Studies On The Hydrothermal Conversion Process Of Heavy-metal Highly-accumulated Plants

Remediating and safely utilizing heavy metal contaminated soil has become an urgent problem with the increasingly serious problem of the heavy metal pollution.Phytoremediation has been proved to be one of the main remediation approaches for heavy metal contaminated soil because of its eco-friendly,cost-effective and remediating in situ.However,this process produced a large amount of heavy-metal highly-accumulated biomass.Nowadays,the main technologies to deal with heavy-metal highly-accumulated plants involve incineration,ashing,composting,compaction landfill,pyrolysis and liquid extraction,etc.However,there are some disadvantages existed during the process:low utilization rate of biomass energy and separation rate of heavy metals,easy to cause the secondary pollution and so on.The hydrothermal conversion can convert biomass into bio-fuel and achieve the purpose of separating heavy metals from biomass at the same time.Additionally,the separation rate of heavy metals and utilization rate of biomass energy are high without secondary pollution.Therefore,it is of great importance to treat heavy-metal highly-accumulated plants using hydrothermal conversion,and its principle is to achieve the goal of the harmlessness,reduction and recycling.In this work,hydrothermal conversion of the heavy-metal highly-accumulated plants was systematically studied.Three representative plants,Sedum alfredii Hance of hyperaccumulator,Elsholtzia splendens of heavy metal tolerant plant and rice straw of heavy metal low-accumulation crop,were selected.Composition of raw materials,reaction process and product bio-oil were studied.The influences on the separation of heavy metals,the synthesis of bio-oil,reduction of biomass and general rules of hydrothermal conversion of heavy-metal highly-accumulated plants were investigated.As last,theoretical evidence and experimental basis for hydrothermal conversion of heavy-metal highly-accumulated plant were provided,and industrialization process was also promoted.The main contents are summarized as follows:Firstly,the process of hydrothermal conversion of Sedum alfredii Hance which can accumulate Cd,Cu,Pb,Zn was studied.The moisture content,element and biomass composition were analyzed.The influences of 14 additives with different alkalinity,concentration of H2O2,raw materials/solvent ratio,reaction time and temperature on hydrothermal conversion of Sedum alfredii Hance were investigated.The results indicated that additives and H2O2 have no influence on the separation rate of heavy metals.Higher than 0.10 of raw materials/solvent ratio contributed to higher yield of heavy metals in solid phase.The yields of Cd,Cu,Pb and Zn in solid phase were 90±4%,95 ±5%,100 ± 6%and 80 ±6%,respectively,at 320 ? for 1 h with the 0.15 of raw materials/solvent ratio.The mass of plant was reduced to 18.8%(1.9%of the fresh plant).21.5%yield of bio-oil was also achieved with a high calorific value of 38.1 MJ/kg.The efficient conversion of hyperaccumulator was realized with the characteristic of recycling,harmlessness and reduction.And a two-step process combining liquid extraction and hydrothermal conversion was proposed based on the analysis of extraction efficiency,mass distribution and composition of production in solid and liquid phase after extraction by Na2EDTA.Secondly,the process of hydrothermal conversion of Elsholtzia splendens which can accumulate Cu was studied.The moisture content,element and biomass composition were analyzed.The influences of 2 additives(KOH,K2CO3),raw materials/solvent ratio,reaction time and temperature on hydrothermal conversion of Elsholtzia splendens were investigated.The results indicated that additives have no influence on the separation rate of Cu.Higher than 0.15 of raw material to/solvent ratio contributed to higher yield of Cu in solid phase.The yield of Cu was 95±2%at 320 ? for 20 min with the 0.15 of raw materials/solvent ratio.The mass of plant was reduced to 17.5%(13.8%of the fresh plant).25.0%yield of bio-oil was also achieved with a high calorific value of 35.2 MJ/kg.The efficient conversion of tolerant plant was realized with the characteristic of recycling,harmlessness and reduction.Thirdly,the process of hydrothermal conversion of rice straw,which is a kind of Cd low-accumulation crop was studied The moisture content,element and biomass composition were analyzed.The influences of 14 additives with different alkalinity,raw materials/solvent ratio,reaction time and temperature on hydrothermal conversion of rice straw were investigated.The results indicated that almost all Cd concentrated in solid phase without additives.Higher raw materials/solvent ratio contributed to higher yield of Cd in solid phase.All Cd concentrated in solid phase at 290 ? for 1 h with the 0.15 of raw materials/solvent ratio.The mass of plant was reduced to 22.5%(21.5%of the fresh plant).22.0%yield of bio-oil was also achieved with a high calorific value of 36.7 MJ/kg.The efficient conversion of low-accumulation crop was realized with the characteristic of recycling,harmlessness and reduction.Finally,the mechanism of the separation of heavy metals during hydrothermal conversion of heavy-metal highly-accumulated plants was studied.Using reducing monosaccharide(glucose,fructose,xylose)and the non-hyperaccumulating eotype Sedum alfredii Hance,and Cd,Cu,Pb and Zn nitrate solution as reaction medium,the influences of model materials,mole ratios of model materials/heavy metal ions,reaction time and times of adding model materials were investigated.The results indicated that the conversions of heavy metal ions presented the rule of Cu>Pb>Cd>Zn,which was consistent with the oxidation order of the metal ions.Conversions of heavy metal ions increased gradually as the mole ratios of model materials/metal ions increased when using reducing monosaccharide,more than 90%of heavy metal ions were converted when the ratio was up to 600.The non-hyperaccumulating eotype Sedum alfredii Hance has always been able to convert metal ions completely.Conversion of metal ions increased gradually as the extension of reaction time,99%of the heavy metal ions were converted after 8 h,which was consistent with the results of hydrothermal conversion of heavy-metal highly-accumulated plants.Moreover,conversions of the metal ions increased with the increasing of the times of adding the model materials,more than 96%of the heavy metal ions were converted after adding 3 times of model materials.Based on the results,the mechanism of separation of heavy metals was inferred:Firstly,the coordination bonds between heavy metals and metal ligands broke down under near/super critical water,which converted the heavy metals into ions.Then,the heavy metal ions were reduced by the reducing agent in plants as well as reducing materials produced from the decomposition of the biomass into elemental state.As a result,the heavy metals concentrated in the solid residue.