Research On Phosphorus Retention Characteristics And Application Effects Of New Phosphorus-locking Agents

Nowadays,eutrophication has been one of the most severe water environmental problems around the world,and the release of nutrient elements from sediments plays a vital role in maintaining continuous eutrophication of water bodies.Phosphorus（P）is a key limiting nutrient element of eutrophication,most of the externally imported P is stored in sediments.Therefore,it is necessary to prevent the excessive release of P from sediments to the water column during the treatment of eutrophic water bodies.The phoslocks are the core of in situ passivation techniques,which have been widely applied in the treatment of P high-risk sediments.The performance of a phoslock determines the effect,cost and ecological and environmental risk of sediment treatment,which is still the hot spot and difficulty of in situ passivation techniques.Based on the investigation and assessment of the risk of internal nutrient release from Hongchaojiang（HCJ）and Niuweiling（NWL）,the P retention characteristics of a new phoslock prepared by high-temperature synthesis of aluminium oxides and calcium oxides were explored with the batch experiments,and its the practical application effect was estimated through static release experiments.The conclusions were as follows:1.The NaOH-P（P extracted by NaOH）was the highest in sediments,accounting for42-51%of the total extracted P,followed by BD-P（reducible P,exctracted by Na₂S₂O₄）and Res-P（residual P）.The organic nitrogen（Org-N）content of the sediment was the highest,accounting for 92-95%of the total N,followed by KCl-N（N extracted by KCl）and H₂SO₄-N（N extracted by H₂SO₄）.The total phosphorus（TP）and total nitrogen（TN）in HCJ sediments were lower than that in NWL sediments.The content of TP and soluble reactive phosphorus（SRP）in overlying water in the NWL were higher than HCJ,while the content of TN and NH₄⁺-N were similar.The content of most nutrients（TP,SRP,TN and NH₄⁺-N）in the overlying water at 25℃were higher than that at 15℃.The fundamental factors that impact the apparent release flux of TN,NH₄⁺-N,TP and SRP were related to the adsorption characteristics of sediments（i.e.extracting P with water,exchangeable P,total inorganic N or pH）and redox properties（i.e.iron content,reducible P or redox potential of sediments）or ones induced by sediment microorganisms（i.e.total organic carbon,organic P,etc.）.The factors responsible for the apparent release fluxes of N and P from sediments were relevant to their species and temperature.The apparent release flux of N had a limited influence on that of P.The apparent release fluxes of N and P were obviously increased when the temperature was raised from 15 to25℃.The maximum P retention of the new phoslock highly depended on acidification time.When the phoslock was acidified for 12 h,its maximum P retention reached to 30.5 mg·g^-1,and the corresponding value increased to 38.6 mg·g^-1if acidification time was increased to 24 h,meaning that they were much higher than other commercial phoslocks currently reported.2.The phoslock had the highest P retention under the natural sediment pH.Excessively high ionic strength（1.0M KCl）could decrease the maximum P retention of phoslock,however,the low ionic strength similar to that in natural sediments had less effect on P retention,especially when the phoslock to bind P was far from saturated,in which the effect of ionic strength could be ignored.The dynamic results showed the retention of P by the new phoslock was time-consuming,which more than 120 h was required to obtain equilibrium.The retention of P to the phoslock was an endothermic and spontaneous process.Raising reaction temperature was conducive to the phoslock to retain P.3.The addition of new phoslock with different amounts on the surface sediments with high P risk significantly decreased the P concentration in the overlying water（p<0.05）.The release fluxes of sediment P were calculated to be negative,compared with the treatment without phoslock,indicating that the phoslock prevented the P release from sediments,but also promoted the transfer of P from the overlying water to sediments.The effect of phoslock addition on P passivation was further confirmed by the decreases of P in interstitial water and available P in sediments.Addition of the phoslock effectively decreased interstitial water P and available P,especially available P,the reduction magnitude of which approached to 50%,compared with the treatment without phoslock.