| With the constant development of the economy and society,the issues of water pollution and water security are becoming vital limiting factors that limit global environmental security and development.It’s urgent to find practical and effective wastewater treatment technology for degradable substance removing especially with the coming of degradable pollutants in aquatic environment.Constructed wetland（CW）is a kind of ecological wastewater treatment technology that combines the roles of substrates,plants and microbes to realize the purification of sewage.CW has been widely used due to the advantages of small investment,low cost and well treatment performance.However,traditional CW could not realize effective pollutants removal with the appearance of degradable organic pollutants in water environment such as PAHs.PAHs tend to accumulate in wetland substrate based on the property of high hydrophobicity.Thus,it’s very important to enhance pollutants removal ability of wetland substrates for the long-term stable operation of CW and water security guarantee.Based on above problems,this paper picked phenanthrene as typical PAHs and aimed to enhance pollutants removal effect of wetland substrates and prepare metal matrix composite.The ability of substrates fixation and specific adsorption were proposed to be strengthened by revealing the special interactions of metallic elements with PAHs.Novel CWs with Mn oxides were developed to realize the promotion of adsorption ability and pollutants removal by oxidation through Mn cycle,aiming to better study the role and influence of metal recycling for wetland pollutants removal.From the point of catalytic oxidation,pollutants oxidation process by Mn oxides was proposed to be accelerated by the preparation and addition of artificial electron shuttle composite to promote metal cycling process in CW.Mechanisms of electron shuttle for metal cycling were well studied and the influence and role of electron shuttle for plants and microbes were further analyzed.The preparation,properties and catalytic oxidation fuction of natural electron shuttle composite as well as their effects and mechanism for metal cycling process in Mn oxides-based CW were well studied.What’s more,the influence of natural electron shuttle composite on each component of CW and pollutants removal mechanism was investigated.The main conclusions obtained are as follows:（1）Metal matrix composite could increase the adsorption and fixation of PHE by promoting complex absorption,hydrophobic adsorption and electrostatic attraction.What’s more,constructed wetlands filled with manganese oxides-coated sands（B-CW）could also realize the oxidation removal of PHE by Mn cycle.B-CW enhanced the removal efficiencies of pollutants.The removal efficiencies of phenanthrene（PHE）,NH4+-N and TP were 96.64%,81.80%and 127%higher than that of control,respectively;removal efficiencies of NO3--N were rose from 79.00%to 92.1 8%.B-CW based on metal cycling performed 1.71 ug/g PHE total fixation capacity,while the PHE total fixation capacity in copper-based CW without metal cycling was 2.84 ug/g which was contributed to the degradation of PHE.Based on the identification and analyses of degradation products,the transformation pathway for the degradation of PHE was given:PHE first formed 1-hydroxy-2-naphthoic acid,and then these intermediates formed trans-2-carboxybenzalpyruvic acid and were cleared to be CO2 and H2O by TCA cycle based on the observation of phthalic acid.According to the results of elements peak fitting before and after experiment,there were obvious variation of Mn valence state in Mn-based CW combined with metal copper material without element cycling,which verified the electrons transformance between different Mn components.Highlycharged Mn components increased most in CWs with electron shuttles,guaranteeing the well oxidability of Mn oxides and continuous Mn cycle.More Mn-oxidizing bacteria were detected in B-CW substrates,which could realize well transformation of low-charged Mn components to highly ones and make sence on the accomplishment of Mn cycle.However,the total relative abundance of PAHs degrading bacteria in copperbased CW was 2.41 times as many as that of B-CW,due to the microbial screening of copper-based material by more abundance of PHE fixation capacity.（2）Electron shuttle composite accelerated elements cycle process in B-CW,promoted total oxidizability of wetland and realized removal of organic pollutants.Biochar（BC）supported Ru（Ru/BC）and ABTS（ABTS/BC）were induced in B-CWs as electron shuttles.The removal efficiencies of PHE in CWs with Ru/BC and ABTS/BC reached 94.61%and 95.51%,and the efficient removal of P was also realized through enhanced biological P removal of Candidatus Accumulibacter phosphatis by more electron acceptors.The main roles of heterogeneous electron shuttle composite were adsorption and fixation and catalyzing oxidation of Mn cycle.The substrates of Ru/BC-B performed highest fixed capacity of 9.92 μg/g,which was 2.86 times and 2.95 times respectively more than that of BC-B and ABTS/BC-B.The total relative abundance of PAH degrading bacteria in Ru/BC-B-CW was 106.67%and 180.48%higher than that of ABTS/BC-B-CW and BC-B-CW.ABTS/BC-B performed the best oxidation-reduction ability with oxidation removal capacity of 5.47 mg,29.31%and 17.38%higher than that of BC-B and Ru/BC-B.The total relative abundance of Mnoxidizing bacteria was highest with the CWs containing ABTS/BC-B,which was 1.72 and 3.58 times more than Ru/BC-B-CW and BC-B-CW.The amounts of Fe-oxidizing microbes in ABTS/BC-B-CW was 19.16%and 38.97%higher than that in Ru/BC-BCW and BC-B-CW.Served as electron shuttle,ABTS performed best catalytic action for oxidation.（3）Humic acid/biochar（HA/BA）boosted complexation-flocculation process and promoted PHE fixation and oxidation in Mn-based CWs through the role of HA of natural electron shuttle.Humic acid was well combined with biochar to form natural electron shuttle composite.A monolayer of HAs first formed on material appearance by anabolic effect,and then HA molecules were polymerized through intermolecular hydrophobic interactions.Stable HA/BA material was prepared with well electric capacity and successfully simulated the main existing form of HA adlayers in environment.In control CWs,HA reduced the average PHE concentration of effluent by 26.58%,which mainly due to its role of complexation-flocculation for PHE and increase of hydrophobic contaminants removal from water.For B-CW with Mn/Fe cycle,HA could further enhanced pollutants removal due to the role of electron shuttle.Element of Mn and Fe both performed stable cyclic reaction process during the operation period of CW.B-CW with HA/BA performed the highest Mn proportion of high-state components and average oxidation state of Mn oxides and the oxidation properties were enhanced.HA/BA increased the relative abundance of PAH-degrading microbes in Mn-based CWs from 2.18%to 4.79%and the proportion of Fe-oxidizing bacteria was increased by 3.80 times.The biodegradation of PAHs was accelerated by the increasing surface solubility and disseminative mass transfer,which could promote their availability to microorganisms.The relative abundance of PAH-degrading microbes was increased by 6.29 times.However,the occupation of adsorption site of substrates by HA inhibited the removal of N in CWs.The increased proportion of effluent TN caused by HA decreased from 51.79%to 40.81%.It could be found that the usage of birnessite could effectively weaken the inhibiting effect of HA for N removal in CWs.CW with element cycle combined with HA could realize improved hydrophobic organic pollutants removal,at the same time,the whole wetland biodegradability was boosted and the negative influence caused by HA could be weakened.HA has been recently regard as an effective biostimulation way to accelerate bioremediation of hydrophobic organic contaminants.The addition of HA could be an effective biostimulation way to promote pollutants removal of CW with metal element cycle. |
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