Genotypic Differences In Effectiveness Of Phosphorus Removal From Eutrophic Water Under Floating Culture Conditions And Some Mechanisms

Water eutrophication has become one of the most important environmental problems all over the world.With occurrence of water eutrophication,the frequency and extentof alga bloom has been increased greatly,especially the harm of toxic alga bloom has seriously embarrassed the normal function of water body,continue development of society and economy.Nitrogen(N)and phosphorus(P)are the two main nutrient pollutants which affect water eutrophication easily,however,P is considered to be the limiting nutrient for water eutrophication.Thus,how to remove P from eutrophic water effectively and at low cost has become the key issue for developing technologies for controlling and remediate water eutrophication.In resent years,ecological remediation methods have become new and hot study point. Ecological remediation of eutrophic water is largely dependent on the ability of the plants used to remove the nutrients.In this paper,different seasonal plant species were compared for their abilities of removing P from eutrophic water,and some mechanisms behind genotypic difference in P uptake and removal from eutrophic water were studied,using indoor and field experimental technic platforms.The main innovative results are summarized as follows:1.In order to screen out plant species with highest P removal efficiency,at different seasons(warm season and cool season),six plant species(e.i.Pistia stratiotes Linn, Secale cereale L.,Pistia stratiotes Linn.,,Canna flaccida,Hydrocharis dubia,Lolium multVlorum Lam)were selected to compare their efficiency of P removal and other pollutant removal from eutrophicated water in summer(with high temperature)and in winter(with low temperature).The results showed that Pistia stratiotes Linn.and Secale cereale L.were the two species with highest efficiency for removing P and other pollutants in summer and winter,respectively.After the experiment,total phosphorus(TP)was decreased on average by around 50%,CODMn by 35%,BOD₅ by 55%,and Chla concentration by 40%.Alga bloom in treated eutrophic water was restrained effectively,pH was decreased to around 7.0,and water transparence was improved markedly.When the plants were grown in eutrophic water with low P (0.01mg/L)and high P(0.50 mg/L),Pistia stratiotes Linn.and Secale cereale L.were found to be more suitable for remediating high P-polluted eutrophic water because they showed the highest efficiency for P removal and other pollutant removal,follwed by Canna flaccida and Silphium perfoliatum Linn..Whereas Hydrocharis dubia and Lolium muItVlorum Lam.showed the lowest P removal efficiency,but suitable for growing at low P-polluted eutrophic water.2.By comparing the characteristics of P uptake by the four plant species(two efficient ones Pistia stratiotes Linn and SecaIe cereale L,and two inefficient ones Hydrocharis dubia and Lolium multVlorum Lam),we found that Pistia stratiotes Linn.and Secale cereale L.had higher Vmax and Km of phosphate absorption than Hydrocharis dubia and Lolium muitVlorum Lam.indicating that Pistia stratiotes Linn.and Secale cereale L.had higher ability absorbing phosphate.However higher Km indicated lower affinity of the roots,and Pistia stratiotes Linn.and Secale cereale L.were proper to gown in high P-polluted water.In contrast,Hydrocharis dubia and Lolium multVlorum Lam.had lower Km values than Pistia stratiotes Linn.and Secale cereale L.,implying they are suitable to grow in low P-polluted water.The results also showed that the highest P removal efficiency was always observed at the growth stage of 5-20 days of these four different plant species(Pistia stratiotes Linn.,Secale cereale L.,Hydrocharis dubia and Lolium multVlorum Lam.).Secondly period with high P removal efficiency was found at the growth stage of 20-35 days.3.The results from the study of P removal with relation to phosphatase activities among the four plant species(Pistia stratiotes Linn.,Secale cereale L.,Hydrocharis dubia,Lolium multVlorum Lam)showed that that acid phosphatase activity(APA) had different response to P supplied levels.At low P supply(0.01mg/L),leaf APA of all the four different plant species were enhanced obviously,but different upon plant species.The extent of increase in leaf APA due to low P stress was greater in Hydrocharis dubia and Lolium multVlorum Lam.than those in Pistia stratiotes Linn. and Secale cereale L..Similarly,root APA of all the four plant species were enhanced obviously when they were subjected to low P stress.The enhancement of root APA could greatly improve P bioavailability and use efficiency.Thus it can be concluded that the two contrast plant species,i.e.Hydrocharis dubia and Lolium multVlorum Lam.,could grow in low P-polluted water better than Pistia stratiotes Linn.and Secale cereale L.4.The study of different environmental factors influencing P removal efficiency by the plant species showed thatâ‘ The best N:P ratio which can induce the highest P removal efficiency was 7.5:1 when Pistia stratiotes Linn.and Secale cereale L.were under low P(0.01 mg/L)treatment,however,the best N:P ratio which can induce the highest P removal efficiency was 2.5:1 when Pistia stratiotes Linn.and Secale cereale L.were under high P(0.50 mg/L)treatment;â‘¡EM could remove N and P nutrients from eutrophicated water very effectively especially when it was combined with plants.Pistia stratiotes Linn.and Secale cereale L.could grow and remove P faster, and control alga bloom more effectively when they were combined with EM;â‘¢The optimal temperature for growing well and removing P from eutrophic water was 25℃for Pistia stratiotes Linn.while 15℃for Secale cereale L.;â‘£The optimal pH was around 6.7 both for Pistia stratiotes Linn.and Secale cereale L.to grow well and remove P from eutrophic water;⑤Continuous aeration was found to be the optimal aeration mode for the floating hydrophyte Pistia stratiotes Linn.because it was much closer to the natural environment of floating hydrophytes.However,intermittent aeration was observed to be most suitable for Secale cereale L.to effectively remove P from water.The two plant species showed higher P removal efficiency and purifying effectiveness when they were supplied with the optimal aeration mode,respectively.5.The fates of P removal from eutrophicated water in the process of phytoremediation included plant absorption,root adsorption,sediment adsorption(mainly including flocculation and deposition)and P volatilization(PH₃)under reductive state.In this study,we took Pistia stratiotes Linn.and Secale cereale L.which had higher P removal efficiency as experiment materials.The contribution ratios of these four fates of P removal in the total phosphorus(TP)removal from eutrophicated water were respectively studied through simulating ideal ecological phytoremediation system. The study results showed that the two main fates of P removal were plant absorption and sediment adsorption whose contribution ratios respectively were 23-58%and 27%-51%in TP removal;Secondly was root adsorption whose contribution ratio was 13%-28%;The lowest contribution ratio was observed in the track of P volatilization under reductive state whose contribution ratio was only lower than 1.5%,thus,this contribution in TP removal was very small and could be neglected.