Transformation And Fate Of Phenolic Pollutants In Soil Under Different Redox Conditions

Phenolic compounds are a class of natural compounds and industrial products as well as the degradation products of polycyclic aromatic hydrocarbons and biphenyls,which make them a common type of pollutants in the environment.Soil is an important sink of pollutants in the environment,and thus the environmental processes of phenolic pollutants in soil,such as migration,transformation,and immobilization,are of significance for the evaluation of their environmental risks.Previous studies have demonstrated that phenolic pollutants could be easily transformed in the environment.However,the long-term fate of phenolic pollutants and the characteristics of their residuals in the soil are unclear.Additionally,the fate of phenolics during redox alteration has not been studied yet.Formation of non-extractable residues（NERs）is an important transformation pathway of pollutants in soil.Nevertheless,the underlying mechanism of NER formation of phenolic pollutants and the composition,distribution and stability of the formed NERs are lack of exploration.This dissertation explored the environmental fate and behavior of two representative phenolic pollutants in soil:2,4,6-tribromophenol（TBP）and catechol that are easily to be reduced and oxidized,respectively.Using C-14 radioactive and C-13 stable isotope tracing techniques,we investigated degradation,transformation,mineralization and NER formation of these two phenolic pollutants under continuous oxic,continuous anoxic,and the alternation of anoxic and oxic conditions.Especially,chemical structure and stability of NERs were further studied.Besides,the effects of humic substances（HS）and metal ions with variable valences on the fate of phenolic pollutants were investigated.Results of this dissertation provides new insights for evaluating the environmental risks of phenolic pollutants in soil.In Chapter 2,the fate of TBP in soil under continuous anoxic,oxic,and alternation of anoxic to oxic conditions were studied.We found that TBP could be reduced or oxidized under anoxic or oxic conditions,respectively.TBP was degraded,mineralized and transformed to NERs under all the three redox conditions.Dissipation of TBP in soil followed a first-order kinetic model and the half-lives of TBP were 17 or 5.2 days under the continuous anoxic or oxic conditions,respectively.After 60 days of cultivation,TBP was mineralized（13%for continuous anoxic conditions and 26%for oxic conditions）and transformed to NERs（62%for anoxic conditions and 70%for oxic conditions）.In the starting phase of cultivation,metabolites of TBP gradually increased under both conditions.However,degradation rates of the metabolites gradually became greater than their production rates from day 30,resulting in only 5%of radioactivity remained as metabolites in the end.Formation of NERs was the main dissipation pathway of TBP in soil.Dissipation and transformation of TBP under oxic conditions was significantly faster than that under anoxic conditions.When alternated from anoxic to oxic conditions,dissipation,transformation,and mineralization of TBP were significantly promoted.Interestingly,formation of NERs was positively correlated to the dissipation of TBP in the soil,indicating that the hydrophobic TBP likely bound to HS during its dissipation and transformation.The results showed that although TBP was degraded and mineralized in oxic and anoxic soils with short half-lives,TBP was not completely removed from the soils but a significant amount of NERs formed.When soil conditions changed,the formed NERs might be released and become bioavailable,which may pose potential risks to soil environment.In Chapter 3,we studied the degradation and humification of catechol by the reactive oxygen radicals（ROS）generated from redox alternation of HAs.HAs had the capacity to produce ROS when first reduced by H₂/Pd then followed by O₂ oxidation.The results demonstrated that the generated ROS not only transformed catechol,but also promoted the covalent binding of catechol on HAs,which is a pathway for NER formation as well as humification of phenolics（i.e.,HS-phenol polymerization）.In the HA_red-Air treatment,70%of the catechol was transformed,which was significantly higher that the other treatment groups:HA_org-Air（43%）,HA_red-N₂（25%）and HA_org-N₂（19%）.This may result from ROS formation in the HA_red-Air group,as confirmed by the electronic paramagnetic resonance analysis.During reaction,catechol was transformed to hydrophobic transformation products（HTPs）but more to NERs.For this reason,redox alternation of HAs induced humification of catechol.The amount of NERs formed was positively correlated to the transformation of catechol.Metal ions with variable valences have the potential of oxidation and reduction reactions,and thus may influence HA oxidation by competing electron acceptors or donors.Therefore,the effect of Cu（Ⅱ）and Fe（Ⅱ）presence on the transformation and humification of catechol was studied.Cu（Ⅱ）and Fe（Ⅱ）enhanced transformation of catechol in HA_red-N₂ and HA_neu-N₂ treatments,even at the very beginning of the reactions.However,during oxidation,Cu（Ⅱ）suppressed oxidative transformation