| Soil organic matter is one of the most important soil components. It has a profound effect onthe physical, chemical and biological functions of soil, including aggregate stability, water-holdingcapacity, cation exchange capacity, nutrient retention and release, etc. Humic substances representthe largest constituent (60-80%) of soil organic matter, responsible for many complex chemicalreactions in soil. One of the most striking features of humic substances is their strong interactionwith metal ions. The adsorption of metal ions on humic substances has long been known to playan important role in controlling the behavior and fate of trace metals in the environment. Aimingat a better understanding of the structural characteristics of humic substances, and its mechanismof interactions with metal cations, this paper researched the adsorption of Cu2+on different soilsand humic substances derived from different soils and organic matters. The results as below:1、Adsorption process of Cu2+on phaepzem, chernozem and dark-brown soil were studied bythe batch equilibrium method under different pH, ionic strength, temperature and contact time.This analysis provides practical theoretic data and foundation on Cu2+application and pollutionprevention. The results showed that three types of soil adsorption of Cu2+increased with theincrease of pH. However, Cu2+adsorption decreased with increasing ionic strength. Withtemperature and initial Cu2+concentration increased in the solution, the absorption ratio of Cu2+gained in all samples.Phaepzem fits the Freundlich formula better while chernozem anddark-brown soil fit the Langmuir model best. Thermodynamic parameters showed that theadsorption of Cu2+on three types of soil is spontaneous, endothermic and increasingly disorderedprocess. The adsorption kinetics of three types of soil is similar. Amounts of absorbed Cu2+rapidincreased at the beginning of adsorption, and the rate of increase slowed down with the increasingof time and got to a equilibrium finally. The adsorption kinetics of Cu2+on phaepzem andchernozem can well described by Elovich formula while parabolic diffusion equation fitdark-brown soil better.2、Results of the elemental analysis and solid-state13C CPMAS NMR techniques for thestructural characteristics of bulk humin and its AS-humin and AIS-humin fractions fromphaepzem and chernozem are different.Among the three humic components, bulk humin was themost aliphatic and most hydrophobic, AS-humin was the least aliphatic, and AIS-humin was theleast alkylated. The differences in the structural characteristics showed that the functions of thesehumic components in the soil environment differed. Further studies on the basis of low ash level should be conducted.3、Results of the adsorption of Cu2+on humin and humic acid derived from phaepzemshowed that chemical compositions of humin differed from those of corresponding humic acid.The former was more humified, aliphatic, hydrophobic, and less polar compared with the latter.The adsorption amounts of humin for Cu2+were lower than that of humic acid. The maximumadsorption capacities of Cu2+on humin and humic acid were227.3and243.9mg g-1at298K,respectively. The adsorption data could be better described Freundlich equation with respect toequation. The increase in temperature had a positive effect on the sorption process. The adsorptionof Cu2+on humin and humic acid was a spontaneous, endothermic, and increasingly disorderedprocess. Both humin and humic acid had energetically heterogeneous surfaces for the adsorptionof Cu2+, and the surface energy heterogeneity of humic acid was greater than that of humin.4、The structural and Cu2+adsorption/desorption characteristics of humin from forest soilwere different from those of humic acid. Compared with humic acid, humin contained more alkylC, carbohydrate C and phenolic C, and less methoxyl C, aryl C and carbonyl C. Humin exhibited alower adsorption capacity and a higher adsorption reversibility for Cu2+compared with humic acid.Therefore, humin plays important role in controlling the fate, transport and bioavailability of Cu2+in the environment.5、The adsorption of Cu2+from aqueous solution onto HA-like fractions isolated from cattlemanure (CHA), peat (PHA), and leaf litter (LHA) as a function of contact time, pH, ion strength,and initial concentration was studied using the batch method. Synchrotron-based X-ray absorptionspectroscopy (XAS) was used to examine the coordination environment of the Cu2+adsorbed byHA-like fractions on a molecular level. Moreover, the chemical compositions of the isolatedHA-like fractions were characterized by elemental analysis and solid-state13C nuclear magneticresonance (NMR) spectroscopy. The kinetic data showed that the adsorption equilibrium can beachieved within8h. The maximum adsorption capacities of2+on CHA, PHA, and LHA were229.4,210.4, and197.7mg g-1, respectively. The adsorption process followed thepseudo-second-order kinetics equation and fitted the Langmuir isotherm model well. Theadsorption of Cu2+on HA-like fractions increased with the increase in pH from2to7, andmaintained a high level at pH>7. The adsorption of Cu2+was also strongly influenced by the lowionic strength of0.01to0.2mol L-1NaNO3, but was weakly influenced by high ionic strength of0.4to1mol L-1NaNO3. The Cu2+adsorption on HA-like fractions may be mainly attributed to ionexchange and surface complexation. XAS results revealed that the binding site and oxidation stateof Cu adsorbed on HA-like fractions surface did not change at the initial Cu2+concentrations of15to40mg L-1. For all the Cu2+adsorption samples, each Cu atom was surrounded by4O/N atoms with Cu-O distance of1.95in the first coordination shell. However, a decreasing tendency forthe coordination number of Cu on HA-like fractions surface was observed with the increase ininitial Cu2+concentrations. The structural differences of the three HA-like fractions samples wereresponsible for their distinct Cu2+adsorption capacities. These results are important to achievebetter understanding of the behavior of Cu2+in soil and water bodies with the addition of organicmaterials. |
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