Research On Ecological Restoration Of Heavy Metals And Anaerobic Fermentation Resource Utilization Of Alfalfa

Aiming at the problems of the current serious heavy metal pollution and energy shortage,this study was conducted using system biology approach.In this study,alfalfa was used for phytoremediation to remove heavy metals in a mining area.The effects of activated carbon addition on the biogas production and mechanism of anaerobic fermentation with cow manure and alfalfa were analyzed.The risk of the organic fertilizers produced by fermentation as well as the effects of fertilizers on plant growth was evaluated.The results are as follows:（1）The concentrations of heavy metals in the soil of the surveyed mining area increased with mining activities.The content of Zn and the ecological risk of Cd were highest among the heavy metals.Alfalfa accumulated most species of the metals.The enriched metals were mainly concentrated in the aerial part of plant.The accumulation of Zn by alfalfa was higher than other metals while the accumulation of Cd was lowest.The absorbed content of Fe,Cu and Ni by alfalfa was positively correlated to their contents in the soil.Alfalfa was suitable for heavy metal remediation.（2）The cumulative biogas yield of anaerobic fermentation with alfalfa and cow manure was 47.16 mL/gTS.The addition of activated carbon significantly increased the cumulative biogas yield to 108.04 mL/gTS.The activated carbon increased the content and stability of CH4 while inhibited the production of NH4+-N.It promoted the production of COD and VFA,which caused the decrease of pH.The surface structure of activated carbon was conducive to the growth of microorganisms.Prevotella₇,Bacteroides,Ruminococcus₁ were the dominant bacterial genera in the fermenter.Methanobacterium and Methanosarcina were the dominant archaeal genera.The microorganisms on the activated carbon accelerated the decomposition of hemicellulose and increased the yields of VFA.Direct interspecies electron transfer composed of Petrimonas and Methanosarcina on activated carbon effectively improved the CH4 production.The adsorption capacity of activated carbon increased with the fermentation progressed.The adsorption of Fe was more than other metals.pH effected the metals speciations.The surface of activated carbon was rich in pore structure and was covered by a large amount of microorganisms after anaerobic fermentation.A large number of carboxylic acid group was detected on the activated carbon.The infrared peak of the organic matter was strong at the beginning of the fermentation and weakened with the fermentation progressed.（3）The contents of heavy metals in the alfalfa fermented organic fertilizers did not exceed the standard level.The content of Zn with the addition of activated carbon exceeded the standard level.However,Zn was mainly in the residual fraction,whose risk of pollution was reduced.Due to the low content of nitrogen in the raw materials,the total nutrition of the biogas fertilizers were inadequate too.Fertilization of biogas residue from alfalfa promoted the growth of maize seedlings,while soaking seeds in biogas slurry inhibited maize germination slightly.The acceleration by biogas residue from cow manure,wine residue and maize&wine groups was significant higher than other groups.This study investigated the phytoremediation feasibility and resourceful utilization of alfalfa comprehensively.The results showed that alfalfa achieved the goals of heavy metal removal and energy production efficiently during in the whole process.Alfalfa used for phytoremediation was able to adsorb most species of metals except Cd.The anaerobic fermentation with metal contaminated alfalfa and cow manure was characterized by considerable biogas production capacity.The biogas production was further strengthened by adding activated carbon.Activated carbon effectively promoted the degradation of the substrate,stabilized the metal content in the fermentation system and provided an adhesion environment for the growth of microorganisms.The biogas residues from fermentation improved the growth of maize seedlings.This study provided a comprehensive solution for the treatment of soil heavy metals and the clean energy production.It provided a theoretical basis for optimizing the anaerobic fermentation process and exploring the fermentation mechanism.