Research On Effects On Internal Phosphorus Of Black-odour Sediment By Adding CaO₂

At present,the in-situ remediation technology of contaminated sediment has attracted much attention.CaO₂ acts as an oxygen release agent（ORC）,which can improve the anaerobic environment of the sediment and effectively suppress the release of endogenous phosphorus.It has received extensive attention in sediment repair.However,there are still many doubts about how to add CaO₂.Therefore,this study started with the method of CaO₂ dosing,with endogenous phosphorus solidification as the research goal,in order to elucidate the mechanism of action of CaO₂ in the conversion of endogenous phosphorus forms,and explored the role of CaO₂ dosing method in river repair.In this paper,sediments from a typical black-odour channel in Suzhou was studied,and CaO₂ was added by three different methods（covering,injecting and mixing）to repair the sediment.The effects on P in the overlying water and the distribution of different sedimentary P fractions were explored,and the optimal dosage method was screened out.Taking a typical black-odour riverbed sediment and CaO₂ as experimental materials in laboratory,indoor simulated cultivation methods were used to determine the contents of various phosphorus in overlying water,pore water and sediment with Rhizon water samplers and other instruments.Data and analysis of the conversion between the phosphorus forms,to filter out the best way to vote.After determining the optimal dosing method,CaO₂ was also used as experimental material for a typical black-odour riverbed sediment in Suzhou.The outdoor simulated method was used to investigate the mechanism of CaO₂ induced endogenous phosphorus transformation in the sediment and its mechanism and effects on the bioavailability of endogenous phosphorus.The inhibition of sedimentary phosphorus（P）was investigated under the CaO₂with 3 addition ways.The overlying water and sediments came from a black-odor channel in S City.The results show that the value of pH and DO in the overlying water increased under the 3 addition ways of CaO₂.In the 20d experiment period,under the conditions of injection and coverage,the average DO concentration of the overlying water was 6.35mg·L^-11 and 12.18mg·L^-1,respectively,which indicated that the injection could delay the oxidation of CaO₂ and be favor of creating the micro-oxidation environment in the sediment.The concentrations and the trend of TP and DIP in the overlying water were similar under the injection and covering,which were lower than that in the mixing condition.However,under the covering,the DIP concentration in the pore water was the highest,up to 120.33-142.18 times,compared with the injection.On the contrary,under the injection conditions,the DIP concentration in the interstitial water was the lowest(average value was 0.02 mg·L^-1),and the DIP concentration in the overlying water was also low(average value was 0.06 mg·L^-1),indicating that CaO₂under the injection condition did have an inhibitory effect on the release of endogenous phosphorus,which was closely related to the micro-oxidation environment it creates.The sedimentary P fractions also confirmed this.The explanation may be that the addition of CaO₂ accelerated the mineralization of the organophosphate and the formation of the Fe-Al/P and Ca-P,which enhanced the retention of sedimentary P.Point injection of CaO₂ to repair urban black-odour sediment was carried out in the outdoor.The transformation of sedimentary phosphorus（P）fractions by point injection of CaO₂ was investigated in this study.The results showed that under the injection condition,when the dosage of CaO₂ was 0.254 kg·m^-2,the addition of CaO₂ directly improved the appearance of water body,reduced the odor and increased the dissolved oxygen（DO）in the overlying water as well as its pH.Simultaneously,the concentrations of total organic carbon（TOC）,ammonia nitrogen（NH₄⁺-N）,total phosphorus（TP）and dissolved inorganic P（DIP）in the overlying water were sharply reduced by the point injection of CaO₂,indicating that this accelerated the mineralisation rate of organic matter.It also favoured the formation and maintenance of the micro-oxidation environment in the deep sediments.NH₄Cl-P,Fe/Al-P and Ca-P increased significantly,while the Res-P decreased sharply,especially close to the injection point of CaO₂,accelerating the transformation of P.These results can all be attributed to the oxidation of organic P and the adsorption of dissolved P from the overlying water.Additionally,the NaHCO₃-extractable phosphorus（Olsen-P）and algal available phosphorus（AAP）were reduced,and their percentages in Ca-P and Fe/Al-P also decreased,especially close to the injection point of CaO₂.The bioavailable P decreased from 25.6%（the initial）to 17.2%（the mean for the different distances）after the point injection of CaO₂,when the sum of the Olsen-P and AAP was used to calculate the bioavailable P.This indicates that point injection of CaO₂ favours not only the transformation of P species but also inhibition of the bioavailable P.This is attributable to acceleration of the transformation from bioavailable to inert P.After preliminary accounting,the CaO₂pharmaceutical dosage cost is about 14.73 RMB/m²,which is far lower than the cost of the pharmaceutical application of the system using Phoslock?（54.67 RMB/m²）.All results show that point injection of CaO₂ has a good application prospect in inhibiting endogenous phosphorus release and repairing the black-odour sediments.