| Polycyclic aromatic hydrocarbons (PAHs), mainly formed from the incombustion of fossil hydrocarbons and biomass, are typical persistent organic pollutants (POPs) in the environment. PAHs have attracted widespread attention around the world because of their strong hydrophobicity, potential bioaccumulation and carcinogenic properties. Biosorption is a physic-chemical process involved in the sorption of a chemical substance in/on a biological matrix/surface. Biosorption has great potential to remove persistent organic pollutants such as PAHs in aqueous solution. Finding cheap and efficient biosorption materials as well as modification methods and illustrating the structure-function relationship in biosorption of organic pollutants has been a hot topic. Now more studies focused on microorganism and algae biomass as biosorbents, while only a few studies on plant residues as cheap biosorbents were investigated. The structure-function relationship in biosorption of PAHs and the contribution of organic components to biosorption are not yet clearly elucidated. In this dissertation, the advances on biosorption of organic pollutants were reviewed. Bamboo wood, pine wood, pine needle and pine bark were chosen and modified by acid hydrolysis, producing four modified plant residues, and study the sorption capacity of PAHs by biosorbents before and after modification. Tea leaves was selected as a model biomass and modified via three chemical treatment including Soxhlet extraction, saponification and acid hydrolysis to obtain different organic components of biomass and study the respective contribution of organic components in biomass residues to biosorption of PAHs. Complete tea leaves was selected and isolated by enzymatic hydrolysis to obtain tea leaves abaxial and adaxial surface cuticles and discuss the impact of plant surface microstructure on biosorption. All samples were characterized by elemental analysis, Fourier transform infrared spectroscopy and scanning electron microscopy. The relationship between the structure of biosorbents and sorption capacities of PAHs was illustrated, and the biosorption mechanisms of PAHs by raw and modified plant residues in wastewater were analyzed. These observations provide a scientific basis and technical support for the application of plant residue materials to PAHs removal in actual wastewater. The main original conclusions and innovations of this work are drawn as follows:(1) Elucidation the structure characteristics of plant residues on its sorption capacity of PAHs. The sorption isotherm of PAHs by plant residues was a predominant partition process, while all de-sugared samples exhibited more nonlinear isotherms. The Kd values (L/kg) of the four raw plant residues for phenanthrene ranked in the order of bamboo wood (2896L/kg)<pine bark (5445L/kg)<pine needle (6370L/kg)<pine wood (6754L/kg). The adsorption kinetics of PAHs by raw and modified plant samples obeyed the pseudo second-order kinetic model. The sorption rate (k2) of PAHs to the raw samples were relatively faster than that of the modified samples, attributed to the increase of aromaticity of de-sugared samples as well as the decrease of polarity. Negative correlation of Kd values with sugar content for four raw plant residues as well as positive correlation of Kd values with aromaticity (H/C) were obtained. The positively linear correlation between log Koc and log Kow/of PAHs were also observed. The powerful sorption capacity of aromatic domains was seriously suppressed by the coexisting polysaccharide component; therefore, the consumption of polar components through acid hydrolysis was suggested as a promising choice to enhance the sorption capability of plant residues (6-18folds for phenanthrene,6-8folds for naphthalene and pyrene and5-8folds for acenaphthene). The de-sugared pine wood presented the highest sorption capacity for PAHs, attributed to its lowest polarity and highest aromaticity.(2) The relationship between the removal of PAHs by different plant biomass fractions and natural organic material chemistry structure was illustrated. The amorphous cellulose components play a regulating role on the sorption kinetics, capacity and mechanism of biomass fractions.With the removal of extractable lipids and polymer lipids step by step (T1â†'T2â†'T3), the polarity and aromaticity of biosorbents gradually increased. After acid hydrolysis, the polarities of all samples dropped markedly and aromaticity enhanced notably. Sorption isotherms of phenanthrene and pyrene to the tea leaf fractions fit well to the Freundlich model. The sorption isotherm of phenanthrene and pyrene by bulk tea leaves (T1) and dewaxed fractions (T2) were practically linear, indicating a partition process, while the de-sugared samples (T4-T6) exhibited more nonlinear isotherms. The adsorption kinetics of PAHs by tea leaf fractions obeyed the pseudo second-order kinetic model. The sorption rate of raw, de-waxed and de-waxed-nonsaponifiable tea leaf fractions (T1-T3) were relatively faster, mainly because the high polarity surfaces of T1-T3samples may strongly interact with water to give loose to the structure of organic region and diffused easily. While the sorption of de-sugared samples (T4-T6) presented a relative slow rate, mainly because the polarity of sorption phase was low, difficult to hydrate and swell by water molecules. Meanwhile, the sorption phases became condensed for aromatic components. The SEM figures indicated that after wax extraction treatment, the surface of de-waxed fraction became smoothly, with some grooves appearing on some parts of the region. With saponification treatment (T2â†'T3), the non-saponifiable fraction (T3) became thinner. Performing further acid hydrolysis treatment upon T3sample, it can be seen that skeleton structure in T3disappeared and changed into some fragmentary particles, which proved that aromatic components existed in T3were surrounded by polysaccharides. Establishment of the relationship between the structures of tea leaf fractions and sorption capacities would provide a scientific basis to design novel biosorbent for environmental applications.(3) The effect of surface microstructure of tea leaves adaxial and abaxial surface cuticles on sorption of PAHs has been studied preliminarily. The structure characteristics of tea leaves adaxial and abaxial surface cuticles were analyzed by using FTIR-ATR and SEM methods. Tea leaves abaxial surface cuticle contained more waxes and aromatic groups and less polysaccharide component, presenting a higher sorption capacity. Sorption isotherms fit well to the Freundlich model, indicating a partition process. The structure of tea leaves adaxial inner surface and abaxial inner surface, adaxial outer surface and abaxial outer surface were similar, while the structure of adaxial abaxial outer and inner surfaces were quite different. Inner surfaces contained more aromatic groups and polysaccharide component, while outer surfaces contained more waxes and polymerizable lipids. Tea leaves adaxial and abaxial surfaces both contained abundant waxes, presenting some holes and accessible to the internal surface. Abaxial surface cuticle presented a lot stomatas structures. The structure difference of tea leaves adaxial and abaxial cuticles will determine the way to enter into the plant. |
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