Effect Of Biochar On The Adsorption Behavior Of Polycyclic Aromatic Hydrocarbons Onto Oil-contaminated Loess Soil

The fertility of the loess soil can be improved and increased by adding the exogenous substances, such as straw burning matter and biochar. The understanding of the adsorption mechanism of loess soil by adding the exogenous substances will further reveal the mechanism of transfer and transformation for petroleum contaminants into loess soil in the cold and arid region in the northwestern China. Based on the adsorption of petroleum pollutants onto loess soil in this study, the exogenous substances(BC-ash, BC-200, BC-400, BC-600) were characterized via thermal gravimetric analysis(TG-DTG), elemental analysis, fourier transform infrared spectroscopy(FTIR), the surface area detection and scanning electron microcopytometer(SEM). The effect of the different exogenous substances on the adsorption of loess was investigated. Furthermore, the adsorption properties of polycyclic aromatic hydrocarbons(PAHs) onto diesel-contaminated loess soil were discussed in detail. Besides, the contribution of the different adsorption behavior such as surface adsorption and distribution onto diesel-contaminated loess soil was analyzed. The main conclusions of this dissertation are as follows:(1) TG-DTG, FTIR spectroscopy showed that the rate of weight loss for wheat straw rises to a maximum of 33.73% when the pyrolytic temperature at 350 ℃, the chemical bond was breaking and fatty alkyl chains was disappearing in the pyrolytic process of straw. The straw ash yield and ash is 12%, 64.78%, respectively with increasing pyrolytic temperature. The yield of biochar was reduced, when the content of ash increases. The biochar become “carbon-rich particles”, and its aromaticities(H/C ratio) and hydrophobicity(O/C ratio) were enhanced dramatically, while its polarity [(O+N)/C] were decreased sharply. The distinct surface characteristics of biochar from the wheat straw regulated by the pyrolytic temperature, and the hole-wall of biochar got thin gradually with increasing pyrolytic temperature. Furthermore, the internal pore structure of biochar increased gradually, and then a large number of micropores of the biochar were appeared.(2) Most of the adsorption of naphthalene on loess soil occurred within 10 h and the adsorption capacity of naphthalene increased slowly after 10 h, and then the adsorption process reached equilibrium after 24 h. The adsorption of naphthalene onto loess soil followed the pseudo-second-order kinetic model, the adsorption process were controlled by the external liquid film diffusion, surface adsorption, intra-particle diffusion and so on. The adsorption isotherms of naphthalene onto loess soil were linear in the experiments condition, and distribution plays a key role. The adsorption behavior is described better by Henry and Freundlich isotherm. The adsorption of naphthalene onto loess soil is a spontaneous endothermic process, when the temperature rises. The capacity of naphthalene adsorption on loess increased, besides, the entropy increasing was the main driving force. It was obvious that the naphthalene was able to be adsorbed effectively by loess soil. But there was little difference for the sorption capacity of naphthalene between the two loess soils. The saturated capacity of adsorption onto soil from LZ was higher than those onto soil from JYG.(3) As seen from the whole sorption process, the adsorption of naphthalene onto loess soil by adding straw ash can reach equilibrium within 24 h. It can be divided into two stages, such as rapid sorption stage(10 h ahead) and slow sorption stage(10 h afterward). At the beginning of adsorption, the naphthalene arrived at the surface of sorbents fast. Then adsorbed naphthalene on surface migrated to the inner of inorganic and organic structure via pore diffusion mechanism, which controlled the sorption process. The increase of initial concentration enhance the collision frequency between the loess/straw ash particle surface adsorption sites and naphthalene, resulting in enlargement of adsorption capacity. The results proved that the sorption processes of naphthalene on the different sorbents could be fitted well with the pseudo-first-order model. The adsorption of loess soil by adding straw ash is mainly a physical adsorption process. The adsorption is a spontaneous and endothermic process with increased entropy, and the increase of temperature would benefit the adsorption process. The naphthalene sorption onto the soils by adding straw ash was less than the loess soils itself. This may be attributed to the competition of organic matter or other soil components with naphthalene on the surface binding sites of straw ash.(4) It was obvious that the naphthalene was able to be adsorbed effectively onto loess soil by adding the biochar. The adsorption capacity of the naphthalene onto loess soil by adding biochar was significantly improved. The adsorption process could reach equilibrium within 20 h, but the rapid adsorption process was still 10 h. The kinetic adsorption process was in line with pseudo-second-order model. The adsorption was multimedia reaction in the complex soil system, which was controlled by a variety of mechanisms. The nonlinear adsorption of naphthalene onto loess soil were enhanced by adding biochar, the adsorption process was influenced by the porous structure and aromatic of biochar. The higher pyrolytic temperature of biochar, the more significant of nonlinear adsorption process onto loess soil. The pickling process can largely remove the ash, but significantly changed the amorphous active component of biochar. The difference of the adsorption properties that adsorbed onto loess soil by adding biochar under different pyrolytic temperature were caused by pickling process.(5) The adsorption of naphthalene onto diesel-contaminated loess soil reached equilibrium within 72 h. The presence of diesel in loess prolonged the naphthalene adsorption kinetics process on loess soil. The presence of diesel oil had a significant influence on the sorption of naphthalene, resulting in a drastic decrease of adsorption with increasing oil content in the system. It also indicated that the naphthalene adsorption onto loess soil was suppressed, which may be attributed to the competition of diesel with naphthalene for the surface binding sites. The adsorption behavior of naphthalene onto diesel-contaminated loess soil was nonlinear adsorption process controlled by both physical adsorption and chemisorptions. The addition of exogenous substances increased the contribution of surface adsorption. However, the role of surface adsorption and distribution of diesel pollution were significantly reduced on loess, and a separate adsorption phase of diesel has some impact on the whole adsorption process.