and humification of catechol in HA_red-Air treatment for quenching ROS.Except for 0.2 m M,Cu（Ⅱ）had no significant effect in HA_neu-Air treatment.In contrast to Cu（Ⅱ）,Fe（Ⅱ）did not induce significant effect on catechol transformation in both HA_red-Air and HA_neu-Air treatments.This may be attributed to a simultaneous consumption of the generated ROS.But overall,Fe（Ⅱ）addition promoted catechol transformation in the HA suspensions.The results indicated that redox alteration of HAs might contribute largely to the abiotic dissipation of phenolic pollutants and herewith represent a significant alternative to photochemical reaction in environments with low microbial activity,e.g.,at low temperature.More importantly,the incorporation of catecholic residues into humic substances during alteration of redox state revealed a chemical humification process of phenolic compounds in natural.This may be relevant in wetlands,soils,and sediments where the water table fluctuates.The results provide insights into the significant impacts of redox conditions at oxic-anoxic boundaries on the fate of phenolic pollutants and carbon cycling in the environment.In Chapter 4,the long-term fate of catechol in oxic soil and characteristics of the formed NERs were studied.Based on the C-14 radioactive and C-13 stable isotope tracing techniques,we found that except for mineralization,catechol was almost all transformed to NERs（>99%）after 4.1 years of incubation.The ring carbons of catechol were transformed in soil into carboxyl or carbonyl groups of HS and mainly distributed in humin via physico-chemical entrapment.After the 4.1 years of incubation,catecholic radioactivity was distributed in ¹⁴CO₂（48.0%）,water-soluble fraction（1.3%）,alkaline extractable fraction（24.3%）,and humin 27.8%.Namely,most of the residual catechol was found in the HS,which is in agreement with the abiotic binding of catechol and HAs in Chapter 3.Mineralization of catechol in oxic soil followed a double first-order kinetic model.The model indicated that catechol was transformed into two groups of soil carbon with different degradability,one is relative labile to degradation（17.2%）and the other is relatively recalcitrant（82.8%）,with a half-life of 27.8 days and 4.75 years,respectively.During the cultivation（or aging）,the amount of HA-NERs decreased while the humin-NERs increased,indicating that mineralization of catechol and/or its metabolites may be attributed to HA-NERs.The silylation analysis of humin-NERs showed that almost all the catecholic residues（>99%）were bound to the humin molecules by covalent bondings.In addition,catecholic carbon was heterogeneously distributed in HS and mainly distributed in molecules with weight of 0.2-20 k Da or in HAs with molecular weight of 1-20 k Da,as analyzed by¹⁴C-HP-GPC of the alkaline extractable fraction.The solid state ¹³C-NMR(DP MAS¹³C-NMR)analysis indicated that the catecholic NERs were mainly in the form of benzene ring and partly in the form of carbonyl or carboxyl carbon in the soil.The catecholic NERs could still be mineralized in fresh soil.Nevertheless,the residual catecholic carbons present in the residual bulk soil（i.e.,with presence of mineral components）mineralized at much lower rates in the fresh soil than those catecholic carbons present in separated soil organic fractions,e.g.,HA and humin fractions,indicating that catecholic residues present together with mineral components were more stable.Since catechol is an intermediate product of various pollutants undergone microbial degradation,such as phenols,anilines,polycyclic aromatic hydrocarbons and biphenyls,the characteristics of catecholic NERs could also indicate the NERs of these pollutants to some extent.Although the catecholic NERs were bound to HS via covalent bondings（especially in the most stable humin fraction）,these NERs might become available when the soil condition changed and thus pose potential risk to the environment.In summary,based on the C-14 radioactive and C-13 stable isotope tracing techniques,this dissertation revealed that the main fate of phenolic pollutants in soil is the formation of NERs,there is a new way of humification of small phenolic molecules.NER formation of phenolic pollutants could be promoted by both reductive and oxidative degradation.The formed NERs were mainly unevenly bound to HS via covalent bondings.Formation of NERs was positively correlated to the transformation of the phenolic pollutants.Microorganisms and redox alternation of HAs play an important role in the fate of phenolic pollutants in the environment via biotic and abiotic pathways,respectively.Different phenolic pollutants exhibited different distribution preferences of NERs in HS:NERs of TBP were mainly distributed in HAs while NERs of catechol were mainly distributed in humin.As a part of HS,NERs are protected physic-chemically in the soil.The characteristics of NERs determines their stability in the soil.HA-NERs of phenolic pollutants were more easily to be mineralized than the humin-NERs.The revealed mechanisms for NER formation of phenolic pollutants and their stability analysis in this dissertation would be of significance for assessing the environmental risks of these pollutants.