Study On The Formation Of Humic Substances And The Change Of Heavy Metals Fractions During Co-composting Of Agricultural Waste And Sediment

In recent years,a large amount of substandard industrial wastewater and industrial waste discharge,heavy metal mining and smelting,excessive application of pesticides and fertilizers have caused the heavy metal pollution in Xiangjiang River Sediment increasingly serious.Heavy metal pollution has attracted great attention due to its non-degradability,bioaccumulation and toxicity.As a promising bioremediation technology,composting has been successfully and widely used to remediate the soil which contaminated with chlorophenols,petroleum,pesticides,polycyclic aromatic hydrocarbons,and especially for heavy metals contaminated soil.The traditional composting way has a long composting period,low degradation efficiency of organic matter in agricultural waste,and unsatisfactory passivation effect on heavy metals.Therefore,it is of great importance to optimize the composting process to improve the passivation effect of heavy metals and compost quality.In addition,humic substances,the product of organic matter degradation during composting,can enhance composting quality and soil fertility and has a chelating ability on heavy metals.Therefore,a comprehensive understanding of the relationship between humic substances and the bioavailability of heavy metal can provide some useful information and guidance for improving the passivation effect of composting on heavy metals.Therefore,in this paper,a series of studies about the influence of Phanerochaete Chrysosporium(P.Chrysosporium)inoculation on the distribution of heavy metals(Pb,Cd,Cu and Zn)during co-composting of agricultural waste and river sediment were carried out.The passivation of P.Chrysosporium inoculation on different heavy metals was clarified,the relationship between humic substances and the passivation of heavy metal was preliminarily revealed,and the contribution of different humic substances components to heavy metal passivation was quantitatively.Besides,in order to further improve the passivation of composting on heavy metals by promoting the humic substances formation and humification degree,a series of optimizations were carried out on the formation mode of humus,and the influence of humus precursors on humus synthesis was explored.In addition,different physico-chemical factors have different effects on the bioavailability of different heavy metals in composting process,and different heavy metals also have different responses to the same physico-chemical factor due to their different properties.Therefore,this study also employs the way of multivariate statistical analysis to explore the relationship between the bioavailability of different heavy metals and physico-chemical factors during composting process,and to determine the types of significant factors that affecting the bioavailability of heavy metals(Pb,CD,Cu and Zn),and further to measure the explanation for the variation of each heavy metals by each significant factor without interference of other factors.These studies can provide some useful information and theoretical guidance for optimizing composting process to repair different heavy metals polluted soil or sediments by regulating physical and chemical factors.The specific research works and main results of this paper include the following five aspects:(1)P.Chrysosporium was inoculated into the co-composting of agricultural waste and river sediment,and to study its effects on the bioavailability of heavy metals(Pb,CD,Cu and Zn).The results showed that composting had a certain passivation on the four heavy metals,and followed the order:Cd>Zn?Cu>Pb.While the inoculation of P.Chrysosporium was greatly enhanced the passivation on Cu,Pb and Cd during 60days co-composting.The effects in the three metals followed the order:Cu>Cd>Pb.There were no differences for Zn whether inoculation with P.chrysosporium or not.This may be due to the different passivation mechanisms of the four heavy metals.It may also be related to the fact that one of the original materials for composting was Xiangjiang River sediment that contaminated by various heavy metals,which makes the composting environment very complex.The results of redundancy analysis(RDA)implied that more than 4/5 of the variation of all forms data for each heavy metal was explained by all significant canonical axes,and the lowest was also 94.6%.The inoculation of P.chrysosporium could change the significant parameters for each heavy metal and the RDA model explanatory power was also enhanced.In addition,organic matter(OM)plays a predominant advantage in the passivation of heavy metals.The inoculation of P.chrysosporium can weakens the effect of p H,but strengthens the effect of dehydrogenase activity(DA).That may be due to the inoculated with P.chrysosporium mainly affected the transformation of organic materials by influencing the microorganism's activity,and then enables the passivation of heavy metals mainly through the chelation affected by organic humus rather than the chemical precipitation affected by p H.Moreover,Pearson correlation analysis showed that the bioavailability factors(BF)was positively correlated with OM,while negatively correlated with p H.(2)The composting period was set at 256 days to further explore the passivation of heavy metal(Cd)during the long-term composting,and the effect of P.chrysosporium inoculation on the formation of humic substances(HS)and the passivation t of humic substances on Cd co-composting of agricultural waste and river sedimen were also studied.These results revealed that composting had a persistent passivation on the Cd in a long-term.In the first 60 days of composting,the RES-Cd increased by 16.77%and 17.83%in piles A(without inoculation)and B(with inoculation),respectively.From 60 days to 256 days of composting,the RES-Cd increased by only 7.45%and 9.21%in piles A and B,respectively.Therefore,an ideal and economical composting period was about 60 days in terms of time cost and operation benefit.The correlation analysis suggested that the Res-Cd in both pile A and B exhibited a significantly positive correlation with humic acid(HA)(P<0.01)and