Whole Process Research On Ecological Remediation And Bioenergy Production

The human beings are facing the stress from resource,population and environment.The technological innovation and sustainable development are the common goal of all countries.The present study proposed the strategy of combining ecological remidation and bioenengy production.This study was carried out in the whole process from the responses of plants to heavy metal,nitrogen?N?and phosphorus?P?pollution in soil and water,to the bioenergy production from bioenergy crops and aquatic plants.Plant morphology and physiology were investigated to assess the impact of environmental stress on plant productivity,bioenergy potential of varied bioenergy crops,threshold of metal tolerance as well as the appilication predi ction.Moreover,the mechanisms of heavy metal on the anaerobic fermentation process were researched,so did the impact factors.The ojective of this study was to provide references for further managing the ecological remediation process,improving the utilization of lignocellulose and the efficiency of bioenergy production.The results showed that:?1?During the process of ecological remediation,plants were able to tolerate certain ranges of pollutants.The productivity was imapcted which further influenced the following bioenergy productin.1)During the entire growth period,the low concentrations of Cd enhanced the growth of energy plants while high concentrations inhibited the growth.All the five energy plants showed high potential of Cd tolerance with the tolerant thresholds of 30,10,50,10 and 10 mg/kg for maize,sweet sorghum,canola,wheat and oat,respectively.The main organ that Cd accumulated in the plants was root.Nevertheless,the high aerial biomass guaranteed the extraction efficiency of Cd.2)The morphology of aquactic plants showed responses to nitrogen and phosphorus pollution.The ability of plant adsorption varied with species.The leaf of cattail was able to adsorb nitrogen from water and resulted in higher leaf nitrogen contents and wider leaves and shoot.The growth of reed was not sensitive to nitrogen and phosphorus pollution.Leaves of reed contained higher nitrogen and phosphorus than shoot.The growth of lotus leaves was sensitive to nitrogen and phosphorus pollution.Its morphological parameters declined with the increase of total nitrogen concentrations.3)The plants accilimated to photoinhibition caused by ambient pressure.The productivity of plants varied a lot with the growth stages of plants.In order to accimilate photoinhibition,plants adjusted the growth of leaves as well as the structure of chloroplasts.The growth stages were brought forward or delayed by different stress,which caused the modification of starch contents and biomass.The impact of heavy metal was significant during the seedling stage,where the net photosynthetic rate?NPR?was found to be related with the initial Cd concentrations in the soil?R2=0.8511,p<0.05?.After the seedling stage,the plants showed acclimation to the Cd pollution.The NPR of reed was higher than that of cattail and was not siginificantly different between summer and autumn.However,the NPR of lotus leaves varied a lot in summer and autumn,and was cooresponding to the nitrogen and phosphorus concentrations in water.Before flowering stage,the contents of VS?6.33%TS?,cellulose?1.12%TS?and hemicelloluse?0.99%TS?of Arabidopsis plants were increased due to Cd pollution while the percentage of lignin?0.65%TS?was decreased,which would benefit for the followed anaerobic fermentation.The fresh maize straw was found to produce more biogas?130%?,better stability and shorter fermentation process than the dry maize straw,i.e.fresh maize straw was more suitable for biogas production than dry straw.?2?There is large potential of producing bioenergy from the biomass collected after ecological remediation.The bioreutilization process was impacted by heavy metal,pretreatment and temperature.1)Cd concentrations in the biomass in the certain range stimulated the biogas production.The threshold of Cd concentrations that thermophilic fermentation?55.0±1.0 °C?with mixed straw and cow dung as substrate can tolerate was 1 mg/L basing on the analysis of the relationship between the calculated Cd concentrations and biogas yields.Within the thresholds,the cumulative biogas yields of Cd contained groups were not less than the control group.2)Acid pretreatment increased the VS,cellulose,hemicellulose contents of the substrate,and then enhanced the degradation of cellulose and hemicellulose,increased the process stability and finally increased the biogas yields.3)Cd addition increased the peak daily biogas yields?245%?and prolonged the period of peak biogas stage.It also enhanced the efficiency of transferring CO2 into CH4.Cd addition reduced the ORP and increased the p H values in the fermenter,which increased the stability of fermentation system.Moreover,the degradation of cellulose was enhanced by increased acitivity of cellulase.This then resulted in the higher VFA concentrations,especially acetic acid?acetic acid > butyrate>propionic acid> valeric acid?,which guaranteed the supplement of precursor for CH4 production.The acitivites of dehydrogenase and co-enzyme F420 were increased as well.Eventually,Cd addition increased the yields?405%?and the percentage of CH4 in biogas?6%?.4)The main phases that temperature impacted on the fermentation were the acidification and acetification.Better hydrolysis efficiency at the beginning of fermentation,better balance and stability during the process were recorded during thermophilic fermentation,which then increased the biogas production.Lower temperature went against with the hydrolysis at the beginning and inhibited the acidification and acetocylation.?3?The aquatic biomass collected from ecological remediation process showed large bioenergy potential.Acid pretreatement,addition of Fe²⁺and Ni²⁺ improved the biodegradation and biogas production.1)The phosphoric acid pretreatment efficiently reduced the lignin content?9.54%?in the reed straw,destroyed the structure of lignocellulose.It then promoted the degradation of cellulose and hemicellulose during the fermentation process,enhanced the stability of fermentation and finally increased biogas production?610%?.2)Fe²⁺ addition increased the cumulative biogas yields by extending the peak period with high daily biogas yields.Meanwhile,the methane?CH 4?contents in the Fe²⁺ added groups were generally higher than the control group.The activity of dehydrogenase was enhanced during the start-up stage.The cellulase activity was increased in the earlier stage which increased the available organic matters for fermentation.The degradation of organic matters was improved,resulting in the exhaustion of COD and high biogas producing efficiency.3)Ni²⁺ addition increased the cumulative biogas yields by improving the efficiency of peak stage and the yields after peak stage.At the start-up stage of fermentation?4th day?,the activity of cellulase was enough for supporting the hydrolysis and providing the organic matters for acetification and methanogensis.The bigas yields were mainly depended on the e ffect of Ni²⁺ addition on dehydrogenase activity.After the start-up stage,the impact of Ni²⁺ addition on biogas production was mainly dependent on its effect on cellulase activities.The present study was conducted with the concept of whole process control and combined the bioenergy production with environmental pollution control.This study investigated the possibility of producing bioenergy from heavy metal contaminated land and nitrogen and phosphorus polluted water.Meanwhile,this study provided references for harvesting time of the biomass and the threshold of heavy metal ion.The mechanisms at different fermentation stages and impact factors gave information for further study.The results of this work provided new insights for future research on bioenergy production and solving the energy crisis problems.