Adsorption And Desorption Mechanism Of Phosphorus In Reed Wetland Soil Around Bohai Se

Phosphorus is the main pollutant that causes offshore water eutrophication.Coastal wetland is located in the key zone at the intersection of land and sea,showing a strong ability to remove pollutants,and is the source,sink and converter of phosphorus cycle in the basin.A large area of reed wetland developed in Bohai Rim plays an important role in phosphorus geochemical cycle and balance in the basin.In this study,the main reed wetland soil around the Bohai Sea was sampled for further investigation.Through adsorption kinetics and isothermal adsorption/desorption,phosphorus adsorption/desorption parameters were obtained.Through piecewise fitting of adsorption model,morphological changes of phosphorus before and after adsorption and adsorption thermodynamics,the main preservation mechanism of phosphorus in different wetland soils was discussed.The research results are expected to provide scientific basis for wetland phosphorus storage,wetland protection and management.The main research results are as follows:（1）The rule of phosphorus adsorption and desorption in wetlands around Bohai SeaLangmuir model fitting results showed that the maximum adsorption capacity（Q_m）of soil phosphorus in the Bohai Rim wetlands ranged from 217 to 1000 mg·kg^-1,There was no significant difference in Q_m values among Lai zhou Bay coastal wetlands,Bohai Bay coastal wetlands and Liaodong Bay coastal wetlands.However,the Q_m size of the Liao he Delta wetland was in the order of core area(1000 mg·kg^-1)>buffer area(370mg·kg^-1)>experimental area(217 mg·kg^-1).The variation of Q_m in the Yellow River Delta wetland also showed a similar pattern,namely,the core area(969 mg·kg^-1)>buffer area(454 mg·kg^-1)>experimental area(357 mg·kg^-1).The adsorption binding energy（K_L）and maximum buffer capacity（MBC）also showed similar changes with Q_m.From the point of view of phosphorus desorption rate,take the Yellow River Delta wetland as an example:core area（45±0.02%）<buffer area（73±0.05%）<experimental area（83±0.04%）.Therefore,wetland restoration can improve the maximum storage capacity of soil phosphorus and reduce the desorption release of soil phosphorus,which is conducive to the storage of wetland phosphorus.（2）Mechanism of phosphorus adsorption and desorption in wetlands around Bohai SeaThe adsorption thermodynamics showed that the adsorption activation energy（E_a）of phosphorus in the Bohai-rim wetland soil ranged from 17.49 to 114.64 k J·mol^-1.Both the Yellow River Delta wetland core area(114.6 k J·mol^-1)and the Liao he River Delta wetland core area(87.7 k J·mol^-1)were higher than 83.7 k J·mol^-1,indicating that the phosphorus adsorption mechanism of these two wetlands was mainly chemical adsorption.E_a of other wetlands was less than 83.7 k J·mol^-1,indicating that physical adsorption and chemisorption existed simultaneously,and physical adsorption was the main method.The Gibbs free energy（△G）of the phosphorus adsorption reaction is-111.4～-26.6 k J·mol^-1,and the reaction type is consistent with the conclusion of E_a.The phosphorus adsorbed by soil in the Bohai Rim wetland was mainly converted into exchangeable phosphorus（E_X-P）,which increased from 21 mg·kg^-1 to 268 mg·kg^-1before adsorption and decreased to 75 mg·kg^-1 after desorption.The mean values of Fe-Al-bound phosphorus（Fe/Al-P）and calcium-bound phosphorus（Ca-P）increased from 85 mg·kg^-1 to 244 mg·kg^-1 and 234 mg·kg^-1 to 356 mg·kg^-1 respectively before adsorption,but the changes were relatively small after desorption.The changes of phosphorus morphology indicate that non-obligate adsorption（E_X-P）,obligate adsorption（Fe/Al-P）and precipitation reaction（Ca-P）exist simultaneously in the Bohai Rim wetland,and the former two mechanisms contribute more.The Langmuir model was used to divide the reaction process of wetland soil phosphorus into three stages:fast adsorption,slow adsorption and precipitation.The critical values of fast and slow adsorption range from 22.8 to 103.2 mg·L^-1.When the P concentration exceeds 282.3～392.4 mg·L^-1,the P transition from adsorption reaction to precipitation reaction.The results of P saturation index showed that hydroxyapatite was the main precipitating product.In conclusion,wetland around Bohai Sea has strong phosphorus adsorption capacity,wetland restoration significantly improves the storage potential of soil phosphorus,and reduces the desorption release of soil phosphorus.The phosphorus adsorption of wetland soil was mainly physical adsorption,and the proportion of chemical adsorption increased due to wetland restoration.In conclusion,coastal wetlands have strong phosphorus storage potential,and this study can provide scientific basis for wetland protection and utilization.