Sorption Characteristics And Mechanisms Of Bio-chars With Organic Contaminants In Water

Environmental black carbons(BCs),such as chars and soots,are widely found in soil,sediment,and aerosols.Quantification and characterization of environmental BCs in sorption of neutral organic contaminants(NOCs)have been pursued with many investigators.Environmental BCs stem mainly from the incomplete combustion of biomass and fossil fuel.The burning of vegetation is usually the primary cause of char formation in forest and agricultural soils.To investigate the sorptive behavior of the BC, some surrogate materials such as wood charcoals and crop residue-derived chars have been prepared under various conditions and tested.Since the char contains non-carbonized matter,the term bio-char was used instead of charcoal.The bio-char (biomass-derived black carbon)is a ubiquitous form of black carbon in soils capable of sequestrating other substances in terrestrial ecosystems because of its recalcitrance to decomposition and mineralization.The combined adsorption and partition effects of bio-chars with varying fractions of non-carbonized organic matter have not been clearly defined.The primary objective of this dissertation is to demonstrate the hypotheses that the relative contributions of adsorption with carbonized organic matter(COM)and partition with non-carbonized organic matter(NOM)of bio-chars are quantitatively regulated by the transitional compositions of the bio-char with different pyrolytic temperature. Toward this end,a series of bio-chars were produced by the pyrolysis of pine needles,a model for the forest litter and crop straw,a representive of agricultural biomass,under a wide range of temperatures(100-700℃,referred as P100-P700 and RC100-RC700). The bio-chars were characterized via elemental analysis,BET-N₂ surface area,Fourier Transform Infrared Spectroscopy,Thermogravimetry(TG-DTG),and scanning electron microscopy.Sorption of nonpolar(naphthalene)and polar(nitrobenzene,m-dinitro-benzene,and p-nitrotoluene)aromatic compounds on bio-chars and an activated carbon from water were compared.The quantitative correlation of the bio-chars structural characteristics and sorption mechanisms were discussed.The observations offers a theoretical gist for predicting sorption of organic pollutant with COM-NOM complexes, and also provide an engineering reference to design a super-biosorbent for environmental application.The main original conclusions of this work are drawn: (1)The bio-chars exhibit powerful sorptive capability for organic pollutants in water.Bio-chars' surface area and structural characteristics regulated by the pyrolytic temperature dominate their sorption capacities and mechanisms with organic pollutants. With increasing pyrolytic temperature,the bio-chars' aromaticities(H/C ratio)and specific surface areas increased dramatically,and polarity index[(O+N)/C ratio] decreased sharply,correspondingly the "soft-matter" was evolved into "hard-matter". Sorption parameters(N and logK_f)are linearly related to sorbent aromaticities.(2)Sorption mechanisms of bio-chars are evolved from partitioning-dominant at low pyrolytic temperatures(e.g.,P100 and RC100)to adsorption-dominant at higher pyrolytic temperatures(e.g.,P700 and RC700).The quantitative contributions of adsorption and partition are determined by the relative carbonized and non-carbonized fractions and their surface and bulk properties.(3)Partition and adsorption contributions are quantitatively correlated with the structural characteristics of bio-chars for the first time as we known.The partition contributions(K_p)are dominated by the compatibility and accessibility of the partition medium and the tested solute's polarities.The partition of bio-chars with low pyrolytic temperature(100-300℃)originates from an amorphous aliphatic fraction,which is enhanced with a reduction of the substrate polarity;for bio-chars with high pyrolytic temperature(400-700℃),the partition occurs with a condensed aromatic core that diminishes with a further reduction of the polarity.Adsorption mechanism of bio-chars includes surface coverage and specific interaction.Adsorption contributions are depended on the compatibility of properties(polarities and dimensions)of organic molecule with the structural characteristics(surface polarity and pore volume)of adsorption medium.The adsorption component exhibits a transition from a polarity-selective(P200-P400)to a porosity-selective(P500-P600)process,and displays no selectivity with P700 and AC in which the adsorptive saturation capacities are comparable to predicted values based on the monolayer surface coverage of molecule.