Removal Of Dissolved Petroleum Hydrocarbons From Sea Water By Biochar Loaded With N-TiO₂

Marine oil spill accidents occur frequently due to increase of oil demand and transportation around the world.Oil film has the most direct negative impact on the marine environment,thus a series of emergency measures are proposed to remove it in order to maximize the removal of pollution sources in a short period of time after an oil spill accident.However,a certain amount of Dissolved Petroleum Hydrocarbons will still remain in the seawater after emergency treatment which will still pose a serious threat to the marine ecosystem under the influence of marine water conservancy conditions.In addition,the toxic substances that are difficult to be degraded are likely to accumulate in the food chain and ultimately endanger human health.Therefore oil spill remediation agent with low cost,small environmental side effects and high DPH removal efficiency should be developed immediately.In this paper,Biochar（BC）was prepared using agricultural and forestry waste as raw materials under different conditions of pyrolysis temperature and pyrolysis time.N-doped titanium dioxide（N-TiO₂）was prepared by sol-gel method using urea as nitrogen source.Biochar,which has good adsorption properties for DPH,was used as a carrier and N-TiO₂ was loaded on the BC（N-TiO₂@BC）by calcining method.The effects of preparation conditions and reaction conditions on the removal of DPH from seawater by N-TiO₂ and N-TiO₂@BC were studied respectively.The experimental data were fitted by kinetic equations,The morphology and structure of the samples were characterized by SEM,XRD,UV-Vis and XPS and so on.The removal mechanism of BC,N-TiO₂ and N-TiO₂@BC for DPH were investigated.The main research contents and results are as follows:（1）BC was prepared by Oxygen-Limited Pyrolysis method.The types of raw materials and pyrolysis temperature had a great influence on the properties of BC,while the pyrolysis time had little effect.The DPH adsorption performance of BC prepared from the three raw materials was ranked from high to low:pine sawdust biochar>corncob biochar>maize straw biochar,the pyrolysis temperature and pyrolysis time was 500°C,and 2 h respectively.The BC prepared at 500°C has the highest specific surface area and total pore volume with complete cellulose cracking and well-developed pore structure.The types and contents of functional groups on the surface of BC are abundant,especially oxygen-containing and Carbon-containing functional groups.The C element content in BC accounts for more than 80%and the C/H can reach 30.71,indicating that the prepared BC has strong aromaticity and hydrophobicity,so it has good adsorption capacity for DPH.（2）The saturated adsorption capacities of 500-PBC and 500-CBC for DPH in seawater were 16.08 mg/g and 13.62 mg/g,respectively,and the adsorption process conformed to the pseudo-second-order kinetic model（R²>0.999）.The intraparticle diffusion model indicated that the adsorption process of BC for DPH was controlled by surface adsorption as well as intraparticle diffusion and occupancy of adsorption sites.The adsorption process of BC for DPH is more conformed to the Freundlich adsorption isotherm model（R²>0.964）within the studied reaction temperature,and moreover,it was an exothermic reaction in which with a decrease in entropy and can proceed spontaneously,and the temperature reduction is conducive to the progress of the adsorption.（3）Urea was used as the nitrogen source,and the optimal conditions of preparing N-TiO₂by sol-gel method were as follows:calcination at 500℃for 2 h,the doping ratio of N element was 0.5while the p H of the system was 4.0 during the preparation.The characterization results show that the prepared TiO₂and N-TiO₂ are mainly anatase crystal structure,the doping of N element will generate O-Ti-N bond but will not change the crystal structure of TiO₂.The doping of N element can make the surface of TiO₂ is rougher,the specific surface area is larger,and the particle size distribution is mainly concentrated between 70-150 nm.the forbidden band gap of N-TiO₂ is calculated to be 2.93 e V according to the UV-vis spectrum.（4）The removal efficiency of DPH was 61.5%after 3 h of reaction when the experimental temperature was 25℃,the dosage of N-TiO₂ was 1.2 g·L^-1and the seawater salinity was 30‰which was 1.86 times of TiO₂ removal efficiency.In addition,the reaction process conforms to pseudo-first-order kinetics,and the apparent rate constant of the reaction is 0.0057 min^-1.（5）Based on the preparation conditions of N-TiO₂,the optimal preparation conditions of the repairing agent N-TiO₂@BC were discussed as follows:the calcination temperature was500℃,the mass ratio of N-TiO₂ in the repairing agent was 0.5,and PBC is the supported substrate.The characterization results show that the repairing agent retains the rich oxygen-containing functional groups of PBC,a certain graphene structure andπ-πbonds,and the better adsorption capacity of DPH can promote its removal of DPH.（6）The removal efficiency of DPH was 83.41%after 3 h of reaction when the experimental temperature was 5℃,the dosage of N-TiO₂@BC was 1.0 g·L^-1 and the seawater salinity was30‰which was 3.31 times and 2.52 times of BC and TiO₂ removal efficiency,respectively.Besides the reaction process conforms to pseudo-first-order kinetics,and the apparent rate constant of the reaction is 0.0078 min^-1.Furthermore,the pseudo-first-order kinetic equation was used to fit the data of N-TiO₂ and N-TiO₂@PBC-0.5 remove DPH from seawater under different reaction conditions.The functional relationship between the total apparent reaction rate constant under each condition and the influencing factors such as reaction temperature（T）,dosage of sample（n）and salinity of sea water（S） were established.（7）Radical trapping experiments showed that h⁺and·O₂^—played a major role in the removal of DPH from seawater,while·OH contributed relatively little.