affinity for proton to the water molecules-formed strong hydrogen bonds with carboxyl or hydroxyl on biochar surfaces,and induced a high sorption coefficient Kd when pH approaching the pKa.Similarly,the amido(pKa1)and sulfonamido(pKa2)of sulfonamides aslo formed hydrogen bonds at pH where they were protonated or deprotonated,and more interestingly,these two ionsiable groups could cooporatedly form hydrogen bonds at pH around(pKa1+pKa2)/2 to cause an additional sorption.Contribution of hydrogen bonding interactions to overall sorption of anilines,phenols and sulfonamides was up to 22.8-82.7%.