Research On The Passivation Mechanism Of Heavy Metals During Pig Manure Composting By Adding Green Synthesis Of Iron Nanoparticles

China has the largest pig farm in the world,producing about 4 billion tons of pig manure every year.Manure has a huge impact on the health of pigs,but also causes serious heavy metals and other environmental safety problems when used or discarded directly.Green synthesis of iron nanoparticles(G-n Fe)was prepared with green tea extract as reducing agent.Then,G-n Fe was applied to the aerobic compost of pig manure to study the influence of G-n Fe on the process of compost and the passivation of heavy metals.Finally,the passivated compost was tested in pot experiments.The main research conclusions are as follows.(1)The effect of G-n Fe on aerobic composting of pig manure was studied.We found that the addition of G-n Fe promoted the process of thermostatic aerobic composting of pig manure.When the concentration of G-n Fe was 200 m L/kg,the highest degradation rate of VS reached 20.1 %.The addition of G-n Fe promoted the maturity and the growth of humus in compost.Meanwhile,G-n Fe also promotes the release of available phosphorus from pig manure compost,accompanied by phosphate production.High-throughput analysis showed that the microbial diversity and community richness of the compost was the best when the addition amount of G-n Fe was 200 m L/kg.G-n Fe increased the microbial community richness of acid-producing bacteria Clostridium sensu stricto?1 and Turicibacter throughout the compost,confirming that the highest levels of organic degradation occur in compost.(2)The aerobic composting of pig manure with Giant Bacillus was conducted,and it was found that the addition of G-n Fe promoted the fermentation process of composting microorganisms.High-throughput DNA sequencing found that G-n Fe could improve the microbial diversity of compost,and G-n Fe had a certain promoting effect on the growth of Bacillus,with the best effect when the concentration was 500 m L/kg.when the addition of G-n Fe was 500 m L/kg,the increase of available phosphorus was the highest,reaching 36.5%.At the same time FTIR analysis also found that the increase of humus in composting process is obvious.(3)The passivation of heavy metals copper(Cu)and cadmium(Cd)by adding G-n Fe in the aerobic composting process of pig manure was studied.Morphological analysis of five-step extraction method(FSEM)showed that G-n Fe promoted the increase of organic bound and residuals of heavy metals during pig manure composting.When the concentration of G-n Fe was 200 m L/kg,the highest increase was 469.88 % for OM-Cu and 3329.84 % for RES-Cu.When G-n Fe concentrations were 500 m L/kg,RES-Cd increased from 0.0016 % to 55.7 %.Correspondingly,EXC-Cu?CAR-Cu ?EXC-Cd and CAR-Cd decreased significantly during composting.The residue samples were analyzed by XPS,XRD,SEM-mapping and FTIR.The results show that the passivation of copper and cadmium in compost is highly correlated with the content of phosphorus,and the phosphorus in the residue basically exists in the form of phosphate.Combined with the increased concentration of available phosphorus,Cu and Cd in the residue state is most likely to be passivated in the form of phosphate precipitation or phosphate coprecipitation.(4)Pot experiments were carried out to verify the passivated compost products,and it was found that the enrichment of heavy metals Cu,Cd and Zn in Chinese cabbage was the least when G-n Fe compost was applied.Pot experiments proved that the passivating effect of heavy metals in G-n Fe compost products reduced the plant toxicity of heavy metals.When the addition amount of G-n Fe is 200 m L/kg,the passivation effect was the best,and the content of heavy metals in roots,stems and leaves was the lowest.