Dynamic Interaction Of Rice Straw With Soil Minerals And The Effect On Copper Sorption

Soil organic matter and minerals are rarely present alone in natural systems,in which most of them are associated together to form organo-mineral composites.Straw returning is the main sources of soil organic matter in cropland soils,and the decomposition dynamics of straw applied to the soil are closely related to mineral composition.The interactions between soil organic matter and minerals not only affect the transformation of organic matter,but also have an important effect on the environmental chemical behavior of heavy metals in soils.In this study,rice straw and the selected five common soil minerals(montmorillonite,kaolinite,gibbsite,hematite and goethite)were used as materials to study the dynamical changes of materials in the process of rice straw decomposition and the organic-inorganic interactions during co-incubation with soil minerals.The materials(solid organic matter,humic acid and organo-mineral composites)formed during rice straw decomposition were characterized by various technologies including atomic force microscopy(AFM),scanning electron microscopy(SEM),acid-base potential titration,X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR),three-dimensional excitation-emission matrix fluorescence spectroscopy(3D-EEM)and solid-state 13 C nuclear magnetic resonance spectroscopy(NMR).The sorption of Cu on examined materials was investigated using a combination of batch sorption-desorption experiments,isothermal titration calorimetry(ITC)and X-ray photoelectron spectroscopy(XPS).The main results are as follows:1.During the period of rice straw decomposition from 1 to 12 months,the content of total organic carbon in the solid organic matter decreased significantly,which decreased from 351.7 g kg-1 to 301.3 g kg-1;while that of humus substances increased significantly,which increased from 49.7 g kg-1 to 74.9 g kg-1.No obvious change in the content of carboxyl was found,while that of phenolic group increased from 0.73 mmol g-1 to 1.07 mmol g-1.The specific surface area and the pore volume increased,while the average pore size decreased.The formed humus substances were mainly humic acid.Amorphous humic acid particles exhibited aggregation state.The contents of carboxyl and hydroxyl in humic acid gradually increased with increasing duration of the decomposition.The later-formed humic acid had higher degrees of oxidation and aromaticity,and increased particle dispersion compared to the early-formed humic acid.2.During the process of co-incubation with montmorillonite,kaolinite,gibbsite and goethite,the decomposition of rice straw was significantly promoted by the presence of goethite.After decomposition for 18 months,the highest maturity,the highest loss of organic carbon and humic acid predominated in humus substances were observed in the residues formed in the presence of goethite.The residues formed in the presence of montmorillonite,kaolinite and gibbsite showed relatively low maturity,relatively less loss of organic carbon,and no obvious difference among the three treatments.3.The humic acid formed in the presence of goethite(Go-HA)had smaller particle size,higher carboxyl and hydroxyl content,and higher degrees of oxidation and aromaticity than those of the humic acid formed in the absence of minerals(Ck-HA).While the humic acid formed in the presence of montmorillonite(M-HA),kaolinite(K-HA)and gibbsite(Gi-HA)showed large particle size and poor dispersibility,and the contents of carboxyl and hydroxyl,and the degrees of oxidation and aromaticity were lower than those of Ck-HA.4.The content of total organic carbon in organo-mineral composites increased in the order organo-gibbsite(O-G)composites < organo-kaolinite(O-K)composites < organo-hematite(O-H)composites < organo-montmorillonite(O-M)composites.With increasing interaction time from 1 to 12 months,the total organic carbon content increased in O-G composites,but decreased in the other three types of organo-mineral composites.The amount of humus substances increased gradually in each type of organo-mineral composite,and humic acid was the predominant fraction.Aliphatic carbon was the mainly form of organic carbon in O-M,O-K and O-G composites,the aromaticity of organic carbon in O-M composites tended to decrease while that in O-K and O-G composites tended to increase with increasing interaction time.The specific surface area and the total concentration of surface active sites of O-M composites were lower than those of pure montmorillonite,while the other three types of organo-mineral composites had larger specific surface area and higher surface active sites compared to pure minerals.In addition,the crystalline mineral particles can be destroyed,dissolved and combined with organic matter during the process of co-incubation,which decreased the purity and the crystallinity of the mineral phase in organo-mineral composites.5.At p H 5.0,the sorption isotherms of Cu on solid organic matter,humic acid and organo-mineral composites were well fitted by the Langmuir equation,and the sorption kinetics were well described by the pseudo-second-order model.Moreover,the thermodynamic analysis indicated that Cu sorption was a spontaneous,endothermic process,and an increase in the randomness at the solid-solution interface.In the aspect of sorption capacity,the maximum amount of Cu sorption on solid organic matter and humic acid from rice straw decomposed for 1,3,6 and 12 months increased with increasing the duration of straw decomposition.The maximum amount of Cu sorption on O-M composites was lower than that on pure montmorillonite,and the sorption capacity of later-formed O-M composites tended to decrease;while the maximum amount of Cu sorption on other three types of organo-mineral composites were higher than that on pure minerals,and the sorption capacity of later-formed composites tended to increase.The maximum amount of Cu sorption on humic acid from rice straw decomposition in the absence and presence of different minerals increased in the order M-HA < K-HA < Gi-HA < Ck-HA < Go-HA.In the aspect of sorption rate,the sorption rates of Cu on solid organic matter and humic acid from different stages of decomposition showed the opposite trend,in which that on solid organic matter tended to decrease while that on humic acid tended to increase with increasing decomposition time.The sorption rates of Cu on organo-mineral composites were lower than that on pure minerals.6.At p H 5.0,the sorption of Cu on solid organic matter was the consequence of ion exchange and surface complexation,and the contribution of surface complexation to Cu sorption in later-formed solid organic matter was higher than that in early-formed solid organic matter.The sorption of Cu on humic acid was mainly through the complexation with the carboxyl groups in humic acid to form inner-sphere complexes.The sorption of Cu on organo-mineral composites was the result of joint action of non-specific sorption and specific sorption,in which the proportions of non-specific sorption were higher in O-M and O-K composites,while those of specific sorption were higher in O-G and O-H composites.