humic acids-bound Cd(HA-Cd),but negative correlation with fulvic acid(FA),and had no significant correlation with water-soluble Cd,which indicated that the contribution of humic substances to heavy metal passivation mainly came from the content of HA-Cd rather than the content of FA-Cd that contained in water-soluble Cd.In addition,there was a significant positive correlation between humification index and RES-Cd,which indicated that the higher humification degree was,the more conducive to the passivation of Cd.During the first 60 days of composting,the percentages of HA-Cd/T-Cd in piles A and B increased from 8.68%to 16.45%and 8.91%to 17.69%,respectively.Then they had a little drop to 14.99%and 16.56%,respectively.The slightly decrease of HA-Cd was likely due to the competitive between other heavy metals in sediments and Cd for the binding site of HA.FTIR spectroscopy and EEM fluorescence spectroscopy analysis demonstrated that the inoculation of P.chrysosporium could promote the chelating capability of HA to Cd and accelerating the maturity of compost.(3)Fenton-like system was established by using Fe₃O₄ nanomaterials(NMs)and P.chrysosporium or oxalic,and applied to the co-composting of agricultural waste and river sediment to explore its effect on the formation of humic substances and the bioavailability of heavy metals.The results revealed that the application of such Fenton-like system to composting increased the yield of·OH,accelerated the degradation of organic matter,enhanced the activity of enzyme and promoted the formation of HA.In addition,the application of Fenton-like process significantly promoted the passivation of Cd and Cu,while not shown obvious enhancement for Pb.Redundancy analyses(RDA)manifested that the application of such process enhanced the influence of humic substances on the bioavailability of Cd,Cu,and Pb,but weakened the effect of OM on the bioavailability of Cd,Cu,and Pb.This may be due to the humic substances generated in the degradation of OM played a major role in the passivation of heavy metals.In addition,EEM fluorescence analysis suggested that the application of Fenton-like process caused the red shift of peak B and peak C,which was correlated to the raising of hydroxyl,alkoxy,carbonyl,amino and carboxyl groups in the structure of HA fluorophore.This indicates that the application of Fenton-like process can enhance the stability of humic substances structure and the quality of compost products.This research provides a new perspective and way for composting to remediate heavy metals pollution.(4)The aim of this part was to investigate the effect of P.chrysosporium combined with Mn O₂ on humic substances formation and heavy metal forms during co-composting of rice straw and river sediments.The results showed that the application of P.chrysosporium combined with Mn O₂ to composting process could significantly improve the formation of HA and the passivation of heavy metal.This may be resulted from the inoculation of P.chrysosporium could accelerated the degradation of lignocellulose into simple small molecular organics,and Mn O₂ could not only act as an enzyme promoter to accelerate the degradation of lignocellulose into small molecular organics,but also catalyze the polymerization of low molecular weight substances such as carbonyl and amino groups to form HA,so as to promote the passivation of heavy metals.In the whole composting process,P.chrysosporium combined with Mn O₂ showed different passivation effects on Cd,Cu,and Zn.The passivation of Cu was mainly achieved by increasing the content of RES-Cu,while Cd and Zn were mainly through the decrease of EXC content,which may be related to the initial distribution of different heavy metal forms and the different affinity of heavy metals to HA.The results of RDA analysis showed that the inoculation of P.chrysosporium enhanced the effect of DA,while the effect of OM weakened,but the interaction between OM,p H,and DA was significantly improved.That may be resulted from the inoculation of P.chrysosporium could promoted the activity of microorganisms,and thus accelerated the degradation of OM.In addition,when Mn O₂or P.chrysosporium were added to the composting system alone,the effect of p H on the formation and transformation of humic subatances was strengthened.When the addition of Mn O₂ and the inoculation of P.chrysosporium were combined,the effect of p H on the formation of humic substances was weakened,but the effect of DA was strengthened,and the interaction between OM and DA was significantly promoted.(5)The effects of humic substances(sugars,amino acids and phenols)on the formation of humic substances and the bioavailability of heavy metals(Cd,Cu,Pb,and Zn)were investigated.The results showed that phenols had the greatest promoting effect on the formation of humic substances,especially for HA,while sugars had the least.That may be due to the majority of sugars were consumed by microorganisms as growth substrates,and only a small part was used to synthesize HA.However,phenols were difficult to decompose because its structure was more complex than that of sugars,so most of phenols were used to synthesize HA.These results indicated that there was a competitive relationship between the growths of microorganisms and HA formation for the utilization of small molecule organic compounds(such as carbohydrate,amino acids,carbonyls and amines)produced by the degradation of organic matter during composting.However,sugars had the best passivation effect on heavy metals(Cd,Cu,Pb and Zn),followed by phenols and amino acids.That may be resulted from the addition of glucose increases the substrate and energy for microorganisms during composting,thus promoting the growth of microorganisms.In turn,microorganisms improve the passivation of heavy metals(Cd,Cu,Pb and Zn)through the extracellular adsorption,intracellular accumulation,and the formation of extracellular polymer.In addition,the results of the elemental composition and atomic ratios of HA showed that the addition of HS precursor increased the C/H atomic ratio,but decreased the C/N atomic ratio.These results suggested that the addition of HS precursor promoted the chemical stability of HA in the final compost product.