Fabrication And Modification Of Activated Carbons From Biomass In Constructed Wetland And Their Adsorption Mechanisms Towards Heavy Metal Ions

As a kind of ecological wastewater treatment technology,constructed wetland has advantages of low capital cost,stable operation and easy management,which has been successfully applied for wastewater reuse,ecological restoration and wastewater treatment in developing regions.As one of the most important components of constructed wetland,plants can assimilate directly pollutants,provide oxygen through radial oxygen loss,and facilitate microbial activities by providing habitats.Along with the application of constructed wetland,large abundant of biomass is producing annually from the withered plants,which results in the problems of resources waste and second pollution in constructed wetland without reasonable disposal and treatment.Thus,it is necessary to explore efficient recovery and utilization technologies of the biomass.Heavy metal pollution in water environment has been regarded as a universal problem due to its high toxicity,persistence,bioaccumulation,carcinogenicity,and teratogenicity.Adsorption onto activated carbon(AC)is an effective way for treatment of heavy metal-polluted wastewater,but its wide application is limited by the relatively high preparation cost and low adsorption ability.Therefore,this study aims to explore new activation or facile modification methods to enhance the adsorption abilities of wetland plants-based ACs towards heavy metal ions.Based on the well-developed aeration tissues of wetland plants,they were utilized as carbon precursors to fabricate ACs.The effect of polymerization of phosphorus oxyacids at high temperatures on the textural property and surface chemistry of AC was investigated to further study the mechanisms of phosphoric acid activation and to find primary factors for controlling the formation of surface functionality of AC.Carbohydrates and polyhydric alcohols were employed as precursors to prepare ACs via phosphoric acid activation to verify that carbohydrates and alcohols hydrolyzed from lignocellulosic materials duing phosphoric acid impregnation promoted the creation of functional groups.Organic phosphates activations were explored to develop ACs from lotus stalk,and they were promising adsorbents for Pb(II)removal with excellent adsorption abilities.Pentaerythritol was employed to modify AC during phosphoric acid activation to promote the formation of organic phosphates,and this method simplified the conventional modification method and significantly improved the Ni(?)adsorption ability of AC.In situ modification with tartaric acid of AC during phosphoric acid activation improved the formation of organic phosphates and pore-forming effect,and this method maintained the porosity of AC and enhanced Cr(VI)removal performance of AC.(1)The porosity and functionality of ACs were determined by the polymerization degree of phosphoric acid at different latitudes and impregnation effect of lignocellulosic materials.AC-H3PO4(SBET,1114 m2/g)and AC-H4P2O7(940 m2/g)exhibited better developed porosities than AC-(HPO3)n(279 m2/g)and AC-H3PO4(125 m2/g).H4P2O7 yielded the strongest corrosion and hydrolysis for lotus stalk,resulting in the higher content of functional groups of AC-H4P2O7(4.156 mmol/g)than AC-H3PO4(2.486 mmol/g).(HPO3)n maintained the structure of AC as its starting material after activation.AC-H3PO3 had the most dimensions of crystallites perpendicular to aromatic layers.It suggested that polymerization of phosphoric acid promoted the depolymerization and hydrolysis of lignocellulosic materials,and the formation of low-polymerized organic phosphates enhanced the creation of functional groups for ACs.(2)The creation of functional groups for ACs was induced by low-polymerized organic phosphates during phosphoric acid activation.Carbohydrates-based ACs had much lower surface area(12-698 m2/g)than Phragmites australis-based AC(1057 mg),but they exhibited 60-75%higher contents of functional groups.The monosaccharide yielded AC with higher functional groups content than disaccharide and polysaccharide,hence some chain-like polyhydric alcohols with lower stability were utilized as carbon precursors.Xylitol was used as precursor to optimize the preparation parameters,and the optimized conditions were activation temperature of 350? and phosphoric acid to xylitol impregnation ratio of 1.5 considering their yields and functional groups contents.Different polyhydric alcohols were used to prepare ACs at the optimized conditions,and the produced ACs exhibited 50-90%more surface functional groups than Phragmites australis-based AC.These results demonstrated that the formation of organic phosphates improved the creation of functional groups of AC during phosphoric acid activation.(3)Organic phosphates activations was used to fabricate ACs from lotus stalk with low cost and high Pb(?)adsorption abilities.Seven phosphate and phosphite esters activations dramatically enhanced the porosity,yield and surface functionality of ACs.The ACs with phosphate esters activations showed higher surface functional groups contents,and AC with trimethyl phosphate activation had the highest value(6.00 mmol/g).It was suggested that trimethyl phosphate and tributyl phosphate activations produced ACs with the better properties considering their yields and functional groups contents.ACs with trimethyl phosphate(345 m2/g)and tributyl phosphate(541 m2/g)activations exhibited much lower surface area than AC with phosphoric acid activation(1418 m2/g),but they contained 70-80%higher functional groups,and had 65-75%higher Pb(?)adsorption capacities.These results indicated that Pb(?)adsorption was controlled by the surface chemistry of ACs,and the main adsorption mechanisms were electrostatic attraction,cation exchange and surface ccomplexation.(4)The Ni(?)adsorption ability was improved by a simplified modification of lotus stalk-based AC with pentaerythritol during phosphoric acid activation.Pentaerythritol promoted the formation of organic phosphates,and hence enhanced the creation of functional groups for AC.These reactions resulted in the formation of highly cross-linked structure,and then dramatically decreased the porosity of the resulting AC.Nevertheless,the surface oxygen atomic content and functional groups content of AC after modification were improved 80%.Under three ionic strengths(0,100,and 1000 mM NaCl),the Ni(?)adsorption capacities of the modified AC were improved 30-60%.It was also indicated that Ni(?)adsorption onto ACs was controlled by chemical adsorption,and the major mechanisms were cation exchange,electrostatic attraction and surface complexation.(5)The Cr(?)removal performance was enhanced by in situ modification of Zizania caduciflora based-AC with tartaric acid during phosphoric acid activation and functional groups blocking.The optimized additive amount of tartaric acid was 30 mmol/10 g precursor.ACs with and without modification had slightly differences in porosities,but the modified AC contained 60%more surface functional groups,and had 20-30%higher Cr(?)removal efficiency than original AC.After reducing the electrostatic attraction between the Cr(?)and ACs' surfaces by blocking their carboxyl and hydroxyl groups,the Cr(?)removal was further enhanced 10-15%.About 80%Cr adsorbed on the carbon's surface was present as Cr(?),which indicated that the mechanisms for Cr(?)removal by ACs were "Cr(?)adsorption,Cr(?)reduction and Cr(?)adsorption".