Research On Phosphorus Removal Performance And Mechanism Of The Red Soil Substrate Fillings In Constructed Wetlands

Constructed wetland is a kind of ecological processing method. More often, theabsorption of the substrates is the major guiding factor used in constructed wetlandsto remove phosphorus. However, the phosphorus removal effect of the ordinary soil orgravel substrates in wetlands is not satisfying. As a result, the research tries to developthe red soil widely in Erhai area as the substrate to absorb phosphorus in wetlands,aiming at providing effective help for controlling the phosphorus pollution in Erhaiarea. In addition, the research also intends to provide some references in controllingand handling phosphorus pollution in the eutrophicated lake.First of all, the research studies the features of isothermal adsorption of red soilby carrying out the test of absorption’s effectiveness in phosphorous removal and testof isothermal adsorption. Besides, the research investigates into the feasibility ofapplying red soil as the phosphorous removal substrate in constructed wetlands andexplores into the effects of the initial concentration of phosphorous solution, theparticle size of red soil, the amount of added red soil, PH value of the solution andother factors on the phosphorous removal by absorption. The results indicate that redsoil has sound phosphorous removal effect by absorption. According to Langmuirequation, we get that the theoretical amount of saturation absorption of red soil tophosphorus is4692.04mg/kg. The effects of the additive amount and the PH value ofthe solution on the phosphorous removal of the red soil’s absorption become moresignificant. With the increase of the additive amount, the red soil’s equilibrium absorption capacity of phosphate in the water body reduces significantly withremarkable increase of the phosphorous removal efficiency. With the increasing PHvalue, the red soil’s equilibrium absorption capacity of phosphorus and absorptionefficiency gradually decline. Through the dynamics test of phosphorus removal byabsorption, the research studies the dynamic characteristics of the red soil’sphosphorus removal by absorption. Besides, the research adopts the parameters ofrelevant thermodynamics functions to describe the thermodynamics characteristics ofred soil’s phosphorus removal by absorption. The results of the test show thatpseudo-second-order kinetic equation can more accurately describe the dynamicsprocess of red soil’s absorbing phosphorus than first-order kinetic equation,double-constant equation and Elovich equation. The relative errors of both red soil’sequilibrium absorption capacity of phosphorus Xecalculated by thepseudo-second-order kinetic equation and the actual equilibrium absorption capacity±4%are less than±4%. In addition, ΔG and ΔH of the system of red soil’sphosphorous removal by absorption are both less than0, i.e., the red soil’s absorptionof phosphorus is a simultaneous and heat-releasing process. Through the dynamicabsorption test, the research investigates into the major affecting factors on red soilparticles’ absorbing phosphorus. The research also applies the dynamic absorptioncurve to describe the process of red soil particles’ phosphorous removal by absorptionand quantitatively analyzes the effect of red soil particles’ phosphorous removal byabsorption. By analyzing the results, we can find that the increasing initialconcentration of the solution, the accelerating speed of water inflow, the decliningthickness of substrates and the increasing PH value all will lead to the reduction of thepenetration time of the dynamic curve of red soil particles’ phosphorous removal byabsorption. When we apply red soil particles into the substrates of the wetlands toabsorb and remove the phosphorus, the phosphorus concentration of the wetlandinflow should not be too low. Besides, the slow-down of the inflow speed can helpextend the stay duration, thus improving the effect of phosphoros removal effect. Theincrease of the thickness of the packed bed can provide more active substances andabsorption sites. In addition, to adjust the PH value of the inflow into an acid status can effectively enhance the red soil particles’ phosphorous removal ability byabsorption. Finally, through analyzing the changes of the contents of variousphosphorus before and after the red soil’s saturation absorption, the research exploresinto the red soil’s phosphorus removal mechanism by absorption, i.e., the red soil’sabsorption of phosphorus is mainly chemical absorption. The essence of the reactionis that the phosphorus radical and the activate substances of Al and Fe in red soil reactwith each other, thus absorbing the phosphorus.