Research On The Complex Toxicity Mechanism Of Biochar And Various Pollutants

Biochar(BC)has been widely used due to its unique physicochemical properties,and its environmental abundance is much higher than other engineered materials.Most of the current studies have been focused on BC’s applications,such as soil remediation,heavy metal and organic pollutant removal in water bodies.However,the environmental risks of BC in large quantities have not been explored much.Furthermore,BC decomposes in the environment and becomes nano-BC,which has higher surface area and reactivity,and greater mobility in the environment.Thus nano-BCs may carry the pollutants in the environment,resulting in unknown combined toxic effects.The toxic effects when nanoBC is combined with multiple pollutants are rarely reported.In addition,BC naturally ages in the environment,and how the aging process affects the environmental risks of BC and BC-contaminant complexes is also unknown.Therefore,in this thesis,we prepared various BCs from different sources and temperatures as model materials to adsorb one or more heavy metal ions,and then investigated the cytotoxic effects and related mechanism of BC-metal ion complexes.The effect of aging process on the combined cytotoxicity of BC-metal ion complexes was also investigated.The study in this paper is mainly divided into three parts,as follows:1.BC adsorbed with contaminants usually produces different cytotoxic effects than BC without contaminants.However,how the adsorption of multiple contaminants alters the cytotoxicity of BC and the mechanisms that regulate such toxicity remain unexplored.By preparing various BCs(BP300-BC,BP500-BC,CS300-BC and CS500-BC)from two different materials(banana peel(BP)and corn stalk(CS))at two pyrolysis temperatures(300 °C and 500 °C),we systematically investigated the combined cytotoxic effects and related mechanisms of contaminant-containing BC in human normal rectal mucosal cells(FHC).We found that the reduction of Cr(Ⅵ)to Cr(Ⅲ)occurred on BC,which was higher on BC prepared at 300 ℃ than that on BC prepared at 500 ℃,probably attributing to the large amount of organic residues on BC prepared at low temperature.The presence of Cu(Ⅱ)promoted the release of Cr on BC inside cells,which was more obvious on BC prepared at 500 ℃.The changes in the valence states of Cr and release rate of Cr from BCs prepared at different pyrolysis temperatures together led to a decrease in the cytotoxicity of BP300-BC-Cr(Ⅵ)-Cu(Ⅱ),but an increase in the cytotoxicity of BP500-BC-Cr(Ⅵ)-Cu(Ⅱ)compared with the same amount of mixture of Cr(Ⅵ)-Cu(Ⅱ).Our results provide insight into how the pyrolysis temperature and the adsorbed multiple contaminants affect the cytotoxicity of BC in complex with contaminants,thus providing a theoretical basis for safe application of BC in the future.2.The valence states of Cr and As in the environment are easily transformed,and the toxicity of different forms of Cr and As varies greatly.Therefore,we investigated the interaction between BC and Cr or As and its effect on the combined toxicity of BC-Cr/BCAs complex in FHC cells.Our results showed that redox properties during the interaction between BC and Cr or As was a key factor in the combined cytotoxicity.Specifically,when Cr(Ⅵ)was adsorbed on the surface of BC,most of Cr(Ⅵ)was reduced to Cr(Ⅲ).Only Cr(Ⅲ)was released from BP500-BC-Cr(Ⅲ)in both culture medium and artificial lysosomal solution,but both Cr(Ⅲ)and Cr(Ⅵ)could be released from BP500-BC-Cr(Ⅵ).In contrast,As(Ⅲ)was partially oxidized to As(Ⅴ)when As(Ⅲ)is adsorbed on the BC surface.Notably,although both As(Ⅲ)and As(Ⅴ)were present on the surface of BC,only As(Ⅴ)was released from both BP500-BC-As(Ⅲ)and BP500-BC-As(Ⅴ)at low rates in the culture medium and artificial lysosomal solution.These findings suggest that the transformation of contaminants of the same element in different valence state might be a key factor of the combined toxicity.The different redox reactions between Cr/As and BC and the bioaccumulation of Cr and As together lead to enhanced cytotoxicity of BP500-BC-Cr(Ⅲ)and BP500-BC-As(Ⅴ),while reduced cytotoxicity of BP500-BC-Cr(Ⅵ)and BP500-BC-As(Ⅲ),compared to the same amount of metal ions.This suggests that the changes in valence state of heavy metal due to redox reaction is one of the parameters to be considered when studying the toxic effects of different heavy metal contaminants in complex with BC.3.BC undergoes an aging process when applied to the environment.The aging process leads to significant changes in the physicochemical properties of BC,such as elemental composition,ionic content,surface morphology,acidity,and surface area,which in turn affects its adsorption behavior towards contaminants and the combined cytotoxicity.We prepared aged BC(A-BP500-BC,O-BP500-BC)from BP500-BC by acidification and oxidation treatments simulating the natural chemical aging process.Our results showed that BP500-BC did not significantly alter the cytotoxicity of As(Ⅲ)-Cu(Ⅱ)due to high release rates of As and Cu as well as increased As uptake and partial oxidation of As(Ⅲ)to the less toxic As(Ⅴ).A-BP500-BC and O-BP500-BC converted more As(Ⅲ)to the less toxic As(Ⅴ),and their lower As release rate and lower cellular uptake played a synergistic role in reducing the combined cytotoxicity.Our findings emphasize the importance of aging process and adsorbed multiple contaminants in regulating the combined cytotoxicity of BC and contaminant complexes.In conclusion,in this thesis,we systematically investigated the environmental behavior of BC-metal ion complexes and their combined toxic effects at the cellular level in terms of the preparation conditions of BC,the valence s tates of heavy metals,and the interactions between multiple heavy metals on BC.We call for a deeper study of the combined cytotoxicity mechanism between BC and heavy metal contaminants to provide a theoretical basis for the safe use of BC.