FeCl₃ Modified Hydrochar And Its Effect On Nitrogen And Phosphorus Removal In Bioretention Tank

In recent years,the problem of surface runoff pollution caused by rainfall has become more and more serious.Biomass energy is a kind of renewable energy with a large content.In this paper,a new type of adsorbent hydrotchar is produced by using agricultural waste biomass,and it is used as an adsorption for bioretention tank to remove nitrogen and phosphorus pollutants.As a cheap and energy-saving adsorption material,hydrochar has broad application prospects in the treatment of pollutants in water.The corn cob,rice straw and rice bran are used as raw materials.The reaction process sets up different reaction temperatures(180°C,220°C,260°C)and different reaction time(3h,6h,9h)and modified with ferric chloride to change surface properties of hydrochar.The products have been selected to be the most suitable hydrochar for use as a nitrogen and phosphorus removal adsorbent.By comparing the hydrochar yield and its adsorption capacity for phosphate,it is considered that the hydrochar produced by using rice bran as a raw material at 180°C for 3 hours and iron-modified has the best phosphorus removal effect.The removal rate of phosphorus is 86.26%.The differences in surface properties between modified and unmodified hydrochar at 180°C-3h-rice bran are compared by scanning electron microscopy(SEM),energy spectrum analysis,specific surface area and pore size analysis,and zeta potential.The results shows that the modified hydrochar has a 60%increase in specific surface area before unmodified,and the specific surface area after modification is 27.78 m~2/g;the pore volume is increased by 32%,and the pore volume after modification is 0.0963 cm~3/g.From SEM results it shows that the carbon microspheres are present on the surface of the modified carbon,and iron elements are attached.The zeta potential is negative and the electronegativity increased with the increase of pH.Comparing the modified hydrochar and before modification(under the same production conditions),the modified hydrochar can effectively adsorb the soluble phosphate in water,and can hardly be used for phosphorus removal before modification.Meanwhile,through the adsorption performance test,the optimal dosage of hydrochar to adsorb phosphate is 8g/L and the initial pH value of the best adsorption solution is 7.The adsorption of phosphate by hydrochar conforms to the Langmuir adsorption isotherm and pseudo-secondary adsorption kinetics.The saturated adsorption capacity is 1.025 mg/g.The bioretention tank simulated reaction column filled with hydrochar material is subjected to dry-wet alternate operation test.The nitrogen and phosphorus removal capacities of the simulated reaction column are compared at different filtration rates.The results shows that the removal rate of total phosphorus is 82.51%(4cm/h filtration rate)and 77.36%(6cm/h filtration rate),the removal rate of ammonia nitrogen is 97.30%(4cm/h filtration rate)and 97.46%(6cm/h filtration rate),nitrate removal rate is 29.50%(4cm/h filtration rate)and 24.33%(6cm/h filtration rate).The nitrite nitrogen is little accumulation in the effluent and there is organic matter leaching.The total nitrogen removal rate is 58.12%(4cm/h filtration rate)and 53.31%(6 cm/h filtration rate).The removal effect decreases with the extension of the drought period.When changing the total phosphorus concentration in the influent it resultes in a decrease in the total phosphorus removal rate.With the increase of influent water filtration rate,the removal rates of ammonia nitrogen,nitrate nitrogen and total phosphorus all decrease slightly.It can be seen that the hydrochar can be used for the nitrogen and phosphorus removal,and has a good effect on the removal of total phosphorus and ammonia nitrogen.The best filtration rate of nitrogen and phosphorus removal in bioretention tank is 4 cm/h.