Effects Of Humic Substances On Heavy Metals Occurrence Mode In Contaminated Soil And The Growth Of Maize

With the rapid development of Chinese industry and agriculture,heavy metals have entered the soil through industrial"three wastes"emissions,excessive application of pesticides and fertilizers,and sewage irrigation,causing a series of environmental problems.In the remediation of heavy metal pollution in agricultural land,the addition of chemical amendments to the soil is a simple,quick,and cost-effective remediation technology.It can not only stabilize the heavy metals,but also ensure the economics production capacity of agricultural land.The purpose of this study was to investigate the effects of humic substance?HS?from different sources on the heavy metal species and the growth of maize in soils contaminated by heavy metals.Firstly,sodium hydroxide?NaOH?was used to extract soluble humic substance from peat soil?PHS?and soluble humic substance from weathered lignite?LHS?.And elemental analysis,fourier transform infrared spectroscopy?FTIR?and¹³C nuclear magnetic resonance spectroscopy(¹³C NMR)were used to analyze the chemical composition and functional group of the two HS.Then,PHS and LHS at different concentrations?0,100,200,500,1000 mg/kg?were added to heavy metal-contaminated soils under the condition of indoor soil culture.And the effects of exogenous HS on the contents of effective heavy metals?Cd,Cu,Zn,Pb?extracted by diethyltriamine pentaacetic acid extraction method and the contents of different morphological forms extracted by Tessier continuous extraction method were investigated.Lastly,the effects of LHS and PHS on soil carbon and nitrogen contents,and on growth of maize and heavy metals absorption at different plant parts were studied through pot experiments.The results showed as follows:1.Two HS contained similar functional groups but differ in functional group content.Both of them contained more aliphatic carbon chains and had less aromatic carbon structure.The aromatic carbon functional group occupies a dominant position in LHS,resulting in its molecules having higher aromaticity and hydrophobicity than PHS;while the functional groups in PHS were more balanced and the most abundant functional group was carbon oxidation functional group.2.Compared with the control,soil pH was significantly increased with various PHS and LHS concentrations.Exogenous addition of LHS and PHS at different concentrations reduced the bioavailable levels of Cd,Cu,Zn,Pb in soil with some extent.At the same concentration,PHS had better passivation effect on heavy metals in soil.In addition,the application of exogenous HS caused changes in the morphology of soil Cd,Cu,Zn,and Pb.On the whole,exogenous HS addition significantly reduced the exchangeable content and carbonate bound state content of heavy metals,while the organic bound state content that was more difficult to be absorbed by plants was significantly increased.3.Both PHS and LHS significantly increased soil organic carbon and water soluble carbon contents.Comparing the variability of soil organic carbon to nitrogen ratio?TOC/TN?and water-soluble carbon to nitrogen ratio?WSOC/WSTN?,the latter showed higher variability.Furthermore,maize biomass was significantly increased when adding 200 mg/kg HS compared to the control.The effects of the two HS on the plant height and root length were different.PHS with more hydrophilic mainly affected the plant height,while LHS with more hydrophobic mainly promoted the root growth of maize.4.Compared with the control,when the application concentration of PHS was100 mg/kg,the content of Cd in the aboveground part of maize was significantly decreased;when the application concentration of PHS was 200 mg/kg,the contents of Cd,Cu,Pb in the underground part and the contents of Cd,Cu,Zn,Pb in the aboveground part of maize were significantly decreased.When the concentration of PHS was 500 mg/kg,the content of Zn in the underground part of maize was significantly reduced.Whereas when adding 200 mg/kg LHS,the contents of Zn and Pb in the underground part of maize were significantly reduced.When adding 500mg/kg LHS,the contents of Cd,Cu,Pb in the underground part and the contents of Cd and Zn in the aerial part of maize were significantly reduced.These showed that the content of heavy metals in different parts of maize varied due to the different concentrations of PHS and LHS.